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161+ Induction Motor MCQ Questions and Answers | Electrical Engineering

December 12, 2021 , ,
161+ Induction Motor MCQ Questions and Answers | Electrical Engineering

161+ Induction Motor MCQ Questions and Answers for Electrical Engineering

Search Description: Practice 161+ Induction Motor MCQ Questions and Answers with short explanations. Useful for Electrical Engineering students, diploma exams, ITI, GATE, SSC JE, RRB JE and technical interviews.

Introduction

Induction motor is one of the most important topics in Electrical Machines. It is widely used in industries, pumps, fans, compressors, conveyors, machine tools and many other electrical drives. Because of its simple construction, rugged operation, low maintenance and good efficiency, the three-phase induction motor is often called the workhorse of industry.

This post contains Induction Motor MCQ questions and answers arranged from easy to hard level. These objective questions cover construction, slip, rotor frequency, torque, power factor, starters, speed control, losses, tests, circle diagram, squirrel-cage motor, slip-ring motor, cogging, crawling and braking. Each question includes a short explanation so that students can revise the concept quickly.

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Table of Contents

Quick Notes on Induction Motor

  • An induction motor works on the principle of electromagnetic induction.
  • The rotating magnetic field is produced by the three-phase stator supply.
  • The rotor speed is always less than synchronous speed in normal motoring operation.
  • Slip is necessary for torque production in an induction motor.
  • Squirrel-cage motors are simple, rugged and require less maintenance.
  • Slip-ring motors are preferred where high starting torque and speed control are required.

Easy Induction Motor MCQs

These questions are useful for beginners and cover basic construction, slip, rotor, stator and starting concepts.

Question 1. Which of the following component is usually fabricated out of silicon steel ?

  1. A. Bearings
  2. B. Shaft
  3. C. Stator core
  4. D. None of the above

Answer: C. Stator core

Explanation: The correct option is C. Stator core is the standard concept used in induction motor theory.

Question 2. The frame of an induction motor is usually made of

  1. A. silicon steel
  2. B. cast iron
  3. C. aluminium
  4. D. bronze

Answer: B. cast iron

Explanation: The correct option is B. cast iron is the standard concept used in induction motor theory.

Question 3. The shaft of an induction motor is made of

  1. A. stiff
  2. B. flexible
  3. C. hollow
  4. D. any of the above

Answer: A. stiff

Explanation: The correct option is A. stiff is the standard concept used in induction motor theory.

Question 4. The shaft of an induction motor is made of

  1. A. high speed steel
  2. B. stainless steel
  3. C. carbon steel
  4. D. cast iron

Answer: C. carbon steel

Explanation: The correct option is C. carbon steel is the standard concept used in induction motor theory.

Question 5. In an induction motor, no-load the slip is generally

  1. A. less than 1%
  2. B. 1.5%
  3. C. 2%
  4. D. 4%

Answer: A. less than 1%

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 6. In medium sized induction motors, the slip is generally around

  1. A. 0.04%
  2. B. 0.4%
  3. C. 4%
  4. D. 14%

Answer: C. 4%

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 7. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to

  1. A. reduce windage losses
  2. B. reduce eddy currents
  3. C. reduce accumulation of dirt and dust
  4. D. reduce magnetic hum

Answer: D. reduce magnetic hum

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 8. In case the air gap in an induction motor is increased

  1. A. the magnetising current of the rotor will decrease
  2. B. the power factor will decrease
  3. C. speed of motor will increase
  4. D. the windage losses will increase

Answer: B. the power factor will decrease

Explanation: The correct option is B. the power factor will decrease is the standard concept used in induction motor theory.

Question 9. Slip rings are usually made of

  1. A. copper
  2. B. carbon
  3. C. phospor bronze
  4. D. aluminium

Answer: C. phospor bronze

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 10. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be

  1. A. 200 Hz
  2. B. 50 Hz
  3. C. 2 Hz
  4. D. 0.2 Hz

Answer: C. 2 Hz

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 11. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

  1. A. Ns
  2. B. s.N,
  3. C. (l-s)Ns
  4. D. (Ns-l)s

Answer: C. (l-s)Ns

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 12. The efficiency of an induction motor can be expected to be nearly

  1. A. 60 to 90%
  2. B. 80 to 90%
  3. C. 95 to 98%
  4. D. 99%

Answer: B. 80 to 90%

Explanation: The correct option is B. 80 to 90% is the standard concept used in induction motor theory.

Question 13. The number of slip rings on a squirrel cage induction motor is usually

  1. A. two
  2. B. three
  3. C. four
  4. D. none

Answer: D. none

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 14. The starting torque of a squirrel-cage induction motor is

  1. A. low
  2. B. negligible
  3. C. same as full-load torque
  4. D. slightly more than full-load torque

Answer: A. low

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 15. A double squirrel-cage induction motor has

  1. A. two rotors moving in oppsite direction
  2. B. two parallel windings in stator
  3. C. two parallel windings in rotor
  4. D. two series windings in stator

Answer: C. two parallel windings in rotor

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

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Question 16. Star-delta starting of motors is not possible in case of

  1. A. single phase motors
  2. B. variable speed motors
  3. C. low horse power motors
  4. D. high speed motors

Answer: A. single phase motors

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 17. The term 'cogging' is associated with

  1. A. three phase transformers
  2. B. compound generators
  3. C. D.C. series motors
  4. D. induction motors

Answer: D. induction motors

Explanation: Cogging and crawling are abnormal effects mainly related to slot combinations and harmonics.

Question 18. In case of the induction motors the torque is

  1. A. inversely proportional to (Vslip)
  2. B. directly proportional to (slip)2
  3. C. inversely proportional to slip
  4. D. directly proportional to slip

Answer: D. directly proportional to slip

Explanation: Torque in an induction motor depends on rotor current, rotor power factor, slip and supply voltage.

Question 19. An induction motor with 1000 r.p.m. speed will have

  1. A. 8 poles
  2. B. 6 poles
  3. C. 4 poles
  4. D. 2 poles

Answer: B. 6 poles

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 20. The good power factor of an induction motor can be achieved if the average flux density in the air gap is

  1. A. absent
  2. B. small
  3. C. large
  4. D. infinity

Answer: B. small

Explanation: Induction motors draw magnetizing current, so their power factor changes with load and is low at no-load.

Question 21. An induction motor is identical to

  1. A. D.C. compound motor
  2. B. D.C. series motor
  3. C. synchronous motor
  4. D. asynchronous motor

Answer: D. asynchronous motor

Explanation: The correct option is D. asynchronous motor is the standard concept used in induction motor theory.

