Descriptive type Questions
*Marked Questions are optional.
1. Describe the constructional details of a commutator machine.
2. *Explain how the commutator keeps the armature mmf stationary in space, along the inter polar axis.
3. *In a Dc machine if 𝑇𝑒 = 𝐾𝑡 ∅𝐼𝑎 and 𝐸𝑔 = 𝐾𝑒 ∅𝜔, prove that 𝐾𝑡 = 𝐾𝑒 .
4. Derive the expression for torque developed in a dc machine using Bil concept and explain various types
of torque.
5. “Emf induced in a DC motor armature is known as Back emf”. Justify it.
6. Explain the process of armature reaction in a DC machine.
7. Explain the effects of armature reaction.
8. *Explain the effect of armature mmf on field mmf with the help of developed diagram.
9. What is interpole and describe its working?
10. Describe the process by which the flash over in a dc machine can be avoided.
11. Explain the procedure to determine OCC. Why does the slope of OCC changes after a certain value of
field current.
12. A series generator is connected to Bus-Bar, if the prime mover fails in which direction the machine will
continue to rotate. Justify your answer.
13. Explain, why does the terminal voltage in a shunt generator drops more rapidly compare to a separately
excited machine.
14. Explain the process of voltage build up in a DC shunt generator. What are the factors that affect the
voltage build up.
15. Give a comparison study of external characteristics of various type of dc generator.
16. Why the external characteristic of a dc shunt generator does turns back as the generator overloaded?
Explain, with appropriate diagram how the internal characteristics can be drawn from it.
17. *Draw and explain the construction and working detail of a 3-point starter. Write its advantages and
disadvantages.
18. *Draw and explain the construction and working detail of a 4-point starter. Write its advantages and
disadvantages.
19. Derive the speed-torque characteristics of all types of DC generator. Give a comparison assuming
i) Same torque at no-load
ii) Same torque at rated-load
20. Why the pole core has a smaller cross-section, but the pole shoe has a wider cross-section?
21. Why in a 4 pole machine, the adjacent poles are having opposite polarity, rather to have diametrically
opposite poles being of opposite polarity?
22. Draw the magnetic circuit of 4-pole machine.
23. Why a commutator and brush arrangement is is necessary for operation of DC machine?
24. Why field poles are electromagnets, not permanent magnets? What can be the advantage and
disadvantage?
25. With the help of a neat diagram show the power division in a Dc generator.
26. Because of the dc magnetic field, there is no hysteresis loss in the stator. Can it be the same for the rotor
also? Justify your answer.
27. Why is it desirable to have the brush width much greater than the width of one commutator segment?
28. Interpoles help in making the flux density uniform over the pole face. Is it true? Justify your answer.
29. Is it true that the compensating winding is connected in series with the armature winding? Give reasons
for your answer.
30. What should be the polarity of the interpoles for a dc generator? Reason it out.
31. Shifting the brush to the magnetic neutral axis is one of the remedies for improving commutation. What
are the drawbacks associated with this?
32. Carbon brushes are better than copper brushes. Justify.
33. *What is the role of a dummy coil?
34. *What is a split coil?
35. *What is the function of an equalizer ring?
36. *What do you mean by reactance voltage in a DC machine? How it is neutralize?
37. *Explain the term ‘commutation period’.
38. Why are interpole windings connected in series with the armature circuit?
39. What are the causes of sparking in a dc machine?
40. What is regulation? Can it be negative for a dc generator?
41. The shunt field winding has high resistance whereas series field has less. Why?
42. In which way does saturation help us in a dc generator and how does it affect adversely?
43. A dc shunt generator is self protecting from short circuits. Justify.
44. A dc shunt generator fails to build up voltage. What can be the probable reasons?
45. The external characteristic of a shunt generator shows a reversal of behavior after a certain load current.
Why it happens? Can it be obtained for a separately excited dc generator also?
46. During no load test for tracing the O.C.C., in order to vary the flux, should we connect the field circuit
with a rheostatic connection or potentiometric connection? Justify your answer.
47. What is meant by the saturation curve? What test must be performed to determine data for its
construction?
48. What is the significance of air gap line?
49. “In every generator motor action occurs and vice versa “. Justify.
50. What is torque? What is the source of torque in a dc motor?
51. How is dc motor reversed?
52. What is the significance of ‘Back Emf’?
53. How does a dc motor adjust itself to match the mechanical load?
54. A shunt generator and a series generator with same voltage rating are operating at no load condition with
rated speed. Should the output voltages be the same or different? Justify your answer.
55. A dc shunt generator builds up a voltage of 220 volts while running in clockwise direction. If it is
rotated at the same speed but in anti clockwise direction, then with all other things remaining same, what
will be the output voltage? Give reasons for your answer.
56. A dc shunt machine has 4 terminals brought out of the machine. Is it possible to distinguish who are the
armature terminals and who are the field winding terminals?
57. What is critical resistance? What importance does it carry for a shunt generator? Does it carry the same
importance for a separately excited machine also?
58. What do you mean by level compound dc generator?
59. What is the effect on terminal voltage in a differentially compound dc generator in full load, if its series
field is shorted?
60. Arrange the generators according to their voltage regulation.
61. What are the factors that affect the shape of the magnetization curve of a D.C machine?
62. Why losses occur in pole shoes where as not in pole core?
63. Why pole tips in a dc machine are are chamfered?
64. What is meant by speed losses in DC machines?
