Something unpleasant that I discovered with my AC ammeter.

There is a lot more current going from the contactors to the terminals
on the rectifier, than is going from the transformer (A1, B1, C1) to
the rectifier. 60A per leg at 100 A welding current.

I think that I misunderstood current flows for my configuration. I
will have to replace wires going from the rectifier to contactors,
with something heavier or will use triple wire.

Everywhere else the currents are quite manageable for 10 gauge, 200C
rated military hookup wire. (about 20 or so amps per wire at 100A
welding current)

i

On Mon, 29 May 2006 05:38:24 GMT, Ignoramus27812 wrote:
I am working on converting an old Hobart CyberTig into a welder/plasma
cutter that would support several welding modes (CC, CV, TIG, Stick
etc).

I am trying to do at least a little bit every night.

The neighbor for whom I cut her refrigerator in half (so she could
take it out of the basement) took my son to a museum for half a day,
so I had some more time today.

What I was working on lately is a contactor system that switches my
three phase transformer (six secondaries, two paralleled on each
phase), from low voltage "parallel delta" at 65 VAC, to "series Wye"
at 240V.

You can see this thing he

http://igor.chudov.com/projects/Weld...r/dscf0133.jpg


When one set (three top contactors) are closed, I have a parallel
Wye. When another set (two bottom contactors) are closed, I have a
"series Wye".

Here's a picture of how I labeled leads of the transformer:

http://igor.chudov.com/projects/Weld...ifier/xfmr.jpg

Here's a picture of the actual transformer:

http://igor.chudov.com/projects/Weld...lding-xfmr.jpg

Here's what leads are closed by what contactors:

Transformer:

A1 B1 C1
A2 B2 C2
A3 B3 C3
A4 B4 C4

================================================= =====================
3 phase, 67VAC, Delta:

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

(A1, B4) (B1, C4) (C1, A4)

Contactors:

A1 B1 C1 A1 B1 C1 A4 B4 C4
A3 B3 C3 B2 C2 A2 A2 B2 C2
================================================= =====================
3 phase, 220 VAC, Wye:

(A2, A3) (B2, B3) (C2, C3)
(A4, B4, C4)

Contactors:

A3 B3 C3 ========
A2 B2 C2 A4 B4 C4
================================================= =====================


Anyway, the bottom line is that that subsystem is working. If I
energize one set, I get low voltage (90 VDC OCV), high current, if I
energize another, I get high voltage (300 VDC OCV).

Made some welding beads today just to make sure that low voltage works
okay. Now (unlike before where I did not have paralleled secondaries)
I can easily go up to 200 amps.

I have to manually plug one or another lead for these contactors,
instead of using a switch, and here's why: I bought (but have not yet
received) a nice microcontroller, which I will try to program so it
can do whatever I want as far as welding sequencing, etc. It has built
in relays. So, I will let it control when to use high voltage or low
voltage. I would not use a manual switch.

My current (temporary) control panel looks like this:

http://igor.chudov.com/projects/Weld...r/dscf0134.jpg

(you can see welder energized at 80 VDC OCV). You also see a low
voltage high voltage switch, which I have not yet wired (see above).

I have also decided to stay limited to 200 amps for now.

i