For a project I am working with a three-phase AC brushless motor, connected to a motor driver. The driver allows digital control over the motor in position, velocity or torque modes.

Below are a few points leading up the final question

My understanding of three-phase motors in one sentence: the three phases over a number of coils produce a rotating magnetic field, that rotates the motor shaft with the same speed (synchronous).

  • So the speed of the motor is completely determined by the frequency of the three-phase input. Correct?
  • So the motor driver varies the frequency of the three-phase power going into the motor. Correct?
  • So when the motor is stalling but not off, the frequency in the three phases simply drops to zero, resulting in three different fixed voltages. Correct?

Then how does the motor driver make the motor exert a target torque, independent of the rotational speed?

  • 1
    \$\begingroup\$ Is this a synchronous, or as you imply, an induction motor? That affects the relationship between the rotating field and the shaft speed. \$\endgroup\$
    – Phil G
    Feb 28, 2019 at 17:07
  • 1
    \$\begingroup\$ "AC brushless motors... Like DC motors... are permanent-magnet synchronous motors, or PMSMs, that rely upon magnets built into the rotor." - motioncontrolonline.org/products/ac-brushless-motors.cfm \$\endgroup\$ Feb 28, 2019 at 17:16
  • \$\begingroup\$ Yeah, I think I got some terms confused: I am working with an AC brushless synchronous motor. So it not an induction motor. I'll modify the post. \$\endgroup\$
    – Roberto
    Feb 28, 2019 at 17:59
  • \$\begingroup\$ There are motors with rotating magnets, rotating coils with PM etc and VFD with vector control, but basically forward torque and current control comes with current feedback and RPM feedback by tach or current sensing to regulate the commutated multilevel PWM supply output voltage to regulate current and voltage while accelerating to target speed at some 80% of kV/RPM per pole with 100% at no load.. Stall detection protection is needed. \$\endgroup\$ Feb 28, 2019 at 18:03

1 Answer 1


Torque is given by the strength of the magnetic field, which is proportional to current, and the phase offset between the current position and the set position.

If the motor driver advances the phase of the three phase voltage (i.e. the set position), but the motor will not turn, torque will drop back to zero at the point where the electromagnetic field is exactly opposite the permanent magnets' field, then rise again in the opposite direction (because the shortest way from the current to the set position is now to go backwards), go to zero when the set position reaches the current position (phase angle is zero) and then rise again in forward direction.

For a motor that is held in place, torque will follow a sine wave pattern (more or less, the geometry of the fields is not exactly uniform).

  • \$\begingroup\$ Thanks, that makes sense. Do you think it's likely an advanced motor driver could prevent this, by acting on the rotation speed? \$\endgroup\$
    – Roberto
    Feb 28, 2019 at 18:03
  • \$\begingroup\$ @Roberto, yes, motor drivers measure back EMF from the motor and adjust or warn accordingly (the voltage-to-current curve is different depending on how much energy is transferred to the rotor). \$\endgroup\$ Feb 28, 2019 at 18:13

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