Abstract : This paper analyzes the application characteristics of variable frequency speed control technology in constant pressure water supply, proposes a design scheme for a constant pressure water supply system combining PLC and frequency converter, and introduces the system composition and working principle. Keywords : frequency converter, constant pressure water supply system 1 Introduction Currently, most large high-rise buildings and residential communities still use traditional fixed-frequency three-phase AC motors to drive loads, and use elevated water tanks or pressure tanks for pressurization. Due to the influence of seasonal, diurnal, and temporal variations, water consumption varies greatly, the water pressure in the pipe network is not constant, and there is an imbalance between water supply and consumption. A constant pressure water supply device using PLC and frequency converter can solve many of these problems. This design uses a control scheme of one frequency converter to control multiple water pumps (one-to-N), which has the advantages of stable water supply system operation, low impact on the power grid, and energy saving. 2. Pump Control Characteristics The pump characteristic control curve is shown in Figure 1: If the flow rate decreases from Q1 to Q2, the constant speed pump head will increase to B. Since the constant pressure line is at CP, through feedback control, the frequency converter operates at a reduced frequency, and the pump decelerates from n2 to maintain at point n1, thus maintaining constant pressure CP. This reduces the head in the "BB" section, saving energy. Based on the actual needs of the pipeline network, the pipeline pressure is set. The system is equipped with a pressure sensor and PLC, outputting a standard signal to adjust the frequency converter's output frequency, controlling the motor and pump speeds, achieving closed-loop automatic pressure control with the given pressure as the target. [b][align=center]For more details, please click: High-Rise Water Supply Control System Based on PLC + Frequency Converter[/align][/b]