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BASIC ELECTRICAL ENGINEERING
Short Answer Questions
1. What is meant by charge?Charge is an electrical property of
the atomic particles which matter consists. The charge of
anelectron is so small. Charge in motion represents current. The
unit of charge is coulomb.
2. What is meant by Current?
The flow of free electrons in a conductor is called current.
Unit is ampere (A).I = Q/t
3. What is meant by Voltage?
The poterntial difference between two points is called as
voltage. Unit is Vol
ts (V).V=W/Q ,
W=work done in joules &Q = charge in coulombs
4. State Ohm’s Law.
The potential difference across any two ends of a conductor is
directly proportional to the currentflowing between the two ends
provided the temperature of the conductor remains constant.
5. State Kirchoff’s Voltage Law KVL states that the
algebraic sum of voltages in a closed path is zero.
6. State Kirchoff’s current Law.
KCL states that the algebraic sum of currents in a node is
zero.
7. Give notes on Nodal Analysis.• KCL is used.
• No: of equations = n-1, n=no: of nodes
8. Give notes on Mesh Analysis.• KVL is used
• Here mesh currents are found.
9. Give short notes on resistor.It is a property of a substance
which opposes the flow of electrons. It is denoted by R and its
unit
is Ohm ( )
10. Distinguish between a Branch and a node of a circuit.A pair
of network which connects the various points of the network is
called branch . A point at
which two or more elements are joined together is called
node.
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11. Distinguish between a mesh and a loop of a circuit.A mesh is
a loop that does not contain other loops. All meshes are loop, but
all loops are not
meshes. A loop is any closed path of branches
12. Write down the formula for a star connected network is
converted into a delta
network?RA=( R1 R2)/( R1 +R2+ R3)RB=( R1 R3)/(
R1 +R2+ R3)
RC=( R2 R3)/( R1 +R2+ R3)
13. Write down the formula for a delta connected network is
converted into a star
network?
R1=( RARB+RBRC+RCRA)/RC
R2=( RARB+RBRC+RCRA)/RB R3=( RARB+RBRC+RCRA)/RA
14. Define line currents and phase currents?• The currents
flowing in the lines are called as line currents
• The currents flowing through phase are called phase
currents
15. Define line voltage and phase voltage?The voltage across one
phase and neutral is called line voltage & the voltage between
two lines
is called phase voltage
16. Give the phase value & Line value of a star connected
system.
17. Give the phase value and line valued of a delta connected
system.
18. What is the power equation for a star connected system?
19. What is the power equation for a delta connected system?
20. What is meant by Real power?Real power means the useful
power transfer from source to load. Unit is watts.
21. What is meant by apparent power?
Apparent power is the product of voltage and current and it is
not true power. Unit is VA
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22. What is reactive power?
If we consider the circuit as purely inductive the output power
is reactive power. Its unit is VAR
23. Define Instrument.
Instrument is defined as a device for determining the value or
magnitude of a quantity orvariable.
24. Mention the two main differences between an ammeter and a
voltmeter.Ammeter Voltmeter
It is a current measuring device It is a voltage measuring
deviceAlways connected in series with circuit Always connected in
parallel with circuit
The resistance is very small The resistance is very high
25. Give short notes on resistor.It is a property of a substance
which opposes the flow of electrons. It is denoted by R and its
unit
is Ohm ( )
26. What is control system?A system consists of a number of
components connected together to perform a specific function .
In a system when the output quantity is controlled by varying
the input quantity then the systemis called control system.
27. What are the two major types of control system?
The two major types of control system are open loop and closed
loop
28. .Define open loop control system.The control system in which
the output quantity has no effect upon the input quantity are
called
open loop control system. This means that the output is not
feedback to the input for correction.
29 .Define closed loop control system.The control system in
which the output has an effect upon the input quantity so as to
maintain the
desired output value are called closed loop control system
30. Mention the errors in Moving iron instruments.• Hysteresis
error
• Temperature error • Stray magnetic field
error
• Frequency error • Eddy current
error
31. Mention any two precautions to be taken while using an
Ammeter.
• It should never be connected across any source.
• The polarity must be observed correctly.
• First use the highest range and then decrease the voltage
range until the sufficient deflection isobtained.
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32. Give some applications of DC motor.Shunt : driving constant
speed, lathes, centrifugal pumps, machine tools, blowers and
fans,
reciprocating pumps
Series : electric locomotives, rapid transit systems, trolley
cars, cranes and hoists, conveyors
Compound : elevators, air compressors, rolling mills, heavy
planners
33. Define slip.S = (Ns – Nr)/Ns
Where,Ns = synchronous speed in rpm.
Nr = rotor speed in rpmS = Slip
34. Define synchronous speed.It is given by Ns = 120f / p
rpm.Where
Ns = synchronous speed,p = no. of stator poles,
f = supply frequency in Hz
35. Why a single phase induction motor does not self start?When
a single phase supply is fed to the single phase induction motor.
Its stator winding
produces a flux which only alternates along one space axis. It
is not a synchronously revolvingfield, as in the case of a 2 or
3phase stator winding, fed from 2 or 3 phase supply.
36. Is Induction motor runs with synchronous speed or not.
Induction motor never runs with synchronous speed. It will stop
if it tries to achieve synchronousspeed.
37. Define Form factor and Crest factor.
Form factor= RMS valueAverage Value
Crest(peak) factor=Maximum ValueRMS value
38.Which type of instrument is called as universal
instrument?
The moving iron instrument are known as universal instruments,
because these instruments canbe used for AC and DC.
39. What are the applications of MI instruments?
i) Used as multi-range ammeters and voltmeters.ii) Used as in
expensive indicators such as charging and discharging current
indicators in
automobiles.iii)Extensively used in industries for measurement
of AC voltage and current where errors of the
order of 5% to 10% are acceptable.
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40. What is meant by eddy current damping?
When the conductor moves in a magnetic field an emf is induced
in it and if a closed path isprovided ,a current flows known as
eddy current. This current intersect with the magnetic field to
produce an electromagnetic torque , which opposes the deflecting
torque.
41. .How is electrical power measured?i) Using Voltmeter-ammeter
method for DC circuits.
ii)Using Watt meters for AC circuits.
42. .What do you mean by compensation coil in a wattmeter?By
connecting a compensating coil in series with a pressure coil ,The
error caused by the
pressure coil flowing in the current coil can be
neutralized.
43. What are the three types of power used in an a.c circuit?i)
Real power or active power P=EI cos
ii) Reactive power Q=EI siniii) Apparent power,S=EI
44. Define average value.
The average value of an alternating current is that value of
steady direct current which transfersthe same charge as the
alternating current flowing for the same time.
45. Define RMS value.
The effective value of an alternating current is that value of
steady ,direct current which producesthe same heat as that produced
by the alternating current when passed which produces the same
heat as that produced by the alternating current when passed
through the same resistance for thesame interval of time.
46. Define reactive power.
The power consumed by a pure reactance (XL or Xc ) in
a a.c circuit is called reactive power. Theunit is VAR. Q=EIsin
.
47. What is the basic principle of a dc generator?
Basic principle of a dc generator is Faraday’s law of
electromagnetic induction.That is whenevera conductor is moved
in a magnetic field dynamically induced emf is produced in that
conductor.
48 .What is the purpose of interpoles in modern d.c machine?
In modern d.c machines commutating poles or interpoles are
provided to improve commutation.
49. What is the use of commutator and brush in a d.c machine?The
commutator converts the alternating emf into unidirectional or
direct emf. The brushes are
mainly used to collect current from the commutator.
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50. What is a d.c series motor?In a d.c series motor,the field
winding is connected in series with the armature.The field
winding
should have less number of turns of thick wire.
51. Why a series motor cannot be started without any load?
Series motor cannot be started without any load because under no
load condition the startingtorque is less and motor rotates at
dangerous speed and may be damaged.
52. What is meant by transformer?The transformer is a static
piece of apparatus by means of which electricalenergy is
transformed
from one circuit to another with desired change in voltage and
current , without any change inthe frequency.It works on the
principle of mutual induction.
