MULTIPLE CHOICE QUESTIONS ON THREE PHASE INDUCTION MOTOR 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
1. (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
2. 2 Hz
3.The shaft of an induction motor is made of high speed steel stainless steel carbon steel
cast iron
3. carbon steel
4.The number of slip rings on a squirrel cage induction motor is usually two three four none
4. 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
5. 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
6. 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
7. directly proportional to slip
8.The term 'cogging' is associated with three phase transformers compound generators D.C. series motors induction motors
8. 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
9. single phase motors
10.Slip rings are usually made of copper carbon phospor bronze aluminium
10. phospor bronze
11.Which of the following methods is easily applicable to control the speed of the squirrelcage 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
11. By changing the number of stator poles
12.The frame of an induction motor is usually made of silicon steel cast iron
aluminium bronze
12. cast iron
13.An induction motor is identical to D.C. compound motor D.C. series motor synchronous motor asynchronous motor
13. 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
14. 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
15. harmonics develped in the motor
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
1.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 Explanation :
2.In the circle diagram for induction motor, the diameter of the circle represents
slip rotor current running torque line voltage
ANSWER : : rotor current Explanation :
3.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 Explanation :
4.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 Explanation :
5.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
6.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 Explanation :
7.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) Explanation :
8.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 Explanation :
9.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 Explanation :
10.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 Explanation :
11.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 Explanation :
12.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 Explanation :
13.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 Explanation :
14.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 Explanation :
15.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 Explanation :
16.When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered
in series in parallel in series-parallel in parallel with stator
ANSWER : : in parallel Explanation :
17.Slip ring motor is recommended where
speed control is required frequent starting, stopping and reversing is required high starting torque is needed all above features are required
ANSWER : : all above features are required Explanation :
18.The 'crawling" in an induction motor is caused by
high loads low voltage supply improper design of machine harmonics developed in the motor
ANSWER : : harmonics developed in the motor Explanation :
19.It is advisable to avoid line-starting of induction motor and use starter because
motor takes five to seven times its full load current it will pick-up very high speed and may go out of step it will run in reverse direction starting torque is very high
ANSWER : : motor takes five to seven times its full load current Explanation :
20.Low voltage at motor terminals is due to
inadequate motor wiring poorely regulated power supply any one of the above
none of the above
ANSWER : : any one of the above Explanation :
21.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 Explanation :
22.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 Explanation :
23.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 Explanation :
24.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 Explanation :
1.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 Explanation :
2.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 Explanation :
3.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 Explanation :
4.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 Explanation :
5.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 Explanation :
6.5 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. Explanation :
7.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 Explanation :
8.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
Explanation :
9.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 Explanation :
10.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 Explanation :
11.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 Explanation :
12.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 Explanation :
13.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
Explanation :
14.A 3-phase slip ring motor has
double cage rotor wound rotor short-circuited rotor any of the above
ANSWER : : wound rotor Explanation :
15.The circle diagram for an induction motor cannot be used to determine
efficiency power factor frequency output
ANSWER : : efficiency Explanation :
16.The shape of the torque/slip curve of induction motor is
parabola hyperbola
rectangular parabola straigth line
ANSWER : : rectangular parabola Explanation :
17.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 Explanation :
18.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 Explanation :
19.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 Explanation :
20.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
21.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 Explanation :
22.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
1.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 Explanation :
2.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 Explanation :
3.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 Explanation :
4.The rotor of an induction motor runs at
synchronous speed below synchronous speed above synchronous speed any of the above
ANSWER : : below synchronous speed
Explanation :
5.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
7.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 Explanation :
8.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 Explanation :
9.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 Explanation :
10.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 Explanation :
11.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 Explanation :
12.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 Explanation :
13.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 Explanation :
14.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
Explanation :
15.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
17.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 Explanation :
18.In induction motor, percentage slip depends on
supply frequency supply voltage copper losses in motor none of the above
ANSWER : : copper losses in motor Explanation :
19.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 Explanation :
20.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 Explanation :
21.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