Question 22. The injected e.m.f. in the rotor of induction motor must have

  1. A. zero frequency
  2. B. the same frequency as the slip fre-quency
  3. C. the same phase as the rotor e.m.f.
  4. D. high value for the satisfactory speed control

Answer: B. the same frequency as the slip fre-quency

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 23. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor ?

  1. A. By changing the number of stator poles
  2. B. Rotor rheostat control
  3. C. By operating two motors in cascade
  4. D. By injecting e.m.f. in the rotor circuit

Answer: A. By changing the number of stator poles

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 24. The crawling in the induction motor is caused by

  1. A. low voltage supply
  2. B. high loads
  3. C. harmonics develped in the motor
  4. D. improper design of the machine

Answer: C. harmonics develped in the motor

Explanation: Cogging and crawling are abnormal effects mainly related to slot combinations and harmonics.

Question 25. The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type

  1. A. and
  2. B. both
  3. D. none of the above

Answer: C.

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 26. The torque developed in the cage induction motor with autostarter is

  1. A. k/torque with direct switching (6) K x torque with direct switching
  2. C. K2 x torque with direct switching
  3. D. k2/torque with direct switching

Answer: C. K2 x torque with direct switching

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 27. When the equivalent circuit diagram of double squirrel-cage induction motor is constructed the two cages can be considered

  1. A. in series
  2. B. in parallel
  3. C. in series-parallel
  4. D. in parallel with stator

Answer: B. in parallel

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 28. It is advisable to avoid line-starting of induction motor and use starter because

  1. A. motor takes five to seven times its full load current
  2. B. it will pick-up very high speed and may go out of step
  3. C. it will run in reverse direction
  4. D. starting torque is very high

Answer: A. motor takes five to seven times its full load current

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 29. Stepless speed control of induction motor is possible by which of the following methods ?

  1. A. e.m.f. injection in rotor eueuit
  2. B. Changing the number of poles
  3. C. Cascade operation
  4. D. None of the above

Answer: B. Changing the number of poles

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 30. Rotor rheostat control method of speed control is used for

  1. A. and
  2. C. both
  3. D. none of the above

Answer: B.

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 31. In the circle diagram for induction motor, the diameter of the circle represents

  1. A. slip
  2. B. rotor current
  3. C. running torque
  4. D. line voltage

Answer: B. rotor current

Explanation: Induction motor tests provide equivalent circuit data and performance parameters such as losses, current and power factor.

Question 32. For which motor the speed can be controlled from rotor side ?

  1. A. and
  2. C. Both
  3. D. None of the above

Answer: B.

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 33. If any two phases for an induction motor are interchanged

  1. A. the motor will run in reverse direction
  2. B. the motor will run at reduced speed
  3. C. the motor will not run
  4. D. the motor will burn

Answer: A. the motor will run in reverse direction

Explanation: The correct option is A. the motor will run in reverse direction is the standard concept used in induction motor theory.

Question 34. An induction motor is

  1. A. self-starting with zero torque
  2. B. self-starting with high torque
  3. C. self-starting with low torque
  4. D. non-self starting

Answer: C. self-starting with low torque

Explanation: The correct option is C. self-starting with low torque is the standard concept used in induction motor theory.

Question 35. The maximum torque in an induction motor depends on

  1. A. frequency
  2. B. rotor inductive reactance
  3. C. square of supply voltage
  4. D. all of the above

Answer: D. all of the above

Explanation: Torque in an induction motor depends on rotor current, rotor power factor, slip and supply voltage.

Question 36. In three-phase squirrel-cage induction motors

  1. A. rotor conductor ends are short-circuited through slip rings
  2. B. rotor conductors are short-circuited through end rings
  3. C. rotor conductors are kept open
  4. D. rotor conductors are connected to insulation

Answer: B. rotor conductors are short-circuited through end rings

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 37. In a three-phase induction motor, the number of poles in the rotor winding is always

  1. A. zero
  2. B. more than the number of poles in stator
  3. C. less than number of poles in stator
  4. D. equal to number of poles in stator

Answer: D. equal to number of poles in stator

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 38. DOL starting of induction motors is usually restricted to

  1. A. low horsepower motors
  2. B. variable speed motors
  3. C. high horsepower motors
  4. D. high speed motors

Answer: A. low horsepower motors

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 39. The speed of a squirrel-cage induction motor can be controlled by all of the following except

  1. A. changing supply frequency
  2. B. changing number of poles
  3. C. changing winding resistance
  4. D. reducing supply voltage

Answer: C. changing winding resistance

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 40. The 'crawling" in an induction motor is caused by

  1. A. high loads (6) low voltage supply
  2. C. improper design of machine
  3. D. harmonics developed in the motor

Answer: D. harmonics developed in the motor

Explanation: Cogging and crawling are abnormal effects mainly related to slot combinations and harmonics.

Question 41. The power factor of an induction motor under no-load conditions will be closer to

  1. A. 0.2 lagging
  2. B. 0.2 leading
  3. C. 0.5 leading
  4. D. unity

Answer: A. 0.2 lagging

Explanation: Induction motors draw magnetizing current, so their power factor changes with load and is low at no-load.

Question 42. The 'cogging' of an induction motor can be avoided by

  1. A. proper ventilation
  2. B. using DOL starter
  3. C. auto-transformer starter
  4. D. having number of rotor slots more or less than the number of stator slots (not equal)

Answer: D. having number of rotor slots more or less than the number of stator slots (not equal)

Explanation: Cogging and crawling are abnormal effects mainly related to slot combinations and harmonics.

Question 43. If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as

  1. A. humming
  2. B. hunting
  3. C. crawling
  4. D. cogging

Answer: C. crawling

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

Question 44. Slip of an induction motor is negative when

  1. A. magnetic field and rotor rotate in opposite direction
  2. B. rotor speed is less than the syn-chronous speed of the field and are in the same direction
  3. C. rotor speed is more than the syn-chronous speed of the field and are in the same direction
  4. D. none of the above

Answer: C. rotor speed is more than the syn-chronous speed of the field and are in the same direction

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 45. Size of a high speed motor as compared to low speed motorfor the same H.P. will be

  1. A. bigger
  2. B. smaller
  3. C. same
  4. D. any of the above

Answer: B. smaller

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 46. A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor

  1. A. will continue running burning its one phase
  2. B. will continue running burning its two phases
  3. C. will stop and carry heavy current causing permanent damage to its winding
  4. D. will continue running without any harm to the winding

Answer: A. will continue running burning its one phase

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

Question 47. A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor

  1. A. will continue running burning its one phase
  2. B. will continue running burning its two phase
  3. C. will stop and carry heavy current causing permanent damage to its winding
  4. D. will continue running without any harm to the winding

Answer: C. will stop and carry heavy current causing permanent damage to its winding

Explanation: The correct option is C. will stop and carry heavy current causing permanent damage to its winding is the standard concept used in induction motor theory.

