501 MCQ Questions on DC Machine

51. Which of the following is the standard requirement of a dc armature winding? [UPPCL JE 2014]

(a) It should be a closed one.
(b) It should be a lap winding.
(c) It should be a wave winding.
(d) It should be either a wave winding or a lap winding.

52. The poorest voltage regulation exists in case of which of the following generators? [UPPCL JE – 2014]

(a) Compound generators
(b) Shunt generators
(c) Series generators
(d) High generators

53. What would be the value of pole pitch if there are 80 conductors and 8 poles? [UPPCL JE – 2014]

(a) 640
(c) 80
(b) 64
(d) 10

54. What is the value of flux in the section of yoke, if the pole flux is 4 weber? [UPPCL JE 2014]

(a) 2ɸ
(b) √ɸ
(c) ɸ/2
(d) ɸ

55. The transformer that should never have the secondary open-circuited when primary is energised is

(a) power transformer.
(b) voltage transformer.
(c) auto transformer.
(d) current transformer.

56. Speed of a DC motor is

(a) inversely proportional to the air gap flux.
(b) directly proportional to the air-gap flux.
(c) inversely proportional to the square of air-gap flux.
(d) not related to the air-gap flux. [UPPCL JE – 2014]

57. What is the relationship between the speed (N) and armature current (I_a) in case of a dc-series motor? [UPPCL JE – 2014]

(a) N α 1/I_a
(b) N α √I_a
(c) N α I_a²
(d) N = I_a

58. What is the value of reflection coefficient for a short circuit line? [UPPCL JE 2014]

(a) 1
(b) 0
(c) -1
(d) 0.5

59. The method of speed control of DC shunt motors used for applications where a very wide range sensitive speed control is required, is:

(a) Ward-Leonard System
(b) Multiple Voltage Control
(c) Tapped Field Control
(d) Rheostatic Control [NMRC JE-2017]

60. The DC shunt motor is running with a certain load. The effect of adding an external resistance in field circuit is to

(a) increase the motor speed.
(b) stop the motor.
(c) reduce the motor speed.
(d) reduce the armature current of the motor. [NMRC JE-2017]

61. In DC motor, the speed depends upon

(a) Applied voltage alone
(b) Back emf alone
(c) Back emf and flux
(d) Flux only [NMRC JE-2017]

62. Three point starter can be used for

(a) both shunt and compound motors.
(b) shunt motor only.
(c) series motor only.
(d) compound motor only. [NMRC JE-2017]

63. Voltage equation of DC motor is given by

(a) V = E_b + I_aR_a
(b) V = E_b – I_aR_a
(c) V = E_bI_a –  R_a
(d) V = E_bI_a + R_a [NMRC JE-2017]

64. DC motor recommended for locomotive drive is

(a) DC series motor.
(b) DC long shunt compound motor.
(c) DC shunt motor.
(d) DC short shunt compound motor. [NMRC JE-2017]

65. Brushes are provided in DC machine for

(a) smooth rotation.
(b) preventing sparking.
(c) providing a path for flow of current.
(d) reducing the losses. [NMRC JE-2017]

66. A DC shunt motor is running at 1200 rpm, when excited with 220V dc. Neglecting the losses and saturation, the speed of the motor when connected to a 175 V supply is

(a) 70 rpm
(c) 1050 rpm
(b) 900 rpm
(d) None of these

67. Armature shunting method of speed control of dc shunt motor is preferred over armature resistance method due to

(a) better speed regulation.
(b) less loss in external resistance.
(c) simplicity in control circuit.
(d) reduced cost of controller.

68. Carbon brushes are used in electric motors to

(a) brush off carbon deposit on the commutator .
(b) provide a path for flow of current.
(c) prevent overheating of armature windings.
(d) prevent sparking during commutation.

69. The function(s) of pole shoes in DC machine is/are to

(a) support the field coils.
(b) reduce the reluctance of the magnetic path.
(c) spread out the flux to achieve uniform flux distribution in the air gap.
(d) all of the above.

70. The condition for a maximum power output from dc motor is

 (a) E_b = V
(b) E_b = V/2
(c) E_b = 0
(d) E_b = V/√2 [SSC JE 2010]

71. Fleming’s left hand rule is applicable to

(a) dc generator.
(c) alternator.
(b) dc motor.
(d) transformer.

72. DC motors are still preferred for use in

(a) electric excavators steel mills and cranes.
(b) lathes and machine tools.
(c) flour mills and jaw crushers.
(d) paper industry.

73. The number of parallel paths in the armature winding of a 4-pole, wave connected d.c. machine having 28 coil side is

(a) 28
(c) 4
(b) 14
(d) 2

74. Which of the following parameters is changed to increase the speed of DC motor more than its base speed?

(a) Flux
(c) Field
(b) Voltage
(d) Firing Angle [UPRVUNL AE 2016]

75. The back e.m.f of a motor at the moment of starting

(a) Zero
(c) Minimum
(b) Maximum
(d) Optimum

76. In a d.c. series motor, the torque developed is 20 Nm at 10 A. If the load current is doubled, the new torque will be

(a) 60 Nm
(b) 45 Nm
(c) 80 Nm
(d) 100 Nm

77. The torque speed characteristic of a Repulsion motor same as the following DC motor characteristic

(a) separately excited.
(b) shunt.
(c) series.
(d) compound.

