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Saturday, 25 November 2017

Some questions and answers about DC Generators

1. Laminations of core are generally made of
(a) case iron
(b) carbon
(c) silicon steel
(d) stainless steel
Ans: c
2. Which of the following could be lamina-proximately the thickness of laminations of a D.C.
machine ?
(a) 0.005 mm
(b) 0.05 mm
(c) 0.5 m
(d) 5 m
Ans: c
3. The armature of D.C. generator is laminated to
(a) reduce the bulk
(b) provide the bulk
(c) insulate the core
(d) reduce eddy current loss


Ans: d
4. The resistance of armature winding depends on

(a)          length of conductor
(b)         cross-sectional area of the conductor
(c)          number of conductors
(d)         all of the above
Ans: d
 
5. The field coils of D.C. generator are usually made of
(a)  mica
(b)  copper
(c)   cast iron
(d)  carbon
Ans: b
6. The commutator segments are connected to the armature conductors by means of
(a)  copper lugs
(b)  resistance wires
(c)   insulation pads
(d)  brazing
Ans: a
7. In a commutator
(a)    copper is harder than mica
(b)    mica and copper are equally hard
(c)     mica is harder than copper
(d)    none of the above
Ans: c
8. In D.C. generators the pole shoes are fastened to the pole core by
(a)  rivets
(b)  counter sunk screws
(c) brazing
(d) welding
Ans: b
9. According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of

(a)  motion of conductor
(b)  lines of force
(c)   either of the above
(d) none of the above
Ans: b
10. Fleming's right-hand rule regarding direction of induced e.m.f., correlates
(a)  magnetic flux, direction of current flow and resultant force
(b)  magnetic flux, direction of motion and the direction of e.m.f. induced
(c)   magnetic field strength, induced voltage and current
(d)  magnetic flux, direction of force and direction of motion of conductor
Ans: b
11. While applying Fleming's right-hand rule to And the direction of induced e.m.f., the thumb points towards (a) direction of induced e.m.f.
(b)  direction of flux
(c)   direction of motion of the conductor if forefinger points in the direction of generatede.m.f.
(d)  direction of motion of conductor, if forefinger points along the lines of flux
Ans: d
12. The bearings used to support the rotor shafts are generally
(a)  ball bearings
(b)  bush bearings
(c)   magnetic bearmgs
(d)  needle bearings
Ans: a
13. In D.C. generators, the cause of rapid brush wear may be
(a)  severe sparking
(b)  rough commutator surface
(c)   imperfect contact
(d)  any of the above
Ans: d
14. In lap winding, the number of brushes is always
(a)  double the number of poles
(b)  same as the number of poles

(a)  half the number of poles
(b)  two
Ans: b
15. For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?
(a)  Lap winding
(b)  Wave winding
(c)   Either of (a) and (b) above
(d)  Depends on other features of design
Ans: b
16. In a four-pole D.C. machine
(a)  all the four poles are north poles
(b)  alternate poles are north and south
(c)   all the four poles are south poles
(d)  two north poles follow two south poles
Ans: b
17. Copper brushes in D.C. machine are used
(a)    where low voltage and high currents are involved
(b)    where high voltage and small cur-rents are involved
(c)     in both of the above cases
(d)    in none of the above cases
Ans: a
18. A separately excited generator as compared to a self-excited generator
(a)  is amenable to better voltage con-trol
(b)  is more stable
(c)   has exciting current independent of load current(d) has all above features
Ans: d
19. In case of D.C. machines, mechanical losses are primary function of (a) current
(b)  voltage
(c)   speed
(d)  none of above
Ans: c
20. Iron losses in a D.C. machine are independent of variations in
(a)  speed
(b)  load
(c)   voltage
(d)  speed and voltage
Ans: b
21. In D.C. generators, current to the external circuit from armature is given through
(a)  commutator
(b)  solid connection
(c)   slip rings
(d)  none of above
Ans: a
23. Brushes of D.C. machines are made of
(a)  carbon
(b)  soft copper
(c)   hard copper
(d)  all of above
Ans: a
24.              If B is the flux density, I the length of conductor and v the velocity of conductor, theninduced e.m.f. is given by
(a)Blv
(b)Blv2
(c)Bl2v
(d)Bl2v2
Ans: a
25.              In case of a 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be
(a)  4
(b)  8
(c)   16
(d)  32 Ans: b
26. The material for commutator brushes is generally
(a)  mica
(b)  copper

