How does an AC solenoid valve works???

AC solenoids employ shading coils. These coils are transformer coupled to the main solenoid coil, and the coupled current lags the solenoid current by 90 degrees. This shading coil current is enough to hold the movable iron core in place when the ac current goes to zero. Interestingly, the direction of the current through the main coil does not affect the movement of the core, only the magnitude of the current is important. The following nice visual description should make it clear. They don't mention it, but some ac solenoids have built-in rectification and filtering to yield dc. But as the following web site explains, that isn't fundamentally necessary.

http://www.cartage.org.lb/en/themes/sciences/physics/Electromagnetism/Magnetostatics/Electromagnetic/Solenoid/Solenoid.htm

That is a Keeper Link for sure.

One addition, You will hear a 60 Cycle Hum when direct AC is the driving force,

Also, on very critical cycle timing, a DC Coil will always act the same electriclly while an AC Coil can have differences in Armature movement due to the cycling On-Off. I've never come across the problem but have read about critical timing and repeatability operations being affected.