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Vacuum-cleanable case fan with generation-activated brake

 
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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.

36/207

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.

notexactly, May 04 2017

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