February 13, 2017

37 Interview Questions and Answers on DC Generators - Part-3

Q. 1. How may the number of parallel paths in an armature be increased?

Ans. By increasing the number of magnetic poles.

Q. 2. How are brushes connected in a d.c. generator?

Ans. Usually, all positive brushes are connected together and all the negative brushes together (figure-a.).

Q. 3. What is meant by armature reaction?

Ans. It is the effect of armature magnetic field on the distribution of flux under main poles of a generator. The armature magnetic field has two effects:
  1. It demagnetises or weakens the main flux and
  2. It cross-magnetizes or distorts it.

Figure - (a)

Q. 4. What is the effect of this distortion on the operation of the machine?

Ans. It acts as a magnetic drag on the armature which consequently requires more power to turn it.

Q. 5. How can field distortion be remedied?

Ans. By using compensating windings which are embedded in the slots in the pole-shoe and are connected in series with the armature.

Q. 6. What is meant by normal neutral plane?

Ans. It is a plane which passes through the axis of the armature perpendicular to the magnetic field of the generator when there is no flow of current through the armature.

Q. 7. What is the importance of this plane in the working of the machine?

Ans. It gives the position where brushes would be placed to prevent sparking during the operation of the generator where the main pole field not distorted by armature field and were there no self-induction in the coils.

Q. 8. How do you differentiate between normal neutral plane?

Ans. The NNP is the position of zero induction and hence minimum sparking assuming no field distortion i.e. on no-load.  It is perpendicular to magnetic axis.  NA is the position of zero induction and hence minimum sparking with distorted field i.e. when generator is on load.

Q. 9. How do you define ‘commutating plane?

Ans. It is the plane which passes through the axis of the armature and through centre of contact of the brushes as shown in Figure (b).

Q. 10. What is the angle of lead?

Ans. It is the angle between the NNP and the commutating plane.

Q. 11. What affects this angle?

Ans. For sparkless commutation, the angle of lead varies directly with load. Its value can be kept small by making main pole field considerably more powerful than the armature field.

Q. 12. What is the best way of minimizing eddy currents in an armature?

Ans. Lamination.

Q. 13. How should the armature be laminated for the purpose?

Ans. It should be laminated at right angle to its axis.

Q. 14. How does field distortion affect communication?

Ans. The neutral plane no longer coincides with the normal neutral plane but is advanced by a certain angle in the direction of rotation of the armature.

Q. 15. Should the brushes of a loaded generator be placed in the neutral plane?

Ans. No.

Q. 16. Why not?

Ans. The brushes must be advanced by a certain angle (called brush lead) beyond the neutral plane to prevent sparking.

Q. 17. What causes sparking at the brushes?

Ans. It is due to the self-induction of the coil undergoing commutation.

Q. 18. What is the standard direction of rotation of the d.c. generators?

Ans. Clockwise when viewed from the end opposite to the driven end.

Q. 19. What is meant by build-up of a generator?

Ans. It means the gradual increase in the generator voltage to its maximum value after the generator is started from rest.

Q. 20. How should a generator be started?

Ans. It is usually brought up to speed with the help of the driving engine called prime-mover.

Q. 21. How should a shunt or compound generator be started?

Ans. Such machines excite best when all switches controlling the external circuit are open.

Q. 22. How about a series generator?

Ans. In this case, the external circuit must be closed otherwise the generator will not build-up.

Q. 23. What is the procedure for shunting down a generator?

Ans. First, the load should be gradually reduced, if possible, by easing down the driving engine, then when the generator is supplying little or no current, the main switch should be opened. When the voltmeter reads almost zero, then brushes should be raised from the commutator.

