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 range 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 for motor voltage loss and undervoltage protection AC voltage relays for winding motor braking and reverse control AC voltage relays for DC motor reverse rotation And the DC voltage relay for reverse braking, etc.

② Current 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.
③Time relay This is a relay with a certain time interval from receiving the signal to the action of the actuator, such as a relay used to delay the switching of the starting resistance, the energy consumption braking of the motor and the program control of the production process when starting the motor.
④The thermal relay is a relay used for the overload and phase failure protection of the AC motor.
⑤Temperature relay is a relay used for overheating protection or temperature control of various equipment.
⑥ Speed ​​relay is a relay for monitoring the motor speed and steering changes.