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Objective Type Questions

(A)       Choose the Correct Answer:

1.   Which of the following components is usually fabricated out of silicon steel?
a)                 Bearings
b)                Shaft
c)                 Stator core
d)                None of the above.
2.   The frame of an induction motor is usually made at’
a)                 silicon steel
b)                cast iron
c)                 aluminium
d)                bronze.
3.   The shaft of an induction motor must be
a)                 Stiff
b)                flexible
c)                 hollow
d)                any of the above.
4.   The shaft of an induction motor is made of
a)                 high speed steel
b)                stainless steel
c)                 carbon steel
d)                cast iron.
5.   In an induction motor, on no-load the slip is generally
a)                 less than 1%
b)                1.5%
c)                 2%
d)                4%.
6.   In medium sized induction motors, the slip is generally around
a)                 0.04%
b)                0.4%
c)                 4%
d)                14%.
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.
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.
9.   Slip rings are usually made of
a)                 copper
b)                carbon
c)                 phosphor bronze
d)                aluminium.
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
11.   If N, is the synchronous speed and s the slip, then actual running speed of an induction motor will be
a)                 Ns
b)                s.Ns
c)                 (1- s)Ns
d)                (Ns -1)s.
12.   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%.

13.   The number of slip rings on a squirrel cage induction motor is usually
a)                 two
b)                three
c)                 four
d)                none.
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.
15.   A double squirrel-cage induction motor has
a)                 two rotors moving in opposite direction
b)                two parallel windings in stator
c)                 two parallel windings in rotor
d)                two series windings in stator.
16.   The ratio of starting torque to normal torque in case of a star-delta starter will be
a)                 0.35
b)                0.67
c)                 1.07
d)                1.35.
17.   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.
18.   The terms ‘cogging’ is associated with
a)                 three phase transformers
b)                compound generators
c)                 D.C. series motors
d)                induction motors.
19.   In case of the induction motors the torque is
a)                 inversely proportional to ( )
b)                directly proportional to (slip)2
c)                 inversely Proportional to slip
d)                directly Proportional to slip.
20.   Stepless speed control of induction motor is possible by which of the following methods?
a)                 E.m.f. injection in rotor circuit
b)                Changing the number of poles
c)                 Cascade operation
d)                None of the above.

21.   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.
22.   In the circle diagram for induction motor, the diameter of the circle represents
a)                 Slip
b)                rotor current
c)                 running torque
d)                line voltage.
23.   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.
24.   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.
25.   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.
26.   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.
27.   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.

28.   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.
29.   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.
30.   No-load current in an induction motor is nearly
a)                 0.1 per cent of full-load current
b)                1 to 5 per cent of full-load current
c)                 15 to 25 per cent of full-load current
d)                50 to 60 per cent of full-load current
31.   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.
32.   The ‘crawling’ in an induction motor is caused by
a)                 high loads
b)                low voltage supply
c)                 improper design of machine
d)                harmonics developed in the motor.
33.   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.
34.   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).
35.   One phase of a three-phase partly loaded induction motor power supply got opened up while the motor was in motion. This is likely to result in
a)                 stalling of the motor
b)                burning of one phase winding of the motor
c)                 burning of all three phases of winding of the motor
d)                undisturbed running of motor.

36.   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.
37.   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 full-load current
d)                starting torque is very high.
38.   The speed characteristics of an induction motor closely resemble the speed-load 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.
39.   In a slip-ring induction motor, resistance is connected in rotor phases
a)                 to increase starting torque
b)                to limit starting current
c)                 to limit the starting current and increase the starting torque
d)                none of the above.
40.   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.
41.   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 .

42.   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.
43.   A 3-phase slip ring motor has
a)                 double cage rotor
b)                wound rotor
c)                 short-circuited rotor
d)                any of the above.

44.   The torque developed in an induction motor is nearly proportional to
(a)            1/v
(b)             V
(c)             V2
(d)            1/V2
where V = applied voltage.
45.   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:
46.   In a three-phase induction motor
a)                 iron losses in stator will be negligible as compared to that in rotor
b)                iron losses in motor will be negligible 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.
47.   In case of 3-phase induction motors, plugging means
a)                 putting 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.
48.   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.
49.   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.
50.   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 is the same as that while running.
51.   The circle diagram for an induction motor cannot be used to determine
a)                 efficiency
b)                power factor
c)                 frequency
d)                output.
52.   The current taken by a three-phase induction motor during plugging will be
a)                 normal full-load current
b)                more than blocked rotor current
c)                 equal to blocked rotor current
d)                less than blocked rotor current.
53.   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.
54.   Lubricant used for ball bearing is usually
a)                 graphite
b)                grease
c)                 mineral oil
d)                molasses.
55.   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.
56.   Which of the following can be reduced by skewing in induction motor?
a)                 Noise
b)                Vibration
c)                 Synchronous cusps
d)                All of the above.
57.   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.
58.   The torque developed by a 3-phase induction motor least depends on
a)                 rotor current
b)                rotor power factor
c)                 rotor e.mf.
d)                 shaft diameter.
59.   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.

