Synchronous Machines: Why Can't Induction Machines Run at Synchronous Speed?

[smruti]

synchronous machine can run at synchronous speed but why can't induction machine run at synchronous speed?

 

[Hesch]

why can't induction machine run at synchronous speed?

Asynchronous motors contain a rotor with a squirrel cage. When the cage has a different rotational speed than the speed of magnet field from the stator, an emf will be induced in cage, which results in a current. This current magnetizes the rotor, and a torque arises.

The difference in speeds is called the slip, which by synchronous speed = 0. No slip → no emf → no current → no magnetic field in rotor → no torque.

Synchronous motors can only run at synchronous speed.

 

[Babadag]

The force exercised on a wire through which a current flows in a magnetic field it is F=K*I*B- B=magnetic flux density [induction].Based on Newton second law the action [force]=reaction[force] that means this force acts on both-stator and rotor and rotor rotates.
In a synchronous machine we have a magnetic field in the rotor.
In asynchronous machine we haven’t. In order to create a magnetic field we need current. If the rotor run with the same velocity as the magnetic field produced by stator –as Hesch explained it very well-no current will appear in the rotor winding-squirrel cage or wounded rotor or else- that means no force will act on the rotor.
The speed difference-the slip=[synchronous speed-rotor speed]/synchronous speed-will produce a current at a low frequency =slip*supply frequency. Higher slip higher force.
As the load is increasing the required force increases and then the slip increases.

 

[Hesch]

Higher slip higher force.
As the load is increasing the required force increases and then the slip increases.

Well, yes, with some limitations. Typical torque characteristic for an asynchronous motor:

 

[Babadag]

Thank you, Hesch.I agree with you of course.