1. Flashing is used to __________
a) temporarily excited field from a battery source for large DC generator
b) temporarily excited field from a battery source for small DC generator
c) temporarily excited field from a battery source for large DC motor
d) temporarily excited field from a battery source for large DC motor
Explanation: It is a method to give a kick start to dc machine when its residual magnetism is vanished.
2. A series generator, having external characteristics which is a straight line through 0 to 50 V at 200 A is connected as a booster, between a station bus-bar and feeders of 0.5 ohm resistors. The voltage at the far end of feeder at current of 50 A is?
a) 25 V
b) 17 V
c) 8 V
d) 50 V
Explanation: V = voltage drop-voltage added up by boosters
= (0.3*50 – 50*50/200)
= 15-12.50
= 2.5 V.
3. The number of parallel paths in the armature is increased by __________
a) increasing number of magnetic poles
b) decreasing number of magnetic poles
c) lap number of magnetic poles
d) using more brushes
Explanation: It is increased by increasing magnetic poles which give parallel path to the magnetic circuit.
4. A dc series generator as armature and field connections are reversed, the generator __________
a) stops
b) opposite direction
c) same direction
d) none of the mentioned
Explanation: It happens due to non build up voltage.
5. _____________ is responsible for mechanical power output of a DC motor.
a) Electrical input power
b) Any of the mentioned
c) Air-gap flux
d) Armature emf
Explanation: Primarily it is the electrical input power that starts the motoring action.
6. The armature mmf affects __________
a) commutation
b) generated voltage
c) torque
d) any of the mentioned
Explanation: Armature reaction thus armature mmf affects all of the mentioned above.
7. Armature reaction in a dc machine is __________
a) cross magnetizing in nature
b) magnetizing in nature
c) demagnetizing in nature
d) none of the mentioned
Explanation: It is cross magnetizing in nature which happens due to interaction between stator and field mmf.
8. The air-gap flux density waveform has decreased flux under one pole tip and measured under the other is due to __________
a) cross magnetization
b) magnetization
c) demagnetization
d) saturation of iron magnetic circuit
Explanation: The flux density increases under one pole tip while it decreases under the other due to the saturation of iron magnetic circuit.
9. The flux distortion caused by cross magnetizing armature reaction is more pronounced in a __________
a) dc shunt motor
b) dc series motor
c) any of the mentioned
d) differential Compound Motors
Explanation: Since field excitation remains substantially constant, while aarmature mmf reaches higher values at loading.
10. Which method needed to limit cross-magnetizing effect?
a) by increasing reluctance of cross flux path
b) by chamfering the pole faces
c) by compensating winding
d) any of the mentioned
Explanation: Any of the given methods will help to neutralize the armature reaction effect