Alternator Multiple Choice Questions and Answers PDF

1. Armature winding is one in which

(a) emf is induced by the leakage flux.
(b) emf is induced by the main or working flux.
(c) working or main flux is created by the field current.
(d) working or main flux is created by the armature current.

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2. Armature winding in ac machines

(a) is always closed one.
(b) is always open one.
(c) may be either closed one giving delta connections or open one giving star connections.

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3. The stator of modern alternators are wound for ………. phase groups.

(a) 180°
(b) 120°
(c) 60°
(d) 240°

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4. Concentrated winding has

(a) one coil per phase.
(b) two coils per phase.
(c) one coil per pair of poles per phase.
(d) one coil side per pole pair per phase.

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5. For a two layer winding the number of stator slots is equal to the number of

(a) poles.
(b) conductors.
(c) coil sides.
(d) coils.

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6. For a concentrated winding the polar group of each phase is ………… electrical degrees apart.

(a) 360
(b) 180
(c) 120
(d) 60

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7. Consider the following statements regarding the design of the distributed armature winding in a 3-phase alternator:

  1. It reduces the phase belt harmonics.
  2. It increases utilization of armature iron and copper.
  3. It increases rigidity and mechanical strength of the winding.
  4. It reduces copper in the overhang of the winding.

Which of these statements arc correct?
(a) 1, 3 and 4.
(b) 2, 3 and 4.
(c) 1, 2 and 3.
(d) 1, 2, 3 and 4. [I.E.S. E.E.-II, 2000]

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8. The windings for an alternator are

I. 36 slots, 4-poles, span 1 to 3
II. 72 slots, 6-poles, span 1 to 10
III. 96 slots, 6-poies, span 1 to 12
The windings having pitch factor of more than 0.9 are
(a) I and II only.
(b) II and III only.
(c) l and III only.
(d) I, II and III. [U.P.S.C. I.E.S. 1992]

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9. Distributed winding is preferred over concentrated winding as it

(a) reduces noise.
(b) reduces the machine size.
(c) reduces the amount of copper required.
(d) improves the generated emf waveform and adds rigidity and mechanical strength to the winding.

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10. In a synchronous alternator, which of the following coils wil! have emf closer to sine wave form ?

(a) Concentrated winding in full pitch coils.
(b) Concentrated winding in short pitch coils.
(c) Distributed winding in full pitch coils.
(d) Distributed winding in short pitch coils. [U.P.S.C. I.E.S. 1992]

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11. If the space flux distribution is non-sinusoidal, emf induced in the distributed winding will be

(a) more sinusoidal in comparison to the flux distribution.
(b) less sinusoidal in comparison to the flux distribution.
(c) of the same shape as that of flux distribution.
(d) none of the above.

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12. The armature winding of a 2-pole, 3-phase alternator for each phase is distributed in a number of slots per phase. The rms value of the voltage per phase is less than the rms value of the voltage per coil multiplied by the number of coils in series because the

(a) rms value of the voltage in different coils of the phase is different. (b) equal rms voltages in different coils of the phase has mutual phase difference.
(c) maximum values of the induced voltages in different coils of the phase are different.
(d) different coils of the phase pass through different saturated regions of the magnetic circuit. [U.P.S.C. I.E.S. 1993]

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13. Consider the following statements: The effects of slot harmonics are effectively reduced by

  1. skewing the slots.
  2. fractional slot winding.
  3. short-chorded winding.
  4. distributed winding.

Of these statements
(a) 1 and 2 are correct.
(b) 2, 3 and 4 are correct.
(c) 1, 3 and 4 are correct.
(d) 1 and 3 are correct. [U.P.S.C. I.E.S. 1998]

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14. The short-pitch winding is preferred over full-pitch winding for an alternator because it

(a) gives improved waveform of the induced emf as the distorting harmonics can be reduced or totally eliminated.
(b) reduces the inductance of the winding.
(c) increases total induced emf.
(d) both (a) and (b).

