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Getting Started for Beginners - Analysis of Inverter Circuit Principles If you want to do a good job in inverter maintenance, of course it is very important to understand the basic knowledge of inverters, and it is also urgent. Let’s share the basic knowledge of inverter maintenance below. After everyone has read it, if there are any mistakes, please correct me. If you think it is okay, support it and give me some encouragement! Introduction to Frequency Converter Maintenance - Circuit Analysis Diagram For frequency converter repair, it is not enough to only understand the above basic circuits. It is also necessary to have a deep understanding of the following main circuits. The main circuit is mainly composed of a rectifier circuit, a current limiting circuit, a filter 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_image001.jpg 1) Drive circuit The drive circuit is to photoelectrically isolate and amplify the six PWM signals generated by the CPU in the main control circuit, and then provide drive signals as the commutation device (inverter module) of the inverter circuit. The various requirements for the drive circuit vary depending on the commutation device. At the same time, some developers have developed many special drive modules suitable for various commutation devices. Some brands and models of inverters directly use dedicated drive modules. However, most frequency converters use drive circuits. From the perspective of repair, here is an introduction to the more typical drive circuit. Figure 2.2 is a more common drive circuit (see Figure 2.3 for drive circuit power supply). file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image002.jpg file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image003.jpg The drive circuit consists of an isolation amplifier circuit, a drive amplifier circuit and a drive circuit power supply. The three upper-side drive circuits are three independent drive power circuits, and the three lower-side drive circuits are a common drive power circuit. 2) Protection circuit When an abnormality occurs in the frequency converter, in order to minimize the loss caused by the abnormality of the frequency converter, or even reduce it to zero. Every 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 functions, manufacturers have tried their best to make good articles. In this way, the diversity and complexity of the inverter protection circuit are formed. There are conventional detection and protection circuits and software comprehensive protection functions. Some inverter drive circuit modules, intelligent power modules, rectifier and inverter combination modules, etc., all have internal protection functions. 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_image004.jpg 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 2.5 Structural diagram of Fuji G11 switching power supply circuit. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image005.jpg The DC high-voltage P terminal is added to the primary end of the high-frequency pulse transformer, and the switching regulator tube is connected in series to the other primary end of the pulse transformer, and then connected to the DC high-voltage N terminal. The switching tube is turned on and off periodically, changing the primary DC voltage into a rectangular wave. It is coupled to the secondary by the pulse transformer, and then after rectification and filtering, the corresponding DC output voltage is obtained. It also 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 controller (PLC) or host computer, human-machine interface, etc., signals must be transmitted to each other through the communication interface. Figure 2.6 is the communication interface circuit of LG inverter. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image006.jpg When communicating with the frequency converter, the two-wire RS485 interface is usually used. The same goes for Siemens inverters. The two lines are used to transmit and receive signals respectively. After the frequency converter 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 main control board of the frequency converter mainly refers to this part of the circuit, as well as the signal anti-interference circuit. 5) External control circuit The external control circuit of the inverter mainly refers to frequency setting voltage input, frequency setting current input, forward rotation, reverse rotation, jog and stop operation control, and multi-speed speed control. The frequency setting voltage (current) input signal enters the CPU through the A/D conversion circuit in the frequency converter. Some other controls are passed to the CPU through the optocoupler isolation of the input circuit in the frequency converter. In the following article, I have uploaded the maintenance knowledge about frequency converters for everyone to share! Based on your suggestions and support for me, I will now contribute some of the most basic and basic knowledge about frequency converters to everyone. Frequency converter switching power supply circuit Frequency converter switching power supply mainly includes input grid filter, input rectification filter, converter, output rectification filter, control circuit and protection circuit. The switching power supply circuit of our company's product is as shown below, which is a switching circuit composed of UC3844: The switching power supply mainly has the following characteristics: 1. Small size and light weight: Since there is no power frequency converter, the volume and weight are 20~30% of the linear power supply 2. Small power consumption and high efficiency: The power transistor works in a switching state, so the upper power consumption of the transistor is small and the conversion efficiency is high, generally 60~70%, while the linear power supply is only 30~40% file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image007.jpg Diode limiting circuit limiter is an op amp circuit with nonlinear voltage transmission characteristics. Its characteristics are: When the input signal voltage is within a certain range, the circuit is in a linear amplification state with a constant amplification factor. Beyond this range, it enters a nonlinear region and the amplification factor is close to zero or very low. The requirements for inverter circuit design are also very high. To be a good inverter maintenance technician, it is also very important to understand it. 1. The diode parallel limiter circuit diagram is shown below: file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image008.jpg 2. The diode series limiting circuit is shown in the figure below: file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image009.jpg The composition of the inverter control circuit is shown in Figure 1. The control circuit consists of the following circuits: Frequency and voltage calculation circuit, main circuit voltage and current detection circuit, motor speed detection circuit, drive circuit that amplifies the control signal of the calculation circuit, and protection circuit for the inverter and motor. