> while driving a motor with PWM, many say to set the freq 
> above the audible
> hearing range, but in many cases isnt the motor loud 
> enough that it covers
> the harmonics created by the PWM drive?

For some values of "many".
You can definitely have motors "sing" extremely audibly with 
PWM.
This may not be a problem, depending on your application. In 
a few applications it may even be an advantage if it adds 
techo-appeal to your product. Fairly niche target audience* 
though :-).  * In this case, unlike many where the word is 
misused, AUDience is entirely correct.



            Russell


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