On Wed, 21 Feb 2007 07:47:39 +1100, Franc Zabkar wrote:

Brushless DC fans use Hall effect sensors for electronic commutation.
If you must chop the supply to these types of motors, then you should
ensure that the PWM frequency is much lower than the fan's rotational
frequency, otherwise the speed control logic will be disturbed.


Not any of the fans I've ever come across. They usually use a chip to
generate AC out of the DC supply and drive an AC motor.