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image010.jpg Within the dotted line in Figure 1, the non-speed detection circuit is open-loop control. A speed detection circuit is added to the control circuit, that is, a speed command is added, which can control the speed of the asynchronous motor for more precise closed-loop control. 1) The operation circuit compares 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) The voltage and current detection circuit is electrically isolated from the main circuit to detect voltage, current, etc. 3) The drive circuit is a circuit that drives the main circuit components. It is isolated from the control circuit and turns the main circuit components on and off. 4) I/0 input and output circuit In order to facilitate the human-computer interaction of the frequency converter, the frequency converter has a variety of input signals (such as operation, multi-speed operation, etc.), as well as various internal parameter output signals (such as current, frequency, protection action drive, etc.). 5) The speed detection circuit uses the signal of the speed detector (TG, PLG, etc.) installed on the asynchronous electric shaft machine as the speed signal and sends it to the calculation loop. According to the instructions and calculations, the motor can run at the command speed. 6) The protection circuit detects the voltage, current, etc. of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent the inverter and asynchronous motor from being damaged, the inverter stops working or the voltage and current values are suppressed. The protection circuit in the inverter control circuit can be divided into two types: inverter protection and asynchronous motor protection. The protection functions are as follows. HCPL-316J characteristics of the inverter drive circuit. HCPL-316J is an IGBT gate drive optocoupler produced by Agilent. It integrates a collector-emitter voltage undersaturation detection circuit and a fault status feedback circuit, which provides guarantee for the reliable operation of the drive circuit. Its characteristics are: Compatible with CMOS/TYL level ； Optical isolation, fault status feedback ； Switching time maximum 500ns ； “Soft” IGBT turn-off ； Under-saturation detection and under-voltage lockout protection ； Overcurrent protection function ； Wide operating voltage range (15～30V) ； User configurable automatic reset, automatic shutdown. The DSP is combined with the coupler to drive the IGBT, making the IGBT VCE undersaturation detection compact, low-cost and easy to implement, while meeting a wide range of safety and regulation needs. Implementation of HCPL-316J protection function HCPL-316J has built-in rich IGBT detection and protection functions, making the drive circuit design more convenient, safe and reliable. The following details the working principles of undervoltage lockout protection (UVLO) and overcurrent protection.: (1) IGBT undervoltage lockout protection (UVLO) function. During the power-on process, the chip supply voltage gradually rises from 0V to the maximum value. If the chip has an output at this time, which will cause the IGBT gate voltage to be too low, then it will work in the linear amplification zone. The undervoltage lockout protection function (UVLO) of the HCPL316J chip can solve this problem. When the voltage value between VCC and VE is less than 12V, the output is low level to prevent the IGBT from overheating and burning due to operating in the linear operating area. For a detailed schematic diagram, see the UVLO part in Figure 1. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image011.jpg Figure 1 Internal schematic diagram of HCPL-316J (2) IGBT over-current protection function HCPL-316J has an over-current protection function for IGBT. It implements protection actions by detecting the conduction voltage drop of IGBT. It can also be seen from the figure that there is a fixed 7V level inside it. When the detection circuit is working, it compares the detected voltage drop across the C to E poles of the IGBT with the built-in 7V level. When it exceeds 7V, the HCPL-316J chip outputs a low level to turn off the IGBT. At the same time, an error detection signal is fed back to the input side through the on-chip optocoupler so that corresponding solutions can be taken. When the IGBT is turned off, the voltage across its C to E poles must exceed 7V, but at this time, the overcurrent detection circuit fails and the HCPL-316J chip will not report a fault signal. In fact, due to the tube voltage drop of the diode, the chip takes protective action when the voltage between the C and E electrodes of the IGBT is less than 7V. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image012.jpg The entire circuit board functions as an optocoupler isolation amplifier circuit. Its core part is the chip HCPL-316J, in which XPWM1 and XCLEAR are generated by the controller (DSP-TMS320F2812) * The signal is output to HCPL-316J, and at the same time the IGBT fault signal FAULT generated by HCPL-316J * to the controller. At the same time, a push-pull output circuit composed of NPN and PNP is connected to the output end of the chip in order to improve the output current capability and match the IGBT driving requirements. When the output terminal VOUT of HCPL-316J is high level, the upper tube (T1) of the push-pull circuit is turned on and the lower tube (T2) is turned off. The output terminal of the three-terminal voltage regulator block LM7915 is added to the IGBT gate (VG1). The IGBT VCE is 15V and the IGBT is turned on. When the HCPL-316J output terminal VOUT output is low level, the upper tube (T1) is turned off, the lower tube (T1) is turned on, VCE is -9V, and the IGBT is turned off. The above is the turning on and off process of IGBT.