An AC-AC converter , also known as a frequency converter, is a process that directly converts AC power from the mains grid at a fixed frequency into AC power with an adjustable frequency through power semiconductor circuits. Unlike ordinary frequency converters, it does not have a stage for rectifying AC to DC and then inverting it back to AC; it uses an AC-AC conversion structure. This technology is generally used in large-power installations.
A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the motor's power supply.
A frequency converter mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit. The frequency converter adjusts the output voltage and frequency by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, the frequency converter has many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
AC-DC-AC frequency converter circuit schematic analysis
The working principle of an AC-DC-AC frequency converter is to use microelectronic devices, power electronic devices, and control technology to first rectify the mains frequency power supply into DC power through diodes, and then use power electronic devices to invert the DC power into AC power with an adjustable frequency. The working principle diagram of an AC-DC-AC frequency converter is shown below:
As shown in the diagram, the frequency converter consists of a main circuit (including a rectifier, an intermediate DC link, and an inverter) and a control circuit. The functions of each part are as follows:
1. A rectifier's function is to rectify three-phase (or single-phase) AC power into DC power. In SPWM frequency converters, full-wave rectifier circuits are mostly used. In most medium and small capacity frequency converters, uncontrolled rectifier diodes or diode modules are used as rectifier devices.
2. An inverter functions oppositely to a rectifier, converting direct current (DC) into alternating current (AC) with variable voltage and frequency to achieve variable frequency speed control for AC motors. Inverter circuits consist of switching devices, mostly employing a bridge circuit, often called an inverter bridge. In an SPWM inverter, the switching devices are controlled by the SPWM modulation signal in the control circuit, converting DC into three-phase AC.
3. Control Circuit: This part of the circuit consists of arithmetic circuits, detection circuits, drive circuits, protection circuits, etc., and generally uses large-scale integrated circuits.
AC-DC-AC frequency converters are quite common, consisting of three parts: a rectifier, a filtering system, and an inverter. The rectifier is either a diode-based 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 the rectifier; it converts 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. AC-DC-AC frequency converters can be further divided into voltage-type and current-type based on 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 applied 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.
