If a controller includes regenerative braking, then there is a potential problem if a battery connection fails during braking. This can happen on a golf buggy if the batteries are not properly secured. If it happens then the controller will automatically increase the regenerated voltage in an attempt at regenerating braking energy into a missing battery. If the regenerated voltage is not clamped in some way then the regenerated voltage will simply rise until something fails. Overvoltage protection is this clamping mechanism.
So the controller inhibits braking at the clamping level (approx 50v): it does this by speeding up the motor so it is not braked. The same effect will happen if excessive voltage is applied to the controller: the motor will run in attempt to reduce the applied voltage.
Of course with a disconnected battery all control is lost at the clamping level.
This feature limits the regenerated over voltage and is fitted on all controllers having regenerative braking.
Generally its level is limited by what the MOSFETs can handle, so it’s set to 47v on 12v, 24v and 36v controllers (which use 55v MOSFETs) and 68v on the 48v models (which use 75v MOSFETs)