Also, I agree with Gunner, yank out the mechanical vari-speed drive and run

Its seems aweful silly to remove a prefectly good working Mechanical VS
drive. Yes, it makes a little noise and it wastes a little power but it
works just find and give more flexability than using the VFD alone.
I have a VFD on my mill and DC motors with speed controllers on my
lathes but I don't think they give quite enough adjustment range to be
used with only one belt speed.
chuck