triac speed control
Hello
You can use an gate control usng an moc 3020.
You just need to detect zero cross and control the angle of firirng the semiconductor.
if you chosse an angle of 180 degres you will have 0 v on charge a long you decreasse the angle the tensio goes Up .
An better solution if use an pwm control and 4 igbt and define the frequancy that you want your fan runs, its more expensive but the result are best.
One way or other I recomend you to read some electronis power books.
Then you should easy to understand.
Regards