EE 6351 ELECTRICAL DRIVES AND CONTROL 3 0 0 100 OBJECTIVE 1. To understand the basic concepts of different types of electrical machines and their Performance. To study the different methods of starting D.C motors and induction motors. To study the conventional and solid-state drives. 8 INTRODUCTION Basic Elements – Types of Electric Drives – factors influencing the choice of electrical drives – heating and cooling curves – Loading conditions and classes of duty – Selection of power rating for drive motors with regard to thermal overloading and Load variation factors 4. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES Types of D.C Motor starters – Typical control circuits for shunt and series motors – Three phase squirrel cage and slip ring induction motors. 10 Speed control of three phase induction motor – Voltage control, voltage / frequency control, slip power recovery scheme – Using inverters and AC voltage regulators – applications. TOTAL : 45 McGraw- TEXT BOOKS 1. 2. REFERENCES 1.PILLAI.S.K “A first course on Electric drives”, Wiley Eastern Limited, 1998 2.M.D.SINGH, K.B.KHANCHANDANI, “Power Electronics”, Tata McGraw-Hill, 1998 H.Partab, “Art and Science and Utilisation of electrical energy”, Dhanpat Rai and Sons, 1994 VEDAM SUBRAHMANIAM, “Electric Drives (concepts and applications)”, Tata Hill, 2001 NAGRATH.I.J. & KOTHARI.D.P, “Electrical Machines”, Tata McGraw-Hill, 1998 5. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES Speed control of DC series and shunt motors – Armature and field control, Ward-Leonard control system - Using controlled rectifiers and DC choppers –applications. 10 3. STARTING METHODS Mechanical characteristics – Speed-Torque characteristics of various types of load and drive motors – Braking of Electrical motors – DC motors: Shunt, series and compound - single phase and three phase induction motors. 8 2. DRIVE MOTOR CHARACTERISTICS 9 www.Vidyarthiplus.com
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EE 6351 ELECTRICAL DRIVES AND CONTROL 3 0 0 100
OBJECTIVE
1.
To understand the basic concepts of different types of electrical machines and their Performance. To study the different methods of starting D.C motors and induction motors. To study the conventional and solid-state drives.
8 INTRODUCTION
Basic Elements – Types of Electric Drives – factors influencing the choice of electrical drives – heating and cooling curves – Loading conditions and classes of duty – Selection of power rating for drive motors with regard to thermal overloading and Load variation factors
4. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES
Types of D.C Motor starters – Typical control circuits for shunt and series motors – Three phase squirrel cage and slip ring induction motors.
10
Speed control of three phase induction motor – Voltage control, voltage / frequency control, slip power recovery scheme – Using inverters and AC voltage regulators – applications.
TOTAL : 45
McGraw-
TEXT BOOKS
1.
2.
REFERENCES
1.PILLAI.S.K “A first course on Electric drives”, Wiley Eastern Limited, 1998 2.M.D.SINGH, K.B.KHANCHANDANI, “Power Electronics”, Tata McGraw-Hill, 1998 H.Partab, “Art and Science and Utilisation of electrical energy”, Dhanpat Rai and Sons, 1994
VEDAM SUBRAHMANIAM, “Electric Drives (concepts and applications)”, Tata Hill, 2001 NAGRATH.I.J. & KOTHARI.D.P, “Electrical Machines”, Tata McGraw-Hill, 1998
5. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES
Speed control of DC series and shunt motors – Armature and field control, Ward-Leonard control system - Using controlled rectifiers and DC choppers –applications.
10
3. STARTING METHODS
Mechanical characteristics – Speed-Torque characteristics of various types of load and drive motors – Braking of Electrical motors – DC motors: Shunt, series and compound - single phase and three phase induction motors.
8
2. DRIVE MOTOR CHARACTERISTICS 9
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UNIT-I
INTRODUCTION TO ELECTRICAL DRIVES
Drives are employed for systems that require motion control – e.g. transportation
system, fans, robots, pumps, machine tools, etc. Prime movers are required in drive systems to
provide the sources: diesel engines, petrol engines, hydraulic motors, electric motors etc.
movement or motion and energy that is used to provide the motion can come from various
Drives that use electric motors as the prime movers are known as electrical drives.
