[Al Skierkiewicz]

Brian, Joe and Dr. Joe,
These functions are more attributable to the segment spacing of the commutator vs motor speed vs PWM switching frequency. Each motor type will respond differently and the overall inductance of the windings will play a part as well. Often linearity is affected as much by the mechanical frictions within the motor and the load at low RPM as with any of the other variables. The "hum" in motors controlled by Victor controllers, I believe, is directly attributable to the low PWM frequency and short pulse duration. I have often observed teams who failed to calibrate the controllers and were supplying minimum pulse widths to their motor at rest. These short pulses were sufficient to bump the motor armature but could not supply enough current to overcome the friction in the drive train. One of the advantages to using 12 volt PWM is to get the motor moving while giving some speed control at low RPM. I expect the higher switching frequency to produce less hum in future systems. The upcoming season will give us some data on linearity under real world loads, stress on the motor in forms of heating, brush and commutator arcing, RF interference, etc. It will be interesting.