# Three phase induction motor MCQ | Electrical Machine question and answers

1) WHEN A THREE PH ASE INDUCTION MOTOR IS ROTATING AT STEADY STATE. THEN IF THE LOAD IS INCREASED THEN THE SLIP WILL…….. AND THE EFFICIENCY WILL……………
• a. Decrease, decrease.
• b. Decrease, increase.
• c. Increase, increase.
• d. Increase, decrease.

2) A 3-phase IM runs at a slip 5% at normal voltage. For the same load torque, if supply voltage reduced to 80% of normal voltage, the motor slip will be.......
• a. 1.5 %
• b. 6.2 %
• c. 3.9 %
• d. None of these answers

3) frequency of rotor circuit of a standard 3-phase IM is……..at starting and……..at nominal operating speed.
• A. Maximum, maximum.
• B. Minimum, minimum.
• C. Maximum, minimum.
• D. Minimum, maximum.

4) For a three phase induction motor, the slip at maximum developed torque is
• A. Equal to that of max. rotor output power and less than that of max. airgap power
• B. Equal to that of max. rotor input power and greater than that of max. rotor output power
• C. Equal to that of max. rotor input power and less than that of max. developed power
• D. Equal to that of max. rotor output power and greater than that of max. rotor input power

5) A 6-pole, 3-phase IM develops the maximum torque at 1000 rpm when operated from a 60 Hz supply. Rotor resistance per phase is 1.2 ohm. Neglecting stator impedance, the speed at which it will develop the maximum torque when operate at 50 Hz is ----------
• a. 1000 rpm
• b. 833.33 rpm
• c. 800 rpm
• d. None of these answer

6) for standard 3-phase IM, the motor direction of motion is:
• A. Opposite to that of the rotating field.
• B. Same as that of the rotating field.
• C. Depends on supply voltage phase sequence.
• D. Depends on supply voltage phases.

7) In a 3-phase IM, maximum torque of 300 Nm occur at slip of 10%. The torque at slip of 5% would be…..
• a. 120 Nm
• b. 240 Nm
• c. 150 Nm
• d. 300Nm

8) In a 3-phase IM, maximum torque of 300 Nm occur at slip of 10%. The torque at slip of 5% would be…..
• a. 120 Nm
• b. 240 Nm.
• c. 150 Nm
• d. 300 Nm

9) A wound rotor induction motor can be distinguished from squirrel cage induction motor by--------------------
• a. presence of slip-rings
• b. shaft diameter
• c. frame’s structure
• d. all of the above

10) When a three phase induction motor is rotating at steady state. Then if the load is decreased then the efficiency will--------------and slip the will-------------
• a. Decrease, increase.
• b. Increase, decrease.
• c. Decrease, decrease.
• d. Increase, increase.

11) A 3-phase induction motor has a rotor resistance of 0.02 ohm and standstill reactance of 0.1 ohm per phase. The value of total resistance of a starter for the rotor circuit for maximum torque to be exerted at starting------------
• a. 0.02 Ω
• b. 0.06 Ω
• c. 0.04 Ω
• d. 0.08 Ω
12) A 3-phase IM runs at slip of 5% at normal voltage. For the same load torque, if supply voltage reduces to 80% of nominal voltage, the motor slip will be--------------------
• a. 7.8 %
• b. 4%
• c. None of these answer
• d. 3.2%

13) Torque developed by a 3-phase, 400 V, induction motor is 200 Nm. If the supply voltage is reduced to 200 V, the developed torque will be-------------------

14) Torque developed by a 3-phase, 400 V, induction motor is 100 Nm. If the supply voltage is reduced to 200 V, the developed torque will be
• a. 15 Nm
• b. 150 Nm
• c. 25 Nm
• d. 50 Nm

15) The rotor conductors of 3-phase squirrel cage induction motors are-----------
• a. short circuited through resistors.
• b. short-circuited through slip rings
• c. kept open
• d. short-circuited through end rings

16) Under locked rotor condition, the rotor circuit frequency of a 3-phase induction motor connected to a 50 Hz supply will be------------
• a. 30 Hz
• b. 60 Hz
• c. 50 Hz
• d. 3 Hz

17) A 3-phase, 6-pole induction motor is supplied from a 3-phase, 400 V, 50 Hz supply. If it is operating at full-load with 0.04 slip,The speed of the revolving field relative to the stator structure is----------------------
• a. 40 rpm
• b. None of these answers
• c. 1000 rpm
• d. zero rpm

18) Assume that for an induction motor, all losses are taken in consideration. But stator copper losses is equal to rotor copper loss, then the efficiency is---------------------
• a. s
• b. 1-s
• c. None of these answer
• d. (1-s)/(1+s)

## More Three phase induction motor MCQ to practece

In a 3-phase slip ring induction motor, if some resistance is added in the rotor circuit.
• a. its both starting torque and maximum torque will increase
• b. its starting torque will decrease and maximum torque will increase
• c. its starting torque will increase but the maximum torque will remain the same
• d. its starting torque will remain the same but maximum torque will increase

For standard three phase induction motor, the speed of rotoating magnetic field in the rotor is---------
• a. Synchronous speed (Ns)
• b. None of these answers
• c. The motor speed (Nm)
• d. Relative speed (Ns-Nm)

The power input to a 400 V, 50 Hz, 4-pole, 3-phase induction motor running at 1440 rpm is 40 kW. The stator losses are 1 kW, the rotor copper loss will be------------------
• a. 1560
• b. 1600
• c. 1450
• d. 1500

If the electrical torque in IM equal 300 Nm and the slip decreased from 5% to 3% and rotor resistance decreased by the same ratio then the torque will be--------------
• a. 300 Nm
• b. 350 Nm
• c. 250Nm
• d. None of these answer

In a double-cage IM, if it is assumed that the inner-cage has a small cross section and the outer-cage has a large cross section, then the rotor current flows mainly throuth---------------------
• a. Outer cage at starting and inner cage at normal speed
• b. At inner cage all the time
• c. None of these answer
• d. Outer cage at normal speed and inner cage at starting

in a 3 phase double cage induction motor , the rotor flows mainly through
• i. lower cage at starting and Upper cage at normal speed
• ii. Upper cage at starting and Upper cage at normal speed
• iii. Upper cage at starting and lower cage at normal speed
• iv. lower cage at starting and lower cage at normal speed