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Reasons why the inverter must be rectified

Jan 14, 2021

Reasons why the inverter must be rectified

The fact is that there are inverters that do not require a rectifier unit, so-called AC-AC inverters. However, most of the AC-DC-AC inverters in the market are those containing rectifier units. This is completely a pattern formed by technology and market competition to a certain extent. AC-DC-AC inverters are produced at lower prices and are more expensive to use. Reliable and mature, so everyone uses it. In fact, this is in line with some laws of human scientific research.

For example, our voices are now digitized and turned into simple 0-1 codes, then transmitted to a remote place, and then turned into real voices. Because simple things are easy to quantify and process, we will linearize complex curves, and then use linearized things to approximate complex actual links.

The AC-DC-AC inverter first converts the alternating current into direct current, and then inverts it into alternating current through IGBT chopping. It is easier to process the input direct current when chopping, because it is linear, from calculus From a logical point of view, as long as it is divided into many small blocks, the cumulative effect is the same as a sine wave, and IGBT devices can only be turned on and off by themselves, so processing the signal of the block is more suitable.

Therefore, the AC is changed to DC first. It seems that there is one more process. In fact, "sharpening the knife without cutting wood by mistake" is much easier anyway. In addition, the rectifier modules and capacitors are relatively traditional and mature electronic devices, and the price is relatively cheap, but the volume is a little larger.

AC-DC-AC frequency converters are relatively common and consist of three parts: rectifier, filter system and inverter. The rectifier is a diode three-phase bridge-type uncontrolled 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 the opposite of the rectifier. It exchanges constant direct current into Adjustable voltage, adjustable frequency alternating current.

The intermediate filtering link uses capacitors or reactors to filter the rectified voltage or current. AC-DC-AC inverters can be divided into voltage type and current type according to the difference of the intermediate DC filtering links. Because of the control method and Various factors such as hardware design, voltage-type inverters are widely used. They are used in inverters in the industrial automation field (using variable voltage and variable frequency VVVF control, etc.) and uninterrupted power supplies in the IT and power supply fields (ie UPS, using constant voltage and constant Frequency CVCF control) have applications.

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

However, during the commutation process, the phenomenon that the two switches are turned on or off at the same time is not allowed, which is more difficult to implement. To put it plainly, the algorithm is immature. The low maximum output voltage capability of matrix converters and high device withstand voltage are also a big disadvantage of such converters. In addition, although it does not need a rectifier unit, it has 6 more switching devices than an AC-DC-AC inverter. At present, it is still much more expensive in terms of cost.