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.

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.

Gary