Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Gold Member

    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


    User Avatar

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

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted