ELECTRICAL DRIVES AND CONTROLS

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ELECTRICAL DRIVES AND CONTROLS

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

131305
-

ELECTRICAL DRIVES & CONTROLS

III SEM
-

MECHANICAL & CHEMICAL ENGINEERING

PART A (Q & A)


UNIT 1
-

INTRODUCTION

1. What is meant by el
ectrical drives?


Systems employed for motion control are called "Drives" and many employ any of


the prime movers such as, diesel or petrol engines, gas or steam turbines, hydraulic


motors and electric motors for supplying mechanical energy for mot
ion control.


Drives employing electrical motors are known as "Electrical drives".

2 . What are the different types of drives?


1. Group drive


2. Individual drive


3. Multi motor drive

3. What are the different types of electrical drives?


1.

DC drives


2. AC drives

4. What are the advantages of electric drives?


1. They have flexible control characteristics. The steady state and dynamic


characteristics of electrical drives can be shaped to satisfy load requirements.


2. Driv
es can be provided with automatic fault detection systems. Programmable


logic controllers and computers can be employed to automatically control the


drive operations in a desired sequence.


3. They are available in wide range of torque, sp
eed and power.


4. It can operate in all the four quadrants of speed
-
torque plane. Electric baking


gives smooth deceleration and increases life of the equipment compound to other


forms of braking.


5. Control gear enquired for speed c
ontrol, starting and braking is usually simple


and easy to operate.

5 . Mention the different factors for the selection of electric drives.


1. Steady state operation requirements


2. Transient operation requirements


3. Requirement
s related to the source


4. Capital and running cost, maintenance needs, life.


5. Environment and location


6. Reliability

6. What are the parts of electrical drives?


1. Electrical motors and load

Page 1

ELECTRICAL DRIVES AND CON
TROLS

2. Power modulator

3. Sources

4. Control unit

5. Sensing unit

7. What are the applications of electrical drives?


1. Paper mills


2. Electric traction


3. Cement mills


4. Steel mills

8. What are the different types of classes of
duty?









Continuous duty

Short time duty operation of motorMain classes of duties

Intermittent periodic duty

Intermittent periodic duty with starting

Intermittent periodic duty with starting & braking

Continuous duty with intermittent periodic loadi
ng

Continuous duty with starting & braking

Continuous duty with periodic load changes

9. Define equivalent current method.


The motor selected should have a current rating more than or equal to the equal


current. It is also necessary to check the over
load capacity of the motor. This method


of determining the power rating of motor is known as the equivalent current method.

10. What is meant by cooling time constant?


It is defined as the ratio between C and A. Cooling time constant is denoted as



τ’
= C/A

Where C = amount of heat required to rise the temperature of the motor body by 1 degree


o

Celsius in J C.

A = amount of heat dissipated by the motor per unit time per degree Celsius in

J
/S/
o
C.

11. What are the assumptions made while pe
rforming heating & cooling calculation of an


electric motor?

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.

Page 2

ii.

iii
.


ELECTRICAL DRIVES AND CONTROLS

12.
What are the factors that influence the choice of electrical drives?

1. Shaft power & speed

2. Power range

3. Starting torque

4. Maintenance

5. Total purchase cost

6. Influence on power supply

7. Availability

8. Natu
re of electric supply

9. Types of drive

10.Service cost

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

Speed range

Efficiency

Influence on the supply network

Special competence

Cost of energy losses

Environment

Accessibility

Nature of load

Electrical Characterist
ics

Service capacity & rating

13. 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 starti
ng 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.

14. What is meant by “s
hort 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 (P
sh
) that is much higher than P, if the temperature rise is

the consideration.

15. 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 droo
ping characteristic

so that on load there is a considerable speed drop.

16. What is short time duty?


In short time duty the period of operation is so short that the temperature rise of

the motor does not reach its final steady value and the period o
f rest is so long that the

motor returns to cold conditions.

17. What is heating time constant?


Heating time constant is defined as the time taken by the machine to attain 0.632

of its final steady temperature rise. The heating time constant of th
e machine is the index

of the time taken by the machine to attain its final steady temperature rise.

UNIT 2
-

DRIVE MOTOR CHARACTERISTICS

18. What is meant by mechanical characteristics?


A curve is drawn between speed
-
torque. This characteristic is call
ed mechanical


characteristics.


