I'm in the UK and have a standard single-phase 240V mains supply.
I've got a spot welder marked variously as 380V (on the handheld welder) and 400V (on the dedicated control box). It is not, however, 3-phase : internally it's wired across 2 phases which wil give a single-phase 380V supply. The control box has internal wiring that can be reconfigured to 240V but the welding unit itself is fixed at 380.
To run this from my 240 supply I'd therefore require a 240-380 transformer of 5kVA or so, and an outlet capable of supplying about 24A.
I could also reconfigure the control box and underrun the welder by 30%. This might be adequate.
However, it seems to me that I could replace the 240-380 transformer with a tool transformer configured with 240V in and with the 110V out boosting the 240. Furthermore, the tool transformer will only be providing 1/3 of the total output so can be a relatively cheap 3k3VA unit.
I''m sure this sounds hairy but I can't actually see a flaw in it. What do you think ?
(Note : I'm aware that the tool transformer has an earth at the centretap. The arrangement described would put this at 220+55 above neutral so it should not be connected to earth. I could connect the 110V winding to neutral rather than live and I would then have a 220-0-110 supply).
I'm quite capable of connecting, testing and using this safely, but I'm wondering if I've missed anything.
postnote :
(I see there is a similar question
https://www.eevblog.com/forum/renewable-energy/converting-a-safety-isolating-transformer-to-an-autotransforer-for-460vac/ which suggests it's OK, though the answers there have mainly concentrated on creatiung a 400x1.4 DC bus)