Electrical
Circuits &Machines
UNIT IV: AC
Motors
MCQ’S
-Prof. Tushar V.
Kafare
BSIOTR Wagholi
1. Which of
the following component is usually fabricated out of silicon steel?
(a) Bearings
(b) Shaft
(c)
Statorcore
(d) None of
the above
Ans: c
2. The frame
of an induction motor is usually made of
(a) silicon
steel
(b) cast
iron
(c)
aluminium
(d) bronze
Ans: b
3. The shaft
of an induction motor is made of
(a) stiff
(b) flexible
(c) hollow
(d) any of
the above
Ans: a
4. The shaft
of an induction motor is made of
(a) high
speed steel
(b)
stainless steel
(c) carbon
steel
(d) cast iron
Ans: c
5. In an
induction motor, no-load the slip is generally
(a) less
than 1%
(b) 1.5%
(c) 2%
(d) 4%
Ans: a
6. In medium
sized induction motors, the slip is generally around
(a) 0.04%
(b) 0.4%
(c) 4%
(d) 14%
Ans: c
7. In
squirrel cage induction motors, the rotor slots are usually given slight skew
in order
to
(a) reduce
windage losses
(b) reduce
eddy currents
(c) reduce
accumulation of dirt and dust
(d) reduce
magnetic hum
Ans: d
8. In case
the air gap in an induction motor is increased
(a) the
magnetising current of the rotor will decrease
(b) the
power factor will decrease
(c) speed of
motor will increase
(d) the
windage losses will increase
Ans: b
9. Slip
rings are usually made of
(a) copper
(b) carbon
(c) phospor
bronze
(d)
aluminium
Ans: c
10. A
3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f.
will
be
(a) 200 Hz
(b) 50 Hz
(c) 2 Hz (d)
0.2 Hz
Ans: c
11. In Ns is
the synchronous speed and s the slip, then actual running speed of an
induction
motor will be
(a) Ns
(b) s.N,
(c) (l-s)Ns
(d) (Ns-l)s
Ans: c
The
efficiency of an induction motor can be expected to be nearly
(a) 60 to
90%
(b) 80 to
90%
(c) 95 to
98%
(d) 99%
Ans: b
13. The
number of slip rings on a squirrel cage induction motor is usually
(a) two
(b) three
(c) four
(d) none
Ans: d
14. The
starting torque of a squirrel-cage induction motor is
(a) low
(b)
negligible
(c) same as
full-load torque
(d) slightly
more than full-load torque
Ans: a
15. A double
squirrel-cage induction motor has
(a) two
rotors moving in oppsite direction
(b) two
parallel windings in stator
(c) two
parallel windings in rotor
(d) two
series windings in stator
Ans: c
16.
Star-delta starting of motors is not possible in case of
(a) single
phase motors
(b) variable
speed motors
(c) low
horse power motors
(d) high
speed motors
Ans: a
17. The term
'cogging' is associated with
(a) three
phase transformers
(b) compound
generators
(c) D.C.
series motors
(d)
induction motors
Ans: d
18. In case
of the induction motors the torque is
(a)
inversely proportional to (Vslip)
(b) directly
proportional to (slip)2
(c)
inversely proportional to slip
(d) directly
proportional to slip
Ans: d
19. An
induction motor with 1000 r.p.m. speed will have
(a) 8 poles
(b) 6 poles
(c) 4 poles
(d) 2 poles
Ans: b
20. The good
power factor of an induction motor can be achieved if the average flux
density in
the air gap is
(a) absent
(b) small
(c) large
(d) infinity
Ans: b
21. An
induction motor is identical to
(a) D.C.
compound motor
(b) D.C.
series motor
(c)
synchronous motor
(d)
asynchronous motor
Ans: d
22. The
injected e.m.f. in the rotor of induction motor must have
(a) zero
frequency
(b) the same frequency as the
slip fre-quency
(c) the same
phase as the rotor e.m.f.
(d) high
value for the satisfactory speed control
Ans: b
23. Which of
the following methods is easily applicable to control the speed of the
squirrel-cage
induction motor ?
