Electric Motor Phase Correction Question
Bob
power is generated from a rotary converter. This motor is not happy
with my home made power and trips the heaters. Normal load is about 9
Amps under load but the one leg pulls 13-14A and trips the heaters.
The contactor is a small GE unit with integral heaters and I have them
set on max which is 14A IIRC.
The motor checks out just fine at the local motor shop. Coil
resistances are all equal. There are no excessive mechanical loads on
the motor. I have narrowed it to the current overload on one leg
only.
I'll test with an added load on the RPC to see if that helps, but in
the mean time; can I use a capacitor to adjust the current draw on
that leg down? If so what MF values? I think that the caps in my
surplus are all 600V rated.
Thanks,
Bob.
Tim Wescott
It sounds like the voltages on the three legs aren't balanced, or
perhaps the three legs are grounded in one configuration (delta or Y),
in the phase converter and in the other in the motor.
I'd be surprised if it was a grounding issue, but the more screwed up
systems of various kinds I repair, the less surprised I get when I find
them.
If the problem is voltage balance, you could find it by measuring
between pairs of the three legs with a volt meter. If it's a phase
imbalance (which it would almost have to be) you'd either need a phase
meter (which would be a power company or an industrial electrician
thing) or an oscilloscope.
Did the motor shop actually run the motor, or just do static checks?
You could have something perverse like a shorted winding in a motor coil
that would only show up if you put AC on the thing.
Do you have any other three-phase motors running? If all are fine
except for the one motor then it's more likely a problem with that
motor, or a deficiency in both the motor and the system (like the
colliding grounds that I posited earlier), than it is with your converter.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Lloyd E. Sponenburgh
news:LtOdnb6LaoS0j1fW...@web-ster.com:
> You could have something perverse like a shorted winding in a motor coil
> that would only show up if you put AC on the thing.
>
And that, like a shorted turn in a transformer, will result in excessive
current through that one winding. It doesn't have to be a "dead short";
just ONE turn shorted can do that sort of thing.
LLoyd
Tim Wescott
Yup. And one shorted winding won't show up if you do DC resistance checks.
Martin H. Eastburn
wire to another winding. It might be that you need to have a smaller
three phase motor to balance the rotary and then switch in the big one.
Rotaries generate the third and the small starter motor might be just
what you need - the mag field in it will supply some surge energy needed.
It will also load the wild one and have it better under control when
the contactor is snapped on.
I run a rotary when I use my grinders. I have it running a few minutes
before I kick in the grinder. That might help.
Martin
pent...@yahoo.com
wrote:
You don't give any information on your home brew rotary
converter but it it's the usual type without "run" capacitors,
for 230v 60Hz and a 7.5 HP motor, your need to add about 200 to
250uF run capacitor to the phantom phase.
This should be enough to bring the current balance within
acceptqble limits.. Some further improvement can be made by
adding a second capacitor from phantom phase to the other side of
the supply and messing about with the relative size of the two
capacitors but the complication is rarely justified. Incidentally
the"obviously symmetrical" arrangement of two equal capacitors is
not useful - most or all of the capacitance needs to be in the
top run capacitor.
You are absolutely right in aiming for current balance.
Voltage balance only gives geneneral indication. It is current
balance that results in minimum I squared R loss and minimum
full load power drawn from the supply.
Jim
Ignoramus2187
> I have a 7.5 HP TEFC Motor on a hot high pressure washer. My 3 phase
> power is generated from a rotary converter. This motor is not happy
> with my home made power and trips the heaters. Normal load is about 9
> Amps under load but the one leg pulls 13-14A and trips the heaters.
A 7.5 HP motor is supposed to pull 3.4*7.5=25A.
> The contactor is a small GE unit with integral heaters and I have them
> set on max which is 14A IIRC.
>
> The motor checks out just fine at the local motor shop. Coil
> resistances are all equal. There are no excessive mechanical loads on
> the motor. I have narrowed it to the current overload on one leg
> only.
What are the respective voltages?
> I'll test with an added load on the RPC to see if that helps, but in
> the mean time; can I use a capacitor to adjust the current draw on
> that leg down? If so what MF values? I think that the caps in my
> surplus are all 600V rated.
Yes, you can do so. But I would eliminate other possibilities first
and measure voltages.
i
Jon Elson
> I have a 7.5 HP TEFC Motor on a hot high pressure washer. My 3 phase
> power is generated from a rotary converter. This motor is not happy
> with my home made power and trips the heaters. Normal load is about 9
> Amps under load but the one leg pulls 13-14A and trips the heaters.
> The contactor is a small GE unit with integral heaters and I have them
> set on max which is 14A IIRC.
>
per HP. So, I'd expect a 7.5 Hp 3-phase motor to draw roughly 22 A per
line on 240 V. Ohhh, pressure washer -- must be some of those inflated
HP readings. So, it probably is really a 3 Hp motor.
Jon
Gunner Asch
Ayup..my big Hotsy pressure washer..with the diesel/kero burner..only
has a 1.5 hp pressure pump.
And the fucking motor just died..damnit! Ive got a TON of 3 phase
motors..but nothing that big in 110/220 single
phase...sob...whimper..snivel.....