Question 48. Low voltage at motor terminals is due to

  1. A. inadequate motor wiring
  2. B. poorely regulated power supply
  3. C. any one of the above
  4. D. none of the above

Answer: C. any one of the above

Explanation: The correct option is C. any one of the above is the standard concept used in induction motor theory.

Question 49. In an induction motor the relationship between stator slots and rotor slots is that

  1. A. stator slots are equal to rotor slots
  2. B. stator slots are exact multiple of rotor slots
  3. C. stator slots are not exact multiple of rotor slots
  4. D. none of the above

Answer: C. stator slots are not exact multiple of rotor slots

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

Intermediate Induction Motor MCQs

These questions are suitable for diploma, ITI, engineering semester exams and technical interviews.

Question 50. Slip ring motor is recommended where

  1. A. speed control is required (6) frequent starting, stopping and reversing is required
  2. C. high starting torque is needed
  3. D. all above features are required

Answer: D. all above features are required

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

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Question 51. As load on an induction motor goes on increasing

  1. A. its power factor goes on decreasing
  2. B. its power factor remains constant
  3. C. its power factor goes on increasing even after full load
  4. D. its power factor goes on increasing upto full load and then it falls again

Answer: D. its power factor goes on increasing upto full load and then it falls again

Explanation: The correct option is D. its power factor goes on increasing upto full load and then it falls again is the standard concept used in induction motor theory.

Question 52. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move

  1. A. in the opposite direction as the direction of the rotating field
  2. B. in the same direction as the direction of the field
  3. C. in any direction depending upon phase squence of supply
  4. D. None of the above

Answer: B. in the same direction as the direction of the field

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

Question 53. It is advisable to avoid line starting of induction motor and use starter because

  1. A. it will run in reverse direction
  2. B. it will pick up very high speed and may go out of step
  3. C. motor takes five to seven times its fullload current
  4. D. starting torque is very high

Answer: C. motor takes five to seven times its fullload current

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 54. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines

  1. A. D.C. series motor
  2. B. D.C. shunt motor
  3. C. universal motor
  4. D. none of the above

Answer: B. D.C. shunt motor

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 55. Which type of bearing is provided in small induction motors to support the rotor shaft ?

  1. A. Ball bearings
  2. B. Cast iron bearings
  3. C. Bush bearings
  4. D. None of the above

Answer: A. Ball bearings

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 56. A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will

  1. A. stall after sometime
  2. B. stall immediately
  3. C. continue to run at lower speed without damage
  4. D. get heated and subsequently get damaged

Answer: D. get heated and subsequently get damaged

Explanation: The correct option is D. get heated and subsequently get damaged is the standard concept used in induction motor theory.

Question 57. 5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest ?

  1. A. 730 r.p.m.
  2. B. 960 r.p.m.
  3. C. 1440 r.p.m.
  4. D. 2880 r.p.m.

Answer: A. 730 r.p.m.

Explanation: The correct option is A. 730 r.p.m. is the standard concept used in induction motor theory.

Question 58. A 3-phase slip ring motor has

  1. A. double cage rotor (6) wound rotor
  2. C. short-circuited rotor
  3. D. any of the above

Answer: B.

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 59. The starting torque of a 3-phase squirrel cage induction motor is

  1. A. twice the full load torque
  2. B. 1.5 times the full load torque
  3. C. equal to full load torque
  4. D. None of the above

Answer: B. 1.5 times the full load torque

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 60. Short-circuit test on an induction motor cannot be used to determine

  1. A. windage losses
  2. B. copper losses
  3. C. transformation ratio
  4. D. power scale of circle diagram

Answer: A. windage losses

Explanation: Induction motor tests provide equivalent circuit data and performance parameters such as losses, current and power factor.

Question 61. In a three-phase induction motor

  1. A. iron losses in stator will be negligible as compared to that in rotor (6) iron losses in motor will be neg¬ligible as compared to that in rotor
  2. C. iron losses in stator will be less than that in rotor
  3. D. iron losses in stator will be more than that in rotor

Answer: D. iron losses in stator will be more than that in rotor

Explanation: The correct option is D. iron losses in stator will be more than that in rotor is the standard concept used in induction motor theory.

Question 62. In case of 3-phase induction motors, plugging means

  1. A. pulling the motor directly on line without a starter
  2. B. locking of rotor due to harmonics
  3. C. starting the motor on load which is more than the rated load
  4. D. interchanging two supply phases for quick stopping

Answer: D. interchanging two supply phases for quick stopping

Explanation: The correct option is D. interchanging two supply phases for quick stopping is the standard concept used in induction motor theory.

Question 63. Which is of the following data is required to draw the circle diagram for an induction motor ?

  1. A. Block rotor test only
  2. B. No load test only
  3. C. Block rotor test and no-load test
  4. D. Block rotor test, no-load test and stator resistance test

Answer: D. Block rotor test, no-load test and stator resistance test

Explanation: Induction motor tests provide equivalent circuit data and performance parameters such as losses, current and power factor.

Question 64. In three-phase induction motors sometimes copper bars are placed deep in the rotor to

  1. A. improve starting torque
  2. B. reduce copper losses
  3. C. improve efficiency
  4. D. improve power factor

Answer: A. improve starting torque

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 65. In a three-phase induction motor

  1. A. power factor at starting is high as compared to that while running
  2. B. power factor at starting is low as compared to that while running
  3. C. power factor at starting in the same as that while running
  4. D. None of the above

Answer: B. power factor at starting is low as compared to that while running

Explanation: The correct option is B. power factor at starting is low as compared to that while running is the standard concept used in induction motor theory.

Question 66. The value of transformation ratio of an induction motor can be found by

  1. A. open-circuit test only
  2. B. short-circuit test only
  3. C. stator resistance test
  4. D. none of the above

Answer: B. short-circuit test only

Explanation: The correct option is B. short-circuit test only is the standard concept used in induction motor theory.

Question 67. The power scale of circle diagram of an induction motor can be found from

  1. A. stator resistance test (6) no-load test only
  2. C. short-circuit test only
  3. D. noue of the above

Answer: C. short-circuit test only

Explanation: Induction motor tests provide equivalent circuit data and performance parameters such as losses, current and power factor.