(a) By
changing the number of stator poles
(b) Rotor
rheostat control
(c) By
operating two motors in cascade
(d) By
injecting e.m.f. in the rotor circuit
Ans: a
24. The
crawling in the induction motor is caused by
(a) low
voltage supply
(b) high
loads
(c)
harmonics develped in the motor
(d) improper
design of the machine
(e) none of
the above
Ans: c
25. The
auto-starters (using three auto transformers) can be used to start cage
induction
motor of the
following type
(a) star
connected only
(b) delta
connected only
(c) (a) and
(b) both
(d) none of
the above
Ans: c
26. The
torque developed in the cage induction motor with autostarter is
(a) k/torque
with direct switching
(6) K x
torque with direct switching
(c) K2 x
torque with direct switching
(d)
k2/torque with direct switching
Ans: c
27. When the
equivalent circuit diagram of doouble squirrel-cage induction motor is
constructed
the two cages can be
considered
(a) in
series
(b) in
parallel
(c) in
series-parallel
(d) in
parallel with stator
Ans: b
28. It is
advisable to avoid line-starting of induction motor and use starter because
(a) motor
takes five to seven times its full load current
(b) it will
pick-up very high speed and may go out of step
(c) it will
run in reverse direction
(d) starting
torque is very high
Ans: a
29. Stepless
speed control of induction motor is possible by which of the following
methods ?
(a) e.m.f.
injection in rotor eueuit
(b) Changing
the number of poles
(c) Cascade
operation
(d) None of
the above
Ans: b
30. Rotor
rheostat control method of speed control is used for
(a)
squirrel-cage induction motors only
(b) slip
ring induction motors only
(c) both (a)
and (b)
(d) none of
the above
Ans: b
31. In the
circle diagram for induction motor, the diameter of the circle represents
(a) slip
(b) rotor
current
(c) running
torque
(d) line
voltage
Ans: b
32. For
which motor the speed can be controlled from rotor side ?
(a)
Squirrel-cage induction motor
(b)
Slip-ring induction motor
(c) Both (a)
and (b)
(d) None of
the above
Ans: b
33. If any
two phases for an induction motor are interchanged
(a) the
motor will run in reverse direction
(b) the
motor will run at reduced speed
(c) the
motor will not run
(d) the
motor will burn
Ans: a
34. An
induction motor is
(a)
self-starting with zero torque
(b)
self-starting with high torque
(c)
self-starting with low torque
(d) non-self
starting
Ans: c
35. The
maximum torque in an induction motor depends on
(a)
frequency
(b) rotor
inductive reactance
(c) square
of supply voltage
(d) all of
the above
Ans: d
36. In
three-phase squirrel-cage induction motors
(a) rotor
conductor ends are short-circuited through slip rings
(b) rotor
conductors are short-circuited through end rings
(c) rotor
conductors are kept open
(d) rotor
conductors are connected to insulation
Ans: b
37. In a
three-phase induction motor, the number of poles in the rotor winding is always
(a) zero
(b) more
than the number of poles in stator
(c) less
than number of poles in stator
(d) equal to
number of poles in stator
Ans: d
38. DOL
starting of induction motors is usually restricted to
(a) low horsepower
motors
(b) variable
speed motors
(c) high
horsepower motors
(d) high
speed motors
Ans: a
39. The
speed of a squirrel-cage induction motor can be controlled by all of the
following
except
(a) changing
supply frequency
(b) changing
number of poles
(c) changing
winding resistance
(d) reducing
supply voltage
Ans: c
40. The
'crawling" in an induction motor is caused by
(a) high
loads
(6) low
voltage supply
(c) improper
design of machine
(d)
harmonics developed in the motor
Ans: d
41. The
power factor of an induction motor under no-load conditions will be closer to
(a) 0.2
lagging
(b) 0.2
leading
(c) 0.5
leading
(d) unity
Ans: a
42. The
'cogging' of an induction motor can be avoided by
(a) proper
ventilation
(b) using
DOL starter
(c)
auto-transformer starter
(d) having
number of rotor slots more or less than the number of stator slots (not equal)
Ans: d
43. If an
induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the
normal
speed, the phenomenon will be termed as
(a) humming
(b) hunting
(c) crawling
(d) cogging
Ans: c
44. Slip of
an induction motor is negative when
(a) magnetic
field and rotor rotate in opposite direction
(b) rotor
speed is less than the syn-chronous speed of the field and are in the same
direction
(c) rotor
speed is more than the syn-chronous speed of the field and are in the same
