MCQ on DC motor 21-40

21. The main disadvantage of the Ward-Leonard control method is
(a) high initial cost
(b) high maintenance cost
(c) low efficiency at Hght loads
(d) all of the above✔

22. Regenerative method of braking is based on that
(a) back e.m.f. is less than the applied voltage
(b) back e.m.f. is equal to the applied voltage✔
(c) back e.m.f. of rotor is more than the applied voltage
(d) none of above

23. The hysteresis loss in a D.C. machine least depends on
(a) Frequency of magnetic reversals
(b) Maximum value of flux density
(c) Volume and grade of iron
(d) Rate of flow of ventilating air✔

24. In a D.C. generator all of the following could be the effects of iron losses except
(a) Loss of efficiency
(b) Excessive heating of core
(c) Increase in terminal voltage✔
(d) Rise in temperature of ventilating air

25. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?
(a) hysteresis loss
(b) field copper loss
(c) armature copper loss✔
(d) eddy current loss

26. Which of the following loss in a D.C. generator varies significantly with the load current ?
(a) Field copper loss
(b) Windage loss
(c) Armature copper loss✔
(d) None of the above

27. Torque developed by a D.C. motor depends upon
(a) magnetic field
(b) active length of the conductor
(c) current flow through the conductors
(d) all above factors✔

28. D.C. shunt motors are used for driving
(a) trains
(b) cranes
(c) hoists
(d) machine tools✔

29. In a manual shunt motor starter
(a) over load relay is connected in series and no volt relay in parallel with the load.✔
(6) over load relay is connected in parallel and no volt relay in series with the load
(c) over load relay and no volt relay are both connected in series with the load
(d) over load relay and no volt relay are both connected in parallel with the load

30. Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?
(a) Providing laminations in armature core
(b) Providing laminations in stator
(c) Using non-magnetic material for frame
(d) Using material of low hysteresis co-efficient for armature core material✔

31. 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
(d)all of above

32. For which types of D.C. motor, dynamic braking is generally used ?
(a) Shunt motors
(b) Series motors
(c) Compound motors
(d) All of the above✔

33. Which method of braking is generally used in elevators ?
(a) Plugging✔
(b) Regenerative braking
(c) Rheostatic braking
(d) None of the above

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. Which D.C. motor has got maximum self relieving property ?
(a) Series motor✔
(6) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor