Electrical Machines Questions and Answers – Machine Applications

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Machine applications”.

1. Synchronous-induction coupled motors are required for ____________
a) high starting torque applications
b) high pull out torque applications
c) high running torques applications
d) none of the mentioned
View Answer

Answer: a
Explanation: Induction motor provides high starting torque with proper values of slip ring terminal resistance. Coupled along with the synchronous motor it acts as an independent set and be employed in high torque application.
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2. Which of the following can be used for braking purposes in electric trains?
a) Induction generator
b) Induction motor
c) Dc series motor
d) Dc differentially compounded generator
View Answer

Answer: a
Explanation: Induction generator runs at a speed greater than the synchronous speed. To apply in braking purposes the induction generator can be run in the opposite direction to that of the operating machine direction and stopped.

3. A synchronous motor absorbing 60 kW is connected in parallel with an continuous load of 240 kW operating at 0.8 power factor lag.What is the kVAR supplied by the synchronous motor to obtain a total power factor of 0.9 lagging?
a) 35, leading
b) 35, lagging
c) 145, leading
d) None of the mentioned
View Answer

Answer: a
Explanation: Total power = 240 + 60 = 300 kW
cos(phi) = 0.90. Total reactive power = P*tan(phi) = 300*tan(25.8) = 180 kVAR Load kVAR = 240*tan(36.87) = 180 kVAR
kVAR supplied by the motor = 145-180 = -35 or k3 kVAR leading.
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4. The most widely used application of ac series motor is ____________
a) electric traction upto 1600 kW
b) paper mills
c) welding
d) none of the mentioned
View Answer

Answer: d
Explanation: Ac series motor is dc series motor connected to ac supply having an unidirectional torque, which is not very high in magnitude.It is widely applicable for small torque requirements like portable drilling machines, mixers etc.

5. Universal motor is used in vacuum cleaners, table fans and portable drilling machine.
a) True
b) False
View Answer

Answer: a
Explanation: The universal motor is dc series motor with ac supply with smaller torque. So it can be used for lower torque applications.
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6. _______ generator is used in arc welding purposes.
a) Differential compound dc
b) Dc series
c) Cumulative compounded dc
d) Shunt
View Answer

Answer: a
Explanation: The external characteristics of the differentially compound generator have minimum voltage for the high current voltages. This is best harnessed feature for a high current requirement by the welding application.

7. The dc series generator is employed as ____________
a) a booster to maintain constant voltage at the end of the feeder
b) for supplying traction load
c) for battery bank charging
d) none of the mentioned
View Answer

Answer: a
Explanation: Referring to the external characteristics of dc series generator shows that the terminal end voltage increases as the loading increases from no load to full load. In feeders due to variable requirements the voltage drops while transferring when loading is raised.
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8. If it asked by the customer to provide tappings on the transformer, the most efficient and economical way to provide it at ____________
a) middle of hv winding
b) phase end of hv winding
c) neutral end of hv winding
d) middle of lv winding
View Answer

Answer: a
Explanation: Tappings are provided to tap different levels of voltages at the high voltage end of the transformer. Providing it as low voltage side has no use and it is not economical.

9. In a single phase induction regulator, the voltage boost is maximum when armature mmf and rotor mmf make an angle of ____________
a) zero degrees
b) 45 degrees
c) 90 degrees
d) 180 degrees
View Answer

Answer: a
Explanation: To obtain a maximum of the voltage in induction regulator, the stator mmf and rotor mmf should be stationary and aligned to each other.
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10. An alternator is to be used for generation purpose in an aircraft. The most suited operating frequency of the alternator should be ____________
a) of 400 Hz to remove extra bulk
b) of 400 Hz to reduce losses
c) of 50 Hz to remove extra bulk
d) none of the mentioned
View Answer

Answer: a
Explanation: The components to be used in aircrafts are made to operate at very high frequencies so that the size of the components to be used is reduced.

11. A 3-phase synchronous alternator is connected to the infinite bus is over excited, that corresponds to ______________
a) capacitor
b) inductor
c) variable inductor
d) any of the mentioned
View Answer

Answer: a
Explanation: As the machine is over excited, it will act as source of reactive power and it is positive.
Q=V (Ecos (delta)-V)/X.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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