direction
(d) none of
the above
Ans: c
45. Size of
a high speed motor as compared to low speed motorfor the same H.P. will be
(a) bigger
(b) smaller
(c) same
(d) any of
the above
Ans: b
46. A
3-phase induction motor stator delta connected, is carrying full load and one
of its
fuses blows
out. Then the motor
(a) will
continue running burning its one phase
(b) will
continue running burning its two phases
(c) will
stop and carry heavy current causing permanent damage to its winding
(d) will
continue running without any harm to the winding
Ans: a
47. A
3-phase induction motor delta connected is carrying too heavy load and one of
its
fuses blows
out. Then the
motor
(a) will
continue running burning its one phase
(b) will
continue running burning its two phase
(c) will
stop and carry heavy current causing permanent damage to its winding
(d) will
continue running without any harm to the winding
Ans: c
48. Low
voltage at motor terminals is due to
(a)
inadequate motor wiring
(b) poorely
regulated power supply
(c) any one
of the above
(d) none of
the above
Ans: c
49. In an
induction motor the relationship between stator slots and rotor slots is that
(a) stator
slots are equal to rotor slots
(b) stator
slots are exact multiple of rotor slots
(c) stator
slots are not exact multiple of rotor slots
(d) none of
the above
Ans: c
50. Slip
ring motor is recommended where
(a) speed
control is required
(6) frequent
starting, stopping and reversing is required
(c) high
starting torque is needed
(d) all
above features are required
Ans: d
51. As load
on an induction motor goes on increasing
(a) its power factor goes on
decreasing
(b) its
power factor remains constant
(c) its
power factor goes on increasing even after full load
(d) its
power factor goes on increasing upto full load and then it falls again
Ans: d
52. If a
3-phase supply is given to the stator and rotor is short circuited rotor will
move
(a) in the
opposite direction as the direction of the rotating field
(b) in the
same direction as the direction of the field
(c) in any
direction depending upon phase squence of supply
Ans: b
53. It is
advisable to avoid line starting of induction motor and use starter because
(a) it will
run in reverse direction
(b) it will
pick up very high speed and may go out of step
(c) motor
takes five to seven times its fullload current
(d) starting
torque is very high
Ans: c
54. The
speed characteristics of an induction motor closely resemble the speedload
characteristics
of which of the
following
machines
(a) D.C.
series motor
(b) D.C.
shunt motor
(c)
universal motor
(d) none of
the above
Ans: b
55. Which
type of bearing is provided in small induction motors to support the rotor
shaft
? (
a) Ball
bearings
(b) Cast
iron bearings
(c) Bush
bearings
(d) None of
the above
Ans: a
56. A pump
induction motor is switched on to a supply 30% lower than its rated voltage.
The pump
runs. What will
eventually
happen ? It will
(a) stall
after sometime
(b) stall
immediately
(c) continue
to run at lower speed without damage
(d) get
heated and subsequently get damaged
Ans: d
57. 5 H.P.,
50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m.
Which motor
will be the
costliest ?
(a) 730 r.p.m.
(b) 960
r.p.m.
(c) 1440
r.p.m.
(d) 2880
r.p.m.
Ans: a
58. A
3-phase slip ring motor has
(a) double
cage rotor
(6) wound
rotor
(c)
short-circuited rotor
(d) any of
the above
Ans: b
59. The
starting torque of a 3-phase squirrel cage induction motor is
(a) twice
the full load torque
(b) 1.5
times the full load torque
(c) equal to
full load torque
Ans: b
60.
Short-circuit test on an induction motor cannot be used to determine
(a) windage
losses
(b) copper
losses
(c)
transformation ratio
(d) power
scale of circle diagram
Ans: a
61. In a
three-phase induction motor
(a) iron
losses in stator will be negligible as compared to that in rotor
(6) iron
losses in motor will be neg¬ligible as compared to that in rotor
(c) iron
losses in stator will be less than that in rotor
(d) iron
losses in stator will be more than that in rotor
Ans: d
62. In case
of 3-phase induction motors, plugging means
(a) pulling
the motor directly on line without a starter
(b) locking
of rotor due to harmonics
(c) starting
the motor on load which is more than the rated load
(d) interchanging two supply
phases for quick stopping
Ans: d
63. Which is
of the following data is required to draw the circle diagram for an induction
motor ?
