ln 4th sem.pdf - Government Polytechnic Nabarangpur

AANNAALLOOGG EELLEECCTTRROONNIICCSS

AANNDD

OOPPAAMMPP

4th SEM ELECTRICAL ENGG.

Under SCTE&VT, Odisha

CHAPTER - 1 [ PAGE – 1. 1 ]

[P-N JUNCTION DIODE ]

DEFINITION:-

When a p-type semiconductor is suitably joined to n-type semiconductor, the contact

surface is called p-n Junction.

FORMATION OF PN JUNCTION

In actual practice, the characteristic properties of PN junction will not be apparent if a p-

type block is just brought in contact with n-type block.

It is fabricated by special techniques and one common method of making PN junction is

called Alloying.

In this method, a small block of indium (trivalent impurity) is placed on an n-type

germanium slab as shown in Fig (i). The system is then heated to a temperature of about

500ºC. The indium and some of the germanium melt to form a small puddle of molten

germanium-indium mixture as shown in Fig (ii). The temperature is then lowered and

puddle begins to solidify.

Under proper conditions, the atoms of indium impurity will be suitably adjusted in the

germanium slab to form a single crystal.

The addition of indium overcomes the excess of electrons in the n-type germanium to

such an extent that it creates a p-type region.

As the process goes on, the remaining molten mixture becomes increasingly rich in

indium. When all germanium has been re-deposited, the remaining material appears as

indium button which is frozen on to the outer surface of the crystallized portion as

shown in Fig (iii).

PROPERTIES OF PN JUNCTION

To explain PN junction, consider two types of materials: -

1) P-Type-P-type semiconductor having –ive acceptor ions and +ive charged holes.

2) N-Type -N-type semiconductor having +ive donor ions and –ive free electrons.

P-type has high concentration of holes & N-type has high concentration of electrons.

The tendency for the free electron to diffuse over p-side and holes to n-side process is

called Diffusion.

[ PAGE – 1. 2 ]

When a free electron move across the junction from n-type to p-type, positive donor ions

are removed by the force of electrons. Hence positive charge is built on the n-side of the

junction. Similarly negative charge establish on p-side of the junction.

When sufficient no of donor and accepter ions gathered at the junction, further diffusion

is prevented.

Since +ive charge on n-side repel holes to cross from p-side to n-side, similarly –ive

charge on p-side repel free electrons to cross from n-type to p-type.

Thus a barrier is set up against further movement of charge carriers is hole or electrons.

This barrier is called as Potential Barrier/ Junction Barrier (V0) and is of the order 0.1

to 0.3 volt. This prevents the respective majority carriers for crossing the barrier region.

This region is known as Depletion Layer.

The term depletion is due to the fact that near the junction, the region is depleted (i.e.

emptied) of charge carries (free electrons and holes) due to diffusion across the junction.

It may be noted that depletion layer is formed very quickly and is very thin compared to

the n region and the p-region.

Once pn junction is formed and depletion layer created, the diffusion of free electrons

stops. In other words, the depletion region acts as a barrier to the further movement of

free electrons across the junction.

The positive and negative charges set up an electric field as shown in fig below.

The electric field is a barrier to the free electrons in the n-region.

There exists a potential difference across the depletion layer and is called barrier

potential (V0). The barrier potential of a p-n junction depends upon several factors

including the type of semiconductor material, the amount of doping and temperature.

The typical barrier potential is approximately: - For Si, V0 = 0.7 V, For Ge, V0 = 0.3 V.

PN JUNCTION UNDER FORWARD BIASING

When external D.C. voltage applied to the junction is in such a direction that it cancels

the potential barrier, thus permitting current flow, it is called Forward Biasing.

To apply forward bias, connect positive terminal of the battery to p-type and negative

terminal to n-type as shown in fig below.

[ PAGE – 1. 3 ]

The applied forward potential establishes an electric field which acts against the field due

to potential barrier. Therefore, the resultant field is weakened and the barrier height is

reduced at the junction.

As potential barrier voltage is very small (0.1 to 0.3 V), therefore, a small forward

voltage is sufficient to completely eliminate the barrier.

Once the potential barrier is eliminated by the forward voltage, junction resistance

becomes almost zero and a low resistance path is established for the entire circuit.

Therefore, current flows in the circuit. This is called Forward Current.

With forward bias to PN junction, the following points are worth noting :

(i) The potential barrier is reduced and at some forward voltage (0.1 to 0.3 V), it is

eliminated altogether.

(ii) The junction offers low resistance (called forward resistance, Rf) to current flow.

(iii) Current flows in the circuit due to the establishment of low resistance path. The

magnitude of current depends upon the applied forward voltage.

CURRENT FLOW IN A FORWARD BIASED PN JUNCTION:-

It is concluded that in n-type region, current is carried by free electrons whereas in p-type

region, it is carried by holes. However, in the external connecting wires, the current is

carried by free electrons.

PN JUNCTION UNDER REVERSE BIASING

When the external D.C. voltage applied to the junction is in such a direction that

potential barrier is increased, it is called Reverse Biasing.

To apply reverse bias, connect negative terminal of the battery to p-type and

positive terminal to n-type.

[ PAGE – 1. 4 ]

It is clear that applied reverse voltage establishes an electric field which acts in the same

direction as the field due to potential barrier.

Therefore, the resultant field at the junction is strengthened and the barrier height is

increased as shown in fig below .

The increased potential barrier prevents the flow of charge carriers across the junction.

Thus, a high resistance path is established for the entire circuit and hence the current

does not flow.

With reverse bias to PN junction, the following points are worth noting:

(i) The potential barrier is increased.

(ii) The junction offers very high resistance (Reverse Resistance Rr) to current flow.

(iii)No current flows in the circuit due to the establishment of high resistance path.

VOLT-AMPERE CHARACTERISTICS OF PN JUNCTION:-

Volt-ampere or V-I characteristic of a pn junction (also called a crystal or semiconductor

diode) is the curve between voltage across the junction and the circuit current.

Usually, voltage is taken along x-axis and current along y-axis. Fig. shows the circuit

arrangement for determining the V-I characteristics of a pn junction.

The characteristics can be studied under three heads namely:

1) Zero external voltage

2) Forward Bias

3) Reverse Bias.

ZERO EXTERNAL VOLTAGE: -

When the external voltage is zero, i.e. circuit is open at K; the potential barrier at the

junction does not permit current flow.

Therefore, the circuit current is zero as indicated by point O in Fig.

(II) FORWARD BIAS: -

[ PAGE – 1. 5 ]

With forward bias to the pn junction i.e. p-type connected to positive terminal and n-type

connected to negative terminal, the potential barrier is reduced.

At some forward voltage (0.7 V for Si and 0.3 V for Ge), the potential barrier is

altogether eliminated and current starts flowing in the circuit.

From now onwards, the current increases with the increase in forward voltage.

Thus, a rising curve OB is obtained with forward bias as shown in Fig. From the forward

characteristic, it is seen that at first (region OA), the current increases very slowly and

the curve is non-linear.

It is because the external applied voltage is used up in overcoming the potential barrier.

However, once the external voltage exceeds the potential barrier voltage, the pn junction

behaves like an ordinary conductor.

Therefore, the current rises very sharply with increase in external voltage (region AB on

the curve). Here the curve is almost linear.

(III) REVERSE BIAS:-

With reverse bias to the pn junction i.e. p-type connected to negative terminal and n-type

connected to positive terminal, potential barrier at the junction is increased.

Therefore, the junction resistance becomes very high and practically no current flows

through the circuit.

However, in practice, a very small current (of the order of μA) flows in the circuit with

reverse bias as shown in the reverse characteristic.

This is called Reverse Saturation Current (Is) and is due to the minority carriers.

It may be recalled that there are a few free electrons in p-type material and a few holes in

n-type material.

These undesirable free electrons in p-type and holes in n-type are called minority

carriers. Therefore, a small current flows in the reverse direction.

[ PAGE – 1. 6 ]

If reverse voltage is increased continuously, the kinetic energy of electrons (minority

carriers) may become high enough to knock out electrons from the semiconductor atoms.

At this stage breakdown of the junction occurs, characterized by a sudden rise of reverse

current and a sudden fall of the resistance of barrier region. This may destroy the

junction permanently.

Note: -The forward current through a p-n junction is due to the majority carriers

produced by the impurity.

However, reverse current is due to the minority carriers produced due to breaking of

some covalent bonds at room temperature.

IMPORTANT TERMS: -

(i) BREAKDOWN VOLTAGE: - It is the minimum reverse voltage at which pn junction

breaks down with sudden rise in reverse current.

(ii) KNEE VOLTAGE: - It is the forward voltage at which the current through the junction

starts to increase rapidly.

(iii) PEAK INVERSE VOLTAGE (PIV):- It is the maximum reverse voltage that can be

applied to the pn junction without damage to the junction. If the reverse voltage across the

junction exceeds its PIV, the junction may be destroyed due to excessive heat. The peak

inverse voltage is of particular importance in rectifier service.

(iv) MAXIMUM FORWARD CURRENT:- It is the highest instantaneous forward current

that a pn junction can conduct without damage to the junction. Manufacturer’s data sheet

usually specifies this rating. If the forward current in a pn junction is more than this rating,

the junction will be destroyed due to overheating.

(v) MAXIMUM POWER RATING: - It is the maximum power that can be dissipated at

the junction without damaging it. The power dissipated at the junction is equal to the

product of junction current and the voltage across the junction. This is a very important

consideration and is invariably specified by the manufacturer in the data sheet.

DC LOAD LINE:-

The line obtained by joining the maximum values of Ic and Vce in the output

characteristics of a CE configuration transistor is known as the DC Load Line.

PN JUNCTION BREAKDOWN:-

Electrical break down of semiconductor can occur due to two different phenomena.

Those two phenomena are

1. Zener breakdown

2. Avalanche breakdown

ZENER BREAKDOWN:-

A properly doped crystal diode which

has a sharp breakdown voltage is known

as a Zener Diode.

[ PAGE – 1. 7 ]

It has already been discussed that when the reverse bias on a crystal diode is increased, a

critical voltage, called Breakdown Voltage is reached where the reverse current increases

sharply to a high value.

The breakdown region is the knee of the reverse characteristic as shown in Figure.

The satisfactory explanation of this breakdown of the junction was first given by the

American scientist C. Zener.

The breakdown voltage is sometimes called Zener Voltage and the sudden increase in

current is known as Zener Current. The breakdown or Zener voltage depends upon the

amount of doping. If the diode is heavily doped, depletion layer will be thin and

consequently the breakdown of the junction will occur at a lower reverse voltage.

On the other hand, a lightly doped diode has a higher breakdown

voltage. Fig. shows the symbol of a Zener diode. It may be seen that it

is just like an ordinary diode except that the bar is turned into z-shape.

PROPERTIES OF ZENER DIODE:-

The following points may be noted about the Zener diode:

A Zener diode is like an ordinary diode except that it is properly doped to have a sharp

breakdown voltage. A Zener diode is always reverse connected i.e. it is always reverse

biased. A Zener diode has sharp breakdown voltage, called Zener voltage VZ.

When forward biased, its characteristics are just those of ordinary diode.

The Zener diode is not immediately burnt just because it has entered the breakdown

region. As long as the external circuit connected to the diode limits the diode current to

less than burn out value, the diode will not burn out.

Zener diode operated in this region will have a relatively constant voltage across it,

regardless of the value of current through the device. This permits the Zener diode to be

used as a Voltage Regulator.

WORKING/OPERATION OF ZENER BREAKDOWN:-

When the reverse voltage across the pn junction diode increases, the electric field across

the diode junction increases (both internal & external).

This results in a force of attraction on the negatively charged electrons at junction.

This force frees electrons from its covalent bond and moves those free electrons to

conduction band. When the electric field increases (with applied voltage), more and more

electrons are freed from its covalent bonds.

This results in drifting of electrons across the junction and electron hole recombination

occurs. So a net current is developed and it increases rapidly with increase in electric

field. Zener breakdown phenomena occurs in a pn junction diode with heavy doping &

thin junction (means depletion layer width is very small).

Zener breakdown does not result in damage of diode since current is only due to drifting

of electrons, there is a limit to the increase in current as well.

AVALANCHE BREAKDOWN:-

[ PAGE – 1. 8 ]

Avalanche breakdown occurs in a p-n junction diode which is moderately doped and has

a thick junction (means its depletion layer width is high).

Avalanche breakdown usually occurs when we apply a high reverse voltage across the

diode (obviously higher than the zener breakdown voltage,say Vz).

By increasing the applied reverse voltage, the electric field across junction will keep

increasing. If applied reverse voltage is Va and the depletion layer width is d, then the

generated electric field can be calculated as Ea =Va/d.

This generated electric field exerts a force on the electrons at junction and it frees them

from covalent bonds. These free electrons will gain acceleration and it will start moving

across the junction with high velocity.

This results in collision with other neighboring atoms. These collisions in high velocity

will generate further free electrons. These electrons will start drifting and electron-hole

pair recombination occurs across the junction. This results in net current which rapidly

increases.

From the above fig we can see that avalanche breakdown occurs at a voltage (Va) which

is higher than zener breakdown voltage (Vz).

It is because avalanche phenomena occurs in a diode which is moderately doped and

junction width (say d) is high where as zener break down occurs in a diode with heavy

doping and thin junction (here d is small).

The electric field that occur due to applied reverse voltage (say V) can be calculated as E

= V/d. So in a Zener breakdown, the electric field necessary to break electrons from

covalent bond is achieved with lesser voltage than in avalanche breakdown due to thin

depletion layer width.

[ PAGE – 1. 9 ]

In avalanche breakdown, the depletion layer width is higher and hence much more

reverse voltage has to be applied to develop the same electric field strength (necessary

enough to break electrons free).

CLIPPING CIRCUITS

The circuit with which the waveform is shaped by removing (or clipping) a portion of the

applied wave is known as a clipping circuit.

Clippers find extensive use in radar, digital and other electronic systems.

Although several clipping circuits have been developed to change the wave shape, we

concentrate only on diode clippers.

These clippers can remove signal voltages above or below a specified level.

The important diode clippers are:-

1. Positive clipper and negative clipper

2. Biased positive clipper and biased negative clipper

3. Combination clipper.

POSITIVE CLIPPER

A positive clipper is that which removes the positive half-cycles of the input voltage.

The positive clipper is of two types

1. Positive series clipper

2. Positive shunt clipper

The below Fig. shows the typical circuit of a positive shunt clipper using a diode.

Here the diode is kept in parallel with the load.

During the positive half cycle, the diode ‘D’ is forward biased and the diode acts as a

closed switch. This causes the diode to conduct heavily.

This causes the voltage drop across the diode or across the load resistance RL to be zero.

Thus output voltage during the positive half cycles is zero.

During the negative half cycles of the input signal voltage, the diode D is reverse biased

and behaves as an open switch. Consequently the entire input voltage appears across the

diode or across the load resistance RL if R is much smaller than RL

Actually the circuit behaves as a voltage divider with an output voltage of -[RL / R+ RL] Vmax ≅ -Vmax ( Taking or assuming when RL >> R).

NEGATIVE CLIPPER

[ PAGE – 1. 10 ]

A negative clipper is that which removes the positive half-cycles of the input voltage.

The negative clipper is of two types

1. Negative series clipper

2. Negative shunt clipper

The below Fig. shows the typical circuit of a negative shunt clipper using a diode.

During the negative half cycle, the diode ‘D’ is forward biased and the diode acts as a

closed switch. This causes the diode to conduct heavily.

This causes the voltage drop across the diode or across the load resistance RL to be zero.

Thus output voltage during the negative half cycles is zero.

During the positive half cycles of the input signal voltage, the diode D is reverse biased

and behaves as an open switch. Consequently the entire input voltage appears across the

diode or across the load resistance RL if R is much smaller than RL

Actually the circuit behaves as a voltage divider with an output voltage of [RL / R+ RL] Vmax ≅ Vmax ( Taking or assuming when RL >> R).

BIASED POSITIVE CLIPPER

When a small portion of the positive half cycle is to be removed, it is called a biased

positive clipper. The circuit diagram and waveform is shown in the figure below.

During negative half cycle, when the input signal voltage is negative, the diode ‘D’ is

reverse-biased. This causes it to act as an open-switch. Thus the entire negative half

cycle appears across the load, as illustrated by output waveform.

During positive half cycle, when the input signal voltage is positive but does not exceed

battery the voltage ‘V’, the diode ‘D’ remains reverse-biased and most of the input

voltage appears across the output.

[ PAGE – 1. 11 ]

When during the positive half cycle of input signal, the signal voltage becomes more

than the battery voltage V, the diode D is forward biased and so conducts heavily. The

output voltage is equal to ‘+ V’ and stays at ‘+ V’ as long as the magnitude of the input

signal voltage is greater than the magnitude of the battery voltage, ‘V’.

Thus a biased positive clipper removes input voltage when the input signal voltage

becomes greater than the battery voltage.

BIASED NEGATIVE CLIPPER

When a small portion of the negative half cycle is to be removed, it is called a biased

negative clipper. The circuit diagram and waveform is shown in the figure below.

During positive half cycle, when the input signal voltage is positive, the diode ‘D’ is

reverse-biased. This causes it to act as an open-switch. Thus the entire positive half cycle

appears across the load, as illustrated by output waveform.

During negative half cycle, when the input signal voltage is negative but does not exceed

battery the voltage ‘V’, the diode ‘D’ remains reverse-biased and most of the input

voltage appears across the output.

When during the negative half cycle of input signal, the signal voltage becomes more

than the battery voltage V, the diode D is forward biased and so conducts heavily. The

output voltage is equal to ‘-V’ and stays at ‘-V’ as long as the magnitude of the input

signal voltage is greater than the magnitude of the battery voltage, ‘V’.

Thus a biased negative clipper removes input voltage when the input signal voltage

becomes greater than the battery voltage.

COMBINATION CLIPPER:-

Combination clipper is employed when a portion of both positive and negative of each

half cycle of the input voltage is to be clipped (or removed) using a biased positive and

negative clipper together. The circuit for such a clipper is given in the figure below.

[ PAGE – 1. 12 ]

For positive input voltage signal when input voltage exceeds battery voltage +V1 diode

D1 conducts heavily while diode D2 is reversed biased and so voltage +V1 appears across

the output. This output voltage +V1 stays as long as input signal voltage exceeds +V1.

On the other hand for the negative input voltage signal, the diode D1 remains reverse

biased and diode D2 conducts heavily only when input voltage exceeds battery voltage

V2 in magnitude.

Thus during the negative half cycle the output stays at -V2 so long as the input signal

voltage is greater than -V2.

APPLICATIONS OF CLIPPER:-

There are numerous clipper applications however, in general, clippers are used to

perform one of the following two functions:

(i) CHANGING THE SHAPE OF WAVEFORM: - Clippers can alter the shape of a

waveform. For example, a clipper can be used to convert a sine wave into a rectangular

wave, square wave etc. They can limit either the negative or positive alternation or both

alternations of an a.c. voltage.

(ii) CIRCUIT TRANSIENT PROTECTION:- Transients can cause considerable

damage to many types of circuits e.g., a digital circuit. In that case, a clipper diode can be

used to prevent the transient form reaching that circuit.

CLAMPER CIRCUITS:-

A clamping circuit is used to place either the positive or negative peak of a signal at a

desired level. The dc component is simply added or subtracted to/from the input signal.

The clamper is also referred to as an IC restorer and ac signal level shifter.

A clamp circuit adds the positive or negative dc component to the input signal so as to

push it either on the positive side.

The clamper is of two types :-

1. Positive clamper

2. Negative clamper

The circuit will be called a positive clamper, when the signal is pushed upward side by

the circuit and the negative peak of the signal coincides with the zero level.

The circuit will be called a negative clamper, when the signal is pushed downward by the

circuit and the positive peak of the input signal coincides with the zero level.

[ PAGE – 1. 13 ]

For a clamping circuit at least three components — a diode, a capacitor and a resistor are

required. Sometimes an independent dc supply is also required to cause an additional

shift. The important points regarding clamping circuits are:

1. The shape of the waveform will be the same, but its level is shifted either upward or

downward,

2. There will be no change in the peak-to-peak or r.m.s value of the waveform due to the

clamping circuit. Thus, the input waveform and output waveform will have the same

peak-to-peak value that is, 2Vmax. This is shown in the figure above. It must also be noted

that same readings will be obtained in the ac voltmeter for the input voltage and the

clamped output voltage.

3. There will be a change in the peak and average values of the waveform. In the figure

shown above, the input waveform has a peak value of Vmax and average value over a

complete cycle is zero. The clamped output varies from 2 Vmax and 0 (or 0 and -2Vmax).

Thus the peak value of the clamped output is 2Vmax and average value is Vmax.

4. The values of the resistor R and capacitor C affect the waveform.

5. The values for the resistor R and capacitor C should be determined from the time

constant equation of the circuit, t = RC. The values must be large enough to make sure

that the voltage across capacitor C does not change significantly during the time interval

the diode is non-conducting. In a good clamper circuit, the circuit time constant t = RC

should be at least ten times the time period of the input signal voltage. It is advantageous

to first consider the condition under which the diode becomes forward biased.

POSITIVE CLAMPER:-

Consider a negative clamping circuit, a circuit that shifts the original signal in a vertical

downward direction.

The diode D will be forward biased and the capacitor C is charged with the polarity

shown, when an input signal is applied.

During the negative half cycle of input, the output voltage will be equal to the barrier

potential of the diode, V0 and capacitor is charged to (V – V0).

[ PAGE – 1. 14 ]

During the positive half cycle, the diode becomes reverse-biased and acts as an open-

circuit. Thus, there will be no effect on the capacitor voltage.

The resistance R, being of very high value, cannot discharge C a lot during the positive

portion of the input waveform.

Thus during positive input, the output voltage will be the sum of the input voltage and

capacitor voltage = +V + (V — V0) = +(2 V – V0) .

The value of the peak-to-peak output will be the difference of the negative and positive

peak voltage levels is equal to (2V-V0) - V0 = 2 V.

NEGATIVE CLAMPER:-

Consider a negative clamping circuit, a circuit that shifts the original signal in a vertical

downward direction.

The diode D will be forward biased and the capacitor C is charged with the polarity

shown, when an input signal is applied.

During the positive half cycle of input, the output voltage will be equal to the barrier

potential of the diode, V0 and capacitor is charged to (V – V0).

During the negative half cycle, the diode becomes reverse-biased and acts as an open-

circuit. Thus, there will be no effect on the capacitor voltage.

The resistance R, being of very high value, cannot discharge C a lot during the negative

portion of the input waveform.

Thus during negative input, the output voltage will be the sum of the input voltage and

capacitor voltage = – V – (V — V0) = – (2 V – V0) .

The value of the peak-to-peak output will be the difference of the negative and positive

peak voltage levels is equal to V0 - [-(2V-V0)] = 2 V.

APPLICATIONS OF CLAMPER:-

Clamping circuits are often used in television receivers as dc restorers in the TV receiver

They also find applications in storage counters, analog frequency meter, capacitance meter,

divider and stair-case waveform generator.

---------------- ALL THE BEST -------------------------- ALL THE BEST ----------------

CHAPTER - 2 [ PAGE – 1. 15 ]

------------------- [SPECIAL SEMICONDUCTOR DEVICES] ----------------------

THERMISTOR

Thermistor is the contraction of the term Thermal Resistor.

It is generally composed of semiconductor materials. Most thermistors have a negative

coefficient of temperature that is their resistance decreases with the increases of

temperature.

This high sensitivity to temperature changes makes thermistors extremely useful for

precision temperature measurement, control and compensation.

The temperature measurement of thermistor ranges from -60 0C to 150 0C and the

resistance of thermistor ranges from 0.5Ω to 0.75MΩ. It exhibits highly non-linear

characteristics of resistance versus temperature.

CONSTRUCTION

These thermistors are composed of sintered mixture of metallic oxides such as

Manganese, Nickel, Cobalt, Copper, Iron and Uranium.

These may be in the form of beads or rods or discs or probes.

The relation between resistance and absolute temperature of a thermistor can be

represented as

RT1=RT2 exp[β(1/T1)-(1/T2)]

Where RT1=resistance of thermistor at absolute temperature T1 K

RT2=resistance of thermistor at absolute temperature T2K

And β=a constant depending on the material of the thermistor (usually it

ranges from 3500 K to 4500 K).

FEATURES

These are compact, rugged and inexpensive and have good stability when properly aged.

Measuring current is maintained at a value as low as possible so that self-heating of

thermistors is avoided otherwise errors are introduced on account of changes of

resistance caused by self-heating.


------------------- [SPECIAL SEMICONDUCTOR DEVICES] ----------------------

THERMISTOR

Thermistor is the contraction of the term Thermal Resistor.

It is generally composed of semiconductor materials. Most thermistors have a negative

coefficient of temperature that is their resistance decreases with the increases of

temperature.

This high sensitivity to temperature changes makes thermistors extremely useful for

precision temperature measurement, control and compensation.

The temperature measurement of thermistor ranges from -60 0C to 150 0C and the

resistance of thermistor ranges from 0.5Ω to 0.75MΩ. It exhibits highly non-linear

characteristics of resistance versus temperature.

CONSTRUCTION

These thermistors are composed of sintered mixture of metallic oxides such as

Manganese, Nickel, Cobalt, Copper, Iron and Uranium.

These may be in the form of beads or rods or discs or probes.

The relation between resistance and absolute temperature of a thermistor can be

represented as

RT1=RT2 exp[β(1/T1)-(1/T2)]

Where RT1=resistance of thermistor at absolute temperature T1 K

RT2=resistance of thermistor at absolute temperature T2K

And β=a constant depending on the material of the thermistor (usually it

ranges from 3500 K to 4500 K).

FEATURES

These are compact, rugged and inexpensive and have good stability when properly aged.

Measuring current is maintained at a value as low as possible so that self-heating of

thermistors is avoided otherwise errors are introduced on account of changes of

resistance caused by self-heating.

[ PAGE – 2. 2 ]

The upper operating limit of temperature for thermistor is dependent on physical changes

in the material.

For thermistor the Response time can vary from fraction of second to minute depending

on the size of detecting mass and thermal capacity of the thermistor.

Response time varies inversely with dissipation factor.

APPLICATIONS

It is used for measurement and control of temperature and for temperature compensation.

It is used for measurement of power at high frequency. It is also used for thermal

conductivity.

Thermistor is used for measurement of level, flow and pressure of liquid, composition of

gases and vaccum measurement. It is used for providing time delay.

BARRETERS

Barreters are the short length wires with fine diameters with operating range around

1500C.

SENSORS

A sensor is a device that detects events or changes in quantities and provides a

corresponding output, generally as an electrical or optical signal; for example,

a thermocouple converts temperature to an output voltage.

Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps

which dim or brighten by touching the base, besides innumerable applications of which

most people are never aware.

With advances in micro machinery and easy to use microcontroller platforms, the uses of

sensors have expanded beyond the more traditional fields of temperature, pressure or

flow measurement.

Moreover, analog sensors such as potentiometers and force-sensing resistors are still

widely used. Applications include manufacturing and machinery, airplanes and

aerospace, cars, medicine and robotics.

A sensor's sensitivity indicates how much the sensor's output changes when the input

quantity being measured changes.

For instance, if the mercury in a thermometer moves 1 cm when the temperature changes

by 1 °C, the sensitivity is 1 cm/°C .

Sensors need to be designed to have a small effect on what is measured; making the

sensor smaller often improves this and may introduce other advantages.

Technological progress allows more and more sensors to be manufactured on

a microscopic scale as microsensors using MEMS technology.

In most cases, a microsensor reaches a significantly higher speed and sensitivity

compared with macroscopic approaches.

ZENER DIODE:-

[ PAGE – 2. 3 ]

A properly doped crystal diode which has a sharp breakdown voltage is known as a

Zener Diode.

It has already been discussed that when the reverse bias on a crystal diode is increased, a

critical voltage, called Breakdown Voltage is reached where the reverse current

increases sharply to a high value.

The breakdown region is the knee of the reverse characteristic as shown in Fig.

The satisfactory explanation of this breakdown of the junction was first given by the

American scientist C. Zener.

The breakdown voltage is sometimes called Zener Voltage and the sudden increase in

current is known as Zener Current.

The breakdown or Zener voltage depends upon the amount of doping. If the diode is

heavily doped, depletion layer will be thin and consequently the breakdown of the

junction will occur at a lower reverse voltage.

On the other hand, a lightly doped diode has a higher breakdown voltage.

The given figure shows the symbol of a Zener diode. It may be seen that it is just like an

ordinary diode except that the bar is turned into z-shape.

The following points may be noted about the Zener diode:

1. A Zener diode is like an ordinary diode except that it is properly doped to have a

sharp breakdown voltage.

2. A Zener diode is always reverse connected i.e. it is always reverse biased.

3. A Zener diode has sharp breakdown voltage, called Zener voltage VZ.

4. When forward biased, its characteristics are just those of ordinary diode.

5. The Zener diode is not immediately burnt just because it has entered the breakdown

region.

As long as the external circuit connected to the diode limits the diode current to less than

burn out value, the diode will not burn out.

[ PAGE – 2. 4 ]

Zener diode operated in this region will have a relatively constant voltage across it,

regardless of the value of current through the device. This permits the Zener diode to be

used as a Voltage Regulator.

TUNNEL DIODE:-

Under normal forward bias operation, as voltage begins to increase, electrons at first

tunnel through the very narrow p–n junction barrier because filled electron states in the

conduction band on the n-side become aligned with empty valence band hole states on

the p-side of the p-n junction.

As voltage increases further these states become more misaligned and the current drops –

this is called negative resistance because current decreases with increasing voltage.

As voltage increases yet further, the diode begins to operate as a normal diode, where

electrons travel by conduction across the p–n junction, and no longer by tunneling

through the p–n junction barrier.

The most important operating region for a tunnel diode is the negative resistance region.

When used in the reverse direction, tunnel diodes are called back diodes (or backward

diodes) and can act as fast rectifiers with zero offset voltage and extreme linearity for

power signals (they have an accurate square law characteristic in the reverse direction).

Under reverse bias, filled states on the p-side become increasingly aligned with empty

states on the n-side and electrons now tunnel through the pn junction barrier in reverse

direction.

In a conventional semiconductor diode, conduction takes place while the p–n junction is

forward biased and blocks current flow when the junction is reverse biased. This occurs

up to a point known as the “reverse breakdown voltage” when conduction begins (often

accompanied by destruction of the device).

In the tunnel diode, the dopant concentrations in the p and n layers are increased to the

point where the reverse breakdown voltage becomes zero and the diode conducts in the

reverse direction.

However, when forward-biased, an

odd effect occurs called quantum

mechanical tunnelling which gives

rise to a region where an increase in

forward voltage is accompanied by

a decrease in forward current.

In the current voltage characteristics

of tunnel diode, we can find a

negative slope region when forward

bias is applied.

[ PAGE – 2. 5 ]

Quantum mechanical tunneling is responsible for the phenomenon and thus this device is

named as tunnel diode.

The doping is very high so at absolute zero temperature the Fermi levels lies within the

bias of the semiconductors. When no bias is applied any current flows through the

junction.

PIN DIODE:-

The PIN diode can be shown diagrammatically as being a PN junction, but with an

intrinsic layer between the PN and layers.

The intrinsic layer of the PIN diode is a layer without doping, and as a result this

increases the size of the depletion region - the region between the P and N layers where

there are no majority carriers. This change in the structure gives the PIN diode its unique

properties.

Basic PIN diode structure

The PIN diode operates in exactly the same way as a normal diode.

The only real difference is that the depletion region, that normally exists between the P

and N regions in an unbiased or reverse biased diode is larger.

In any PN junction, the P region contains holes as it has been doped to ensure that it has

a predominance of holes.

Similarly the N region has been doped to contain excess electrons.

The region between the P and N regions contains no charge carriers as any holes or

electrons combine As the depletion region has no charge carriers it acts as an insulator.

Within a PIN diode the depletion region exists, but if the diode is forward biased, the

carriers enter the depletion region (including the intrinsic region) and as the two carrier

types meet, current starts to flow.

When the diode is forward biased, the carrier concentration, i.e. holes and electrons is

very much higher than the intrinsic level carrier concentration.

Due to this high level injection level, the electric field extends deeply (almost the entire

length) into the region.

This electric field helps in speeding up of the transport of charge carriers from p to n

region, which results in faster operation of the diode, making it a suitable device for high

frequency operations.

A PIN diode obeys the standard diode equation for low frequency signals.

[ PAGE – 2. 6 ]

At higher frequencies, the diode looks like an almost perfect (very linear, even for large

signals) resistor.

There is a lot of stored charge in the intrinsic region.

At low frequencies, the charge can be removed and the diode turns off.

At higher frequencies, there is not enough time to remove the charge, so the diode never

turns off. The PIN diode has a poor reverse recovery time.

The high-frequency resistance is inversely proportional to the DC bias current through

the diode.

A PIN diode, suitably biased, therefore acts as a variable resistor.

This high-frequency resistance may vary over a wide range (from 0.1 ohm to 10 kΩ in

some cases the useful range is smaller, though).

The wide intrinsic region also means the diode will have a low capacitance when reverse

biased.

In a PIN diode, the depletion region exists almost completely within the intrinsic region.

This depletion region is much larger than in a PN diode, and almost constant-size,

independent of the reverse bias applied to the diode.

This increases the volume where electron-hole pairs can be generated by an incident

photon.

PIN DIODE USES AND ADVANTAGES

The PIN diode is used in a number of areas as a result of its structure proving some

properties which are of particular use.

1. HIGH VOLTAGE RECTIFIER: The PIN diode can be used as a high voltage

rectifier. The intrinsic region provides a greater separation between the PN and N

regions, allowing higher reverse voltages to be tolerated.

2. RF SWITCH: The PIN diode makes an ideal RF switch. The intrinsic layer

between the P and N regions increases the distance between them. This also decreases

the capacitance between them, thereby increasing he level of isolation when the diode

is reverse biased.

3. PHOTODETECTOR: As the conversion of light into current takes place within the

depletion region of a photdiode, increasing the depletion region by adding the

intrinsic layer improves the performance by increasing he volume in which light

conversion occurs.

---------------- ALL THE BEST -------------------------- ALL

THE BEST ----------------


[rectifiers]

INTRODUCTION: -

For reasons associated with economics of generation and transmission, the electric power available is

usually an A.C. Supply. The supply voltage varies sinusoidal and has a frequency of 50 Hz. It is used

for lighting, heating and electric motors.

But there are many applications (e.g. electronic circuits) where D.C. supply is needed. When such a

D.C. Supply is required, the mains A.C. Supply is rectified by using Crystal Diodes.

The following two rectifier circuits can be used: -

(i) Half-wave rectifier (ii) Full-wave rectifier

HALF-WAVE RECTIFIER:-

In half-wave rectification, the rectifier conducts current only during the positive half-cycles of input

A.C. Supply.

The negative half-cycles of A.C. Supply is suppressed i.e. during negative half-cycles, no current is conducted and hence no voltage appears across the load.

Therefore, current always flows in one direction

through the load though after every half-cycle

(Input Wave form) (Half wave Rectifier Circuit) (Output voltage wave) (Output Current)

Circuit Details: -

The above Fig shows the circuit where a single crystal diode acts as a half-wave rectifier.

The A.C. Supply to be rectified is applied in series with the diode and load resistance RL. Generally,

A.C. Supply is given through a transformer.

The use of transformer permits two advantages.

Firstly, it allows us to step up or step down the A.C. input voltage as the situation demands.

Secondly, the transformer isolates the rectifier circuit from power line and thus reduces the risk

of electric shock.

OPERATION:-

The A.C. voltage across the secondary winding AB changes polarities after every half-cycle.

During the positive half-cycle of input A.C. voltage, end A becomes positive w.r.t. end B. This makes

the diode forward biased and hence it conducts current.

During the negative half-cycle, end A is negative w.r.t. end B. Under this condition, the diode is reverse biased and it conducts no current.

Therefore, current flows through the diode during positive half-cycles of input A.C. voltage only; it is

blocked during the negative half-cycles. In this way, current flows through load RL always in the same

direction. Hence D.C. output is obtained across RL.

It may be noted that output across the load is pulsating D.C. These pulsations in the output are further

smoothened with the help of filter circuits discussed later.

Disadvantages : -

(i) The pulsating current in the load contains alternating component whose basic frequency is equal to

the supply frequency. Therefore, an elaborate filtering is required to produce steady direct current.

(ii) The A.C. supply delivers power only half the time. Therefore, the output is low.

FULL-WAVE RECTIFIER: -

[ PAGE – 3. 2 ]

In full-wave rectification, current flows through the load in the same direction for both half-cycles of

input A.C. voltage. This can be achieved with two diodes working alternately.

For the positive half- cycle of input voltage, one diode supplies current to the load and for the negative half-cycle, the other diode does so ; current being always in the same direction through the load.

Therefore, a full-wave rectifier utilizes both half-cycles of input A.C. voltage to produce the D.C. output.

The following two circuits are commonly used for full-wave rectification: -

(i) Centre-tap full-wave rectifier (ii) Full-wave bridge rectifier

CENTRE-TAP FULL-WAVE RECTIFIER:-

Circuit Details: -

The circuit employs two diodes D1 and D2 as shown in Fig below. A centre tapped secondary winding

AB is used with two diodes connected so that each uses one half-cycle of input A.C. voltage.

In other words, diode D1 utilizes the A.C. voltage appearing across the upper half (OA) of secondary winding for rectification while diode D2 uses the lower half winding OB.

Circuit Operation: -

During the positive half-cycle of secondary voltage, the end A of the secondary winding becomes

positive and end B negative. This makes the diode D1 forward biased and diode D2 reverse biased.

Therefore, diode D1 conducts while diode D2 does not. The conventional current flow is through diode D1, load resistor RL and the upper half of secondary winding as shown by the dotted arrows.

During the negative half-cycle, end A of the secondary winding becomes negative and end B positive.

Therefore, diode D2 conducts while diode D1 does not. The conventional current flow is through diode

D2, load RL & lower half winding shown by solid arrows.

It may be seen that current in the load RL is in the same direction for both half-cycles of input A.C. voltage. Therefore, D.C. is obtained across the load RL.

(Input Wave form) (Centre-Tap Full-Wave Rectifier Circuit) (Output wave form)

Advantages:-

(i) The D.C. output voltage and load current values are twice than that of a half wave rectifier. (ii) The ripple factor is much less (0.482) than that of half rectifier (1.21).

(iii) The efficiency is twice (81.2%) than that of half wave rectifier (40.6%).

Disadvantages:-

(i) It is difficult to locate the centre tap on the secondary winding. (ii) The D.C. output is small as each diode utilizes only one-half of the transformer secondary voltage.

(iii) The diodes used must have high peak inverse voltage.

FULL-WAVE BRIDGE RECTIFIER: -

Circuit Details: -

The need for a centre tapped power transformer is eliminated in the bridge rectifier.

It contains four diodes D1, D2, D3 and D4 connected to form bridge as shown in Fig below.

The A.C. supply to be rectified is applied to the diagonally opposite ends of the bridge through the

transformer.

Between other two ends of the bridge, the load resistance RL is connected.

[ PAGE – 3. 3 ]

(Input Wave Form) (Full-Wave Bridge Rectifier Circuit) (Output wave form)

CIRCUIT OPERATION :-

During the positive half-cycle of secondary voltage, the end P of the secondary winding becomes

positive and end Q negative.

This makes diodes D1 and D3 forward biased while diodes D2 and D4 are reverse biased.

Therefore, only diodes D1 and D3 conduct. These two diodes will be in series through the load RL as

shown in Fig. below. The conventional current flow is shown by dotted arrows. It may be seen that

current flows from A to B through the load RL.

During the negative half-cycle of secondary voltage, end P becomes negative and end Q positive. This

makes diodes D2 and D4 forward biased whereas diodes D1 and D3 are reverse biased.

Therefore, only diodes D2 and D4 conduct. These two diodes will be in series through the load RL as shown in Fig. below. The current flow is shown by the solid arrows.

It may be seen that again current flows from A to B through the load i.e. in the same direction as for the positive half-cycle. Hence, D.C. output is obtained across load RL.

(Full-Wave Bridge Rectifier Circuit in +ve Half Cycle) (Full-Wave Bridge Rectifier Circuit -ve Half Cycle)

Advantages: -

(i) The need for centre-tapped transformer is eliminated.

(ii) The output is twice that of the centre-tap circuit for the same secondary voltage.

(iii) The PIV is one-half that of the centre-tap circuit (for same D.C. output).

Disadvantages: -

(i) It requires four diodes. (ii) Internal resistances high.

Mathematical Derivation for Rectification Efficiency for HALF WAVE rectifier : - The ratio of d.c. power output to the applied input a.c. power is known as rectifier efficiency i.e.,

Consider a half-wave rectifier shown in Fig.

Let v = Vmsin θ be the alternating voltage that appears across

the secondary winding. Let rf and RL be the diode resistance

and load resistance respectively.

The diode conducts during positive half-cycles of a.c. supply

while no current conduction takes place during negative half-

cycles.

dc

2

rNC

I

0

G

rNC

√2

L

OUTPUT D.C. POWER :-

The output current is pulsating direct current. Therefore, in order to find

D.C. power, average current has to be found out.

Average Value = Area Under The €urve Over a cycle

= ∫G i d

[ PAGE – 3. 4 ]

Iav = Idc = 1 ∫

G i d =

1

Bace

∫G Vm si n d =

VN

0 2n

∫G

SIN8 d 2G 0 2G 0 rf+ RL 2G(rf+ RL ) 0

= Vm si n [−c os]G = VN × [(−c osπ) − (−c os0)] rf+ RL

= Vn

0

× 2 = VN

2G(rf+ RL )

× 1 = IN [∵ IN = VN ] 2G(rf+ RL) (rf+ RL ) n n (rf+ RL )

∴ D.C. Power, Pdc = I2 × RL = ( Im2

) × R G

INPUT A.C. POWER: -

The A.C. power input is given by : Pac = I2 (r f+ RL

∴ Pac = (Im)2 × (r f + RL )

) For a half-wave rectified wave, Irms = Im/2

∴ Rectifier efficiency = d.c.output power = ( (IN/G )2 × RL

) = 0.406RL

= 0.406RL

a.c.input power (IN/2 )2 (rf+ RL) rf+ RL 1+ rf

RL

The efficiency will be maximum if rf is negligible as compared to RL.

∴ Max. Rectifier Efficiency for HALF WAVE Rectifier = 40.6%

It shows that in half-wave rectification, maximum of 40.6% of a. c. power is converted into d. c. power.

NOTE: - Irms = [ 1 ∫2G

i 2 d] ½ = [ 1 ∫G

I2 sin28 d + 1 ∫2G

0 d] ½

2 = [ N ∫

G 1–coc28

d] ½

2G 0

I2 sin2 ½

2G 0 N

I2

sin2G

2G G

½ I2

2G 0 2

½ I2 ½ Im Im

= [ N 4G

[ − ]G] 2

= [ N 4G

[ π − 0 − 2

+ s in0]] = [ N 4G

x π ] = [ N ] 4 =

2 Irms =

2

Similarlly, Vrms = Vm/2 for Half Wave and For Full Wave Rectifier Irms = Im/√2 and Vrms = Vm/√2

Mathematical Derivation for Rectification Efficiency for FULL WAVE Rectifier : - Fig. shows the process of full-wave rectification.

Let v = Vmsinθ be the a.c. voltage to be rectified. Let rf and RL be

the diode resistance and load resistance respectively.

Obviously, the rectifier will conduct current through the load in the

same direction for both half-cycles of input a.c. voltage. The

instantaneous current i is given by :

D.C. OUTPUT POWER.

i = v (rf+ RL)

= VN cin8 (rf+

RL)

The output current is pulsating direct current. Therefore, in order to find the d.c. power, average current has to be found out. For a full wave rectifier the average value or dc value can be found like half wave ,

Idc = 2Im

∴ D.C. power output, Pdc = I2 × RL = (2Im)2 × RL dc G

A.C. INPUT POWER.

The a.c. input power is given by : Pac = I2 (r f+ RL)

For a full-wave rectified wave, we have, Irms = Im/√2 ∴ Pac = (Im)2(r f + RL )

∴ Full-wave rectification efficiency is

η = Pdc =

(2IN/n)2RL =

8

×

RL

=

0.812 RL =

0.8 12

Pac (IN)2 (rf+ R ) n2

√2

(rf+ RL) rf+ RL 1+ rf

RL

The efficiency will be maximum if rf is negligible as compared to RL. ∴ Maximum efficiency = 81.2%

This is double the efficiency due to half-wave rectifier. Therefore, a full-wave rectifier is twice as

effective as a half-wave rectifier.

L

2 G

RIPPLE FACTOR: -

[ PAGE – 3. 5 ]

The output of a rectifier consists of a d.c. component and an a.c. component (also known as ripple).

The a.c. component is undesirable and accounts for the

pulsations in the rectifier output.

The effectiveness of a rectifier depends upon the magnitude

of a.c. component in the output; the smaller this component,

the more effective is the rectifier.

Ripple mean unwanted ac signal present in the rectified

output.

The ratio of R.M.S. value of A.C. component to the D.C.

component in the rectifier output is known as ripple factor

i.e.

Mathematical Analysis.

The output current of a rectifier contains d.c. as well as a.c. component.

By definition, the effective (i.e. r.m.s.) value of total load current is given by :

Irms = JIdc2

+ Iac2

Or Iac = JIrNC

2 − Idc

2

Dividing throughout by Idc, we get,

Iac

Idc

= 1

Idc

JIrNC2 − Idc

2 (But Iac/ Idc is the ripple factor.)

∴ Ripple f ac tor =1 JI

2 − I

2 = J( Irms )2 − 1 Idc

rNC dc Idc

(i) For half-wave rectification: -

In half-wave rectification, Irms = Im/2 ; Idc = Im/π

∴ Ripple f ac tor =J( IN /2)2 − 1 = 1. 21 IN/n

It is clear that a.c. component exceeds the d.c. component in the output of a half-wave rectifier.

This results in greater pulsations in the output.

Therefore, half-wave rectifier is ineffective for conversion of a.c. into d.c. (ii) For full-wave rectification: -

In full-wave rectification, Irms = IN √

; Idc = 2IN

∴ Ripple f ac tor =J(IN /√2)2 − 1 = 0. 48 i.e. effective a.c.component = 0.48 2IN/n d.c.component

This shows that in the output of a full-wave rectifier, the d.c. component is more than the a.c. component. Consequently, the pulsations in the output will be less than in half-wave rectifier.

For this reason, full-wave rectification is invariably used for conversion of a.c. into d.c.

Peak Inverse Voltage (PIV) : -

The maximum value of reverse voltage occurs at the peak of the input cycle, which is equal to Vm.

This maximum reverse voltage is called peak inverse voltage (PIV). Thus the PIV of diode : -

a) For Half Wave = Vm., b) For Center Tapped =2Vm and c) For Bridge Rectifier = Vm .

Transformer Utilization Factor (TUF) : -

It may be defined as the ratio of d.c. power delivered to the load and the a.c. rating of the transformer secondary.

Thus,

For half wave rectifier, TUF = 0.287; Center taped rectifier, TUF = 0.693; Bridge rectifier, TUF = 0.812.

The TUF is very useful in determining the rating of a transformer to be used with rectifier circuit.

TUF = Pdc / Pac

Vdc = 2Vm/π = 0.636 Vm and Idc = 2Im/π = 0.636 Im

Vdc = Vm/π = 0.318 Vm and Idc = Im/π = 0.318 Im

[ PAGE – 3. 6 ]

Average Value of Voltage & Current for HALF WAVE Rectifiers: -

If Vm= Maximum value of the a.c. input voltage, then the average or d.c. value of the output voltage and

current is given by

Average Value of Voltage & Current for FULL WAVE Rectifiers: -

If Vm= Maximum value of the a.c. input voltage, then the average or d.c. value of the output voltage and

current is given by

Output Frequency of Half Wave Rectifier: -

The output frequency of a half-wave rectifier is equal to the input frequency (50 Hz). Recall how a

complete cycle is defined.

A waveform has a complete cycle when it repeats the same wave pattern over a given time.

Thus in Fig. (i), the a.c. input voltage repeats the same wave pattern over 0° – 360°, 360° – 720° and so on.

In Fig. (ii), the output waveform also repeats the same wave pattern over 0° – 360°, 360° – 720° and so on.

This means that when input a.c. completes one cycle, the output half wave

rectified wave also completes one cycle.

In other words, for the half wave rectifier the output frequency is equal to

the input frequency i.e. fout = fin

For example, if the input frequency of sine wave applied to a half-wave

rectifier is 100 Hz, then frequency of the output wave will also be 100 Hz.

Output Frequency of Full Wave Rectifier: -

The output frequency of a full-wave rectifier is double the input

frequency.

As a wave has a complete cycle when it repeats the same pattern.

In Fig. (i), the input a.c. completes one cycle from 0° – 360°.

However, in Fig. (ii) full-wave rectified wave completes two cycles in

this period.

Therefore, output frequency is twice the input frequency i.e. fout = 2fin

For example, if the input frequency to a full-wave rectifier is 100 Hz,

then the output frequency will be 200 Hz.

COMPARISON OF RECTIFIERS: -

[ PAGE – 3. 7 ]

FILTER CIRCUITS:-

Generally, a rectifier is required to produce pure D.C. supply for using at various places in the electronic

circuits.

However, the output of a rectifier has pulsating character i.e. it contains A.C. and D.C. components.

The A.C. component is undesirable and must be kept away from the load.

To do so, a filter circuit is used which removes (or filters out) the A.C. component and allows only the

D.C. component to reach the load.

A filter circuit is a device which removes the A.C. component of rectifier output but allows the D.C.

component to reach the load.

A filter circuit is generally a combination of inductors (L) and capacitors (C).

The filtering action of L and C depends upon the basic electrical principles.

A capacitor offers infinite reactance to d.c.

We Know that XC= 1/2πfC. But for D.C., f = 0.

∴ XC= 1/2πfC = 1/ 2π x 0 x C = ∞ (Means Capacitor shows infinite reactance to DC)

Hence, a Capacitor does not allow d.c. to pass through it.

We know XL= 2πfL. For d.c., f = 0

∴ XL = 2π x 0 x L = 0 (Means Inductor shows zero reactance to DC)

Hence Inductor passes d.c. quite readily.

A Capacitor passes A.C. but does not pass D.C. at all. On the other hand, an Inductor opposes A.C. but allows D.C. to pass through it.

It then becomes clear that suitable network of L and C can effectively remove the A.C. component, allowing the D.C. component to reach the load.

(Pulsating D.C) (Filter Circuit) (Pure D.C)

Types Of Filter Circuits:-

There are different types of filter circuits according to their construction. The most commonly used filter circuits are : -

Inductive Filter or Series Inductor,

Capacitor Filter or Shunt Capacitor,

Choke Input Filter or LC Filter and

Capacitor Input Filter or π-Filter.

Inductive Filter Or Series Inductor:-

(Rectified output Pulsating d.c) (Inductive Filter Circuit) (Output of Inductive Filter)

Fig. (ii) Shows a typical Inductive filter circuit. It consists of an Inductor L placed across the rectifier output in series with load RL.

[ PAGE – 3. 8 ]

The choke (Inductor with iron core) offers high opposition to the passage of a.c. component but no

opposition to the d.c. component.

The result is that most of the a.c. component appears across the choke while whole of d.c. component passes through the choke on its way to load. This results in the reduced pulsations at Load resistance RL.

Capacitor Filter Or Shunt Capacitor:-

(Rectified output Pulsating d.c) (Capacitor Filter Circuit) (Output of Capacitor Filter)

Fig. (ii) Shows a typical capacitor filter circuit. It consists of a capacitor C placed across the rectifier

output in parallel with load RL.

The pulsating direct voltage of the rectifier is applied across the capacitor. As the rectifier voltage increases, it charges the capacitor and also supplies current to the load.

At the end of quarter cycle [Point A in Fig. (iii)], the capacitor is charged to the peak value Vm of the rectifier voltage.

Now, the rectifier voltage starts to decrease. As this occurs, the capacitor discharges through the load

and voltage across it decreases as shown by the line AB in Fig. (iii).

The voltage across load will decrease only slightly because immediately the next voltage peak comes and recharges the capacitor.

This process is repeated again and again and the output voltage waveform becomes ABCDEFG. It may be seen that very little ripple is left in the output.

Moreover, output voltage is higher as it remains substantially near the peak value of rectifier output voltage.

The capacitor filter circuit is extremely popular because of its low cost, small size, little weight and good characteristics.

Choke Input Filter Or LC Filter:-

(Rectified output Pulsating d.c) (Choke Input Filter Circuit) (Output of Choke Input Filter)

Fig. shows a typical choke input filter circuit. It consists of a choke L connected in series with the rectifier output and a filter capacitor C across the load.

Only a single filter section is shown, but several identical sections are often used to reduce the pulsations as effectively as possible.

The pulsating output of the rectifier is applied across terminals 1 and 2 of the filter circuit.

As discussed before, the pulsating output of rectifier contains a.c. and d.c. components. The choke offers

high opposition to the passage of a.c. component but negligible opposition to the d.c. component.

The result is that most of the a.c. component appears across the choke while whole of d.c. component passes through the choke on its way to load. This results in the reduced pulsations at terminal 3.

[ PAGE – 3. 9 ]

At terminal 3, the rectifier output contains d.c. component and the remaining part of a.c. component

which has managed to pass through the choke.

Now, the low reactance of filter capacitor bypasses the a.c. component but prevents the d.c. component to flow through it. Therefore, only d.c. component reaches the load.

In this way, the filter circuit has filtered out the a.c. component from the rectifier output, allowing d.c. component to reach the load.

Capacitor Input Filter or π-Filter:-

(Rectified output Pulsating d.c) (Capacitor Input or π- Filter Circuit) (Output of π-Filter)

Fig. shows a typical capacitor input filter or π-filter. It consists of a filter capacitor C1 connected across

the rectifier output, a choke Lin series and another filter capacitor C2 connected across the load.

Only one filter section is shown but several identical sections are often used to improve the smoothing

action. The pulsating output from the rectifier is applied across the input terminals (i.e. terminals 1 & 2)

of the filter.

The filtering action of the three components viz C1 , L and C2 of this filter is described below :

(a) The filter capacitor C1 offers low reactance to a.c. component of rectifier output while it offers

infinite reactance to the d.c. component. Therefore, capacitor C1 bypasses an appreciable amount of a.c.

component while the d.c. component continues its journey to the choke L.

(b) The choke L offers high reactance to the a.c. component but it offers almost zero reactance to the

d.c. component. Therefore, it allows the d.c. component to flow through it, while the un bypassed a.c.

component is blocked.

(c) The filter capacitor C2 bypasses the a.c. component which the choke has failed to block. Therefore,

only d.c. component appears across the load and that is what we desire

---P N GOUDA----ALL THE BEST ------------------ ALL THE BEST---- P N GOUDA ----


[TRANsIsTORs]

INTRODUCTION:-

When a third doped element is added to a crystal diode in such a way that two PN junctions are formed,

the resulting device is known as a Transistor.