(c)  cast iron
(d)  carbon
Ans: d
27. The insulating material used between the commutator segments is normally
(a)  graphite
(b)  paper
(c)   mica
(d)  insulating varnish
Ans: c
28. In D.C. generators, the brushes on commutator remain in contact with conductors which
(a)          lie under south pole
(b)         lie under north pole
(c)          lie under interpolar region
(d)         are farthest from the poles
Ans: c
29. If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be
(a)          demagnetisation only
(b)         cross magnetisation as well as magnetisation
(c)          crossmagnetisation as well as demagnetising
(d)         cross magnetisation only
Ans: c
30. Armature reaction of an unsaturated D.C. machine is
(a)  crossmagnetising
(b)  demagnetising
(c)   magnetising
(d)  none of aboveAns: a
31. D.C. generators are connected to the busbars or disconnected from them only under the floating condition
(a)  to avoid sudden loading of the primemover
(b)  to avoid mechanicaljerk to the shaft
(c)   to avoid burning of switch contacts
(d)  all above
Ans: d

32. Eddy currents are induced in the pole shoes of a D.C. machine due to
(a)  oscillating magnetic field
(b)  pulsating magnetic flux
(c)   relative rotation between field and armature
(d)  all above
Ans: c
33. In a D.C. machine, short-circuited field coil will result in
(a)            odour of barning insulation
(b)            unbalanced magnetic pull producing vibrations
(c)             reduction of generated voltage for which excitation has to be increased to maintain the voltage (d) all above Ans:
34. Equilizer rings are required in case armature is
(a)  wave wound
(b)  lap wound
(c)   delta wound
(d)  duplex wound
Ans: b
35. Welding generator will have
(a)  lap winding
(b)  wave winding
(c)   delta winding
(d)  duplex wave winding
Ans: a
36. In case of D.C. machine winding, number of commutator segments is equal to
(a)  number of armature coils
(b)  number of armature coil sides
(c)   number of armature conductors
(d)  number of armature turns
Ans: a
37. For a D.C. machines laboratory following type of D.C. supply will be suitable
(a)          rotary converter
(b)         mercury are rectifier
(c)          induction motor D.C. generator set
synchronous motor D.C. generator set
Ans: c
38. The function of pole shoes in the case of D.C. machine is (a) to reduce the reluctance of the magnetic path
(b)  to spread out the flux to achieve uniform flux density
(c)   to support the field coil
(d)  to discharge all the above functions
Ans: d
39. In the case of lap winding resultant pitch is
(a)  multiplication of front and back pitches
(b) division of front pitch by back pitch(c) sum of front and back pitches
(d) difference of front and back pitches Ans: d
40. A D.C. welding generator has
(a)  lap winding
(b)  wave moving
(c)   duplex winding
(d)  any of the above
Ans: a
41. 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 
Ans: d
42. 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
Ans: a
43. 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
Ans: a
44. 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 Ans: d
45. 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
Ans: c
46. 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
Ans: b
47. 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
Ans: c
48. 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
Ans: b
49. 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 
Ans: b



50. 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
Ans: c
51. The e.m.f. generated by a shunt wound D.C. generator isE. 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
Ans: b
52. 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 aboveAns:
53. The armature core of a D.C. generator is usually made of
(a)          silicon steel
(b)         copper
(c)          non-ferrous material
(d)         cast-iron
Ans: a
9.63. 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
Ans: d
54. Open circuited armature coil of a D.C. machine is
(a)  identified by the scarring of the commutator segment to which open circuited coil isconnected
(b)  indicated by a spark completely around the commutator
(c)   both (a) and (b)
(d)none of the above


