I put a VFD onto the existing 3ph motor in a surface grinder, and they weren't cheap, although that was awhile ago and they seem to be a lot less expensive now, for example on AliExpress. The one's I've seen are often 220v in and out but now I see them in 110v too. I've found a number of 3ph motors that were very inexpensive..... I got some at a metal salvage yard 3/4hp 3ph for $2-3 each (by the pound). I got a 5hp and a 10hp 3ph motor for free.