Polyphase Induction Motors Mcqs – (Electrical Engineering) MCQs Latest For FPSC, PPSC, NTS, KPPSC, SPSC & Other Tests
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Insertion of resistance in the stator of an induction motor___________?
A. increases the starting torque
B. decreases the starting torque
C. increases the load torque
D. none of the above
The starting torque of a three phase induction motor can be increased by__________?
A. increasing current
B. increasing slip
C. both A. and (b)
D. none of the above
The rotor of an induction motor runs at__________?
A. above synchronous speed
B. below synchronous speed
C. synchronous speed
D. any of the above
In a star-delta starter of an induction motor___________?
A. reduced voltage is applied to the stator
B. resistance is inserted in the stator
C. resistance is inserted in the rotor
D. applied voltage perl stator phase is 57.7% of the line voltage
Slip ring motor is preferred over squirrel cage induction motor where___________?
A. high starting torque is required
B. heavy pull out torque is required
C. load torque is heavy
D. all of the above
Reduced voltage starter can be used with_____________?
A. squirrel cage induction motor only but not with slip ring motor
B. slip ring motor only but not with squirrel cage induction motor
C. squirrel cage as well as slip ring induction motor
D. none of the above
A squirrel cage induction motor is not selected when___________?
A. maintenance cost is to be kept low
B. initial cost is the main consideration
C. higher starting torque is the main consideration
D. all above considerations are involved
Less maintenance troubles are experienced in case of___________?
A. squirrel cage induction motor
B. slip ring induction motor
C. both A. and (b)
D. none of the above
In a squirrel cage motor the induced e.m.f. is___________?
A. dependent on the number of slots
B. dependent on the shaft loading
C. slip times the stand still e.m.f. induced in the rotor
D. none of the above
The purpose of using short-circuit gear is___________?
A. to short circuit the rotor at slip rings
B. to short circuit the stator phase of motor to form star
C. to short circuit the starting resistances in the starter
D. none of the above
Temperature of the stator winding of a three phase induction motor is obtained by___________?
A. thermometer method
B. resistance rise method
C. embedded temperature method
D. all above methods
For driving high inertia loads best type of induction motor suggested is_____________?
A. slip ring type
B. squirrel cage type
C. Both of the above
D. none of the above
Insertion of resistance in the rotor of an induction motor to develop a given torque?
A. increases the rotor current
B. decreases the rotor current
C. rotor current becomes zero
D. rotor current rernains same
Insertion of reactance in the rotor circuit_______?
A. reduces starting torque as well as maximum torque
B. increases starting torque but maxi-mum torque decreases
C. increases starting torque but maxi-mum torque remains unchanged
D. increases starting torque as well as maximum torque
An induction motor can run at synchronous speed when____________?
A. it is run in reverse direction
B. it is run on load
C. it is run on voltage higher than the rated voltage
D. e.m.f. is injected in the rotor circuit
The low power factor of induction motor is due to___________?
A. stator reactance
B. rotor leakage reactance
C. the reactive lagging magnetizing current necessary to generate the magnetic flux
D. all of the above
In induction motor, percentage slip depends on__________?
A. supply voltage
B. supply frequency
C. copper losses in motor
D. none of the above
When /?2 is the rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be_____________?
A. sR2 = X2
B. sR2X2 = 1
C. R2 = sX2
D. R2 = s2X2
In case of a double cage induction motor, the inner cage has__________?
A. high inductance arid low resistance
B. low inductance and low resistance
C. low inductance and high resistance
D. high inductance and high resistance
In an induction motor if air-gap is increased__________?
A. the power factor will be low
B. bearing friction will reduce
C. windage losses will be more
D. copper loss will reduce In an induction motor
The torque developed by a 3-phase induction motor least depends on__________?
A. rotor power factor
B. rotor current
C. rotor e.m.f.
D. shaft diameter
Which motor is preferred for use in mines where explosive gases exist ?
