Abstract This paper discusses anti-interference measures that can be taken in the main circuit of a variable frequency speed control system, and how to use reactors and filters to suppress conducted emission interference. Keywords Variable frequency drive, filter, reactor, anti-interference With the development of power electronics technology, microelectronics technology and control theory, variable frequency drives (VFDs) have been widely used in various motor speed control fields (such as various military weapon servo systems) due to their advantages of high-efficiency drive performance, good control characteristics and superior energy-saving characteristics. As a power electronic device, the core components of the VFD, such as internal electronic components and control chips, are susceptible to external electrical interference. The rectifier and inverter sections of the VFD have steep rising and falling edges during operation, and their input and output voltages and currents contain rich harmonic pollution and high-frequency noise, making them a serious source of radio frequency interference. Electromagnetic interference caused by this conducted and radiated interference can lead to malfunctions in communication and sensitive numerical control circuits, thereby deteriorating the electromagnetic environment. Therefore, when a variable frequency speed control system is put into operation, it is necessary to prevent external interference to the VFD and suppress the electromagnetic interference generated by the VFD speed control system to the outside world. In practical applications, inverter harmonic interference is frequently encountered. Therefore, the design of anti-interference measures (especially how to address the issue of conducted emissions exceeding CE101 and CE102 standards) is a crucial topic in the design of variable frequency speed control systems. 1. Transmission Paths and Hazards of Interference in Variable Frequency Speed ​​Control Systems External interference in variable frequency speed control systems mainly originates in three main circuits with sufficient power flow: the inverter's input circuit, the braking circuit connected to the DC circuit, and the inverter's output circuit. As interference sources, their interference paths are generally classified as conducted, radiated, and simultaneously generated by both conducted and radiated interference, as shown in Figure 1. As can be seen from the figure, conducted interference generated by the variable frequency speed control system feeds back to the power grid, significantly impacting the electronic equipment connected to the power input. Simultaneously, conducted interference generated at the output significantly increases the copper and iron losses of the directly driven motor, affecting its operating characteristics. Radiated interference generated by the inverter has a strong polluting effect on surrounding electronic receiving equipment. 2. Interference Suppression Measures for Variable Frequency Speed ​​Control Systems When formulating interference suppression measures, it is essential to first identify the interference source and the failure mechanism of interference occurrence. Prioritize source suppression. When analyzing failure mechanisms, first distinguish whether the interference is conducted or radiated, and for conducted interference, further differentiate between common-mode and differential-mode. This allows for the use of simple, effective, and low-cost suppression techniques to address the interference specifically. During inverter operation, the nonlinear components such as rectification and inversion exhibit steep current slopes on rising and falling edges in switching states, generating high-order harmonics and high-frequency noise, primarily through conducted emissions and electromagnetic field radiation. In the electromagnetic compatibility design of variable frequency speed control systems, key technologies include shielding, grounding, and filtering, as shown in Figure 2. For electromagnetic field radiation interference generated by the inverter, shielding of the inverter itself and its output lines is generally used, with separate wiring for output lines and other weak signal lines. Other nearby sensitive circuits should also ideally be shielded. For conducted emission interference from the inverter causing CE101 and CE102 to exceed limits, a combination of reactors, filters, and proper grounding is used to control spectral components other than the useful signal. Conducted emission interference is also a relatively complex and difficult-to-control interference mode. In engineering practice, equipment with high electromagnetic compatibility requirements generally needs to focus on solving conducted emission interference. Practice has proven that properly selecting filters and reactors to use with frequency converters can effectively reduce conducted emission interference and improve the power factor of the frequency converter, thus effectively solving CE101 and CE102 problems. 2.1 Installing a reactor on the input side of the frequency converter Installing an AC or DC reactor on the input side of the frequency converter is a multi-functional anti-interference measure. Utilizing the characteristic of the reactor to suppress the rate of change of current, the input current changes from discontinuous to continuous, suppressing harmonic interference on the input side, suppressing external voltage interference to the frequency converter, and also suppressing input circuit impulse interference. Figure 3 shows the current waveform after connecting a reactor on the input side. As can be seen from Figure 3, without the reactor connected, the AC and DC currents are discontinuous. After installing the reactor, the AC current becomes continuous, and the DC current is synthesized from the three-phase current on the AC side, which is also continuous. After the current becomes continuous, the deviation between the actual current and the equivalent sine wave (dashed line) decreases, meaning harmonics become smaller and harmonic interference is suppressed. The working mechanism of a DC reactor differs slightly from that of an AC reactor; its function is to directly ensure the continuity of the DC-side current. After installing a DC reactor, the AC-side phase current remains continuous, and the deviation between the AC-side phase current and the sine curve (dashed line) is not significant. Compared to the case without a reactor, harmonic interference is still well suppressed. Both AC and DC reactors can effectively suppress impulse interference; both can effectively reduce the instantaneous current value of rectifier devices; both can effectively suppress harmonic interference from the frequency converter to the electronic equipment connected to the power input terminal, with AC reactors having a better suppression capability than DC reactors; and both can improve the power factor on the power supply side, with DC reactors having a better improvement capability than AC reactors. In engineering practice, one type of reactor can be selected for connection to the input circuit according to the specific conditions of the equipment. In the main circuit, the AC reactor is connected after the AC contactor in the input circuit, with each phase connected in series. The DC reactor is installed in the DC circuit. 2.2 Installing filters on the input and output sides of the frequency converter. As can be seen from Figure 3, harmonic components still remain in the input current after the reactor is installed. To further suppress these harmonic components, a power supply filter needs to be connected on the input side. This type of power supply filter is a low-pass filter composed of n-type and T-type composite links of L and C elements. Using this type of power supply filter can effectively suppress conducted interference fed back to the power grid along the power line and improve the electromagnetic interference immunity of the equipment. In engineering practice, in places with special requirements for electromagnetic compatibility, in the design of anti-interference measures for the variable frequency speed control system, while installing a power supply filter on the input side of the frequency converter, a regular power supply filter is also installed at the equipment power supply. This design has a very significant effect on suppressing harmonic interference. The input-side power supply filter is installed before the frequency converter. If an input-side AC reactor is installed, the filter should be installed after the reactor. Installing a power filter on the output side of the frequency converter can reduce the high-order harmonic components in the output current and also reduce the additional torque caused by high-order harmonic currents in the motor, thus improving the operating characteristics of the motor. Due to the special internal circuit structure of the frequency converter, the output power filter and the input power filter have different structures and cannot be used interchangeably. 3. Conclusion Frequency converters are widely used in various motor speed control fields, but they also introduce serious harmonic pollution to the power grid and cause severe electromagnetic interference to surrounding electrical equipment. Appropriately selecting filters and reactors to match the frequency converter in the main circuit of the frequency converter speed control system can effectively suppress the conducted interference of the frequency converter to the power grid and load, reduce the energy of radiated interference from input and output wires, improve the electromagnetic environment quality around the frequency converter, and ensure that the frequency converter meets the electromagnetic compatibility requirements with surrounding sensitive electronic equipment. References: 1. Zhang Yanbin. Variable Frequency Speed ​​Control Technology (2nd Edition). Beijing: Machinery Industry Press, 2002. 2. Wang Shu. Design and Application of Variable Frequency Speed ​​Control System. Beijing: Machinery Industry Press, 2005