Well I’m not sure what your exact setup is or what you are willing to replace. If you are talking of replacing a stepped belt 1P fixed RPM motor on a mill or lathe with a VFD & new 3P motor it can be done. You will have some retiring and matching to fit the new motor to the old frame. I’d use a 1.5hp 3P to replace a 1hp 1P. (2 to replace a 1.5) this is because when you are all done you will likely leave the belt changer in one position and VF drive the motor from near 0RPM to max RPM with a pot. With a new motor expect 0Hz to 120Hz from the controller. In this scenario you will be loosing some torque at the low RPM end and the added hp will help fill in.
Here is an example I converted a couple years ago.
Taiwanese JET 12 x 40 engine lathe with 3 step belt speed changer on gear head and 2 step belt on motor circa 1985. 1HP 120/240 1P motor, conventional carriage mounted drum switch for F/Off/R controlling motor windings. Strip off motor and all wiring to drum switch.
Automation Direct 2HP Durapulse GS3 VFD ($300), big external braking resistor for 1.5HP motor, Leeson 1.5HP 3P metric shaft inverter rated motor($~$150), new split wedge shive for motor, 2HP 3P 24v contactor, 1.5HP overload fuse block, 2P 2HP input choke to keep noise out of the rest of the shop, 10K 10 turn pot for speed control, PBs for start, stop, EStop of controller.
I built all into an old breaker box. Drum SW now provides only logic to controller to F/R/stop. This controller can be programmed to stop spindle in like 2-3turns if you want!
I can go from cutoff speeds to 1000 RPM with the spin of a knob like those 1940’s Monarch vacuum tube DC controller lathes with a 1knob speed control!
Ive seen a lot of designs on the internet. Many are unsafe. Many of the cheap Chinese VFD controllers do not contain breaking transistors, even though there are external terminals for breaking resistors.! Many designs don’t include motor overload protection, somehow assuming a VFD provides that, (most don’t). Many conversions don’t add an Estop button, even though almost all controllers have an input for one. All of the VFD controllers I have worked with, (<5hp) say never disconnect motor under load. I can’t speak for 100HP units.
Also 3P motors are not created equal. Standard motors can be problematic at low speed under load. They don’t cool well and can overheat. So up sizing helps with raw steel at low speed. Or look for inverter rated motors that are designed for this application.
But before you start, ask your self whether you want to build machines or use machines. I’m in the latter camp and want any mods to work forest time and for a long time.