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
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Jim Wilson wrote:

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
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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:

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Grant Erwin 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.
Jon
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Jon Elson wrote...

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
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Jon Elson 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.
-- Greg
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Greg Dermer wrote...

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
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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
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Bob wrote...

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

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

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
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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
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I gather a "pony motor" is used just to start the idler? In place of capacitors? Pretty neat!
Maybe the bearings are getting hot? Have you run the pony motor by itself for a while, to see how hot it gets under its own no-load power?
You might also try amp-probing the current draw of the idler with and without the pony attached. You'd have to sort of guess-timate what portion of the extra current draw of the idler was due to purely mechanical factors (inertia, friction, etc.), and what might be due to possible internal currents, etc. of the pony motor. It's an interesting conceptual problem--if you put another pony motor on the idler, w/ no heat build up, you still wouldn't be able to ascertain if the heat buildup in the first pony was due to mechanical or electrical factors! Only if you did something like changed the bearings in the troublesome pony motor, and the problem went away, could you know for sure. -- Mr. P.V.'d formerly Droll Troll

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Proctologically Violated wrote...

In place of a starting capacitor. The current surge to cap start a 20HP idler is hell. The pony gets it up to speed, then you switch the contactors so the idler takes over. Very little starting current surge. The idler is balanced, so the run and power factor correction caps are still present.

Nope. The heat comes from the windings.

Yes. It warms to a normal degree, but doesn't get hot. It's a significant difference.

It would be a pain, and I don't see a good reason to do it. The laws of physics and thermodynamics convince me that the heat coming from the pony motor must be supplied by the idler mechanically, no matter what. And that translates into a greater load on the idler's electrical supply.
Jim
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On Wed, 22 Feb 2006 18:17:17 -0700, Jim Wilson

This is exactly what a capacitor run pony motor would do. See if any capacitors are hidden away.
, Jim
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(Another) Jim wrote...

*click* <-- sound of a light bulb turning on
I thought a cap run motor had two caps. This one has only one. I think its a permanent-split-capacitor motor.
But your point is well made. The PSC-type motors always keep the start/run capacitor in the start winding circuit. It never drops out. So, in that regard, the start winding of a PSC motor is just like the run winding of a cap run motor, except for the significantly higher winding resistance.
Wonder what will happen if I also remove the pony motor capacitor from the start winding circuit when the contactor opens... We'll find out. Wish I'd used a three-pole contactor for that pony motor, now. :-)
I *still* can't figure out why the thing gets hot, though, with essentially no load.
Thanks!
Jim
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Itgets hot because of current traveling through windings. The current charges and discharges the capacitor repeatedly and heats up the wires.
That's my guess, but it is just about the only possibility that makes sense to me.
An easy fix would be to wire in a relay or whatever to disconnect the run capacitor when it is not necessary (when the pony does not produce power). You can experiment with just a wire nut and see if this (disconnecting run cap) would in fact help. If it does, as I hope would be the case, then you can wire in another relay or use an extra pole on the contactor that you use to start the pony.
i
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Jim Wilson wrote:

Absolutely. I think this is what I suggested last month.

Yes, I think that will fix it. The capacitors are exactly what you do when building an induction alternator, which some people do to generate AC power from waterwheels and such.

It gets hot because the capacitor is building up the slight remanent field of the rotor into a fully-excited rotor, and the flux in the stator is about the same as when the motor is connected to the line. The iron loss in the stator is what is making it hot.
Jon
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On Thu, 23 Feb 2006 08:51:29 -0700, Jim Wilson

In fact you've been quite lucky - if your RPC motor had driven the pony motor a bit faster you would have had a funeral pyre!
A capacitor loaded, driven induction motor acts as an unstable induction generator. As the speed increases from zero the residual magnetism in the rotor first generates a very small output voltage. Little current flows in the capacitor because the frequency is low and the voltage is small.
However the small amount of current that flows is of leading power factor and this increases the rotor magnetic field so that the output voltage increases more rapidly than the speed. This is positive feedback so that quite small further increases in speed lead to disproportionate increases in output voltage.
If the speed is high enough, or the other variable - the capacitor is large enough, the output rises uncontrollably until limited by magnetic saturation in the stator and rotor teeth. Very large currents flow in the windings to provide the power for the saturated iron losses.
This effect is occasionally used for electronic braking. A large capacitor permanently connected across one winding of an induction motor can produce spectacularly short stopping times when power is removed and it induction generates. Capacitor size needs to be 5 to 10 times the value needed for unity power factor correction.
Disconnecting or short circuiting the capacitor when power is removed will clear the problem. Another possibility (if you're using belt drive) is to use a slight step up ratio to the RPC. A 10 to 20% drop in pony motor driven speed should be enough to kill most of the regeneration.
Jim
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(The other) Jim wrote...

As expected, disconnecting the pony run cap did the trick. Mystery solved. Everything works perfectly, and even better, I understand why the problem occurred in the first place.
One curiosity that someone raised was how much the pony heating was mechanically loading the idler. Switching out the capacitor reduces the line current into the electrically unloaded idler from 8.4A to 5.2A. Assuming the idler's power factor correction is perfect, that works out to about 768W, or a tad over the pony motor's 1HP rating. So, the pony was working about as hard as it could to slow down the idler without burning itself up.

Actually, I had used a variable sheave on the pony motor and adjusted the diameter to match the fixed idler sheave. Since both motors run at the same nominal speed, the pony wasn't being overdriven by much, just whatever the difference in slip is. That's probably why I avoided the "funeral pyre."
Thanks, Jim, Jon, and everyone else who weighed in on this. It was an interesting excursion, and I'm jazzed with the results!
Cheers!
Jim
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