Electrical Machines (Speed Control of D.C. Motors)

1. The speed of a d.c. motor can be controlled by varying
(a) its flux per pole
(b) resistance of armature circuit
(c) applied voltage
(d) all of the above

2. The most efficient method of increasing the speed of a 3.75 kW d.c. shunt motor would be
the ...........method.
(a) armature control
(b) flux control
(c) Ward-Leonard
(d) tapped-field control

3. Regarding Ward-Leonard system of speed control which statement is false ?
(a) It is usually used where wide and very sensitive speed control is required.
(b) It is used for motors having ratings from 750 kW to 4000 kW
(c) Capital outlay involved in the system is right since it uses two extra machines.
(d) It gives a speed range of 10 : 1 but in one direction only.
(e) It has low overall efficiency especially at light loads.

4. In the rheostatic method of speed control for a d.c. shunt motor, use of armature divertor makes the method
(a) less wasteful
(b) less expensive
(c) unsuitable for changing loads
(d) suitable for rapidly changing loads

5. The chief advantage of Ward-Leonard system of d.c. motor speed control is that it
(a) can be used even for small motors
(b) has high overall efficiency at all speeds
(c) gives smooth, sensitive and wide speed control
(d) uses a flywheel to reduce fluctuations in power demand

6. The flux control method using paralleling of field coils when applied to a 4-pole series d.c. motor can give ........... speeds.
(a) 2
(b) 3
(c) 4
(d) 6

7. The series-parallel system of speed control of series motors widely used in traction work gives a speed range of about
(a) 1 : 2
(b) 1 : 3
(c) 1 : 4
(d) 1 : 6

8. In practice, regenerative braking is used when
(a) quick motor reversal is desired
(b) load has overhauling characteristics
(c) controlling elevators, rolling mills and printing presses etc.
(d) other methods can not be used.

9. Statement 1. A direct-on-line (DOL) starter is used to start a small d.c. motor because
Statement 2. it limits initial current drawn by the armature circuit.
(a) both statement 1 and 2 are incorrect
(b) both statement 1 and 2 are correct
(c) statement 1 is correct but 2 is wrong
(d) statement 2 is correct but 1 is wrong

10. Ward-Leonard system of speed control is NOT recommended for
(a) wide speed range
(b) constant-speed operation
(c) frequent motor reversals
(d) very low speeds

11. Thyristor chopper circuits are employed for
(a) lowering the level of a d.c. voltage
(b) rectifying the a.c. voltage
(c) frequency conversion
(d) providing commutation circuitry

12. An invertor circuit is employed to convert
(a) a.c. voltage into d.c. voltage
(b) d.c.voltage into a.c. voltage
(c) high frequency into low frequency
(d) low frequency into high frequency

13. The phase-control rectifiers used for speed of d.c. motors convert fixed a.c. supply voltage into
(a) variable d.c. supply voltage
(b) variable a.c. supply voltage
(c) full-rectified a.c. voltage
(d) half-rectified a.c. voltage

14. If some of the switching devices in a convertor are controlled devices and some are diodes, the convertor is called
(a) full convertor
(b) semiconvertor
(c) solid-state chopper
(d) d.c. convertor

15. A solid-state chopper converts a fixed-voltage d.c. supply into a
(a) variable-voltage a.c. supply
(b) variable-voltage d.c. supply
(c) higher-voltage d.c. supply
(d) lower-voltage a.c. supply

16. The d.c. motor terminal voltage supplied by a solid-state chopper for speed control purposes varies...........with the duty ratio of the chopper
(a) inversely
(b) indirectly
(c) linearly
(d) parabolical

Answers
1. d      2. b      3. d      4. d      5. c      6. b      7. c      8. b      9. c      10. b
11. a    12. b    13. a    14. b    15. b    16. c


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