On Tue, 26 Aug 2003 22:57:37 GMT, "Bob Swinney"
wrote:

Gary sez:

"Why do you think you can count the same resistance twice? The current only
goes through it once, regardless of whether it is being used as a motor or a
generator."

He is not counting anything twice. Go back and read the post carefully.
You can rely on anything that Jim says as gospel.

I respect both contributors, agree with Gary here. Loss is I^2 R
regardless of which way the current is going. Power out is power in
minus I^2 R, whether power "in" is electrical or mechanical.

I think the reason the stepper motor wimps out at higher speeds has to
do with coil inductance. Reactance grows with frequency which is
proportional to speed. The same issue limits how fast they can be
driven with given available voltage.

I wonder if a brushless DC motor would commutate without external
excitation. Something has to supply the electronics that makes it
brushless.

What are you going to spin it with, Eric? Most electric-start
engines (including outboards) already have alternators capable of a
good deal more than 100 watts.

My local surplus store sometimes has DCPM brush-type 12-volt motors.
I tested a couple in my collection today. One has resistance of 0.121
ohms, so at 8.33 amps (100 watts at 12 volts out) the loss would be
8.4 watts and the efficiency would be 100/108.4 or 92%. The price
sticker on it says $5.95 but I've had it a while. Ya never know
what the Ax-Man will have on any given day, but if you'd like I'd be
glad to make the selfless sacrifice (snort!) of visiting the Ax-Man
to see what he might have for you. This particular motor weighs 3
lb. I'm near Minneapolis.