[Richard Wallace]

Originally Posted by KenWittlief

... DC motors tend to be in the 90% effeciency range, so a motor that is rated at 1/4 HP (180W) will require 200W to run, and you might get 162W out when you use it as a generator.

Dream on, Ken.

Larger machines, rated for industrial duty at, say, 20 HP and higher typically achieve 90% or better efficiency.

Fractional horsepower, intermittent-duty purpose built motors like the CIM (link is to the datasheet provided by the manufacturer) typically run at efficiencies around 65%. For example, operating as a motor, the CIM's rated output at maximum efficiency is 154 Watts at 4614 RPM, and at this load it draws 19.8 Amperes from a 12V supply. So the electrical power input is 12 x 19.8 = 238 Watts and the efficiency is Pmech/Pelec = 154/238 = 65%.

The CIM could be operated as a generator at the same speed, current and torque (with the current and torque in the opposite polarity), and under those conditions its power losses would be similar; i.e., about 238 - 154 = 84 Watts. This operating point would require 154 Watts input (mechanical) power and the electrical output would be 154 - 84 = 70 Watts, so the efficiency would be Pelec/Pmech = 70/154 = 45%. Efficiency could be improved by running at higher RPM (i.e., at higher output voltage), and might get back up to about 65% at speeds above ~5700 RPM.

Of course the bearings won't last long running like that. But remember, the CIM was designed for intermittent duty as a winch motor, and its endurance is rated (again, refer to the manufacturer's data) for 6000 seconds total operating life at nominal rated power; i.e., at 4 lbf-in torque, 4320 RPM. That's roughly equivalent to 100 minutes (50 FRC matches) of relatively hard (27Amp) driving. We all know that the CIM typically lasts longer, but those are the ratings.