Frequency converter

Introduction

The frequency conversion speed regulation essentially provides a frequency controllable power supply to the AC asynchronous motor, and the device that can realize this function is called a frequency converter.

classification

The frequency converter in the variable frequency speed control system can be generally divided into two types: AC-AC inverter and AC-DC-AC inverter. The AC-AC inverter is also called a direct inverter or a cycloconverter. It directly converts the AC power of the grid into an AC output with adjustable voltage and frequency.

The AC-DC-AC inverter is also called the indirect inverter. It converts the AC power into a variable-amplitude DC voltage (rectifier) ​​through the controllable rectifier, and then changes the DC voltage to the frequency through the inverter. AC output (inverter). The AC-DC-AC inverter has already formed a general-purpose product and is widely used.

The AC-DC-AC inverter can be divided into two types: current type and voltage type. The intermediate DC link of the current type inverter uses an inductance element as a filter element. The outstanding advantage of this type of frequency converter is that it can easily return electrical energy to the AC grid when the motor is in a regenerative state. Its disadvantage is that the filtering of the AC component of the voltage output by the inductor component to the rectifier is greatly affected by the load. This type of frequency converter is mainly used in large-capacity transmissions with frequent acceleration and deceleration. At present, the application surface is not as wide as the voltage type.

The intermediate DC link of the voltage type inverter uses a capacitor element as a filter element. This structure can obtain a smooth DC voltage and is supplied to the inverter. This structure is less affected by the load and achieves good performance from no load to full load. Its disadvantage is that when the motor is in the regenerative power generation state, the electric energy fed back to the DC side is difficult to feed back to the AC grid, and must be solved by using corresponding circuits.

basic component

The frequency converter is composed of a rectifier, an intermediate DC link, an inverter, and a control circuit. Among them, the intermediate DC link is responsible for filtering out the AC component of the rectifier output, and obtaining pure DC power to the inverter. The control circuit is composed of detection circuit, signal input, signal output, power tube drive, calculation of various control signals, etc. At present, this part of the link is mainly completed by a microcomputer control system composed of high-performance microcomputer and peripheral equipment.