Objective tests on d.c. Motor

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OBJECTIVE TESTS

1. In a d.c. motor, undirectional torque is produced with the help of

(a) brushes

(b) commutator

(c) end-plates

(d) both (a) and (b)

2. The counter e.m.f. of a d.c. motor

(a) often exceeds the supply voltage

(b) aids the applied voltage

(c) helps in energy conversion

(d) regulates its armature voltage

3. The normal value of the armature resistance of a

d.c. motor is

(a) 0.005

(b) 0.5

(c) 10

(d) 100

(Grad. I.E.T.E. June 1987)

4. The Eb/V ratio of a d.c. motor is an indication of its

(a) efficiency

(b) speed regulation

(c) starting torque

(d) Running Torque

(Grad. I.E.T.E. June 1987)

5. The mechanical power developed by the arma- ture of a d.c. motor is equal to

(a) armature current multiplied by back e.m.f.

(b) power input minus losses

(c) power output multiplied by efficiency

(d) power output plus iron losses

6. The induced e.m.f. in the armature conductors of a d.c. motor is

(a) sinusoidal

(b) trapezoidal

(c) rectangular

(d) alternating

7. A d.c. motor can be looked upon as d.c. genera- tor with the power flow

(a) reduced

(b) reversed

(c) increased

(d) modified

8. In a d.c. motor, the mechanical output power actually comes from

(a) field system

(b) air-gap flux

(c) back e.m.f.

(d) electrical input power

9. The maximum torque of d.c. motors is limited by

(a) commutation

(b) heating

(c) speed

(d) armature current

10. Which of the following quantity maintains the same direction whether a d.c. machine runs as a generator or as a motor ?

(a) induced e.m.f.

(b) armature current

(c) field current

(d) supply current

11. Under constant load conditions, the speed of a

d.c. motor is affected by

(a) field flux

(b) armature current

(c) back e.m.f.

(d) both (b) and (c)

12. It is possible to increase the field flux and, at the same time, increase the speed of a d.c. motor provided its .......... is held constant.

(a) applied voltage

(b) torque

(c) Armature circuit resistance

(d) armature current

13. The current drawn by a 120 - V d.c. motor of armature resistance 0.5 W and back e.m.f. 110 V is .......... ampere.

(a) 20

(b) 240

(c) 220

(d) 5

14. The shaft torque of a d.c. motor is less than its armature torque because of .......... losses.

(a) copper

(b) mechanical

(c) iron

(d) rotational

15. A d.c. motor develops a torque of 200 N-m at 25 rps. At 20 rps it will develop a torque of

.......... N-m.

(a) 200

(b) 160

(c) 250

(d) 128

16. Neglecting saturation, if current taken by a series motor is increased from 10 A to 12 A, the percentage increase in its torque is ........ percent

(a) 20

(b) 44

(c) 30.5

(d) 16.6

17. If load on a d.c. shunt motor is increased, its speed is decreased due primarily to

(a) increase in its flux

(b) decrease in back e.m.f.

(c) increase in armature current

(d) increase in brush drop

18. If the load current and flux of a d.c. motor are held constant and voltage applied across its armature is increased by 10 per cent, its speed will

(a) decrease by about 10 per cent

(b) remain unchanged

(c) increase by about 10 per cent

(d) increase by 20 per cent.

19. If the pole flux of a d.c. motor approaches zero, its speed will

(a) approach zero

(b) approach infinity

(c) no change due to corresponding change in back e.m.f.

(d) approach a stable value somewhere between zero and infinity.

20. If the field circuit of a loaded shunt motor is suddenly opened

(a) it would race to almost infinite speed

(b) it would draw abnormally high armature current

(c) circuit breaker or fuse will open the circuit before too much damage is done to the motor

(d) torque developed by the motor would be reduced to zero.

21. Which of the following d.c. motor would be suit- able for drives requiring high starting torque but only fairly constant speed such as crushers ?

(a) shunt

(b) series

(c) compound

(d) permanent magnet

22. A d.c. shunt motor is found suitable to drive fans because they require

(a) small torque at start up

(b) large torque at high speeds

(c) practically constant voltage

(d) both (a) and (b)

23. Which of the following load would be best driven by a d.c. compound motor ?

(a) reciprocating pump

(b) centrifugal pump

(c) electric locomotive

(d) fan

24. As the load is increased, the speed of a d.c. shunt motor

(a) increases proportionately

(b) remains constant

(c) increases slightly

(d) reduces slightly

25. Between no-load and full-load, .......... motor develops the least torque

(a) series

(b) shunt

(c) cumulative compound

(d) differential compound

26. The Ta/Ia graph of a d.c. series motor is a

(a) parabola from no-load to overload

(b) straight line throughout

(c) parabola throughout

(d) parabola upto full-load and a straight line at overloads.

27. As compared to shunt and compound motors, series motor has the highest torque because of its comparatively .......... at the start.

(a) lower armature resistance

(b) stronger series field

(c) fewer series turns

(d) larger armature current

28. Unlike a shunt motor, it is difficult for a series motor to stall under heavy loading because

(a) it develops high overload torque

(b) its flux remains constant

(c) it slows down considerably

(d) its back e.m.f. is reduced to almost zero.

29. When load is removed, .......... motor will run at the highest speed.

(a) shunt

(b) cumulative-compound

(c) differential compound

(d) series

30. A series motor is best suited for driving

(a) lathes

(b) cranes and hoists

(c) shears and punches

(d) machine tools

31. A 220 V shunt motor develops a torque of 54 N-m at armature current of 10 A. The torque produced when the armature current is 20 A, is

(a) 54 N-m

(b) 81 N-m

(c) 108 N-m

(d) None of the above

(Elect. Machines, A.M.I.E. Sec. B, 1993)

32. The d.c. series motor should never be switched on at no load because

(a) the field current is zero

(b) The machine does not pick up

(c) The speed becomes dangerously high

(d) It will take too long to accelerate.

(Grad. I.E.T.E. June 1988)

33. A shunt d.c. motor works on a.c. mains

(a) unsatisfactorily

(b) satisfactorily

(c) not at all

(d) none of the above

(Elect. Machines, A.M.I.E. Sec. B, 1993)

34. A 200 V, 10 A motor could be rewound for 100 V, 20 A by using .......... as many turns per coil of wire, having .......... the cross-sectional area.

(a) twice, half

(b) thrice, one third

(c) half, twice

(d) four times, one-fourth

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