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The differences between BLDC and PMSM are very small and for what I have seen while reading about it, is not clear to many of us. Most of the articles I have read say that the main difference is that the BEMF of a BLDC motor looks more trapezoidal and the one for PMSM looks more sinusoidal, however I can't understand what is the difference on their construction; is it the wounding? their geometry?...

The main reason for my question is that I want to understand which control method should be used on each motor to have the best performance of if any control method can be used on both.

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In a BLDC motor, the windings are concentrated on salient poles, so producing a voltage waveform that's more trapeziodal than sinusoidal.

In a PMSM, the windings are distributed over several poles, so producing a voltage waveform that's more more sinusoidal.

These differences mean that BLDC motors are slightly cheaper to produce, but may be slightly less efficient, noisier and have more cogging, than PMSMs.

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  • \$\begingroup\$ Thanks for your answer. My concern now is about the control method that sould be used. I understand that it would be a trapezoidal commutation for BLDC and a sinusoidal commutation for PMSM, but I just found a solution from Allegro MicroSystems that uses sinusoidal drive for a BLDC... which makes it confusing. \$\endgroup\$ – E. Mac Oct 23 '18 at 13:27
  • \$\begingroup\$ While there's some effort put into making the emf waveform trapezoidal on BLDCs, it often ends up somewhere between the two, resulting in there being little difference in the efficiency of operation using either sine or trap - but sine drives tend to be quieter operating, which is a factor in some applications. \$\endgroup\$ – Phil G Oct 23 '18 at 14:14
  • \$\begingroup\$ The voltage produced by a BLDC vs a PMSM running as a generator may indeed be shown by either sinusoidal or trapezoidal OUTPUT. This has NOTHING to do with how you elect to drive the motors. You can drive with any 3 phase signal you want (Trap or Sine) including PWM. The PMSM will typically give smother positional between pole positions, so is excellent for servo applications (3 phase stepper motor), while the typical BLDC will have non-linear spacing of the angular positions when used as a servo. The differences are actually less significant than you think if used in a closed loop servo. \$\endgroup\$ – Jack Creasey Oct 23 '18 at 17:16

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