53. What are the different types of single phase motor?i)Single
phase induction motorii)Single phase synchronous motor.
iii)Single phase series motor
54. What are the two types of rotors of an induction motor?i)
Squirrel cage rotor
ii)Slip ring or wound rotor
5 Marks Questions
1. A 3 φ 4 pole 50 hz induction motor runs at 1460 r.p.m. find
its % of slip. Solution
N s = 120f/p= 120*50/4
= 1500r.p.m.
Running speed of motorn= 1460r.p.m.Slip S=( N s – N)/
N s*100
=(1500-1460) x 100 / 1500= 2.667%
2. Explain the working principle of Transformer.
A Transformer is a device that transfers electrical energy from
one circuit to another by
electromagnetic induction (transformer action).
The electrical energy is always transferred without a
change in frequency, but mayinvolve changes in magnitudes of
voltage and current. Because a transformer works on
the principle of electromagnetic induction, it must be used with
an input source voltagethat varies in amplitude.
There are many types of power that fit this description;
for ease of explanation andunderstanding, transformer action will
be explained using an ac voltage as the input
source. The amount of power used by the load of an
electrical circuit is equal to the current in the
load times the voltage across the load, or P = EI. If, for
example, the load in an electrical
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circuit requires an input of 2 amperes at 10 volts (20 watts)
and the source is capable ofdelivering only 1 ampere at 20 volts,
the circuit could not normally be used with this
particular source.
However, if a transformer is connected between the source and
the load, the voltage can
be decreased (stepped down) to 10 volts and the current
increased (stepped up) to 2
amperes. Notice in the above case that the power remains
the same. That is, 20 volts times 1
ampere equals the same power as 10 volts times 2 amperes.
A Transformer consists of the following parts• A primary coil or
winding.
• A secondary coil or winding.• A core that supports the coils
or windings.
The primary winding is connected to a 50 hertz ac voltage
source. The magnetic field(flux) builds up (expands) and collapses
(contracts) about the primary winding.
The expanding and contracting magnetic field around the primary
winding cuts thesecondary winding and induces an alternating
voltage into the winding.
This voltage causes alternating current to flow through
the load.
The voltage may be stepped up or down depending on the
design of the primary and
secondary windings.
3. Calculate the amount of resistance (R) in a circuit, given
values of voltage (E) and
current (I):
The amount of resistance (R) offered by the lamp
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4. Calculate the amount of voltage supplied by a battery, given
values of current (I) and
resistance (R):
the amount of voltage provided by the battery
5. Calculate the electric power in the given circuit and discuss
the effect of increasing thebattery voltage.
The formula for determining the power in an electric circuit: by
multiplying the voltage in"volts" by the current in "amps" we
arrive at an answer in "watts." Let's apply this to the given
circuit.
In the above circuit, we know we have a battery voltage of 18
volts and a lamp resistance of 3 _.Using Ohm's Law to determine
current, we get:
Now that we know the current, we can take that value and
multiply it by the voltage to determine
power:
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6. What is meant by DEFLECTING TORQUE ?
The deflecting torque is produced by making use of one of
the magnetic, chemical,electrostatic and electromagnetic induction
effects of current or voltage and causes the
moving system of the instrument to move from its zero position
when the instrument is
connected in an electrical circuit to measure the electrical
quantity. The method of producing this torque depend upon the
type of instrument. In attracting the
type of instrument, this torque to equal toTd = 1/2 I
2 dL/dθ
Whereas in Pmmc instrumentsTd = Bilur
Where B - magnetic densityi - current flowing
l - length of coilu - number of turn
r - radius of coil
7. Find the voltage across each resistors in the following
circuit.
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8. The effective resistance of two resistors connected in series
is 100 . When connected in
parallel, then effective value in 24 ohm’s. Determine the
value of two resistorsSeries R1+R2=100 => R2 =100 - R1
R1R2/R1+R2 = 24
R1R2/100 = 24R1R2 =2400R1 (100-R1) = 2400
100 R1-R12-2400 = 0
R12-100 R1 + 2400 = 0
(R1-60)(R1-40) = 0Therefore R1 = 60; R1 = 40
When R1 = 60 ; R2 = 100 – 60 = 40
When R1 = 40 ; R2 = 100 - 40 = 60
9. Find the Req between two points A & B.
1/Req = ½+1/3+1/3 = 1.17 (Req = 1/1.17= 0.8547)
1/Req = 2+.85+4Req = 7.2
10. Explain about Kirchoffs voltage and current laws.
Kirchhoff’s Current Law The sum of current flowing towards a
function is equal to the current flowing away from it.
Consider a function formed by 6 conductors. The current in these
conductors are i1, i2, .i6.Some
of these currents are flowing towards a 8 other’s away from
A
According to Kirchhoff’s Law,
i1+i4+i5+i6 = i2+i3 (Flowing towards) (Flowing away
from A)
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Kirchhoff’s Voltage Law (II Law)
In a closed circuit, the sum of the potential drops is equal to
thesum of the potential resistance
ABCDA forms a closed circuit.From A B, We have a potential
drop of IR1.
From D A, We have a potential drop of V.Sum of potential
drops = IR1+IR2+IR3
Potential rise from D A =VIR1+IR2+IR3 = V
11. Explain the principle of operation of DC Motor.
When a current passes through a conductor, lines of
magnetic force (flux) are generatedaround the conductor.
The direction of the flux is dependent on the direction of the
current flow .
In terms of conventional current flow (positive to negative)
then, using your right handpoint your thumb in the direction of the
current flow and your fingers will wrap around
the conductor in the same direction of the flux lines.
On the side of the conductor where the lines of flux
oppose each other, the magnetic fieldwill be made weaker.
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On the side of the conductor where the lines of flux are
not opposing each other, themagnetic field will be made
stronger.
Because of the strong field on one side of the conductor
and a weak field or, the otherside, the conductor will be pushed
into the weaker field.
The armature is connected to the commutator which rides
along the brushes which are
connected to a DC power source. The current from the DC
power source flows from the positive lead, through the brush
labeled A1 through one commutator section, through the armature
coil, through the othercommutator section, through the brush
labeled A2 and back to the negative lead .
This current will generate lines of flux around the
armature and affect the lines of flux inthe air gap.
On the side of the coil where the lines of flux oppose
each other, the magnetic field willbe made weaker.
On the side of the coil where the lines of flux are riot
opposing each other, the magneticfield is made stronger.
Because of the strong field on one side of the coil and the weak
field on the other side,the coil will be pushed into the weaker
field and, because the armature coil is free torotate, it will
rotate.
The torque available at the motor shaft (turning effort)
is determined by the magneticforce (flux) acting on the armature
coil and the distance from the renter of rotation that
force is. The flux is determined by the current flowing
through the armature coil and strength of
the field magnets.
12. Explain the working principle of three phase induction
motor.
In three phase induction motor, the magnetic field
generated by the stator rotates in the accase.
Three electrical phases are introduced through terminals,
each phase energizing an
individual field pole.
When each phase reaches its maximum current, the magnetic field
at that pole reaches a
maximum value.
As the current decreases, so does the magnetic field.
Since each phase reaches itsmaximum at a different time within a
cycle of the current, that field pole whose magnetic
field is largest is constantly changing between the three poles,
with the effect that themagnetic field seen by the rotor is
rotating.
The speed of rotation of the magnetic field, known as the
synchronous speed, depends onthe frequency of the power supply and
the number of poles produced by the statorwinding.
For a standard 60 Hz supply, as used in the United States, the
maximum synchronousspeed is 3,600 rpm.
In the three phase induction motor, the windings on the
rotor are not connected to apower supply, but are essentially short
circuits.
The most common type of rotor winding, the squirrel cage
winding, bears a strong
resemblance to the running wheel used in cages for pet
gerbils.
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When the motor is initially switched on and the rotor is
stationary, the rotor conductorsexperience a changing magnetic
field sweeping by at the synchronous speed.
From Faraday's law, this situation results in the induction of
currents round the rotorwindings; the magnitude of this current
depends on the impedance of the rotor windings.
