If you"ve got a mill like mine where the drive and feed motors are all 3 phase then you can"t use a static converter as they will handle only one motor at a time. A far as a VFD is concerned I have heard the same about them but can"t say for sure.
That said a rotary converter isn"t that hard to build if you take a little time to search the web and find plans for one. Between Craigslist and a relative that gave me some old motor contactors and capacitors, I built one using a brand new 10HP Teco-Westinghouse motor and the mentioned capacitors, for less than $250. I run a Supermax mill with it that has a 5 HP main drive, the feed motor, and the coolant pump, all with no problems.
That said if you find a company that does work on municipal wells, etc (like the place Dad works) they will often pull "blown up" pump controllers out and simply replace them -vs- trying to repair them. Usually even the small ones contain at least one motor starter and a start and run capacitors. In other words you should be able to get pretty much everything needed from them, relatively cheap, if you ask, just like I did. An alternative is a salvage yard, etc that takes in industrial equipment with electrical cabinets full of motor controllers, which are the expensive items needed. The capacitors are relatively cheap from places like Surplus Center, etc.
In the end, a little research and a few hours putting parts together, and you can have a rotary converter that would ordinarily cost you $800 for not much than the cost of a good, new/used three phase motor.