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The mobo does not care what the speed is - it only cares what the TEMPERATURE of the device is that the fan is cooling. This is counted by the mobo for speed measurement, but is NOT used for speed control. The motor generates a speed signal consisting of 5VDC pulses (two per revolution) sent back to the mobo header on Pin #3. The voltage supplied by the mobo on Pin #2 varies from 12 VDC max (full speed) down to about 5 VDC (min) because any lower voltage might cause the fan to stall and fail to re-start until the voltage is raised significantly. There are two main designs of fan widely used today (there are others, too).
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