Since the conditions for motor action are now fulfilled, that
is, current carryingconductors are found in a magnetic field, the
rotor experiences a torque and starts to turn.
The rotor can never rotate at the synchronous speed
because there would be no relativemotion between the magnetic field
and the rotor windings and no current could be
induced.
The induction motor has a high starting torque.
13. Explain the working principle of single phase induction
motor.
Single phase induction motor has only one stator winding (main
winding) and operateswith a single-phase power supply.
In all single-phase induction motors, the rotor is the
squirrel cage type.
The single-phase induction motor is not
self-starting.
When the motor is connected to a single-phase power
supply, the main winding carries an
alternating current.
This current produces a pulsating magnetic field.
Due to induction, the rotor is energized. As the main magnetic
field is pulsating, thetorque necessary for the motor rotation is
not generated.
This will cause the rotor to vibrate, but not to
rotate.
Hence, the single phase induction motor is required to
have a starting mechanism that canprovide the starting kick for the
motor to rotate.
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The starting mechanism of the single-phase induction motor is
mainly an additional statorwinding (start/ auxiliary winding) as
shown in Figure.
The start winding can have a series capacitor and/or a
centrifugal switch.
When the supply voltage is applied, current in the main
winding lags the supply voltagedue to the main winding
impedance.
At the same time, current in the start winding leads/lags
the supply voltage depending onthe starting mechanism
impedance.
Interaction between magnetic fields generated by the main
winding and the startingmechanism generates a resultant magnetic
field rotating in one direction.
The motor starts rotating in the direction of the
resultant magnetic field.
Once the motor reaches about 75% of its rated speed, a
centrifugal switch disconnects thestart winding.
From this point on, the single-phase motor can maintain
sufficient torque to operate on itsown.
14. Explain the working principle of single phase Energy
Meter.
An electric meter or energy meter is a device that
measures the amount of electricalenergy supplied to or produced by
a residence, business or machine.
The most common type is a kilowatt hour meter.
When used in electricity retailing, the utilities record the
values measured by these metersto generate an invoice for the
electricity.
They may also record other variables including the time
when the electricity was used.
Modern electricity meters operate by continuously measuring the
instantaneous voltage(volts) and current (amperes) and finding the
product of these to give instantaneous
electrical power (watts) which is then integrated against time
to give energy used (joules,kilowatt-hours etc).
The meters fall into two basic categories,
electromechanical and electronic.
The energy meter operates by counting the revolutions of
an aluminium disc which ismade to rotate at a speed proportional to
the power.
The number of revolutions is thus proportional to the
energy usage.
It consumes a small amount of power, typically around 2
watts.
The metallic disc is acted upon by two coils.
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One coil is connected in such a way that it produces a
magnetic flux in proportion to thevoltage and the other produces a
magnetic flux in proportion to the current.
The field of the voltage coil is delayed by 90 degrees
using a lag coil.
This produces eddy currents in the disc and the effect is
such that a force is exerted on thedisc in proportion to the
product of the instantaneous current and voltage.
A permanent magnet exerts an opposing force proportional to the
speed of rotation of thedisc - this act as a brake which causes the
disc to stop spinning when power stops being
drawn rather than allowing it to spin faster and faster.
This causes the disc to rotate at a speed proportional to
the power being used.
The type of meter described above is used on a
single-phase AC supply.
Different phase configurations u se additional voltage
and current coils.
The aluminium disc is supported by a spindle which has a worm
gear which drives the
register. The register is a series of dials which record the
amount of energyused. The dials may be of the cyclometer
type, an odometer-like display that is easy to read
where for each dial a single digit is shown through a window in
the face of the meter, or
of the pointer type where a pointer indicates each digit.
It should be noted that with the dial pointer type,
adjacent pointers generally rotate inopposite directions due to the
gearing mechanism.
10 Marks Questions
1. DETERMINE THE EQUIVALENT RESISTANCE BETWEEN TERMINALS A &
B
SOLUTION:
50 & 12.5 ARE PARALLEL
50*12.5 / 50+12.5 = 10STEP – I
q g p
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STEP – II
20*30/20+30 = 12
STEP – III
60*20/60+20 = 15
STEP – I V
STEP – V
RAB = 50
q g p
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2. Find the mesh currents in the following network
Solution:
The solution of -1 amp for I2 means that our initially
assumed direction of current was incorrect.
In actuality, I2 is flowing in a counter-clockwise
direction at a value of (positive) 1 amp:
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3. Explain the working of a moving iron type instruments.
These instruments are widely used in laboratories and
switch board at commercialfrequencies because these are cheaper in
cost, robust in construction and can be
manufactured with required accuracy.
These are generally of two types:-1. The attraction type.
2. The repulsion type.
The attraction type instrument operate on the principle of
attraction of a single piece ofsoft iron into a magnetic field and
repulsion type instrument operate on the principle of
repulsion of two adjacent iron pieces magnified by the same
magnetic field.
Repulsion type instrument are more sensitive than
attraction type instrument as inrepulsion type instrument large
separating torque is developed by having two iron
element positional class together inside the field coil where
the magnetizing effect ismaximum.
In both type of these instruments, the current under
measurement is passed through a coilof wire.
This current carrying coil set up the necessary field
depending on the magnitude of thecurrent to be measured.
The coil may be of a few turns of very heavy conductor or
of many turns of fine wire.
The instrument to be used as an ammeter is provided with a coil
of few turns of thickwire in order to have low resistance and carry
large current and that to be used as a
voltammeter is provided with a coil of large number of turns of
wire in order to have highresistance and draw as small current as
possible.
4. Derive the expression for torque produced in moving iron
instrument.
Let L be the self inductance corresponding to a total angular
deflection of q radians and changein inductance be dL corresponding
to small change in deflection angel dq due to small change in
current. The change in energy of magnetic field,dw =
Td dθ
Since change in energy dE = workdone, dwTd dθ = ½ I
2dL
Td = ½ I2dL/dθ
where I is in amperes, L is in Henry and θ is in Radians.
Thus torque is proportional to the square of the instrument
current and to the rate of change ofinductance with deflection.
5. An energy meter revolves 10 revolutions of disc for unit of
energy. Find the number of
revolutions made by it during an hour when connected across when
connected 20A at 210V
and 0.8 power factor leading. If energy meter revolves 350
revolutions, find the % error.
Answer.
Energy consumed in one hour = VI cos φ / 1000 = 210 x 20 x
0.8 / 1000= 3.360 kwh.
The number of revolution the meter should make it is correct
:
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=3.360 x registration const in revolution per kwh= 3.360 x
100
= 336
Number of revolution actually made = 350
% error = (350-336) x 100 / 350% error = 0.1466 %
6. Explain how following torque are produced in pmmc instrument
and attracted type
moving iron instruments
1. Deflecting torque
2. Control torque
3. Damping torque
1. DEFLECTING TORQUE:- The deflecting torque is produced by
making use of one of themagnetic, chemical, electrostatic and
electromagnetic induction effects of current or voltage and
causes the moving system of the instrument to move from its zero
position when the instrumentis connected in an electrical circuit
to measure the electrical quantity. The method of producing
this torque depend upon the type of instrument. In attracting
the type of instrument, this torque toequal to
Td = 1/2 I2 dL/dθ
Whereas in Pmmc instrumentsTd = BilurWhere B - magnetic
density
i - current flowingl - length of coil
u - number of turnr - radius of coil
2. CONTROLLING TORQUE:- The magnitude of the movement to the
moving system would
be somewhat indefinite under the influence of deflecting torque
unless some controlling torqueexist. This torque opposes the
deflecting torque and increases with increase in deflection of
the
moving system without controlling system the irrespective
magnitude of current and moreover,once deflected it would not
return to its zero position on removing the current. In attraction
type
instrument it is produced by spring control and in PMMC too it
would be produced by springcontrol.
3. DAMPING TORQUE:- This torque is also necessary to avoid
oscillation of the moving
system about it's final deflected position owing to the inertia
of the moving parts and to bring themoving system to rest in it's
final deflected position quickly.
