Synchronous Generator Questions and Answers PDF

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

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

2. In a synchronous generator

(a) the armature mmf leads the air-gap flux and air-gap flux leads the field mmf.
(b) the armature mmf leads the air gap flux and the air-gap flux lags behind the field mmf.
(c) the armature mmf lags behind the air gap flux and air- gap flux lags behind the field mmf.
(d) the armature mmf lags behind the air-gap and air-gap flux leads the field mmf.

3. Armature reaction effect is/are ………. on the main field flux.

(a) distorting (or cross-magnetizing) as well as demagnetizing.
(b) only assisting
(c) only demagnetizing
(d) only distorting

4. The armature reaction effect in a synchronous machine depends on

(a) load current.
(b) power factor of the load.
(c) speed of the machine.
(d) both (a) and (b).

5. In a synchronous generator, the effect of cross-magnetization is to make the generated voltage

(a) truely sinusoidal.
(b) non-sinusoidal.
(c) free from harmonics.
(d) none of these.

6. In an alternator, the armature mmf will he in phase with the main field flux only when the load is purely

(a) resistive.
(b) inductive.
(c) capacitive.

7. In an alternator, the armature reaction is considered to be equivalent to a fictitious

(a) reactance.
(b) resistance.
(c) impedance.
(d) admittance.

8. A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is

(a) magnetizing.
(b) demagnetizing.
(c) cross-magnetizing.
(d) ineffective. [GATE E.E. 2006]

9. Armature reaction AT of a synchronous generator supplying power at rated voltage with zero power factor lagging is

(a) Magnetizing
(b) Demagnetizing
(c) Cross-magnetizing
(d) Both magnetizing and cross-magnetizing [U.P.S.C. I.E.S. 2010]

10. The synchronous reactance is the

(a) reaction due to armature reaction of the machine.
(b) reactance due to leakage flux.
(c) combined reactance due to leakage flux and armature reaction.
(d) reactance either due to armature reaction or leakage flux. [I.E.S. E.E. II 2003]

11. Drop in terminal voltage of an alternator due to armature reaction is countered by

(a) damper winding.
(b) effect of saliency.
(c) increased prime mover output.
(d) automatic voltage regulator. [A.M.I.E. Sec B. Electrical Machines Winter 1994]

12. The saturated synchronous reactance of an alternator is its …….. unsaturated synchronous reactance.

(a) more than
(b) less than
(c) equal to
(d) none of these

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

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

14. When the load (inductive) it thrown off, the terminal voltage will

(a) increase.
(b) decrease.
(c) remain unchanged.
(d) none of these.

15. As the leading power factor of the load of an alternator decreases, the magnitude of generated voltage required to give rated terminal voltage

(a) increases.
(b) decreases.
(c) remains unchanged.

16. In a synchronous generator delivering lagging power factor load

(a) the excitation emf leads terminal voltage by the power angle.
(b) the excitation emf lags the terminal voltage by the power angle.
(c) excitation emf leads the terminal voltage by the power factor angle.
(d) none of the above. [A.M.I.E. Sec B. Electrical Machines Summer 1995]

17. The power factor on which an alternator operates depends on the

(a) speed of the prime-mover.
(b) nature of the load being supplied.
(c) armature losses.
(d) copper losses.

18. Unbalanced 3-phase stator currents cause

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

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

(a) always +ve.
(b) always -ve.
(c) either of the above or zero.
(d) none of the above.

20. A leading pf load on an alternator implies that its voltage regulation shall be

(a) positive.
(b) negative.
(c) zero.
(d) any one of these. [A.M.I.E. Electrical Science Summer 1994]

21. The effect of leading power factor on the voltage regulation of an alternator is

(a) increasing in nature.
(b) decreasing in nature.
(c) maintained at constant value.
(d) oscillating in nature. [A.M.I.E. Sec B. Winter 1996]

22. The leakage reactance of a three-phase alternator is determined by performing

(a) open-circuit and zero power factor tests.
(b) zero power factor and slip tests.
(c) open- short-circuit tests.
(d) short-circuit and slip tests. [U.P.S.C. I.E.S. 2003]

23. The ‘synchronous impedance method’ of finding the voltage regulation of a cylindrical rotor alternator is generally considered as

(a) a pessimistic method because saturation is not considered.
(b) an optimistic method because saturation is not considered.
(c) a fairly accurate method even if power factor is not taken into account while determining synchronous impedance.
(d) a fairly accurate method when power factor is taken into account while determining synchronous impedance. [U.P.S.C. I.E.S. 2002]

24. The regulation obtained by synchronous impedance method is always higher than actual value because

(a) synchronous reactance is assumed variable while it is not.
(b) effective armature resistance is assumed constant.
(c) synchronous reactance is assumed constant while it is not.
(d) field current is increased to give short-circuit current about twice of full-load current.

25. The short-circuit characteristic of an alternator is

(a) always linear.
(b) always nonlinear.
(c) either of (a) or (b).
(d) none of these.

26. Under short-circuit conditions, the power factor of an alternator is

(a) unity.
(b) almost zero lagging.
(c) almost zero leading.
(d) none of the above.

 

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