How does the frequency converter realize frequency conversion?

The frequency converter is actually an inverter. It first converts alternating current into direct current, and then uses electronic components to convert direct current into alternating current. High-power frequency converters generally use silicon controlled rectifier, and a frequency adjustable device is set to make the frequency adjustable within a certain range. Used to control the number of revolutions of the motor, so that the number of revolutions can be adjusted within a certain range. Frequency converters are widely used in the speed regulation of AC motors. Frequency conversion speed regulation technology is an important development direction of modern electric drive technology. With the development of power electronics technology, the frequency conversion technology of alternating current has changed from. The inverter not only has the advantages of stable speed regulation, wide range, high efficiency, low starting current and stable operation, but also has obvious energy saving effect. Therefore, AC variable frequency speed regulation has gradually replaced the traditional slip speed regulation, pole-changing speed regulation, DC speed regulation and other speed regulation systems, and is more and more widely used in metallurgy, textile, printing and dyeing, air conditioning, hood production line, construction, water supply and other fields. Generally divided into rectifier circuit, smoothing circuit, control circuit and inverter circuit.

1. rectifier circuit

The function of rectifier circuit is to convert AC power supply into DC power supply. The rectifier circuit is usually a single rectifier module.

2. Smoothing circuit

The smoothing circuit includes 6 times frequency pulsating voltage of power supply in rectifier and rectified DC voltage. In addition, the pulsating current generated by the inverter will also change the DC voltage. In order to suppress voltage fluctuation, the pulsating voltage (current) is absorbed by inductance and capacitance. In general, the DC part of the general inverter has a margin for the main circuit, so a simple capacitor filter smoothing circuit is used instead of the inductor.

3. Control circuit

At present, 16-bit and 32-bit single chip microcomputer or DSP are basically used as the control core of frequency conversion governor, thus realizing full digital control.

Frequency converter is a speed regulating device with adjustable output voltage and frequency. The loop that provides control signals is called the main control circuit, which consists of the following circuits: frequency and voltage operation circuit, voltage and current detection circuit of the main circuit, and speed detection circuit of the motor. The control signal of the arithmetic circuit is sent to the "drive circuit" and "protection circuit" of the inverter and motor.

The control mode adopted by the frequency converter, that is, speed control, suppression control, PID or other modes.

4 inverter circuit

The inverter circuit is opposite to the rectifier circuit. The inverter circuit converts the DC voltage into an AC voltage with a desired frequency, and turns on and off the power switching devices of the upper bridge and the lower bridge at a certain time. Therefore, three-phase AC voltage with phase difference of 1.20 can be obtained at output terminals u, v and w.