I've seen plenty of questions wondering what happens when voltage is applied that would exceed the RPM rating, but what happens on the voltage/current side when something external - the wind spinning blades attached to the motor - spins the motor faster than its rated RPM? Does it overheat? Does the voltage eventually saturate? I'm planning on using a permanent magnet motor, but this question applies to any motor.
All PMDC motors generate V proportional to RPM [kRPM/V] . The rated maximum power will be at approximately 82% of its maximum speed but will continue to rise with overheating above if load continues.
While the max RPM depends on the mechanical and eddy current losses in the magnets at some frequency. This maximum powerpoint (MPP), of course depends on the load current, I .
All static structures have a resonant frequency including moving parts like bearings and the risk of imbalance or approaching those resonant frequencies with stored inertial energy rising means a high risk of fatigue and catastrophic failure . Therefore all structural resonances must be much greater f than the excitation frequency is its harmonics.
Thus shunting the generator to act as a speed brake into wasted energy and inertial flap speed brakes must be designed into the system to handle worst case winds expected in the next 100 years to prevent these failures.
The motor loads will increase with conduction current losses I^2*DCR and eddy currents will rise with f^2 above max and thus liquid cooling may be needed or simply rely on increasing the drag on the blades with speed flaps and possibly an inertial clutch brake . The mechanical solutions were how we did a 20m tall egg beater type wind power Gen in 1975.
If the mechanical brakes failed and the structure held together somehow with guy wires AND the electrical system to brake failed then a 3rd protection system is needed for safety. Otherwise the voltage could increase possibly enough to breakdown and arc , if there was not at least a 300% safety margin on insulation.
Large synchronous wind turbines however use a transmission to speed up the RPM and the PMAC generator requires a drive specifically designed for PM motors, similar to flux vector drives for ac induction motors, in that the drive uses current-switching techniques to control motor torque — and simultaneously controls both torque and flux current via mathematically intensive transformations between one coordinate system and another in order to keep in phase with the grid frequency and phase while allowing prop angle to harness more power. Therefore they are only designed to run at constant speed unless starting or stopping or shifting slowly with the grid. These are generally Betz type steerable turbines, so they can control speed by direction error with wind but still must be able to survive a near miss of tornado.
This question cannot be answered without making assumptions about motor type and the behavior of the electrical source which is driving.
As a general rule, most motors can potentially act as generators when overdriven by an external force. But there are a lot of details.
Let us assume you have a permanent magnet synchronous motor (PMSM) driven by an AC source with fixed frequency and voltage. When a mechanical force tries to make the rotor spin faster than the synchronous frequency, current will flow backwards. Instead of acting as a motor, the motor will act as a generator. This mode is called "regeneration." Torque will still be present, but it will be act to slow the rotor. If the torque becomes too high, the current may become very large, and the rotor may even spin faster than synchronous speed (which would likely be catastrophic in large system).
An AC induction motor driven by an AC source with fixed frequency and voltage will behave fairly similarly to the PMSM, except that the induction motor does not operate strictly at the synchronous speed. When mechanically over-driven, it will speed up a bit, and when its speed exceeds the synchronous speed of the AC source, it will also operate in regenerative mode. But, again, there are limits to how much torque can be applied by the external force. At some point, current will become very high, and some form of failure will occur. The stator may overheat if nothing else breaks first.
Brushed DC motors also can be over-driven and also go into regeneration.
A BLDC motor may be essentially identical in construction to a PMSM. But when it is driven as a BLDC, then how it responds to being over-driven is really a matter of how it is controlled. It can be controlled in such a way that it goes into regeneration, but not all controllers are designed to do this.
If you wish to harvest energy from the wind, you really need to match a lot of things. In order to harvest energy from the wind, the blades need to spin at just the right speed. If they are too fast, they will be adding to the wind and wasting electrical power. If they are too slow, they will be stalled, and not converting power efficiently. This variable speed gives rise to a need for a variable voltage also. The voltage applied to the motor (no matter which type) must be effectively varied as the speed changes. But if you want the power to charge a battery or run an appliance, it probably needs to be converted into a fixed voltage prior to use. So there has to be some form of conversion.
Another option would be to use an AC induction motor with variable pitch fan blades. The pitch of the blades can be adjusted so that over some range of wind-speed, the motor is always in regeneration. There would have to be a lot of other control, and a reduction gear so the fan blades can spin slowly, etc. But it might be the easiest way if the goal is to put power into the electrical grid.