chapter 9_dc generators

7/29/2019 Chapter 9_dc Generators

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DC GENERATORS

1. Lamination of core are generally made of

A. cast iron B. carbon C. silicon steel D. stainless steel

2. Which of the following could be approximately the thickness of laminations of a DC machine?

A. 0.005 mm B. 0.05 mm C. 0.5 m D. 5 mm

3. The armature of DC generator is laminated to

A. reduce the bulk C. insulate the core

B. provide passage for cooling air D. reduce eddy current loss

4. The resistance of armature winding depends on

A. length of conductor C. number of conductors

B. cross-sectional area of the conductor D. all of the above

5. The field coils of DC generators are usually made of

A. mica B. copper C. cast iron D. carbon

6. The commutator segments are connected to the armature conductors by means of

A. copper lugs B. resistance wires C. insulation pads D. brazing

7. In a commutator

A. copper is harder that mica C. mica is harder than copper

B. mica and copper are equally hard D. none of the above

8. In DC generators the pole shoes are fastened to the pole core by

A. rivets B. counter sunk screws C. brazing D. welding

9. According to Flemings right-hand rule for finding the direction of induced emf, when middle finger

points in the direction of induced emf, forefinger will point in the direction of

A. motion of conductor C. either of the above

B. lines of force D. none of the above

10. Flemings right-hand rule regarding direction of induced emf, correlates

A. magnetic flux, direction of current flow and resultant force

B. magnetic flux, direction of motion and direction of emf induced

C. magnetic field strength, induced voltage and current


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D. magnetic flux, direction of force and direction of motion of conductor

11. While applying Flemings right-hand rule to find the direction of induced emf, the thumb points

towards

A. direction of induced emf

B. direction flux

C. direction of motion of the conductor if forefinger points in the direction of generated emf

D. direction of motion of conductor, if forefinger points along the line of flux

12. The bearings used to support the rotor shafts are generally

A. ball bearings B. bush bearings C. magnetic bearings D. needle bearings

13. In DC generators, the cause of rapid brush wear may be

A. severe speaking C. imperfect contact

B. rough commutator surface D. any of the above

14. In lap winding, the number of brushes is always

A. double the number of poles C. half the number of poles

B. same as the number of poles D. two

15. For a DC generator when the number of poles and the number of armature conductors is fixed, then

which winding will give the higher emf?

A. lap winding C. Either of (a) and (b) above

B. wave winding D. depends on other features of design

16. In a four-pole DC machine

A. all the four poles are north poles C. all the four poles are south poles

B. alternating poles are north and south D. two north poles follow two south poles

17. Copper brushes in DC machine are used

A. where low voltage and high current are involved

B. where high voltage and small currents are involved

C. in both of the above cases

D. in none of the above cases

18. A separately excited generators as compared to a self excited generators

A. is amenable to better voltage control C. has exciting current independent of load current


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B. is more stable D. has all above features

19. In case of DC machines, mechanical losses are primary function of

A. current B. voltage C. speed D. none of above

20. Iron losses in a DC machine are independent of variation in

A. speed B. load C. voltage D. speed and voltage

21. In DC generators, current to the external circuit from armature is given through

A. commutator B. solid connection C. slip rings D. none of above

22. Requirements of speed at which machine is driven is

A. more critical in the case of alternators

B. more critical in the case of DC generators

C. equally critical in the case of alternators as well as DC generators

23. Brushes of DC machines are made of

A. carbon B. soft copper C. hard copper D. all of above

24. In DC generators, on no load

A. magnetic neutral axis moves from geometrical neutral axis in the opposite direction of rotation

B. magnetic neutral axis coincides with geometrical neutral axis

C. magnetic neutral axis moves from geometrical neutral axis in the direction of rotation

D. none of the above

25.If B is the flux density, the length of conductor and the velocity of conductor, then induced emf is

given by

A. B. C. D.

26. In case of a 4-pole DC generator provided with a two layer lap winding with sixteen coils, the pole

pitch will be

A. 4 B. 8 C. 16 D. 32

27. The material of commutator brushes is generally

A. mica B. copper C. cast iron D. carbon

28. The insulating material used between the commutator segments is normally

A. graphite B. paper C. mica D. insulating varnish


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29. In a DC generator, if p be the number of poles and N be the rpm of rotor, then the frequency of

