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Old 03-16-2019, 09:20 PM   #21
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Thinking a little more: If you have one transformer, it is probably a wye secondary with no neutral connected. If so, a primary winding failure could cause a similar problem.

In either case, it would seem to be a transformer problem IF NOTHING ELSE HAS CHANGED.
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Old 03-17-2019, 12:04 AM   #22
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Quote:
Originally Posted by varmit View Post
If one of the transformer delta secondaries has failed, you could still measure the correct voltage on all three phases. The failed winding could cause a phase shift that would cause reverse rotation.

Motor current would probably be higher also.
motor temp would also be higher as well
If a motor lost a phase while running it would still keep on running but would get very hot
the op's situation is a bit conspicuous!
what he changed is a light and it should not have had any effect on the 3 phase power system.
seeing that nothing else has changed points out to a poco issue even if they stated hasn't been changed but who knows?
its possibly a transformer issue.
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Old 03-17-2019, 04:48 AM   #23
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If one of the transformer delta secondaries has failed, you could still measure the correct voltage on all three phases. The failed winding could cause a phase shift that would cause reverse rotation.
That’s what I was thinking. Looking back at my post I should have said lost a winding not lost a phase. I know a three phase motor won’t start if it lost a phase, but loosing a winding makes it go from closed delta to open delta.

With an open delta you will still read 480 volts phase to phase. I thought phase to ground was different. Like 480 to ground on two phases and zero on the last phase? <edit: maybe I was thinking corner grounded>

That’s why I was wondering if her checked voltage with the main open, so he wasn’t reading a back feed through three phase loads.

Only one delta transformer on the pole? That part I don’t get. So what are they doing there? Grabbing two of the transformers from the other bank?

Wouldn’t you need at least two transformers for the delta bank (not borrowing any from another bank? It is common to see a single phase center tapped transformer to feed one customer 120/240 and the added transformers for the small three phase customer. Like a four unit apartment next to a tire shop.
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Old 03-17-2019, 07:25 AM   #24
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How does the missing leg cause a phase shift? I don't see how phase rotation changes with the open delta.

Say you have AB BC and CA windings. They are 120 degrees apart. Say you lose CA winding and AC is now coming from AB and BA. OK. The voltages are all the same. The grounded corner if any is the same.

Aren't AB and BC still 120 degrees apart, and in the same direction, because that comes from upstream? I would think if BC was AB + 120 before, it must be the same after.
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Old 03-17-2019, 11:07 AM   #25
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It will be interesting to here what the problem turns out to be.

Reading this

https://www.electricalpereview.com/o...er-connection/

It appears the Voltage phase angle doesn’t change but the phase current does. They are using a perfect power factor in their formulas though.

To me though, if nothing changed on the utility side, my guess is a faulty winding. I’m leaning that way more than a phase reversal bandit!
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Old 03-17-2019, 11:10 PM   #26
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How does the missing leg cause a phase shift? I don't see how phase rotation changes with the open delta.

Say you have AB BC and CA windings. They are 120 degrees apart. Say you lose CA winding and AC is now coming from AB and BA. OK. The voltages are all the same. The grounded corner if any is the same.

Aren't AB and BC still 120 degrees apart, and in the same direction, because that comes from upstream? I would think if BC was AB + 120 before, it must be the same after.
It would be determined by where the open winding or winding jumper was located. Say you lost the incoming "C" portion of the AC winding, The jumper from "A" could still energize the AC winding, but would be in series through the "A" winding. This would reverse the polarity of the "C" winding.
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Old 03-17-2019, 11:18 PM   #27
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Quote:
Originally Posted by varmit View Post
If one of the transformer delta secondaries has failed, you could still measure the correct voltage on all three phases. The failed winding could cause a phase shift that would cause reverse rotation.
Sorry Varmit, but I don't see what you're talking about either.

I've never come across this in countless failures.


Keep it simple ... Poco usually comes clean when you call them out on it. Maintenance guy just hides in the corner till the heat cools down

They did some work ... now you're not an electrician, but a detective
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Old 03-28-2019, 09:19 PM   #28
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Originally Posted by foothillselectrical View Post
I agree with every theory posed here, including that "random Phase reversals cannot just happen." But, I am the only one who does work in these facilities and, other than the light fixture replacement previously mentioned, I haven't done any work in these two particular buildings in months. And the only two men who can authorize work to be done, haven't had anything done.


I am truly stumped on this one. The POCO is sending out a couple of engineers Monday to see if they can determine if something has changed on their side, but, as stated earlier, I can physically see the primary jumpers from the 208V transformer to the 480V transformer. And the 208V side is running correctly. I have checked rotation on several motors in facilities fed from the same utility line, including a different building yesterday a half mile away, and all is well in those facilities. I do not believe that the utility has done any work on the lines.

As far as the air compressor running backwards, it is a 2 stage piston compressor. I do not believe there is a check valve leaking by as it has held pressure for the several days this issue has been going on. And, I don't think it could cause the motor to start backward if it was leaking because it should only compress the piston to the bottom of it's stroke.

Thank you to everyone who has provided input on this.
What did you/they come up with and find?
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Old 03-29-2019, 01:29 PM   #29
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What did you do to correct the situation? Was it just those 3 machines running backwards? When the compressor runs backwards does it become a vacuum pump?
When a piston type compressor runs backwards for to long it burns up because the cooling fan on the flywheel is going backwards. Don't ask how I know this. It pumps air like normal. I think the valves are timed at TDC.
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Old 03-29-2019, 08:16 PM   #30
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When a piston type compressor runs backwards for to long it burns up because the cooling fan on the flywheel is going backwards. Don't ask how I know this. It pumps air like normal. I think the valves are timed at TDC.
The valves are reed style so no timing, no camshaft. When the piston goes down the inlet opens, going up the exhaust opens. IR compressors turn the opposite direction of the rest of compressor world, and it trips up a lot of installers. Iv'e ran into numerous compressors rolling the wrong direction and while they were hotter, they generally survived the abuse.
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Old 03-29-2019, 09:32 PM   #31
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The valves are reed style so no timing, no camshaft. When the piston goes down the inlet opens, going up the exhaust opens. IR compressors turn the opposite direction of the rest of compressor world, and it trips up a lot of installers. Iv'e ran into numerous compressors rolling the wrong direction and while they were hotter, they generally survived the abuse.

reed valve pumps can run in reverse and still function, to counteract the heat issue building in air flow guides around the pump housing allows better control of airflow and heat transfer.
for a setup like that it wouldn't matter which direction the motor runs.
losing a winding on a motor thats running will continue to run but it will be hotter.
now if it had an interuption of power it would not rotate but just hum.
if this motor was on a loaded belt conveyor (elevator type) unless it was equipped with a brake motor the weight on the conveyor could rotate the motor backwards enough for the power to run it up.(Ive seen happen this on an old batch elevator that was not equipped with a brake motor)
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Old 03-30-2019, 01:59 AM   #32
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When a piston type compressor runs backwards for to long it burns up because the cooling fan on the flywheel is going backwards. Don't ask how I know this. It pumps air like normal. I think the valves are timed at TDC.
I was KIDDING. It doesn't do much good for scroll compressors either. How do they time reed valves? (Again, just kidding). When the flywheel fan runs backwards, does it create a negative pressure? jk.
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Old 04-12-2019, 09:23 PM   #33
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OP, did they ever figure it out?
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Old 05-28-2019, 09:53 PM   #34
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Bump here, but what was the resolution to the problem? Enquiring minds want to know.
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