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.
Electrical – What happens when a motor is spun faster than its rated RPM from an external force
motorpowerrpmvoltage
Best Answer
PMDC
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.
PMAC
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.