[TCML] Potential transformer, ballast and input voltage

[SusaX2 X2 <susax2@xxxxxxxxx>]

Hi everyone,

as a replacement for my molten 4-MOT stack I'm getting a potential
transformer.
With this, I immediately stumbled upon 2 problems.
First, the PT needs 100V and mains is 230V.
Second, the existing ballast has too much DC resistance

By coincidence I found a cheapo stickwelder (topcraft tcl1500,) which had a
larger E core and a thicker wire, compared to the old ballast. Current is
limited by adjusting the position of the shunt laminations, specified
40-140A (probably less), open voltage 45-48V.

So I got this idea to rewind the secondary and combine this with the
adjustable current limiting funtion.

The 48V winding was removed and I noticed that the coil didn't provide
enough space for the new winding. The primary coil form though did, so I
rewound the primary wire onto the narrower coil form. Maybe few windings
less, but still respecting the original turns/volt ratio which is about 1,
about the same as in MOT's.
(By the way, nowadays MOT's and hobby welders are all wound with aluminium
wire...)

I had a spool of 1,5mm building wire left, just enough length to wind a
bifilar secondary with 110 turns. Both wires parallel give an open voltage
of 98-112V, depending on the shunt position. And 194-222V for both wires in
series.

But I still had some doubts if it would provide enough current limiting, so
I tested the transformer on the largest (and last) 230V MOT with it's
secondary open and shorted. With the shunts all the way in, secondary
current is 0.7A open and 6A shorted. Turning the shunts all the way out,
secondary current is 2A idle and 13A shorted.

Has anyone tried something similar before and how well (or not) did it work?

Also a question about potential transformers, since the one I'll get is my
first PT. It's only 6000:100V and the low voltage winding has a 600VA max.
thermal rating, however the datasheet specifies a threefold for 6 minutes.
I know these things are designed to take at least 120V before saturating.
I'm presuming it's safe to take 2200VA for a short time. Is this correct,
will it melt down or can it be pushed much higher?

Cheers,
Mark
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