- #1
smruti
- 20
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synchronous machine can run at synchronous speed but why can't induction machine run at synchronous speed?
Synchronous Machines: Why Can't Induction Machines Run at Synchronous Speed?
- Thread starter smruti
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Induction machines cannot run at synchronous speed due to their reliance on slip, which is the difference between the rotor speed and the synchronous speed of the magnetic field. In asynchronous motors, the rotor does not have a magnetic field until a current is induced, which occurs only when there is slip. If the rotor matches the synchronous speed, no current flows, resulting in no magnetic field and consequently no torque. Synchronous machines, on the other hand, maintain a magnetic field in the rotor, allowing them to operate at synchronous speed. The discussion emphasizes that increased load leads to higher slip, which in turn increases the required torque and current in induction machines.
- #2
Hesch
Gold Member
- 921
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smruti said:
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.
- #3
Babadag
- 611
- 162
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.
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.
- #4
Hesch
Gold Member
- 921
- 152
Babadag said:
Well, yes, with some limitations. Typical torque characteristic for an asynchronous motor:
- #5
Babadag
- 611
- 162
Thank you, Hesch.I agree with you of course.