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Wednesday, 9 March 2016

Three Phase Induction Motor - Multiple Choice Questions

Three Phase Induction Motor - Multiple Choice Questions Points : three phase induction motor, multiple choice questions and answers, electrical objective type question answers, mcqs 1. In a 3-phase induction motor, the rotor speed is ……… the synchronous speed.
(i) greater than
(ii) smaller than
(iii) equal to
(iv) none of the above

2. A 4-pole, 50 Hz induction motor operates at 5% slip. The frequency of e.m.f. induced in the rotor will be ..............
(i) 25Hz
(ii) 50Hz
(iii) 2.5 Hz
(iv) none of the above

3. The air-gap between stator and rotor of a 3-phase induction motor ranges from …………
(i) 2 cm to 4 cm
(ii) 0.4mm to 4 mm
(iii) 1 cm to 2 cm
(iv) 4 cm to 6 cm

4. A high starting torque can be obtained in a 3-phase induction motor by …………..
(i) increasing rotor resistance
(ii) decreasing rotor resistance
(iii) increasing rotor reactance
(iv) none of the above

5. If the frequency of 3-phase supply to the stator of a 3-phase induction motor is increased, then synchronous speed ………….
(i) is decreased
(ii) is increased
(iii) remains unchanged
(iv) none of the above

6. The rotor winding of a 3-phase wound rotor induction motor is generally …………. connected.
(i) star
(ii) delta
(iii) partly star and partly delta
(iv) none of the above

7. In a wound rotor motor, the rotor winding is wound for poles ………… that of stator winding.
(i) equal
(ii) eater than
(iii) small than
(iv) none of the above

8. When an induction motor is running at full-load, rotor reactance is ……… rotor resistance.
(i) comparable to
(iii) very large compared to
(iii) large compared to
(iv) none of the above

9. When a 3-phase induction motor is at no load, the slip is
(i) 1
(ii) 0.5
(iii) practically zero
(iv) 02

10. The stator of a 3-phase induction motor produces ………. magnetic field.
(i) steady
(ii) rotating
(iii) alternating
(iv) none of the above

11. If a 4-pole induction motor has a synchronous speed of 1500 r.p.m., then, supply frequency is …………
(i) 50Hz
(ii) 25 Hz
(iii) 60 Hz
(iv) none of the above

12. The advantage of wound rotor motor is that …………….
(i) it is inexpensive
(ii) it requires less maintenance
(iii) external resistance can be inserted in the rotor circuit
(iv) none of the above

13. The rotor current in a 3-phase induction motor is …………. slip.
(i) inversely proportional to
(ii) directly proportional to
(iii) independent of
(iv) none of the above

14. The starting torque of a 3-phase induction motor is …………. supply voltage.
(i) independent of
(ii) directly proportional to
(iii) directly proportional to square of
(iv) none of the above

15. A 3-phase induction motor is …………..
(i) essentially a constant-speed motor
(ii) a variable speed motor
(iii) very costly
(iv) not easily maintainable

16. When the rotor of a 3-phase induction motor is blocked, the slip is …………..
(i) zero
(ii) 05
(iii) 0.1
(iv) 1

17. An induction motor is preferred to a d.c. motor because it …………….
(i) provides high starting torque
(ii) provides fine speed control
(iii) has simple and rugged construction
(iv) none of the above

18. The reactance of the rotor circuit of a 3-phase induction motor is maximum at ……………
(i) no-load
(ii) full-load
(iii) half full-Load
(iv) starting

19. The torque characteristic of a 3-phase induction motor is similar to that of …………….
(i) d.c. series motor
(ii) d.c. shunt motor
(iii) d.c. differentially compounded motor
(iv) d.c. cumulatively compounded motor

20. The synchronous speed of a 3-phase induction motor having 20 poles and connected to a 50 Hz source is ………..
(i) 600 r.p.m
(ii) 1000 r.p.m
(iii) 1200 rpm
(iv) 300 r.p.m