magnetic reversals will be

A. B. C. D.

30. For generating large current on DC generators which winding is generally preferred?

A. progressive wave winding C. retrogressive wave winding

B. lap winding D. current depends on design

31. The purpose of providing dummy coils in a generator is

A. to enhance flux density C. to provide mechanical balanced for the rotor

B. to amplify voltage D. to reduce eddy current

32. In a DC generator, the armature reaction results in

A. demagnetization of the centers of poles

B. magnetization of inter-poles

C. demagnetization of the leading pole tip and magnetization of the trailing pole tip

D. magnetization of the leading pole tip and demagnetization of the trailing pole tip

33. In a DC generator in case the brushes are moved so as to bring them in magnetic neutral axis, then,

there will be

A. demagnetization C. cross-magnetization as well as demagnetization

B. cross-magnetization D. none of the above

34. The polarity of a DC generator can be reversed by

A. reversing the field current C. reversing field current as well as direction of rotation

B. increasing field current D. any of the above

35. In Dc generators, the brushes on commutator remain in contact with conductors which

A. lie under South Pole C. lie under interpolar region

B. lie under North Pole D. are farthest from the poles

36. In a lap wound DC generator having p as number of poles and Z as number of conductors, the

maximum number of equalizer rings will be

A. Z B. p C. D.


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37. If brushes of a DC generator are moved in order to bring these brushes in magnetic neutral axis,

there will be

A. demagnetization only

B. cross magnetization as well as magnetization

C. cross magnetization as well as demagnetization

D. cross magnetization only

38. Armature reaction of an unsaturated DC machine is

A. cross-magnetizing C. magnetizing

B. demagnetizing D. none of above

39. DC generators are connected to the busbars or disconnected from them only under floating condition

A. to avoid sudden loading of the prime mover

B. to avoid mechanical jerk to the shaft

C. to avoid burning of switch contacts

D. all above

40. Eddy currents are induced in the pole shoes of a DC machine due to

A. oscillating magnetic field C. relative rotation between field and armature

B. pulsating magnetic flux D. all above

41. In a Dc machine, short-circuited field coil will result in

A. odour of burning 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

42. Equalizer rings are required in case armature is

A. wave wound B. lap wound C. delta wound D. duplex wound

43. Welding generator will have

A. lap winding B. wave winding C. delta winding D. duplex wave winding

44. In case of DC machine winding, number of commutator segments is equal to

A. number of armature coils C. number of armature conductors


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B. number of armature coils sides D. number of armature turns

45. For a DC machine laboratory following type of DC supply will be suitable

A. rotary converter C. induction motor DC generator set

B. mercury are rectifier D. synchronous motor DC generator set

46. The function of pole shoes in the case of DC machines 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

47. In the case of lap winding resultant pitch is

A. multiplication of front and back pitches C. sum of front and back pitches

B. division of front pitch by back pitch D. difference of front and back pitches

48. A DC welding generators has

A. lap winding B. wave moving C. duplex winding D. any of the above

49. Which of the following statement about DC generators is false?

A. compensating winding in a DC machine helps in commutation

B. in a DC 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. equalizing bus bars are used with parallel running of DC shunt generators

50. The demagnetizing component of armature reaction in a DC generator

A. reduces generator emf C. reduces interpoles flux density

B. increases armature speed D. results in sparkling trouble

51. Magnetic field in a DC generator is produced by

A. electromagnets B. permanent magnet C. both (a) and (b) D. none of the above

52. The number of brushes in a commutator depends on

A. speed of armature C. voltage

B. type of winding D. amount of current to be collected

53. Compensating windings are used in DC generators


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A. mainly to reduce the eddy currents by providing local short-circuits

B. the provide path for circulation of cooling air

C. to neutralize the cross-magnetizing effect of the armature reaction


54. Which of the following components of a DC generator plays vital role for providing direct current of a

DC generator?

