Abstract: This paper discusses the anti-interference and interference characteristics of AC frequency converter systems and proposes methods 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 more and more widespread. Among various speed regulation methods, AC frequency conversion speed regulation technology has been recognized at home and abroad 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 is "electromagnetic compatibility". It refers to the ability of electrical equipment to work well in an electromagnetic environment and not to generate electromagnetic interference that other equipment working in this environment cannot accept. 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, in an AC-DC-AC voltage-type inverter, the input side consists of a rectifier and filter circuit (as shown in Figure 2-2). Charging current only occurs in the rectifier bridge when the power supply line voltage U2 is greater than the voltage UD across the capacitor. This charging current always appears near the amplitude of the power supply voltage, exhibiting a discontinuous impulse wave form (as shown in Figure 2-1). It has high odd harmonic content, especially the 5th and 7th harmonics, as shown in Table 1 (using the Siemens MM3 inverter as an example). Output voltage and current waveform: Most inverters use PWM modulation in their inverter bridge (as shown in Figure 2-2), and their output voltage is a series of rectangular waves with a sinusoidal duty cycle (as shown in Figure 2-3). Parasitic capacitance Cp exists in the motor cable and inside the motor. Therefore, the switching wings of the inverter's PWM output voltage waveform generate 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 diagram, Ze represents the ground impedance, and Zn represents the impedance between the power cable and ground. The voltage drop caused by harmonic current flowing through these two impedances will affect other equipment on the same power grid, causing interference. 3. Propagation Modes of Interference Signals The input and output currents of frequency converters contain many high-order harmonic components, which will propagate their energy in various ways, forming interference signals to other equipment. Generally speaking, the propagation modes of interference signals are as follows: 1) Circuit coupling mode: that is, propagation through the power network. 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. This is the main propagation mode of frequency converter input current interference signals. 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 induction modes: electromagnetic induction mode: that is, induction through inductance. This is the main propagation mode of current interference signals. Electrostatic induction mode: that is, induction through inter-line capacitance. This is the main propagation mode of voltage interference signal. 3) Air radiation mode: that is, radiation into the air in the form of electromagnetic waves. This is the main propagation mode of high frequency harmonic content. 4. Anti-interference measures of frequency converter 1) Reactor (as shown in Figure 4-1): In the input circuit of frequency converter, the proportion of low frequency harmonic content (5th-11th, etc.) is relatively high. In addition to potentially interfering with the normal operation of other equipment, they also consume a lot 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 current. AC reactor (input line reactor): connected in series between the power supply and the input side of the frequency converter (La in Figure 4-1). The main functions of the input line reactor are as follows: (1) Reduce the harmonics generated by the frequency converter and increase the power supply impedance at the same time. (2) Absorb and weaken the impact of surge voltage, current and main power supply voltage spikes generated by nearby equipment on the frequency converter. (3) Reduce the impact of power supply voltage imbalance on the inverter DC reactor (smoothing reactor): connected in series between the rectifier bridge and the filter capacitor (as shown in Figure 4-1 Ld), its main function is to reduce the high-order harmonic components in the inverter input current, and can improve the power factor by suppressing harmonic current. Filter; In the input and output circuit 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 devices in various ways. The filter is a method to reduce the higher-frequency harmonic components (as shown in Figure 4-2). Input filter: There are usually two types (1) Line filter: mainly composed of inductor coils (as shown in Figure 4-2 F11). It mainly reduces the higher-frequency harmonic current by increasing the impedance of the line at high frequencies. (2) Radiation filter: mainly composed of high-frequency capacitors (as shown in Figure 4-2 F12, or Δ connection). It will absorb the high-frequency harmonic components with radiation energy. This greatly reduces the high-frequency current flowing back to the power supply. (Alternatively, a dedicated "radio interference suppression filter" provided by the inverter manufacturer can be used). Output filter: Primarily composed of inductor coils (as shown by F0 in Figure 4-2). It effectively reduces high-order harmonic components in the output current, not only providing anti-interference but also reducing the additional torque caused by high-order harmonic currents in the motor. Note: When the output filter is constructed using an LC circuit, the side of the capacitor connected within the filter must be connected to the motor. 3) Use of shielded cables and proper wiring: Interference signals propagated through induction can be eliminated in the following ways. Use of shielded motor cables: For high-frequency interference, if the high-order harmonic interference current Is has a reasonable path, the high-frequency interference can be suppressed. If unshielded cables are used, the high-order harmonic interference current Is flows back to the inverter along an uncertain path, generating a high-frequency component voltage drop in this circuit, affecting and interfering with other equipment. To ensure that the high-order harmonic interference current Is flows back to the inverter along a defined path, shielded motor cables (as shown in Figure 4-3) are required. The cable shield must be connected to both the inverter housing and the motor housing. When the high-order harmonic interference current Is must return to the inverter, the shield forms an effective path. This prevents the high-order harmonic interference current from causing 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). Shielding signal cables increases interference immunity (as shown in Figure 4-4): When the inverter is the target of interference, the high-order harmonic interference current Is can enter the inverter through potential and coupling capacitance, generating a voltage drop across impedance Zi, leading to noise interference. The most effective method to address this 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 directly grounded inside the inverter, with the other end grounded through a small high-frequency capacitor (e.g., 3.3nF/3000V). When the differential-mode voltage across the shield is not high and connected to the same ground wire, both ends of the shield can also be directly grounded. Signal lines and their return lines should be twisted together to reduce interference caused by inductive coupling. The closer the twist is to the terminal, the better. Double-shielded twisted-pair cable should be used for analog signal transmission lines. Different analog signal lines should be routed independently, with their own shielding layers to reduce inter-line coupling. Do not place different analog signals on the same common return line. Low-voltage digital signals are best routed using double-shielded twisted-pair cable, but single-shielded twisted-pair cable can also be used. Good grounding and proper wiring: 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 particularly 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 preferred due to their lower impedance at high frequencies. Proper wiring is also crucial; 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 anti-interference and interference resistance of equipment are very important issues. Currently, EMC has become a major cause of system failures. One of the principles 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 converters.