This is a new type of electronics device which can able to amplify a weak signal in a fashion comparable

and often superior to that realized by vacuum tubes.

A transistor consists of two PN junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types. Hence Transistor is classified into two types, namely: -

(i) n-p-n transistor (ii) p-n-p transistor

An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type.

However, a p-n-p transistor is formed by two p-sections separated by a thin section of n-type as shown in

Figure below.

NAMING: - A transistor has two pn junctions. As discussed later, one junction is forward biased and the other is

reverse biased.

The forward biased junction has a low resistance path whereas a reverse biased junction has a high resistance path.

The weak signal is introduced in the low resistance circuit and output is taken from the high resistance circuit. Therefore, a transistor transfers a signal from a low resistance to high resistance.

The prefix ‘trans’ means the signal transfer property of the device while ‘istor’ classifies it as a solid element in the same general family with resistors.

NAMING THE TRANSISTOR TERMINALS:-

A transistor (PNP or NPN) has three sections of doped semiconductors.

The section on one side is the emitter and the section on the opposite side is the collector.

The middle section is called the base and forms two junctions between the emitter and collector.

(i) Emitter: -

The section on one side that supplies charge carriers (electrons or holes) is called the emitter.

The emitter is always forward biased w.r.t. base so that it can supply a large number of majority

carriers.

The emitter (p-type) of PNP transistor is forward biased and supplies hole charges to its junction

with the base. Similarly the emitter (n-type) of NPN transistor has a forward bias and supplies free

electrons to its junction with the base.

(ii) Collector: -

The section on the other side that collects the charges is called the collector. The collector is always

reverse biased. Its function is to remove charges from its junction with the base.

The collector (p-type) of PNP transistor has a reverse bias and receives hole charges that flow

in the output circuit. Similarly the collector (n-type) of NPN transistor has reverse bias & receives

electrons. (iii)Base: -

The middle section which forms two PN-junctions between emitter & collector is called base.

The base-emitter junction is forward biased, allowing low resistance for the emitter circuit.

[ PAGE – 4. 2 ]

The base-collector junction is reverse biased and provides high resistance in the collector circuit.

TRANSISTOR SYMBOL:-

WORKING OF NPN TRANSISTOR (NPN): - The NPN transistor with forward bias to emitter- base junction & reverse bias to collector-base junction.

The forward bias causes the electrons in the n-type emitter to flow towards the base.

This constitutes the emitter current IE. As these

electrons flow through the p-type base, they tend to

combine with holes.

As the base is lightly doped and very thin, therefore,

only a few electrons (less than 5%) combine with

holes to constitute base current IB.

The remainders (more than 95%) cross over into the

collector region to constitute collector current IC.

In this way, almost the entire emitter current flows in the collector circuit.

It is clear that emitter current is the sum of collector and base currents i.e. IE = IB + IC

WORKING OF PNP TRANSISTOR (PNP): -

Fig. shows the basic connection of a PNP transistor.

The forward bias causes the holes in the p-type

emitter to flow towards the base.

This constitutes the emitter current IE.

As these holes cross into n-type base, they tend to

combine with the electrons.

As the base is lightly doped and very thin, therefore,

only a few holes (less than 5%) combine with the

electrons. The remainder (more than 95%) cross into

the collector region to constitute collector current IC.

In this way, almost the entire emitter current flows in the collector circuit.

[ PAGE – 4. 3 ]

It may be noted that current conduction within PNP transistor is by holes. However, in the external

connecting wires, the current is still by electrons

TRANSISTOR CONNECTIONS:- There are three leads in a transistor such as emitter, base and collector terminals.

However, when a transistor is to be connected in a circuit, we require four terminals; two for the input and two for the output.

This difficulty is overcome by making one terminal of it in common to both input and output terminals.

The input is fed between this common terminal and one of the other two terminals.

The output is obtained between the common terminal and the remaining terminal.

So a transistor can be connected in a circuit in the following ways:- (i) Common Base connection (ii) Common Emitter connection (iii) Common Collector

connection

(i) Common Base Connection

In this circuit arrangement, input is applied between emitter and base and output is taken from

collector and base.

Here, base of the transistor is common to both input and output circuits and hence the name

Common Base connection. A Common Base NPN and PNP in figure below.

(ii) Common Emitter Connection In this circuit arrangement, input is applied between base and emitter and output is taken from

the collector and emitter.

Here, emitter of the transistor is common to both input and output circuits and hence the name

Common Emitter connection. A Common Emitter NPN and PNP transistor circuit is shown in figure

below.

(iii) Common Collector Connection In this circuit arrangement, input is applied between base and collector while output is taken

between the emitter and collector.

Here, collector of the transistor is common to both input and output circuits and hence the name

Common Collector connection. A Common Collector NPN and PNP in figure below.

[ PAGE – 4. 4 ]

TRANSISTOR CHARACTERISTICS:-1) Characteristics of Common Base Connection

The complete electrical behavior of a transistor can be described by stating the interrelation of the

various currents and voltages.

These relationships can be conveniently displayed graphically and the curves thus obtained are known as the characteristics of transistor.

The most important characteristics of common base connection are input characteristics and output

characteristics.

A) Input Characteristics:-

It is the curve between emitter current IE & emitter-base

voltage VBE at constant collector-base voltage VCB.

The emitter current is generally taken along y-axis and

emitter-base voltage along x-axis. Fig. Shows the input

characteristics of a typical transistor in CB arrangement.

The following points may be noted from these

characteristics :

The emitter current IE increases rapidly with small increase in emitter-base voltage VEB. It means that input resistance is very small.

The emitter current is almost independent of collector-

base voltage VCB. This leads to the conclusion that

emitter current (and hence collector current) is almost independent of collector voltage.

Input Resistance: - It is the ratio of change in emitter-base voltage (ΔVEB) to the resulting change in

emitter current (ΔIE) at constant collector-base voltage (VCB) i.e.

In fact, input resistance is the opposition offered to the signal current. As a very small VEB is sufficient to

produce a large flow of emitter current IE, thus, input resistance is quite small, of the order of a few

ohms.

B) Output Characteristics:-

It is the curve between collector current IC & collector-base voltage VBC at constant emitter current IE.

Generally, collector current is taken along y-axis and collector-base voltage along x-axis.

The fig. shows the input and output characteristics of a typical transistor in CB arrangement.

The following points may be noted from characteristics :

The collector current IC varies with VCB only at very

low voltages (< 1V). The transistor is never operated in this region.

When the value of VCB is raised above 1 − 2 V, the

collector current becomes constant as indicated by

straight horizontal curves. It means that now IC is

independent of VCB and depends upon IE only. This

is consistent with the theory that the emitter current

[ PAGE – 4. 5 ]

flows almost entirely to the collector terminal. The transistor is always operated in this region.

A very large change in collector-base voltage produces only a tiny change in collector current. This

means that output resistance is very high.

Output Resistance: - It is the ratio of change in collector-base voltage (ΔVCB) to the resulting change

in collector current (ΔIC) at constant emitter current i.e.

The output resistance of CB circuit is very high, of the order of several tens of kilo-ohms.

2) Characteristics of Common Emitter Connection:-

The important characteristics of this circuit arrangement are the input characteristic and output

characteristic.

(Circuit Arrangement for studying Common Emitter Connection of Transistor)

A) Input Characteristics:-

It is the curve between base current IB & base-emitter voltage VBE at constant collector-emitter volt

VCE. The input characteristics of a CE connection can be

determined by the circuit shown in Fig. Keeping VCE constant

(Let 10 V), note the base current IB for various values of VBE.

Then plot the readings obtained on the graph, taking IB along

y-axis and VBE along x-axis. This gives the input

characteristic at VCE = 10V as shown in Fig.

The following points may be noted from the characteristics :

The characteristic resembles that of a forward biased diode curve. This is expected since the base-emitter section of

transistor is a diode and it is forward biased.

As compared to CB arrangement, IB increases less rapidly

with VBE. Therefore, input resistance of a CE circuit is

higher than that of CB circuit.

Input Resistance: - It is the ratio of change in base-emitter voltage (ΔVBE) to the change in base

current (ΔIB) at constant VCE. The value of input resistance for CE circuit is of the order of a few

hundred ohms

B) Output Characteristics: -

It is the curve between collector current IC and collector-emitter voltage VCE at constant base current IB.

The output characteristics of CE circuit can be drawn with the help of above circuit arrangement in Fig.

Keeping the base current IB fixed at some value say, 5 µA,

note the collector current IC for various values of VCE.

Then plot the readings on a graph, taking IC along y-axis and

VCE along x-axis.

This gives the output characteristic at IB = 5 µA as shown in

Fig. The test can be repeated for IB= 10 µA to obtain the new

output characteristic as shown in Fig.

Following similar procedure, a family of output characteristics

can be drawn as shown in Fig.

The following points may be noted from the characteristics:

[ PAGE – 4. 6 ]

(i) The collector current IC varies with VCE for VCE between 0 and 1V only. After this, IC becomes

almost constant & independent of VCE.

This value of VCE upto which IC changes with VCE is called the knee voltage (Vknee). The transistors

are always operated in the region above knee voltage.

(ii) Above knee voltage, IC is almost constant. However, a small increase in IC with increasing VCE is caused by the collector depletion layer getting wider and capturing a few more majority carriers

before electron-hole combinations occur in the base area.

(iii) For any value of VCE above knee voltage, the collector current IC is approximately equal to β ×

IB

Output Resistance: - It is the ratio of change in collector-emitter voltage (ΔVCE) to the change in

collector current (ΔIC) at constant IB i.e.

It may be noted that whereas the output characteristics of CB circuit are horizontal, they have noticeable

slope for the CE circuit.

Therefore, output resistance of CE circuit is less than that CB circuit. Its value is of the order of 50 kΩ.

3) Characteristics of Common Collector Connection:-

In a Common Collector circuit connection the load resistor connected from emitter to ground, so the

collector tied to ground even though the transistor is connected in a manner similar to the CE connection.

Hence there is no need for a set of common-collector characteristic to choose the parameters of the circuit. The output characteristic of the CC configuration is same as CE configuration.

For CC Connection the output characteristic are plot of IE versus VCE for a constant value of IB.

There is an almost unnoticeable change in the vertical scale of IC of the CE connection if IC is replaced

by IE for CC connection.

The input circuit of CC connection, the CE characteristic is sufficient to obtain the required information.

Hence Common Collector circuit connection is known as Emitter Follower.

CURRENT AMPLIFICATION FACTORS: - (It is the ratio of output current to input current)

1) Common Base Connection:-

In a common base connection, the input current is the Emitter Current IE and output current is the

Collector Current IC.

Hence the ratio of change in collector current to the change in emitter current at constant collector-

base voltage VCB is known as current amplification factor for CB Connection and is denoted as α

(Alpha).

Practical values of α in commercial transistors range from 0.9 to 0.99.

2) Common Emitter Connection:-

In a common emitter connection, the input current is the Base Current IB and output current is the

Collector Current IC.

Hence ratio of change in collector current (IC) to the change in base current (IB) at constant collector-

emitter voltage VCE is known as current amplification factor for CE Connection and denoted as β (Beta).

Usually, its value ranges from 20 to 500.

3) Common Collector Connection:-

In a common collector connection, the input current is the Emitter Current IB and output current is

the Emitter Current IE.

Hence the ratio of change in emitter current to the change in base current at constant VCC is known as

current amplification factor for CC Connection and is denoted as γ (Gamma).

This circuit provides about the same current gain as the common emitter circuit as ΔIE ≈ ΔIC.

[ PAGE – 4. 7 ]

RELATION AMONG DIFFERENT CURRENT AMPLIFICATION FACTORS:-

1) Relation between α and β :-

As, þ = ΔI€ =

ΔI€ =

ΔI€ /ΔIE =

a As, a = ΔI€ =

ΔI€ =

ΔI€ /ΔIE =

þ

ΔIB ΔIE–ΔI€ 1–ΔI€ /ΔIE 1–a ΔIE ΔIB+ΔI€ 1+ΔI€ /ΔIB 1+þ

2) Relation between α and γ :-

As, γ = ΔIE =

ΔIE =

ΔIE /ΔIE =

1 As, α =

ΔI€ =

ΔIE–ΔIB =

ΔIE /ΔIB–1 =

y–1

ΔIB ΔIE–ΔI€ 1–ΔI€/ΔIE 1–a ΔIE ΔIE ΔIE /ΔIB y

3) Relation between β and γ :-

As, γ = ΔIE

= ΔIB+ΔI€

= ΔIB

+ ΔI€

= 1 + þ As, þ = ΔI€ = ΔIE–ΔIB = ΔIE - ΔIB = γ-1

ΔIB ΔIB ΔIB ΔIB ΔIB ΔIB ΔIB ΔIB

4) Relation between α, β and γ:-

As, þ = a 1–a

= α X 1

1–a

= α x γ

COMPARISON OF TRANSISTOR CONNECTIONS:-

Out of the three transistor connections, the Common Emitter Circuit is the most efficient.

It is used in about 90 to 95 per cent of all transistor applications.

The main reasons for the widespread use of this circuit arrangement are :

(i) High current gain. (ii) High voltage and power gain. (iii) Moderate output to input impedance ratio.

AMPLIFIER:-

The device which increases the strength of a weak signal is known as Amplifier. This can achieve by use

of Transistor. It may be classified according to the number of stage of amplification, Such as:-

Single Stage Transistor Amplifier: - When only one transistor with associated circuitry is used for

amplifying a weak signal, the circuit is known as Single Stage Transistor Amplifier.

Multi stage Transistor Amplifier:-When a transistor circuit containing more than one stage of

amplification is known as Multi stage Transistor Amplifier.

SINGLE STAGE TRANSISTOR AMPLIFIER:-

A single stage transistor amplifier has one transistor, bias circuit and other auxiliary components.

When a weak A.C. signal is given to the base of transistor, a small base current starts flowing.

Due to transistor action, a much larger (β times the base current) current flows through the collector load RC.

As the value of RC is quite high (usually 4-10 kΩ), therefore, a large voltage appears across RC.

Thus, a weak signal applied in the base circuit appears in amplified form in the collector circuit.

It is in this way that a transistor acts as an amplifier. [Transistor as an Amplifier]


∴ þ = y + 1 ∴ y = 1 + þ y

∴ a = y−1 1

1−a ∴ y =

a

1–a ∴ þ = þ

1+þ ∴ a =

∴ þ = a X γ

ΔIE=ΔIB+ ΔIC


[TRANSISTOR CIRCUITS]

TRANSISTOR BIASING:-

The basic function of transistor is to do amplification. The weak signal is given to the base of the

transistor and amplified output is obtained in the collector circuit.

One important requirement during amplification is that only the magnitude of the signal should increase

and there should be no change in signal shape.

This increase in magnitude of the signal without any change in shape is known as faithful amplification.

For this we have to provide input circuit (i.e. base-emitter junction) remains forward biased and output

circuit (i.e. collector-base junction) remains reverse biased at all times.

To achieve faithful amplification the following basic conditions must be satisfied:-

(i) Proper zero signal collector current

(ii) Minimum proper base-emitter voltage (VBE) at any instant

(iii) Minimum proper collector-emitter voltage (VCE) at any instant

The fulfillment of these will ensure that transistor works over the active region of the output

characteristics.

The proper flow of zero signal collector current and the maintenance of proper collector-emitter voltage during the passage of signal is known as Transistor Biasing.

The basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction properly reverse biased during the application of signal.

This can be achieved with a bias battery or associating a circuit with a transistor.

The second method i.e. with a bias battery or associating a circuit with a transistor is more efficient and

is frequently employed.

The circuit which provides transistor biasing is known as biasing circuit. The transistor biasing is very

essential for the proper operation of transistor in any circuit.

NEED OF TRANSISTOR BIASING:-

(i) It should ensure proper zero signal collector current.

(ii) It should ensure that VCE does not fall below 0.5 V for Ge transistors and 1 V for Si transistors.

(iii) It should ensure the stabilization of operating point.

STABILISATION: -

The process of making operating point independent of temperature changes or variations in transistor

parameters is known as Stabilization.

NEED FOR STABILIZATION:- Stabilization of the operating point is necessary due to the

following reasons :

(i) Temperature dependence of IC

(ii) Individual variations

(iii) Thermal runaway

The self-destruction of an unsterilized transistor is known as Thermal Runaway.

STABILITY FACTOR :- The rate of change of collector current IC w.r.t. the collector leakage current ICO [ = ICEO] at constant β

and IB is called stability factor i.e.

B

B

[ PAGE – 5. 2 ]

METHODS OF TRANSISTOR BIASING:-

In the transistor amplifier circuits drawn so far biasing was done with the aid of a battery VBB which was

separate from the battery VCC used in the output circuit. However, for simplicity and economy, it is

desirable that transistor circuit should have a single source of supply the one in the output circuit (i.e.

VCC).

The following are the most commonly used methods of obtaining transistor biasing from one source of

supply:

(i) Base resistor method

(ii) Biasing with collector-feedback resistor

(iii) Voltage-divider bias

In all these methods, the same basic principle is employed i.e. required value of base current (and hence

IC) is obtained from VCC in the zero signal conditions.

The value of collector load RC is selected keeping in view that VCE should not fall below 0.5 V for

germanium transistors and 1V for silicon transistors.

BASE RESISTOR METHOD:-

In this method, a high resistance RB (several hundred kΩ) is

connected between the base and +ve end of supply for npn transistor

and between base and negative end of supply for pnp transistor.

Here, the required zero signal base current is provided by VCC and it

flows through RB. It is because now base is positive w.r.t. emitter i.e.

base-emitter junction is forward biased.

The required value of zero signal base current IB (and hence IC = βIB) can be made to flow by selecting the proper value of base resistor RB.

Circuit analysis:-

It is required to find the value of RB so that required collector

current flows in the zero signal conditions.

Let IC be the required zero signal collector current.

∴ IB = IC /β

Considering the closed circuit ABENA and applying Kirchhoff's voltage law, we get,

VCC = IB RB + VBE

IB RB = VCC - VBE

R = Vcc – VBE ......................................................................................... (i)

IB

As VCC and IB are known and VBE can be seen from the transistor manual, therefore, value of RB can found from exp. (i).

Since VBE is generally quite small as compared to VCC, the former can be neglected with little error.

Thus Equation (i) becomes, R = V€€ IB

It may be noted that VCC is a fixed known quantity and IB is chosen at some suitable value. Hence, RB can always be found directly, and for this reason, this method is sometimes called fixed-bias method.

Advantages :

(i) This biasing circuit is very simple as only one resistance RB is required. (ii) Biasing conditions can easily be set and the calculations are simple.

Disadvantages :

(i) This method provides poor stabilization. (ii) The stability factor is very high.

BIASING WITH FEEDBACK CIRCUIT:-

In this method, one end of RB is connected to the base and the other end to the collector. Here, the

required zero signal base current is determined not by VCC but by the collector-base voltage VCB. It is

clear that VCB forward biases the base-emitter junction and hence base current IB flows through RB. This

causes the zero signal collector current to flow in the circuit.

[ PAGE – 5. 3 ]

Circuit Analysis:-

The required value of RB needed to give the zero signal current IC can be determined as follows.

From the above circuit diagram, VCC = IC RC + IB RB + VBE

Alternately, VCE = VBE + VCB or VCB = VCE - VBE

Advantages :-

(i) It is a simple method as it requires only one resistance RB.

(ii) This circuit provides some stabilization of the operating point than fixed bias method.

Disadvantages:-

(i) The circuit does not provide good stabilization.

(ii) This circuit provides a negative feedback which reduces the gain of the amplifier.

(iii)This will reduce the base current and hence collector current.

VOLTAGE DIVIDER BIAS METHOD:-

This is the most widely used method of providing biasing and stabilization to a transistor. In this

method, two resistances R1 and R2 are connected across the supply voltage VCC and provide biasing.

The emitter resistance RE provides stabilization. The name ‘‘voltage divider’’ comes from the voltage

divider formed by R1 and R2. The voltage drop across R2 forward biases the base- emitter junction. This

causes the base current and hence collector current flows in the zero signal conditions.

Circuit analysis:-

Suppose that the current flowing through resistance R1 is I1. As base current IB is very small, therefore, it can be assumed with reasonable accuracy that current flowing through R2 is also I1.

(i) Collector current IC :-

Applying Kirchhoff's voltage law to the base circuit

V2 = VBE + VE = VBE + IE RE

; ; (Voltage Divider Bias Circuit)

Thus IC in this circuit is almost independent of transistor parameters and hence good stabilization is

ensured. Due to this reason the potential divider bias has become universal method for providing

transistor biasing.

(ii) Collector-emitter voltage (VCE): -

Applying Kirchhoff's voltage law to the collector side,

VCC = IC RC + VCE+ IE RE = IC RC + VCE+ IC RE (As IC IE)

So, VCC = IC (RC + RE) + VCE => VCE = VCC - IC (RC + RE)

Advantages : -

In this circuit, excellent stabilization is provided by RE. Consider the Following Equation,

V2 = VBE+ ICRE

Suppose the collector current IC increases due to rise in temperature. This will cause the voltage drop across emitter resistance RE to increase.

---P N GOUDA----ALL THE BEST ------------------ ALL THE BEST---

- P N GOUDA ----

A.C. load, RAC = (RCRL/ (RC+RL)

∴ Total maximum collector-emitter voltage, VCE MAX = VCE+ IC RAC


----------------------- [TRANSISTOR AMPLIFIERS OSCILLATORS ---------------------

D.C. AND A.C. EQUIVALENT CIRCUITS: - Various circuit currents. It is useful to mention

the various currents in the complete amplifier

circuit. These are shown in the circuit of Fig.

(i) Base Current: - When no signal is applied in

the base circuit, D.C. base current IB flows due to

biasing circuit. When A.C. signal is applied, A.C.

base current ib also flows.

Therefore, with the application of signal, Total

Base Current iB is given by: iB= IB+ ib

(ii) Collector Current: - When no signal is

applied, a D.C. collector current IC flows due to

biasing circuit. When A.C. signal is applied, A.C.

collector current ic also flows.

Therefore, the Total Collector Current iC is given

by: - iC= IC+ ic Where IC= β IB= zero signal collector current and ic= β ib= collector current due to signal.

(iii) Emitter Current:- When no signal is applied, a D.C. emitter current IE flows. When A.C. signal is

applied, A.C. Emitter Current ie also flows. Therefore the Total Emitter Current is : - iE= IE+ ie It is useful to keep in mind that: IE= IB+ IC and ie= ib+ ic.

But base current is usually very small, therefore, as a reasonable approximation, IE ≈ IC and ie ≈ ic.

D. C. Equivalent Circuit: - In order to draw the equivalent D.C. circuit, the

following two steps are applied to the transistor circuit:-

(a) Reduce all A.C. sources to zero.

(b) Open all the capacitors.

Referring D.C. Equivalent Circuit

D.C. Load RDC= RC+ RE & VCC = VCE+ IC (RC+ RE)

The maximum value of VCE will occur when there is no collector current i.e. IC= 0.

∴

The maximum collector current will flow when VCE= 0.

∴

A.C. Equivalent Circuit: - In order to draw A.C.

equivalent circuit, the following two steps are

applied to the transistor circuit:

(a) Reduce all D.C. sources to zero (i.e. VCC= 0).

(b) Short all the capacitors.

Referring A.C. Equivalent circuit A.C. load equal

to RC || RL i.e.

Maximum positive swing of A.C. collector-

emitter voltage = IC × RAC

Maximum positive swing of A.C. collector current = VCE/RAC

∴ Total maximum collector current, IC MAX = IC+ VCE/RAC

Maximum IC=VCC/ (RC+RE)

Maximum VCE= VCC

LOAD LINE ANALYSIS: -

[ PAGE – 6. 2 ]

In the transistor circuit analysis, it is generally required to determine the collector current for various

collector-emitter voltages.

One of the methods can be used to plot the output characteristics and determine the collector current at any desired collector-emitter voltage.

However, a more convenient method, known as load line method can be used to solve such problems.

This method is quite easy and is frequently used in the analysis of transistor applications.

D.C. LOAD LINE: - It is the line on the output characteristics of a transistor circuit which gives the values of IC and VCE corresponding to zero signal or D.C. conditions.

Consider a common emitter NPN transistor circuit where no signal is applied. Therefore, D.C. conditions

prevail in the circuit. The output characteristics of this circuit

are shown in Fig.

The value of collector-emitter voltage VCE at any time is given

by ; VCE = VCC– IC RC Or IC RC = VCC - VCE

Or IC = VCC/ RC - VCE/ RC

Or IC = (-1/ RC) VCE + VCC/ RC ( ≡ Y= mX + C)

As VCC and RC are fixed values, therefore, it is a first degree

equation and can be represented by a straight line on the output

characteristics. This is known as D.C. Load Line.

To add load line, we need two end points of the straight line.

These two points can be located as under:

(i) When the collector current IC= 0, then collector-emitter voltage is maximum and is equal to VCC

i.e. Max. VCE= VCC– ICRC = VCC (As IC = 0)

This gives the first point B (OB =VCC) on the collector-emitter

voltage axis as shown in Fig.

(ii) When collector-emitter voltage VCE = 0, the collector current

is maximum and is equal to VCC/RC

i.e. VCE= VCC− IC RC or 0 = VCC− IC RC

∴ Max. IC= VCC/RC

This gives the second point A (OA =VCC/RC) on the collector current axis as shown in Fig.

By joining these two points, D.C. Load Line AB is constructed.

(II) A.C. LOAD LINE. This is the line on the output

characteristics of a transistor circuit which gives the values of iC

and vCE when signal is applied.

Referring back to the transistor amplifier shown in Fig., its A.C.

equivalent circuit as far as output circuit is concerned is as

shown in Fig.

To add A.C. load line to the output characteristics, we again require two end points: –

1. One maximum collector-emitter voltage point (VCE MAX) and

2. Other is maximum collector current point. (IC MAX)

Under the application of A.C. signal, these values are Maximum

collector-emitter voltage, VCE MAX = VCE+ IC RAC.

This locates the point C of the A.C. load line on the collector-emitter voltage axis.

Maximum collector current, IC MAX = IC+ VCE/RAC

This locates the point D of A.C. load line on the collector-current axis.

By joining points C and D, the A.C. Load Line CD is constructed.

[ PAGE – 6. 3 ]

OPERATING POINT: -

The zero signal values of IC and VCE are known as the Operating point.

It is called operating point because the variations of IC and VCE take

place about this point when signal is applied.

It is also called quiescent (silent) point or Q-Point because it is the

point on IC –VCE characteristic when the transistor is silent i.e. in

the absence of the signal.

Suppose in the absence of signal, the base current is 5µA. Then IC

and VCE conditions in the circuit must be represented by some

point on IB = 5 µA characteristic.

But IC and VCE conditions in the circuit should also be represented

by some point on the d. c. load line AB.

The point Q where the load line and the characteristic intersect is the only point which satisfies both these conditions. Therefore, the

point Q describes the actual state of affairs in the circuit in the zero signal conditions and is called the

operating point. Referring to Fig, for IB = 5 µA, the zero signal values are :

VCE = OC volts IC = OD mA

It follows, therefore, that the zero signal values of IC and VCE (i.e. operating

point) are determined by the point where d.c. load line intersects at proper

base current curve.

THE LEAKAGE CURRENT:-

The current is due to the movement of minority carriers is known as Leakage Current.

In Common Base Connection of Transistor the leakage current ICBO is the

Collector-Base current with emitter open.

Similarly, In Common Emitter Connection the leakage current ICEO is the Collector-Emitter Current with open Base.

Expression for collector current in Common Base Connection is given by,

Expression for collector current in Common Emitter Connection is given by,

Or

MULTI STAGE TRANSISTOR AMPLIFIER:-

The output from a single stage amplifier is usually insufficient to drive an output device. In other words,

the gain of a single amplifier is inadequate for practical purposes.

Consequently, additional amplification over two or three stages is necessary. To achieve this, the output

of each amplifier stage is coupled in some way to the input of the next stage.

The resulting system is referred to as multistage amplifier.

A transistor circuit containing more than one stage of amplification is known as multistage transistor

amplifier.

In a multistage amplifier, a number of single amplifiers are connected in cascade arrangement i.e. output of first stage is connected to the input of the second stage through a suitable coupling device and so on.

The purpose of coupling device (e.g. a capacitor, transformer etc.) is

(i) to transfer A.C. output of one stage to the input of the next stage and

(ii) to isolate the D.C. conditions of one stage from the next stage.

The name of the amplifier is usually given after the type of coupling used. e.g.

[ PAGE – 6. 4 ]

IMPORTANT TERMS:-

Gain: - The ratio of the output electrical quantity to the input one of the amplifier is called its gain.

The gain of a multistage amplifier is equal to the product of gains of individual stages.

Frequency response: - The curve between voltage gain and signal frequency of an amplifier is known

as frequency response.

Decibel gain: - Although the gain of an amplifier can be expressed as a number, yet great practical

importance to assign it a unit.

The unit assigned is bel or decibel (db). The common logarithm (log to the base 10) of power gain is known as bel power gain i.e (1 bel=10 db.)

Bandwidth: - The range of frequency over which the voltage gain is equal to or greater than 70.7% of

the maximum gain is known as bandwidth.

From the fig. it is clear that for any frequency lying between f1 and f2, the gain is equal to or greater than

70.7% of the maximum gain.

Therefore, f1− f2 is the bandwidth. It may be seen that f1 and f2 are the limiting frequencies. The f1 is called lower cut-off frequency and f2 is known as upper cut-off frequency.

R-C COUPLED TRANSISTOR AMPLIFIER:-

This is the most popular type of coupling because it is cheap and provides excellent audio fidelity over a

wide range of frequency. It is usually employed for voltage amplification.

Fig shows two stages of an RC coupled amplifier. A coupling capacitor CC is used to connect the output of first stage to the base (i.e. input) of the second stage and so on.

As the coupling from one stage to next is achieved by a coupling capacitor followed by a connection to a shunt resistor, therefore, such amplifiers are called Resistance - Capacitance coupled amplifiers.

The resistances R1, R2 and RE form the biasing and stabilization network. The emitter bypass capacitor

offers low reactance path to the signal. Without it, the voltage gain of each stage would be lost.

The coupling capacitor CC transmits A.C. signal but blocks D.C. This prevents D.C. interference between various stages and the shifting of operating point.

[Circuit Diagram of RC Coupled Transistor Amplifier]

OPERATION: -

When A.C. signal is applied to the base of the first transistor, it appears in the amplified form across its

collector load RC.

The amplified signal developed across RC is given to base of next stage through coupling capacitor CC. The second stage does further amplification of the signal.

[ PAGE – 6. 5 ]

In this way, the cascaded (one after another) stages amplify the signal and the overall gain is

considerably increased.

It may be mentioned here that total gain is less than the product of the gains of individual stages.

It is because when a second stage is made to follow the first stage, the effective load resistance of first

stage is reduced due to the shunting effect of the input resistance of second stage.

This reduces the gain of the stage which is loaded by the next stage

FREQUENCY RESPONSE R-C COUPLED TRANSISTOR AMPLIFIER:

Fig shows the frequency response of a typical RC coupled amplifier. It is clear that voltage gain drops

off at low (< 50 Hz) and high (> 20 kHz) frequencies whereas it is uniform over mid-frequency range

(50 Hz to 20 kHz).

This behaviour of the amplifier is briefly explained below:-

(i) At low frequencies (< 50 Hz):- At this stage the reactance of coupling capacitor CC is quite high and

hence very small part of signal will pass from one stage to the

next stage. Moreover, CE cannot shunt the emitter resistance RE

effectively because of its large reactance at low frequencies.

These two factors cause a falling of voltage gain at low

frequencies.

(ii) At high frequencies (> 20 kHz):-At this stage the reactance

of CC is very small and it behaves as a short circuit. These

increases the loading effect of next stage and serves to reduce

the voltage gain.

Moreover, at high frequency, capacitive reactance of base-

emitter junction is low which increases the base current. This

reduces the current amplification factor β. Due to these two reasons, the voltage gain drops off at high

frequency. [Frequency Response Curve of RC Coupled Amp]

(iii) At mid-frequencies (50 Hz to 20 kHz):- At this stage the voltage gain of the amplifier is constant.

The effect of coupling capacitor in this frequency range is such so as to maintain a uniform voltage gain.

Thus, as the frequency increases in this range, reactance of CC decreases which tends to increase the

gain. However, at the same time, lower reactance means higher loading of first stage and hence lower

gain. These two factors almost cancel each other, resulting in a uniform gain at mid-frequency.

ADVANTAGES:-

(i) It has excellent frequency response. The gain is constant over the audio frequency range which is the

region of most importance for speech, music etc.

(ii) It has lower cost since it employs resistors and capacitors which are cheap.

(iii) The circuit is very compact as the modern resistors and capacitors are small and extremely light.

DISADVANTAGES:-

(i) The RC coupled amplifiers have low voltage and power gain. It is because the low resistance

presented by the input of each stage to the preceding stage decreases the effective load resistance (RAC)

and hence the gain.

(ii) They have the tendency to become noisy with age, particularly in moist climates.

(iii) Impedance matching is poor. It is because the output impedance of RC coupled amplifier is several

hundred ohms whereas the input impedance of a speaker is only a few ohms. Hence, little power will be

transferred to the speaker.

APPLICATIONS:-

The RC coupled amplifiers have excellent audio fidelity over a wide range of frequency. Therefore, they

are widely used as voltage amplifiers e.g. in the initial stages of public address system.

If other type of coupling (e.g. transformer coupling) is employed in the initial stages, this results in frequency distortion which may be amplified in next stages.

However, because of poor impedance matching, RC coupling is rarely used in the final stages.

[ PAGE – 6. 6 ]

CIRCUIT DIAGRAM FOR OTHER TYPE OF COUPLING ARE GIVEN BELOW:-

(Transformer Coupled Transistor Amplifier) (Direct Coupled Transistor Amplifier)

Comparison of Different Types of Coupling:-

[FEED BACK AMPLIFIER] INTRODUCTION:-

A practical amplifier has a gain of nearly one million i.e. its output is one million times the input.

Consequently, even a casual disturbance at the input will appear in the amplified form in the output.

The noise in the output of an amplifier is undesirable and must be kept to as small a level as possible.

The noise level in amplifiers can be reduced considerably by the use of negative feedback i.e. by

injecting a fraction of output in phase opposition to the input signal.

The object of this chapter is to consider the effects and methods of providing negative feedback in transistor amplifiers.

FEEDBACK:-

The process of injecting a fraction of output energy of some device back to input is known as feedback.

Depending upon whether the feedback energy aids or opposes the input signal, there are two basic types

of feedback in amplifiers viz Positive Feedback and Negative Feedback.

Positive Feedback. When the feedback energy

(voltage or current) is in phase with the input signal

and thus aids it, it is called positive feedback. This is

illustrated in Fig.

Both amplifier and feedback network introduce a

phase shift of 180°. The result is a 360° phase shift

around the loop, causing the feedback voltage Vf to be

in phase with the input signal Vin.

The positive feedback increases the gain of the

amplifier. However, it has the disadvantages of increased distortion and instability.

Therefore, positive feedback is not often employed in amplifiers.

[ PAGE – 6. 7 ]

One important use of positive feedback is in oscillators. If positive feedback is sufficiently large, it leads

to oscillations. As a matter of fact, an oscillator is a device that converts d.c. power into a.c. power of

any desired frequency.

(ii) Negative Feedback. When the feedback energy

(voltage or current) is out of phase with the input

signal and thus opposes it, it is called negative

feedback. This is illustrated in Fig.

As you can see, the amplifier introduces a phase shift

of 180° into the circuit while the feedback network is

so designed that it introduces no phase shift (i.e., 0°

phase shift). The result is that the feedback voltage Vf

is 180° out of phase with the input signal Vin.

Negative feedback reduces the gain of the amplifier. However, the advantages of negative feedback are: reduction in distortion, stability in gain, increased bandwidth & improved input and output impedances.

It is due to these advantages that negative feedback is frequently employed in amplifiers.

PRINCIPLES OF NEGATIVE VOLTAGE FEEDBACK IN AMPLIFIERS:-

A feedback amplifier has main two parts such as an amplifier and a feedback circuit.

The feedback circuit usually consists of resistors and returns a fraction of output energy back to the input.

Fig. shows the principles of negative voltage feedback in an

amplifier. Typical values have been assumed to make the

treatment more illustrative.

The output of the amplifier is 10 V. The fraction mv of this

output i.e.100 mV is feedback to the input where it is applied

in series with the input signal of 101 mV.

As the feedback is negative, therefore, only 1 mV appears at the input terminals of the amplifier.

Referring to Fig., we have,

Gain of amplifier without feedback, Av = (10 V)/(1 mV) = 10,000

Fraction of output voltage feedback, mv = (100 mV)/10V = 0.01

Gain of amplifier with negative feedback, Avf = 10V/101mV = 100

The following points are worth noting:-

When negative voltage feedback is applied, the gain of the amplifier is reduced. Thus, the gain of above amplifier without feedback is 10,000 whereas with negative feedback, it is only 100.

When negative voltage feedback is employed, the voltage actually applied to the amplifier is extremely small. In this case, the signal voltage is 101 mV and the negative feedback is 100 mV so that voltage applied at the input of the amplifier is only 1 mV.

In a negative voltage feedback circuit, the feedback fraction mv is always between 0 and 1.

The gain with feedback is sometimes called closed-loop gain while the gain without feedback is called

open-loop gain. These terms come from the fact that amplifier and feedback circuits form a “loop”.

When loop is “opened” by disconnecting feedback circuit from I/P, amplifier's gain Av ,[open-loop gain]

When the loop is “closed” by connecting the feedback circuit, gain decreases to Avf [“closed-loop” gain]

GAIN OF NEGATIVE VOLTAGE FEEDBACK AMPLIFIER:-

Consider the negative voltage feedback amplifier shown in Fig.

The gain of the amplifier without feedback is Av.

Negative feedback is then applied by feeding a fraction mv of the

output voltage e0 back to amplifier input.

Therefore, the actual input to the amplifier is the signal voltage eg

minus feedback voltage mv e0 i.e.,

Actual input to amplifier = (eg − mve0)

Avf = Av

1+ Avmv

[ PAGE – 6. 8 ]

The output e0 must be equal to the input voltage (eg − mve0) multiplied by gain Av of the amplifier

i.e. (eg − mve0) Av = e0 Aveg − Av mv e0 = e0

e0 + Av mv e0 = Aveg e0 (1 + Av mv) = Aveg

But e0/eg is the voltage gain of the amplifier with feedback.

∴ Voltage gain with negative feedback is

It may be seen that the gain of the amplifier without feedback is Av. However, when negative voltage

feedback is applied, the gain is reduced by a factor 1 + Av mv.

It may be noted that negative voltage feedback does not affect the current gain of the circuit.

ADVANTAGES OF NEGATIVE VOLTAGE FEEDBACK:-

The following are the advantages of negative voltage feedback in amplifiers:-

Gain Stability. An important advantage of negative voltage feedback is that the resultant gain of the amplifier can be made independent of transistor parameters or the supply voltage variations.

Avf = Æv

1+ Æv Nv

For negative voltage feedback in an amplifier to be effective, the designer deliberately makes the product

Avmv much greater than unity. Therefore, in the above relation, 1 can be neglected as compared to Avmv

and the expression becomes:

Avf = Æv

= 1

Æv Nv Nv

It may be seen that the gain now depends only upon feedback fraction mv i.e., on the characteristics of

feedback circuit. As feedback circuit is usually a voltage divider (a resistive network), therefore, it is

unaffected by changes in temperature, variations in transistor parameters and frequency. Hence, the gain

of the amplifier is extremely stable.

(ii) Reduces non-linear Distortion. A large signal stage has non-linear distortion because its voltage

gain changes at various points in the cycle. The negative voltage feedback reduces the nonlinear distortion in large signal amplifiers.

It can be proved mathematically that:

Dvf = D

1+ Æv Nv

Where D = distortion in amplifier without feedback Dvf = distortion in amplifier with negative feedback

Thus by applying negative voltage feedback to an amplifier, distortion is reduced by a factor 1 + Av mv.

(iii)Improves Frequency Response. As feedback is usually obtained through a resistive network,

therefore, voltage gain of the amplifier is independent of signal frequency.

The result is that voltage gain of the amplifier will be substantially constant over a wide range of signal

frequency. The negative voltage feedback, therefore, improves the frequency response of the amplifier.

(iv)Increases Circuit Stability. The output of an ordinary amplifier is easily changed due to variations in ambient temperature, frequency and signal amplitude.

This changes the gain of the amplifier, resulting in distortion. However, by applying negative voltage feedback, voltage gain of the amplifier is stabilized or accurately fixed in value.

This can be easily explained. Suppose the output of a negative voltage feedback amplifier has increased

because of temperature change or due to some other reason.

This means more negative feedback since feedback is being given from the output. This tends to oppose the increase in amplification and maintains it stable.

The same is true should the output voltage decrease. Consequently, the circuit stability is considerably increased.

(v) Increases input impedance and decreases output impedance. The negative voltage feedback increases the input impedance and decreases the output impedance of amplifier. Such a change is

profitable in practice as the amplifier can then serve the purpose of impedance matching.

eg 1+ Avmv

eO = Av

∴ Z′in = Zin (1 + Aν mν)

∴ Z′out =

[ PAGE – 6. 9 ]

FEEDBACK CIRCUIT:-

The function of the feedback circuit is to return a fraction

of the output voltage to the input of the amplifier.

Fig. shows the feedback circuit of negative voltage feedback amplifier.

It is essentially a potential divider consisting of resistances R1 and R2.

The output voltage of the amplifier is fed to this potential divider which gives the feedback voltage to the input.

Referring to Fig. it is clear that :

Voltage across R1 = ( e0

Feedback fraction, mv = =

INPUT & OUTPUT IMPEDANCE OF NEGATIVE FEEDBACK AMPLIFIER :-

(a) Input impedance. The increase in input impedance with negative voltage feedback can be

explained by referring to Fig.

Suppose the input impedance of the amplifier is Zin without feedback and Z′in with negative feedback.

Let us further assume that input current is i1.

Referring to Fig., we have,

eg − mve0 = i1Zin

Now eg = (eg − mν e0) + mv e0

= (eg − mve0) + Aν mν (eg − mve0) [∵ e0 = Aν (eg − mve0)]

= (eg − mve0) (1 + Aνmν)

= i1Zin (1 + Aν mν) [∵ eg − mv e0 =

i1Zin] Or = Zin (1 + Aν mν)

But = Z′in, the input impedance of the amplifier with

negative voltage feedback.

It is clear that by applying negative voltage feedback, the input impedance of the amplifier is increased by a factor 1 + Aνmv. As Aνmv is much greater than unity.

Therefore, input impedance is increased considerably. This is an advantage, since the amplifier will now present less of a load to its source circuit.

(b) Output impedance. Following similar line, we can show that output impedance with negative voltage feedback is given by :

Where Z′out = output impedance with negative voltage feedback

Zout = output impedance without feedback

It is clear that by applying negative feedback, the output impedance of the amplifier is decreased by a

factor 1 + Aνmν.

This is an added benefit of using negative voltage feedback.

With lower value of output impedance, the amplifier is much better suited to drive low impedance loads.

[ PAGE – 6. 10 ]

[AUDIO POWER AMPLIFIERS] INTRODUCTION:-

A practical amplifier always consists of a number of stages that amplify a weak signal until sufficient

power is available to operate a loudspeaker or other output device.

The first few stages in this multistage amplifier have the function of only voltage amplification. However, last stage is designed to provide maximum power. This final stage is known as power stage.

Transistor Audio Power Amplifier: -

A transistor amplifier which raises the power level of signals having audio frequency range is known as

transistor Audio Power Amplifier. Generally last stage of a multistage amplifier is the power stage.

The power amplifier differs from all the previous stages in that here a concentrated effort is made to obtain maximum output power.

A transistor that is suitable for power amplification is generally called a power transistor.

DIFFERENCE BETWEEN VOLTAGE AND POWER AMPLIFIERS

The difference between the two types is really one of degree; it is a question of how much voltage and

how much power.

A voltage amplifier is designed to achieve maximum voltage amplification. It is, however, not

important to raise the power level.

On the other hand, a power amplifier is designed to obtain maximum output power.

1) Voltage Amplifier. The voltage gain of an amplifier is given by : Av = β × Rc Rin

In order to achieve high voltage amplification, the following features are incorporated in such

amplifiers:

The transistor with high β( >100) is used in the circuit. i.e. Transistors are employed having thin base.

The input resistance Rin of transistor is sought to be quite low as compared to the collector load RC.

A relatively high load RC is used in the collector. To permit this condition, voltage amplifiers are

always operated at low collector currents (≈ mA). If the collector current is small, we can use large

RC in the collector circuit

2) Power Amplifier. A power amplifier is required to deliver a large amount of power and as such it has

to handle large current.

In order to achieve high power amplification, the following features are incorporated in such amplifiers:

The size of power transistor is made considerably larger in order to dissipate the heat produced in the transistor during operation.

The base is made thicker to handle large currents. In other words, transistors with comparatively

smaller β are used.

Transformer coupling is used for impedance matching.

[ PAGE – 6. 11 ]

PERFORMANCE QUANTITIES OF POWER AMPLIFIERS

The prime objective for a power amplifier is to obtain maximum output power. Since a transistor, like

any other electronic device has voltage, current and power dissipation limits, therefore, the criteria for a

power amplifier are : Collector Efficiency, Distortion & Power Dissipation Capability

Collector efficiency.

The main criterion for a power amplifier is not the power gain rather it is the maximum a.c. power output. Now, an amplifier converts d.c. power from supply into a.c. power output.

Therefore, the ability of a power amplifier to convert d.c. power from supply into a.c. output power is a

measure of its effectiveness. This is known as collector efficiency and may be defined as under :

The ratio of a.c. output power to the zero signal power (i.e. d.c. power) supplied by the battery of a power amplifier is known as collector efficiency.

Distortion. The change of output wave shape from input wave shape of amplifier is called Distortion.

Power Dissipation Capability. The ability of a power transistor to dissipate heat is known as power

dissipation capability.

CLASSIFICATION OF POWER AMPLIFIERS

Transistor power amplifiers handle large signals. Many of them are driven by the input large signal that

collector current is either cut-off or is in the saturation region during a large portion of the input cycle.

Therefore, such amplifiers are generally classified according to their mode of operation i.e. the portion

of the input cycle during which the collector current is expected to flow. On this basis, they are

classified as

(i) Class A power amplifier (ii) Class B power amplifier (iii) Class C power amplifier

CLASS A POWER AMPLIFIER. If the collector current flows at all times during the full cycle of

the signal, the power amplifier is known as class A power amplifier.

The power amplifier must be biased in such a way that no part of the signal is cut off. Fig (i) shows

circuit of class A power amplifier. Note that collector has a transformer as the load which is most

common for all classes of power amplifiers.

The use of transformer permits impedance matching, resulting in the transference of maximum power to

the load e.g. loudspeaker. Fig (ii) shows the class A operation in terms of a.c. load line.

The operating point Q is so selected that collector current flows at all times throughout the full cycle of

the applied signal. As the output wave shape is exactly similar to the input wave shape, therefore, such

amplifiers have least distortion.

However, they have the disadvantage of low power output and low collector efficiency (about 35%).

CLASS B POWER AMPLIFIER: - If the collector current flows only during the positive half-cycle of

the input signal, it is called a class B power amplifier.

In class B operation, the transistor bias is so adjusted that zero signal collector current is zero i.e. no

biasing circuit is needed at all.

During the positive half-cycle of the signal, the input circuit is forward biased and hence collector

current flows. However, during the negative half-cycle of the signal, the input circuit is reverse biased

and no collector current flows.

[ PAGE – 6. 12 ]

Fig. shows the class B operation in terms of a.c. load line.

The operating point Q shall be located at collector cut off voltage.

It is easy to see that output from a class B amplifier is

amplified half-wave rectification.

In a class B amplifier, the negative half-cycle of the signal is cut off and hence a severe distortion occurs.

However, class B amplifiers provide higher power output

and collector efficiency (50 − 60%).

Such amplifiers are mostly used for power amplification

in push-pull arrangement.

In such an arrangement, 2 transistors are used in class B

operation. One transistor amplifies the positive half cycle of the signal while the other amplifies the

negative half-cycle.

CLASS C POWER AMPLIFIER. If the collector current flows for less than half-cycle of the input

signal, it is called class C power amplifier.

In class C amplifier, the base is given some negative bias so that collector current does not flow just

when the positive half-cycle of the signal starts.

Such amplifiers are never used for power amplification. However, they are used as tuned amplifiers i.e.

to amplify a narrow band of frequencies near the resonant frequency.

EXPRESSION FOR COLLECTOR EFFICIENCY

For comparing power amplifiers, collector efficiency is the main criterion. The greater the collector

efficiency, the better is the power amplifier. Now, Collector Efficiency, η = a.c. powe r output = PO

d.c. powe r input Pdc

Where Pdc = VCC IC & PO = VCE IC in which VCE is the r.m.s. value of signal output voltage and Ic is

the r.m.s. value of output signal current.

In terms of peak-to-peak values, the a.c. power output can be expressed as: P = [(0.5×0.707) v ][(0.5 ×0.707) i ] =

Vce(p—p) × ic(p—p)

o ce(p − p) c(p − p) 8

∴ Collector η = Vce(p—p) × ic(p—p) 8 Vcc Ic

IMPORTANT POINTS ABOUT CLASS-A POWER AMPLIFIER : -

(i) A Transformer coupled class A power amplifier has a maximum collector efficiency of 50% i.e., maximum of 50% d.c. supply power is converted into a.c. power output.

In practice, the efficiency of such an amplifier is less than 50% (about 35%) due to power losses in the

output transformer, power dissipation in the transistor etc.

(ii) The power dissipated by a transistor is given by :

Pdis = Pdc − Pac

Where Pdc= available d.c. power

& Pac = available a.c. power

So, In class A operation, Transistor must dissipate less heat when signal is applied therefore runs cooler.

(iii) When no signal is applied to a class A power amplifier, Pac= 0. ∴ Pdis= Pdc

Thus in class A operation, maximum power dissipation in the transistor occurs under zero signal

conditions.

Therefore, the power dissipation capability of a power transistor (for class A operation) must be at least

equal to the zero signal rating.

(iv) When a class A power amplifier used in final stage, it is called single ended class A power

amplifier.

PUSH-PULL AMPLIFIER : - [ PAGE – 6. 13 ]

The push-pull amplifier is a power amplifier and is frequently employed in the output stages of

electronic circuits. It is used whenever high output power at high efficiency is required. Fig. shows the

circuit of a push-pull amplifier.

Two transistors Tr1 and Tr2 placed back to back are employed. Both transistors are operated in class B

operation i.e. collector current is nearly zero in the absence of the signal.

The centre tapped secondary of driver transformer T1 supplies equal and opposite voltages to the base

circuits of two transistors. The output transformer T2 has the centre-tapped primary winding. The supply

voltage VCC is connected between the bases and this centre tap.

The loudspeaker is connected across the secondary of this transformer.

CIRCUIT OPERATION.

The input signal appears across the secondary AB of driver transformer. Suppose during the first half-

cycle (marked 1) of the signal, end A becomes positive and end B negative.

This will make the base-emitter junction of Tr1 reverse biased and that of Tr2 forward biased. The circuit will conduct current due to Tr2 only and is shown by solid arrows.

Therefore, this half-cycle of the signal is amplified by Tr2 and appears in the lower half of the primary of

output transformer. In the next half cycle of the signal, Tr1 is forward biased whereas Tr2 is reverse

biased. Therefore, Tr1 conducts and is shown by dotted arrows.

Consequently, this half-cycle of the signal is amplified by Tr1 and appears in the upper half of the output

transformer primary. The centre-tapped primary of the output transformer combines two collector

currents to form a sine wave output in the secondary.

It may be noted here that push-pull arrangement also permits a maximum transfer of power to the Load

through impedance matching. If RL is the resistance appearing across secondary of output transformer,

then resistance R′L of primary shall become:

R′L = (2N1) 2 RL N2

Where N1 = Number of turns between either end of primary winding and centre-tap N2 = Number of secondary turns

ADVANTAGES

1) The efficiency of the circuit is quite high (≈ 75%) due to class B operation. 2) A high a.c. output power is obtained.

DISADVANTAGES

1) Two transistors have to be used. 2) It requires two equal and opposite voltages at the input. Therefore, push-pull circuit requires the use

of driver stage to furnish these signals.

3) If the parameters of the two transistors are not the same, there will be unequal amplification of the

two halves of the signal.

4) The circuit gives more distortion.

5) Transformers used are bulky and expensive.

COMPLEMENTARY-SYMMETRY AMPLIFIER

[ PAGE – 6. 14 ]

By complementary symmetry is meant a principle of assembling push-pull class B amplifier without

requiring centre-tapped transformers at the input and output stages.

Fig. shows the transistor push-pull amplifier using complementary symmetry. It employs one npn and one pnp transistor and requires no centre-tapped transformers.

The circuit action is as follows. During the positive-half of the input signal, transistor T1 (the npn

transistor) conducts current while T2(the pnp transistor) is cutoff.

During the negative half-cycle of the signal, T2 conducts while T1 is cut off. In this way, npn transistor

amplifies the positive half-cycles of the signal while the pnp transistor amplifies the negative half-

cycles of the signal.

Note that we generally use an output transformer (not centre-tapped) for impedance matching.

Advantages: - (1) This circuit does not require transformer. This saves on weight and cost.

(2) Equal and opposite input signal voltages are not required.

Disadvantages: -(1) It is difficult to get a pair of transistors (npn & pnp) having similar characteristics.

(2) We require both positive and negative supply voltages.

HEAT SINK: -

As power transistors handle large currents, they always heat up during operation. Since transistor is a

temperature dependent device, the heat must be dissipated to the surroundings to keep the temperature

within allowed limits.

Usually transistor is fixed on Aluminum metal sheet so that additional heat is transferred to the Al sheet.

The metal sheet that serves to dissipate the additional heat from power transistor is known as Heat

Sink.


[ PAGE – 6. 15 ]

[SINUSOIDAL OSCILLATOR] INTRODUCTION TO OSCILLATOR: -

Many electronic devices require a source of energy at a specific frequency which may range from a few

Hz to several MHz. This is achieved by an electronic device called an oscillator.

Oscillators are extensively used in electronic equipment. For example, in radio and television receivers, oscillators are used to generate high frequency wave (called carrier wave) in the tuning stages.

Audio frequency and radiofrequency signals are required for the repair of radio, television and other

electronic equipment. Oscillators are also widely used in radar, electronic computers and other

electronic devices. Oscillators can produce sinusoidal or non-sinusoidal (e.g. square wave) waves.

SINUSOIDAL OSCILLATORS:-

An electronic device that generates sinusoidal oscillations of desired frequency is known as a

sinusoidal oscillator. Although we speak of an oscillator as “generating” a frequency, it should be

noted that it does not create energy, but merely acts as an energy converter.

It receives D.C. energy and changes it into A.C. energy of our desired frequency.