A. dummy coils B. commutator C. eye bolt D. equalizer rings

55. In a Dc generator the ripples in the direct emf generated are reduced by

A. using conductors of annealed copper

B. using commutator with large number of segments

C. using carbon brushes of superior quality

D. using equalizer rings

56. In DC generators, lap winding is used for

A. high voltage, high current C. high voltage, low current

B. low voltage, high current D. low voltage, low current

57. 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 emf is generator A and B will be

A. 2:3 B. 3:1 C. 3:2 D. 1:3

58. The voltage drop for which of the following types of brush can be expected to be least?

A. Graphite brushes C. Metal graphite brushes

B. Carbon brushes D. None of the above

59. The emf generated by a shunt wound DC generator is E. Now while pole flux remains constant, if the

speed of the generator is doubled, the emf generated will be

A. E/2 B. 2E C. slightly less than E D. E

60. In a DC generator the actual flux distribution depends upon

A. size of air gap C. clearance between tips of the adjacent pole shoes

B. shape the pole shoe D. all of the above

61. The armature core of a DC generator is usually made of

A. silicon steel B. copper C. non-ferrous material D. cast-iron


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62. DC generators generates

A. AC voltage in the armature C. AC superimposed over DC

B. DC voltage in the armature D. none of the above

63. Satisfactory commutation of DC 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

64. Open circuited armature coil of a DC 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)


65. In a DC 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

66. For the parallel operation of two or more DC 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 field should be run in parallel by means of equalizer connection

D. series field of all generators should be either on positive side or negative side of the

armature

E. all condition mentioned above should be satisfied

67. Dc series generator is used

A. to supply traction load

B. to supply industrial load at constant voltage


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C. as a booster to maintain constant voltage at the load end of the feeder

D. for none of the above purpose

68. Following DC generator will be in a position to build up without any residual magnetism in the poles

A. series generator B. shunt generator C. compound generatorD. self-excited generator

69. Interpole flux should be sufficient to

A. neutralize the commutating self-induced emf

B. neutralize the armature reaction flux

C. neutralize both the armature reaction flux as well as commutating emf induced in the coil

D. perform none of the above functions

70. DC generator generally preferred for charging automobile batteries is

A. series generator C. long shunt compound generator

B. shunt generator D. any of the above

71. In a DC generator the number of mechanical degrees and electrical degrees will be the same when

A. rpm is more than 300 C. number of poles is 4

B. rpm is less than 300 D. numbers of poles is 2

72. Permeance is the reciprocal of

A. flux density B. reluctance C. ampere-turns D. resistance

73. In DC 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 emf

74. The emf generated in a DC generator is directly proportional to

A. flux/pole B. speed of armature C. number of poles D. all of the above

75. In a DC generator the magnetic neutral axis coincides with the geometrical neutral axis, when

A. there is no load on the generator C. the generator runs on overload

B. the generator runs on full load D. the generator runs on designed speed


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76. In a DC generator in order to reduce sparking at brushes, the self-induced emf in the coil is

neutralized by all of the following except

A. interpoles C. compensating winding

B. dummy coils D. shifting of axis of brushes

77. In DC generators on no-load, the air gap flux distribution in space is

A. sinusoidal B. triangular C. pulsating D. flat topped

78. A shunt generator running at 1000 rpm has generated emf as 200 V. If the speed increases to 1200

rpm, the generated emf will be nearly

A. 150 V B. 175 V C. 240 V D. 290 V

79. The purpose providing dummy coils in a generator is

A. to reduce eddy current losses C. to amplify voltage

B. to enhance flux density D. to provide mechanical balance for the rotor

80. In a shunt generator the voltage build up is generally restricted is

A. speed limitation B. armature heating C. insulation restriction D. saturation of iron

81. if a DC 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

82. Flashing the field of DC generator means

A. neutralizing residual magnetism

B. creating residual magnetism by a DC source

C. making the magnetic losses of forces parallel

D. increasing flux density by adding extra turns of windings on poles

83. The emf induced in the armature of a shunt generator is 600V. The armature resistance is 0.1 ohm. If

the armature current is 200A, the terminal voltage will be

A. 640V B. 620V C. 600V D. 580V

84. In a DC generator the critical resistance refers to the resistance of

A. brushes B. field C. armature D. load


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85. To achieve sparkless commutation brushes of a DC generator are rocked ahead so as to bring them

