JimboZA:
Just yesterday I tried to run a laptop cooling fan backwards by hooking the battery up back-to-front, and it didn't start.Do brushless DC motors actually even run backwards?
Jim
Inside a computer fan is a control IC - it's actually a small computer with state machine and all. This switches 3 coils on the motor, making the magnet ring turn. For feedback there is sometimes some hall-sensors too. Usually all this circuitry is built in a single component with 3 to 5 legs soldered to a small circuit board. Reversing the polarity on the wires would be like putting in power to the Arduino in reverse - you wouldn't want to do that - right ?!
There is no way of reversing the fan direction without modifying it heavily - and then the fan blades do not work in reverse - look at the profile on them, fan blades and propellers only work the "right way" round.