Abstract : This paper describes the application of a combination of PLC and frequency converter to achieve stable and reliable system operation, realizing truly unattended fully automatic circulating water pump switching and variable frequency operation. Keywords : PLC, frequency converter, pressure transmitter, level sensor, regulator [b][align=center]Application of Frequency Converter and PLC in Constant Voltage Water Supply YANG Hui, L1 Shenjian, SHI Ling f1. Shandong Province Kanglu Energy-efficient Equipment Plant, Jinan 250001; 2. Shandong Hongji Water Technology Co., Ltd., Jinan 250001; 3. Jinan Vocational School, Jinan 250001 [/align][/b] Keywords : PLC, Frequency transducer, Pressure transmitters, Tank sensor, Controller 1 Introduction With the rapid development of variable frequency speed control technology and programmable logic controllers (PLCs), and their wide application, powerful functions, and ease of use, they have become one of the main devices in modern industrial automation. They are widely used in industrial production and their application in other fields (such as civil and home automation) has also developed rapidly. Due to the flexibility and convenience of variable frequency speed control technology and PLCs, they are also widely used in constant pressure water supply systems. Using a PLC as the central control unit, and combining a frequency converter with PID control, the working pressure of the water supply system can be quickly adjusted according to the system status to achieve constant pressure water supply, improving the system's working stability and achieving good control and significant energy-saving effects. [b][align=center]For more details, please click: Application of Frequency Converters and PLCs in Constant Pressure Water Supply[/align][/b]