I have run a phase converter for years now. It has a 7.5hp idler motor. I use it to run a lathe (3hp), mill (1hp), 2 grinders, bandsaw, press, and ironworker. My model has always been to use one machine at a time.
A good friend of mine called yesterday. He got (probably free) a 15hp motor. He wants to bring it over and try to start it in my shop. Just the naked motor, only self-loaded. Do I even have a chance of being able to start it? My phase converter is connected through a 40A breaker via a 60A disconnect box which uses Buss fuses. I don't remember what size fuses are in the disconnect box anymore. I have a little rig I made which uses a 30A disconnect and has a cord with large alligator-type clips which I use to test motors, but I've never even tried to start anything bigger than 7.5hp. I figure we'll fire up my phase converter, maybe adding another motor or two as extra idlers, then spin the big motor with a rope, then quickly connecting it to power. I don't much care if it blows the breaker in my fusebox, but it would be a little bit expensive if it blew a bunch of those Buss fuses. Don't remember but it seems they cost at least $5 each.
My alternatives are to tell him no problem, bring it on and we'll try it, but if any fuses blow, he's got to replace them. Second is to tell him to take the motor to a motor shop (there's a good one I've used before) who ought to spin it up for him and even put it on their megger to look for shorts/opens without charging too much.