You don't say what kind of motor, so I assume it's an asynchronous motor.
Then you may be lucky that it's a 3ph. motor converted to 1ph. by adding a capacitor. (My Emco millhead is one example of this). Then just toss the cap and add a VFD. Check it by comparing resistance over the 3 terminals. If it is the same regardless of which of the 3 terminals you measure between, you can be almost sure it's a 3ph. motor.
A 1ph. + start winding will have a higher resistance over the start winding, and the resistance between 2 of the terminals will usually be the sum of the 2 windings. In other words a delta winding with one winding missing and the remaining 2 are different.
The VFD's are always 3ph. out. Smaller units are 1ph input.