Abstract : To improve the control accuracy and reliability of large-scale variable load constant pressure water supply systems, a control strategy of hierarchical control of multiple water pumps and alternating frequency conversion start-up is proposed. The conventional PID control algorithm is improved based on the characteristics of large-scale variable load water supply systems. Simultaneously, the output and switching issues of the frequency converter are analyzed, and specific switching methods and control circuits are given. This system replaces the direct pressurization method using high towers or pumps, improving water supply quality and achieving significant energy savings. Keywords: Constant pressure water supply; PLC; Frequency converter; Variable frequency speed regulation. PLC-based variable frequency speed regulation constant pressure control is a major method in modern water supply control systems. It utilizes a PLC, sensors, electrical control equipment, frequency converter, and water pump units to form a closed-loop control system, maintaining constant pressure in the water supply network. Due to its high degree of automation, high efficiency and energy saving, safety and hygiene, and convenient maintenance, it is widely used in community water supply and factory liquid water supply control, achieving good control effects and social benefits. 1. Composition of a Variable Frequency Constant Pressure Water Supply System The water supply system described in this paper is applicable to various applications including domestic water, industrial water, and fire protection. Taking a water supply system consisting of five pumps as an example (Figure 1), the variable frequency speed control constant pressure water supply system comprises actuators, signal detection, a PLC control system, a frequency converter, a human-machine interface, upper-level connection, and alarm devices. Its working process is as follows: The PLC first checks whether the water level protection switch in the water tank has activated; otherwise, the frequency converter directly starts the first pump. Simultaneously, a remote pressure gauge measures the water pressure in the outlet pipeline and sends the analog signal to the PLC controller. After comparing this analog signal with the given water pressure value (set upper and lower limits), the PLC controls the frequency output of the frequency converter to adjust the pump speed. When the frequency converter reaches its maximum or minimum, the PLC controls the pump to increase or decrease pressure to achieve constant pressure water supply. This constitutes a pressure closed-loop system based on the set pressure. [b][align=center]For more details, please click: Design of a PLC-Based Variable Frequency Speed ​​Control Constant Pressure Water Supply System[/align][/b]