There are several advantages of electrical drives:
a. Flexible control characteristic – This is particularly true when power electronic
Converters are employed where the dynamic and steady state characteristics of the motor
can be controlled by controlling the applied voltage or current.
b. Available in wide range of speed, torque and power
c. High efficiency, lower noise, low maintenance requirements and cleaner operation
d. Electric energy is easy to be transported.
With the advancement of power electronics, microprocessors and digital electronics,
typical Electric drive systems nowadays are becoming more compact, efficient, cheaper and
versatile this is shown in Figure 2. The voltage and current applied to the motor can be
changed at will by employing power electronic converters. AC motor is no longer limited to
application where only AC source is available, however, it can also be used when the power
source available is DC or vice versa
A typical conventional electric drive system for variable speed application employing
multi-machine system is shown in Figure 1. The system is obviously bulky, expensive,
inflexible and require regular maintenance. In the past, induction and synchronous machines
were used for constant speed applications – this was mainly because of the unavailability of
variable frequency supply.
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COM PONENTS OF ELECTRICAL DRIVES
The main components of a modern electrical drive are the motors, power processor, control
unit and electrical source. These are briefly discussed below
a) Motors : Motors obtain power from electrical sources. They convert energy from
electrical to mechanical - therefore can be regarded as energy converters. In braking mode, the
flow of power is reversed. Depending upon the type of power converters
Electric drives is multi-disciplinary field. Various research areas can be sub-divided from
electric drives as shown in Figure 3.
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used, it is also possible for the power to be fed back to the sources rather than dissipated as
heat
There are several types of motors used in electric drives – choice of type used depends on
applications, cost, environmental factors and also the type of sources available.. Broadly, they
can be classified as either DC or AC motors they can be classified as either DC or AC motors:
Clearly now, the value of s can be changed by the value of Pr. For Pr = 0, the machine is
like a normal machine with a short circuited rotor. As Pr becomes positive, for all other circuit
conditions remaining constant, s increases or in the other words, speed reduces. As Pr
becomes negative, the right hand side of the equation and hence the slip decreases. The
physical interpretation is that we now have an active source connected on the rotor side, which
is able to supply part of the rotor copper losses. When Pr = _I02 2 R2 the entire copper loss is
supplied by the external source. The RHS and hence the slip is zero. This corresponds to
operation at synchronous speed. In general the circuitry connected to the rotor may not be a
simple resistor or a machine but a power electronic circuit, which can process this power
requirement. This circuit may drive a machine or recover power back to the mains. Such
circuits are called static Kramer drives.
4. Pole changing schemes
The latter expression is for the case where the second machine is connected in opposite
phase sequence to the first. The cascade-connected system can therefore run at two possible
speeds. Speed control through rotor terminals can be considered in a much more general way.
Consider the induction machine equivalent circuit, where the rotor circuit has been terminated
with a voltage source Er.
If the rotor terminals are shorted, it behaves like a normal induction machine. This is
equivalent to saying that across the rotor terminals a voltage source of zero magnitude is
connected. Different situations could then be considered if this voltage source Er had a non-
zero magnitude. Let the power consumed by that source be Pr. Then considering the rotor side
circuit power dissipation per phase
Note that while giving the rotor output of the first machine to the stator of the second,
the resultant stator mmf of the second machine may set up an air-gap flux which rotates in the
same direction as that of the rotor, or opposes it. This results in values for speed as
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In case (b) however, the flux lines are out of the poles in A & C. The flux lines will be
then have to complete the circuit by owing into the pole structures on the sides. If, when seen
from the rotor, the pole emanating flux lines is considered as north pole and the pole into
which they enter is termed as south, then the pole configurations produced by these
connections is a two-pole arrangement in fig and a four-pole arrangement. Thus by changing
the terminal connections we get either a two pole air-gap field or a four- pole field. In an
induction machine this would correspond to a synchronous speed reduction in half from case
(a) to case (b). Further note that irrespective of the connection, the applied voltage is balanced
by the series addition of induced emfs in two coils. Therefore the air-gap flux in both cases is
the same. Cases (a) and (b) therefore form a pair of constant torque connections.
Figure: Pole arrangement
Sometimes induction machines have a special stator winding capable of being
externally connected to form two different number of pole numbers. Since the synchronous
speed of the induction machine is given by ns = fs=p (in rev./s) where p is the number of pole
pairs, this would correspond to changing the synchronous speed. With the slip now
corresponding to the new synchronous speed, the operating speed is changed. This method of
speed control is a stepped variation and generally restricted to two steps.
