Basic principles of AC drives

In modern AC speed control systems, whether variable frequency speed control or double-fed speed control is used, a power electronic transformer inverter that can simultaneously control voltage amplitude and frequency is required, commonly known as an AC inverter.

AC inverters can be divided into AC-DC-AC inverters and AC-AC inverters in terms of overall structure. The so-called "AC-DC-AC inverter" is an inverter with an intermediate DC link. Its working principle is to first rectify the industrial frequency AC into DC, and then invert it into AC with adjustable frequency and voltage after passing through the intermediate DC filter link. The so-called "AC-AC inverter" is an inverter that directly converts common frequency AC into AC with adjustable frequency and voltage without the need for an intermediate DC link.

AC-DC-AC and AC-AC inverter principle block diagram

There are many ways to classify AC-DC-AC inverters. For example, based on the nature of the intermediate DC link, they can be divided into voltage type inverters and current type inverters; based on functional use, they can be divided into general-purpose inverters and special-purpose inverters; based on voltage levels, they can be divided into low-voltage inverters, medium-voltage inverters and high-voltage inverters.

Low voltage inverter

In fact, the so-called high voltage and low voltage are relative terms, and different applications have different definitions. As far as the inverter is concerned, my country's low-voltage inverter usually refers to an AC inverter with a rated voltage of less than 690V. The structure of the low-voltage inverter usually adopts a voltage-type structure as shown in the figure below, in which the rectifier part adopts diode uncontrolled rectification, the intermediate filter link is capacitor filtering, and the inverter part adopts a three-phase voltage-type PWM inverter. Since the diode uncontrolled rectification is used as the input power supply, when the motor is in the regenerative braking state, the energy fed back by the load cannot be transmitted to the AC power grid, but can only be stored in the DC side capacitor, which increases the capacitor voltage.