Abstract : With the development of power electronics technology and industrial automatic control technology, AC variable frequency speed control systems have been widely used in the field of industrial motor drives. In addition, due to the powerful functions, ease of use, and high reliability of PLCs, they are often used for field data acquisition and equipment control. This design utilizes a frequency converter and PLC to control the water level in a water tank. Keywords : PLC system control design. Frequency converter technology is a comprehensive technology based on control technology, electronic power technology, microelectronics technology, and computer technology. Compared with traditional AC drive systems, using a frequency converter to control the speed of AC motors has many advantages, such as energy saving, easy speed control of existing motors, efficient continuous speed control over a wide range, and precise speed control. It also facilitates forward and reverse switching of the motor, allows for high-capacity start-stop operation, electric braking, and high-speed drive of the motor. Comprehensive Protection Functions: The inverter boasts robust protection capabilities, capable of detecting various faults during operation and displaying the fault type (such as instantaneous voltage drop in the power grid, phase loss in the power grid, DC overvoltage, power module overheating, motor short circuit, etc.), and immediately blocking the output voltage. This "self-protection" function not only protects the inverter but also protects the motor from damage. PLC Features: First, high reliability and strong anti-interference capability, with a mean time between failures (MTBF) of hundreds of thousands of hours. Furthermore, PLCs employ numerous hardware and software anti-interference measures. Second, simple programming and ease of use. Most PLCs currently use ladder diagram programming with relay control, easily accepted by operators. Some PLCs also feature step ladder instructions designed for specific problems, further simplifying programming. Third, easy design and installation, requiring minimal maintenance. Fourth, suitable for harsh industrial environments, employing an encapsulated design suitable for applications involving vibration, corrosion, and toxic gases. Fifth, convenient connection to external equipment, using detachable terminal blocks with a unified wiring method, providing different terminal functions suitable for various electrical specifications. Sixth, the system boasts comprehensive functions, strong versatility, small size, low energy consumption, and a high performance-price ratio. In this system, to fully utilize energy and meet production needs, motor speed regulation is required. Considering the characteristics of motor starting, running, speed regulation, and braking, LG's IP5A frequency converter is used. The system utilizes an LGMASTER-K series PLC to complete data acquisition and control tasks for the frequency converter, motor, and other equipment. The programming software based on the LGMASTER-K PLC adopts a modular programming method, extensively using code reuse to reduce software development and maintenance. The system utilizes the PLC software design to realize frequency converter parameter setting, fault diagnosis, and motor starting and stopping. [b][align=center]For more details, please click: PLC-based Frequency Converter Liquid Level Control Design[/align][/b]