Page 3

ELECTRICAL DRIVES AND CONTROLS

19. Draw the speed
-
torque

characteristics of dc shunt motor.

20.Draw the speed
-
torque characteristics

of dc series motor.

21. A series motor should never be started without some mec
hanical load why?


When the load current Ia falls to a small value, speed becomes dangerously high.


Hence a series motor should never be started without some mechanical load.

22. What are the different types of dc motor?


I. DC series motor2. Shu
nt motor


3. Compound motor4. Separately excited de motor

23. a)What is meant by electrical characteristics?

A curve is drawn torque and armature current. It is known as electrical characteristics.

24. What is the relation between speed and flux of a dc

motor?


The speed of a dc motor is inversely proportional to field flux.

25. What is the application of dc motor?


DC shunt motor:
-


1. For driving constant speed operations


2. Lathes


3. Centrifugal pumps

DC series motor:
-

1. Elec
tric locomotives

2. Rapid transit systems

3. Trolley cars

4. Cranes and hoists

5. Conveyors

DC compound motor:
-


1. Elevators.


2. Air compressors


3. Rolling mills


4. Heavy planers

Page 4

4. Machine tools

5. Blowers and fans

6. Reciprocating pumps

ELECTRICAL DRIVES AND CONTROLS

26. A dc shunt motor is called as constant speed motor
-
why?


The drop in speed from no
-
load fuel
-
load is small; hence the dc shunt motor is also


called as constant speed motor.

27. What is mean by braking?


Whenever a
n electric drive is disconnected from the supply, the speed of the driving


motor gradually decreases and becomes zero. Braking is a generic term used to


describe a set of operating conditions for electric drive systems. It includes rapid


stopping of
the electric motor holding the motor shaft to a specific position,


maintaining the speed to a desired value of preventing the motor from over speeding.

28. What are the two types of braking?


1. Mechanical braking


2. Electrical braking

29. Wh
at is meant by mechanical braking?


In mechanical braking, the frictional force between the rotating parts and brake drums


provide thee required brake.

30. What is meant by electric braking?


In electric braking, the motor is made to work as generat
or. So it produces a negative


slip and negative torque (braking torque). This is achieved by suitably changing the


electrical connections of the motor.

31. What are the different types of electric braking?


1. Regenerative braking


2. Dynamic b
raking


3. Plugging

32. What are the advantages of electric braking?


1. High efficient method


2. Low maintenance.


3. Braking is very smooth.

33. What is meant by regenerative braking?


In the regenerative braking operation, the motor opera
tors as a generator, while it is


still connected to the supply. Here, the motor speed is greater than the synchronous


speed. Mechanical energy is converted into electrical energy, part of which is returned


to the supply and rest of the energy is l
ast as heat in the winding and bearings.

34. What is meant by dynamic braking?


When an electric motor rotates, a kinetic energy of the motor is converted into electric


energy. This energy is dissipated in resistive elements.

35. What is meant by plug
ging?

Page 5

ELECTRICAL DRIVES AND CONTROLS

The plugging operation can be obtained by changing the polarity of the motor. For a

machine, the phase sequence of the starter windings and dc machines the polarities of

the field or armature terminals.

36. Dra
w the plugging characteristics of dc series and separately excited motor.

37. Draw the dynamic braking characteristics of separately excited and series motor.

38. Draw the regenerative 6raking characteristics of separately excited motor.

39. What are the d
isadvantages of dc machine?


1. Higher cost


2. Higher rotor inertia


3. Maintains problems with commutator and brushes


4. EMI problems


5. Do not permit a machine to operate in dirty and explosive environments.

40. What are the advantages
of squirrel cage induction motor?


1. Rugged


2. Cheaper

Page 6

ELECTRICAL DRIVES AND CONTROLS

3. Lighter

4. More efficient

5. Less maintenance

6. Can operate in explosive and dirty environment.

UNIT 3
-

STARTING METHODS

41. What is the use of a st
arter for dc motor?


The starter mainly used to reduce the starting current of the dc motor. At the time of


starting back e.m.f. is zero. Armature resistance also very small. Hence starting


current is very high through armature.

Ia=
(V
-
Eb)/Ra

42
. What are the different types of DC motor starters?


Three point starter, four point starter for shunt & compound motors and two point


starter for series motor.