A. just ahead of magnetic neutral axis C. just behind the magnetic neutral axis

B. in magnetic neutral axis

86. Armature coil is short-circuited by brushes when it lies

A. along neutral axis C. in any of the above position

B. along field axis D. in none of the above position

87. A cumulative compounded long shunt generator when operating as a motor would be

A. cumulative compounded long shunt C. cumulative compounded short shunt

B. differentially compounded long shunt D. differentially compounded short shunt

88. To avoid formation of grooves in the commutator of a DC 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

89. The following constitute short-circuit in the armature winding

A. insulation failure between two commutator bars

B. insulation failure between two turns of a coil

C. two of more turns of the same coil getting grounded

D. all of the above

90. The rapid wear of brushes takes place due to

A. abrasion from dust D. high mica insulation between commutator bars

B. excessive spring pressure E. all of the above factors

C. rough commutator bars

91. Number of tappings in each equalizer ring is equal to

A. number of pole pairs C. number of parallel paths

B. number of pole D. number of commutator segments

92. A DC generator can be considered as

A. rectifier B. prime mover C. rotating amplifier D. power pump


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93. In any rotating machine that part which houses the conductors in and in which emf induced is to be

utilized is called

A. rotor B. stator C. field D. armature

94. In a DC machine stray loss is the sum of

A. total copper loss and mechanical loss C. shunt field copper loss and mechanical loss

B. armature copper loss and iron loss D. iron loss and mechanical loss

95. Lap winding is composed of

A. any eve numbers of conductors C. that even number which is exact multiple of poles + 2

B. any odd numbers of conductors D. that even number which is exact multiple of poles

96. In a DC generator in case the resistance of the field winding is increased, then output voltage will

A. increase B. decreased C. remain unaffected D. fluctuate heavily

97. An exciter for a turbo generator is a

A. separately excited generator C. series generator

B. shunt generator D. compound generator

98. In case of a flat compounded generator

A. voltage generated is less than the rated voltage

B. generated voltage is proportional to the load on the generator

C. voltage remain constant irrespective of the load

D. speed varies in proportion to the load on the generator

99. Which of the following generator will have negligible terminal voltage while running on no-load?

A. series generator C. compound generator

B. shunt generator D. separately excited generator

100. Which of the following DC generators will be in position to build up without any residual

magnetism in the poles?

A. series generator C. compound generator

B. shunt generator D. none of the above

101. In over compounded generator, full load terminal voltage is

A. almost zero C. more than no-load terminal voltage

B. less than no-load terminal voltage D. equal to no-load terminal voltage


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102. In a level compounded DC generator, full load terminal voltage is

A. negligibly low C. more than no-load terminal voltage

B. equal to no-load terminal voltage D. less than no-load terminal voltage

103. The terminal voltage of a DC shunt generator drops on load because of all of the following reasonexcept

A. armature reaction

B. armature resistance drop

C. field weakening due to armature reaction and armature

D. commutation

104. In a DC generator

A. external resistance = internal characteristics armature reaction

B. internal characteristics = magnetization characteristics ohmic drop

C. external characteristics = magnetization characteristics ohmic drop armature reaction

D. magnetization characteristics = external characteristics

105. A sinusoidal voltage of 5 Hz is applied to the field of a shunt generator. The armature voltage

wave

A. will be zero B. will be of 5 Hz C. will be of 5 x N Hz D. will be of Hz

106. A 220 V DC generator is run at full speed without any excitation. The open circuit voltage will be

A. zero B. about 2 V C. about 50 V D. 220 V

107. In a separately excited generator supplying rated load the armature reaction

A. is always present C. maybe sometimes present

B. is always absent D. none of the above

108. If residual magnetism is present in a DC generator, the induced emf at zero speed will be

A. zero C. the same as rated voltage

B. small D. high

109. Armature reaction in a generator results in

A. demagnetization of leading pole tip and magnetization of trailing pole tip

B. demagnetization of trialing pole tip and magnetization of leading pole tip


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C. demagnetizing the centre of all poles

D. magnetizing the center of all poles

110. Following energized winding of a DC machine should not be opened as it would produce high

inductive voltage which may be dangerous to personnel and may cause its own insulation failure.

A. Series field B. compensating field C. inter pole field D. shunt field

111. Wave winding is compose of

A. any even number of conductors C. even number which is exact multiple of poles + 2

B. any odd number of conductors D. even number which is exact multiple of poles

112. The critical resistance of the DC generator is the resistance of

A. field B. brushes C. armature D. load

113. When two DC series generators are running in parallel, an equalizer bar is used

A. to increase the speed and hence generated emf

B. to increase the series flux

C. so that two similar machines will pass approximately equal current to the load

D. to reduce the combined effect of armature reaction of both machines

114. Which of the following generating machine will offer constant voltage on all loads?

A. self-excited generator C. level compounded generator

B. separately excited generator D. all of the above

115. Which of the following generators will be preferred if they are required to be run in parallel?

A. shunt generators C. compound generators

B. series generators D. none of the above

116. Two generators are running in parallel. One of the generators may run as motor for which of the

following reasons?

