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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

Three phase induction motor MCQ | Electrical Machine question and answers

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

Write your comment about each phrase

  1. The double squirrel cage induction motor has an advantage over a single cage rotor.
  2. The starting torque for the squirrel induction motor is voltage is reduced to one-half per phase. when the applied rated
  3. The starting current of a 3-phase induction motor is supply voltage.
  4. The starting torque of an induction motor is the maximum when the ...
  5. The insertion of resistance in the rotor circuit during the starting of a slip ring induction motor with a rotor resistance starter results in:
  6. By use of double-cage and deep-bar rotors, squirrel-cage motors can be designed to have the good starting characteristics resulting from... .. rotor resistance.


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