If the changes in stator winding connections are made so that the air gap flux remains
constant, then at any winding connection, the same maximum torque is achievable. Such
winding arrangements are therefore referred to as constant-torque connections. If however
such connection changes result in air gap flux changes that are inversely proportional to the
synchronous speeds, then such connections are called constant-horsepower type. The
following figure serves to illustrate the basic principle.
Consider a magnetic pole structure consisting of four pole faces A, B, C, D in fig.
Coils are wound on A & C in the directions shown. The two coils on A & C may be connected
in series in two different ways | A2 may be connected to C1 or C2. A1 with the other terminal
at C then form the terminals of the overall combination. Thus two connections result as shown
in fig.
Now, for a given direction of current flow at terminal A1, say into terminal A1, the
flux directions within the poles are shown in the figures. In case (a), the flux lines are out of
the pole A (seen from the rotor) for and into pole C, thus establishing a two-pole structure.
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Consider, on the other hand a connection as shown in the fig. The terminals T1 and T2
are where the input excitation is given. Note that current direction in the coils now resembles
that of case (b), and hence this would result in a four-pole structure. However, in fig, there is
only one coil-induced emf to balance the applied voltage. Therefore flux in case (c) would
therefore be halved compared to that of case (b) (or case(a), for that matter). Cases (a) and (c)
therefore form a pair of constant horse power connections. It is important to note that in
generating a different pole numbers, the current through one coil (out of two, coil C in this
case) is reversed. In the case of a three-phase machine, the following example serves to
explain this. Let the machine have coils connected as shown [C1 _ C6]
The current directions shown in C1 & C2 correspond to the case where T1; T2; T3 are
supplied with three phase excitation and Ta; Tb & Tc are shorted to each other (STAR point).
The applied voltage must be balanced by induced emf in one coil only (C1 & C2 are parallel).
If however the excitation is given to Ta; Tb& Tc with T1; T2; T3 open, then current
through one of the coils (C1 & C2) would reverse. Thus the effective number of poles would
increase, thereby bringing down the speed. The other coils also face similar conditions.
5. Stator frequency control
The expression for the synchronous speed indicates that by changing the stator
frequency also it can be changed. This can be achieved by using power electronic circuits
called inverters, which convert dc to ac of desired frequency. Depending on the type of
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control scheme of the inverter, the ac generated may be variable-frequency-fixed-
amplitude or variable-frequency- variable-amplitude type. Power electronic control
achieves smooth variation of voltage and frequency of the ac output. This when fed to the
machine is capable of running at a controlled speed. However, consider the equation for
the induced emf in the induction machine.
where N is the number of the turns per phase, _m is the peak flux in the air gap and f is the
frequency. Note that in order to reduce the speed, frequency has to be reduced. If the
frequency is reduced while the voltage is kept constant, thereby requiring the amplitude of
induced emf to remain the same, flux has to increase. This is not advisable since the machine
likely to enter deep saturation. If this is to be avoided, then flux level must be maintained
constant which implies that voltage must be reduced along with frequency. The ratio is held
constant in order to maintain the flux level for maximum torque capability.
Actually, it is the voltage across the magnetizing branch of the exact equivalent circuit
that must be maintained constant, for it is that which determines the induced emf. Under
conditions where the stator voltage drop is negligible compared the applied voltage, is valid.
In this mode of operation, the voltage across the magnetizing inductance in the 'exact'
equivalent circuit reduces in amplitude with reduction in frequency and so does the inductive
reactance. This implies that the current through the inductance and the flux in the machine
remains constant. The speed torque characteristics at any frequency may be estimated as
before. There is one curve for every excitation frequency considered corresponding to every
value of synchronous speed. The curves are shown below. It may be seen that the maximum
torque remains constant.
Figure: Pole change example: three phase
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Equation shows that this maximum value is independent of the frequency. Further ^s! is
independent of frequency. This means that the maximum torque always occurs at a speed
lower than synchronous speed by a fixed difference, independent of frequency. The overall
effect is an apparent shift of the torque-speed characteristic as shown in fig. Though this is the
aim, E is an internal voltage, which is not accessible. It is only the terminal voltage V that we
have access to and can control. For a fixed V, E changes with operating slip (rotor branch
impedance changes) and further due to the stator impedance drop. Thus if we approximate E=f
as V=f, the resulting torque-speed characteristic shown in fig. is far from desirable.