A. the direction of that generator is reversed C. the field of that generator is weakened

B. the speed of that generator is increases D. that generator takes large share of loads

117. A DC generator works on the principle of

A. Lenzs law C. Faradays law of electromagnetic induction

B. Ohms law D. none of the above

118. A series generator can self-excite


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A. only if the load current is zero C. irrespective of the value of load current

B. only if the load current is not zero D. none of the above

119. A shunt generator can self-excite

A. only if the resistance of the field circuit is less than critical value

B. only if the resistance of the field circuit is greater than critical value

C. irrespective of the value of the resistance in the field circuit

120. The terminal voltage of a series generator is 150 V when the load current is 5 A. if the load

current is increased to 10 A, the terminal voltage will be

A. 150 V B. less than 150 V C. greater than 150 V D. none of the above

121. The open circuit voltage of a compound generator is 250 V. At full load the terminal voltage

A. will be less than 250V C. maybe greater or less than 250V

B. will always be 250V D. none of the above

122. Two DC shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50

ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their emfs are

270V and 265V, their bus bar voltage will be

A. 270V B. 267.5V C. 265V D. 257.4V

123. The essential condition for parallel operation of two DC generators is that they have

A. same kW rating C. the same drooping voltage characteristics

B. the same operation rpm D. same percentage regulation

124. In case of DC generators, the armature voltage control is considered as suitable if the machine is

driven

A. at constant load B. at constant current C. at constant torque D. at constant VA

125. When two DC generators are running in parallel an equalizer bar is used

A. to increase the series flux

B. to increase the generated emf

C. to reduce the combined effect of armature reaction of both the machine

D. so that the two identical machines will pass approximately equal currents to the load

126. With a DC generator which of the following regulation is preferred?

A. 100% regulation B. infinite regulation C. 50% regulation D. 1% regulation

127. Which generator would you prefer for feeding long DC transmission lines?


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A. series generator C. over compound generator

B. shunt generator D. flat compound generator

128. Two generators A and B running in parallel are supplying power to a common load of 500 kW.

Generator A has armature resistance equal to half that of B. Which of the following statements is

correct?

A. both generators will share load equally

B. generator A will take load less than the load taken by generator B

C. generator B will take more loads as compared to generator A


129. In a DC generator the critical resistance can be increased by

A. increasing its field resistance C. increasing its speed

B. decreasing its field resistance D. decreasing its speed

130. The number of armature parallel paths in two-pole DC generator having duplex lap winding is

A. 2 B. 4 C. 6 D. 8

131. For both lap and wave windings, there are as many commutator bars as the number of

A. slots B. armature conductor C. winding elements D. poles

132. The series field of a short-shunt DC generator is excited by

A. external current B. armature current C. shunt current D. load current

133. As a result of armature reaction, the reduction in the mutual air gap flux in a DC generator is

approximately

A. 40 percent B. 25 percent C. 10 percent D. 5 percent

134. Shunt generators are most suited for stable parallel operation because of their

A. rising voltage characteristics C. drooping voltage characteristics

B. identical voltage characteristics D. linear voltage characteristics

135. The main factor which leads to unstable parallel operation of flat and over compounded generator

is

A. their rising voltage characteristics

B. unequal number of turns in their series field windings

C. unequal speed regulation of their prime movers

D. unequal series field resistance


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136. If a self excited DC generator after being installed, fails to build up on its first trial run, the first

thing to do is to

A. reverse the field connection C. increase the speed of prime mover

B. increase the field resistance D. check armature insulation resistance

137. If residual magnetism of a shunt generator is destroyed accidentally, it may be restored by

connecting its shunt field

A. in reverse B. to a battery C. to earth D. to an alternator

138. The slight curvature at the lower end of the O.C.C. of a self-excited DC generator is due to

A. high armature speed D. magnetic inertia

B. high field circuit resistance E. none of the above

C. residual pole flux

139. Which one of the following types of generators does not need equalizers for satisfactory parallel

operation?

