Introduction With the development of variable frequency speed control technology and the increasing demands for drinking water quality, variable frequency constant pressure water supply systems have gradually replaced the original water tower water supply systems and are widely used in multi-story residential communities' domestic and fire-fighting water supply systems. However, since new systems often continue to use some old equipment from the original system (such as water pumps), some theoretically unexpected problems often arise in the practice of frequency conversion retrofitting of the original water supply system. The variable frequency control constant pressure water supply system introduced in this article was designed based on the principle of preserving as much of the original equipment as possible during the technical retrofitting of a typical water tower water supply system. This system effectively solves the problem of frequent maintenance of old equipment, demonstrating the technical advantages of variable frequency control constant pressure water supply while effectively saving money. [b]1 System Introduction[/b] The principle of the variable frequency constant pressure water supply system is shown in Figure 1. It mainly consists of a PLC, frequency converter, PID controller, TC time controller, pressure sensor, level sensor, power control circuit, and four water pumps. Users can understand and control the operation of the system through indicator lights, buttons, and selector switches on the control cabinet panel. Pressure sensors installed on the outlet water pipe network convert the outlet pressure signal into a standard 4-20mA signal, which is then sent to a PID controller. After calculation and comparison with the given pressure parameter, a regulation parameter is derived and sent to the frequency converter. The frequency converter controls the pump speed, regulating the system's water supply to maintain the pressure in the water supply network at the given pressure. When water consumption exceeds the capacity of a single pump, an additional pump is added via a PLC controller. Based on water consumption, the PLC controls the increase or decrease in the number of operating pumps and the frequency converter adjusts the pump speed to achieve constant pressure water supply. When the water supply load changes, the input motor voltage and frequency also change accordingly, thus forming a closed-loop control system based on the set pressure. The system is also equipped with a timer controller and a PID controller, enabling timed pump switching (i.e., clock control, implemented by the timer controller) and dual working pressure setting (implemented by both the PID controller and the timer controller). Furthermore, the system has multiple protection functions, especially a hardware/software backup pump function, ensuring timely pump maintenance and normal system water supply. 2. Working Principle 2.1 Operating Modes This system has two operating modes: manual and automatic. ⑴. Manual Operation: Press the button to start or stop the water pumps. Pumps #1-#4 can be controlled separately as needed. This mode is mainly used for maintenance and in case of inverter failure. ⑵. Automatic Operation: After closing the automatic switch, the #1 pump motor is powered on, and the inverter output frequency rises from 0Hz. Simultaneously, the PID controller 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 pump #1 switches from inverter to mains frequency, starting pump #2. The inverter gradually increases the frequency to the given value, and pumps are added in sequence. If water consumption decreases, the first pump started is de-energized, while the system operates smoothly according to the adjustment parameters given by the PID controller. In the event of a momentary power outage, the system stops. After the power is restored, the system automatically resumes operation and then starts pump #1 in automatic mode until it stabilizes at the given water pressure value. The automatic frequency conversion function is the most basic function of this system. The system automatically completes the entire process of soft starting, stopping, and cyclic frequency conversion for multiple pumps. 2.2 Fault Handling 2.2.1 Fault Alarm The system will issue an audible alarm signal in case of phase loss, frequency converter failure, low liquid level, overpressure, or differential pressure. In particular, the system will automatically shut down and issue an audible alarm signal in case of phase loss, frequency converter failure, low liquid level, or overpressure, notifying maintenance personnel to come for repair. Furthermore, in case of frequency converter failure, the system will automatically shut down, at which point it can be switched to manual mode to ensure uninterrupted water supply. 2.2.2 Pump Maintenance For pump maintenance and repair, it is required that a specific pump be shut down at regular intervals under normal system water supply conditions. The system has a forced pump standby function (hardware standby), allowing any pump to be used as a standby pump without affecting normal system operation. To enable pump rotation, the system also has a software standby function (clock control function, implemented by a time controller). The working pump and the standby N pump have a periodic timed switching function, with the period interval set by the time controller: continuously adjustable from 1 hour to 96 hours. 