Abstract: This paper introduces the structure and working principle of a variable frequency constant pressure water supply system based on PLC and frequency converter using serial bus communication. The system automatically adjusts the speed of the water pump motor using variable frequency speed regulation to maintain a constant water supply pressure. During periods of low water consumption, a small flow pump is activated to reduce energy loss. The system can still operate automatically in case of frequency converter failure, ensuring uninterrupted water supply. Furthermore, it can automatically restart after the fault is cleared, achieving unattended fully automatic operation. Keywords: Programmable Logic Controller, Variable Frequency Speed ​​Regulation, Serial Communication, Small Flow Pump. In water supply systems for residential water use, industrial water use, various waterworks, oil fields, oil depots, boiler constant pressure heating, constant pressure water supply sprinkler systems, and fire protection, traditional equipment such as water towers, elevated water tanks, and air pressure boosters are used. These systems not only occupy a large area and require significant equipment investment, but also are difficult to maintain and cannot meet the high-pressure, high-flow-rate rapid water supply needs of high-rise buildings, industry, and fire protection. On the other hand, due to the randomness of water supply, traditional methods cannot guarantee real-time water supply. Furthermore, water pump selection is often based on maximum water supply capacity, while peak water usage is short, leaving the pumps with a large margin of safety for extended periods. This not only results in low pump efficiency and unstable water pressure but also significant energy waste. This paper introduces a variable frequency constant pressure water supply system controlled by a programmable logic controller (PLC). This system solves the problems of tedious manual operation and mental stress while saving energy. 1. System Introduction [align=center] Figure 1[/align] The variable frequency constant pressure water supply control system consists of a PLC controller, a touch screen display, a variable frequency speed controller, a pressure transmitter, a water level transmitter, an AC contactor, and other electrical control equipment, as well as three water pumps (the number of pumps can be set as needed) and one small flow pump, as shown in Figure 1. A pressure transmitter is installed on the main outlet pipe of the water supply system to detect the outlet water pressure, and a level transmitter is installed in the water storage tank. The PLC has an analog input detection module to detect the 4-20mA signals output by the pressure and level transmitters. After PID calculation, the detected pressure signal and the set pressure signal are compared and then the motor speed is adjusted by controlling the output frequency of the frequency converter to maintain a constant water supply pressure. This forms a pressure closed-loop system based on the set pressure. The system automatically detects the water level signal in the tank and compares it with the set low water level limit, outputting a low water level alarm signal or directly stopping the pump. The touch screen display shows information such as power supply voltage, current, frequency converter output frequency, actual water supply pressure, set water supply pressure, and the operating status of each pump. The set water supply pressure and pump operation can be modified online via the touch screen. The system also has multiple protection functions, especially the high-voltage logic hardware interlock function, to ensure normal water supply and allow for unattended operation. 2. Working Principle The system has two operating modes: manual and automatic. 2.1 Manual Operation Mode: In this mode, pressing the start button or stop button allows you to start or stop each pump as needed. This mode is only for maintenance or when the control system malfunctions. 2.2 Automatic Operation Mode: 2.2.1 Startup Procedure: In automatic operation mode, the system first checks the water level in the tank. If the water level meets the set requirement, the AC contactor of pump #1 engages, connecting the motor to the frequency converter. The frequency converter's output frequency starts rising from 0Hz. At this time, the pressure transmitter detects the pressure signal and feeds it back to the PLC. The PLC then controls the frequency output of the frequency converter after PID calculation. If the pressure is insufficient, the frequency rises to 50Hz. After a certain delay, pump #1 switches to the mains frequency, the AC contactor of pump #2 engages, and pump #2 starts using frequency conversion. The frequency gradually increases until the outlet pressure reaches the set pressure. This process is repeated for each pump. 2.2.2 Pump Switching Procedure: If water consumption decreases and the outlet pressure exceeds the set pressure, the PLC controls the frequency converter to reduce the output frequency, thereby reducing the water flow to stabilize the outlet pressure. If the frequency converter output frequency is lower than a certain set value (the water pump outlet frequency, generally 25Hz), but the outlet pressure is still higher than the set pressure value, the PLC starts timing. If the outlet pressure drops to the set pressure within a certain time, the PLC stops timing and continues frequency conversion speed regulation. If the outlet pressure is still higher than the set pressure within a certain time, according to the principle of first-in, first-out, the PLC will stop the mains frequency pump that has been running the longest until the outlet pressure reaches the set value. 