Servo Peak Torque Vs Cont.

[JerryFlyGuy]

My apparent stepper supplier doesn't have stock on the steppers I'd planned on buying so now I'm thinking about servo's again. In calculating what I should be using I came up with a question. I'm wondering which numbers to use to calculate my torque on servo's. The peak torque makes things look pretty good, but the cont. isn't so swell w/out huge gearing reductions [25-30:1]. Can I bank on using peak torque in my calculations or what?

If memory serve's me correct you can't go over 10% duty at peak? Don't know where I got that from.

Jerry

[H500]

The peak torq is the value it can generate for a few 10's of seconds without burning up.

[Geof]

I think if you size your servoes using the continuous output for the load you expect to get when cutting or moving at a constant speed in a rapid motion you will be okay because then you have the short period peak capacity to handle acceleration loads. Actually to be conservative you should probably do your calculations for cutting loads and then get a servo with something like 1.2 times what the calculation gives. On commercial machines the servoes rarely go over 100% during cutting or constant speed motion but nearly always hit 200% or more during acceleration and often this is controlled by the processor with a timeout that kills the servo power if it stays too high too long.

[Jason Ross]

The peak torque is an RMS calculation where the average value has to be in the continuous range. Also most servos are limited to operating at peak for 3 seconds. The 20% rule of thumb is pretty good depending on your knowledge of your application, conservatively you should probably be in the 30% range. It is not just a torque calculation changing from stepper to servo. If you require a 30:1 gear reduction to get the servo to perform I would assume that there is a large inertia mismatch without a gear reduction. For CNC cutting I would like to keep the inertia mismatch under 7:1 and preferable 5:1. My company stocks steppers as well as almost every stepper motor manufacturer. If your supplier does not have it almost anybody you change to will. Do a google search and good luck!

[JerryFlyGuy]

Ha.. well as it turned out I've gone w/ my stepper in the end. It had looked like I wasn't going to be able to get the stepper's I wanted and so I was faced w/ redesigning everything. I was thinking at that point I should move up to servo's. Anyway now its back to steppers. However just to clarify a couple things.

Jason you mentioned that if I need 30:1 there is a large inertia mis-match. I'm not sure what that means but the reason I was planning on 30:1 was simply to duplicate the power I was getting from my steppers. I was calculating from the cont. power curves. In actual fact the 30:1 would put my WAY over what the steppers were putting out I could have gone with 15:1 and had the same power as the steppers [ w/ the steppers at 5:1] but at 30:1 I was getting max power out of the servo at my cutting speed that I needed. It was pretty much way overkill in every way.


Maybe that clears that up a bit.

Jerry