1. Introduction Inverters are power electronic devices containing electronic components and computer chips, making them susceptible to external electrical interference. Furthermore, inverters also contain switching components, oscillation circuits, digital circuits, contacts, and switches, all of which generate continuous interference spectra. This means that when an inverter is put into operation, it must be protected from both external interference and interference from itself—a process commonly referred to as electromagnetic compatibility (EMC). 2. Sources of Inverter Interference 2.1 Interference from the External Power Grid Harmonic interference from the power grid mainly affects the inverter through its power supply. The power grid contains numerous harmonic sources such as various rectifiers, AC/DC converters, electronic voltage regulators, nonlinear loads, and lighting equipment. These loads cause waveform distortion in the voltage and current of the power grid, thus causing harmful interference to other equipment in the grid. If the inverter's power supply is not properly treated after being interfered with by a polluted AC power grid, the grid noise will interfere with the inverter through the power supply circuit. The interference of power supply to the frequency converter is mainly overvoltage, undervoltage, instantaneous power failure; surge, drop; peak voltage pulse; radio frequency interference, etc. (1) Interference of thyristor converter equipment to the 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 of power compensation capacitor to the frequency converter The power department has certain requirements for the power factor of the power users. Therefore, many users adopt the method of centralized capacitor compensation in the substation to improve the power factor. During the transient process of the compensation capacitor being put on or cut off, the network voltage may have a very high peak value, which may cause the rectifier diode of the frequency converter to break down due to excessive reverse voltage. 2.2 Interference of the frequency converter to the outside world The rectifier bridge of the frequency converter is a nonlinear load for the power grid. The harmonics it generates will cause harmonic interference to other electronic and electrical equipment in the same power grid. In addition, most inverters of the frequency converter adopt PWM technology. When working in switching mode and making high-speed switching, a lot of coupling noise is generated. Therefore, the frequency converter is an electromagnetic interference source for other electronic and electrical equipment in the system. The input and output current of the frequency converter contain many high-order harmonic components. In addition to the lower-order harmonics that can constitute power supply reactive power loss, there are also many high-frequency harmonic components. They will spread 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 frequency converter is a diode rectification and capacitor filtering circuit. Obviously, there is a charging current in the rectifier bridge only when the instantaneous value of 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 value of the power supply voltage and is in the form of a discontinuous shock wave. It has a strong high-order harmonic component. Data shows that the 5th and 7th harmonic components in the input current are the largest, accounting for 80% and 70% of the 50Hz fundamental frequency, respectively. (2) Waveforms of output voltage and current Most inverter bridges of frequency converters adopt SPWM modulation, 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. 3. Propagation mode of interference signal Frequency converters can generate harmonics with large power. Due to the large power, they have a strong interference effect on other equipment in the system. Their interference path is consistent with the general electromagnetic interference path, mainly divided into conduction (i.e., circuit coupling), electromagnetic radiation, and inductive coupling. Specifically: First, it generates electromagnetic radiation to the surrounding electronic and electrical equipment; second, it generates electromagnetic noise to the directly driven motor, which increases the iron loss and copper loss of the motor; and conducts interference to the power supply, which is transmitted to other equipment in the system through the power distribution network; finally, the frequency converter generates inductive coupling to other adjacent lines, inducing interference voltage or current. Similarly, interference signals within the system interfere with the normal operation of the frequency converter through the same pathway. 3.1 Circuit Coupling Mode Since the input current is non-sinusoidal, when the capacity of the frequency converter 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 loss and iron loss of the directly driven motor, affecting the operating characteristics of the motor. Obviously, this is the main propagation mode of the frequency converter input current interference signal. 3.2 Inductive Coupling Mode When the input circuit 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: (1) Electromagnetic induction mode, which is the main mode of current interference signal; (2) Electrostatic induction mode, which is the main mode of voltage interference signal. 3.3 Air Radiation Mode That is, radiating into the air in the form of electromagnetic waves, which is the main propagation mode of high-frequency harmonic components. 4. Anti-interference measures for variable frequency speed control systems According to the basic principles of electromagnetic inertia, electromagnetic interference (EMI) requires three elements: an electromagnetic interference source, an electromagnetic interference path, and a system sensitive to electromagnetic interference. To prevent interference, hardware and software anti-interference measures can be used. Hardware anti-interference is the most basic and important measure, generally addressing both "resistance" and "prevention." The general principle is to suppress and eliminate interference sources, cut off the coupling path of interference to the system, and reduce the system's sensitivity to interference signals. Specific measures in engineering include isolation, filtering, shielding, and grounding. 