Short notes
Objective questions (Answers are given below)
- 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) - 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 - 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) - 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) - The mechanical power developed by the armature
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 - The induced e.m.f. in the armature conductors
of a d.c. motor is
(a) sinusoidal (b) trapezoidal
(c) rectangular (d) alternating - A d.c. motor can be looked upon as d.c. generator
with the power flow
(a) reduced (b) reversed
(c) increased (d) modified - 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 - The maximum torque of d.c. motors is limited
by
(a) commutation (b) heating (c) speed (d) armature current
- 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 - 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) - 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 - The current drawn by a 120 – V d.c. motor of
armature resistance 0.5 Ω and back e.m.f. 110 V
is ………. ampere.
(a) 20 (b) 240
(c) 220 (d) 5 - 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 - 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 - 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 - 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 - 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. - 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. - 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. - Which of the following d.c. motor would be suitable
for drives requiring high starting torque but
only fairly constant speed such as crushers ?
(a) shunt (b) series
(c) compound (d) permanent magnet - 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) - 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 - 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 - Between no-load and full-load, ………. motor
develops the least torque
(a) series
(b) shunt
(c) cumulative compound
(d) differential compound
- 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. - 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 - 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. - When load is removed, ………. motor will run at
the highest speed.
(a) shunt
(b) cumulative-compound
(c) differential compound
(d) series - A series motor is best suited for driving
(a) lathes
(b) cranes and hoists
(c) shears and punches
(d) machine tools - 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) - 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) - 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) - 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
- (d) 2. (c) 3.(b) 4. (a) 5. (a) 6. (a) 7. (b) 8. (d) 9. (a) 10. (a) 11. (a) 12.(d) 13. (a) 14. (d) 15. (a) 16. (b) 17. (b) 18. (c) 19. (b) 20. (c) 21. (c) 22. (d) 23.(a) 24. (d) 25. (a) 26. (d) 27. (b) 28. (a) 29. (d) 30. (b) 31. (c) 32. (c) 33. (a) 34.(c).