> The question:  Is there an "optimal" PWM cycle size that I should be using
for
> this kind of DC motor?

No.  It's a balance of tradeoffs.  Slower PWM is better in that the
switching components spend less time in the transition region, thereby
dissapating less power.  It also allows for better resolution using the PIC
hardware.  However, the PWM frequency needs to be fast enough so that the
device being driven "sees" the average voltage instead of the individual
pulses.  In the case of a motor, this is mostly dependent on the
torque/inertia ratio.  Other considerations might be to reduce the magnitude
of small internal vibrations of the coil wires in the motor, or to reduce
audible noise.  For example, a few hundred Hz (or even lower) is fine to
effectively drive many DC motors, but can produce an annoying whine.  In one
project I ended up using 10KHz just for this reason even though the system
functioned fine at 200Hz.

> Does it depend on what speed it is running at?

No.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@embedinc.com, http://www.embedinc.com

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