4.1 Interference Isolation Interference isolation refers to isolating the interference source from the susceptible parts in the circuit, preventing electrical contact between them. In variable frequency speed control systems, isolation transformers are typically used on the power lines between the power supply and amplifier circuits to prevent conducted interference. Noise isolation transformers can be used for power supply isolation. 4.2 Setting up filters The purpose of setting up filters in the system circuit is to suppress interference signals from the frequency converter being conducted through the power lines to the power supply or motor. To reduce electromagnetic noise and losses, an output filter can be installed on the output side of the inverter; to reduce power supply interference, an input filter can be installed on the input side of the inverter. If there are sensitive electronic devices in the line, a power supply noise filter can be installed on the power line to prevent conducted interference. In the input and output circuits of the inverter, 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 Input filters usually come in two types: Line filters: mainly composed of inductor coils. They weaken high-frequency harmonic currents by increasing the impedance of the line at high frequencies. Radiation filters: mainly composed of high-frequency capacitors. They will absorb high-frequency harmonic components with radiating energy. (2) Output filters Output filters are also composed of inductor coils. They can effectively weaken high-order harmonic components in the output current. They not only play an anti-interference role, but also weaken the additional torque caused by high-order harmonic currents in the motor. For anti-interference measures at the inverter output, the following aspects must be noted: Capacitors are not allowed to be connected to the inverter output to avoid generating a large peak charging (or discharging) current at the moment the inverter tube is turned on (off), which could damage the inverter tube; When the output filter is composed of an LC circuit, the side of the filter connected to the capacitor must be connected to the motor side. 4.3 Shielding Interference Sources Shielding interference sources is the most effective way to suppress interference. Usually, the inverter itself is shielded with an iron shell to prevent electromagnetic interference leakage; the output line is best shielded with steel pipe, especially when the inverter is controlled by external signals. The signal line should be as short as possible (generally within 20m), and the signal line should be double-core shielded and completely separated from the main circuit line (AC380V) and control line (AC220V). They must never be placed in the same conduit or cable tray, and the surrounding electronically sensitive equipment lines must also be shielded. To ensure effective shielding, the shielding cover must be reliably grounded. 4.4 Proper Grounding Proper grounding can effectively suppress external interference and reduce the interference of the equipment itself to the outside world. In practical applications, the lack of separation between the system power supply neutral wire (neutral wire) and ground wire (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 the system's stability and reliability. For frequency converters, proper grounding of the main circuit terminals PE (E, G) is crucial for improving the frequency converter's noise suppression capability and reducing interference; therefore, it must be given high priority in practical applications. The cross-sectional area of ​​the frequency converter grounding conductor should generally be no less than 2.5 mm², and its length should be controlled within 20 m. It is recommended that the frequency converter's grounding be separated from the grounding points of other power equipment and not shared. 4.5 Using reactors: The proportion of low-frequency harmonic components (5th, 7th, 11th, 13th, etc.) in the frequency converter's input current is very high. Besides potentially interfering with the normal operation of other equipment, they also consume a large amount of reactive power, significantly reducing the power factor of the line. Introducing reactors in series in the input circuit is an effective method for suppressing low-harmonic currents. According to the different wiring positions, there are mainly two types: (1) AC reactors are connected in series between the power supply and the input side of the inverter. Its main functions are: to improve the power factor to (0.75~0.85) by suppressing harmonic current; to reduce the impact of surge current in the input circuit on the inverter; and to reduce the influence of power supply voltage imbalance. (2) DC reactors are connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, which is to reduce the high-order harmonic components in the input current. However, it is more effective than AC reactors in improving the power factor, which can reach 0.95, and has the advantages of simple structure and small size. 4.6 Reasonable wiring For interference signals that are transmitted through induction, they can be weakened by reasonable wiring. The specific methods are: (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 inverter; (2) The power lines and signal lines of other equipment should avoid being parallel to the input and output lines of the inverter. 5. Conclusion Through the analysis of the sources and propagation paths of interference during the application of frequency converters, practical countermeasures for solving these problems were 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 crucial issue that must be addressed in the design and application of variable frequency speed control systems, and 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 converters themselves. The requirements of industrial sites and the social environment for frequency converters are constantly increasing, and truly "green" frequency converters that meet actual needs will soon emerge. We believe that the EMC problems of frequency converters will definitely be effectively solved.