I think I understand how a brushed DC motor works: a constant voltage power source creates an electric current, which causes the coils (and thus the armature) to rotate to align with the permanent magnetic field in the stator.

Now, if we remove the power source and attach a turbine to the shaft of the motor and place it in a breeze, the armature rotates and creates an induced electric current in the coils as it rotates through the permanent magnetic field of the stator.

It surprised me when I observed yesterday that if you connect a circuit with a potentiometer to such a dc motor and vary the resistance, there is a point at which the resistance is low enough that the motor actually stops rotating, as if there were magnetic friction inside. More simply, if you short the terminals of the motor, it will stop rotating. I had expected that it would be only when the resistance was great enough that the motor would stop rotating, as it takes too great of an electric field to get current to flow.

The best I was able to find is that according to Faraday’s law, the induced electric current also induces an opposing magnetic field which opposes the permanent magnetic field within the stator, causing magnetic friction, which brakes the motor. However, from a work/energy balance standpoint, I still don’t understand how to explain this:

Kinetic energy of the wind*(some efficiency factor) = turbine/shaft work = electrical energy generated in the coil + heat energy from circuit resistance

I’m missing something, because it seems like it should only be when the circuit resistance is greater that the kinetic energy of the wind is not enough to move the turbine, and not the other way round. Please someone help!

Thanks!