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15. Chording and distribution of armature windings for ac machines results in

(a) reduction in air gap mmf harmonics.
(b) reduction in fundaments component or induced emf .
(c) increase in fundamental component of induced emf.
(d) reduction in noise and machine size.
(e) both (a) and (b).

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16. Harmonics in the emf generated in an alternator can be reduced by

(a) skewing the slots.
(b) chamfering the salient pole tips.
(c) using distributed winding
(d) all of the above.

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17. Skew of rotor bar eliminates

(a) the effect of space harmonics.
(b) the entire effect of crawling.
(c) magnetic noise.
(d) virbration due to unequal force developed on rotor. [AMIE. Sec B. Electrical Machines Winter 1996]

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18. The emf generated due to nth harmonic component of flux in an alternator will be …….. the fundamental emf in magnitude.

(a) less than
(b) more than
(c) equal to

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19. The stator of a 3-phase, 6-pole ac machine has 45 slots. The stator winding has 45 coils with a coil span of 6 slots. What type of winding will be selected for this machine?

(a) Double-layer, fractional slot, short-pitched winding.
(b) Single-layer, fractional slot, short-pitched winding.
(c) Single-layer, integral slot, full-pitch winding.
(d) Double-layer, fractional slot, full-pitch winding. [U.PS.C. I.E.S. 2008]

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20. Distributed winding and short chording employed in ac machines will result in

(a) increase in emf and reduction in harmonics.
(b) reduction in emf and increase in harmonics.
(c) increase in both emf and harmonics.
(d) reduction in both emf and harmonics. [GATE E.E. 2008]

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21. To eliminate rth harmonic from the induced emf in a phase of a synchronous machine the pitch of the coil must be

(a) (r – 1)/r th fraction of full pitch.
(b) (2r – 1)/r th fraction of full pitch.
(c) r/(r + 1) & th fraction of full pitch.
(d) 2r/(r + 1) th fraction of full pitch.

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22. It is desirable to eliminate 5th harmonic voltage from the phase voltage of an alternator. The coils should be short pitched by an electrical angle of

(a) 30°
(b) 36°
(c) 72°
(d) 18° [GATE E.E. 2001]

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23. In a synchronous generator, a divided winding rotor is preferable to a conventional winding rotor because of

(a) higher efficiency.
(b) increased steady-state stability limit.
(c) higher short circuit ratio.
(d) batter damping. [A.M.I.E. Sec B. Winter 1996]

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24. Power factor of an alternator driven by constant prime mover input can be changed by changing its

(a) Speed.
(b) Load.
(c) Field excitation.
(d) Phase sequence. [U.P.S.C. I.E.S. 2010]

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25. How can the reactive power delivered by a synchronous generator be controlled?

(a) By changing the prime mover input.
(b) By changing the excitation.
(c) By changing the direction of rotation.
(d) By changing the prime mover speed. [U.P.S.C. I.E.S. 2007]

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26. The rating of the prime mover driving an alternator is determined entirely by its rating.

(a) voltage
(b) current
(c) kW output
(d) speed

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27. When an altemator designed for operation at 60 Hz is operated at 50 Hz

(a) kVA rating will increase in the ratio of 1.2.
(b) operating voltage will reduce in the ratio of 5/6.
(c) operating voltage will increase in the ratio of 1.2
(d) operating voltage will reduce in the ratio of (5/6)2.

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28. The rating of a synchronous machine is usually governed by its

(a) speed.
(b) temperature rise.
(c) weight.
(d) none of these.

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29. The flux set up by the armature current, which does not cross the air gap and takes a different path is called the ………. flux.

(a) leakage
(b) main
(c) cross-magnetizing
(d) demagnetizing

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30. The magnitude of leakage flux depends on

(a) the magnitude of armature current.
(b) phase angle between armature current and terminal voltage.
(c) air gap thickness.
(d) both (a) and (b).

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