Overview The Zhuzhou campus of Central South Forestry University has long relied on a fixed-pressure water supply system using quantitative pumps. This system wastes water and electricity resources and fails to meet the ever-increasing demands of faculty and students for water quality for daily life and teaching. Currently, variable-frequency constant-pressure water supply systems are widely used in production and daily life. This paper applies variable-frequency and automatic control technology to upgrade existing equipment (such as water pumps, reservoirs, and pipe networks) and transforms the original water supply system using frequency conversion. The system automatically adjusts the pump motor speed and accelerates/decelerates the pumps using variable-frequency speed regulation. This changes the previous fixed-pressure pump water supply method, automatically completing soft starts and shock-free switching of the pump groups, ensuring a smooth transition of water pressure. The system can still operate in case of failure, ensuring uninterrupted water supply. It automatically adjusts the water supply volume during short-term water outages and can automatically restart after power failure. Hardware/software backup and clock control functions are used to allow each pump to take turns resting, extending the mechanical life of the equipment. This article introduces a variable frequency drive (VFD) controlled constant pressure water supply system. Designed based on the principle of preserving existing equipment as much as possible, this system effectively solves the problem of frequent maintenance required for older equipment, demonstrating the technical advantages of VFD controlled constant pressure water supply while effectively saving costs. II. System Introduction The original structure of the VFD constant pressure water supply system is shown in Figure 1. It mainly consists of a PLC, a frequency converter, pressure sensors, level sensors, power distribution equipment, and water pumps. Users can understand and control the system's operation through indicator lights, buttons, and selector switches on the control cabinet panel. The system has manual and automatic functions, as well as a water stop mode . ⑴. Manual Operation Pressing the button starts or stops the water pumps, allowing control of pumps 1-4 as needed. This mode is mainly used for maintenance and in case of frequency converter failure. (2) Automatic Operation After the automatic switch is closed, the #1 pump motor is powered on, and the inverter output frequency rises from 0Hz. Simultaneously, the PID control program receives the standard signal from the pressure sensor, calculates and compares it with the given pressure parameter, and sends the adjustment parameter to the inverter. If the pressure is insufficient, the frequency rises to 50Hz, and #1 pump switches from variable frequency to mains frequency. #2 pump is then switched to variable frequency, and the inverter gradually increases the frequency to the given value. Pumps are added in sequence. If water consumption decreases, the system is decelerated starting with the first pump started, while the system operates smoothly according to the adjustment parameters provided by the PID controller. In the event of a momentary power outage, the system shuts down. Once the power is restored, the system automatically resumes operation, then starts #1 pump in automatic mode until it stabilizes at the given water pressure. The automatic frequency conversion function is the most basic function of this system, automatically completing the entire process of soft start, stop, and cyclic frequency conversion for multiple pumps. The water outage mode addresses water supply issues during short-term interruptions by utilizing existing water tanks for time-segmented water supply, employing low-pressure, low-flow methods to ensure basic water needs for faculty and students. Additionally, level sensors continuously monitor tank levels to control the inlet valves and maintain a constant water volume. Manual control is available during system maintenance and tank cleaning. The control principle is as follows: A pressure sensor installed on the outlet pipeline converts the outlet pressure signal into a 4-20mA standard signal, which is then fed into the PLC for PID regulation. The signal is compared with a given pressure parameter to derive an adjustment parameter, which is sent to the frequency converter. The frequency converter controls the pump speed, regulating the system's water supply to maintain pressure within the network at the given level. When water consumption exceeds the capacity of a single pump, the PLC controls a switch to adjust the number of pumps. Based on water consumption, the PLC controls the number of operating pumps and the frequency converter adjusts their speed to achieve constant pressure water supply. When the water supply load changes, the voltage and frequency of the input motor also change accordingly, thus forming a closed-loop control system based on the set pressure. In addition, the system has multiple protection functions, especially a hardware/software backup water pump function, which fully ensures timely maintenance of the water pump and normal water supply of the system. IV. PLC Control System This system uses a Siemens S7-200 series programmable logic controller (PLC) with 24 I/O points. PLC programming uses STEP7-Micro/WIN, a Windows software support tool for SIMATIC PLCs, providing a complete programming environment for offline programming, online connection and debugging, and conversion between ladder diagrams and statement lists. To improve the overall system's cost-effectiveness, the system uses digital inputs/outputs to control motor start/stop, timed switching, soft start, cyclic frequency conversion, and fault alarms, while analog quantities such as motor speed and water pressure are controlled by the PLC's PID control and frequency converter. Its hardware configuration is as follows: (1) PS307 power supply module, input 120/230VAC output 24VDC power supply (2) CPU226, 32KB working memory, integrated 64KB RAM MPI interface can communicate with OP7/DP operation panel (3) SM221 analog input module 8-channel isolated input (4) SM222 analog output module (5) CP communication processing module can realize PPI communication (6) RS485 interface for PLC to connect to computer Five sensor selection The FT-1 pressure sensor and FT-1E liquid level sensor of Hunan Feiteng Electromechanical New Technology Industry Co., Ltd. are used. Both output 4-20mA current signal and output to PLC through optocoupler to obtain pressure and flow information. VI. Selection of Frequency Converter The frequency converter adopted is the TVF7-750-P-T3 series water pump-specific frequency converter from the Times Group. This product has functions such as vector control, overcurrent, overvoltage, undervoltage, frequency converter overheating, frequency converter thermal protection, motor thermal protection, motor overheating, stall, instantaneous IGBT shutdown (BX) protection, fuse blown, external fault, pulse encoder fault, electronic thermal protection, motor stall (V/F), motor overload (V/F), frequency converter overload protection, etc. VII. Precautions (1) Safety issues: In terms of software, multiple protection links are set up to monitor the system status and provide safety alarms. In terms of hardware, a safety chain mechanical protection is set up, consisting of multiple closed contacts, including emergency stop, pressure upper and lower limit switches, water level upper and lower limit switches, motor overheat protection relay, no-load protection, etc. All contacts of the safety chain are normally closed contacts. If any contact is opened, the safety chain will fail and the system will be in a stopped state. The system can only function normally after the fault is eliminated. (2) For drinking water hygiene, the water storage tank must be cleaned and disinfected regularly. Conclusion In the water supply system, the variable frequency speed regulation mode is adopted. The system can automatically adjust the speed of the water pump motor and add or remove pumps according to the actual set water pressure, so that the pressure in the water supply system network is kept at a given value, so as to maximize energy saving, water saving, land saving and cost saving, and keep the system in a reliable operating state to achieve constant pressure water supply; when reducing pumps, the "first start and first stop" switching mode is adopted, which can better ensure the average use of each pump and extend the service life of the equipment compared with the "first start and then stop" mode; at the same time, in view of the actual situation that the three pumps used have been used for many years and need to be regularly inspected, hardware/software backup functions have been added, which effectively extends the service life of the equipment; pressure closed-loop control can keep the service pressure of the water supply network at a given level regardless of any change in the system water consumption, which greatly improves the water supply quality; after the frequency converter fails, it can still ensure uninterrupted water supply, and at the same time realize self-start after the fault is eliminated, which has a certain degree of advancement. If our school adopts this system to replace the original water supply method, it will achieve significant results in terms of saving electricity, water, and money.