First, of all most solenoids are only full on or full off devices, much like a electro-mechanical relay. Trying to control a solenoid with a PWM signal is most likely to be very disappointing, they are not designed to act like linear actuators.
if i send power with less frequency? If i power on the triac in a way to get a 45hz instead of 60hz,
You can't send power with any frequency. That is what the power supply provides.
Why do you keep asking about AC solenoids? These do not have any proprieties that a DC one does not have appart from being powered by AC and AC is much harder to control proportionally.
these kind of valves are AC only or DC version exists too?
in the end i think will use a servo controlled valve that seems capable of doing this (btw, looking over the web i found a solenoid electrovalve capable to open and close in less than 5ms but they costs really a lot, more than 100€)
I can replace the knob with a cogwheel and control it through a servo!
QuoteI can replace the knob with a cogwheel and control it through a servo!You don't get more than one rotation with a servo.If you want to do this then look at a stepping motor. Mind you the speed will not be fast.