Motor is a 2kW varicon
Manual control by potentiometer, cnc control by Mach3
Dont now exactly what I am doing obviously
I take it you mean you set the speed on the potentiometer manually
If that is the case you don't need to set the spindle motor up in motor tuning as Mach3 isn't controlling the spindle.
Do you have a sensor to read the speed ?
Have you set up a spindle pulley in Mach3 with the MIn & Max the same as your motor ? under CONFIG, SPINDLE PULLEYS
Phil