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polyphase induction motors MCQs

Practise focused polyphase induction motors questions with marked answers for exam preparation and subject revision.

polyphase induction motors Mcqs
1. In case the air gap in an induction motor is increased____________?
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A. speed of motor will increase

B. the magnetising current of the rotor will decrease

C. the windage losses will increase

D. the power factor will decrease
2. The stating torque of the slip ring induction motor can be increased by adding__________?
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A. external inductance to the rotor

B. external resistance to the rotor

C. external capacitance to the rotor

D. both resistance and inductance to rotor

3. The starting torque of a squirrel-cage induction motor is___________?
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A. negligible

B. slightly more than full-load torque

C. same as full-load torque

D. low
4. If any two phases for an induction motor are interchanged_______?
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A. the motor will run in reverse direction

B. the motor will burn

C. the motor will not run

D. the motor will run at reduced speed

5. A change of 4% of supply voltage to an induction motor will produce a change of approximately ________?
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A. 4% in the rotor torque

B. 12% in the rotor torque

C. 8% in the rotor torque

D. 16% in the rotor torque
6. In an induction motor, no-load the slip is generally____________?
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A. less than 1%

B. 1.50%

C. 4%

D. 2%

7. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines?
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A. none of the above

B. universal motor

C. D.C. shunt motor

D. D.C. series motor

8. An induction motor can run at synchronous speed when____________?
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A. it is run on voltage higher than the rated voltage

B. it is run on load

C. it is run in reverse direction

D. e.m.f. is injected in the rotor circuit
9. Which of the following component is usually fabricated out of silicon steel ?
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A. None of the above

B. Statorcore

C. Shaft

D. Bearings

10. The maximum torque in an induction motor depends on___________?
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A. square of supply voltage

B. rotor inductive reactance

C. frequency

D. all of the above
11. The term cogging is associated with___________?
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A. induction motors

B. D.C. series motors

C. compound generators

D. three phase transformers

12. A squirrel cage induction motor is not selected when___________?
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A. all above considerations are involved

B. maintenance cost is to be kept low

C. initial cost is the main consideration

D. higher starting torque is the main consideration
13. Temperature of the stator winding of a three phase induction motor is obtained by___________?
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A. all above methods

B. embedded temperature method

C. resistance rise method

D. thermometer method

14. It is advisable to avoid line-starting of induction motor and use starter because__________?
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A. it will run in reverse direction

B. starting torque is very high

C. motor takes five to seven times its full load current

D. it will pick-up very high speed and may go out of step

15. Slip ring motor is preferred over squirrel cage induction motor where___________?
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A. heavy pull out torque is required

B. load torque is heavy

C. high starting torque is required

D. all of the above

16. In a star-delta starter of an induction motor___________?
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A. applied voltage perl stator phase is 57.7% of the line voltage

B. resistance is inserted in the stator

C. resistance is inserted in the rotor

D. reduced voltage is applied to the stator

17. The crawling in an induction motor is caused by___________?
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A. improper design of machine

B. high loads

C. harmonics developed in the motor

D. low voltage supply

18. 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____________?
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A. stall after sometime

B. continue to run at lower speed without damage

C. get heated and subsequently get damaged

D. stall immediately

19. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be___________?
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A. Ns

B. (l-s)Ns

C. (Ns-l)s

D. s.N,

20. In three-phase induction motors sometimes copper bars are placed deep in the rotor to___________?
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A. improve starting torque

B. reduce copper losses

C. improve power factor

D. improve efficiency

21. The crawling in the induction motor is caused by____________?
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A. high loads

B. improper design of the machine

C. none of the above

D. harmonics develped in the motor

E. low voltage supply

22. In induction motor, percentage slip depends on__________?
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A. supply frequency

B. supply voltage

C. none of the above

D. copper losses in motor
23. The number of slip rings on a squirrel cage induction motor is usually____________?
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A. four

B. three

C. two

D. none
24. Insertion of resistance in the stator of an induction motor___________?
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A. increases the starting torque

B. none of the above

C. increases the load torque

D. decreases the starting torque
25. The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type__________?
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A. A. and B. both

