What is a DC Brushless Motor?
Curt Wilson posted on July 24, 2014 |
A DC brushless motor is actually a permanent-magnet synchronous AC motor.

A DC brushless motor is an AC motor. Specifically, it is a permanent-magnet synchronous (AC) motor (PMSM).

So why, you may ask, is it called a DC brushless motor?

The term "DC brushless motor" is a marketing term. The intent was to sell these motors as replacements for DC brush motors in servo applications. It was intended that these motors with accompanying servo drives be drop-in replacements for DC brush motors and their drives. The motor is intended to be operated from a DC bus through electronic inverter circuitry, not directly from AC lines.


Why would you want to replace a DC brush motor with a "DC brushless" motor?
DC brush motors are easy and inexpensive to control because the phase commutation is handled mechanically inside the motor by the orientation of the brushes on the stator to the commutator bar segments on the rotor. 

However, they have several important drawbacks. The brushes wear, requiring periodic replacement, and the wear particles make them inappropriate for use in clean environments. The sparking means they cannot be used in explosive environments. In high-precision applications, the friction makes it hard to settle into position quickly and accurately. The fact that the current is on the rotor, where it is difficult to dissipate the resulting heat, limits the continuous current rating significantly.

Brushless motors, with their windings on the stator, eliminate all of these problems. The commutation of current in the phases must be done electronically. This used to be difficult and expensive, but is now easy, and the motor/drive combination of a given rating is often less expensive than a comparable brush motor/drive pair.


What's the difference between a "DC brushless" motor and an "AC brushless" motor?
Again, these are marketing terms. Both are AC PMSMs. A DC brushless motor is optimized for smoothest operation with square wave (but still alternating) current inputs to the phases. An AC brushless motor is optimized for smoothest operation with sinusoidal alternating current input to the phases.

More descriptive terminology labels the "DC brushless" motors as "trapezoidally wound", with the back EMF and torque curves as a function of angle being (roughly) trapezoidal in shape, and the "AC brushless" motors as "sinusoidally wound", with these curves being sinusoidal.


How should you choose between a DC brushless and an AC brushless motor?
DC brushless motors are optimally controlled with simple switching commutation logic, directing the commands to the appropriate phases based on a low-resolution angle sensor, often Hall-effect. AC brushless motors are optimally controlled with more complex algorithms requiring mathematical capabilities. While the switching logic, usually called "six-step" commutation, is less expensive, it does result in more torque and velocity ripple, particularly at the switching points.

Many applications tried to use six-step commutation to save a few dollars, but with the result of leaving some kind of mark on the part being operated on at every switching point. Switching over to a sinusoidally wound motor with the more sophisticated match sinusoidal commutation instantly fixed the problem. In positioning servo applications, there is already a high-resolution position sensor for the feedback loop, so there is no added cost to using this sensor for commutation as well.


What's the difference between these brushless motors and motors sold as AC synchronous motors?
Ironically, the motors sold as AC synchronous motors have an asynchronous torque generation mechanism as well, and these brushless motors do not. An "AC synchronous motor" is intended to be used with a fixed-frequency supply, usually of 50 or 60 Hz, so needs a separate mechanism to pull into synchronism. (Most commonly, it acts like an induction motor until it locks in at synchronous speed.) A "brushless motor" is intended to be used with some sort of variable-frequency inverter, usually with feedback, so does not need this other mechanism.


About the Author:
Curt Wilson is Vice President of Engineering at Delta Tau Data Systems, an industrial controls firm.

Curt is a member of the Engineering Writers Guild at www.eng-tips.com. He is also an Eng-Tips Forums Fellow, MVP, and member of their Round Table. Follow Curt (cswilson) at http://www.eng-tips.com/userinfo.cfm?member=cswilson.

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