I also did a pwm project during the late 80's and found similar to Bruce that for the motors we were using there was A sharply defined torque cut off at about 6 khz. I never investigated temperature, as the product was a line of electric outboards - Water cooling is so efficient. The cut off showed up as a reduction in top speed. The goal was a 20 % increase in range while trolling over simpler voltage regulating designs. This was easily achived. As a side benefit some of the testers played with the frequency to see if it affected fishing productivity and I believe it did, but was not consistent at any setting. Terry