How to find correct part cooling fans?
If the fan you are using is not defective and if you test another fan that behaves in a similar manner, it's possible your firmware or hardware are the root cause of the problem.

The controller directs the driver to vary the power provided to the fan. It's a method called pulse wave modulation, aka PWM. Full voltage is applied to the fan one hundred percent of the time, for full speed operation.

For fifty percent performance, half of the time full voltage is sent, while half of the time no voltage is sent. The time period is rather short, but I didn't determine that aspect of this answer.

One can find a clear explanation of PWM online but one word is worth one one-thousandth of a picture:



If your fans are not performing properly, either the program embedded into the controller is buggy, or the driver module for that unit is bugging out.