Chapter 6 : DC Machines
71. An amplidyne develops an output voltage of 100
volts at an armature speed of n r.p.m. If its armature is now driven at
2n r.p.m., then its output voltage is
A. 100 volts
B. 200 volts
C. 400 volts
D. 50 volts
...
Answer is (C)
72. A dc shunt motor is rotating in clockwise
direction as viewed from one end. The polarity of connection of the
armature and field winding are reversed simultaneously. The motor will
A. not rotate at all
B. rotate in anti-clockwise direction
C. rotate in clockwise direction.
D. None of above
73. A self-excited d.c. shunt motor is ranning at
no load at 1000 rpm. If the terminal voltage is reduced to 50% of its
original value, the no-load speed of the otor will be?rpm.
A.1000
B.2000
C.200
D.202
...
Answer is (A)
74. the applied voltage of a dc shunt motor is
halved with the load torque doubled, the following approximate changes
will be observed. Speed is
A. doubled and armature current is halved
B. the same and armature current is doubled
C. same and armature current increases four-fold
D. halved and armature current is same
...
Answer is (C)
75. A d.c. shunt motor is running with a certain load. The effect of adding an external resistance in the field circuit is to
A. reduce the motor speed
B. increase the motor speed
C. reduce the armature current of the motor
D. stop the motor
76. Which of the following motor has high starting torque?
A. ac series motor
B. dc series motor
C. induction motor
D. synchronous motor
...
Answer is (B)
77. A d.c. series motor with rated voltage of 230 volts is connected to an a.c. supply of 230 volts. It will
A. run
B. not run
C. run with less efficiency
D. run with more efficiency
...
Answer is (B)
78. There are two 2-pole d.c. machines. Main field
axis of machine A is horizontal and that of B is vertical. It is
preferable to purchase
A. machine A
B. machnie B
C. any of the two machines
D. none of these
79. A differentially compounded d.c. generator is
delivering full-load current a terminal voltage of 220 V. If its series
field winding is short circuited, then its terminal voltage
B. becomes less than 220 V
C. remains the same
D. would reduce to zero.
80. The mechanical power developed by a d.c. motor is maximum when the back e.m.f. is
A. equal to the applied voltage
B. equal to zero
C. equal to half the applied voltage
D. approaching the value of the applied voltage
...
Answer is (B)
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