43. What are the different types of AC motor starters?


Direct on Line starter, Auto

transformer starter, Star
-

Delta starter, Stator resistance


starter for Squirrel cage induction motor and Rotor resistance starter for slip ring


induction motor.

44. What are the starters used for Squirrel cage induction motor?


Direct on Line

starter, Auto transformer starter, Star
-

Delta starter, Stator resistance


starter for Squirrel cage induction motor.

45. Which type of starter is used for slip ring induction motor?


Rotor resistance starter for slip ring induction motor.

46. What

are the applications of DC motor?


Electric traction, Machine tool, Steel mills, Textile mills.

47. What are the advantages of DC drives?


Lower cost, Reliability, Simple control.

48. Give some advantages and disadvantages of D.O.L starter.


A
dvantages:


Highest starting torque


Low cost


Greatest simplicity


Disadvantages:


The inrush current of large motors may cause excessive voltage drop in the weak


power system


The torque may be limited to protec
t certain types of loads.

49. What is the objective of rotor resistance starter (stator rotor starter)?

Page 7

ELECTRICAL DRIVES AND CONTROLS

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.

50. Give the prime purpose of a starter for motors.


when 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 ma
gnitude as to cause


objectionable voltage drop in the lines. So Starters are necessary

51. Why motor take heavy current at starting?


When 3 phase supply is given to the stator of an induction motor, magnetic field


rotating in space at sync
hronous speed is produced. This magnetic field is cut by


the rotor conductors, which are short circuited. This gives to induced current in


them.


Since rotor of an induction motor behaves as a short circuited secondary of a


transformer w
hose primary is stator winding, heavy rotor current will require


corresponding heavy stator balancing currents.
Thus motor draws heavy current


at starting

52. Mention the Starters used to start an Induction motor.



D.O.L Starter (Direc
t Online Starter)



Star
-
Delta Starter



Auto Transformer Starter



Reactance or Resistance starter



Stator Rotor Starter (Rotor Resistance Starter)

53. Draw the Speed
-
Torque characteristics of an Induction motor wi
th various values of

Rotor Resistance.

Rotor Resistance Increasing

T
max

Torque

Speed

Page 8

ELECTRICAL DRIVES AND CONTROLS

54. What are the protective devices in a DC/AC motor Starter.






Over load Release (O.L.R) or No volt coil

Hold on Coil

Ther
mal Relays

Fuses(Starting /Running)

Over load relay


UNIT 4

CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES

55. Write down the speed equation of a dc motor.


N = (V


IaRa) / Kbφ


Where


V = applied voltage


I
a = armature current


Ra = armature resistance


φ
=flux

Kb= constant

56. What are the methods of speed control of dc motors?


Armature resistance control, Flux control, Voltage control.

57. What is meant by armature resistance control?


A contr
oller resistance is connected in series with armature. By varying the controller


resistance R, the potential drop across the armature is varied. Therefore, the motor


speed also varied. This method of speed control only applicable for speed less tha
n no


load speed.

58. What are the advantages and disadvantages of armature resistance control of dc shunt


motor?


Advantages:


Simple method of speed control.


Disadvantages:


1. This method is highly inefficient, because more power is
wasted in controller


resistance.


2. Change in speed with Change in load becomes large.

59. What is meant by flux control method?


The speed of the dc motor can be controlled by varying the field flux. This can be


increasing the speed of the
motor above its rated speed, because the speed is inversely


proportional to the field flux.

60. What are the methods of speed control of dc series motor?


1. Variable resistance in series with motor

Page 9

ELECTRICAL DRIVES AND CONTROLS

2. Flux co
ntrol method


i. Field diverter


ii. Armature diverter


iii. Tapped field control


iv. Paralleling field coils and


v. Series parallel control

61. What is meant Ward Leonard system?


The speed of the dc shunt motor can be controlled by ab
ove and below rated speed


using the system. It consists of motor generator set. The armature voltage control can


be achieved by varying the field of the dc generator. The flux control can be achieved


by varying the field of the dc motor.

62.
Wh
at are the advantages and disadvantages of Ward Leonard system?


Advantages:
-











Full forward and reverse speed can be achieved.

A wide range of speed control is possible.

Power is automatically regenerated to the ac line to the mg se
t when speed

is reduced.

Short time overload capacity is large,

The armature current is smooth.

High initial cost.

