Electrical contactors, sometimes referred to as motor starters or controllers, are devices that control the electrical currents to a motor, welder, or lighting system. The difference between a contactor and a circuit breaker, which also makes and breaks the circuit current, is the operation and duty cycle of the device.

A contactor must be capable of operating one million times mechanically, while a circuit breaker is designed to operate tens of thousands times. A contactor operates as either a normally open or normally closed switch; that is, it has monostable operation, while a circuit breaker has bistable operation.

Both devices provide circuit protection under overload and short-circuit currents, but the sensors that trip the
contactor are set differently from those of the breakers. The contactor may be part of switchgear or an electrical panelboard used to control a machine or lighting.

It can be coordinated with a fuse so that the contactor provides the control and protection while the fuse serves as a backup, in case the contactor fails.

Most industrial power circuits are three phase, as opposed to household circuits, which are single phase. Therefore, a contactor has three sets of contacts to make or break the current in each phase of the circuit, making it a three-phase device.

The sets of contacts for each phase are in the current interrupting part (interrupter chamber) of the contactor. The interrupter chamber can be an air chamber or a vacuum chamber. Contactors that have the contacts in an air chamber are called air (or magnetic air) contactors, and those in which the contacts are in vacuum chamber are called vacuum contactors.

In addition, there are special types of contactors that will control currents in dc circuits. These contactors usually have only one set of contacts, and are of the air magnetic type. The operation of a contactor can be controlled by buttons and handles on the contactor body or remotely.

Air magnetic motor protectors have built-in overload current sensors and short-circuit current instantaneous sensors that trip the contactor. The overload sensor is a thermal device and has detach tripping, while the short-circuit current sensor is a magnetic device and has instantaneous tripping.

Vacuum contactors can be operated manually or remotely by electronic current-sensing devices (e.g., Westinghouse IQ electronic sensors). The current sensors are adjustable so that the proper current-time characteristics for the contactor application is produced.

Contactors are manufactured by many different companies in the United States, Europe, and Asia. Some of these companies are Cutler-Hammer (Westinghouse products), G.E., Joy Manufacturing, Mitsubishi, and Toshiba.

Manufacturers of the vacuum interrupter for the vacuum contactor are Cutler-Hammer (Westinghouse products), G.E., Joy Manufacturing, Jennings, Hitachi, Mitsubishi, and Toshiba. This list of companies
is not complete.

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