Interference and its suppression in the applicatio

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Interference and its suppression in the application of frequency converter

[Abstract] frequency converter is more and more widely used in industrial production, and its interference problem has attracted more and more attention. This paper mainly introduces the source of interference and its propagation path in the application system of frequency converter, puts forward the practical solution of anti-interference, and expounds the specific measures to suppress interference in the design and installation of the application system of frequency converter

1 introduction

frequency converter speed regulation technology is a high-tech technology integrating automatic control, microelectronics, power electronics, communication and other technologies. It has been widely used in all walks of life because of its good speed regulation and energy-saving performance. Because it adopts soft start, it can reduce the mechanical impact of equipment and motor and prolong the service life of equipment and motor. With the rapid development of science and technology, frequency converter has been applied to various fields of industrial control with its characteristics of power saving, energy saving, reliability and high efficiency, such as the application of variable frequency speed regulation in water supply, air conditioning equipment, process control, elevators, machine tools, etc., which ensures the adjustment accuracy, reduces the labor intensity of workers, and improves economic benefits, but also brings some interference problems. The on-site power supply and electrical equipment will have an impact on the frequency converter, and the high-order harmonic generated during the operation of the frequency converter will also interfere with the operation of the surrounding equipment. There are three kinds of interference generated by frequency converter: interference to electronic equipment, interference to communication equipment and interference to radio. The interference to electronic equipment such as computers and automatic control devices is mainly inductive interference; The interference to communication equipment and radio is radioactive interference. If the interference problem of frequency converter is not solved well, not only the system cannot operate reliably, but also the normal operation of other electronic and electrical equipment will be affected. Therefore, it is necessary to discuss the interference in the application system of frequency converter to promote its further popularization and application. The following mainly discusses the interference of frequency converter and its suppression methods

2 main electromagnetic interference sources and ways of variable frequency speed regulation system

2.1 main electromagnetic interference sources

electromagnetic interference, also known as electromagnetic interference (EMI), is the electromagnetic interference caused by external noise and useless signals in reception, which is usually transmitted through circuit and in the form of field. The rectifier bridge of the frequency converter is a non-linear load to the electricity, and the harmonic generated by it will cause harmonic interference to other electronic and electrical equipment of the same electricity. In addition, the inverter of the frequency converter mostly adopts PWM technology. When it works in the switching mode and makes high-speed switching, it produces a lot of coupling noise. Therefore, the frequency converter is an electromagnetic interference source for other electronic and electrical equipment in the system. On the other hand, the harmonic interference in electricity mainly interferes with the frequency converter through the power supply of the frequency converter. There are a large number of harmonic sources in electricity, such as various rectifying equipment, AC/DC exchange equipment, electronic voltage regulation equipment, nonlinear load and lighting equipment. These loads make the voltage and current in the electricity produce waveform distortion, which will cause harmful interference to other equipment in the electricity. After the power supply of the frequency converter is interfered by the polluted AC, if it is not treated, the electrical noise will interfere with the frequency converter through the electrical power circuit. The interference of power supply to frequency converter mainly includes overvoltage, undervoltage and instantaneous power failure; Surge and drop; Spike voltage pulse; Radio frequency interference. Secondly, common mode interference through the control signal line of the frequency converter will also interfere with the normal operation of the frequency converter

2.2 ways of electromagnetic interference

the frequency converter can generate high-power harmonics and have strong interference with other equipment of the system. Its interference path is consistent with the general electromagnetic interference path, mainly including electromagnetic radiation, conduction and inductive coupling. Specifically: ① generate electromagnetic radiation to surrounding electronic and electrical equipment; ② Generate electromagnetic noise to the directly driven motor, increase the iron and copper consumption of the motor, and conduct interference to the power supply, which is transmitted to other equipment of the system through the distribution network; ③ The frequency converter generates inductive coupling to other adjacent lines and induces interference voltage or current. Similarly, the interference signals in the system interfere with the normal operation of the frequency converter in the same way. The following are analyzed separately

(1) electromagnetic radiation

if the frequency converter is not in a fully enclosed metal shell, it can radiate electromagnetic waves through space. Its radiation field strength depends on the current intensity of the interference source, the equivalent radiation impedance of the device and the emission frequency of the interference source. The rectifier bridge of the frequency converter is a nonlinear load for electricity, and the harmonic generated by it is connected. 1. The following experience is summarized through examples: other electronic and electrical equipment connected to the same electricity generate harmonic interference. The inverter bridge of the frequency converter mostly adopts PWM technology. When the expected sum weight is generated according to the given frequency and amplitude command, 7 According to whether the sample has prefabricated cracks or not, it can be divided into: when the switching mode is complex in the conventional fatigue test and fatigue crack growth test, the power spectrum of the output voltage and current is discrete, and there are high-order harmonic groups corresponding to the switching frequency. High carrier frequency and high-speed switching of field controlled switching devices (dv/dt up to 1kV/μ S) or more, which is quite prominent

when the metal shell of the frequency converter has a gap or hole, the radiation intensity is related to the wavelength of the interference signal. When the size of the hole is close to the wavelength of the electromagnetic wave, the interference radiation source will radiate around. The metal objects in the radiation field may also form secondary radiation. Similarly, the external radiation of the frequency converter will also interfere with the normal operation of the frequency converter

