Alternator Quiz

1. In air-crafts, alternators have operating frequency of

(a) 800 Hz
(b) 400 HZ
(c) 60 Hz
(d) 50 HZ

2. Alternators, now-a-days, are designed to have poor regulation because

(a) it reduces the construction cost
(b) it limits the value of short-circuit current
(c) we employ automatic voltage regulators
(d) none of the above

3. Voltage regulation of an alternator is usually

(a) equal to that of power transformer
(b) much lower than that of a power transformer
(c) much higher than that of a power transformer

4. In an alternator, the stator frame serves

(a) to verticate the armature
(b) to hold the armature stampings
(c) to protect the whole machine
(d) as a return path for the flux

5. For a given output steam turbo-alternators are much smaller is size than water turbine alternators because

(a) steam turbo-alternators are built with smaller capacities
(b) steam turbo-alternators have long rotors
(c) steam turbo-alternators run at high speed
(d) all of the above

6. Non-salient pole type of rotor construction is usually provided in the alternators used in

(a) hydropower stations
(b) thermal power stations
(c) either of the above
(d) none of the above

7. In an alternator terminal voltage rise will be more

(a) when leading load is thrown off
(b) when lagging load is thrown off
(c) when unity power factor load is thrown off
(d) none of the above

8. In an alternator short pitch coils are used

(a) to reduce the stray losses
(b) to reduce the size of the machine
(c) to provide accurate phase difference of 120 degrees between each phase
(d) to reduce the harmonics in generated e.m.f.

9. If the input to the prime-mover of an alternator is kept constant but the excitation is increased, then

(a) kVA will be lagging
(b) kVA will be leading
(c) kW will be changed
(d) the power factor of the load remains constant

10. Generated e.m.f. for same field current and double speed will be

(a) same
(b) double
(c) less than double
(d) more than double

11. If the space flux distribution is non-sinusoidal, e.m.f. induced in the distributed winding

(a) will be less sinusoidal than flux distribution
(b) will be more sinusoidal than flux distribution
(c) will be equally non-sinusoidal
(d) none of the above

12. Voltage regulation obtained by synchronous impedance method is

(a) equal to that given by actual tests
(b) less than that given by actual tests
(c) more than that given by actual tests
(d) none of the above

13. In an alternator short-circuit current is limited by

(a) the saturated synchronous impedance
(b) the unsaturated synchronous impedance
(c) either of the above
(d) none of the above

14. In an alternator, armature reaction is considered equivalent to

(a) fictitious impedance
(b) fictitious conductance
(c) fictitious reactance
(d) fictitious resistance

15. Synchronous impedance method gives more regulation as compared to ampere-turn method because

(a) armature reaction is considered negligible
(b) saturation effect is ignored in the synchronous impedance
(c) saturation effect is taken into account in the synchronous impedance method

16. Leakage reactance of the armature of a salient pole synchronous machine which varies with the position of the rotor is due to

(a) leakage flux which remains within the slot
(b) leakage flux of end connections
(c) leakage flux which comes out of slot teeth, crosses the air gap and enters pole faces
(d) none of the above

17. Turbo-rotor is made up of

(a) solid steel forging having milled slots for field winding
(b) large spider to which laminated pole shoes are secured
(c) laminations
(d) none of the above

18. For the alternators operating in parallel, if the load shared by one of them is to be increased, its field excitation is

(a) to be weakened keeping input torque same
(b) to be strengthened keeping input torque same
(c) to be kept constant but input torque should be increased
(d) to be kept constant but input torque should be decreased

19. Which of the following methods is best for finding the voltage regulation?

(a) Synchronous impedance method
(b) M.M.F. method
(c) Pointer triangle method
(d) All are equally good

20. For a 3-phase winding with 5 slots per pole per phase and with coil span of 12 slot pitch, the value of pitch factor is

(a) 0.851
(b) 0.951
(c) 0.98
(d) 1.05

21. The power factor of an alternator is determined by its

(a) prime-mover
(b) excitation
(c) speed
(d) load

22. If the input to the prime mover of an alternator is kept constant but the excitation is changed then

(a) the power factor of the load remains constant
(b) the reactive component of the output is changed
(c) the active component of the output is changed
(d) none of the above

23. For parallel operation, A.C. polyphase alternators must have the same

(a) kVA rating
(b) excitation
(c) speed
(d) voltage rating

24. An alternator running in parallel with other alternators all having automatic voltage regulators is to be taken off the bus. The usual procedure before opening the switch is to

(a) reduce the power fed to the prime-mover
(b) reduce alternator excitation
(c) increase alternator excitation
(d) none of the above

25. Due to which of the following reasons concentrated winding are not used in alternators ?

(a) Concentrated windings increase voltage harmonics.
(b) Concentrated windings decreases induced e.m.f./phase.
(c) Concentrated windings increase copper-to-iron ratio thereby decreasing the capacity.
(d) Concentrated winding require deep slots for accommodation which leads to increased armature leakage reactance.

26. A stationary alternator should not be connected to a live bus-bar because it,

(a) will get short-circuited
(b) will disturb the generated e.m.fs. of other alternators connected in parallel
(c) is likely to run as a synchronous motor
(d) will decrease bus-bar voltage momentarily
(e) none of the above

27. The power drawn by the prime-mover of an alternator, under no-load conditions, goes to

(a) meet copper losses both in armature and rotor windings
(b) produce power in armature
(c) meet no-load losses
(d) produce e.m.f. in armature winding

28. In a larger generator dampers

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

29. The choice of field construction, in an alternator, is basically determined by

(a) generated voltage needed by the customers
(b) comparative cost of the field systems
(c) supply frequency required by the consumers
(d) the kind of energy source available in the geographic location

30. The load between two steam-driven alternators operating in parallel may be adjusted by varying

(a) steam supply to their prime movers
(b) speed of the alternators
(c) field strengths of the alternators
(d) power factors of the alternators

31. In an alternator, the armature reaction influences

(a) generated voltage per phase
(b) waveform of voltage generated
(c) operating speed
(d) windage losses

32. An alternator operating at lower voltage for the same power rating will be

(a) more efficient
(b) costlier
(c) less noisy
(d) larger in size

33. When two alternators are running in parallel their kVAR load share and kW load share are changed by changing their

(a) driving torque, driving torque respectively
(b) driving torque, excitation respectively
(c) excitation, driving torque respectively
(d) excitation and excitation

34. In an alternator one of the advantages of distributing the winding is to

(a) improve voltage waveform
(b) reduce noise
(c) save on copper
(d) reduce harmonics

35. ………. plays an important role in over speed protection of a generator ?

(a) Governor
(b) Differential protection
(c) Over current relay
(d) Alarm

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