I'm nearing the end of a conversion of a Denford Starmill to replace the old spindle with a Chinese 2.2kW (for the ER20 collet size) spindle and controller.

The existing Mach3 conversion that was done before I got the Starmill had a pretty basic parallel BOB with a relay for the spindle and 0-10V speed controller.

Configuring the controller to support FOR and analogue 0-10V, then hooking up the wires appropriately, this seems to work...

HOWEVER...

At full speed, with M3 configured to support full spindle speed of 24k, the controller displays 240000 rpm. Below 24k input on M3, the spindle speed appears to lag behind the programmed setting - getting worse, the further away from full speed you get. e.g. at a commanded 5000rpm, the controller is showing more like 2500 rpm. I can probably live with this for now - I'm assuming the PWM drive on the BOB is less than perfect and I know I could probably remedy this somehow. But, I'd like to ask if anyone else has seen similar behaviour?

More sinister, and of more interest to me, there's some, what i can only think to describe as wow/flutter, with the commanded speed oscillating around the set point at around 5Hz, so, the sound of the spindle (the frequency) oscillates around the set point. This doesn't appear related to spindle speed (i.e. if I double/triple the spindle speed, the frequency obviously changes, but the speed of the wow effect remains reasonably constant). Observing the controller there's no fixed speed - the spindle speed varies somewhere around the set point. (I should try it at 24k rpm - didn't think to try that until I started typing here). Assuming that this could be electrical noise, or earth loop, and reading the manual, there's the setting PD071 - Analogue filter (factory:20, 0-50), I've tried changing this up to 50 which should make the controller less responsive to changes on the analogue input. This appeared to have no effect. I then slaved a 47uF capacitor between the analogue VI input and ground - there was an immediate dip in the spindle speed which recovered quickly as the capacitor charged up, but the sound remained. Has anyone else observed similar, and found a solution?

(the second part concerns me the most - my brain doesn't like noises that fluctuate like that, and I can't help but think there's likely some stress on the spindle with constant accelerations/decelerations.

Any advice?

Mike