Abstract: Taking a small secondary pressurized water plant as an example, this paper introduces the principle, structure, characteristics, and practical application of an automatic constant pressure water supply monitoring and control system implemented by a PLC and a frequency converter. Field operation shows that the system has high reliability, comprehensive protection functions, high automation, and good energy-saving effect, demonstrating significant economic and social benefits. Keywords : Constant pressure water supply; PLC; Frequency converter; Monitoring and control system Currently, most urban water plants are transit stations that process and pressurize water from distant water sources before supplying it to local users. They must ensure that the pressure of the local water supply network remains within a certain range. These water plants often have outdated equipment, backward technology, and significant energy waste. Furthermore, the "oversized engine pulling a small cart" phenomenon caused by random fluctuations in water consumption and large engineering margins results in very low overall efficiency of the pumps and motors. Water plants typically have 5-7 water pump motors (capacity between 75-160kW), which are generally started and stopped manually. Before starting the pump motor, a vacuum must be drawn, and the start-up and shutdown process must be coordinated with the outlet valves to prevent water hammer damage to the pump motor. Therefore, the operation is complex and the level of automation is extremely low, urgently requiring technical upgrades to improve the management level of the water plant's water supply system. 1. System Overview Firstly, addressing the "oversized motor for undersized load" phenomenon in small and medium-sized water plants, variable frequency drive (VFD) technology is adopted to replace valves in regulating pipeline pressure and flow. VFD speed control of the water pumps does not require replacing the original reliable and durable three-phase asynchronous motors, thus achieving the same speed control performance as DC motors with minimal investment. Furthermore, according to the proportional law of water pumps, changing the speed will correspondingly change the pump's flow rate, pressure, and power. The relationships are: flow rate is directly proportional to speed, pressure is directly proportional to the square of the speed, and shaft power is directly proportional to the cube of the speed. Therefore, VFD speed control of the water pump motors has a very significant energy-saving effect. If a 110kW motor is subjected to frequency conversion speed regulation, with an operating time of 4000 hours/year and an electricity price of 0.5 (kW·h), then the electricity cost savings would be 55,000 yuan/year. [b][align=center]For details, please click: Design and Implementation of Constant Pressure Water Supply Automatic Measurement and Control System[/align][/b]