The main circuit of an AC-DC-AC frequency converter consists of four parts: 1. Main circuit; 2. Control circuit; 3. External terminals; and 4. Operation panel.
1. Main Circuit: This is the power conversion section that provides voltage and frequency adjustable power to the asynchronous motor. The main circuit of a frequency converter can be broadly divided into two categories: voltage-source converters, which convert DC from a voltage source to AC, use capacitors for DC circuit filtering; and current-source converters, which convert DC from a current source to AC, use inductors for DC circuit filtering, including:
(1) Rectifier circuit: used to rectify three-phase alternating current into direct current;
(2) Filter circuit: Used to transform the pulsating DC after rectification into a smoother DC through energy storage elements. The filter circuit can also improve the power factor.
(3) Inverter circuit: used to invert DC power into AC power. The most common type is a three-phase bridge inverter circuit composed of 6 inverter modules. The CPU controls the switching on and off of the inverter, which can output three-phase AC power of any frequency.
2. Control circuit: It consists of arithmetic circuits, detection circuits, input/output circuits for control signals, and drive circuits, etc.
3. External terminals: Three-phase power supply terminals for the main circuit, motor terminals, DC reactor terminals, braking unit terminals, and braking resistor terminals;
4. Operation panel: The operation panel is used to set the control functions, parameters and frequency settings of the frequency converter.
An AC-DC-AC current-source inverter refers to an inverter with a smoothing reactor connected in series on the DC side, ensuring a DC level that forms a current source. This facilitates the feedback of load energy to the grid, enables rapid and frequent four-quadrant operation, and allows for closed-loop current control, improving the reliability of the device. It is suitable for single-unit rapid speed control systems. An inverter converts constant-voltage, constant-frequency AC power into adjustable DC power, providing DC power to the inverter through voltage-source or current-source filters. The inverter then converts the DC power into AC power with an adjustable frequency.