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8. Discuss the construction and working of an electro-dynamic
wattmeter with the help of
diagram?
Answer.
This type of instrument is similar in design and principle to
the dynamometer type ammeter andvoltammeter.
WORKING AND CONSTRUCTION:-
When the instrument of this type is used as wattmeter, the fixed
coil which is divided intotwo equal portions in order to provide
them uniform field , is employed as current coil
and moving coil is used as pressure coil.
The fixed coil which is divided into two equal portion in order
to provide them uniformfield, is employed as current coil and the
moving coil is used as pressure coil, i.e the fixed
coil carries the current proportional to the voltage across the
circuit.
A high non inductive resistance is connected in series
with the moving coil in order to
limit current. The magnetic field of the fixed and moving
coil react on one another causing the moving
coil to turn about it's axis.
The movement is controlled by hair springs which also
leads the current into and out ofthe moving element.
Damping is provided by light aluminium moving in an air
dash pot.
The pointer is fixed to the moving coil spindle and moves
over a suitable calibrated scale.
THEORY:-Let us be the supply voltage, i the load current and R
the load resistance of the moving coil
circuit.Current through fixed coil, if = i
Current through moving coil, im = V/Rdeflecting torque,
Td ∝ if im ∝ V/R
For a DC circuit the deflecting torque is thus proportional to
the power and for any circuit with
fluctuating torque. The instantaneous to the instantaneous
power.
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9. Compare merits and demerits of moving iron type instruments
and dynamometer type
instruments. Which one is superior why?
Answer.
1. TORQUE HEIGHT RATIO:- Dynamometer type instruments have equal
small torqueheight ratio.
2. FRICTION ERROR:- Dynamometer type instruments have
considerable friction error.
3. FRICTION LOSS:- Owing to heavy moving system,
dynamometer type instruments havemore friction losses.
4. COST AND SENSITIVITY TO OVERLOAD:- As a result of
measures to reduce thefrictional error, the dynamometer type
instruments are more sensitive to overloads and
mechanical impacts is in comparison to moving iron type
instruments.5. SENSTIVITY:- The sensitivity of dynamometer
instrument is typically very poor due to poor
deflecting torque.6. POWER CONSUMPTION:- Dynamometer type
instrument have comparatively higher
power consumption.7. EFFECT OF STRAY MAGNETIC FIELD:- There
is no effect of stray magnetic field on
moving iron type while dynamometer type are most sensitive
towards it.8. HYSTERISIS AND EDDY CURRENT ERRORS:- Dynamometer
type instruments are free
from these errors while moving iron have these errors.9. EFFECT
OF WAVE FORM:- Dynamometer type instruments are very useful for
accurate
measurement of runs voltage while frequency change serious
errors in AC measurement inmoving iron type instruments.
10. CALIBRATION:- Dynamometer type instruments have same
calibration for AC and DCmeasurements while moving iron type have a
difference between AC and DC calibration.
10 Wh h t i ll d lt t d t ? A i il i t t h
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10. Why shunt is usually used voltmeter and ammeter? A moving
coil instrument has a
resistance of 5 _ and gives full deflection of 100mv. Show how
the instrument may be used
to measure:-
1. voltage upto 50V
2. current upto 10A
Answer.Shunt is usually used in voltmeter and ammeter to extend
the range of voltmeter and ammeters.Rm = 5
Vm = 100mvIm = Vm /Rm = 100mv/5 = 20mA
1. For measuring voltage upto 50V.
Series resistance is used with the instrument whose resistance
is
R = V/Im - Rm = 50/(20 x 10-3
) - 5R = 2.5 x 10
-3 - 5
R = 2495
2. Such resistance of resistance Rf is used to be connected
Rf = Rm /[I/Im - 1]Rf = 5/[10/20
x 10
-3 -1] = 5 x 2/998
Rf = 0.01002004
11. Explain the principle of operation of attraction type moving
iron instruments and
explain how the controlling and damping forces are obtained?
Answer.
The earliest and simplest form of attraction moving iron
instruments uses a solenoid andmoving oval shaped soft iron pinoted
eccentrically.
To this iron a pointer is attached so that it may deflect
along with the moving iron over agraduate scale.
The iron is made of sheet metal specially shaped to give a scale
as nearby uniform aspossible.
The moving iron is drawn into field of solenoid when current
flows through it.
The movement of the iron always from weaker magnetic
field outside the coil into thestronger field inside the coil
regardless the direction of flow of current.
When the current to be measured is passed through the
solenoid, a magnetic field is setup inside the solenoid, which in
turn magnetises the iron.
Thus the iron is attached into the coil causing the spindle and
the pointer to rotate.
So much instruments normally have spring control and pneumatic
damping forces.
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13 Explain the method of temperature control in open loop and
closed loop systems
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13. Explain the method of temperature control in open loop and
closed loop systems.
Temperature controllers are needed in any situation
requiring a given temperature be kept
stable.
This can be in a situation where an object is required to
be heated, cooled or both and toremain at the target temperature
(set point), regardless of the changing environment
around it. There are two fundamental types of temperature
control; open loop and closed loop
control.
Open loop is the most basic form and applies continuous
heating/cooling with no regardfor the actual temperature
output.
It is analogous to the internal heating system in a car. On a
cold day, you may need toturn the heat on to full to warm the car
to 75°.
However, during warmer weather, the same setting would
leave the inside of the car
much warmer than the desired 75°.
Closed loop control is far more sophisticated than open
loop.
In a closed loop application, the output temperature is
constantly measured and adjusted
to maintain a constant output at the desired temperature.
Closed loop control is always conscious of the output signal and
will feed this back into
the control process.
Closed loop control is analogous to a car with internal
climate control. If you set the cartemperature to 75°, the climate
control will automatically adjust the heating (during colddays) or
cooling (during warm days) as required to maintain the target
temperature of 5°.
A temperature controller is a device used to hold a
desired temperature at a specified
value.
The simplest example of a temperature controller is a common
thermostat found inhomes.
For instance, a hot water heater uses a thermostat to
control the temperature of the waterand maintain it at a certain
commanded temperature.
Temperature controllers are also used in ovens.
When a temperature is set for an oven, a controller
monitors the actual temperature inside
of the oven.
If it falls below the set temperature, it sends a signal to
activate the heater to raise thetemperature back to the set
point.
Thermostats are also used in refrigerators So if the temperature
gets too high a controller
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Thermostats are also used in refrigerators. So if the
temperature gets too high, a controllerinitiates an action to bring
the temperature down.
14. Explain about open loop and closed loop control system.
Figure 1 shows an open loop system. A computed force is
applied to the system which isexpected to respond based on the
specifications.
If the system fails to respond correctly (because your
estimates were off) or anunanticipated disturbance acted on it,
then there is no way to correct the course.
On the other hand, figure 2 shows a feed-back system.
The response C(s) is measured using the sensor H(s) and the
resultant is compared withthe input R(s).
The resultant difference (error) is acted upon by the controller
which works on theactuator.
The actuator then applies the required force on the system.
The closed loop thus contains the sensor dynamics, the
controller dynamics, the actuatordynamics in addition to the system
we are interested in.
It should be noted that all measurements have to be done
or converted if necessary intoone unit so that comparison with the
target signal is possible.
Usually, measurements result in currents and voltages.
Hence, this conversion from a mechanical input to an electrical
output is also included inthe sensor, controller and actuator
dynamics.
In designing the full control system the dynamics of all
the components need to be
accounted for.
If the controller is very slow compared to the system, it
will not send the right input at theright time.
In this class, we will assume perfect sensor and actuator
dynamics, i.e., what goes into the
sensor (it is commonly denoted by H(s)) and the actuator comes
out unmodifiedinstantaneously.So we replace them with unity
transfer functions.
15. Explain the relation between voltage, current and
resistance
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p g ,
An electric circuit is formed when a conductive path is
created to allow free electrons to
continuously move.
