D.C. Motors

21. For a D.C. shunt motor if the excitation is changed

  1. torque will remain constant
  2. torque will change but power will remain constant
  3. torque and power both will change
  4. torque, power and speed, all will change

Correct answer: (B)
torque will change but power will remain constant

22. For constant torque drive which speed control method is preferred?

  1. Field control
  2. Armature voltage control
  3. Shunt armature control
  4. Mechanical loading system

Correct answer: (B)
Armature voltage control

23. For starting a D.C. motor a starter is required because

  1. it limits the speed of the motor
  2. it limits the starting current to a safe value
  3. it starts the motor
  4. none of the above

Correct answer: (B)
it limits the starting current to a safe value

24. For the same H.P. rating and full load speed, following motor has poor starting torque

  1. shunt
  2. series
  3. differentially compounded
  4. cumulativelyc'ompounded

Correct answer: (C)
differentially compounded

25. For which of the following applications a D.C. motor is preferred over an A.C. motor?

  1. Low speed operation
  2. High speed operation
  3. Variable speed operation
  4. Fixed speed operation

Correct answer: (C)
Variable speed operation

26. For which types of D.C. motor, dynamic braking is generally used?

  1. Shunt motors
  2. Series motors
  3. Compound motors
  4. All of the above

Correct answer: (D)
All of the above

27. Hopkinson's test on D.C. machines is conducted at

  1. no-load
  2. part load
  3. full-load
  4. overload

Correct answer: (C)
full-load

28. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor

  1. of lower H.P. should be selected
  2. of higher H.P. should be selected
  3. can be used for 50°C ambient temperature also
  4. is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

Correct answer: (D)
is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

29. If a D.C. motor is connected across the A.C. supply it will

  1. run at normal speed
  2. not run
  3. run at lower speed
  4. burn due to heat produced in the field winding by .eddy currents

Correct answer: (D)
burn due to heat produced in the field winding by .eddy currents

30. If a D.C. motor is to be selected for conveyors, which rriotor would be preferred?

  1. Series motor
  2. Shunt motor
  3. Differentially compound motor
  4. Cumulative compound motor

Correct answer: (A)
Series motor

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