Grind wheel motor, use a servo or VFD?

Here's my understanding:

• The VFD with AC motor should be less maintenance (and cost)
• The Servo Controller with DC (even brushless) Servo motor performs better, but costs more.

Where you really get better performance with a servo motor is (well, obviously precise positioning, but...) in a speed sense the very low end speed (sub 2 Hz). A VFD in V/Hz mode is not precisely speed controlled - it will operate at a slightly lower RPM than the frequency you're giving it. You can compensate, but the control is open-loop.

However, with sensorless vector mode, you should get very good speed control, but you'll still have problems with low speed. You can put an encoder on too, and then you get even better speed performance.

The first thing that jumps out at me for your application is stopping. If memory serves, VFDs rely on ramping down the Hz, DC injection braking, resistive braking, or mechanical braking to stop, whereas a servo motor will just decelerate under control (essentially powering in the reverse direction). I have a hunch that a servo motor can probably change speeds faster than a VFD and AC motor. Does the application gain anything by really fast accel and decel times? I would check the specs of the two options to check.

You also need to account for the much larger motor required when moving to a VFD. Servo motors pack a lot more torque into a smaller package.

Also does the current servo system (for the whole CNC) have a common DC bus. They may have put a servo on the wheel simply because it is cheaper to add another axis than add a seperate VFD.