This continuous movement of free electrons through the
conductors of a circuit is called acurrent , and it is often
referred to in terms of "flow," just like the flow of a liquid
through
a hollow pipe. The force motivating electrons to "flow" in a
circuit is called voltage. Voltage is a specific measure of
potential energy that is always relative between two
points.
When we speak of a certain amount of voltage being present in a
circuit, we are referringto the measurement of how
much potential energy exists to move electrons from
oneparticular point in that circuit to another particular
point.
Without reference to two particular points, the term
"voltage" has no meaning.
Free electrons tend to move through conductors with some
degree of friction, or
opposition to motion.
This opposition to motion is more properly called
resistance.
The amount of current in a circuit depends on the amount of
voltage available to motivatethe electrons, and also the amount of
resistance in the circuit to oppose electron flow. Justlike
voltage, resistance is a quantity relative between two points.
For this reason, the quantities of voltage and resistance
are often stated as being"between" or "across" two points in a
circuit.
To be able to make meaningful statements about these quantities
in circuits, we need tobe able to describe their quantities in the
same way that we might quantify mass,
temperature, volume, length, or any other kind of physical
quantity.
For mass we might use the units of "kilogram" or "gram."
For temperature we might usedegrees Fahrenheit or degrees
Celsius.
Here are the standard units of measurement for electrical
current, voltage, and resistance:
16. Explain the construction of DC machine with neat
diagram.
A D.C. machine consists mainly of two part the stationary part
called stator and therotating part called stator.
The stator consists of main poles used to produce
magnetic flux ,commutating poles or
inter-poles in between the main poles to avoid sparking at the
commutator but in the caseof small machines sometimes the
interpoles are avoided and finally the frame or yoke
which forms the supporting structure of the machine.
The rotor consist of an armature a cylindrical metallic
body or core with slots in it toplace armature windings or bars, a
commutator and brush gears.
The magnetic flux path in a motor or generator is show below and
it is called themagnetic structure of generator or motor.
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Cross sectional view of a DC Machine
Frame
Frame is the stationary part of a machine on which the main
poles and commutator poles arebolted and it forms the supporting
structure by connecting the frame to the bed plate. The ring
shaped body portion of the frame which makes the magnetic path
for the magnetic fluxes fromthe main poles and interpoles is called
Yoke.
Yoke
In early days Yoke was made up of cast iron but now it is
replaced by cast steel. This is because
cast iron is saturated by a flux density of 0.8 Wb/sq.m where as
saturation with cast iron steel isabout 1.5 Wb/sq.m.So for the same
magnetic flux density the cross section area needed for caststeel
is less than cast iron hence the weight of the machine too. If we
use cast iron there may be
chances of blow holes in it while casting. so now rolled steels
are developed and these haveconsistent magnetic and mechanical
properties.
End Shields or Bearings
If the armature diameter does not exceed 35 to 45 cm then in
addition to poles end shields or
frame head with bearing are attached to the frame. If the
armature diameter is greater than 1mpedestral type bearings are
mounted on the machine bed plate outside the frame. Thesebearings
could be ball or roller type but generally plain pedestral bearings
are employed. If the
diameter of the armature is large a brush holder yoke is
generally fixed to the frame.
17. Explain the Working of dynamometer type wattmeter?
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The principle of operation of the electrodynamometer-type
wattmeter is the same as that
for dynamo-electric machines.
The deflection torque is produced by the interaction of
two magnetic fluxes.
One of the fluxes is produced by a fixed coil which
carries a current proportional to the
load current and therefore called the current coil. The
other flux is created by a movable coil which carries a current
proportional to the
load voltage and thus called the voltage or potential coil.
A high non-inductive resistance is connected to the
potential coil so that its current isalmost in phase with the load
voltage.
The control torque is provided by a control spring.
In a dynamometer type wattmeter the fixed coil (current coil) is
connected in series withthe load.
This coil is divided in to two parts and they are kept parallel
to each other.
The coil is thick in cross section and has less number of
turns.
The moving coil (pressure coil) is connected across the
load. It is thin in cross - sectionand has hundreds of turns.
It has a non - inductive high resistance in series with it
The wattmeter is an electrodynamic instrument for
measuring the electric power or thesupply rate of electrical energy
of any given circuit.
The device consists of a pair of fixed coils, known as
current coils, and a movable coilknown as the potential coil.
The current coils are connected in series with the circuit,
while the potential coil isconnected in parallel.
Also, on analog wattmeters, the potential coil carries a needle
that moves over a scale toindicate the measurement.
A current flowing through the current coil generates an
electromagnetic field around thecoil.
The strength of this field is proportional to the line current
and in phase with it.
The potential coil has, as a general rule, a high-value resistor
connected in series with itto reduce the current that flows through
it.
The result of this arrangement is that on a dc circuit,
the deflection of the needle is
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proportional to both the current and the voltage, thus
conforming to the equation
W=VA or P=EI.
On an ac circuit the deflection is proportional to the
average instantaneous product ofvoltage and current, thus measuring
true power, and possibly (depending on load
characteristics) showing a different reading to that obtained by
simply multiplying thereadings showing on a stand-alone voltmeter
and a stand-alone ammeter in the same
circuit.
The two circuits of a wattmeter are likely to be damaged by
excessive current.
The ammeter and voltmeter are both vulnerable to overheating -
in case of an overload,their pointers will be driven off scale -
but in the wattmeter, either or even both the
current and potential circuits can overheat without the pointer
approaching the end of thescale!
This is because the position of the pointer depends on the power
factor, voltage andcurrent.
Thus, a circuit with a low power factor will give a low
reading on the wattmeter, evenwhen both of its circuits are loaded
to the maximum safety limit.
Therefore, a wattmeter is rated not only in watts, but
also in volts and amperes.
18. Explain the construction of transformer with neat
diagram.
A transformer is an electrical device used to convert AC
power at a certain voltage levelto AC power at a different voltage,
but at the same frequency.
The construction of a transformer includes a
ferromagnetic core around which multiplecoils, or windings, of wire
are wrapped.
The input line connects to the 'primary' coil, while the output
lines connect to 'secondary'coils.
The alternating current in the primary coil induces an
alternating magnetic flux that'flows' around the ferromagnetic
core, changing direction during each electrical cycle.
The alternating flux in the core in turn induces an alternating
current in each of thesecondary coils.
The voltage at each of the secondary coils is directly
related to the primary voltage by theturns ratio, or the number of
turns in the primary coil divided by the number turns in
thesecondary coil.
For instance, if the primary coil consists of 100 turns
and carries 480 volts and asecondary coil consists of 25 turns, the
secondary voltage is then:
secondary voltage = (480 volts) * (25/100) = 120 volts
Two coils of wire (called windings) are wound on some type of
core material.
In some cases the coils of wire are wound on a
cylindrical or rectangular cardboard form.
In effect, the core material is air and the transformer
is called an AIR-CORETRANSFORMER.
Transformers used at low frequencies, such as 50 hertz
and 400 hertz, require a core oflow-reluctance magnetic material,
usually iron.
This type of transformer is called an IRON-CORE
TRANSFORMER.
Most power transformers are of the ironcore type.
The principle parts of a transformer and their functions
are:
The CORE, which provides a path for the magnetic lines of
flux. Th PRIMARY WINDING hi h i f th
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The PRIMARY WINDING, which receives energy from the ac
source.
The SECONDARY WINDING, which receives energy from the
primary winding anddelivers it to the load.
The ENCLOSURE, which protects the above components from
dirt, moisture, andmechanical damage.
A soft-iron-core transformer is very useful where the
transformer must be physicallysmall, yet efficient.
The iron-core transformer provides better power transfer
than does the air-coretransformer.
A transformer whose core is constructed of laminated sheets of
steel dissipates heatreadily; thus it provides for the efficient
transfer of power.
The majority of transformers you will encounter in Navy
equipment contain laminated
steel cores.
These steel laminations are insulated with a non conducting
material, such as varnish, andthen formed into a core.
It takes about 50 such laminations to make a core an inch
thick.
The purpose of the laminations is to reduce certain losses.
