3 ms·
> Efficiency at 500 to 1000 RPM is important Why is this? Thinking out loud: can't you gear your motor so that it's most efficient at cruising speed for the ve
by 1-more 4y ago
> Efficiency at 500 to 1000 RPM is important
Why is this? Thinking out loud: can't you gear your motor so that it's most efficient at cruising speed for the vehicle? But the problem is that the car still needs to be able to accelerate from zero and some gearings will make that impossible, so having an efficient band at 500—1000 RPM gives you the best chance of being able to accelerate from 0–500 RPM?
- phkahler 4y agoI 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 ;-)