# Electrical Machines Questions and Answers – Physical Concepts of Synchronous Machine Operation

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

1. Floating of synchronous machine on infinite bus means ___________
a) Fa = 0 and machine neither generator nor motor
b) Fa is not zero and machine neither generator nor motor
c) Fa = 0 and machine is not excited
d) frequency is zero

Explanation: Floating means the armature flux is zero and it neither motoring nor generating.

2. If the field poles are ahead of resultant mmf, then the 3-phase synchronous machine is operating as ___________
a) generator
b) motor
c) reluctance motor
d) any of the mentioned

Explanation: Field mmf leads the resultant mmf so it is generator.

3. At steady state operation in a 3-phase synchronous generator. (where Tem is developed electromagnetic torque developed in the machine and Tpm is the prime mover torque.)
a) Tem = Tpm
b) Tem > Tpm
c) Tem < Tpm
d) can not be coupled

Explanation: At steady state the net torque is zero in the machine.
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4. Active power in synchronous machine is proportional to __________
a) torque
b) excitation
c) sinϕ
d) all of the mentioned

Explanation: Active power is proportional to the torque of the synchronous machine.

5. The reactive power in synchronous machine is proportional to __________
a) torque
b) excitation
c) sinϕ
d) all of the mentioned

Explanation: Reactive power depends on the excitation of the synchronous machine.

6. If the synchronous machine is loaded from floating conditions, the alternator begins to operate at __________
b) lagging p.f
c) unity p.f
d) zero p.f

7. If the synchronous machine is loaded from floating conditions, the synchronous motor begins to operate at __________
b) lagging p.f
c) unity p.f
d) zero p.f

Explanation: The resultant flux will be lagging in nature, so after loading it from loading it from floating, alternator will operate at lagging p.f.

8. Reactive power flow in synchronous machine can be controlled by __________
a) varying field excitation
b) varying field current
c) varying field flux
d) any of the mentioned

Explanation: All the alternatives are same, varying the field flux matters at the end to change the reactive power in a synchronous machine.

9. If the 3-phase alternator has 4 poles and has synchronous speed of 120 rad/s. Then the mechanical speed in rad/sec is?
a) 60
b) 240
c) 120
d) 30

Explanation: Mechanical speed = electrical speed*2/Number of Poles.

10. The power load angle characteristic as maximum for a load angle of __________
a) 90°
b) 45°
c) more than 90°
d) 180°

Explanation: As per the parabolic graph, the maxima is obtained at 90°.

11. For running a 750 kW compressor, a synchronous motor is preferred because of __________

```(i) lower noise
(ii) faster build up of pressure
(iii) better power factor
(iv) low starting torque
```

a) (ii), (iii)
b) (i), (ii), (iv)
c) (i), (ii), (iii), (iv)
d) (iii), (iv)

Explanation: It is preferred due to better power factor and low starting torque.

12. A poly phase synchronous motor will be used for the load of ______ over poly phase induction motor.
a) 600 kW 500 rpm
b) 600 kW 1500 rpm
c) 600 kW 3500 rpm
d) 600 kW 1200 rpm

Explanation: A synchronous motor is used at low speed.

13. During the starting of the 3-phase synchronous motor by damper bars, the field winding is usually short circuited so that starting torque is equal to __________
a) induction motor torque plus an additional torque produced by short circuited field winding
b) induction motor torque
c) electromagnetic torque
d) reluctance torque due to rotor saliency

Explanation: Using dampers to start the synchronous motor, it will produce induction motor torque initially along with the torque due to shorted field winding.

14. To start the synchronous motor, its field winding should be __________
a) short circuited
b) kept open
c) connected to dc excitation source
d) any of the mentioned

Explanation: To start the synchronous motor, it is first run as field excitation as zero, so we short circuit them.

15. Squirrel cage bars placed in the rotor pole faces of a 3-phase alternator help in reducing hunting.

```I. above synchronous speed
II. below synchronous speed
III. at synchronous speed
```

a) I, II
b) III
c) I, II, III
d) none of the mentioned

Explanation: This method is based on the operation of the induction motor, hence it will not operate at synchronous speed.

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