The fact is that frequency converters that do not require a rectifier unit exist; these are called AC-AC frequency converters. However, the vast majority of frequency converters on the market are AC-DC-AC frequency converters, which contain a rectifier unit. This situation is entirely due to technological and market competition reaching a certain level. AC-DC-AC frequency converters are cheaper to produce and more reliable and mature in use, so everyone uses them. In fact, this aligns with some patterns in human scientific research.
For example, our voices now need to be digitized, converted into simple 0-1 codes, and then transmitted to distant locations to become real sound. Because simple things are easy to quantify and process, we linearize complex curves and then use linearization to approximate and simulate complex real-world processes.
An AC-DC-AC inverter first converts AC power to DC power, and then inverts it back to AC power through IGBT chopping. It is easier 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 sufficiently small blocks, the cumulative effect is the same as a sine wave. IGBT devices can only be turned on and off, so they are more suitable for processing block signals.
So, converting AC to DC first seems like an extra step, but actually, "sharpening the axe doesn't delay the work," and it makes things much easier. Furthermore, rectifier modules and capacitors are relatively traditional and mature electronic components, making them relatively inexpensive; they're just a bit larger.
AC-DC-AC frequency converters are quite common and consist of three parts: a rectifier, a filter system, and an inverter. The rectifier is either a diode three-phase bridge 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, and its function is exactly the opposite of that of the rectifier. It converts constant DC power into AC power with adjustable voltage and frequency.
The intermediate filtering stage uses capacitors or reactors to filter the rectified voltage or current. AC-DC-AC frequency converters can be divided into voltage-type and current-type types according to the different intermediate DC filtering stages. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. They are used in frequency converters in industrial automation (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, this doesn't mean that AC-AC converters have no future. Matrix converters are a new type of AC-AC direct converter, consisting of nine switch arrays directly connected between the three-phase input and output. Matrix converters have no intermediate DC link, and their output consists of three levels with relatively low harmonic content. Their power circuitry is simple and compact, and they can output sinusoidal load voltages with controllable frequency, amplitude, and phase. The input power factor of a matrix converter is controllable, and it can operate in four quadrants. Although matrix converters have many advantages...
However, the commutation process does not allow for two switches to be simultaneously on or off, making implementation difficult; frankly, the algorithm is not mature. The low maximum output voltage capability and high voltage withstand capability of the components are also significant drawbacks of this type of converter. Furthermore, although it eliminates the need for a rectification unit, it still has six more switching devices than an AC-DC-AC converter, making it considerably more expensive at present.