21. At starting, rotor reactance of a 3-phase induction motor is ………. rotor resistance.
(i) small as compared to
(ii) equal to that of
(iii) large as compared to
(iv) none of the above

22. If the slip of a 3-phase induction motor increases, the p.f. of the rotor circuit
(i) is increased
(ii) is decreased
(iii) remains unchanged
(iv) none of the above

23. The full-load slip of a 3-phase induction motor ranges from ……………
(i) 10% to 20%
(ii) 20% to 30%
(iii) 2% to 5%
(iv) none of the above

24. The maximum voltage is induced in the rotor of a 3 phase induction motor when it …........
(i) runs at no load
(ii) runs at full load
(iii) is blocked
(iv) none of the above

25. 3-phase wound rotor motors are also called …………… motors.
(i) synchronous
(ii) slip ring
(iii) series
(iv) commutator

26. At no-load, the rotor core loss of a 3-phase induction motor is
(i) large
(ii) small
(iii) practically zero
(iv) none of the above

27. The direction of rotation of field in a 3-phase induction motor depends upon …….
(i) number of poles
(ii) magnitude of supply voltage
(iii) supply frequency
(iv) phase sequence of supply voltage

28. Friction and windage loss of a 3-phase induction motor are ……………
(i) maximum at no-load
(ii) minimum at no-load
(iii) remains the same at all loads
(iv) none of the above

29. A 3-phase induction motor is running at 2% slip. If the input to rotor is 1000 W, then mechanical power developed by the motor is ………….
(i) 20W
(ii) 980W
(iii) 500W
(iv) 200W

30. The starting torque of an induction motor is maximum when rotor resistance per phase is ………. rotor reactance/phase.
(i) equal to
(ii) two times
(iii) four times
(iv) none of the above

31. A wound rotor motor is mainly used in applications where …………….
(i) high starting torque is required
(ii) speed control is required
(iii) less costly motor is not required
(iv) high rotor resistance is required during running

32. If the supply voltage of a 3-phase induction motor is increased two times, then, torque is ……….
(i) increased two times
(ii) decreased two times
(iii) increased four times
(iv) decreased four times

33. The conditions of an induction motor at no-load resemble those of a transformer whose secondary is …………
(i) short circuited
(ii) open-circuited
(iii) supplying a variable resistive load
(iv) none of the above

34. The approximate efficiency of a 3-phase, 50 Hz, 4-pole induction motor running at 1350 r.p.m. is ………….
(i) 90%
(ii) 40%
(iii) 65%
(iv) none of the above

35. The speed of a squirrel cage induction motor is changed by ……………..
(i) pole changing
(ii) rheostatic control
(iii) cascade control
(iv) none of the above

36. If the air gap between the rotor and stator of a 3-phase induction motor is increased, then ………..
(i) no-load current is decreased
(ii) leakage reactances are decreased
(iii) leakage reactances are increased
(iv) none of the above

37. In a double squirrel cage induction motor, the outer cage winding has …………….
(i) high inductance
(ii) Low resistance
(iii) high resistance
(iv) none of the above

38. The 3-phase induction motor is so designed that the rotor should have ……….. under running conditions.
(i) high resistance
(ii) high reactance
(iii) large slip
(iv) low resistance

39. A four-speed squirrel cage induction motor uses ……… stator windings.
(i) four
(ii) three
(iii) one
(iv) two

40. The conditions of induction motor on load resemble those of a transformer whose secondary is …………….
(i) open-circuited
(ii) short-circuited
(iii) supplying a variable resistive load
(iv) none of the above

41. Wound rotor motors are less extensively used than squirrel cage motors because …………..
(i) slip rings are required on the rotor circuit
(ii) rotor windings are generally Y-connected
(iii) they are costly and require greater maintenance
(vi) none of the above

42. In a squirrel cage motor, the number of stator slots is ………. rotor slots.
(i) always equal to the number of
(ii) always greater than the number of
(iii) always less than the number of
(iv) either more or less than the number of