Question 68. The shape of the torque/slip curve of induction motor is

  1. A. parabola
  2. B. hyperbola
  3. C. rectangular parabola
  4. D. straigth line

Answer: C. rectangular parabola

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 69. A change of 4% of supply voltage to an induction motor will produce a change of appromimately

  1. A. 4% in the rotor torque
  2. B. 8% in the rotor torque
  3. C. 12% in the rotor torque
  4. D. 16% in the rotor torque

Answer: D. 16% in the rotor torque

Explanation: The correct option is D. 16% in the rotor torque is the standard concept used in induction motor theory.

Question 70. The stating torque of the slip ring induction motor can be increased by adding

  1. A. external inductance to the rotor
  2. B. external resistance to the rotor
  3. C. external capacitance to the rotor
  4. D. both resistance and inductance to rotor

Answer: B. external resistance to the rotor

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 71. A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be

  1. A. 0.01
  2. B. 0.02
  3. C. 0.03
  4. D. 0.04

Answer: D. 0.04

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 72. The complete circle diagram of induetion motor can be drawn with the help of data found from

  1. A. noload test (6) blocked rotor test
  2. C. stator resistance test
  3. D. all of the above

Answer: D. all of the above

Explanation: Induction motor tests provide equivalent circuit data and performance parameters such as losses, current and power factor.

Question 73. In the squirrel-cage induction motor the rotor slots are usually given slight skew

  1. A. to reduce the magnetic hum and locking tendency of the rotor
  2. B. to increase the tensile strength of the rotor bars
  3. C. to ensure easy fabrication
  4. D. none of the above

Answer: A. to reduce the magnetic hum and locking tendency of the rotor

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 74. The torque of a rotor in an induction motor under running condition is maximum

  1. A. at the unit value of slip
  2. B. at the zero value of slip
  3. C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
  4. D. at the value of the slip which makes the rotor reactance half of the rotor

Answer: C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 75. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?

  1. A. The slip of the motor will be 5%
  2. B. The rotor will not run
  3. C. The rotor will run at very high speed
  4. D. The torque produced will be very large

Answer: B. The rotor will not run

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

Question 76. The circle diagram for an induction motor cannot be used to determine

  1. A. efficiency
  2. B. power factor
  3. C. frequency
  4. D. output

Answer: A. efficiency

Explanation: Induction motor tests provide equivalent circuit data and performance parameters such as losses, current and power factor.

Question 77. Blocked rotor test on induction motors is used to find out

  1. A. leakage reactance
  2. B. power factor on short circuit
  3. C. short-circuit current under rated voltage
  4. D. all of the above

Answer: D. all of the above

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 78. Lubricant used for ball bearing is usually

  1. A. graphite
  2. B. grease
  3. C. mineral oil
  4. D. molasses

Answer: B. grease

Explanation: The correct option is B. grease is the standard concept used in induction motor theory.

Question 79. An induction motor can run at synchronous speed when

  1. A. it is run on load
  2. B. it is run in reverse direction
  3. C. it is run on voltage higher than the rated voltage
  4. D. e.m.f. is injected in the rotor circuit

Answer: D. e.m.f. is injected in the rotor circuit

Explanation: Induction motor speed depends mainly on synchronous speed and slip.

Question 80. Which motor is preferred for use in mines where explosive gases exist ?

  1. A. Air motor
  2. B. Induction motor
  3. C. D.C. shunt motor
  4. D. Synchronous motor

Answer: A. Air motor

Explanation: The correct option is A. Air motor is the standard concept used in induction motor theory.

Question 81. The torque developed by a 3-phase induction motor least depends on

  1. A. rotor current
  2. B. rotor power factor
  3. C. rotor e.m.f.
  4. D. shaft diameter

Answer: D. shaft diameter

Explanation: Torque in an induction motor depends on rotor current, rotor power factor, slip and supply voltage.

Question 82. In an induction motor if air-gap is increased

  1. A. the power factor will be low
  2. B. windage losses will be more
  3. C. bearing friction will reduce
  4. D. copper loss will reduce In an induction motor

Answer: A. the power factor will be low

Explanation: The correct option is A. the power factor will be low is the standard concept used in induction motor theory.

Question 83. In induction motor, percentage slip depends on

  1. A. supply frequency
  2. B. supply voltage
  3. C. copper losses in motor
  4. D. none of the above

Answer: C. copper losses in motor

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 84. When /?2 is tne rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running condi-tions will be

  1. A. sR2X2 = 1
  2. B. sR2 = X2
  3. C. R2 = sX2 id) R2 = s2X2
  4. D. None of the above

Answer: C. R2 = sX2 id) R2 = s2X2

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 85. In case of a double cage induction motor, the inner cage has

  1. A. high inductance arid low resistance
  2. B. low inductance and high resistance
  3. C. low inductance and low resistance
  4. D. high inductance and high resis¬tance

Answer: A. high inductance arid low resistance

Explanation: The correct option is A. high inductance arid low resistance is the standard concept used in induction motor theory.

Question 86. The low power factor of induction motor is due to

  1. A. rotor leakage reactance
  2. B. stator reactance
  3. C. the reactive lagging magnetizing current necessary to generate the magnetic flux
  4. D. all of the above

Answer: D. all of the above

Explanation: Induction motors draw magnetizing current, so their power factor changes with load and is low at no-load.

Question 87. Insertion of reactance in the rotor circuit

  1. A. reduces starting torque as well as maximum torque
  2. B. increases starting torque as well as maximum torque
  3. C. increases starting torque but maxi-mum torque remains unchanged
  4. D. increases starting torque but maxi-mum torque decreases

Answer: A. reduces starting torque as well as maximum torque

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 88. Insertion of resistance in the rotcir of an induction motor to develop a given torque

  1. A. decreases the rotor current
  2. B. increases the rotor current
  3. C. rotor current becomes zero
  4. D. rotor current rernains same

Answer: D. rotor current rernains same

Explanation: Torque in an induction motor depends on rotor current, rotor power factor, slip and supply voltage.

Question 89. For driving high inertia loods best type of induction motor suggested is

  1. A. slip ring type
  2. B. squirrel cage type
  3. C. any of the above
  4. D. none of the above

Answer: A. slip ring type

Explanation: The correct option is A. slip ring type is the standard concept used in induction motor theory.

Question 90. Temperature of the stator winding of a three phase induction motor is obtained by

  1. A. resistance rise method
  2. B. thermometer method
  3. C. embedded temperature method
  4. D. all above methods

Answer: D. all above methods

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

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Question 91. The purpose of using short-circuit gear is

  1. A. to short circuit the rotor at slip rings
  2. B. to short circuit the starting resis¬tances in the starter
  3. C. to short circuit the stator phase of motor to form star
  4. D. none of the above

Answer: A. to short circuit the rotor at slip rings

Explanation: The correct option is A. to short circuit the rotor at slip rings is the standard concept used in induction motor theory.

Question 92. In a squirrel cage motor the induced e.m.f. is

  1. A. dependent on the shaft loading
  2. B. dependent on the number of slots
  3. C. slip times the stand still e.m.f. induced in the rotor
  4. D. none of the above

Answer: C. slip times the stand still e.m.f. induced in the rotor

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 93. Less maintenance troubles are experienced in case of

  1. A. and
  2. C. both
  3. D. none of the above

Answer: B.

Explanation: The correct option is B. is the standard concept used in induction motor theory.

Question 94. A squirrel cage induction motor is not selected when

  1. A. initial cost is the main consideration
  2. B. maintenance cost is to be kept low
  3. C. higher starting torque is the main consideration
  4. D. all above considerations are involved

Answer: C. higher starting torque is the main consideration

Explanation: A squirrel-cage induction motor has rotor bars short-circuited by end rings, giving simple and rugged construction.

Question 95. Reduced voltage starter can be used with

  1. A. slip ring motor only but not with squirrel cage induction motor
  2. B. squirrel cage induction motor only but not with slip ring motor
  3. C. squirrel cage as well as slip ring induction motor
  4. D. none of the above

Answer: C. squirrel cage as well as slip ring induction motor

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 96. Slip ring motor is preferred over squirrel cage induction motor where

  1. A. high starting torque is required
  2. B. load torque is heavy
  3. C. heavy pull out torque is required
  4. D. all of the above

Answer: A. high starting torque is required

Explanation: Slip is the difference between synchronous speed and rotor speed expressed as a fraction or percentage of synchronous speed.

Question 97. In a star-delta starter of an induction motor

  1. A. resistance is inserted in the stator
  2. B. reduced voltage is applied to the stator
  3. C. resistance is inserted in the rotor
  4. D. applied voltage per1 stator phase is 57.7% of the line voltage

Answer: D. applied voltage per1 stator phase is 57.7% of the line voltage

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 98. The torque of an induction motor is

  1. A. directly proportional to slip
  2. B. inversely proportional to slip
  3. C. proportional to the square of the slip
  4. D. none of the above

Answer: A. directly proportional to slip

Explanation: Torque in an induction motor depends on rotor current, rotor power factor, slip and supply voltage.

Question 99. The rotor of an induction motor runs at

  1. A. synchronous speed
  2. B. below synchronous speed
  3. C. above synchronous speed
  4. D. any of the above

Answer: B. below synchronous speed

Explanation: The rotor receives power by induction from the stator rotating magnetic field.

Question 100. The starting torque of a three phase induction motor can be increased by

  1. A. and
  2. C. both
  3. D. none of the above

Answer: C. both

Explanation: Starting behavior depends on rotor resistance, applied voltage and slip; starters are used to limit high starting current.

Question 101. Insertion of resistance in the stator of an induction motor

  1. A. increases the load torque
  2. B. decreases the starting torque
  3. C. increases the starting torque
  4. D. none of the above

Answer: B. decreases the starting torque

Explanation: Silicon steel and laminated cores are used to provide a good magnetic path and reduce core losses.

Question 102. The synchronous speed of a 4-pole, 50 Hz induction motor is

  1. A. 750 rpm
  2. B. 1000 rpm
  3. C. 1500 rpm
  4. D. 3000 rpm

Answer: C. 1500 rpm

Explanation: Synchronous speed is Ns = 120f/P = 120 × 50/4 = 1500 rpm.

Question 103. If the rotor speed is 1440 rpm and synchronous speed is 1500 rpm, the slip is

  1. A. 2%
  2. B. 4%
  3. C. 6%
  4. D. 8%

Answer: B. 4%

Explanation: Slip = (Ns - N)/Ns = (1500 - 1440)/1500 = 0.04 or 4%.

Question 104. The rotor frequency of an induction motor is equal to

  1. A. supply frequency
  2. B. slip × supply frequency
  3. C. synchronous speed × slip
  4. D. zero at starting

Answer: B. slip × supply frequency

Explanation: Rotor frequency is fr = s f, where s is slip and f is stator supply frequency.

Question 105. At starting, the slip of a three-phase induction motor is

  1. A. 0
  2. B. 0.5
  3. C. 1
  4. D. greater than 1

Answer: C. 1

Explanation: At starting rotor speed is zero, so slip becomes 1.

Question 106. At synchronous speed, the torque of an induction motor is

  1. A. maximum
  2. B. zero
  3. C. rated
  4. D. double rated

Answer: B. zero

Explanation: At synchronous speed there is no relative speed between rotor and rotating field, so rotor emf and torque become zero.

Question 107. The rotating magnetic field in a three-phase induction motor is produced by

  1. A. rotor current only
  2. B. three-phase stator supply
  3. C. commutator
  4. D. external DC supply

Answer: B. three-phase stator supply

Explanation: A balanced three-phase supply in stator windings produces a rotating magnetic field.

Advanced Induction Motor MCQs

These questions cover torque-slip characteristics, circle diagram, tests, speed control, losses and numerical concepts.

Question 108. The direction of rotation of a three-phase induction motor can be reversed by

  1. A. changing frequency
  2. B. interchanging any two supply phases
  3. C. increasing load
  4. D. reducing voltage

Answer: B. interchanging any two supply phases

Explanation: Interchanging any two phases reverses the phase sequence and hence the direction of rotating field.

Question 109. The speed of an induction motor is always

  1. A. equal to synchronous speed
  2. B. less than synchronous speed in motoring mode
  3. C. greater than synchronous speed in motoring mode
  4. D. zero

Answer: B. less than synchronous speed in motoring mode

Explanation: In motoring operation, rotor must run slightly below synchronous speed to produce torque.

Question 110. The main advantage of a squirrel-cage induction motor is

  1. A. complex construction
  2. B. low maintenance
  3. C. poor efficiency
  4. D. need of brushes

Answer: B. low maintenance

Explanation: Squirrel-cage motors have simple and rugged construction with no brushes or slip rings.

Question 111. A slip-ring induction motor is mainly used where

  1. A. very low starting torque is needed
  2. B. high starting torque and speed control are required
  3. C. no starting current exists
  4. D. single-phase supply is used

Answer: B. high starting torque and speed control are required

Explanation: External rotor resistance can be added in slip-ring motors to improve starting torque and speed control.

Question 112. The starting current of a squirrel-cage induction motor is generally

  1. A. very low
  2. B. zero
  3. C. 5 to 7 times full-load current
  4. D. equal to no-load current

Answer: C. 5 to 7 times full-load current

Explanation: At starting, back emf effect is small and the motor draws high inrush current.

