[Abstract] The interference problem of frequency converters is increasingly attracting attention. This paper mainly introduces the sources and propagation paths of interference in frequency converter applications, proposes anti-interference methods, and elaborates on specific measures to suppress interference in the design of frequency converter application systems. [Keywords] Frequency converter, interference sources, interference suppression measures Introduction Frequency converter speed control technology has been widely used due to its excellent speed control and energy-saving performance. The interference generated by frequency converters is mainly of three types: interference to electronic equipment, communication equipment, and radio, etc. The interference to electronic equipment such as computers and automatic control devices is mainly induced interference, and the interference to communication equipment and radio, etc., is radiated interference. If the interference problem of frequency converters is not properly solved, not only will the system not operate reliably, but it will also affect the normal operation of other electronic and electrical equipment. 1 Electromagnetic Interference Sources and Propagation Paths of Variable Frequency Speed ​​Control Systems 1.1 Main Electromagnetic Interference Sources Electromagnetic interference, also known as electromagnetic disturbance (EMI), is electromagnetic interference caused by external noise and unwanted signals. It usually propagates in the form of circuit conduction and radiation fields. The rectifier bridge of the frequency converter is a nonlinear load to the power grid. Its harmonics will generate harmonic interference to other electronic and electrical equipment on the same power grid. Inverters in frequency converters often use PWM technology. When operating in switching mode and performing high-speed switching, they generate a large amount of coupling noise. Therefore, frequency converters are a source of electromagnetic interference for other electronic and electrical equipment in the system. On the other hand, if the power supply of the frequency converter is affected by interference from a polluted AC power grid, the grid noise will interfere with the frequency converter if not dealt with. The main sources of power supply interference to the frequency converter are: overvoltage, undervoltage, momentary power failure, surge, voltage drop, voltage spikes, and radio frequency interference; secondly, common-mode interference can also interfere with the normal operation of the frequency converter through its control signal lines. 1.2 Propagation Paths of Electromagnetic Interference Frequency converters can generate relatively high-power harmonics. They have a strong interference effect on other equipment in the system. The main interference paths are: electromagnetic radiation, conduction, and inductive coupling. Specifically: it generates electromagnetic radiation to surrounding electronic and electrical equipment; it generates electromagnetic noise to directly driven motors, increasing their iron and copper losses, and conducts interference to the power supply, which is then conducted to other equipment in the system through the power distribution network; the frequency converter generates inductive coupling to other adjacent lines, inducing interference voltage or current. Similarly, interference signals in the system interfere with the normal operation of the frequency converter through the same path. (1) Electromagnetic radiation: If the frequency converter is not fully enclosed in a metal casing, it will radiate electromagnetic waves. The intensity of its radiation field depends on the current intensity of the interference source, the equivalent radiation impedance of the device, and the emission frequency of the interference source. The rectifier bridge of the frequency converter is a nonlinear load to the power grid, and its harmonics generate harmonic interference to other electronic and electrical equipment connected to the same power grid. The inverter of the frequency converter mostly uses PWM technology. When the expected and repetitive switching modes are generated according to the given frequency and amplitude command, the power spectrum of its output voltage and current is discrete and has high-order harmonic groups corresponding to the switching frequency. The radiation interference caused by high carrier frequency and high-speed switching of field-controlled switching devices (dv/dt can reach 1kV/μs) is quite prominent. (2) Electromagnetic conduction: In addition to being emitted to the outside through the wires connected to it, the above electromagnetic interference can also be carried into other circuits through impedance coupling or grounding loop coupling. The propagation path can be very long. A typical path is: the interference signal generated by the frequency converter connected to the industrial low-voltage network enters the medium-voltage network along the distribution transformer and enters the civil low-voltage distribution network along other distribution transformers, making the electrical equipment connected to the civil busbar a remote victim. (3) Inductive coupling: Inductive coupling is between radiation and conduction. When the frequency of the interference source is low, the electromagnetic radiation capability of the interference is quite limited, and the interference source is not directly connected to other conductors, the electromagnetic interference energy can be induced by the input and output wires of the frequency converter and other adjacent wires or conductors. Interference current or voltage is induced in the adjacent wires or conductors. Inductive coupling can occur through capacitive coupling between conductors, or through inductive coupling, or through a mixture of capacitive and inductive coupling. This is related to factors such as the frequency of the interference source and its distance from adjacent conductors. 