# Objective Questions on Alternator

1. A synchronous machine is

By changing the excitation of synchronous machine it can run in any p.f i.e., in over excitation it gives leading p.f, in under excitation it gives lagging p.f and hence in normal excitation it will be in unity p.f. Those are 'v' curves of synchronous machine.

2. Symmetrical waveforms doesn’t having

Symmetrical wave forms doesn’t possess even harmonics. Since the wave shape for voltage generated in generators is mostly symmetrical due to field system and armature coils are symmetrical, therefore generated voltage or electric current have no even harmonics.

3. A winding is having no. of slots is equal to no. of poles, then this winding is called

In concentrating winding no.of slots is equal to no. of poles. If all the no. of conductors / phase are placed in one slot keeping remaining m-1 slots / pole / phase then winding is called concentrated winding. Where m = no. of slots / pole / phase.
In this winding emf waveform is not smooth and it contains harmonics.

4. A winding is having no of slots is greater than no. of poles, then this winding is called

In the distributed winding no. of slots is greater than no. of poles. The voltages induced in conductors of each coil side will be at different phases. Harmonics can be eliminated. The resultant emf waveform is smooth sine wave. It facilitates uniform distribution of heat around the armature.

5. When the no. of slots / pole / phase is fractional, then this winding is called

The no. of slots / pole / phase is fractional then the winding is called fractional winding or fractional slot winding. This winding reduces the high frequency harmonics to emf and mmf wave forms. All though it is appeared to be complicated easy to manufacture.

6. When the no of slots / pole / phase is integer, then this winding is called

If the no.of slots / pole / phase is integer then this winding is called integral slot winding. The no.of slots in an ac machine should always be an integral multiple of no. of phases. This winding may be short or full pitch winding.

7. Which alternator are more suitable for high-speed system?

Salient pole type field structure is used for low speed alternators. Non-salient pole machine has high peripheral speed and sufficient mechanical strength. So it is use for high speed application.

8. The number of poles in turbo alternator are

Under construction.

9. When two alternator are operating in parallel and at perfect synchronization, their synchronizing power will be

The process of connecting the alternators operating in parallel infinite bus bar without any interruption of supply, is called synchronization. At perfect synchronization, their synchronizing power is zero.

10. The stationary alternator must not be connected to live bus bars because it will results _______________ .

The stationary alternator must not be connected to live bus bars because the induced emf is zero at standstill and a short circuit will result.

11. Floating of alternators with bus bars occurs at when

When synchronized, the generated emf of the incoming machine is equal to bus bar voltage. Then the incoming machine is said to be floating on bus bars. At this instant it will neither deliver nor receive any power.

12. Which of following conditions is to be fulfilled, for connecting an alternator to infinite bus?

Conditions to be full filled to connect an alternator to infinite bus:
1. The phase sequence of incoming alternator and the existing system must be same.
2. The frequency of incoming alternator must be same as that of the existing system.
3. The phase voltage of incoming alternator and existing system must be in phase.

13. If a slot consists of one coil side then the winding is called

If a slot consists of one coil side then the winding is called single layer winding. All the single layer windings are full pitch windings. Fractional slot windings can’t be employed for single layer winding.

14. If the slot consists 2 coil sides of different coils is called

If the slot consists 2 coil sides of 2 different coils, one at bottom and another at top then it is called double layer winding. In this winding fractional slot winding can be used, short pitch winding is possible and easier to manufacture and it costs lower.

15. Distributed winding and short chording employed in AC machine will result in

Due to distributed winding and short chording, both emf and harmonics are reduced.

16. What is the distribution factor for a 108 slot, 12 pole, 3 phase winding?

Slots / pole / phase = 108 / ( 3 × 12 ) = 3, Slots angle ( θ ) = 180° / 2 = - 20°, Kd ( distribution factor ) = sin(mθ / 2) / msin(θ/2) = sin( 3× 10° ) / 3 × sin(10°) = 0.96.

17. Fifth harmonic component of the induced voltage in three phase AC generator can be eliminated by using a winding pitch of

Coil span factor for nth harmonic, Kc = cos ½nθ. Where θ is a angle by which the coil is short of full pitch.
Kc = − cos ½5.{1- (4 ⁄ 5)}π = 0. Those fifth harmonic component gets eliminated by using a coil pitch of 4 ⁄ 5.

18. The pitch of arc with 96 stator slots and 6 pole is

Pitch of arc = No. of stator slots / pole = 96 / 6 = 16 .

19. Distributed winding and short chording employed in AC machines will result in

Due to distribution winding and short chording both emf and harmonics are reduced.

20. Factors on which frequency of alternator depends

Ns = 120f / P ;
⇒ f = (P × Ns) / 120.
Where P in no. of poles and Ns is the synchronous speed.