On Mon, 24 Apr 2006 18:58:10 GMT, Glen Walpert wrote:
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!

Not the easiest, but, definitely, not terribly hard. A little bit of
cutting and maybe a little bit of drilling will "get me there".



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.

Well, I have only one controller set.

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.

Makes sense.

(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.

That (about the phase converter) is a great point indeed.

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.

It is a f*&^ing brilliant idea. I will do exactly that! I will try
starting tonight.

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?

Well, my current SCRs are wired with gate towards the common DC+ bus,
so there is no way for the PCTI controller to sense voltage.

I already have enough isolated dual SCRs, adding them to the octagonal
DC+ bar is very easy. Note that the DC+ bar will no longer be
energized, it will only be used for cooling.

If you do not understand what I mean in the above paragraph, it is my
fault, my main message here is that I have all parts for using half
windings.

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

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


I have to admit to it! I think that you know what you are talking
about.

My plan is as follows:

1) use one half of the secondaries as a Wye connected 3 phase source
2) add new dual SCR modules to the octagonal heatsink (that used to
double as DC+ bar)

3) wire everything together with the PCTI modules for some test runs.

If everything works and produces voltage and current, then I could
work on doing a few things such as:

1) reconnecting second half of secondaries
2) making a proper welding control panel.

Glen, do you know anything about plasma cutting? The reason for this
question is that if I put secondaries in series, I would get a power
source that can supply 190V OCV and about 135V continuous voltage.

I could make this transformer switchable from paralleled secondaries
to secondaries in series, with three contactors (not too
expensive). Would it be able to do any plasma cutting?

It would be good to decide on whether to do it, sooner rather than
later. But, first things first, I will definitely try your great idea
about using only hald of the windings "as is".

i