Synchronous Motor MCQ

76. A synchronous motor is supplying its rated load. It excitation is increased

1. the power factor becomes more leading
2. the power factor becomes more lagging
3. the power factor remains constant
4. None of these

77. A rotary converter generally

1. combines the function of an induction motor and a dc generator
2. has a set of slip rings at both ends
3. has one armature and 2 fields
4. is a synchronous motor and a dc generator combined

78. In a synchronous machine, in case the axis of field flux is in line with the armature flux then

1. the machine is working as synchronous motor
2. the machine is working as synchronous generator
3. the machine is said to floating
4. the machine will vibrate violently

79. The speed regulation of a synchronous motor is

1. positive
2. zero
3. negative
4. any of the these

80. The regulation of a synchronous motor is

1. 0.01
2. 1
3. 0.5
4. 0

81. In a synchronous motor the magnitude of back emf E_b, depends on

1. speed of the motor
2. load on the motor
3. both speed and stator flux
4. d.c. excitation only

82. In a synchronous motor with field under excited, the power factor will be

1. lagging
2. leading
3. unity
4. none of these

83. What is the ratio of no-load speed to full load speed of a 200kVA, 12 pole, 220V, 3 phase, 60 Hz synchronous motor?

1. 1.1
2. 1
3. 1.21
4. infinite

84. A synchronous motor is operating on no load at unity power factor, if the field current is increased, the power factor will become

1. Leading and the stator current will decrease
2. Lagging and the stator current will increase
3. Lagging and the stator current will decrease
4. Leading and the stator current will increase

85. The power factor of the input power to a synchronous motor is adjusted by adjusting

1. Magnitude of excitation
2. Magnitude of armature reaction
3. Number of poles
4. None of these

86. For 100% excitation in synchronous motor, the relationship between back-emf E and applied voltage V is

1. E > V
2. E < V
3. E = V
4. none

87. The per unit impedance of a synchronous machine is 0.242. If the base voltage is increased by 1.1 times, the per-unit value will be 

1. 0.200
2. 0.266
3. 0.242
4. 0.220

88. The purpose of having a commutator and brush arrangement in a dc motor is

1. To produce a unidirectional torque
2. To produce a unidirectional current in the armature
3. To help in changing the direction of rotation of the armature
4. To reduce eddy current loss in the armature

89. A cylindrical rotor synchronous motor is switched on to the supply with its field winding shorted on themselves. It will

1. No start
2. Start but not run at synchronous speed
3. Start as an induction motor and then run as a synchronous motor.
4. Start and run as a synchronous motor.

90. A synchronous motor is connected an infinite bus at 1.0 pu voltage and draws 0.6 pu at unity power factor. Its synchronous reactance is 1.0 pu and resistance is negligible. Keeping the excitation voltage the same, the load on the motor is increased such that motor current increased by 20% the operating power factor will become-

1. 0.995 lagging
2. 0.995 leading
3. 0.791 lagging
4. 0.791 leading