If this equation is differentiated with respect to s and equated to zero to find the slip at
maximum torque ^s, we get ^s = _R0 r=(! L0 lr). The maximum torque is obtained by
substituting this value into eqn.
Figure: Torque-speed curves with E=f held constant
This may be seen mathematically as follows. If E is the voltage across the magnetizing branch
and f is the frequency of excitation, then E = kf, where k is the constant of proportionality. If!
= 2_f, the developed torque is given by
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Figure: Torque-speed curves with V=f constant
At low frequencies and hence low voltages the curves show a considerable reduction
in peak torque. At low frequencies (and hence at low voltages) the drop across the stator
impedance prevents sufficient voltage availability. Therefore, in order to maintain sufficient
torque at low frequencies, a voltage more than proportional needs to be given at low speeds.
Another component of compensation that needs to be given is due to operating slip.
With these two components, therefore, the ratio of applied voltage to frequency is not a
constant but is a curve such as that shown in fig. With this kind of control, it is possible to get
a good starting torque and steady state
performance. However, under dynamic conditions, this control is insufficient. Advanced
control techniques such as field- oriented control (vector control) or direct torque control
(DTC) are necessary.
Figure: Voltage boost required for V=f control
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Department of Electrical & Electronics Engineering
Electrical Drives and Control – EE 6351
(For Mechanical Engineering)
Third Semester
2 marks (Questions & Answers)
UNIT – I
INTRODUCTION
1. Define Drive and Electric Drive.
Drive: A combination of prime mover, transmission equipment and mechanical
working load is called a drive Electric drive: An Electric Drive can be defined as an electromechanical device for converting electrical energy to mechanical energy to impart motion to different
machines and mechanisms for various kinds of process control.
I.C Engines, Steam engine, Turbine or electric motors.
4. Give some examples of Electric Drives.
i. ii. iii.
iv.
Driving fans, ventilators, compressors and pumps. Lifting goods by hoists and cranes. Imparting motion to conveyors in factories, mines and warehouses Running excavators & escalators, electric locomotives trains, cars trolley buses, lifts & drum winders etc.
5. What are the types of electric drives?
Group electric drives (Shaft drive), I
iii.
ii.
i. Availability of electric drives over a wide range of power a few
watts to mega watts. Ability to provide a wide range of torques over wide range of speeds. Electric motors are available in a variety of design in order to make them compatible to any type of load.
3. List out some advantages of electric drives.
2. List out some examples of prime movers.
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Individual Drives, Multi motor electric drives.
Classify electric drives based on the means of control. Manual,Semiautomatic, Automatic. 7. What is a Group Electric Drive (Shaft Drive)?
This drive consists of single motor, which drives one or more line shafts supported on bearings. The line shaft may be fitted with either pulleys & belts or gears, by means of which a group of machines or mechanisms may be operated.
6.
Disadvantages:
2. What is an individual electric drive? Give some examples.
In this drive, each individual machine is driven by a separate motor. This motor also imparts motion to various other parts of the machine. Single spindle drilling machine, Lathe machines etc.
3. What is a multi motor electric drive? Give some examples.
In this drive, there are several drives, each of which serves to activate on of the working parts of the driven mechanisms.
There is no flexibility, Addition of an extra machine to the main shaft is difficult. The efficiency of the drive is low, because of the losses occurring in several transmitting mechanisms. The complete drive system requires shutdown if the motor, requires servicing or repair.
The system is not very safe to operate The noise level at the work spot is very high.
Advantages: A single large motor can be used instead of a number of small motors. The rating of the single motor may be appropriately reduced taking into account the diversity factor of loads.
1. What are the advantages and disadvantages of Group drive (Shaft drive)?
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Metal cutting machine tools, paper making machines, rolling mills, traction drive, Traveling cranes etc.,
4. Write about manual control, semiautomatic control & Automatic control?
Manual control: The electric drives with manual control can be as simple as a room fan, incorporating on switch and a resistance for setting the required speed. Semiautomatic control: This control consists of a manual device for giving a certain command (Starting, braking, reversing, change of speed etc.,) and an automatic device that in response to command, operates the drive in accordance with a preset sequence or order.
