For all duty cycles less than the minimum duty cycle, the RPM shall not be greater than the minimum RPM.
The following graphs and definitions show three recommended solutions to handle PWM duty cycles that are less than the minimum operational RPM, as a percentage of maximum.
In a Type A implementation the fan will run at minimum RPM for all PWM duty cycle values less than minimum duty cycle. The minimum fan speed is controlled by design and can not be overridden by the external fan speed controller. See Figure 3
In a Type B implementation the fan will run at minimum RPM for all non-zero PWM duty cycle values less than minimum duty cycle and turn off the motor at 0% PWM duty cycle. See Figure 4.
In Type C implementation the fan will stop running when the current provided to the motor windings is insufficient to support commutation. The fan should not be damaged from this. The fan would also turn off the motor at 0% PWM duty cycle input. See Figure 5.