43. The conditions of induction motor at stands till resemble those of a transformer whose secondary is
(i) short-circuited
(ii) open-circuited
(iii) supplying a variable resistive load
(iv) none of the above

44. Very large 3-phase induction motors (> 25 H.P.) are started ………………..
(i) direct on line
(ii) star-delta starting
(iii) by auto transformer starting
(iv) none of the above

45. The magnetising current drawn by a 3-phase induction motor is about ………. of full-load stator current.
(i) 5%
(ii) 10 to 15%
(iii) 15 to 20%
(iv) 30 to 50%

46. If the rotor slots are made deeper, rotor resistance is ……………….
(i) increased at starting
(ii) decreased at starting
(iii) increased under running conditions
(iv) none of the above

47. The maximum torque of a 3-phase induction motor under running conditions is ………….
(i) inversely proportional to supply voltage
(ii) inversely proportional to rotor reactance at standstill
(iii) directly proportional to rotor resistance
(iv) none of the above

48. One of the speeds of a 2-speed squirrel cage induction motor is 800 r.p.m. (lower speed). The other speed will be …………..
(i) 400 r.p.m.
(ii) 2400 r.p.m.
(iii) 1600 r.p.m.
(iv) 1200 r.p.m.

49. In a squirrel cage induction motor, the number of rotor bars is generally …………….
(i) even number
(ii) equal to stator poles
(iii) prime number
(iv) none of the above

50. For higher efficiency of 3-phase induction motor, the slip should be …………..
(i) large
(ii) very large
(iii) as small as possible
(iv)1

51. In a squirrel cage rotor, the bars are not placed parallel to the shaft but are skewed to have ……………
(i) greater mechanical strength
(ii) less rotor losses
(iii) uniform torque
(iv) none of the above

52. For the same kVA rating, the leakage flux in induction motor is ………… that of transformer.
(i) more than
(ii) less than
(iii) about the same
(iv) none of the above

53. If the slip of induction motor increases, then, current in the stator winding …………..
(i) is increased
(ii) is decreased
(iii) remains unchanged
(iv) none of the above

54. The frequency of rotor current in a 6-pole, 50 Hz, 3-phase induction motor running at 950 r.p.m. is
(i) 25 Hz
(ii) 15 Hz
(iii) 5 Hz
(iv) 005 Hz

55. The rotor of induction motor
(i) always revolves in the direction of stator flux
(ii) always revolves in the direction opposite to stator flux
(iii) insufficient dale
(iv) none of above

56. At starting of a double squirrel cage induction motor …………….
(i) lower cage winding has low reactance
(ii) outer cage winding has good p.f.
(iii) lower winding is more effective
(iv) none of the above

57. In a squirrel cage induction motor, the maximum torque developed under running conditions is …………
(i) equal to starting torque
(ii) full-load torque
(iii) less than starting torque
(iv) much higher than full-load torque

58. An 8-pole alternator runs at 750 r.p.m. and supplies power to a 6-pole induction motor which has a full-load slip of 3%. The full-load speed of the motor is
(i) 1050 r.p.m.
(ii) 970 r.p.m.
(iii) 960 r.p.m.
(iv) 1250 r.p.m.

59. What is the approximate efficiency of a 60Hz, 6-pole, 3-phase induction motor running at 1050 r.p.m.?
(i) 72.5%
(ii) 81.2%
(iii) 76.8%
(iv) 87.5%

60. The no-load speed of an induction motor depends upon
(i) the supply frequency
(ii) the number of its poles
(iii) the maximum flux/phase
(iv) only (i) and (ii)

61. The starting torque of a 3-phase induction motor has maximum value. The rotor power factor is then
(i) 0.8 lag
(ii) 0.5 lag
(iii) 0.707 lag
(iv) none of above

62. An induction motor is so called because its operation depends upon
(i) self-induction
(ii) mutual induction
(iii) eddy currents
(iv) hysteresis

63. The induction motor has lagging power factor during
(i) starting only
(ii) operation only
(iii) both starting and operation
(iv) none of above

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