A. Flat-compound B. Over-compound C. Under-compound D. Series

140. The ___ generator has poorest voltage regulation.

A. shunt B. series C. compound D. over-compound

141. A simple method of increasing the voltage of a DC generator is

A. to increase the length of the armature D. to decrease the speed of rotation

B. to decrease the length of the armature E. any of the above

C. to increase the speed of rotation

142. Which one of the following generators is used for charging the batteries?

A. shunt B. series C. compound D. any of the above

143. An external resistance added in the field of a DC shunt generator will

A. decrease the voltage of the generator D. increase the speed of the generator

B. increase the voltage of the generator E. decrease the speed of the generator

C. increase the power delivered

144. An ideal DC generator has a regulation of

A. 90% B. 70% C. 20% D. zero

145. Which of the following statements is incorrect in DC shunt generators about hysteresis loss?


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A. it is independent of lamination the armature

B. it can be minimize by laminating the armature

C. it depends upon the supply frequency


146. A properly designed DC generator can have an overall efficiency of

A. 60% B. 80% C. 95% D. 100%

147. In DC generator the change in voltage when the load is reduced from rated value to zero,

expressed as a percentage of rated load voltage, is known as

A. efficiency B. regulation C. armature reactance D. loss factor

148. If the no load voltage of a certain generator is 210 V and the rated voltage is 200 V, then the

voltages regulation is

A. 2.5% B. 5% C. 10% D. 15%

149. Which generator cannot start if there is no residual magnetism?

A. series generator C. separately excited generator

B. shunt generator D. all of the above

150. In DC generators, the brushes remain in contact with conductors which

A. lie under South Pole C. lie in the inter-polar gaps

B. lie under North Pole D. none of the above

151. When the shunt field of a compound generator is connected across both series field and

armature. Such a convection is known as

A. short shunt C. cumulative compounding

B. long shunt D. differential compounding

152. Drop in speed of a DC generator due to increase in load can be compensated by

A. cooling the armature C. reducing the load voltage

B. increasing the armature resistance D. increasing the input to the prime mover

153. A dynamo is a machine which converts heat energy into electrical energy. Answer: NO

154. When a dynamo is driven mechanically by a prime mover it is called a generator. Answer: YES

155. A generator works on the principle of statically induced emf. Answer: NO

156. In DC generators, the field is produced by the field magnets which are stationary. Answer: YES


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157. Permanent magnets are used for large DC machines to create magnetic flux. Answer: NO

158. The pole shoe acts as a support to the field coils and spreads out the flux in the air gap.

Answer: YES

159. In small DC machines yokes are made of ____. Answer: cast iron

160. Permeability of cast steel is about twice of cast iron. Answer: YES

161. In small machines the poles are cast integral with the yoke. Answer: YES

162. The purpose of armature is to produce magnetic flux. Answer: NO

163. The armature is made from high permeability silicon-steel stampings. Answer: YES

164. The commutator is a sort of rotating switch placed between the armature and the external circuit.

Answer: YES

165. Brushes are made of aluminum. Answer: NO

166. Number of coils arranged in coil groups is called the ____Answer: winding

167. Open coil winding is that winding which does not close on itself. Answer: YES

168. DC machines employ only open coil winding. Answer: NO

169. Ring type of armature winding is an early from of armature winding. Answer: YES

170. Drum winding may be either single layer or double layer winding. Answer: YES

171. In a lap winding there are only two parallel paths. Answer: NO

172. In a simplex lap winding back pitch and front pitch of all coils remain the same. Answer: YES

173. In a simplex lap winding there are as many parallel paths in the armature as the number poles.

Answer: YES

174. Wave winding is also sometimes called ____ winding. Answer: series

175. Simplex wave windings always have three parallel paths. Answer: NO

176. In simplex wave winding both the pitches, back pitch and front pitch must be even numbers.

Answer: NO

177. In simplex wave winding commutator pitch is equal to average pitch. Answer: YES

178. If the duplex winding has an even number of pair of poles the number of commutator segments

and coils must be even irrespective of whether the average pitch is odd or even. Answer: YES

