Electrical Engineering practice MCQ : Three Phase Induction Motors

51. As load on an induction motor goes on increasing

(a) its power factor goes on decreasing

(b) its power factor remains constant

(c) its power factor goes on increasing even after full load

(d) its power factor goes on increasing upto full load and then it falls again Ans: d

52. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move

(a) in the opposite direction as the direction of the rotating field

(b) in the same direction as the direction of the field

(c) in any direction depending upon phase squence of supplyAns: b

53. It is advisable to avoid line starting of induction motor and use starter because

(a) it will run in reverse direction

(b) it will pick up very high speed and may go out of step

(c) motor takes five to seven times its fullload current

(d) starting torque is very high Ans: c

54. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines (a) D.C. series motor

(b) D.C. shunt motor

(c) universal motor

(d) none of the above

Ans: b

55. 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

Ans: a

56. 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 (a) stall after sometime

(b) stall immediately

(c) continue to run at lower speed without damage

(d) get heated and subsequently get damaged Ans: d

57. 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 ?

(a) 730 r.p.m.

(b) 960 r.p.m.

(c) 1440 r.p.m.

(d) 2880 r.p.m.

Ans: a

58. A 3-phase slip ring motor has

(a) double cage rotor

(6) wound rotor

(c) short-circuited rotor

(d) any of the above

Ans: b

59. The starting torque of a 3-phase squirrel cage induction motor is

(a) twice the full load torque

(b) 1.5 times the full load torque

(c) equal to full load torque

Ans: b

60. Short-circuit test on an induction motor cannot be used to determine

(a) windage losses

(b) copper losses

(c) transformation ratio

(d) power scale of circle diagram Ans: a

61. In a three-phase induction motor

(a) iron losses in stator will be negligible as compared to that in rotor

(6) iron losses in motor will be neg¬ligible as compared to that in rotor

(c) iron losses in stator will be less than that in rotor

(d) iron losses in stator will be more than that in rotor Ans: d

62. In case of 3-phase induction motors, plugging means

(a) pulling the motor directly on line without a starter

(b) locking of rotor due to harmonics

(c) starting the motor on load which is more than the rated load

(d) interchanging two supply phases for quick stopping

Ans: d

63. Which is of the following data is required to draw the circle diagram for an induction motor ?

(a) Block rotor test only

(b) No load test only

(c) Block rotor test and no-load test

(d) Block rotor test, no-load test and stator resistance test Ans: d

64. In three-phase induction motors sometimes copper bars are placed deep in the rotor to

(a) improve starting torque

(b) reduce copper losses

(c) improve efficiency

(d) improve power factor

Ans: a

65. 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 runningAns: b

66. The vafcie of transformation ratio of an induction motor can be found by

(a) open-circuit test only

(b) short-circuit test only

(c) stator resistance test

(d) none of the above

Ans: b

67. The power scale of circle diagram of an induction motor can be found from

(a) stator resistance test

(6) no-load test only

(c) short-circuit test only

(d) noue of the above

Ans: c

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

(a) parabola

(b) hyperbola

(c) rectangular parabola

(d) straigth line

Ans: c

69. A change of 4% of supply voltage to an induction motor will produce a change of appromimately

(a) 4% in the rotor torque

(b) 8% in the rotor torque

(c) 12% in the rotor torque

(d) 16% in the rotor torque

Ans: d

70. The stating torque of the slip ring induction motor can be increased by adding

(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 Ans: b

71. 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

(a) 0.01

(b) 0.02

(c) 0.03

(d) 0.04Ans: d

72. The complete circle diagram of induetion motor can be drawn with the help of data found from (a) noload test

(6) blocked rotor test

(c) stator resistance test

(d) all of the above

Ans: d

73. In the squirrel-cage induction motor the rotor slots are usually given slight skew

(a) to reduce the magnetic hum and locking tendency of the rotor

(b) to increase the tensile strength of the rotor bars

(c) to ensure easy fabrication

(d) none of the above

Ans: a

74. The torque of a rotor in an induction motor under running condition is maximum

(a) at the unit value of slip

(b) at the zero value of slip

(c) at the value of the slip which makes rotor reactance per phase equal to the resistance per phase

(d) at the value of the slip which makes the rotor reactance half of the rotor Ans: c

75. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ? (a) The slip of the motor will be 5%

(b) The rotor will not run

(c) The rotor will run at very high speed

(d) The torque produced will be very large Ans: b

76. The circle diagram for an induction motor cannot be used to determine

(a) efficiency

(b) power factor

(c) frequency

(d) output

Ans: a

77. Blocked rotor test on induction motors is used to find out

(a) leakage reactance

(b) power factor on short circuit

(c) short-circuit current under rated voltage

(d) all of the above

Ans: d

78. Lubricant used for ball bearing is usually

(a) graphite

(b) grease

(c) mineral oil

(d) molasses

Ans: b

79. An induction motor can run at synchronous speed when

(a) it is run on load

(b) it is run in reverse direction

(c) it is run on voltage higher than the rated voltage

(d) e.m.f. is injected in the rotor circuit Ans: d

80. 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

Ans: a

81. The torque developed by a 3-phase induction motor least depends on

(a) rotor current (b) rotor power factor (c) rotor e.m.f.