Overall efficiency low (less than 80%, because of the additional MG set).

Costly foundation and large amount of space is required.

This produ
ces noise.

It requires frequent maintenance

Disadvantages:

63. What are the methods of speed control of induction motors?


Stator voltage control, frequency control, Pole changing method, Cascaded control,


slip power control.

64. What it is meant by

voltage control?


The induction motor speed can be controlled by varying the stator voltage. This can


be done by using auto transformer. Using this method speed below rated speed is only


possible.

65. What is frequency control?


N
=
120f/P


Where N= speed of the motorf = supply frequency


p= number of poles


Speed of the induction motor can be controlled by varying supply frequency as speed


is directly proportional to supply frequency.

66. What are the different types of

slip power control system?

Page 10

ELECTRICAL DRIVES AND CONTROLS

Kramer system, Scherbius system.

67. What is meant by slip power?


The portion of air gap power, which is not converted into mechanical power, is called


slip power. Slip power is n
othing but multiplication of slip
(s)
and air gap
power (P ag)

Slip power = s (P ag)

68. What are the advantages of slip power recovery system?


The slip power can be recovered and fed back to the supply. The overall efficiency


also improved.

69. W
hat are the different types of slip power recovery system?


These are classified two types.


1. Kramer system


i. Conventional Kramer system


ii. Static Kramer system


2. Scherbius system


i. Conventional Scherbius system



ii. Static Scherbius system


iii. DC link static Scherbius system


iv. Cycloconverter Scherbius system

70. What is meant by Kramer system?


The Kramer system is only applicable for sub
-
synchronous speed operation because


the slip p
ower is fed back to the supply.

71. What are advantages of conventional Kramer method?


1) The main advantage of this method is that any speed, within the

working range, can be obtained instead of only two or three, as with other methods o
f

speed control.


2) If the rotor converter is over excited, it will take a leading current

which compensates for the lagging current drawn by SRIM & hence improves the power

factor of the system.

72. What is the function of static Kramer
system?


The slip power is converted into dc by diode bridge rectifier and the DC voltage is

converted into AC by line commutated inverter and fed back to supply. As the slip power

can flow only in one direction, static Kramer drive offers speed control

below

synchronous speed only.

73. Define slip power control. What is meant by slip power recovery system?


In slip ring induction motor the rotor power (slip power can be recover and fed


back to supply or can be used to supply and additional mot
or which is mechanically


coupled to the main motor. This type of drive is known as slip power recovery system


and improves overall efficiency of the system.

Page 11

ELECTRICAL DRIVES AND CONTROLS

74. What is the function of conventional Kramer Sy
stem?


In conventional Kramer system, the slip power is converted into dc by rotary


converter. The dc voltage is fed to dc motor. The dc motor is coupled with slip ring


induction motor. The speed of the SRIM can be controlled by varying the
field


regulator of the de motor.

75. Where static Kramer drive is used?


In large power pump and fan type drives, where speed control within narrow

range and below synchronous speed.

76. What are the advantages of static Kramer system?


1.
The drive system is very efficient and the converter power rating is low,

because it has to handle only the slip power.


2. The drive system has dc machine
-
like characteristics and the control is very

simple.

77. What are applications of static Scher
bius drive system?


1. Multi
-
MW, variable speed pumps/generators.


2. Flywheel energy storage system.

78. What are the advantages and disadvantages of static Scherbius drive?


Advantages:


1. In this method, the problem of commutation near synchronous
speed disappears.


2. The cyclo
-
converter can easily operates as a phase
-
controlled rectifier, supplying dc

current in the rotor and permitting true synchronous machine operation.


3. The near
-
sinusoidal current waves in the rotor, which reduce harmoni
c loss, and a

machine over excitation capacity that permits leading power
factor operation on the

stator side. So the line's power factor is unity.


4. The cyclo
-
converter is to be controlled so that its output frequency tracks precisely

with the slip fr
equency.


Disadvantages:


1. The cyclo
-
converter cost is increases.


2. The control of the Scherbius drive is some what complex.

79. Compare conventional method of Kramer and Scherbius system

Kramer Method

This system consists of SRIM, Rotary

convert
er and dc motor and Induction

generator

Here, the return power is Mechanical

Less cost.

Scherbius Method

This system consists of SRIM, Rotary

converter and dc motor

Here, the return power is mechanical

More cost

80. What is meant by controlled rectifier?



It converts fixed ac voltage into variable dc output voltage.

