Thursday, September 10, 2020

Chapter 6 : DC Machines-Page 7

Chapter 6 : DC Machines

61. The armature reaction of an unsaturated dc machine is
A. non-magnetising  
B. magnetizing
C. demagnetizing
D. cross-magnetising

... Answer is (D)



62. Electromagnetic torque and speed are in opposite directions in the case of
A. shunt generators
B. shunt motors
C. series motors
D. all of these

... Answer is (A)


63. The function of a starter in a dc machine is to
A. avoid excessive current at starting
B. avoid armature reaction
C. control the speed
D. avoid excessive heating

... Answer is (A)



64. In dc machines, the armature reaction mmf is.
A. stationary with respect to the armature
B. rotating with respect to the stator
C. stationary with respect to the stator
D. rotating with respect to brushes

  ... Answer is (C)


65. A cumulatively compound dc generator supplies 15 A at 240 V. No if the series field winding is short circuited, the terminal voltage
A. will remain unaltered at 240 V
B. will rise to 260 V
C. will shoot up to a very high value
D. will reduce below 240 V
Answer is (D)


66. In a dc machine
A. torque and induced emf are produced both in motor and generator
B. torque is produced in motor and emf is induced in generator
C. torque is produced in generator and emf is induced in motor
D. none of these

  ... Answer is (A)



67. Function of the interpoles in the dc generators is to
A. neutralize the reactance voltage
B. make commutation sparkles
C. neutralize cross magnetizing effects of a armature reaction.
D. all the above

  ... Answer is (A)


68. Which of the following is not a cross field machine?
A. Level compound generator with interpoles
B. Amplidyne
C. Rosenberg dynamo
D. Metadyne

... Answer is (A)


69. A d.c. shunt generator has full-load voltage regulation of 10% at rated speed of 1000 r.p.m. If it is now driven at 1250 r.p.m., then its voltage regulation at full load would

A. be more than 10%
B. be less than 10%
C. remain unchanged
D. be 12.5%
  ... Answer is (B)


70. In d.c. machines, constant loss is composed of
A. iron loss and mechanical loss
B. friction windage (FW) and iron loss and field circuit loss
C. iron loss and field circuit loss
D. FW and stray load losses

  ... Answer is (B)



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