301 MCQ Questions on Induction Motors

51. For a single-phase capacitor start induction motor which of the following statements is valid?

(a) The capacitor is used for power factor improvement.
(b) The direction of rotation can be changed by reversing the main winding terminals.
(c) The direction of rotation cannot be changed.
(d) The direction of rotation can be changed by interchanging the supply terminals.

52. The type of single-phase induction motor having the highest power factor at full load is

(a) Shaded-pole type.
(b) Split-phase type.
(c) Capacitor-start type.
(d) Capacitor-run type.

53. In case of a split-phase motor, the phase shift between currents in the two windings is around

(a) 30°
(b) 70°
(c) 90°
(d) 120°

54. Among single-phase induction motors, which one of the following motors has the least starting torque in per unit terms?

(a) Split-phase motor
(b) Capacitor motor
(c) Capacitor start motor
(d) Shaded pole motor

55. The capacitor-start single-phase induction motor develops much larger starting torque in comparison to the split-phase motor because the use of capacitors in the auxiliary winding enables

(a) Provision of larger number of turns in the auxiliary winding.
(b) A larger starting current to be drawn from the supply.
(c) The torque-slip characteristic in general to get a shape to give a large starting torque.
(d) The current division between the main and auxiliary winding to be in such a way angle between them near to 90° for the development of large starting torque.

56. In a shaded pole induction motor, the motor runs from the

(a) Shaded portion to the unshaded portion of the pole while the flux in the former leads that of the latter.
(b) Shaded portion to the unshaded portion of the pole while the flux in the former lags that in the latter.
(c) Unshaded portion to the shaded portion while the flux in the former leads that in the latter.
(d) Unshaded portion to the shaded portion while the flux in the former lags that in the latter.

57. The capacitor in a single-phase inductor motor is used for

(a) Improving the power factor.
(b) Improving the starting torque.
(c) Starting the motor.
(d) Reducing the harmonics on Motors.

58. Which one of the following statements is correct?

When a single phase induction motor is excited with single phase AC voltage, the magnetic field set up is equivalent to
(a) Two fields, rotating in opposite directions with different speeds.
(b) Two fields, rotating in synchronous speed in opposite direction.
(c) Two fields, rotating in synchronous speed.
(d) Two fields, rotating in the same directions but at different speeds.

59. A single-phase induction motor is running at N rpm. Its synchronous speed is N_s. If its slip w.r.t. forward field is ‘s’, the slip w.r.t. the backward field is

(a) s
(b) -s
(c) 1 – s
 (d) 2 – s

60. If the frequency of a 3-phase supply to the stator of a 3-phase induction motor is increased, then synchronous speed

(a) Is decreased.
(b) Is increased.
(c) Remains unchanged.
(d) None of the above.

61. If a 4-pole induction motor has a synchronous speed of 1500 rpm then supply frequency is

(a) 50 Hz
(b) 25 Hz
(c) 60 Hz
(d) 100 Hz

62. The direction of rotation of field in a 3-phase induction motor depends upon

(a) Number of poles.
(b) Magnitude of supply voltage.
(c) Supply frequency.
(d) Phase sequence of supply voltage.

63. The shaft of an induction motor is made of

(a) Stiff
(b) Flexible
(c) Hollow
 (d) Any of the above

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

(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 decrease.

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

(a) Low.
(b) Negligible.
(c) Same as full-load torque.
(d) Slightly more than full-load torque.

66. The term ‘cogging’ is associated with

(a) Three phase transformers
(b) Compound generators
(c) D.C. series motors
(d) Induction motors

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

(a) Zero frequency.
(b) The same frequency as the slip frequency
(c) The same phase as the rotor e.m.f.
(d) High value for the satisfactory speed control.

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

(a) The motor will run in reverse direction.
(b) The motor will run at reduced speed.
(c) The motor will not run.
(d) The motor will burn.

69. Slip of an induction motor is negative when

(a) Magnetic field and rotor rotate in opposite direction.
(b) Rotor speed is less than the synchronous speed of the field and are in the same direction.
(c) Rotor speed is more than the synchronous speed of the field and are in the same direction.
(d) None of the above.

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

(a) Bigger
(b) Smaller
(c) Same
 (d) Any of the above

71. Which type of bearing is provided in small induction motors to support the rotor shaft?

(a) Ball bearings
(b) Cast iron bearings
(c) Bush bearings
(d) None of the above

72. In a three-phase induction motor

(a) Power factor at starting is high as compared to that while running.
(b) Power factor at starting is low as compared to that while running.
(c) Power factor at starting in the same as that while running.
(d) None of the above.

73. The shape of the torque/slip curve of induction motor is

(a) Parabola
(b) Hyperbola
(c) Rectangular parabola
(d) Straight line

74. Which motor is preferred for use in mines where explosive gases exist

(a) Air motor
(b) Induction motor
(c) D.C. shunt motor
(d) Synchronous motor

75. The torque developed by a single-phase motor at starting is

(a) More than the rated torque.
(b) Rated torque.
(c) Less than the rated torque.
(d) Zero.

