Abstract: This paper mainly introduces the formation, sources, and pathways of interference in frequency converters, as well as countermeasures to prevent interference and several effective anti-interference measures in practical applications. Keywords: Frequency converter, electromagnetic interference, anti-interference In various industrial control systems, with the widespread use of power electronic devices such as frequency converters, electromagnetic interference (EMI) is becoming increasingly serious, and corresponding anti-interference design technology (i.e., electromagnetic compatibility, EMC) has become increasingly important. Interference in frequency converter systems can sometimes directly cause hardware damage to the system. Sometimes, although it cannot damage the hardware, it often causes the microprocessor's system program to run out of control, leading to control failure and thus causing equipment and production accidents. Therefore, how to improve the anti-interference capability and reliability of the system is an important aspect that cannot be ignored in the research and application of automation devices, and it is also one of the keys to the application and promotion of computer control technology. When discussing the anti-interference problem of frequency converters, we must first understand the sources and propagation methods of interference, and then take different measures against these interferences. I. Sources of Frequency Converter Interference The first source is interference from the external power grid. Harmonic interference in the power grid mainly interferes with the frequency converter through the power supply of the frequency converter. There are many harmonic sources in the power grid, such as various rectifiers, AC/DC converters, electronic voltage regulators, nonlinear loads, and lighting equipment. These loads cause waveform distortion of voltage and current in the power grid, which in turn causes harmful interference to other equipment in the power grid. If the power supply of the frequency converter is interfered with by the polluted AC power grid and is not treated, the power grid noise will interfere with the frequency converter through the power grid power supply circuit. The interference of the power supply to the frequency converter mainly includes (1) overvoltage, undervoltage, instantaneous power failure (2) surge, drop (3) voltage spike (4) radio frequency interference. 1. Interference of thyristor converter equipment to frequency converter When there is a large capacity thyristor converter in the power supply network, since the thyristor is always conducting for part of the half-cycle of each phase, it is easy to cause the network voltage to have a notch and the waveform to be severely distorted. This may cause the rectifier circuit on the input side of the frequency converter to be damaged by a large reverse recovery voltage, which may lead to the input circuit breakdown and burnout. 2. Interference from Power Compensation Capacitors to Frequency Converters: Power companies have certain requirements for the power factor of electricity users. Therefore, many users adopt centralized capacitor compensation methods in substations to improve the power factor. During the transient process of switching the compensation capacitors on or off, the network voltage may experience very high peak values, which may cause the rectifier diodes of the frequency converter to break down due to excessive reverse voltage. Secondly, there is the interference from the frequency converter itself to external systems. The rectifier bridge of the frequency converter is a non-linear load for the power grid, and the harmonics it generates cause harmonic interference to other electronic and electrical equipment on the same grid. Furthermore, most frequency converter inverters use PWM technology, which generates a large amount of coupling noise when operating in switching mode and making high-speed switching. Therefore, the frequency converter is an electromagnetic interference source for other electronic and electrical equipment in the system. The input and output currents of the frequency converter contain many high-order harmonic components. In addition to the lower-order harmonics that constitute power supply reactive power loss, there are also many high-frequency harmonic components. These will propagate their energy in various ways, forming interference signals to the frequency converter itself and other equipment. (1) Waveform of input current The input side of the inverter is a diode rectifier and capacitor filter circuit. Obviously, there is a charging current in the rectifier bridge only when the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor. Therefore, the charging current always appears near the amplitude of the power supply voltage and is in the form of a discontinuous shock wave. It has a strong high-order harmonic component. According to relevant data, the 5th and 7th harmonic components in the input current are the largest, accounting for 80% and 70% of the 50Hz fundamental wave, respectively. (2) Waveform of output voltage and current Most inverters use SPWM modulation for their inverter bridge, and their output voltage is a series of rectangular waves with a duty cycle distributed according to a sine law. Due to the inductive nature of the stator winding of the motor, the stator current is very close to a sine wave. However, the harmonic components that are equal to the carrier frequency are still relatively large. II. Propagation Mode of Interference Signals Inverters can generate high-power harmonics. Due to their high power, they are highly interfering with other equipment in the system. Their interference paths are consistent with those of general electromagnetic interference, mainly consisting of conduction (i.e., circuit coupling), electromagnetic radiation, and inductive coupling. Specifically: First, they generate electromagnetic radiation to surrounding electronic and electrical equipment; second, they generate electromagnetic noise to the directly driven motor, increasing the iron and copper losses of the motor; and conduct interference to the power supply, which is then transmitted to other equipment in the system through the power distribution network; finally, the inverter generates inductive coupling with other adjacent lines, inducing interference voltage or current. Similarly, interference signals within the system interfere with the normal operation of the inverter through the same path. (1) Circuit coupling mode, i.e., propagation through the power supply network. Since the input current is non-sinusoidal, when the inverter capacity is large, it will cause distortion of the network voltage, affecting the operation of other equipment. At the same time, the conducted interference generated at the output end will significantly increase the copper and iron losses of the directly driven motor, affecting the motor's operating characteristics. Obviously, this is the main propagation mode of the inverter input current interference signal. (2) Inductive coupling: When the input or output circuit of the frequency converter is very close to the circuit of other equipment, the high-order harmonic signals of the frequency converter will be coupled to other equipment through induction. There are two types of induction: a) Electromagnetic induction, which is the main way of current interference signals; b) Electrostatic induction, which is the main way of voltage interference signals. (3) Air radiation: that is, electromagnetic waves are radiated into the air, which is the main way of propagation of high-frequency harmonic components. III. Anti-interference measures for variable frequency speed control system According to the basic principle of electromagnetic properties, electromagnetic interference (EMI) requires three elements: electromagnetic interference source, electromagnetic interference path, and system sensitive to electromagnetic interference. To prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-interference is the most basic and important anti-interference measure of the application system. It generally starts from both resistance and prevention to suppress interference. Its general principle is to suppress and eliminate interference sources, cut off the coupling channel of interference to the system, and reduce the sensitivity of the system to interference signals. Specific measures in engineering can be isolation, filtering, shielding, grounding and other methods. 