2.2.3 Starting a Small Flow Pump: For residential water supply or other water supply systems with highly time-dependent water usage, a small flow pump can be installed. For example, between midnight and 5 a.m., residential water usage is low. A 30kW water pump needs to operate at around 25Hz for an extended period to maintain water pressure. This consumes over ten kilowatts of electricity and operates at a low frequency, significantly impacting the motor's lifespan. If a small-flow water pump of around 5kW is installed in the system, it can operate at a higher frequency while consuming significantly less electricity to maintain water pressure. The small-flow pump's power is typically 1/4 to 1/6 of the main pump's power, with the same head. 2.2.4 Water Level Detection: During automatic water supply, the PLC monitors the water level in the tank in real time. If the water level is below the set alarm level, a buzzer sounds a water shortage alarm; if the water level is below the set shutdown level, all pumps stop to prevent dry running and a shutdown alarm is issued; if the water level is above the set upper limit, the electric valve on the water supply pipe to the tank is automatically shut off. 2.2.5 Automatic Start-up Sometimes power outages may occur suddenly. If no one is on duty, the system may fail to start after power is restored, resulting in a water outage. Therefore, this system is equipped with an automatic variable frequency start-up mode after power is restored. After power is restored, the PLC will issue a command, the buzzer will sound a warning, and then the #1 pump will start automatically using variable frequency until it is running stably at the given water pressure value. 2.2.6 Fire Alarm When a fire alarm signal is detected, the system will immediately operate according to the fire pressure. 2.2.7 Fault Handling Variable Frequency Fault Considering the redundancy design principle, water supply must not be interrupted even if the variable frequency drive (VFD) fails. When the VFD suddenly fails, the buzzer will sound an alarm, the PLC will issue a command to stop all water pumps, and then the #1 pump will run at mains frequency (if the pump power is greater than 37KW, a reduced-voltage start or other start-up method is required). After a certain delay, the #2 pump will then start running at mains frequency based on pressure changes. At this time, the PLC will switch between mains frequency pumps according to the actual water pressure changes. When situations such as water tank dryness causing shutdown, motor undervoltage, overvoltage, phase reversal, or motor failure occur, an alarm will sound via a buzzer. If conditions permit, a MODEM module can be added to remotely contact on-duty personnel via the communication port when the inverter or motor malfunctions, notifying them to come for repairs. An alarm signal will also be issued before automatic restart after all faults are resolved and normal operation is restored. 3. Control System Configuration and Software Programming 3.1 Control System Hardware Configuration 3.1.1 PLC Selection The programmable controller uses a SIEMENS S7-200 series CPU-226 main unit with 40 I/O points (24 input points and 16 output points), two RS-485 communication/programming ports, and supports PPI, MPI, and free-mode communication protocols. Free-mode communication is a distinctive feature of the S7-200 PLC, allowing users to define their own communication protocols. This free-mode communication allows the PLC to be easily connected to the inverter and touchscreen in this system. The analog input uses the EM231 module with 4-channel 12-bit A/D analog input, which has high precision. PLC programming uses STEP7-Micro/WIN programming software, which provides a complete programming environment, can perform offline programming and online connection and debugging, and can realize the mutual conversion between ladder diagrams and statement lists. 