Hi Tom, I use an 15hp American Rotary brand phase converter on a 10hp CNC mill, and it has performed excellent.
You should be good to go with a 50hp converter. With a lathe, you normally double the spindle HP for the converter. Lathes draw more load on the converter .
My phase convert seems to use very little electricity, i have seen no impact from it on my electric bill.
Here is a link to a video of my machine running if you are interested....
FLASH VIDEO