polyphase induction motors MCQs – Page 3

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

polyphase induction motors Mcqs
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_____________?
comments icon0

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______________?
comments icon0

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____________?
comments icon0

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___________?
comments icon0
A. phospor bronze

B. copper

C. carbon

D. aluminium

45. In an induction motor if air-gap is increased__________?
comments icon0
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 ?
comments icon0
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_____________?
comments icon0
A. carbon steel

B. cast iron

C. stainless steel

D. high speed steel

48. Low voltage at motor terminals is due to____________?
comments icon0

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_____________?
comments icon0

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_____________?
comments icon0
A. slip ring type

B. none of the above

C. any of the above

D. squirrel cage type

51. 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____________?
comments icon0
A. crawling

B. cogging

C. humming

D. hunting

52. Stepless speed control of induction motor is possible by which of the following methods ?
comments icon0

A. e.m.f. injection in rotor eueuit

B. Changing the number of poles

C. None of the above

D. Cascade operation

53. In a three-phase induction motor, the number of poles in the rotor winding is always__________?
comments icon0

A. less than number of poles in stator

B. equal to number of poles in stator

C. zero

D. more than the number of poles in stator

54. The low power factor of induction motor is due to___________?
comments icon0

A. stator reactance

B. all of the above

C. rotor leakage reactance

D. the reactive lagging magnetizing current necessary to generate the magnetic flux

55. A double squirrel-cage induction motor has______________?
comments icon0

A. two parallel windings in stator

B. two parallel windings in rotor

C. two rotors moving in oppsite direction

D. two series windings in stator

56. The power scale of circle diagram of an induction motor can be found from__________?
comments icon0

A. stator resistance test

B. short-circuit test only

C. no-load test only

D. noue of the above

57. The purpose of using short-circuit gear is___________?
comments icon0

A. none of the above

B. to short circuit the rotor at slip rings

C. to short circuit the starting resistances in the starter

D. to short circuit the stator phase of motor to form star

58. Slip of an induction motor is negative when_____________?
comments icon0
A. rotor speed is more than the synchronous speed of the field and are in the same direction

B. rotor speed is less than the synchronous speed of the field and are in the same direction

C. magnetic field and rotor rotate in opposite direction

D. none of the above

59. An induction motor is__________?
comments icon0

A. self-starting with high torque

B. self-starting with zero torque

C. self-starting with low torque

D. non-self starting

60. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be____________?
comments icon0

A. 200 Hz

B. 0.2 Hz

C. 50 Hz

D. 2 Hz