Re: Controllable torque electric motor questions

Richard J Kinch wrote:

Tim Wescott writes:


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.


The point is, the *stall* torque developed at slow speeds isn't affected by friction.

No. Not unless you're looking at the torque developed by the armature of the motor, which is a singularly useless place to be looking when what you care about is the output of a 100:1 gear box.

At the output of the gearbox, the torque will be

torque = (motor torque) * (gear ratio) - (gearbox friction)

As a linear instead of rotational example, air cylinders develop force = area x pressure, without regard to friction.

No. Not unless you're looking at the force developed at the face of the piston, which is a singularly useless place to be looking when what you care about is the force developed by the cylinder assembly.

At the output of your hypothetical air cylinder the force will be

force = area x pressure - friction.

If friction is much bigger than area x pressure then you have to regard it a great deal, or resign yourself to poorly working machinery.

To the OP:

Sorry for the confusion. This is one of those special USENET moments when you have to read the posts, think a bit, and figure out who's right. Have fun.

--

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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