Tach motor

I recently dug out my treasure box, in it, there are two old pc fan motors. They all have 3 wires, red black and yellow. At first, I thought they were PWM motor, deeper research, I found out that they are tach motors. Now I am trying to think of an experiment on Pi or Arduino. Any suggestions? By the way, the only thing I can think of a tach motor is the tachometer in cars, what are the application of it? Why would a pc fan use a tach motor? Thanks.

My guess is that the tacho is used for speed control and the fans are not some small 5 inch electronic cooling fans.

In a PC cooling is critical. A stalled/stuck fan can be flagged by the OS and possibly handled in hardware (e.g. reducing clocking to reduce heat generation.) The tach output gives the system insight into the fan's status.

A modern CPU will simply shutdown on over-temperature. They have thermal sensors all over the die,
you can get apps to plot the temperatures. Its normal for task schedulers to use temperature data
to select a core I believe - keeping even temperatures across the entire die reduces thermal stress and
aging.

Older CPUs didn’t have any thermal sensing and would destroy themselves without cooling I suspect,
hence the need for fan monitoring.

The typical failure mode for a fan is gradual hardening/contamination of the oil in the bearings leading to friction,
slower running, gradually reducing effectiveness till one day it just stalls completely.

Thank you all for the replies.
So I just want to make sure I understand the tach motor better, please tell me if I understand correctly.

  1. A tach motor, the yellow wire is for output only, it outputs the revs of the motor. Correct?
  2. The output can be hooked up to any analog input pins on Pi or Arduino. Correct?
  3. The wire has nothing to do with PWM, it does not control the motor speed what so ever, Correct?
  4. Does it mean the tach motor has a hall effect sensor built in?

Thanks.

af1812:
Thank you all for the replies.
So I just want to make sure I understand the tach motor better, please tell me if I understand correctly.

  1. A tach motor, the yellow wire is for output only, it outputs the revs of the motor. Correct?
  2. The output can be hooked up to any analog input pins on Pi or Arduino. Correct?
  3. The wire has nothing to do with PWM, it does not control the motor speed what so ever, Correct?
  4. Does it mean the tach motor has a hall effect sensor built in?

Thanks.

  1. Yes.
  2. Check the datasheet for the fan. Typically, the "FG" (frequency generator) output is an open-collector switched output whose frequency corresponds to the speed of the fan. Often it will generate N pulses per revolution where N is equal to the number of poles in the fan. Not always though. Check the datasheet.
  3. Correct.
  4. It might, or it might be using the BDC logic commutation to pulse the TACH output stage.

Do you have an oscilloscope? If so, hook it up, put a 10K pull-up on the tach output and have a look at it.

Thank for the reply.
"Do you have an oscilloscope?" I wish I had.
Do you know some tutorials the can read the output either for pi or the Arduino?

Connect a +5 V powered resistor and an LED to the mysterious output and rotate the fan by hand and see if some blinking occur. Use a 1 k or higher resistor value. Or use only the resistor and a multimeter connected across Mr Mystery.