Automatic control: The electric drives with automatic control have a control gear, without manual devices
Power Supply
Motor
Geared Coupling
Mechanical Laod
A load diagram is the diagram which shows graphically the variation of torque acting on the electric drive. The motor of the electric drive has to overcome the load torque expressed as a function of time.
Types:
One for the static or steady state process Other for the dynamic process, when the dynamic components of torque are induced by the inertia of the motor & load. (Instantaneous speed, acceleration, Torque & power) as a function of time are required to draw…..
14. What are the types Drive systems?
13. What is a load diagram? What are its types? What are required to draw a load diagram?
Speed & Torque Control
12. What are the Typical elements of an Electric Drive?
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Electric Drives Electromechanical Drives
Mechanical Drives Hydraulic drives.
15. Give an expression for the losses occurring in a machine.
The losses occurring in a machine is given by W = Wc + x2 Wv Where Wc = Constant losses Wv = Variable losses at full load X = load on the motor expressed as a function of rated load.
ii.
iii.
17. What are the factors that influence the choice of electrical drives? 1. Shaft power & speed11. Speed range 2. Power range12. Efficiency 3. Starting torque13.Influence on the supply network 4. Maintenance14. Special competence 5. Total purchase cost15. Cost of energy losses 6. Influence on power supply16. Environment 7. Availability17. Accessibility 8. Nature of electric supply18. Nature of load 9. Types of drive19. Electrical Characteristics 10.Service cost20. Service capacity & rating
18. Indicate the importance of power rating & heating of electric drives.
Power rating: Correct selection of power rating of electric motor is of economic interest as it is associated with capital cost and running cost of drives. Heating: For proper selection of power rating the most important consideration is the heating effect
i. The machine is considered to be a homogeneous body having a uniform temperature gradient. All the points at which heat generated have the same temperature. All the points at which heat is dissipated are also at same temperature. Heat dissipation taking place is proportional to the difference of temperature of the body and surrounding medium. No heat is radiated. The rate of dissipation of heat is constant at all temperatures.
16. What are the assumptions made while performing heating & cooling calculation of an electric motor?
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of load. In this connection various forms of loading or duty cycles have to be considered.
19. How heating occurs in motor drives?
The heating of motor due to losses occurring inside the motor while converting the electrical power into mechanical power and these losses occur in steel core, motor winding & bearing friction.
20. What are the classes of duties? 1. Continuous duty 2. Short time duty operation of motor Main classes of duties 3. Intermittent periodic duty 4. Intermittent periodic duty with starting 5. Intermittent periodic duty with starting & braking 6. Continuous duty with intermittent periodic loading 7. Continuous duty with starting & braking 8. Continuous duty with periodic load changes
23. Why the losses at starting is not a factor of consideration in a continuous duty motor? While selecting a motor for this type of duty it is not necessary to give importance to the heating caused by losses at starting even though they are more than the losses at rated load. This is because the motor does not require frequent starting it is started only once in its duty cycle and the losses during starting do not have much influence on heating.
What is meant by “short time rating of motor”? Any electric motor that is rated for a power rating P for continuous operation can be loaded for a short time duty (Psh) that is much higher than P, if the temperature rise is the consideration.
24.
22. List out some applications for which continuous duty is required. Centrifugal pumps, fans, conveyors & compressors
1. 2. 3. 4. 5. 6.
Reversible &non reversible in controlled constant speed Reversible and non reversible step speed control Reversible and non reversible smooth speed control Constant predetermined position control Variable position control Composite control.
21. How will you classify electric drives based on the method of speed control?
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What is meant by “load equalization”? In the method of “load Equalization” intentionally the motor inertia is increased by adding a flywheel on the motor shaft, if the motor is not to be reversed. For effectiveness of the flywheel, the motor should have a prominent drooping characteristic so that on load there is a considerable speed drop.
26. How a motor variable load ? 1. Method of 2. Method of 3. Method of 4. Method of
rating is determined in a continuous duty and
Average losses equivalent power equivalent current equivalent Torque
25.
27. Draw the heating & cooling curve of an electric motor.
29. Write down the heat balance equation. Heat balance equation is given by Ghd0 + S0 .dt = p.dt
28. What are the various function performed by an electric drive?
1. Driving fans, ventilators, compressors & pumps etc., 2. Lifting goods by hoists & cranes 3. Imparting motion to conveyors in factories, mines & warehouses and 4. Running excavators & escalators, electric locomotives, trains, cars, trolley buses and lifts etc.
