[jackbauer]

Regards the batteries i have no idea thats being honest. Something ain't right. 1.5khz would be fine for those devices. Anything higher will cause the waveform to become trapezoidal. 3us is the time it takes the device to go from fully off to fully on with the right driving current into the base. My old igbts switch in half that time. Remember also that a darlington is basically a current driven current amplifier whereas a fet or igbt is a voltage driven device. My guess would be that the revolt driver stage does not like the transistor and vice versa.

Spinning the wheels requires only a very small current so you "get away" with a non ideal driver. when you then load the motor and it draws more amps the fun starts. my 2c.