Abstract : This paper introduces the application of the frequency converter LOGO! in a constant pressure water supply control system. The system uses the LOGO! frequency converter and soft starter as its core, and a PID controller collects the water pressure signal to adjust the pump speed so that the actual speed follows the rated speed. The control system structure, interface composition, and software design are presented. Actual operation results show that the scheme is effective. Keywords : frequency converter; LOGO!; soft starter 0 Introduction In traditional water pump control systems, the pump motor operates at a constant speed. To achieve constant water pressure, the pump must be frequently started and stopped. Frequent starting and stopping not only reduces the motor's lifespan and increases the equipment failure rate, but also results in inconsistent water pressure. Simply using a frequency converter often cannot meet the needs of production and daily life. The control system combining a frequency converter and a soft starter effectively solved the above problems. Actual operation showed good results. [b]1 Control System Design 1.1 Control System Principle[/b] The control system uses one frequency converter to drive two water pumps alternately. The pumps automatically switch within one week using LOGO!. When the pumps are working, the pressure transmitter installed in the water-using equipment converts the water pressure signal into an electrical signal and transmits it to the PID controller built into the frequency converter. The PID controller adjusts the pump speed (frequency) to make the actual speed follow the rated speed. If the actual speed is less than the rated speed, the frequency converter output frequency increases, the pump speed increases, and the water pressure increases. If a running pump fails, the integrated protection device sends a signal to LOGO!, and all contactors are de-energized. LOGO! immediately and automatically controls the frequency converter to start the other pump. This pump continues to run until the failed pump is repaired, and then they alternate operation. If the frequency converter fails, the soft starter immediately starts the running pump at the power frequency. 2 The water pumps also switch at fixed times, and the control system schematic is shown in Figure 1. [b][align=center]For details, please click: Application of Frequency Converter—LOGO in Constant Pressure Water Supply Control System[/align][/b]