Old phase convertors were mechanical devices.I haven't seen one in years. Today you can buy a small solid state converter for a few hundred bucks.. I think they make models up to 5 HP.They work fine for motor like a mill that doesn't start with a load on it.Phase converters don't supply true 3 phase power but a modified single phase power. This is not something I'm real familar with as I do mostly power distribution work.Most wireman look at phase converters like putting a 4 bbl carb on single barrel adapter.I Have installed the new solid state ones a few times for friends. They were satisified with the operation.