hello chaps, i wonder if anyone could help with a minor emergency i have.

i have an electric fan in a scene that is initially off. it gets turned on at a certain point and spins continuosly until a button for the faster rotation is pushed.

my problem is this, i cant get the fan to stop slowing down. i've keyframed the start of the rotation and the point where it is to speed up and the end point. all this seems to do is make the fan slowly pick up speed, stop the get faster as it passes the second keyframe. is there an easy way to do this like with a piece of script or something? tried a google which spat back nothing so please guys, help me...