Abstract : This paper introduces a method for technically upgrading a factory's water supply system using a variable frequency drive (VFD). It focuses on the application of the VFD's built-in PID control function and the setting and debugging methods for related parameters. Keywords : VFD; PID; closed-loop control; technical upgrade 0 Introduction The factory's water supply system (as shown in Figure 1) uses a deep well pump to draw water from underground and pump it into a 600 m³ reservoir. A booster pump then supplies the water to the refrigeration workshop for cooling. These two pumps are driven by two three-phase asynchronous motors (100 kW each), neither of which has speed regulation. Because the original design allowed for a water extraction capacity greater than the water consumption, excess water overflowed from the top overflow hole of the reservoir and was discharged outside the factory, wasting both water and electricity. This system uses two Kangwo CVF-P1-4T1100 high-performance, high-power VFDs to control the water extraction pump motor and the booster pump motor respectively. A closed-loop variable frequency speed control system is adopted to control the water level in the pool within a certain limit loop, and to maintain a constant water pressure using the water pipe network. [b]1 Variable Frequency Retrofit Scheme 1.1 Control System of the Water Pump Motor[/b] Here, the signal processor amplifies the output signal of the level sensor and converts it into a reliable switching signal (such as the on/off signal of a relay contact); simultaneously, it converts the upper limit water level arrival signal (equivalent to a normally open signal) into a normally closed signal, or the lower limit water level shortage signal (equivalent to a normally closed signal) into a normally open signal. [b][align=center]For details, please click: Technical Retrofit of Water Supply System Using Variable Frequency Speed ​​Controller[/align][/b]