BASIC ELECTRONICS
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CONDUCTION IN SEMICONDUCTORS
The branch of engineering which deals with the flow of Electrons
through vacuum, gas or semiconductor is called
Electronics.
Electronics essentially deals with electronic devices and their
utilization.
Atomic Structure
Atom is the basic building block of all the elements. It
consists of the centralnucleus of positive charge around which
small negatively charged particles called
electrons revolve in different paths or orbits.
An Electrostatic force of attraction between electrons
and the nucleus holds upelectrons in different orbits.
Electrostatic force.
+
Centrifugal force.
Figure1.1. Atomic structure
Nucleus is the central part of an atom and contains
protons and neutrons. A protonis positively charged particle, while
the neutron has the same mass as the proton,
but has no charge. Therefore ,nucleus of an atom is positively
charged.
atomic weight = no. of protons + no. of neutrons
An electron is a negatively charged particle having negligible
mass. The chargeon an electron is equal but opposite to that on a
proton. Also the number ofelectrons is equal to the number of
protons in an atom under ordinary conditions.
Therefore an atom is neutral as a whole.
atomic number = no. of protons or electrons in an
atom
The number of electrons in any orbit is given by 2n
2
where n is the number of theorbit.
For example I orbit contains 2x12
=2 electrons
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For example, I orbit contains 2x1 =2 electrons
II orbit contains 2x22 = 8 electrons
III orbit contains 2x32 = 18 electrons and so on
The last orbit cannot have more than 8 electrons.
The last but one orbit cannot have more than 18
electrons.
Positive and negative ions
Protons and electrons are equal in number hence if an atom loses
an electron ithas lost negative charge therefore it becomes
positively charged and is referred aspositive ion.
If an atom gains an electron it becomes negatively
charged and is referred to asnegative ion.
Valence electrons
The electrons in the outermost orbit of an atom are known as
valence electrons .
The outermost orbit can have a maximum of 8 electrons.
The valence electrons determine the physical and chemical
properties of amaterial.
When the number of valence electrons of an atom is less
than 4, the material isusually a metal and a conductor. Examples
are sodium, magnesium and
aluminium, which have 1,2 and 3 valence electrons
respectively.
When the number of valence electrons of an atom is more
than 4, the material isusually a non-metal and an insulator.
Examples are nitrogen, sulphur and neon,
which have 5,6 and 8 valence electrons respectively.
When the number of valence electrons of an atom is 4 the
material has both metaland non-metal properties and is usually a
semi-conductor. Examples are carbon,silicon and germanium.
Free electrons
The valence electrons of different material possess different
energies. The greaterthe energy of a valence electron, the lesser
it is bound to the nucleus.
In certain substances, particularly metals, the valence
electrons possess so muchenergy that they are very loosely attached
to the nucleus
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Conduction band
Forbidden gap
Valence band
energy that they are very loosely attached to the nucleus.
The loosely attached valence electrons move at random
within the material andare called free electrons.
The valence electrons, which are loosely attached to the
nucleus, are known as freeelectrons.
Energy bands
In case of a single isolated atom an electron in any orbit has
definite energy.
When atoms are brought together as in solids, an atom is
influenced by the forcesfrom other atoms. Hence an electron in any
orbit can have a range of energiesrather than single energy. These
range of energy levels are known as Energy
bands.
Within any material there are two distinct energy bands
in which electrons mayexist viz Valence band and conduction
band.
Energy level
Figure1.2 Energy level diagram
The range of energies possessed by valence electrons is
called valence band .
The range of energies possessed by free electrons is
called conduction band.
Valence band and conduction band are separated by an
energy gap in which no
electrons normally exist this gap is called forbidden gap.
Electrons in conduction band are either escaped from their atoms
(free electrons) or onlyweakly held to the nucleus. Thereby by the
electrons in conduction band may be easily
moved around within the material by applying relatively small
amount of energy. (either
by increasing the temperature or by focusing light on the
material etc. ) This is the reasonwhy the conductivity of the
material increases with increase in temperature.
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y y p
But much larger amount of energy must be applied in order to
extract an electron from
the valence band because electrons in valence band are usually
in the normal orbit arounda nucleus. For any given material, the
forbidden gap may be large, small or non-existent.
Classification of materials based on Energy band theory
Based on the width of the forbidden gap, materials are broadly
classified as conductors,
Insulators and semiconductors.
(a) Conductor (b) Insulator (c) Semiconductor
Conductors
Conductors are those substances, which allow electric current to
pass throughthem.
Example: Copper, Al, salt solutions, etc.
In terms of energy bands, conductors are those substances in
which there is noforbidden gap. Valence and conduction band overlap
as shown in fig (a).
For this reason, very large number of electrons are
available for conduction evenat extremely low temperatures. Thus,
conduction is possible even by a very weak
electric field.
Insulators
Insulators are those substances, which do not allow
electric current to passthrough them.Example: Rubber, glass, wood
etc.
In terms of energy bands, insulators are those substances in
which the forbiddengap is very large.
Conduction
band
overlap
Valence
band
Conduction
band
Valence
band
Conduction
band
Valence
band
ForbiddenGa E =1eV
Forbidden
Gap
EG =6eV
Thus valence and conduction band are widely separated as
shown in fig (b).Therefore insulators do not conduct electricity
even with the application of a large
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y pp g
electric field or by heating or at very high temperatures.
Semiconductors
Semiconductors are those substances whose conductivity lies in
between that of aconductor and Insulator.Example: Silicon,
germanium, Cealenium, Gallium, arsenide etc.
In terms of energy bands, semiconductors are those
substances in which theforbidden gap is narrow.
Thus valence and conduction bands are moderately separated as
shown in fig(C).
In semiconductors, the valence band is partially filled,
the conduction band is alsopartially filled, and the energy gap
between conduction band and valence band isnarrow.
Therefore, comparatively smaller electric field is
required to push the electronsfrom valence band to conduction band
. At low temperatures the valence band is
completely filled and conduction band is completely empty.
Therefore, at verylow temperature a semi-conductor actually behaves
as an insulator.
Conduction in solids
Conduction in any given material occurs when a voltage of
suitable magnitude isapplied to it, which causes the charge
carriers within the material to move in a
desired direction.
This may be due to electron motion or hole transfer or
both.
Electron motion
Free electrons in the conduction band are moved under the
influence of the appliedelectric field. Since electrons have
negative charge they are repelled by the negative
terminal of the applied voltage and attracted towards the
positive terminal.
Hole transfer
Hole transfer involves the movement of holes.
Holes may be thought of positive charged particles and as such
they movethrough an electric field in a direction opposite to that
of electrons.
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I I+ +
V V
(a) Conductor (b) Semiconductor
Flow of electrons Flow of electrons
Flow of current Flow of holes
Flow of current
In a good conductor (metal) as shown in fig (a) the current flow
is due to freeelectrons only.
In a semiconductor as shown in fig (b). The current flow
is due to both holes andelectrons moving in opposite
directions.
The unit of electric current is Ampere (A) and since the flow of
electric current isconstituted by the movement of electrons in
conduction band and holes in valence
band, electrons and holes are referred as charge carriers.
Classification of semiconductors
Semiconductors are classified into two types.
a) Intrinsic semiconductors.
b)
Extrinsic semiconductors.
a) Intrinsic semiconductors
A semiconductor in an extremely pure form is known
as Intrinsic
semiconductor.
Example: Silicon, germanium.
Both silicon and Germanium are tetravalent (having 4
valence electrons).
Each atom forms a covalent bond or electron pair bond
with the electrons ofneighboring atom. The structure is shown
below.
Silicon or Germanium
Valence electron
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Covalent bond
Figure1.3. Crystalline structure of Silicon (or Germanium)
At low temperature
At low temperature, all the valence electrons are tightly
bounded the nucleushence no free electrons are available for
conduction.
The semiconductor therefore behaves as an Insulator at
absolute zerotemperature.
At room temperature
Free electron
Valence electron
Holes
Figure 1.4. Crystalline structure of Silicon (or Germanium) at
room temperature
At room temperature, some of the valence electrons gain
enough thermal energyto break up the covalent bonds.
