Which Servo Motor To Buy?

[lerman]

I'm looking at Glentek servos http://www.glentek.com/dcsel.html and trying to chose a motor for my Bridgeport clone conversion. Price is an important consideration, and I don't have price info yet.

I've pretty much decided that the GM4020 series servo with a continuous stall torque of 23 lb-in should do the job. But there are three motors in the series with top speeds of 2200 to 4000 RPM. Since they all have the same torque, the higher speed motor has higher peak horsepower, but the same horsepower at lower speeds.

The motor mounts I'm using have a 2:1 ratio and the ballscrews are five pitch. So, the 2200 rpm motor would give rapids of 220 inches per minute and the 4000 rpm motor would give rapids of 400 inches per minute. Either of these would be fine.

Since the armature inertias are the same for these motors, I assume that the difference is more turns of smaller wire.

So, aside from the maximum speeds, what other tradeoffs are there among the three motors in this group? (Assuming they are about the same price.)

Thanks,

Ken

[unterhaus]

The fastest one takes a lot more amperage, so you likely will have to spend more on amps. Since the armature resistance is lower by the same proportion as the amps are higher, it seems that the faster motor actually has fewer turns of a larger wire. This makes sense, as then the back emf will be lower at a given rpm, and you will thus be able to turn faster for a given voltage.


Did you check with them what the price is? You might have to rethink your approach given your budget.

[aghobby]

From experience, it will be hard to beat the servo from www.ajaxcnc.com...


(I'm not affiliated in any way to this website, just have good service from previously purchased item...)

[unterhaus]

Actually, I was going to mention Ajax if you have the budget to buy new. I'm guessing that for the price of the motors (forget the amps) from Glentech you'd be close to a system from Ajax. If you are going to piece things together, there are alternatives that are much more economical, starting with Gecko or Rutex, Mach 2 or EMC, and scrounged motors.

But I'm guessing on the price.

[lerman]

The reason I'm looking at Glentek is that Dwayne Elrod suggested them as a possibility. I don't have pricing yet.

I'm familiar with Ajax. That's a little more than I'd like to spend.

I'm also considering some Reliance motors (on Ebay). They're Nema 34 motors with 3/8 inch shafts. The XYZ axis I'll be using take Nema 42 motors with 5/8 shafts, so I'd have to make some adapters. If I can spend a little more money and avoid that, I think it would make sense. Particularly if I decide to go with larger motors in the future.

Ken

[unterhaus]

I'd be curious what you find out about prices. I just priced a new nema 24 brushless motor from Aerotech and it was almost $2000. They are high end motors, but it wouldn't suprise me to pay $1000 for a NEMA 42 motor. There used to be a web site that sold Baldor servos which are some of the lowest priced motors around, and a NEMA 42 motor was still several hundred bucks. A name brand encoder is going to run you a coupla hundred bucks by itself. That's why people are happy buying three for $400 from those guys on ebay. A good place to buy servos if you don't want to scrounge on ebay is servo systems www.servosystems.com
They cost a lot more than ebay, but they sell surplus, so the prices aren't too bad. There are also surplus dealers that have motors, I don't have any names at hand.

I went Ebay brushless, so I have three different motors and two different kinds of amps. Not quite the worst situation possible, but close.