Indeed it's frequency modulation, I got more focused on the fact that it's a PWM output in the PIC that I'm using.

I haven't tried slowing down the ramp, the above is purely speculation with no real proof. I was simply curious whether there's some method to quickly judge what the max acceleration limit is, based maybe on the load torque requirements. I can roughly judge what the torque required by the load is, but inertia is a bit more complicated. I think this needs further re-thinks.

Cheers Jonathan.

Regards,
T.