Most Synchronous Motors

81. The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as

  1. pull-up torque
  2. pull-in torque
  3. pull-out torque
  4. none of the above

Correct answer: (B)
pull-in torque

82. The maximum power developed in a synchronous motor occurs at a coupling angle of

  1. 30°
  2. 60°
  3. 90°
  4. 180°

Correct answer: (B)
90°

83. The maximum power developed in a synchronous motor will depend on

  1. the rotor excitation only
  2. the supply voltage only
  3. the rotor excitation and supply volt-age both
  4. the rotor excitation, supply voltage and maximum value of coupling angle (90°)

Correct answer: (D)
the rotor excitation, supply voltage and maximum value of coupling angle (90°)

84. The maximum speed variation in a 3-phase synchronous motor is

  1. 10 per cent
  2. 6 per cent
  3. 4 per cent
  4. 2. per cent
  5. zero

Correct answer: (E)
zero

85. The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as

  1. locked-rotor torque
  2. synchronous torque
  3. pull up torque
  4. reluctance torque

Correct answer: (A)
locked-rotor torque

86. The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as

  1. slip torque
  2. pull-out torque
  3. breaking torque
  4. synchronising torque

Correct answer: (D)
synchronising torque

87. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of

  1. zero degree
  2. two degrees
  3. five degrees
  4. ten degrees

Correct answer: (C)
five degrees

88. The minimum armature current of the synchronous motor corresponds to operation at

  1. zero power factor leading
  2. unity power factor
  3. 0.707 power factor lagging
  4. 0.707 power factor leading

Correct answer: (B)
unity power factor

89. The net armature voltage of a synchronous motor is equal to the

  1. vector sum of Eb and V
  2. arithmetic sum of Eb and V
  3. arithmetic difference of Eb and V
  4. vector difference of Eh and V

Correct answer: (D)
vector difference of Eh and V

90. The operating speed of a synchronous motor can be changed to new fixed value by

  1. changing the load
  2. changing the supply voltage
  3. changing frequency
  4. using brakes

Correct answer: (C)
changing frequency

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