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'[EE]: AC Power problem -pulling hair out, not much'
2002\07\16@111518 by Chris Loiacono

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For the power people out there (or anyone who may venture an explanation):

I have a large 3-phase transformer connected to a 208V  line (pri) to get
480 on the secondary in order to test one of my PIC based controllers. The
controller depends upon the timing between phases to determine phase angles
to output properly timed firing pulses.

Everything on the control board is working, ie; zero-cross detectors, PIC &
timing h/w, etc... etc... yet the result appears to be off time, since the
output is not in proportion with the input.

When checking the actual 3-phase line w/ a scope, it seems that I have three
phases that are all 120 degrees apart, but one of them is inverted. Of
course, this would throw off the timing for that phase by 180 degrees.

I have never seen this before, and am wondering if the transformer is in
some way responsible. The primary side is wired as a wye - "N" at center.
The secondary is a pure delta, no "N" or "G".
Can anyone offer an explanation as to how this phase can possibly become
inverted?????

Thanks in advance for any ideas that may get my back away from the wall.

Chris

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2002\07\16@114015 by Alan B. Pearce

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>I have never seen this before, and am wondering if the
>transformer is in some way responsible. The primary side
>is wired as a wye - "N" at center. The secondary is a
>pure delta, no "N" or "G". Can anyone offer an explanation
>as to how this phase can possibly become inverted?????

The appropriate winding, either primary or secondary, is wired back to
front? I do not have any experience with 3 phase, so do not know what the
implications are, but imagine that the transformer could be noisy
acoustically and/or draw a lot of current in this situation.

It may be worth making a winding on each of the three phases using a half
dozen turns of wire if you can squeeze them in and looking at the magnetic
flux relationship once you know you have three windings all in the same
phase. A half dozen turns should give enough signal to look at on a scope
for this purpose.

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2002\07\16@114426 by Quentin

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Chris Loiacono wrote:

> I have never seen this before, and am wondering if the transformer is in
> some way responsible. The primary side is wired as a wye - "N" at center.
> The secondary is a pure delta, no "N" or "G".
> Can anyone offer an explanation as to how this phase can possibly become
> inverted?????
That would be my bet as well. Who ever manufactured your tfm made a
booboo.
Say your secondary has three matching coils with wires A1 and A2, B1 and
B2, C1 and C2. A2 should be connected to B1 and that is a phase. B2 is
connect to C1 and that is another phase, and C2 is connected to A1 and
that is the third phase. It is for instance quite possible that B2 is
connected to C2 and C1 is connected to A1 instead, thus reversing the
third phase.

Quentin

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2002\07\16@152714 by Chris Loiacono

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OK, well, I'll follow up with Square D.

Now for an additional thought: If my scope has the BNC shields grounded, how
can I check phase-to-phase, or rather, across two corners of a Delta with
it?
Someone suggested removing the ground pin from the scope line cord cap, but
this goes against my better judgement....

Chris

> {Original Message removed}

2002\07\16@154630 by Dwayne Reid

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At 03:30 PM 7/16/02 -0400, Chris Loiacono wrote:
>OK, well, I'll follow up with Square D.
>
>Now for an additional thought: If my scope has the BNC shields grounded, how
>can I check phase-to-phase, or rather, across two corners of a Delta with
>it?
>Someone suggested removing the ground pin from the scope line cord cap, but
>this goes against my better judgement....

Not a good plan in this case.

I'd use 2 channels in differential mode (1+2 with 2 inverted).  Use Line
trigger mode.  You will be able to see the phase relationship easily.

You may need to calibrate the 2 vertical channels: all I do is connect them
both to the same point, then adjust the variable gain control on 1 channel
for minimum signal visible on the CRT.

dwayne

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2002\07\16@154952 by Barry Gershenfeld

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If it's a two-channel scope you should have a differential
mode available.

In this mode you invert one channel and add the two together
(usually, those buttons are separate.)

Barry


At 03:30 PM 7/16/02 -0400, you wrote:
>OK, well, I'll follow up with Square D.
>
>Now for an additional thought: If my scope has the BNC shields grounded, how
>can I check phase-to-phase, or rather, across two corners of a Delta with
>it?
>Someone suggested removing the ground pin from the scope line cord cap, but
>this goes against my better judgement....
>
>Chris
>
>> {Original Message removed}

2002\07\16@220559 by Bob Ammerman

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If your Delta is truly floating as you indicated in a prior post (no N or G
line) then you can temporarily ground one corner and measure the other two
relative to it.

Bob Ammerman
RAm Systems

----- Original Message -----
From: "Chris Loiacono" <.....chrisKILLspamspam@spam@MAIL2ASI.COM>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Tuesday, July 16, 2002 3:30 PM
Subject: Re: [EE]: AC Power problem -pulling hair out, not much left!!!!


