Lee I have used rotary phase converters and VFD's.......anybody that reccommends a rotary phase converter over a VFD has probably never used a modern VFD.
I used to have a 3HP 3phase lathe that I ran on a rotary phase converter, when the lathe wasn't being used the noise of the motor on the rotary phase converter was always in the background also when the rotary phase converter was started I could see the lights dim in the workshop.
With a VFD (which I have been using for over 7 years) I have the best of everthing, no background noise, no power surge (lights dimming) the ability to change speed on the fly and virtually no power usage when the machine is idle. Some talk about the robustness of VFD's well Ihave a 2HP VFD that all I do is turn it on and start it, I have done all the machine on and off's with the machine controls, I've even occasionally left that VFD powered on and in the run state with no load for days at a time......this VFD is still running with absolutely no problem 7 years after buying it.
My first VFD was originally bought to run a lathe, now I use one on a lathe, drill press, Bridgeport, cylindrical grinder, compressor, and T&C grinder, 3phase machines are more common and also cheaper to buy due to the lack of 3phase supply to most households and home workshops.
I had a friend who converted his 3HP 3phase lathe to single phase after running my lathe he wishes he had just installed a VFD........Ian