I have a Hobart CyberTig 200 DC-S welder. It is 3 phase, however, the
fan motor in it is single phase. (110V). I verified that the welder
powers up on 3 phase 240V. (after I rewired it).

My question is, is it safe to run it on single phase 240V (provided
that I select the right wires to power signal transformer and fan). If
so, what would be a safe reduced output amperage for welding. For 3
phase, it is rated 200 A continuous, I believe.

It would be great if that was possible, at least for small projects.
I could use some 8/3 cable that I have, not worry about the phase
converter etc.

i