Adding a PMAC motor into the discussion is something you should have mentioned in the beginning, it's a different animal.
In practicality, the limit of the voltage coming back off of the motor is the limit of what the VFD will allow. Every VFD has a threshold of maximum DC bus voltage based upon the working tolerances of the components and circuit dielectrics. Once the DC bus voltage gets to that point, the VFD will shut itself off, disabling the transistors and stopping the flow of current back into the bus. On many of the 480V VFDs I'm familiar with that threshold is between 800V and 900VDC on the bus (varies by mfr.). The point at which the Dynamic Braking Resistor is triggered is of course much less than that.
Now, in the case of a PM AC motor, there is a potential issue. With a standard induction motor, if the VFD turns off it would cease to regenerate once the stator power is cut off, because it is the stator that excites the rotor to allow regeneration. With permanent magnets in the rotor, the motor can continue to regenerate voltage potential onto the lines between it and the VFD. Under normal machine conditions if you are driving a load and stopping it, the load speed will not have been higher than the motor was designed for, so would not present a problem. But in theory IF the motor can be driven OVER it's design speed by the load with the VFD off, even though there is no path for current flow, there may be issues with conductor insulation and termination flashover etc. The voltage profile is V/kRPM based on the design. So if you have a PMAC motor designed for 360VAC at 1800RPM, it is designed for 200V/RPM. That means if you over speed it to 5000RPM, the voltage will rise to 1000VAC. This is a known concern for using PMAC motors in overhauling load applications and they are NOT recommended for that purpose.
Now, where can that happen? A case in point is a long down hill conveyor where the motor was used in permanent regen mode to RETARD the belt speed. In that case if it were a basic induction motor and VFD, and the VFD trips off, the motor ceases to become a generator. But if you were to replace that induction motor with a PMAC motor, if the drive trips and the belt is allowed to run away, it may cause a fire in the conductors or controller because the regen voltage would run away as well, causing a flashover. So in either case you need a backup emergency mechanical brake, however if the brake fails the PMAC motor can cause a fire in addition to the other problems too, whereas the induction motor would not.
