In September 2003, Shanghai Quyang Business Center-Fire-fighting booster pump frequency conversion transformation, using LG inverter, PLC and pressure sensor to form a pressure closed loop system to achieve unattended operation.
　The principle of variable frequency constant pressure water supply system is mainly composed of PLC, frequency converter, PID regulator, TC time controller, pressure sensor, liquid level sensor, power control circuit and multiple pumps. The user understands and controls the operation of the system through the indicator lights, buttons and changeover switches on the control cabinet panel.
Through the pressure sensor installed on the outlet pipe network, the outlet pressure signal is converted into a standard signal of 4-20mA and sent to the PID regulator. After calculation and comparison with the given pressure parameter, an adjustment parameter is obtained and sent to the inverter. The frequency converter controls the speed of the water pump, adjusts the water supply volume of the system, and keeps the pressure in the water supply system pipe network at a given pressure; when the water volume exceeds the water supply volume of one pump, the pump is added through the PLC controller. According to the water consumption, the PLC controls the increase or decrease of the number of working pumps and the frequency converter to adjust the speed of the water pumps to realize constant pressure water supply. When the water supply load changes, the voltage and frequency of the input motor also change, thus forming a closed-loop control system based on the set pressure.