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63.   How many different speeds can be obtained in cascade control of two induction motors?
a)                 2
b)                4
c)                 8
d)                16.

64.   In case of a double cage induction motor, the inner cage has
a)                 high inductance and low resistance
b)                low inductance and high resistance
c)                 low inductance and low resistance
d)                high inductance and high resistance.
65.   Open circuit test is usually conducted on
a)                 slip ring motors only
b)                wound rotor motors only
c)                 either of (a) and (b) above
d)                none of the above.
66.   When two motors of 12 and 8 poles respectively are connected to a 50-Hz supply, which of the follow in no-load speed cannot be obtained?
a)                 300 r.p.m .
b)                450 r.p.m.
c)                 500 r.p.m.
d)                750 r.p.m.
e)                 1500 r.p.m.
67.   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.
68.   During back to back test
a)                 both machines run as induction motors
b)                both machines run as induction generators
c)                 one machine runs as induction motor and the other as induction generator .
d)                one machine runs as induction motor and the other as synchronous generator.
69.   Phase advancers are used with induction motors to
a)                 reduce noise
b)                reduce vibrations
c)                 reduce copper losses
d)                improve power factor.
70.   During dynamic braking of induction motor
a)                 stator connections are reversed
b)                motor is used as induction generator
c)                 stator winding is excited by direct current
d)                fictitious load is applied on motor.
71.   The operation of repeated starting and stopping of an induction motor for exact positioning of the driven equipment is called
a)                 crawling
b)                balancing
c)                 plugging
d)                jogging.

71.   In moist surroundings which frame of motor will be preferred?
a)                 Open type
b)                Totally enclosed fan cooled
c)                 Drip proof
d)                Splash proof.
72.   The principle of operation of a three-phase induction motor closely resembles to that of which of the following?
a)                 Synchronous motor
b)                D.C. series motor
c)                 Transformer with short-circuited secondary
d)                Transformer with open-circuited secondary.
73.   Under which of the following conditions a squirrel-cage induction motor would be preferred over a wound rotor induction motor?
a)                 When external voltage is to be necessarily introduced into the rotor
b)                When wide range of speed control is desired
c)                 When cost is the major consideration
d)                When higher starting torque is required.
74.   Which of the following statement is not valid when a squirrel-cage induction motor operates under no-load?
a)                 Rotor induced e.m.f. is low
b)                Rotor current is low
c)                 Power factor is low
d)                Slip is low.

ANSWERS


1.   (c)
2.   (b)
3.   (a)
4.   (c)
5.   (a)
6.   (c)
7.   (d)
8.   (b)
9.   (c)
10.                (c)
11.                (c)
12.                (b)
13.                (d)
14.                (a)
15.                (c)
16.                (a)
17.                (a)
18.                (d)
19.                (d)
20.                (b)
21.                (b)
22.                (b)
23.                (b)
24.                (a)
25.                (c)
26.                (d)
27.                (b)
28.                (d)
29.                (a)
30.                (d)
31.                (c)
32.                (d)
33.                (a)
34.                (d)
35.                (d)
36.                (c)
37.                (c)
38.                (b)
39.                (b)
40.                (a)
41.                (d)
42.                (a)
43.                (b)
44.                (c)
45.                (a)
46.                (d)
47.                (d)
48.                (d)
49.                (a)
50.                (b)
51.                (a)
52.                (b)
53.                (d)
54.                (b)
55.                (d)
56.                (d)
57.                (a)
58.                (d)
59.                (a)
60.                (b)
61.                (c)
62.                (b)
63.                (a)
64.                (a)
65.                (b)
66.                (d)
67.                (c)
68.                (d)
69.                (c)
70.                (d)
71.                (b)
72.                (c)
73.                (c)
74.                (d)

(B) Say ‘Yes’ or ‘No’:

1.   Three-phase induction ‘motor has a low efficiency.
2.   An induction motor is simply an electric transformer whose magnetic circuit is separated by an air gap into two relatively movable portions, one carrying the primary and the other the secondary winding.
3.   Frames of electrical machines house the stator core.
4.   The number of slots in the rotor should always be equal to the number of slots in the stator.
5.   For large and heavy rotors journal bearings are used.
6.   The slips rings are made of aluminium.
7.   The difference between the synchronous speed and rotor speed is known as slip,
7
9.   Starting torque is inversely proportional to the square of the applied voltage.
10.                Stator iron loss is practically constant.
8
    
12.                Circle diagram of an induction motor can be drawn by using the data obtained from no-load test, short-circuit test and. stator resistance test.
13.                The synchronous speed of the nth order harmonic is 1/nth of the synchronous speed of fundamental.
14.                ‘Cogging’ of squirrel-cage motors can be easily overcome by making the number of rotor slots prime to the number of stator slots.
15.                A double squirrel-cage motor has two independent squirrel-cage windings on the rotor, each having its own set of slots.

ANSWERS

 

1.   No
2.   Yes
3.   Yes
4.   No
5.   Yes
6.   No
7.   Yes
8.   Yes
9.   No
10. Yes
11. No
12. Yes
13. Yes
14. Yes
15.  Yes