The frequency of oscillations depends upon the constants of the device. It may be mentioned here that

although an alternator produces sinusoidal oscillations of 50Hz, it cannot be called an oscillator.

Firstly, An alternator is a mechanical device having rotating parts whereas an oscillator is a non- rotating electronic device.

Secondly, An alternator converts Mechanical Energy into A.C. Energy while an oscillator converts

D.C. Energy into A.C. energy.

Thirdly, An alternator cannot produce high frequency oscillations whereas an oscillator can produce

oscillations ranging from a few Hz to several MHz.

ADVANTAGES

Although oscillations can be produced by mechanical devices (e.g. alternators), but electronic

oscillators have the following advantages:

An oscillator is a non-rotating device. Consequently, there is little wear and tear and hence longer life.

Due to the absence of moving parts, the operation of an oscillator is quite silent.

An oscillator can produce waves from small (20 Hz) to extremely high frequencies (> 100 MHz).

The frequency of oscillations can be easily changed when desired.

It has good frequency stability i.e. frequency once set remains constant for considerable period of time.

It has very high efficiency.

TYPES OF SINUSOIDAL OSCILLATIONS:-

Sinusoidal oscillations can be of two types viz Damped Oscillations and Undamped Oscillations.

(i) Damped Oscillations: - The electrical oscillations whose amplitude goes on decreasing with time are called damped oscillations. Fig (i) Shows waveform of damped

electrical oscillations.

Obviously, the electrical system in which these oscillations are

generated has losses and some energy is lost during each oscillation.

Further, no means are provided to compensate for the losses and

consequently the amplitude of the generated wave decreases gradually.

It may be noted that frequency of oscillations remains unchanged

since it depends upon the constants of the electrical system.

(ii) Undamped Oscillations. The electrical oscillations whose amplitude remains constant with time are

called undamped oscillations. Fig. (ii) Shows waveform of undamped

electrical oscillations.

Although the electrical system in which these oscillations are being

generated has also losses, but now right amount of energy is being

supplied to overcome the losses.

Consequently, amplitude of generated wave remains constant. It should

be emphasized that an oscillator is required to produce undamped

electrical oscillations for utilizing in various electronics equipment.

OSCILLATORY CIRCUIT: -

[ PAGE – 6. 16 ]

A circuit which produces electrical oscillations of any desired frequency is known as an Oscillatory

Circuit or Tank Circuit.

A simple oscillatory circuit consists of a capacitor (C) and inductance coil (L) in parallel as shown in Fig. This system can produce electrical oscillations of frequency determined by the values of L and C.

To understand how this comes about, suppose the capacitor is charged from a d.c. source with a polarity as shown in Fig. (i).

(i) In the position shown in Fig (i), the upper plate of capacitor has deficit of electrons and the lower

plate has excess of electrons. Therefore, there is a voltage across the capacitor and the capacitor has

electrostatic energy.

(ii) When switch Sis closed as shown in Fig (ii), the capacitor will discharge through inductance and the

electron flow will be in the direction indicated by the arrow.

This current flow sets up magnetic field around the coil. Due to the inductive effect, the current builds up slowly towards a maximum value.

The circuit current will be maximum when the capacitor is fully discharged. At this instant, electrostatic

energy is zero but because electron motion is greatest (i.e. maximum current), the magnetic field energy

around the coil is maximum. This is shown in Fig (ii).

Obviously, the electrostatic energy across the capacitor is completely converted into magnetic field

energy around the coil.

(iii) Once the capacitor is discharged, the magnetic field will begin to collapse and produce a counter

e.m.f. According to Lenz's law, the counter e.m.f. will keep the current flowing in the same direction.

The result is that the capacitor is now charged with opposite polarity, making upper plate of capacitor

negative and lower plate positive as shown in Fig (iii).

(iv) After the collapsing field has recharged the capacitor, the capacitor now begins to discharge; current now flowing in the opposite direction.

Fig (iv) shows capacitor fully discharged and maximum current flowing. The sequence of charge and discharge results in alternating motion of electrons or an oscillating current.

The energy is alternately stored in the electric field of the capacitor (C) and the magnetic field of the

inductance coil (L). This interchange of energy between L and C is repeated over and again resulting in

the production of oscillations.

UNDAMPED OSCILLATIONS FROM TANK CIRCUIT:-

As discussed before, a tank circuit produces damped oscillations. However, in practice, we need

continuous undamped oscillations for the successful operation of electronics equipment.

In order to make the oscillations in the tank circuit undamped, it is necessary to supply correct amount

of energy to tank circuit at proper time intervals to meet the losses.

Therefore, in order to make the oscillations in the tank circuit undamped,

the following conditions must be fulfilled :

(i) The amount of energy supplied should be such so as to meet the losses in the tank circuit & the a.c. energy removed from the circuit by the load.

(ii) The applied energy should have the same frequency as tank circuit.

(iii) The applied energy should be in phase with the oscillations set up in the tank circuit.

If these conditions are fulfilled, the circuit will produce continuous undamped output as shown in Fig.

f =

mvAv = 1

[ PAGE – 6. 17 ]

Therefore, the amplifier will produce sinusoidal output with no external signal source. The following

points may be noted carefully :

A transistor amplifier with proper positive feedback will work as an oscillator.

The circuit needs only a quick trigger signal to start the oscillations.

Once the oscillations have started, no external signal source is needed.

In order to get continuous undamped output from the circuit, the following condition must be met:

Where Av = Voltage Gain of Amplifier without Feedback and mv = Feedback Fraction

This relation is called Barkhausen Criterion.

ESSENTIALS OF TRANSISTOR OSCILLATOR: -

Fig shows the block diagram of an oscillator. Its essential components are : -

Tank circuit. It consists of inductance coil (L) connected in parallel with capacitor (C).

The frequency of oscillations circuit depend upon the values

of inductance of the coil and capacitance of the capacitor.

(ii) Transistor Amplifier. The transistor amplifier receives

D.C. power from the battery and changes it into a.c. power for

supplying to the tank circuit.

The oscillations occurring in the tank circuit are applied

to the input of the transistor amplifier. Because of the

amplifying properties of the transistor, we get increased

output of these oscillations.

This amplified output of oscillations is due to the D.C. power supplied by the battery.

The output of the transistor can be supplied to the tank circuit to meet the losses.

(iii) Feedback Circuit. The feedback circuit supplies a part of collector energy to the tank circuit in

correct phase to aid the oscillations i.e. it provides positive feedback.

DIFFERENT TYPES OF TRANSISTOR OSCILLATORS:-

A transistor can work as an oscillator to produce continuous undamped oscillations of any desired

frequency if tank and feedback circuits are properly connected to it.

All oscillators under different names have similar function i.e., they produce continuous undamped

output. However, the major difference between these oscillators lies in the method by which energy is

supplied to the tank circuit to meet the losses.

The following are the transistor oscillators commonly used at various places in electronic circuits:

(i) Tuned Collector Oscillator (ii) Colpitt’s Oscillator (iii) Hartley Oscillator (iv) Phase Shift Oscillator (v) Wien Bridge Oscillator (vi) Crystal Oscillator

TUNED COLLECTOR OSCILLATOR:-

Fig shows circuit of tuned collector oscillator. It contains

tuned circuit L1 - C1 in the collector and hence the name.

The frequency of oscillations depends upon the values

of L1 and C1 and is given by :

The feedback coil L2 in the base circuit is magnetically

coupled to the tank circuit coil L1. In practice, L1 and L2 form the primary and secondary of the

transformer respectively.

The biasing is provided by potential divider arrangement. The capacitor C connected in the base circuit

provides low reactance path to the oscillations.

Circuit Operation. When switch S is closed, collector current starts increasing and charges the

capacitor C1. When this capacitor is fully charged, it discharges through coil L1, setting up oscillations

of frequency determined by above equation.

f =

f =

[ PAGE – 6. 18 ]

These oscillations induce some voltage in coil L2 by mutual induction. The frequency of voltage in

coil L2 is the same as that of tank circuit but its magnitude depends upon the number of turns of L2

and coupling between L1 and L2.

The voltage across L2 is applied between base and emitter and appears in the amplified form in the

collector circuit, thus overcoming the losses occurring in the tank circuit.

The number of turns of L2 and coupling between L1 and L2 are so adjusted that oscillations across L2

are amplified to a level just sufficient to supply losses to the tank circuit.

It may be noted that the phase of feedback is correct i.e. energy supplied to the tank circuit is in phase

with the generated oscillations. A phase shift of 180º is created between the voltages of L1 and L2 due to

transformer action.

A further phase shift of 180º takes place between base-emitter and collector circuit due to transistor

properties. As a result, the energy feedback to the tank circuit is in phase with the generated oscillations.

COLPITT’S OSCILLATOR:-

Fig shows a Colpitt's oscillator. It uses two

capacitors and placed across a common inductor L and the

centre of the two capacitors is tapped.

The tank circuit is made up of C1, C2 and L. The frequency

of oscillations is determined by the values of C1, C2 and L and

is given by ;

Where CT =

Note that C1− C2− L is also the feedback circuit that

produces a phase shift of 180°.

Circuit Operation. When the circuit is turned on, the capacitors C1 and C2 are charged. The capacitors discharge through L, setting up oscillations of frequency determined by exp.(i).

Output voltage of the amplifier appears across C1 and feedback voltage is developed across C2.

The voltage across it is 180° out of phase with the voltage developed across C1 (Vout) as shown in Fig.

It is easy to see that voltage feedback (voltage across C2) to the transistor provides positive feedback.

A phase shift of 180° is produced by transistor and a further

phase shift of 180° is produced by C1− C2 voltage divider. In

this way, feedback is properly phased to produce continuous

undamped oscillation.

HARTLEY OSCILLATOR:-

The Hartley oscillator is similar to Colpitt’s oscillator with

minor modifications. Instead of using tapped capacitors, two

inductors L1 and L2 are placed across a common capacitor C and

the centre of the inductors is tapped as shown in Fig.

The tank circuit is made up of L1, L2 and C. The frequency of oscillations is determined by the values

of L1, L2 and C and is given by :

……….. (i)

Where LT =L1 + L2 + 2M & M= Mutual inductance between L1 & L2

Circuit Operation. When the circuit is turned on, the capacitor is charged. When this capacitor is fully charged, it discharges through coils L1 and L2 setting up oscillations of frequency determined by equ (i).

The output voltage of the amplifier appears across L1 and feedback

voltage across L2. The voltage across L2 is 180° out of phase with the voltage developed across L1 (Vout)

as shown in Fig.

It is easy to see that voltage feedback (i.e., voltage across L2) to transistor provides positive feedback.

A phase shift of 180° is produced by the transistor & further phase shift of 180° is produced by L1 − L2

voltage divider. In this way, feedback is properly phased to produce continuous undamped oscillations.

PRINCIPLE OF PHASE SHIFT OSCILLATORS:-

[ PAGE – 6. 19 ]

One desirable feature of an oscillator is that it should feedback energy of correct phase to

the tank circuit to overcome the losses occurring in it.

In the oscillator circuits discussed so far, the tank circuit employed inductive (L) and capacitive

(C) elements. In such circuits, a phase shift of 180º was obtained due to inductive or capacitive

coupling and a further phase shift of 180º was obtained due to transistor properties.

In this way, energy supplied to the tank circuit was in phase with the generated oscillations.

The oscillator circuits employing L-C elements have two general drawbacks.

Firstly, they suffer from frequency instability and poor waveform. Secondly, they cannot be used

for very low frequencies because they become too much bulky and expensive.

Good frequency stability and waveform can be obtained from oscillators employing

resistive and capacitive elements. Such amplifiers are called R-C or phase shift oscillators and have

the additional advantage that they can be used for very low frequencies.

In a phase shift oscillator, a phase shift of 180º is obtained with a phase

shift circuit instead of inductive or capacitive coupling.

A further phase shift of 180º is introduced due to the transistor properties.

Thus, energy supplied back to the tank circuit is assured of correct phase.

Phase shift Circuit. A phase-shift circuit essentially consists of an R-C network.

Fig (i) shows a single section of RC network. From the elementary theory of

electrical engineering, it can be shown that alternating

voltage V′1across R leads the applied voltage V1 by φº. The

value of φ depends upon the values of R and C.

If resistance R is varied, the value of φ also changes. If R

were reduced to zero, V′1 will lead V1 by 90º i.e. φ= 90º.

However, adjusting R to zero would be impracticable

because it would lead to no voltage across R.

Therefore, in practice, R is varied to such a value that makes V′1 to lead V1 by 60º.

Fig (ii) shows the three sections of RC network. Each section produces a phase shift of 60º.

Consequently, a total phase shift of 180º is produced i.e. voltage V2 leads the voltage V1 by 180º.

PHASE SHIFT OSCILLATOR:-

Fig. shows the circuit of a phase shift oscillator. It consists of a conventional single transistor amplifier

and a RC phase shift network.

The phase shift network consists of three sections R1C1, R2C2 and R3C3. At some particular frequency

f0, the phase shift in each RC section is 60º so that total phase-shift produced by the RC network is 180º.

The frequency of oscillations is given by:

Where R1=R2 =R3 = R & C1= C2 = C3 =C

Circuit Operation. When the circuit is switched on, it produces oscillations of frequency determined

by exp. (i). The output E0 of the amplifier is fed back to RC feedback network.

This network produces a phase shift of 180º and a voltage Ei appears at its output which is applied

to the transistor amplifier.

Obviously, the feedback fraction m= Ei/E0. The

feedback phase is correct. A phase shift of 180º

is produced by the transistor amplifier.

A further phase shift of 180º is produced by the

RC network. As a result, the phase shift around

the entire loop is 360º.

Advantages

It does not require transformers or inductors.

It can be used to produce very low frequencies.

The circuit provides good frequency stability.

f0 =

f =

[ PAGE – 6. 20 ]

Disadvantages

It is difficult for the circuit to start oscillations as the feedback is generally small.

The circuit gives small output.

WIEN BRIDGE OSCILLATOR:-

The Wien-bridge oscillator is the standard oscillator circuit for all frequencies in the range of 10 Hz to

about 1 MHz. It is the most frequently used type of audio oscillator as the output is free from

circuit fluctuations and ambient temperature.

Fig. shows the circuit of Wien bridge oscillator. It is essentially a two-stage amplifier with R-C bridge

circuit. The bridge circuit has the arms R1C1, R3, R2C2 and tungsten lamp Lp.

Resistances R3 and Lp are used to stabilize the amplitude of the output. The transistor T1 serves as an

oscillator and amplifier while the other transistor T2 serves as an inverter (to produce 180º phase shift).

The circuit uses positive and negative feedbacks. The positive feedback is through R1C1, C2R2

to the transistor T1. The negative feedback is through the voltage divider to the input of transistor T2.

The frequency of oscillations is determined by the series element R1C1 and parallel element R2C2

of the bridge.

If R1 = R2 = R and C1 = C2 = C, then, f =

When the circuit is started, bridge

circuitproduces oscillations of frequency

determined.

The two transistors produce a total phase shift

of 360º so that proper positive feedback is

ensured.

The negative feedback in the circuit ensures

constant output. This is achieved by the

temperature sensitive tungsten lamp Lp. Its

resistance increases with current.

Should the amplitude of output tend

to increase, more current would provide

more negative feedback.

The result is that the output would return

to original value.

A reverse action would take place if the output tends to decrease.

Advantages

(i) It gives constant output. (ii) It works quite easily.

(iii) Overall gain is high due to two transistors.

(iv) The frequency of oscillations can be easily changed by using a potentiometer.

Disadvantages

(v) It requires two transistors & large number of components. (vi) It cannot generate very high frequencies.

LIMITATIONS OF LC AND RC OSCILLATORS:-

The LC and RC oscillators discussed so far have their own limitations. The major problem in such

circuits is that their operating frequency does not remain strictly constant. There are two principal

reasons for it viz.,

(i) As the circuit operates, it will warm up. Consequently, the values of resistors and inductors, which

are the frequency determining factors in these circuits, will change with temperature.

This causes the change in frequency of the oscillator.

(ii) If any component in the feedback network is changed, it will shift the operating frequency

of the oscillator. However, in many applications, it is desirable and necessary to maintain the

frequency constant with extreme low tolerances.

[ PAGE – 6. 21 ]

It is apparent that if we employ LC or RC circuits, a change of temperature may cause the frequencies

of adjacent broadcasting stations to overlap.

In order to maintain constant frequency, piezoelectric crystals are used in place of LC or RC circuits.

Oscillators of this type are called crystal oscillators.

The frequency of a crystal oscillator changes by less than 0.1% due to temperature and other changes.

Therefore, such oscillators offer the most satisfactory method of stabilizing the frequency and are used

in great majority of electronic applications.

PIEZOELECTRIC CRYSTALS:-

Certain crystalline materials, namely, Rochelle salt, quartz and tourmaline exhibit the piezoelectric

effect i.e., when we apply an a.c. voltage across them, they vibrate at the frequency of the applied

voltage. Conversely, when they are compressed or placed under mechanical strain to vibrate, they

produce an a.c. voltage.

Such crystals which exhibit piezoelectric effect are called piezoelectric crystals. Of the

variouspiezoelectric crystals, quartz is most commonly used as it is inexpensive & readily available in nature.

Quartz Crystal. Quartz crystals are generally used in crystal oscillators

becauseof their great mechanical strength and simplicity of manufacture.

The natural shape of quartz crystal is hexagonal as shown in Fig. The threeaxes

are shown: the z-axis is called the optical axis, the x-axis is called the electrical

axis and y-axis is called the mechanical axis.

Quartz crystal can be cut in different ways. Crystal cut perpendicular to the x-

axis is called x-cut crystal whereas that cut perpendicular to y-axis is called

y- cut crystal. The piezoelectric properties of a crystal depend upon its cut.

Frequency of Crystal. Each crystal has a natural frequency like a pendulum.

The natural frequency f of a crystal is given by: f = K t

Where,

K = Constant that depends upon the cut & t = Thickness of the crystal.

It is clear that frequency is inversely proportional to crystal thickness. The thinner the

crystal, the greater is its natural frequency and vice-versa.

However, extremely thin crystal may break because of vibrations. This puts a limit to the frequency

obtainable. In practice, frequencies between 25 kHz to 5 MHz have been

obtained with crystals.

WORKING OF QUARTZ CRYSTAL:-

In order to use crystal in an electronic circuit, it is placed

between two metal plates. The arrangement then forms a capacitor with

crystal as the dielectric as shown in Fig.

If an a.c. voltage is applied across the plates, the crystal will start vibrating at the frequency of

appliedvoltage. However, if the frequency of the applied voltage is made equal to the natural frequency of

the crystal, resonance takes place and crystal vibrations reach a maximum value.

This natural frequency is almost constant. Effects of temperature change can be eliminated by mounting

the crystal in a temperature-controlled oven as in radio and television transmitters.

---P N GOUDA----ALL THE BEST ------------------ ALL

THE BEST---- P N GOUDA ----

CHAPTER - 7

[ PAGE – 7. 1 ]

--------------------------- [FIELD EFFECT TRANSISTOR (FET)] ------------------------ INTRODUCTION: -

In the previous chapters, we have discussed the circuit applications of an ordinary transistor. In this type

of transistor, both holes and electrons play part in the conduction process. For this reason, it is

sometimes called a Bipolar Transistor.

The ordinary or bipolar transistor has two principal disadvantages. First, it has low input impedance

because of forward biased emitter junction. Secondly, it has considerable noise level.

Although low input impedance problem may be improved by careful design and use of more than one

transistor, yet it is difficult to achieve input impedance more than a few mega ohms.

The field effect transistor (FET) has, by virtue of its construction and biasing, large input impedance

which may be more than 100 mega ohms.

The FET is generally much less noisy than the ordinary or bipolar transistor. The rapidly expanding

FET market has led many semiconductor marketing managers to believe that this device will soon

become the most important electronic device, primarily because of its integrated-circuit applications.

CLASSIFICATION OF FIELD EFFECT TRANSISTORS: -

Other types of C-MOS also There Such as: -CMOS, VMOS, LDMOS etc.

DIFFERENTIATION BETWEEN BJT & FET : -

FET BJT

It means Field Effect Transistor Means Bipolar Junction Transistor

Its three terminals are Source, Gate & Drain Its terminals are Emitter, Base & Collector.

It is Unipolar devices i.e. Current in the

device is carried either by electrons or holes.

It is Bipolar devices i.e. Current in the device

is carried by both electrons and holes.

It is Voltage controlled device. i.e. Voltage at

the gate or drain terminal controls the amount of current flowing through the devices.

It is Current controlled device. i.e. Base

Current controls the amount of collector current flowing through the devices.

It has very High Input Resistance and Low

Output Resistance.

It has very Low Input Resistance and High

Output Resistance.

Low noisy operation High noisy operation

It is Longer Life & High Efficiency. It is Shorter Life & Low Efficiency.

It is much simpler to fabricate as IC and

occupies less space on IC.

It is comparatively difficult to fabricate as IC

and occupies more space on IC then FET.

It has Small gain bandwidth product. It has Large gain bandwidth product.

It has higher switching speed. It has higher switching speed.

N-Channel JFE

T P-Channel

FET

Depletion

Type

N-Channel

P-Channel MOSF

ET Enhance

me

nt Type

N-Channel

P-Channel

JUNCTION FIELD EFFECT TRANSISTOR (JFET) : -

[ PAGE – 7. 2 ]

A junction field effect transistor is a three terminal semiconductor device in which current conduction is

by one type of carrier i.e., electrons or holes.

In a JFET, the current conduction is either by electrons or holes and is controlled by means of an electric field between the gate electrode and the conducting channel of the device.

The JFET has high input impedance and low noise level.

CONSTRUCTIONAL DETAILS.

A JFET consists of a p-type or n-type silicon bar containing two pn junctions at the sides as shown in Fig.

The bar forms the conducting channel for the charge

carriers. If the bar is of n-type, it is called n-channel

JFET as shown in Fig (i) and if the bar is of p-type, it is

called a p-channel JFET as shown in Fig (ii).

The two pn junctions forming diodes are connected

internally & a common terminal called gate is taken out.

Other terminals are source and drain taken out from the

bar as shown. Thus a JFET has essentially three

terminals viz., Gate (G), Source (S) & Drain (D).

JFET POLARITIES: - Fig (i) shows n-channel JFET polarities whereas

Fig (ii) shows the p-channel JFET polarities.

Note that in each case, voltage between gate and source is such that the gate is reversing biased.

This is the normal way of JFET connection.

The drain & source terminals are interchangeable

i.e., either end can be used as source and the

other end as drain.

The following points may be noted:

The input circuit (i.e. gate to source) of a JFET is reverse biased. This means that the device has

high input impedance.

The drain is so biased w.r.t. source that drain current ID flows from the source to drain.

In all JFETs, source current IS is equal to the drain current i.e. IS = ID.

WORKING PRINCIPLE OF JFET:-

Principle: - Fig. shows the circuit of n-

channel JFET with normal polarities. Note that the gate is reverse biased.

The two pn junctions at the sides form two

depletion layers. The current conduction by

charge carriers (i.e. free electrons in this

case) is through the channel between the two

depletion layers and out of the drain.

The width and hence resistance of this channel can be controlled by changing the input voltage VGS.

The greater the reverse voltage VGS, the wider will be the depletion layers and narrower will be the

conducting channel. The narrower channel means greater resistance and hence source to drain current

decreases. Reverse will happen should VGS decrease.

Thus JFET operates on the principle that width and hence resistance of the conducting channel can be

varied by changing the reverse voltage VGS.

In other words, the magnitude of drain current (ID) can be changed by altering VGS.

Working: - The working of JFET is as under :

(i) When voltage VDS is applied between drain & source terminals and voltage on the gate is zero [See

the above Fig (i)], the two pn junctions at the sides of the bar establish depletion layers.

The electrons will flow from source to drain through a channel between the depletion layers.

[ PAGE – 7. 3 ]

The size of these layers determines width of the channel & hence current conduction through the bar.

(ii) When a reverse voltage VGS is applied between the gate and source [See Fig (ii)], the width of the depletion layers is increased.

This reduces the width of conducting channel, thereby increasing the resistance of n-type bar. Consequently, the current from source to drain is decreased.

On the other hand, if the reverse voltage on the gate is decreased, the width of the depletion layers also decreases. This increases the width of the conducting channel and hence source to drain current.

It is clear from the above discussion that current from source to drain can be controlled by the application of potential (i.e. electric field) on the gate.

For this reason, the device is called field effect transistor. It may be noted that a p-channel JFET

operates in the same manner as an n-channel JFET except that channel current carriers will be the holes

instead of electrons and the polarities of VGS and VDS are reversed.

JFET AS AN AMPLIFIER :- Fig shows JFET amplifier circuit. The weak signal is applied between

gate and source and amplified output is obtained in the drain-source

circuit. For the proper operation of JFET, the gate must be negative

w.r.t. source i.e., input circuit should always be reverse biased.

This is achieved either by inserting a battery VGG in the gate circuit or by a circuit known as biasing circuit. [Schematic Symbol of JFET]

In the present case, we are providing biasing by the battery VGG. A small change in the reverse bias on

the gate produces a large change in drain current.

This fact makes JFET capable of raising the strength of a weak signal.

During the positive half of signal, the reverse bias on the gate

decreases. This increases the channel width and hence the drain current.

During the negative half-cycle of the signal, the reverse voltage on the gate increases. Consequently, the drain current decreases.

The result is that a small change in voltage at the gate produces a large change in drain current.

These large variations in drain current produce large output across the load RL. In this way, JFET acts as an amplifier

OUTPUT CHARACTERISTICS OF JFET

The curve between drain current (ID) and drain-source voltage (VDS) of

a JFET at constant gate source voltage (VGS) is known as output

characteristics of JFET.

Fig shows circuit for determining output characteristics of JFET.

Keeping VGS fixed at some value, say 1V, the drain source voltage is changed in steps.

Corresponding to each value of VDS, the drain current ID is noted.

A plot of these values gives output characteristic of JFET at VGS= 1V.

Repeating similar procedure, output characteristics at other gate-source voltages can be drawn. Fig.

shows a family of output characteristics.

The following points may be noted from the characteristics:

(i) At first, the drain current ID rises rapidly with drain-source voltage VDS but then becomes constant.

The drain-source voltage above which drain current becomes constant is known as pinch off voltage. Thus in Fig. OA is the pinch off voltage VP.

(ii) After pinch off voltage, the channel width becomes so narrow that depletion layers almost touch each other.

The drain current passes through the small passage between these layers.

Thus increase in drain current is very small with VDS above pinch off

voltage.

Consequently, drain current remains constant. The characteristics resemble that of a pentode valve.

Transconductance, gfs = AID at constant VDS AVGS

A.C. Drain Resistance, rd = A VDS at constant VGS A ID

d

fs

Amplification Factor, µ = AVDS at constant ID AVGS

[ PAGE – 7. 4 ]

IMPORTANT TERMS : -

1. Shorted-Gate Drain Current (IDSS): -

It is the drain current with source short-circuited to gate (i.e. VGS = 0) and drain voltage (VDS) equal to

pinch off voltage. It is sometimes called zero-bias current.

2. Pinch Off Voltage (VP) : -

It is the minimum drain-source voltage at which the drain current essentially becomes constant.

3. Gate-Source Cut Off Voltage VGS (off): -

It is the gate-source voltage where the channel is completely cut off & the drain current becomes zero.

PARAMETERS OF JFET: - Like vacuum tubes, a JFET has certain parameters which determine its performance in a circuit. The

main parameters of JFET are: - (i) A.C. drain resistance (ii) Transconductance (iii) Amplification factor.

(i) A.C. Drain Resistance (rd). Corresponding to the a.c. plate resistance, we have a.c. drain resistance

in a JFET. It may be defined as follows :

It is the ratio of change in drain-source voltage (ΔVDS)to the change in drain current(ΔID) at constant

gate-source voltage i.e.

For instance, if a change in drain voltage of 2 V produces a change in drain current of 0.02 mA, then,

a.c. drain resistance, r = 2V 0.02 mA

= 100 kΩ

Referring to the output characteristics of a JFET in Fig., it is clear that above the pinch off voltage, the

change in ID is small for a change in VDS because the curve is almost flat.

Therefore, drain resistance of a JFET has a large value, ranging from 10 kΩ to 1 MΩ.

(ii) Transconductance ( gfs) : -The control that the gate voltage has over the drain current is measured

by transconductance gfs & is similar to transconductance gm of the tube. It may be defined as follows: -

It is the ratio of change in drain current (ΔID) to the change in gate-source voltage(ΔVGS) at constant

drain-source voltage i.e.

The transconductance of a JFET is usually expressed either in mA/volt or micro mho. As an example, if

a change in gate voltage of 0.1 V causes a change in drain current of 0.3 mA, then, Transconductance,

g = 0.3 mA

0.1 V =3mA/V = 3×10−3 A/V or mho or S(Siemens)=3×10−3 × 106 µ mho = 3000 µ mho (or μS)

(iii) Amplification Factor ( µ ). It is the ratio of change in drain-source voltage (ΔVDS) to the change

in gate-source voltage(ΔVGS) at constant drain current i.e.

Amplification factor of a JFET indicates how much more control the gate voltage has over drain current

than has the drain voltage.

For instance, if the amplification factor of a JFET is 50, it means that gate voltage is 50 times as effective as the drain voltage in controlling the drain current.

Amplification Factor = A.C. Drain Resistance × Transconductance

[ PAGE – 7. 5 ]

RELATION AMONG JFET PARAMETERS: - The relationship among JFET parameters can be established as under :

We know µ = AVDS AVGS

Multiplying the numerator and denominator on R.H.S. by ΔID, we get,

µ = OVDS

× OID

= OVDS

× OID

OVGS OID OID OVGS

JFET BIASING: - For the proper operation of n-channel JFET, gate must be negative w.r.t. source. This can be achieved

either by inserting a battery in the gate circuit or by a circuit known as biasing circuit.

The latter method is preferred because batteries are costly and require frequent replacement.

1. Bias Battery: - In this method, JFET is biased by a bias battery VGG. This battery ensures that gate is

always negative w.r.t. source during all parts of the signal.

2. Biasing circuit: -The biasing circuit uses supply voltage VDD to provide the necessary bias. Two

most commonly used methods are (i) Self-Bias (ii) Potential Divider Method.

SELF-BIAS FOR JFET : - Fig shows the self-bias method for n-channel JFET. The resistor

RS is the bias resistor.

The d.c. component of drain current flowing through RS produces the desired bias voltage.

Voltage across RS, VS = ID RS

Since gate current is negligibly small, the gate terminal is at d.c.

ground i.e., VG = 0.

∴ VGS = VG − VS = 0 − ID RS or VGS = − ID RS

Thus bias voltage VGS keeps gate negative w.r.t. source.

Operating point: -

The operating point (i.e., zero signals ID & VDS) can be easily

determined. Since the parameters of the JFET are usually known, zero signal ID can be calculated from

the following relation :

ID = IDSS (1 - AVGS ) 2 AVGS(of f)

Also VDS = VDD − ID (RD + RS)

Thus d.c. conditions of JFET amplifier are fully specified i.e. operating point for the circuit is (VDS, ID).

Also, RS = | VGS | | ID |

Note that gate resistor RG does not affect bias because voltage across it is zero. Midpoint Bias: - It is often desirable to bias a JFET near the midpoint of its transfer characteristic

curve where ID = IDSS/2. When signal is applied, the midpoint bias allows a maximum amount of drain

current swing between IDSS and 0.

It can be proved that when VGS = VGS (off) / 3.4, midpoint bias conditions are obtained for ID. I = I (1 - OVGS ) 2 = I (1 -

OVGS(oFF) /3.4 ) 2 = 0.5 I

D DSS OVGS(oFF) DSS OVGS(oFF)

DSS

To set drain voltage at midpoint (VD = VDD/2), select a value of RD to

produce the desired voltage drop.

JFET with Voltage-Divider Bias :- Fig shows potential divider method of biasing a JFET. This circuit is

identical to that used for a transistor.

The resistors R1 and R2 form a voltage divider across drain supply VDD.

The voltage V2 (= VG) across R2 provides the necessary bias.

V2 = VG = VDD × R2 R1+ R2

µ = rd × gfs

Now V2 = VGS + ID RS Or VGS = V2 - ID RS

[ PAGE – 7. 6 ]

The circuit is so designed that ID RS is larger than V2 so that VGS is negative. This provides correct bias voltage. We can find the operating point as under:

ID = V2– VGC

Rc and VDS = VDD − ID (RD + RS)

Although the circuit of voltage-divider bias is a bit complex, yet the advantage of this method of biasing is that it provides good stability of the operating point.

The input impedance Zi of this circuit is given by ; Zi = R1 || R2

JFET Connections: - There are three leads in a JFET viz., source, gate and drain terminals. However, when JFET is to be

connected in a circuit, we require four terminals;

two for the input and two for the output.

This difficulty is overcome by making one

terminal of the JFET common to both input and

output terminals. Accordingly, a JFET can be

connected in a circuit in the following three ways:

Common Source connection

Common Gate connection

Common Drain connection

The common source connection is the most widely

used arrangement. It is because this connection

provides high input impedance, good voltage gain

and moderate output impedance.

However, the circuit produces a phase reversal i.e., output signal is 180° out of phase with the input signal. Fig. shows a common source n-channel JFET amplifier.

Note that source terminal is common to both input and output.

JFET Applications : - The high input impedance and low output impedance and low noise level make JFET far superior to the

bipolar transistor. Some of the circuit applications of JFET are :

As a Buffer amplifier

As Phase-shift oscillators

As RF amplifier

---P N GOUDA----ALL THE BEST ------------------ ALL

THE BEST---- P N GOUDA ----

[ PAGE – 8. 1 ]

CHAPTER - 8

OPERATIONAL AMPLIFIERS

INTRODUCTION

The operational amplifier is an extremely efficient and versatile device. Its applications span the broad

electronic industry filling requirements for signal conditioning, special transfer functions, analog

instrumentation, analog computation, and special systems design. The analog assets of simplicity and

precision characterize circuits utilizing operational amplifiers.

OP-AMP BASICS

Operational amplifiers are convenient building blocks that can be used to build amplifiers, filters, and even

an analog computer. Op-amps are integrated circuits composed of many transistors & resistors such that the

resulting circuit follows a certain set of rules. The most common type of op-amp is the voltage feedback

type and that's what we'll use.

The schematic representation of an op-amp is shown to the left. There are

two input pins (non-inverting and inverting), an output pin, and two power

pins. The ideal op-amp has infinite gain. It amplifies the voltage difference

between the two inputs and that voltage appears at the output. Without

feedback this op-amp would act like a comparator (i.e. when the non-

inverting input is at a higher voltage than the inverting input the output

will be high, when the inputs are reversed the output will be low).

NON-INVERTING AMPLIFIER:

No current flows into the input, Rin = ∞ The output adjusts to bring Vin- to the same voltage as Vin+.

Therefore Vin- = Vin and since no current flows into Vin- the same current must flow through R1 & R2.

Vout is therefore VR1 + VR2 = Vin- + IR2 = Vin- + (Vin/R1)R2.

INVERTING AMPLIFIER[ PAGE – 8. 2 ]

Because no current flows into the input pins there can’t be any voltage drop across the R1

R2. Vin+ is therefore at 0V(this is called a virtual ground). The output will adjustsuch that

Vin- is at zero volts. This makes Rin = R1 (not ∞). The current through R1 & R2 have to be

the same since no current goes into the input pins.

Therefore I = Vin/R1. Vout = Vin+ - IR2 = 0 - (Vin/R1)R2. Therefore Vout = -Vin(R2/R1)

SUMMING AMPLIFIER:

Since Vin- is a virtual ground adding V2 and R2 (and V3 & R3) doesn't change the current flowing through

R1 from V1. Each input contributes to the output using the following equation:

Vout = -V1(R4/R1) - V2(R4/R2) - V3(R4/R3).

The input impedance for the V1 input is still R1, similarly V2's input impedance is R2 and V3's is R3. Most

of the time the parallel combination of R1-R4 isn't used and Vin+ is grounded.

DIFFERENCE AMPLIFIER:[ PAGE – 8. 3 ]

You can work out the gain as before using the two rules (no current flows into the inputs,

and the output will adjust to bring Vin- to Vin+). The result is Vout = 2(V2-V1)*(R2/R1).

Also, Rin(-) = R1, Rin(+) = R1 + R2.

COMMON-MODE OP- AMP

These type of op-amp have common mode voltage to both terminals.

It means without connecting the same voltage at both the terminal we may connect one voltage or either

inverting or non-inverting terminal and other is connected with short to that voltage.

COMMON MODE REJECTION RATIO

Common mode rejection ratio which is defined as the ratio of differential gain to common mode

CMRR = Ad / Acm

Ad = V0/ Vd , Acm=V0/Vcm

As the gain is generally high so CMRR is used to express as a logarithmic gain function

CMRRR= 20log Ad/Acm

OPERATIONAL - AMPLIFIER WITH FEEDBACK

[ PAGE – 8. 4 ]

[ PAGE – 8. 5 ]

[ PAGE – 8. 6 ]

[ PAGE – 8. 7 ]

VOLTAGE SUBSTRACTOR

Generally Subtraction of signals are being performed by substracting one signal from another signal.

These types of subtractor are always used in analog signals.

[ PAGE – 8. 8 ]

Voltage across terminal A can be found by using voltage division rule and we know that voltage across A is

equals to the B so VA = VB

VA= V2 .R2/ R2+R3 = VB

Applying nodal analysis in terminal B the equation becomes

(VB – V 1)/R1 + (VB-V0) / RF = 0

VB/R1 + (VB/RF – V1/R1) = V0/RF

VB (1/R1 + 1/RF) –V1/R1 = V0/RF

But we know that VB= V2 .R2/ R2+R3

(V2 .R2/ R2+R3) [(RF+R1)/R1 .RF] – V1 /R1 = V0/RF

(V2 .R2/ R2+R3) [(RF+ R1)/R1] – V1. RF/R1 = V0

V0= (V2 .R2/ R2+R3) [1+ RF/R1] – (V1. RF)/ R1

If we put RF= R1 =R2 =R3 = 1 KΩ

The output voltage V0 becomes

V0 = V2 – V1

SLEW RATE

It is the ratio of change in output voltage to change in time

S.R = ∆ V0 / ∆ T (V/µs)

---------------- ALL THE BEST -------------------------- ALL THE BEST ----------------

PPRREEPPAARREEDD BBYY:: -- 1. Er. DEBI PRASAD PATNAIK

[Sr. Lecture, Dept of ETC, UCP ENGG. SCHOOL, Berhampur]

2. Er. PARAMANANDA GOUDA

[Lecturer (PT), Dept of ETC, UCP ENGG. SCHOOL, Berhampur]

Electrical Measurement & Instrumentation

Semester-4th

CONTENTS

Measuring instruments

Analog ammeters and voltmeters

Wattmeter and measurement of power

Energy meters and measurement of energy

Measurement of speed, frequency and power factor

Measurement of Resistance, Inductance& Capacitance

Sensors And Transducer

Oscilloscope

CHAPTER-1

MEASURING INSTRUMENTS

1. ACCURACY- The closeness with which an instrument reading approaches the true value

of the quantity being measured is called accuracy. Accuracy is determined as the maximum

amount by which the result differs from the true value.

2. PRECISION- The term precise means means clearly or sharply defined. Precision is a

measure of reproducibility of measurement.

3. ERRORS- The deviation or change of the value obtained from measurement from the desired

standard value.

Mathematically,

Error = Obtained Reading/Value – Standard Reference Value.

There are three types of error. They are as follows:-

GROSS ERRORS-This are the error due to humans mistakes such as careless reading

mistakes in recoding observation incorrect application of an instrument.

SYSTEMATIC ERROR-A constant uniform deviation of an instrument is as systematic

error. There are two types of systematic error.

a) STATIC ERROR

Thestatic error of a measuring instrument is the numerical different between the true

value of a quantity and its value as obtained by measurement.

b) DYNAMIC ERROR-

1. It is the different between true value of a quantity changing with and value

indicated by the instrument.

2. The Dynamic Errors are caused by the instrument not responding fast enoughto

follow the changes in the measured value.

RANDOM ERROR-The cause of such error is unknown or not determined in the

ordinary process of making measurement.

TYPES OF STATIC ERROR

1. INSTRUMENTAL ERROR- Instrumental error are errors inherent in mastering

instrument because of the mechanical construction friction is bearing in various moving

component. It can be avoided by

a. Selecting a suitable instrument for the particular measurement.

b. Applying correction factor after determining the amount of

instrumental error.

2. ENVIROMENTAL ERROR –Environmental error are due to conditions external tothe

measuring device including condition al in the area surrounding the instrument such as

effect of change in temperature , humidity or electrostatic field it can be avoided

a. Providing air conditioning.

b. Use of magnetic shields.

3. OBSERVATIONAL ERROR- The errors introduced by the observer. These errors are

caused by habits of the observers like tilting his/her head too much while reading a

“Needle – Scale Reading”.

Measuring instruments are classified according to both the quantity measured by the

instrument and the principle of operation.

There are three general principles of operation:

electromagnetic, which utilizes the magnetic effects of electric currents;

electrostatic, which utilizes the forces between electrically-charged conductors;

Electro-thermic, which utilizes the heating effect

Electric measuring instruments and meters are used to indicate directly the value of

current,voltage, power or energy.An electromechanical meter (input is as an electrical signal

results mechanical force or torqueas an output) that can be connected with additional suitable

components in order to act as anammeters and a voltmeter.The most common analogue

instrument or meter is the permanent magnet moving coilinstrument and it is used for measuring

a dc current or voltage of an electric circuit.

4. SENSITIVITY- Sensitivity can be defined as a ratio of a change output to the change

input at steady state condition.

5. RESOLUTION- Resolutions the least increment value of input or output that can be

detected, caused or otherwise discriminated by the measuring device.

6. TOLETANCE- Tolerance refers to the total allowable error within an item. This is

typically represented as a +/- value off of a nominal specification. Products can become

deformed due to changes in temperature and humidity, which lead to material expansion and

contraction, or due to improper feedback from a process control device. As such, it's

necessary to take errors into consideration with regard to design values in the manufacturing

and inspection processes.

CLASSIFICATION OF MEASURING INSTRUMENT-

The instrument may be classified as

1. Absolute and secondary Instrument

2. Analog and digital Instrument

3. Mechanical, Electrical and Electronics Instruments.

4. Manual and automatic instruments

5. Self contained and remote indicating instruments

6. Self operated and power-operated instrument

7. Deflection and Null output instrument.

1. Absolute instrument & Secondary instruments:

Absolute instrument measures the process variable directly from the process without the use of

conversion. Such instruments do not require comparison with any other standard. The tangent galvanometer is an example for the absolute instrument. These instruments are used as standards in

labs and institution.

Secondary instrument: These instruments are so constructed that the deflection of such instruments

gives the magnitude of the electrical quantity to be measured directly. These instruments required

to calibrated with respect to the standard instrument. These instruments are usually used in practice.

Secondary instruments are classified as:

1. Indicating instrument: Those instruments that measure and indicates the magnitude of the

electricity. The indications are given by a pointer moving over a calibrated scale. Ordinary

ammeters, voltmeters, wattmeters, frequency meters, power factor meters, etc., fall into this

category.

2. Integrating instrument: Integrating instruments are those which measure the total amount of

either quantity of

electricity (ampere-hours) or electrical energy supplied over a period of time. The ampere-hour

meters and energy meters fall in this class.

3. Recording instrument: These instruments record continuously the variation of the magnitude of

the electric quantity for a definite period of time. Such instruments are generally used in

powerhouses where the current, voltage, power, etc., are to be maintained within a certain

acceptable limit.

2. Analog and digital Instrument

Analog instrument: The signals of an analog unit vary in a continuous fashion and can take on an

infinite number of values in a given range. Fuel gauge, ammeter and voltmeters, wristwatch,

speedometer fall in this category.

Digital Instruments: Signals that vary in discrete steps and that take a finite number of different

values in a given range are digital signals and the corresponding instruments are of digital

type. Digital instruments have some advantages over analog meters, in that they have high accuracy

and high speed of operation. Digital multimeter is an example for the digital instrument.

3. Mechanical, Electrical and Electronics Instruments.

Mechanical instrument: Mechanical instruments are very reliable for static and stable conditions.

As they use mechanical parts these instruments cannot faithfully follow the rapid changes which

are involved in dynamic instruments. But they are cheaper in cost and durable.

Electrical Instruments: When the instrument pointer deflection is caused by the action of some

electrical methods then it is called an electrical instrument. The time of operation of an electrical

instrument is more rapid than that of a mechanical instrument. This mechanical movement has

some inertia due to which the frequency response of these instruments is poor.

Electronic Instruments: Electronic instruments use semiconductor devices. They are very fast in

response. In electronic devices, since the only movement involved is that of electrons, the

response time is extremely small owing to very small inertia of the electrons. With the use of

electronic devices, a very weak signal can be detected by using pre-amplifiers and amplifiers.

4. Manual and automatic instruments

In case of Manual instruments services of an operator are required. Example: Measurement of

temperature by a resistance thermometer incorporating a Wheatstone bridge in its circuit.

In an Automatic Instrument an operator is not required. Example: Measurement of temperature by mercury-in-glass thermometer.

5. Self contained and remote indicating instruments

A Self contained instrument has all its different elements in one physical assembly.

In a Remote Indicating Instrument the primary sensing element may be located at an adequate

long distance from the secondary indicating element. Such type of instrument are finding wide use

in the modern instrumentation technology.

6. Self operated and power-operated instrument

Self-operated instruments don’t need any outside power for its working. The output energy is

supplied wholly or almost wholly by the input measurand. Dial indicating type instruments belong

to this category.

Power operated instrument need external power for its working. External power can electric

current, hydraulic or pneumatic energy. In such cases, the input signal supplies only an

insignificant portion of the output power.

7. Deflection and Null output instrument.

In a deflection-type instrument, the deflection of the instrument indicates the measurement of the

unknown quantity. The measurand quantity produces some physical effect which deflects or

produces a mechanical displacement in the moving system of the instrument.

An opposite effect is built in the instrument which opposes the deflection or the mechanical

displacement of the moving system. The balance is achieved when opposing effect equals the

actuating cause producing the deflection or the mechanical displacement. Permanent Magnet

Moving Coil (PMMC), Moving Iron (MI), etc., type instruments are examples of this category.

In Null type instruments, a zero or null indication leads to the determination of the magnitude of

the measurand quantity. The null condition depends upon some other known conditions. These are

more accurate and highly sensitive as compared to deflection-type instruments. A dc potentiometer

is a null- type instrument.

TYPES OF FORCES/TORQUES ACTING IN MEASURING INSTRUMENTS

1. DEFLECTING TORQUE/FORCE:

The defection of any instrument is determined by the combined effect of thedeflecting

torque/force, control torque/force and damping torque/force.The value of deflecting torque must

depend on the electrical signal to be measured.This torque/force causes the instrument movement

to rotate from its zero position.

MAGNITUDE EFFECT

When a current passes through the coil, it produces a imaginary bar magnet. When a soft-iron

piece is brought near this coil it is magnetized. Depending upon the current direction the poles

are produced in such a way that there will be a force of attraction between the coil and the soft

iron piece. This principle is used in moving iron attraction type instrument.

If two soft iron pieces are place near a current carrying coil there will be a force of repulsion

between the two soft iron pieces. This principle is utilized in the moving iron repulsion type

instrument.

FORCE BETWEEN A PERMANENT MAGNET AND A CURRENT CARRYING COIL

When a current carrying coil is placed under the influence of magnetic field produced by a

permanent magnet and a force is produced between them. This principle is utilized in the moving

coil type instrument.

FORCE BETWEEN TWO CURRENT CARRYING COIL

When two current carrying coils are placed closer to each other there will be a force of repulsion

between them. If one coil is movable and other is fixed, the movable coil will move away from

the fixed one. This principle is utilized in electrodynamometer type instrument.

2. CONTROLLING TORQUE/FORCE:

This torque/force must act in the opposite sense to the deflecting torque/force, and themovement

will take up an equilibrium or definite position when the deflecting andcontrolling torque are

equal in magnitude.The Spiral springs or gravity usually provides the controlling torque.

When the external signal to be measured by the instrument is removed, the pointer should return

back to the zero position. This is possibly due to the controlling force and the pointer will be

indicating a steady value when the deflecting torque is equal to controlling torque.

Td = Tc

SPRING CONTROL

Two springs are attached on either end of spindle. The spindle is placed in jewelled bearing, so

that the frictional force between the pivot and spindle will be minimum. Two springs are

provided in opposite direction to compensate the temperature error. The spring is made of

phosphorous bronze. When a current is supply, the pointer deflects due to rotation of the spindle.

While spindle is rotate, the spring attached with the spindle will oppose the movements of the

pointer. The torque produced by the spring is directly proportional to the pointer deflectionθ.

TC𝖺θ

The deflecting torque produced Td proportional to ‘I’. When TC Td , the pointer will come

to a steady position. Therefore

I

3. DAMPING TORQUE/FORCE:

A damping force is required to act in a direction opposite to the movement of the moving

system.This brings the moving system to rest at the deflected position reasonably quickly

withoutany oscillation or very small oscillation.

To damp out the oscillation is quickly, a damping force is necessary.

This force is produced by different systems.

(a) Air friction damping

(b) Fluid friction damping

(c) Eddy current damping

CHAPTER- II

ANALOG AMMETERS & VOLTMETERS

PERMANENT MAGNET MOVING COIL (PMMC) INSTRUMENT

One of the most accurate type of instrument used for D.C. measurements is PMMC

instrument.

CONSTRUCTION:

The moving coil and permanent magnet are the main part of the PMMC instrument. The parts of

the PMMC instruments are explained below in details.

Moving Coil – The coil is the current carrying part of the instruments which is freely moved

between the stationary field of the permanent magnet. The current passes through the coil deflects it

due to which the magnitude of the current or voltage is determined. The coil is mounted on the

rectangular former which is made up of aluminium. The former increases the radial and uniform

magnetic field between the air gaps of the poles. The coil is wound with the silk cover copper wire

between the poles of a magnet.

The coil is mounted on the rectangular former which is made up of aluminium. The former

increases the radial and uniform magnetic field between the air gaps of the poles. The coil is wound

with the silk cover copper wire between the poles of a magnet.

Magnet System – The PMMC instrument using the permanent magnet for creating the stationary

magnets. The Alcomax and Alnico material are used for creating the permanent magnet because

this magnet has the high coercive force (The coercive force changes the magnetisation property of

the magnet). Also, the magnet has high field intensities.

Control – In PMMC instrument the controlling torque is because of the springs. The springs are

made up of phosphorous bronze and placed between the two jewel bearings. The spring also

provides the path to the lead current to flow in and out of the moving coil. The controlling torque is

mainly because of the suspension of the ribbon.

Damping – The damping torque is used for keeping the movement of the coil in rest. This damping

torque is induced because of the movement of the aluminium core which is moving between the

poles of the permanent magnet.

Pointer & Scale – The pointer is linked with the moving coil. The pointer notices the deflection of

the coil, and the magnitude of their deviation is shown on the scale. The pointer is made of the

lightweight material, and hence it is easily deflected with the movement of the coil. Sometimes the

parallax error occurs in the instrument which is easily reduced by correctly aligning the blade of the

pointer.

PRINCIPLE OF OPERATION

When D.C. supply is given to the moving coil, D.C. current flows through it. When the

current carrying coil is kept in the magnetic field, it experiences a force. This force produces a

torque and the former rotates. The pointer is attached with the spindle. When the former rotates,

the pointer moves over the calibrated scale. When the polarity is reversed a torque is produced in

the opposite direction. The mechanical stopper does not allow the deflection in the opposite

direction. Therefore the polarity should be maintained with PMMC instrument. If A.C. is

supplied, a reversing torque is produced. This cannot produce a continuous deflection. Therefore

this instrument cannot be used in A.C.

TORQUE DEVELOPED BY PMMC

The deflecting torque induces because of the movement of the coil. The deflecting torque is

expressed by the equation shown below.

Where, I- Current through the coil, N – Number of turns of coil

B – flux density in the air gap

L, d – the vertical and horizontal length of the side

The spring provides the restoring torque to the moving coil which is expressed as

Where K = Spring constant.

For final deflection,

By substituting the value of equation (1) and (3) we get,

The above equation shows that the deflection torque is directly proportional to the current passing

through the coil.

ERRORS IN PMMC INSTRUMENT:

In PMMC instruments the error occurs because of the ageing and the temperature effects of the

instruments. The magnet, spring and the moving coil are the main parts of the instruments which

cause the error. The different types of errors of the instrument are explained below in details.

1. Magnet – The heat and vibration reduce the lifespan of the permanent magnet. This treatment

also reduced the magnetism of the magnet. The magnetism is the property of the attraction or

repulsion of the magnet. The weakness of the magnet decreases the deflection of the coil.

2. Springs – The weakness of the spring increases the deflection of moving coil between the

permanent magnet. So, even for the small value of current, the coil show large deflection. The

spring gets weakened because of the effect of the temperature. One degree rise in temperature

reduces the 0.004 percent life of the spring.

3. Moving Coil – The error exists in the coil when their range is extended from the given limit by

the use of the shunt. The error occurs because of the change of the coil resistance on the shunt

resistance. This happens because the coil is made up of copper wire which has high shunt resistance

and the shunt wire made up of Magnin has low resistance.

To overcome from this error, the swamping resistance is placed in series with the moving coil. The

resistor which has low-temperature coefficient is known as the swamping resistance. The

swamping resistance reduces the effect of temperature on the moving coil.

Advantages of PMMC Instruments

The following are the advantages of the PMMC Instruments.

1. The scale of the PMMC instruments is correctly divided.

2. The power consumption of the devices is very less.

3. The PMMC instruments have high accuracy because of the high torque weight ratio.

4. The single device measures the different range of voltage and current. This can be done by the use

of multipliers and shunts.

5. The PMMC instruments use shelf shielding magnet which is useful for the aerospace applications.

Disadvantages of PMMC Instruments

The following are the disadvantages of the PMMC instruments.

1. The PMMC instruments are only used for the direct current. The alternating current varies with

the time. The rapid variation of the current varies the torque of the coil. But the pointer can not

follow the fast reversal and the deflection of the torque. Thus, it cannot use for AC.

2. The cost of the PPMC instruments is much higher as compared to the moving coil instruments.

The moving coil itself provides the electromagnetic damping. The electromagnetic damping

opposes the motion of the coil which is because of the reaction of the eddy current and the

magnetic field.

RANGES OF PMMC INSTRUMENT:

DC ammeter:

1. Without shunt- 0/5 micro amperes upto0/30 microamperes

2. With internal Shunt- upto 0/2000 amperes

3. With external Shunt- upto 0/5000 amperes

DC Voltmeters:

1. Without series resistance- 0/100 milli-volts

2. With series resistance- upto 0/5000 amperes

MOVING IRON (MI) INSTRUMENTS

One of the most accurate instruments used for both AC and DC measurement is moving iron

instrument. There are two types of moving iron instrument.