B. none of the above

C. delta connected only

D. star connected only

26. In the squirrel-cage induction motor the rotor slots are usually given slight skew__________?
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A. none of the above

B. to ensure easy fabrication

C. to reduce the magnetic hum and locking tendency of the rotor

D. to increase the tensile strength of the rotor bars

27. In case of the induction motors the torque is___________?
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A. inversely proportional to slip

B. directly proportional to slip

C. inversely proportional to (Vslip)

D. directly proportional to (slip)2

28. When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered__________?
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A. in parallel with stator

B. in series

C. in parallel

D. in series-parallel

29. The torque developed in the cage induction motor with autostarter is___________?
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A. K2 x torque with direct switching

B. k2/torque with direct switching

C. K x torque with direct switching

D. k/torque with direct switching

30. In an induction motor the relationship between stator slots and rotor slots is that_____________?
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A. none of the above

B. stator slots are not exact multiple of rotor slots

C. stator slots are exact multiple of rotor slots

D. stator slots are equal to rotor slots

31. Insertion of resistance in the rotor of an induction motor to develop a given torque?
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A. increases the rotor current

B. rotor current rernains same

C. rotor current becomes zero

D. decreases the rotor current

32. Reduced voltage starter can be used with_____________?
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A. squirrel cage as well as slip ring induction motor

B. squirrel cage induction motor only but not with slip ring motor

C. slip ring motor only but not with squirrel cage induction motor

D. none of the above

33. In a three-phase induction motor__________?
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A. power factor at starting in the same as that while running

B. power factor at starting is high as compared to that while running

C. power factor at starting is low as compared to that while running
34. 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__________?
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A. 0.03

B. 0.02

C. 0.01

D. 0.04
35. A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor_______________?
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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 without any harm to the winding

D. will continue running burning its two phases

36. In case of a double cage induction motor, the inner cage has__________?
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A. low inductance and low resistance

B. low inductance and high resistance

C. high inductance and high resistance

D. high inductance arid low resistance
37. Blocked rotor test on induction motors is used to find out?
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A. all of the above

B. power factor on short circuit

C. leakage reactance

D. short-circuit current under rated voltage

38. The rotor of an induction motor runs at__________?
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A. any of the above

B. above synchronous speed

C. synchronous speed

D. below synchronous speed
39. The vafcie of transformation ratio of an induction motor can be found by__________?
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A. short-circuit test only

B. none of the above

C. stator resistance test

D. open-circuit test only

40. Rotor rheostat control method of speed control is used for____________?
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A. squirrel-cage induction motors only

B. slip ring induction motors only

C. both A. and (b)

D. none of the above

41. When /?2 is the rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be_____________?
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A. sR2 = X2

B. R2 = s2X2

C. R2 = sX2

D. sR2X2 = 1

42. A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor______________?
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A. will continue running without any harm to the winding

B. will continue running burning its one phase

C. will continue running burning its two phase

D. will stop and carry heavy current causing permanent damage to its winding
43. It is advisable to avoid line starting of induction motor and use starter because____________?
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A. it will pick up very high speed and may go out of step

B. starting torque is very high

C. motor takes five to seven times its full load current

D. it will run in reverse direction

44. Slip rings are usually made of___________?
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A. phospor bronze

B. copper

C. carbon

D. aluminium

45. In an induction motor if air-gap is increased__________?
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A. the power factor will be low

B. windage losses will be more

C. copper loss will reduce In an induction motor

D. bearing friction will reduce

46. 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 ?
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A. 730 r.p.m.

B. 960 r.p.m.

C. 1440 r.p.m.

D. 2880 r.p.m.

47. The shaft of an induction motor is made of_____________?
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A. carbon steel

B. cast iron

C. stainless steel

D. high speed steel

48. Low voltage at motor terminals is due to____________?
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A. none of the above

B. inadequate motor wiring

C. any one of the above

D. poorely regulated power supply

49. DOL starting of induction motors is usually restricted to_____________?
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A. variable speed motors

B. high speed motors

C. high horsepower motors

D. low horsepower motors
50. For driving high inertia loads best type of induction motor suggested is_____________?
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A. slip ring type

B. none of the above

C. any of the above

D. squirrel cage type