Dimmers are NOT suited for resistive loads like (fan)motors. Please do not do this: it's dangerous. Apart from that, you will -possibly over time- break your dimmer and potentially even the fan motor. Plus the behaviour of the speed regulating will not be as expected: you may experience speed fluctuations, not getting full speed, the motor not spinning up when starting at low dimmer level, the dimmer heating up, etc. But most importantly: when you switch the dimmer off, the fan motor will continue spinning, sending energy back into the dimmer. This can cause damage to the dimmer and even start a fire!
The proper alternative is buying a motor regulator like for example sonoff's ifan03.