Main circuit
The main circuit is the power conversion part that provides voltage regulation and frequency regulation power supply for asynchronous motor, and the main circuit of the frequency converter can be roughly divided into two categories: the voltage type is the frequency converter that converts the DC of the voltage source into AC, and the filter of the DC loop is the capacitor. The current type is a frequency converter that converts the DC of the current source into AC, and its DC loop filtering is inductor. It consists of three parts: a "rectifier" that converts power frequency power into DC power, a "flat wave loop" that absorbs voltage pulsations generated by the converter and inverter, and an "inverter" that converts DC power into AC power.
Rectifier
A large number of converters use diodes, which convert power frequency power to DC power. It is also possible to form a reversible converter with two sets of transistor converters, which can be regenerated due to its reversible power direction.
Flat wave circuit
The rectified DC voltage of the rectifier contains a pulsation voltage that is 6 times the frequency of the power supply, and the pulsating current generated by the inverter also causes the DC voltage to fluctuate. To suppress voltage fluctuations, inductors and capacitors are used to absorb pulsating voltages (currents). The device capacity is small, and if there is a margin between the power supply and the main circuit component device, a simple flat wave loop can be used without the inductor.
Inverter
In contrast to the rectifier, the inverter converts the DC power into AC power at the required frequency, and turns the 6 switching devices on and off at a determined time to obtain a 3-phase AC output. Using a voltage-type PWM inverter as an example, the switching time and voltage waveform are shown.
The control circuit is a circuit that provides control signals to the main circuit of asynchronous motor power supply (voltage, frequency adjustable), which has frequency and voltage "operation circuit", the main circuit "voltage, current detection circuit", motor "speed detection circuit", the "drive circuit" that amplifies the control signal of the operation circuit, and the "protection circuit" of the inverter and motor.
(1) Arithmetic circuit: compare the external speed, torque and other instructions 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 loop potential to detect voltage, current, etc.
(3) Drive circuit: the circuit that drives the main circuit device. It is isolated from the control circuit, so that the main circuit device is turned on and off.
(4) Speed detection circuit: the signal of the speed detector (TG, PLG, etc.) installed on the asynchronous motor shaft machine is the speed signal, sent to the operation loop, and the motor can run at the command speed according to the instructions and calculations.
(5) Protection circuit: detect the voltage, current, etc. of the main circuit, and when an abnormality such as overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor.