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As the title says, does a sinusoidal input voltage make a BLDC motor nearly into a PMSM?

From Sensorless Trapezoidal Control of BLDC Motors, it follows that a BLDC motor has a trapezoidal back emf while a PMSM has sinusoidal back EMF. But if I use a sinusoidal commutation, will the back EMF of a BLDC motor still be trapezoidal?

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  • $\begingroup$ I think you mean PMSM. The excitation voltage doesn't affect the back EMF, though it will affect the current due to their difference, and hence the torque. So I believe the answer is "no". $\endgroup$ – Brian Drummond Nov 19 '16 at 20:49
  • $\begingroup$ Yea, sorry, I somehow missed the right spelling, I mean PMSM. What I have seen a lot is, that people use the model of a PMSM motor to describe a BLDC motor mathematically, but that seems to be wrong. Especially if we assume a sinusoidal drive, the torque will not be constant for a BLDC motor, but It's nearly constant. When can we assume that a BLDC motor has nearly the characteristics of a PMSM? $\endgroup$ – HansPeterLoft Nov 20 '16 at 8:41
  • $\begingroup$ My prev comment is misleading, sorry, the back EMF will still be trapezoidal so the answer to the specific question is "yes". To the question in the comment : I don't know what you mean by "nearly" the same characteristics. I would expect either motor to work with either driver, but with larger torque ripple (and drive current ripple) and that will affect ultimate performance (top speed, ability to pull heavy loads) etc. $\endgroup$ – Brian Drummond Nov 20 '16 at 13:38
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The back EMF waveshape of a brushless DC motor is a function of the motor geometry. It has nothing to do with how the motor is driven. There may be other signals superimposed on the back EMF, but the back EMF, by definition, is how it acts as a generator.

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