(d) shaft diameter

Ans: d

82. In an induction motor if air-gap is increased

(a) the power factor will be low

(b) windage losses will be more

(c) bearing friction will reduce

(d) copper loss will reduce In an induction motorAns: a

83. In induction motor, percentage slip depends on

(a) supply frequency

(b) supply voltage

(c) copper losses in motor

(d) none of the above

Ans: c

84. When /?2 is tne rotor resistance, .X2 the rotor reactance at supply frequency

and s the slip, then the condition for maximum torque under running condi-tions will be

(a) sR2X2 = 1 (b) sR2 = X2

(c) R2 = sX2 id) R2 = s2X2

Ans: c

85. In case of a double cage induction motor, the inner cage has

(a) high inductance arid low resistance

(b) low inductance and high resistance

(c) low inductance and low resistance

(d) high inductance and high resis¬tance Ans: a

86. The low power factor of induction motor is due to

(a) rotor leakage reactance

(b) stator reactance

(c) the reactive lagging magnetizing current necessary to generate the magnetic flux (d) all of the above

Ans: d

87. Insertion of reactance in the rotor circuit

(a) reduces starting torque as well as maximum torque

(b) increases starting torque as well as maximum torque

(c) increases starting torque but maxi-mum torque remains unchanged

(d) increases starting torque but maxi-mum torque decreases Ans: a

88. Insertion of resistance in the rotcir of an induction motor to develop a given torque

(a) decreases the rotor current

(b) increases the rotor current

(c) rotor current becomes zero

(d) rotor current rernains same

Ans: d

89. For driving high inertia loods best type of induction motor suggested is

(a) slip ring type

(b) squirrel cage type

(c) any of the above

(d) none of the above

Ans: a

90. Temperature of the stator winding of a three phase induction motor is obtained by

(a) resistance rise method

(b) thermometer method

(c) embedded temperature method

(d) all above methods

Ans: d

91. The purpose of using short-circuit gear is

(a) to short circuit the rotor at slip rings

(b) to short circuit the starting resis¬tances in the starter

(c) to short circuit the stator phase of motor to form star

(d) none of the above

Ans: a

92. In a squirrel cage motor the induced e.m.f. is

(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

Ans: c

93. Less maintenance troubles are experienced in case of

(a) slip ring induction motor

(6) squirrel cage induction motor

(c) both (a) and (b)

(d) none of the above

Ans: b

94. A squirrel cage induction motor is not selected when

(a) initial cost is the main consideration

(b) maintenance cost is to be kept low

(c) higher starting torque is the main consideration

(d) all above considerations are involved Ans: c

95. Reduced voltage starter can be used with

(a) slip ring motor only but not with squirrel cage induction motor

(b) squirrel cage induction motor only but not with slip ring motor

(c) squirrel cage as well as slip ring induction motor

(d) none of the above

Ans: c

96. Slip ring motor is preferred over squirrel cage induction motor where

(a) high starting torque is required

(b) load torque is heavy

(c) heavy pull out torque is required

(d) all of the above

Ans: a

97. In a star-delta starter of an induction motor

(a) resistance is inserted in the stator

(b) reduced voltage is applied to the stator

(c) resistance is inserted in the rotor

(d) applied voltage per1 stator phase is 57.7% of the line voltage Ans: d

98. The torque of an induction motor is

(a) directly proportional to slip

(b) inversely proportional to slip

(c) proportional to the square of the slip

(d) none of the above

Ans: a

99. The rotor of an induction motor runs at

(a) synchronous speed

(b) below synchronous speed

(c) above synchronous speed

(d) any of the above

Ans: b

100. The starting torque of a three phase induction motor can be increased by

(a) increasing slip

(b) increasing current

(c) both (a) and (b)

(d) none of the above

Ans: c

101. Insertion of resistance in the stator of an induction motor

(a) increases the load torque

(b) decreases the starting torque

(c) increases the starting torque

(d) none of the above
Ans: b