I asked about how can i automaticaly enable cooling under 8000 RPM, not how to prevent going under 8000 RPM completely. I am already doing this. Reason why i added external cooling is that i want to use my spindle in low RPM, because it's very useful when cutting plastics (even with 1 flute endmill and fast). I can simply set relay to switch the fan on, when the spindle is running, but i have very powerful fan (from 1U server) and it's loud, so i don't want to run it when over 8000 RPM and spindle fan is sufficient.
Not really. It happens even on accelerating and it does not have anything to do with braking. It's just auxiliary relay configured to switch when spindle is at defined frequency...