# MCQ On Induction Motor: Q & A On Induction Motors

MCQ On Induction Motor. This section has Questions and Answers related to Induction Motors in Electrical Engineering.

## MCQ On Induction Motor

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

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. any of the above
D. none of the above

A. slip ring type

Insertion of resistance in the rotor 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 remains same

D. rotor current remains same

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

A. reduces starting torque as well as maximum torque

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

D. all 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. sR2X2 = 1
B. sR2 = X2
C. R2 = sX2
D. R2 = s2X2

C. R2 = sX2

Less maintenance troubles are experienced in case of ?

A. slip ring induction motor
B. squirrel cage induction motor
C. both A. and (b)
D. none of the above

B. squirrel cage induction motor

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 resistance

A. high inductance arid low resistance

MCQ On Induction Motor

The purpose of using short-circuit gear is ?

A. to short circuit the rotor at slip rings
B. to short circuit the starting resistances in the starter
C. to short circuit the stator phase of motor to form star
D. none of the above

A. to short circuit the rotor at slip rings

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

D. 0.04

The power scale of circle diagram of an induction motor can be found from ?

A. stator resistance test
C. short-circuit test only
D. noue of the above

C. short-circuit test only

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

A. Air motor

A change of 4% of supply voltage to an induction motor will produce a change of approximately ?

A. 4% in the rotor torque
B. 8% in the rotor torque
C. 12% in the rotor torque
D. 16% in the rotor torque

D. 16% in the rotor torque

The circle diagram for an induction motor cannot be used to determine ?

A. efficiency
B. power factor
C. frequency
D. output

A. efficiency

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

B. short-circuit test only

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

D. all of the above

MCQ On Induction Motor

The complete circle diagram of induetion motor can be drawn with the help of data found from ?

B. blocked rotor test
C. stator resistance test
D. all of the above

D. all of the above

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

A. improve starting torque

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

A. to reduce the magnetic hum and locking tendency of the rotor

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

C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase

Which is of the following data is required to draw the circle diagram for an induction motor ?

A. Block rotor test only
C. Block rotor test and no-load test
D. Block rotor test, no-load test and stator resistance test

D. Block rotor test, no-load test and stator resistance test

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

B. The rotor will not run

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

A. windage losses

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

C. will stop and carry heavy current causing permanent damage to its winding

MCQ On Induction Motor

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.

A. 730 r.p.m.

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

A. will continue running burning its one phase

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

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. Cast iron bearings
C. Bush bearings
D. None of the above

A. Ball bearings

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

B. D.C. shunt motor

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

C. motor takes five to seven times its full load current

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

C. crawling

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
D. None of the above

B. in the same direction as the direction of the field

MCQ On Induction Motor

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)

D. having number of rotor slots more or less than the number of stator slots (not equal)

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 up to full load and then it falls again

D. its power factor goes on increasing up to full load and then it falls again

The power factor of an induction motor under no-load conditions will be closer to ?

A. 0.2 lagging
D. unity

A. 0.2 lagging

In the circle diagram for induction motor, the diameter of the circle represents ?

A. slip
B. rotor current
C. running torque
D. line voltage

B. rotor current

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
D. None of the above

B. Changing the number of poles

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

A. motor takes five to seven times its full load current

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

B. in parallel

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

D. equal to number of poles in stator

MCQ On Induction Motor

The torque developed in the cage induction motor with autostarter is ?

A. k/torque with direct switching
B. K x torque with direct switching
C. K2 x torque with direct switching
D. k2/torque with direct switching

C. K2 x torque with direct switching

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

C. A. and B. both

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

A. By changing the number of stator poles

The term cogging is associated with ?

A. three phase transformers
B. compound generators
C. D.C. series motors
D. induction motors

D. induction motors

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

C. (l-s)Ns

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