About a month ago, I posted about some weirdness with my RPC pony motor.
It's a salvaged pool pump motor, and it gets hot when the idler is doing
the driving. Here are some important details:

1) The idler and pony motors have separate contactors, and whenever one
is engaged, the other is not.

2) The pony motor gets hot when the idler is running and the pony motor
contactor is disengaged.

3) I have confirmed that both contactors are working properly, and that
both pairs of contacts on the pony motor contactor are indeed open when
the motor gets hot. There is no current flowing in the lines to the pony
motor.

Jon Elson suggested an interesting experiment, which I finally got 'round
to this afternoon. This post would be a reply to his message if that
article hadn't already been deleted from my newsreader.

Anyway, Jon's idea was to put a pair of 120V incandescent bulbs in series
with each other and across the pony motor. I did. Lo and behold, my pony
motor is an excellent generator. The lamps light up just as if I had
plugged them into an outlet. :-(

The voltage supplied to the pony motor via the contactor is 240V
according to my voltmeter. When the pony motor is acting as a generator,
it supplies 245VAC. There's no sag in voltage when I hook up the lights,
so the field in the rotor must be pretty substantial.

I still don't understand why the pony motor gets hot, though. With the
motor leads open, there's no load. Since it does no work, why is it
heating? Losses shouldn't be *that* bad.

Now, I need to decide what, if anything, to do about it. I'm inclined to
do nothing at this point. The motors are enclosed in the same box and the
20HP idler's integral blower moves a substantial amount of air across the
pony. The box is ventilated so most of the heat gets exhausted into the
room. I haven't measured the temperature, but my hand tells me the pony
motor case is probably under 160F or so (maybe as hot as a coffee pot).
So far, it hasn't gotten warm enough for the pony motor's thermal
protection to kick in, so I don't expect any real damage, just wasted
energy. Any thoughts on this?

This is all just temporary, by the way, as Tucson Electric Power has
deigned to supply my shop with a 200A 3-phase service ("soon"), just for
the asking. At no cost to me, either, except I have to buy a new meter
base and panel. There's a drive-by neener, eh? :-)

Cheers!

Jim