VFD Operation Principle
Currently, most general-purpose variable-frequency drives are of AC-DC-AC type, with voltage inverter drives being particularly common. The main circuit diagram of the variable-frequency drive, which is the core circuit, consists of a rectifying circuit (AC-DC conversion), a DC filtering circuit (energy consumption circuit), and an inverter circuit (DC-AC conversion). It also includes components such as current limiting circuits, braking circuits, and control circuits, of course.
1. Rectifier circuit
Figure 1.2 illustrates that the rectifier circuit of a general-purpose variable frequency inverter consists of a three-phase bridge rectifier. Its function is to rectify the industrial frequency power supply, smooth it through an intermediate DC link, and then provide the necessary DC power to the inverter circuit and control circuit. A three-phase AC power supply typically passes through an absorption capacitor and a surge arrester network to the input of the rectifier bridge. The network's role is to absorb high-frequency harmonic signals and surge overvoltages from the AC power grid, thereby preventing damage to the inverter. When the power supply voltage is three-phase 380V, the large reverse voltage of the rectifying devices is generally 1200-1600V, and the large rectifying current is twice the rated current of the inverter.
2. Filter Circuit
The inverter's load is an asynchronous motor with inductive load characteristics. Whether the asynchronous motor is in electric or generator mode, there will always be exchange of reactive power between the DC filter circuit and the asynchronous motor. This reactive energy needs to be buffered by the energy storage elements in the DC intermediate circuit. Additionally, the voltage and current output from the three-phase rectifier bridge are DC pulse voltage and current. To reduce fluctuations in DC voltage and current, the DC filter circuit serves to filter the output of the rectifier circuit.
Large capacity aluminum electrolytic capacitors in the DC filter circuit of general-purpose variable-frequency inverters are typically composed of a series and parallel combination of capacitors to achieve the required voltage withstand and capacitance. Additionally, due to the significant variability in the capacitance of electrolytic capacitors, this will cause them to have unequal voltages. Therefore, each capacitor is paralleled with a resistor of equal value to maintain a uniform voltage, eliminating the impact of variability, which consequently severely limits the lifespan of the capacitors and, in turn, the variable-frequency inverter.
3. Inverter circuit
The function of the inverter circuit is to convert the direct current output from a DC circuit into an alternating current power supply with adjustable frequency and voltage under the control of the control circuit. The output of the inverter circuit is the output of the inverter, making the inverter circuit one of the core circuits of the inverter, playing a very important role.
The common inverter circuit structure utilizes a three-phase bridge inverter composed of six power switching devices (GTR, IGBT, GTO, etc.). By regulating the conduction and switching-off of the inverter's power switching devices, any frequency of three-phase AC output can be achieved.
