I play around with an old 3-wire CPU fan from a dell server PC. It is this thing: 3612KL-04W-B66 https://www.coolingsurplus.com/index.php?option=com_content&view=article&id=86:3612kl-04w-b66&catid=6:92mm-fans&Itemid=7
It was mounted at the end of heat pipe over the CPU. It was pulling the air from the CPU to the outside of the case.
I found a datasheet here (https://www.electronicsdatasheets.com/manufacturers/nmb-technologies/parts/3612kl04wb66) but this particular model B66 is not in it, only B50 and B60 and other round numbers and they are all 2-wire fans.
According to some info I found about dell fan wiring, black wire should be GND, red wire is +12V and white wire is TACH feedback.
As I understand the basics of 3-wire brushless fans, I can control the speed by varying the voltage (10.5 - 13.2 V according to datasheet) or low frequency PWM the 12 V input.
As a less experienced fan user I would expect the PWM method to cause the fan to slow down while running at full speed at 12 V. But the fan acts in the opposite way, at least for me.
It spins up to speed for a short period after connecting the positive wire to 12V PSU (made from ATX PSU), then it slows down to very low speed.
When I connect and disconnect the positive wire manually in short succession (0.5 Hz by my fingers judgment) the fan speeds up significantly.
I ask you guys for some advise how to actually control the speed of this fan, roughly and manually.
Edit: I managed to resolve the mistery behind the low RPM at 12 V continous. There is some speed controll built into the fans board. I can see a small part coming out from the side of the stator case. Might be a temperature sensor. I pointet a heat gun at it from behind the air flow. The fan revived to significant speed :-) I am stil curious, what is the white wire for (seems not to be TACH, only bias on the analog scope) and how to get rid of the internal speed control
Edit: Shorting the leads of the termistor (youtube.com/watch?v=AVovy7UWDQ0, is exactly the same fan by the way) lets the fan run at full speed and I can control RPM now by lowering the voltage. The question about the function of the white wire stays in the room :-)