This breaking up of covalent bonds sets the electrons
free and are available for
conduction.
When an electron escapes from a covalent bond and becomes
free electrons avacancy is created in a covalent bond as shown in
figure above. Such a vacancy is
ll d H l It i iti h d d th i fl f l t i
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called Hole. It carries positive charge and moves under the
influence of an electricfield in the direction of the electric
field applied.
Numbers of holes are equal to the number of electrons
since, a hole is nothing butan absence of electrons.
Extrinsic Semiconductor
When an impurity is added to an Intrinsic semiconductor
its conductivity changes.
This process of adding impurity to a semiconductor is called
Doping and the
impure semiconductor is called extrinsic semiconductor.
Depending on the type of impurity added, extrinsic
semiconductors are furtherclassified as n-type and p-type
semiconductor.
n-type semiconductor
free e-
Figure 1.5 n-type semiconductor
Fermi level
Si
Si
Si
Si
Si
As
Si Si Si
Conduction band
Forbidden gap
Valence band
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Figure1.6 Energy band diagram for n-type semiconductor
When a small current of Pentavalent impurity is added to
a pure
semiconductor it is called as n-type semiconductor.
Addition of Pentavalent impurity provides a large number
of free electrons in asemiconductor crystal.
Typical example for pentavalent impurities are Arsenic,
Antimony andPhosphorus etc. Such impurities which produce n-type
semiconductors are known
as Donor impurities because they donate or provide free
electrons to thesemiconductor crystal.
To understand the formation of n-type semiconductor, consider a
pure siliconcrystal with an impurity say arsenic added to it as
shown in figure 1.5.
We know that a silicon atom has 4 valence electrons and
Arsenic has 5 valenceelectrons. When Arsenic is added as impurity
to silicon, the 4 valence electrons of
silicon make co-valent bond with 4 valence electrons of
Arsenic.
The 5th Valence electrons finds no place in the covalent
bond thus, it becomes freeand travels to the conduction band as
shown in figure. Therefore, for each arsenic
atom added, one free electron will be available in the silicon
crystal. Though eacharsenic atom provides one free electrons yet an
extremely small amount of arsenic
impurity provides enough atoms to supply millions of free
electrons.
Due to thermal energy, still hole election pairs are generated
but the number of freeelectrons are very large in number when
compared to holes. So in an n-type
semiconductor electrons are majority charge carriers and holes
are minority chargecarriers . Since the current conduction is
pre-dominantly by free electrons( -vely charges)
it is called as n-type semiconductor( n-
means – ve).
p-type semiconductor
hole
Si
Si
Si
Si
Si
Ga
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Figure 1.7 p-type semiconductor
Fermi level
Figure 1.8 Energy band diagram for p-type semiconductor
When a small amount of trivalent impurity is added to a
pure semiconductor
it is called p-type semiconductor.
The addition of trivalent impurity provides large number of
holes in the
semiconductor crystals.
Example: Gallium, Indium or Boron etc. Such impurities which
produce p-typesemiconductors are known as acceptor impurities
because the holes created canaccept the electrons in the semi
conductor crystal.
To understand the formation of p-type semiconductor, consider a
pure silicon crystal with
an impurity say gallium added to it as shown in figure 1.7.
We know that silicon atom has 4 valence electrons and
Gallium has 3 electrons.When Gallium is added as impurity to
silicon, the 3 valence electrons of galliummake 3 covalent bonds
with 3 valence electrons of silicon.
The 4th
valence electrons of silicon cannot make a covalent bond
with that ofGallium because of short of one electron as shown
above. This absence of
electron is called a hole. Therefore for each gallium atom added
one hole iscreated, a small amount of Gallium provides millions of
holes.
Conduction band
Forbidden gap
Valence band
Due to thermal energy, still hole-electron pairs are generated
but the number of holes arevery large compared to the number of
electrons. Therefore, in a p-type semiconductor
holes are majority carriers and electrons are minority carriers.
Since the current
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holes are majority carriers and electrons are minority carriers.
Since the currentconduction is predominantly by hole( + charges) it
is called as p-type semiconductor( p
means +ve)
Drift and Diffusion current
The flow of current through a semiconductor material is normally
referred to as one ofthe two types.
Drift current
If an electron is subjected to an electric field in free
space it will accelerate in a
straight line form the – ve terminal to the + ve
terminal of the applied voltage. However in the case of
conductor or semiconductor at room temperature, a free
electrons under the influence of electric field will move
towards the +ve terminal
of the applied voltage but will continuously collide with atoms
all the ways asshown in figure 1.9.
Electron drift due to field
conduction
when electricfield is
present
+Conduction
Applied voltage when no electricfield is applied
Figure 1.9
Each time, when the electron strikes an atom, it rebounds in a
random directionbut the presence of electric field doesnot stop the
collisions and random motion.As a result the electrons drift in a
direction of the applied electric field.
The current produced in this way is called as Drift current and
it is the usual kindof current flow that occurs in a conductor.
Diffusion current
The directional movement of charge carriers due to their
concentration gradient produces a component of current known
as Diffusion current.
semiconductor
The mechanism of transport of charges in a semiconductor
when no electric fieldis applied called diffusion. It is
encountered only in semiconductors and is
normally absent in conductors.
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y
heavy concentration of less concentrationelectrons electrons
Diffusion current
Even distribution
Net diffusion current is zero
With no applied voltage if the number of charge carriers
(either holes orelectrons) in one region of a semiconductor is less
compared to the rest of the
region then there exist a concentration gradient.
Since the charge carriers are either all electrons or all holes
they sine polarity ofcharge and thus there is a force of repulsion
between them.
As a result, the carriers tend to move gradually or
diffuse from the region ofhigher concentration to the region of
lower concentration. This process is called
diffusion and electric current produced due to this process is
called diffusion
current.
This process continues until all the carriers are evenly
distributed through thematerial. Hence when there is no applied
voltage, the net diffusion current will bezero.
Fermi-level
Fermi level indicates the level of energy in the forbidden
gap.
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1. Fermi-level for an Intrinsic semiconductor
Energy level
EC
Ef
Forbidden gapEV
EC Conduction band energy level
EV Valence band energy level
Ef fermi level
We know that the Intrinsic semiconductor acts as an insulator at
absolute zerotemperature because there are free electrons and holes
available but as the
temperature increases electron hole pairs are generated and
hence number ofelectrons will be equal to number of holes.
Therefore, the possibility of obtaining an electron in the
conduction band will be
equal to the probability of obtaining a hole in the valence
band.
If Ec is the lowest energy level of Conduction band and
Ev is the highest energylevel of the valence band then the fermi
level Ef is exactly at the center of these
two levels as shown above.
2. Fermi-level in a semiconductors having impurities
(Extrinsic)
a) Fermi-level for n-type Semiconductor
Conduction band
Fermilevel
Valence band
Let a donar impurity be added to an Intrinsic
semiconductor then the donarenergy level (ED) shown by the dotted
lines is very close to conduction
band energy level (Ec).
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Therefore the unbonded valence electrons of the impurity atoms
can veryeasily jump into the conduction band and become free
electros thus, atroom temperature almost all the extra electrons of
pentavalent impurity
will jump to the conduction band.
The donar energy level (ED) is just below conduction band
level (Ec) asshown in figure1.10(a). Due to a large number of free
electrons, theprobability of electrons occupying the energy level
towards the conduction
band will be more hence, fermi level shifts towards the
conduction band.
EC
EDmoves Ef upward
EV
ED Energy level of donar impurity
Figure 1.10 (a) Energy level diagram for n-type
semiconductor
b)
Fermi-level for P-type semiconductor
Let an acceptor impurity be added to an Intrinsic
semiconductor then the acceptorenergy level (Ea) shown by dotted
lines is very close to the valence band shown
by dotted lines is very close to the valence band energy level
(Ev).
Therefore the valence band electrons of the impurity atom
can very easily jump
into the valence band thereby creating holes in the valence
band.
