According to the mode of actuation, a distinction is made between direct-acting valves, internally piloted valves, and externally piloted valves. A further distinguishing feature is the number of port connections or the number of flow paths (e.g. 2-way or 3-way solenoid valve).


Solenoid valve coils are available in all the commonly used AC and DC voltages. The low power consumption, in particular with the smaller solenoid systems, means that control via solid state circuitry is possible.

The magnetic force available increases as the air gap between the core and plug nut decreases, regardless of whether AC or DC is involved. An AC solenoid system has a larger magnetic force available at a greater stroke than a comparable DC solenoid system.

The current consumption of an AC solenoid is determined by the inductance. With increasing stroke the inductive resistance decreases and causes an increase in current consumption. This means that at the instant of de-energization, the current reaches its maximum value. The opposite situation applies to a DC solenoid where the current consumption is a function only of the resistance of the windings. A time-based comparison of the energization characteristics for AC and DC solenoids is shown in Fig. 9. At the moment of being energized, i.e. when the air gap is at its maximum, solenoid valves draw much higher currents than when the core is completely retracted, i.e., the air gap is closed. This results in a high output and increased pressure range. In DC systems, after switching on the current, flow increases relatively slowly until a constant holding current is reached. These valves are therefore, only able to control lower pressures than AC valves at the same orifice sizes. Higher pressures can only be obtained by reducing the orifice size and, thus, the flow capability.