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Does the brushless servo motor need to measure the phase voltage? What is the pu

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My friend and I discussed about the voltage measurement of the servo drive. The point we are arguing about is whether it is necessary to measure the three-phase voltage? What is the purpose of measurement?

David Hess:
The phase between the stator and rotor has to be measured in one way or another for proper commutation.

Well, you're generating the voltages. If you trust your drive, why measure them? And if you don't trust them, something is wrong in your concept.


--- Quote from: David Hess on December 02, 2021, 08:03:33 pm ---The phase between the stator and rotor has to be measured in one way or another for proper commutation.

--- End quote ---

I'm inclined to ask why?  My train of thought follows Benta's - the brushless motors I've seen have all been relatively small, and whatever feed back was used, was what I would call "positional" - the controller would drive the motor until whatever was being moved reached the desired location - if it stalled or jammed it would be apparent from the lack of a position change.

BLDC PMSM and Stepper motors are all pretty much the same.

Most stepper motors are 2 phase, but 3-phase (ans also 5-phase) stepper motors also exist.

If you take a BLDC motor, and push a DC current to one or two of it's coils, then the motor will "lock" in a fixed position, just like a stepper motor. Current (power) consumption without rotation (= no output power) equals to very low efficiency.

All these motors have the highest efficiency if there is a 90 degree phase difference between the mechanical and the electric position of the magnetic fields, and it is the job of the motor controller to try to maintain this 90 degree phase difference, and to do that it needs some form of feedback.

This feedback is commonly done with Hall sensors (for "BLDC" motors) or with an encoder ( for "PMSM", "Servo" and "Steppermotors with feedback".

It can also be done "sensorless", by measuring the back EMF of the motor coil that does not have current. A limitation of this method is that there is no back EMF (or too little) when the motor is rotating too slow, but it can work nicely for some applications such as for example motors of quad copters.
This technique is called "FOC" (Field Oriented Control) and there is a lot of literature written about this.

Even Brushed DC motors are very similar. The main difference with brushed DC motors is that this phase shift is maintained by the mechanical position of the brushes.


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