Attraction type

Repulsion type

ATTRACTION TYPE M.I. INSTRUMENT

CONSTRUCTION:

It consists of a flat cylindrical coil. The moving iron is a flat disc or oval shaped disc,

pivoted on a spindle. A pointer is attached to the spindle which moves on a calibrated scale. The

controlling force is obtained by gravity control system. The damping force is provided by air

friction with the help of light aluminium piston attached to the moving system

PRINCIPLE OF OPERATION

The current to be measured is passed through the fixed coil. As the current is low through

the fixed coil, a magnetic field is produced. By magnetic induction the moving iron gets

magnetized. The north pole of moving coil is attracted by the south pole of fixed coil. Thus the

deflecting force is produced due to force of attraction. Since the moving iron is attached with the

spindle, the spindle rotates and the pointer moves over the calibrated scale. But the force of

attraction depends on the current flowing through the coil.

REPULSION TYPE MOVING IRON INSTRUMENT

CONSTRUCTION:

The repulsion type instrument has a hollow fixed iron attached to it .

The moving iron is connected to the spindle. The pointer is also attached to the spindle in

supported with jeweled bearing.

PRINCIPLE OF OPERATION:

When the current flows through the coil, a magnetic field is produced by it. So both fixed

iron and moving iron are magnetized with the same polarity, since they are kept in the same

magnetic field. Similar poles of fixed and moving iron get repelled. Thus the deflecting torque is

produced due to magnetic repulsion. Since moving iron is attached to spindle, the spindle will

move. So that pointer moves over the calibrated scale. Damping: Air friction damping is used to

reduce the oscillation. Control: Spring control is used.

Ranges of Ammeter and Voltmeter

1. For a given moving-iron instrument the ampere-turns necessary to produce full-scale deflection

are constant.

2. One can alter the range of ammeters by providing a shunt coil with the moving coil. 3. Voltmeter range may be altered connecting a resistance in series with the coil. Hence the same

coil winding specification may be employed for a number of ranges.

Advantages

1. The instruments are suitable for use in AC and DC circuits.

2. The instruments are robust, owing to the simple construction of the moving parts.

3. The stationary parts of the instruments are also simple.

4. Instrument is low cost compared to moving coil instrument.

5. Torque/weight ratio is high, thus less frictional error.

Disadvantages

DYNAMOMETER (OR) ELECTROMAGNETIC MOVING COIL INSTRUMENT

Dynamometer type measuring instruments are similar to PMMC instrument. Except that the permanent

magnetic field coil is replaced by a coil which carries the current to be measured. They have precision

grade accuracy both for ac and dc measurements

CONSTRUCTION:

A fixed coil is divided in to two equal half. The moving coil is placed between the

two half of the fixed coil. Both the fixed and moving coils are air cored. So that the hysteresis

effect will be zero. The pointer is attached with the spindle. In a non metallic former the

moving coil is wounded.

Control: Spring control is used.

Damping: Air friction damping is used.

1. Scale not uniform.

2. For low voltage range, the power consumption is higher. 3. The errors are caused due to hysteresis in the iron of the operating system and due to stray

magnetic field.

4. In case of AC measurements, change in frequency causes serious error. 5. With the increase in temperature the stiffness of the spring decreases.

Errors

Error due to variation in temperature.

Error due to friction is quite small as torque-weight ratio is high in moving coil instruments.

Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient

magnetic screening is essential to reduce this effect.

Error due to variation of frequency causes change of reactance of the coil and also changes

the eddy currents induced in neighbouring metal.

Deflecting torque is not exactly proportional to the square of the current due to non-linear

characteristics of iron material.

PRINCIPLE OF OPERATION:

When the current flows through the fixed coil, it produced a magnetic field, whose flux

density is proportional to the current through the fixed coil. The moving coil is kept in between

the fixed coil. When the current passes through the moving coil, a magnetic field is produced by

this coil. The magnetic poles are produced in such a way that the torque produced on the

moving coil deflects the pointer over the calibrated scale. This instrument works on AC and DC.

When AC voltage is applied, alternating current flows through the fixed coil and moving coil.

When the current in the fixed coil reverses, the current in the moving coil also reverses. Torque

remains in the same direction. Since the current i1 and i2 reverse simultaneously. This is because

the fixed and moving coils are either connected in series or parallel.

Errors in dynamometer type instruments

Frictional Error: Since the coils are air-cored, therefore the magnetic field produced is of

small strength. So they require a large number of ampere-turns to create necessary

deflecting torque. This result in the heavy moving system. Therefore small torque-weight

ratio. Thus the frictional losses in dynamo type instruments are somewhat larger as

compared to other instruments.

Temperature errors: Since the operation of dynamo type instrument required considerable

power, self heating in these instrument is appreciable. The error due to self heating may be

much as 1% of full scale deflection.

Error Due to Stray Magnetic field: Since the operating magnetic field produced by

the fixed coil. In these instruments is somewhat weaker in comparison to that in

the instrument of other type. The operation of these instruments is more sensitive to the

stray magnetic field.

Frequency error: The change in frequency causes error

Due to change in reactance of operating coil.

Due to change in magnitude of Eddy current setup in the metal part of the instrument

near to operating portion.

Advantages of Dynamometer type instrument

As the instrument has Square Law response so can be used on both the dc as well as on AC.

These instruments are free from hysteresis and Eddy current errors. It is because of absence

of iron in the operating part of the instrument.

Ammeter up to 10A and voltmeter up to 600V can be constructed with precision grade

accuracy.

Dynamo type voltmeter are useful for accurate measurement of rms value of voltage

irrespective of waveform.

Because of Precision grade accuracy and same calibration for DC and AC measurement

instruments are used as transfer and calibration instruments.

Disadvantage of Dynamometer type instrument

The scale is not uniform as the instrument uses Square Law response. These instruments

have small torque-weight ratio so the friction error is considerable.

Owing to heavy moving system friction losses in these instruments are somewhat more than

those in other instruments.

As a result of measures taken to reduce the frictional errors, their cost is more in

comparison to moving iron and PMMC instruments. They are more sensitive to overload

and mechanical impact and are to be handled with care.

Adequate screening of the movements against the stray magnetic field is essential.

The sensitivity of the instrument is typically very low due to poor deflecting torque. The

sensitivity of dynamo type wattmeter is 10 to 30 per volt in comparison to the sensitivity of

20-kilo-ohm per volt in case of D’Arsnoval movement.

The power consumption of this instrument is comparatively high because of their

construction.

Ranges:

Ammeter: 1. With fixed and moving coil in series- 0/0.01A-0/0.05 A

2. With moving coil shunted or parallel connection- upto 0/30A.

Voltmeter: Upto 0-750 volts

EXTENSION OF RANGE OF INSTRUMENT BYTHE USE OF SHUNT AND MULTIPLIER

AMMETER CONNECTION

Shunts are used for the range extension of ammeters. A shunt is a low-value resistance having

minimum temperature co-efficient and is connected in parallel with the ammeter whose range is to

be extended. The combination is connected in series with the circuit whose current is to be

measured.

The ratio of maximum current (with shunt) to the full-scale deflection current (without shunt) is

known as the ‘multiplying power’ or ‘multiplying factor’ of the shunt.

Example: A moving coil ammeter reading up to 1 ampere has a resistance of 0.02 ohm. How

could this instrument be adopted to read current up to 100 amperes.

Solution: In this case,

Full-scale deflection current of the ammeter, Im = 1 A Line current to be measured, I = 100 A

Resistance of ammeter, Rm = 0.02 ohm Let, the required shunt resistance = S

As seen from Figure, the voltage across the instrument coil and the shunt resistance is the same

since

both are joined in parallel.

∴ ImRm = SIs = S(I − Im)

or S = ImRm/(I – Im)

= 1×0.02/(100 – 1) = 0.02/99 = 0.000202 Ans.

VOLTMETER CONNECTION

Multipliers are used for the range extension of voltmeters. The multiplier is a non-inductive high-

value resistance connected in series with the instrument whose range is to be extended. The

combination is connected across the circuit whose voltage is to be measured.

Example: A moving coil voltmeter reading upto 20 mV has a resistance of 2 ohms. How this

instrument can be adopted to read voltage upto 300 volts.

Solution: In this case, Voltmeter resistance, Rm = 2 ohm

Full-scale voltage of the voltmeter, ν = Rm Im = 20 mV = 0.02 V

Full-scale deflection current, Im = v/Rm = 0.02/2 = 0.01 A Voltage to be measured, V = 300 V

Let the series resistance required = R

Then as seen from figure, the voltage drop across R is V – ν

R Im = V – ν

or R = (V – v)/Im

or R = (300 – 0.02)/0.01 = 299.98/0.01 = 29998 ohms Ans.

RECTIFIER TYPE INSTRUMENT

A rectifier type instrument measures alternating electrical signal by means of D.C measuring

instrument. As the name implies, this instrument first rectifies an A.C signal to D.C then measures.

Although it measures the rectified A.C signal (D.C signal), but the scale of the instrument is

calibrated for A.C. The sensitivity of D’ Arsonval instrument is quite high. But a D’Arsonval

instrument can only measure to D.C. So, to utilize the sensitivity of D’Arsonval movement for A.C,

we use a rectifier type instrument.

Rectifier Elements for Rectifier Type Instrument

To convert A.C. to D.C, a rectifier instrument must have some rectifier elements.

At the low-frequency range, the instrument uses copper oxide or selenium cells for rectification

purpose. Again at higher frequencies, we use germanium or silicon diodes.

The copper oxide rectifier element has a peak inverse voltage (PIV) about 2 volts. On the other

hand, the selenium element has PIV of 10 volts. Also, both of these rectifier elements have very

low current handling capacity. Therefore, for rectification purpose, these elements have become

obsolete in the modern era.

Besides it, a germanium diode has a peak inverse voltage (PIV) of about 300 volts. In addition to

that, the current carrying capacity of a germanium diode is about 100 mA. Also, PIV of silicon

diode is about 1000 volts with a current rating of 5000 mA. Therefore these germanium and silicon

diodes have become most suitable choice as rectifier elements for the purposes.

Advantages of Rectifier Type Instrument

1. This instrument can measure an electrical signal of very low frequency to radio frequency.

2. The instrument is also capable of measuring electrical signals up to several mega Hz.

3. The sensitivity of the instrument is much higher than that of any other type of A.C

measuring instruments. Actually, for achieving very high sensitivity in A.C measurements,

we use rectifier type instruments.

Characteristic of Rectifier Type Instrument

1. A rectifier type instrument is an economical and suitable mean of A.C measurement.

2. It can measure the electrical signal at an audio frequency range.

3. The sensitivity of the instrument is much higher than a commonly used A.C measuring

instrument. It has been found that the sensitivity of a general rectifier type instrument is

around 50 times higher than of a dynamometer type instrument or a moving iron type

instrument.

4. Generally, D’ Arsonval movement is uniform; therefore a rectifier type instrument generally

has a linear scale.

5. The sensitivity of the instrument is in order of 1000 to 2000 ohms/volt.

6. The power consumption of a rectifier type instrument is high because of the resistance of

the rectifier elements.

7. Shunting of the rectifier instrument is not practical because the resistance of rectifier

elements changes with the temperature and the current flowing through it.

8. The rectifier type instrument is capable of measuring a very tiny current of microampere

range. Again it can also be capable of measuring current in the milliampere range. But we

do not construct a rectifier type instrument beyond for the current of 15mA. Because it

requires special sized rectifier elements and the size of the instruments becomes

impracticality large.

INDUCTION TYPE INSTRUMENT

The operation of Induction type instruments depends on the production of torque due to reaction

between two magnetic fluxes having some phase difference or reaction between flux of an AC

magnet and the eddy current induced by this flux. This instrument having an aluminum disc

(or aluminum drum) in the magnetic field. Hence, the changing flux links with the

aluminum disc. As a result, the flux induces an eddy current on the disc. This eddy current

interacts with the flux which has induced it. Consequently, there is a mechanical torque

acting on the disc. This mechanical torque rotates the disc. These type of instruments are used

only for AC measurements.

Torque in Induction Type Instrument

So, the torque depends on two factors. The first one is the strength of the field of the electromagnet.

The second one is the value of eddy current on the disc. Of course, the torque is proportional to the

strength of the magnetic field. Also, it is proportional to the eddy current. Again, the strength of the

magnetic field depends on the current of the electromagnet. On the other hand, the value of eddy

current depends on the strength of the magnetic field. So, we can say, the value of eddy current also

depends on the current of the electromagnet.

So, the torque acting on the disc is directly proportional to the square of the current of the

electromagnet. In an induction type instrument, we directly feed the measuring current into the coil

of the electromagnet. Therefore, the deflecting torque is directly proportional to the square of the

measuring current.

Let us consider the flux, produced by the electromagnet is

The phase angle between that flux and induced eddy current is α. Hence, we can write the

expression of the eddy current as

Again, the instantaneous torque is directly proportional to the instantaneous eddy current

and the flux. Hence, we can write,

So, the mean torque is as follows,

The above expression tells that the torque is zero if α is 90°. Hence to obtain resulting torque it is

necessary to produce an eddy current which is either less than or more than 90ᵒ out of phase with

flux ∅. 𝖺 is the phase angle between the flux and eddy current.

So, there must be some means in induction type instrument to prevent this phase angle

from being 90°. We can achieve this by two methods listed below.

1. Split-phase type and

2. Shaded pole type

1. SPLIT-PHASE TYPE INDUCTION TYPE INSTRUMENT:

In this arrangement, there are two AC magnets P1 and P2 connected in series. The winding in P2 is

shunted by a resistance R. The current in the P2 winding lags with respect to the total current. This

helps to develop the necessary phase angle 𝖺 between the two fluxes. Eddy current damping is used

in this type of instrument.

2. SHADDED POLE TYPE INDUCTION TYPE INSTRUMENT:

Shaded pole type induction instrument uses a single winding to produce flux. The flux

produced by this winding is split up into two fluxes, having phase difference with respect to each

other. The phase difference is usually 40 to 50 degrees and can be varied by varying the size of

shading band. This is done by making a narrow slot in the poles of electromagnet. A copper strip is

placed around the smaller of the two areas formed by t he slot. This copper shading band acts as a

short circuited secondary winding.

The exciting coil is placed on the poles and a current proportional to current or voltage

being measured is passed through it. An aluminium disc which is mounted on a spindle is inserted

in the air gap of the electromagnet. The spindle carries a pointer and has a control sring attached to

it. The controlling torque is provided by this spring only.

Damping is provided by a permanent magnet placed at the opposite side of the

electromagnet, so that the disc can be used for production of both deflecting and damping torque.

CHAPTER-2

MEASUREMENT OF POWER

DYNAOMETER TYPE WATTMETER

A dynamometer type wattmeter primarily consists of two coils called fixed coil and moving

coil. The fixed coil is splitted into two equal parts, which are placed parallel to each other. The two

fixed coils are air-cored to avoid hysteresis effects when used on AC.

The fixed coil is connected in series with the load and carries the circuit current. It is,

therefore, called the current coil. The moving coil is pivoted between the two parts of the fixed coil

and is mounted on a spindle.

A pointer is attached to the spindle, which gives deflection. The moving coil is connected in

parallel with the load and carries the current proportional to the voltage. It is, therefore, called the

potential coil.Generally, a high resistance is connected in series with the moving coil to limit the

current through it. By limiting the current, the moving coil is made lightweight, which in turn

increases the sensitivity of the instrument.

The springs provide the controlling torque. They also serve the additional purpose of leading the

current into and out of the moving coil. Air friction damping is employed in such instruments.

Dynamometer Type Wattmeter Working

We use the wattmeter for power measurements. Its current coil is connected in series with the load,

carries the load current, and the potential coil, connected in parallel with the load, carries the

current proportional to the voltage across the load.

The fixed coil produces a field Fm, and moving coil creates a field Fr. The field Fr tries to come in

line with the main field Fm, which provides a deflecting torque on the moving coil.

Thus, the pointer attached to the spindle of the moving coil deflects. This deflection is controlled by

the controlling torque produced by the springs. Also read Power Measurements in Three Phase

Circuits.

Advantages and Disadvantages of Dynamometer Type Wattmeter

Advantages:

It can be used both on AC and DC circuits.

It has a uniform scale.

We can obtain a high degree of accuracy through careful design.

Disadvantages:

At low power factors, the inductance of the potential coil causes serious errors.

The reading of the instrument may be affected by stray fields acting on the moving coil. To

prevent it, magnetic shielding is provided by enclosing the instrument in an iron case.

Errors in Dynamometer Type Wattmeter

Errors in this type of wattmeter:

1. Error due to potential coil inductance: The inductance of the potential coil is liable to cause an

error in the reading of the wattmeter. Because of this error, the wattmeter gives a high reading on

lagging power factor and low reading on leading power factor.

The high non-inductive resistance connected in series with the coil swamps the phasing effect of

the potential coil inductance.

2. Error due to power loss in the potential coil or current coil: Another possible error in the

indicated power may be due to some voltage drop in the current coil or the current taken by the

potential coil.

We can overcome this defect by using an additional compensating winding. This winding is

connected in series with the potential coil and so placed that it produces a field in the opposite

direction to that of the current coils.

3. Error due to eddy currents: The alternating field of fixed or current coil induces eddy currents in

the solid metal parts which set up their own magnetic field. This alters the magnitude and phase of

the magnetic field, causing deflection.

Thus an error is introduced in the instrument reading. To reduce this error, the solid metal parts are

placed far away from the current coil as possible.

4. Error due to the stray magnetic field: The dynamometer type wattmeter has a relatively weak

operating field; therefore, stray fields affect the reading of this instrument considerably and cause

serious errors.

Hence, this type of instrument must be shielded against stray magnetic fields try using iron cases or

providing thin iron shields over the working parts.

Range

Current circuit 0 – 0.25A to 0 – I00A without employing CTs.

Potential circuit 0 – 5 V to 0 – 750 V without using PTs.

INDUCTION TYPE WATTMETER

The induction type wattmeter is used to measure a.c power only.

Principle of Induction type wattmeter:

The principle of operation of an induction wattmeter is same as that of induction ammeters and

voltmeters i.e. induction principle. However, it differs from induction ammeter or voltmeter in so

far that separate two coils are used to produce the rotating flux in place of one coil with phase split

arrangement.

Construction of Induction type wattmeter:

The principle parts of an induction wattmeter are as shown in the fig below. It consists of two

laminated electromagnets. One electromagnet, called shunt magnet is connected across supply and

carries current proportional to the applied voltage. The coil of this magnet is made highly inductive

so that the current in it lags behind the supply voltage by 90 degrees. The other electromagnet,

called series magnet is connected in series with supply and carries the load current. The coil of this

magnet is made highly non inductive so that the angle of lag or lead is determined fully by the load.

A thin aluminium disc mounted on the spindle is placed in between the two magnets so that it cuts

the fluxes of both the magnets. The controlling torque is provided by spiral springs. The damping is

electromagnet and is usually provided by a permanent magnet embracing the aluminium disc. Two

or more closed copper rings, called shading rings are provide on the central limb of the shunt

magnet. By adjusting the position of these rings, the shunt magnet flux can be made to lag behind

supply voltage by exactly 90degrees.

Working of Induction type wattmeter:

When the wattmeter is connected in the circuit to measure a.c power, the shunt magnet carries

current proportional to the supply voltage and the series magnet carries the load current. The two

fluxes produced by the magnets induce eddy currents in the aluminium disc. The interaction

between the fluxes and eddy currents produce the deflecting torque on the disc, causing the pointer

connected to the moving system to move over the scale.

Vector diagram

Deflecting torque of Induction type wattmeter:

let V = Applied voltage

Ic = Load current carried by the series magnet

Iv = Current carries by the shunt magnet

cos a = Lagging power factor of the load

The vector diagram of this wattmeter is shown in the fig below. The current Iv in the shunt magnet

lags the applied voltage V by 90 degrees and so does the flux av produced by it. The current Ic in

the series magnet is the load current and hence lags behind the applied voltage by a'. The flux

ac produced by this current Ic is in phase with it. Therefore the two currents Ic in the current coil

and Iv in the voltage coil and also corresponding fluxes av and ac are (90 - a') apart.

The flux ac induces the eddy currents iv in the aluminium disc which lags behind the flux by

90degrees. Similarly, flux ac induces eddy currents ic which again lags behind flux ac by 90

degrees.

Mean deflecting torque, Td proportional ac sin (90 - a )

Td proportional V I cos a

Td proportional a.c power

Since control is by springs, therefore

Tc proportional deflection

For steady deflected position, Td = Tc

Deflection proportional power

Hence, such instruments have uniform scale.

CHAPTER 4- MEASUREMENT OF ENERGY

SINGLE PHASE INDUCTION TYPE ENERGYMETERS:

Induction type energy meter consists of the following components:

Figure 1

BASIC OPERATION:

Figure 2

ERROS IN ENERGYMETER:

CHAPTER-5

MEASUREMENT OF SPEED, FREQUENCY AND POWER FACTOR

DIGITAL TACHOMETER

The technique employed in measuring the speed of a rotating shaft is similar to the

technique used in a conventional frequency counter, except that the selection of the gate period is

in accordance with the rpm calibration. Let us assume that the rpm of a rotating shaft is R. Let P

be the number of pulses produced by the pickup for one revolution of the shaft. Therefore, in one

minute the number of pulses from the pickup is R x P. Then, the-frequency of the signal from the

pickup is (R x P)/60. Now, if the gate period is G s the pulses counted are (R x P x G)/60. In order

to get the direct reading in rpm, the number of pulses to be counted by the counter is R. So we

select the gate period as 60/ P, and the counter counts (Rx P x 60)/ 60P = R pulses and we can

read the rpm of the rotating shaft directly. So, the relation between the gate period and the number

of pulses produced by the pickup is G = 60/P. If we fix the gate period as one second (G= 1 s),

then the revolution pickup must be capable of producing 60 pulses per revolution. Figure shows a

schematic diagram of a digital tachometer.

MECHANICAL RESONANCE TYPE FREQUENCYMETER:

CHAPTER -6

MEASUREMENT OF RESISTANCE, INDUCTANCE, CAPACITANCE

Explain the working of Wheatstone Bridge(Measurement of

Resistance)

For measuring accurately any electrical resistance Wheatstone bridge is widely used.

There are two known resistors, one variable resistor and one unknown resistor connected in

bridge form as shown below. By adjusting the variable resistor the electric current through the

Galvanometer is made zero. When the electric current through the galvanometer becomes zero,

the ratio of two known resistors is exactly equal to the ratio of adjusted value of variable

resistance and the value of unknown resistance. In this way the value of unknown electrical

resistance can easily be measured by using a Wheatstone Bridge.

Wheatstone Bridge Theory

The general arrangement of Wheatstone bridge circuit is shown in the figure below. It

is a fourarms bridge circuit where arm AB, BC, CD and AD are consisting of electrical

resistances P, Q, Sand R respectively. Among these resistances P and Q are known fixed

electrical resistances andthese two arms are referred as ratio arms. An accurate and sensitive

Galvanometer is connectedbetween the terminals B and D through a switch S2. The voltage

source of this Wheatstonebridge is connected to the terminals A and C via a switch S1 as shown.

A variableresistor S isconnected between point C and D. The potential at point D can be varied

by adjusting the valueof variable resistor. Suppose electric current I1 and electric current I2 are

flowing through thepaths ABC and ADC respectively. If we vary the electrical resistance value

of arm CD the valueof electric current I2 will also be varied as the voltage across A and C is

fixed. If we continue toadjust the variable resistance one situation may comes when voltage drop

across the resistor Sthat is I2.S is becomes exactly equal to voltage drop across resistor Q that is

I1.Q. Thus thepotential at point B becomes equal to the potential at point D hence potential

difference betweenthese two points is zero hence electric current through galvanometer is nil.

Then the deflection inthe galvanometer is nil when the switch S2 is closed.

Here in the above equation, the value of S and P ⁄ Q are known, so value of R can easily

be determined. The electrical resistances P and Q of the Wheatstone bridge are made of definite

ratio such as 1:1; 10:1 or 100:1 known as ratio arms and S the rheostat arm is made continuously

variable

from 1 to 1,000 Ω or from 1 to 10,000 Ω.

MAXWELLS BRIDGE:

This bridge is used to find out the self inductor and the quality factor of the circuit. As it

is basedon the bridge method (i.e. works on the principle of null deflection method), it gives

veryaccurateresults. Maxwell bridgeis an AC bridge so before going in further detail let us know

moreaboutthe ac bridge.Let us now discuss Maxwell's inductor bridge. The figure shows the

circuit diagram ofMaxwell's inductor bridge.

Maxwells Bridge

In this bridge the arms bc and cd are purely resistive while the phase balance depends on the

arms ab and ad.

Here l1 =Unknown inductor of r1.

l2 =Variable inductor ofresistanceR2.

r2 =variable electricalresistance

As we have discussed in ac bridge according to balance condition, we have at balance point

We can vary R3 and R4 from 10 ohms to 10,000 ohms with the help of resistance box.

MAXWELL'S INDUCTANCE CAPACITANCE BRIDGE

In this Maxwell Bridge, the unknown inductor is measured by the standard variable capacitor.

Circuit of this bridge is given below.

Maxwell's Inductance Capacitance Bridge

Advantages of Maxwell's Bridge

(1) The frequency does not appear in the final expression of both equations, hence it

isindependent of frequency.

(2) Maxwell's inductor capacitance bridge is very useful for the wide range of measurementof

inductor at audio frequencies.

Disadvantages of Maxwell's Bridge

(1) The variable standard capacitor is very expensive.

(2) The bridge is limited to measurement of low quality coils (1 < Q < 10) and it is also

unsuitable

for low value of Q (i.e. Q < 1) from this we conclude that a Maxwell bridge is used suitable only

for medium Q coils.

SCHERING BRIDGE THEORY

This bridge is used to measure to the capacitance of the capacitor, dissipation factor

andmeasurement of relative permittivity. Let us consider the circuit of Schering bridge as

shownbelow

Schering Bridge

Here, c1 is the unknown capacitance whose value is to be determined with series

electricalresistance r1.

c2 is a standard capacitor.

c4 is a variable capacitor.

r3 is a pure resistor (i.e. non inductive in nature).

And r4 is a variable non inductive resistor connected in parallel with variable capacitor c4.

Now the supply is given to the bridge between the points a and c. The detector is

connectedbetween b and d. From the theory of ac bridges we have at balance condition

Application:

This bridge is used to measure to the capacitance of the capacitor, dissipation factor

andmeasurement of relative permittivity.

TRANSDUCERS AND SENSORS

METHOD OF SELECTING TRANSDUCERS

While selecting the proper transducer for any applications, or ordering the transducers the

following specifications should be thoroughly considered.

1) Ranges available

2)Squaring System

3)Sensitivity

4) Maximum working temperature

5) Method of cooling employed

6) Mounting details

7) Maximum depth

8) Linearity

and hysteresis

9) Output for zero input

10) Temperature co-efficient of zero drift

11) Natural Frequency.

ADVANTAGES OF ELECTRICAL TRANSDUCERS

1. Very small power is required for controlling the electrical or electronic system

2. The electrical output can be amplified to any desired level

3. Mass inertia effects are reduced to minimum possible.

4. The size and shape of the transducers can be suitably designed to achieve the

optimum weight and volume

5. The output can be indicated and recorded remotely at a distance from the sensing

medium .

6. The outputs can be modified to meet the requirements of the indicating or controlling

equipment.

RESISITIVE TRANSDUCERS

The resistance of a conductor is expressed by a simple equation that involves a few

physical quantities . The relationship is given by

R= ρL/A

Where , R= resistance, Ω

ρ = Resistivity of conductor materials, Ω-m

L= Length of conductor, m

A = Cross sectional area of the conductor,m2

Any method of varying one of the quantities involved in the above relationship can be the

designedbasis of an electrical resistance transducer. There are a number of ways in

whichresistance can bechanged by a physical phenomenon.The translational and

rotationalpotentiometer which work on the basis of change in the value ofresistance with change

in length of the conductor can be used for measurement of translational orrotary displacements.

The resistivity of materials changes with the change of temperature thus causing a change

ofresistance. This property may be used for measurement of temperature.In a resistance

transducer an indication of measured physical quantity is given by a change in theresistance. It

may be classified as follows

1. Mechanically varied resistance - POTENTIOMETER

2. Thermal resistance change – RESISTANCE THERMOMETER

3. Resistivity change - RESISTANCE STRAIN GAUGE

STRAIN GAUGE

INTRODUCTION

When a metal conductor is stretched or compressed , its resistance changes on account of

the fact thatboth length and diameter of conductor change . The value of resistivity of conductor

also changes.When it is strained it’s property is called piezo-resistance .Therefore , resistance

strain gauges arealso known as piezo- resistive gauges .The strain gauge is a measurement

transducer for measuring strain and associated stress inexperimental stress analysis.

TYPES

Four types of Strain gauges are :

1. Wire –wound strain gauge

2. Foil-type strain gauge

3. Semiconductor strain gauge

4. Capacitive strain gauge.

WORKING PRINCIPLE

Strain gauges work on the principle that the resistance of a conductor or a semiconductor

changeswhen strained .This property can be used for measurement of displacement, force and

pressure .When a strain gauge is subjected to tension (positive strain) it’s length increases while

it’s crosssectional area decreases. Since the resistance of a conductor is proportional to it’s length

and inverselyproportional to it’s area of cross section, The resistance of the gauge increases with

positive strain .Strain gauges are most commonly used in wheat –stone bridge circuits to measure

the change ofresistance of grid of wire for calibration proposes; the ‘GAUGE FACTOR’ is

defined as the ratio ofper unit change in resistance to per unit change in length.

i.e , Gauge factor (Gf) = ΔR/R ÷ ΔL/L

Where, ΔR = corresponding change in resistance, R

ΔL = Change in length per unit length, L

R= ρL/A

Where, R= resistance, Ω

ρ = Resistivity of conductor materials, Ω-m

L= Length of conductor, m

A = Cross sectional area of the conductor, m2

L.V.D.T

LVDT is a passive inductive transducer and is commonly employed to measure force(or

weight,pressure and acceleration etc. Which depend on force )in terms of the amount and

direction ofdisplacement of an object.

WORKING PRINCIPLE

When the core is in the centre (called reference position ) the induced voltages E1 and E2 are

equal andopposite. Hence they cancel out and the output voltages V0 is zero.When the external

applied force moves the core towards the coil S2 ,E2 is increased but E1 isdecreased in

magnitude though they are still antiphase with each other. The net voltage available is(E2-E1)

and is in phase with E2.

Similarly , When movable core moves towards coil S1, E1>E2 and Vo = E1-E2 and is in phase

with E1.

ADVANTAGES

1. It gives a high output and therefore many a times there is no need for intermediate

amplification

devices.

2. The transducer possess a high sensitivity as high as 40V/mm

3. It shows a low hysteresis and hence repeatability is excellent under all conditions.

4. Most of the LVDTs consume a power of less than 1W.

5. Less friction and less noise

DISADVANTAGES

1. These transducers are sensitive to stray magnetic fields but shielding is possible .This is done

byproviding magnetic shields with longitudinal slots.

2. Relatively large displacements are required for appreciable differential output.

3.Several times, the transducer performance is affected by vibrations.

APPLICATIONS

1. Measurement of material thickness in hot strip or slab steel mills

2. In accelerometers.

3. Jet engine controls in close proximity to exhaust gases.

CHAPTER 8- OSCILLOSCOPE

BASIC PRINCIPLE OF OSCILLOSCOPE.

A CRO (Cathode-Ray Oscilloscope), or DSO ( Digital Storage Oscilloscope), is a typeof

electronic test instrument that allows observation of constantly varyingsignalvoltages, usually as

a two-dimensional plot of one or more signals as a functionof time.

BLOCK DIAGRAM OF OSCILLOSCOPE & SIMPLE CRO.

The block diagram of simple CRO is as shown in figure below.Herethe Oscilloscopes

areused to observe the change of an electrical signal over time, such that voltage and

timedescribe a shape which is continuously graphed against a calibrated scale. Theobserved

waveform can be analyzed for such propertiesasamplitude, frequency,rise time,time interval,

distortion and others. Modern digital instruments may calculate and displaythese properties

directly. Originally, calculation of these values required manuallymeasuring the waveform

against the scales built into the screen of the instrument.

The oscilloscope can be adjusted so that repetitive signals can be observed as a continuousshape

on the screen. A storage oscilloscope allows single events to be captured by theinstrument and

displayed for a relatively long time, allowing human observation of events toofast to be directly

perceptible.Oscilloscopes are used in the sciences, medicine, engineering, and

telecommunicationsindustry. General-purpose instruments are used for maintenance of electronic

equipmentand laboratory work. Special-purpose oscilloscopes may be used for such purposes as

analyzing an automotive ignition system or to display the waveform of the heartbeat asan

electrocardiogram.

DUAL TRACE CRO:

The block diagram of dual trace oscilloscope which consist of following steps,

1. Electronics gun (single)

2. Separate vertical input channels ( Two)

3. Attenuators

4.pr-amplifiers

5. Electronic switch.

The two separate input signals can be applied to single electron gun with the helpof

electronic switching it Produces a dual trace display .Each separate vertical inputchannel are uses

separate attenuators and pr-amplifier stages, so the amplitude of eachsignal can be

independentlycontrolled. Output of the pr-amplifiers is given to theelectronic switch, which

passes one signal at a time into the main vertical amplifier of theoscilloscope.The time base-

generator is similar to that of single input oscilloscope.By using switch S2 the circuit can be

triggered on either A or Bchannel, waveforms, or an external signal, or on line frequency. The

horizontalamplifier canbefed from sweep generator or from channel B by switching S1. When

switch S, is in channelB, itsoscilloscope operates in the X-Y mode in which channel A acts as

the vertical inputsignal andchannelBasthe horizontal inputsignal.

From the front panel several operating modes can be selected for display, like channel

Bonly,channel A only, channels B and A as two traces, and signals A + B, A - B, B ~ A or - (A +

B)as a single trace. Two types of common operating mode are there for the

electronicswitch,namely,

1.Alternatemode

2.Chopmode.

UNIT II UNIT-I

DC GENERATORS

Principle of Operation of a D.C. Generator

All the generators work on a principle of dynamically induced e.m.f. This principle nothing but the

Faraday’s law of electromagnetism induction. It states that, ‘whenever the number of magnetic lines of

force i.e. flux linking with a conductor or a coil changes, an electromotive force is set up in that conductor

or coil.’ The change in flux associated with the conductor can exist only when there exists a relative

motion between a conductor and the flux. The relative motion can be achieved by rotating conductor with

respect to flux or by rotating flux with respect to a conductor. So a voltage gets generated in a conductor,

as long as there exists a relative motion between conductor and the flux. Such an induced e.m.f. which is due to the physical movement of coil or conductor with respect to flux or

movement of flux with respect to coil or conductor is called dynamically induced e.m.f. Key Point: So a generating action requires following basic components to exist,

i) The conductor or a coil ii) The relative motion between conductor and flux.

In a particular generator, the conductors are rotated to cut the magnetic flux, keeping flux stationary. To

have a large voltage as the output, the number of conductors are connected together in a specific manner,

to form a winding. This winding is called armature winding of a d.c. machine. The part on which this

winding is kept is called armature of a d.c. machine. To have the rotation of conductors, the conductors

placed on the armature are rotated with the help of some external device. Such an external device is called

a prim mover. The commonly used prim movers are diesel engines, steam engines, steam turbines, water

turbines etc. The necessary magnetic flux is produced by current carrying winding which is called field

winding. The direction of the induced e.m.f. can be obtained by using Fleming’s right hand role. Single Loop DC Generator

Figure: Single Loop Generator In the figure above, a single loop of conductor of rectangular shape is placed between two opposite poles

of magnet. Let's us consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field

about its own axis ab. When the loop rotates from its vertical position to its horizontal position, it cuts the

flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines

there will be an emf induced in these both of the sides (AB and BC) of the loop.

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Figure: Single Loop Generator

As the loop is closed there will be a current circulating through the loop. The direction of the current can

be determined by Flemming's right hand Rule. This rule says that if you stretch thumb, index finger and

middle finger of your right hand perpendicular to each other, then thumbs indicates the direction of

motion of the conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and

middle finger indicates the direction of flow of current through the conductor. Now if we apply this right hand rule, we will see at this horizontal position of the loop, current will flow

from point A to B and on the other side of the loop current will flow from point C to D.


Now if we allow the loop to move further, it will come again to its vertical position, but now upper side of

the loop will be CD and lower side will be AB (just opposite of the previous vertical position). At this

position the tangential motion of the sides of the loop is parallel to the flux lines of the field. Hence there

will be no question of flux cutting and consequently there will be no current in the loop. If the loop rotates

further, it comes to again in horizontal position. But now, said AB side of the loop comes in front of N

pole and CD comes in front of S pole, i.e. just opposite to the previous horizontal position as shown in the

figure beside.

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Here the tangential motion of the side of the loop is perpendicular to the flux lines, hence rate of flux

cutting is maximum here and according to Flemming's right hand Rule, at this position current flows from

B to A and on other side from D to C. Now if the loop is continued to rotate about its axis, every time the

side AB comes in front of S pole, the current flows from A to B and when it comes in front of N pole, the

current flows from B to A. Similarly, every time the side CD comes in front of S pole the current flows

from C to D and when it comes in front of N pole the current flows from D to C.

If we observe this phenomena in different way, it can be concluded, that each side of the loop comes in

front of N pole, the current will flow through that side in same direction i.e. downward to the reference

plane and similarly each side of the loop comes in front of S pole, current through it flows in same

direction i.e. upwards from reference plane. From this, we will come to the topic of principle of DC

generator. Now the loop is opened and connected it with a split ring as shown in the figure below. Split

ring are made out of a conducting cylinder which cuts into two halves or segments insulated from each

other. The external load terminals are connected with two carbon brushes which are rest on these split slip

ring segments.

Working Principle of DC Generator

Fig: Commutation action

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It is seen that in the first half of the revolution current flows always along ABLMCD i.e. brush no 1 in

contact with segment a. In the next half revolution, in the figure the direction of the induced current in the

coil is reversed. But at the same time the position of the segments a and b are also reversed which results

that brush no 1 comes in touch with the segment b. Hence, the current in the load resistance again flows

from L to M. The wave from of the current through the load circuit is as shown in the figure. This current

is unidirectional.

Fig: Output waveform of generator This is basic working principle of DC generator, explained by single loop generator model. The position

of the brushes of DC generator is so arranged that the change over of the segments a and b from one

brush to other takes place when the plane of rotating coil is at right angle to the plane of the lines of force.

It is so become in that position, the induced emf in the coil is zero.

Construction of a DC Machine:

A DC generator can be used as a DC motor without any constructional changes and vice versa is also

possible. Thus, a DC generator or a DC motor can be broadly termed as a DC machine. These basic

constructional details are also valid for the construction of a DC motor. Hence, let's call this point as

construction of a DC machine instead of just 'construction of a DC generator.

Figure 1: constructional details of a simple 4-pole DC machine

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The above figure shows constructional details of a simple 4-pole DC machine. A DC machine consists of

two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not

only provides mechanical strength to the whole assembly but also carries the magnetic flux

produced by the field winding.

2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry

field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support

field coils and (ii) spread out the flux in air gap uniformly.

Figure 2: Pole Core and Poles Shoes representation

3. Field winding: They are usually made of copper. Field coils are former wound and placed on

each pole and are connected in series. They are wound in such a way that, when energized, they

form alternate North and South poles.

4. Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to

carry armature winding. The armature is built up of thin laminated circular steel disks for

reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling

purposes. Armature is keyed to the shaft.

Figure 3: Armature of DC machine

5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The

armature conductors are insulated from each other and also from the armature core. Armature

winding can be wound by one of the two methods; lap winding or wave winding. Double layer

lap or wave windings are generally used. A double layer winding means that each armature slot

will carry two different coils.

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Figure 4: Armature Winding/coil of DC machine

6. Commutator and brushes: Physical connection to the armature winding is made through a

commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the

current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in

providing current to the armature conductors. A commutator consists of a set of copper segments

which are insulated from each other. The number of segments is equal to the number of armature

coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.

Brushes are usually made from carbon or graphite. They rest on commutator segments and slide

on the segments when the commutator rotates keeping the physical contact to collect or supply

the current.

Figure 5: Commutator of DC machine

Armature Winding Terminology:

Now we are going to discuss about armature winding in details. Before going through this section, we

should understand some basic terms related to armature winding of DC generator.

Pole Pitch:

The pole pitch is defined as peripheral distance between centers of two adjacent poles in DC machine.

This distance is measured in term of armature slots or armature conductor come between two adjacent

pole centers. Pole Pitch is naturally equal to the total number of armature slots divided by the number of

poles in the machine.

If there are 96 slots on the armature periphery and 4 numbers of poles in the machine, the numbers of

armature slots come between two adjacent poles centres would be 96/4 = 24. Hence, the pole pitch of that

DC machine would be 24.

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As we have seen that, pole pitch is equal to total numbers of armature slots divided by total numbers of

poles, we alternatively refer it as armature slots per pole.

Coil side:

Coil of dc machine is made up of one turn or multi turns of the conductor. If the coil is made up of single

turn or a single loop of conductor, it is called single turn coil. If the coil is made up of more than one turn

of a conductor, we refer it as a multi-turn coil. A single turn coil will have one conductor per side of the

coil whereas, in multi turns coil, there will be multiple conductors per side of the coil. Whatever may be

the number of conductors per side of the coil, each coil side is placed inside one armature slot only. That

means all conductors of one side of a particular coil must be placed in one single slot only. Similarly, we

place all conductors of opposite side of the coil in another single armature slot.

Coil Span

Coil span is defined as the peripheral distance between two sides of a coil, measured in term of the

number of armature slots between them. That means, after placing one side of the coil in a particular slot,

after how many conjugative slots, the other side of the same coil is placed on the armature. This number

is known as coil span.

Figure: Armature windings

If the coil span is equal to the pole pitch, then the armature winding is said to be full - pitched. In this

situation, two opposite sides of the coil lie under two opposite poles. Hence emf induced in one side of

the coil will be in 180o phase shift with emf induced in the other side of the coil. Thus, the total terminal

voltage of the coil will be nothing but the direct arithmetic sum of these two emfs. If the coil span is less

than the pole pitch, then the winding is referred as fractional pitched. In this coil, there will be a phase

difference between induced emf in two sides, less than 180o. Hence resultant terminal voltage of the coil

is vector sum of these two emf’s and it is less than that of full-pitched coil.

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In practice, coil pitch (or Span) as low as eight tenth of a Pole Pitch, is employed without much serious

reduction in emf. Fractional pitched windings are purposely used to effect substantial saving in copper of

the end connection and for improving commutation.

Pitch of Armature Winding

Back Pitch (YB)

A coil advances on the back of the armature. This advancement is measured in terms of armature

conductors and is called back pitch. It is equal to the number difference of the conductor connected to a

given segment of the commutator.

Front Pitch (YF)

The number of armature conductors or elements spanned by a coil on the front is called front pitch.

Alternatively, we define the front-pitch as the distance between the second conductor of the next coil

which connects the front, i.e., commutator end of the armature. In other words, it is the number difference

of the conductors connected together at the back end of the armature. We are showing both front and back

pitches for a lap, and a wave windings in the figure below.

Resultant Pitch (YR)

It is the distance between the beginning of one coil and the beginning of the next coil to which it is

connected. As a matter of precautions, we should keep in mind that all these pitches, though normally

stated concerning armature conductors, are also times of armature slots or commutator bars.

Commutator Pitch (YC)

Commutator pitch is defined as the distance between two commutator segments which two ends of same

armature coil are connected. We measure commutator pitch in term of commutator bars or segment.

Single Layer Armature Winding

We place armature coil sides in the armature slots differently. In some arrangement, each one side of an

armature coil occupies a single slot. In other words, we place one coil side in each armature slot. We refer

this arrangement as single layer winding.

Two Layer Armature Winding

In other types of armature winding, arrangement two coil sides occupy every armature slot; one occupies

upper half, and another one occupies the lower half of the slot. We so place the coils in two layers

winding that if one side occupies upper half, then another side occupies the lower half of some other slot

at a distance of one coil pitch away.

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Armature Winding of A DC Machine

Based on type of winding connections we classified armature winding of a dc machine into two types. These winding connections are same for DC generator & DC motor. Types of Windings in DC Machine, 1. Lap winding. 2. Wave winding.

Lap winding of a DC Machine

In this type of winding the completing end of one coil is connected to a commutator segment and to the

start end of adjacent coil located under the same pole and similarly all coils are connected. This type of

winding is known as lap because the sides of successive coils overlap each other.

Lap winding may be simplex (single) or multiplex (duplex or triplex) winding. In simplex lap winding the

connection of the winding is that there are as many parallel paths as there are number of poles. Whereas for duplex, the number of parallel paths are equal to twice that of the number of poles and for

triplex it is thrice. For this reason, the lap winding is called multiple or parallel winding. The sole

purposes of such type of windings are, (a) To increase the number of parallel paths enabling the armature current to increase i.e., for high current

output. (b) To improve commutation as the current per conductor decreases.

Notes on Lap winding

1. The coil or back pitch YB must be approximately equal to pole pitch i.e., YB = Z/P.

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2. The back pitch and front pitch are odd and are of opposite sign. They differ from each other by 2m, where m = 1,2,3 for simplex, duplex, and triplex respectively.

i.e., YB = YF ± 2m

When YB > YF i.e., YF + 2m then the winding progresses from left to right and such a winding is

known as progressive winding. If YB < YF i.e., YB = YF - 2m then the winding progresses from

right to left and such a winding is known as retrogressive winding. 3. The average pitch,YAVE=( YB + YF )/2.

4. Resultant pitch, YR is always even as difference between two odd numbers is even and is equal to

2m. 5. Commutator pitch, YC = m i.e., , 2, 3, 4 etc. for simplex, duplex, triplex, quadruplex etc. 6. Number of parallel paths = mP. Where, m = multiplicity.

Example: For instance, the number of parallel paths for a 6-pole duplex lap winding is given by 6

x 2 = 12 paths. 7. The total number of poles are equal to the total number of brushes. 8. If Ia is the total armature current, then current per parallel path is Ia /P. 9. Lap winding is used for low voltage and high current machines.

Wave winding of a DC Machine In wave winding the coils which are carrying current in one direction are connected in series circuit and

the carrying current in opposite direction are connected in another series circuit. A wave winding is

shown in figure.

If after passing once around the armature the winding falls in a slot to the left of its starting point then

winding is said to be retrogressive. If it fails one slot to the right it is progressive.

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Notes on Wave winding The following are the important points to be remembered pertaining to wave winding,

1. Both pitches YB and YF are odd and of same sign. 2. Back and front pitches may be equal or differ by 2 and are merely equal to pole pitch.

3. Resultant pitch, YR = YF + YB = (Z ± 2)/2 P = Number of poles Z = Total number of conductors.

4. Commutator pitch, YC = YA (Average pitch)

YC =(Number of commutator bars ± 1)/(Number of pair of poles). 5. Number of parallel paths are equal to 2m,where m is the multiplicity. 6. The number of brushes required are two irrespective of the number of poles. 7. If Ia is the total armature current then current carried by each path or conductor is Ia/2. 8. Since a wave winding is a series winding, it is used for high voltage and low current machine. Emf Equation of a DC Generator

As the armature rotates, a voltage is generated in its coils. In the case of a generator, the emf of rotation is

called the Generated emf or Armature emf and is denoted as Er = Eg. In the case of a motor, the emf of

rotation is known as Back emf or Counter emf and represented as Er = Eb. The expression for emf is

same for both the operations. I.e., for Generator as well as for Motor

Derivation of EMF Equation of a DC Machine – Generator and Motor

Let,

• P – Number of poles of the machine

• ϕ – Flux per pole in Weber.

• Z – Total number of armature conductors.

• N – Speed of armature in revolution per minute (r.p.m).

• A – Number of parallel paths in the armature winding.

In one revolution of the armature, the flux cut by one conductor is given as

Time taken to complete one revolution is given as

Therefore, the average induced e.m.f in one conductor will be

Putting the value of (t) from Equation (2) in the equation (3) we will get

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The number of conductors connected in series in each parallel path = Z/A.

Therefore, the average induced e.m.f across each parallel path or the armature terminals is given by the

equation shown below.

Where n is the speed in revolution per second (r.p.s) and given as

For a given machine, the number of poles and the number of conductors per parallel path (Z/A) are

constant. Hence, the equation (5) can be written as

Where, K is a constant and given as

Therefore, the average induced emf equation can also be written as

Where K1 is another constant and hence induced emf equation can be written as

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Where ω is the angular velocity in radians/second is represented as

Thus, it is clear that the induced emf is directly proportional to the speed and flux per pole. The polarity

of induced emf depends upon the direction of the magnetic field and the direction of rotation. If either of

the two is reverse the polarity changes, but if two are reversed the polarity remains unchanged.

This induced emf is a fundamental phenomenon for all the DC Machines whether they are working as a

generator or motor.

If the machine DC Machine is working as a Generator, the induced emf is given by the equation shown

below.

Where Eg is the Generated Emf

If the machine DC Machine is working as a Motor, the induced emf is given by the equation shown

below.

In a motor, the induced emf is called Back Emf (Eb) because it acts opposite to the supply voltage.

Types of DC Generators – Separately Excited and Self Excited

The DC generator converts the electrical power into electrical power. The magnetic flux in a DC machine

is produced by the field coils carrying current. The circulating current in the field windings produces a

magnetic flux, and the phenomenon is known as Excitation. DC Generator is classified according to the

methods of their field excitation.

By excitation, the DC Generators are classified as Separately excited DC Generators and Self-excited DC

Generators. There is also Permanent magnet type DC generators. The self-excited DC Generators are

further classified as Shunt wound DC generators; Series wound DC generators and Compound wound DC

generators. The Compound Wound DC generators are further divided as long shunt wound DC

generators, and short shunt wound DC generators.

The field pole of the DC generator is stationary, and the armature conductor rotates. The voltage

generated in the armature conductor is of alternating nature, and this voltage is converted into the direct

voltage at the brushes with the help of the commutator.

The detailed description of the various types of generators is explained below.

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Permanent Magnet type DC Generator

In this type of DC generator, there is no field winding is placed around the poles. The field produced by

the poles of these machines remains constant. Although these machines are very compact but are used

only in small sizes like dynamos in motorcycles, etc. The main disadvantage of these machines is that the

flux produced by the magnets deteriorates with the passage of time which changes the characteristics of

the machine.

Separately Excited DC Generator

A DC generator whose field winding or coil is energized by a separate or external DC source is called a

separately excited DC Generator. The flux produced by the poles depends upon the field current with the

unsaturated region of magnetic material of the poles. i.e. flux is directly proportional to the field current.

But in the saturated region, the flux remains constant.

The figure of self-excited DC Generator is shown below.

Separately Excited DC Generator

Here,

Ia = IL where Ia is the armature current and IL is the line current.

Terminal voltage is given as

If the contact brush drop is known, then the equation (1) is written as

The power developed is given by the equation shown below.

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Power output is given by the equation (4) shown above.

Self Excited DC Generator

Self-excited DC Generator is a device, in which the current to the field winding is supplied by the

generator itself. In self-excited DC generator, the field coils mat be connected in parallel with the

armature in the series, or it may be connected partly in series and partly in parallel with the armature

windings.

The self-excited DC Generator is further classified as

1. Shunt Wound Generator 2. Series Wound Generator 3. Compound Wound Generator

1. Shunt Wound Generator

In a shunt wound generator, the field winding is connected across the armature winding forming a

parallel or shunt circuit. Therefore, full terminal voltage is applied across it. A very small field current Ish,

flows through it because this winding has many turns of fine wire having very high resistance Rshof the

order of 100 ohms.

The connection diagram of shunt wound generator is shown below.

Shunt Wound DC Generator

Shunt field current is given as

Where Rsh is the shunt field winding resistance.

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The current field Ish is practically constant at all loads. Therefore, the DC shunt machine is considered to

be a constant flux machine.

Armature current is given as

Terminal voltage is given by the equation shown below.

If the brush contact drop is included, the equation of the terminal voltage becomes

2. Series Wound Generator

A series-wound generator the field coils are connected in series with the armature winding. The series

field winding carries the armature current. The series field winding consists of a few turns of wire of thick

wire of larger cross-sectional area and having low resistance usually of the order of less than 1 ohm

because the armature current has a very large value.

Its convectional diagram is shown below.

Series Wound DC Generator

Series field current is given as

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Rse is known as the series field winding resistance.


If the brush contact drop is included, the terminal voltage equation is written as

The flux developed by the series field winding is directly proportional to the current flowing through it.

But it is only true before magnetic saturation after the saturation flux becomes constant even if the current

flowing through it is increased.

3. Compound Wound Generator

In a Compound Wound Generator, there are two sets of the field winding on each pole. One of them is

connected in series having few turns of thick wire, and the other is connected in parallel having many

turns of fine wire with the armature windings. In other words, the generator which has both shunt and

series fields is called the compound wound generators.

If the magnetic flux produced by the series winding assists the flux produced by the shunt winding, then

the machine is said to be cumulative compounded. If the series field flux opposes the shunt field flux,

then the machine is called the differentially compounded.

It is connected in two ways. One is a long shunt compound generator, and another is a short shunt

compound generator. If the shunt field is connected in parallel with the armature alone then the machine

is called the short compound generator. In long shunt compound generator, the shunt field is connected in

series with the armature. The two types of generators are discussed below in details.

Long Shunt Compound Wound Generator

In a long shunt wound generator, the shunt field winding is parallel with both armature and series field

winding. The connection diagram of long shunt wound generator is shown below.

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Long Shunt Compound Wound Generator





Short Shunt Compound Wound Generator

In a Short Shunt Compound Wound Generator, the shunt field winding is connected in parallel with the

armature winding only. The connection diagram of short shunt wound generator is shown below.

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Short Shunt Compound Wound Generator





In this type of DC generator, the field is produced by the shunt as well as series winding. The shunt field

is stronger than the series field. If the magnetic flux produced by the series winding assists the flux

produced by the shunt field winding, the generator is said to be Cumulatively Compound Wound

generator.

If the series field flux opposes the shunt field flux, the generator is said to be Differentially Compounded.

Voltage buildup in self excited Generator or Dc Shunt Generato

A self excited DC generator supplies its own field excitation . A self excited generator shown in figure is

known as a shunt generator because its field winding is connected in parallel with the armature. Thus the

armature voltage supplies the field current.

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This generator will build up a desired terminal voltage. Assume that the generator in figure has no load

connected to it and armature is driven at a certain speed by a prime mover. we shall study the condition

under which the voltage buildup takes place. Due to this residual flux, a small voltage Ear will be

generated. It is given by

This voltage is of the order of 1V or 2V . It causes a current If to flow in the field winding in the

generator. The field current is given by

This field current produces a magneto motive force in the field winding, which increases the flux. This

increase in flux increases the generated voltage Ea. The increased armature voltage Ea increases the

terminal voltage V. with the increase in V, the field current If increases further. This in turn increases Φ

and consequently Ea increases further. The process of the voltage buildup continues. Figure shows the

voltage buildup of a dc shunt generator.

