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I have a BLDC motor with sinusoidal windings that I drive with FOC technique. With my recent knowledge earned by implementating FOC, I got confused with the typical motor constant given in manufacturer datasheet Kt. With the FOC algorithm I drive the motor using the Id and Iq variables where I try to control Id to go to 0 Amp and to maximize Iq. Id is the current component that generates a magnetic field that is parallel to the rotor magnetic field, thus not generating torque. Iq is the current component that generates a magnetic field that is orthogonal to the rotor magnetic field, thus maximizing the motor torque. To compute those virtual variables I uses the phase current Ia, Ib and Ic.

My question is the following :

Considering the well-know torque equation torque = Kt * I, which current value should I choose to use this equation.

Should it be the DC current consummed ? Should it be the Iq current which is the value producing the torque ?

Knowing that most of the time the current fed to the motor is not entirely used to produce an orthogonal magnetic field (true for FOC and six step commutation), it would mean that this equation is only an estimation of the best case scenario when current use is optimal.

I have the feeling that most documentation refers to the DC current fed to the motor but without consideration of the commutation/control technique used on the particular motor.

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  • \$\begingroup\$ If you're using FOC you can independently control the d-axis and q-axis flux. Even with a permanent magnet machine you can do flux weakening by "fighting" the magnet's flux. However, you need both in order to produce torque, you can't let the d-axis flux go to zero. The torque constant will vary based on how you choose to control the d-axis flux. The stated torque constant probably assumes that the d-axis flux is the nominal permanent magnet flux. \$\endgroup\$
    – John D
    Commented Aug 18, 2023 at 15:44
  • \$\begingroup\$ Thanks for your answer. In typical FOC, the Id current is controlled to reach 0 Amp. I don't get why we can't let the d-axis flux go to zero. Do you have a hint or documentation link to help me understand ? And so you confirm that torque constant equation is actually true only in specific condition ? \$\endgroup\$
    – Victo Rien
    Commented Aug 22, 2023 at 7:11
  • \$\begingroup\$ You can let the Id current go to zero in a permanent magnet machine because the d-axis flux is supplied by the magnets. In that case the specified torque constant is likely accurate. However you don't HAVE to let the d-axis current go to zero. You can fight the magnet flux to get higher speed (lower BEMF) but lower torque for example. In that case the torque constant will be different from the nominal. I don't have a reference handy, but if I can find one I'll link it here. \$\endgroup\$
    – John D
    Commented Aug 22, 2023 at 16:50

1 Answer 1

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As for torque equation you must use Iq. Id must be Zero, i.e. magnetic flux is supported by permanent magnets only. Iq is presented current vector peak value - thus, you must take Kt unit as Nm/Apeak as well.

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