On Tue, 20 Feb 2007 21:01:49 GMT, AZ Nomad
put finger to keyboard and composed:

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.

According to the following article, a BLDC motor is essentially an AC
motor.

http://en.wikipedia.org/wiki/Brushless_DC_motor

"BLDC motors have come to dominate many applications: Consumer devices
such as computer hard drives, CD/DVD players, and PC cooling fans use
BLDC motors almost exclusively."

"A brushless DC motor (BLDC) is an AC synchronous electric motor that
from a modeling perspective looks very similar to a DC motor.
Sometimes the difference is explained as an electronically-controlled
commutation system, instead of a mechanical commutation system,
although this is misleading, as physically the two motors are
completely different."

"Because the controller must direct the rotor rotation, the controller
needs some means of determining the rotor's orientation/position
(relative to the stator coils.) Some designs use Hall effect sensors
or a rotary encoder to directly measure the rotor's position. Others
measure the back EMF in the undriven coils to infer the rotor
position, eliminating the need for separate Hall effect sensors..."

"Although BLDC motors are practically identical to permanent magnet AC
motors, the controller implementation is what makes them DC. While AC
motors feed sinusoidal current simultaneously to each of the legs,
(with an equal phase distribution), DC controllers only approximate
this by feeding full positive and negative power to two of the legs at
a time."

- Franc Zabkar
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