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Join Date: Apr 2011
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Pole Power

09/18/2017 11:29 PM

Out in the oil patch in west Texas they have many remote spots running 480 volt three phase pumps. This is a disconnect for one such application, (sorry could'nt figure out how to get the picture rotated) I'm trying to figure out how this works. The 'B' phase has a copper bar to ground yet they can run a three phase motor. This site had some other problem, the power was reported as down. when I got there I checked the voltages and currents. Here are the findings:

Voltage Phase to Phase

A-B 481 - 485

B-C 377 - 425

A -C 409 - 430

The voltages were fluctuating between those values, the voltages to ground were 0 on 'B' of course and I forgot to record A and C. What really threw me was the load. It was a 20 HP motor running a PD pump at 1179 rpm and it was running fine. The load currents were 25A on 'A', 26A on 'B' and 'C' was 1.5A and would jump up to 8A. I wanted to investigate further but it was running fine and we had other site issues to get to so we had to leave. No one reported any other problems. How can that work? What was going on? Thanks

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#1

Re: pole power

09/18/2017 11:37 PM

It's likely a corner grounded Delta setup with a third leg phantom phase.

If so they can cheat the system and only use two fuses and it would also explain the odd voltage and current readings.

Does the site only have two transformers on the power pole rather than three? If so then what I described is what it likely has.

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#2

Re: pole power

09/19/2017 12:20 AM
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Commentator

Join Date: Apr 2011
Posts: 76
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#4
In reply to #2

Re: pole power

09/20/2017 2:18 AM

Thanks. that was what I needed to see. I still don't understand why I see no current on the 'C' leg, but maybe they had switched B and C at the starter. I see now how it should be and next time I'm out there I'll look it over closer. And I'll check out what transformers are there. Thanks again folks.

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#5
In reply to #4

Re: pole power

09/21/2017 9:45 PM

If it's running off of two transformers in a phantom phase setup the phase winding A-C in the diagram would be missing to which when doing current and voltage readings from point to point it's possible to get some very odd phase shift effects between each phase that can result in some false voltage measurements.

Part of it has to do with the fact that in a 3 phase system, although theoretically all three phases should be 120 degrees off from each other, they can in fact drift a fair amount depending on what loading, or even feedback, effects may be present on any one of the three phases at any time from anywhere else in the system.

As I came to learn three phase theory the way it was first explained to me is to think of each phase (as shown in the picture) as a actual spring where their connection points can move around to some degree by either stretching or compressing the individual springs of each phase.

Ideally they want to all stay in the correct balance but in reality stuff gets pushed and pulled whenever there is a loading change in the overall system.

The more you compress or stretch one the more the other two have to move and compress or stretch to hold the new positions thusly the ideal equilateral triangle diagram gets a bit wonky if things get unbalanced far enough.

Now with a phantom phase system one whole leg (spring) in the triangle is missing to which allows the other two to move a lot further with less influences than normal thusly making for some rather odd phantom voltage measurement issues that may come and go as other unbalanced loads in the system shift their respective influences around.

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#3

Re: pole power

09/19/2017 10:16 AM

Yes, you have one phase at ground reference potential. The relative potentials still hold. This would not be my preferred way to set up a 20 HP motor, but it still works, and no one got jolted so far.

If one fuse blows what are your safety implications?

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