(a) Block
rotor test only
(b) No load
test only
(c) Block rotor
test and no-load test
(d) Block
rotor test, no-load test and stator resistance test
Ans: d
64. In
three-phase induction motors sometimes copper bars are placed deep in the rotor
to
(a) improve
starting torque
(b) reduce
copper losses
(c) improve
efficiency
(d) improve
power factor
Ans: a
65. In a
three-phase induction motor
(a) power
factor at starting is high as compared to that while running
(b) power
factor at starting is low as compared to that while running
(c) power
factor at starting in the same as that while running
Ans: b
66. The
vafcie of transformation ratio of an induction motor can be found by
(a)
open-circuit test only
(b)
short-circuit test only
(c) stator
resistance test
(d) none of
the above
Ans: b
67. The
power scale of circle diagram of an induction motor can be found from
(a) stator
resistance test
(6) no-load
test only
(c)
short-circuit test only
(d) noue of
the above
Ans: c
68. The
shape of the torque/slip curve of induction motor is
(a) parabola
(b)
hyperbola
(c)
rectangular parabola
(d) straigth line
Ans: c
69. A change
of 4% of supply voltage to an induction motor will produce a change of
appromimately
(a) 4% in
the rotor torque
(b) 8% in
the rotor torque
(c) 12% in
the rotor torque
(d) 16% in
the rotor torque
Ans: d
70. The
stating torque of the slip ring induction motor can be increased by adding
(a) external
inductance to the rotor
(b) external
resistance to the rotor
(c) external
capacitance to the rotor
(d) both
resistance and inductance to rotor
Ans: b
71. A 500
kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m.
on
full load.
The machine has 6 poles. The slip of the machine will be
(a) 0.01
(b) 0.02
(c) 0.03
(d) 0.04
Ans: d
72. The
complete circle diagram of induetion motor can be drawn with the help of data
found from
(a) noload
test
(6) blocked
rotor test
(c) stator
resistance test
(d) all of
the above
Ans: d
73. In the
squirrel-cage induction motor the rotor slots are usually given slight skew
(a) to
reduce the magnetic hum and locking tendency of the rotor
(b) to
increase the tensile strength of the rotor bars
(c) to
ensure easy fabrication
(d) none of
the above
Ans: a
74. The
torque of a rotor in an induction motor under running condition is maximum
(a) at the
unit value of slip
(b) at the zero value of slip
(c) at the
value of the slip which makes rotor reactance per phase equal to the resistance
per phase
(d) at the
value of the slip which makes the rotor reactance half of the rotor
Ans: c
75. What
will happen if the relative speed between the rotating flux of stator and rotor
of
the
induction motor is zero ?
(a) The slip
of the motor will be 5%
(b) The
rotor will not run
(c) The
rotor will run at very high speed
(d) The
torque produced will be very large
Ans: b
76. The
circle diagram for an induction motor cannot be used to determine
(a)
efficiency
(b) power
factor
(c)
frequency
(d) output
Ans: a
77. Blocked
rotor test on induction motors is used to find out
(a) leakage
reactance
(b) power
factor on short circuit
(c)
short-circuit current under rated voltage
(d) all of
the above
Ans: d
78.
Lubricant used for ball bearing is usually
(a) graphite
(b) grease
(c) mineral
oil
(d) molasses
Ans: b
79. An
induction motor can run at synchronous speed when
(a) it is
run on load
(b) it is
run in reverse direction
(c) it is
run on voltage higher than the rated voltage
(d) e.m.f.
is injected in the rotor circuit
Ans: d
80. Which
motor is preferred for use in mines where explosive gases exist ?
(a) Air motor
(b)
Induction motor
(c) D.C.
shunt motor
(d)
Synchronous motor
Ans: a
81. The
torque developed by a 3-phase induction motor least depends on
(a) rotor
current
(b) rotor
power factor
(c) rotor
e.m.f.
