In fluid technology electro-mechanical signal converters used to actuate valves are often referred to as solenoids. These are not permanent magnets, but are electromagnets.
An electrical input signal (voltage or current) generates, via the magnetic field in a coil, a mechanical output signal (distance or force). Solenoids can be designed either as linear solenoids (switching solenoid), as proportional solenoids or as control solenoids. In operation linear solenoids only move between their two end positions, i.e. the valves operated by them are switched from one position to another. If the mechanical output signal is to be changed continuously, proportional or control solenoids are required. All three types of solenoid common today apply a compressive force. The return to the initial position is generally via spring force. In fluid technology DC solenoids (voltage or current) and AC solenoids (voltage or current) are available as linear solenoids. They differ in particular in the stroke-force curve (Figure E 23) and with it the switch-on time. The switch-off time is dependent on the unlocking time and the spring force (Table E 1).
AC solenoids are primarily manufactured as dry switching solenoids (switching in air) as well as sealed (switching in oil). While the solenoids switching in air require between the valve and solenoid a seal that causes friction, the sealed solenoid does not need a seal. The sealed DC solenoid is becoming increasingly standard in hydraulics.
Figure E 23: Stroke-force curve of an AC solenoid (a), DC solenoid (b) and proportional solenoid (c)
Figure Table E 1: Comparison of switching times, AC and DC solenoid