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Synchronous machine

  1. May 20, 2015 #1
    synchronous machine can run at synchronous speed but why can't induction machine run at synchronous speed???
     
  2. jcsd
  3. May 20, 2015 #2

    Hesch

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    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.
     
  4. May 21, 2015 #3
    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.
     
  5. May 21, 2015 #4

    Hesch

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

    graphics1.png
     
  6. May 21, 2015 #5
    Thank you, Hesch.I agree with you of course.:smile:
     
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