We have a system involving a fan blowing air through a box which contains a small (but constant) heat source and a temperature sensor. The fan speed is controlled with PWM.
As PWM (and hence fan speed) is increased, the temperature is initially observed to decrease as expected. However above a certain PWM value, increasing fan speed appears to actually have the opposite effect and the temperature is observed to increase. Does anyone have any intuition for what could be causing this?