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An inverter includes a rectifier circuit and a pulse voltage waveform generation circuit. The input AC power is rectified by the converter and smoothing circuit to be converted into DC voltage, which is then transformed by the inverter into pulse voltages of different widths (referred to as pulse width modulation voltage, PWM). By using this PWM voltage to drive the motor, it is possible to adjust the motor’s torque and speed. This working principle leads to three types of electromagnetic interference: 1. Harmonic interference. The rectifier circuit generates harmonic currents, and these harmonic currents cause voltage drops across the impedance of the power supply system, resulting in a distortion of the voltage waveform. Such distorted voltages pose a disturbance to many electronic devices, as most of them can only operate under sine wave voltage conditions. A common form of voltage distortion is the flattening of the peaks of the sine wave. When the harmonic current remains constant, voltage distortion is more severe in the case of weak power supplies. This type of interference is characterized by affecting devices connected to the same power grid, regardless of the distance between those devices and the frequency converter. 2. RF conducted emission interference: Since the load voltage is pulsed, the current drawn by the inverter from the power grid is also pulsed. This pulsed current contains a large amount of high-frequency components, resulting in RF interference. The characteristic of this interference is that it affects devices connected to the same power grid, regardless of the distance between those devices and the inverter. 3. Radio frequency radiation interference: Radio frequency radiation interference originates from the input and output cables of the frequency converter. In the case of RF conducted emission interference mentioned above, when there is an RF interference current in the input cable of the frequency converter, the cable acts as an antenna, which inevitably leads to electromagnetic wave radiation and thus radiative interference. The PWM voltage transmitted over the inverter’s output cable also contains a large amount of high-frequency components, which generate electromagnetic wave radiation and cause interference. The characteristic of radiation interference is that the interference becomes more severe when other electronic devices are placed near the frequency converter. According to the basic principles of electromagnetism, three elements are necessary to generate electromagnetic interference: an electromagnetic interference source, a path for the interference to propagate, and a system sensitive to that electromagnetic interference. To prevent interference, hardware-based and software-based anti-interference methods can be used. Among them, hardware interference resistance is the most fundamental and important measure for combating interference. Interference is generally suppressed from two aspects: suppression and emission. The overall principle is to suppress and eliminate the sources of interference, cut off the pathways through which interference affects the system, and reduce the system’s sensitivity to interference signals. Specific measures in engineering include methods such as isolation, filtering, shielding, and grounding. The following are the main steps to address interference at the site: 1. Use software-based anti-interference measures: Specifically, this involves using the human-machine interface of the inverter to reduce its carrier frequency, adjusting it to an appropriate level. If this method does not work, then the following hardware anti-interference measures must be adopted. 2. Ensure proper grounding: Through on-site investigations, it can be seen that the grounding conditions at the site are not ideal. Proper grounding not only enables the system to effectively suppress external interference but also reduces the interference generated by the equipment itself; it is the most effective measure for addressing interference issues with frequency converters. Specifically, the following points need to be fulfilled: (1) The main circuit terminal E of the inverter must be grounded. This grounding can share the same ground connection as the motor connected to the inverter, but it cannot share the same ground connection with other devices; a separate grounding electrode must be used, and this grounding point should be placed as far away as possible from the grounding points of low-voltage equipment. At the same time, the cross-sectional area of the inverter grounding wire should be no less than 4 mm2, and its length should be kept within 20 meters. (2) For the ground wires of other mechanical and electrical equipment, the protective grounding and the working grounding should be connected to separate grounding electrodes, which should then be connected to the electrical grounding point in the distribution cabinet. The ground for the control signals and the ground for the main circuit wires should also be provided with separate grounding electrodes, which should then be connected to the electrical grounding point in the distribution cabinet. 