Alternator MCQ Question Answer

1. Salient pole type alternators are generally used on

(a) low voltage alternators
(b) hydrogen cooled prime-movers
(c) high speed prime-movers
(d) low and medium speed prime-movers

2. Turbo-alternators are generally used to run at

(a) 500 R.P.M.
(b) R.P.M.
(c) 5000 R.P.M.
(d) 16000 R.P.M.

3. The rotor preferred for alternators applied to hydraulic turbines are

(a) salient pole type
(b) cylindrical rotor type
(c) solid rotor type
(d) any of the above

4. Which of the following prime-mover is least efficient?

(a) Gas turbine
(b) Petrol engine
(c) Diesel engine
(d) Steam engine

5. In which coil the harmonic component of the generated e.m.f. will be more?

(a) Full pitch coil
(b) Short pitch coil
(c) Long pitch coil
(d) Same in all coils

6. In case of turbo-alternators the rotor is usually made of

(a) cast iron
(b) forged steel
(c) laminated stainless steel
(d) manganese steel

7. The number of poles in turbo-alternators is usually

(a) 2
(b) 4
(c) 6
(d) 8

8. In huge alternators, the moving part is

(a) brushes
(b) poles
(c) armature
(d) none of the above

9. When a three-phase alternator supplies capacitive load, armature reaction will be

(a) opposing the main magnetic field
(b) helping the main magnetic field
(c) none of the above

10. Where the terminal voltage of an alternator falls on throwing of the load, it indicates that the load is purely

(a) resistive
(b) capacitive
(c) inductive
(d) none of the above

11. In case of a uniformly distributed winding, the value or distribution factor is

(a) 0.65
(b) 0.76
(c) 0.85
(d) 0.995

12. Unbalanced three-phase stator currents cause

(a) vibrations
(b) heating of rotor
(c) double frequency currents in the rotor
(d) all of the above

13. In a large generator, dampers

(a) reduce frequency fluctuations
(b) reduce voltage fluctuations
(c) increase stability
(d) none of the above

14. In an alternator, the armature reaction will be completely magnetizing in case the load power factor is

(a) zero lagging
(b) zero leading
(c) 0.860
(d) unity

15. In a star connected armature winding

(a) phase voltage contains 3rd harmonic e.m.f. but line voltage does not contain it
(b) phase voltage contains 3rd harmonic e.m.f. and line voltage also contains third harmonic component
(c) phase voltage does not contain third harmonic e.m.f. and line voltage also does not contain it
(d) none of the above

16. The slip rings employed in a 3-phase alternator in hydro station are insulated for

(a) low voltage
(b) very low voltage
(c) full armature voltage
(d) extra high tension voltage

17. In an alternator, for a coil having a span of 2/3 of pole pitch, the coil span-factor will be

(a) 0.866
(b) 0.707
(c) 0.6
(d) 0.2

18. Excessive windage loss and noise occur with

(a) cylindrical rotors
(b) salient pole rotors
(c) either of the above
(d) none of the above

19. Regulation of an alternator supplying leading load is

(a) always negative
(b) always positive
(c) sometimes positive and sometimes negative depending upon load and power factor

20. Which harmonic will be totally eliminated in an alternator by using fractional pitch of 4/5 ?

(a) Third
(b) Fifth
(c) Seventh
(d) Ninth

21. Dirt accumulation in generators can cause all of all of the following except

(a) low power factor
(b) poor voltage regulation
(c) flashovers
(d) overheating

22. If an alternator is operating at leading power factor, then it can he concluded that

(a) the alternator is under- excited
(b) the alternator is over-excited
(c) the torque angle of the alternator has negative value
(d) the residual magnetism of the poles is zero

23. Which fractional pitch will eliminate the seventh harmonic from the voltage waveform of an alternator ?

(a) 6/7
(b) 7/8
(c) 5/6
(d) None of the above

24. Damper winding on alternator results in all of the following except

(a) increases instability of machine
(b) elimination of harmonics
(c) absorption of energy of oscillations when operating in parallel with other alternators
(d) suppression of spontaneous hunting when supplying power to transmission line with high resistance to reactance ratio

25. In a cylindrical rotor how much portion of the rotor is wound

(a) One third
(b) Half
(c) Two third
(d) Full

26. The speed of an alternator is changed from 3000 r.p.m. to 1500 r.p.m. The generated e.m.f./phase will become

(a) one fourth
(b) half
(c) double
(d) unchanged

27. Zero power factor method for an alternator is generally used to determine

(a) synchronous impedance of alternator
(b) efficiency of alternator
(c) voltage regulation of the alternator
(d) none of the above

28. In an alternator the armature reaction is mainly influenced by

(a) power factor of the load
(b) short- circuit ratio
(c) speed of the alternator
(d) total current drawn

29. Regulation of an alternator supplying resistive or inductive load is

(a) always negative
(b) always positive
(c) either of the above
(d) none of the above

30. To obtain sinusoidal flux distribution

(a) chamfering of poles is done in turbo alternators only
(b) chamfering of poles is done in salient pole alternators only
(c) either of the above
(d) none of the above

31. Which of the following is the common synchronous speed in R.P.M. between 50 Hz and be 60 Hz alternators ?

(a) 200
(b) 300
(c) 600
(d) 900

32. Salient pole type rotors as compared to cylindrical pole type are

(a) small in diameter as well as axial length
(b) large in diameter as well as axial length
(c) smaller in diameter and larger in axial length
(d) larger in diameter and smaller in axial length

33. Which of the following relays come into operation in the event of the failure of prime-mover connected to the generator ?

(a) Buchholz relay
(b) Reverse power relay
(c) Differential relay
(d) All of the above

34. In order to reduce the harmonics in the e.m.f. generated in an alternator

(a) winding is well distributed
(b) slots are skewed
(c) salient pole tips are chamfered
(d) all of the above

35. The permissible duration for which a generator of rated frequency 50 Hz can run at 46 Hz is

(a) one cycle
(b) one second
(c) one minute
(d) none of the above

36. Due to which of the following reasons, for aircraft alternators high frequency is used ?

(a) To reduce the bulk
(b) To compensate for high speeds
(c) To compensate for high altitudes
(d) To free the systems from external disturbance

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