"Eric R Snow" wrote in message
...
If a motor is used as a generator will it put out about the same
wattage as it consumes when used as a motor?
Thanks,
Eric

Well, no. The power it consumes is the mechanical power it puts out, plus
electrical and mechanical losses. If you put the same mechanical power in,
using it as a generator, you have losses going the *other* way, too. So the
difference in power in versus power out, for a fixed value of mechanical
power in and out, is the sum of those two sets of losses -- the losses it
has as a motor, plus the losses it has as a generator.

The efficiency of the motor depends on its design and size. A small DC motor
used as a generator starts off in the hole, because its efficiency as a
*motor* isn't all that great (maybe 70%). And, at the very best, you have
equal losses working the other way. Big, quality motors are much more
efficient -- into the 90%+ range.

To make an efficient generator out of a brush-type permanent-magnet DC
motor, you may have to shift the phase angle of the brushes slightly. I have
here in the mess I call my shop an old DC motor that was modified by someone
to allow fine adjustments of the brushes for that express purpose.
Unfortunately, it's a series/parallel-wound electromagnet motor, which
requires a lot of knowledge to get it properly excited.

My understanding is that small, modern, permanent magnet DC motors run at
something close to their motor efficiency when run as generators, but not
quite.

Ed Huntress