DC Machines Questions and Answers

1. Which of the following are true in respect of interpoles in dc machines?

1. Reduce armature reaction effects in interpolar region.
2. Windings are connected in series with armature.
3. Have the same dimensions as main poles.

Select the correct answer using the code given below:
(a) 1 and 2 only.
(b) 2 and 3 only.
(c) 1 and 3 only.
(d) I, 2 and 3.

2. Consider the following statements about commutating poles which are fitted on most large dc shunt motors.

1. The commutating poles are placed in geometric neutral planes and their number is usually equal to the number of main poles.
2. The winding on the commutating poles is connected in series with the shunt field winding on the main poles.
3. The polarity of the commutating pole must be that of the next main pole further ahead.
4. The commutating poles neutralize the reactance voltage produced in the coil undergoing commutation.

(a) 1, 2 and 3 are correct.
(b) 1 and 4 are correct.
(c) 2, 3 and 4 are correct.
(d) 1, 2 and 4 are correct.

3. The compensating windings are placed in the slots in the pole faces to neutralize the

(a) cross-magnetizing effect.
(b) demagnetizing effect.
(c) both of the demagnetizing and cross-magnetizing effects.
(d) armature reaction MMF under the pole faces and avoid flash over between positive and negative brushes.

4. In large sized dc machines the compensating windings are provided to

(a) compensate for the decrease in speed due to increase of load.
(b) neutralize the voltage induced in the coil due to high fluctuation of load.
(c) reduce eddy current losses by providing local short-circuits.
(d) none of the above.

5. Compensating winding in a dc machine is placed on

(a) armature in slots.
(b) yoke in the interpolar gap.
(c) yoke in the pole faces.
(d) partly in armature slots and partly in pole faces.

6. Compensating windings are employed in dc motors which are likely to operate

(a) at a constant speed over wide range of load.
(b) with rapidly changing loads of wide range.
(c) over wide range of speed by field excitation control.
(d) both (b) and (c).

7. In a dc machine, which of the following statements is true?

(a) Compensating winding is used for neutralizing armature reaction while interpole winding is used for producing residual flux.
(b) Compensating winding is used for neutralizing armature reaction while interpole winding is used for improving commutation.
(c) Compensating winding is used for improving commutation while interpole winding is used for neutralizing armature reaction.
(d) Compensating winding is used for improving commutation while interpole winding is used for producing residual flux.

8. The compensating winding in a dc machine

(a) is located in armature slots for compensation of the armature reaction.
(b) is located on commutating poles for improving commutation.
(c) is located on pole shoes for avoiding the flashover at the commutator surface.
(d) is located on pole shoes to avoid the sparking at the brushes.

9. In dc machines, the interpoles are provided

(a) for mechanical balancing of machines.
(b) to counteract the demagnetizing effect of armature MMF in the commutating zone.
(c) to counteract the reactance voltage and the cross-magnetizing effect of armature MMF in the commutating zone.
(d) to make commutation sparkless.
(e) both (c) and (d).

10. The commutating flux produced by interpole must be proportional to

(a) armature current.
(b) field current.
(c) both armature and field currents.
(d) none of the above.

11. When the shunt field of a dc compound generator is connected across both the series field and armature. Such a connection is known as

(a) short shunt.
(b) long shunt.
(c) cumulative compounding.
(d) differential compounding.

12. The series field of a short shunt dc generator is excited by current.

(a) armature
(b) shunt field
(c) load[/expander_maker]
(d) I_L + I_sh

13. With the increases in field excitation of a dc generator, its generated emf

(a) increases.
(b) decreases.
(c) remains constant.
(d) goes on increasing proportionately.
(e) increases up to a limit and then remains almost constant.

14. The slight curvature at the lower end of the OCC of a self excited dc generator is due to

(a) magnetic inertia.
(b) residual flux.
(c) high speed.
(d) high field resistance.

15. The fall in speed of a dc generator due to increase in load can be corrected by

(a) cooling the armature.
(b) increasing the excitation.
(c) reducing the load voltage.
(d) increasing the input to the prime mover.

16. The magnetic field which initially induces emf in the armature in a self excited dc generator is because of

(a) permanent magnets.
(b) armature rotation.
(c) residual magnetism.
(d). none of the above.

17. A generator may lose residual magnetism due to

(a) heating.
(b) vibrations.
(c) over excitation.
(d) any of these.

18. In dc generators, the residual magnetism is of the order of

(a) 2.5%
(b) 10%
(c) 15%
(d) 25%

19. In comparison to self excited generator, a separately excited generator

(a) is more reliable.
(b) is amenable to better voltage regulation.
(c) has exciting current independent of load characteristic.
(d) has all of the above features.

20. A separately excited dc generator has an open-circuit voltage E_o for a certain value of field current. If the field current is halved, the open-circuit voltage will be

(a) E₀/2.
(b) less than E₀/2
(c) may be E₀/2 or more than E₀/2
(d) 2 E₀

21. The voltage at the terminals of a dc series generator running at rated rpm and no load will be

(a) more than the rated voltage.
(b) full rated voltage.
(c) a very small voltage.
(d) zero

22. A dc series generator is provided with divertor and is delivering its rated current. If the divertor switch is opened, the terminal voltage will

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

23. If the field circuit resistance of a dc shunt generator exceeds its critical value, the generator

(a) fails to build up.
(b) builds up a very high voltage.
(c) exceeds its current capacity.
(d) produces power beyond its rating.

24. A shunt generator has a critical field resistance of 200 Ω at a speed of 800 rpm. If the speed of the generator is increased to 1.000 rpm, what is the change in the critical field resistance of the generator?

(a) Decreases to 160 Ω
(b) Remains the same at 200 Ω
(c) Increases to 250 Ω
(d) Increases to 312.5 Ω

25. Consider the following statements related to a shunt generator:

1. The full-load voltage is lower than no-load voltage.
2. The above lowering is partly due to voltage drop in armature resistance.
3. The field current does not change with load and has no effect on generated voltage.

Which of the statements given above are correct?
(a) 1, 2 and 3.
(b) Only 1 and 2.
(c) Only 2 and 3.
(d) Only 1 and 3.

26. The factor that is not responsible for drop in terminal voltage of a dc shunt generator on load is

(a) commutation.
(b) armature reaction.
(c) armature resistance drop.
(d) field weakening due to (b) and (c).

27. In a dc shunt generator, the voltage buildup is generally restricted due to

(a) armature heating.
(b) insulation restrictions.
(c) saturation of iron.
(d) speed limitation.

28. Consider the following statements regarding the magnetization characteristics of a dc generator:

1. It represents the variation of field flux with field current.
2. Variation of open-circuit terminal voltage with field current is independent of speed.
3. It is nonlinear from zero to rated field current.
4. It helps to determine critical field resistance at a given speed.
5. It always starts from the origin.

Which of these statements are correct?
(a) 1 and 4.
(b) 1, 2 and 5.
(c) 2, 3 and 4.
(d) 3 and 5.

29. Out of the following factors for a dc machine

(i) Interpole (ii) Armature resistance (iii) Armature (iv) Reduction in field current (v) Effect of armature reaction
The factors that are responsible for decrease in terminal voltage of a shunt generator are
(a) (i), (ii) and (iv).
(b) (ii), (iii) and (v).
(c) (i), (iv) and (v).
(d) (ii), (iv) and (v).

30. An ideal dc generator has a regulation of ………… per cent.

(a) zero
(b) 10
(c) 25
(d) 100

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