Electromagnet operated with DC. The DC solenoid only reaches its nominal current some time after switching on. For this reason the force initially increases more slowly, and it therefore switches more slowly than the AC solenoid (Figure G 18).
Advantages:
- Given corresponding design, very high switching figures are also possible,
- The armature can be maintained in any intermediate position without the current (the force) increasing excessively.
Disadvantages:
- Relatively long switching times,
- Relatively low attraction force,
- As DC solenoids primarily operate with 24 V, a transformer and rectifier are required.
It is possible to accelerate the increase in current by means of fast excitation or by means of over-excitation(Figure G 19).
In relation to the layout a differentiation is made between:
- Dry DC solenoids on which the armature moves in an air gap. The armature axis opening must therefore be sealed against the oil in the valve using a seal. This situation signifies friction and hysteresis;
- Sealed DC solenoids on which the armature moves in a pressure-tight tube that comprises a steel section and a non-ferrous section for reasons related to the magnetic flux. The armature axis does not need to be sealed (Figure G 20). This design is primarily used these days.
Figure G 18: Voltage, current and stroke curves for DC solenoids
Figure G 19: Switching time reduction with quick excitation and over-excitation
Figure G 20: Sealed DC solenoid