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