On Mon, 24 Apr 2006 02:49:10 GMT, Ignoramus17838
wrote:

Glen, I have physical access to all 12 leads: I have three legs, two
secondaries on each, and two leads on each secondary, all plainly
visible and physically accessible.

Some secondaries are connected in parallel to form the existing "6
phase rectifier", but I could disconnect them.

Here's the schematic of my welding xfmr:

http://igor.chudov.com/tmp/firing/xfmr.jpg

Here's its actual pictu

http://igor.chudov.com/tmp/firing/welding-xfmr.jpg

I got around to looking at the picture; not the easiest connections to
rewire with those brazed neutral bussbars!


Right now, the following points are connected:

A3, B3, C3;

A2, B2, C2;

These connected pieces are connected to the interphase transformer.

I want to rewire it by disconnecting A3, B3, and C3 from one another,
and by connecting

(A3, A1); (A2, A4)
(B3, B1); (B2, B4)
(C3, C1); (C2, C4)

Then I will have a common point of (A2, A4, B2, B4, C2, C4); and the
wye leads (A1, A3), (B1, B3), (C1, C3).

Is this wrong?

i

That is the correct way to rewire the transformer secondary as single
3-phase Y (and not what I thought you were planning), but I thought
you said you wanted to leave your existing 6-phase rectifier connected
and operational, and rewiring your transformer as 3-phase will
preclude that. This also gets rid of the current balancing action of
the interphase transformer, which is of course completely
disconnected.

Glen, just to make sure that you are aware, I was given a three phase
firing system by PCTI. As a sign of appreciation for certain activity
of mine (algebra.com). Wiring it into a existing 3 phase supply is
next to trivial. All I have to do is supply SCRs and potentiometers
and switches. I already have it sitting on my desk.

Here it is:

http://www.pcti.com/3phfiring.htm

The question that we are discussing is how to make my power supply
compatible with this PCTI system.

So, yes, I was hoping that I could simply use my "6 phase rectifier"
to be controlled by this PCTI system. As of now, it does not look like
I could do it, because it is really a 6 phase system with different
phase angles that necessitate different firing angles.

Your 6-phase rectifier is two 3-phase rectifiers in parallel, with 30
degrees phase difference between them, so you could use a pair of the
PCTI controllers for your pair of 3-phase rectifiers.

Why not use the installed rectifier as wired, and build a new
controller for it, adaptable for either CC or CV? If you have
problems you can just reconnect the original controller.

Well, I think that reconnecting the xfmr (which is easily reversed) is
quite easy, will take a couple of evenings. And I already have a well
working SCR controller, made for rough industrial conditions, by company
that is.

If you are willing to abandon the existing rectifier entirely then
transformer rewiring and a standard 3-phase SCR rectifier module
should work. But it seems like the hard way to do the job to me.

It is hard if we assume that I need to make a SCR controller in either
case. But it is not my situation, my situation is that I have a super
nice controller for a 3 phase bridge.

Also, Glen, if I wire this transformer as a Wye vs. Delta, is it true
that I would get different voltages phase to phase? I have a suspicion
that the side effect of my Wye wiring would be increase of voltage by
sqrt( 3 ). Is that true?

The transformer winding connections you have shown above are still Wye
connected (you still have a neutral), but if you are planning to
connect your new rectifier in Delta (and not use the neutral) then the
line-to-line voltage will be sqrt(3) times the line to neutral voltage
seen by the original rectifier. You could reduce the line-to-line
voltage to the same as your present line-to-neutral voltage by
rewiring the transformer secondary as Delta, with no neutral. (The
winding voltages will stay the same.) But since you are using a motor
based rotary converter with inherent voltage imbalances a Delta
connection could result in excessive circulating current - you would
want to measure this with a current shunt in the loop before using a
Delta winding connection.

How about starting out by using 1/2 of your transformer secondary
windings only; either one of the two Wye connected windings provides
3-phase power as is. If it works and you decide you need full power
instead of half power, get another control module or rewire the
transformer.

How about temporarily disconnecting half of your transformer and
rectifiers, so that you have only a 3-phase rectifier, and you can use
your existing SCRs and wiring too?

Thanks, I always listen very closely to what you say.

Fine, just don't always assume I know what I am talking about :-).