polyphase induction motors MCQs – Page 2

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

polyphase induction motors Mcqs
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____________?
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A. crawling

B. cogging

C. humming

D. hunting

52. Stepless speed control of induction motor is possible by which of the following methods ?
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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__________?
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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___________?
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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______________?
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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__________?
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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___________?
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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_____________?
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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__________?
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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____________?
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A. 200 Hz

B. 0.2 Hz

C. 50 Hz

D. 2 Hz
61. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor _____________?
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A. By injecting e.m.f. in the rotor circuit

B. Rotor rheostat control

C. By changing the number of stator poles

D. By operating two motors in cascade

62. The good power factor of an induction motor can be achieved if the average flux density in the air gap is___________?
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A. absent

B. large

C. small

D. infinity

63. The shape of the torque/slip curve of induction motor is__________?
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A. rectangular parabola

B. straigth line

C. parabola

D. hyperbola

64. The starting torque of a 3-phase squirrel cage induction motor is____________?
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A. twice the full load torque

B. 1.5 times the full load torque

C. equal to full load torque

65. The starting torque of a three phase induction motor can be increased by__________?
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A. increasing slip

B. increasing current

C. both A. and (b)

D. none of the above

66. Size of a high speed motor as compared to low speed motor for the same H.P. will be__________?
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A. any of the above

B. smaller

C. bigger

D. same

67. A 3-phase slip ring motor has____________?
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A. double cage rotor

B. wound rotor

C. any of the above

D. short-circuited rotor

68. Short-circuit test on an induction motor cannot be used to determine___________?
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A. copper losses

B. transformation ratio

C. windage losses

D. power scale of circle diagram

69. In a squirrel cage motor the induced e.m.f. is___________?
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A. dependent on the shaft loading

B. dependent on the number of slots

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

D. none of the above

70. Which motor is preferred for use in mines where explosive gases exist ?
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A. Induction motor

B. Air motor

C. Synchronous motor

D. D.C. shunt motor

71. The speed of a squirrel-cage induction motor can be controlled by all of the following except_____________?
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A. changing winding resistance

B. changing number of poles

C. reducing supply voltage

D. changing supply frequency

72. Which is of the following data is required to draw the circle diagram for an induction motor ?
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A. Block rotor test, no-load test and stator resistance test

B. Block rotor test and no-load test

C. Block rotor test only

D. No load test only

73. The circle diagram for an induction motor cannot be used to determine____________?
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A. efficiency

B. power factor

C. output

D. frequency

74. For which motor the speed can be controlled from rotor side ?
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A. Squirrel-cage induction motor

B. Slip-ring induction motor

C. Both A. and (b)

D. None of the above

75. The efficiency of an induction motor can be expected to be nearly____________?
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A. 80 to 90%

B. 60 to 90%

C. 99%

D. 95 to 98%

76. Insertion of reactance in the rotor circuit_______?
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A. increases starting torque but maxi-mum torque decreases

B. increases starting torque as well as maximum torque

C. reduces starting torque as well as maximum torque

D. increases starting torque but maxi-mum torque remains unchanged

77. The torque of an induction motor is___________?
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A. proportional to the square of the slip

B. directly proportional to slip

C. inversely proportional to slip

D. none of the above

78. The frame of an induction motor is usually made of____________?
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A. cast iron

B. aluminium

C. silicon steel

D. bronze

79. The cogging of an induction motor can be avoided by____________?
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A. auto-transformer starter

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

C. using DOL starter

D. proper ventilation

80. The torque developed by a 3-phase induction motor least depends on__________?
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A. shaft diameter

B. rotor power factor

C. rotor current

D. rotor e.m.f.

81. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move___________?
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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
82. In a three-phase induction motor_______________?
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A. iron losses in motor will be negligible as compared to that in rotor

B. iron losses in stator will be more than that in rotor

C. iron losses in stator will be negligible as compared to that in rotor

D. iron losses in stator will be less than that in rotor

83. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to__________?
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A. reduce magnetic hum

B. reduce eddy currents

C. reduce accumulation of dirt and dust

D. reduce windage losses

84. As load on an induction motor goes on increasing______________?
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A. its power factor remains constant

B. its power factor goes on decreasing

C. its power factor goes on increasing even after full load

D. its power factor goes on increasing up to full load and then it falls again
85. In case of 3-phase induction motors, plugging means__________?
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A. pulling the motor directly on line without a starter

B. interchanging two supply phases for quick stopping

C. locking of rotor due to harmonics

D. starting the motor on load which is more than the rated load

86. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
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A. The torque produced will be very large

B. The slip of the motor will be 5%

C. The rotor will run at very high speed

D. The rotor will not run
87. The torque of a rotor in an induction motor under running condition is maximum__________?
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A. at the unit value of slip

B. at the value of the slip which makes the rotor reactance half of the rotor

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

D. at the zero value of slip

88. In case the air gap in an induction motor is increased?
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A. the magnetising current of the rotor will decrease

B. the power factor will decrease

C. speed of motor will increase

D. the windage losses will increase

89. Lubricant used for ball bearing is usually___________?
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A. graphite

B. mineral oil

C. grease

D. molasses

90. The injected e.m.f. in the rotor of induction motor must have__________?
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A. zero frequency

B. the same phase as the rotor e.m.f.

C. high value for the satisfactory speed control

D. the same frequency as the slip frequency
91. In three-phase squirrel-cage induction motors____________?
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A. rotor conductors are connected to insulation

B. rotor conductors are kept open

C. rotor conductors are short-circuited through end rings

D. rotor conductor ends are short-circuited through slip rings

92. Slip ring motor is recommended where____________?
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A. frequent starting, stopping and reversing is required

B. high starting torque is needed

C. speed control is required

D. all above features are required
93. Which type of bearing is provided in small induction motors to support the rotor shaft ?
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A. Ball bearings

B. Cast iron bearings

C. None of the above

D. Bush bearings

94. Less maintenance troubles are experienced in case of___________?
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A. slip ring induction motor

B. squirrel cage induction motor

C. both A. and (b)

D. none of the above

95. In the circle diagram for induction motor, the diameter of the circle represents____________?
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A. rotor current

B. running torque

C. line voltage

D. slip

96. In medium sized induction motors, the slip is generally around___________?
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A. 0.04%

B. 4%

C. 0.40%

D. 14%

97. An induction motor is identical to_________?
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A. synchronous motor

B. D.C. compound motor

C. D.C. series motor

D. asynchronous motor
98. An induction motor with 1000 r.p.m. speed will have____________?
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A. 2 poles

B. 6 poles

C. 8 poles

D. 4 poles

99. The power factor of an induction motor under no-load conditions will be closer to_____________?
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A. 0.2 lagging

B. 0.5 leading

C. unity

D. 0.2 leading

100. Star-delta starting of motors is not possible in case of____________?
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A. single phase motors

B. high speed motors

C. low horse power motors

D. variable speed motors