From the point of view, it can be roughly understood that its main principle is that it needs to work according to the voltage signal, and the action of the contact is determined according to the magnitude of the coil voltage. During installation and use, the coil needs to be connected in parallel with the load. The voltage relay can be divided into AC and DC according to the coil voltage, and can be divided into overvoltage and undervoltage according to the operating voltage. Therefore, their functions will also be different. Among them, the overvoltage is that when the coil voltage is within the rated value range, the armature will not make any pull-in action. On the contrary, it will perform the action.
The AC overvoltage relay plays the role of overvoltage protection in the circuit. Undervoltage is when the coil voltage reaches or exceeds the rated value of the coil, the armature will make the pull-in action, and the coil voltage will be lower than the 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 figure below (left is overvoltage and undervoltage).