OCC Characteristics of DC generator

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The effect of magnetic saturation in the pole faces limits the terminal voltage of the generator to a steady

state value. We have assumed that the generator is no load during the buildup process. The following equations

describe the steady state operation.

Since the field current If in a shunt generator is very small, the voltage drop If Ra can be neglected, and V= Ea

The Ea versus If curve is the magnetization curve shown in figure

For the field circuit

V = If Rf

The straight line given by V = If Rf is called the field-resistance line.

The field resistance line is a plot of the voltage If Rf across the field circuit versus the field If. The slop

this line is equal to the resistance of the field circuit.

The no-load terminal voltage V0 of thr generator. thus, the intersection point P of the magnetization curve

and the field resistance line gives the no-load terminal voltage V0(bP) and the corresponding field current

(Ob). Normally , in the shunt generator the voltage buildup to the value given by the point P. at this point

Ea = If Rf = V0.

If the field current corresponding to point P is increase further , there is no further increase in the terminal

voltage.

The no-load voltage is adjusted by adding resistance in series with shunt field. This increase slope of this

line causing the operating point to shift at lower voltage

The operating point are graphical solution of two simultaneous equation namely , the magnetization curve

and field resistance line . A graphical solution is preferred due to non-linear nature of magnetization

curve. Self excited generator are designed to obtain no-load voltage from 50% to 125% of the rated value while

varying the added resistance in field circuit from maximum to zero value.

Critical Field Resistance: Figure below shows the voltage buildup in the dc shunt generator for various resistances of the field

circuit.

Fig: Determination of critical resistance

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A decrease in the resistance of the field circuit reduces the slope of the field resistance line result in

higher voltage. If the speed remains constant, an increase in the resistance of field circuit increases the

slop of field resistance line, resulting in a lower voltage. If the field circuit resistance is increased to Rc

which is terminal as the critical resistance of the field, the field resistance line becomes a tangent to the

initial part of the magnetization curve. When the field resistance is higher than this value, the generator

fails to excite. Critical Speed:

Figure shows the variation of no-load voltage with fixed Rf and variable speed of the armature.

Fig: Determination of critical speed

The magnetization curve varies with the speed and its ordinate for any field current is proportional to the

speed of the generator. all the points on the magnetization curve are lowered, and the point of intersection

of the magnetization curve and the field resistance line moves downwards. at a particular speed, called the

critical speed, the field resistance line becomes tangential to the magnetization curve. below the critical

speed the voltage will not build up. In Brief, the following condition must be satisfied for voltage buildup in a self-excited generator. There must be sufficient residual flux in the field poles.

1. the field terminal should be connected such a way that the field current increases flux in the

direction of residual flux. 2. The field circuit resistance should be less than the critical field circuit resistance.

If there is a no residual flux in the field poles, Disconnected the field from the armature circuit and apply

a dc voltage to the field winding. this process is called Flashing the field. It will induce some residual flux

in the field poles.

Causes for Failure to Self Excite and Remedial Measures

There may be one or more of the following reasons due which a self excited generator may fail to build

up voltage.

1. No residual magnetism

The start of the buildup process needs some residual magnetism in the magnetic circuit of the generator. If

there is little or no residual magnetism. because of inactivity or jarring in shipment, no voltage will be

induced that can produce field current.

2. Reversal of Field Connections

The voltage induced owing to residual magnetism acts across the field and results in flow or current in the

field coils in such a direction as to produce magnetic flux in the same direction as the residual flux.

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Reversal of connections of the field winding destroys the residual magnetism which causes the generator

failure to build up voltage.

3. In case of dc series wound generators

The resistance in the load circuit may be more than its critical resistance, which may be due to

( i ) ( ii ) ( ii i )

( iv )

4. In case of

(a) (b) (c)

Remedy

open-circuit

high resistance of load circuit

faulty contact between brushes and commutator

and commutator surface dirty or greasy. shunt wound generator

the resistance of the shunt field circuit may be greater than the critical resistance;

the resistance in the load circuit may be lower than the critical resistance; the

speed of rotation may not be equal to rated one.

In case the generator is started up for the first time, it may be that no voltage will be built up either

because the poles have no residual magnetism or the poles have retained some residual magnetism but the

field winding connections are reversed so that the magnetism developed by the field winding on start has

destroyed the residual magnetism and the machine can not "build up". In both the cases, the field coils

must be connected to a dc source (a storage battery) for a short while to magnetise the poles. The

application of external source of direct current to the field is called flashing of the field.

Armature Reaction in DC Machines

In a DC machine, two kinds of magnetic fluxes are present; 'armature flux' and 'main field flux'. The

effect of armature flux on the main field flux is called as armature reaction.

MNA And GNA

EMF is induced in the armature conductors when they cut the magnetic field lines. There is an axis (or,

you may say, a plane) along which armature conductors move parallel to the flux lines and, hence, they

do not cut the flux lines while on that plane. MNA (Magnetic Neutral Axis) may be defined as the axis

along which no emf is generated in the armature conductors as they move parallel to the flux lines.

Brushes are always placed along the MNA because reversal of current in the armature conductors takes

place along this axis.

GNA (Geometrical Neutral Axis) may be defined as the axis which is perpendicular to the stator field

axis.

Armature Reaction

The effect of armature reaction is well illustrated in the figure below.

Consider, no current is flowing in the armature conductors and only the field winding is energized (as

shown in the first figure of the above image). In this case, magnetic flux lines of the field poles are

uniform and symmetrical to the polar axis. The 'Magnetic Neutral Axis' (M.N.A.) coincides with the

'Geometric Neutral Axis' (G.N.A.).

The second figure in the above image shows armature flux lines due to the armature current. Field poles

are de-energised

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Fig: Armature reaction

Now, when a DC machine is running, both the fluxes (flux due to the armature conductors and flux due to

the field winding) will be present at a time. The armature flux superimposes with the main field flux and,

hence, disturbs the main field flux (as shown in third figure the of above image). This effect is called as

armature reaction in DC machines.

The Adverse Effects Of Armature Reaction:

1. Armature reaction weakens the main flux. In case of a dc generator, weakening of the main flux

reduces the generated voltage. 2. Armature reaction distorts the main flux, hence the position of M.N.A. gets shifted (M.N.A. is

perpendicular to the flux lines of main field flux). Brushes should be placed on the M.N.A.,

otherwise, it will lead to sparking at the surface of brushes. So, due to armature reaction, it is hard to

determine the exact position of the MNA

For a loaded dc generator, MNA will be shifted in the direction of the rotation. On the other hand, for a

loaded dc motor, MNA will be shifted in the direction opposite to that of the rotation.

How To Reduce Armature Reaction? Usually, no special efforts are taken for small machines (up to few kilowatts) to reduce the armature

reaction. But for large DC machines, compensating winding and interpoles are used to get rid of the ill

effects of armature reaction.

Compensating winding: Now we know that the armature reaction is due to the presence of armature

flux. Armature flux is produced due to the current flowing in armature conductors. Now, if we place

another winding in close proximity of the armature winding and if it carries the same current but in the

opposite direction as that of the armature current, then this will nullify the armature field. Such an

additional winding is called as compensating winding and it is placed on the pole faces. Compensating

winding is connected in series with the armature winding in such a way that it carries the current in

opposite direction.

Interpoles: Interpoles are the small auxiliary poles placed between the main field poles. Winding on the

interpoles is connected in series with the armature. Each interpole is wound in such a way that its

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magnetic polarity is same as that of the main pole ahead of it. Interpoles nullify the quadrature axis

armature flux.

Demagnetizing and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the

Fig.1.

Fig. 1 Conductors which are responsible for producing demagnetizing and distortion effects

The brushes are lying along the new position of MNA which is at angle θ from GNA. The

conductors in the region AOC = BOD = 2θ at the top and bottom of the armature are carrying current in

such a direction as to send the flux in armature from right to left. Thus these conductors are in direct

opposition to main field and called demagnetizing armature conductors.

The remaining armature conductors which are lying in the region AOD and BOC carry current in

such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field

flux. Hence these conductors are called cross magnetizing armature conductors which will cause

distortion in main field flux.

These conductors are shown in the Fig. 2

Fig. 2 Conductors which are responsible for producing Cross magnetization ffects

Calculation of Demagnetizing and Cross Magnetizing Amp-Turns

Let us the number of demagnetizing and cross magnetizing amp-turns. 25 | P a g e

Let Z = Total number of armature conductors

P = Number of poles

I = Armature conductor current in Amperes

= Ia/2 for simplex wave winding

= Ia/P for simplex lap winding

θm = Forward lead of brush in mechanical degrees.

The conductors which are responsible for demagnetizing ampere-turns are lying in the region

spanning 4 θm degrees. The region is between angles AOC and BOD, as shown in the Fig. 2.

... Total number of armature conductors lying in angles AOC and BOD.

Since two conductors from one turn,

The conductor which are responsible fro cross magnetizing ampere turns are lying between the angles

AOD and BOC, as shown in the Fig.2.

Total armature-conductors / pole = Z/P

From above we have found an expression for demagnetizing conductors per pole.

Since two conductors from one turn,

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If the brush shift angle is given in electrical degrees then it should be converted into

mechanical degrees by using the relation,

Example :

A wave wound 4 pole d.c. generator with 480 armature conductors supplies a current of 144 A. The

brushes are given an actual lead of 10o. Calculate the demagnetizing and cross magnetizing amp turns

per pole.

Solution :

P = 4, Z = 480, Ia = 144 A

For wave wound,

I = Ia/2 = 144/2 = 72 A

θm = 10o

Compensating Winding and Interpoles in DC Generator

In DC compound machine setup by armature current opposes magnetic field flux, this is known as armature reaction. The armature reaction has two effects (i) Demagnetizing effect and (ii) Cross magnetizing effect. Demagnetizing effect weakens the main field flux which in turn decreases the induced e.m.f (as E ∝ Ø)). To overcome this effect a few extra turns/poles are added in series to main field winding. This creates a series field which serves two purposes,

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(i) It helps to neutralize the demagnetizing effect of armature reaction.

(ii) If wound for cumulative compounded machine the electrical performance will be improved.

Compensating winding:

All armature conductors placed under the main poles region produces e.m.f which is at right angle (90°)

to the main field e.m.f. This e.m.f causes distortion in main field flux. This is known as cross magnetizing

effect. To minimize the cross magnetizing effect compensating winding is used. This compensating

winding produces an m.m.f which opposes the m.m.f produced by armature conductors.

This objective is achieved by connecting compensating winding in series with armature winding. In

absence of compensating winding, cross magnetizing effect causes sparking at the commutators and short

circuiting the whole armature winding.

Let, Zc = Number of compensating conductors/pole

Za = Number of active armature conductors/pole

Ia = Armature current.

ZcIa= Za (Ia/A)

Where,

Ia/A=Armature current/conductor

Zc= Za/A

Compensating Winding Disadvantages

This winding neutralizes the cross magnetizing effect due to armature conductors only but not due to

interpolar region. This winding is used in large machine in which load is fluctuating.

Interpoles

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Cross magnetizing effect in interpolar region is by interpoles (also known as compoles (or) commutating

poles). These interpoles are small in size and placed in between the main poles of yoke. Like

compensating winding, interpoles are also connected in series with armature winding such that the m.m.f

produced by them opposes the m.m.f produced by armature conductor in interpolar region. In generators,

the interpole polarity is same as that of main pole ahead such that they induce an e.m.f which is known as

commutating or reversing e.m.f. This commutating e.m.f minimizes the reactance e.m.f and hence sparks

or arcs are eliminated.

Fig: Intepoles

Compensating winding and Interpoles are used for same purpose but the difference between them is,

interpoles produce e.m.f for neutralizing reactance e.m.f whereas compensating winding produces an

m.m.f which opposes the m.m.f produced by conductors.

COMMUTATION:

The process of reversal of current in the short circuited armature coil is called ‘Commutation’. This

process of reversal takes place when coil is passing through the interpolar axis (q-axis), the coil is short

circuited through commutator segments. Commutation takes place simultaneously for ‘P’ coils in a lap-

wound machine and two coil sets of P/2 coils each in a wave-wound machine.

The process of commutation of coil ‘B’ is shown below. In figure ‘1.29’ coil ‘B’ carries current from left

to right and is about to be short circuited in figure ‘1.30’ brush has moved by 1/3 rd of its width and the

brush current supplied by the coil are as shown. In figure ‘1.31’ coil ‘B’ carries no current as the brush is

at the middle of the short circuit period and the brush current in supplied by coil C and coil A. In figure

‘1.32’ the coil B which was carrying current from left to right carries current from right to left. In fig

‘1.33’ spark is shown which is due to the reactance voltage. As the coil is embedded in the armature slots,

which has high permeability, the coil posses appreciable amount of self inductance. The current is

changed from +I to –I. So due to self inductance and variation in the current from +I to –I, a voltage is

induced in the coil which is given by L dI/dt. Fig ‘1.34’ shows the variation of current plotted on the time

axis. Sparking can be avoided by the use of interpoles or commutating-poles.

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Fig: Commutation process

METHODS OF IMPROVING COMMUTATION:

Methods of Improving Commutation: There are two practical ways of improving commutation i.e. of

making current reversal in the short-circuited coil as sparkless as possible. These methods are known as (i) resistance commutation and (ii) emf. commutation (which is done with the help of either brush lead or

interpoles, usually the later).

Resistance Commutation: This method of improving commutation consists of replacing low-resistance

Cu brushes by comparatively high-resistance carbon brushes. When current I from coil C reach the

commutator segment b, it has two parallel paths open to it. The first part is straight from bar ‘b’ to the

brush and the other parallel path is via the short-circuited coil B to bar ‘a’ and then to the brush. If the Cu

brushes are used, then there is no inducement for the current to follow the second longer path, it would

preferably follow the first path. But when carbon brushes having high resistance are used, then current I

coming from C will prefer to pass through the second path. The additional advantages of carbon brushes

are that (i) they are to some degree self lubricating and polish the commutator and (ii) should sparking

occur, they would damage the commutator less than when Cu brushes are used. But some of their minor

disadvantages are: (i) Due to their high contact resistance (which is beneficial to sparkless commutation)

a loss of approximately 2 volt is caused. Hence, they are not much suitable for small machines where this

voltage forms an appreciable percentage loss. (ii) Owing to this large loss, the commutator has to be made

somewhat larger than with Cu brushes in order to dissipate heat efficiently without greater rise of

temperature. (iii) because of their lower current density (about 7-8 A/cm2 as compared to 25-30 A/cm2

for Cu brushes) they need larger brush holders.

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Fig: Resistance Commutation

EMF Commutation: In this method, arrangement is made to neutralize the reactance voltage by

producing a reversing emf in the short-circuited coil under commutation. This reversing emf, as the name

shows, is an emf in opposition to the reactance voltage and if its value is made equal to the latter, it will

completely wipe it off, thereby producing quick reversal of current in the short-circuited coil which will

result in sparkless commutation. The reversing emf may be produced in two ways: (i) either by giving the

brushes a forward lead sufficient enough to bring the short-circuited coil under the influence of next pole

of opposite polarity or (ii) by using Interpoles. The first method was used in the early machines but has

now been abandoned due to many other difficulties it brings along with.

Interpoles of Compoles: These are small poles fixed to the yoke and spaced in between the main poles.

They are wound with comparatively few heavy gauge Cu wire turns and are connected in series with the

armature so that they carry full armature current. Their polarity, in the case of a generator, is the same as

that of the main pole ahead in the direction of rotation. The function of interpoles is two-fold: (i) As their

polarity is the same as that of the main pole ahead, they induce an emf in the coil (under commutation)

which helps the reversal of current. The emf induced by the compoles is known as commutating or

reversing emf. The commutating emf neutralizes the reactance emf thereby making commutation

sparkless. With interpoles, sparkless commutation can be obtained up to 20 to 30% overload with fixed

brush position. In fact, interpoles raise sparking limit of a machine to almost the same value as heating

limit. Hence, for a given output, an interpole machine can be made smaller and, therefore, cheaper than a

non-interpolar machine. As interpoles carry armature current, their commutating emf is proportional to

the armature current. This ensures automatic neutralization of reactance voltage which is also due to

armature current. (ii) Another function of the interpoles is to neutralize the cross-magnetising effect of

armature reaction. Hence, brushes are not to be shifted from the original position.

Fig: Interpoles of Compoles

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OF as before, represents the mmf due to main poles. OA represents the crossmagnetising mmf due to

armature. BC which represents mmf due to interpoles, is obviously in opposition to OA, hence they

cancel each other out. This cancellation of crossmagnetisation is automatic and for all loads because both

are produced by the same armature current. The distinction between the interpoles and compensating

windings should be clearly understood. Both are connected in series and thier m.m.fs. are such as to

neutralize armature reaction. But compoles additionally supply mmf for counteracting the reactance

voltage induced in the coil undergoing commutation. Moreover, the action of the compoles is localized;

they have negligible effect on the armature reaction occurring on the remainder of the armature periphery.

Characteristics of DC Generators

Generally, following three characteristics of DC generators are taken into considerations: (i) Open Circuit

Characteristic (O.C.C.), (ii) Internal or Total Characteristic and (iii) External Characteristic. These

characteristics of DC generators are explained below.

1. Open Circuit Characteristic (O.C.C.) (E0/If)

Open circuit characteristic is also known as magnetic characteristic or no-load saturation characteristic.

This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a

given fixed speed. The O.C.C. curve is just the magnetization curve and it is practically similar for all

type of generators. The data for O.C.C. curve is obtained by operating the generator at no load and

keeping a constant speed. Field current is gradually increased and the corresponding terminal voltage is

recorded. The connection arrangement to obtain O.C.C. curve is as shown in the figure below. For shunt

or series excited generators, the field winding is disconnected from the machine and connected across an

external supply.

Fig: Circuit for OCC

Now, from the emf equation of dc generator, we know that Eg = kɸ. Hence, the generated emf should be

directly proportional to field flux (and hence, also directly proportional to the field current). However,

even when the field current is zero, some amount of emf is generated (represented by OA in the figure

below). This initially induced emf is due to the fact that there exists some residual magnetism in the field

poles. Due to the residual magnetism, a small initial emf is induced in the armature. This initially induced

emf aids the existing residual flux, and hence, increasing the overall field flux. This consequently

increases the induced emf. Thus, O.C.C. follows a straight line. However, as the flux density increases,

the poles get saturated and the ɸ becomes practically constant. Thus, even we increase the If further, ɸ

remains constant and hence, Eg also remains constant. Hence, the O.C.C. curve looks like the B-H

characteristic.

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The above figure shows a typical no-load saturation curve or open circuit characteristics for all types of

DC generators.

2. Internal or Total Characteristic (E/Ia)

An internal characteristic curve shows the relation between the on-load generated emf (Eg) and the

armature current (Ia). The on-load generated emf Eg is always less than E0 due to the armature reaction.

Eg can be determined by subtracting the drop due to demagnetizing effect of armature reaction from no-

load voltage E0. Therefore, internal characteristic curve lies below the O.C.C. curve.

3. External Characteristic (V/IL)

An external characteristic curve shows the relation between terminal voltage (V) and the load current

(IL). Terminal voltage V is less than the generated emf Eg due to voltage drop in the armature circuit.

Therefore, external characteristic curve lies below the internal characteristic curve. External

characteristics are very important to determine the suitability of a generator for a given purpose.

Therefore, this type of characteristic is sometimes also called as performance characteristic or load

characteristic.

Internal and external characteristic curves are shown below for each type of generator. Characteristics of Separately Excited DC Generator:

If there is no armature reaction and armature voltage drop, the voltage will remain constant for any load

current. Thus, the straight line AB in above figure represents the no-load voltage vs. load current IL. Due

to the demagnetizing effect of armature reaction, the on-load generated emf is less than the no-load

voltage. The curve AC represents the on-load generated emf Eg vs. load current ILi.e. Internal

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characteristic (as Ia = IL for a separately excited dc generator). Also, the terminal voltage is lesser due to

ohmic drop occurring in the armature and brushes. The curve AD represents the terminal voltage vs. load

current i.e. external characteristic.

Characteristics of DC Shunt Generator

To determine the internal and external load characteristics of a DC shunt generator the machine is allowed

to build up its voltage before applying any external load. To build up voltage of a shunt generator, the

generator is driven at the rated speed by a prime mover. Initial voltage is induced due to residual

magnetism in the field poles. The generator builds up its voltage as explained by the O.C.C. curve. When

the generator has built up the voltage, it is gradually loaded with resistive load and readings are taken at

suitable intervals. Connection arrangement is as shown in the figure below.

Fig: Circuit for External characteristics of shunt generator

Unlike, separately excited DC generator, here, IL≠Ia. For a shunt generator, Ia=IL+If. Hence, the internal

characteristic can be easily transmitted to Eg vs. IL by subtracting the correct value of Iffrom Ia.

During a normal running condition, when load resistance is decreased, the load current increases. But, as

we go on decreasing the load resistance, terminal voltage also falls. So, load resistance can be decreased

up to a certain limit, after which the terminal voltage drastically decreases due to excessive armature

reaction at very high armature current and increased I2R losses. Hence, beyond this limit any further

decrease in load resistance results in decreasing load current. Consequently, the external characteristic

curve turns back as shown by dotted line in the above figure.

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Characteristics of DC Series Generator

The curve AB in above figure identical to open circuit characteristic (O.C.C.) curve. This is because in

DC series generators field winding is connected in series with armature and load. Hence, here load current

is similar to field current (i.e. IL=If). The curve OC and OD represent internal and external characteristic

respectively. In a DC series generator, terminal voltage increases with the load current. This is because, as

the load current increases, field current also increases. However, beyond a certain limit, terminal voltage

starts decreasing with increase in load. This is due to excessive demagnetizing effects of the armature

reaction.

Characteristics Of DC Compound Generator

The above figure shows the external characteristics of DC compound generators. If series winding amp-

turns are adjusted so that, increase in load current causes increase in terminal voltage then the generator is

called to be over compounded. The external characteristic for over compounded generator is shown by the

curve AB in above figure.

If series winding amp-turns are adjusted so that, the terminal voltage remains constant even the load

current is increased, then the generator is called to be flat compounded. The external characteristic for a

flat compounded generator is shown by the curve AC.

If the series winding has lesser number of turns than that would be required to be flat compounded, then

the generator is called to be under compounded. The external characteristics for an under compounded

generator are shown by the curve AD.

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Parallel Operation of D.C. Generators:

Here this explains you the parallel operation of dc generators and load sharing among them for the

continuous power supply. In a DC power plant, power is usually supplied from several generators of

small ratings connected in parallel instead of from one large generator. This is due to the following

reasons:

(i) Continuity of service:

If a single large generator is used in the power plant, then in case of its breakdown, the whole plant will

be shut down. However, if power is supplied from a number of small units operating in parallel, then in

case of failure of one unit, the continuity of supply can be maintained by other healthy units.

(ii) Efficiency:

Generators run most efficiently when loaded to their rated capacity. Electric power costs less per kWh

when the generator producing it is efficiently loaded. Therefore, when load demand on power plant

decreases, one or more generators can be shut down and the remaining units can be efficiently loaded.

(iii) Maintenance and repair:

Generators generally require routine-maintenance and repair. Therefore, if generators are operated in

parallel, the routine or emergency operations can be performed by isolating the affected generator while

the load is being supplied by other units. This leads to both safety and economy.

(iv) Increasing plant capacity:

In the modern world of increasing population, the use of electricity is continuously increasing. When

added capacity is required, the new unit can be simply paralleled with the old units. In many situations, a

single unit of desired large capacity may not be available. In that case, a number of smaller units can be

operated in parallel to meet the load requirement. Generally, a single large unit is more expensive.

(v) Non-availability of single large unit:

In many situations, a single unit of desired large capacity may not be available. In that case, a number of

smaller units can be operated in parallel to meet the load requirement. Generally, a single large unit is

more expensive.

Connecting Shunt Generators in Parallel:

The generators in a power plant are connected in parallel through bus-bars. The bus-bars are heavy thick

copper bars and they act as +ve and -ve terminals. The positive terminals of the generators are connected

to the +ve side of bus-bars and negative terminals to the negative side of bus-bars.

Fig. (3.15) shows shunt generator 1 connected to the bus-bars and supplying load. When the

load on the power plant increases beyond the capacity of this generator, the second shunt generator 2 is

connected in parallel with the first to meet the increased load demand. The procedure for paralleling

generator 2 with generator 1 is as under:

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Parallel operation of shunt generators

(i) The prime mover of generator 2 is brought up to the rated speed. Now switch S4 in the field circuit of

the generator 2 is closed.

(ii) Next circuit breaker CB-2 is closed and the excitation of generator 2 is adjusted till it generates a

voltage equal to the bus-bars voltage.This is indicated by voltmeter V2.

(iii) Now the generator 2 is ready to be paralleled with generator 1. The main switch S3 is closed, thus

putting generator 2 in parallel with generator 1. Note that generator 2 is not supplying any load because

it's generated e.m.f. is equal to bus-bars voltage.The generator is said to be “floating” (i.e., not supplying

any load) on the bus-bars.

(iv) If generator 2 is to deliver any current, then it's generated voltage E should be greater than the bus-

bars voltage V. In that case, the current supplied by it is I = (E - V)/Ra where Ra is the resistance of the

armature circuit. By increasing the field current (and hence induced e.m.f. E), the generator 2 can be made

to supply the proper amount of load.

(v) The load may be shifted from one shunt generator to another merely by adjusting the field excitation.

Thus if generator 1 is to be shut down, the whole load can be shifted onto generator 2 provided it has the

capacity to supply that load. In that case, reduce the current supplied by generator 1 to zero (This will be

indicated by ammeter A1) open C.B.-1 and then open the main switch S1.

Load Sharing of two generators:

The load sharing between shunt generators in parallel can be easily regulated because of their drooping

characteristics.The load may be shifted from one generator to another merely by adjusting the field

excitation.Let us discuss the load sharing of two generators which have unequal no-load voltages.

Let E1, E2 = no-load voltages of the two generators

R1, R2 = their armature resistances

V = common terminal voltage (Bus-bars voltage)

then I1 = (E1 - V)/R1 and I2= (E2-V)/R2

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Thus the current output of the generators depends upon the values of E1 and E3.These values may be

changed by field rheostats.The common terminal voltage (or bus-bars voltage) will depend upon

(i) the e.m.f.s of individual generators and

(ii) the total load current supplied.

It is generally desired to keep the bus bars voltage constant. This can be achieved by adjusting the field

excitations of the generators operating in parallel.

Compound Generators in Parallel:

Under-compounded generators also operate satisfactorily in parallel but over compounded generators will

not operate satisfactorily unless their series fields are paralleled. This is achieved by connecting two

negative brushes together as shown in Fig. (3.16) (i). The conductor used to connect these brushes is

generally called equaliser bar. Suppose that an attempt is made to operate the two generators in Fig. (3.16) (ii) in parallel without an equalizer bar. If, for any reason, the current supplied by generator 1 increases

slightly, the current in its series field will increase and raise the generated voltage.

This will cause generator 1 to take more load. Since total load supplied to the system is constant,

the current in generator 2 must decrease and as a result, its series field is weakened. Since this effect is

cumulative, the generator 1 will take the entire load and drive generator 2 as a motor. Under such

conditions, the current in the two machines will be in the direction shown in Fig. (3.16) (ii). After

machine 2 changes from a generator to a motor, the current in the shunt field will remain in the same

direction, but the current in the armature and series field will reverse.

Thus the magnetising action, of the series field opposes that of the shunt field. As the current taken

by the machine 2 increases, the demagnetizing action of series field becomes greater and the resultant

field becomes weaker. The resultant field will finally become zero and at that time machine 2 will short

circuit machine 1, opening the breaker of either or both machines.

Fig: Parallel operation of compound generators

When the equalizer bar is used, a stabilizing action exists? And neither machine tends to take the

entire load. To consider this, suppose that current delivered by generator 1 increase. The increased current

will not only pass through the series field of generator 1 but also through the equalizer bar and series field

of generator 2.Therefore, the voltage of both the machines increases and the generator 2 will take a part of

the load.

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PARALLEL OPERATION OF DC SERIES GENERATOR

The interesting thing about the parallel operation of DC series generator is that DC series generators are

not usually employed for supply of power. Instead DC series motors are arranged in parallel to operate as

DC series generators during Electric Braking.

• Series generators are rarely used in industry for supplying loads. Some applications like electric

braking may employ them and operate two or more series generates in parallel.

• The excitation of the machine I increase, increasing the load current delivered. As the total

current is I the current supplied by machine II reduces, so also its excitation and induced emf.

Fig: Parallel operation of Series generators

• Thus machine I takes greater and greater fraction of the load current with machine II shedding its

load. Ultimately the current of machine II becomes negative and it also loads the first machine.

• Virtually there is a short circuit of the two sources, the whole process is thus highly unstable. One

remedy is for a problem as this is to make the two fields immune to the circulating current

between the machines.

• With the equalizer present, a momentary disturbance does not put the two machines out of action.

• A tendency to supply a larger current by a machine strengthens the field of the next machine and

increases its induced emf . This brings in stable conditions for operation rapidly.

Use of Equalizer Bars:

Here comes the use of equalizer bars in the parallel operation of DC series generators. The possibility of

reversal of either machine can be prevented by preventing the flow of circulating current produced due to

inequalities of induced emfs of the machines through the series field winding.

• This Aim can be achieved by connecting a heavy copper bar of negligible resistance across the

two machines as shown in the figure.

• Now the circulating current does not affect the field winding, but it get confined to the armature

and the equalizing bars.

• Now if the armature current increases, the terminal voltage drop occurs and the original condition

is restored.

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Cross connection of Field windings:

If the field coils are cross-connected when the series motors are connected in parallel, then any increase in

the current of the armature of generator 1, the increased current flows occurs through the field coil of

generator 2. This increases the electromotive force of generator, which opposes the change in load sharing

trying to stabilize the operation of the two generators at the original operating condition itself. Thus it will

give more positive and better operation than equalizer connection.

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UNIT II

DC MOTORS AND TESTING

WORKING PRINCIPLE OF A DC MOTOR The DC motor is the device which converts the direct current into the mechanical work. It works on the

principle of Lorentz Law, which states that “the current carrying conductor placed in a magnetic and

electric field experience a force”. And that force is called the Lorentz force. The Fleming left-hand rule

gives the direction of the force.

Fleming Left Hand Rule

If the thumb, middle finger and the index finger of the left hand are displaced from each other by an angle

of 90°, the middle finger represents the direction of the magnetic field. The index finger represents the

direction of the current, and the thumb shows the direction of forces acting on the conductor.

Fig: Flemings left hand rule

The formula calculates the magnitude of the force,

Before understanding the working of DC motor first, we have to know about their construction. The

armature and stator are the two main parts of the DC motor. The armature is the rotating part, and the

stator is their stationary part. The armature coil is connected to the DC supply.

The armature coil consists the commutators and brushes. The commutators convert the AC induces in the

armature into DC and brushes transfer the current from rotating part of the motor to the stationary

external load. The armature is placed between the north and south pole of the permanent or

electromagnet.

For simplicity, consider that the armature has only one coil which is placed between the magnetic field

shown below in the figure A. When the DC supply is given to the armature coil the current starts flowing

through it. This current develops their own field around the coil. Figure B shows the field induces around

the coil.

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Fig: magnetic field induces around the coil.

By the interaction of the fields (produces by the coil and the magnet), resultant field develops across the

conductor. The resultant field tends to regain its original position, i.e. in the axis of the main field. The

field exerts the force at the ends of the conductor, and thus the coil starts rotating.

Fig: Field produced due to poles alone

Let the field produces by the main field be Fm, and this field rotates in the clockwise direction. When the

current flows in the coil, they produce their own magnetic field says Fr. The field Fr tries to come in the

direction of the main field. Thereby, the torque act on the armature coil.

Fig: Field produced due to conductors alone

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The actual DC motor consists a large number of armature coils. The speed of the motor is directly

proportional to the number of coils used in the motor. These coils are kept under the impact of the

magnetic field.

The one end of the conductors are kept under the influence of north pole, and the other end is kept under

the influence of the South pole. The current enters into the armature coil through the north pole and move

outwards through the south pole. When the coil moves from one brush to another, at the same time the

polarity of the coil also changes. Thus, the direction of the force or torque acting on the coil remains

same.

The torque induces in the coil become zero when the armature coil is perpendicular to the main field. The

zero torque means the motor stops rotating. For solving this problem, the number of armature coil is used

in the rotor. So if one of their coils is perpendicular to the field, then the other coils induce the torque.

And the rotor moves continuously.

Also, for obtaining the continuous torque, the arrangement is kept in such a way that whenever the coils

cut the magnetic neutral axis of the magnet the direction of current in the coils become reversed. This can

be done with the help of the commutator.

Back Emf and its Significance in DC Motor

When a dc voltage V is applied across the motor terminals, the armature starts rotating due to the torque

developed in it.

As the armature rotates, armature conductors cut the pole magnetic field, therefore, as per law of

electromagnetic induction, an emf called back emf is induced in them.

The back emf (also called counter emf) is given by

where, P=number of poles of dc motor

Φ= flux per pole

Z=total number of armature conductors

N=armature speed

A=number of parallel paths in armature winding

As all other parameters are constant, therefore, Eb ∝ N

As per Lenz's law, "the induced emf always opposes the cause of its

production”. Here, the cause of generation of back emf is the rotation of

armature. Rotation of armature is due to armature torque. Torque is due

to armature current and armature current is due to supply dc voltage V.

Therefore, the ultimate cause of production of Eb is the supply voltage V.

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Therefore, back emf is always directed opposite to supply voltage V.

Significance of back emf in dc motor

(1) As the back emf opposes supply voltage V, therefore, supply voltage has to force current through the

armature against the back emf, to keep armature rotating. The electric work done in overcoming and

causing the current to flow against the back emf is converted into mechanical energy developed in the

armature.

It follows, therefore, that energy conversion in a dc motor is only possible due to the production of back

emf.

Mechanical power developed in the armature = EbIa

(2) Back emf makes dc motor a self-regulating motor i.e Eb makes motor to adjust Ia automatically as per

the load torque requirement. Lets see how.

From the motor figure,

V and Ra are fixed, therefore, armature current Ia dpends on back emf, which in turn depends on speed of

the motor.

(a) when the motor is running at no-load, small torque ( Ta=KIa ) is required by the motor to overcome

friction and windage. Therefore, a small current is drawn by the motor armature and the back emf is

almost equal to the supply voltage.

(b) If the motor is suddenly loaded, the load torque beomes greater than the armature torque and the

motor starts to slow down. As motor speed decreases, back emf decreases and therefore, armature current

starts increasing. With increasing Ia , armature torque increases and at some point it becomes equal to the

load torque. At that moment, motor stops slowing down and keeps running at this new speed.

(c) If the load on the motor is suddenly reduced, the driving torque becomes more than the load torque

and the motor starts accelerating. As the motor speed increases, back emf increases and therefore,

armature current decreases. Due to this reducing armature current, armature developed torque decreases

and at some point becomes equal to the load torque. That point onwards, motor will stop accelerating and

will start rotating uniformly at this new slightly increased speed.

So, this shows how important is back emf in dc motor. Without back emf, the electromagnetic energy

conversion would not have been possible at the first place.

Power Equation of a D.C. Motor

The voltage equation of a d.c. motor is given by,

V = Eb + Ia Ra

Multiplying both sides of the above equation by Ia we get,

V Ia = Eb Ia + Ia 2 Ra

This equation is called power equation of a d.c. motor.

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VIa = Net electrical power input to the armature measured in watts.

Ia2Ra = Power loss due the resistance of the armature called armature copper loss.

So difference between VIa and Ia2Ra i.e. input - losses gives the output of the armature.

So Eb Ia is called electrical equivalent of gross mechanical power developed by the armature.

This is denoted as Pm.

... Power input to the armature - Armature copper loss = Gross mechanical power

developed in the armature.

Condition for Maximum Power

For a motor from power equation it is known that,

Pm = Gross mechanical power developed = Eb Ia

= VIa - Ia 2Ra

For maximum Pm, dPm/dIa = 0

... 0 = V - 2IaRa

... Ia = V/2Ra i.e. IaRa = V/2

Substituting in voltage equation,

V = Eb + IaRa = Eb + (V/2)

... Eb = V/2 .................Condition for maximum power

Key Point : This is practically impossible to achieve as for this, current required is much more than its

normal rated value. Large heat will be produced and efficiency of motor will be less than 50 %.

TORQUE EQUATION OF A DC MOTOR

When a DC machine is loaded either as a motor or as a generator, the rotor conductors carry current.

These conductors lie in the magnetic field of the air gap. Thus, each conductor experiences a force. The

conductors lie near the surface of the rotor at a common radius from its centre. Hence, a torque is

produced around the circumference of the rotor, and the rotor starts rotating.

When the machine operates as a generator at a constant speed, this torque is equal and opposite to that

provided by the prime mover. When the machine is operating as a motor, the torque is transferred to the

shaft of the rotor and drives the mechanical load. The expression is same for the generator and motor.

When the current carrying current is placed in the magnetic field, a force is exerted or it which exerts

turning moment or torque F x r. This torque is produced due to the electromagnetic effect, hence is called

Electromagnetic torque. The torque which is produced in the armature is not fully used at the shaft for

doing the useful work. Some part of it where lost due to mechanical losses. The torque which is used for

doing useful work in known as the shaft torque.

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Since,

Multiplying the equation (1) by Ia we get

Where,

VIa is the electrical power input to the armature.

I2aRa is the copper loss in the armature.

We know that,

Total electrical power supplied to the armature = Mechanical power developed by the armature +

losses due to armature resistance

Now, the mechanical power developed by the armature is Pm.

Also, the mechanical power rotating armature can be given regarding torque T and speed n.

Where n is in revolution per seconds (rps) and T is in Newton-Meter.

Hence,

But,

Where N is the speed in revolution per minute (rpm) and

Where, n is the speed in (rps).

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Therefore,

So, the torque equation is given as

For a particular DC Motor, the number of poles (P) and the number of conductors per parallel path (Z/A)

are constant.

Where,

Thus, from the above equation (5) it is clear that the torque produced in the armature is directly

proportional to the flux per pole and the armature current. Moreover, the direction of electromagnetic

torque developed in the armature depends upon the current in armature conductors. If either of the two is

reversed the direction of torque produced is reversed and hence the direction of rotation. But when both

are reversed, and direction of torque does not change.

Shaft Torque

In a DC Motor whole of the electromagnetic torque (T) developed in the armature is not available on the

shaft. A part of it is lost to overcome the iron and mechanical (friction and windage) losses. Therefore,

shaft torque (Tsh) is somewhat less than the torque developed in the armature.

Definition: Thus, in the case of DC motors, the actual torque available at the shaft for doing useful

mechanical work is known as Shaft Torque. It is so called because it is available on the shaft of the

motor. It is represented by the symbol Tsh. The output of the motor is given by the equation shown below

where Tsh is the shaft torque in r.p.s and the N is the rotation of the motor in r.p.m. The shaft torque is

expressed as

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The difference between the armature torque and the shaft torque ( Ta – Tsh ) is known as the lost torque

and is due to the formation of the torque.

Brake Horse Power (B.H.P)

In the case of the motor, the mechanical power available at the shaft is known as Brake Horse Power. If

Tsh is the shaft torque in Newton Meter and N is the speed in r.p.m then,

The output brake horsepower is given by the equation (1) shown above.

TYPES OF DC MOTOR

A Direct Current Motor, DC is named according to the connection of the field winding with the armature.

Mainly there are two types of DC Motors. First, one is Separately Excited DC Motor and Self-excited DC

Motor. The self-excited motors are further classified as Shunt wound or shunt motor, Series wound or

series motor and Compound wound or compound motor.

The dc motor converts the electrical power into mechanical power is known as dc motor. The

construction of the dc motor and generator are same. But the dc motor has the wide range of speed and

good speed regulation which in electric traction.The working principle of the dc motor is based on the

principle that the current carrying conductor is placed in the magnetic field and a mechanical force

experience by it.

The DC motor is generally used in the location where require protective enclosure, for example, drip-

proof, the fireproof, etc. according to the requirements. The detailed description of the various types of

the motor is given below.

Separately Excited DC Motor

As the name signifies, the field coils or field windings are energized by a separate DC source as shown in

the circuit diagram shown below.

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Separately Excited DC Motor

Self Excited DC Motor

As the name implies self-excited, hence, in this type of motor, the current in the windings is supplied by

the machine or motor itself. Self-excited DC Motor is further divided into shunt wound, and series wound

motor. They are explained below in detail.

Shunt Wound Motor

This is the most common types of DC Motor. Here the field winding is connected in parallel with the

armature as shown in the figure below.

Shunt Wound DC Motor

The current, voltage and power equations for a shunt motor are written as follows.

By applying KCL at the junction A in the above figure.

The sum of the incoming currents at A = Sum of the outgoing currents at A.

Where,

I is the input line current

Ia is the armature current

Ish is the shunt field current

49 | P a g e

Equation (1) is the current equation.

The voltage equations are written by using Kirchhoff’s voltage law (KVL) for the field winding circuit.

For armature winding circuit the equation will be given as

The power equation is given as

Power input = mechanical power developed + losses in the armature + loss in the field.

Multiplying equation (3) by Ia we get the following equations.

Where,

VIa is the electrical power supplied to the armature of the motor.

Series Wound Motor

In the series motor, the field winding is connected in series with the armature winding. The connection

diagram is shown below.

Series Wound Motor

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By applying the KCL in the above figure

Where,

Ise is the series field current

The voltage equation can be obtained by applying KVL in the above figure

The power equation is obtained by multiplying equation (8) by I we get

Power input = mechanical power developed + losses in the armature + losses in the field

Comparing the equation (9) and (10), we will get the equation shown below.

Compound Wound Motor

A DC Motor having both shunt and series field windings is called a Compound Motor. The connection

diagram of the compound motor is shown below.

Compound Motor

The compound motor is further subdivided as Cumulative Compound Motor and Differential

Compound Motor. In cumulative compound motor the flux produced by both the windings is in the same

direction, i.e.

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In differential compound motor, the flux produced by the series field windings is opposite to the flux

produced by the shunt field winding, i.e.

The positive and negative sign indicates that direction of the flux produced in the field windings.

CHARACTERISTICS OF DC MOTORS

Generally, three characteristic curves are considered important for DC motors which are,

(i) Torque vs. armature current, (ii) (ii) Speed vs. armature current and (iii) (iii) Speed vs. torque.

These are explained below for each type of DC motor. These characteristics are determined by keeping

the following two relations in mind.

Ta ∝ ɸ.Ia and N ∝ Eb/ɸ These above equations can be studied at - emf and torque equation of dc machine. For a DC motor, magnitude of the back emf is given by the same emf equation of a dc generator i.e. Eb = PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ Eb/ɸ Characteristics Of DC Series Motors

Torque Vs. Armature Current (Ta-Ia)

This characteristic is also known as electrical characteristic. We know that torque is directly proportional to the product of armature current and field flux, Ta ∝ ɸ.Ia. In DC series motors, field winding is connected in series with the armature, i.e. Ia = If. Therefore, before magnetic saturation of the field, flux ɸ is directly proportional to Ia. Hence, before magnetic saturation Ta α Ia2. Therefore, the Ta-Ia curve is parabola for smaller values of Ia. After magnetic saturation of the field poles, flux ɸ is independent of armature current Ia. Therefore, the torque varies proportionally to Ia only, T ∝ Ia.Therefore, after magnetic saturation, Ta-Ia curve becomes a straight line.

The shaft torque (Tsh) is less than armature torque (Ta) due to stray losses. Hence, the curve Tsh vs Ia lies

slightly lower.

In DC series motors, (prior to magnetic saturation) torque increases as the square of armature current,

these motors are used where high starting torque is required.

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http://www.electricaleasy.com/2014/01/basic-working-of-dc-motor.html


http://www.electricaleasy.com/2012/12/emf-and-torque-equation-of-dc-machine.html

http://www.electricaleasy.com/2014/01/losses-in-dc-machine.html

Speed Vs. Armature Current (N-Ia) We know the relation, N ∝ Eb/ɸ

For small load current (and hence for small armature current) change in back emf Eb is small and it may

be neglected. Hence, for small currents speed is inversely proportional to ɸ. As we know, flux is directly

proportional to Ia, speed is inversely proportional to Ia. Therefore, when armature current is very small

the speed becomes dangerously high. That is why a series motor should never be started without some

mechanical load.

But, at heavy loads, armature current Ia is large. And hence, speed is low which results in decreased back

emf Eb. Due to decreased Eb, more armature current is allowed.

Speed Vs. Torque (N-Ta)

This characteristic is also called as mechanical characteristic. From the above two characteristics of DC

series motor, it can be found that when speed is high, torque is low and vice versa.

Characteristics Of DC Shunt Motors

Torque Vs. Armature Current (Ta-Ia) In case of DC shunt motors, we can assume the field flux ɸ to be constant. Though at heavy loads, ɸ

decreases in a small amount due to increased armature reaction. As we are neglecting the change in the

flux ɸ, we can say that torque is proportional to armature current. Hence, the Ta-Ia characteristic for a dc

shunt motor will be a straight line through the origin.

Since heavy starting load needs heavy starting current, shunt motor should never be started on a heavy

load. Speed Vs. Armature Current (N-Ia)

As flux ɸ is assumed to be constant, we can say N ∝ Eb. But, as back emf is also almost constant, the speed should remain constant. But practically, ɸ as well as Eb decreases with increase in load. Back emf Eb decreases slightly more than ɸ, therefore, the speed decreases slightly. Generally, the speed decreases only by 5 to 15% of full load speed. Therefore, a shunt motor can be assumed as a constant speed motor. In speed vs. armature current characteristic in the following figure, the straight horizontal line represents the ideal characteristic and the actual characteristic is shown by the dotted line.

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Characteristics Of DC Compound Motor

DC compound motors have both series as well as shunt winding. In a compound motor, if series and

shunt windings are connected such that series flux is in direction as that of the shunt flux then the motor is

said to be cumulatively compounded. And if the series flux is opposite to the direction of the shunt flux,

then the motor is said to be differentially compounded. Characteristics of both these compound motors are

explained below.

(a) Cumulative compound motor Cumulative compound motors are used where series characteristics are required but the load is likely to

be removed completely. Series winding takes care of the heavy load, whereas the shunt winding prevents

the motor from running at dangerously high speed when the load is suddenly removed. These motors have

generally employed a flywheel, where sudden and temporary loads are applied like in rolling mills.

(b) Differential compound motor

Since in differential field motors, series flux opposes shunt flux, the total flux decreases with increase in

load. Due to this, the speed remains almost constant or even it may increase slightly with increase in load

(N ∝ Eb/ɸ). Differential compound motors are not commonly used, but they find limited applications in

experimental and research work.

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SPEED CONTROL OF DC MOTOR:

The dc motor converts the mechanical power into dc electrical power. One of the most important features

of the dc motor is that their speed can easily be control according to the requirement by using simple

methods. Such type of control is impossible in an AC motor.

The concept of the speed regulation is different from the speed control. In speed regulation, the speed of

the motor changes naturally whereas in dc motor the speed of the motor changes manually by the operator

or by some automatic control device. The speed of the DC Motor is given by the relation shown below.

The equation (1) that the speed is dependent upon the supply voltage V, the armature circuit resistance Ra

and the field flux ϕ, which is produced by the field current.

For controlling the speed of DC Motor, the variation in voltage, armature resistance and field flux is taken

into consideration. There are three general methods of speed control of a DC Motor. They are as follows.

1. Variation of resistance in the armature circuit.This method is called Armature Resistance or

Rheostatic control. 2. Variation in field flux.This method is known as Field Flux Control. 3. Variation in applied voltage.This method is also known as Armature Voltage Control.

The detailed discussion of the various method of controlling the speed is given below.

Armature Resistance Control of DC Motor

Shunt Motor

The connection diagram of a shunt motor of the armature resistance control method is shown below. In

this method, a variable resistor Re is put in the armature circuit. The variation in the variable resistance

does not effect the flux as the field is directly connected to the supply mains.

Fig: Connection diagram of a shunt motor of the armature resistance control method

The speed current characteristic of the shunt motor is shown below.

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Fig: Speed current characteristic of the shunt motor

Series Motor:

Now, let us consider a connection diagram of speed control of the DC Series motor by the armature

resistance control method.

Fig: Diagram of speed control of the DC Series motor

By varying the armature circuit resistance, the current and flux both are affected. The voltage drop in the

variable resistance reduces the applied voltage to the armature, and as a result, the speed of the motor is

reduced.

The speed–current characteristic of a series motor is shown in the figure below.

Fig: Speed–current characteristic of a series motor

When the value of variable resistance Re is increased, the motor runs at a lower speed. Since the variable

resistance carries full armature current, it must be designed to carry continuously the full armature

current.

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Disadvantages of Armature Resistance Control Method A large amount of power is wasted in the external resistance Re.

Armature resistance control is restricted to keep the speed below the normal speed of the motor and increase in the speed above normal level is not possible by this method.

For a given value of variable resistance, the speed reduction is not constant but varies with the motor load.

This speed control method is used only for small motors.

Field Flux Control Method of DC Motor

Flux is produced by the field current. Thus, the speed control by this method is achieved by control of the

field current.

Shunt Motor

In a Shunt Motor, the variable resistor RC is connected in series with the shunt field windings as shown in

the figure below. This resistor RC is known as a Shunt Field Regulator.

Fig: Shunt Field Regulator

The shunt field current is given by the equation shown below.

The connection of RC in the field reduces the field current, and hence the flux is also reduced. This

reduction in flux increases the speed, and thus, the motor runs at speed higher than the normal speed.

Therefore, this method is used to give motor speed above normal or to correct the fall of speed because of

the load.

The speed-torque curve for shunt motor is shown below.

Fig: speed-torque curve for shunt motor

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Series Motor

In a series motor, the variation in field current is done by any one method, i.e. either by a diverter or by a

tapped field control.

By Using a Diverter:

A variable resistance Rd is connected in parallel with the series field windings as shown in the figure

below.

Fig: Diverter is connected in parallel with the series field windings

The parallel resistor is called a Diverter. A portion of the main current is diverted through a variable

resistance Rd. Thus, the function of a diverter is to reduce the current flowing through the field winding.

The reduction in field current reduces the amount of flux and as a result the speed of the motor increases.

Tapped Field Control:

The second method used in a series motor for the variation in field current is by tapped field control. The

connection diagram is shown below.

Fig: Tapped Field Control

Here the ampere turns are varied by varying the number of field turns. This type of arrangement is used in

an electric traction system. The speed of the motor is controlled by the variation of the field flux. The

speed-torque characteristic of a series motor is shown below.

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Fig: Speed-torque characteristic

Advantages of Field Flux Control The following are the advantages of the field flux control method.

This method is easy and convenient.

As the shunt field is very small, the power loss in the shunt field is also small.

The flux cannot usually be increased beyond its normal values because of the saturation of the iron.

Therefore, speed control by flux is limited to the weakening of the field, which gives an increase in speed.

This method is applicable over only to a limited range because if the field is weakened too much, there is

a loss of stability.

Armature Voltage Control of DC Motor

In armature voltage control method the speed control is achieved by varying the applied voltage in the

armature winding of the motor. This speed control method is also known as Ward Leonard Method,

which is discussed in detail under the topic Ward Leonard Method or Armature Voltage Control.

Ward Leonard Method Of Speed Control Or Armature Voltage Control

Ward Leonard Method of speed control is achieved by varying the applied voltage to the armature. This

method was introduced in 1891. The connection diagram of the Ward Leonard method of speed control of

a DC shunt motor is shown in the figure below.

In the above system, M is the main DC motor whose speed is to be controlled, and G is a separately

excited DC generator.The generator G is driven by a 3 phase driving motor which may be an induction

motor or a synchronous motor. The combination of AC driving motor and the DC generator is called the

Motor-Generator (M-G) set.

The voltage of the generator is changed by changing the generator field current. This voltage when

directly applied to the armature of the main DC motor, the speed of the motor M changes. The motor field

current Ifm is kept constant so that the motor field flux ϕm also remains constant. While the speed of the

motor is controlled, the motor armature current Ia is kept equal to its rated value.

The generated field current Ifg is varied such that the armature voltage Vtchanges from zero to its rated

value. The speed will change from zero to the base speed. Since the speed control is carried out with the

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rated current Ia and with the constant motor field flux, a constant torque is directly proportional to the

armature current, and field flux up to rated speed is obtained. The product of torque and speed is known

as power, and it is proportional to speed. Thus, with the increase in power, speed increases automatically.

The Torque and Power Characteristic is shown in the figure below.

Hence, with the armature voltage control method, constant torque and variable power drive is obtained

from speed below the base speed. The Field flux control method is used when the speed is above the base

speed. In this mode of operation, the armature current is maintained constant at its rated value, and the

generator voltage Vt is kept constant. The motor field current is decreased and as a result, the motor field flux also decreases.This means that

the field is weakened to obtain the higher speed. Since VtIa and EIa remain constant, the electromagnetic

torque is directly proportional to the field flux ϕm and the armature current Ia. Thus, if the field flux of

the motor is decreased the torque decreases. Therefore, the torque decreases, as the speed increases. Thus, in the field control mode, constant power

and variable torque are obtained for speeds above the base speed. When the speed control over a wide

range is required, a combination of armature voltage control and field flux control is used. This

combination permits the ratio of maximum to minimum speed available speeds to be 20 to 40. For closed

loop control, this range can be extended up to 200. The driving motor can be an induction or synchronous motor. An induction motor operates at a lagging

power factor. The synchronous motor may be operated at a leading power factor by over-excitation of its

field. Leading reactive power is generated by over excited synchronous motor. It compensates for the

lagging reactive power taken by other inductive loads. Thus, the power factor is improved. A Slip ring induction motor is used as p prime mover when the load is heavy and intermittent. A flywheel

is mounted on the shaft of the motor. This scheme is known as Ward Leonard-Ilgener scheme. It prevents

heavy fluctuations in supply current. When the Synchronous motor is acting as a driving motor, the fluctuations cannot be reduced by

mounting a flywheel on its shaft, because the synchronous motor always operates at a constant speed. In

another form of Ward Leonard drive, non-electrical prime movers can also be used to drive the DC

generator. For example – In DC electric locomotive, DC generator is driven by a diesel engine or a gas turbine and

ship propulsion drives. In this system, Regenerative braking is not possible because energy cannot flow in

the reverse direction in the prime mover.

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Advantages of Ward Leonard Drives

The main advantages of the Ward Leonard drive are as follows:- • Smooth speed control of DC motor over a wide range in both the direction is possible. • It has an inherent braking capacity. • The lagging reactive volt-amperes are compensated by using an overexcited synchronous motor

as the drive and thus, the overall power factor improves.

• When the load is intermittent as in rolling mills, the drive motor is an induction motor with a

flywheel mounted to smooth out the intermittent loading to a low value.

