Im interested in buying or designing an alternator/electrical generator that increases torque as rpms increase – is there an alternator design that increases resistance as rpm/angular velocity increases? The alternator is for an exercise bike and would simulate real life pedal resistances.
I intend to collect the energy (however insignificant it is) to do something small like charge my cellphone.
Best Answer
What you are asking about is a generator that presents "viscous friction" to the rotating shaft. That means the torque is proportional to the rotation rate. Yes, this can be done with a electric generator. Torque is proportional to the current, and open circuit voltage is proportional to rotation rate. A resistor is the right load to get torque proportional to rotation rate, at least it will be as close as the generator is efficient.
For more fancy control and to harvest the energy, you need what is essentially a switching power supply on the output of the generator. The controller of this power supply has a input that tells it the rotation speed. It can then put any load it wants to on the generator output to get the desired torque. Usually the generator output is bucked to a higher voltage. This voltage is not regulated except that it gets whatever power the generator puts out at the current operating point.
Another switcher then keeps this voltage within some range so that the first switcher can continue to dump power onto it as it sees fit. The second switcher usually produces a well regulated lower supply. It can also handle excess power in various ways, including shunting it to a resistor when there is nothing else to do with it. If you want to get really fancy, you could dump any excess power back onto the AC power line. However, that would take considerable electronics, has regulatory and safety issues, and will certainly not be worth it economically.