3.1.2 Selection of frequency converter and control mode In the traditional frequency converter control system, the start/stop of the frequency converter is controlled by the PLC through the switch output, and the frequency of the frequency converter is controlled by the PLC through the analog output port outputting 0~5 (10)V or 4~20mA signals. This requires the purchase of relatively expensive analog output port modules of PLC. The detection of frequency converter faults is only by the PLC reading the fault alarm contacts of the frequency converter. It only knows that the frequency converter has a fault, but it is not clear what the specific fault is. The operator needs to check the alarm information of the frequency converter and then read the frequency converter manual to find out. This is too difficult for general duty personnel. Therefore, in this system, the PLC controls the frequency converter via serial communication. The selected frequency converters are either SIEMENS MM series or ABB ACS-400 series fan/pump dedicated frequency converters, which have RS-485 communication interfaces and offer high cost-effectiveness. The PLC communicates with the frequency converter via a free communication port to control its operation, reads parameters such as voltage, current, power, frequency, cumulative running time, and all alarm information including overvoltage, overcurrent, and overload, and displays these parameters on the touchscreen. This method offers higher reliability than controlling the frequency converter via an external port, saves valuable PLC I/O ports, and obtains a large amount of information from the frequency converter. 3.1.3 Touchscreen The touchscreen is the PWS series manufactured by HITCH in Taiwan. It has abundant driver programs and can easily communicate with the S7-200 series PLC via serial port. The touchscreen displays the operating status of each pump, the actual outlet water pressure of the pipeline network, the set pressure, the voltage, current, power of the frequency converter, and various fault information. Operators can easily set parameters such as domestic water supply pressure, fire water supply pressure, frequency converter acceleration/deceleration time, and the online/maintenance status of each pump online via the touchscreen, and can control the operation of each pump through the touchscreen. 3.1.4 Control Circuit Design In the control circuit design, attention was paid to the automatic/manual switching of the system, the frequency converter contactor and power frequency contactor of each pump, and the electrical interlocking of the frequency converter contactors of each pump to prevent the simultaneous connection of power frequency and frequency converter power supplies for one pump or multiple pumps to the frequency converter power supply. The main control loop of the system is shown in Figure 2. [align=center] Figure 2[/align] 3.2 PLC Software Programming The PLC in the entire system has both digital and analog inputs/outputs, as well as communication with the frequency converter and touchscreen. Therefore, a modular structure is adopted in the PLC control software programming. Various functional program modules are organically combined through the main program, making the system stable and reliable in operation. In the main program, the PLC is powered on and initialized, detecting the status information of each part of the system. If there is an alarm, a warning is issued first. If there is no alarm, the system starts variable frequency drive (VFD) pump #1 (if pump #1 is disconnected from the system, the pump with the lowest number is started) to monitor the outlet water pressure in real time and perform PID calculations to control the output frequency of the VFD to maintain a constant water supply pressure. If the outlet water pressure is still lower than the set pressure after a delay of a few seconds when the VFD frequency reaches 50Hz, pump #1 is switched to mains frequency, and pump #2 is started to maintain a constant pressure, and so on. If the outlet water pressure exceeds the set pressure, the VFD reduces the output frequency to stabilize the outlet water pressure. If the VFD output frequency is lower than the set water pump outlet frequency but the outlet water pressure is still higher than the set pressure value, after a delay, the mains frequency pump that has been running the longest among the running pumps is stopped according to the principle of first-in, first-out, until the outlet water pressure reaches the set value. If only one pump in the system is operating at a variable frequency and its frequency is lower than the set outlet frequency for a continuous period of time, the main pump operating at the variable frequency will be shut down and a small flow pump will be put into operation, which will protect the main pump motor and save energy. 4. Conclusion In the water supply system, the variable frequency speed regulation mode can automatically adjust the speed of the pump motor or add or remove pumps according to the actual water pressure change, so as to achieve constant pressure water supply and save energy and reduce consumption; the system adds a small flow pump to extend the service life of the main pump motor. The frequency converter can still run automatically after a failure, which basically ensures uninterrupted water supply. At the same time, the frequency converter is controlled by the communication between the PLC and the frequency converter, which has certain advanced features. At present, more than a dozen sets of the system have been put into use, and the operation is good. The operation is intuitive and convenient, and the energy saving effect is obvious, which has been well received by users. References: (1) Chen Liding, Wu Yuxiang, Su Kaicai Electrical Control and Programmable Controller [M] Guangzhou South China University of Technology Press (2) Xie Jianying, Jia Chun Microcomputer Control Technology [M] Beijing National Defense Industry Press