Drawbacks of Classical Ward Leonard System

The Ward Leonard system with rotating Motor Generator sets has following drawbacks. • The Initial cost of the system is high as there is a motor generator set installed, of the same rating

as that of the main DC motor. • Larger size and weight. • Requires large floor area • Costly foundation • Maintenance of the system is frequent. • Higher losses. • Lower efficiency. • The drive produces more noise. •

Applications of Ward Leonard Drives

The Ward Leonard drives are used where a smooth speed control of the DC motors over a wide range in

both the directions is required. Some of the examples are as follows:- • Rolling mills • Elevators • Cranes • Paper mills • Diesel-electric locomotives • Mine hoists

Solid State Control or Static Ward Leonard System

Now a days Static Ward Leonard system is mostly used. In this system, the rotating motor-generator (M- G) set is replaced by a solid state converter to control the speed of the DC motor. Controlled Rectifiers

and choppers are used as a converter. In the case of an AC supply, controlled rectifiers are used to convert fixed AC supply voltage into a

variable AC supply voltage. In the case of DC supply, choppers are used to obtain variable DC voltage

from the fixed DC voltage.

STARTING OF DC MOTORS

A starter is a device to start and accelerate a motor. A controller is a device to start the motor, control

and reverse the speed of the DC motor and stop the motor. While starting the DC motor, it draws the

heavy current which damages the motor. The starter reduces the heavy current and protects the system

from damage.

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Need of Starters for DC Motors

The dc motor has no back EMF. At the starting of the motor, the armature current is controlled by the

resistance of the circuit. The resistance of the armature is low, and when the full voltage is applied at the

standstill condition of the motor, the armature current becomes very high which damage the parts of the

motor.

Because of the high armature current, the additional resistance is placed in the armature circuit at starting.

The starting resistance of the machine is cut out of the circuit when the machine gains it speeds. The

armature current of a motor is given by

Thus, Ia depends upon E and Ra, if V is kept constant. When the motor is first switched ON, the

armature is stationary. Hence, the back EMF Eb is also zero. The initial starting armature

current Ias is given by the equation shown below.

Since, the armature resistance of a motor is very small, generally less than one ohm. Therefore,

the starting armature current Ias would be very large. For example – if a motor with the armature

resistance of 0.5 ohms is connected directly to a 230 V supply, then by putting the values in the

equation (2) we will get.

This large current would damage the brushes, commutator and windings.

As the motor speed increases, the back EMF increases and the difference (V – E) go on

decreasing. This results in a gradual decrease of armature current until the motor attains its stable

speed and the corresponding back EMF. Under this condition, the armature current reaches its

desired value. Thus, it is found that the back EMF helps the armature resistance in limiting the

current through the armature.

Since at the time of starting the DC Motor, the starting current is very large. At the time of

starting of all DC Motors, except for very small motors, an extra resistance must be connected in

series with the armature. This extra resistance is added so that a safe value of the motor is

maintained and to limit the starting current until the motor has attained its stable speed.

The series resistance is divided into sections which are cut out one by one, as the speed of the

motor rises and the back EMF builds up. The extra resistance is cut out when the speed of the

motor builds up to its normal value.

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3 POINT STARTER

3 Point Starter is a device whose main function is starting and maintaining the speed of the DC

shunt motor. The 3 point starter connects the resistance in series with the circuit which reduces

the high starting current and hence protects the machines from damage. Mainly there are three

main points or terminals in 3 point starter of DC motor. They are as follows

• L is known as Line terminal, which is connected to the positive supply.

• A is known as the armature terminal and is connected to the armature windings.

• F or Z is known as the field terminal and is connected to the field terminal windings.

The 3 Point DC Shunt Motor Starter is shown in the figure below

It consists of a graded resistance R to limit the starting current. The handle H is kept in the OFF

position by a spring S. The handle H is manually moved, for starting the motor and when it

makes contact with resistance stud one the motor is said to be in the START position. In this

initial start position, the field winding of the motor receives the full supply voltage, and the

armature current is limited to a certain safe value by the resistance (R = R1 + R2 + R3 + R4).

Working of 3 Point Starter

The starter handle is now moved from stud to stud, and this builds up the speed of the motor until

it reaches the RUN position. The Studs are the contact point of the resistance. In the RUN

position, three main points are considered. They are as follows.

• The motor attains the full speed.

• The supply is direct across both the windings of the motor.

• The resistance R is completely cut out.

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The handle H is held in RUN position by an electromagnet energised by a no volt trip coil

(NVC). This no volt trip coil is connected in series with the field winding of the motor. In the

event of switching OFF, or when the supply voltage falls below a predetermined value, or the

complete failure of supply while the motor is running, NVC is energised. The handle is released

and pulled back to the OFF position by the action of the spring. The current to the motor is cut

off, and the motor is not restarted without a resistance R in the armature circuit. The no voltage

coil also provides protection against an open circuit in the field windings.

The No Voltage Coil (NVC) is called NO-VOLT or UNDERVOLTAGE protection of the

motor. Without this protection, the supply voltage might be restored with the handle in the RUN

position. The full line voltage is directly applied to the armature. As a result, a large amount of

current is generated.

The other protective device incorporated in the starter is the overload protection. The Over Load

Trip Coil (OLC) and the No Voltage Coil (NVC) provide the overload protection of the motor.

The overload coil is made up of a small electromagnet, which carries the armature current. The

magnetic pull of the Overload trip coil is insufficient to attract the strip P, for the normal values

of the armature current

When the motor is overloaded, that is the armature current exceeds the normal rated value, P is

attracted by the electromagnet of the OLC and closes the contact aa thus, the No Voltage Coil is

short-circuited, shown in the figure of 3 Point Starter. As a result, the handle H is released, which

returns to the OFF position, and the motor supply is cut off.

To stop the motor, the starter handle should never be pulled back as this would result in burning

the starter contacts. Thus, to stop the motor, the main switch of the motor should be opened.

Drawbacks of a 3 Point Starter

The following drawbacks of a 3 point starter are as follows:-

• The 3 point starter suffers from a serious drawback for motors with a large variation of

speed by adjustment of the field rheostat.

• To increase the speed of the motor, the field resistance should be increased. Therefore,

the current through the shunt field is reduced.

• The field current may become very low because of the addition of high resistance to

obtain a high speed.

• A very low field current will make the holding electromagnet too weak to overcome the

force exerted by the spring.

• The holding magnet may release the arm of the starter during the normal operation of the

motor and thus, disconnect the motor from the line. This is not a desirable action.

Hence, to overcome this difficulty, the 4 Point Starter is used.

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4 POINT STARTER

A 4 Point Starter is almost similar in functional characteristics like 3 Point Starter. In the absence

of back EMF, the 4 Point Starter acts as a current limiting device while starting of the DC motor.

4 Point Starter also acts a protecting device.

The basic difference in 4 Point Starter as compared to 3 Point Starter is that in this a holding coil

is removed from the shunt field circuit. This coil after removing is connected across the line in

series with a current limiting resistance R. The studs are the contact points of the resistance

represented by 1, 2, 3, 4, 5 in the figure below. The schematic connection diagram of a 4 Point

Starter is shown below.

Fig: 4 Point Starter

The above arrangement forms three parallel circuits. They are as follows:-

• Armature, starting the resistance and the shunt field winding.

• A variable resistance and the shunt field winding.

• Holding coil and the current limiting resistance.

With the above three arrangements of the circuit, there will be no effect on the current through

the holding coil if there is any variation in speed of the motor or any change in field current of

the motor. This is because the two circuits are independent of each other.

The only limitation or the drawback of the 4 point starter is that it cannot limit or control the high

current speed of the motor. If the field winding of the motor gets opened under the running

condition, the field current automatically reduces to zero. But as some of the residual flux is still

present in the motor, and we know that the flux is directly proportional to the speed of the motor.

Therefore, the speed of the motor increases drastically, which is dangerous and thus protection is

not possible. This sudden increase in the speed of the motor is known as High-Speed Action of

the Motor.

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Nowadays automatic push button starters are also used. In the automatic starters, the ON push

button is pressed to connect the current limiting starting resistors in series with the armature

circuit. As soon as the full line voltage is available to the armature circuit, this resistor is

gradually disconnected by an automatic controlling arrangement.

The circuit is disconnected when the OFF button is pressed. Automatic starter circuits have been

developed using electromagnetic contactors and time delay relays. The main advantage of the

automatic starter is that it enables even the inexperienced operator to start and stop the motor

without any difficulty.

LOSSES IN DC MACHINE

The losses that occur in a DC Machine is divided into five basic categories. The various losses

are Electrical or Copper losses (I2R losses), Core losses or Iron losses, Brush losses, Mechanical

losses, Stray load losses. These losses are explained below in detail.

Fig: Classification of losses in DC machnes

Electrical or Copper Losses in dc machine

These losses are also known as Winding losses as the copper loss occurs because of the

resistance of the windings. The ohmic loss is produced by the current flowing in the windings.

The windings that are present in addition to the armature windings are the field windings,

interpoles and compensating windings.

Armature copper losses = Ia2Ra where Ia is armature current, and Ra is the armature resistance.

These losses are about 30 percent of the total full load losses.

In shunt machine, the Copper loss in the shunt field is I2shRsh, where Ish is the current in the

shunt field, and Rsh is the resistance of the shunt field windings. The shunt regulating resistance

is included in Rsh.

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In a series machine, the copper loss in the series windings is I2

seRse, where, Ise is the current

through the series field windings, and Rse is the resistance of the series field windings.

In a Compound machine, both the shunt and the series field losses occur. These losses are almost

20 percent of the full load losses.

Copper losses in the interpole windings are written as Ia2Ri where Ri is the resistance of the

interpole windings.

Copper loss in the compensating windings if any is Ia2Rc where Rc is the resistance of

compensating windings.

Magnetic Losses or Core Losses or Iron Losses in dc machine

The core losses are the hysteresis and eddy current losses. These losses are considered almost

constant as the machines are usually operated at constant flux density and constant speed. These

losses are about 20 percent of the full load losses.

Brush Losses in dc machine

Brush losses are the losses taking place between the commutator and the carbon brushes. It is the

power loss at the brush contact point. The brush drop depends upon the brush contact voltage

drop and the armature current Ia. It is given by the equation shown below.

The voltage drop occurring over a large range of armature currents, across a set of brushes is

approximately constant If the value of brush voltage drop is not given than it is usually assumed

to be about 2 volts. Thus, the brush drop loss is taken as 2Ia.

Mechanical Losses in dc machine

The losses that take place because of the mechanical effects of the machines are known as

mechanical losses. Mechanical losses are divided into bearing friction loss and windage loss. The

losses occurring in the moving parts of the machine and the air present in the machine is known

as Windage losses. These losses are very small.

Stray Losses in dc machine

These losses are the miscellaneous type of losses. The following factors are considered in stray

load losses.

• The distortion of flux because of armature reaction.

• Short circuit currents in the coil, undergoing commutation.

These losses are very difficult to determine. Therefore, it is necessary to assign the reasonable

value of the stray loss. For most machines, stray losses are taken by convention to be one percent

of the full load output power.

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EFFICIENCY OF DC GENERATOR

Efficiency is simply defined as the ratio of output power to the input power. Let R = total

resistance of the armature circuit (including the brush contact resistance, at series winding

resistance, inter-pole winding resistance and compensating winding resistance). The efficiency of

DC generator is explained below in the line diagram.

Fig: Power flow diagram

• I is the output current

• Ish is the current through the shunt field

• Ia is the armature current = I + Ish

• V is the terminal voltage.

Total copper loss in the armature circuit = Ia2Rat

Power loss in the shunt circuit = VIsh (this includes the loss in the shunt regulating resistance).

Mechanical losses = friction loss of bearings + friction loss at a commutator + windage loss.

Core losses = hysteresis loss + eddy current loss

Stray loss = mechanical loss + core loss

The sum of the shunt field copper loss and stray losses may be considered as a combined

fixed (constant) loss that does not vary with the load current I.

Therefore, the constant losses (in shunt and compound generators) = stray loss + shunt

field copper losses.

SWINBURNE’S TEST

Swinburne’s Test is an indirect method of testing of DC machines. In this method the losses are measured

separately and the efficiency at any desired load is predetermined. Machines are tested for finding out

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losses, efficiency and temperature rise. For small machines direct loading test is performed. For large

shunt machines, indirect methods are used like Swinburne’s or Hopkinson’s test.

The machine is running as a motor at rated voltage and speed. The connection diagram for DC shunt

machine is shown in the figure below.

Fig: Swinburne’s Test

Let

V be the supply voltage

I0 is the no-load current

Ish is the shunt field current

Therefore, no load armature current is given by the equation shown below.

No-load input = VI0

The no-load power input to the machine supplies the following, as given below.

• Iron loss in the core

• Friction losses in the bearings and commutators.

• Windage loss

• Armature copper loss at no load. When the machine is loaded, the temperature of the armature winding and the field winding increases due to I

2R losses.

For calculating I2R losses hot resistances should be used. A stationary measurement of resistances at room temperature of t

degree Celsius is made by passing current through the armature and then field from a low voltage DC supply. Then the heated resistance, allowing a temperature rise of 50⁰C is found. The equations are as follows:-

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Where, α0 is the temperature coefficient of resistance at 0⁰C Therefore,

Stray loss = iron loss + friction loss + windage loss = input at no load – field copper loss – no load

armature copper loss

Also, constant losses

If the constant losses of the machine are known, its efficiency at any other load can be determined as

follows.

Let I be the load current at which efficiency is required.

Efficiency when the machine is running as a Motor.

Therefore, total losses is given as

The efficiency of the motor is given below.

Efficiency when the machine is running as a Generator.

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Therefore, total losses is given as

The efficiency of the generator is given below.

Advantages of Swinburne’s Test:

The main advantages of the Swinburne’s test are as follows:-

• The power required to test a large machine is small. Thus, this method is an economical and

convenient method of testing of DC machines.

• As the constant loss is known the efficiency can be predetermined at any load.

Disadvantages of Swinburne’s Test:

• Change in iron loss is not considered at full load from no load. Due to armature reaction flux is

distorted at full load and, as a result, iron loss is increased.

• As the Swinburne’s test is performed at no load. Commutation on full load cannot be determined

whether it is satisfactory or not and whether the temperature rise is within the specified limits or

not.

Limitations of Swinburne’s Test:

• Machines having a constant flux are only eligible for Swinburne’s test. For examples – shunt

machines and level compound generators.

• Series machines cannot run on light loads, and the value of speed and flux varies greatly. Thus,

the Swinburne’s Test are not applicable for series machines

•

BRAKE TEST ON DC SHUNT MOTOR: Brake test is a method of finding efficiency of dc motors. We took dc shunt motor as running

machine. Brake test also called as direct loading test of testing the motor because loading will be

applied directly on shaft of the motor by means of a belt and pulley arrangement.

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Test Requirements:

1. DC shunt motor 2. Water-cooled pulley 3. Spring balance

Procedure of Brake Test on DC Shunt Motor: 1. By adjusting the handle of the pulley take different readings of the spring balance. 2. The tension in the belt can be adjusted using the handle. The tension in kg can be obtained from

the spring balance readings. 3. Adjusting the load step by step till full load, number of readings can be obtained. By increasing the

load is slowly, adjust to get rated load current. 4. The power developed gets wasted against the friction between belt and shaft. Due to the braking

action of belt the test is called brake test. 5. The speed can be measured by tachometer. Thus all the motor characteristics can be plotted.

Calculation of Brake Test on DC Shunt Motor

Let R (or) r= Radius of pulley in meters

N = Speed in R.P.M.

W1 = spring balance reading on tight side in kg

W2 = spring balance reading on slack side in kg

So, net pull on the belt due to friction at the pulley is the difference between the two spring balance

readings.

Net pull on the rope = (W - S) kg = (W - S) X 9.81 newtons ......(1)

As radius R and speed N are known, the shaft torque developed can be obtained as,

Tsh = Net pull X R = (W - S) X 9.81 X R .....(2)

Now let, V = Voltage applied in volts

I = Total line current drawn in amps.

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As we know V and I are input parameters of dc motors in brake test.

Then,

Pin=V.I Watts..... (3)

We have output and input. Then why late go and find the efficiency of dc shunt motor.

Efficiency (η)=Output/Input [No units]

From equation (2) & (3)

Advantages of Brake Test on DC Shunt Motor: 1. Actual efficiency of the motor under working conditions can be found out. 2. Brake test is simple and easy to perform. 3. It is not only for dc shunt motor, also can be performed on any type of D.C. motor.

Disadvantages of Brake Test on DC Shunt Motor: 1. In brake test due the belt friction lot of heat will be generated and hence there is large dissipation

of energy. 2. Cooling arrangement is necessary to minimize the heat. Mostly in our laboratories we use water as

cooling liquid. 3. Convenient only for small rated machines due to limitations regarding heat dissipation

arrangements. 4. Power developed gets wasted hence brake test method is little expensive. 5. The efficiency observed is on lower side.

HOPKINSON’S TEST

Hopkinson’s Test is also known as Regenerative Test, Back to Back test and Heat Run Test. In

Hopkinson Test, two identical shunt machines are required which are coupled both mechanically and

electrically in parallel. One is acting as a motor and another one as a generator. The input to the motor is

given by the supply mains.

The mechanical output of motor drives the generator, and the electrical output of the generator is used in

supplying the input to the motor. Thus, the output of each machine acts as an input to the other machine.

When both the machines are running on the full load, the supply input is equal to the total losses of the

machines. Hence, the power input from the supply is very small.

The Circuit Diagram of the Hopkinson’s Test is shown in the figure below.

Supply is given and with the help of a starter, the machine M starts and work as a motor. The switch S is

kept open. The field current of M is adjusted with the help of rheostat field RM, which enables the motor

to run at rated speed. Machine G acts as a generator. Since the generator is mechanically coupled to the

motor, it runs at the rated speed of the motor.

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Fig: Hopkinson’s Test

The excitation of the generator G is so adjusted with the help of its field rheostat RG that the voltage

across the armature of the generator is slightly higher than the supply voltage. In actual the terminal

voltage of the generator is kept 1 or 2 volts higher than the supply voltage.

When the voltage of the generator is equal and of the same polarity as the of the busbar supply voltage,

the main switch S is closed, and the generator is connected to the busbars. Thus, both the machines are

now in parallel across the supply. Under this condition, when the machines are running parallel, the

generator are said to float. This means that the generator is neither taking any current nor giving any

current to the supply.

Now with the help of a field rheostat, any required load can be thrown on the machines by adjusting the

excitation of the machines with the help of field rheostats.

Let,

• V be the supply voltage

• IL is the line current

• Im is the input current to the motor

• Ig is the input current to the generator

• Iam is the motor armature current

• Ishm is the motor shunt field current

• Ishg is the generator shunt field current

• Ra is the armature resistance of each machine

• Rshm is the motor shunt field resistance

• Rshg is the generator shunt field resistance

• Eg is the generator induced voltage

• Em is the motor induced voltage or back emf

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Since the field flux is directly proportional to the field current.

Thus, the excitation of the generator shall always be greater than that of the motor.

Calculation of the Efficiency of the Machine by Hopkinson’s Test

• Power input from the supply = VIL = total losses of both the machines

• Armature copper loss of the motor = I2am Ra

• Field copper loss of the motor = I2shm Rshm

• Armature copper loss of the generator = I2ag Ra

• Field copper loss of the generator = = I2shg Rshg

The constant losses Pc like iron, friction and windage losses are assumed to be equal and is written as

given below. Constant losses of both the machines = Power drawn from the supply – Armature and shunt copper losses

of both the machines.

Assuming that the constant losses known as stray losses are divided equally between the two machines.

Total stray loss per machine = ½ PC

Efficiency of the Generator

• Output = VIag

• Constant losses for generator is given as PC/2

• Armature copper loss = I2ag Ra

• Field copper loss = I2shg Rshg

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The Efficiency of the generator is given by the equation shown below

Efficiency of the Motor

• Constant losses of the motor is given as PC/2

• Armature copper loss = I2am Ra

• Field copper loss = I2shm Rshm

The Efficiency of the motor is given by the equation shown below

Advantages of Hopkinson’s Test

The main advantages of using Hopkinson’s test are as follows:-

• This method is very economical. • The temperature rise and the commutation conditions can be checked under rated load conditions. • Stray losses are considered, as both the machines are operated under rated load conditions. • Large machines can be tested at rated load without consuming much power from the supply. • Efficiency at different loads can be determined.

Disadvantage of Hopkinson’s Test The main disadvantage of this method is the necessity of two practically identical machines for

performing the Hopkinson’s test. Hence, this test is suitable for large DC machines.

FIELD’S TEST:

This is one of the methods of testing the D.C. series motors. Unlike shunt motors, the series motor

cannot be tested by the methods which area available for shunt motors as it is impossible to run the motor

on no-load. It may run at dangerously high speed on no load. In case of small series motors brake test may

be employed.

The series motors are usually tested in pairs. The field test is applied to two similar series motors

which are coupled mechanically. The connection diagram for the test is shown in the Fig. 1.

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Fig. 1 Field test

As shown in the Fig. 1 one machine is made to run as a motor while the other as a generator which is

separately excited. The fields of the two machines are connected in series so that both the machines are

equally excited. This will make iron losses same for the two machines. The two machines are running at

the same speed. The generator output is given to the variable resistance R.

The resistance R is changed until the current taken by motor reaches full load value. This will be

indicated by ammeter A1. The other readings of different meters are then recorded.

Let V = Supply voltage

I1 = Current taken by motor

I2 = Load current

V2 = Terminal p.d. of generator

Ra, Rse = Armature and series field resistance of each machine

Power taken from supply = VI1

Output obtained from generator = V2 I2

Total losses in both the machines, WT = VI1 - V2 I2

Armature copper and field losses, WCU = ( Ra + 2 Rse ) I12 + I2

2 Ra

Total stray losses = WT - WCU

Since the two machines are equally excited and are running at same speed the stray loses are equally

divided.

For Motor;

Input to motor = V1 I1

Total losses = Aramture Cu loss + Field Cu loss + Stray loss

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= I12 ( Ra + Rse) + Ws

Output of motor = Input - Total losses = V1 I1 - [ I12 ( Ra + Rse) + Ws ]

For Generator:

Efficiency of generator is of little importance because it is running under conditions of separate

excitation. Still it can be found as follows.

Output of generator = V2 I2

Field Cu loss = I12 Rse

Armature Cu loss = I22 Ra

Total losses = Armature Cu loss + Field Cu loss + Stray loss

= I22 Ra + I1

2 Rse + Ws

Input to generator = Output + Total losses = V2 I2 + [ I22 Ra + I1

2 Rse + Ws ]

The important point to be noted is that this is not regenerative method though the two machines are

mechanically coupled because the generator output is not fed back to the motor as in case of Hopkinson's

test but it is wasted in load resistance.

RETARDATION TEST OR RUNNING DOWN TEST

This method is generally employed to shunt generators and shunt motors. From this method we can

get stary losses. Thus if armature and shunt copper losses at any given load current are known then

efficiency of a machine can be easily estimated.

The machine whose test is to be taken is run at a speed which is slightly above its normal speed. The

supply to the motor is cut off while the field is kept excited. The armature consequently slows down and

its kinetic energy is used in supplying the rotational or stray losses which includes iron, friction and

winding loss.

If I is the amount of inertia of the armature ans is the angular velocity.

Kinetic energy of armature = 0.5 Iω2

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... Rotational losses, W = Rate of change of kinetic energy

Fig. 2 Retardation test

Angular velocity, ω = (2 πN)/60

Thus, to find the rotational losses, the moment of inertia I and dN/dt must be known. These quantities can be found as follows;

1.1 Determination of dN/dt

The voltmeter V1 which is connected across the armature will read the back e.m.f. of the motor. We

know that back e.m.f. is proportional to speed so that voltmeter is calibrated to read the speed directly.

When motor is cut off from the supply, the speed decrease in speed is noted with the help of stop

watch. A curve showing variation between time and speed which is obtained from voltmeter which is

suitably calibrated is shown in the Fig. 3.

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Fig. 3 Determination of dN/dt

At any point C corresponding to normal speed, a tangent AB is drawn. Then

The value obtained from above can be substituted in the expression for W which can give the

rotational looses.

1.2 Determination of moment of inertia (I):

Method 1: Using Flywheel

The armature supply is cut off and time required for definite change in speed is noted to draw the

corresponding curve as we have drawn in previous case. This curve is drawn considering only armature of

the machine. Now a flywheel with known moment of the inertia say is I1 keyed onto the shaft and the

same curve is drawn again. The slowing down time will be extended as combined moment of inertia of

the two is increased.

For any given speed (dN/dt1) and (dN/dt2) are determined same as previous case. It can be seen that

the losses in both the cases are almost same as addition of flywheel will not make much difference to the

losses.

In the first case where flywheel is not there then,

Adding the flywheel to the motor armature in second case we get,

Method 2: without using Flywheel

In this method time is noted for the machine to slow down by say 5 % considering the armature

alone. The a retarding torque either mechanical or electrical is applied. Preferably electrical retarding

torque is applied and time required to slow down by 5% is noted again. The method by which electrical

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torque can be provided is shown in the Fig. 1 in which the switch S after disconnecting from the supply is

thrown to terminals 1'2'. The machine then gets connected to a non-inductive load resistance RL. The

power drawn by this resistance will acts as a retarding torque on the armature which will make it slow

more quickly.

The additional loss in the resistance will be equal to product of ammeter reading and the average

reading of the voltmeter (for a fall of 5% of voltmeter reading, the time is noted.) The ammeter reading is

also changing so its average reading is taken. Thus the additional losses is Ia2 (Ra + R). Let t1 be the time

when armature is considered alone and t2 be the time when armature is connected across a load resistance,

V be average voltage across R and Ia be the average current and W' is additional retarding electrical

torque supplied by motor.

If dN i.e. change in speed is same in two cases then

Here dN/dt1 is rate of change in speed without extra load whereas dN/dt2 is rate change in speed

with extra electrical load which provides retarding torque.

UNIT IV

SINGLE PHASE TRANSFORMERS

WORKING PRINCIPLE OF A TRANSFORMER:

The basic principle on which the transformer works is Faraday’s Law of Electromagnetic Induction or

mutual induction between the two coils. The working of the transformer is explained below. The

transformer consists of two separate windings placed over the laminated silicon steel core.

The winding to which AC supply is connected is called primary winding and to which load is connected

is called secondary winding as shown in the figure below. It works on the alternating current only because

an alternating flux is required for mutual induction between the two windings.

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Fig: Simple Transformer

When the AC supply is given to the primary winding with a voltage of V1, an alternating flux ϕ sets up in

the core of the transformer, which links with the secondary winding and as a result of it, an emf is induced

in it called Mutually Induced emf. The direction of this induced emf is opposite to the applied voltage V1,

this is because of the Lenz’s law shown in the figure below

Fig: Transformer Symbol

Physically, there is no electrical connection between the two windings, but they are magnetically

connected. Therefore, the electrical power is transferred from the primary circuit to the secondary circuit

through mutual inductance. The induced emf in the primary and secondary windings depends upon the

rate of change of flux linkage that is (N dϕ/dt).

dϕ/dt is the change of flux and is same for both the primary and secondary windings. The induced emf E1

in the primary winding is proportional to the number of turns N1 of the primary windings (E1 ∞ N1).

Similarly induced emf in the secondary winding is proportional to the number of turns on the secondary

side. (E2 ∞ N2).

Transformer on DC supply: As discussed above, the transformer works on AC supply, and it cannot work not DC supply. If the rated

DC voltage is applied across the primary winding, a constant magnitude flux will set up in the core of the

transformer and hence there will not be any self-induced emf generation, as for the linkage of flux with

the secondary winding there must be an alternating flux required and not a constant flux.

According to Ohm’s Law

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The resistance of the primary winding is very low, and the primary current is high. So this current is much

higher than the rated full load primary winding current. Hence, as a result, the amount of heat produced

will be greater and therefore, eddy current loss (I2R) loss will be more. Because of this, the insulations of

the primary windings will get burnt, and the transformer will be damaged.

Turn Ratio: It is defined as the ratio of primary to secondary turns.

If N2 > N1 the transformer is called Step up transformer

If N2 < N1 the transformer is called Step down transformer

Transformation Ratio: The transformation ratio is defined as the ratio of the secondary voltage to the primary voltage. It

is denoted by K.

As (E2 ∞ N2 and E1 ∞ N1)

Ideal Transformer: Definition: The transformer which is free from all types of losses is known as an ideal transformer. It is an

imaginary transformer which has no core loss, no ohmic resistance and no leakage flux. The ideal

transformer has the following important characteristic.

1. The resistance of their primary and secondary winding becomes zero. 2. The core of the ideal transformer has infinite permeability. The infinite permeable means less

magnetising current requires for magnetising their core. 3. The leakage flux of the transformer becomes zero, i.e. the whole of the flux induces in the core of

the transformer links with their primary and secondary winding. 4. The ideal transformer has 100 percent efficiency, i.e., the transformer is free from hysteresis and

eddy current loss.

The above mention properties are not possible in the practical transformer. In an ideal transformer, there

is no power loss. Therefore, the output power is equal to the input power.

Since El ∞ N2 and E1 ∞ N1, also E1 is similar to V1 and E2 is similar to V2

Therefore, transformation ratio will be given by the equation shown below

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https://circuitglobe.com/what-is-a-transformer.html

The primary and the secondary currents are inversely proportional to their respective turns.

Behaviour of Ideal Transformer:

Consider the ideal transformer shown in the figure below. The voltage source V1is applied across the

primary winding of the transformer. Their secondary winding is kept open. The N1 and N2 are the

numbers of turns of their primary and secondary winding.

The current Im is the magnetizing current flows through the primary winding of the transformer. The

magnetizing current produces the flux φm in the core of the transformer. As the permeability of the core is infinite, the flux of the core link with both the primary and secondary winding of the transformer.

Fig: Ideal Trasfomer

The flux link with the primary winding induces the emf E1 because of self-induction. The direction of the

induced emf is inversely proportional to the applied voltage V1. The emf E2 induces in the secondary

winding of the transformer because of mutual induction.\

Phasor Diagram of Ideal Transformer: The phasor diagram of the ideal transformer is shown in the figure below. As the coil of the primary

transformer is purely inductive the magnetising current induces in the transformer lag 90º by the input

voltage V1. The E1 and E2 are the emf induced in the primary and secondary winding of the transformer.

The direction of the induces emf inversely proportional to the applied voltage

Phasor Diagram of an Ideal Transformer

Point to Remember The input energy of the transformer is equal to their output energy. The power loss in the

ideal transformer becomes zero.

EMF EQUATION OF A TRANSFORMER:

When a sinusoidal voltage is applied to the primary winding of a transformer, alternating flux ϕm sets up

in the iron core of the transformer. This sinusoidal flux links with both primary and secondary winding.

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The function of flux is a sine function. The rate of change of flux with respect to time is derived

mathematically.

The derivation of EMF Equation of the transformer is shown below. Let

• ϕm be the maximum value of flux in Weber • f be the supply frequency in Hz

• N1 is the number of turns in the primary winding

• N2 is the number of turns in the secondary winding Φ is the flux per turn in Weber

Fig: Flux waveform

As shown in the above figure that the flux changes from + ϕm to – ϕm in half a cycle of 1/2f

seconds. By Faraday’s Law

Let E1 is the emf induced in the primary winding

Where Ψ = N1ϕ

Since ϕ is due to AC supply ϕ = ϕm Sinwt

So the induced emf lags flux by 90 degrees. Maximum valve of emf

But w = 2πf

Root mean square RMS value is

Putting the value of E1max in equation (6) we get

Putting the value of π = 3.14 in the equation (7) we will get the value of E1 as

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Similarly

Now, equating the equation (8) and (9) we get

The above equation is called the turn ratio where K is known as transformation ratio. The equation (8) and (9) can also be written as shown below using the relation

(ϕm = Bm x Ai) where Ai is the iron area and Bm is the maximum value of flux density.

For a sinusoidal wave

Magnetic Leakage flux: In a transformer it is observed that, all the flux linked with primary winding does not get linked with

secondary winding. A small part of the flux completes its path through air rather than through the core (as

shown in the fig at right), and this small part of flux is called as leakage flux or magnetic leakage in

transformers. This leakage flux does not link with both the windings, and hence it does not contribute to

transfer of energy from primary winding to secondary winding. But, it produces self induced emf in each

winding. Hence, leakage flux produces an effect equivalent to an inductive coil in series with each

winding. And due to this there will be leakage reactance.

Fig: Magnetic Leakage flux

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(To minimize this leakage reactance, primary and secondary windings are not placed on separate legs,

refer the diagram of core type and shell type transformer from construction of transformer.)

Practical Transformer with Resistance and Leakage Reactance In the following figure, leakage reactance and resistance of the primary winding as well as secondary

winding are taken out, representing a practical transformer.

Fig; Practical Transformer with Resistance And Leakage Reactance

Where, R1 and R2 = resistance of primary and secondary winding respectively

X1 and X2 = leakage reactance of primary and secondary winding resp.

Z1 and Z2 = Primary impedance and secondary impedance resp.

Z1 = R1 + jX1 ...and Z2 = R2 + jX 2 .

The impedance in each winding lead to some voltage drop in each winding. Considering this voltage drop

the voltage equation of transformer can be given as –

V1 = E1 + I1(R1 + jX1 ) --------primary side

V2 = E2 - I2(R2 + jX2 ) --------secondary side

where, V1 = supply voltage of primary winding

V2 = terminal voltage of secondary winding

E1 and E2 = induced emf in primary and secondary winding respectively.

Resistance and Reactance of the Transformer: The Resistance of the transformer is defined as the internal resistance of both primary and secondary

windings. In an actual transformer, the primary and the secondary windings have some resistance

represented by R1 and R2 and the reactances by X1 and X2. Let K be the transformation ratio. To make

the calculations easy the resistances and reactances can be transferred to either side that means either all

the primary terms are referred to the secondary side, or all the secondary terms are referred to the primary

side. The resistive and the reactive drops in the primary and secondary side are represented as follows

• Resistive drop in the secondary side = I2R2 • Reactive drop in the secondary side = I2X2 • Resistive drop in the primary side = I1R1 • Reactive drop in the primary side = I1X1

Primary Side Referred to Secondary Side

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Since the transformation ratio is K, primary resistive and reactive drop as referred to secondary side will

be K times, i.e., K I1R1 and K I1X1 respectively. If I1 is substituted equal to KI2 then we have primary

resistive and reactive drop referred to secondary side equal to K2I2R1 and K2I2X1 respectively.

The Total resistive drop in a transformer

Total reactive drop in a transformer

The term

represent the equivalent resistance and reactance of the transformer referred to the secondary side.

TRANSFORMER ON NO-LOAD CONDITION: When the transformer is operating at no load, the secondary winding is open circuited, which means there

is no load on the secondary side of the transformer and, therefore, current in the secondary will be zero,

while primary winding carries a small current I0 called no load current which is 2 to 10% of the rated

current. This current is responsible for supplying the iron losses (hysteresis and eddy current losses) in the

core and a very small amount of copper losses in the primary winding. The angle of lag depends upon the

losses in the transformer. The power factor is very low and varies from 0.1 to 0.15.

Fig: Transformer is operating at no load

The no load current consists of two components

• Reactive or magnetizing component Im

(It is in quadrature with the applied voltage V1. It produces flux in the core and does not

consume any power)

• Active or power component Iw, also known as working component

(It is in phase with the applied voltage V1. It supplies the iron losses and a small amount

of primary copper loss)

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The following steps are given below to draw the phasor diagram 1. The function of the magnetizing component is to produce the magnetizing flux, and thus, it will

be in phase with the flux. 2. Induced emf in the primary and the secondary winding lags the flux ϕ by 90 degrees.

3. The primary copper loss is neglected, and secondary current losses are zero as I2 = 0.

Therefore, the current I0 lags behind the voltage vector V1 by an angle ϕ0 called no-load power

factor angle shown in the phasor diagram above.

4. The applied voltage V1 is drawn equal and opposite to the induced emf E1 because the

difference between the two, at no load, is negligible.

5. Active component Iw is drawn in phase with the applied voltage V1.

6. The phasor sum of magnetizing current Im and the working current Iw gives the no load current I0.

Fig: Phasor diagram drawn

TRANSFORMER ON LOAD CONDITION:

When the transformer is on loaded condition, the secondary of the transformer is connected to load. The

load can be resistive, inductive or capacitive. The current I2 flows through the secondary winding of the

transformer. The magnitude of the secondary current depends on the terminal voltage V2 and the load

impedance. The phase angle between the secondary current and voltage depends on the nature of the load.

Operation of the Transformer on Load Condition

The Operation of the Transformer on Load Condition is explained below

• When secondary of the transformer is kept open, it draws the no-load current from the main supply. The

no-load current induces the magneto motive force N0I0 and this force set up the flux Φ in the core of the

transformer. The circuit of the transformer at no load condition is shown in the figure below.

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Fig: Operation of the Transformer on Load Condition

• When the load is connected to the secondary of the transformer, the I2current flows through their

secondary winding. The secondary current induces the magnetomotive force N2I2 on the secondary

winding of the transformer. This force set up the flux φ2 in the transformer core. The flux φ2 oppose the

flux φ, according to Lenz’s law

Fig: Operation of the Transformer on Load Condition

• As the flux φ2 opposes the flux φ, the resultant flux of the transformer decreases and this flux reduces the induces EMF E1. Thus, the strength of the V1 is more than E1 and an additional primary current I’1

drawn from the main supply. The additional current is used for restoring the original value of the flux in

the core of the transformer so that the V1 = E1. The primary current I’1 is in phase opposition with the

secondary current I2. Thus, it is called the primary counter balancing current. • The additional current I’1 induces the magnetomotive force N1I’1. And this force set up the flux φ’1. The

direction of the flux is same as that of the φ and it cancels the flux φ2 which induces because of the MMF

N2I2

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Now, N1I1’ = N2I2

Therefore,

• The phasor difference between V1 and I1 gives the power factor angle ϕ1 of the primary side of the

transformer. • The power factor of the secondary side depends upon the type of load connected to the transformer. • If the load is inductive as shown in the above phasor diagram, the power factor will be lagging, and if the

load is capacitive, the power factor will be leading.The total primary current I1 is the vector sum of the

current I0 and I1’. i.e

Phasor Diagram of Transformer on Inductive Load: The phasor diagram of the actual transformer when it is loaded inductively is shown below

Phasor Diagram of the Transformer on Inductive Load

Steps to draw the phasor diagram • Take flux ϕ a reference

• Induces emf E1 and E2 lags the flux by 90 degrees. • The component of the applied voltage to the primary equal and opposite to induced emf in the

primary winding. E1 is represented by V1’.

• Current I0 lags the voltage V1’ by 90 degrees.

• The power factor of the load is lagging. Therefore current I2 is drawn lagging E2 by an angle ϕ2.

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• The resistance and the leakage reactance of the windings result in a voltage drop, and hence

secondary terminal voltage V2 is the phasor difference of E2and voltage drop.

V2 = E2 – voltage drops

I2 R2 is in phase with I2 and I2X2 is in quadrature with I2. • The total current flowing in the primary winding is the phasor sum of I1’ and I0. • Primary applied voltage V1 is the phasor sum of V1’ and the voltage drop in the primary winding. • Current I1’ is drawn equal and opposite to the current

I2 V1 = V1’ + voltage drop

I1R1 is in phase with I1 and I1XI is in quadrature with I1. • The phasor difference between V1 and I1 gives the power factor angle ϕ1 of the primary side of

the transformer. • The power factor of the secondary side depends upon the type of load connected to the transformer. • If the load is inductive as shown in the above phasor diagram, the power factor will be lagging, and if the

load is capacitive, the power factor will be leading. Where I1R1 is the resistive drop in the primary

windings

I2X2 is the reactive drop in the secondary winding

Phasor Diagram of Transformer on Capacitive Load

The Transformer on Capacitive load (leading power factor load) is shown below in the

phasor diagram.

Phasor Diagram of the Transformer on Capacitive Load

Steps to draw the phasor diagram at capacitive load

• Take flux ϕ a reference • Induces emf E1 and E2 lags the flux by 90 degrees. • The component of the applied voltage to the primary equal and opposite to induced emf in the

primary winding. E1 is represented by V1’. • Current I0 lags the voltage V1’ by 90 degrees.

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• The power factor of the load is leading. Therefore current I2 is drawn leading E2 • The resistance and the leakage reactance of the windings result in a voltage drop, and hence secondary

terminal voltage V2 is the phasor difference of E2and voltage drop.

V2 = E2 – voltage drops

I2 R2 is in phase with I2 and I2X2 is in quadrature with I2. • Current I1’ is drawn equal and opposite to the current I2 • The total current I1 flowing in the primary winding is the phasor sum of I1’ and I0. • Primary applied voltage V1 is the phasor sum of V1’ and the voltage drop in the primary winding.

V1 = V1’ + voltage drop

I1R1 is in phase with I1 and I1XI is in quadrature with I1. • The phasor difference between V1 and I1 gives the power factor angle ϕ1 of the primary side of

the transformer. • The power factor of the secondary side depends upon the type of load connected to the transformer.

EQUIVALENT CIRCUIT OF A TRANSFORMER:

The equivalent circuit diagram of any device can be quite helpful in predetermination of the behaviour of

the device under the various condition of operation. It is simply the circuit representation of the equation

describing the performance of the device.

The simplified equivalent circuit of a transformer is drawn by representing all the parameters of the

transformer either on the secondary side or on the primary side. The equivalent circuit diagram of the

transformer is shown below

Fig: Equivalent circuit diagram of a transformer

Let the equivalent circuit of a transformer having the transformation ratio K = E2/E1

The induced emf E1 is equal to the primary applied voltage V1 less primary voltage drop.This voltage

causes current I0 no load current in the primary winding of the transformer. The value of no-load current

is very small, and thus, it is neglected. Hence, I1 = I1’. The no load current is further divided into two

components called magnetizing current (Im) and working current (Iw).

These two components of no-load current are due to the current drawn by a noninductive resistance R0

and pure reactance X0 having voltage E1 or (V1 – primary voltage drop).

The secondary current I2 is

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The terminal voltage V2 across the load is equal to the induced emf E2 in the secondary winding less

voltage drop in the secondary winding.

Equivalent Circuit when all the Quantities are referred to Primary side: In this case to draw the equivalent circuit of the transformer all the quantities are to be referred to the

primary as shown in the figure below

Fig: Circuit Diagram of Transformer when all the Secondary Quantities are Referred to Primary Side

The following are the values of resistance and reactance given below

Secondary resistance referred to primary side is given as

The equivalent resistance referred to primary side is given as

Secondary reactance referred to primary side is given as

The equivalent reactance referred to primary side is given as

Equivalent Circuit when all the Quantities are referred to Secondary side: The equivalent circuit diagram of the transformer is shown below when all the quantities are referred to

the secondary side.

Fig: Circuit Diagram of Transformer When All the Primary Quantities are Referred to Secondary Side

The following are the values of resistance and reactance given below

Primary resistance referred to secondary side is given as

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The equivalent resistance referred to secondary side is given as

Primary reactance referred to secondary side is given as

The equivalent reactance referred to secondary side is given as

No load current I0 is hardly 3 to 5% of full load rated current, the parallel branch consisting of

resistance R0 and reactance X0 can be omitted without introducing any appreciable error in the behavior

of the transformer under the loaded condition. Further simplification of the equivalent circuit of the transformer can be done by neglecting the

parallel branch consisting R0 and X0. The simplified circuit diagram of the transformer is shown below

Fig: Simplified Equivalent Circuit Diagram of a Transformer

VOLTAGE REGULATION OF A TRANSFORMER:

Definition: The voltage regulation is defined as the change in the magnitude of receiving and sending the

voltage of the transformer. The voltage regulation determines the ability of the transformer to provide the

constant voltage for variable loads.

When the transformer is loaded with continuous supply voltage, the terminal voltage of the transformer

varies. The variation of voltage depends on the load and its power factor. Mathematically, the voltage

regulation is represented as

Where,

E2 – secondary terminal voltage at no load

V2 – secondary terminal voltage at full load

The voltage regulation by considering the primary terminal voltage of the transformer is expressed as,

Let us understand the voltage regulation by taking an example explained below

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If the secondary terminals of the transformer are open circuited or no load is connected to the secondary

terminals, the no-load current flows through it. If the no current flows through the secondary terminals of

the transformer, the voltage drops across their resistive and reactive load become zero. The voltage drop

across the primary side of the transformer is negligible.

If the transformer is fully loaded, i.e., the load is connected to their secondary terminal, the voltage drops

appear across it. The value of the voltage regulation should always be less for the better performance of

transformer.

Fig: Equivalent Circuit of transformer

From the circuit diagram shown above, the following conclusions are made

• The primary voltage of the transformer is always greater than the emf induces on the primary side. V1>E1

• The secondary terminal voltage at no load is always greater than the voltage at full load condition.

E2>V2 By considering the above circuit diagram, the following equations are drawn

The approximate expression for the no-load secondary voltage for the different types of load is

For inductive load:

Where,

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For Capacitive load:

ALL DAY EFFICIENCY OF A TRANSFORMER:

Definition: All day efficiency means the power consumed by the transformer throughout the day. It is

defined as the ratio of output power to the input power in kWh or Wh of the transformer over 24 hours.

Mathematically, it is represented as

All day efficiency of the transformer depends on their load cycle. The load cycle of the transformer means

the repetitions of load on it for a specific period.

The ordinary or commercial efficiency of a transformer define as the ratio of the output power to the input

power.

What is the need of All Day Efficiency? Some transformer efficiency cannot be judged by simple commercial efficiency as the load on certain

transformer fluctuates throughout the day. For example, the distribution transformers are energised for 24

hours, but they deliver very light loads for the major portion of the day, and they do not supply rated or

full load, and most of the time the distribution transformer has 50 to 75% load on it.

As we know, there are various losses in the transformer such as iron and copper loss. The iron loss takes

place in the core of the transformer. Thus, the iron or core loss occurs for the whole day in the distribution

transformer. The second type of loss known as copper loss takes place in the windings of the transformer

also known as the variable loss. It occurs only when the transformers are in the loaded condition.

Hence, the performance of such transformers cannot be judged by the commercial or ordinary efficiency,

but the efficiency is calculated or judged by All Day Efficiency also known as operational efficiency or

energy efficiency which is computed by energy consumed during 24 hours.

POLARITY TEST OF TRANSFORMER: Polarity means the direction of the induced voltages in the primary and the secondary winding of the

transformer. If the two transformers are connected in parallel, then the polarity should be known for the

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proper connection of the transformer. There are two types of polarity one is Additive, and another is

Subtractive.

Additive Polarity: In additive polarity the same terminals of the primary and the secondary windings of

the transformer are connected

Subtractive Polarity: In subtractive polarity different terminals of the primary and secondary side of the

transformer are connected.

Explanation with Connection Diagram:

Each of the terminals of the primary as well as the secondary winding of a transformer is alternatively

positive and negative with respect to each other as shown in the figure below. Let A1 and A2 be the

positive and negative terminal respectively of the transformer primary and a1, a2 are the positive and

negative terminal of the secondary side of the transformer.

If A1 is connected to a1 and A2 is connected to a2 that means similar terminals of the transformer are

connected, then the polarity is said to be additive. If A1 is connected to a2 and A2 to a1, that means the

opposite terminals are connected to each other, and thus the voltmeter will read the subtractive polarity.

Fig: Polarity test

It is essential to know the relative polarities at any instant of the primary and the secondary terminals for

making the correct connections if the transformers are to be connected in parallel or they are used in a

three phase circuit.

In the primary side, the terminals are marked as A1 and A2 and from the secondary side the terminals are

named as a1 and a2. The terminal A1 is connected to one end of the secondary winding, and a voltmeter is

connected between A2 and the other end of the secondary winding.

When the voltmeter reads the difference that is (V1 – V2), the transformer is said to be connected with

opposite polarity know as Subtractive polarity and when the voltmeter reads (V1 + V2), the transformer is

said to have additive polarity.

Steps to Perform Polarity Test: • Connect the circuit as shown in the above circuit diagram figure and set the autotransformer to zero

position. • Switch on the single phase supply

• Records the values of the voltages as shown by the voltmeter V1, V2 and V3.

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• If the reading of the V3 shows the addition of the value of V1 and V2 that is V2= V1+V2 the transformer

is said to be connected in additive polarity.

• If the reading of the V3 is the subtraction of the readings of V1 and V2, then the transformer is said to be

connected in subtractive or negative polarity.

RESISTANCE MEASUREMENT:

This test is a verification that proper sizes of conductors have been used and that the joints have

been made properly. Since this test is indicative in nature, there is no tolerance applicable to the measured

resistances. Resistances of the windings are measured by using 'Resistance Bridge'.

This test also serves two other important testing functions:

(a) The measured resistance is used for obtaining I2 R, which is used in the 'Load loss' test. (b) Measurements of cold resistance and hot resistance are used for calculation of temperature rise of

windings during the Temperature Rise Test.

The measurement of resistance is done at room temperature but corrected to a reference temperature

which is 20 degrees higher than the temperature class of the unit. For example : the reference temperature

is 75 0C for 55 0C rise oil-filled units, or is 85 0C for 65 0C rise units. For dry type transformers the

typical rises are 800C, 115 0C and 150 0C.

OPENCIRCUIT AND SHORT CIRCUIT TESTONTRANSFORMER The open circuit and short circuit test are performed for determining the parameter of the transformer like

their efficiency, voltage regulation, circuit constant etc. These tests are performed without the actual

loading and because of this reason the very less power is required for the test. The open circuit and the

short circuit test gives the very accurate result as compared to the full load test.

Open Circuit Test:

The purpose of the open circuit test is to determine the no-load current and losses of the transformer

because of which their no-load parameter are determined. This test is performed on the primary winding

of the transformer. The wattmeter, ammeter and the voltage are connected to their primary winding. The

nominal rated voltage is supplied to their primary winding with the help of the ac source.

Fig: Circuit Diagram of Open Circuit Test on Transformer

The secondary winding of the transformer is kept open and the voltmeter is connected to their terminal.

This voltmeter measures the secondary induced voltage. As the secondary of the transformer is open the

no-load current flows through the primary winding.

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The value of no-load current is very small as compared to the full rated current. The copper loss occurs

only on the primary winding of the transformer because the secondary winding is open. The reading of

the wattmeter only represents the core and iron losses. The core loss of the transformer is same for all

types of loads.

Calculation of open circuit test:

Let,

W0 – wattmeter reading

V1 – voltmeter reading

I0 – ammeter reading

Then the iron loss of the transformer Pi = W0 and

The no-load power factor is

Working component Iw is

Putting the value of W0 from the equation (1) in equation (2) you will get the value of working

component as

Magnetizing component is

No load parameters are given below

Equivalent exciting resistance is

Equivalent exciting reactance is

The iron losses measured by the open circuit test are used for calculating the efficiency of the transformer.

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Short Circuit Test: The short circuit test is performed for determining the below mention parameter of the transformer.

• It determines the copper loss occur on the full load. The copper loss is used for finding the efficiency

of the transformer. • The equivalent resistance, impedance, and leakage reactance are known by the short circuit test.

The short circuit test is performed on the secondary or high voltage winding of the transformer. The

measuring instrument like wattmeter, voltmeter and ammeter are connected to the High voltage

winding of the transformer. Their primary winding is short circuited by the help of thick strip or

ammeter which is connected to their terminal.

The low voltage source is connected across the secondary winding because of which the full load current

flows from both the secondary and the primary winding of the transformer. The full load current is

measured by the ammeter connected across their secondary winding.

The circuit diagram of the short circuit test is shown below

Fig: Circuit Diagram of Short Circuit Test on Transformer

The low voltage source is applied across the secondary winding which is approximately 5 to 10% of the

normal rated voltage. The flux is set up in the core of the transformer. The magnitude of the flux is small

as compared to the normal flux.

The iron loss of the transformer depends on the flux. It is less occur in the short circuit test because of the

low value of flux. The reading of the wattmeter only determines the copper loss occur on their windings.

The voltmeter measures the voltage applied to their high voltage winding. The secondary current induces in the transformer because of the applied voltage.

Calculation of Short Circuit Test Let,

Wc – Wattmeter reading

V2sc – voltmeter reading

I2sc – ammeter reading

Then the full load copper loss of the transformer is given by

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Equivalent resistance referred to secondary side is

Equivalent impedance referred to the secondary side is given by

The Equivalent reactance referred to the secondary side is given by

The voltage regulation of the transformer can be determined at any load and power factor after knowing

the values of Zes and Res.

In the short circuit test the wattmeter record, the total losses including core loss but the value of core loss

are very small as compared to copper loss so, the core loss can be neglected.

BACK-TO-BACK TEST (SUMPNER’S TEST) ON TRANSFORMER

Definition: The full load test on a small transformer is very convenient, but on the large transformer, it is

very difficult. The maximum temperature rise in a large transformer is determined by the full load test.

This test is called, back-to-back test, regenerative test or Sumpner’s test.

The suitable load which absorbs the full load power of a large transformer will not easily be available.

Hence a large amount of energy will be wasted. The back-to-back test determines the maximum

temperature rise in a transformer, and hence the load is chosen according to the capability of the

transformer.

Back to Back Test Circuit:

The two identical transformers are used for the back to back test. Consider the Tr1and Tr2 are the primary

windings of the transformer connected parallel to each other. The nominal rated voltage and frequency is

supplied to their primary winding. The voltmeter and ammeter are connected on their primary side for the

measurement of the input voltage and current. The secondary winding of the transformer is connected in series with the each other but with opposite

polarity. The voltmeter V2 is connected to the terminal of the secondary winding for the measurement of

the voltage. The series opposition of the secondary winding is determined by connecting there any two terminals; the

voltmeter is connected across their remaining terminals. If it is connected in series opposition, the

voltmeter gives the zero reading. The open terminal is used for measuring the parameter of the

transformer.

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The above figure shows that the terminal B and C are connected to each other, and the voltage is

measured across the terminal A and D.

Finding Losses with Sumpner's Test on Transformer The secondaries of the transformer are so connected that their potentials are in opposition to each other.

This would so if VAB = VCD and A is joined to C whilst B is joined to D. In that case, there would be no

secondary current flowing around the loop formed by the two secondaries. T is an auxiliary low-voltage

transformer [Regulation Transformer].which can be adjusted to give a variable voltage and hence current

in the secondary loop circuit. By proper adjustment of T, full-load secondary current I2 can be made to

flow as shown. It is seen, that I2 flows from D to C and then from A to B. Flow of I1 is confined to the

loop FEJLGHMF and it does not pass through W1. Hence, W1continues to read the core loss and W2

measures full-load Cu loss (or at any other load current value I2). Obviously, the power taken in is twice the losses of a single transformer.

i.e. copper loss per transformer PCu = W2/2. i.e. iron loss per transformer Pi = W1/2.

From results of sumpner's test, the full load efficiency of each transformer can be given as

Advantages of Sumpner's Test 1. Low power is required to conduct this test. Because no external load is connecting 2. Full load copper losses and iron losses of both transformers determined. 3. Increase in transformer temperature can be found.

Determination of Temperature Rise The temperature rise of the transformer is determined by measuring the temperature of their oil after

every particular interval of time. The transformer is operating back to back for the long time which

increases their oil temperature. By measuring the temperature of their oil the withstand capacity of the

transformer under high temperature is determined.

