On Tue, 02 May 2006 15:00:13 GMT, Ignoramus5749
wrote:


A little update, I took out the old rectifier from my Cyber Tig, which
freed up a lot of space, for a nice installation of isolated case dual
SCRs.

I will hopefully install the heatsink and SCRs in a couple of days. It
will need some brackets to fit into the space available.


Sounds good so far. Do you have a MIG torch for testing under load
yet?

No, all I have is TIG equipment...

So I assume you are scanning ebay for a deal?

An alternative to transformer reconfiguration with a bunch of large
contactors would be a patch panel with the transformer windings and
load connections brought out to studs on an insulating panel, with
copper bussbar links and a few jumper cables able to switch to any of
the 5 configurations (6-phase, and 3-phase wye and delta both with
series or parallel winding connections). Compare the time difference
in assembly with the likely time spent changing configurations, and of
course parts on hand ...

Well, I already bought five 50a contactors for $9.99 each. I drew some
schematics, they should be enough to switch between parallel delta,
vs. series wye (for plasma).

50 A is in the ballpark but seems a bit light for your transformer;
assuming a nominal rating of 200 A, 100 A per 3-phase section, 100 /
sqrt(3) = 58 A per winding. Probably close enough.

Your original rectifier did not make very effective use of the
transformer secondary windings, as each winding was used in one
direction only. With your full wave bridge each winding is used in
both directions, reducing copper losses by 1/sqrt(2) IIRC, so you
could use about 82 A per phase from one of the two windings for 82 *
sqrt(3) = 140 A RMS total welding current. (If you did that for both
secondary windings at the same time you would probably overload the
transformer primary, which was designed for the half wave rectifier
configuration). So you could do reasonable load testing with your
existing single winding configuration. Also you get the same VA with
either delta or wye, so the delta config of one winding would be good
for 140 * sqrt(3) = 245 A. With both windings paralleled the delta
connection will provide 200 * sqrt(3) = 346 A, limited by the
transformer priimary.

clip

Sorry about that Glen, I changed the Subject of this post.

Thanks!