Short notes
Objective questions (Answers are given below)
- 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 - 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 - 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. - 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 - 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 - 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 - 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 - 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. - 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 - 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 - 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
- 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 - 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 - 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 - 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 - 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) parabolically
- d 2. b 3. d 4. d 5. c 6. b 7. c 8. b 9. c 10. b 11. a
- b 13. a 14. b 15. b 16. c