MCQ on DC generator 21-40

21. Which of the following statement about D.C. generators is false ?
(a) Compensating winding in a D.C. machine helps in commutation
(b) In a D. C. generator interpoles winding is connected in series with the armature winding
(c) Back pitch and front pitch are both odd and approximately equal to the pole pitch
(d) Equilizing bus bars are used with parallel running of D.C. shunt generators✔

22. The demagnetising component of armature reaction in a D.C. generator
(a) reduces generator e.m.f.✔
(b) increases armature speed
(c) reduces interpoles flux density
(d) results in sparking trouble

23. Magnetic field in a D.C. generator is produced by
(a) electromagnets✔
(b) permanent magnets
(c) both (a) and (b)
(d) none of the above

24. The number of brushes in a commutator depends on
(a) speed of armature
(b) type of winding
(c) voltage
(d) amount of current to be collected✔

25. Compensating windings are used in D.C. generators
(a) mainly to reduce the eddy currents by providing local short-circuits
(b) to provide path for the circulation of cooling air
(c) to neutralise the cross-magnetising effect of the armature reaction✔
(d) none of the above

26. Which of the following components of a D.C, generator plays vital role for providing direct current of a D.C. generator ?
(a) Dummy coils
(b) Commutator✔
(c) Eye bolt
(d) Equilizer rings

27. In a D.C. generator the ripples in the direct e.m.f. generated are reduced by
(a) using conductor of annealed copper
(b) using commutator with large number of segments
(c) using carbon brushes of superior quality✔
(d) using equiliser rings

28. In D.C. generators, lap winding is used for
(a) high voltage, high current
(b) low voltage, high current✔
(c) high voltage, low current
(d) low voltage, low current

29. Two generators A and B have 6-poles each. Generator A has wave wound armature while generator B has lap wound armature. The ratio of the induced e.m.f. is generator A and B will be
(a) 2 : 3
(b) 3 : 1✔
(c) 3 : 2
(d) 1 : 3

30. The voltage drop for which of the following types of brush can be expected to be least ?
(a) Graphite brushes
(b) Carbon brushes
(c) Metal graphite brushes✔
(d) None of the above

31. The e.m.f. generated by a shunt wound D.C. generator is E. Now while pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be
(a) E/2
(b) 2E✔
(c) slightly less than E
(d) E

32. In a D.C. generator the actual flux distribution depends upon
(a) size of air gap
(b) shape of the pole shoe
(c) clearance between tips of the ad¬jacent pole shoes
(d) all of the above✔

33. The armature core of a D.C. generator is usually made of
(a) silicon steel✔
(b) copper
(c) non-ferrous material
(d) cast-iron

34. Satisfactory commutation of D.C. machines requires
(a) brushes should be of proper grade and size
(b) brushes should smoothly run in the holders
(c) smooth, concentric commutator properly undercut
(d) all of the above✔

35. Open circuited armature coil of a D.C. machine is
(a) identified by the scarring of the commutator segment to which open circuited coil is connected
(b) indicated by a spark completely around the commutator
(c) both (a) and (b)✔
(d) none of the above

36. For the parallel operation of two or more D.C. compound generators, we should ensure that
(a) voltage of the incoming generator should be same as that of bus bar
(b) polarity of incoming generator should be same as that of bus bar
(c) all the series fields should be run in parallel by means of equilizer connection
(d) series fields of all generators should be either on positive side or negative side of the armature✔

37. In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when
(a) r.p.m. is more than 300
(b) r.p.m. is less than 300
(c) number of poles is 4
(d) number of poles is 2✔

38. Permeance is the reciprocal of
(a) flux density
(b) reluctance✔
(c) ampere-turns
(d) resistance

39. The e.m.f. generated in a D.C. generator is directly proportional to
(a) flux/pole
(b) speed of armature✔
(c) number of poles
(d) all of the above

40. In D.C. generators on no-load, the air gap flux distribution in space is
(a) sinusoidal
(b) triangular
(c) pulsating
(d) flat topped✔