An electric solenoid is a type of switch that is used to control a wide variety of mechanical processes. It is similar in construction to an electric motor with the exception that the metallic central shaft of the solenoid is moved in and out of its enclosure by the application of a magnetic field that is created when electrical power is applied to a copper winding that surrounds the shaft. Types of solenoids are used in automobiles for various purposes, such as mechanical clutches to engage a starter motor assembly that meshes with a flywheel, as controls for vacuum and air valve systems, and in fuel injectors.
Miniature versions of solenoids are built into doorbells to engage a small armature that rings a bell when the doorbell button is pushed, and in thousands of other micro-control systems. Other precise applications include for pinball machine flipper controls and for door locks and controls to close some doors automatically. Small electric solenoid components have very little force that they are capable of applying, so usually they act as an electric solenoid actuator which engages stronger mechanical systems to close doors or otherwise move heavy objects.
The purpose of an electric solenoid centers around any need for rapid mechanical control of a system though the application of electrical power. This gives it a diversity of uses in both heavy-duty machinery and fine electro-mechanical circuits. A solenoid's parts are essentially simple electromagnet parts consisting of a copper coil winding that produces a magnetic field when electrical power is applied to it, and a central movable shaft, usually made of a magnetic metal like steel or iron. Through the principle of induction discovered by Michael Faraday in 1831, the coil winding produces a magnetic field that pulls or pushes on the shaft. The shaft is usually spring-loaded to keep it in a non-contact position until power is applied to the electric solenoid, and, once it has done its job as an actuator, power is cut to it and it withdraws to a neutral position in the mechanical assembly.
Since using an electric solenoid depends entirely on the application, they vary greatly in size and power needs. Solenoids can be powered either by alternating current (AC) or direct current (DC), as both will produce induction in a wire winding. The mechanical components that operate the shaft are usually referred to as the armature assembly and make up the moving parts in the electric solenoid. The axial stroke is the amount of distance that the shaft will move when activated, and usually ranges from between 0.022 to 0.1 inches (0.559 to 2.54 millimeters). Return springs act as a mechanical means for resetting the solenoid to a neutral position, and the coil winding that surrounds the armature assembly is known as the stator assembly.