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Introduction to Inverter Repair -- Circuit Analysis Diagrams For repairing inverters, it is far from sufficient to only understand the basic circuits mentioned above; a thorough understanding of the following key circuits is also necessary. The main circuit is primarily composed of a rectifier circuit, a current-limiting circuit, a filtering circuit, a braking circuit, an inverter circuit, and a detection and sampling circuit. Figure 2.1 is its structure diagram. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image002.jpg 1) Drive circuit: The drive circuit takes the six PWM signals generated by the CPU in the control circuit, isolates and amplifies them optically, and then provides drive signals for the switching devices (inverter modules) of the inverter circuit. Yes, the various requirements for the drive circuit vary depending on the type of converter device. At the same time, some developers have developed many dedicated drive modules suitable for various converter devices. Some brands and models of frequency converters use dedicated drive modules directly. However, most frequency converters use drive circuits. From a repair perspective, a typical drive circuit is introduced here. Figure 2.2 shows a more common driving circuit (the power supply for the driving circuit is shown in Figure 2.3). file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image004.jpg file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image006.jpg The drive circuit consists of an isolation amplification circuit, a drive amplification circuit, and a power supply for the drive circuit. The three upper bridge arm drive circuits are three independent drive power supply circuits, while the three lower bridge arm drive circuits share one common drive power supply circuit. 2) Protection circuit: When an abnormality occurs in the inverter, this circuit is used to minimize, or even eliminate, the losses caused by such abnormalities. Inverter manufacturers of every brand place great emphasis on protection functions, striving to add more of them and improve their effectiveness. In the field of inverter protection functions, manufacturers go to great lengths to develop effective solutions. Thus, the diversity and complexity of inverter protection circuits are created. It has conventional detection and protection circuits, as well as software-based comprehensive protection functions. Some frequency converters have protection functions built into their drive circuit modules, smart power modules, rectifier-inverter combination modules, and so on. The circuit shown in Figure 2.4 is a typical overcurrent detection protection circuit. It consists of three parts: current sampling, signal isolation amplification, and signal amplification output. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image008.jpg 3) Switching power supply circuit: The switching power supply circuit provides low-voltage power to the operation panel, main control board, drive circuits, and fans, among other components. Figure 2.5 Schematic diagram of the circuit composition of the Fuji G11 switching power supply. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image010.jpg The DC high-voltage terminal P is connected to the primary side of the high-frequency pulse transformer; the switching control transistor is connected in series with the other primary side of the pulse transformer, which is then connected to the DC high-voltage terminal N. The switching transistor turns on and off periodically, converting the primary DC voltage into a rectangular wave. It is coupled to the secondary side via a pulse transformer, and after rectification and filtering, the corresponding DC output voltage is obtained. It also samples and compares the output voltage in order to control the pulse width modulation circuit, thereby changing the pulse width and stabilizing the output voltage. 4) Communication circuit on the main control board: When the frequency converter is controlled by a programmable logic controller (PLC), a host computer, a human-machine interface, etc., signals must be transmitted to each other through communication interfaces. Figure 2.6 shows the communication interface circuit of the LG inverter. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image012.jpg When communicating via frequency modulators, a two-wire RS485 interface is typically used. The same is true for Siemens frequency converters. The two lines are used for transmitting and receiving signals respectively. Before transmitting the signal after receiving it, the inverter passes both types of signals through integrated circuits such as buffer A1701 and 75176B, in order to ensure good communication quality. Therefore, the communication interface circuit on the inverter’s main control board refers mainly to this part of the circuit, as well as the circuitry for reducing signal interference. 5) External control circuit: The external control circuit of the frequency converter refers mainly to the voltage input for frequency setting, the current input for frequency setting, controls for forward rotation, reverse rotation, jog operation, and shutdown, as well as multi-speed control. The frequency setting voltage (current) input signal enters the CPU via the A/D conversion circuit inside the inverter. Some other controls are transmitted to the CPU through optocoupler isolation in the input circuit of the frequency converter.