How does a motherboard determine the RPM of a connected fan?

The sensor wire sends a signal back from the fan which the motherboard reads.

Some times if the fan is to slow in the past some motherboards have interpreted the speed as 40k + RPM.
 
From what I understand fans utilise something called a "hall effect sensor"

Simplest way I can describe it is;

On the fans hub(or main body) there are small magnets (one on more) the tachometer wire connects to another magnet or copper probe connected to the housing of the fan. Each time the magnet makes contact with the sensor it produces a pulse which can be read. These pulse are then multiples by 60 to produce rpm(presuming your mobo updates per second)

For example the hall sensor detects 3 pulses in a second useing a single magnet the motherboard would register this as 180rpm.

If a system contains more than one magnet(normally on faster fans) they may contain a chip to simplify the revolutions for the motherboard(never seen them myself) so each magnet is opposite each other each revolution provides two pulses indicating a single revolution of the hub.

More magnets apparently make a fan more stable (less buffering from a wonky fan) which also increases the sensitivity of the tachometer feedback.

I'm not amazing at explaining things so I hope this makes sense. If anyone knows this to be wrong please say as I'm here to learn:)
 
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