J. P. Gilliver (John) wrote:
In message , T writes:
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A "fan" is also know as a "reservable generator". If you
cause it to spin, you will create a current. This can

Not always - only if it has permanent magnets.
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Yes, they have permanent magnets.

http://pcbheaven.com/wikipages/How_B...s_Motors_Work/

And when you see a closeup of the controller board,
the schematic doesn't match the "Internet schematics"
you might see. This one seems to have three transistors
and a Hall probe, rather than just two transistors.

http://tipperlinne.com/fan-tach.htm

*******

If you expect to extract energy from a BLDC fan...

http://www.instructables.com/id/Old-...in-10-Minutes/

"To get the induced current from the motor used
as a generator, you must remove this IC"

Yes, the controller board gets in the way of
obtaining an AC signal from the coils. Or at least,
it would prevent efficient energy extraction.

*******

When you switch off a PC, the fan goes from "motor" to
"generator", instantly. That means, if you have a 1500RPM
fan, then it can tolerate "generator" mode at 1500 RPM
for a short interval (as it comes to a stop on its own
pretty quickly).

On an electric lawnmower, when you switch off, the mower
goes from "motor" to "Generator" too. To make the mower
blade stop faster, the coils are shorted when you
switch off, so the "generator" action dumps into a
dead short. If you take the cover off the mower and
watch the brushes, the sparks are larger on shutdown,
than in normal operation. (I changed the brushes on my
electric lawnmower last summer, and got to observe
them in action.)

Paul