DC Motor Question Bank

1. In an electromechanical energy conversion devices (e.g. generators and motors) a small air gap is left between the stator and the rotor in order to

(a) reduce the reluctance of the magnetic path.
(b) increase flux density in the air gap.
(c) permit mechanical clearance.
(d) avoid saturation of field.

2. The EMF induced in a conductor rotating in a bipolar field is

(a) DC
(b) AC
(c) DC and AC both
(d) None of these.

3. A conductor is rotating within a magnetic field. At which of the positions do the peak voltages occur?

(a) At right angles to the axis of magnetic field.
(b) Along the axis of the magnetic field.
(c) At 45^o angles of the magnetic field.
(d) Anywhere.

4. EMF induced in a coil rotating in a uniform magnetic field will be maximum when the

(a) flux linking with the coil is maximum.
(b) rate of change of flux linkage is minimum.
(c) rate of change of flux linkage is maximum.
(d) rate of cutting flux by the coil sides is minimum.

5. In a DC motor the windage loss is proportional to

(a) supply voltage
(b) square of the supply voltage
(c) square of the flux density.
(d) square of the armature speed.

6. Which of the following are the variable losses in a rotating machine?

(a) Core losses and mechanical losses.
(b) Core losses and stray load losses.
(c) Copper losses and core losses.
(d) Copper losses and stray load loss.

7. Stray losses are sum of:

(a) iron and mechanical losses.
(b) copper and iron losses.
(c) copper and mechanical losses.
(d) none of these.

8. Generally the no-load losses of an electrical machine is represented in its equivalent circuit by a

(a) parallel resistance with a low value.
(b) series resistance with a low value.
(c) parallel resistance with a high value.
(d) series resistance with a high value.

9. Neglecting all losses, how is developed torque (T) of a DC separately excited motor, operating under constant terminal voltage, related to its output power (P)?

(a) T α √P
(b) T α P
(c) T² α P³
(d) T is independent of P.

10. The duty cycle of a motor is

Load               Duration
20 kW            120 s
10 kW            120 s
Stopped        280 s
The continuous rating of motor suitable for above application would be
(a) 20 kW
(b) 16.4 kW
(c) 14.14 kW
(d) 9 kW

11. In a DC machine

(a) the current and the EMF in armature conductors are alternating while those at the terminals are unidirectional.
(b) the current and the EMF in armature conductors are unidirectional while those at the terminals are alternating.
(c) the current and the EMF in armature conductors and at the terminals are unidirectional.
(d) the EMF in armature conductors and at the terminals is alternating while current there is unidirectional.

12. Laminated yoke in a DC motor can reduce

(a) speed regulation
(b) iron loss.
(c) temperature rise.
(d) sparking on load.

13. Pole shoe of a DC machine is laminated for the purpose of

(a) decreasing hysteresis loss.
(b) decreasing eddy current loss.
(c) decreasing both hysteresis loss and eddy current loss.
(d) manufacturing ease.

14. Poles of DC machine are often laminated to

(a) reduce pulsation loss.
(b) reduce armature reaction.
(c) reduce iron weight.
(d) dissipate more heat.

15. The ventilating ducts in most of the DC machines are

(a) radial.
(b) longitudinal.
(c) radial as well as longitudinal.
(d) none of these.

16. The armature of a DC machine is laminated

(a) to reduce hysteresis loss.
(b) to reduce eddy current loss.
(c) to reduce the mass.
(d) to reduce the inductance.

17. The armature of a DC machine is made of silicon steel laminations to

(a) to reduce hysteresis loss only.
(b) to reduce eddy current loss only.
(c) to increase the permeability.
(d) to reduce hysteresis and eddy current loss.

18. In a DC machine, the armature is always on the rotor unlike an AC machine where it could be either on the stator or the rotor, because

(a) commutation would otherwise not be possible.
(b) armature reaction demagnetizing action would otherwise be more.
(c) otherwise it would not be possible to place compensating winding in pole shoes.
(d) self-starting of DC motor would not be possible otherwise.

19. The teeth in the armature of a DC machine are sometimes skewed. This is done to reduce

(a) hysteresis loss.
(b) copper loss.
(c) eddy current loss.
(d) vibrations.

20. Wave winding is employed in a DC machine of

(a) high current and low voltage rating.
(b) low current and high voltage rating.
(c) high current and high voltage rating.
(d) low current and low voltage rating.

21. If two 8-pole DC machines of identical armatures are wound, one with lap winding and the other with wave winding, then

(a) wave wound machine will have more rated current and more voltage.
(b) lap wound machine will have more rated voltage and more current.
(c) lap wound machine will have more rated voltage and less current.
(d) wave wound machine will have more rated voltage and less current.

22. In lap winding, the number of brushes is always ……… the number of poles.

(a) double
(b) same as
(c) half
(d) none of these

23. A P – pole lap wound DC machine had an armature current I_a. The conductor current in the armature winding is

(a) I_a
(b) I_a/P
(c) PI_a
(d) None of the above

24. A lap wound DC machine has 400 conductors and 8 poles. The voltage induced per conductor is 2 V. The machine generates a voltage of

(a) 100 V
(b) 200 V
(c) 400 V
(d) 800 V

25. If in a 6 pole lap wound DC machine, the air gap under each pole is not the same, this is likely to result in

(a) higher terminal voltage.
(b) reduced eddy currents.
(c) unequal currents in parallel paths.
(d) none of the above.

26. The number of parallel paths for a 4 – pole duplex lap winding will be

(a) 2
(b) 4
(c) 6
(d) 8

27. Width of carbon brush should be equal to

(a) less than the width of one commutator segment.
(b) the width of 1 to 2 commutator segments.
(c) the width of 2 to 3 commutator segments.
(d) the width of more than 3 commutator segments.

28. For a 4 – pole machine wave winding is impossible with ……… armature conductors.

(a) 30
(b) 32
(c) 34
(d) 38

29. In a DC machine, for the same value of ɸ, Z and N; which one of the following statement is correct?

(a) Armature EMF is more with wave winding than with lap winding.
(b) Armature EMF is less with wave winding than with lap winding.
(c) Armature EMF depends on whether the machine is running as a motor or a generator.
(d) Armature EMF is the same as long as the flux density in the air gap remains the same.

30. The dummy coils in DC machines are useful to

(a) increase the efficiency.
(b) improve the commutation.
(c) reduce the cost of the machine.
(d) maintain mechanical balance of armature.

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