Induction motor - -Electrical Engineering (MCQ) questions and answers

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

A. Bearings
B. Shaft
C. Winding
D. Stator core

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

A. silicon steel
B. aluminium
C. cast iron
D. bronze

3. The shaft of an induction motor is made of

A. flexible
B. hollow
C. stiff
D. any of the above

4. The shaft of an induction motor is made of

A. high speed steel
B. carbon steel
C. stainless steel
D. cast iron

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

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

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

A. 0.04%
B. 0.4%
C. 14%
D. 4%

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

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

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

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

9. Slip rings are usually made of

A. phospor bronze
B. copper
C. carbon
D. aluminium

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

A. 200 Hz
B. 50 Hz
C. 2 Hz
D. 0.2 Hz

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

A. Ns
B. s.N,
C. (Ns-l)s
D. (l-s)Ns

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

A. 60 to 90%
B. 95 to 98%
C. 80 to 90%
D. 99%

13. . A double squirrel-cage induction motor has

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

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

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

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

A. two
B. three
C. four
D. none

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

A. variable speed motors
B. low horse power motors
C. high speed motors
D. single phase motors

17.The term ‘cogging’ is associated with

A. induction motors
B. three phase transformers
C. compound generators
D. D.C. series motors

18. In case of the induction motors the torque is

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

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

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

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

A. absent
B. infinity
C. small
D. Large

21. An induction motor is identical to

A. D.C. compound motor
B. D.C. series motor
C. synchronous motor
D. asynchronous motor

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

A. zero frequency
B. the same phase as the rotor e.m.f.
C. the same frequency as the slip frequency
D. high value for the satisfactory speed control

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

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

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

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

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

A. star connected only
B. delta connected only
C. none of the above
D. (a) and (b) both

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

A. k/torque with direct switching
B. K x torque with direct switching
C. k2/torque with direct switching
D. K2 x torque with direct switching

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

A. in parallel
B. in series
C. in series-parallel
D. in parallel with stator

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

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

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

A. Changing the number of poles
B. e.m.f. injection in rotor circuit
C. Cascade operation
D. None of the above

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

A. squirrel-cage induction motors only
B. Synchronous motor
C. slip ring induction motors only
D. both (a) and (c)

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

A. slip
B. running torque
C. line voltage
D. rotor current

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

A. Squirrel-cage induction motor
B. Double cage motor
C. Slip-ring induction motor
D. Both (a) and (b)

33. An induction motor is

A. self-starting with zero torque
B. self-starting with high torque
C. self-starting with low torque
D. non-self starting

34. If any two phases for an induction motor are interchanged

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

35. The maximum torque in an induction motor depends on

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

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

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

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

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

38. DOL starting of induction motors is usually restricted to

A. variable speed motors
B. high horsepower motors
C. high speed motors
D. low horsepower motors

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

A. changing winding resistance
B. changing supply frequency
C. changing number of poles
D. reducing supply voltage

40. The ‘crawling” in an induction motor is caused by

A. high loads
B. low voltage supply
C. harmonics developed in the motor
D. improper design of machine

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

A. 0.2 leading
B. 0.5 leading
C. unity
D. 0.2 lagging

42. The ‘cogging’ of an induction motor can be avoided by

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

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

A. humming
B. hunting
C. crawling
D. cogging

44. Slip of an induction motor is negative when

A. magnetic field and rotor rotate in opposite direction
B. rotor speed is more than the synchronous speed of the field and are in the same direction
C. rotor speed is less than the synchronous speed of the field and are in the same direction
D. none of the above

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

A. bigger
B. same
C. any of the above
D. smaller

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

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

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

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

48. Low voltage at motor terminals is due to

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

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

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

50. As load on an induction motor goes on increasing

A. its power factor goes on decreasing
B. its power factor remains constant
C. its power factor goes on increasing up to full load and then it falls again
D. its power factor goes on increasing even after full load

51. Slip ring motor is recommended where

A. frequent starting, stopping and reversing is required
B. speed control is required
C. high starting torque is needed
D. all above features are required

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

A. in the opposite direction as the direction of the rotating field
B. in any direction depending upon phase squence of supply
C. in the same direction as the direction of the field
D. Will not move at all

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

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

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

A. D.C. series motor
B. D.C. shunt motor
C. universal motor
D. none of the above

55. Which type of bearing is provided in small induction motors to support the rotor shaft ? None of the above

A. Cast iron bearings
B. Bush bearings
C. Both A and B
D. Ball bearings

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

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

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 ?

A. 730 r.p.m.
B. 960 r.p.m.
C. 1440 r.p.m.
D. 2880 r.p.m.

58. A 3-phase slip ring motor has

A. double cage rotor
B. short-circuited rotor
C. any of the above
D. wound rotor

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

A. 1.5 times the full load torque
B. equal to full load torque
C. twice the full load torque
D. Lower than full load torque

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

A. copper losses
B. transformation ratio
C. windage losses
D. power scale of circle diagram