1. The so-called interference isolation refers to isolating the interference source and the susceptible part from the circuit so that they do not have electrical contact. In the variable frequency speed control transmission system, isolation transformers are usually used on the power lines between the power supply and the amplifier circuit to prevent conducted interference. The power isolation transformer can be a noise isolation transformer. 2. The purpose of setting up filters in the system circuit is to suppress interference signals from the frequency converter to the power supply and the motor through the power lines. To reduce electromagnetic noise and loss, output filters can be set on the output side of the frequency converter; to reduce interference to the power supply, input filters can be set on the input side of the frequency converter. If there are sensitive electronic devices in the circuit, power noise filters can be set on the power lines to prevent conducted interference. In the input and output circuits of the frequency converter, in addition to the lower harmonic components mentioned above, there are many high-frequency harmonic currents, which will spread their energy in various ways and form interference signals to other devices. Filters are the main means of weakening high-frequency harmonic components. According to the different locations of use, they can be divided into: (1) Input filters usually have two types: a. Line filters are mainly composed of inductor coils. It weakens high-frequency harmonic currents by increasing the impedance of the line at high frequencies. b. The radiation filter is mainly composed of high-frequency capacitors. It will absorb high-frequency harmonic components with radiant energy. (2) The output filter is also composed of inductor coils. It can effectively weaken high-order harmonic components in the output current. It not only plays the role of anti-interference, but also weakens the additional torque caused by high-order harmonic currents in the motor. For anti-interference measures at the output of the inverter, the following aspects must be noted: a. Capacitors are not allowed to be connected to the output of the inverter, so as to avoid generating a large peak charging (or discharging) current at the moment the inverter tube is turned on (turned off), which will damage the inverter tube; b. When the output filter is composed of LC circuit, the side of the filter connected to the capacitor must be connected to the motor side. 3. Shielding the interference source is the most effective way to suppress interference. Inverters are typically shielded with iron casings to prevent electromagnetic interference leakage. Output lines should ideally be shielded with steel conduits, especially when the inverter is controlled by external signals. Signal lines should be as short as possible (generally within 20m) and double-core shielded, completely separated from the main circuit lines (AC380V) and control lines (AC220V). They must never be placed in the same conduit or cable tray. Surrounding electronically sensitive equipment lines also require shielding. For effective shielding, the shielding cover must be reliably grounded. 4. Proper grounding effectively suppresses external interference and reduces interference from the equipment itself. In practical applications, the lack of separation between the system power neutral (neutral) and ground (protective ground, system ground), and the chaotic connection of the control system shielding ground (control signal shielding ground and main circuit conductor shielding ground), significantly reduces system stability and reliability. For inverters, proper grounding of the main circuit terminals PE (E, G) is crucial for improving noise suppression and reducing interference; therefore, it must be given high priority in practical applications. The cross-sectional area of ​​the inverter grounding conductor should generally be no less than 2.5 mm2, and the length should be controlled within 20 m. It is recommended that the grounding of the inverter be separated from the grounding point of other power equipment and not share a common ground. 5. The proportion of low-frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the input current of the inverter is very high. In addition to potentially interfering with the normal operation of other equipment, they also consume a large amount of reactive power, which greatly reduces the power factor of the line. Connecting a reactor in series in the input circuit is an effective way to suppress low-harmonic currents. Depending on the wiring position, there are two main types: (1) The reactor is connected in series between the power supply and the input side of the inverter. Its main functions are: a. To increase the power factor to (0.75-0.85) by suppressing harmonic currents; b. To reduce the impact of surge current in the input circuit on the inverter; c. To reduce the impact of power supply voltage imbalance. (2) The DC reactor is connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, which is to weaken the high-order harmonic components in the input current. However, it is more effective than the AC reactor in improving the power factor, which can reach 0.95, and has the advantages of simple structure and small size. 6. Reasonable wiring For interference signals that are propagated by induction, reasonable wiring can be used to weaken them. Specific methods include: (1) The power lines and signal lines of the equipment should be kept as far away as possible from the input and output lines of the frequency converter; (2) The power lines and signal lines of other equipment should avoid being parallel to the input and output lines of the frequency converter; IV. Conclusion Through the analysis of the sources and propagation paths of interference in the application of frequency converters, practical countermeasures to solve these problems are proposed. With the continuous application of new technologies and theories in frequency converters, paying attention to the EMC requirements of frequency converters has become a problem that must be faced in the design and application of variable frequency speed control transmission systems, and it is also one of the keys to the application and promotion of frequency converters. These problems existing in frequency converters are expected to be solved through the functions and compensation of the frequency converter itself. The requirements for frequency converters in industrial sites and the social environment are constantly increasing, and truly "green" frequency converters that meet actual needs will soon be available. We believe that the EMC problem of frequency converters will be effectively solved. References: (1) Frequency Converter Application Manual, edited by Wu Zhongzhi and Wu Jialin, Machinery Industry Press (2) Practical Application of Frequency Converter Speed ​​Regulation, written by Zhang Yanbin, Machinery Industry Press (3) Electromagnetic Compatibility Principles and Design, written by Wang Dinghua, University of Electronic Science and Technology of China Press