Page 12

ELECTRICAL DRIVES AND CONTROLS

81. What is meant by electric drive?


An electric motor together with its control equipment and energy
-
transmitting


device forms an electric d
rive. A ceiling
-
fan motor with its speed regulator but


without blades is an example of electric device.

82. What is meant by electric
-
drive system?


An electric drive together with its working machines constitutes an electric drive


system.

83:
What is meant by dc drives?


A dc motor speed can be controlled by using power
-
electronic converters. It is called


as dc drives.

84. What are the different types of dc drives?


Single phase dc drives, three phase dc drives, Chopper drives.

85. Wh
at are the two basic methods of speed control of dc motors?


The dc motor speed controlled by (1) armature voltage control (below base speed)


(2) Flux control (above base speed).

86. What are the different types of single phase dc drives?


Single

phase half controlled dc drives.


Single phase full controlled dc drives.

87. What are the different types of chopper?


1. First quadrant chopper
-
motoring control


2. Second quadrant chopper
-
braking controls


3. Two quadrant chopper



4. Four quadrant chopper.

88. What is meant by duty cycle?


Duty cycle is defined as the ratio between on time of chopper and total time of


chopper


α= T
on
/T

89. What are the advantages of dc chopper drives?


Dc chopper drives has the advant
ages of high efficiency, flexibility in control, light


weight, small size, quick response and regeneration down to very low speed.

90. What are the different methods of speed control of induction motors?


1. Stator voltage control2. Voltage/frequenc
y control (v/f)


3. Rotor resistance control4. Slip energy recovery control.

91. What is stator voltage control?

Page 13

ELECTRICAL DRIVES AND CONTROLS

Three phase induction motor speed can be controlled by varying the stator voltage.

This stator volt
age can be varied by using ac voltage controllers. This method of

speed control of induction motor is called as stator voltage control. It is also called

soft start.

92. What is meant by ac voltage controller?


Ac voltage controller means, it converts f
ixed ac voltage into variable ac voltage


without change in supply of frequency.

93. What are advantages of stator voltage control?


1. The control is very simple


2. More compact and less weight


3. Its response time is quick


4. Th
is is an economical method

94. What are disadvantages of stator voltage control?


1. The input power factor is very low.


2. Voltage and current waveforms are highly distorted due to harmonics, which


affects the efficiency of the machine.



3. Operating efficiency is low as resistance losses are high.


4. Performance is poor under running conditions at low speeds.

95. Define base speed.


The base speed is defined as the synchronous speed corresponding to the rated


frequency.

96. What is meant by stator frequency control?


Three phase induction motor speed can be controlled by varying the stator frequency.


The variable stator frequency can be obtained by inverters circuit.

97. What is meant by v/f control?


If the rat
io of voltage to frequency is kept constant; the flux also remains constant. At


low frequency, the air gap flux is reduced due to the drop in the stator impedance and


the voltage has to be increased to maintain the torque level. This type of contro
l is


usually known as volts/hertz (v/f) control. The voltage at variable frequency can be


obtained from three
-
phase inverter or cyclo
-
converter.

98. What are main features of v/f control?


The maximum torque should be constant & the starting cur
rent is also constant.

99. What type of motor is applicable for slip power recovery system?


Slip ring induction motor

100. What is slip?

Page 14

ELECTRICAL DRIVES AND CONTROLS

The difference between the synchronous speed Ns and the actual speed N of
the rotor

is known as slip.

101. What is uncontrolled rectifier?


A diode is an uncontrolled rectifier, which converts fixed ac to fixed dc.

102. What is rheostatic braking?


In rheostatic braking the armature is disconnected from the supply and is
connected

as a variable resistance. The braking is controlled by varying the series resistance.

103. What are the applications of induction motors?


Domestic applications like washing machines, refrigeration, air conditioners, etc.

pumps and compresso
rs. Industrial applications such as lifts, cranes, pumps, lathes,

machines tools etc.

104. What is a chopper?

A chopper is a DC to DC converter. The fixed voltage of a dc source can be

converted into an adjustable average voltage across a load by inserting

a high speed

switch between the dc source and the load. The high speed switch is called the chopper.

105. What is an individual drive?


A drive which is allotted for each process equipment is known as individual drive.

