On AC coils, the current is determined not only by the ohmic resistance of the windings, but also by the inductive resistance (reactance). The inductive resistance is significantly influenced by the position of the solenoid core. If the solenoid core has dropped out, the inductive resistance is lower and the coil current is higher. The current is thus higher in the pick-up phase than the coil current in the holding phase. As compared to a DC coil, coil resistance is far lower with the same voltage value being applied. Consequently, a coil should never be operated without a solenoid core when operating with AC voltage. Otherwise, there is a risk of the coil overheating and burning out after a few minutes.
Functional principle of an AC solenoid