Never simple is it! Mach, machine halts

[Jonathan]

How will accelerating too fast cause the DC bus to go overvoltage?

I was referring to both the ramp up and down settings, so clearly that comment is regarding negative acceleration...

Also, how is accelerating a geared machine too quickly with a VFD any different from turning the motor on with a switch?

If the user is using the original motor, then it's not much different, however it's not uncommon for people to get a bigger motor than the machine was designed for, which could potentially achieve higher acceleration than the original motor. I guess it was hardly worth mentioning in this context. Also, in general it is sensible to warn to be conservative with these settings as they could cause problems later on - for instance on a lathe swapping to a larger chuck or faceplate can greatly increase the inertia (due to power 4 relationship) and as a result cause the drive to cut out.