> Then for certain applications there's low frequency.  In a recent bench t=
est
> driving a Delta AUB0812HH fan, with an ATmega328, a 7.8 Hz PWM was
> sufficient to operate the fan and not cause audible noise.

But don't try it with a coreless motor e.g. a Faulhaber. As various model r=
ailroaders have found when fitting a digital decoder, if you don't use a hi=
gh frequency drive (typically 20kHz is available), but use a lower frequenc=
y drive as used for iron cored motors, then you shake the windings apart as=
 the mass is low enough to physically oscillate at the drive frequency.



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