Question

Induction motor rotor always rotate at a speed ________ than synchronous speed
1. same
2. greater
3. less
4. infinite

Answer 1

Correct Answer - Option 3 : less

• Synchronous speed is the speed of rotation of the magnetic field in a rotary machine. An induction motor always runs at a speed less than synchronous speed.
• The rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor rotate. But due to the lagging of rotor flux current with respect to stator flux current, the rotor will never reach its rotating magnetic field speed i.e. the synchronous speed.
• If the motor is running at synchronous speed, then the torque generated by the motor equal to zero. If the rotor runs at synchronous speed as the field of the stator, this will cause the slip to become zero, then there will be no torque generated in the rotor.

 

Important Points:

In a three-phase induction motor,

• The stator is stationary, and the rotor is a rotating part
• Both the stator field and rotor field rotate with synchronous speed
• The rotor rotates with a speed less than synchronous speed
• The difference between the synchronous speed and the rotor speed is known as slip speed.