106. What are the disadvantages

of an individual drive?


Initial cost is high.

107. What are the classifications of drives?


The classifications of drives are a) Group Drive b) Individual Drive c) Multi

motor drive

108. What are the advantages of group drive?


a) Initial

cost is less


b) Group drive system is useful because all operation are stopped simultaneously


c) Less space is required in group drive


d) Maintenance cost is less

109. Define continuous duty


Continuous duty is defined as the load

that may be carried by the machine for an

indefinite time without the temperature rise of any part exceeding the maximum

permissible value.

110. What are the advantages of star delta starter?


a) Cheaper than the auto
-
transformer starter


b) Sim
ple arrangement


c) Commonly employed for both small and medium size motors


Page 15

ELECTRICAL DRIVES AND CONTROLS

111. What is the function of Over Load Release present in the starter?


If there is any overload conditions, then the motor wi
ll draw large current. This

large current will flow through the over load release coil. Due to this, the electromagnet

gets energized and pulls the iron piece upward which short circuits the coils of the hold

on
electromagnet. The hold on electromagnet get
s de
-
energized and therefore the starter

arm returns to the off position, thus protecting the motor against overload.

112. What is the function of No
-
Load release present in the starter?


If there is no load or low load, the speed of DC series motor

will be dangerously

high. During this condition, no load release makes the control arm to return to OFF

position and prevent the motor from over speeding.

113. Draw the torque slip characteristics of an Induction motor.

PART
-
B



UNIT
-
1

1. Explain the factors governing the selection of motors. (16)

2. Discuss in detail the determination of power rating of motors. (16)

3. (i) Explain the different types of loading of drives. (8)


(ii) Explain the choice of selection of the

motor for different loads. (8)

4. (i) Describe the simplifications based on which the heating and cooling


calculations of an electric motor are made. (3)

Page 16

ELECTRICAL DRIVES AND CONTROLS


(ii) Establish the heating time constant and the he
ating curves. (13)

5. (i) Compare the D.C and A.C drives. (6)


(ii) Write a brief note on classes of duty for an electric motor. (10)

6. Draw the typical temperature rise
-
time curve and derive the equation for


temperature rise in an electric drive. (16
)

7. Explain the loading of an electric motor and its duty cycle with a simple


diagram. (16)

8. Explain in detail about the various types of electric drives. (16)

9. A 100 kW motor, having rated temperature rise of 60°C, has full
-
load


efficiency of 8
0% and the maximum efficiency occurs at 85% full load. It


has thermal time constants of 80 minutes and 65 minutes. It is cyclically


loaded, 120% of full load for one hour and 50% of full load for the next


hour. Find the temperature rise after 3 ho
urs. (16)

10. The thermal time constant and final steady temperature of a motor on


continuous running is 30 minutes and 60°C. Find out the temperature.


i) After 15 minutes at this load.


ii) After 1 hour at this load.


iii) If t
emperature rise at 1 hour rating is 60°C, find the maximum


steady temperature.


iv) What will be the time required to increase the temperature


from 40°C to 60°C at 1 hour rating. (16)

UNIT
-
2

1. (i) List out the advantages and disadva
ntages of electrical braking over


mechanical braking. (8)


(ii) Discuss any one method of electrical braking of DC Machines. (8)

2. Explain the Speed
-
Torque characteristics of three phase induction motor


with neat diagrams. (16)

3. Explain about

the speed
-
torque characteristics of a DC Shunt Motor with


suitable graph and equations. (16)

4. Explain about the quadrantal diagram of speed
-
torque characteristics for


a motor driving hoist load. (16)

5. Explain how an induction motor is brought to

stop by (i) Plugging and (ii)


dynamic braking. (16)

6. Explain the various methods of braking of induction motors. (16)

7. Draw and explain various load characteristics of DC Shunt Motor. (16)

8. Explain Rheostat braking in DC Series Motor and Plugging

in DC Shunt


Motor. (16)

9. Explain various methods of braking of DC Shunt Motors with neat

Page 17

ELECTRICAL DRIVES AND CONTROLS


diagrams. (16)

10. Explain Various methods of braking of DC Series Motors with neat


diagrams. (16)

11. (i) Explain th
e speed


torque curve of single phase induction motors in


detail. (8)


(ii) Explain the method of regenerative braking employed in DC


Motors.(8)

12. Explain about the speed
-
torque characteristics of a DC Compound Motor


with suitable graph and eq
uations. (16)

13. A 220V shunt Motor has an armature resistance of 0.062
Ω and with full


field has an emf of 215V at a speed of 960 rpm, the motor is driving an


overhauling load with a torque of 172 Nm. Calculate the minimum speed


at which the motor can hold the load by means of regenerative braking.