3 PLC Control System This system uses an Omron SYSMAC CPM2A series programmable logic controller (PLC) with 60 I/O points. PLC programming uses OMRON CX-Programmer, a 32-bit Windows software support tool for Omron 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 input/output to control motor start/stop, timed switching, soft start, cyclic frequency conversion, and fault alarms. Analog quantities such as motor speed and water pressure are controlled by a PID controller and a frequency converter. The pump switching diagram is shown in Figure 2. Initially, if neither hardware nor software backups are available (hardware takes priority if both are available), pump #1 starts with frequency conversion, its speed increasing from 0 with the frequency. If the frequency converter reaches 50Hz while the water pressure is still at the lower limit, after a delay (to avoid malfunctions due to interference), pump #1 switches to mains frequency operation, and the frequency converter stops at 50Hz and returns to 0Hz. Pump #2 then starts with frequency conversion. If the water pressure is still insufficient, pumps #3 and #4 are started sequentially, following the same pump switching process. If pump #1 is initially on standby, pump #2 is started directly. The speed increases from 0 rpm as the frequency rises. If the frequency converter reaches 50 Hz and the water pressure is still at the lower limit, pump #2 switches to mains frequency operation after a delay. Simultaneously, the frequency converter drops from 50 Hz to 0 Hz, and pump #3 starts. If the water pressure is still insufficient, pump #4 is started, and the pump switching process is the same. If both pumps #1 and #2 are on standby, pump #3 is started directly, and the pump switching process is similar. Similarly, if all three pumps (assuming #1, #2, and #3) are running, and pump #3's frequency converter speed drops to 0 Hz, but the water pressure is still at the upper limit, pump #1 is stopped after a delay, and the frequency converter rapidly increases from 0 Hz. If the water pressure remains at the upper limit after this, pump #2 is stopped after a delay. This switching process effectively reduces frequent pump starts and stops. Simultaneously, the frequency converter rapidly adjusts the water pressure before the actual pipe network reacts to water pressure fluctuations, ensuring a smooth transition and effectively preventing short-term water outages for high-rise residents. Previously, variable frequency constant pressure water supply systems typically stopped the variable frequency pump and then switched the frequency converter to the pump already running at the fixed frequency for adjustment when water pressure was high. While theoretically more advanced than directly switching to fixed frequency, this method easily leads to frequent pump starts and stops, reducing equipment lifespan. In this system, the fixed frequency pump is stopped directly, and the frequency converter rapidly adjusts the pressure. With proper parameter settings, a shock-free switching of the pump set can be achieved, ensuring a smooth water pressure transition and effectively preventing large-scale water pressure fluctuations and short-term water shortages when water pressure is too low, thus improving water supply quality. [b]4 Precautions[/b] To ensure stable system operation, several parameters require special attention: ⑴. Frequency Conversion to Power Frequency Switching Time (TMC): Setting the TMC ensures that during the pump's transition from frequency conversion to power frequency, the corresponding AC contactors for both frequency conversion and power frequency operation of the same pump will not engage simultaneously, potentially damaging the frequency converter. Additionally, to avoid excessive starting current during power frequency startup and its impact on the power grid, the TMC must be as small as possible within permissible limits. ⑵. Upper and Lower Limit Frequency Duration (TH and TL): The frequency of the frequency converter increases with the increase in water consumption in the pipeline network. This system determines whether to add (or remove) a pump based on whether the frequency converter reaches its upper (lower) limit and maintains it for a certain period. This determination time is TH (TL). If the set value is too high, the system cannot respond quickly to changes in water consumption; if the set value is too low, changes in water consumption may lead to frequent pump additions and removals. Both situations will affect the quality of constant pressure water supply. **5 Conclusion** The variable frequency drive (VFD) operation mode in the water supply system allows for automatic adjustment of the pump motor speed or pump addition/reduction based on the actual set water pressure, maintaining the pressure in the water supply network at a given value. This maximizes energy, water, land, and cost savings, ensuring reliable system operation and achieving constant pressure water supply. The "first-start, first-stop" switching method for pump reduction, compared to "first-start, then-stop," ensures more even pump usage, extending equipment lifespan. Furthermore, considering the four pumps have been in use for many years and require regular maintenance, hardware/software backup functions have been added, effectively extending equipment lifespan. Pressure closed-loop control ensures the service pressure of the water supply network remains constant regardless of changes in water consumption, significantly improving water quality. Even after a VFD failure, uninterrupted water supply is guaranteed, and automatic restart after fault clearance is achieved, demonstrating advanced technology. The system is currently in use with significant results. **References** 1. OMRON CPM2A Programmable Controller Operation Manual. 2 OMRON Programmable Logic Controller CPM1/CPM1A/CPM2A/CPM2C/SRM1 (-V2) Programming Manual.