Question 113. A star-delta starter reduces starting voltage per phase to

  1. A. 100% of line voltage
  2. B. 57.7% of line voltage
  3. C. 33% of line voltage
  4. D. zero

Answer: B. 57.7% of line voltage

Explanation: In star connection, phase voltage is line voltage divided by √3, about 57.7%.

Question 114. In a star-delta starter, starting torque becomes approximately

  1. A. three times DOL torque
  2. B. same as DOL torque
  3. C. one-third of DOL torque
  4. D. zero

Answer: C. one-third of DOL torque

Explanation: Torque is proportional to voltage squared; reduced phase voltage gives about one-third starting torque.

Question 115. The torque of an induction motor is approximately proportional to supply voltage

  1. A. directly
  2. B. inversely
  3. C. squared
  4. D. independent

Answer: C. squared

Explanation: Induction motor torque is approximately proportional to V².

Question 116. If supply voltage is reduced by 10%, torque becomes approximately

  1. A. 90%
  2. B. 81%
  3. C. 100%
  4. D. 50%

Answer: B. 81%

Explanation: Torque varies as V², so 0.9² = 0.81 or 81%.

Question 117. The power factor of an induction motor at no-load is low because of

  1. A. large active current
  2. B. magnetizing current
  3. C. zero flux
  4. D. high mechanical output

Answer: B. magnetizing current

Explanation: At no-load, the motor draws mainly magnetizing current, which is highly lagging.

Question 118. The power factor of an induction motor improves when

  1. A. load increases from no-load to rated load
  2. B. load becomes zero
  3. C. frequency becomes zero
  4. D. slip becomes zero

Answer: A. load increases from no-load to rated load

Explanation: With load, active current component increases, improving the power factor.

Question 119. The air gap in an induction motor is kept small to

  1. A. increase magnetizing current
  2. B. reduce magnetizing current
  3. C. increase noise
  4. D. increase leakage

Answer: B. reduce magnetizing current

Explanation: A small air gap reduces reluctance and magnetizing current.

Question 120. Skewing of rotor bars helps to reduce

  1. A. cogging and noise
  2. B. rated voltage
  3. C. supply frequency
  4. D. stator resistance

Answer: A. cogging and noise

Explanation: Skewed rotor bars reduce magnetic locking, cogging, and magnetic hum.

Question 121. Crawling in induction motors is mainly due to

  1. A. harmonics
  2. B. high bearing friction
  3. C. open circuit stator
  4. D. low copper loss

Answer: A. harmonics

Explanation: Crawling is caused by space harmonics, especially the 7th harmonic.

Question 122. Cogging is also called

  1. A. magnetic locking
  2. B. dynamic braking
  3. C. regeneration
  4. D. slip control

Answer: A. magnetic locking

Explanation: Cogging occurs when rotor and stator teeth magnetically lock and the motor fails to start.

Question 123. Blocked rotor test of an induction motor is similar to

  1. A. open-circuit test of transformer
  2. B. short-circuit test of transformer
  3. C. polarity test
  4. D. temperature test only

Answer: B. short-circuit test of transformer

Explanation: The blocked rotor test is analogous to transformer short-circuit test.

Question 124. No-load test of an induction motor is used to determine

  1. A. no-load losses and magnetizing branch
  2. B. short-circuit current only
  3. C. rotor resistance only
  4. D. synchronous speed only

Answer: A. no-load losses and magnetizing branch

Explanation: No-load test gives information about no-load current, core loss, friction, and windage loss.

Question 125. The mechanical losses of an induction motor include

  1. A. stator copper loss
  2. B. rotor copper loss
  3. C. friction and windage loss
  4. D. core loss only

Answer: C. friction and windage loss

Explanation: Mechanical losses mainly include bearing friction and windage loss.

Question 126. Rotor copper loss in an induction motor is equal to

  1. A. s × rotor input
  2. B. rotor output/s
  3. C. stator input
  4. D. zero at starting

Answer: A. s × rotor input

Explanation: Rotor copper loss is slip times rotor input power.

Question 127. Mechanical power developed in rotor is equal to

  1. A. rotor input
  2. B. rotor input minus rotor copper loss
  3. C. stator copper loss
  4. D. core loss

Answer: B. rotor input minus rotor copper loss

Explanation: Part of air-gap power is lost as rotor copper loss; the remainder becomes mechanical power developed.

Question 128. The ratio of rotor copper loss to mechanical power developed is

  1. A. s/(1-s)
  2. B. 1/s
  3. C. (1-s)/s
  4. D. s²

Answer: A. s/(1-s)

Explanation: Rotor copper loss : mechanical power developed = s : (1-s).

Question 129. At maximum torque, rotor resistance and rotor reactance relation is

  1. A. R2 = sX2
  2. B. R2 = X2/s
  3. C. R2 = 0
  4. D. X2 = 0

Answer: A. R2 = sX2

Explanation: For running maximum torque, condition is R2 = sX2, where X2 is standstill rotor reactance.

Question 130. Increasing rotor resistance in a slip-ring induction motor

  1. A. improves starting torque up to a limit
  2. B. always decreases starting torque
  3. C. has no effect
  4. D. stops rotating field

Answer: A. improves starting torque up to a limit

Explanation: External rotor resistance improves starting torque and starting power factor up to an optimum value.

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Question 131. External resistance cannot be added in the rotor circuit of

  1. A. slip-ring motor
  2. B. wound-rotor motor
  3. C. squirrel-cage motor
  4. D. phase-wound motor

Answer: C. squirrel-cage motor

Explanation: A squirrel-cage rotor is permanently short-circuited by end rings, so external rotor resistance cannot be inserted.

Question 132. For variable frequency speed control, the ratio generally kept constant is

  1. A. V/f
  2. B. I/R
  3. C. P/V
  4. D. R/X only

Answer: A. V/f

Explanation: V/f is kept nearly constant to maintain air-gap flux.

Question 133. If frequency is reduced without reducing voltage, the motor may suffer from

  1. A. over-fluxing
  2. B. zero current
  3. C. reverse rotation only
  4. D. no torque

Answer: A. over-fluxing

Explanation: Reducing frequency at same voltage increases flux and may saturate the core.

Question 134. The most common modern method for induction motor speed control is

  1. A. VFD control
  2. B. manual pulley control
  3. C. only star-delta control
  4. D. fuse control

Answer: A. VFD control

Explanation: Variable Frequency Drives provide smooth and efficient speed control.

Question 135. Induction motor speed control by changing poles is known as

  1. A. pole changing method
  2. B. slip braking
  3. C. plugging
  4. D. rheostatic heating

Answer: A. pole changing method

Explanation: Changing the number of stator poles changes synchronous speed.