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Determination of Iron Loss

The wattmeter W1 measures the power loss which is equal to the iron loss of the transformer. For

determining the iron loss, the primary circuit of the transformer is kept closed. Because of the primary

closed circuit, no current flows through the secondary windings of the transformer. The secondary

winding behaves like an open circuit. The wattmeter is connected to their secondary terminal for the

measurement of iron loss.

Determination of Copper Loss The copper loss of the transformer is determined when the full load current flows through their primary

and secondary windings. The additional regulating transformer is used for exciting the secondary

windings. The full load current flows from the secondary to the primary winding. The wattmeter W2

measures the full load copper loss of the two transformers.

PARALLEL OPERATION OF TRANSFORMERS

Sometimes, it becomes necessary to connect more than one transformer’s in parallel, for example, for

supplying excess load of the rating of existing transformer. If two or more transformers are connected to a

same supply on the primary side and to a same load on the secondary side, then it is called as parallel

operation of transformers.

Necessity of Parallel Operation of Transformers: Why parallel operation of transformers is needed? Increased Load: When load is increased and it exceeds the capacity of existing transformer,

another transformer may be connected in parallel with the existing transformer to supply the increased load.

Non-availability of large transformer: If a large transformer is not available which can meet the total requirement of load, two or more small transformers can be connected in parallel to increase the capacity.

Increased reliability: If multiple transformers are running in parallel, and a fault occurs in one transformer, then the other parallel transformers still continue to serve the load. And the faulty transformer can be taken out for the maintenance.

Transportation is easier for small transformers: If installation site is located far away, then transportation of smaller units is easier and may be economical.

Conditions for parallel operation of Transformers: There are various conditions that must fulfill for the successful operation of transformers as follows.

1. The line voltage ratio of two transformers must be equal. 2. The per unit impedance of each transformer should be equal and they should have same ratio of

equivalent leakage reactance to the equal resistance(X/R). 3. The transformers should have same secondary winding polarity. 4. The Transformers should have same phase sequence (Three phase transformer) 5. The transformers should have the zero relative phase replacement between the secondary line

voltages.(Three phase transformers)

1) The line voltage ratios of the two transformers must be equal This condition is used to avoid the inequality EMF induction at the two secondary windings. If the two

transformers connected in parallel have slightly different voltage ratios, then due the inequality of induced

emfs in the secondary voltages, a circulating current will flow in a loop format in the secondary windings.

This current is greater than the no load current and will be quite high due to less leakage impedance

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during load. When the secondary windings are loaded, this circulating current will tend to unequal

loading on two transformers and one transformer may be over loaded and another may be less loaded.

2) Equal per unit leakage impedance If the ratings or line voltages are equal their per unit leakage impedance’s should be equal in order to have

equal load sharing of the both transformers. If the ratings are unequal then the transformer which has less

rating will draw more current and it leads to unequal load sharing. It may also lead to mismatch in line

voltages due to voltage drops. In other words, for unequal ratings, the numerical values of their

impedance’s should be in inverse proportional to their ratings to have current in them in line with their

ratings. A difference in the ratio of the reactance value to the resistance value of the impedance results in different

phase angles of the currents carried by the two parallel transformers. Due to this phase angle difference

between voltage and current, one transformer may be working on high power factor and another

transformer may be working on lower factor. Hence real power sharing is not proportional between the

two transformers.

3) The transformers should have same secondary winding polarity The transformers should be properly connected with regards to their polarity. If they are connected with

in correct polarities then the two emf’s induced in the secondary winding which are in parallel, will act

together and produce a short circuit between the two of them. Total loss of power supply and high

damage to the transformers.

4) The Transformers should have same phase sequence In case of three winding transformers in-addition to the above conditions the phase sequence of line

voltages of the both transformers must be identical for parallel operation. If the phase sequence is not

correct in every voltage cycle each pair of phases will get shorted

5) The transformers should have zero relative phase displacement between the secondary line

voltages This condition indicates that the two secondary line voltages should have zero phase displacement which

avoids UN-intended short circuit between the phases of two windings. There are four groups which in to which the three phase windings connections are classified: Group 1: Zero phase displacement (Yy0,Dd0,Dz0)

Group2: 1800 Phase displacement (Yy6,Dd6,Dz6)

Group3: -300 Phase displacement (Ys1,Dy1,Yz1)

Group4: +300 Phase displacement(Yd11,Dy11,Yz11)

The letters Y,D and z represents the Star, Delta and zigzag type winding connections. In order to have

zero phase displacement of secondary side line voltages, the transformers belonging to the same group

can be paralleled. For example with Yd1 and Dy1 can be paralleled. The transformers of groups 1 and 2

can be paralleled with their own group where as the transformers of group 3 and 4 can be paralleled by

revering the phase sequence of one of them. For example a transformer with Yd11 connection of group 4

can be paralleled with that having Dy1 connection by reversing the phase sequence of both primary and

secondary terminals of the Dy1 transformer.

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UNIT V

THREE PHASE TRANSFORMERS

THREE PHASE TRANSFORMER Three phase transformers are more economical for supplying large loads and large power distribution.

Even though most of the utilization equipments are connected by the single phase transformers, these are

not preferred for large power distribution in the aspect of economy.

The three phase power is used in almost all fields of electrical power system such as power generation,

transmission and distribution sectors, also all the industrial sectors are supplied or connected with three

phase system. Therefore, to step-up (or increase) or step-down (or decrease) the voltages in the three

phase systems, three phase transformers are used. As compared with the single phase transformer, there

are numerous advantages with 3 phase transformer such as smaller and lighter to construct for the same

power handling capacity, better operating characteristics, etc.

Fig: Three Phase Transformer

Three phase transformers are used to step-up or step-down the high voltages in various stages of power

transmission system. The power generated at various generating stations is in three phase nature and the

voltages are in the range of 13.2KV or 22KV. In order to reduce the power loss to the distribution end, the

power is transmitted at somewhat higher voltages like 132 or 400KV. Hence, for transmission of the

power at higher voltages, three phase step-up transformer is used to increase the voltage. Also at the end

of the transmission or distribution, these high voltages are step-down to levels of 6600, 400, 230 volts,

etc. For this, a three phase step down transformer is used. A three phase transformer can be built in two ways; a bank of three single phase transformers or single

unit of three phase transformer.

The former one is built by suitably connecting three single phase transformers having same ratings and

operating characteristics. In this case if the fault occurs in any one of the transformers, the system still

retained at reduced capacity by other two transformers with open delta connection. Hence, continuity of

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the supply is maintained by this type of connection. These are used in mines because easier to transport

individual single phase transformers.

Fig: Three phase transformer

Instead of using three single phase transformers, a three phase bank can be constructed with a single three

phase transformer consisting of six windings on a common multi-legged core. Due to this single unit,

weight as well as the cost is reduced as compared to three units of the same rating and also windings, the

amount of iron in the core and insulation materials are saved. Space required to install a single unit is less

compared with three unit bank. But the only disadvantage with single unit three phase transformer is if the

fault occurs in any one of the phase, then entire unit must be removed from the service.

Construction of Three Phase Transformers: A three phase transformer can be constructed by using common magnetic core for both primary and

secondary windings. As we discussed in the case of single phase transformers, construction can be core

type or shell type. So for a bank of three phase core type transformer, three core type single phase

transformers are combined. Similarly, a bank of three phase shell type transformer is get by properly

combining three shell type single phase transformers. In a shell type transformer, EI laminated core

surrounds the coils whereas in core type coil surrounds the core.

Core Type Construction: In core type three phase transformer, core is made up of three limbs or legs and two yokes. The magnetic

path is formed between these yokes and limbs. On each limb both primary and secondary windings are

wounded concentrically. Circular cylindrical coils are used as the windings for this type of transformer.

The primary and secondary windings of one phase are wounded on one leg. Under balanced condition, the

magnetic flux in each phase of the leg adds up to zero. Therefore, under normal conditions, no return leg

is needed. But in case of unbalanced loads, high circulating current flows and hence it may be best to use

three single phase transformers.

Fig: Core Type transformer

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Shell Type Construction: In shell type, three phases are more independent because each phase has independent magnetic circuit

compared with core type transformer. The construction is similar to the single phase shell type

transformer built on top of another. The magnetic circuits of this type of transformer are in parallel. Due

to this, the saturation effects in common magnetic paths are neglected. However, shell type constructed

transformers are rarely used in practice.

Fig: Shell Type

Working of Three Phase Transformers: Consider the below figure in which the primary of the transformer is connected in star fashion on the

cores. For simplicity, only primary winding is shown in the figure which is connected across the three

phase AC supply. The three cores are arranged at an angle of 120 degrees to each other. The empty leg of

each core is combined in such that they form center leg as shown in figure.

Fig: Working of Three Phase Transformer

Working of a transformer: When the primary is excited with the three phase supply source, the currents IR, IY and IB are starts

flowing through individual phase windings. These currents produce the magnetic fluxes ΦR, ΦY and ΦB

in the respective cores. Since the center leg is common for all the cores, the sum of all three fluxes are

carried by it. In three phase system, at any instant the vector sum of all the currents is zero. In turn, at the

instant the sum of all the fluxes is same. Hence, the center leg doesn’t carry any flux at any instant. So

even if the center leg is removed it makes no difference in other conditions of the transformer.

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Likewise, in three phase system where any two conductors acts as return for the current in third

conductor, any two legs acts as a return path of the flux for the third leg if the center leg is removed in

case of three phase transformer. Therefore, while designing the three phase transformer, this principle is

used. These fluxes induce the secondary EMFs in respective phase such that they maintain their phase angle

between them. These EMFs drives the currents in the secondary and hence to the load. Depends on the

type of connection used and number of turns on each phase, the voltage induced will be varied for

obtaining step-up or step-down of voltages.

THREE PHASE TRANSFORMER CONNECTIONS:

Three phase transformer connections in three phase system, the three phases can be connected in either

star or delta configuration. In case you are not familiar with those configurations, study the following

image which explains star and delta configuration. In any of these configurations, there will be a phase

difference of 120° between any two phases.

Three Phase Transformer Connections Windings of a three phase transformer can be connected in various configurations as (i) star-star, (ii)

delta-delta, (iii) star-delta, (iv) delta-star, (v) open delta and (vi) Scott connection. These configurations

are explained below. Star-Star (Y-Y) Star-star connection is generally used for small, high-voltage transformers. Because of star

connection, number of required turns/phase is reduced (as phase voltage in star connection is 1/√3 times of line voltage only). Thus, the amount of insulation required is also reduced.

The ratio of line voltages on the primary side and the secondary side is equal to the transformation ratio of the transformers.

Line voltages on both sides are in phase with each other.

This connection can be used only if the connected load is balanced.

Delta-Delta (Δ-Δ) This connection is generally used for large, low-voltage transformers. Number of required

phase/turns is relatively greater than that for star-star connection.

The ratio of line voltages on the primary and the secondary side is equal to the transformation ratio of the transformers.

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This connection can be used even for unbalanced loading.

Another advantage of this type of connection is that even if one transformer is disabled, system can continue to operate in open delta connection but with reduced available capacity.

Star-Delta OR Wye-Delta (Y-Δ) The primary winding is star (Y) connected with grounded neutral and the secondary winding is

delta connected.

This connection is mainly used in step down transformer at the substation end of the transmission line.

The ratio of secondary to primary line voltage is 1/√3 times the transformation ratio.

There is 30° shift between the primary and secondary line voltages.

Delta-Star OR Delta-Wye (Δ-Y) The primary winding is connected in delta and the secondary winding is connected in star with

neutral grounded. Thus it can be used to provide 3-phase 4-wire service.

This type of connection is mainly used in step-up transformer at the beginning of transmission line.

The ratio of secodary to primary line voltage is √3 times the transformation ratio.

There is 30° shift between the primary and secondary line voltages.

Above transformer connection configurations are shown in the following figure.

Fig: Three phase transformer connections

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THREE PHASE TO SIX PHASE CONVERSION

In certain applications like thyristors and rectifiers six phase supply is required. Therefore it

becomes necessary to convert three phase a.c. supply into six phase. By using three identical single phase

transformers suitably interconnected this can be achieved. The primary winding is connected in delta

whereas its secondary winding is split up into two halves. Thus conversion from 3 phase to six phase can

be obtained by having two similar secondary windings for each of the primaries of the three phase

transformer. This is showing in the Fig. 1.

The three phase supply is given to primaries of the three transformers and six phase output can be

obtained from the six secondaries as shown. There are many ways of connecting these secondaries. Some

of them are i) double delta ii) double star iii) dimetrical. The dimetrical connection is generally used in

practice.

Fig. 1 The double delta connection

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Fig. 2 The double delta connection

As seen from the Fig.2 this arrangement of secondaries connections are taken from points 1, 3, 5 and

in the second set, the connection are taken from points 2, 4 and 6.

The double star connection for obtaining 6 phase supply is shown in the Fig.3.

Fig. 3 The double star connection

Six equal voltages with a phase displacement of is obtained which is indicated in the phasor diagram

shown in the Fig. 4. The primary windings may be either connected in delta or in star. If the primaries are

connected in star then care must be taken that the neutral point of the star connection must be connected

to the neutral of the generator to avoid the difficulties due to floating neutral and triple frequency currents.

Fig. 4 Phasor diagram shown

In the case of six phase rectifier, the star point or the neutral point formed by the coils serves as the

neutral point of the d.c. supply from the rectifier.

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The most commonly used connection is the dimetrical connection for three phase to six phase

transformers. It is shown in the Fig. 5. On each single phase transformer there is only one winding on the

secondary side. The two secondary leads are connected to diametrically opposite points on the armature

windings of the synchronous converter. The middle points of the secondary windings may be connected

together to form a neutral for the three wire circuit. The high voltage or primary windings are either

connected in star or delta. But delta connection is generally preferred because of the triple frequency

harmonics of voltages which are introduced in star-star connection.

Fig. 5 Dimetrical connection

The corresponding phasor diagram is shown in the Fig.6.

Fig. 6 Phasor diagram

OPEN DELTA CONNECTION:

In three phase systems, the use of transformers with three windings (or legs) per side is common. These

three windings are often connected in delta or star, resulting in common transformer configurations such

as delta-delta or delta-star. An open delta transformer is a special arrangement which uses only two

windings.

Transformer Configuration

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The illustration shows how an open delta transformer is connected. On both the primary and secondary

there are only two windings. Even in this configuration, it is still possible to transform a three phase

voltage.

Open Delta Transformer

Open delta transforms are not the commonly used. Typically they would be used for small loads where

cost is important. Alternatively, they could be used as an emergency measure, should one winding only of

a transformer fail. Sometimes you may hear an open delta transformer referred to as a V-connection transformer.

Power Delivered: Sometimes the power delivered by an open delta transformer is compared to that of an equivalent three

winding transformer. Typically figures like having 57.7% of the capacity of an equivalent three winding

transformer or 87% of two transformers (same winding size) are quoted. While you can think of the

transformer in this manner, it is more fruitful not to consider comparisons but to the necessary

calculations on the open delta transformer.

Consider the illustration, showing the output from both a close delta and open delta transformer. Note,

that in the delta connection to line current is √3 times the phase current, whereas in the open delta, they

are the same. The transformer output power (in VA) is for a balanced transformer system for the closed delta

connection (using phase current), this give:

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VA=3VLIph And for the open delta connection:

VA= Iph

Taking the ratio of open delta to closed delta power, gives:

=0.577(or 57.7%) Open delta transformers are three phase devices, with only two windings on each of the primary and

secondary sides. While cheaper than a conventional three winding transformer, the open delta will only

deliver 57.7% of the power of a conventional transformer (not two thirds, 66.7% as may be expected).

There is limited adoption of open delta transformers, although they can be useful in certain situations.

Power Factor with Two Transformers in 3-Phase Open-Delta Configuration:

The power factor of a typical T/F is 0.866, so the max output (KVA) of a T/F will be 86.6% of its rated

capacity. Suppose we have three transformers each having the same rating (50 KVA). Then the total

capacity is equal to 150 * 0.866=130 KVA and not 150 KVA. (The factor 0.866 is called the Utility

Factor.)

If one transformer is removed, the total output of two T/Fs will be 100 * 0.866=86.6 KVA. From this

calculation, we see that output capacity is reduced when a Delta-Delta configuration is operated as an

Open-Delta connection, i.e. with one T/F removed.

Let say the two T/Fs of Open-Delta configuration are supplying 3-phase power to a load having power

factor Cosα. The power factor of one T/F is Cos(30-α) and the other is Cos(30+α).

Power (T/F1)=KVA Cos(30-α) and

Power(T/F2)= KVACos(30+α)

• Case (i): When phase angle (α=0) is zero, mean load p.f=1 - each transformer has power factor =

Cos30 = 0.866 • Case (ii):When phase angle is 30(α=30), mean load p.f=0.866, one T/F will have p.f of cos(30-30) =

Cos0 = 1 and the other will be operating at p.f of Cos(30+30) = Cos60 = 0.866 • Case(iii):When phase angle is 60 (α=60), mean load p.f=0.5, then one T/F will have p.f of Cos(30-60)

= 0.866 and other will have a p.f of Cos(30+60) = Cos90 = 0.

From the third scenario we conclude that if we have a transformer Open-Delta configuration in which one

T/F is transferring power to an entire load at a p.f of 0.866, then the other one will not provide any load.

Applications of Open-Delta configurations:

• Widely used for small 3-phase loads. • Used in applications where the current load is less, and may increase in future. • There is no relative shift in output, so can be used for unbalanced loads and where harmonics are

involved.

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SCOTT CONNECTION OF TRANSFORMERS: Definition: The Scott-T Connection is the method of connecting two single phase transformer to perform

the 3-phase to 2-phase conversion and vice-versa. The two transformers are connected electrically but not

magnetically. One of the transformers is called the main transformer, and the other is called the auxiliary

or teaser transformer.

The figure below shows the Scott-T transformer connection. The main transformer is centre tapped at D

and is connected to the line B and C of the 3-phase side. It has primary BC and secondary a1a2. The teaser

transformer is connected to the line terminal A and the centre tapping D. It has primary AD and the

secondary b1b2

The identical, interchangeable transformers are used for Scott-T connection in which each transformer has

a primary winding of Tp turns and is provided with tapping at 0.289Tp , 0.5Tpand 0.866 Tp.

Phasor Diagram of Scott Connection Transformer

The line voltages of the 3-phase system VAB, VBC, and VCA which are balanced are shown in the figure

below. The same voltage is shown as a closed equilateral triangle. The figure below shows the primary

windings of the main and the teaser transformer.

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The D divides the primary BC of the main transformers into two halves and hence the number of turns in

portion BD = the number of turns in portion DC = Tp/2.The voltage VBD and VDC are equal, and they are

in phase with VBC.

The voltage between A and D is

The teaser transformer has the primary voltage rating that is √3/2 or 0.866 of the voltage ratings of the

main transformer. Voltage VAD is applied to the primary of the teaser transformer and therefore the

secondary of the voltage V2tof the teaser transformer will lead the secondary terminal voltage V2m of the

main transformer by 90º as shown in the figure below.

Fig: secondary Phasor diagram

Then,

For keeping the voltage per turn same in the primary of the main transformer and the primary of the teaser

transformer, the number of turns in the primary of the teaser transformer should be equal to √3/2Tp.

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Thus, the secondaries of both transformers should have equal voltage ratings.The V2t and V2m are equal

in magnitude and 90º apart in time; they result in the balanced 2-phase system.

Position of Neutral Point (N)

The primary of the two transformers may have a four wire connection to a 3-phase supply if the tapping N

is provided on the primary of the teaser transformer such that

The voltage across AN = VAN = phase voltage = Vl/√3.

Since the voltage across the portion AD.

the voltage across the portion ND

The same voltage turn in portion AN, ND and AD are shown by the equations,

The equation above shows that the neutral point N divides the primary of the teaser transformer in ratio.

AN:ND=2:1

Applications of Scott Connection: The following are the applications of the Scott-T connection.

1. The Scott-T connection is used in an electric furnace installation where it is desired to operate two

single-phase together and draw the balanced load from the three-phase supply. 2. It is used to supply the single phase loads such as electric train which are so scheduled as to keep

the load on the three phase system as nearly as possible.

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3. The Scott-T connection is used to link a 3-phase system with a two–phase system with the flow of

power in either direction.

The Scott-T connection permits conversions of a 3-phase system to a two-phase system and vice versa.

But since 2-phase generators are not available, the converters from two phases to three phases are not

used in practice.

AUTOTRANSFORMER

A Transformer, in which a part of the winding is common to both the Primary and Secondary circuit, is

called an Auto Transformer. It shall be noted that in Two Winding Transformer, Primary and Secondary

windings are electrically isolated but in Auto Transformer, the two windings are not electrically isolated.

Why do we need to go for Auto Transformer?

We have some advantages of auto-transformer over normal two winding transformers.

1. Autotransformers usually smaller in size, because one winding is eliminated. 2. As size is small cost also low(so cheap in cost) 3. As the winding is same so leakage reactance will be less. 4. Increased kVA rating.

PRINCIPLE OF OPERATION OF AUTOTRANSFORMER: The principle of operation of the transformer is the same as the one of the common transformer, and then

the relation between input and output voltages and input and output currents and the ratio of number of

turns between the primary and the secondary winding is the same.

The currents of the primary and secondary windings are flowing on the opposite directions, so the total

current flowing through the common part of the winding is equal to the difference between the current on

the low-voltage winding and the current on the high-voltage winding.

A simplified diagram of a Step-down Auto Transformer is shown in figure below.

Fig: Auto transformer

119 | P a g e

As shown in the figure above, N1 and N2 are the number of turns between winding AB and AC

respectively. If a voltage V1is applied across AB, an exciting current will start flowing through the full

winding. Therefore, voltage per turn in winding AB is V1/N1and hence voltage across CB is (V1/N1)N2.

As the load current is I2and the current taken from the source is I1, neglecting losses

Input Power = Output Power

V1I1CosØ1= V2I2CosØ2 ………………….(1)

Assuming internal impedance drop and losses to be negligible, power factor for primary and secondary

will be almost same.

CosØ1 = CosØ2

Therefore from equation (1),

V1I1 = V2I2

So,

V2 / V1= I1 / I2 = N2 / N1 =k(say)

In our case of Step down Auto Transformer, k<1

Again, since the direction of flow of load current is in the opposite to the current flowing in the Primary

because of Lenze’s Law, hence the current flowing in winding BC = (I2-I1)

ICB = (I2 – I1)

Now the primary MMF = IAC (N1-N2)

= I1 (N1– N2)

= N1I1– N2I1

But N1 / N2= I2 / I1

So, N1I1= N2I2

Therefore,

Now the primary MMF = N2I2– N2I1

= (I2– I1) N2

= ICB N2 = Secondary MMF

Thus we see that in Auto Transformer, Transformer action take place between winding AC and BC. Thus

VA of winding AC will be transferred to winding BC by Transformer action. Therefore,

120 | P a g e

Transformed VA = VAB IAB

= (V1-V2) I1 ………………………(2)

Now, total VA input = V1 I1

Therefore,

Transformed VA / Input VA = [(V1-V2) I1] / V1I1

= 1 – V2/V1

= 1-k

Thus we see that out of total input VA, only a part of it is transformed by Transformer action and

remaining is therefore transferred by conduction.

So,

Conducted VA = Total Input – Transformed VA

= V1I1 – (V1-V2) I1 …………………From equation (1)

= V2I1 So,

Conducted VA / Input VA = V2I1/ V1I1

= V2/V1= k Thus in an Auto Transformer, a part of input power is delivered to the load by Transformer action while

the remaining is by conduction method. Power transfer because of conduction method is possible in Auto

Transformer because a part of winding is common to both the primary as well as secondary circuit.

EQUIVALENT CIRCUIT OF AUTOTRANSFORMER:

Fig: Auto transformer on load

As mentioned earlier the magnetizing current can be neglected, for simplicity. Writing the Kirchoff’s

equation to the primary and secondary

121 | P a g e

we have V1 = E1 + I1(r1 + jxl1) − (I2 − I1)(r2 + jxl2)

Note that the resistance r1 and leakage reactance xl1 refer to that part of the winding where only the primary current flows. Similarly on the load side we have, E2 = V2 + (I2 − I1)(r2 + jxl2)

The voltage ratio V1 : V2 = E1 : E2 = T1 : T2 = a where T1 is the total turns of the primary. Then E1 =

aE2 and I2 = aI1

The magnetization branch can now be hung across the mains for completeness. The above equivalent

circuit can now be compared with the approximate equivalent circuit of a two winding case Re = r1 + a 2

r2 and Xe = xl1 + a 2xl2. Thus in the case of an auto transformer total value of the short circuit

impedance is lower and so also the percentage resistance and reactance. Thus the full load regulation is

lower. Having a smaller value of short circuit impedance is sometimes considered to be a disadvantage.

That is because the short circuit currents become very large in those cases. The efficiency is higher in

auto transformers compared to their two winding counter part at the same load. The phasor diagram of

operation for the auto transformer drawing a load current at a lagging power factor angle of θ2 is shown

in Fig. 32. The magnetizing current is omitted here again for simplicity.

Fig: Equivalent Circuit From the foregoing study it is seen that there are several advantages in going in for the autotransformer

type of arrangement. The voltage/current transformation and impedance conversion aspects of a two

winding transformer are retained but with lesser material (and hence lesser weight) used. The losses are

reduced increasing the efficiency. Reactance is reduced resulting in better regulation characteristics. All

these benefits are enhanced as the voltage ratio approaches unity. The price that is required to be paid is

loss of electrical isolation and a larger short circuit current (and larger short circuit forces on the winding).

122 | P a g e

Fig: Phasor Diagram

Advantages of Auto transformer:

• Less costly • Better regulation • Low losses as compared to ordinary two winding transformer of the same rating.

Disadvantages of Auto transformer: There are various advantages of the auto transformer, but then also one major disadvantage, why auto

transformer is not widely used, is that

• The secondary winding is not insulated from the primary winding. If an auto transformer is used to supply low voltage from a high voltage and there is a break in the

secondary winding, the full primary voltage comes across the secondary terminal which is dangerous

to the operator and the equipment. So the auto transformer should not be used to for interconnecting

high voltage and low voltage system. • Used only in the limited places where a slight variation of the output voltage from input voltage is

required.

Applications of Auto transformer:

• It is used as a starter to give upto 50 to 60% of full voltage to the stator of a squirrel cage induction

motor during starting. • It is used to give a small boost to a distribution cable, to correct the voltage drop. • It is also used as a voltage regulator • Used in power transmission and distribution system and also in the audio system and railways.

TAP-CHANGING TRANSFORMERS The change of voltage is affected by changing the numbers of turns of the transformer provided with taps.

For sufficiently close control of voltage, taps are usually provided on the high voltage windings of the

transformer. There are two types of tap-changing transformers

123 | P a g e

1. Off-load tap changing transformer 2. On-load tap changing transformer

Off-load tap-changing transformer:

In this method, the transformer is disconnected from the main supply when the tap setting is to be

changed. The tap setting is usually done manually. The off load tap changing transformer is shown in the

figure below

On-load tap-changing transformer:

In order that the supply may not be interrupted, on-load tap changing transformer is used. Such a

transformer is known as a tap-changing under load transformer. While tapping, two essential conditions

are to be fulfilled.

• The load circuit should not be broken to avoid arcing and prevent the damage of contacts. • No parts of the windings should be short–circuited while adjusting the tap.

The tap changing employing a center tapped reactor R show in the figure above. Here S is the diverter

switch, and 1, 2, 3 are selector switch. The transformer is in operation with switches 1 and S closed. To

change to tap 2, switch S is opened, and 2 is closed. Switch 1 is then opened, and S closed to complete the

tap change. It is to be noted that the diverter switch operates on load, and no current flows in the selector

switches during tap changing. During the tap change only half of the reactance which limits the

124 | P a g e

It is to be noted that the diverter switch operates on load, and no current flows in the selector switches

during tap changing. During the tap change, only half of the reactance which limits the current is

connected in the circuit.

HARMONICS IN THREE PHASE TRANSFORMERS The harmonic is the distortion in the waveform of the voltage and current. It is the integral multiple of

some reference waves. The harmonic wave increases the core and copper loss of the transformer and

hence reduces their efficiency. It also increases the dielectric stress on the insulation of the transformer.

In a three-phase transformer, the non-sinusoidal nature of magnetising current produces sinusoidal flux

which gives rise to the undesirable phenomenon. The phase magnetising currents in transformer should

contain third harmonics and higher harmonics necessary to produce a sinusoidal flux.

Fig: Harmonics in transformers

If the phase voltage across each phase is to remain sinusoidal, then the phase magnetising currents

must be of the following form.

It is seen from equation (1), (2), and (3) that the third harmonics in the three currents are co-phase, that is

they have the same phase. The fifth harmonics have different phases.

Delta Connection

125 | P a g e

Let the IAO, IBO and the ICO represent the phase magnetising current in a delta connection. The

line currents can be found by subtracting two phases current. For examples,

The third harmonic present in the phase magnetising current of three phase transformer is not present in the line current. The third harmonic components are co-phase and hence cancel out in the line. The third harmonic components are flows round the closed loop of the delta.

The delta connection only allows a sinusoidal flux and voltage with no third harmonic current in the

transmission line. For this reason majority of the 3-phase transformer has delta connected windings and in

places where it is not convenient to have an either primary or secondary connected in delta, a tertiary

winding is provided. The tertiary windings carry the circulating third harmonics current required by the

sinusoidal flux in each limb of the core.

In a delta connection, the voltage acting around the closed delta is,

This is a third harmonic voltage and it will circulate a third harmonic current round the closed loop of the

delta

126 | P a g e

Star connection

If IAO, IBO and ICO, represents the phase magnetising current in a star connection,

Where In is the current in the neutral wire.

The harmonics above the seventh be neglected. The equation (6) shows that under the balanced condition

the current flow in the neutral wire is the third harmonic current. The magnitude of the third harmonics

current is thrice the magnitude of each third phase current. The thirds harmonic current produced

inductive interference with communication circuit. If the supply to the star connection is three wires,

neutral current must be zero and therefore

Thus, it is seen that the three wire star connection suppresses the flow of harmonic and magnetising

currents. For a four wire star-connected system, the in phase third harmonic current flow in the neutral

wire.

Similarly, the third balance phase voltage containing harmonics can be written as

127 | P a g e

The equation (7), (8) and (9) shows that the third harmonics in the three phase voltage have the same

phase. The line voltage in a star connection can be obtained by subtracting two phase voltages. For

example

From equation (10) it is seen that the third harmonics is not present in the line to line voltage of a

star connection. This applies to all triplers harmonics.

128 | P a g e

LECTURE NOTES ON

GENERATION, TRANSMISSION AND

DISTRIBUTION (Th. 4)

A COURSE IN 4TH SEMESTER OF DIPLOMA IN ELECTRICAL

ENGINEERING

Department of Electrical Engineering GOVT. POLYTECHNIC, NABARANGPUR

DISCLAIMER This document does not claim any originality and cannot be used as a substitute for

prescribed textbooks. The matter presented here is prepared by the author for their

respective teaching assignments by referring the text books and reference books. Further,

this document is not intended to be used for commercial purpose and the committee

members are not accountable for any issues, legal or otherwise, arising out of use of this

document.

SYLLABUS

OBJECTIVES :

After completion of this subject the student will be able to:

(i) Different schemes of power generation with their block diagram.

(ii) Mechanical and electrical design of transmission lines and numerical problems.

(iii) Types of cables and their methods of laying and testing.

(iv) Different schemes of distribution with problem solving

(v) Different types of sub-stations.

(vi) Economic aspects of power supply system with problem and type of tariff of

electricity.

Course content

1. GENERATION OF ELECTRICITY

1.1Elementary idea on generation of electricity from Thermal, Hydel, Nuclear,

Power station.

1.2 Introduction to Solar Power Plant (Photovoltaic cells).

1.3 Layout diagram of generating stations.

2. TRANSMISSION OF ELECTRIC POWER

2.1Layout of transmission and distribution scheme.

2.2Voltage Regulation & efficiency of transmission.

2.3State and explain Kelvin’s law for economical size of conductor.

2.4Corona and corona loss on transmission lines.

3. OVER HEAD LINES

3.1 Types of supports, size and spacing of conductor.

3.2 Types of conductor materials.

3.3 State types of insulator and cross arms.

3.4 Sag in overhead line with support at same level and different level.

(Approximate formula effect of wind, ice and temperature on sag)

3.5 Simple problem on sag.

4. PERFORMANCE OF SHORT & MEDIUM LINES

4.1. Calculation of regulation and efficiency.

5. EHV TRANSMISSION

5.1 EHV AC transmission.

5.1.1. Reasons for adoption of EHV AC transmission.

5.1.2. Problems involved in EHV transmission.

5.2 HV DC transmission.

5.2.1. Advantages and Limitations of HVDC transmission system.

6. DISTRIBUTION SYSTEMS

6.1 Introduction to Distribution System.

6.2 Connection Schemes of Distribution System: (Radial, Ring Main and Inter

connected system)

6.3 DC distributions.

6.3.1 Distributor fed at one End.

6.3.2 Distributor fed at both the ends.

6.3.3 Ring distributors.

6.4 AC distribution system.

6.4.1. Method of solving AC distribution problem.

6.4.2. Three phase four wire star connected system arrangement.

7. UNDERGROUND CABLES

7.1 Cable insulation and classification of cables.

7.2 Types of L. T. & H.T. cables with constructional features.

7.3 Methods of cable lying.

7.4 Localization of cable faults: Murray and Varley loop test for short circuit fault /

Earth fault.

8. ECONOMIC ASPECTS

8.1 Causes of low power factor and methods of improvement of power factor in

power system.

8.2 Factors affecting the economics of generation: (Define and explain)

8.2.1 Load curves.

8.2.2 Demand factor.

8.2.3 Maximum demand.

8.2. Load factor.

8.2.5 Diversity factor.

8.2.6 Plant capacity factor.

8.3 Peak load and Base load on power station.

9 TYPES OF TARIFF

9.1. Desirable characteristic of a tariff.

9.2. Explain flat rate, block rate, two part and maximum demand tariff. (Solve

Problems)

10 SUBSTATION

10.1 Layout of LT, HT and EHT substation.

10.2 Earthing of Substation, transmission and distribution lines.

Learning resources:

Sl.No Title of the Book Name of Author Publisher

1.

Principles of Power

System V. K. Mehta S Chand

2.

A text book of

Power

A Chakrabarti, M L

Soni,

Dhanpat Rai

& Co

System Engineering P V Gupta, U S

Bhatnagar

3.

A course of electrical

power S. L. Uppal

Khanna

publisher

system

4.

Power System

Engineering

D. P. Kothari, IJ

Nagrath TMH

INTRODUCTION TO POWER SYSTEM

BASIC OF ELECTRIC POWER

Electric power is the product of two quantities: current and voltage. These two quantities can vary with respect to time (AC power) or can be kept at constant levels (DC power).Most refrigerators, air conditioners, pumps and industrial machinery use AC power whereas most computers and digital equipment use DC power.

AC power has the advantage of being easy to transform between voltages and is able to be generated and utilised by brushless machinery. DC power remains the only practical choice in digital systems and can be more economical to transmit over long distances at very high voltages (see HVDC)

The ability to easily transform the voltage of AC power is important for two reasons: Firstly, power can be transmitted over long distances with less loss at higher voltages. So in power systems where generation is distant from the load, it is desirable to step-up (increase) the voltage of power at the generation point and then step-down (decrease) the voltage near the load. Secondly, it is often more economical to install turbines that produce higher voltages than would be used by most appliances.

http://en.wikipedia.org/wiki/Electric_current

http://en.wikipedia.org/wiki/Electric_potential

http://en.wikipedia.org/wiki/AC_power

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/HVDC

STRUCTURE OF POWER SYSTEM

COMPONENTS OF POWER SYSTEM Single Line Diagram: “A single line diagram is diagrammatic of power system in which the components are

represented by their symbols and the interconnection between them is shown by straight lines”.

ELEMENTS OF POWER SYSTEM

Power transformers: Power transformers are used generation and transmission network for stepping-up the

voltage at generating station and stepping-down the voltage for distribution. Auxiliary transformers supply power to auxiliary equipments at the substations.

Current transformers (CT): The lines in substations carry currents in the order of thousands of amperes. The

measuring instruments are designed for low value of currents. Current transformers are connected in lines to supply measuring instruments and protective relays.

Potential transformers (PT): The lines in substations operate at high voltages. The measuring instruments are designed

for low value of voltages. Potential transformers are connected in lines to supply measuring instruments and protective relays. These transformers make the low voltage instruments suitable for measurement of high voltages. For example a 11kV/110V PT is connected to a power line and the line voltage is 11kV then the secondary voltage will be 110V.

Circuit breaker (CB): Circuit breakers are used for opening or closing a circuit under normal as well as abnormal

(faulty) conditions. Different types of CBs which are generally used are oil circuit breaker, air-blast circuit breaker, and vacuum circuit breaker and SF6 circuit breaker.

Isolators or Isolating switches: Isolators are employed in substations to isolate a part of the system for general

maintenance. Isolator switches are operated only under no load condition.

GENERATION OF ELECTRICAL ENERGY Three most widely found generating stations – thermal, hydel and nuclear plants in our

country and elsewhere.

THERMAL POWER PLANT The theory of thermal power station or working of thermal power station is very simple.

A power generation plant mainly consists of alternator runs with help of steam turbine. The steam is obtained from high pressure boilers. Generally in India, bituminous coal, brown coal and peat are used as fuel of boiler.

For better understanding we furnish every step of function of a thermal power station as follows,

1. First the pulverized coal is burnt into the furnace of steam boiler. 2. High pressure steam is produced in the boiler. 3. This steam is then passed through the super heater, where it further

heated up. 4. This supper heated steam is then entered into a turbine at high speed. 5. In turbine this steam force rotates the turbine blades that means here in

the turbine the stored potential energy of the high pressured steam is converted into mechanical energy.

1. After rotating the turbine blades, the steam has lost its high pressure,

passes out of turbine blades and enters into a condenser. 2. In the condenser the cold water is circulated with help of pump which

condenses the low pressure wet steam. 3. This condensed water is then further supplied to low pressure water

heater where the low pressure steam increases the temperature of this feed water, it is then again heated in a high pressure heater where the high pressure of steam is used for heating.

4. The turbine in thermal power station acts as a prime mover of the alternator.

https://www.electrical4u.com/alternator-or-synchronous-generator/

https://www.electrical4u.com/steam-boiler-working-principle-and-types-of-boiler/

https://www.electrical4u.com/steam-boiler-furnace-grate-firebox-combustion-chamber-of-furnace/


A typical Thermal Power Station Operates on a Cycle which is shown below.

The working fluid is water and steam. This is called feed water and steam cycle. The ideal Thermodynamic Cycle to which the operation of a Thermal Power Station closely resembles is the RANKINE CYCLE.

HYDEL POWER PLANT

In hydroelectric power station the kinetic energy developed due to gravity in a falling water from higher to lower head is utilised to rotate a turbine to produce electricity. The potential energy stored in the water at upper water level will release as kinetic energy when it falls to the lower water level. This turbine rotates when the following water strikes the turbine blades. To achieve a head difference of water hydroelectric electric power station are generally constructed in hilly areas.

https://www.electrical4u.com/rankine-cycle/

There are only six primary components required to construct a hydroelectric power plant. These are dam, pressure tunnel, surge tank, valve house, penstock, and powerhouse.

1. The dam is an artificial concrete barrier constructed across the way of the river. The catchment area behind the dam creates a huge water reservoir.

2. The pressure tunnel takes water from the dam to the valve house. 3. In the valve house, there are two types of valves available. The first one is main

sluicing valve and the second one is an automatic isolating valve. The sluicing valves control the water flowing to the downstream and automatic isolating valves stop the water flow when the electrical load is suddenly thrown off from the plant.

4. The penstock is a steel pipeline of suitable diameter connected between the valve house and powerhouse. The water flows down from upper valve house to lower powerhouse through this penstock only.

5. In the powerhouse there are water turbines and alternators with associated step up transformers and switchgear systems to generate and then facilitate transmission of electricity.

6. At last, we will come to the surge tank. The surge tank is also a protective accessory associated with hydroelectric power plant. It is situated just before the valve house. The height of the tank must be greater than the head of the water stored in the water reservoir behind the dam. This is an open top water tank.

The purpose of this tank is to protect the penstock from bursting out when suddenly turbine refuses to take water

The main advantage of an electric power plant is that it does not require any fuel. It only requires water head which is naturally available after the construction of the

required dam. No fuel means no fuel cost, no combustion, no generation of flue gases, and no

pollution in the atmosphere. Due to the absence of fuel combustion, the hydroelectric power plant itself is very neat

and clean.

NUCLEAR POWER PLANT We can generate electrical power by means of nuclear power. In nuclear power station,

generates electrical power by nuclear reaction. Here, heavy radioactive elements such as Uranium (U235) or Thorium (Th232) are subjected to nuclear fission. This fission is done in a special apparatus called reactor.

The basic principle of a nuclear power station is the same as a conventional thermal power station. The only difference is that, instead of using heat generated due to coal combustion, here in a nuclear power plant, the heat generated due to nuclear fission.


https://www.electrical4u.com/step-up-transformer/

https://www.electrical4u.com/step-up-transformer/

https://www.electrical4u.com/power-plants-types-of-power-plant/

https://www.electrical4u.com/electric-power-single-and-three-phase/

https://www.electrical4u.com/thermal-power-generation-plant-or-thermal-power-station/

https://www.electrical4u.com/thermal-power-generation-plant-or-thermal-power-station/

A nuclear power station has mainly four components.

1. Nuclear reactor In a nuclear reactor, Uranium 235 is subjected to nuclear fission. It controls the

chain reaction that starts when the fission is done. The chain reaction must be controlled otherwise the rate of energy released will be fast, there may be a high chance of explosion.

The heat released during a nuclear reaction is carried to the heat exchanger by means of coolant consist of sodium metal.

2. Heat exchanger In a heat exchanger, the heat carried by sodium metal is dissipated in water and

water is converted to high-pressure steam here. After releasing heat in water the sodium metal coolant comes back to the reactor by means of a coolant circulating pump.

3. Steam turbine In a nuclear power plant, the steam turbine plays the same role as a coal power

plant. The steam drives the turbine in the same way. After doing its job, the exhaust steam comes into a steam condenser where it is condensed to provide space to the steam behind it.

4. Alternator An alternator, coupled with a turbine, rotates and generates electrical power, for

utilization. The output from the alternator is delivered to the bus-bars through a transformer, circuit breakers, and isolators.

https://www.electrical4u.com/steam/


Advantages of Nuclear Power Station

1. As we said, the fuel consumption in this power station is quite low and hence, the cost for generating a single unit of energy is quite less than other conventional power generation methods. The amount of nuclear fuel required is also less.

2. A nuclear power station occupies a much smaller space compared to other conventional power stations of the same capacity.

3. This station does not require plenty of water, hence it is not essential to construct plant near-natural sources of water. This also does not require a huge quantity of fuel; hence it is also not essential to construct the plant near a coal mine or the place where good transport facilities are available. Because of this, the nuclear power station can be established very near to the load center.

4. There are large deposits of nuclear fuel globally therefore such plants can ensure the continued supply of electrical energy for coming thousands of years.

Disadvantages of Nuclear Power Plant

1. The fuel is not easily available and it is very costly. 2. The initial cost of constructing a nuclear power station is quite high. 3. Erection and commissioning of this plant are much complicated and

sophisticated than other conventional power stations. 4. The fission by-products are radioactive in nature, and it may cause high

radioactive pollution. 5. The maintenance cost is higher and the manpower required to run a

nuclear power plant is quite higher since specialist trained people are required. 6. The sudden fluctuation of load cannot be met up efficiently by the

nuclear plants. 7. As the by-products of the nuclear reactions are highly radioactive, it is a

very big problem for the disposal of these by-products. It can only be disposed of deep inside the ground or in a sea away from the seashore.

SOLAR POWER PLANT

A solar power plant is any type of facility that converts sunlight either directly, like Photovoltaic, or indirectly, like Solar Thermal plants, into electricity.

Photovoltaics

Photovoltaic power plants use large areas of photovoltaic cells, known as PV or solar cells, to directly convert sunlight into usable electricity. These cells are usually made from silicon alloys and are the technology most people have become familiar with - chances are you may have one on your roof.

The panels themselves come in various forms:

- Crystalline solar panels - As the name suggests these types of panels are made from crystalline silicon. They can be either monocrystalline or poly- or multi-crystalline. As a rule of thumb monocrystalline versions are more efficient (about 15-20%) but more expensive than

https://www.dw.com/en/how-does-a-solar-power-plant-work/a-5073142

https://interestingengineering.com/egypt-will-soon-be-home-to-the-worlds-largest-solar-park

http://energyinformative.org/best-solar-panel-monocrystalline-polycrystalline-thin-film/

http://energyinformative.org/best-solar-panel-monocrystalline-polycrystalline-thin-film/

their alternatives (tend to be 13-16% efficient) but advancements are closing the gap between them over time.

- Thin-film solar panels - These types of panels consist of a series of films that absorb light in different parts of the EM spectrum. They tend to be made from amorphous silicon (aSi), cadmium telluride (CdTe), cadmium sulfide (CdS), and copper indium (gallium) diselenide.

Working Principle

Most solar PV panels are made from semiconductor materials, usually some form of silicon. When photons from sunlight hit the semiconductor material free electrons are generated which can then flow through the material to produce a direct electrical current. This is known as the photo-effect in physics. The DC current then needs to be converted to alternating current (AC) using an inverter before it can be directly used or fed into the electrical grid.

https://medium.com/@solar.dao/how-pv-solar-plants-work-a-beginners-guide-79f085b8ee88

https://energyeducation.ca/encyclopedia/Photovoltaic_effect

TRANSMISSION OF ELECTRIC POWER

MOST ECONOMICAL SIZE OF CONDUCTOR It has already been shown that maximum stress in a cable occurs at the surface of the

conductor. For safe working of the cable, dielectric strength of the insulation should be more than the maximums tress. Rewriting the expression for maximum stress, we get,

The values of working voltage V and internal sheath diameter D have to be kept fixed at certain values due to design considerations. This leaves conductor diameter d to be the only variable in exp.(i). For given values of V and D, the most economical conductor diameter will be one for which gmax has a minimum value. The value of gmax will be minimum when d loge D/d is maximum i.e.

Most economical conductor diameter is

and the value of gmax under this condition is

CORONA When an alternating potential difference is applied across two conductors whose spacing

is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called corona.

The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference.

The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and the radio noise.

Factors Affecting Corona The phenomenon of corona is affected by the physical state of the atmosphere as well as

by the conditions of the line. The following are the factors upon which corona depends: (i)Atmosphere As corona is formed due to ionisation of air surrounding the conductors, therefore, it is

affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than normal and as such corona occurs at much less voltage as compared with fair weather.

(ii) Conductor size. The corona effect depends upon the shape and conditions of the conductors. The rough

and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage. Thus a stranded conductor has irregular surface and hence gives rise to more corona that a solid conductor.

(iii) Spacing between conductors. If the spacing between the conductors is made very large as compared to their diameters,

there may not be any corona effect. It is because larger distance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.

(iv) Line voltage. The line voltage greatly affects corona. If it is low, there is no change in the condition of

air surrounding the conductors and hence no corona is formed. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

Important Terms The phenomenon of corona plays an important role in the design of an overhead

transmission line. Therefore, it is profitable to consider the following terms much used in the analysis of corona effects:

(i)Critical Disruptive Voltage It is the minimum phase-neutral voltage at which corona occurs. Consider two conductors

of radii r cm and spaced d cm apart. If V is the phase-neutral potential, then potential gradient at the conductor surface is given by:

In order that corona is formed, the value of g must be made equal to the breakdown strength of air. The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.) and is denoted by go. If Vc is the phase-neutral potential required under these conditions, then,

(ii) Visual critical voltage It is the minimum phase-neutral voltage at which corona glow appears all along the line

conductors. It has been seen that in case of parallel conductors, the corona glow does not begin at the disruptive voltage Vc but at a higher voltage Vv, called visual critical voltage. The phase-neutral effective value of visual critical voltage is given by the following empirical formula : where mV is another irregularity factor having a value of 1·0 for polished conductors and 0·72 to 0·82 for rough conductors.

(iii) Power loss due to corona Formation of corona is always accompanied by energy loss which is dissipated in the form

of light, heat, sound and chemical action. When disruptive voltage is exceeded, the power loss due to corona is given by:

Advantages and Disadvantages of Corona Corona has many advantages and disadvantages. In the correct design of a high voltage

overhead line, a balance should be struck between the advantages and disadvantages.

Advantages (i) Due to corona formation, the air surrounding the conductor becomes conducting

and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.

(ii) Corona reduces the effects of transients produced by surges.

Disadvantages (vii) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line. (viii) Ozone is produced by corona and may cause corrosion of the conductor due to

chemical action. (ix) The current drawn by the line due to corona is non-sinusoidal and hence no

sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighbouring communication lines.

Methods of Reducing Corona Effect It has been seen that intense corona effects are observed at a working voltage of 33 kV or

above. Therefore, careful design should be made to avoid corona on the sub-stations or bus-bars rated for 33 kV and higher voltages otherwise highly ionized air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment. The corona effects can be reduced by the following methods

(i)By increasing conductor size. By increasing conductor size, the voltage at which corona occurs is raised and hence

corona effects are considerably reduced. This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in transmission lines.

(ii) By increasing conductor spacing By increasing the spacing between conductors, the voltage at which corona occurs is

raised and hence corona effects can be eliminated. However, spacing cannot be increased too much otherwise the cost of supporting structure (e.g., bigger cross arms and supports) may increase to a considerable extent.

OVERHEAD LINE CONDUCTORS Commonly used conductor materials: The most commonly used conductor materials for over head lines are copper, aluminium,

steel cored aluminium, galvanised steel and cadmium copper. The choice of a particular material will depend upon the cost, the required electrical and mechanical properties and the local conditions. All conductors used for overhead lines are preferably stranded in order to increase the flexibility. In stranded conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires. Thus, if there are n layers, the total number of

individual wires is 3n(n + 1) + 1. In the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiralled in opposite directions so that layers are bound together.

TYPES OF CONDUCTOR

1.Copper Copper is an ideal material for overhead lines owing to its high electrical conductivity and

greater tensile strength. Copper has high current density i.e., the current carrying capacity of copper per unit of X-

sectional area is quite large. This leads to two advantages. Firstly, smaller X-sectional area of conductor is required and secondly, the area offered by the conductor to wind loads is reduced. However, due to its higher cost and non-availability, it is rarely used for these purposes. Nowadays the trend is to use aluminium in place of copper.

2. Aluminium Aluminium is cheap and light as compared to copper but it has much smaller conductivity

and tensile strength. The relative comparison of the two materials is briefed below: (i) The conductivity of aluminium is 60% that of copper. The smaller conductivity of

aluminium means that for any particular transmission efficiency, the X-sectional area of conductor must be larger in aluminium than in copper. For the same resistance, the diameter of aluminium conductor is about 1·26 times the diameter of copper conductor. The increased X-section of aluminium exposes a greater surface to wind pressure and, therefore, supporting towers must be designed for greater transverse strength. This often requires the use of higher towers with consequence of greater sag.

(ii) The specific gravity of aluminium (2·71 gm/cc) is lower than that of copper (8·9 gm/cc).Therefore, an aluminium conductor has almost one-half the weight of equivalent copper conductor. For this reason, the supporting structures for aluminium need not be made so strong as that of copper conductor.

(iii) Aluminium conductor being light, is liable to greater swings and hence larger cross-arms are required.

3. Steel cored aluminium Due to low tensile strength, aluminium conductors produce greater sag. This prohibits

their use for larger spans and makes them unsuitable for long distance transmission. In order to increase the tensile strength, the aluminium conductor is reinforced with a core of galvanised steel wires. The composite conductor thus obtained is known as steel cored aluminium and is abbreviated as A.C.S.R. (aluminium conductor steel reinforced).

Steel-cored aluminium conductor consists of central core of galvanized steel wires surrounded by a number of aluminium strands. Usually, diameter of both steel and aluminium wires is the same.

Advantages: (i) The reinforcement with steel increases the tensile strength but at the same time keeps

the composite conductor light. Therefore, steel cored aluminium conductors will produce smaller sag and hence longer spans can be used.

(ii) Due to smaller sag with steel cored aluminium conductors, towers of smaller heights can be used.

4. Galvanised steel Steel has very high tensile strength. Therefore, galvanised steel conductors can be used

for extremely long spans or for short line sections exposed to abnormally high stresses due to climatic conditions. They have been found very suitable in rural areas where cheapness is the main consideration. Due to poor conductivity and high resistance of steel, such conductors are not suitable for transmitting large power over a long distance. However, they can be used to advantage for transmitting a small power over a small distance where the size of the copper conductor desirable from economic considerations would be too small and thus unsuitable for use because of poor mechanical strength.

5. Cadmium copper The conductor material now being employed in certain cases is copper alloyed with

cadmium. An addition of 1% or 2% cadmium to copper increases the tensile strength by about 50% and the conductivity is only reduced by 15% below that of pure copper. Therefore, cadmium copper conductor can be useful for exceptionally long spans. However, due to high cost of cadmium, such conductors will be economical only for lines of small X-section i.e., where the cost of conductor material is comparatively small compared with the cost of supports.

INSULATOR Insulating Materials The main cause of failure of overhead line insulator, is flash over, occurs in between line

and earth during abnormal over voltage in the system. During this flash over, the huge heat produced by arcing, causes puncher in insulator body. Viewing this phenomenon the materials used for electrical insulator, has to posses some specific properties.

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Properties of insulating material 1. The materials generally used for insulating purpose is called insulating material.

For successful utilization, this material should have some specific properties as listed below-1.It must be mechanically strong enough to carry tension and weight of conductors.

2. It must have very high dielectric strength to withstand the voltage stresses in High Voltage system.

3. It must possess high Insulation Resistance to prevent leakage current to the earth.

4. There physical as well as electrical properties must be less affected by changing temperature

TYPES OF INSULATOR

There are mainly three types of insulator likewise

1. Pin Insulator 2. Suspension Insulator 3. Strain insulator 4. Shackle insulator 5. Stay Insulator

In addition to that there are other two types of electrical insulator available mainly for low voltage application, e.i. stay insulator and shackle insulator.

1. Pin Type Insulators

As the name suggests, the pin type insulator is secured to the cross-arm on the pole. There is a groove on the upper end of the insulator for housing the conductor. The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor. Pin type insulators are used for electric power at voltages up to 33 kV. Beyond operating voltage of 33 kV, the pin type insulators become too bulky and hence uneconomical.

Causes of Insulator Failures:

Insulators are required to withstand both mechanical and electrical stresses. The latter type is primarily due to line voltage and may cause the breakdown of the insulator. The electrical breakdown of the insulator can occur either by flash-over or puncture. In flashover, an arc occurs between the line conductor and insulator pin (i.e., earth) and the discharge jumps across the air gaps, following shortest distance.


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In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulator. In case of puncture, the discharge occurs from conductor to pin through the body of the insulator. When such breakdown is involved, the insulator is permanently destroyed due to excessive heat. In practice, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage. The ratio of puncture strength to flashover voltage is known as safety factor.