Q. 24. What are the indications and causes of an overloaded generator?

Ans. A generator is said to be overloaded if a greater output is taken from it that it can safely carry. Overloading is indicated by (i) excessive sparking at brushes and (ii) overheating of the armature and other parts of the generator. Most likely causes of overloading are :
  1. Excessive voltage-as indicated by the voltmeter or the increased brilliancy of the pilot lamp. This could be due to over-excitation of field magnets or too high speed of the engine.
  2. Excessive current–which could be due to bad feeding of the load.
  3. Reversal of polarity–this happens occasionally when the series or compound-wound generators are running in parallel.  Polarity reversal occurs during stopping by the current from the machines at work.
  4. Short-circuit or ground in the generator itself or in the external circuit.

Figure - (b)

Q. 25. Mention and explain the various causes for the failure of the generator to build up.

Ans. Principal causes due to which a generator may fail to excite are :
  1. Brushes not properly adjusted–if brushes are not in their proper positions, then whole of the armature voltage will not be utilized and so would be insufficient to excite the machine.
  2. Defective contacts-unclean contacts may interpose large resistance in the path of the exciting current and reduce it to such a small value that it fails to excite the machine.
  3. Incorrect adjustment of regulators–in the case of shunt and compound generators, it is possibly that the resistance of field regulator may be too high to permit the passage of sufficient current through the field windings.
  4. Speed too low–in the case of shunt–and compound-wound generators, there is certain critical armature speed below which they will not excite.
  5. Open-circuit–in the case of series machines.
  6. Short-circuit–in the generator of external circuit.
  7. Reversed field polarity–usually caused by the reversed connections of the field coils.
  8. Insufficient residual magnetism–The trouble normally occurs when the generator is new. It can be remedied by passing a strong direct current through the field coils.

Q. 26. How do we conclude that connections between field coils and armature are correct?

Ans. If the generator builds up when brought to full speed. If it does not, then connections are reversed.

Q. 27. When a generator loses its residual magnetism either due to lighting or short circuit, how can it be made to build up?

Ans. By temporarily magnetisng the main poles with the help of current from an external battery.

Q. 28. Can a generator be reversed by reversing the connections between the armature and field coils?

Ans. No, because if these connections are reversed, the generator will not build up at all.

Q. 29. Will a generator build up if it becomes reversed?

Ans. Yes.

Q. 30. Then, what is the objection to a reversed generator?

Ans. Since the current of such a reversed generator is also reversed, serious trouble can occur if attempt is made to connect it in parallel with other machines which are not reversed.

Q. 31. What are the two kinds of sparking produced in a generator?

Ans. One kind of sparking is due to bad adjustment of brushes and the other due to bad condition of the commutator. The sparking of the first are bluish whereas those of the other are reddish in colour.

Q. 32. What is the probable reason if sparking does not disappear in any position when brushes are rocked around the commutator?

  1. The brushes may not be separated at correct distance.
  2. The neutral plane may not be situated in the true theoretical position on the commutator due to faulty winding.

Q. 33. What is the permissible rise of temperature in a well-designed generator?

Ans. 27°C above the surrounding air.

Q. 34. What are the causes of hot bearings?

  1. Lack of oil 
  2. Belt too tight 
  3. Armature not centered with respect of pole pieces
  4. Bearing too tight or not in line.

Q. 35. What causes heating of armature?

  1. Eddy currents.
  2. Moisture which almost short-circuits the armature.
  3. Unequal strength of magnetic poles.
  4. Operation above rated voltage and below normal speed.

Q. 36. What is the commutator pitch of a 4-pole d.c. armature having 49 commutator bars?

Ans. Yc  = (49 ± 1)/2 = 24 or 25.

Q. 37. Will it make any difference if lower figure of 24 is selected in preference to other?

Ans. Yes.  Direction of armature rotation would be reversed.

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Source: A Textbook of Electrical Technology by BL Theraja, A K Theraja, Sedha R S_Volume_II

Read More:
  1. 16 Common Interview Questions and Answers in Generators - Part-1
  2. 21 Common Interview Question and Answers in AC Generator - Part-2


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