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Inverter knowledge: why rectifier and then inverter?

2020-12-14

Special frequency converter supplier reminds you: in the daily industrial control production, there is a frequency converter without rectifier unit, which is the so-called AC-AC converter. But the vast majority of the market are AC-DC-AC converters, that is to say, they contain rectifier units. This is a pattern formed by technology and market competition to a certain extent. The price of AC-DC-AC converters is lower and the use is more reliable and mature, so we all use it. In fact, this is also in line with some laws of human scientific research.


For example, our voice now has to be digitized into a simple 0-1 code, then transmitted to the distance, and then turned into real sound. Because simple things are easy to quantify and process, we will linearize complex curves, and then use linearized things to approximate and simulate complex practical links.


The AC-DC-AC converter first converts AC into DC, and then reverses it into AC through IGBT chopping. It is easier to process the input DC during chopping because it is linear. From the perspective of calculus, as long as it is divided into many small blocks, the cumulative effect is the same as that of sine wave. However, IGBT devices can only be turned on and off, So it is more suitable to process the signal of the block.


Therefore, it seems that there is an extra process to turn AC into DC. In fact, it is much easier to "sharpen the knife and cut wood". In addition, rectifier module, capacitor these are more traditional mature electronic devices, the price is relatively cheap, but the volume is a little larger.


AC-DC-AC converter is common, which consists of rectifier, filter system and inverter. The rectifier is a diode three-phase bridge rectifier or a fully controlled rectifier composed of high-power transistors. The inverter is a three-phase bridge circuit composed of high-power transistors. Its function is just opposite to that of the rectifier. It exchanges constant direct current into AC with adjustable voltage and adjustable frequency.


The intermediate filter is used to filter the voltage or current after rectification by capacitor or reactor. AC-DC-AC converter can be divided into voltage type and current type according to the difference of intermediate DC filtering link. Due to various factors such as control method and hardware design, voltage type inverter is widely used. It is used in frequency converter of industrial automation field (using variable voltage variable frequency VVVF control) UPS (constant voltage and constant frequency CVCF control) and uninterruptible power supply (UPS) in the field of it and power supply are applied.


Of course, it is not to say that the cycloconverter has not developed. Matrix converter is a new type of AC-AC direct converter, which consists of nine switch arrays directly connected to three-phase input and output. The matrix converter has no intermediate DC link, and its output consists of three levels, so the harmonic content is relatively small; its power circuit is simple and compact, and can output sinusoidal load voltage with controllable frequency, amplitude and phase; the input power factor of matrix converter is controllable and can work in four quadrants, although matrix converter has many advantages.


However, in the process of commutation, two switches are not allowed to turn on or turn off at the same time, so it is difficult to realize the phenomenon. Matrix converter has low maximum output voltage capability and high device withstand voltage. In addition, although it does not need the rectifier unit, it has six more switching devices than the AC-DC-AC converter.


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