Abstract : This paper introduces the basic functions of a frequency converter (VFD). The VFD's target is not the speed of a motor, but rather the flow rate and pressure of fluid machinery such as fans and water jets. By controlling the frequency of the VFD, these physical quantities are controlled to achieve set values. This paper addresses a practical problem by using a VFD to achieve constant pressure water supply. Keywords : Frequency converter, variable frequency speed regulation, constant pressure water supply. 1. Introduction The original water supply system was provided by a pneumatic water supply device, consisting of two 7.5KW water pumps, a control box, a pressure tank, and a water pool. It had simple pump start/stop and general protection functions, and the pressure could be set between 0.2-0.4Mpa. Because it could not maintain a completely constant pressure, the water pumps had to be started and stopped every 8-10 minutes. The frequent start/stop of the pumps greatly shortened the lifespan of the pumps and control contacts, resulting in a high failure rate, frequent maintenance, and frequent contactor replacements, seriously affecting the domestic water supply. The requirements for the modification of the water supply control system are: A. Constant pressure water supply, with arbitrarily settable pressure. B. Variable frequency speed regulation and soft start. C. Two pumps operate alternately, with fully automatic switching. D. It has anti-cavitation protection. E. It has overcurrent, undervoltage, and phase loss protection functions. 2. Basic Concepts of Frequency Converters A frequency converter is a device that uses the characteristic that the synchronous speed of an AC motor changes with the frequency of the motor's stator voltage to achieve speed regulation of the motor. The earliest form of frequency converter used a rotating variable frequency generator set as a variable frequency power supply to power AC motors, mainly for speed regulation of asynchronous motors. With the development of power semiconductor devices, static variable frequency power supplies have become the main form of frequency converters. [b][align=center]For more details, please click: Apartment Building Water Supply Control System Renovation[/align][/b]