Ans: c
55. In a D.C. machine, fractional pitch winding is used
(a)          to increase the generated voltage
(b)         to reduce sparking
(c)          to save the copper because of shorter end connections
 (d) due to (b) and (c) above 
Ans:
56. 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 thearmature
(e)  all conditions mentioned above should be satisfied
Ans: d
57. D.C. series generator is used
(a)         to supply traction load
(b)        to supply industrial load at constant voltage
(c)         voltage at the toad end of the feeder
(d)        for none of the above purpose 
Ans: c
58. Following D.C. generator will be in a position to build up without any residual magnetism in the poles
 (a)           series generator
(b)         shunt generator
(c)          compound generator
(d)         self-excited generator
Ans: d
59. Interpole flux should be sufficient to
(a)  neutralise the commutating self induced e.m.f.
(b)  neutralise the armature reaction flux
(c)   neutralise both the armature reaction flux as well as commutating e.m.f. induced in thecoil
(d)  perform none of the above functions
Ans: c
60. D.C. generator generally preferred for charging automobile batteries is
(a)        series generator
(b)       shunt generator
(c)    long shunt compound generator


(a)        any of'the above
Ans: c
61. 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
Ans: d
62. Permeance is the reciprocal of
(a)  flux density
(b)  reluctance
(c)   ampere-turns
(d)  resistance
Ans: b
63. In D.C. generators the polarity of the interpoles
(a)  is the same as that of the main pole ahead
(b)  is the same as that of the immediately preceding pole
(c)   is opposite to that of the main pole ahead
(d)  is neutral as these poles do not play part in generating e.m.f.Ans: a
64. 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
Ans: b
65. In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when
(a)  there is no load on|he generator
(b)  the generator runs on full load
(c)   the generator runs on overload
(d)  the generator runs on designed speedAns: a
66. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except
(a) interpoles


(b)  dummy coils
(c)  compensating winding
(d)   shifting of axis of brushes
Ans: b
67. In D.C. generators on no-load, the air gap flux distribution in space is
(a) sinusoidal 
(b) triangular
(c)   pulsating
(d)  flat topped
Ans: d
68. A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed increases to 1200 r.p.m., the generated e.m.f. will be nearly
(a)  150 V
(b)  175 V
(c)   240 V
(d)  290 V
Ans: c
69. The purpose of providing dummy coils in a generator is
(a)  to reduce eddy current losses
(b)  to enhance flux density
(c)   to amplify voltage
(d)  to provide mechanical balance for the rotor
Ans: d
70. In a shunt generator the voltage build up is generally restricted by
(a)  speed limitation
(b)  armature heating
(c)   insulation restrictions
(d)  saturation of ironAns:
71. If a D.C. generator fails to build up the probable cause could not be
(a) imperfect brush contact
(b)  field resistance less than the critical resistance
(c)   no residual magnetism in the generator
(d)  faulty shunt connections tending to reduce the residual magnetism Ans: b
72. Flashing the field of D.C. generator means
(a)  neutralising residual magnetism
(b) creating residual magnetism by a D.C. source

(c)  making the magnetic losses of forces parallel
(d)  increasing flux density by adding extra turns of windings on poles
Ans: b
73. The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be (a) 640 V
(b)  620 V
(c)   600 V
(d)  580 V
Ans: d
74. In a D.C. generator the critical resistance refers to the resistance of
(a)  brushes
(b)  field
(c)   armature
(d)  load
Ans: b
75. To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them
(a)  just ahead of magnetic neutral axis
(b)  in magnetic neutral axis
(c)   just behind the magnetic neutral axis
Ans: a
76. Armature coil is short circuited by brushes when it lies
(a)          along neutral axis
(b)         along field axis
(c)          in any of the above positions
(d)         in none of the above positions 
Ans: a
77. A cumulatively compounded long shunt generator when operating as a motor would be (a) cumulatively compounded long shunt
(b)  differentially compounded long shunt
(c)   cumulatively compounded short shunt
(d)  differentially compounded short shunt 
Ans: b
78. To avoid formation of grooves in the commutator of a D.C. machine
(a)  the brushes of opposite polarity should track each other
(b)  the brushes of same polarity should track each other
(c) brush position has no effect on the commutator grooving
Ans: a
Some questions and answers about DC Generators

Some questions and answers about DC Generators
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