there:
https://ths.gardenweb.com/forums/load/wiring/msg1116265815118.html
itsunclebill said "Many fans use a shaded pole motor" and he explains they are compatible with triac-based variable-speed controls, and why they can work erraticaly.
I tried and it worked for me.
A triac switches off when the current is zero. So it needs a subtantial current in order to stay on. The voltage control unit I bought is rated for min-max: 40W-160W, I understand that a load less than 40W wont make enough current to keep the triac on.
These cheap voltage control unit are ment to be use with resistive loads (ie. lights), this means it only lacks a snubber for inductive loads (this helps switching off the triac). So I added a R-C accross the triac, it reduces the noise in the fan and allows for very low speed.
Snubber ratings depends on the fan power, mine is a 240V 56W, I've used conservative values: 100ohm 1/2W-0.1µF 400V.