This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Losses and efficiency”.
1. For the given traction application using a dc series motor for a starting time ’t’ is applied. If this method is replaced by a series-parallel control, giving 50% time for each series and parallel. The saving in the starting energy of _________
a) 25%
b) zero
c) 50%
d) 75%
View Answer
Explanation: Initially let the energy utilized was 100% for time t.Then for 50% duration for series, energy used is 50%. For parallel operation for the same circuit, the resistance gets halved assuming the same machine. So, for 50% of the parallel operation, 25% of energy is only utilized. Hence total saving is 25%.
2. If two series motors are identically coupled. One is running as motor while other as generator. For this combination, the iron losses and frictional losses are found identical for _________
a) identical speed and excitation
b) identical speed
c) identical rating and construction
d) none of the mentioned
View Answer
Explanation: For same iron and frictional losses, same frequency should be there. For that same speed must be run.
3. Hysteresis loss in a dc machine is _____ of rate of flow of air and _____ on frequency of _____
a) independent, dependent, magnetic reversal
b) independent, dependent, operation
c) dependent, independent, magnetic reversal
d) none of the mentioned
View Answer
Explanation: Hysteresis loss in a dc machine is independent of rate of flow of air and depends on frequency of magnetic reversal.
4. In dc machine iron losses cause _________
a) heating in core
b) loss in efficiency
c) rise in temperature of ventilating air
d) all of the mentioned
View Answer
Explanation: The iron losses cause heating of the core which causes reduction inefficiency and cooling air gets heated up.
5. For squirrel cage and slip ring induction motor, cooling methods is efficient in _________
a) squirrel cage induction motor
b) slip ring induction motor
c) both of the motors
d) none of the mentioned
View Answer
Explanation: As the squirrel cage induction motor has more ventilation and more space as compares to slip ring induction motor, there will be more efficient ways to cool down the squirrel cage motor.
6. If one of the phases of the supply breaks down, then the connected three phase induction motor _________
a) continues to run as a single phase induction motor, provided load does does not increase beyond 57.7%
b) stops operating after few seconds
c) continues to run as single phase induction motor
d) continues to run as two phase induction motor
View Answer
Explanation: When one of the phases breaks down then the other two phases will supply the rated current but the load should be reduced to 57.7%.
7. If one of the phases of RYB supply gets broken, then the temperature rise of the induction motor _________
a) remains same as before
b) reduces as compared to the normal operating temperature
c) reduces to temperature rise of corresponding single phase induction motor
d) increases
View Answer
Explanation: The current flowing in the armature will remain same so to maintain the flux requirements of the machine. Hence the temperature rise will also remain same as the current remains same.
8. A three phase induction motor is connected to the infinite bus operating at the normal conditions. There occurs an unbalancing in the supply, leading to _________
a) unequal heating losses
b) stopping of motor
c) increase in lower
d) none of the mentioned
View Answer
Explanation: The unbalancing in the supply phase will create an unbalanced distribution of the current in the phases, thereby unequal heating.
9. A peaky voltage supply is given to the 3 phase power transformer of the connection power system. These results in _________
a) reduction in iron losses
b) reduction in copper losses
c) increase in iron losses
d) reduction in noise
View Answer
Explanation: The flux wave will be sinusoidal in nature as the emf is peaky in nature. Sine wave has least losses when compared to other wave forms.
10. The iron losses in a saturated three phase alternator is lesser than the non-saturated three phase alternator.
a) True
b) False
View Answer
Explanation: The non-saturated reactance is greater than saturated reactance. So the reactive losses or iron losses will reduce due lesser contribution of the same.
11. The hydroelectric plants in the industry are best suited with _________
a) closed circuit air cooling
b) hydrogen gas
c) direct water
d) all of the mentioned
View Answer
Explanation: The closed circuit is so used to reuse the air in the system so that to save water again in the cooling system.
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