In the manual for a Scott Drive motor controller, they have a parameter you can set, called Motor Type. Its description reads

The motor type determines the method used to calculate the reference Id, Iq currents. The four supported motor types are Brushless DC (BLDC), Permanent Magnet Synchronous Machine (PMSM), Interior Permanent Magnet Synchronous Machine (IPMSM), and Induction.

I understand that Induction is the odd one from the group, as its drive algorithm is completely different. However I have a hard time assimilating the difference between the rest (since they are all permanent magnet machines).

Googling around I found:

  • BLDC is the one with trapezoidal back-emf, meaning simplified driving (it can be an AC square-wave)
  • PMSM has sinusoidal back-emf, so it needs sine-shaped drive
  • IPMSM is only different in that the magnets are buried in the rotor (for added strength). The back-emf should be sine (right?). As such I don't get how the drive algorithm differs from PMSM.

How is the drive algorithm different between PMSM and IPMSM? Is it only minor tuning constants?

  • \$\begingroup\$ Unlike traditional forums, Stack Exchange is for specific questions only - there does not seem to be a specific, concisely answerable question in your post, just a broad curiosity. \$\endgroup\$ Commented Oct 12, 2020 at 20:40
  • 1
    \$\begingroup\$ I realized that when reading back my question. Sorry about that. I've pin-pointed a more-specific question, as I only care about PMSM vs IPMSM, but if anyone likes to include differences wrt BLDC (esp. if my understanding is wrong) that'll be of course gladly appreciated. \$\endgroup\$
    – anrieff
    Commented Oct 12, 2020 at 20:52

1 Answer 1


A perminant magnet synchronous machine (PMSM) must be excited with an AC current for torque to be developed.

A Brush less DC (BLDC) PMSM has a backEMF profile which is trapizoidal in shape to maximise the peak torque but also to simplify the drive algorithm as they can be excited with a simple quasi square wave controller.

A Brush less AC (BLAC) machine has a sinusoidal backEMF profile and thus should be excited with a sinus current waveform

There are three types of BLAC machines

Surface Permanent Magnet (SPM). The rotor magnets are bonded to the rotor

Interior Permanent Magnet synchronous machines (IPMSM) where the magnets are mounted in slots of the rotor

Flux switching Permanent Magnet (FSPM). A cross between an SR and a IPMSM.

Fundamentally they are controlled exactly the same way. However due to the fact there is a reluctance component associated with the IPMSM, the machines requires a slight amount of phase advance to provide a balanced torque profile. The motor also benefits from reluctance torque and thus can be more efficient than a SPM as it can produce slightly higher torque for the same current. Likewise they can operate at higher velocities due to the magnets are encased by the rotor

  • \$\begingroup\$ I believe another advantage of some buried magnet designs is that field weakening is easier. I suspect this is the main reason the new Tesla motor uses that topology, as it reduces the cost of the power electronics. \$\endgroup\$
    – Jon
    Commented Oct 14, 2020 at 12:41
  • \$\begingroup\$ Hello, I wonder is there a community dedicated to advance motor design? I'd be excited to join. \$\endgroup\$ Commented Apr 9, 2023 at 3:44

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