Inverter circuit structure and basic circuit analysis

The inverter circuit structure is mainly composed of rectifier circuit, current limiting circuit, filter circuit, brake circuit, inverter circuit and detection sampling circuit.
1) Drive circuit
The drive circuit is to provide drive signals to the inverter device (inverter module) of the inverter circuit after the six PWM signals generated by the CPU in the main control circuit are photoelectrically isolated and amplified.
The various requirements for the drive circuit vary depending on the inverter device. At the same time, some developers have developed many dedicated drive modules suitable for various inverter devices. Some brands and models of inverters directly use dedicated drive modules. However, most inverters use drive circuits. From the perspective of repair, a more typical drive circuit is introduced here. Figure 1 is a more common drive circuit (see Figure 2 for the drive circuit power supply). The drive circuit consists of an isolation amplifier circuit, a drive amplifier circuit and a drive circuit power supply. The three upper bridge arm drive circuits are three independent drive power supply circuits, and the three lower bridge arm drive circuits are a common drive power supply circuit.
2) Protection circuit
When the inverter is abnormal, in order to minimize the loss caused by the abnormality, or even reduce it to zero. Each brand of inverter attaches great importance to the protection function, and tries to increase the protection function and improve the effectiveness of the protection function.
In the field of inverter protection function, manufacturers can be said to have done their best to make good articles. In this way, the diversity and complexity of the inverter protection circuit are formed. There are conventional detection protection circuits and software integrated protection functions. Some inverter drive circuit modules, intelligent power modules, rectifier inverter combination modules, etc. have internal protection functions.
The circuit shown in Figure 3 is a typical overcurrent detection protection circuit. It consists of three parts: current sampling, signal isolation and amplification, and signal amplification output.
3) Switching power supply circuit The
switching power supply circuit provides low-voltage power to the operation panel, main control board, drive circuit, fan and other circuits. Figure 4 is a structural diagram of the Fuji G11 switching power supply circuit.
The DC high voltage P end is added to the primary end of the high-frequency pulse transformer, and the switch adjustment tube is connected in series to the other primary end of the pulse transformer, and then connected to the DC high voltage N end. The switch tube is periodically turned on and off, so that the primary DC voltage is converted into a rectangular wave. It is coupled to the secondary by the pulse transformer, and then rectified and filtered to obtain the corresponding DC output voltage. It samples and compares the output voltage to control the pulse width modulation circuit to change the pulse width to stabilize the output voltage.
4) Communication circuit on the main control board
When the inverter is controlled by a programmable (PLC) or a host computer, a human-machine interface, etc., signals must be transmitted to each other through a communication interface. Figure 5 is the communication interface circuit of the LG inverter.
When the inverter communicates, a two-wire RS485 interface is usually used. The same is true for Siemens inverters. The two lines are used to transmit and receive signals respectively. After the inverter receives the signal and before transmitting the signal, both signals pass through integrated circuits such as buffers A1701 and 75176B to ensure good communication effects. Therefore, the communication interface circuit on the inverter main control board mainly refers to this part of the circuit, as well as the signal anti-interference circuit.
5) External control
circuit The inverter external control circuit mainly refers to the frequency setting voltage input, frequency setting current input, forward, reverse, jog and stop operation control, and multi-speed control. The frequency setting voltage (current) input signal enters the CPU through the A/D conversion circuit in the inverter. Some other controls are transmitted to the CPU through the optocoupler isolation of the input circuit in the inverter.