Polyphase Induction Motors

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

1. zero frequency
2. the same frequency as the slip frequency
3. the same phase as the rotor e.m.f.
4. high value for the satisfactory speed control

View answer

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

1. rotor leakage reactance
2. stator reactance
3. the reactive lagging magnetizing current necessary to generate the magnetic flux
4. all of the above

View answer

73. The maximum torque in an induction motor depends on

1. frequency
2. rotor inductive reactance
3. square of supply voltage
4. all of the above

View answer

74. The number of slip rings on a squirrel cage induction motor is usually

1. two
2. three
3. four
4. none

View answer

75. The power factor of an induction motor under no-load conditions will be closer to

1. 0.2 lagging
2. 0.2 leading
3. 0.5 leading
4. unity

View answer

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

1. stator resistance test
2. no-load test only
3. short-circuit test only
4. noue of the above

View answer

77. The purpose of using short-circuit gear is

1. to short circuit the rotor at slip rings
2. to short circuit the starting resistances in the starter
3. to short circuit the stator phase of motor to form star
4. none of the above

View answer

78. The rotor of an induction motor runs at

1. synchronous speed
2. below synchronous speed
3. above synchronous speed
4. any of the above

View answer

79. The shaft of an induction motor is made of

1. stiff
2. flexible
3. hollow
4. any of the above

View answer

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

1. parabola
2. hyperbola
3. rectangular parabola
4. straigth line

View answer