Fan speed control is tricky. Typically the speed controls built into units is done by having multiple poles or taps. Trying to do this externally proves difficult and dangerous.
Although you can get away with it, if you use the wrong type of speed controller, you could be seriously limiting the life of your fan or risking external damage. If its cheap or has a variable knob, chances are its the bad kind.
Resistive or switching controllers such as Rheostats, Potentiometers, triac etc. Should be avoided. They add additional stress through heat or harmonic distortion and can seriously degrade the life of the unit or add fire risk.
If you insist on having a fan speed controller, ensure its a step capacitor type or variac. There are variable frequency which is the best of all. But that would cost a lot more just for the added functionality of full speed control.