78. A DC machine is provided with both inter pole winding (IPW) and compensating winding (CPM). With respect to armature

(a) both IPW and CPW are in parallel.
(b) both IPW and CPW in series.
(c) IPW is in series CPW is in parallel.
(d) IPW is in parallel and IPW is in series.

79. The function of equalizing ring in lap wound DC generator is

(a) to avoid SC current.
(b) to neutralise the armature reaction.
(c) to help get sparkless commutation.
(d) all the above.

80. In a level compound generator, the series field AT are

(a) in direct opposition to the shunt field AT.
(b) in the same direction as the shunt field AT.
(c) at 90° to the shunt filed AT.
(d) placed on inter pole.

81. In series parallel control of dc series motor the total field turns are N, then

(a) AT parallel = 2AT series.
(b) AT parallel = AT series.
(c) AT parallel = 1/2 AT series.
(d) AT parallel = 1/4 AT series.

82. Field control of a DC shunt motor gives

(a) Constant torque drive
(b) Constant kW drive
(c) Constant speed drive
(d) Variable speed driver

83. Which of the motor is used for rolling mills?

(a) DC shunt motor.
(b) DC commulatively compound motor.
(c) DC series motor.
(d) DC differentially compound motor.

84. What will happen if the supply terminals of DC shunt motor are inter changed?

(a) motor will stop.
(b) motor will run at its normal speeds in the same direction as it was running.
(c) the direction of rotation will reverse.
(d) motor will run at speeds lower than the normal speeds in the same direction.

85. Which of the following test can be conducted on other than shunt machines?

(a) Swinburne test.
(b) Retardation test.
(c) Field’s test.
(d) Blocked rotor test.

86. During the regenerative breaking energy is

(a) dissipated in resistor.
(b) returned to the supply lines.
(c) stored in the form KE.
(d) all the above.

87. The field poles and armature core of a d.c. generator are laminated in order to reduce

(a) hysteresis loss.
(b) eddy current loss.
(c) weight.
(d) speed. [UPPCL JE – 2007]

88. The mechanical power developed in a d.c. motor is equal to

(a) power input – core losses.
(b) power input – mechanical losses.
(c) armature current × counter e.m.f.
(d) armature current x supply voltage. [UPPCL JE – 2007]

89. An electric train employing a d.c. series motor is running at a fixed speed. When a sudden drop in voltage of supply takes place, then this results in

(a) drop in speed and rise in current.
(b) rise in speed and drop in current.
(c) rise in speed and rise in current.
(d) drop in speed with current unaltered. [UPPCL JE – 2007]

90. For D.C. shunt motor, speed control by armature resistance variations is best suited for

(a) constant power drive.
(b) variable power drive.
(c) constant torque drive.
(d) variable torque drive. [UPRVUNL AE-2016]

91. If number of poles in lap wound d.c. generator are doubled, the generated e.m.f. will be

(a) increased by a factor of 2.
(b) decreased by a factor of 2.
(c) increased by a factor of 4.
(d) unchanged. [UPPCL JE 2007]

92. A DC series motor has linear magnetization characteristics and negligible armature resistance. The motor speed is

(a) Directly proportional to T
(b) Inversely proportional to T
(c) Directly proportional to T²
(d) Inversely proportional to T²
Where T = load torque

93. A 4-pole dc generator runs at 1500 rpm. The frequency of current in armature winding is

(a) Zero
(b) 25 Hz
(c) 50 Hz
(d) 100 Hz [UPPCL JE – 2007]

94. For a d.c. series motor, which of the following expression is correct assuming torque (T) versus armature current (I_a) characteristics unsaturated? [UPPCL JE – 2007]

(a) T α √I_a
(b) T α I_a
(c) T α -I_a
(d) T α I_a²

95. A d.c. series motor should never be started at

(a) normal load condition.
(b) full load condition.
(c) no load condition.
(d) slightly overload condition. [UPPCL JE – 2007]

96. In a d.c. generator, 8 parallel paths and 16 brushes for collection of current are used. If voltage drop per brush is 1 V, then reduction in the induced e.m.f. will be

(a) 2 V
(b) 4 V
(c) 8 V
(d) 16 V [UPPCL JE – 2007]

97. For DC motor magnets ………… is preferred.

(a) tungsten steel
(b) silicon steel
(c) carbon steel
(d) iron cobalt alloy [UPPCL JE – 2016]

98. The speed control of dc shunt motor in both directions can be obtained by

(a) armature resistance control method.
(b) armature voltage control method.
(c) field diverter method.
(d) ward leonard method. [UPPCL AE 2016]

99. In which of the following methods of speed control of DC series motor, “Field ampere-turns are adjusted in steps by varying the number of turns included in the circuit”?

(a) Diverter field control
(b) Tapped field control
(c) Series-parallel control
(d) Rheostatic control [UPPCL AE – 2016]

100. The armature resistance of a DC motor is 0.4 Ω, the supply voltage is 200 V and the back e.m.f. is 198 V at full speed. The armature current is

(a) 4A
(b) 8A
(c) 5A
(d) 0.5A [UPPCL AE – 2016]