76. In a shaded pole single-phase motor, the revolving field is produced by the use of

(a) Inductor
(c) Resistor
(b) Capacitor
(d) Shading coils

77. A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value. It will

(a) Start and then stop.
(b) Start and run slowly.
(c) Start and run at rated speed.
(d) Not start at all

78. Which of the following motors is inherently self starting?

(a) Split motor
(b) Shaded-pole motor
(c) Reluctance motor
(d) None of these

79. Burning out of windings is due to

(a) Short circuited capacitor.
(b) Capacitor value having changed.
(c) Open circuiting of capacitor.
(d) None of the above.

80. The power factor of a single-phase induction motor is usually

(a) Lagging
(b) Always leading
(c) Unity
(d) Unity to 0.8 leading

81. A universal motor can run on

(a) A.C. only
(b) D.C. only
(c) either A.C. or D.C.
(d) none of the above

82. In a two value capacitor motor, the capacitor used for running purposes is

(a) Air capacitor
(b) Paper spaced oil-filled type
(c) Ceramic type
(d) A.c. electrolytic type

83. The speed of a squirrel cage induction motor is changed by

(a) Pole changing
(b) Rheostatic control
(c) Cascade control
(d) None of these

84. Which of the following motor is use in a situation where load increases with speed?

(a) Induction motor
(b) Three-phase series motor
(c) Schrage motor
(d) Hysteresis motor

85. Which of the following statements regarding two value capacitor motor is incorrect?

(a) It is a reversing motor.
(b) It is preferred to permanent-split single-value capacitor motor where frequent reversals are required.
(c) It has low starting as well as rushing currents.
(d) It has high starting torque.

86. In a universal motor, the most common cause of brush sparking is

(a) Open armature winding.
(b) Shorted armature winding.
(c) Shorted field winding.
(d) All of the above.

87. In a A.C. series motor armature coils are usually connected to commutator

(a) Through resistance
(b) Through reactances
(c) Through capacitors
(d) Solidly

88. A repulsion motor is equipped with

(a) Slip rings
(b) Commutator
(c) Both (a) and (b)
(d) None of the above

89. A.C. series motor as compared to D.C. series motor has

(a) Smaller brush width
(b) Less number of field turns
(c) More number of armature turns
(d) All of the above

90. Speed control of a universal motor is achieved by

(a) Varying field flux with tapped field winding.
(b) Connecting rheostat in series.
(c) Applying variable voltage by means of silicon controlled rectifier.
(d) All of the above methods.

91. Deep bar rotor construction is used in three phase induction motors to mainly

(a) Control speed
(b) Control power factor
(c) Increase starting torque
(d) None of these [DMRC JE-2016]

92. If a 3-phase, 40V, 50Hz, 4 pole induction motor is running at a slip of 5% then the relative speed of rotor field with respect to stator filed is

(a) Zero
(b) 75 rpm
(c) 142.5 rpm
(d) 1500 rpm [DMRC JE-2016]

93. A 3-phase induction motor is running at slip ‘s’. If its two supply leads are interchanged, then the operating slip at that instant will be:

(a) 2s
(b) (1-s)
(c) (2-s)
 (d) Zero [DMRC JE-2016]

94. Skewing of rotor bars eliminate the

(a) Effect of cogging
(b) Entire effect of crawling
(c) Magnetic noise
(d) Vibration due to unequal force developed on rotor [DMRC JE-2016]

95. A 3-phase delta connected squirrel cage induction motor when started with a DOL starter has a starting torque of 600 Nm. Its starting torque when star delta starter is used

(a) 600 Nm
(b) 200 Nm
(c) 300 Nm
(d) 1200 Nm [DMRC JE-2016]

96. Which of the following motor has squirrel cage winding on the stator:

(a) 3 phase squirrel cage induction motor
(b) Single phase squirrel cage pump motor
(c) Single phase ceiling fan induction motor
(d) Single phase table fan induction motor [DMRC JE-2016]

97. What is the frequency of an alternator, if A = number of poles and B = revolution made per second?

(a) AB Hz
(b) AB/4 Hz
(c) AB/2 Hz
(d) None of these

98. When the load on an induction motor is increased from no load to full load:

(a) Both slip and power factor decrease
(b) Both slip and power factor increase
(c) Slip decreases and power factor increases
(d) Slip increases and power factor decreases [UPPCL JE – 2014]

99. Which of the following statements is correct regarding the actual voltage which is used for setting up the useful flux in the air gap of a 3- phase induction motor? [UPPCL JE 2014]

(a) It is equal to applied voltage
(b) It is greater than applied voltage
(c) It is less than applied voltage
(d) It is equal to rotor induced emf

100. Among the following types of single phase induction motor which one has the highest power factor at full load? [UPPCL JE – 2014]

(a) Shaded pole type
(b) Split-phase type
(c) Capacitor-start type
(d) Capacitor-run type