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

[Ziya]

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?

 

[Hesch]

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

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

B_Iq has angle to B_rotor = 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 B_stat, that is perpendicular to the B_rotor.

 

[Cdz]

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

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

B_Iq has angle to B_rotor = 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 B_stat, that is perpendicular to the B_rotor.

Thank you. I got it now.
I am one year too late

 

[jim hardy]

It may help you to imagine a simplified machine with a one turn armature and permanent magnet field
from a thread last year

that old thread addressed armature reaction
but it's a short hop from there to torque.
https://www.physicsforums.com/threads/armature-reaction-drop.826513/#post-5191404