Overview of working principle:

The main circuit is the power conversion part that provides voltage and frequency regulation power supply to the asynchronous motor. 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 to 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 to AC, and the DC loop filter is an inductance.

It consists of three parts, a "rectifier" that converts power frequency power to DC power, and a "flat wave loop rectifier" that absorbs the voltage pulsation generated by the converter and inverter uses a large number of diode converters. The power frequency power supply is converted into a DC power supply. Two sets of transistor converters can also be used to form an inverter, which can be regenerated due to its reversible power direction.

The smoothing circuit contains the pulsating voltage of 6 times the frequency of the power supply in the DC voltage rectified by the rectifier. In addition, the pulsating current generated by the inverter also fluctuates the DC voltage. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). When the capacity of the device is small, if the power supply and the main circuit constitute a device with a margin, the inductor can be omitted and a simple smooth wave circuit can be used.

Inverter is the opposite of rectifier. Inverter converts DC power to AC power of the required frequency, and turns on and off the 6 switching devices at the determined time to get 3-phase AC output. Taking the 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 that supplies power to the asynchronous motor (adjustable voltage and frequency). It has a frequency and voltage "operation circuit", a "voltage and current detection circuit" of the main circuit, and a "speed detection" of the motor. "Circuit" is composed of a "driving circuit" that amplifies the control signal of the arithmetic circuit, and a "protection circuit" of the inverter and the motor.

(1) Calculation circuit: compare the 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: It is isolated from the main circuit potential to detect voltage and current.

(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 used as the speed signal, which is sent to the calculation circuit, and the motor can be operated at the command speed according to the command and calculation.

(5) Protection circuit: Detect the voltage, current, etc. of the main circuit. When an overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor.