On Wed, 27 Aug 2003 14:56:12 GMT, "Bob Swinney" wrote:
Seems we are having basic math problems again. Look at it this way (which
is presumably what Jim Pentagrid meant). You have a power rating for a
motor. The question was, how much of the *motor's power rating* is
available if it is run as a generator?

Assume the motor is 80% efficient and that it is exactly as capable of being
a generator as it was a motor; i.e., also 80%. Take the input power rating
for the motor X 80%. This is the input power rating of that motor ran as a
generator. Incidentally that amount of power is input as mechanical motion.
Now, the motor-as-generator, is 80% efficient. The mechanical input power
is converted to electrical power at 80%. That would be 80 X 80 or aprox.
64% of the original input power rating for the motor.

No. You can't count the efficiency twice. The motor is 80% efficient, so
it can convert 80% of the electrical power fed to it to mechanical motion.
Conversely, when used as a generator, it can convert 80% of the mechanical
power fed to it to electrical power.

In both cases, the missing 20% goes to heating the armature resistance
(ignoring bearing and windage losses). You can't *count that twice* because
it can't be used as a motor and a generator *simultaneously*. It is used as
one or the other, and in each case, efficiency is 80%, *not* 64%.

Lets run some numbers. Suppose we have a motor rated at 1 hp electrical
(746 watts) input. If it is 80% efficient, then it will produce 0.8 hp (596.8
watts) on its output shaft. Now if we decide to use it as a generator, we
feed it 1 hp (746 watts) mechanical on its *input* shaft, and then we can
extract 0.8 hp (596.8 watts) electrical on its *output* terminals. All we've
done is show that, as an electrical machine, the motor is reversible.
Efficiency is the same either way.

Now I'll grant that if you were silly enough to only feed the generator
with 0.8 hp on its input shaft, you'd only get 477.4 watts out of it. But
that doesn't mean its efficiency has been reduced to 64%. You just
arbitrarily decided not to feed in enough power to get 596.8 watts out
at the generator's 80% conversion efficiency.

Gary