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Computer fans use to run at 12Volt, but most, as for today, use to allow 9Volt or even less to slow down the fan speed (RPM, Revolutions Per Minute).

In cases of too low voltage, the fan stops, but I can see it "trying to start again". For example: my Tacens 9dB fan stops at about 10 Volts, but to start it again, 10.5Volt is not enough, and the engine tries to move the fan (I can see a small movement as an "attempt" to move) each 1-5 seconds, but it does not succees, so the fan keeps at 0 RPM.

Maybe that "attempt" to move could damage the internal engine of the fan when it last for hours or days?

UPDATE: Why would I want to stop (some of) my fans:

  • Less noise when temperatures are low and fans are not needed.
  • Dust reduction (fans acumulate it).
  • Less consumption.
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  • It is possible that the fan could overheat if stalled. It would depend on its heat dissipation abilities when stalled in relation to the wattage it's absorbing when stalled due to low voltage. – Daniel R Hicks Jun 10 '14 at 1:14
  • How could I know, @DanielRHicks ? Just by touching the engine (supposed to be at the center of any fan)? Or maybe its specifications could tell? – Sopalajo de Arrierez Jun 10 '14 at 1:37
  • how/why is your fan getting undervolted? – Journeyman Geek Jun 10 '14 at 1:44
  • @JourneymanGeek: How: many motherboards allow voltage regulation for fan management (at BIOS you can often set values like 11Volts... etc), so programs like Almico Speedfan or MSI AfterBurner can use this capability. Why: Noise (and dust acumulation) reduction. – Sopalajo de Arrierez Jun 10 '14 at 9:48
  • ahh, ok. I can think of a few ways its doable, none of which seem dangerous. Noctua ships a speed reducer that's basically a little resistor, for example. I cannot think of any reason, this would cause a fan to fail off hand. Not having startup current on a motor ALSO sounds like a bad thing, so... I'm not sure – Journeyman Geek Jun 10 '14 at 13:56
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A brushless DC fan has four stator coils and a permanent magnet in the rotor. A hall sensor detects which pole of the permanent magnet is facing towards it and it, in turn toggles power between 2 stator coils opposite of eachother, thereby attracting or repelling the permanent magnet and creating a rotational movement.

Whenever a fan cannot rotate because of undervoltage or being mechanically blocked, the same coils will continue to be powered for a prolonged period. Some of this energy is converted into heat, most of it is converted into a constant electromagnetic field.

Whilst not designed for this, chances of a coil melting are slim. Also, when undervolted, the energy in the coils is low enough to barely attract the rotor, let alone melt.

This website has a page that explains the inner workings of a PC fan.

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  • Thanks you. A good answer, I think. Allow me a question, even when this is not the case for PC computers: would it be very different in a not brushless fan? – Sopalajo de Arrierez Jun 10 '14 at 15:38

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