Thursday, September 10, 2020

Chapter 6 : DC Machines-Page 8

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

... Answer is (A)


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.

... 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
Answer is (B)


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

... Answer is (C)


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

A. becomes more than 220 V
B. becomes less than 220 V
C. remains the same
D. would reduce to zero.
  ... Answer is (A)


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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