Abstract: The widespread application of frequency converters (VDCs) in industrial control has greatly facilitated process regulation. However, due to harmonic issues during VDC operation, strong electromagnetic interference can occur to electrical instruments and control equipment, leading to control system malfunction. This paper analyzes the causes of PCL control system malfunction caused by VDC operation in a reclaimed water process and briefly describes corresponding solutions. Keywords: Frequency converter; Interference; Harmonics; Control system General-purpose frequency converters, with their advantages of intelligence, digitalization, and networking, are widely used in field processes where technical parameters need to be controlled by speed regulation. Especially for fan and pump loads, using general-purpose VDCs for speed control can conveniently achieve flow control while achieving significant energy savings. However, due to problems such as harmonics, load mismatch, and heat generation, improper measures in actual use can cause serious interference to the industrial control system, even paralyzing it. 1. Interference of Frequency Converter on Industrial Control System The wastewater reuse process at Taihua Group's water plant uses a PCL centralized control system. The process includes a wastewater lifting unit. Due to the conservative selection of the lift pump, its head and flow rate are far greater than the actual technical specifications. The lift pump is started using a soft starter, so controlling the head and flow rate relies solely on the outlet valve, which is inconvenient for process control and consumes a lot of energy. To solve this problem, a frequency converter was installed to control the speed of the lift pump, thereby regulating the effluent flow rate and reducing energy consumption. During individual and load testing, the lift pump worked normally and could fully meet process control requirements. However, during integrated testing with the PLC control system, it was found that as soon as the frequency converter started, all field instruments on the central control room monitoring screen would malfunction, and the process could not be controlled. Only when the frequency converter was running at its mains frequency could everything return to normal. 2. Interference Analysis and Corresponding Measures The superficial reason for the PLC industrial control system failure is the use of a frequency converter, which caused interference to a part of the industrial control system—the field instruments. In production sites, equipment is typically situated in harsh environments created by high-voltage circuits and equipment. Although most instruments incorporate technical measures in their circuit design and material selection to enhance their anti-interference capabilities, these performances are still significantly influenced by the use of high-voltage equipment, the instrument's installation location, the sophistication of the power supply network, and the signal channel configuration. [b][align=center]For more details, please click: Interference from Frequency Converters to Industrial Control Systems[/align][/b]