Q. 1. How may the number of parallel paths in an armature be increased?
Q. 2. How are brushes connected in a d.c. generator ?
Q. 3. What is meant by armature reaction?
- It
or weakens the main flux anddemagnetises - It cross-
magnetizes or distorts it.
Figure - (a) |
Q. 4. What is the effect of this distortion on the operation of the machine?
Q. 5. How can field distortion be remedied?
Q. 6. What is meant by normal neutral plane?
Q. 7. What is the importance of this plane in the working of the machine?
Q. 8. How do you differentiate between normal neutral plane?
Q. 9. How do you define ‘commutating plane?
Q. 10. What is the angle of lead?
Q. 11. What affects this angle?
Q. 12. What is the best way of minimizing eddy currents in an armature?
Q. 13. How should the armature be laminated for the purpose?
Q. 14. How does field distortion affect communication?
Q. 15. Should the brushes of a loaded generator be placed in the neutral plane?
Q. 16. Why not?
Q. 17. What causes sparking at the brushes?
Q. 18. What is th e standard direction of rotation of the d.c. generators ?
Q. 19. What is meant by build-up of a generator?
Q. 20. How should a generator be started?
Q. 21. How should a shunt or compound generator be started?
Q. 22. How about a series generator?
Q. 23. What is the procedure for shunting down a generator?
Q. 24. What are the indications and causes of an overloaded generator?
- 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.
- Excessive current–which could be due to bad feeding of the load.
- 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.
- 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.
- Brushes not properly adjusted–if
are not in their proper positions, thenbrushes of the armature voltage will not be utilized and so would be insufficient to excite the machine.whole - 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.
- Incorrect adjustment of regulators–in the case of shunt and compound generators, it is
that the resistance of field regulator may be too high to permit the passage of sufficient current through the field windings.possibly - Speed too low–in the case of shunt–and compound-wound generators, there is certain critical armature speed below which they will not excite.
- Open-circuit–in the case of series machines.
- Short-circuit–in the generator of external circuit.
- Reversed field polarity–usually caused by the reversed connections of the field coils.
- 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?
Q. 27. When a generator loses its residual magnetism either due to lighting or short circuit, how can it be made to build up?
Q. 28. Can a generator be reversed by reversing the connections between the armature and field coils?
Q. 29. Will a generator build up if it becomes reversed?
Q. 30. Then, what is the objection to a reversed generator?
Q. 31. What are the two kinds of sparking produced in a generator?
Q. 32. What is the probable reason if sparking does not disappear in any position when brushes are rocked around the commutator?
- The brushes may not be separated at
.correct distance - 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?
Q. 34. What are the causes of hot bearings?
- Lack of oil
- Belt too tight
- Armature not centered with respect of pole pieces
- Bearing too tight or not in line.
Q. 35. What causes heating of armature?
- Eddy currents.
- Moisture which almost short-circuits the armature.
- Unequal strength of magnetic poles.
- 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?
Q. 37. Will it make any difference if lower figure of 24 is selected in preference to other ?
Source: A Textbook of Electrical Technology by BL Theraja, A K Theraja, Sedha R S_Volume_II
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