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The difference between PMSM and BLDC
What's the difference between permanent magnet synchronous motor and brushless permanent magnet DC motor in control?
By Anonymous on 11 May, 2003 - 8:51 pm

What's the difference between permanent magnet synchronous motor and brushless permanent magnet DC motor in control?

By Curt Wilson on 12 May, 2003 - 2:45 pm

These are really the same thing. The term "brushless DC motor" is really a marketing term to encourage people to think of the motor with its associated drive as a drop-in replacement for a brush DC motor and its drive.

The early motors sold as brushless DC motors tended to be "trapezoidally wound" (i.e. to have trapezoidal back EMF profiles) so they could be reasonably commutated by simple switching techniques from the DC bus (usually called six-step commutation). These days, most of these motors are "sinusoidally wound" and optimally controlled with a more sophisticated sinusoidal commutation algorithm.

To further compound the confusion, some companies refer to trapezoidally wound motors as "brushless DC servos" and to sinusoidally wound motors as "brushless AC servos". I consider this a horrible distinction in terminology, because both motors require AC inputs to move in one direction; it's just that the shape of the optimal input is a little different.

Curt Wilson
Delta Tau Data Systems

By Curt Wuollet on 12 May, 2003 - 3:25 pm

If I understand the question, one is self commutated and emulates a DC PM motor with electronic commutation instead of brushes. The
other relies on AC excitation for rotation. One is DC only and the other AC only. The control methods follow this difference.


By Curt Wilson on 15 May, 2003 - 2:08 pm

No -- both motors rely on AC excitation for rotation. However, if it is sold as a "PM DC brushless motor", it will probably also be sold with a drive that creates the AC waveforms from a DC source, often based on rotor position feedback. If it is sold as an "AC synchronous motor", it will probably be used straight from the AC lines.

Now, if it is sold as a "PM DC brushless motor", it will probably have a feedback device installed. More significantly, if it is sold as an "AC synchronous motor", it may also have an asynchronous torque generation mechanism (like that of an induction motor) to bring it up to synchronous speed. But once in synchronicity, the principles of operation are the same for both motor types.

They will tend to be used differently, with the frequency and direction sense of the AC inputs to the "PM DC brushless motor" being varied often, with sometimes a zero frequency (aka DC) being used to hold position against a load torque.

Curt Wilson
Delta Tau Data Systems

By Curt Wuollet on 16 May, 2003 - 3:20 pm

Hi Curt

You are correct of course, but what I was implying was that the PM Brushless, at least in small sizes, often includes the drive
internally, looking externally like a DC motor. However, we may be thinking of two different types of motors. I'll certainly yield to Delta Tau's experience in motion control.



By Alex Ruderman on 14 May, 2003 - 3:34 pm

It is also a matter of position feedback. DC (trapez) brushless relies on coarse Hall effect position sensors (60 el.deg resolution).

AC (sine) brushless uses resolver (electrical minimachine) with resolver-to-digital converter or optical encoder as accurate position sensors.

Most popular encoders are incremental (not absolute) ones. Therefore, there is a problem of AC brushless position initialization (also called phasing search or start-up) if no position Hall sensors (or special commutation signals from incremental encoder - additional wiring) are available.

Conventional initialization (phasing search) may cause long and rapid oscillating start-up rotor movement unacceptable for many applications. Also the known initialization methods are problematic in the presence of load torque.

In the paper: Doo-Hee Jung, In-Joong Ha, "An Efficient Method for Identifying the Initial Position of a PMSM with an Incremental Encoder", IEEE Trans. on Ind. Electronics, Vol. 45, No.4, August 1998, pp. 682-685, ( the authors suggest initialization procedure for a constant load torque.

It is based on providing (small) test sinusoidal torque disturbances and observing motor angle sinusoidal steady-state responses. At least, three test points are required. Test points selection is not that trivial because the close-loop system may become unstable.

This approach seems too complicated and burdensome (test settling time, steady state trial movement, stability etc.) to gain industry recognition.

More intelligent simple and accurate initialization methods in the presence of unknown (not necessary constant - time invariant position dependent) load torque (payload, cogging etc) were recently developed.


In simple terms, in a bldc u have the rotor which is the permanent maganet and its sets up the armature flux while the stator is fed by 3 phase supply to give us the field flux. u can do it by a PWM. there are some set relations for back emf and torque which can be found out from any literature. as matter of fact these 3 phase supply can be both square wave or sine... although former is preferred.

By Miroslaw Wlas on 20 August, 2003 - 8:52 pm

BLDC control looks like DC motor - speed and current PI regulators, PMSM control is AC control system like field oriented control.

Don't be confuse about the different of both. You could see the complete explanation in IEEE Transaction on Industrial Electronics, Vol. 35, No. 4, November 1988. "Modelling Permanent Magnet Motor Drives".

email to :

By Sean Fountain on 23 May, 2012 - 5:08 pm

This question echoes one that arose while I was studying motor types for a PM generator to Motor/Gen conversion.

In my situation, an out-runner PM generator is being converted with as minimal alteration as possible into an extremely power dense motor/generator. The application is around 10kW 6sec burst, very small physical size compared to industry motors.

Anyway, while studying a great deal of literature on induction, synchronous, doubly-fed, and BLDC motors, I noticed that if mechanical configurations or winding diagrams for Synchronous and BLDC motors were held side by side, with the control algorithm ignored, they were identical.

I even had some texts swear that they were different (mostly older texts from early 80s that were based on partial control research from the time).

I came to a similar conclusion as the answer provided above, BLDC is just a name for a Synchronous AC motor who's speed is varied while under load... this is primarily an application difference, as a synchronous motor is rarely varied in speed through its own drive/generative action, based on the research I have recently done.

Maybe BLDC was thought up by brushed-DC guys building synchronous motors and not wanted to call them that? Either way, control and sensing strategy, and application, seem to be the largest determination in literature for what name is used.

Just my two cents, I defer to the greater experience represented in earlier responses.