Abstract: This paper discusses the anti-interference and interference characteristics of AC frequency converter systems and proposes measures for preventing and resisting interference. Keywords: Interference, Electromagnetic compatibility, Frequency conversion. With the development of China's economy and the progress of science and technology, the application of AC frequency conversion speed regulation is becoming increasingly widespread. Among various speed regulation methods, AC frequency conversion speed regulation technology has been recognized both domestically and internationally as the most ideal and promising speed regulation method. When factories and equipment use AC speed regulation, harmonic interference will be generated on both the power supply side and the motor side of the frequency converter. On the one hand, when the frequency converter is running, it is necessary to prevent it from being subjected to external electromagnetic interference; on the other hand, it is necessary to prevent the generation of high-order harmonic interference to other external equipment, which is the so-called "EMC". I. What is EMC? EMC stands for "Electromagnetic Compatibility". It refers to the ability of electrical equipment to operate well in an electromagnetic environment and not generate electromagnetic interference that is unacceptable to other equipment operating in this environment. The International Electrotechnical Commission (IEC) defines electromagnetic compatibility (EMC) as: "Electromagnetic compatibility is a function of electronic equipment that enables it to perform its functions in an electromagnetic environment without generating intolerable interference." China's national standard for "Electromagnetic Compatibility" defines it as: "Equipment or systems that function normally in their electromagnetic environment without causing unacceptable electromagnetic interference to anything in that environment." Clearly, EMC has a dual meaning: immunity to interference and interference resistance. II. Frequency Converters and EMC Generally, electrical equipment must have the ability to suppress both high-frequency and low-frequency interference. High-frequency interference mainly includes electrostatic discharge, pulse interference, and electromagnetic fields at emission frequencies; while low-frequency interference mainly refers to power supply voltage fluctuations, undervoltage, and frequency instability. Frequency converters typically operate in industrial environments where high electromagnetic interference (EMI) may exist; they are both noise sources and potential noise receivers. 1. External Interference Received by the Frequency Converter: 1) Interference from Transistor Converters: When there are large-capacity thyristor converters in the power supply network, the thyristors are always conducting for part of each phase half-cycle, which can easily cause fluctuations in the network voltage (as shown in Figure 1-1). This can cause the rectifier circuit on the input side of the frequency converter to experience a large reverse recovery voltage, potentially damaging it. 2) Interference from the Switching On and Off of Compensation Capacitors: When centralized capacitor compensation is used in the substation of the power supply line to improve the power factor, the network voltage may experience very high peak values ​​during the transient process of switching on and off the compensation capacitors (as shown in Figure 1-2). As a result, the rectifier diodes of the frequency converter may experience excessive reverse voltage and break down. 2. External Interference from the Frequency Converter: 1) Frequency Converter Current Waveform: The input and output currents of the frequency converter contain strong high-order harmonic components, which will interfere with other control equipment and affect the normal operation of other equipment. Input current waveform: For example, the input side of an AC-DC-AC voltage-type inverter consists of a rectifier and filter circuit (as shown in Figure 2-2). Charging current only occurs in the rectifier bridge when the line voltage U2 of the power supply is greater than the voltage UD across the capacitor. Figure 1-1 shows the distortion caused by thyristor commutation. Figure 1-2 shows the voltage distortion when the compensation capacitor is connected. The distortion always appears near the amplitude of the power supply voltage, exhibiting a discontinuous shock wave form (as shown in Figure 2-1). It has a high level of odd harmonic components, especially the 5th and 7th harmonics, as shown in Table 1 (using the Siemens MM3 inverter as an example). Figure 2-1 shows the input current waveform. Figure 2-3 shows the output voltage waveform. Output voltage and current waveforms: Most inverters use PWM modulation for their inverter bridge (as shown in Figure 2-2), and their output voltage is a series of rectangular waves with a sinusoidal duty cycle distribution (as shown in Figure 2-3). Parasitic capacitance Cp exists in the motor cable and inside the motor. Therefore, the switching wing of the inverter's PWM output voltage waveform generates a high-frequency pulse current Is through the parasitic capacitance, making the inverter a source of harmonic interference. Since the source of the harmonic current Is is the inverter, it must flow back to the inverter. In the figure, Ze is the ground impedance, and Zn is the impedance between the power cable and the ground. The voltage drop caused by the harmonic current flowing through these two impedances will affect other equipment on the same power grid, causing interference. Table 1 Figure 2-2 Flow direction of inverter noise current 3. Propagation mode of interference signal The input and output currents of the inverter contain many high-order harmonic components, which will propagate their energy in various ways, forming interference signals to other equipment. Generally speaking, the propagation mode of interference signals is as follows: 1) Circuit coupling mode: that is, propagation through the power network. Since the input current is non-sinusoidal, when the inverter capacity is large, it will cause the network voltage to be distorted, affecting the operation of other equipment. This is the main propagation mode of inverter input current interference signals. 2) Inductive Coupling: When the inverter's input or output circuit is very close to the circuits of other equipment, the inverter's high-order harmonic signals will be coupled to other equipment through induction. There are two types of induction: electromagnetic induction and electrostatic induction. Electrostatic induction occurs through line capacitance. This is the main propagation method for current interference signals. 