The main circuit of an AC-DC-AC frequency converter consists of three parts: the rectifier main circuit, the intermediate circuit, and the inverter circuit.
1. The main circuit 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 convert DC voltage to AC, with capacitors used for DC circuit filtering; and current-source converters convert DC current to AC, with inductors used for DC circuit filtering.
2. The intermediate circuit, a converter with rectifier diodes, transforms the mains frequency power supply into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can perform regenerative operation due to its reversible power direction.
4. Inverter: Unlike rectifiers, inverters convert DC power into AC power at the required frequency. By switching six switching devices on and off at predetermined times, a three-phase AC output can be obtained. The switching time and voltage waveform are shown as an example using a voltage-source PWM inverter.
An AC-AC inverter directly converts AC power from the power grid into AC power with adjustable voltage and frequency. It features a simple circuit design, high efficiency, and is economical for low-speed, high-capacity applications. Its maximum frequency is typically only 1/3 to 1/2 of the power supply frequency, making it suitable for low-frequency, high-capacity speed control systems. Unlike ordinary inverters, it does not have a step of rectifying AC to DC and then inverting it back to AC; it uses an AC-AC conversion structure. This technology is generally used in large-power devices.
AC-DC-AC frequency converter performance characteristics:
1. The main circuit rectifier transformer can be one of six double-winding rectifier transformers or two three-phase three-fracture structure transformers.
2. The motor and converter adopt a double Y-type connection structure to form a 12-pulse waveform, which greatly reduces harmonics. Through switching cabinet switching, each transformer and converter can operate independently at full load or half speed, greatly increasing applicability and flexibility.
3. The synchronous motor excitation adopts irreversible thyristor three-phase full-bridge control. In addition to a high-capacity varistor absorption circuit, the protection circuit is also equipped with a dedicated fast protection circuit for diode BOD detection and thyristor switching control.
