Electrical Engineering practice mcq : DC Motor (Part 3)

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71. If I2 be the armature current, then speed of a D.C. shunt motor is

(a) independent of Ia

(b) proportional to la

(c) varies as (Ia)

(d) varies as la

Ans: a

72. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will

(a) remain unchanged

(6) reduce to one-fourth value

(c) increase four folds

(d) be doubled

Ans: a

73. At the instant of starting when a D.C. motor is put on supply, it behaves like

(a) a highly resistive circuit

(6) a low resistance circuit

(c) a capacitive circuit

(d) none of the above

Ans: b

74. The speed of a D.C. motor can be varied by varying

(a) field current

(b) applied voltage

(c) resistance in series with armature

(d) any of the above
Ans: d

75. Which one of the following is not necessarily the advantage of D.C. motors over A.C.

motors ?

(a) Low cost

(b) Wide speed range

(c) Stability

(d) High starting torque.

Ans: a

76. For a D.C. shunt motor if the excitation is changed (a) torque will remain constant

(b) torque will change but power will remain constant

(c) torque and power both will change

(d) torque, power and speed, all will change
Ans: b

77. Which motor has the poorest speed control?

(a) Differentially compounded motor

(b) Cumulatively compounded motor

(c) Shunt motor

(d) Series motor

Ans: d

78. The plugging gives the

(a) zero torque braking

(b) smallest torque braking

(c) highest torque braking

(d) none of the above

Ans: c

79. The armature voltage control of D.C. motor provides

(a) constant voltage drive

(b) constant current drive

(c) constant torque drive

(d) none of the above

Ans: c

80. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor (a) of lower H.P. should be selected

(6) of higher H.P. should be selected

(c) can be used for 50°C ambient temperature also

(d) is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor Ans: d

81. If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking

(a) regenerative

(b) plugging

(c) dynamic braking

(d) none of the above

(e) any of the above
Ans: b

82. Which of the following motors one will choose to drive the rotary compressor ?

(a) D.C. shunt motor

(b) D.C. series motor

(c) Universal motor

(d) Synchronous motor

Ans: d

83. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will

(a) increase

(b) decrease

(c) remain same
(d) become zero
Ans: a

84. Why are the D.C. motors preferred for traction applications ?

(a) Torque and speed are inversely proportional to armature current

(b) Torque is proportional to armature current

(c) Torque is proportional to square root of armature current

(d) The speed is inversely proportional to the torque and the torque is proportional to square of armature current
Ans: d

85. Which of the following motors is usually used in house-hold refrigerators ?

(a) D.C. shunt motor

(b) D.C. series motor

(c) Single phase induction motor (split phase start or induction run motor)

(d) Reluctance motor

(e) Synchronous motor

Ans: c

86. Which of the following motors is most suitable for signalling devices and many kinds of timers ?

(a) D.C. shunt motor

(b) D.C. series motor

(c) Induction motor

(d) Reluctance motor
Ans: d

87. Which motor should not be started on no-load ?

(a) Series motor

(b) Shunt motor

(c) Cumulatively compounded motor

(d) Differentially compounded motor.
Ans: a

88. Ward-Leonard control is basically a

(a) voltage control method

(b) field divertor method

(c) field control method

(d) armature resistance control method
Ans: a

89. For constant torque drive which speed control method is preferred ?

(a) Field control

(b) Armature voltage control

(c) Shunt armature control

(d) Mechanical loading system

Ans: b

90. In Ward-Leonard control the lower limit of speed is imposed by

(a) residual magnetism of the generator

(b) core losses of motor

(c) mechanical losses of motor and gen¬erator together

(d) all of the above

Ans: a

91. The main disadvantage of the Ward-Leonard control method is

(a) high initial cost

(b) high maintenance cost

(c) low efficiency at Hght loads

(d) all of the above

Ans: d

92. Regenerative method of braking is based on that

(a) back e.m.f. is less than the applied voltage

(b) back e.m.f. is equal to the applied voltage

(c) back e.m.f. of rotor is more than the applied voltage

(d) none of the above

Ans: b

93. The hysteresis loss in a D.C. machine least depends on

(a) Frequency of magnetic reversals

(b) Maximum value of flux density

(c) Volume and grade of iron

(d) Rate of flow of ventilating air

Ans: d

94. In a D.C. generator all of the following could be the effects of iron losses except (a) Loss of efficiency

(b) Excessive heating of core

(c) Increase in terminal voltage

(d) Rise in temperature of ventilating air
 Ans: c

95. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?

