Polyphase Induction Motors

81. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines

  1. D.C. series motor
  2. D.C. shunt motor
  3. universal motor
  4. none of the above

Correct answer: (B)
D.C. shunt motor

82. The speed of a squirrel-cage induction motor can be controlled by all of the following except

  1. changing supply frequency
  2. changing number of poles
  3. changing winding resistance
  4. reducing supply voltage

Correct answer: (C)
changing winding resistance

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

  1. twice the full load torque
  2. 1.5 times the full load torque
  3. equal to full load torque
  4. any of the above

Correct answer: (B)
1.5 times the full load torque

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

  1. low
  2. negligible
  3. same as full-load torque
  4. slightly more than full-load torque

Correct answer: (A)
low

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

  1. external inductance to the rotor
  2. external resistance to the rotor
  3. external capacitance to the rotor
  4. both resistance and inductance to rotor

Correct answer: (B)
external resistance to the rotor

86. The term 'cogging' is associated with

  1. three phase transformers
  2. compound generators
  3. D.C. series motors
  4. induction motors

Correct answer: (D)
induction motors

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

  1. rotor current
  2. rotor power factor
  3. rotor e.m.f.
  4. shaft diameter

Correct answer: (D)
shaft diameter

88. The torque developed in the cage induction motor with autostarter is

  1. k/torque with direct switching
  2. K x torque with direct switching
  3. K2 x torque with direct switching
  4. k2/torque with direct switching

Correct answer: (C)
K2 x torque with direct switching

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

  1. at the unit value of slip
  2. at the zero value of slip
  3. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
  4. at the value of the slip which makes the rotor reactance half of the rotor

Correct answer: (C)
at the value of the slip which makes rotor reactance per phase equal to the resistance per phase

90. The torque of an induction motor is

  1. directly proportional to slip
  2. inversely proportional to slip
  3. proportional to the square of the slip
  4. none of the above

Correct answer: (A)
directly proportional to slip

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