The feedback unit supplier reminds you that there are frequency converters that do not require rectification units, known as AC-AC frequency converters. However, the vast majority of the market consists of AC-DC-AC frequency converters, which contain rectifier units. This is a pattern formed by a certain degree of technological and market competition. AC-DC-AC frequency converters are cheaper to produce and more reliable and mature to use, so everyone uses them. In fact, this is also in line with some laws of human scientific research.
For example, our voices now have to be digitized, converted into simple 0-1 codes, and then transmitted to distant places before becoming real sounds. Because simple things are easy to quantify and process, we tend to linearize complex curves and then use linearized methods to approximate and simulate complex real-world processes.
AC-DC-AC frequency converter first converts AC power into DC power, and then converts it back into AC power through IGBT chopping. It is relatively easy to process the input DC power 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 a sine wave. IGBT devices can only be turned on and off, so they are more suitable for processing block signals.
So first, turn the AC into DC, which may seem like an additional process, but in fact, 'sharpening the knife doesn't miss the chopping wood', it's much easier anyway. In addition, rectifier modules and capacitors are relatively traditional and mature electronic devices, which are relatively cheaper in price, only slightly larger in size.
AC-DC-AC frequency converters are quite common, consisting of a rectifier, a filtering system, and an inverter. The rectifier is a fully controlled rectifier composed of a diode three-phase bridge uncontrolled rectifier or a high-power transistor, while the inverter is a three-phase bridge circuit composed of high-power transistors. Its function is exactly opposite to that of the rectifier, which exchanges constant DC power into adjustable voltage and frequency AC power.
The intermediate filtering stage uses capacitors or reactors to filter the rectified voltage or current. According to the different intermediate DC filtering stages, AC-DC-AC frequency converters can be divided into two types: voltage type and current type. Due to various factors such as control methods and hardware design, voltage type inverters are widely used. They are applied in industrial automation frequency converters (using variable voltage variable frequency VVVF control, etc.) and uninterruptible power supplies (UPS, using constant voltage constant frequency CVCF control) in IT and power supply fields.
Of course, it does not mean that the development of AC-AC frequency converters has ceased. Matrix frequency converter is a new type of AC-DC-AC direct frequency converter, consisting of nine switch arrays directly connected between the three-phase input and output. The matrix converter does not have an intermediate DC link, and its output consists of three levels with relatively low harmonic content; Its power circuit is simple, compact, and can output sine load voltage with controllable frequency, amplitude, and phase; The input power factor of a matrix converter is controllable and can operate in four quadrants, although matrix converters have many advantages.
However, during its commutation process, it is not allowed for two switches to conduct or turn off simultaneously, which is difficult to implement. Simply put, the algorithm is not mature. A major drawback of matrix converters is their low maximum output voltage capability and high device voltage tolerance. In addition, although it does not require rectification units, it has 6 more switching devices than AC-DC-AC frequency converters.