Roy,

    It won't ever be perfect.  The response of the analog output on the CNC4PC board is not perfectly linear, and neither is the response of the VFD.  If you can manage to be within 10-15% worst case, you're probably doing pretty good.  Mine varies 5-10% from day to day.  But it really doesn't matter.  If getting precise RPM is important, then you have to go to some kind of closed-loop control.

Regards,
Ray L.

Hi Roy,

I was talking to a guy at Newton Tessla here in the UK when making the purchasing decision for the Mitsubishi inverter..... He had experience of the brown & pestall and said that it didn't have a linear output and that it was all a bit of a fudge.  So it seems probable that you have found the limits of this VFD.  In comparison, I find that the speed on my setup is about +- 20 rpm throughout the speed range, as measured by a mechanical tacho.
Paul