The inverter is composed of a main circuit, a rectifier, a flat wave circuit, and an inverter.
1) Main circuit:
The main circuit is the power conversion part that provides voltage regulation and frequency modulation power supply for the asynchronous motor, and the main circuit of the inverter can be roughly divided into two categories: the voltage type is the inverter that converts the DC of the voltage source into AC, and the filter of the DC loop is the capacitor. The current type is an inverter that converts the DC of the current source into AC, and its DC loop filter is an inductor. It consists of three parts: a "rectifier" that converts the power supply into DC power, a "flat wave circuit" that absorbs the voltage ripples generated by the converter and inverter, and an "inverter" that converts DC power into AC power.
2) Rectifier:
A large number of diode converters are used, which convert the power frequency power supply into DC power supply. It is also possible to form a reversible converter with two sets of transistor converters, which can be regenerated due to the reversible power direction.
3) Flat wave circuit
The rectified DC voltage of the rectifier contains a pulsating 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. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb pulsating voltages (currents). If the capacity of the device is small, if there is a margin between the power supply and the main circuit components, a simple flat-wave circuit can be used without inductance.
4) Inverter
Contrary to the rectifier, the inverter converts the DC power into the AC power of the required frequency, and turns on and off the 6 switching devices at a determined time to obtain a 3-phase AC output. Take a voltage-type PWM inverter as an example to show the switching time and voltage waveform.
The control circuit is a circuit that provides control signals for the main circuit of the asynchronous motor power supply (voltage and frequency adjustable), which is composed of the "operation circuit" of frequency and voltage, the "voltage and current detection circuit" of the main circuit, the "speed detection circuit" of the motor, the "driving circuit" that amplifies the control signal of the operation circuit, and the "protection circuit" of the inverter and the motor.
(1) Calculation 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 circuit potential to detect voltage, current, etc.
(3) Drive circuit: the circuit that drives the main circuit device. It is isolated from the control circuit to make the main circuit device on and off.
(4) Speed detection circuit: the signal of the speed detector (TG, PLG, etc.) installed on the shaft of the asynchronous motor is the speed signal, which is sent to the operation loop, and the motor can be made to run according to the command speed according to the instruction and operation.
(5) Protection circuit: detect the voltage, current, etc. of the main circuit, and prevent damage to the inverter and asynchronous motor when overload or overvoltage and other abnormalities occur.
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