I mentioned that efficiency isn't a good measure. We need low losses at all operating point, and the "cruise" points in particular. The problem with measuring e

I mentioned that efficiency isn't a good measure. We need low losses at all operating point, and the "cruise" points in particular. The problem with measuring efficiency is that at zero speed (starting from a stop) or at zero torque (coasting) your output power is zero (torque times speed) and since there will be at least some input energy that means output/input = 0. Because of this, all motor efficiency maps show poor performance along both axis and a big region of "good" at higher power. There is a steep gradient of efficiency between those extremes and small changes in losses can really shift the "efficiency" a lot. I like to plot Torque on the X axis and losses (in watts or kW) on the Y axis. Make a series of curves at different speeds (each curve is at a constant speed). These curves intersect the Y axis at zero torque, so friction can easily be seen on the Y axis. The curves have a parabolic shape (I^2 copper loss) forming a sort of butterfly. If the wings spread out at the tips (curves separate more at high torque) there may be magnetic issues. These maps are really nice, are easy to compare, and never mention efficiency at all ;-)