179. ____ coils are placed in slots to preserve the balance of the machine but are not electrically

connected to the rest of the winding. Answer: Dummy


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180. When the field coils are excited from storage battery or from a separate DC source, the generator

is called a self-excited generator. Answer: NO

181. Self-excited generators are those whose field coils are excited by the generator itself.

Answer: YES

182. In a shunt generator the shunt field current is equal to armature current. Answer: NO

183. The action of the armature mmf on the main mmf is termed the armature ___. Answer: reaction

184. When a machine operates at no-load, there exists in it only the mmf of the main poles.

Answer: YES

185. The armature magnetic field demagnetizes the main flux and distorts it. Answer: YES

186. Cross-magnetizing component is parallel to the main field. Answer: NO

187. Demagnetizing component is at right angles to the main field. Answer: NO

188. Demagnetizing ampere-turns/pole = . Answer: NO

189. Cross-magnetizing ampere-turns/pole = . Answer: YES

190.. Answer: NO

191. The function of compensating winding is to neutralize the demagnetizing effect of armature

reaction. Answer: NO

192. The compensating winding adds considerably to the winding cost of the machine and doubles the

armature copper loss. Answer: YES

193. A compensating winding is used if the machine is subject to violent fluctuations in loads.

Answer: YES

194. Number of armature ampere-turns/pole for compensating winding = 0.2 x armature ampere-

turns/pole. Answer: NO

195. Commutation means the process of current collection by the brush or the changes which take

place in a coil during the period of short-circuit by a brush. Answer: YES

196. The main cause which makes the current reversal impossible in the specified period is the

production of self-induced emf in the coil undergoing commutation. Answer: YES

197. In resistance commutation method low-resistance copper brushes are replaced by comparative

high resistance ____ brushes. Answer: carbon


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198. Carbon brushes have ___ contract resistance. Answer: high

199. The reversing emf can be produced by using interpoles or compoles. Answer: YES

200. Equalizer connections are mostly used in ___ winding. Answer: lap

201. No load saturation characteristic is also known as open circuit characteristics. Answer: YES

202. Internal or total characteristic is of interest mainly to designer. Answer: YES

203. Internal characteristic is also referred to as performance characteristics. Answer: NO

204. External characteristics given relation between the terminal voltage and ___ current.

Answer: load

205. The open circuit characteristics is obtained by calculating the resistance drop for a new values of

current and adding this to the voltage shown by external characteristics. Answer: NO

206. The great advantage of separate excitation over all other forms of excitation is that the current is

entirely independent of load current in the armature. Answer: YES

207. Field circuit resistance above the critical field resistance will fail to produce build-up of voltage in

the generator. Answer: YES

208. The term voltage-regulation is used to indicate the degree of change in armature voltage

produced by application of load. Answer: YES

209. A shunt generator find application in electric traction where dynamic braking is employed.

Answer: NO

210. An over-compound generator is one whose terminal voltage falls with the application of load.

Answer: NO

211. In case of compound wound generators the degree of compounding may be adjusted by means

of diverter which shunts the series field. Answer: YES

212. A flat compound generator has a load voltage characteristics in which the no-load and full load

voltages are unequal. Answer: NO

213. The ___ compound generator is defined as that compounding produced when the series field

mmf opposes the shunt field mmf. Answer: differential

214. Series generators are used to charge batteries. Answer: NO

215. A shunt generator is used as an arc welding generator. Answer: NO

216. Generators are run in parallel to ensure continuity of service only. Answer: NO

217. Power sources are rarely duplicated in home or automobile service, but unusually are in air-craft,

marine, rail and industrial use. Answer: YES


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218. A parallel circuit is defined as one in which different voltage exists across each unit as the

paralleling point. Answer: NO

219. One of the conditions of paralleling DC generators is that the polarities of the generators must be

the same or the connection must be interchanged until they are. Answer: YES

220. The prime movers that drive the generators to be paralleled should have similar and stablerotational speed characteristics. Answer: YES

221. Whenever generators are in parallel their + ve and ve terminals are respectively connected to

the + ve and ve sides of the bus-bars. Answer: YES

222. Under-compounded generators (those with drooping characteristics curves) may be operated

satisfactorily in parallel in exactly the same manner as shunt generators. Answer: YES

223. While operating over-compounded generators in parallel an equalizer must be connected to the

armature side of the series field on the side of same polarity for each machine. Answer: YES

224. The equalizer connection must have a very high resistance. Answer: NO

225. The series fields must have resistances directly proportional to the ratings of the respective

generators. Answer: NO

chapter 9_dc generators

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