Re: Controllable torque electric motor questions

Richard J Kinch wrote:

Tim Wescott writes:


Gearing efficiency _does_ matter at zero RPM. A gearbox's stated efficiency loss is almost entirely due to frictional losses, which you need to take into account going from the torque at the motor to the torque at the shaft.


If nothing's moving then frictional loss is by definition zero.

True, but if your mechanism is designed to not move you need neither motor nor gearbox. As soon as things start moving the gearing efficiency _does_ matter a _lot_. Why? because Colombic friction is not a viscous drag that is somehow proportional to velocity, it is a constant torque that must be overcome.

I suppose that while your statement that "gearing efficiency doesn't matter at zero rpm" is technically correct, you left out the "but it matters for _any_ non-zero speed"

Maybe you mean, "stiction"?

No, I wanted to avoid that question entirely if I could. But if you have to study up on it this may help: http://www.wescottdesign.com/articles/Friction/friction.html.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

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