So Nearly there - Mach3 and VFD Spindle Speed.

[davegrennan]

Well I put the scope on the 0-10v rail last night and immediately the RPMS shot up from the 23.6K to the full 24K RPM. I assume the scope is acting like a parallel resistor and effectively reducing the resistance across the vfd. Could be extra inductive resistance on the wires going from the BOB to the VFD? I'll replace that with quality shielded cable later and add a ferrite bead for good measure.

The scope didn't give me any further clues except the signal looked a little bit noisy. Might try a smoothing capacitor if the shielded cable doesn't help.

Cheers

Dave.

[Clive S]

Well the scope should not have loaded the circuit at all. I take it you have a separate daughter board on that BOB a small cap might help though. ..Clive

[davegrennan]

Ahh the sweet smell of success. Simply replacing the cable with shielded cable did the trick. I had to do a little bit of poking about with the pulley speed setting to get the intermediate values to fall into line, but now they all agree within around 20rpms. Thats good enough for me.

Thanks JAZZ and Clive for all your help.

BTW Clive there is no daughter board, the BOB has the PWM output directly on the board.

[Clive S]

Ok Well I don't think that I help much but Its good to bounce off people to get to the conclusion. Good luck ..Clive