Jim Wilson wrote:

Time for a new rotary phase converter. This one is 20 HP. It's balanced,
but to avoid the monstrous starting surge current, I spin the idler up to
speed with a 1HP single-phase pony motor before kicking in the idler
contactor.

Here's the strange part: while the idler is running the pony motor
housing gets hot. I don't understand why. It's a continuous-duty rated
motor with good bearings. It's an old pool-pump motor that had a bad
start cap.

Now, it *seems* like the heat is coming from the stators -- what you'd
expect if the motor were powered up and driving a load. That is, the heat
isn't concentrated around the bearings, but seems to be coming from the
body of the motor. But, whenever the idler contactor is engaged, the pony
motor contactor is disengaged. I double-checked and there is no line
current into the pony motor when the idler is running.

WTF, mate? Eddy currents? Leftover heat from startup slowly making its
way out to the case? ???


Very interesting. I have an idea to test. next time you run it, set it
up so you can
connect a pair of 120 V 100W light bulbs, in series, to the motor
terminals for
a few seconds. (A normal wall plug might be a good way to do this.) First,
observe whether the lamps light up. Second, see if the motor still gets
hot.
(I say 2 lamps in series assuming the motor is a 220 V single-phase motor.)

The only way the motor can get hot, other than windage loss which ought to
be really small, is if the rotor stays magnetized. I'm wondering if the
wrong
kind of steel laminations were used in the rotor, allowing it to keep a
pretty
strong remanent field. It shouldn't, in general, keep the rotor field
after the
stator is de-energized. But, as a pool pump motor, it would be unlikely
to ever
be spun by mechanical force, so who cares?

If there is any possibility the starting switch inside the motor is sticking
closed, that could cook the motor and the start cap. Since this motor is
only started, but never run for more than a few seconds, you might not be
aware the starting switch is stuck - yet. If it IS sticking, then the
motor will
be recirculating current between the run and start windings through the
start cap. I would expect the start cap to pop fairly quickly in this case,
though.

If the lamps blink and then go out while still connected to the now
idling pony
motor, you have just successfully demagnetized the rotor, and it just
about CAN'T
get hot, now.


Jon