It is easy to find BLDC motor manufacturers on google, but it is difficult to find PMSM. Maybe there is a terminology issue here. There are many "Brushless Servo Motors" or "Synchronous Servo Motors" on google, do manufacturers use these terminologies to refer to PMSM motor?

In theory: BLDC has concentrated stator windings and trapezoidal back EMF, the controling method is called "6-step commutation". PMSM has distributed stator windings and sinusoidal back EMF, the controling method is called "FOC".

In reality: Most so called BLDC motors on the market have sinusoidal back EMF, and can be controlled by the same FOC method as PMSM motor. But I think they are still BLDC motor, not PMSM.

My question is: Is "Brushless/Synchronous Servo Motors" more like the theoretical PMSM motor? What does manufacturer call PMSM motor?

Thank you.

  • \$\begingroup\$ It's kind of Ironic that you come here to find out the differences between PMSM and BLDC and you can't find the term, but no where do you actually define it. Put the definitions of PMSM in your write up. \$\endgroup\$ – placeholder Oct 28 '14 at 14:31
  • \$\begingroup\$ Specifically, why do you need a PMSM and why is a BLDC motor insufficient? \$\endgroup\$ – Brian Drummond Oct 28 '14 at 14:31
  • \$\begingroup\$ PMSM = Permanent Magnet Synchronous Motor. This exact question came up a couple of days ago. \$\endgroup\$ – Brian Drummond Oct 28 '14 at 14:32

The terminology for brushless, permanent magnet motors is confusing.

If you look in academic/technical literature like IEEE papers, then generally BLDC refers to brushless PM motors that have a trapezoidal back-emf and is driven by a six-step, trapezoidal drive, while PMSM refers to brushless PM motors that have a sinusoidal back-emf and are driven by sinusoidal waveforms. Be aware that brushless motors with a trapezoidal back-emf can be driven by sinusoidal waveforms and vice versa. And also be aware that trapezoidal and sinusoidal back-emf's are ideals and you can never really get either one. Of course, I've also seen IEEE papers that refer to BLAC motors and use other terminology, so this isn't strict across the board.

Industry hasn't really adopted this terminology completely. You often will see companies refer to BLDC motors, as you've already pointed out. And generally by BLDC they mean exactly what the academics mean - a brushless motor with a trapezoidal back-emf. However, I've also seen these referred to DC brushless (DCB) motors, brushless PM (BPM) motors, or even PMSM's.

With what academic literature refers to as PMSM's, I've seen them called PMSM's, brushless AC (BLAC) motors, AC servo motors, brushless servomotor (BLSM) and others.

Some manufacturers may not make a distinction between the 2 because in reality it isn't an either/or thing. You can't make a brushless motor with a perfect trapezoidal back-emf and you can't make one with a perfect sinusoidal back-emf. Your best bet is to talk directly to manufacturers and tell them what you want to do and they will guide you in the right direction.

In reality: Most so called BLDC motors on the market have sinusoidal back EMF, and can be controlled by the same FOC method as PMSM motor. But I think they are still BLDC motor, not PMSM.

This may or may not be true. In my experience, BLDC motors do not have sinusoidal back-emf; they are much closer to trapezoidal. Keep in mind that we are talking about the phase back-emf, not the line-to-line back-emf. Sometimes the line-to-line back-emf looks close to sinusoidal while the phase back-emf doesn't.

  • \$\begingroup\$ Thank you for your thorough explaining. I think BLAC(PMSM) has smoother performance than BLDC, is it true? \$\endgroup\$ – Ron Mao Oct 29 '14 at 0:44
  • \$\begingroup\$ +1. Good explanations. Even though explanations probably make it worse :-) \$\endgroup\$ – Russell McMahon Oct 29 '14 at 13:20
  • \$\begingroup\$ @RonMao - The "smoothness" of the performance of a brushless motor depends on how much torque ripple it has. Torque ripple is caused by cogging torque, a mismatch between back-emf shape and current shape, harmonics, non-ideal commutation and other factors. Ideally, you can get ripple-free torque with both BLDC (square-wave back-emf and currents) and BLAC (sinusoidal back-emf and currents) motors/drives, but in practice it is easier to manufacture motors and drives that produce sinusoidal back-emf's and currents, so BLAC's are generally used when you need "smooth performance." \$\endgroup\$ – Eric Oct 29 '14 at 15:18

For starters:

  1. PMSM: Permanent Magnet Synchronous Machine
  2. BLDC: Brushless Direct Current Machine
  3. BLAC: Brushless Alternating Current Machine

Here is the thing. These three types of machines are essentially the same. PMSM is an umbrella term covering BLDC and BLAC as these two machine types are synchronous machines and differ only in their magnetic circuits (producing a trapzoidal shaped BackEMF or sinus)

You can control a BLDC machine with a SVM sinus controller and equally a BLAC machine with a quasi-squarewave controller. They may not achieve the same performance results (as the stator flux will not match the airgap flux) but they will work


This is NOT my area of expertise. But I don't think there is a terminology issue. I think there are a lot more applications for so-called brushless DC motors than so-called permanent magnet synchronous motors. Consequently, there are more offerings of them. One small tip, if you go to a manufacturer's website, the PMSM's are under AC motors, and the BLDC's are under DC motors, or their own category.


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