Common computer fans should not be spun up to extreme
speeds with compressed air
or vacuum cleaners, because going too fast can damage
them. (Also, you allegedly
shouldn't use a vacuum cleaner on a computer anyway,
because the dust flowing past
the electronics can zap them, but somehow cleaning
electronics with compressed air is
100% OK.)
A possible improvement to the fans that could prevent
damage is as follows: A 3-phase
bridge rectifier is connected to the motor. (Computer
fans pretty much universally use
3-phase brushless motors with integrated drive
electronics.) The output of the rectifier
is connected to a muscle wire actuator. The muscle wire
actuator has a brake pad on it.
When the fan is turned on, the brake circuit is disabled
by a depletion-mode MOSFET
or other suitable transistor in series with its output. This
allows the motor driver to
drive the motor normally. When the fan is off, the
transistor loses the signal holding it
in the open state, enabling the brake.
When the fan is spun by airflow from a vacuum cleaner
or blower, the motor acts as a
generator, and current flows through the muscle wire.
Initially, it just acts as an electric
brake (a short circuit on the generator), but if the forced
spinning goes on too long or
goes too fast, the current heats the muscle wire, causing
it to contract, applying the
brake pad to the rotor. This prevents overspeed while
the fan is turned off. The brake
will deactivate as the muscle wire cools.
This has the possible disadvantage that you wouldn't as
easily be able to test a non-
installed or powered-off fan's bearings by blowing on it.
Due to the expense of the
mechanism, though, these would probably only be used
in high-reliability hardware
that needs to operate in a dusty environment and be
cleaned frequently.
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ETA, 2017-05-05: I was just thinking about how this
could be applied in hydroelectric turbines and such, and I
realized that there are already mechanical devices that
do pretty much the same thing, namely centrifugal
governors and centrifugal brakes. Hmmm.