Electric relays are an automatic electrical appliance that is suitable for long-distance connection and disconnection of AC and DC small-capacity control circuits and for control, protection and signal conversion in electric drive systems. The input quantity of the relay is usually electricity such as current, voltage, etc., or it can be non-electrical quantity such as temperature, pressure, speed, etc. The output quantity is the electrical signal or the parameter change of the output circuit when the contact moves.

The characteristic of the relay is that when the change of its input quantity reaches a certain program, the output quantity will change step by step. Control relays have a wide variety of uses and are traditionally divided into the following categories according to their different input quantities:

① Voltage relay It is a relay that acts according to the change of circuit voltage, such as AC and DC voltage relays used for motor voltage loss and undervoltage protection AC voltage relays used for winding motor braking and reverse control AC voltage relays used for DC motor reverse rotation And the DC voltage relay for reverse braking, etc.

②Current relay It is a relay that acts according to the change of circuit current and is used for overload and short circuit protection of motors and other loads, as well as magnetic field control or loss of field protection of DC motors.