(2) conducted

in addition to the above electromagnetic interference, it can also be transmitted to other circuits through impedance coupling or grounding loop coupling. Compared with radiated interference, it can spread far away. The typical transmission path is that the interference signal generated by the frequency converter connected to the industrial low-voltage network will enter the medium voltage network along the distribution transformer, and finally enter the civil low-voltage distribution network along other distribution transformers, making the electrical equipment connected to the civil distribution bus a long-range victim

(3) inductive coupling

inductive coupling is the third transmission path between radiation and conduction. When the frequency of the interference source is low, the electromagnetic wave radiation capacity of the interference is quite limited, and the interference source is not directly connected with other conductors, but the electromagnetic interference energy can be inductively coupled with other adjacent conductors or conductors through the input and output conductors of the frequency converter, and the interference current or voltage can be induced in the adjacent conductors or conductors. Inductive coupling can occur in the form of capacitive coupling between conductors, or in the form of inductive coupling or a mixture of capacitance and inductance, which is related to the frequency of the interference source and the distance from adjacent conductors

3 anti electromagnetic interference measures

according to the basic principle of electromagnetism, the formation of electromagnetic interference (EMI) must have three elements: electromagnetic interference source, electromagnetic interference path, and system sensitive to electromagnetic interference. To prevent interference, anti-interference measures of hardware and software can be adopted. Among them, hardware anti-interference is the most basic and important anti-interference measure. Generally, anti-interference and anti-interference are used to suppress interference. The general principle is to suppress and eliminate interference sources, cut off the coupling channel of interference to the system, and reduce the sensitivity of the system to interference signals. For specific measures, isolation, filtering, shielding, grounding and other methods can be adopted in the project

(1) isolation

the so-called interference isolation refers to isolating the interference source from the easily interfered part of the circuit so that they do not have electrical connection. In the variable frequency drive system, the isolation transformer is usually used on the power line between the power supply and the amplifier circuit to avoid conducted interference. The noise isolation transformer can be used as the power isolation transformer

(2) filtering

the purpose of setting the filter is to suppress the interference signal from the frequency converter to the power supply and motor through the power line. To reduce electromagnetic noise and loss, an output filter can be set at the output side of the frequency converter. In order to reduce the interference to the power supply, an input filter can be set at the input side of the frequency converter. If there is sensitive electronic equipment in the line, a power noise filter can be set on the power line to avoid conducted interference

(3) shielding

shielding interference sources is the most effective way to suppress interference. Usually, the frequency converter itself is shielded with an iron shell to prevent electromagnetic interference leakage. It is better to shield the output line with steel pipe, especially when the frequency converter is controlled by external signal, the signal line shall be as short as possible (generally within 20m), and the signal line shall be double core shielded and completely separated from the main circuit and control circuit. It shall not be placed in the same piping or trunking, and the surrounding electronic sensitive equipment lines shall also be shielded. In order to make the shield effective, the shield must be reliably grounded

(4) grounding

practice has proved that grounding is often an important means to suppress noise and prevent interference. A good grounding method can restrain the coupling of internal noise to a great extent, prevent the invasion of external interference, and improve the anti-interference ability of the system. The grounding methods of the frequency converter include multi-point grounding, one-point grounding and bus grounding, which should be adopted according to the specific situation. Attention should be paid not to interfere with the equipment due to poor grounding

single point grounding means that in a circuit or device, only one physical point is defined as the grounding point. Good performance at low frequency; Multipoint grounding means that each grounding point in the device is directly connected to the nearest grounding point. Good performance at high frequency; Hybrid grounding refers to the common mode of single point grounding and multi-point grounding according to the signal frequency and the length of grounding wire. The frequency converter itself has a special grounding terminal PE end, which must be grounded for the sake of safety and noise reduction. It is not allowed to connect the ground wire to the shell of the electrical equipment, and it can be connected to the zero line without closing the oil delivery valve. One end of the thicker short wire can be connected to the PE end of the grounding terminal, and the other end is connected to the grounding electrode. The value of the grounding resistance

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