Question 136. The synchronous speed is inversely proportional to

  1. A. number of poles
  2. B. supply voltage
  3. C. rotor resistance
  4. D. load current

Answer: A. number of poles

Explanation: Ns = 120f/P, so synchronous speed decreases when poles increase.

Question 137. A 2-pole, 50 Hz motor has synchronous speed of

  1. A. 750 rpm
  2. B. 1000 rpm
  3. C. 1500 rpm
  4. D. 3000 rpm

Answer: D. 3000 rpm

Explanation: Ns = 120 × 50 / 2 = 3000 rpm.

Question 138. A 6-pole, 50 Hz motor has synchronous speed of

  1. A. 500 rpm
  2. B. 750 rpm
  3. C. 1000 rpm
  4. D. 1500 rpm

Answer: C. 1000 rpm

Explanation: Ns = 120 × 50 / 6 = 1000 rpm.

Question 139. The slip of an induction motor is usually expressed in

  1. A. volts
  2. B. percent
  3. C. henry
  4. D. tesla

Answer: B. percent

Explanation: Slip is commonly written as a percentage of synchronous speed.

Question 140. When load on an induction motor increases, slip generally

  1. A. increases
  2. B. decreases
  3. C. becomes zero
  4. D. becomes negative always

Answer: A. increases

Explanation: More load requires more torque, so rotor slows slightly and slip increases.

Question 141. Negative slip occurs during

  1. A. motoring
  2. B. generating operation
  3. C. starting
  4. D. blocked rotor test

Answer: B. generating operation

Explanation: When rotor speed exceeds synchronous speed in the same direction, slip becomes negative and the machine acts as a generator.

Question 142. Plugging of an induction motor is obtained by

  1. A. removing supply
  2. B. interchanging two stator phases
  3. C. reducing bearing friction
  4. D. opening rotor bars

Answer: B. interchanging two stator phases

Explanation: Plugging reverses rotating field by interchanging two phases, producing braking torque.

Question 143. Regenerative braking in induction motor occurs when rotor speed is

  1. A. less than synchronous speed
  2. B. equal to zero
  3. C. greater than synchronous speed
  4. D. equal to half speed

Answer: C. greater than synchronous speed

Explanation: Above synchronous speed, induction machine feeds power back to the supply.

Question 144. A single-phase induction motor is not self-starting because

  1. A. it has no winding
  2. B. single-phase supply produces pulsating field
  3. C. rotor is open circuit
  4. D. frequency is zero

Answer: B. single-phase supply produces pulsating field

Explanation: A single-phase supply produces a pulsating magnetic field, not a starting rotating field.

Question 145. The starting winding in a single-phase induction motor is used to

  1. A. create phase difference and starting torque
  2. B. reduce insulation
  3. C. stop the rotor
  4. D. remove main winding

Answer: A. create phase difference and starting torque

Explanation: Auxiliary winding helps create a rotating field for starting.

Question 146. Capacitor-start motors have

  1. A. high starting torque
  2. B. zero starting torque
  3. C. only DC supply
  4. D. no auxiliary winding

Answer: A. high starting torque

Explanation: A capacitor improves phase shift and starting torque.

Question 147. The rotor bars of squirrel-cage motors are usually made of

  1. A. copper or aluminium
  2. B. mica only
  3. C. wood
  4. D. rubber

Answer: A. copper or aluminium

Explanation: Rotor bars are commonly made from aluminium or copper.

Question 148. Induction motor stator core is laminated to reduce

  1. A. eddy current loss
  2. B. shaft friction
  3. C. load torque
  4. D. air gap

Answer: A. eddy current loss

Explanation: Laminations increase electrical resistance to eddy currents and reduce core loss.

Question 149. The stator winding of a three-phase induction motor is placed in

  1. A. stator slots
  2. B. shaft
  3. C. bearings
  4. D. end rings

Answer: A. stator slots

Explanation: Stator conductors are placed in slots on the inner surface of the stator core.

Question 150. End rings in a squirrel-cage rotor are used to

  1. A. short-circuit rotor bars
  2. B. insulate rotor bars
  3. C. increase frequency
  4. D. reduce stator current to zero

Answer: A. short-circuit rotor bars

Explanation: End rings connect rotor bars at both ends and form closed circuits.

Question 151. A wound rotor induction motor has

  1. A. slip rings and brushes
  2. B. commutator only
  3. C. no rotor winding
  4. D. permanent magnets

Answer: A. slip rings and brushes

Explanation: Wound rotor motors use slip rings and brushes to connect external resistance.

Question 152. The rated output of an induction motor is usually expressed in

  1. A. kW or horsepower
  2. B. kVAR only
  3. C. Hz only
  4. D. ohms only

Answer: A. kW or horsepower

Explanation: Motor output power is usually given in kW or HP at the shaft.

Question 153. The efficiency of an induction motor is output power divided by

  1. A. input power
  2. B. slip
  3. C. frequency
  4. D. rotor speed

Answer: A. input power

Explanation: Efficiency is the ratio of useful output power to electrical input power.

Question 154. A low power factor causes

  1. A. higher current for same power
  2. B. zero current
  3. C. less apparent power
  4. D. no copper loss

Answer: A. higher current for same power

Explanation: For the same real power, low power factor increases line current.

Question 155. The full-load slip of a normal induction motor is generally

  1. A. very small
  2. B. 100%
  3. C. more than 50%
  4. D. negative

Answer: A. very small

Explanation: Normal induction motors operate with small slip at full load.

Question 156. The rotor current frequency at standstill is

  1. A. zero
  2. B. supply frequency
  3. C. twice supply frequency
  4. D. one-third supply frequency

Answer: B. supply frequency

Explanation: At standstill slip is 1, so rotor frequency equals stator supply frequency.

Question 157. The rotor current frequency during normal running is

  1. A. very low compared with supply frequency
  2. B. always 50 Hz
  3. C. zero only
  4. D. higher than supply frequency

Answer: A. very low compared with supply frequency

Explanation: At small slip, rotor frequency is s times supply frequency and is therefore low.

Question 158. The main field in an induction motor rotates at

  1. A. synchronous speed
  2. B. rotor speed
  3. C. zero speed
  4. D. twice rotor speed

Answer: A. synchronous speed

Explanation: The stator rotating magnetic field rotates at synchronous speed.

Question 159. The air-gap power of an induction motor is transferred from

  1. A. stator to rotor
  2. B. shaft to supply only
  3. C. bearings to frame
  4. D. cooling fan to shaft

Answer: A. stator to rotor

Explanation: Electromagnetic power crosses the air gap from stator field to rotor.

Question 160. The main reason for using starters in large induction motors is to

  1. A. limit starting current
  2. B. increase supply frequency
  3. C. remove rotor
  4. D. make motor DC operated

Answer: A. limit starting current

Explanation: A starter reduces high starting current and protects supply and motor.

