View Full Version : AC Brushless vs DC Brushless

02-26-2004, 12:11 AM
Hey ppl,

I have been doing extensive reading lately, online and offline, and I have been looking for drivers that can interface with AC brushless motors and my computer.

In most of my reading, I saw that AC brushless and DC brushless motors are actually the same motor. They are named different not because there is anything different between them, it is because the driver used to run them works differently. Now, I am hoping this is true, because then I might be able to attach some encoders to my AC brushless motors and run them with Rutex 992H drivers.

ANyone know if this is right or not?

Here are 2 references that I saw recently;

Koford (http://www.koford.com/acdc.pdf)
Thermwood (http://www.thermwood.com/twood_site/pages/tech_services/retro_pdfs/5_axis/Siemens.pdf)



03-05-2004, 11:55 AM
H3..,You will usually need some kind of commutation feed back on DC brushless motors usually they use 3 hall effect sensors, or equivalent on an encoder track. I am not familiar with the Rutex drives as to what they take for commutation feedback if any.
PS.I forgot to point out that although both AC &DC brushless are basically three phase motors, there is a difference in the feedback method, AC brushless usually have a resolver for sinusoidal feedback and DC brushless have hall effect sensors as previously mentioned.

03-24-2004, 06:27 PM
This is something that's still a bit unclear to me too.

As you say, the difference is in the driver.

AC motors use sinusoidal commutation, and needs a high resolution feedback device (be it resolver or encoder) for this to work.

DC brushless use hall effect devices for feedback and use trapezoidal commutation.

I don't know if there's anything in the motors that makes them more or less suited to one or the other. Perhaps we should ask the motor guru (Jon Elson) at the Yahoo CAD_CAM_etc group???


03-25-2004, 02:33 AM
Hi Guys

First let me start by saying that there is no difference between an AC servo motor and drive and a DC brushless motor and drive. They are the same thing. Just a different way at looking at things.

There is numerous ways for the drive to manage comunitation but in the end the way the drive controls the motor is the same.

Whether the Rutex 992H drives can control you motors is dependent on the communitation feedback type, and of cause, voltage and current specs.

From reading the spec sheet on the Rutex 992H I am impressed with it.



03-25-2004, 02:45 AM
Hey ppl,

So, if I decide NOT to use resolvers or hall sensors and just use regular agilent type encoders, I should be able to drive a AC and a DC brushless motor with the same drive?

Is this right?



03-25-2004, 10:29 AM
I seems there is still a bit of confusion over the two type's of AC servo, AC brushless & AC sinusoidal, basically the motors themselves are the same (3 ph motors) but each type are fitted with a particular type of commutaion feedback which makes each type require a different type of amplifier, they are not interchangeable.
The two types are Technically described as DC brushless (square wave current) with 3 hall effect type (or equiv) commutation devices and sometimes call six step servo because of the square wave type signal to the motor. The other is Sinusoidal current and will have a resolver/encoder used for commutation. The motor voltage/current resembles a true 3 ph signal. Which can be varied in amplitude and freq. There are DC brushless amplifiers on the market which will drive either DC brushless or true DC servo's.
AC sinusoidal require a unique type amplifier.
You may also come across the term DC trapezoidal, but this is an old DC brushless type which is not usually used today.