RPC pony motor getting hot?
 

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? ???

Jim

RPC pony motor getting hot?
 

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

RPC pony motor getting hot?
 

Normally a pony motor is only used to spin up the idler motor, then it is
mechanically disconnected and shut off. This is often done by a hinge/belt/lever
arrangement, whereby the user tilts the pony motor away from the idler motor
until it's almost far enough, then slips on the V belt, then tilts the pony
motor away a bit farther until the belt is tight, then, holding tension,
switches on the pony motor with his free hand, then switches on the idler motor,
then switches off the pony motor then relaxes the lever and lets the belt flop
off. I had never heard of keeping it electrically powered throughout. I'd just
try reconfiguring your converter a little so you can shut off the pony motor.

GWE

Jon Elson wrote:



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

RPC pony motor getting hot?
 

Grant Erwin wrote:
Normally a pony motor is only used to spin up the idler motor, then it
is mechanically disconnected and shut off. This is often done by a
hinge/belt/lever arrangement, whereby the user tilts the pony motor away
from the idler motor until it's almost far enough, then slips on the V
belt, then tilts the pony motor away a bit farther until the belt is
tight, then, holding tension, switches on the pony motor with his free
hand, then switches on the idler motor, then switches off the pony motor
then relaxes the lever and lets the belt flop off. I had never heard of
keeping it electrically powered throughout. I'd just try reconfiguring
your converter a little so you can shut off the pony motor.

His pony motor is NOT electrically powered after the spin-up, at least
that's what he says. It is mechanically connected so it keeps spinning
when the RPC is on. But, that shouldn't make it get hot.

Jon

RPC pony motor getting hot?
 

Jon Elson wrote...

His pony motor is NOT electrically powered after the spin-up, at least
that's what he says. It is mechanically connected so it keeps spinning
when the RPC is on. But, that shouldn't make it get hot.

Right, on both points.

I'll try your light-bulb experiment next free moment. Probably Sunday,
since we're framing the new shop tomorrow, and I'll be beat by the end of
the day.

Thanks,

Jim

RPC pony motor getting hot?
 

Jon Elson wrote:
Grant Erwin wrote:

Normally a pony motor is only used to spin up the idler motor, then it
is mechanically disconnected and shut off. This is often done by a
hinge/belt/lever arrangement, whereby the user tilts the pony motor
away from the idler motor until it's almost far enough, then slips on
the V belt, then tilts the pony motor away a bit farther until the
belt is tight, then, holding tension, switches on the pony motor with
his free hand, then switches on the idler motor, then switches off the
pony motor then relaxes the lever and lets the belt flop off. I had
never heard of keeping it electrically powered throughout. I'd just
try reconfiguring your converter a little so you can shut off the pony
motor.


His pony motor is NOT electrically powered after the spin-up, at least
that's what he says. It is mechanically connected so it keeps spinning
when the RPC is on. But, that shouldn't make it get hot.

Jon


I didn't see where he explicitly said he disconnected the pony motor.
*IF* it's still connected to the line, and *if* the belting arrangement
will run the pony motor faster than synchonous speed while being driven
by the idler motor, then it will act like an induction generator and be
loaded by the power line, causing heating.

-- Greg

RPC pony motor getting hot?
 

Jim,
My units are built the same way; pony and rcp motor remain connected.
Pony stays cool. There must be something electrical in the wiring
allowing voltage to the pony motor.
Bob

RPC pony motor getting hot?
 

Greg Dermer wrote...

I didn't see where he explicitly said he disconnected the pony motor.
*IF* it's still connected to the line, and *if* the belting arrangement
will run the pony motor faster than synchonous speed while being driven
by the idler motor, then it will act like an induction generator and be
loaded by the power line, causing heating.

In the third paragraph of the original post, I said "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."

Wherefor the conundrum.

Jim

RPC pony motor getting hot?
 

Bob wrote...

My units are built the same way; pony and rcp motor remain connected.
Pony stays cool. There must be something electrical in the wiring
allowing voltage to the pony motor.

Two wires attached, L1 and L2. No other connections. Ammeter reads zero
on both wires. (Yes, the ammeter is working correctly.)

Jim

RPC pony motor getting hot?
 

Jim Wilson wrote:
Bob wrote...


My units are built the same way; pony and rcp motor remain connected.
Pony stays cool. There must be something electrical in the wiring
allowing voltage to the pony motor.


Two wires attached, L1 and L2. No other connections. Ammeter reads zero
on both wires. (Yes, the ammeter is working correctly.)

Jim

Well, then. The bearings in the pony motor must be bad, eh? Friction causes heat ..

GWE

RPC pony motor getting hot?
 

Grant Erwin wrote...

The bearings in the pony motor must be bad, eh? Friction causes heat ..

As indicated in my first post, that was my second thought, after a
possible shorted pony contactor. But the bearings do pass the "spin
freely and sound good" test. Moreover, the end caps and motor shaft were
cooler than the body of the motor, and given the laws of thermodynamics,
I think it's safe to conclude that the heat isn't coming from the
bearings.

In retrospect, I think I noticed the heat after a long series of start
and stop cycles. (I had been tweaking the balancing caps.) And it's a
heck of a job for that 1-horse motor to spin up the idler -- it doesn't
have much starting torque. In fact, it needs a rope pull to get the ball
rolling. After that, spin-up takes around three seconds.

Maybe it was simply built-up heat in the laminations that the fan didn't
have time to dissipate because of the short run times. I'm hoping to give
it a good long test on Monday, and resolve the question for certain.
Today, we're working on the new shop. It's going up fast, but still takes
priority while my beer-priced friends are over (G).

Cheers!

Jim