AC MACHINES
1. A synchronous generator connected to an infinite bus is over-excited. Considering only the reactive power, from the point of view of the system, the machine acts as
a) capacitor
b) inductor
c) resistor
d) none of the above
ans. a
2. A synchronous motor operates at 0.8 pf lagging. If the field current of the motor is continuously increased
a) the power factor decreases up to a certain value of the field current and thereafter it increases
b) the armature current increases up to a certain value of the field current and thereafter it decreases
c) the power factor increases up to a certain value of the field current and thereafter it decreases
d) the armature current decreases up to a certain value of filed current and thereafter it increases
e) both c and d
ans. e
3. A cylindrical rotor synchronous motor is switched on the dc supply with its field winding's shorted on themselves. It will
a) not start
b) start but not run at synchronous speed
c) start as an induction motor and then run as synchronous motor
d) start and run as a synchronous motor
ans. b
4. Two 3-phase Y-connected alternators are to be paralleled to a set of common busbars. The armature has a per phase synchronous reactance of 1.7 ohm and negligible armature resistance. The line voltage of the first machine is adjusted to 3300 V and that of the second machine is adjusted to 3200 V. The machine voltages are in phase at eh instant they are paralleled. Under this condition, the synchronizing current per phase will be
a) 16.98 A
b) 29.41 A
c) 33.96 A
d) 58.82 A
ans. a
5. A power station consists of two synchronous generators A and B ratings of 250 MVA and 500 MVA with inertia 1.6 pu and 1 pu respectively on their own base MVA ratings. The equivalent per unit inertia constant for the system on 100 MVA common base is
a) 2.6
b) 0.615
c) 1.625
d) 9.0
ans. d
6. A 3-phase,4-pole, squirrel cage induction motor to
a) increase the strength of rotor bars.
b) reduce the magnetic hum and locking tendency of the rotor
c) economize on the copper to be used.
d) provide ease of fabrication.
ans. b
7. The rotor of 3-phase, 5 kW, 400 V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no-load steady-state speed when this motor is connected to 400 V, 50 Hz supply is
a) 1500 rpm
b) 500 rpm
c) 0 rpm
d) 1000 rpm
ans. c
8. The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with a 3-phase sinusoidal voltage having phase sequence A-B-C. For counterclockwise rotation of the motor, the phase sequence of the power supply should be
a) B-C-A
b) C-A-B
c) A-C-B
d) B-C-A and C-A-B
ans. b
9. If a 400V,50Hz star-connected, 3-phase, squirrel cage induction motor is operated from a 400V, 75Hz supply, the torque that the motor can now provide while drawing rated current from the supply.
a) reduces
b) increases
c) remains the same
d) increases or reduces depending upon the rotor resistance.
ans. a
10. A 3-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque of 300% with respect to rated voltage and rated frequency. Neglect the stator resistance and rotational losses. The value of slip for maximum torque is
a) 13.48%
b) 16.42%
c) 18.92 %
d) 26.79%
ans. d
a) capacitor
b) inductor
c) resistor
d) none of the above
ans. a
2. A synchronous motor operates at 0.8 pf lagging. If the field current of the motor is continuously increased
a) the power factor decreases up to a certain value of the field current and thereafter it increases
b) the armature current increases up to a certain value of the field current and thereafter it decreases
c) the power factor increases up to a certain value of the field current and thereafter it decreases
d) the armature current decreases up to a certain value of filed current and thereafter it increases
e) both c and d
ans. e
3. A cylindrical rotor synchronous motor is switched on the dc supply with its field winding's shorted on themselves. It will
a) not start
b) start but not run at synchronous speed
c) start as an induction motor and then run as synchronous motor
d) start and run as a synchronous motor
ans. b
4. Two 3-phase Y-connected alternators are to be paralleled to a set of common busbars. The armature has a per phase synchronous reactance of 1.7 ohm and negligible armature resistance. The line voltage of the first machine is adjusted to 3300 V and that of the second machine is adjusted to 3200 V. The machine voltages are in phase at eh instant they are paralleled. Under this condition, the synchronizing current per phase will be
a) 16.98 A
b) 29.41 A
c) 33.96 A
d) 58.82 A
ans. a
5. A power station consists of two synchronous generators A and B ratings of 250 MVA and 500 MVA with inertia 1.6 pu and 1 pu respectively on their own base MVA ratings. The equivalent per unit inertia constant for the system on 100 MVA common base is
a) 2.6
b) 0.615
c) 1.625
d) 9.0
ans. d
6. A 3-phase,4-pole, squirrel cage induction motor to
a) increase the strength of rotor bars.
b) reduce the magnetic hum and locking tendency of the rotor
c) economize on the copper to be used.
d) provide ease of fabrication.
ans. b
7. The rotor of 3-phase, 5 kW, 400 V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no-load steady-state speed when this motor is connected to 400 V, 50 Hz supply is
a) 1500 rpm
b) 500 rpm
c) 0 rpm
d) 1000 rpm
ans. c
8. The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with a 3-phase sinusoidal voltage having phase sequence A-B-C. For counterclockwise rotation of the motor, the phase sequence of the power supply should be
a) B-C-A
b) C-A-B
c) A-C-B
d) B-C-A and C-A-B
ans. b
9. If a 400V,50Hz star-connected, 3-phase, squirrel cage induction motor is operated from a 400V, 75Hz supply, the torque that the motor can now provide while drawing rated current from the supply.
a) reduces
b) increases
c) remains the same
d) increases or reduces depending upon the rotor resistance.
ans. a
10. A 3-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque of 300% with respect to rated voltage and rated frequency. Neglect the stator resistance and rotational losses. The value of slip for maximum torque is
a) 13.48%
b) 16.42%
c) 18.92 %
d) 26.79%
ans. d
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