Abstract : Due to their energy-saving, reliable, and efficient characteristics, frequency converters are widely used in speed control systems. However, system interference is becoming increasingly serious. This paper focuses on the types of interference generated by frequency converters used in the electric equipment of coal preparation plants and the corresponding anti-interference measures to ensure the normal operation of the system. Keywords : Frequency converter; Interference; Prevention 0 Introduction With the rapid development of science and technology, frequency converters are widely used in the automatic control systems of coal preparation plants. Variable frequency speed control systems are a new type of programmable and high-precision control system. It can complete complex speed regulation work, execute programmable control instructions, and automatically identify the operating status and optimal operating parameters of the controlled object. Because the system itself adopts large-scale digital circuits, microprocessors, and programmable control technology, its input setpoints, output controls, and internal system parameter settings can achieve high-precision consistency when using digital signal processing. However, this also brings some interference problems, some of which are so severe that they affect the normal operation of the PLC and instruments in the coal preparation plant's centralized control system. If the interference problem of the frequency converter is not properly solved, the system cannot operate reliably. 1. Basic Types of Frequency Converter Interference (1) Power Supply Interference Frequency conversion speed regulation is the fastest-growing motor speed regulation method in electrical engineering. Frequency conversion control has the advantages of better control linearity and energy saving compared with voltage regulation control. In particular, the intelligent frequency converter with microprocessor developed in recent years has been widely used due to its significantly improved performance. However, its disadvantage of easily interfering with weak electrical signals has become increasingly prominent. Although frequency converter manufacturers have done a lot of work in this regard, they still cannot completely solve the problem. Frequency converters often generate different high-order harmonics when they are working. These high-order harmonics can cause damage to the power grid. Although current frequency converters are equipped with reactors at the front end, which can filter out some high-order harmonics within a certain range, they cannot play a role in some higher-order harmonics. That is, these higher-order harmonics are easy to pollute the working power supply of PLC and instruments. PLC and instruments have relatively weak signals and have high requirements for working power supply. Polluted working power supply will inevitably cause instability of PLC and instrument signals, resulting in the inability of centralized control and density control to work properly. (2) Radio frequency interference In order to prevent interference, reactors are often configured at the back end of the frequency converter. However, due to the characteristics of the frequency converter itself, it controls the speed of the motor by changing the frequency. During the operation of the frequency converter, especially during the speed regulation process, the power cable is very easy to emit radio frequency signals that fluctuate in strength. If the laying path of the PLC and instruments and other weak signal cables is the same as the laying path of the power cable of the frequency converter, it is also very easy to be interfered with, and this interference is very difficult to deal with. 2 Interference of frequency converter (1) Central control system PLC The PLC of the central control system is the central hub of the coal preparation plant. Its normal operation is the key to the normal production of the whole plant. In practice, it was found that the operation of the frequency converter will interfere with the output modules of some PLC systems, causing the output signal of the output module to not act according to the logic program of the PLC, but to act frequently and irregularly. After the frequency converter stops running, the output module returns to normal. Based on the above situation, it is determined that the pollution of the power grid by the frequency converter has infected the working power supply of the PLC system. (2) Instruments Whether the instruments of the coal preparation plant's control system can work normally plays a decisive role in the quality of coal. In the actual production process. The operation of the frequency converter will interfere with the ultrasonic level gauge, causing the ultrasonic level gauge signal to fluctuate or even crash. Checking the parameters of the ultrasonic level gauge shows that the interference coefficient is greater than the required interference coefficient. After power is cut off and then restored, the level gauge can run again. (3) Industrial television system Modern coal preparation plants are mostly equipped with industrial television monitoring systems. The radio frequency interference formed by the power cable of the frequency converter to the video signal of the industrial television system is also very serious, resulting in blurry images and ripples, affecting the monitoring of key positions and key areas by the industrial television system. 3 Anti-interference measures of frequency converter According to the interference type of the frequency converter, different treatment methods are proposed, which can basically solve the harm caused by interference. (1) Prevention and control of power supply interference Usually, an isolation transformer is set on the power line between the power supply and the amplifier circuit to isolate the interference source and the susceptible part, suppress the intrusion of high-order harmonics to the maximum extent, and reduce the pollution of high-order harmonics to the control power supply. (2) Prevention of radio frequency interference: To prevent radio frequency interference from the inverter power cable, on the one hand, armored cables should be used as much as possible to reduce radio frequency intensity; on the other hand, the installation path of the inverter power cable should not be mixed with control cables and signal cables, especially not with video cables of industrial television systems. The inverter power cable should be installed separately, and crosses should be made as much as possible where they cross. If these two aspects are done, radio frequency interference can be avoided. (3) Anti-interference measures for PLC system: If the anti-interference measures of the control system power supply are well done and the PLC system module is not interfered with, anti-interference treatment of the PLC system is not required. Due to the different electrical parameters of different PLC manufacturers when designing the modules, the anti-interference measures of the PLC system output module are slightly different. The main measure is to connect a set of RC absorption circuits in parallel at both ends of the output point to play a filtering role, which can filter out the interference of the inverter to the output power supply. The parameters of the RC absorption circuit may be different for different PLCs, and can be adjusted according to the site conditions. (4) Anti-interference measures for ultrasonic level gauges. In practical application, it has been proven that the DC 24 V working power supply of ultrasonic level gauges in coal preparation plants is easily interfered with by frequency converters, and this interference is not easy to deal with. Using a power supply provided by a different transformer than the same frequency converter is one solution. Using a battery-type working power supply can also avoid interference from the frequency converter, but these two methods are neither economical nor convenient to maintain. Another way to solve the problem of the liquid level measurement in coal preparation plants not being interfered with by the frequency converter is to use a pressure sensor instead of an ultrasonic level gauge, and convert the pressure signal into a liquid level signal by measuring the pressure inside the tank. Practice has proven that this method is easy to install, easy to debug and adjust, has small error, is easy to maintain, and is not easily interfered with by the frequency converter. Practice has proven that by adopting the above anti-interference measures, the interference of frequency converters on the centralized control system and density control system in coal preparation plants can be basically solved, meeting the design and production requirements. 4 Conclusion In conclusion, anti-interference measures for frequency converters are a complex system engineering project. With the gradual enrichment of practical experience and the improvement, development and perfection of the device design, the interference problem of frequency converters will be reduced, providing conditions for the promotion and application of frequency conversion technology. References : [1] Pi Zhuangxing, et al. System Design and Application Examples of Programmable Logic Controllers [M]. Beijing: China Machine Press, 2001. [2] Li Liangren, et al. Variable Frequency Speed ​​Regulation Technology and Application [M]. Beijing: Electronic Industry Press, 2004. [3] Shang Yongde, Jiang Hua, Jia Hongfu. Application of Variable Frequency Speed ​​Regulation in Coal Preparation Plant [J]. Coal Mine Machinery, 2004, 25(8): 113-114.