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