Swede, AC servo's or brushless DC have a lower inertia, higher RPM rating than brushed DC, Brushed usually run a max of 3000rpm tops, whereas BLDC and AC run typical 6000 to 8000 rpm.
A VFD will not run a AC servo, I had no luck when I tried it. I would keep an eye out on ebay if possible for a matched AC/BLDC servo motor amp combination with analogue input , and either a potentiometer for control or an analogue output from the CNC controller if it has one, if it does you can use S and M codes for control.
Al
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.