# Electrical Machines Questions and Answers – Synchronizing Power and Synchronizing Torque

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This set of Electrical Machines Multiple Choice Questions & Answers focuses on “Synchronizing Power and Synchronizing Torque”.

1. The coupling angle or load angle of a synchronous motor is defined as the space angle between __________
a) rotor and stator poles of opposite polarity
b) rotor and stator poles of same polarity
c) rotor and stator teeth
d) rotor and resultant magnetic field

Explanation: It is measured between stator and rotor poles of opposite polarity.

2. What is Synchronizing power?

```(i) Stiffness coefficient of electromagnetic coupling between stator and rotor
(ii) Stability measure of synchronous machine
(iii) Power consumption factor
```

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

Explanation: Synchronizing power is stiffness coefficient representation as well the a measure of stability.

3. Large stiffness coefficient represents ____________
a) motor speed remains practically constant
b) large fluctuations due to mechanical load
c) motor speed is variable to changes
d) no such significance with speed

Explanation: Large stiffness represents the tight coupling in the machine and more stability.

4. An over excited synchronous machine is more stable than under excited.
a) True
b) False

Explanation: Synchronizing power is directly proportional to the excitation.
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5. Consider the below remarks for an alternator.

```(i) An over excited machine is more stiff
(ii) Machine with longer air-gap is less stable than one with smaller air-gap
```

a) (i) is True, (ii) is False
b) (i) is False, (ii) is True
c) (i) is True, (ii) is True
d) (i) is False, (ii) is False

Explanation: Larger the air gap smaller than the reactance.

6. As load angle is increased, degree of stability __________
a) reduces
b) increases
c) remains same
d) no change in stability

Explanation: Synchronizing power = EV*cosδ/X.

7. Synchronizing power is __________
a) transient in nature
c) pulsating at small frequencies
d) constant for a machine

Explanation: It is transient in nature and determines stability while disturbances.

8. Which of the following makes synchronizing power to come to play in the machine?
a) Disturbance in power mover
b) Disturbance in field current
d) All of the mentioned

Explanation: Any disturbance will lead to transients to arise and so the synchronizing power.

9. Which of the following is/are used in synchronous machines to maintain mechanical stability?
a) Damper winding
b) Interpole winding
c) Compensating winding
d) Equalizer rings

Explanation: Damper windings are the dummy windings which help to stablize the machine during transient instability.

10. For a 4-pole 3 phase, 400V alternator has synchronizing power of 300 units. Then the synchronizing power per mechanical degree is?
a) 300
b) 1200
c) 900
d) 600

Explanation: P(sy, mech) = Poles* P(sy,ele)/2
P(sy,ele) = 300 units.

11. Electromagnetic torque is present in rotating machines when ____________
a) both stator and rotor windings carry current
b) rotor windings carry current
c) stator windings carry current
d) any of the mentioned

Explanation: Synchronous machine is a doubly excited machine.

12. The operation of a synchronous motor operating on constant excitation across infinite bus will not be stable if power angle δ?
a) exceeds internal angle θ
b) is less than θ
c) is more than θ/2
d) is less than θ/2

Explanation: Power angle should not exceed θ, to operate under normal conditions.

13. In a synchronous motor the back emf peak set up in the stator depends on ___________
a) rotor excitation
b) supply voltage
c) rotor speed

Explanation: It will depend on the excitation.

14. Variation in the dc excitation of a synchronous motor causes variation in ___________
a) armature current and power factor
b) power factor
c) armature current
d) speed

Explanation: Armature current and power factor both will vary by varying the dc excitation.

15. Which motor can conveniently operate at lagging as well as leading power factor?
a) Synchronous motor
b) Slip ring induction motor
c) Squirrel cage induction motor
d) Stepper motor

Explanation: Its the synchronous motor which can favourably operate both at leading and lagging power factor.

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