A contactor is a switch. Instead of a handleoperated, movable blade, a contactor uses contact blocks of silver-coated copper, which are forced together to make (close) or are separated to break (open) a circuit.
The common wall-mounted light switch is a small, mechanically operated contactor.
A relay is an electrically operated contactor. Most contactors are operated by means of an electromagnet that causes the contacts to close. They open either by spring action or by gravity.
Contactor terminology is somewhat different from that of switches. Its condition when deenergized is its normal state. Thus, a contactor whose contacts are open when the coil is not energized is referred to as normally open (NO), whereas a contactor whose contacts are closed when deenergized is referred to as normally closed (NC).
Units intended for motor connection are called motor starters. Current, voltage, and number of poles have the same nomenclature for contactors and relays as for switches.
The great advantage of contactors over switches is their facility for remote control. Switches must be manually thrown—or, in very special cases, thrown by a motor.
The magnetic contactor, by contrast, is inherently a remotely controlled device, making it ideal for myriad control functions. It can be controlled by a manual or remote push button or by automatic devices such as timers, float switches, thermostats, pressure switches, and so on.
Because control can be both remote and automatic, contactors are universally used in the control of lighting,
heating, air conditioning, motors, and the like.
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