A contactor is an electrically controlled switch used for switching an electrical power circuit for example to provide power to a motor or remove power from a motor. A contactor is typically controlled by a circuit that has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch.

There is a variety of contractors in the market, we shall describe the functionalities of contactors based on Siemens Sirius type contactors. The following diagram shows how we can make connection/disconnection of supply electricity when we apply control voltage (24V DC) to A1 and A2 then line 1<>2 and 3<>4 get connected, on the other hand, R1<>R2 and R3<>R4 are disconnected.

The control voltage can be applied by the PLC program or by any other programming circuit that can provide a 24V supply. If we want to control the motor (forward and reverse direction) easily we can build it as shown in the following video.

Usages of contactor in an automation system :