On Tue, 25 Apr 2006 13:26:48 GMT, Glen Walpert wrote:
On Mon, 24 Apr 2006 19:25:02 GMT, Ignoramus21673
wrote:

clip

(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.

Mabey, I am not sure it would actually work out to be a problem, just
something to check.

OK, I will keep this in mind. My phase converter is 17.5 HP idler
capacity. When I loaded it with full 200 amps of welder output
(shorted leads), the third leg sagged to about 190 volts.

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.

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.

Sounds like a good plan to me, I trust you'll let us know how it goes.

I already started, the first thing that I am working on is mounting
the SCR modules.

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".

Alas, I know next to nothing about plasma cutting. Never even tried
one. They weren't all that common when I was welding a lot in the
early 70's. I think it is a constant current process with the arc
confined to the torch and not transferred to the workpiece as in
plasma welding, but that is just a foggy memory of something I may
have read 20 years ago. Perhaps you can pick a particular torch and
find out what it needs.

I see. Well... If I connect my secondaries with each pair in series,
wye, it should give me about 208 volts AC to play with. I think that
it would result in about 300 volts OCV and 200 volts welding power.

But first things first, I want to make it work as a welder first to
get the proof of concept.

i