OK, I finally got around to hooking up a big ol' 3ph. welder to my phase convertor to see how it works.. It works pretty good. The welder is an Idealarc DC-600, it's a CV/VV DC only machine rated to be good for 600 amps at 100% duty cycle ('tis a manly welder, me buckos..) and the phase convertor is a 3hp, balanced and pf corrected. I'm using it as a stick welder to start out, it already had the cables hooked up when it came to me so that seemed the process to try (I'll probably try it with the wire feeder someday, I'll post the results if I do). I burned 1/8" 7018 for about a half hour and checked the phase convertor, it was hot but not excessively so- I could touch it, but not leave my fingers on it for long, this with an ambient temp. at the convertor of about 90F. Another point I learned is that you want the leads for the control transformer to go to the line connections that come from your single phase- if you put the transformer on the generated phase the voltage will drop enough when you strike an arc to turn the machine off. Also, if anyone's curious, this is a transformer machine with SCRs in the rectifier, I'd figure that a machine with ordinary diodes in the rectifier would work at least as well as this.
To sum it up- it works:-)
John