How a solenoid works ?

When current flows through an electrical conductor, a magnetic field builds up around it. This magnetic field grows in size if the current intensity is increased. Magnetic fields exert an attractive force on workpieces made from iron, nickel or cobalt. This force increases as the magnetic field grows.

Structure of a solenoid A solenoid has the following structure:

• The current-carrying conductor is wound in the shape of a coil (air-cored coil). The overlaying of the magnetic field lines of all the coil windings (see Figure 3.3) amplifies the magnetic field.

• An iron core is placed in the coil. When an electric current flows, the iron is additionally magnetised. This enables a much stronger magnetic field to be generated at the same current intensity than with an air-cored coil.

Both of these features ensure that a solenoid exerts a strong force on ferrous materials even when the current intensity is low.

Applications of solenoids

In electropneumatic control systems, solenoids are primarily used to influence the switching position of valves, relays or contactors. To explain how this happens, we will use the example of a spring-return directional control valve:

• When an electric current flows through the solenoid coil, the valve piston is actuated.

• When the current flow is interrupted, a spring pushes the valve piston back into its initial position.