A. Air motor
B. Synchronous motor
C. D.C. shunt motor
D. Induction motor
Lubricant used for ball bearing is usually___________?
A. mineral oil
B. grease
C. graphite
D. molasses
The circle diagram for an induction motor cannot be used to determine____________?
A. efficiency
B. frequency
C. power factor
D. output
Blocked rotor test on induction motors is used to find out?
A. power factor on short circuit
B. leakage reactance
C. short-circuit current under rated voltage
D. all of the above
The complete circle diagram of induction motor can be drawn with the help of data found from__________?
A. blocked rotor test
B. noload test
C. stator resistance test
D. all of the above
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 ensure easy fabrication
C. to increase the tensile strength of the rotor bars
D. none of the above
The torque of a rotor in an induction motor under running condition is maximum__________?
A. at the zero value of slip
B. at the unit 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
What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
A. The rotor will run at very high speed
B. The rotor will not run
C. The slip of the motor will be 5%
D. The torque produced will be very large
The stating torque of the slip ring induction motor can be increased by adding__________?
A. external capacitance to the rotor
B. external resistance to the rotor
C. external inductance to the rotor
D. both resistance and inductance to rotor
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. 0.02
B. 0.01
C. 0.03
D. 0.04
The power scale of circle diagram of an induction motor can be found from__________?
A. no-load test only
B. stator resistance test
C. short-circuit test only
D. noue of the above
A change of 4% of supply voltage to an induction motor will produce a change of approximately ________?
A. 8% in the rotor torque
B. 4% in the rotor torque
C. 12% in the rotor torque
D. 16% in the rotor torque
The shape of the torque/slip curve of induction motor is__________?
A. hyperbola
B. parabola
C. rectangular parabola
D. straigth line
The vafcie of transformation ratio of an induction motor can be found by__________?
A. stator resistance test
B. short-circuit test only
C. open-circuit test only
D. none of the above
In a three-phase induction motor__________?
A. power factor at starting in the same as that while running
B. power factor at starting is low as compared to that while running
C. power factor at starting is high as compared to that while running
In three-phase induction motors sometimes copper bars are placed deep in the rotor to___________?
A. improve starting torque
B. improve efficiency
C. reduce copper losses
D. improve power factor
Which is of the following data is required to draw the circle diagram for an induction motor ?
A. No load test only
B. Block rotor test only
C. Block rotor test and no-load test
D. Block rotor test, no-load test and stator resistance test
In case of 3-phase induction motors, plugging means__________?
A. locking of rotor due to harmonics
B. pulling the motor directly on line without a starter
C. starting the motor on load which is more than the rated load
D. interchanging two supply phases for quick stopping
In a three-phase induction motor_______________?
A. iron losses in stator will be negligible as compared to that in rotor
B. iron losses in stator will be less than that in rotor
C. iron losses in motor will be negligible as compared to that in rotor
D. iron losses in stator will be more than that in rotor
Short-circuit test on an induction motor cannot be used to determine___________?
A. windage losses
B. transformation ratio
C. copper losses
D. power scale of circle diagram
The starting torque of a 3-phase squirrel cage induction motor is____________?
A. equal to full load torque
B. 1.5 times the full load torque
C. twice the full load torque
A 3-phase slip ring motor has____________?
A. short-circuited rotor
B. wound rotor
C. double cage rotor
D. any of the above
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. 1440 r.p.m.
C. 960 r.p.m.
D. 2880 r.p.m.
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 immediately
B. stall after sometime
C. continue to run at lower speed without damage
D. get heated and subsequently get damaged
Which type of bearing is provided in small induction motors to support the rotor shaft ?
A. Ball bearings
B. Bush bearings
C. Cast iron bearings
D. None of the above
The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines?
A. universal motor
B. D.C. shunt motor
C. D.C. series motor
D. none of the above
It is advisable to avoid line starting of induction motor and use starter because____________?
