Keywords: Frequency converter; Interference fault; Countermeasures Abstract: This article analyzes and summarizes common interference faults of frequency converters and proposes corresponding solutions. 1. Introduction Frequency converters, as a high-efficiency and energy-saving motor speed control device, are increasingly widely used in factories due to their high performance-price ratio. As is well known, a frequency converter consists of a rectifier circuit, a filter circuit, and an inverter circuit. Both the rectifier and inverter circuits use semiconductor switching elements, and the control uses PWM control. This determines that the input and output voltages and currents of the frequency converter contain many high-order harmonic components in addition to the fundamental frequency. These high-order harmonic components will cause distortion of the mains voltage waveform and generate radio interference waves, which will adversely affect surrounding equipment, including the motor driven by the frequency converter. At the same time, due to the use of frequency converters, high-order harmonic components will be generated in the mains power supply voltage. When thyristor rectifiers are operating in the mains power supply, they will cause distortion of the power supply waveform. In addition, surge voltages generated by lightning strikes, the switching on and off of power transformers, or the switching on and off of electrical appliances will also distort the power supply waveform. When this distorted power grid supplies power to the frequency converter, it will have an adverse effect on the frequency converter. This article analyzes the above phenomena and proposes measures to reduce these adverse effects. [b]2. External Interference to the Frequency Converter[/b] The main interferences of the power supply to the frequency converter are overvoltage, undervoltage, instantaneous power loss; surges, drops; voltage spikes; and radio frequency interference. If the power supply of the frequency converter is subjected to harmonic interference from a polluted AC power grid and is not treated, the grid noise will interfere with the frequency converter through the power grid's power circuit. On the input circuit side of the frequency converter, the AC voltage is converted into DC voltage. This is the rectifier circuit, which is often referred to as "grid pollution". Since this DC voltage is output to the subsequent circuits after being smoothed by the filter capacitor, the power supplied to the frequency converter is actually the charging current of the filter capacitor, which distorts the input voltage waveform. (1) The power grid contains various rectifiers, AC/DC converters, electronic voltage regulators, nonlinear loads, and lighting equipment, which are a large number of harmonic sources. These loads in the power grid cause waveform distortion of voltage and current, which in turn causes harmful interference to other equipment in the power grid. For example, when there is a large-capacity thyristor converter in the power supply network, the thyristor is always conducting for part of the half-cycle of each phase, which easily causes the network voltage to have a notch and the waveform to be severely distorted. This may cause the rectifier circuit on the input side of the inverter to be damaged by a large reverse recovery voltage, which may lead to the input circuit breakdown and burnout. (2) Interference of power compensation capacitors to inverters The power sector has certain requirements for the power factor of the power users. Therefore, many users use centralized capacitor compensation in substations to improve the power factor. During the transient process of the compensation capacitor being put on or cut off, the network voltage may have a very high peak value, which may cause the rectifier diodes of the inverter to break down due to excessive reverse voltage. (3) Electromagnetic interference (EMI) is electromagnetic interference caused by external noise and unwanted signals during reception. It is usually transmitted through circuit conduction and propagated in the form of a field [2], that is, radiated into the air in the form of electromagnetic waves. Its radiation field strength depends on the current intensity of the interference source, the equivalent radiation impedance of the device, and the transmission frequency of the interference source. For the interference generated by items (1) and (2), an AC reactor can be connected in series in the inverter input circuit. Its impedance at the fundamental frequency is negligible. However, for high-frequency interference signals with higher frequencies, it presents a very high impedance and can effectively suppress the interference. For the interference signal of item (3), it is mainly weakened by absorption. An absorption capacitor is usually added to the power input terminal of the inverter. A dedicated "radio interference filter" can also be added to further weaken the interference signal. 3. Interference of the inverter to peripheral equipment and countermeasures As mentioned above, the inverter can generate high-order harmonics in the input power supply voltage. Meanwhile, in addition to the fundamental frequency, the output voltage and current of the frequency converter also contain many high-order harmonic components, which will spread their energy in various ways. These high-order harmonics will have adverse effects on the surrounding equipment. Among them, the distortion of the power supply will cause other equipment on the same power supply to malfunction, overheat, noise and vibration; the generated radio interference waves will interfere with the radio receiving devices such as TVs, radios, and mobile phones around the frequency converter, and in severe cases, they will not work properly; it will interfere with the external control signals of the frequency converter. After these control signals are interfered with, they cannot accurately and normally control the operation of the frequency converter, causing the motor driven by the frequency converter to generate noise, vibration and heat. (1) Interference caused by equipment connected to the same power supply When the capacity of the frequency converter is large, it will cause the network voltage to be distorted, and the interference will be transmitted to other circuits through impedance coupling or grounding loop coupling. To eliminate or weaken the interference caused