Contactors are mainly composed of electromagnets and contacts. Contactors with larger capacity also have arc extinguishing devices. When the coil is not electrified, the disconnected contact is called the constant open contact, and the closed contact is called the constant closed contact. When the coil is electrified, the electromagnet generates suction and attracts the armature, closing the normally open contacts and disconnecting the normally closed contacts. When the coil loses power, the electromagnet loses its suction, the open contacts are often disconnected, and the closed contacts are often closed, thus realizing the circuit breaking.

Selection of contactors should meet the following requirements:

(1) The rated working voltage of the main contact of the contactor shall be greater than or equal to the rated voltage of the load.

(2) The rated working current of the main contact of the contactor should be greater than or equal to the current of the load circuit. If the motor is controlled by a contactor, the rated current of the main contact of the contactor is generally reduced to one level.

(3) The breaking capacity and overload current tolerance of the contactor should be higher than the current value that may occur in the circuit.

(4) The types of contactors used should be adapted to the requirements of control objects and operating conditions.