Abstract: This article introduces the application of frequency converters in DCS systems of the fermentation industry, the selection of filtering components, and precautions during installation. Keywords: Frequency converter, electromagnetic interference, electromagnetic compatibility 1 Introduction The fermentation industry is an important part of modern bioengineering, especially new fermentation industries such as pharmaceuticals, monosodium glutamate, citric acid, and xanthan gum, which have developed rapidly in recent years. Since the production of raw materials is essential, fermentation tanks are indispensable. With the continuous increase in product output and new varieties, the stirring speed of fermentation tanks is required to vary, necessitating different adjustments at different times. In recent years, we have carried out numerous equipment upgrades in the fermentation industry nationwide to meet production process requirements and save resources, especially given the large loads, high power consumption, and long fermentation cycles. The application of frequency converters in fermentation tanks is becoming increasingly widespread, providing good production and process benefits for industrial automation control in this industry. However, with the continuous improvement of automation, the degree of power pollution from automated equipment is also increasing, leading to stronger interference with automatic control systems. Therefore, the requirements for power filtering and purification to obtain a relatively stable and green power supply are becoming increasingly stringent. With the development of power electronics technology and the increasing sophistication of household appliances, industrial electrical appliances, and computer networks, the electromagnetic environment has become increasingly complex and deteriorating. This has led to increased attention from governments and manufacturers worldwide to the electromagnetic compatibility (EMC), electromagnetic interference (EMI), and electromagnetic seismic activity (EMS) issues of electrical and electronic products. EMC is a crucial quality indicator for electronic and electrical products, affecting not only the product's reliability and safety but also potentially the normal operation of other equipment and systems, and impacting electromagnetic environment protection. To ensure stable and reliable operation of electronic equipment and reduce electromagnetic pollution, more and more countries are implementing EMC standards, especially in European countries where EMC performance has become a legally mandated indicator and a mandatory requirement for electronic product manufacturers. Internationally, there are relatively clear laws and regulations governing the design and application of electromagnetic compatibility (EMC or EMI), with specific provisions for interference and being interfered with by electronic equipment, as well as the harmonic content of power supplies. Because my country's EMC technology started relatively late, its theoretical level, technical level, and manufacturing of related products (testing instruments, shielding materials, filters, etc.) are far behind those of developed countries, resulting in a relatively lagging practice. However, in some industrial production automation scenarios, the significance of electromagnetic compatibility is already quite evident. Some electronic devices are extremely sensitive to electromagnetic interference, to the point of malfunctioning. 2. Types of Noise and Countermeasures 1) The main paths of interference propagation are conduction and radiation. Conducted interference: The root cause of interference is unnecessary changes in voltage/current, which are directly transmitted to other devices through wires, causing harm. Radiated interference: The phenomenon is inseparable from antennas. According to the "antenna principle," if the length of the wire is equal to the wavelength, electromagnetic waves are easily generated. In short, when the length of the equipment and wires is shorter than the wavelength, the main problem is conducted interference; when their length is longer than the wavelength, the main problem is radiated interference. In addition, there are some brief high-energy pulse interferences in the environment. These interferences are very harmful to electronic equipment and are generally called transient interference. Transient interference can enter the equipment through cables or affect the equipment in the form of broadband radiated interference. 2) Voltage/current changes transmitted through conductors exist in two states: common mode and differential mode. Power lines, signal lines, and communication lines used for exchanging signals with other devices or peripherals typically have at least two conductors. These two conductors act as a round-trip line for transmitting power or signals, but there is another conductor, the ground wire, between them. Interference voltage and current can occur in two ways: differential mode, where both conductors act as round-trip lines; and common mode, where both conductors act as the outgoing line and the ground wire acts as the return line. For differential mode voltage, the voltage across one conductor is (voltage between lines)/2, and the voltage across the other conductor is (voltage between lines)/2, thus they are balanced. However, for common mode voltage, the voltage across both conductors is the same. When both modes exist simultaneously, the voltages of the two conductors relative to ground are different. Therefore, when the voltage or current to ground of two conductors are different, the components of the two modes can be determined by the following methods: UN=（U1-U2）/2UC=（U1+U2）/2 IN=（I1-I2）/2IC=（I1+I2）/2 3) Grounding is an important issue for electronic equipment: 1. Grounding provides a common reference zero potential for all circuits in the entire circuit system, meaning there is no potential difference between the grounds of each circuit, ensuring stable operation of the circuit system. 2. It prevents interference from external electromagnetic fields. Grounding the chassis provides a discharge path for transient interference and allows the large amount of charge accumulated on the chassis due to electrostatic induction to be discharged through the earth. In addition, for the shielding of the circuit, if a suitable grounding is selected, a good shielding effect can be obtained. 3. It ensures safe operation. When electromagnetic induction from direct lightning occurs, it can prevent damage to electronic equipment. When poor insulation of the equipment causes grounding, it can prevent electric shock. In short, grounding is one of the main methods to suppress noise and prevent interference. A well-designed grounding system can prevent unwanted interference and emissions at a very low cost. 4) Cables are efficient electromagnetic wave receiving and radiating antennas, but they are also excellent channels for interference. Using shielded cables may solve the cable radiation problem, but when using shielded cables, proper grounding of the shielding layer is crucial for preventing cable interference. Incorrect grounding point selection and other issues will cause interference problems with the shielded cable. In addition, the cable arrangement also has a significant impact on the product's electromagnetic interference. Coupling between cables and loops formed by cable wiring are important components of cable electromagnetic interference design. 