Hi Simon,
I was looking to upgrade as a preventative measure rather than replace burned out FET's. In the end I decided to stick with the installed originals. I made this decision on the basis that the 36v controllers have been less problematic than the older 48v controller. In addition I've limited the current of my 36v 500w to 16 amps which should mean less heat generated in the controller and I've changed the controller to limit current while allowing 100% speed in all PAS levels. I've also fitted a cadence sensor. The 500w motor is wound to produce more torque than the 250w motor I'm told. Maybe I've wasted all that extra torque by limiting the current to 16A, but I'm able to get over 4 hrs riding from a single charge and get higher average speeds than I was without the motor.
I change down at 80rpm and I change up at 90rpm. (my preferred cadence, although I often hit over 100rpm for short periods) This helps to reduce the load on the motor especially when climbing. I find I can climb a 25% hill using PAS level 7 of 9 in bottom gear (42t bling ring and 36t rear sprocket) @ 80rpm comfortably and the motor barely gets warm. This equates to 7.5mph. I’ve limited PAS level 7 to 62% of max current, so that’s 16amps x 62% = 9.92amps or 357watts @ 36volts. I guess I add another 200watts to that in order to power a 20kilo bike + 72 kilo rider up a 25% gradient @ 7.5mph.
No sign of the FET's giving any problems after 1000km of hilly riding.
Hope that helps. If you need some new FET's to replace burned out ones, try eclipsebikes.com He might be able to source some for you.