Conduction band
Fermi level
Valence band
E
Conduction band
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EC
EfEA
EV
EA Energy level of acceptor impurity
Figure 1.11 (b) Energy level diagram for P-type
semiconductor
The acceptor energy level (EA) is just above the valence band
level as shown infigure 1.11 (b).
Due to large number of holes the probability of holes
occupying the energy level
towards the valence band will be more and hence, the fermi level
gets shiftedtowards the valence band.
HALL EFFECT
If a piece of metal or semiconductor carrying a current I is
placed in a transverse
magnetic field B then an electric field E is induced in the
direction perpendicular to both Iand B. This phenomenon is known as
Hall effect.
Y(+ve)
Surface-2
+ + + + + + + +d
I VHw
X (+ve)
BSurface -1
Z (+ve)
Fermi level
Valence band
Hall effect is normally used to determine whether a
semi-conductor is n-type or p-type.
To find whether the semiconductor is n-type or p-type
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i) In the figure. above, If I is in the +ve X direction
and B is in the +ve Zdirection, then a force will be exerted on the
charge carriers (holes and
electrons) in the – ve Y direction.
ii) This force is independent of whether the charge
carriers are electrons or holes.Due to this force the charge
carriers ( holes and electrons) will be forced
downward towards surface – 1 as shown.
iii) If the semiconductor is N-type, then electrons will
be the charge carriers and
these electrons will accumulate on surface – 1 making
that surface – velycharged with respect to surface
– 2. Hence a potential called Hall voltageappears
between the surfaces 1 and 2.
iv)
Similarly when surface – 1 is positively charged with
respect to surface – 2,then the semiconductor is of
P-type. In this way, by seeing the polarity of Hall
voltage we can determine whether the semiconductor is of P-type
or N-type.
Applications of Hall effect
Hall effect is used to determine,
carrier concentration, conductivity and mobility.
The sign of the current carrying charge.
Charge density.
It is used as magnetic field meter.
Carrier lifetime (τ)
In a pure semiconductor, we know that number of holes are equal
to the number of
electrons. Thermal agitation however, continues to produce new
hole electron pairs whileother hole-electron pair disappear as a
result of recombination.
On an average, a hole will exist for τp second and an
electron will exist for τn secondbefore recombination. This
time is called the carrier lifetime or Mean lifetime.
The average time an electron or hole can exist in the free state
is called carrier lifetime.
SEMICONDUCTOR DIODE
Wh t i d t t i l i it bl j i d t t i d t th
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When a p-type semiconductor material is suitably joined to
n-type semiconductor the
contact surface is called a p-n junction. The p-n junction is
also called as semiconductordiode.
p n
(b)
(a) Depletion region
Fig. 2.0 (a) p-n junction Fig 2.0 (b) symbolic
representation
The left side material is a p-type semiconductor having
– ve acceptor ions and+vely charged holes. The right
side material is n-type semiconductor having +ve
donor ions and free electrons.
Suppose the two pieces are suitably treated to form pn junction,
then there is atendency for the free electrons from n-type to
diffuse over to the p-side and holesfrom p-type to the n-side .
This process is called diffusion.
As the free electrons move across the junction from n-type to
p-type, +ve donorions are uncovered. Hence a +ve charge is built on
the n-side of the junction. Atthe same time, the free electrons
cross the junction and uncover the – ve acceptorions by
filling in the holes. Therefore a net – ve charge is
established on p-side ofthe junction.
When a sufficient number of donor and acceptor ions is
uncovered furtherdiffusion is prevented.
Thus a barrier is set up against further movement of
charge carriers. This is calledpotential barrier or junction
barrier Vo. The potential barrier is of the order of 0.1
to 0.3V.
Note: outside this barrier on each side of the junction,
the material is still neutral. Onlyinside the barrier, there is a
+ve charge on n-side and – ve charge on p-side. This
region iscalled depletion layer.
+ +
2.1 Biasing: Connecting a p-n junction to an external d.c.
voltage source is calledbiasing.
1 Forward biasing
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1.
Forward biasing
2. Reverse biasing
1. Forward biasing
When external voltage applied to the junction is in such
a direction that it cancelsthe potential barrier, thus permitting
current flow is called forward biasing.
To apply forward bias, connect +ve terminal of the battery to
p-type and – veterminal to n-type as shown in fig.2.1
below.
The applied forward potential establishes the electric field
which acts against thefield due to potential barrier. Therefore the
resultant field is weakened and the
barier height is reduced at the junction as shown in fig.
2.1.
Since the potential barrier voltage is very small, a
small forward voltage issufficient to completely eliminate the
barrier. Once the potential barrier iseliminated by the forward
voltage, junction resistance becomes almost zero and a
low resistance path is established for the entire circuit.
Therefore current flows inthe circuit. This is called forward
current.
p n
no external field
External field
Fig.2.1 forward biasing of p-n junction
2. Reverse biasing
When the external voltage applied to the junction is in
such a direction thepotential barrier is increased it is called
reverse biasing.
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potential barrier is increased it is called reverse biasing.
To apply reverse bias, connect – ve terminal
of the battery to p-type and +veterminal to n-type as shown in
figure below.
The applied reverse voltage establishes an electric field which
acts in the samedirection 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.2.2.
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 hencecurrent does not flow.
p n
external field
no external field
Fig.2.2 Reverse biasing of p-n junction
2.2 Volt- Ampere characteristics(V-I)
R
Adiode
V V
(i)
IF(mA)
Break over
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Voltage
VR Knee voltage VF
IR(μA)
(ii)
Fig. 2.3 V-I characteristics of p-n junction diode.
(i) Circuit diagram
(ii) Characteristics
The V-I characteristics of a semiconductor diode can be
obtained with the help ofthe circuit shown in fig. 2.3 (i)
The supply voltage V is a regulated power supply, the
diode is forward biased in
the circuit shown. The resistor R is a current limiting
resistor. The voltage acrossthe diode is measured with the help of
voltmeter and the current is recorded using
an ammeter.
By varying the supply voltage different sets of voltage
and currents are obtained.By plotting these values on a graph, the
forward characteristics can be obtained.
It can be noted from the graph the current remains zero till the
diode voltageattains the barrier potential.
For silicon diode, the barrier potential is 0.7 V and for
Germanium diode, it is 0.3V. The barrier potential is also called
as knee voltage or cur-in voltage.
The reverse characteristics can be obtained by reverse biasing
the diode. It can benoted that at a particular reverse voltage, the
reverse current increases rapidly.
This voltage is called breakdown voltage.
2.3 Diode current equation
The current in a diode is given by the diode current
equation
I = I0( eV/ηV
T – 1)
Where, I------ diode currentI0------ reverse saturation
current
V------ diode voltageη------- semiconductor constant
=1 for Ge, 2 for Si.VT------ Voltage equivalent of temperature=
T/11,600 (Temperature T is in Kelvin)
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Note----- If the temperature is given in 0C then it can be
converted to Kelvin by the helpof following relation,
0C+273 = K
2.4 Diode equivalent circuit
It is generally profitable to replace a device or system by its
equivalent circuit. Once the
device is replaced by its equivalent circuit, the resulting
network can be solved bytraditional circuit analysis technique.
switch rfIf
VF Vo VF
(i) (ii )
Fig.2.4 Diode equivalent circuit. (i) symbol (ii) equivalent
circuit
The forward current If flowing through the diode causes a
voltage drop in its internal
resistance rf . Therefore the forward voltage VF
applied across the actual diode has toovercome
1. potential barrier Vo
2.
internal drop If rf
Vf = Vo + If rf
For silicon diode Vo=0.7V whereas for Germanium diode Vo = 0.3
V.For ideal diode rf =0.
2.4.1 Basic Definitions
1.Knee voltage or Cut-in Voltage.
It is the forward voltage at which the diode starts
conducting.
2. Breakdown voltageIt is the reverse voltage at which the diode
(p-n junction) breaks down with sudden risein reverse current.
3. Peak-inverse voltage (PIV)It is the max. reverse voltage that
can be applied to a p-n junction without causing
damage to the junction.
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If the reverse voltage across the junction exceeds its
peak-inverse voltage, then the junction ex