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?
Best Answer
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).