Hi Paul,
you are free to ask as many questions as you like. As long as your postings deliver detailed information of what you do there is a high chance to get answers each time.
If the details what you have done are missing your potential helpers have to ask back. To have to ask back for details is annoying. It is not annoying to read 20 sentences of a detailed description . If you format your posting into paragraphs. like this one.
I meant the following:
If you have no knowledge about electronics (in spite which you have some knowledge about electronics ) and no knoweldge about programming there will be a lot of questions.
To emphasize that: It is totally OK to be a newbee about electronics and programming.
Anyway it is a good idea to inform the forum about your knowledge level.
From what you have asked so far I can conclude that you are still a newbee about electronics.
@ JCA34F :
You are right backwards through the ESC there won't be coming out a voltage. I made the practical test with a RC-airplane brushless motor: If you rotate it and measure between two wires a AC-voltage is generated.
Though a brushless fan has a plus and minus wire because the ESC that generates the 3.phase AC is build in. If th fan can be opened and the wires to the coils are accessibe this would be a way to get your hands on the AC-voltage. Though this "signal" from the motor-cols needs conditioning
before you can feed it into an Arduino.
So what Blackfin suggested will be easier if a tacho-output is there.
I did a quick search about re-using an old harddisc-spindel-motor
and found this one
At low rpms the generated voltage is pretty low 0.4V. Hard to use for measuring windspeed.
But the video shows interesting basic principles how a 3-phase AC-motor works and how you determine the wiring and how to transform a 3-phase AC into DC.
best regards Stefan