Frequency converter according to the method of frequency conversion is divided into AC-AC type frequency converter and alternating - direct type frequency converter, a AC-AC type frequency converter can be directly converted into frequency, voltage can be controlled by AC, so called direct type frequency converter, we talk about AC/DC frequency converter today.

I. Working Principle

The working principle of AC-AC frequency converter is that the three-phase industrial frequency power supply is controlled by several sets of phase control switches to directly generate the required variable voltage and frequency conversion power supply, the advantage of high efficiency, the energy can be convenient to return to the grid, the biggest disadvantage of its output must be less than the output frequency of the input power supply 1 / 3 or 1/2 of the highest frequency, otherwise the output waveform is too poor, the motor generates jitter, can not work.

This process is usually divided into the following steps:

1. Rectifier

The input power supply of AC/AC frequency converter is usually 220V or 380V, firstly, the input voltage will be raised to a proper level by transformer, then the input AC voltage will be converted to DC voltage (pulse voltage) by rectifier circuit (e.g., diode converter), and will be turned into a relatively smooth DC voltage by DC filter.

2. Frequency converter

After rectification and filtering, the frequency converter converts the DC voltage into a controllable AC voltage through an frequency converter circuit. 

In this process, the frequency converter through a certain frequency converter control methods (such as voltage control and frequency control), so that the output power supply is rendered as a sinusoidal wave, square wave or even triangular wave and other waveforms.

3. Control

 The frequency converter switches the electronic switches and controls the output frequency and voltage through the control chip. 

There are various control methods, such as open-loop control, closed-loop control, vector control and so on. 

Among them, the closed-loop control mode has more stable performance and higher switching frequency, which controls the output power of the frequency converter by feeding back the current, voltage, position and other signals output from the frequency converter to ensure that the output voltage and current are within the required precision range and stable.

II.Features of AC/DC frequency converter

AC-AC frequency converter is a converter that directly converts a higher fixed-frequency voltage to a lower frequency and a variable output voltage without a DC intermediate link, and the characteristics of this frequency converter include:

1. Because it is a direct conversion, so it has a higher efficiency than the general frequency converter;

2. Because of its AC output voltage, is directly from the AC input voltage wave some part of the envelope of the composition, so when the output frequency is much lower than the frequency of the input AC power supply, the output waveform is better;

3.The frequency converter is naturally phase shifted according to the grid voltage over zero, and an ordinary thyristor can be used;

4. Because of the characteristics of the circuit composition method, the number of thyristor elements used is higher; fifth, the power factor is lower, especially in low-speed operation is even lower, and proper compensation is required.