A. starting torque is very high
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. it will run in reverse direction
If a 3-phase supply is given to the stator and rotor is short circuited rotor will move___________?
A. in any direction depending upon phase squence of supply
B. in the same direction as the direction of the field
C. in the opposite direction as the direction of the rotating field
As load on an induction motor goes on increasing______________?
A. its power factor remains constant
B. its power factor goes on decreasing
C. its power factor goes on increasing even after full load
D. its power factor goes on increasing up to full load and then it falls again
Slip ring motor is recommended where____________?
A. high starting torque is needed
B. frequent starting, stopping and reversing is required
C. speed control is required
D. all above features are required
In an induction motor the relationship between stator slots and rotor slots is that_____________?
A. stator slots are exact multiple of rotor slots
B. stator slots are equal to rotor slots
C. stator slots are not exact multiple of rotor slots
D. none of the above
Low voltage at motor terminals is due to____________?
A. poorely regulated power supply
B. inadequate motor wiring
C. any one of the above
D. none of the above
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 two phase
B. will continue running burning its one phase
C. will stop and carry heavy current causing permanent damage to its winding
D. will continue running without any harm to the winding
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 without any harm to the winding
C. will stop and carry heavy current causing permanent damage to its winding
D. will continue running burning its two phases
Size of a high speed motor as compared to low speed motor for the same H.P. will be__________?
A. same
B. smaller
C. bigger
D. any of the above
Slip of an induction motor is negative when_____________?
A. rotor speed is less than the synchronous speed of the field and are in the same direction
B. magnetic field and rotor rotate in opposite direction
C. rotor speed is more than the synchronous speed of the field and are in the same direction
D. none of the above
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. hunting
B. humming
C. crawling
D. cogging
The ‘cogging’ of an induction motor can be avoided by____________?
A. using DOL starter
B. proper ventilation
C. auto-transformer starter
D. having number of rotor slots more or less than the number of stator slots (not equal)
The power factor of an induction motor under no-load conditions will be closer to_____________?
A. 0.2 lagging
B. 0.5 leading
C. 0.2 leading
D. unity
The ‘crawling” in an induction motor is caused by___________?
A. low voltage supply
B. high loads
C. improper design of machine
D. harmonics developed in the motor
The speed of a squirrel-cage induction motor can be controlled by all of the following except_____________?
A. changing number of poles
B. changing supply frequency
C. changing winding resistance
D. reducing supply voltage
DOL starting of induction motors is usually restricted to_____________?
A. low horsepower motors
B. high horsepower motors
C. variable speed motors
D. high speed motors
In a three-phase induction motor, the number of poles in the rotor winding is always__________?
A. zero
B. less than number of poles in stator
C. more than the number of poles in stator
D. equal to number of poles in stator
In three-phase squirrel-cage induction motors____________?
A. rotor conductors are kept open
B. rotor conductors are short-circuited through end rings
C. rotor conductor ends are short-circuited through slip rings
D. rotor conductors are connected to insulation
The maximum torque in an induction motor depends on___________?
A. square of supply voltage
B. rotor inductive reactance
C. frequency
D. all of the above
An induction motor is__________?
A. self-starting with high torque
B. self-starting with zero torque
C. self-starting with low torque
D. non-self starting
If any two phases for an induction motor are interchanged_______?
A. the motor will run in reverse direction
B. the motor will not run
C. the motor will run at reduced speed
D. the motor will burn
Rotor rheostat control method of speed control is used for____________?
A. squirrel-cage induction motors only
B. slip ring induction motors only
C. both
D. none
For which motor the speed can be controlled from rotor side ?
A. Squirrel-cage induction motor
B. Slip-ring induction motor
C. Both
D. None
In the circle diagram for induction motor, the diameter of the circle represents____________?
A. running torque
B. rotor current
C. slip
D. line voltage
Stepless speed control of induction motor is possible by which of the following methods ?
A. Cascade operation
B. Changing the number of poles
C. e.m.f. injection in rotor eueuit
D. None of the above
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. starting torque is very high
D. it will run in reverse direction
When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered__________?
A. in series-parallel
B. in parallel
C. in series
D. in parallel with stator
The torque developed in the cage induction motor with autostarter is___________?
A. K x torque with direct switching
B. k/torque with direct switching
C. K2 x torque with direct switching
D. k2/torque with direct switching
The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type__________?
A. delta connected only
B. star connected only
C. A. and B. both
D. none of the above
The crawling in the induction motor is caused by____________?
A. high loads
B. low voltage supply
C. harmonics develped in the motor
D. improper design of the machine
E. none of the above
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. By operating two motors in cascade
C. Rotor rheostat control
D. By injecting e.m.f. in the rotor circuit
An induction motor is identical to_________?
A. D.C. series motor
B. D.C. compound motor
C. synchronous motor
D. asynchronous motor
The injected e.m.f. in the rotor of induction motor must have__________?
A. the same phase as the rotor e.m.f.
B. the same frequency as the slip frequency
C. zero frequency
D. high value for the satisfactory speed control
In case of the induction motors the torque is___________?
A. directly proportional to (slip)2
B. inversely proportional to (Vslip)
C. inversely proportional to slip
D. directly proportional to slip
The good power factor of an induction motor can be achieved if the average flux density in the air gap is___________?
A. large
B. small
C. absent
D. infinity
A double squirrel-cage induction motor has______________?
A. two parallel windings in stator
B. two rotors moving in oppsite direction
C. two parallel windings in rotor
D. two series windings in stator
Star-delta starting of motors is not possible in case of____________?
A. single phase motors
B. low horse power motors
C. variable speed motors
D. high speed motors
The starting torque of a squirrel-cage induction motor is___________?
A. low
B. same as full-load torque
C. negligible
D. slightly more than full-load torque
In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be___________?
A. s.N,
B. Ns
C. (l-s)Ns
D. (Ns-l)s
Slip rings are usually made of___________?
A. carbon
B. copper
C. phospor bronze
D. aluminium
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. the windage losses will increase
D. speed of motor will increase
In squirrel cage induction motors, the rotor slots are usually given slight skew in order to__________?
A. reduce eddy currents
B. reduce windage losses
C. reduce accumulation of dirt and dust
D. reduce magnetic hum
In medium sized induction motors, the slip is generally around___________?
A. 0.4%
B. 0.04%
C. 4%
D. 14%
Which of the following component is usually fabricated out of silicon steel ?
A. Shaft
B. Bearings
C. Statorcore
D. None of the above
An induction motor with 1000 r.p.m. speed will have____________?
A. 4 poles
B. 6 poles
C. 8 poles
D. 2 poles
The term ‘cogging’ is associated with___________?
A. compound generators
B. three phase transformers
C. D.C. series motors
D. induction motors
The number of slip rings on a squirrel cage induction motor is usually____________?
A. three
B. two
C. four
D. none
The efficiency of an induction motor can be expected to be nearly____________?
A. 95 to 98%
B. 80 to 90%
C. 60 to 90%
D. 99%
A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be____________?
A. 50 Hz
B. 200 Hz
C. 2 Hz
D. 0.2 Hz
In case the air gap in an induction motor is increased?
A. speed of motor will increase
B. the power factor will decrease
C. the magnetising current of the rotor will decrease
D. the windage losses will increase
In an induction motor, no-load the slip is generally____________?
A. less than 1%
B. 2%
C. 1.5%
D. 4%
The shaft of an induction motor is made of_____________?
A. stainless steel
B. high speed steel
C. carbon steel
D. cast iron
The torque of an induction motor is___________?
A. directly proportional to slip
B. proportional to the square of the slip
C. inversely proportional to slip
D. none of the above
The frame of an induction motor is usually made of____________?
A. aluminium
B. cast iron
C. silicon steel
D. bronze