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UNIT – II ELECTRICAL MOTOR CHARACTERISTICS
1. 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 revolving field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.
2. What is meant by plugging?
The N-T curve moves towards the right when the voltage is increased. This can be achieved by means of additional resistance in the armature circuit or by using thyristor power converter.
6. Compare electrical and mechanical braking.
Mechanical Brakes require frequent maintenance Not smooth Can be applied to hold the system at any position torque.
Electrical very little maintenance smooth cannot produce holding
5. What is the effect of variation of armature voltage on N-T curve and how it can be achieved?
Dynamic or Rheostatic braking, Counter current or plugging and Regenerative braking
4. What are the different types of electric braking?
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.
3. Give some applications of DC motor.
The plugging operation can be achieved by changing the polarity of the motor there by reversing the direction of rotation of the motor. This can be achieved in ac motors by changing the phase sequence and in dc motors by changing the polarity.
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7. When does an induction motor behave to run off as a generator?
When the rotor of an induction motor runs faster than the stator field, the slip becomes negative. Regenerative braking occurs and the K.E. of the rotating parts is return back to the supply as electrical energy and thus the machine generates power.
8. Define slip.
S = Ns – Nr Ns Where, Ns Nr S
9. Define synchronous speed.
In the regenerative braking operation, the motor operates as a generator, while it is still connected to the supply here, the motor speed is grater that the synchronous speed. Mechanical energy is converter into electrical energy, part of which is returned to the supply and rest as heat in the winding and bearing.
6. 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.
5. What is meant by regenerative braking?
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 revolving field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.
10. Why a single phase induction motor does not self start?
It is given by Ns = 120f / p rpm. WhereNs = synchronous speed, p = no. of stator poles, f = supply frequency in Hz
= synchronous speed in rpm. = rotor speed in rpm = Slip
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7. Compare electrical and mechanical braking.
Mechanical Brakes require frequent maintenance Not smooth Can be applied to hold the system at any position torque.
Electrical very little maintenance smooth cannot produce holding
8. Differentiate cumulative and differential compound motors.
Cumulative The orientation of the series flux aids the shunt flux shunt flux
9. What is meant by mechanical characteristics?
A curve drawn between the parameters speed and torque.
differential series flux opposes
2. Mention the Starters used to start an Induction motor.
D.O.L Starter (Direct Online Starter) Star-Delta Starter Auto Transformer Starter Reactance or Resistance starter Stator Rotor Starter (Rotor Resistance Starter)
3. What are the protective devices in a DC/AC motor Starter.
Over load Release (O.L.R) or No volt coil Hold on Coil Thermal Relays Fuses(Starting /Running) Over load relay
Two point Starter Three point Starter Four point Starter
1. Mention the Starters used to start a DC motor.
UNIT – III
STARTING METHODS
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4. Is it possible to include/ Exclude external resistance in the rotor of a Squirrel cage induction motor?. Justify
No it is not possible to include/ Exclude external resistance in the rotor of a Squirrel cage induction motor because, the rotors bars are permanently short circuited by means of circuiting rings (end rings) at both the ends. i.e. no slip rings to do so.
5. Give the prime purpose of a starter for motors.
when induction motor is switched on to the supply, it takes about 5 to 8 times full load current at starting. This starting current may be of such a magnitude as to cause objectionable voltage drop in the lines. So Starters are necessary
6. Why motor take heavy current at starting?
To include resistance in the rotor circuit there by reducing the induced rotor current at starting. This can be implemented only on a slip ring induction motor.
9. Why squirrel cage induction motors are not used for loads requiring high starting torque?
Squirrel cage motors are started only by reduced voltage starting methods which leads to the development of low starting torque at starting. This is the reason Why squirrel cage induction motors are not used for loads requiring high starting torque.
10. How reduced voltage starting of Induction motor is achived?.
8. What is the objective of rotor resistance starter (stator rotor starter)?
7. What are the methods to reduce the magnitude of rotor current (rotor induced current) at starting?. By increasing the resistance in the rotor circuit By reducing the magnitude of rotating magnetic field i.e by reducing the applied voltage to the stator windings.
