1. Introduction With the increasing maturity of frequency converter technology, variable frequency speed control technology has been widely applied in various fields. Variable frequency speed control constant pressure water supply equipment, with its advantages of energy saving, safety, and high-quality water supply, plays a significant role in community and factory water supply control. Based on the needs of a washing machine inlet solenoid valve manufacturer, and in order to provide a reference constant pressure water source for the factory's solenoid valve performance testing production line, this paper designs a fully automatic constant pressure water supply system using PLC control technology and variable frequency speed control technology. This system can well meet production needs and has high water pressure accuracy. To avoid wasting water resources, the system also has an automatic water circulation function. 2 System Composition and Implementation Principle The basic control strategy for constant pressure water supply is as follows: a programmable logic controller (PLC) and a variable frequency speed control device are used to form a control system for optimized control of the pump group's speed operation and automatic adjustment of the number of pumps in operation, completing closed-loop control of the water supply pressure. That is, the speed of the pump motor and the number of pumps are automatically adjusted according to the actual set water pressure, automatically compensating for changes in water consumption to ensure that the pressure of the water supply network remains at the set value. This not only meets the production water supply requirements but also saves energy, ensuring the system operates reliably and achieving constant pressure water supply. [align=center] Figure 1: Block Diagram of Constant Pressure Water Supply System Control Principle[/align] The variable frequency speed control constant pressure water supply system consists of a frequency converter, pump group motor, water supply network, water storage tank, intelligent PID controller, pressure transmitter, PLC control unit, etc. The control system principle diagram is shown in Figure 1. The function of the frequency converter is to provide a variable frequency power supply to the motor, realizing stepless speed regulation of the motor, thereby allowing continuous changes in the water pressure of the network. Simultaneously, the frequency converter can also act as a soft starter for the motor, limiting the starting current of the motor. The pressure transmitter detects the water pressure in the pipe network. The intelligent PID controller implements PID regulation of the pipe network water pressure. The PLC control unit is the execution device for pump group management and also the drive control for the frequency converter, automatically adjusting the number of other fixed-frequency pumps in operation based on actual water consumption changes. The application of frequency converters and PLCs facilitates smooth and continuous adjustment of pump speed. The pump motor achieves variable-frequency soft start, eliminating the impact on the power grid, electrical equipment, and mechanical equipment, extending the service life of electromechanical equipment. 3. Control System Hardware Design This system uses three sets of motor-pumps to provide constant pressure water supply to the water network. Each motor can operate in either variable-frequency or fixed-frequency mode, but only one motor operates in variable-frequency speed control mode at a time. During operation, the selection can be made according to the actual situation. The frequency converter continuously adjusts the pump speed based on changes in actual water pressure, achieving constant water pressure by regulating the flow rate. In addition, the programmable controller rationally switches between pumps based on the current water supply situation, adding or removing pumps in a timely manner to achieve optimal matching. 3.1 Main Control Circuit Design The main circuit of the electrical control system consists of three motors, m1, m2, and m3. Contactors km1, km2, and km3 control the variable frequency or mains frequency operation of motors m1, m2, and m3, respectively. fr1, fr2, and fr3 are thermal relays for overload protection of the three water pump motors. qs1, qs2, qs3, and qs4 are the isolating switches for the frequency converter and the main circuit of the three water pump motors, respectively. fu1 is the fuse for the main circuit. VVVF is a general-purpose frequency converter. 3.2 Intelligent PID Regulator and Frequency Converter Wiring Diagram The frequency converter selected is the SHF series from Sanken Lida Electric Co., Ltd., with power ratings of 7.5kW, 1.5kW, and 15kW. The frequency converter adopts analog control. The frequency converter enables soft start functionality for the motors and water pumps. Figure 2 shows the wiring diagram of the PID regulator and frequency converter. The frequency converter adjusts the output frequency in a timely manner according to the output signal of the PID controller, changing the speed of the motor and water pump, regulating the water supply of the system, and stabilizing the pressure in the water supply network at the set pressure value. [align=center] Figure 2 Wiring diagram of PID controller and frequency converter[/align] 3.3 PLC Input/Output Address Allocation Based on the analysis of the control system, this system uses Delta Electronics' DVP60ES00T2 PLC for control, with a total of 60 input/output points, including 36 input points and 24 transistor output points. It is AC powered, and its performance indicators such as ambient temperature, shock resistance, and noise resistance all meet the requirements. The attached table is the PLC input/output address allocation table. [align=center]Attached Table: PLC Input/Output Address Allocation Table[/align] 4 System Software Design The system software design includes PLC program design and frequency converter function parameter setting. This section mainly discusses the PLC program design. The PLC programming includes both manual and automatic control programming. The manual part controls the motor's operation and stop at the mains frequency via buttons, primarily for system debugging or maintenance. When the selector switch is set to "automatic," the system enters automatic operation mode, where the PLC and frequency converter jointly control the activation or deactivation of each motor, and its operation mode (mains frequency or variable frequency). The water supply system has three pump sets. After determining the number of pump sets to be activated based on the water pressure, only the first motor activated uses variable frequency speed control, while the other motors activated later run at full speed at the mains frequency. The switching process between pump sets is implemented by the logic control unit (PLC). Assuming the pump activation sequence is P1, P3, and P2, when the water supply equipment starts working, the variable frequency pump P1 starts first. When the network water pressure reaches the set value, the frequency converter's output frequency stabilizes at a certain value. When water consumption increases, water pressure decreases. The pressure transmitter converts the changed actual main pipe pressure signal at the water outlet into a standard signal of 4-20 mA and sends it to the intelligent PID controller. The PID controller calculates and sends the adjustment parameters to the frequency converter, causing the frequency converter's output frequency to increase, the water pump speed to increase, and the water pressure to rise. If water consumption increases significantly, causing the frequency converter's output frequency to reach its maximum value (50 Hz), and the water supply pressure still cannot reach the set value, after a 20-second delay, the PID controller sends a control signal to start one of the fixed-frequency pumps (P3 pump) via the PLC control unit. If the water supply pressure still cannot reach the set value, after another 20-second delay, the fixed-frequency pump continues to start. Conversely, when water consumption decreases and the water supply pressure exceeds the set value, the frequency converter's output frequency decreases, the water pump speed decreases, and when the frequency converter's output frequency reaches its minimum value (30 Hz), after a 20-second delay, a command is issued to reduce one fixed-frequency pump, and so on for the other pumps. 5 Conclusion The self-designed variable frequency speed regulation constant pressure water supply system is simple to implement, low in cost, and reliable in operation since its commissioning, with good control effect. This is mainly reflected in: (1) Stable water supply pressure within 0.01 MPa; (2) High efficiency and energy saving; the system is managed by PLC and frequency converter, effectively solving the energy-saving problem when the motor is lightly loaded or unloaded under different water consumption conditions; (3) High degree of automation; no personnel are required to monitor the system; automatic protection and alarm signals are issued in case of fault; (4) Automatic water circulation, achieving effective water saving. Furthermore, the system uses PLC control, making it easy to modify the program at any time to change the working conditions and meet different control requirements, exhibiting great flexibility and versatility, and possessing certain promotional and application value.