brushless vs brushed servos - which is better and why

[runinbymdnt]

I am building a large all metal cnc router table. I have both brushless and brushed motors in stock that I can choose to use. I like the brushless but both have the power to do what I need them to do.

Neither have encoders, both have 50 amp 24-48v dc controlers.

The use will be light industrial, not hobby.

Can you, been there done that, machine builders help me decide what to use? and more importantly, why?

Thanks
JK

[Karl_T]

The only issue I'm aware of is brush wear. Important on a machine that runs 24 - 7. Brushless can be far more expensive. As you already own them, I guess cost is not the issue.

Karl

[daedalus]

Brush servos are usually cheaper, but suffer from brush wear and increased em noise (due to arcing with brush contacts). Brushless are great for running life and noise, but require more complex commutation electronics, so the drives are usually more expensive.

One thing to consider with brushless dc is cogging torque at low speeds, this can be reduced by using sinusoidal (AC) commutated drives, but these tend to be more expensive.

[runinbymdnt]

Thanks for the replies
The next question is on the drives. The drives I have with these are new, state of the art, current limited (at 50 amps), reversable, small Industrial vehicle speed controlers. They require 0-5v input for speed control.

From what I have told you here, does it sound like I could use them as servo drives?

Is there a way to test or identify their usefulness?

I have been hanging around this forum for a while now. I have learned so much I cant believe it. With resources like this forum, ignorance is difficult to maintain.

Thanks again JK

[ger21]

If they are 0 to 5V, how would you reverse them? Most servo drives would take a + and - voltage to have forward and reverse. If I'm wrong, someone please correct me.

[NC Cams]

In addition to the 0-5v speed input, you'd better have a separate direction bit or else things could get real interesting.

That or else it is purely a unidirectional speed controller which would make it a challenge to use in CNC where you typically do need bidirectional drive capability.

[Al_The_Man]

I suspect the input could be ±5v, otherwise as the others say, you will need reversing method.
As to Brushless, there are two basic types DC brushless and Sinusoidal. I have found the DC brushless noisier than DC brushed because of the large pulses when commutated, they also do exihibit some cogging if used as direct drive at low speeds, as has been said.
AC sinusoidal are extremelly quiet, smooth and excellent at low speed.
If you show a link or post info on the drives, more can be said as to how usefull for servo use etc.
Al.

[runinbymdnt]

We currently use a seperate imput for reverse but I think they are configurable to except -5 to +5v . That would be called a wig wag throttle and would be used on say a pallet jack. I will check that out.

Is the imput voltage of -5 to +5v exceptable? What do other drivers use?

I would gear them down as the rpms are too high to direct drive, that should help with any cogging issue, however these motors are very smooth and quiet.
I have used and disgarded some very noisey brushless motors. I looked long and hard to find these for my vehicle aplication and buy them and thier controlers in large quantities so the price is good.

Great info. The magic of the "black box" in a cnc machine is starting to make sense. Bye bye to a little more ignorance.

Thanks again JK

[Al_The_Man]

Most analogue controllers use ±10vdc but can be scaled. Check to see if there are other options of control, It depends on what you intend using as control, for example, most Software/Parallel port systems use step & direction in place of analogue.
It depends if you are using them in a CNC application or a stand-alone system.
For precision positioning etc, you will also need encoder feedback.
Al.

[ger21]

You can convert the step and dirction to analog with these.
http://www.skyko.com/products/

[runinbymdnt]

Thanks Al
I new I would have to add encoders, that should be no problem.
I dont understand the stand alone system??? I am building a large steel and aluminum cnc router to start, then I think I will cnc my mill, why not. I always thought cnc was black magic or rocket science, something I would never have. I now know I was mistaken. It's realy quite straight forward as I have spent a couple dozen hours reading on this site and others.

Is there a simple explaination of step and direction as opposed to analog?

If using what I have gets too crazy, I will buy what is needed to simplify things. I want a good system when I am done.

I have been fabricating a wide range of things for most of my life. Everything changed for me 2 years ago when I took 6 months off and taught myself to solid model using solid edge software. Wow. That has me off in a new direction.....digital. I still have alot to learn and am loving it.

Thanks for the help JK

[runinbymdnt]

you guys are fast.
You are answering my questions before I can ask them!!!!!!

Thanks Gerry

JK