Ahum....sniff
Gunner
"First Law of Leftist Debate
The more you present a leftist with factual evidence
that is counter to his preconceived world view and the
more difficult it becomes for him to refute it without
losing face the chance of him calling you a racist, bigot,
homophobe approaches infinity.
This is despite the thread you are in having not mentioned
race or sexual preference in any way that is relevant to
the subject." Grey Ghost
Wes
>And the fucking motor just died..damnit! Ive got a TON of 3 phase
>motors..but nothing that big in 110/220 single
>phase...sob...whimper..snivel.....
If you have time, it might be the perfect oppertunity to explore rewinding motors. If you
could master that, your money worries would be over.
Wes
Gunner Asch
wrote:
GACK!!!! Thats insane! There is too much Wierd Stuff! inside of those
little gray cans. Id go mad..mad I say!...mad as ..well..a Leftwinger.
Shoot me first!
Gunner, pondering on the subject...hummmm....
Ignoramus9593
i
Gunner Asch
It runs for about a minute..then kicks out the breaker built into the
motor. Without any sort of load. At full speed.
So its not the start windings, its not the start cap.
It must be a short in the motor windings
Gunner
Don Foreman
wrote:
>On Mon, 19 Apr 2010 19:28:35 -0400, Wes <ClutchAtL...@Gmail.com>
>wrote:
>
>>Gunner Asch <gunne...@gmail.com> wrote:
>>
>>>And the fucking motor just died..damnit! Ive got a TON of 3 phase
>>>motors..but nothing that big in 110/220 single
>>>phase...sob...whimper..snivel.....
>>
>>If you have time, it might be the perfect oppertunity to explore rewinding motors. If you
>>could master that, your money worries would be over.
>>
>>Wes
>
>
>GACK!!!! Thats insane! There is too much Wierd Stuff! inside of those
>little gray cans. Id go mad..mad I say!...mad as ..well..a Leftwinger.
>
>Shoot me first!
>
>Gunner, pondering on the subject...hummmm....
My neighbor at the lake rewinds motors, as does his wife. Well, they
both did when starting their business anyway. Now they have employees
that do most of it, Marilyn tends the office. Their motor rewinding
business has done well because they've served their customers well.
Their customers are farmers who irrigate and a surprising number of
small machine shop and production companies in rural MN. Lots of
motors that are important to production. Getting them back up and
running ASAP is what he does.
They work hard, wouldn't know how to do otherwise. He also farms a
little, though I can't imagine why.
They are well-to-do. Their "cabin" is a freakin' palace, but they're
good neighbors and we've become friends.
They are politically active and strongly opinionated, but not
offensive about it. We're pretty much apolitical.
Neither of them have any technical training that I know of,other than
OJT some time, some where. They're definitely bright, industrious
and ambitious.
They have prospered rewinding motors up to 200 HP in rural MN.
http://www.merchantcircle.com/business/Kent.Electric.Motor.Repair.2.320-843-2418
Gunner Asch
While it sounds very interesting..frankly...I have no idea about how its
done and there is no one close that could teach me.
Bakersfield, the heart of the oil fields, has two very very small motor
rewinders and both buildings are probably no bigger than my front room.
So evidently the oil companies buy new motors when one fails.
But it is a hell of an idea.
Gunner
Wes
>While it sounds very interesting..frankly...I have no idea about how its
>done and there is no one close that could teach me.
>
>Bakersfield, the heart of the oil fields, has two very very small motor
>rewinders and both buildings are probably no bigger than my front room.
>So evidently the oil companies buy new motors when one fails.
>
>But it is a hell of an idea.
Free reading:
http://www.archive.org/details/armaturewindingm00brayrich
Wes
Gunner Asch
wrote:
Book marked for later today! Thanks!!
Lloyd E. Sponenburgh
news:af9qs514flm0mmesp...@4ax.com:
> So its not the start windings, its not the start cap.
>
> It must be a short in the motor windings
I've lost the origins of this thread. Is this a single-phase, capacitor
start motor?
If so, it could run at full speed with the start winding still connected,
and if the centrifugal switch didn't cut out, it would overheat and stop.
(without there being a short in the windings)
LLoyd
Ignoramus3512
> On Mon, 19 Apr 2010 20:32:49 -0500, Ignoramus9593
><ignora...@NOSPAM.9593.invalid> wrote:
>
>>Gunner, what do you mean motor died?
>>
>>i
>
> It runs for about a minute..then kicks out the breaker built into the
> motor. Without any sort of load. At full speed.
>
> So its not the start windings, its not the start cap.
>
> It must be a short in the motor windings
>
i
Gunner Asch
Ok...so how to I check?
Lloyd E. Sponenburgh
> Ok...so how to I check?
>
> Gunner
>
>
Deliberately disconnect the start winding from the centrifugal switch.
Wind a cord around the shaft, and "pull-start" it (very, very damned soon
after you apply power).
If it runs without kicking the over-temp, it's the start switch that is
failing to open, and that should be _really_ easy to fix, since it's
purely mechanical.
The other alternative is just to open it up and fiddle with the switch.
It could be corroded and jammed, or just welded shut from a bad high-
current "make" at some time.
LLoyd
Gunner Asch
Thanks Lloyd..Ill check it out. Its NOT a THHC motor..so it does have
openings into the motor housing and it does spend its time outdoors in
the open. So the switch could indeed be stuck