This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Methods of Excitation in DC Machines”.
1. Consider a DC generator running at the rated speed of 2000 rpm, suddenly there is an insulator falls on the field circuit and breaks it. Then __________
a) the motor stops in a few rounds
b) it continues to run but as dc motor
c) it continues to run as a motor
d) none of the mentioned
View Answer
Explanation: As the field circuit gets broken there is interruption in the flux to the dc machine, and as a result the motor stops in few seconds.
2. If a self excited DC generator is failed to run, this refers to ________________
a) zero residual voltage
b) field MMF, that it is not cumulative
c) resistance is greater than critical resistance
d) any of the mentioned
View Answer
Explanation: Any of the reasons are valid for no build up of an emf.
3. The self-excited dc generator with gradual build up of residual voltage and EMF corresponds to __________
a) positive feedback
b) negative feedback
c) saturation
d) any of the mentioned
View Answer
Explanation: It responds to a positive feedback because the gain is always more than one in the linear region.
4. The critical resistance refers to __________
a) the resistance above which machine does not excite
b) the resistance below which machine does not excite
c) the resistance at which machine does not excite
d) any of the mentioned
View Answer
Explanation: After taking a glance at the magnetization curve, it can be concluded that the emf will not build up if the resistance is greater than critical resistance.
5. If a DC shunt generator fails to start, the most primitive action would be __________
a) reversing field connection to armature
b) reversing direction of rotation
c) any of the mentioned applicable ways
d) none of the mentioned
View Answer
Explanation: The first measure is to check if the field winding as properly connected with additive polarity so that the flux is aiding in nature.
6. 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
View Answer
Explanation: It is a method to give a kick start to dc machine when its residual magnetism is vanished.
7. 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
View Answer
Explanation: V = voltage drop-voltage added up by boosters
= (0.3*50 – 50*50/200)
= 15-12.50
= 2.5 V.
8. 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
View Answer
Explanation: It is increased by increasing magnetic poles which give parallel path to the magnetic circuit.
9. 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
View Answer
Explanation: It happens due to non build up voltage.
10. _____________ 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
View Answer
Explanation: Primarily it is the electrical input power that starts the motoring action.
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