D.C. Motors

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

  1. this will make armature to take heavy current, possibly burning it
  2. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
  3. nothing will happen to motor
  4. motor will come to stop

Correct answer: (A)
this will make armature to take heavy current, possibly burning it

32. If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

  1. nothing will happen to the motor
  2. this will make armature to take heavy current, possibly burning it
  3. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
  4. motor will run at very slow speed

Correct answer: (C)
this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

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

  1. independent of Ia
  2. proportional to la
  3. varies as (Ia)
  4. varies as la

Correct answer: (A)
independent of Ia

34. If the field of a D.C. shunt motor gets opened while motor is running

  1. the speed of motor will be reduced %
  2. the armature current will reduce
  3. the motor will attain dangerously high speed 1
  4. the motor will continue to nuvat constant speed

Correct answer: (C)
the motor will attain dangerously high speed 1

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

  1. increase
  2. decrease
  3. remain same
  4. become zero

Correct answer: (A)
increase

36. If the supply voltage for a D.C. motor is increased, which of the following will decrease?

  1. Starting torque
  2. Operating speed
  3. Full-load current
  4. All of the above

Correct answer: (C)
Full-load current

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

  1. regenerative
  2. plugging
  3. dynamic braking
  4. none of the above

Correct answer: (B)
plugging

38. In a D.C. generator all of the following could be the effects of iron losses except

  1. Loss of efficiency
  2. Excessive heating of core
  3. Increase in terminal voltage
  4. Rise in temperature of ventilating air

Correct answer: (C)
Increase in terminal voltage

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

  1. yoke
  2. commutator
  3. armature conductors
  4. armature rotor

Correct answer: (D)
armature rotor

40. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

  1. 100% of the previous value
  2. 50% of the previous value
  3. 25% of the previous value
  4. 10% of the previous value

Correct answer: (C)
25% of the previous value

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