The unit that gives the lowest resistance will give the most power. I choose the Green Ametek as typical and plotted electrical data on it.
The above chart shows that as voltage increases the RPM increases. This chart compares running as a motor and running as a generator or running in reverse direction as motor and generator. The brushes are slightly offset for optimum power in one direction. As current generation increases the voltage goes down causing a new curve to be drawn.
Offered by Mike.