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.

Bob Swinney



"Gary Coffman" wrote in message
.. .
On Tue, 26 Aug 2003 23:57:15 +0100, wrote:
On Tue, 26 Aug 2003 07:12:47 -0700, Eric R Snow
wrote:
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

Only if it's 100% efficient. The simplest case to look at is a low
speed permanent magnet DC motor where the armature resistance is the
major loss component.
With an 80% efficient motor, 20% of the input power is lost in
armature resistance. Operated at the same speed as a generator, 80% of
the original motor input power is available as generated output.
However it now has to travel through the armature resistance before it
reaches the output terminals and this loses a further 20% of the
available power.
For the same internal heating, an 80% efficient machine used at
the same speed as a generator will deliver about 0.8 x 0.8 = 64% of
its rated motor input power.


Gary
Greetings Bob,
Maybe you can tell me what is going on with a stepper I just measured.
It is rated at 5 volts, 5 ohms, and 7.5 degrees. When spun by hand the
resistance to turning is quite high. Spun faster by hand it of course
gets way easier to turn. Spinning the shaft in the lathe, with no
load, gets voltage readings from 3 volts @ 330 rpm to 14 volts @ 1800
rpm. Those voltage readings are no load. Using a lamp as the load the
current measured @ 1800 rpm is .5 amps. At 550 rpm it is about.3 amps.
The lamp is a twelve volt lamp. It doesn't glow at all. When the
voltage is measured with the lamp connected it drops to .2 volts. Id s
it because there are so many poles on the rotor that it behaves this
way? If all the coils were wired in parallel and the rotor replaced
with one with just 4 poles would this improve the situation?
Thanks,
Eric