Solenoids come in many forms, flavors, and functions. Sometimes they are called contactors in the electrical realm.
Solenoids rely on electromotive force to move plungers. Those plungers then close a given electrical circuit, or open or close fluid pathways through a valve body.
Electrical Solenoids
Electrical Solenoids can be thought of as really big relays. In a relay, there is a small coil pulling a small paddle to close 2 contacts, and those contacts are usually limited to 10-15 amps or so continuous duty, 20-30 amps burst duty. In a solenoid, the coil is typically 1.5″ or so in diameter and the plunger/contactor is even larger – usually on the scale of 2 inches in diameter – enabling a flow of 150-200 amps continuous duty and 300-400 amps burst duty.
While there are benefits to solenoids, as with anything there are trade-offs. The first is heat. Solenoids can get very warm due to the amount of current that the coil uses to pull the plunger shut, and the ability to shed that heat can be a significant mechanical limitation. The second tradeoff is that Solenoids only come in SPST format, usually a Make-On-Energize.
Example: Engine Starter Assembly
The example shown below is of a NipponDenso style automotive car starter.
A cable, usually 4 AWG or larger, connects one contact post directly to the battery. A jumper, usually 8 gauge, connects the other contact post to the electric motor. The motor is then grounded by the casing.
In the image below, there is a shrouded connector which is the +12 volt trigger for the solenoid coil, which also grounds to the casing. The brass bolt sticking up is where the battery cable (typ. 0-4 AWG) connects.
The starter itself is 3 components: the electric motor, the gearbox, and the solenoid.
In the image below, a replacement set of contacts and a new plunger are shown. These photos were taken during the rebuild of a starter.
In the image below, you can see the new contacts installed in the housing. In this orientation, the top contact uses an 8-AWG wire (shown) to power the electric motor. The bottom contact is where the battery cable attaches.
In the photo below, the worn components are mocked up to illustrate how the plunger connects the 2 contacts to complete the circuit.
Fluid Solenoids
Fluid solenoids operate very much the same as electrical solenoids. The only difference is that instead of mating a contactor to contacts, the plunger is opening or pinching off a fluid circuit – sometimes both at the same time.
There are many more types of fluid solenoids then electrical solenoids – this is because a device called a “spool valve” can be used to open and close many fluid circuits at a time. This is how automatic transmissions work – applying pressures to specific clutch diaphragms to engage certain gearsets.