3 EMC Filtering and Selection of Filtering Components a) Resistors Resistors are the most commonly used components on PCBs, and resistors are also a limiting factor in EMI applications. The limitations in the frequency domain depend on the resistor material used. Due to the additional inductance of wire-wound resistors, wire-wound resistors are not suitable for high-frequency applications. Thin-film resistors contain some inductance, but because of their low lead inductance, they can sometimes be used in high-frequency applications. b) Capacitors Capacitors are commonly used for decoupling, filtering, bypassing, and voltage regulation of power buses. Below the self-resonant frequency, the capacitor maintains its capacitive behavior; above the self-resonant frequency, the capacitor exhibits inductive behavior, which can be described by the formula Xc = 1/2πC, where Xc is the capacitive reactance in ohms (Ω); F is the frequency in Hertz (Hz); and C is the capacitance in Farads (F). c) Inductors: Inductors are also commonly used to control EMI. As the frequency increases, the inductive reactance of the inductor increases linearly, which can be described by the formula XL = 2πL. Common-mode inductors (also called common-mode chokes): Connecting one end of this common-mode inductor to the interference source and the other end to the device being interfered with, and usually using it with a capacitor, forms a low-pass filter, which can control the common-mode EMI signal on the line to a very low level. This circuit can suppress the input of external EMI signals and attenuate the EMI signals generated by the line itself during operation, effectively reducing the intensity of EMI. d) Ferrite rings: When inductors cannot be used for high frequencies, using ferrite rings is a good solution. Ferrite materials are ferromagnetic or iron-nickel alloys. These materials have high high-frequency permeability and high-frequency impedance, while exhibiting minimal inter-wound capacitance, making them suitable for high-frequency applications. At low frequencies, they have low inductance and low line loss; at high frequencies, they are primarily reactive and frequency-dependent. Ferrite magnetic rings are considered "energy-consuming devices." They dissipate high-frequency energy as heat, which can only be explained by resistive rather than inductive characteristics. e) Filters: A filter is a two-port network consisting of inductors, capacitors, and common-mode inductors forming a passive low-pass network. A basic circuit diagram is shown below. In general filters, the common-mode choke primarily filters out low-frequency common-mode interference. At high frequencies, due to the presence of parasitic capacitance, the common-mode choke's interference suppression effect is reduced, and the common-mode filter capacitor becomes the primary support. 4. System Introduction Henan Xinxiang Huaxing Pharmaceutical Co., Ltd. adopts a fully automated DCS control system manufactured by Beijing Kangtuo Bioengineering Co., Ltd. This system provides comprehensive monitoring of the temperature, pressure, flow rate, dissolved oxygen (DO), and pH value of each fermenter. It also provides fully automated control of the feeding and discharging processes during fermentation, including liquid sugar, phenylacetic acid, and ammonium sulfate. Three sensors detect these processes and send electrical signals to the microcomputer control system. The microcomputer control system then sends pulse signals (+5V) based on the detected voltage (or current) values ​​to control the opening and closing of solenoid valves (the solenoid valves operate at +36V), thus achieving the replenishment of feed materials. Each fermenter thus has six solenoid valves with three feed/discharge controls, three sensors, and two detection instruments. All are remotely monitored via a microcomputer, and the data is displayed on a large screen wall. The system diagram is as follows: 5. Electromagnetic Interference Problem Analysis and Solutions The inverter input power supply uses a filter manufactured by Taiyuan Yifute Electronics Co., Ltd. Internally, it uses a high-permeability ferrite core and iron powder core, coupled with a certain capacitor, forming an LC filter. This filter removes the high-order harmonics (within a certain frequency band) generated by the inverter, ensuring that nearby or same-power-grid electrical equipment is not interfered with and can operate normally. Its schematic diagram is shown below. [align=center] Figure 1 shows the schematic diagram of the input filter circuit[/align] The inverter output power supply filter uses an inductor (L) filter to suppress conducted interference at the inverter output and reduce low-frequency radiated interference on the output line, thereby reducing the electromagnetic noise of the directly driven motor and significantly reducing the copper and iron losses of the motor. Its schematic diagram is shown in Figure 2. [align=center]Figure 2 shows the schematic diagram of the output filter circuit[/align] Causes of Interference in Frequency Converters[/align]Figure 3 shows the main circuit diagram of the frequency converter[/align] The main circuit of a frequency converter is generally an AC-DC-AC mode (see Figure 3). An external 380V/50Hz power supply is uncontrolled rectified into a DC voltage signal by a three-phase bridge circuit, filtered by a filter capacitor, and inverted into a variable-frequency AC signal by a high-power transistor switching element. In the rectifier circuit, the waveform of the input current is an irregular rectangular wave. The waveform is decomposed into a fundamental wave and various harmonics according to Fourier series, and the higher harmonics will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by a PWM carrier signal. For GTR high-power inverter elements, the PWM carrier frequency is 2-3kHz, while the PWM carrier frequency of IGBT high-power inverter elements can reach up to 15kHz. Similarly, the output circuit current signal can also be decomposed into a fundamental wave containing only a sine wave and other harmonics, and the higher harmonic current directly interferes with the load. Furthermore, high-order harmonic currents radiate into space through cables, interfering with nearby electrical equipment. 6. Conclusion With the recognition of numerous companies, wind-powered frequency converters have been widely applied in the fermentation industry, providing a relatively complete solution to the problems related to automation control and electromagnetic compatibility of frequency converters in the fermentation industry. As my country's frequency converter market continues to expand, the significance of electromagnetic compatibility will become even more widespread, and its application prospects are very optimistic.