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Why does q-axis stator current control torque in 3-ph IM?

  1. Aug 16, 2015 #1
    I know that decoupling the 3 phase currents we get the id and iq (in rotor frame) currents which control the flux and torque respectively. But where does the relation come from?
     
  2. jcsd
  3. Aug 16, 2015 #2

    Hesch

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    I think ( don't know for sure ) that if we have an instant stator current, Istat, that induces a B-field ( Bstat ) which has an angle α to Brotor,
    then the Istat is decomposed into Id = Istat * cos α , Iq = Istat * sin α.

    In this way Id will not yield any torque, because BId has an angle to Brotor = 0.

    BIq has angle to Brotor = 90°, so Iq will yield a torque.

    That's why a vector-controlled system tries to control Id to be zero, thus increasing the efficiency of the motor.

    Controlling a synchronous motor, you don't need these Id's and Iq's: You just measure the angular position of the rotor by means of an encoder, then induce a current in the stator that will create a Bstat, that is perpendicular to the Brotor.
     
    Last edited: Aug 16, 2015
  4. Jul 25, 2016 #3

    Cdz

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    Thank you. I got it now.
    I am one year too late :smile:
     
  5. Jul 26, 2016 #4

    jim hardy

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    2016 Award

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