Electronics

Brushless DC fan speed can be varied using pulse-width modulation. The typical pulse-width modulation control scheme inserts a power switch in series with the fan. In such applications, an increase in the active duty cycle causes a corresponding increase in fan speed (i.e., fan speed is proportional to t_ON/(t_ON + t_OFF). Conventional wisdom states that pulse-width modulation duty cycle alone determines operating speed (for example, the fan speed runs at 50% of maximum when the pulse-width modulation duty cycle is 50%). In reality, the fan operates at a higher percentage of full speed for any given duty cycle. The resulting difference between actual fan speed and the corresponding pulse-width modulation duty cycle (herein referred to as Speed Error) can be problematic in open loop fan control systems.

http://ww1.microchip.com/downloads/en/AppNotes/00772b.pdf