What are the differences or advantages/ disadvantages of dc brushed, dc brushless or ac servomotors?

Question:
I would like to upgrade / retrofit my CNC Series 1 boss 5 to servo base gecko drive mach3 machine. What are the differences or advantages and disadvantages of dc brushed, dc brushless or ac servomotors?

Answer:

First, Gecko drives will only run brush-type motors. Brushless motors have no commutator or brushes, so they need less maintenance. But, also, the brushless motors have the windings epoxied into the housing, where the heat can escape, while brush motors have all the windings (read – heat generation) in the rotor, therefore the heat has a REALLY hard time getting out. Also, iron and copper are heavier, by a lot, that rare-earth magnets, so the brushless motor has a lighterrotor — less inertia.

When talking about common servo motors, there is no distinction between DC brushless and AC, pretty much ALL servo motors are permanent magnet rotor, 3-phase stator motors. There are some special systems using flux vector drives to employ AC induction motors for servo purposes, but you are quite unlikely to run into one of those.

DC brush motors are VERY standard, if you can obtain the Kv and Kt ratings of the motor (Kv is volts/1000 RPM, Kt is your favorite unit of torque per Amp) and the peak and continuous current rating, you have everything you need for servo drive selection, proper drive ratio matching, etc.

Brushless motors add another whole layer of complexity. The alignment between the commutation encoder and the motor windings is not standard across all brands, and some makers like Yaskawa have proprietary encoder schemes. Others have no commutation encoder at all, and the drive needs to be programmed to know the encoder resolution. Some brushless motors handle 6-step commutation fine, others humm LOUDLY at certain speeds, and need to be driven by sinusoidal drive waveforms to avoid that.

To sum up, if you want to use a brand-name brushless motor with drives sold by that company, everything will work fine. If you want to mix and match servo motors and drives off eBay, you’d better be a motors guru!

Advertisements

3 thoughts on “What are the differences or advantages/ disadvantages of dc brushed, dc brushless or ac servomotors?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s