by equipment connected to the same power supply, an AC reactor can be connected in series at the input end of the frequency converter and a DC reactor can be inserted on the rectifier side of the frequency converter. A filter can also be inserted at the power input terminal of the inverter, as shown in Figure 1 below: The LC filter is a passive filter. It consists of a reactor and a capacitor to form a resonant circuit for high-order harmonics, thereby achieving the purpose of absorbing high-order harmonics. The working principle of the active filter is: by detecting high-order harmonics in the current, and according to the detection results, inputting a current with the opposite phase to the high-order harmonic components to weaken the high-order harmonics. (2) Regarding the generated radio interference waves, most inverters currently use PWM control. The inverter output signal is a high-frequency switching signal. The inverter's output voltage and output current contain high-order harmonics, which generate radio interference waves through electrostatic induction and electromagnetic induction. Some of these interference waves are conducted through wires, while others are directly radiated into the air by electromagnetic waves and electric fields. Metal objects in the radiation field may also form secondary radiation. Similarly, external radiation from the inverter can also interfere with the normal operation of the inverter. Suppressing radio interference waves conducted by wires can be achieved by using noise filter transformers to insulate against higher harmonics; inserting reactors to increase the impedance to higher harmonic components; and inserting filters at the input of the frequency converter. Suppressing radiated radio interference waves is more difficult than suppressing conducted radio interference waves. The magnitude of this radio interference depends on the structure of the frequency converter itself and many other factors such as the length of the motor cable. The motor wires can be shortened as much as possible, and twisted-pair measures can be used to reduce impedance; the input and output lines of the frequency converter can be shielded with iron pipes; the frequency converter housing can be properly grounded; reactors can be connected in series at the input and output of the frequency converter, and filters can be inserted. (3) Regarding the noise interference generated, since the frequency converter uses PWM control, the output voltage waveform of the frequency converter is not a sine wave, and the current passing through the motor inevitably contains many harmonics. The resonance between the harmonic frequency of the frequency converter output and the natural frequency of the rotor increases the noise near the natural frequency of the rotor. The harmonic components of the frequency converter output increase the noise of the core, housing, shaft frame, etc., near their natural frequencies. Therefore, when using a frequency converter to speed control the motor, the motor windings and core generate noise due to harmonic components. Usually, when using a frequency converter to drive the motor, the noise generated by the motor is 5-10 dB higher than the noise generated by direct drive from the grid power supply. The measures that can be taken to suppress noise are: ① Select a low-noise frequency converter with a high carrier frequency using IGBT or similar inverter modules. Select a dedicated motor for the frequency converter and insert a reactor between the frequency converter and the motor to reduce the high-order harmonics generated by the PWM control mode. ② Insert a filter between the frequency converter and the motor that can convert the output waveform into a sine wave. ③ Select a low-noise reactor. (4) For the vibration interference generated, when using a frequency converter to speed control the motor, the same reasons as noise will cause the motor to vibrate. In particular, the pulsating torque generated by the lower-order high-order harmonics will bring greater vibration to the torque output of the motor. If the mechanical system resonates with this vibration, the vibration will be more severe. The following measures can usually be taken to reduce vibration: ① Strengthen the rigidity of the mechanical structure and change the rigid connection to a strong connection. ② Insert a reactor in series between the frequency converter and the motor. ③ Reduce the output voltage-frequency ratio of the frequency converter. ④ Change the carrier frequency of the frequency converter. When the frequency converter adjusts the speed of the motor, if the speed range is large, the resonant frequency of the mechanical system should be measured first, and then the frequency jump function of the frequency converter should be used to avoid these resonant frequencies. If there is a torque margin, U/f can be given a smaller value. (5) For interference that causes the motor to overheat, the frequency converter is used to adjust the speed of the motor. Due to the high-order harmonics, even for the same motor, running at the same frequency, the motor will increase the current by 5% to 10%. The motor temperature will naturally increase. In addition, the cooling fan of the ordinary motor is installed on the motor shaft. When running at low speed continuously, the cooling capacity of its own cooling fan is insufficient, and the motor overheats. The countermeasures for motor overheating are as follows: ① Equip the motor with a separate cooling fan and change the self-cooling type to the external cooling type. Increase cooling capacity during low-speed operation. ② Select a larger capacity motor. ③ Use a motor specifically designed for frequency converters. ④ Change the speed control scheme to avoid continuous low-speed operation of the motor. With the increasing level of electrical automation in factories, various interferences are also increasing. Only by having a deep understanding of the interference problems of frequency converters and taking corresponding measures can we reduce the mutual harm between them and ensure the normal operation of production and the stability of equipment to a greater extent. References [1] Li Zixian, et al. Application, Maintenance and Repair of Frequency Converters [M]. Beijing: Earthquake Press, 2005. [2] Zhang Liuyi. Interference Sources and Anti-interference Measures in the Use of Frequency Converters [J]. Metallurgical Journal, 2007, (1) 13-14.