0 Introduction All buildings in our college are no more than 6 stories high, and water consumption varies significantly throughout the day. The original pump room had three 22kW water pumps. The number of pumps and valve openings were determined based on normal water usage, with one pump running normally and two to three running during peak hours. This resulted in unstable pressure, causing great inconvenience to students and staff, and the problem has worsened in recent years. In October 2002, the school underwent a comprehensive renovation. The renovated water supply system not only met water requirements but also achieved significant energy savings. [b]1 System Design 1.1 Main Circuit[/b] Based on actual operating conditions and cost savings, we selected a one-to-three scheme using one Mitsubishi FR-A540E 22kW frequency converter to control three 22kW motors. The system control device is a PLC controller, as the PLC can not only realize the logic control of the pump group but also perform digital PID regulation of the system, real-time monitoring of various operating parameters and control points, and fault alarms. A typical block diagram of an automatic constant pressure water supply system is shown in Figure 1. The system automatically adjusts the number of operating water pumps and the operating frequency of the variable frequency pumps under PLC control based on changes in user water consumption, ensuring constant pressure water supply across the network. The system converts the detected network water pressure signal into an electrical signal via a pressure transmitter installed on the main outlet pipe. This signal is input to the PLC's input module, where it undergoes PID calculations with the pressure setpoint signal to determine the optimal operating parameters. The output module then outputs logic control commands and the frequency setpoint of the variable frequency pump to control the number of pumps in operation and adjust their operating parameters. Furthermore, it performs soft start and soft switching for each pump according to CP commands. [b][align=center]For more details, please click: PLC-controlled Variable Frequency Speed ​​Water Supply System[/align][/b]