4. Shielding interference sources – Shielding interference sources is a very effective way to suppress interference. Typically, the inverter itself is shielded with an iron casing to prevent the leakage of electromagnetic interference. However, it is best to shield the output wires of the inverter using steel pipes, and magnetic rings should also be installed – wound in parallel for 3–4 turns – to help suppress higher harmonics. Especially when the inverter is controlled by external signals (4–20mA signals sent from a controller), it is necessary to keep such control signal wires as short as possible (usually within 20 meters), and they must use shielded twisted pairs, while being completely separated from the main circuit wires (AC380) and the control wires (AC220V). Furthermore, the wiring for electronic sensitive devices in the system also requires shielded twisted pair, especially for pressure signals. Furthermore, all signal wires in the system must never be placed in the same conduit or cable tray as the main circuit wires and control wires. For the shielding to be effective, the shielding layer must be reliably grounded. 5. The specific methods for proper wiring are as follows: (1) The power and signal cables of the equipment should be kept as far away as possible from the input and output cables of the inverter; (2) The power and signal cables of other equipment should avoid running parallel to the input and output cables of the inverter ； If the above methods still do not work, then proceed with the following methods. 6. Isolation of interference: Isolation of interference refers to separating the source of interference from the parts that are susceptible to interference through electrical means, so as to prevent any electrical connection between them. Typically, an isolation transformer is used on the power supply line between the power supply, controllers, and amplifier circuits such as transmitters, in order to prevent conductive interference; a noise isolation transformer can be employed for this purpose. 7. Install filters in the system circuit. The purpose of device filters is to suppress interference signals from being transmitted from the inverter, via the power lines, and interfering with the power supply and the motor. To reduce electromagnetic noise and losses, an output filter can be installed on the output side of the inverter; to minimize interference with the power supply, an input filter can be installed on the input side of the inverter. If there are sensitive electronic devices in the circuit, such as controllers and transmitters, a power noise filter can be installed on their power supply lines to prevent the transmission of interference. Filters can be classified according to their location of use as follows: (1) There are generally two types of input filters: a) Line filters: These are mainly composed of inductive coils, and they reduce high-frequency harmonic currents by increasing the impedance of the circuit at high frequencies. b. Radiation filter: It is mainly composed of high-frequency capacitors, which absorb the harmonic components with very high frequencies that possess radiant energy. (2) The output filter is also composed of an inductor coil. It can effectively reduce the higher harmonic components in the output current. It not only serves to resist interference but also reduces the additional torque caused by harmonic currents generated by high-order harmonics in the motor. Regarding the anti-interference measures at the output of the inverter, the following points must be taken into account: a) Capacitors are not allowed to be connected to the output of the inverter, as this could result in very high charging (or discharging) currents at the moment when the inversion tubes turn on (off), thereby damaging those tubes; b) When the output filter is composed of an LC circuit, the side of the filter where the capacitor is connected must be connected to the motor side. 8. Use of reactors: In the input current of inverters, harmonic components with lower frequencies (such as the 5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) account for a significant proportion. In addition to potentially interfering with the proper operation of other devices, these harmonics consume a large amount of reactive power, thereby significantly reducing the power factor of the circuit. Inserting a reactor in the input circuit is an effective method to suppress lower harmonic currents. Depending on the wiring location, there are mainly the following two types: (1) AC reactor: connected in series between the power supply and the input side of the inverter. Its main functions are: a) improving the power factor to (0.75–0.85) by suppressing harmonic currents; b) reducing the impact of inrush currents in the input circuit on the inverter; c) mitigating the effects of power supply voltage imbalance. (2) DC reactor: Connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple: it reduces the higher 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 it has advantages such as a simple structure and small size. Figure 1 is a typical solution for addressing inverter interference. As shown in the figure, the anti-interference measures for the frequency converter mainly include installing AC reactors and filters at the input side of the frequency converter, using shielded cables for both the input and output connections, and connecting the shielding layers of all cables to the protective ground of the reactor, filter, frequency converter, and motor. This grounding point is kept separate from other grounding points, with sufficient distance between them. At the same time, the signal cable and the power cable of the frequency converter should not be laid parallel to each other. Furthermore, to prevent the inverter from interfering with signals and control circuits, separate isolated power supplies must be used to power the controllers, instruments, and industrial computers.