> OK, well, I'll follow up with Square D.
>
> Now for an additional thought: If my scope has the BNC shields grounded,
how
> can I check phase-to-phase, or rather, across two corners of a Delta with
> it?
> Someone suggested removing the ground pin from the scope line cord cap,
but
> this goes against my better judgement....
>
> Chris
>
> > {Original Message removed}

2002\07\17@032540 by Alan B. Pearce

face picon face
>>Now for an additional thought: If my scope has the BNC shields grounded,
how
>>can I check phase-to-phase, or rather, across two corners of a Delta with
>>it?
>>Someone suggested removing the ground pin from the scope line cord cap,
but
>>this goes against my better judgement....
>
>Not a good plan in this case.
>
>I'd use 2 channels in differential mode (1+2 with 2 inverted).  Use Line
>trigger mode.  You will be able to see the phase relationship easily.

Not necessarily a good idea either at the voltages coming out of the
transformer. I believe you will exceed the common mode voltage range of the
inputs, and this level of voltage may also exceed the maximum input level
for single ended operation of each input.

Get a pair of 230V to 6 or 12V transformers. Connect both to the same pahse,
and get the outputs in phase on the oscilloscope. Now you know which end of
the primary winding one transformer is the "in phase" end relative to your
other (now reference) transformer. Move this transformer connection round
the other points you want to test, checking the signal phases as you go.

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2002\07\17@141200 by Peter L. Peres

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Maybe you have unexpected leakage somewhere in the transformer. Imho put
three lightbulbs as permanent loads in the secondary, wired in star, with
the ground from the primary. If they light up equally then maybe their
current can overcome the leakage and keep things straight. This is similar
to a previous posting of mine where I indicated the need for this
(relevant to ticklers or something like that).

Peter

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2002\07\17@141402 by Peter L. Peres

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On Tue, 16 Jul 2002, Quentin wrote:

>That would be my bet as well. Who ever manufactured your tfm made a
>booboo.
>Say your secondary has three matching coils with wires A1 and A2, B1 and
>B2, C1 and C2. A2 should be connected to B1 and that is a phase. B2 is
>connect to C1 and that is another phase, and C2 is connected to A1 and
>that is the third phase. It is for instance quite possible that B2 is
>connected to C2 and C1 is connected to A1 instead, thus reversing the
>third phase.

I have a doubt. What you describe would produce maximum load in the
primary and would cause the device to catch fire unless some breaker would
give. There is a rigid phase relationship between the phases in a 3-phase
network.

Peter

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2002\07\17@153930 by Dwayne Reid

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At 08:24 AM 7/17/02 +0100, Alan B. Pearce wrote:
> >
> >I'd use 2 channels in differential mode (1+2 with 2 inverted).  Use Line
> >trigger mode.  You will be able to see the phase relationship easily.
>
>Not necessarily a good idea either at the voltages coming out of the
>transformer. I believe you will exceed the common mode voltage range of the
>inputs, and this level of voltage may also exceed the maximum input level
>for single ended operation of each input.

The scope probes that I use are rated for 1000Vac continuous.  I assumed
that Chris has similar probes - he works with power electronics.

dwayne

--
Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 18 years of Engineering Innovation (1984 - 2002)
 .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-
    `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

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2002\07\17@232236 by Chris Loiacono

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My cable connection has been down and I haven't been able to reply until
now, but I wanted to say thanks for the help. The transformer, which is 1400
miles away from me, had several improper terminations made by the
electrician who installed it. Once re-wired, I now have properly phased
lines at both ends.

The scope that is being used at that site is an oldie, so I will have to
check to see if it has differential capability. Yeah, Dwayne, probes are OK
for 1kV also, same as on my scope here.

On a side note, troubleshooting 480V controllers over the phone sure is fun.
A well-meaning fellow decided to try to help himself while I was taking a
lunch break (without my cell phone, of course) by switching the gate leads
to two of the 160A 480V SCR's. He did this while I was trying to figure out
what was wrong with the transformer. I'm told they had a nice plasma display
& lots of ozone for a few seconds while a $1700 set of SCR modules and
controller respectively fused and the AC side of the control board found
some new paths for current. I believe the isolation was effective and the
entire digital side of the controller - PIC & all- held up. I will check
this for sure as soon as I receive the damaged parts back. I have seen this
happen on 240V and the SCR's can handle it. On 480 3 phase it's a definite
no-no. Anyway, that's one for my personal record book that makes me not want
to take lunch breaks ever again....I know someone who will never try that
form of trial and error troubleshooting again!

It also leaves me thinking of a way to protect the SCR's the next time
someone tries mixing the same wires...

Thanks again,
Chris
{Quote hidden}

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2002\07\18@155139 by Peter L. Peres

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On Tue, 16 Jul 2002, Chris Loiacono wrote:

>OK, well, I'll follow up with Square D.
>
>Now for an additional thought: If my scope has the BNC shields grounded, how
>can I check phase-to-phase, or rather, across two corners of a Delta with
>it?
>Someone suggested removing the ground pin from the scope line cord cap, but
>this goes against my better judgement....

Leave the ground be and use two probes. You HAVE to make a virtual ground
using one of the methods given (like putting three resistors in a star and
connecting the center to gnd). Floating the instruments on the delta phase
requires that you insulate everything, including yourself. At the power
levels you are delaing with normal leakage in the transformer is enough to
kill you. This means that what you measure may not be what there is in
there, unless you put some load onto the outputs. Some load is 0.1-1% of
the system rating or more.

Peter

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