The function of an inverter is to convert constant voltage and frequency alternating current (AC) into adjustable direct current (DC), providing DC power to the inverter through a voltage-type or current-type filter. The inverter then converts the DC power into AC with an adjustable frequency. A frequency converter mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit.
AC-DC-AC voltage-type frequency converters connect large-capacity filter capacitors in parallel on the DC side to buffer reactive power. The DC power supply impedance is low, forming a voltage source. Energy feedback to the grid is difficult, so they can only be used for regenerative braking. They are suitable for small-capacity and low-frequency speed control systems. PWM frequency converters are characterized by frequency and voltage regulation being completed by the inverter, with diode rectification providing a constant DC voltage. They have a high power factor and fast adjustment speed. The output voltage and current are shaped close to a sine wave, improving motor operating performance such as overheating and torque reduction caused by rectangular waves. They are suitable for speed control systems with single or multiple motors operating in parallel and high dynamic performance requirements.
Frequency converters adjust the voltage and frequency of the output power supply by switching their internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, frequency converters have many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
In AC-DC-AC inverters, based on the nature of the DC power supply during the process of converting the main circuit power supply into DC power supply, inverters are classified into voltage-source inverters and current-source inverters.
Working principle of AC-DC-AC frequency converter
The working principle of an AC-DC-AC inverter is to use microelectronic devices, power electronic devices and control technology to first rectify the power frequency power into DC power through diodes, and then use power electronic devices to invert the DC power into AC power with adjustable frequency.
An AC-DC-AC 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.
Currently, the vast majority of general-purpose frequency converters are AC-DC-AC type, especially voltage converters, which are the core circuit of the frequency converter. They consist of a rectifier circuit (AC-DC switching), a DC filter circuit (energy consumption circuit), and an inverter circuit (DC-AC conversion), as well as current limiting circuits, braking circuits, and control circuits. The frequency of the AC power supply and the number of pole pairs of the motor determine the speed of the three-phase asynchronous motor.
The method of adjusting the motor speed by changing the number of pole pairs is called pole-changing speed control; the method of adjusting the motor speed by changing the AC power frequency is called frequency conversion speed control. A frequency converter converts 50Hz or 60Hz industrial frequency AC power into AC power with a variable frequency to supply the motor.
Working principle of AC-DC-AC frequency converter:
AC-AC frequency converters can generally only reduce the frequency of the input current at the output. As a result, the speed regulation range of AC-AC frequency converters is much smaller than that of AC-DC-AC frequency converters. In addition, AC-AC frequency converters are larger and more expensive, so their application range is very narrow.