2. Suspension Type

For high voltages (>33 kV), it is a usual practice to use suspension type insulators shown in

Figure. Consist of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower. Each unit or disc is designed for low voltage, say 11 kV. The number of discs in series would obviously depend upon the working voltage. For instance, if the working voltage is 66 kV, then six discs in series will be provided on the string.

Advantages of suspension type: Suspension type insulators are cheaper than pin type insulators for voltages

beyond 33 kV. Each unit or disc of suspension type insulator is designed for low voltage, usually

11 kV. Depending upon the working voltage, the desired number of discs can be connected in series.

If anyone disc is damaged, the whole string does not become useless because the damaged disc can be replaced.

The suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.

In case of increased demand on the transmission line, it is found more satisfactory to supply the greater demand by raising the line voltage than to provide another set of conductors. The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs.

The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

3. Strain Insulators

When there is a dead end of the line or there is corner or sharp curve, the line is

subjected to greater tension. In order to relieve the line of excessive tension, strain insulators

are used. For low voltage lines (< 11 kV), shackle insulators are used as strain insulators.

However, for high voltage transmission lines, strain insulator consists of an assembly of

suspension insulators as shown in Figure. The discs of strain insulators are used in the vertical

plane. When the tension in lines is exceedingly high, at long river spans, two or more strings are

used in parallel.

4. Shackle Insulators

In early days, the shackle insulators were used as strain insulators. But now a day, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.

5. Stay Insulator

For low voltage lines, the stays are to be insulated from ground at a height. The insulator used in the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the guy-wire will not fall to the ground. There are


several methods of increasing the string efficiency or improving voltage distribution across different units of a string.

Mechanical design of transmission line – sag and tension calculations for different weather conditions, Tower spotting, Types of towers, Substation Layout (AIS, GIS), Methods of grounding.

MECHANICAL DESIGN OF TRANSMISSION LINE

SAG IN OVERHEAD LINES While erecting an overhead line, it is very important that conductors are under safe

tension. If the conductors are too much stretched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension.

In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag. The difference in level between points of supports and the lowest point on the conductor is called sag.

Following Fig. shows a conductor suspended between two equal level supports A and B. The conductor is not fully stretched but is allowed to have a dip. The lowest point on the conductor is O and the sag is S.

The following points may be noted

(i) When the conductor is suspended between two supports at the same level, it takes the shape of catenary. However, if the sag is very small compared with the span, then sag-span curve is like a parabola.

(ii) The tension at any point on the conductor acts tangentially. Thus tension TO at the lowest Point O acts horizontally as shown in Fig. (ii).

(iii) The horizontal component of tension is constant throughout the length of the wire.

(iv) The tension at supports is approximately equal to the horizontal tension

acting at any point on the wire. Thus if T is the tension at the support B, then T = TO

Conductor Sag and Tension

This is an important consideration in the mechanical design of overhead lines. The conductor sag should be kept to a minimum in order to reduce the conductor material required and to avoid extra pole height for sufficient clearance above ground level. It is also desirable that tension in the conductor should be low to avoid the mechanical failure of conductor and to permit the use of less strong supports.

CALCULATION OF SAG In an overhead line, the sag should be so adjusted that tension in the conductors is within

safe limits. The tension is governed by conductor weight, effects of wind, ice loading and temperature variations. It is a standard practice to keep conductor tension less than 50% of its ultimate tensile strength i.e., minimum factor of safety in respect of conductor tension should be 2.

We shall now calculate sag and tension of a conductor when ( i ) supports are at equal levels and ( ii ) supports are at unequal levels.

(i) When supports are at equal levels Consider a conductor between two equilevel supports A and B with O as the lowest

point as shown in Fig.8.2. It can be proved that lowest point will be at the mid-span.

It can be proved that lowest point will be at the mid-span. Let l = Length of span w = Weight per unit length of conductor T = Tension in the conductor. Consider a point P on the conductor. Taking the lowest point O as the origin, let the co-

ordinates of point P be x and y. Assuming that the curvature is so small that curved length is equal to its horizontal projection ( i.e., OP = x ), the two forces acting on the portion OP of the conductor are :

(a) The weight wx of conductor acting at a distance x/2 from O. (b) The tension T acting at O . Equating the moments of above two forces about point O, we get,

( ii ) When supports are at unequal levels In hilly areas, we generally come across conductors suspended between supports at

unequal levels. Fig.3 shows a conductor suspended between two supports A and B which are at different levels. The lowest point on the conductor is O.

Let,

l = Span length h = Difference in levels between two supports x 1 = Distance of support at lower level (A to O) x 2 = Distance of support at higher level (O to B) T = Tension in the conductor

EFFECT OF WIND AND ICE LOADING

The above formulae for sag are true only in still air and at normal temperature when the

conductor is acted by its weight only. However, in actual practice, a conductor may have ice coating and simultaneously subjected to wind pressure. The weight of ice acts vertically

downwards i.e., in the same direction as the weight of conductor. The force due to the wind is assumed to act horizontally i.e., at right angle to the projected surface of the conductor.

Hence, the total force on the conductor is the vector sum of horizontal and vertical forces as shown in below.

Total weight of conductor per unit length is

Where W = weight of conductor per unit length = conductor material density ⋅ volume per unit length

Wi = weight of ice per unit length = density of ice * volume of ice per unit length

ww = wind force per unit length

= wind pressure per unit area projected area per unit length

When the conductor has wind and ice loading also, the following points may be noted:

i)The conductor sets itself in a plane at an angle to the vertical where

ii)The sag in the conductor is given by

Hence S represents the slant sag in a direction making an angle to the vertical. If no specific mention is made in the problem, then slant slag is calculated by using the above formula.

iii)The vertical sag =

MODELLING AND PERFORMANCE OF TRANSMISSION LINES

Classification of lines - short line, medium line and long line - equivalent circuits, phasor diagram, attenuation constant, phase constant, surge impedance; transmission efficiency and voltage regulation, real and reactive power flow in lines, Power - circle diagrams, surge impedance loading, methods of voltage control; Ferranti effect.

CLASSIFICATION OF LINES - INTRODUCTION The important considerations in the design and operation of a transmission line are the

determination of voltage drop, line losses and efficiency of transmission. These values are greatly influenced by the line constants R, L and C of the transmission line.

In this chapter, we shall develop formulas by which we can calculate voltage regulation, line losses and efficiency of transmission lines.

Firstly, they provide an opportunity to understand the effects of the parameters of the line on bus voltages and the flow of power. Secondly, they help in developing an overall understanding of what is occurring on electric power system.

CLASSIFICATION OF OVERHEAD TRANSMISSION LINES A transmission line has three constants R, L and C distributed uniformly along the whole

length of the line. The resistance and inductance form the series impedance. The capacitance existing between conductors for 1-phase line or from a conductor to neutral for a 3-phase line forms a shunt path throughout the length of the line. Therefore, capacitance effects introduce complications in transmission line calculations.

Depending upon the manner in which capacitance is taken into account, the overhead transmission lines are classified as :

( i) Short transmission lines. When the length of an overhead transmission line is upto about 50 km and the line voltage is comparatively low (< 20 kV), it is usually considered as a short transmission line. Due to smaller length and lower voltage, the capacitance effects are small and hence can be neglected. Therefore, while studying the performance of a short transmission line, only resistance and inductance of the line are taken into account.

( ii) Medium transmission lines. When the length of an overhead transmission line is about 50-150 km and the line voltage is moderately high (>20 kV < 100 kV), it is considered as a medium transmission line. Due to sufficient length and voltage of the line, the capacitance effects are taken into account. For purposes of calculations, the distributed capacitance of the line is divided and lumped in the form of condensers shunted across the line at one or more points.

( iii) Long transmission lines. When the length of an overhead transmission line is more than 150 km and line voltage is very high (> 100 kV), it is considered as a long transmission line. For the treatment of such a line, the line constants are considered uniformly distributed over the whole length of the line and rigorous methods are employed for solution.

It may be emphasised here that exact solution of any transmission line must consider the fact that the constants of the line are not lumped but are distributed uniformly throughout the length of the line.

However, reasonable accuracy can be obtained by considering these constants as lumped for short and medium transmission lines.

Important Terms

While studying the performance of a transmission line, it is desirable to determine its voltage regulation and transmission efficiency. We shall explain these two terms in turn.

( i) Voltage regulation. When a transmission line is carrying current, there is a voltage drop in

the line due to resistance and inductance of the line. The result is that receiving end voltage ( VR) of

the line is generally less than the sending end voltage ( VS ). This voltage drop ( Vs −V R) in the line is expressed as a percentage of receiving end voltage V and is called voltage regulation.

The difference in voltage at the receiving end of a transmission line between conditions of no load and full load is called voltage regulation and is expressed as a percentage of the receiving end voltage.

( ii) Transmission efficiency. The power obtained at the receiving end of a transmission line is generally less than the sending end power due to losses in the line resistance.

The ratio of receiving end power to the sending end power of a transmission line is known as the transmission efficiency of the line

PERFORMANCE OF SINGLE PHASE SHORT TRANSMISSION LINES As stated earlier, the effects of line capacitance are neglected for a short transmission

line. Therefore, while studying the performance of such a line, only resistance and inductance of the line are taken into account. The equivalent circuit of a single phase short transmission line is shown in Fig.

Here, the total line resistance and inductance are shown as concentrated or lumped instead of being distributed. The circuit is a simple a.c. series circuit.

Let, I = load current R = loop resistance i.e., resistance of both conductors

XL= loop reactance

VR = receiving end voltage cos φR = receiving end power factor (lagging) VS= sending end voltage cos φS= sending end power factor

The phasor diagram of the line for lagging load power factor is shown in Fig. From the

right angled triangle ODC, we get,

An approximate expression for the sending end voltage Vs can be obtained as follows.

Draw S perpendicular from B and C on OA produced as shown in Fig. Then OC is nearly equal to OF

OC=OF=OA+AF=OA+AG+GF

=OA+AG+BH

Vs= VR + IR cos φR + I XL sin φR

THREE-PHASE SHORT TRANSMISSION LINES

For reasons associated with economy, transmission of electric power is done by 3-phase system. This system may be regarded as consisting of three single phase units, each wire transmitting one-third of the total power. As a matter of convenience, we generally analyse 3-phase system by considering one phase only. Therefore, expression for regulation, efficiency etc. derived for a single phase line can also be applied to a 3-phase system.

Thus, Vs and VR are the phase voltages, whereas R and XL are the resistance S and inductive reactance per phase respectively.

Fig (i) shows a Y-connected generator supplying a balanced Y-connected load through a transmission line. Each conductor has a resistance of R Ω and inductive reactance of X Ω.

Fig. (ii) shows one phase separately. The calculations can now be made in the same way as for a single phase line.

Effect of Load PF on Regulation and Efficiency

The regulation and efficiency of a transmission line depend to a considerable extent upon the power factor of the load.

1. Effect on regulation. The expression for voltage regulation of a short transmission line is given by :

The following conclusions can be drawn from the above expressions:

( i) When the load p.f. is lagging or unity or such leading that I R cos φR > I XL sin φR , then

voltage regulation is positive i.e. , receiving end voltage VR will be less than the sending end

voltage VS .

( ii) For a given VR and I, the voltage regulation of the line increases with the decrease in p.f. for lagging loads.

( iii) When the load p.f. is leading to this extent that I XL sin φR > I cos φ R , then voltage

regulation is negative i.e. the receiving end voltage VR is more than the sending end voltage VS .

(iv) For a given VR and I, the voltage regulation of the line decreases with the decrease in p.f. for leading loads.

2. Effect on transmission efficiency. The power delivered to the load depends upon the power factor.

It is clear that in each case, for a given amount of power to be transmitted ( P) and

receiving end voltage Power Factor Meter ( V R), the load current I is inversely proportional to the load p.f..

Consequently, with the decrease in load p.f., the load current and hence the line losses are increased. This leads to the conclusion that transmission efficiency of a line decreases with the decrease in load Power Factor Regulator p.f. and vice-versa.

MEDIUM TRANSMISSION LINES In short transmission line calculations, the effects of the line capacitance are neglected

because such lines have smaller lengths and transmit power at relatively low voltages (< 20 kV). However, as the length and voltage of the line increase, the capacitance gradually becomes of greater importance.

Since medium transmission lines have sufficient length (50-150 km) and usually operate at voltages greater than 20 kV, the effects of capacitance cannot be neglected. Therefore, in order to obtain reasonable accuracy in medium transmission line calculations, the line capacitance must be taken into consideration.

The capacitance is uniformly distributed over the entire length of the line. However, in order to make the calculations simple, the line capacitance is assumed to be lumped or concentrated in the form of capacitors shunted across the line at one or more points. Such a treatment of localising the line capacitance gives reasonably accurate results. The most commonly used methods (known as localised capacitance methods) for the solution of medium transmissions lines are :

( i) End condenser method ( ii) Nominal T method ( iii) Nominal π method.

Although the above methods are used for obtaining the performance calculations of medium lines, they can also be used for short lines if their line capacitance is given in a particular problem.

i)End Condenser Method

In this method, the capacitance of the line is lumped or concentrated at the receiving or load end as shown in Fig.This method of localising the line capacitance at the load end overestimates the effects of capacitance. In Fig, one phase of the 3-phase transmission line is shown as it is more convenient to work in phase instead of line-to-line values.

Let

I R= load current per phase R = resistance per phase XL= inductive reactance per phase C = capacitance per phase

cos φR= receiving end power factor ( lagging) VS= sending end voltage per phase

The *phasor diagram for the circuit is shown in Fig Taking the receiving end voltage VR as the reference phasor,

The sending end current Is is the phasor sum of load current IR and capacitive current IC

i.e. ,

Thus, the magnitude of sending end voltage VS can be calculated.

Limitations Although end condenser method for the solution of medium lines is simple to work out

calculations, yet it has the following drawbacks : ( i) There is a considerable error (about 10%) in calculations because the distributed

capacitance has been assumed to be lumped or concentrated. ( ii) This method overestimates the effects of line capacitance.

ii)Nominal T Method In this method, the whole line capacitance is assumed to be concentrated at the middle

point of the line and half the line resistance and reactance are lumped on its either side as shown in Fig.Therefore, in this arrangement, full charging current flows over half the line. In Fig. one phase of 3-phase transmission line is shown as it is advantageous to work in phase instead of line-to-line values.

Let

IR = load current per phase ; R = resistance per phase

XL = inductive reactance per phase ; C = capacitance per phase

cos φR = receiving end power factor ( lagging) ; VS= sending end voltage/phase V1 = voltage across capacitor C

The *phasor diagram for the circuit is shown in Fig. Taking the receiving end voltage VR as the reference phasor, we have,

iii) Nominal π Method

In this method, capacitance of each conductor ( i.e., line to neutral) is divided into two halves; one half being lumped at the sending end and the other half at the receiving end as shown in Fig. It is obvious that capacitance at the sending end has no effect on the line drop. However, its charging current must be added to line current in order to obtain the total sending end current.

Let

IR = load current per phase R = resistance per phase

XL = inductive reactance per phase C = capacitance per phase

cos φR = receiving end power factor ( lagging) VS= sending end voltage per phase

The phasor diagram for the circuit is shown in Fig. Taking the receiving end voltage as the reference phasor, we have,

LONG TRANSMISSION LINES It is well known that line constants of the transmission line are uniformly distributed over

the entire length of the line. However, reasonable accuracy can be obtained in line calculations for short and medium lines by considering these constants as lumped. If such an assumption of lumped constants is applied to long transmission lines (having length excess of about 150 km), it is found that serious errors are introduced in the performance calculations. Therefore, in order to obtain fair degree of accuracy in the performance calculations of long lines, the line constants are considered as uniformly distributed throughout the length of the line. Rigorous mathematical treatment is required for the solution of such lines. Fig shows the equivalent circuit of a 3-phase long transmission line on a phase-neutral basis. The whole line length is divided into n sections, each section having line constants 1 /n th of those for the whole line. The following points may by noted :

( i) The line constants are uniformly distributed over the entire length of line as is actually the case.

( ii) The resistance and inductive reactance are the series elements. ( iii) The leakage suptance ( B) and leakage conductance ( G) are shunt elements. The

leakage suptance is due to the fact that capacitance exists between line and neutral. The leakage conductance takes into account the energy losses occurring through leakage over the insulators or due to corona effect between conductors.

Admittance ( iv) The leakage current through shunt admittance is maximum at the sending end of the

line and decreases continuously as the receiving end of the circuit is approached at which point its value is zero.

DISTRIBUTION SYSTEMS

DISTRIBUTION SYSTEM That part of power system which distributes electric power for local use is known as

distribution system. In general, the distribution system is the electrical system between the substation fed by the Transmission system and the consumer’s meters. It generally consists of feeders, distributors, and service mains.

(i) Feeders A feeder is a conductor which connects the sub-station (or localized generating station)

to the area where power is to be distributed. Generally, no tappings are taken from the feeder so that current in it remains the same throughout. The main consideration in the design of a feeder is the current carrying capacity.

(ii)Distributor A distributor is a conductor from which tappings are taken for supply to the consumers. In

Fig. AB, BC, CD and DA are the distributors. The current through a distributor is not constant because tappings are taken at various places along its length. While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the consumers’ terminals.

(iii) Service mains A service mains is generally a small cable which connects the distributor to the

consumers’ terminals.

REQUIREMENTS OF A DISTRIBUTION SYSTEM A considerable amount of effort is necessary to maintain an electric power supply within

the requirements of various types of consumers. Some of the requirements of a good distribution system are : proper voltage, availability of power on demand and reliability.

( i) Proper voltage. One important requirement of a distribution system is that voltage variations at

consumer’s terminals should be as low as possible. The changes in voltage are generally caused due to the variation of load on the system. Low voltage causes loss of revenue, inefficient lighting and possible burning out of motors.

The statutory limit of voltage variations is ± 6% of the rated value at the consumer’s terminals. Thus, if the declared voltage is 230 V, then the highest voltage of the consumer

should not exceed 244 V while the lowest voltage of the consumer should not be less than 216 V.

( ii) Availability of power on demand. Power must be available to the consumers in any amount that they may require from

time to time. This necessitates that operating staff must continuously study load patterns to predict in advance those major load changes that follow the known schedules.

( iii) Reliability. Modern industry is almost dependent on electric power for its operation. Homes and

office buildings are lighted, heated, cooled and ventilated by electric power. This calls for reliable service. Unfortunately, electric power, like everything else that is man-made, can never be absolutely reliable. However, the reliability can be improved to a considerable extent by ( a) interconnected system ( b) reliable automatic control system ( c) providing additional reserve facilities.

DESIGN CONSIDERATIONS IN DISTRIBUTION SYSTEM

Good voltage regulation of a distribution network is probably the most important factor responsible for delivering good service to the consumers. For this purpose, design of feeders and distributors requires careful consideration.

( i) Feeders. A feeder is designed from the point of view of its current carrying capacity while the

voltage drop consideration is relatively unimportant. It is because voltage drop in a feeder can be compensated by means of voltage regulating equipment at the substation.

( ii) Distributors. A distributor is designed from the point of view of the voltage drop in it. It is because a

distributor supplies power to the consumers and there is a statutory limit of voltage variations at the consumer’s terminals (± 6% of rated value). The size and length of the distributor should be such that voltage at the consumer’s terminals is within the permissible limits.

CLASSIFICATION OF DISTRIBUTION SYSTEMS

A distribution system may be classified according to ;

i)Nature of current According to nature of current, distribution system may be classified as

(a) d.c. Distribution system (b) a.c. Distribution system

Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and more economical than direct current method

ii) Type of construction According to type of construction distribution system may be classified as

(a) Overhead system (b) Underground system.

The overhead system is generally employed for distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the underground system is used at places where overhead construction is impracticable or prohibited by the local laws

(iii) Scheme of connection According to scheme of connection, the distribution system may be classified as

(a) Radial system (b) Ring main system (c) Inter-connected system

CONNECTION SCHEMES OF DISTRIBUTION SYSTEM All distribution of electrical energy is done by constant voltage system. In practice, the

following distribution circuits are generally used : ( i) Radial System.

In this system, separate feeders radiate from a single substation and feed the distributors at one end only. Fig. shows a single line diagram of a radial system for d.c. distribution where a feeder OC supplies a distributor A B at point A . Obviously, the distributor is fed at one end only i.e., point A is this case. Fig ( ii) shows a single line diagram of radial system for a.c. distribution. The radial system is employed only when power is generated at low voltage and the substation is located at the centre of the load.

This is the simplest distribution circuit and has the lowest initial cost. However, it suffers from the following drawbacks :

( a) The end of the distributor nearest to the feeding point will be heavily loaded. ( b) The consumers are dependent on a single feeder and single distributor. Therefore,

any fault on the feeder or distributor cuts off supply to the consumers who are on the side of the fault away from the substation.

( c) The consumers at the distant end of the distributor would be subjected to serious voltage fluctuations when the load on the distributor changes.

Due to these limitations, this system is used for short distances only. ( ii) Ring main system.

In this system, the primaries of distribution transformers form a loop. The loop circuit starts from the substation bus-bars, makes a loop through the area to be served, and returns to the substation. Figure below shows the single line diagram of ring main system for a.c. distribution where substation supplies to the closed feeder LMNOPQRS.

The distributors are tapped from different points M, O and Q of the feeder through distribution transformers. The ring main system has the following advantages :

( a) There are less voltage fluctuations at consumer’s terminals. ( b) The system is very reliable as each distributor is fed via *two feeders. In the event of

fault on any section of the feeder, the continuity of supply is maintained. For example, suppose that fault occurs at any point F of section SLM of the feeder. Then section SLM of the feeder can be isolated for repairs and at the same time continuity of supply is maintained to all the consumers via the feeder SRQPONM.

( iii) Interconnected system.

When the feeder ring is energised by two or more than two generating stations or substations, it is called inter-connected system. Figure below shows the single line diagram of interconnected system where the closed feeder ring ABCD is supplied by two substations S and S at points D and C respectively.

Distributors are connected to points O, P, Q and R of the feeder ring through distribution transformers. The interconnected system has the following advantages :

( a) It increases the service reliability. ( b) Any area fed from one generating station during peak load hours can be fed from the

other generating station. This reduces reserve power capacity and increases efficiency of the system.

TYPES OF D.C. DISTRIBUTORS The most general method of classifying d.c. distributors is the way they are fed by the

feeders. On this basis, d.c. distributors are classified as: ( i) Distributor fed at one end ( ii) Distributor fed at both ends ( iii) Distributor fed at the centre ( iv) Ring distributor.

( i) Distributor fed at one end. In this type of feeding, the distributor is connected to the supply at one end and loads are

taken at different point along the length of the distributor.

Fig. shows the single line diagram of a d.c. distributor A B fed at the end A (also known as

singly fed distributor) and loads I1 , I2 and I3 tapped off at points C, D and E respectively.

The following points are worth noting in a singly fed distributor: ( a) The current in the various sections of the distributor away from feeding point goes on

decreasing. Thus current in section AC is more than the current in section CD and current in section CD is more than the current in section DE.

( b) The voltage across the loads away from the feeding point goes on decreasing. Thus in Fig. the minimum voltage occurs at the load point E.

( c) In case a fault occurs on any section of the distributor, the whole distributor will have to be disconnected from the supply mains. Therefore, continuity of supply is interrupted.

( ii) Distributor fed at both ends.

In this type of feeding, the distributor is connected to the supply mains at both ends and loads are tapped off at different points along the length of the distributor. The voltage at the feeding points may or may not be equal. Fig. shows a distributor A B fed at the ends A and B

and loads of I1 , I2 and I3 tapped off at points C, D and E respectively.

Here, the load voltage goes on decreasing as we move away from one feeding point say A

, reaches minimum value and then again starts rising and reaches maximum value when we

reach the other feeding point B. The minimum voltage occurs at some load point and is never

fixed. It is shifted with the variation of load on different sections of the distributor.

Advantages ( a) If a fault occurs on any feeding point of the distributor, the continuity of supply is

maintained from the other feeding point. ( b) In case of fault on any section of the distributor, the continuity of supply is

maintained from the other feeding point. ( c) The area of X-section required for a doubly fed distributor is much less than that of a

singly fed distributor.

( iii) Distributor fed at the centre. In this type of feeding, the centre of the distributor is connected to the supply mains as

shown in Fig. It is equivalent to two singly fed distributors, each distributor having a common feeding point and length equal to half of the total length.

( iv)Ring mains.

In this type, the distributor is in the form of a closed ring as shown in Figure below. It is equivalent to a straight distributor fed at both ends with equal voltages, the two ends being brought together to form a closed ring. The distributor ring may be fed at one or more than one point.

UNIFORMLY LOADED DISTRIBUTOR FED AT ONE END Fig shows the single line diagram of a 2-wire d.c. distributor A B fed at one end A and

loaded uniformly with i amperes per metre length. It means that at every 1 m length of the distributor, the load tapped is i amperes. Let l metres be the length of the distributor and r ohm be the resistance per metre run.

Consider a point C on the distributor at a distance x metres from the feeding point A as shown in Fig. Then current at point C is

= i l − i x amperes = i ( l − x) amperes Now, consider a small length dx near point C. Its resistance is r dx and the voltage drop

over length dx is d v = i ( l − x) r dx = i r ( l − x) dx Total voltage drop in the distributor upto point C is

The voltage drop upto point B ( i.e. over the whole distributor) can be obtained by putting x = l in the above expression.

∴ Voltage drop over the distributor AB

where i l = I, the total current entering at point A r l = R, the total resistance of the distributor

Thus, in a uniformly loaded distributor fed at one end, the total voltage drop is equal to that produced by the whole of the load assumed to be concentrated at the middle point.

AC DISTRIBUTION Now-a-days electrical energy is generated, transmitted and distributed in the form of

alternating current. One important reason for the widespread use of alternating current in preference to direct current is the fact that alternating voltage can be conveniently changed in magnitude by means of a transformer. Transformer has made it possible to transmit a.c. power at high voltage and utilise it at a safe potential. High transmission and distribution voltages have greatly reduced the current in the conductors and the resulting line losses.

The a.c. distribution system is classified into i. primary distribution system and ii. Secondary distribution system.

i) Primary distribution system. It is that part of a.c. distribution system which operates at voltages somewhat higher than

general utilization and handles large blocks of electrical energy than the average low-voltage consumer uses. The voltage used for primary distribution depends upon the amount of power to be conveyed and the distance of the substation required to be fed. The most commonly used primary distribution voltages are 11 kV, 6·6 kV and 3·3 kV.

Due to economic considerations, primary distribution is carried out by 3- phase, 3-wire system Fig. shows a typical primary distribution system Electric power from the generating station is transmitted at high voltage to the substation located in or near the city.

ii) Secondary distribution system It is that part of a.c. distribution system. The secondary distribution employs 400/230V, 3-

phase, 4wire system. Figure shows a typical secondary distribution system. The primary

distribution circuit delivers power to various substations, called distribution sub-stations. The

substations are situated near the consumers’ localities and contain step-down transformers. At

each distribution substation, the voltage is stepped down to 400V and power is delivered by 3-

phase, 4-wire a.c. system. The voltage between any two phases is 400V and between any phase

and neutralize 230V. The single phase domestic loads are connected between any one phase

and the neutral, whereas 3-phase 400V motor loads are connected across 3-phase lines

directly.

A.C. DISTRIBUTION CALCULATIONS

A.C. distribution calculations differ from those of d.c. distribution in the following respects:

( i) In case of d.c. system, the voltage drop is due to resistance alone. However, in a.c. system, the voltage drops are due to the combined effects of resistance, inductance and capacitance.

( ii) In a d.c. system, additions and subtractions of currents or voltages are done arithmetically but in case of a.c. system, these operations are done vectorially.

( iii) In an a.c. system, power factor (p.f.) has to be taken into account. Loads tapped off form the distributor are generally at different power factors. There are two ways of referring power factor viz

( a) It may be referred to supply or receiving end voltage which is regarded as the reference vector.

( b) It may be referred to the voltage at the load point itself. There are several ways of solving a.c. distribution problems. However, symbolic notation

method has been found to be most convenient for this purpose. In this method, voltages, currents and impedances are expressed in complex notation and the calculations are made exactly as in d.c. distribution.

METHODS OF SOLVING A.C. DISTRIBUTION PROBLEMS In a.c. distribution calculations, power factors of various load currents have to be

considered since currents in different sections of the distributor will be the vector sum of load currents and not the arithmetic sum. The power factors of load currents may be given ( i) w.r.t.

receiving or sending end voltage or ( ii) w.r.t. to load voltage itself. Each case shall be discussed separately.

( i) Power factors referred to receiving end voltage.

Consider an a.c. distributor A B with concentrated loads of I1 and I2 tapped off at C and B

as shown in Fig. Taking the receiving end voltage VB points as the reference vector, let points as

the reference vector, let lagging power factors at C and B be cos φ1 and cos φ 2 w.r.t. VB . Let R1

, X1 and R2 , X2 be the resistance and reactance of sections A C and CB of the distributor.

The vector diagram of the a.c. distributor under these conditions is shown in Fig. Here,

the receiving end voltage VB is taken as the reference vector. As power factors of loads are

given w.r.t. VB , therefore, I1 and I2 lag behind VB by φ1 and φ2 respectively.

( ii) Power factors referred to respective load voltages.

Suppose the power factors of loads in the previous Fig. are referred to their respective

load voltages. Then φ1 is the phase angle between Vc and I1 and φ2 is the phase angle

between VB and I2 . The vector diagram under these conditions is shown in Fig

EHVAC and HVDC TRANSMISSION SYSTEM

EHVAC ( Extra High Voltage Alternating Current) This decrease in load current results in following advantages:

As current gets reduced, size and volume of conductor required also reduces for transmitting the same amount of power.

Voltage drop in line (3IR) reduces and hence voltage regulation of the line is improved. Line losses (3I2R) gets reduced which results in the increase in transmission line

efficiency. Some other advantages of extra high voltage transmission are as under: Power handling capacity of the line increases as we increase the transmission voltage. It

is proportional to the square of operating voltage. The cost related to tower, insulators and different types of equipment are proportional to voltage rather than the square of voltage. Thus the net capital cost of transmission line decreases as voltage increases.

Therefore, a large power can be transmitted with high voltage transmission lines economically.

The total line cost of per MW per km decreases considerably. The operation of EHV AC system is simple, reliable and can be adopted easily. The lines can be easily tapped and extended. Disadvantages of High Voltage Transmission

The major problems in this system are as under.

Corona loss is a big problem at higher voltages. This may further increase in bad weather conditions.

It increases radio interference. The height of towers and insulation increases with increase in transmission voltage. The cost of different types of equipment and switchgear required for transmission

increases with increase in transmission voltage. The high voltage lines produce electrostatic effects which are injurious to human beings

and animals.

HVDC

Merits of HVDC Undersea cables, where high capacitance causes additional AC losses. (e.g., 250 km Baltic

Cable between Sweden and Germany), Endpoint-to-endpoint long-haul bulk power transmission without intermediate.

Increasing the capacity of an existing power grid in situations where additional wires

are difficult or expensive to install Power transmission and stabilization between unsynchronized AC distribution systems.

Connecting a remote generating plant to the distribution grid, for example Nelson

River Bipoler.

https://www.yourelectricalguide.com/2017/07/transmission-line-performance.html



https://www.yourelectricalguide.com/2017/07/corona-effect-in-electrical-transmission-lines.html

Stabilizing a predominantly AC power-grid, without increasing prospective short circuit

current. Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple

phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect. Facilitate power transmission between different countries that use AC at differing

voltages and/or frequencies. Synchronize AC produced by renewable energy sources.

Demerits Circuit breaking Is difficult in D.C circuits, therefore the coast of dc circuit is high.

D.C system does not have step up or step down transformers to change the

voltage level. The coast of converter station is very high. Both ac and dc harmonics are generated. System control stability is quite difficult.

UNDERGROUND CABLE

Electric power can be transmitted or distributed either by overhead system or by underground cables. The underground cables have several advantages such as less liable to damage through storms or lightning, low maintenance cost, less chance of faults, smaller voltage drop and better general appearance. However, their major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with the equivalent overhead system. For this reason, underground cables are employed where it is impracticable to use overhead lines.

UNDERGROUND CABLES An underground cable essentially consists of one or more conductors covered with

suitable insulation and surrounded by a protecting cover. Although several types of cables are available, the type of cable to be used will depend upon the working voltage and service requirements. In general, a cable must fulfil the following necessary requirements:

(i) The conductor used in cables should be tinned stranded copper or aluminium of high conductivity. Stranding is done so that conductor may become flexible and carry more current.

(ii) The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.

(iii) The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.

(iv) The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.

(v) The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.

CONSTRUCTION OF CABLES

Fig shows the general construction of a 3-conductor cable. The various parts are

a)Cores or Conductors A cable may have one or more than one core (conductor) depending upon the type of

service for which it is intended. For instance, the 3- conductor cable shown in Fig. is used for 3-phase service. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable.

b) Insulation Each core or conductor is provided with a suitable thickness of insulation, the thickness of

layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound.

c)Metallic sheath. In order to protect the cable from moisture, gases or other damaging liquids (acids or

alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation as shown in Fig.

d) Bedding. Over the metallic sheath is applied a layer of bedding which consists of a fibrous material

like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

e) Armouring. Over the bedding, armouring is provided which consists of one or two layers of galvanized

steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be done in the case of some cables.

f) Serving. In order to protect armouring from atmospheric conditions, a layer of fibrous material

(like jute) similar to bedding is provided over the armouring. This is known as serving. It may not be out of place to mention here that bedding, armouring and serving are only

applied to the cables for the protection of conductor insulation and to protect the metallic sheath from Mechanical injury.

INSULATING MATERIALS FOR CABLES The satisfactory operation of a cable depends to a great extent upon the characteristics of

insulation used. Therefore, the proper choice of insulating material for cables is of considerable importance. In general, the insulating materials used in cables should have the following

Properties (i) High insulation resistance to avoid leakage current. (ii) High dielectric strength to avoid electrical breakdown of the cable. (iii) High mechanical strength to withstand the mechanical handling of cables. (iv) Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture

tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like lead sheath.

(v) Non-inflammable. (vi) Low cost so as to make the underground system a viable proposition. (vii) Unaffected by acids and alkalis to avoid any chemical action. No one insulating

material possesses all the above mentioned properties. Therefore, the type of insulating material to be used depends upon the purpose for which the cable is required and the quality of insulation to be aimed at..

Rubber Rubber may be obtained from milky sap of tropical trees or it may be produced from oil

products. It has relative permittivity varying between 2 and 3, dielectric strength is about 30 kV/mm and resistivity of insulation is 1017 cm. Although pure rubber has reasonably high insulating properties, it suffers form some major drawbacks viz., readily absorbs moisture, maximum safe temperature is low (about 38ºC), soft and liable to damage due to rough handling and ages when exposed to light. Therefore, pure rubber cannot be used as an insulating material.

Vulcanised India Rubber (V.I.R.) It is prepared by mixing pure rubber with mineral matter such as zinc oxide, red lead etc.,

and 3 to 5% of sulphur. The compound so formed is rolled into thin sheets and cut into strips. The rubber compound is then applied to the conductor and is heated to a temperature of about 150ºC. The whole process is called vulcanisation and the product obtained is known as vulcanised India rubber. Vulcanised India rubber has greater mechanical strength, durability and wear resistant property than pure rubber. Its main drawback is that sulphur reacts very quickly with copper and for this reason, cables using VIR insulation have tinned copper conductor. The VIR insulation is generally used for low and moderate voltage cables.

Impregnated paper It consists of chemically pulped paper made from wood chippings and impregnated with

some compound such as paraffinic or naphthenic material. This type of insulation has almost superseded the rubber insulation. It is because it has the advantages of low cost, low capacitance, high dielectric strength and high insulation resistance. The only disadvantage is that paper is hygroscopic and even if it is impregnated with suitable compound, it absorbs moisture and thus lowers the insulation resistance of the cable. For this reason, paper insulated cables are always provided with some protective covering and are never left unsealed. If it is required to be left unused on the site during laying, its ends are temporarily covered with wax or tar. Since the paper insulated cables have the tendency to absorb moisture, they are used where the cable route has a few joints. For instance, they can be profitably used for distribution at low voltages in congested areas where the joints are generally provided only at the terminal apparatus. However, for smaller installations, where the lengths are small and joints are required at a number of places, VIR cables will be cheaper and durable than paper insulated cables.

Varnished cambric It is a cotton cloth impregnated and coated with varnish. This type of insulation is also

known as empire tape. The cambric is lapped on to the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent. As the varnished cambric is hygroscopic, therefore, such cables are always provided with metallic sheath. Its dielectric strength is about 4 kV/mm and permittivity is 2.5 to 3.8.

Polyvinyl chloride (PVC) This insulating material is a synthetic compound. It is obtained from the polymerization of

acetylene and is in the form of white powder. For obtaining this material as a cable insulation, it is compounded with certain materials known as plasticizers which are liquids with high boiling point. The plasticizer forms a gell and renders the material plastic over the desired range of temperature. Polyvinyl chloride has high insulation resistance, good dielectric strength and mechanical toughness over a wide range of temperatures. It is inert to oxygen and almost inert to many alkalies and acids. Therefore, this type of insulation is preferred over VIR in extreme environmental conditions such as in cement factory or chemical factory. As the mechanical properties (i.e., elasticity etc.) of PVC are not so good as those of rubber, therefore, PVC insulated cables are generally used for low and medium domestic lights and power installations.

CLASSIFICATION OF CABLES Cables for underground service may be classified in two ways according to (i) the type of insulating material used in their manufacture (ii) the voltage for which they are manufactured. However, the latter method of

classification is generally preferred, according to which cables can be divided into the following groups:

Low-tension (L.T.) cables — upto 1000 V High-tension (H.T.) cables — upto 11,000 V Super-tension (S.T.) cables — from 22 kV to 33 kV Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV Extra super voltage cables — beyond 132 kV

A cable may have one or more than one core depending upon the type of service for

which it is intended. It may be (i) single-core (ii) (ii) two-core (iii) (iii) three-core (iv) (iv) four-core etc.

For a 3-phase service, either 3-single-core cables or three-core cable can be used depending upon the operating voltage and load demand. Fig. shows the constructional details of a single-core low tension cable. The cable has ordinary construction because the stresses developed in the cable for low voltages (up to 6600 V) are generally small. It consists of one circular core of tinned stranded copper (or aluminium) insulated by layers of impregnated paper. The insulation is surrounded by a lead sheath which prevents the entry of moisture into the inner parts. In order to protect the lead sheath from corrosion, an overall serving of compounded fibrous material (jute etc.) is provided. Single-core cables are not usually armoured in order to avoid excessive sheath losses. The principal advantages of single-core cables are simple construction and availability of larger copper section.

Cable For 3-Phase In practice, underground cables are generally required to deliver 3-phase power. For the

purpose, either three-core cable or three single core cables may be used. For voltages upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons. However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables areused. The following types of cables are generally used for 3-phase service :

1. Belted cables — upto 11 kV

2. Screened cables — from 22 kV to 66 kV 3. Pressure cables — beyond 66 kV.

1. Belted Cables These cables are used for voltages upto 11kV but in extraordinary cases, their use may be

extended upto 22kV. Fig.3 shows the constructional details of a 3-core belted cable. The cores are insulated from each other by layers of impregnated paper.

Another layer of impregnated paper tape, called paper belt is wound round the grouped insulated cores. The gap between the insulated cores is filled with fibrous insulating material (jute etc.) so as to give circular cross-section to the cable. The cores are generally stranded and may be of non circular shape to make better use of available space. The belt is covered with lead sheath to protect the cable against ingress of moisture and mechanical injury. The lead sheath is covered with one or more layers of armouring with an outer serving (not shown in the figure).The belted type construction is suitable only for low and medium voltages as the electro static stresses developed in the cables for these voltages are more or less radial i.e., across the insulation.

2.Screened Cables These cables are meant for use up to 33 kV, but in particular cases their use may be

extended to operating voltages up to 66 kV. Two principal types of screened cables are H-type cables and S.L. type cables.

(i)H-type Cables This type of cable was first designed by H. Hochstetler and hence the name. Fig. shows

the constructional details of a typical 3-core, H-type cable. Each core is insulated by layers of impregnated paper. The insulation on each core is covered with a metallic screen which usually consists of a perforated aluminum foil. The cores are laid in such a way that metallic screens

Make contact with one another. An additional conducting belt (copper woven fabric tape) is Wrapped round the three cores. The cable has no insulating belt but lead sheath, bedding, armouring and serving follow as usual. It is easy to see that each core screen is in electrical contact with the conducting belt and the lead sheath. As all the four screens (3 core screens and one conducting belt) and the lead sheath are at earth potential, therefore, the electrical stresses are purely radial and consequently dielectric losses are reduced. Two principal advantages are claimed for H-type cables. Firstly, the perforations in the metallic screens assist in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids (vacuous spaces) in the dielectric is eliminated. The voids if present tend to reduce the breakdown strength of the cable and may cause considerable damage to the paper insulation. Secondly, the metallic screens increase the heat dissipating power of the cable.

(Ii) S.L. Type cables

Fig. shows the constructional details of a 3-core S.L. (separate lead) type cable. It is basically H-type cable but the screen round each core insulation is covered by its own lead sheath. There is no overall lead sheath but only armouring and serving are provided. The S.L. type cables have two main advantages over H-type cables. Firstly, the separate sheaths minimize the possibility of core-to-core breakdown. Secondly, bending of cables becomes easy due to the elimination of overall lead sheath. However, the disadvantage is that the three lead sheaths of S.L. cable are much thinner than the single sheath of H-cable and, therefore, call for greater care in manufacture

3. Pressure cables For voltages beyond 66 kV, solid type cables are unreliable because there is a danger of

breakdown of insulation due to the presence of voids. When the operating voltages are greater than 66 kV, pressure cables are used. In such cables, voids are eliminated by increasing the pressure of compound and for this reason they are called pressure cables. Two types of pressure cables viz oil-filled cables and gas pressure cables are commonly used.

(i)Oil-filled cables. In such types of cables, channels or ducts are provided in the cable for oil circulation. The

oil under pressure (it is the same oil used for impregnation) is kept constantly supplied to the channel by means of external reservoirs placed at suitable distances (say 500 m) along the route of the cable. Oil under pressure compresses the layers of paper insulation and is forced in to any voids that may have formed between the layers. Due to the elimination of voids,

oil-filled cables can be used for higher voltages, the range being from 66 kV up to 230 kV. Oilfilled cables are of three types viz., single-core conductor channel, single-core sheath channel and three-core filler-space channels.

Fig. shows the constructional details of a single-core conductor channel, oil filled cable. The oil channel is formed at the center by stranding the conductor wire around a hollow cylindrical steel spiral tape. The oil under pressure is supplied to the channel by means of external reservoir. As the channel is made of spiral steel tape, it allows the oil to percolate between copper strands to the wrapped insulation. The oil pressure compresses the layers of paper insulation and prevents the possibility of void formation. The system is so designed that when the oil gets expanded due to increase in cable temperature, the extra oil collects in the reservoir. However, when the cable temperature falls during light load conditions, the oil from the reservoir flows to the channel. The disadvantage of this type of cable is that the channel is at the middle of the cable and is at full voltage w.r.t. earth, so that a very complicated system of joints is necessary. Fig. shows the constructional details of a single core sheath channel oil-filled cable. In this type of cable, the conductor is solid similar to that of solid cable and is paper insulated. However, oil ducts are provided in them etallic sheath as shown. In the 3-core oil-filler cable shown in Fig. the oil ducts are located in the filler spaces. These channels are composed of perforated metalribbon tubing and are at earth potential.

(ii)Gas Pressure Cable The voltage required to set up ionization inside a void increases as the pressure is

increased. Therefore, if ordinary cable is subjected to a sufficiently high pressure, the ionization can be altogether eliminated. At the same time, the increased pressure produces radial compression which tends to close any voids. This is the underlying principle of gas pressure cables.

Fig Shows the section of external pressure cable designed by Hochstetler, Vogal and Bowden. The construction of the cable is similar to that of an ordinary solid type except that it is of triangular shape and thickness of lead sheath is 75% that of solid cable. The triangular section reduces the weight and gives low thermal resistance but the main reason for triangular shape is that the lead sheath acts as a pressure membrane. The sheath is protected by a thin metal tape. The cable is laid in a gas-tight steel pipe. The pipe is filled with dry nitrogen gas at 12 to 15 atmospheres. The gas pressure produces radial compression and closes the voids that may have formed between the layers of paper insulation. Such cables can carry more load current and operate at higher voltages than a normal cable. Moreover, maintenance cost is small and the nitrogen gas helps in quenching any flame. However, it has the disadvantage that the overall cost is very high.

Dielectric Stress In Cable

Under operating conditions, the insulation of a cable is subjected to electrostatic forces. This is known as dielectric stress. The dielectric stress at any point in a cable is in fact the potential gradient (or electric intensity) at that point. Consider a single core cable with core diameter d and a internal sheath diameter D. As proved in Art 8, the electric intensity at a point x metres from the centre of the cable is

By definition, electric intensity is equal to potential gradient. Therefore, potential gradient g at a point x meters from the Centre of cable is

As proved, potential difference V between conductor and sheath is

Substituting the value of Q from exp. (ii) in exp. (i), we get,

It is clear from exp. (iii) that potential gradient varies inversely as the distance x. Therefore, potential gradient will be maximum when x is minimum i.e., when x = d/2 or at the surface of the conductor. On the other hand, potential gradient will be minimum at x = D/2 or at sheath surface. Maximum potential gradient is

The variation of stress in the dielectric is shown in Fig.14. It is clear that dielectric stress is maximum at the conductor surface and its value goes on decreasing as we move away from the conductor. It may be noted that maximum stress is an important consideration in the design of a cable. For instance, if a cable is to be operated at such a voltage that maximum stress is 5 kV/mm, then the insulation used must have a dielectric strength of at least 5 kV/mm, otherwise breakdown of the cable will become inevitable.

ECONOMIC ASPECT

Power Factor

Power factor is a measurement defined as the ratio of real power to total power. In other

words, power factor measures the percentage of power that is being used for useful work.

Causes of Low Power Factor

Low power factor usually is caused by inductive loads, such as:

Electric motors

Transformers

Arc welders

HVAC systems

Molding equipment

Presses

High-intensity discharge lighting

Unlike resistive loads (i.e., incandescent lights, electric heaters, cooking ovens), which involve a more direct conversion to useful work in the form of heat energy, inductive loads operate off of the magnetic field that is created by reactive power.

Benefits of Improving Power Factor

There are many benefits to improving a low power factor, including:

A smaller utility bill.

An increase in electrical system capacity.

Fewer voltage fluctuations.

Correction of power factor

While low power factor can cause a significant increase in your plant expenses and a

decrease in your system’s efficiency, you can take several steps to help correct your power

factor, including:

Minimizing the operation of idling or lightly loaded inductive equipment,

particularly motors

Replacing defunct motors with energy-efficient ones, and operating these near

their rated capacity

Avoiding operating your equipment above its rated voltage

Installing capacitors to decrease the amount of reactive power used

Demand factor

The ratio of the maximum coincident demand of a system, or part of a system, to the total connected load of the system

Load factor

Load factor is defined as the ratio of the average load over a given period to the maximum demand (peak load) occurring in that period. In other words, the load factor is the ratio of energy consumed in a given period of the times of hours to the peak load which has occurred during that particular period.

Load Curve

Load curve or chronological curve is the graphical representation of load (in kW or MW) in proper time sequence and the time in hours. It shows the variation of load on the power station. When the load curve is plotted for 24 hours a day, then it is called daily load curve. If the one year is considered then, it is called annual load curve.

Diversity Factor

Diversity factor is defined as the ratio of the sum of the maximum demands of the various part of a system to the coincident maximum demand of the whole system. The maximum demands of the individual consumers of a group do not occur simultaneously. Thus, there is a diversity in the occurrence of the load.

Capacity Factor

The capacity factor is defined as the ratio of the total actual energy produced or supply over a definite period, to the energy that would have been produced if the plant (generating unit) had operated continuously at the maximum rating. The capacity factor mainly depends on the type of the fuel used in the circuit.

BASE LOAD AND PEAK LOAD

Base load is the minimum level of electricity demand required over a period of 24 hours. It is needed to provide power to components that keep running at all times (also referred as continuous load).

Peak load is the time of high demand. These peaking demands are often for only shorter durations. In mathematical terms, peak demand could be understood as the difference between the base demand and the highest demand.

Base Load Power plants

Plants that are running continuously over extended periods of time are said to be base load power plant.

Examples of base load power plants are:

1. Nuclear power plant

2. Coal power plant

3. Hydroelectric plant

4. Geothermal plant

5. Biogas plant

6. Biomass plant

7. Solar thermal with storage

8. Ocean thermal energy conversion

Peak Load Power plants

To cater the demand peaks, peak load power plants are used. They are started up whenever there is a spike in demand and stopped when the demand recedes.

Examples of gas load power plants are:

1. Gas plant

2. Solar power plants

3. Wind turbines

4. Diesel generators

ELECTRICITY TARIFF

Electricity Tariffs

The amount of money frame by the supplier for the supply of electrical energy to various types of consumers in known as an electricity tariff. In other words, the tariff is the methods of charging a consumer for consuming electric power. The tariff covers the total cost of producing and supplying electric energy plus a reasonable cost.

The electricity tariffs depends on the following factors

Type of load

Time at which load is required.

The power factor of the load.

The amount of energy used.

Objectives of an Electricity Tariff

1. Recovery of cost of producing electrical energy at the power station. 2. Recovery of cost on the capital investment in transmission and

distribution systems. 3. Recovery of cost of operation and maintenance of the supply of electrical

energy. For example, metering equipment, billing, etc. 4. A suitable profit on the capital investment.

Electricity Tariff Characteristics

1. Proper return. The tariff should be structured in such a way that it guarantees the proper return from each consumer. The total receipts from the consumers must be equal to the cost of producing and supplying electrical energy plus the reasonable profit.

2. Fairness. The tariff must be fair so that each and every consumer is satisfied with the cost of electrical energy. Thus, a consumer who consumes more electrical energy should be charged at a lower rate than a consumer who consumes little energy..

3. Simplicity. The tariff should be simple and consumer-friendly so that an ordinary consumer can easily understand.

4. Reasonable profit. The profit element in the tariff should be reasonable. An electric supply company is a public utility company and generally enjoys the benefits of a monopoly.

5. Attractive. The tariff should be attractive so that it can attract a large number of consumers to use electricity

Types of tariffs:

1. Flat demand rate tariff – The flat demand rate tariff is expressed by the equation C = Ax. In this type of tariff, the bill of the power consumption depends only on the maximum demand of the load. The generation of the bill is independent of the normal energy consumption.

This type of tariff is used on the street light, sign lighting, irrigation, etc., where the working hours of the equipment are unknown. The metering system is not used for calculating such type of tariffs.

3. Block meter rate tariff – In this type of tariff, the energy consumption is distinguished into blocks. The per unit tariff of the individual block is fixed. The price of the block is arranged in the decreasing order. The first block has the highest cost, and it goes on decreasing accordingly.

The price and the energy consumption are divided into three blocks. The first few units of energy at a certain rate, the next at a slightly lower rate and the remaining unit at a very lower rate.

3. Two-part tariff – In such type of tariff, the total bill is divided into two parts. The first one is the fixed charge and the second is the running charge. The fixed charge is because of the maximum demand and the second charge depends on the energy consumption by the load.

The factor A and B may be constant and vary according to some sliding.

4. Maximum demand tariff – This is also a two-part tariff.

The low power factor increases the KVA rating of the load.

The peak load and seasonal tariffs both are used for reducing the idle or standby capacity of the load.

SUBSTATION

Introduction The assembly of apparatus used to change some characteristic ( e.g. voltage, a.c. to d.c.,

frequency, p.f. etc. ) of electric supply is called a sub-station. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations. It is, therefore, essential to exercise utmost care while designing and building a sub-station.

The following are the important points which must be kept in view while laying out a sub-station :

( i ) It should be located at a proper site. As far as possible, it should be located at the centre of gravity of load.

( ii ) It should provide safe and reliable arrangement. For safety, consideration must be given to the maintenance of regulation clearances, facilities for carrying out repairs and maintenance, abnormal occurrences such as possibility of explosion or fire etc. For reliability, consideration must be given for good design and construction, the provision of suitable protective gear etc.

(iii) It should be easily operated and maintained. (iv) It should involve minimum capital cost.

Classification of Sub-Stations There are several ways of classifying sub-stations. However, the two most important ways

of classifying them are according to (1) service requirement and (2) constructional features.

1.According to service requirement A sub-station may be called upon to change voltage level or improve power factor or

convert a.c. power into d.c. power etc. According to the service requirement, sub-stations may be classified into :

i)Transformer sub-stations. Those sub-stations which change the voltage level of electric supply are called

transformer sub-stations. These sub-stations receive power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such sub- stations. Most of the sub-stations in the power system are of this type.

( ii ) Switching sub-stations These sub-stations do not change the voltage level i.e. incoming and outgoing lines have

the same voltage. However, they simply perform the switching operations of power lines. (iii) Power factor correction sub-stations. Those sub-stations which improve the power factor of the system are called power factor

correction sub-stations. Such sub-stations are generally located at the receiving end of transmission lines. These sub-stations generally use synchronous condensers as the power factor improvement equipment.

( iv ) Frequency changer sub-stations Those sub-stations which change the supply frequency are known as frequency changer

sub-stations. Such a frequency change may be required for industrial utilisation. ( v ) Converting sub-stations Those sub-stations which change a.c. power into d.c. power are called converting sub-

stations. These sub-stations receive a.c. power and convert it into d.c power with suitable apparatus to supply for such purposes as traction, electroplating, electric welding etc.

( vi ) Industrial sub-stations Those sub-stations which supply power to individual industrial concerns are known as

industrial sub-stations.

2. According to constructional features A sub-station has many components ( e.g. circuit breakers, switches, fuses, instruments

etc.) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-stations are classified as : ( i ) Indoor sub-station ( ii Outdoor sub-station ( iii ) Underground sub-station ( iv ) Pole-mounted sub-station

( i ) Indoor sub-stations For voltages upto 11 kV, the equipment of the sub-station is installed indoor because of

economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages upto 66 kV.

( ii ) Outdoor sub-stations For voltages beyond 66 kV, equipment is invariably installed outdoor. It is because for

such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.

(iii) Underground sub-stations In thickly populated areas, the space available for equipment and building is limited and

the cost of land is high. Under such situations, the sub-station is created underground.

(iv) Pole-mounted sub-stations This is an outdoor sub-station with equipment installed overhead on H-pole or 4-pole

structure. It is the cheapest form of sub-station for voltages not exceeding 11kV (or 33 kV in some cases). Electric power is almost distributed in localities through such sub- stations. For complete discussion on pole-mounted sub-station,

Substation Layout

1. LT substation

2. HT substation

3. EHV substation

ln 4th sem.pdf - Government Polytechnic Nabarangpur

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