(a) hysteresis loss

(b) field copper loss

(c) armature copper loss

(d) eddy current loss

Ans: c

96. Which of the following loss in a D.C. generator varies significantly with the load current ?

(a) Field copper loss

(b) Windage loss

(c) Armature copper loss

(d) None of the above

Ans: c

97. Torque developed by a D.C. motor depends upon

(a) magnetic field

(b) active length of the conductor

(c) current flow through the conductors

(d) number of conductors

(e) radius of armature

(f) all above factors

Ans: f

98. D.C. shunt motors are used for driving

(a) trains

(b) cranes

(c) hoists

(d) machine tools

Ans: d

99. In a manual shunt motor starter

(a) over load relay is connected in series and no volt relay in parallel with the load

(6) over load relay is connected in paral¬lel and no volt relay in series with the load

(c) over load relay and no volt relay are both connected in series with the load

(d) over load relay and no volt relay are both connected in parallel with the load
Ans: a

100. Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?

(a) Providing laminations in armature core

(b) Providing laminations in stator

(c) Using non-magnetic material for frame
(d) Using material of low hysteresis co-efficient for armature core material

Ans: d






101. Which of the following loss in a D.C. generator is dissipated in the form of heat?

(a) Mechanical loss

(b) Core loss

(c) Copper loss

(d) All of the above

Ans: d

102. Which of the following losses are significantly reduced by laminating the core of a D.C. generator ?

(a) Hysteresis losses

(b) Eddy current losses

(c) Copper losses

(d) Windage losses

Ans: b

103. The total losses in a well designed D.C. generator of 10 kW will be nearly

(a) 100 W

(b) 500 W

(c) 1000 W

(d) 1500 W

Ans: b

104. The condition for maximum efficiency for a D.C. generator is

(a) eddy current losses = stray losses

(b) hysteresis losses = eddy current losses

(c) copper losses = 0

(d) variable losses = constant losses Ans: d

105. D.C. generators are normally designed for maximum efficiency around

(a) full-load

(b) rated r.p.m.

(c) rated voltage

(d) all of the above

Ans: a

106. In a D.C. generator, the iron losses mainly take place in

(a) yoke

(b) commutator

(c) armature conductors

(d) armature rotor

Ans: d

107. D.C. generators are installed near the load centres to reduce

(a) iron losses

(b) line losses

(c) sparking

(d) corona losses

Ans: b

108. The purpose of retardation test on D.C. shunt machines is to find out

(a) stray losses

(b) eddy current losses

(c) field copper losses

(d) windage losses

Ans: a

109. Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?

(a) Swinburne's test

(b) Hopkinson's test

(c) Field test

(d) Brake test

Ans: c

110. Hopkinson's test on D.C. machines is conducted at

(a) no-load

(b) part load

(c) full-load

(d) overload

Ans: c

111. During rheostat braking of D.C. series motors

(a) motor is run as a generator

(b) motor is reversed in direction (c) motor is run at reduced speed

Ans: a

112. For which types of D.C. motor, dynamic braking is generally used ?

(a) Shunt motors

(b) Series motors

(c) Compound motors

(d) All of the aboveAns: d

113. Which method of braking is generally used in elevators ?

(a) Plugging

(b) Regenerative braking

(c) Rheostatic braking

(d) None of the above

Ans: a

114. In variable speed motor

(a) a stronger commutating field is needed at low speed than at high speed

(b) a weaker commutating field is needed at low speed than at high speed

(c) same commutating field is needed at low speed than at high speed

(d) none of the above is correct

Ans: b

115. When the armature of a D.C. motor rotates, e.m.f. induced is (a) self-induced e.m.f.

(b) mutually induced e.m.f.

(c) back e.m.f.

(d) none of the above

Ans: c

116. Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control

(a) drum type controller is used

(b) three point starter is used

(c) four point starter is used

(d) all above can be used

Ans: a

117. If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

(a) this will make armature to take heavy current, possibly burning it

(b) this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

(c) nothing will happen to motor

(d) motor will come to stop

Ans: a

118. D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the

(a) series motor
(b) shunt motor

(c) compound motor

(d) any of the above

Ans: a

119. D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this

(a) series motor (b) shunt motor

(c) compound motor

(d) any of the above
Ans: c

120. Which D.C. motor has got maximum self relieving property ?

(a) Series motor

(6) Shunt motor

(c) Cumulatively compounded motor

(d) Differentially compounded motor
Ans: a

121. In the D.C. motor the iron losses occur in

(a) the field

(b) the armature

(c) the brushes

(d) the commutator

Ans: b

122. The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by

(a) reducing the field current

(b) decreasing the armature current

(c) increasing the armature current

(d) increasing the excitation current

(e) none of the above methods

Ans: a

123. One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven

(a) runs as a generator

(b) does not run as a generator
(c) also runs as a motor comes to stop after sometime
Ans: a