Electrical Machines Questions and Answers – Synchronous Machine Applications

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

1. One of the most prominent component of base loads is ___________
a) synchronous generator
b) induction generator
c) dc series generator
d) all of the mentioned
View Answer

Answer: a
Explanation: Synchronous machines are the main generating section of the power plants.
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2. ______ alternator is used in thermal stations and ______ alternator is used in hydel plants.
a) Cylindrical-rotor, salient pole
b) Salinet pole, cylindrical-rotor
c) Salient pole, Reluctance
d) Cylindrical-rotor, cylindrical-rotor
View Answer

Answer: a
Explanation: Due to salient poles, the machine will not run at very large speed and it becomes less rugged.

3. Synchronous compensators are mainly employed for ___________
a) compensating reactive power
b) compensating real power
c) improving power factor
d) compensating field flux
View Answer

Answer: a
Explanation: Synchronous compensator is a source for the reactive power.
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4. We prefer induction motor over synchronous motor due to ___________
a) no requirement of dc supply
b) starting problems
c) sensitive to environment
d) all of the mentioned
View Answer

Answer: a
Explanation: All the listed factors are disadvantages for using a synchronous motor.

5. The condition which allows the torques to be taken care for choosing a synchronous motor is/are?
a) starting torque
b) pull-in torque
c) pull-out torque
d) all of the mentioned
View Answer

Answer: d
Explanation: All the performance torques are needed to know before using any synchronous motor.
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6. A synchronous machine with its field winding on stator and polyphase armature winding on rotor. At steady state, its air gap field is?
a) stationary w.r.t. stator
b) rotating at double the speed Ns w.r.t. rotor
c) rotating in direction opposite to rotor
d) all of the mentioned
View Answer

Answer: d
Explanation: All are correct.

7. A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is?
a) stationary w.r.t. stator
b) rotating at double the Ns w.r.t. rotor
c) rotating at -Ns w.r.t. rotor
d) stationary w.r.t. rotor.
View Answer

Answer: d
Explanation: As the field is on rotor itself, the sir gap will be stationary w.r.t. rotor.
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8. Choose the appropriate synchronous generators used in the thermal power stations.

(i) cylindrical rotor
(ii)slip rings and brushes
(iii)laminated rotor
(iv)stator slots in the multiple of 6
(v)salient rotor

a) (i), (ii), (iv)
b) (ii), (iii), (v)
c) (i), (iii)
d) (ii), (iii), (iv), (v)
View Answer

Answer: a
Explanation: Thermal plants use cylindrical rotor synchronous machines.
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9. The practical speed ranges for the thermal, nuclear and hydel power plants is (in rpm)?
a) 3000, 3000, 300
b) 3000, 300, 300
c) 1500, 1500, 3000
d) 1000, 500, 3000
View Answer

Answer: a
Explanation: Thermal and nuclear power plants have high speed requirements and hydel power plants have salient pole generators so the speed is also less.

10. Synchronous motor has been preferred for driving the loads requiring high power at low speeds.
a) True
b) False
View Answer

Answer: a
Explanation: Synchronous motor acts a capacitor when operated at leading p.f. with no load.

11. The zero power characteristics used in the Potier plot can be obtained by loading the alternator using ___________
a) synchronous motor
b) lamp load
c) dc motor
d) induction motor
View Answer

Answer: a
Explanation: To obtain mmf and emf drops the loading is done as a synchronous motor.

12. Below are the statements about cylindrical rotor alternator.

A. E.m.f. generated by armature reaction lags armature current by 90°.
B. Air gap voltage leads the field flux by 90°.

a) Only A is true
b) Only B is true
c) A is true, B is false
d) A and B both are true
View Answer

Answer: d
Explanation: Both the statements are right.

13. Out of the given statements about the cylindrical rotor alternator, mark the correct.

A. Air gap voltage lags the field flux by 90°    
B. Armature reaction mmf lags the field flux by (90°+ internal p.f. angle).

a) A is false, B is true
b) Only A is true
c) Only B is true
d) Both A and B are true
View Answer

Answer: a
Explanation: Air gap voltage will lead the field flux by 90° in a cylindrical-rotor alternator.

14. The power factor of an alternator under short circuit condition will be almost near ____________
a) zero lagging
b) zero leading
c) unity
d) depends on the type of the alternator
View Answer

Answer: a
Explanation: During short circuit condition, the magnetizing current flows to provide the core losses compensation. So the pf is slightly zpf lagging.

15. In a 3-phase cylindrical rotor alternator the _________
a) field mmf leads air gap flux, air gap flux leads armature flux
b) field mmf leads air gap flux, air gap flux lags armature flux
c) armature mmf leads air gap flux and air gap flux leads field flux
d) armature mmf leads air gap flux and air gap flux lags field flux
View Answer

Answer: a
Explanation: Check the phasor diagram.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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