Magnetic induction with varying rpm: Dealing with excess energy
Don't get confused! RPM is the rotation speed measure. Rotation speed is proportional to the Electormotion Force, which is voltage (Volts), not power (mW).

Power is voltage times current (A) . What current will your generator provide? Well, in wide range, it will be the exact current you take, nothing more, nothing less. That's it.

If you come closer to the maximum power that your source can provide, the rotation will slow down reducing the EMF (voltage), and depending on your circuit the current may get lower, or try to keep the power constant and completely choke the rotation.