MCQ on DC motor 81-100

81. For which of the following applications a

D.C. motor is preferred over an A.C. motor ?
(a) Low speed operation
(b) High speed operation
(c) Variable speed operation✔
(d) Fixed speed operation

82. In D.C. machines the residual magnetism is of the order of
(a) 2 to 3 per cent✔
(6) 10 to 15 per cent
(c) 20 to 25 per cent
(d) 50 to 75 per cent

83. Which D.C. motor is generally preferred for cranes and hoists ?
(a) Series motor✔
(b) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor

84. Three point starter can be used for
(a) series motor only
(b) shunt motor only
(c) compound motor only
(d) both shunt and compound motor✔

85. Sparking, is discouraged in a D.C. motor because
(a) it increases the input power con-sumption
(b) commutator gets damaged✔
(c) both (a) and (b)
(d) none of the above

86. Speed control by Ward Leonard method gives uniform speed variation
(a) in one direction
(b) in both directions✔
(c) below normal speed only
(d) above normal speed only.

87. Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have
to be
(a) level compounded
(b) under compounded
(c) cumulatively compounded✔
(d) differentially compounded

88. Following motor is used where high starting torque and wide speed range control is required.
(a) Single phase capacitor start
(b) Induction motor
(c) Synchronous motor
(d) D.C. motor✔

89. In a differentially compounded D.C. motor, if shunt field suddenly opens
(a) the motor will first stop and then run in opposite direction as series motor✔
(b) the motor will work as series motor and run at slow speed in the same direction
(c) the motor will work as series motor and run at high speed in the same direction
(d) the motor will not work and come to stop

90. Which of the following motor has the poorest speed regulation ?
(a) Shunt motor
(b) Series motor✔
(c) Differential compound motor
(d) Cumulative compound motor

91. Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
(a) D.C. series motor✔
(b) D.C. shunt motor
(c) induction motor
(d) all of above motors

92. As -the load is increased the speed of D.C. shunt motor will
(a) reduce slightly✔
(b) increase slightly
(c) increase proportionately
(d) remains unchanged

93. The armature torque of the D.C. shunt motor is proportional to
(a) field flux only
(b) armature current only✔
(c) both (a) and (b)
(d) none of the above

94. Which of the following method of speed control of D.C. machine will offer minimum efficiency ?
(a) Voltage control method
(b) Field control method
(c) Armature control method✔
(d) All above methods

95. Usually wide and sensitive speed control is desired in case of
(a) centrifugal pumps
(b) elevators
(c) steel rolling mills
(d) colliery winders✔

96. The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is
(a) 2.36%
(b) 4.76%✔
(c) 6.77%
(d) 8.84%

97. The armature voltage control of D.C. motor provides
(a) constant torque drive✔
(b) constant voltage drive
(c) constant current drive
(d) none of the above

98. As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit
(a) a resistance is connected in series with armature✔
(b) a resistance is connected parallel to the armature
(c) armature is temporarily open circuited
(d) a high value resistor is connected across the field winding

99. The speed of a D.C. shunt motor can be increased by
(a) increasing the resistance in armature circuit
(b) increasing the resistance in field circuit✔
(c) reducing the resistance in the field circuit
(d) reducing the resistance in the armature circuit

100. 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