D.C. Motors Multiple Choice Questions And Answers In English -04

1. During rheostat braking of D.C. series motors
(A) motor is run as a generator
(B) motor is reversed in direction
(C) motor is run at reduced speed
Ans: A

2. Which method of braking is generally used in elevators ?
(A) Plugging
(B) Regenerative braking
(C) Rheostatic braking
(D) None of the above
Ans: A

3. In variable speed motor
(A) a stronger commutating field is needed at low speed than at high speed
(B) a weaker commutating field is needed at low speed than at high speed
(C) same commutating field is needed at low speed than at high speed
(D) none of the above is correct
Ans: B

4. Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control
(A) drum type controller is used
(B) three point starter is used
(C) four point starter is used
(D) all above can be used
Ans: A

5. If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens
(A) this will make armature to take heavy current, possibly burning it
(B) this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
(C) nothing will happen to motor
(D) motor will come to stop
Ans: A

6. D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the
(A) series motor
(B) shunt motor
(C) compound motor
(D) any of the above
Ans: A

7. Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have to be
(A) level compounded
(B) under compounded
(C) cumulatively compounded
(D) differentially compounded
Ans: C

8. D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this
(A) series motor
(B) shunt motor
(C) compound motor
(D) any of the above
Ans: C

9. Which one of the following is not the function of pole shoes in a D.C. machine ?
(A) To reduce eddy current loss
(B) To support the field coils
(C) To spread out flux for better uniformity
(D) To reduce the reluctance of the magnetic path
Ans: A

10. In the D.C. motor the iron losses occur in
(A) the field
(B) the armature
(C) the brushes
(D) the commutator
Ans: B

11. The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by
(A) reducing the field current
(B) decreasing the armature current
(C) increasing the armature current
(D) increasing the excitation current
(e) none of the above methods
Ans: A

12. One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven
(A) runs as a generator
(B) does not run as a generator
(C) also runs as a motor comes to stop after sometime
Ans: A

13. For the same H.P. rating and full load speed, following motor has poor starting torque
(A) shunt
(B) series
(C) differentially compounded
(D) cumulativelyc’ompounded
Ans: C

14. When the armature of a D.C. motor rotates, e.m.f. induced is
(A) self-induced e.m.f.
(B) mutually induced e.m.f.
(C) back e.m.f.
(D) none of the above
Ans: C

15. In case of conductively compensated D.C. series motors, the compensating winding is provided
(A) as separately wound unit
(6) in parallel with armature winding
(C) in series with armature winding
(D) in parallel with field winding
Ans: C

16. If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
(A) Starting torque
(B) Operating speed
(C) Full-load current
(D) All of the above
Ans: C

17. The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
(A) 4.0
(B) 2.0
(C) 1.0
(D) 0.5
Ans: D

18. The condition for maximum power in case of D.C. motor is
(A) back e.m.f. = 2 x supply voltage
(B) back e.m.f. = | x supply voltage
(C) supply voltage = | x back e.m.f.
(D) supply voltage = back e.m.f.
Ans: B

19. For which of the following applications a D.C. motor is preferred over an A.C. motor ?
(A) Low speed operation
(B) High speed operation
(C) Variable speed operation
(D) Fixed speed operation
Ans: C

20. In D.C. machines the residual magnetism is of the order of
(A) 2 to 3 per cent
(6) 10 to 15 per cent
(C) 20 to 25 per cent
(D) 50 to 75 per cent
Ans: A