From the point of view of the name, it can be roughly understood that its main principle is to work according to the voltage signal, and to determine the action of the contact according to the magnitude of the coil voltage. When the coil is installed and used, it needs to form a parallel connection with the load. The voltage relay can be divided into AC and DC according to the coil voltage, and overvoltage and undervoltage according to the operating voltage. Therefore, their functions will be different. The overvoltage is that when the coil voltage is within the rated value range, the armature will not perform any pull-in action but will perform the action if it exceeds.
The AC over-voltage relay acts as an over-voltage protection in the circuit. Under-voltage occurs when the coil voltage reaches or exceeds the coil's rated value. When the armature is released immediately, the undervoltage relay mainly plays the role of undervoltage protection in the circuit. The symbol of the voltage relay is shown in the following figure (the left is the overvoltage and the undervoltage).