(d) shaft
diameter
Ans: d
82. In an
induction motor if air-gap is increased
(a) the
power factor will be low
(b) windage
losses will be more
(c) bearing
friction will reduce
(d) copper
loss will reduce In an induction motor
Ans: a
83. In
induction motor, percentage slip depends on
(a) supply
frequency
(b) supply
voltage
(c) copper losses
in motor
(d) none of
the above
Ans: c
84. When /?2
is tne rotor resistance, .X2 the
rotor
reactance at supply frequency
and s the
slip, then the condition for maximum torque under running condi-tions will be
(a) sR2X2 =
1 (b) sR2 = X2
(c) R2 = sX2
id) R2 = s2X2
Ans: c
85. In case
of a double cage induction motor, the inner cage has
(a) high
inductance arid low resistance
(b) low
inductance and high resistance
(c) low
inductance and low resistance
(d) high
inductance and high resis¬tance
Ans: a
86. The low
power factor of induction motor is due to
(a) rotor leakage reactance
(b) stator
reactance
(c) the
reactive lagging magnetizing current necessary to generate the magnetic flux
(d) all of
the above
Ans: d
87.
Insertion of reactance in the rotor circuit
(a) reduces
starting torque as well as maximum torque
(b)
increases starting torque as well as maximum torque
(c)
increases starting torque but maxi-mum torque remains unchanged
(d)
increases starting torque but maxi-mum torque decreases
Ans: a
88. Insertion
of resistance in the rotcir of an induction motor to develop a given torque
(a)
decreases the rotor current
(b)
increases the rotor current
(c) rotor
current becomes zero
(d) rotor
current rernains same
Ans: d
89. For
driving high inertia loods best type of induction motor suggested is
(a) slip
ring type
(b) squirrel
cage type
(c) any of
the above
(d) none of
the above
Ans: a
90.
Temperature of the stator winding of a three phase induction motor is obtained
by
(a)
resistance rise method
(b) thermometer
method
(c) embedded
temperature method
(d) all
above methods
Ans: d
91. The
purpose of using short-circuit gear is
(a) to short
circuit the rotor at slip rings
(b) to short
circuit the starting resis¬tances in the starter
(c) to short
circuit the stator phase of motor to form star
(d) none of
the above
Ans: a
92. In a
squirrel cage motor the induced e.m.f. is
(a)
dependent on the shaft loading
(b) dependent on the number of
slots
(c) slip
times the stand still e.m.f. induced in the rotor
(d) none of
the above
Ans: c
93. Less
maintenance troubles are experienced in case of
(a) slip
ring induction motor
(6) squirrel
cage induction motor
(c) both (a)
and (b)
(d) none of
the above
Ans: b
94. A
squirrel cage induction motor is not selected when
(a) initial
cost is the main consideration
(b)
maintenance cost is to be kept low
(c) higher
starting torque is the main consideration
(d) all
above considerations are involved
Ans: c
95. Reduced
voltage starter can be used with
(a) slip
ring motor only but not with squirrel cage induction motor
(b) squirrel
cage induction motor only but not with slip ring motor
(c) squirrel
cage as well as slip ring induction motor
(d) none of
the above
Ans: c
96. Slip
ring motor is preferred over squirrel cage induction motor where
(a) high
starting torque is required
(b) load
torque is heavy
(c) heavy
pull out torque is required
(d) all of
the above
Ans: a
97. In a
star-delta starter of an induction motor
(a)
resistance is inserted in the stator
(b) reduced
voltage is applied to the stator
(c)
resistance is inserted in the rotor
(d) applied
voltage per1 stator phase is 57.7% of the line voltage
Ans: d
98. The
torque of an induction motor is
(a) directly
proportional to slip
(b)
inversely proportional to slip
(c) proportional to the square
of the slip
(d) none of
the above
Ans: a
99. The
rotor of an induction motor runs at
(a)
synchronous speed
(b) below
synchronous speed
(c) above
synchronous speed
(d) any of
the above
Ans: b
100. The
starting torque of a three phase induction motor can be increased by
(a)
increasing slip
(b)
increasing current
(c) both (a)
and (b)
(d) none of
the above
Ans: c
101.
Insertion of resistance in the stator of an induction motor
(a)
increases the load torque
(b)
decreases the starting torque
(c) increases
the starting torque
(d) none of
the above
Ans: b
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