I know this is an old thread, but for anyone who is interested, I have designed AC motor control circuits using a FET. There are many advantages to using a FET instead of a Triac. One main reason is No Hum (well very little). Triac controlled AC motors are noisy at low speeds, since triacs conduct when voltage reach a certain voltage level (phase delay controlled).
FET controlled is quiet because FET is switched on at each zero crossing and is switched off after a timed period (e.g. on for 5ms of both +VE and -VE half cycles.
If anyone is after some help in designing a quiet AC fully variable speed controller compatible with inductive motors (ideal for fans, but will also work with resistive loads) then post any questions, comments e.t.c.