Frequency converters are generally divided into “AC-DC-AC” frequency conversion and “AC-AC” frequency conversion, and their working principles are very different. “AC-DC-AC” frequency conversion mode is widely used, while “AC-AC” frequency conversion mode is rarely used because of its complex structure. 6kV “AC-AC” frequency conversion and 6kV “AC-DC-AC” frequency conversion are composed of phase-shifting transformer, power module and power frequency bypass. The difference lies in the working principle of power module. In order to briefly explain the working principle of “AC-AC” frequency converter, take low-voltage 380 V “AC-AC” frequency converter as an example.
As shown in the figure, each phase of the “AC-AC” frequency converter power unit is composed of two groups of fully controlled bridge rectifiers. The three-phase power supplies of the two groups of fully controlled bridge rectifiers are the same AC power supply, which are rectified after being reduced to the appropriate voltage by the transformer. In one cycle of the AC power supply, the left rectifier bridge works in the first half cycle, the right rectifier bridge is cut off, and the current flows from the positive pole to the negative pole; In the second half cycle, the right rectifier bridge works, the left rectifier bridge is cut off, and the current flows from the positive pole to the negative pole. The current flowing through the motor winding in a cycle is alternating current, that is, alternating current. The working time of the rectifier bridge can be controlled by changing the conduction angle and conduction time of the thyristor.
The above figure shows the principle of one-phase AC frequency conversion of motor, and three-phase requires three rectifier bridge groups, which requires a total of 36 thyristors. Therefore, the “AC-AC” frequency conversion structure is very complex and has few applications!
The principle of frequency converter to realize AC-AC frequency conversion is not the power, but the principle is basically the same.
First of all, the basic principle is to use a high-power rectifier bridge to rectify AC into DC. If it is single-phase AC, the DC voltage after rectification will not be too high. Even if there is electrolytic capacitor filtering, it will only reduce reactive power. Generally, the DC voltage of 230V will not be exceeded.
If it is three-phase AC, it will generally be rectified to about 530v DC.
Then, the composite tube composed of high-power bipolar triode and field effect transistor, commonly known as IGBT module, turns DC into directional alternating square wave voltage, that is, AC.
When the frequency of the frequency converter increases, the density of the square wave increases and the average voltage increases, because the speed of the AC motor is determined by the frequency and the logarithm of the magnetic poles. The approximate formula of revolutions per minute is: Revolutions = power frequency * 60 / pole pairs. Therefore, when the output frequency of the frequency converter changes, the motor speed will also change.
This is the principle of the main circuit. Of course, what kind of signal the control pole of the power module receives is determined by the main board and the control circuit, which is also more complex. There are many functions that can be set, with its own separate system and chip.
For example, commonly used: swing frequency function, multi-stage speed, simple self-learning function, simple PID, vector converter generally has constant torque function. Of course, each brand of converter has its own operation software, but it is very convenient to operate on the panel, so we ignore its programming software.