Question 161. Which starter gives smooth acceleration for a large induction motor?

  1. A. soft starter
  2. B. direct fuse
  3. C. open switch
  4. D. reverse relay only

Answer: A. soft starter

Explanation: Soft starters gradually increase voltage and reduce mechanical and electrical stress.

Quick Answer Key

This answer key helps in fast revision before exams.

Q. No.OptionCorrect Answer
1CStator core
2Bcast iron
3Astiff
4Ccarbon steel
5Aless than 1%
6C4%
7Dreduce magnetic hum
8Bthe power factor will decrease
9Cphospor bronze
10C2 Hz
11C(l-s)Ns
12B80 to 90%
13Dnone
14Alow
15Ctwo parallel windings in rotor
16Asingle phase motors
17Dinduction motors
18Ddirectly proportional to slip
19B6 poles
20Bsmall
21Dasynchronous motor
22Bthe same frequency as the slip fre-quency
23ABy changing the number of stator poles
24Charmonics develped in the motor
25C
26CK2 x torque with direct switching
27Bin parallel
28Amotor takes five to seven times its full load current
29BChanging the number of poles
30B
31Brotor current
32B
33Athe motor will run in reverse direction
34Cself-starting with low torque
35Dall of the above
36Brotor conductors are short-circuited through end rings
37Dequal to number of poles in stator
38Alow horsepower motors
39Cchanging winding resistance
40Dharmonics developed in the motor
41A0.2 lagging
42Dhaving number of rotor slots more or less than the number of stator slots (not equal)
43Ccrawling
44Crotor speed is more than the syn-chronous speed of the field and are in the same direction
45Bsmaller
46Awill continue running burning its one phase
47Cwill stop and carry heavy current causing permanent damage to its winding
48Cany one of the above
49Cstator slots are not exact multiple of rotor slots
50Dall above features are required
51Dits power factor goes on increasing upto full load and then it falls again
52Bin the same direction as the direction of the field
53Cmotor takes five to seven times its fullload current
54BD.C. shunt motor
55ABall bearings
56Dget heated and subsequently get damaged
57A730 r.p.m.
58B
59B1.5 times the full load torque
60Awindage losses
61Diron losses in stator will be more than that in rotor
62Dinterchanging two supply phases for quick stopping
63DBlock rotor test, no-load test and stator resistance test
64Aimprove starting torque
65Bpower factor at starting is low as compared to that while running
66Bshort-circuit test only
67Cshort-circuit test only
68Crectangular parabola
69D16% in the rotor torque
70Bexternal resistance to the rotor
71D0.04
72Dall of the above
73Ato reduce the magnetic hum and locking tendency of the rotor
74Cat the value of the slip which makes rotor reactance per phase equal to the resistance per phase
75BThe rotor will not run
76Aefficiency
77Dall of the above
78Bgrease
79De.m.f. is injected in the rotor circuit
80AAir motor
81Dshaft diameter
82Athe power factor will be low
83Ccopper losses in motor
84CR2 = sX2 id) R2 = s2X2
85Ahigh inductance arid low resistance
86Dall of the above
87Areduces starting torque as well as maximum torque
88Drotor current rernains same
89Aslip ring type
90Dall above methods
91Ato short circuit the rotor at slip rings
92Cslip times the stand still e.m.f. induced in the rotor
93B
94Chigher starting torque is the main consideration
95Csquirrel cage as well as slip ring induction motor
96Ahigh starting torque is required
97Dapplied voltage per1 stator phase is 57.7% of the line voltage
98Adirectly proportional to slip
99Bbelow synchronous speed
100Cboth
101Bdecreases the starting torque
102C1500 rpm
103B4%
104Bslip × supply frequency
105C1
106Bzero
107Bthree-phase stator supply
108Binterchanging any two supply phases
109Bless than synchronous speed in motoring mode
110Blow maintenance
111Bhigh starting torque and speed control are required
112C5 to 7 times full-load current
113B57.7% of line voltage
114Cone-third of DOL torque
115Csquared
116B81%
117Bmagnetizing current
118Aload increases from no-load to rated load
119Breduce magnetizing current
120Acogging and noise
121Aharmonics
122Amagnetic locking
123Bshort-circuit test of transformer
124Ano-load losses and magnetizing branch
125Cfriction and windage loss
126As × rotor input
127Brotor input minus rotor copper loss
128As/(1-s)
129AR2 = sX2
130Aimproves starting torque up to a limit
131Csquirrel-cage motor
132AV/f
133Aover-fluxing
134AVFD control
135Apole changing method
136Anumber of poles
137D3000 rpm
138C1000 rpm
139Bpercent
140Aincreases
141Bgenerating operation
142Binterchanging two stator phases
143Cgreater than synchronous speed
144Bsingle-phase supply produces pulsating field
145Acreate phase difference and starting torque
146Ahigh starting torque
147Acopper or aluminium
148Aeddy current loss
149Astator slots
150Ashort-circuit rotor bars
151Aslip rings and brushes
152AkW or horsepower
153Ainput power
154Ahigher current for same power
155Avery small
156Bsupply frequency
157Avery low compared with supply frequency
158Asynchronous speed
159Astator to rotor
160Alimit starting current
161Asoft starter
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Frequently Asked Questions on Induction Motor

What is an induction motor?

An induction motor is an AC motor in which rotor current is produced by electromagnetic induction from the stator rotating magnetic field.

Why is slip necessary in an induction motor?

Slip is necessary because relative speed between rotor and rotating magnetic field is required to induce rotor emf and produce torque.

Which induction motor has high starting torque?

A slip-ring induction motor can provide high starting torque by adding external resistance in the rotor circuit.

Why are squirrel-cage induction motors widely used?

They are widely used because they are simple, strong, economical and require less maintenance.

What is crawling in an induction motor?

Crawling is the tendency of an induction motor to run at a very low speed, usually around one-seventh of synchronous speed, due to harmonics.

What is cogging in an induction motor?

Cogging is magnetic locking between stator and rotor teeth, due to which the motor may fail to start.

Conclusion

These 161+ induction motor MCQ questions are useful for building a strong foundation in Electrical Machines. Students preparing for GATE Electrical, SSC JE Electrical, RRB JE, diploma exams, ITI exams and technical interviews should revise these questions regularly. For better results, first try to solve each question yourself and then read the answer explanation.

Suggested Blogger Labels

Induction Motor, Electrical Machines, Electrical Engineering MCQ, Three Phase Induction Motor, GATE Electrical, SSC JE Electrical, RRB JE, Diploma Electrical, Interview Questions

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