A frequency converter mainly consists of a rectifier (AC to DC), a filter, an inverter (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit.
1. Rectifier
Recently, diode converters have been widely used to convert mains frequency power into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can operate regeneratively due to its reversible power direction.
2. Smoothing circuit
The DC voltage after rectification contains a pulsating voltage at six times the frequency of the power supply. Furthermore, the pulsating current generated by the inverter also causes DC voltage fluctuations. To suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). For small device capacities, if the power supply and main circuit components have sufficient capacity, the inductor can be omitted, and a simple smoothing circuit can be used.
3. Inverter
Unlike rectifiers, inverters convert DC power into AC power at the required frequency. By turning 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 using a voltage-source PWM inverter as an example.
4. Main circuit and control circuit
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, which convert DC voltage to AC, and current-source converters, which convert DC current to AC, and current-source converters, which use inductors for DC circuit filtering.
It consists of three parts: a "rectifier" that converts power frequency into DC power, a "smoothing circuit" that absorbs voltage ripples generated in the converter and inverter, and an "inverter" that converts DC power into AC power.
The control circuit is a loop that provides control signals to the main circuit that supplies power (voltage and frequency are adjustable) to the asynchronous motor. It consists of a frequency and voltage "operation circuit", a main circuit "voltage and current detection circuit", a motor "speed detection circuit", a "drive circuit" that amplifies the control signals from the operation circuit, and an inverter and motor "protection circuit".
(1) Operational circuit: It compares and calculates external speed, torque and other commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) Voltage and current detection circuit: isolated from the main circuit potential to detect voltage, current, etc.
(3) Drive circuit: The circuit that drives the main circuit devices. It is isolated from the control circuit to turn the main circuit devices on and off.
(4) Speed detection circuit: The speed signal is sent to the operation circuit using the speed detector (tg, plg, etc.) mounted on the asynchronous motor shaft. According to the instructions and operations, the motor can run at the commanded speed.
(5) Protection circuit: detects the voltage and current of the main circuit. When an overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor, the inverter stops working or suppresses the voltage and current values.
Inverters adjust the voltage and frequency of the output power supply by switching the internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, inverters have many protection functions, such as overcurrent, overvoltage, and overload protection.