Since rotor of an induction motor behaves as a short circuited secondary of a transformer whose primary is stator winding, heavy rotor current will require corresponding heavy stator balancing currents. Thus motor draws heavy current at starting
When 3 phase supply is given to the stator of an induction motor, magnetic field rotating in space at synchronous speed is produced. This magnetic field is cut by the rotor conductors, which are short circuited. This gives to induced current in them.
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D.O.L Starter (Direct Online Starter) Star-Delta Starter Auto Transformer Starter Reactance or Resistance starter
11. Give the relation between line voltage and phase voltage in a (i) Delta connected network (ii) Star connected network Delta connected network: Vphase = Vline Star connected network: Vphase = Vline / √3
14. Compare the Induction motor starters
% of line voltag e applie d
100%
57.7%
80%
Starting current (Is)compared with
D.O.L current(Idol)
Is = Idol
Is = (1/√3)2 Idol
Is =(0.8)2 Idol
Full load current(I )
Is = 6I
Is = 2I
Is = 3.84 I
Starting torque (Ts)compared with
D.O.L Torque(Tdol)
Ts = Tdol
Ts = (1/√3)2 Tdol
Ts = =(0.8)2 Tdol
Full load torque(T)
Ts = 6T
Ts = 2/3T
Ts = 1.28 T
Description of Starter
D.O.L Starter
Star Delta starter
Auto transformer
First stage reduction is due to reduced applied voltage Second stage reduction is due to reduced number of turns
13. Explain double stage reduction of line current in an Auto transformer starter.
Disadvantages: The inrush current of large motors may cause excessive voltage drop in the weak power system The torque may be limited to protect certain types of loads.
12. Give some advantages and disadvantages of D.O.L starter.
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starter 60%
40%
Reactance- resistance starter
64%
Is =(0.6)2 Idol
Is =(0.4)2 Idol
Is = (0.64) Idol 2
Is = 2.16 I
Is = 0.96 I
Is = 2.5 I
Ts = =(0.6)2 Tdol
Ts = =(0.4)2 Tdol
Ts =(0.425)2 Tdol
Ts = 0.72 T
Ts = 0.32 T
Ts = 0.35T
15. Draw the Speed-Torque characteristics of an Induction motor with various values of Rotor Resistance.
Rotor Resistance Increasing
Torque
Speed N = k (V-IaRa)
where V = Terminal Voltage in volts Ia = Armature current in Amps Ra = Armature resistance in ohms flux per pole.
2. What are the ways of speed control in dc motors?
Field control -by varying the flux per pole. Armature control- by varying the terminal voltage
-for above rated speed -for below rated speed
1. Give the expression for speed for a DC motor.
UNIT – IV
CONVENTIONAL SPEED CONTROL
Speed
Tmax
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3. Give the Limitation of field control
a. Speed lower than the rated speed cannot be obtained. b. It can cope with constant kW drives only. c. This control is not suitable to application needing speed reversal.
4. Compensating winding can be used to increase the speed range in field control method
5. What are the 3 ways of field control in DC series motor?
6. What are the main applications of Ward-Leonard system?
7. What are the merits and demerits of rheostat control method?
Impossible to keep the speed constant on rapidly changing loads.
A large amount of power is wasted in the controller resistance.
Loss of power is directly proportional to the reduction in speed. Hence
efficiency is decreased. Maximum power developed is diminished in the same ratio as speed. It needs expensive arrangements for dissipation of heat produced in the controller resistance. It gives speed below normal, not above.
8. What are the advantages of field control method?
More economical, more efficient and convenient.
It is used for colliery winders. Electric excavators In elevators Main drives in steel mills and blooming and paper mills.
Field diverter control Armature diverter control Motor diverter control Field coil taps control Series-parallel control
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It can give speeds above normal speed.
9. Compare the values of speed and torque in case of motors when in parallel and in series.
The speed is one fourth the speed of the motor when in parallel.
The torque is four times that produced by the motor when in parallel.
10. Mention the speed control method employed in electric traction.
Series-parallel speed control.
11. What is the effect of inserting resistance in the field circuit of a dc shunt motor on its speed and torque?
Applied voltage should be maintained constant so as to maintain field strength
12. While controlling the speed of a dc shunt motor what should be done to achieve a constant torque drive?
For a constant supply voltage, flux will decrease, speed will increase and torque will increase.
