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

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
"Jim Wilson" wrote in message
.net...
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

Proctologically Violated©® wrote...
I gather a "pony motor" is used just to start the idler? In place of
capacitors?
Pretty neat!

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.

Maybe the bearings are getting hot?

Nope. The heat comes from the windings.

Have you run the pony motor by itself for a while, to see how hot it gets
under its own no-load power?

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

You might also try amp-probing the current draw of the idler with and
without the pony attached.

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

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

*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

On Wed, 22 Feb 2006 18:17:17 -0700, Jim Wilson
wrote:



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.


The run capacitor is supplying magnetising VARs to the pony motor. Try to
arrange to either:-

1) Open circuit the run capacitor, which will stop the motor being excited as
an induction generator.

or

2) Short the pony motor (yes,really). This will cause the voltage to collapse
and prevent the motor/capacitor combination from self-exciting.


If you have a spare pair of contacts that can do this, or fit another relay
fed from one of the existing ones, your problems should disappear.


Mark Rand
RTFM

Jim Wilson wrote:

(Another) Jim wrote...


This is exactly what a capacitor run pony motor would do.
See if any capacitors are hidden away.



*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.

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


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. :-)

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.


I *still* can't figure out why the thing gets hot, though, with
essentially no load.


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

(The other) Jim wrote...
Disconnecting or short circuiting the capacitor when power is
removed will clear the problem.

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.

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.

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

pony probably has a start cap - I'll bet you find a current path
through the start cap if you look at a schematic - try disconnecitng
cap, see what happens.













On Wed, 22 Feb 2006 18:17:17 -0700, Jim Wilson
wrote:

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
Bill

www.wbnoble.com

to contact me, do not reply to this message,
instead correct this address and use it

will iam_ b_ No ble at msn daught com
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