On Fri, Aug 7, 2009 at 1:50 PM, Chris Loper wrote: > PWM > >> When you are doing PWM and switching the FET on and off thousands of times per second, ... > > Mine operates at 500hz right now. > It looks to me that the switching loss is directly (linearly) related to the PWM frequency. > I don't think the possible hum will be an issue in a go-cart, so > I'm planning to keep the PWM frequency low unless there is a good reason to increase it. > > This is too slow. The inductance of your permanent magnet motor is very low. When the MOSFET is ON, the current rises linearly by i(t) = v * t / L. So if 't' is large then you have high a very high maximum current. If you switch at perhaps >5kHz then the ripple current will be smaller and your motor, wires, batteries, transistors, capacitors will all operate cooler because the peak current level will be lower. -- Martin K. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist