Quick test of controlling the speed of various brushless DC fans using PWM at various frequencies. Speed control of brushless fans using a constant voltage is usually recommended. However, PWM can be used in most cases.
PWM signal into IRFZ44 N-CH MOSFET. Fan +/- to DC power and MOSFET output. Meter displays % duty cycle. Osciloscope top trace (CH1) is input PWM signal. Bottom trace is MOSFET output. Operating frequency is 400-5000Hz.
The fan is still functional at various operating frequencies with little effect on speed. The PWM % determines the speed. At low PMW % (10%), the fan may not startup. In most cases, 25% PWM is required.
Audible switching noise is commom when using PWM at less than 20kHz. Higher operating frequencies may lead to other noise problems.
Recommendation: Use a brushless DC fan with PWM input (4-wire) or constant voltage for speed control. If using PWM, use a higher operating frequency with some noise filtering.
Arduino PWM Fan Controller: https://www.youtube.com/watch?v=Tzn9Rg9jIVQ
Another Arduino PWM Controller: https://www.youtube.com/watch?v=DAn4UguyzfE MOSFET based along with Arduino