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UNIT – V
SOLID STATE SPEED CONTROL
1. What is a controlled rectifier?
A controlled rectifier is a device which is used for converting controlled dc power from a control voltage ac supply.
2. What is firing angle?
The control of dc voltage is achieved by firing the thyristor at an adjustable angle with respect to the applied voltage. This angle is known as firing angle.
3. Give some applications of phase control converters.
4. What is the main purpose of free wheeling diode?
6. What is natural or line commutation?
The commutation which occurs without any action of external force is called natural or line commutation.
7. What is forced commutation?
The commutation process which takes place by the action of an external force is called forced commutation.
8. What is a chopper?
A chopper is essentially an electronic switch that turns on the fixed-voltage dc source for a short time intervals and applies the source potential to motor terminals in series of pulses.
A full converter is a tow quadrant converter in which the voltage polarity of the output can reverse, but the current remains unidirectional because of unidirectional thyristors.
5. What is a full converter?
Free wheeling diode is connected across the motor terminal to allow for the dissipation of energy stored in motor inductance and to provide for continuity of motor current when the thyristors are blocked.
Phase control converters are used in the speed control of fractional kW dc motors as well as in large motors employed in variable speed reversing drives for rolling mills. with motors ratings as large as several MW‟s.
.
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9. What are the two main difficulties of variable frequency system?
Control of Va requires variation of chopper frequency over a wide range. Filter design for variable frequency operation is difficult.
10. At low voltage, a large value of toff makes the motor current discontinuous.
Classify commutation.
Voltage commutation
Current commutation.
12. What is current commutation?
13. What is load commutation?
14. What are the different means of controlling induction motor?
15. What are the two ways of controlling the RMS value of stator voltage?
16. Mention the two slip-power recovery schemes.
Stator voltage control.
Frequency control
Pole changing control.
Slip power recovery control.
Phase control
Integral cycle control
The load current flowing through the thyristor either becomes zero (as in natural or line commutation employed in converters) or is transferred to another device from the conducting thyristor. This is known as load commutation.
A current pulse is forced in the reverse direction through the conducting thyristor. As the net current becomes zero, the thyristor is turned OFF. This is known as current commutation.
A charged capacitor momentarily reverse-bias the conducting thyristor to turn it off. This is known as voltage commutation.
11. What is voltage commutation?
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Static Kramer drive scheme. 17. Give the basic difference between the two slip-power recovery schemes.
The slip is returned to the supply network in scherbius scheme and in Kramer scheme, it is used to drive an auxiliary motor which is mechanically coupled to the induction motor shaft.
18. Write short notes on inverter rectifier.
The dc source could be converted to ac form by an inverter, transformed to a suitable voltage and then rectified to dc form. Because of two stage of conversion, the setup is bulky, costly and less efficient.
Static scherbius scheme
What is electrical power supply system?
The generation, transmission and distribution system of electrical power is called electrical power supply system.
21. What are the 4 main parts of distribution system?
Feeders,
Distributors and
Service mains.
22. What are feeders?
Since a static Kramer system possesses no line commutated inverter, it causes less reactive power and smaller harmonic contents of current than a static scherbius.
20. What are the advantages of static Kramer system,, over static scherbius system?
This drive provides a constant torque control.
If max. slip is denoted by S max, then power rating of diode, inverter and
transformer can be just Smax times motor power rating resulting in a low cost drive.
The scheme has applications in large power fan and pump drives which requires
speed control in anrrow range only.
19. Give the special features of static scherbius scheme.
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Feeders are conductors which connect the stations (in some cases generating stations) to the areas to be fed by those stations.
23. What are the advantages of high voltage dc system over high voltage ac system?
It requires only tow conductors for transmission and it is also possible to transmit the power through only one conductor by using earth as returning conductor, hence much copper is saved.
No inductance, capacitance, phase displacement and surge problem.
There is no skin effect in dc, cross section of line conductor is fully utilized.
24. What do you mean by the term earthing?
25. What are the different methods of providing neutral earthing?
Reactance earthing
Arc suppression coil or Peterson coil earthing.
Resistance earthing
Solid earthing
The term “earthing” means connecting the non-current carrying parts of electrical equipment to the neutral point of the supply system to the general mass of earth in such a manner that at all time an immediate discharge of electrical energy takes place without danger.