A relay is an electrical circuit that turns the circuit on or off according to changes in electrical quantities (such as voltage, current, etc.) or non-electrical quantities (such as heat, time, pressure, speed, etc.) to achieve automatic control and protection of the electric drive. The relay is generally composed of three basic parts: the sensing mechanism, the intermediate mechanism and the actuator. The sensing mechanism transmits the sensed electrical quantity to the time mechanism and compares it with the rated setting value. When the setting value (excess or under) is reached, the intermediate mechanism causes the actuator to act to be turned on or off. Open the controlled circuit. There are many types of relays, which can be divided into control relays and protection relays according to their use. According to the nature of input signals, they can be divided into voltage relays, temperature and current relays, time relays, speed relays, pressure relays and temperature relays. It is an electromagnetic relay, an inductive relay, a thermal relay and an electronic relay; it can be divided into a transient relay and a time delay relay according to the action time.