UNIT
-
3

1. Draw a neat schematic diagram of a three point starter and explain its


working. (16)

2. Draw a neat schematic diagram of a four point starter and explain its


working. (16)

3. Explain with neat circuit diagram, the star
-
de
lta starter method of starting


squirrel cage induction motor. (16)

4. Explain the typical control circuits for DC Series and Shunt motors (16)

5. Explain the different starting methods of three phase squirrel cage


induction motors with neat sketches.

(16)

6. Explain different methods of starting of DC Motors. (16)

7. Explain with neat diagram the starting of three phase slip ring induction


motor. (16)

8. Draw and explain the push
-
button operated direct
-
on line starter for three


phase induction m
otor. (16)

9. Draw and explain the manual auto
-
transformer starter for three phase


induction motor. (16)


UNIT
-
4

1. Explain with neat sketch the chopper control method of speed control of


DC Motors. (16)

2. Explain
with neat sketches about the DC Shunt Motor speed control by


using single phase fully controlled bridge converter. (16)

3. Discuss the Ward
-
Leonard speed control system with a neat circuit


diagram. Also mention its advantages and disadvantages. (16)

4. Explain how the speed of a DC Shunt Motor can be varied both above


and below the speed at which it runs with full field current. (16)

Page 18

ELECTRICAL DRIVES AND CONTROLS

5. (i) Explain with neat sketch the operation of chopper fed DC Series
Motor


drive. Also, derive the expression for average motor current. (10)


(ii) Explain Time ratio control and Current limit control. (6)

6. Explain the speed control schemes of DC Series Motor. (16)

7. Explain the different methods of speed control emp
loyed in DC Shunt


Motor. (16)

8. Explain the control of DC drives using rectifiers and choppers. (16)

9. Explain the single phase half wave converter drive speed control for DC


drive with waveforms. (16)

10. Explain in detail the single phase semi
-
con
verter speed control for DC


drive for separately excited motor. (16)

11. A 500V series motor having armature resistance and field resistance of

0.2 Ω and 0.3 Ω respectively runs at 500 rpm when taking 70A. Assuming

unsaturated field, find out its speed w
hen field diverter of 0.684 Ω is used

constant load torque. (16)

12. A 250V DC Series Motor takes 40A of current when developing a full

load torque at 1500 rpm. Its resistance is 0.5 Ω. If the load torque varies

as the square of the speed determine the res
istance to be connected in

series with the armature to reduce the speed to 122 rpm. Assume the flux

is proportional to the field current.(16)

UNIT
-
5

1.Draw the power circuit arrangement of three phase variable


Frequency inverter for the speed control of
three phase induction


motor and explain its working. (16)

2. Explain the V/f control method of AC drive with neat sketches. (16)

3. Discuss the speed control of AC motors by using three phase AC


Voltage regulators. (16)

4. Explain the speed control sc
hemes of phase wound induction


motors. (16)

5. Explain the concatenation operation of three phase induction


motors.Hence derive the speed experienced for the cascaded set.

6. Explain in detail about Slip power recovery scheme. (16)

7. Explain the differ
ent methods of speed control used in three phase


induction motors. (16)

8. Explain the working of following methods with neat circuit diagram.


i) Kramer system ii) Scherbius system (16)

9. Explain in detail rotor resistance method of speed control of

a slip


Ring induction motor. (16)

Page 19


ELECTRICAL DRIVES AND CONTROLS

10. (i) Explain the operation of Pole changing method of speed


control. (8)


(ii) Explain the pole amplitude modulation method. (8)

11. Explain the static Kramer method a
nd static scherbius method of


Speed control of three phase induction motor. (16)

12. Explain in detail about the various methods of solid state speed


Control techniques by using inverters. (16)

13. Explain the solid state stator voltage control
technique for the


Speed control of three phase induction motor. (16)

14. Explain the various methods of speed control of a three phase


Induction motor when fed through semiconductor devices. (16)

Page 20