3) Airborne Radiation: This occurs through electromagnetic waves, which is the main propagation method for high-frequency harmonics. 4. Inverter Anti-interference Measures 1) Reactors (as shown in Figure 4-1): In the inverter's input circuit, the proportion of lower-frequency harmonics (5th-11th orders, etc.) is relatively 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 to suppress lower-harmonic currents. Figure 4-1 shows the reactors inserted into the inverter. Figure 4-2 shows the filter connection. AC reactor (input reactor): connected in series between the power supply and the input side of the inverter (La in Figure 4-1). The main functions of the input reactor are as follows: (1) Reduce the harmonics generated by the inverter and increase the power supply impedance. (2) Absorb and weaken the impact of surge voltage, current and main power supply voltage spikes generated by nearby equipment on the inverter. (3) Weaken the influence of power supply voltage imbalance on the inverter. DC reactor (smoothing reactor): connected in series between the rectifier bridge and the filter capacitor (Ld in Figure 4-1). Its main function is to weaken the high-order harmonic components in the inverter input current and improve the power factor by suppressing harmonic current. 2) Filter; In the input and output circuits of the inverter, in addition to the lower-order harmonic components mentioned above, there are many higher-frequency harmonic currents, which will form interference signals to other equipment in various ways. The filter is a method to weaken the higher-frequency harmonic components (as shown in Figure 4-2). Input filters: There are usually two types: (1) Line filters: mainly composed of inductors (as shown in F11 in Figure 4-2). It mainly weakens high-frequency harmonic currents by increasing the impedance of the line at high frequencies. (2) Radiation filters: mainly composed of high-frequency capacitors (as shown in F12 in Figure 4-2, or in a delta connection). It absorbs high-frequency harmonic components with radiant energy, greatly reducing the high-frequency current flowing back to the power supply. (A dedicated "radio interference suppression filter" provided by the inverter manufacturer can also be used). Output filters: also mainly composed of inductors (as shown in F0 in Figure 4-2). It can effectively weaken high-order harmonic components in the output current, not only playing an anti-interference role, but also weakening the additional torque caused by high-order harmonic currents in the motor. Note: When the output filter is composed of an LC circuit, the side of the capacitor connected in the filter must be connected to the motor. Figure 4-3 Noise current flow direction with shielded motor cable 3) Use of shielded cables and reasonable wiring: Interference signals propagated by induction can be eliminated by the following methods. Using shielded motor cables: For high-frequency interference, if the higher harmonic interference current Is has a reasonable path, the high-frequency interference can be suppressed. If unshielded cables are used, the higher harmonic interference current Is flows back to the inverter along an uncertain path, generating a high-frequency component voltage drop in this loop, affecting and interfering with other equipment. To ensure that the higher harmonic interference current Is flows back to the inverter along a defined path, shielded motor cables are required (as shown in Figure 4-3). The cable shield must be connected to both the inverter housing and the motor housing, forming an effective path when the higher harmonic interference current Is must return to the inverter. In this way, the higher harmonic interference current will not generate a voltage drop across Ze, and the voltage drop across Zn can be suppressed by the incoming line reactive interference filter (F12 in Figure 4-2). Figure 4-4 illustrates the use of shielded signal cables to enhance interference immunity. When the frequency converter is the target of interference, high-order harmonic interference currents Is can enter the frequency converter through potential and coupling capacitance, generating a voltage drop across impedance Zi, leading to noise interference. The most effective method is to strictly isolate high-frequency interference from the signal cable, and the signal cable shield must be grounded at both ends. Shielded cables are preferred for control cables. Generally, the shield of the control cable should be grounded directly inside the frequency converter, with the other end grounded through a small high-frequency capacitor (e.g., 3.3nF/3000V). When the differential-mode voltage at both ends of the shield is not high and connected to the same ground wire, both ends of the shield can also be directly grounded. Twisting the signal line and its return line together reduces interference caused by inductive coupling. The closer the twist is to the terminals, the better. Double-shielded twisted-pair cables should be used for analog signal transmission lines. Different analog signal lines should be routed independently with their own shields to reduce coupling between lines. Do not place different analog signals on the same common return line. For low-voltage digital signals, double-shielded twisted-pair cable is best, but single-shielded twisted-pair cable can also be used. Good grounding and proper wiring are crucial: ensure all equipment in the cabinet is properly grounded using short, thick grounding wires connected to a common grounding point or grounding busbar. It is especially important that any control equipment connected to the inverter share a common ground, also using short, thick wires. Flat conductors (such as metal mesh) are preferable due to their lower impedance at high frequencies. Proper wiring is also essential; analog and digital signal transmission cables should be routed separately. Avoid sharing the same cable with low-voltage lines and 220VAC power lines. Long-distance parallel routing of motor cables with other cables should be avoided as much as possible. When designing the control cabinet, pay attention to EMC zoning principles and plan different equipment in different zones as much as possible. III. Conclusion The immunity and resistance to interference of equipment are very important issues. Currently, EMC has become a major cause of system failures. One principle of EMC is that "prevention is the most effective and economical solution." Therefore, EMC has become an important issue that cannot be ignored in ensuring the reliable and normal operation of frequency converter equipment.