2 Main measures to resist electromagnetic interference According to electromagnetic principles, the formation of electromagnetic interference (EMI) requires elements such as an electromagnetic interference source, an electromagnetic interference path, and a system sensitive to electromagnetic interference. Hardware and software anti-interference measures can be adopted. Among them, hardware anti-interference measures are the most basic and important. In engineering, isolation, filtering, shielding, grounding and other methods can be used. (1) Isolation: refers to separating the interference source from the susceptible object in the circuit so that they do not have electrical contact. In variable frequency speed control systems, noise isolation transformers are often used on the power lines between the power supply and the amplifier circuit to prevent conducted interference. (2) Filtering: The function of setting up a filter is to suppress the interference signal from the frequency converter to the power supply and motor through the power lines. To reduce electromagnetic noise and loss, an output filter can be set on the output side of the frequency converter. To reduce interference to the power supply, an input filter can be set on the input side of the frequency converter. (3) Shielding: Shielding the source of interference is the most effective way to suppress interference. Inverters are usually shielded with iron shells to prevent electromagnetic interference leakage. Its output lines are best shielded with steel pipes, especially when the inverter is controlled by external signals. The signal lines should be as short as possible (generally within 20m), and the signal lines should be double-core shielded and completely separated from the main circuit and control circuit. They should not be placed in the same pipe or cable tray. The circuits of the surrounding electronically sensitive equipment should also be shielded. To make the shielding effective, the shield must be reliably grounded. (4) Grounding: Grounding is an important means of suppressing noise and preventing interference. A good grounding method can effectively suppress internal noise coupling. It can prevent external interference from intruding and improve the system's anti-interference ability. The grounding methods of inverters are: 1) Single-point grounding: In a circuit or device, there is only one physical point as the grounding point. It has good performance at low frequencies. 2) Multi-point grounding: It means that each grounding point in the device is directly connected to the nearest grounding point. It has good performance at high frequencies. 3) Hybrid grounding is a method that uses both single-point grounding and multi-point grounding according to the signal frequency and the length of the grounding wire. The frequency converter itself has a dedicated grounding terminal (PE terminal), which must be grounded for safety and noise reduction. The ground wire must not be connected to the outer casing of the electrical equipment, nor to the neutral wire. A short, thick wire can be used, with one end connected to the PE terminal and the other end connected to the grounding electrode. The grounding resistance should be <100Ω, and the grounding wire length should be within 20m. The location of the grounding electrode should be carefully selected. When the system requires high anti-interference capability, a power filter can be installed at the power input terminal to reduce interference to the power supply. To suppress harmonic currents on the frequency converter input side and improve the power factor, an AC reactor can be installed at the frequency converter input terminal. To improve the frequency converter output current and reduce motor noise, an AC reactor can be installed at the frequency converter output terminal. Figure 1 shows general anti-interference measures for variable frequency speed control transmission systems. The above measures can be selected appropriately according to the system requirements. If the system contains a control unit (such as a microcomputer), anti-interference measures must be implemented in the software. 3. Some issues to be noted in the design of frequency converter control systems In addition to the above, the following should be noted in the design and application of frequency converter control systems: (1) When arranging equipment, the frequency converter should be arranged separately to minimize electromagnetic radiation interference. For example, the power distribution cabinet should be placed between the frequency converter and the control equipment. (2) An air switch of appropriate capacity can be used as short-circuit protection on the power input side of the frequency converter, but it should not be operated frequently. Since there is a large capacitor in the frequency converter, the discharge process is relatively slow. Frequent operation will cause overvoltage and damage the internal components. (3) The start/stop of the frequency converter speed regulation motor is usually achieved by the control function built into the frequency converter. It should not be achieved through a contactor. Otherwise, frequent operation may damage the internal components. (4) Minimize the connection between the frequency converter and the control system to avoid conducted interference. Except for the necessary control line between the control system and the frequency converter, other such as control power supply should be separated. Since both the control system and the frequency converter require 24V DC power, it should be specially specified in the design or order that two DC power supplies should be used to power the two systems respectively. (5) Pay attention to the interference of the frequency converter with the power grid. The high-order harmonics generated by the frequency converter during operation will affect the power grid, causing severe distortion of the power grid waveform, which may result in a large voltage drop and a very low power factor. High-power frequency converters should pay particular attention to this. The main solution is to use an automatic reactive power compensation device to adjust the power factor. At the same time, a reactor can be added to the power input side of the frequency converter to reduce the impact on the power grid, depending on the specific situation. (6) Except for the dedicated air switch of the frequency converter, other operating switches should not be installed in the frequency converter cabinet to prevent switch noise from invading the frequency converter and causing malfunctions. (7) Pay attention to limiting the minimum speed. At low speeds, the motor noise increases and the motor cooling capacity decreases. If the load torque is large or fully loaded, the motor may burn out. For high-load frequency converter motors that need to run at low speeds, the rated power should be increased or the air cooling should be enhanced. (8) Pay attention to preventing resonance. Since the stator current contains high-order harmonics and the motor torque contains pulsating components, electromagnetic vibration and mechanical vibration resonance may occur, causing equipment failure. The inherent resonance frequency of the load should be found in advance, and the frequency jump function of the frequency converter should be used to avoid the resonance frequency point. 4 Conclusion Through the analysis of interference problems in the operation of frequency converters, practical methods for solving these problems are proposed. With the emergence of new technologies and theories, these problems in frequency converter applications are expected to be solved through the functions and compensation of the frequency converter itself. As the requirements of industrial sites and social environment for frequency converters continue to increase, truly "green" frequency converters that meet actual needs will soon be available. References [1] Han Anrong. 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