As a general rule, DC solenoids are typically preferred over AC solenoids for several reasons. For example, when space constraints are tight, a DC solenoid will usually give better performance in a smaller package than will a comparable AC solenoid. In addition, DC solenoids have the capability of being modified in such a fashion as to prevent the pole faces from making contact at the end of the stroke. In AC solenoid applications, this type of noise dampening would very likely cause premature overheating and failure. AC solenoids require great care to insure precise alignment of the plunger's pole face to the stator pole face. This facilitates the pole faces making contact with as much surface area as possible in the energized state which will reduce the amount of hum or chattering in the AC unit. However, in a similar DC application, a slight gap between the pole pieces at the end of the stroke can have a drastic effect on improving the overall life of the solenoid.
When looking inside the AC solenoid stator cavity at the stator pole piece, there is a small ring inserted into the face of the stator pole. This ring is known as a shading coil and is designed to obtain minimal pulsing in force. What this means is that the coil splits the pole into two separate parts which causes the flux of these parts to be out of phase. If not for this shading coil, the chattering sound that is commonly associated with AC solenoids would be more noticeable.
Bottoming out of the pole pieces during each stroke is a requirement on AC solenoids. Naturally, with this metal-to-metal contact, some deformation of the pole pieces is possible. As the pole pieces deform and less surface area makes contact between the pieces, the AC solenoid hums louder and louder. In contrast, a DC solenoid does not have to have the pole pieces come in contact during each cycle. Rather, if the pole pieces do not make contact, life of the solenoid is extended. Life of an AC solenoid is typically lower than that of a similar DC counterpart.