[b]1 Introduction[/b] In the 1980s and 1990s, frequency converters (VDCs) first entered the Chinese market and have since become widely accepted and applied in metallurgy, textiles, dyeing and printing, tobacco production lines, buildings, water supply, and other fields. Because VDCs offer numerous advantages over traditional speed control methods in electrical drive regulation, such as energy saving, equipment miniaturization, and improved comfort (e.g., elevators, trams), they can achieve automated control, improve control precision, enhance product quality, increase production efficiency, and improve product qualification rates. VDCs will be increasingly widely used in my country in the future. As electronic products, VDCs theoretically have a design lifespan. In practical applications, they sometimes trigger alarms and malfunctions. Their failure probability is shown in Figure 1. Figure 1: Schematic diagram of VDC failures. In practical applications, the failure rate of VDCs is also related to proper use, maintenance, and the operating environment. Figure 1 clearly shows the promising future of VDC repair. Unbalanced output is a common and typical fault in VDCs, which will be briefly discussed here for further discussion among colleagues. 2. Basic Working Principle of Inverter in Frequency Converter The quality of the three-phase (U, V, W) AC output frequency waveform and the degree of voltage balance of the frequency converter directly affect the speed regulation operation and lifespan of the asynchronous motor, and more importantly, the lifespan of the frequency converter. For a repaired frequency converter, the minimum requirement is that the U, V, and W three-phase AC output waveforms meet the requirements and the voltage is balanced. Typically, a frequency converter mainly consists of: a main circuit composed of IGBTs or GTOs and other power switching devices, forming an inverter that provides voltage and frequency regulation power to the asynchronous motor. The voltage, current, and frequency output of this power supply are controlled by control commands from the control loop. These control commands are obtained by calculating external operating commands. For applications requiring precise speed control or rapid response, the calculation should also include signals detected by the frequency converter's main circuit and the drive system for closed-loop control. The protection circuit, in addition to preventing faults caused by overvoltage protection, overcurrent protection, and overheat protection of the main circuit, should also protect the asynchronous motor and drive system, etc. Therefore, inverter faults directly affecting the U, V, and W output main circuit are crucial. Inverters, unlike rectifiers, convert DC power into AC power at the desired frequency by switching the six power switching devices on and off at predetermined times, as shown in Figure 2. Figure 2: Schematic diagram of an inverter. In Figure 2, S1 to S6 form a bridge inverter circuit. The alternating switching of these six switches produces three-phase AC voltages with a phase difference of 2/3π across the U, V, and W phases at the output. Therefore, the voltage waveforms of S1 to S6 in the drive circuit are consistent when they are on and off, which is particularly important for output voltage balance. Figure 3 shows a typical IGBT gate drive circuit encountered during inverter repair. Figure 3: Typical gate drive circuit. When the gate drive circuit is turned on, it outputs a positive gate voltage of 15V. This value is sufficient to fully saturate the IGBT and minimize its conduction losses, while also limiting the short-circuit current and the power stress it causes. When the gate voltage is zero, the IGBT is in the off state to ensure that the IGBT remains off when there is dv/dt noise in the collector-emitter voltage. A turn-off bias voltage needs to be applied to the gate. Using a reverse bias voltage can also reduce turn-off losses. The reverse bias voltage of the H series IGBT is in the range of -5V to 15V. 3 Inverter output imbalance and countermeasures In actual maintenance, the U, V, W output imbalance can be divided into three situations: (1) The inverter display shows (MISSMG MOTO PHASE) output phase loss. If the detection circuit fault is ruled out, the conclusion is that the IGBT module and drive circuit are damaged by direct inspection. The drive circuit also has a problem. The problem can be basically solved by replacing the components on the IGBT module and drive circuit, such as optocouplers, a pair of PNP and NPN drive transistors, electrolytic capacitors, Zener diodes, etc. (2) The phase difference between the outputs U, V, and W of the inverter is about 100V (taking 380V as an example). In the drive circuit, there is no drive voltage or drive signal waveform in one of the drive circuits between S1 and S6. This can be determined by measuring the DC voltage between the output terminals U, V, W and P. (3) The DC voltage between U, V, W and N can be used to find that the drive voltage is abnormal or there is no drive signal waveform. This causes a phase difference caused by one of the phases U, V, and W not working properly. The solution is to check whether the drive circuit voltage is normal, whether the optocoupler is broken, and whether the electrolytic capacitor is leaking. If the waveform of the 6 channels is measured by an oscilloscope and meets the technical requirements, the problem can be solved. Another phenomenon is that the phase difference between the three-phase output AC voltages U, V, and W of the inverter is greater than 3%. Although it can be used, it cannot be used for a long time or under heavy load. This is mainly due to the asymmetry of key components in the drive circuits S1 to S6, such as the technical parameters of transistors, Zener diodes, capacitor depletion, leakage, and current leakage. The wear and tear on the components in the six drive circuits causes some differences in their parameters, resulting in a slight potential difference between the inverter's outputs U, V, and W. While the above situation may be usable, it is technically unacceptable. Our company strives for excellence, screening and aging various components, ensuring that transistor and Zener diode technical parameters are consistent and matched, etc., to guarantee that the drive signals in the drive circuit meet technical requirements. We also ensure that the IGBT module saturates and the conduction time is consistent, guaranteed by the quality of the components. When the repaired inverter is tested under load, the motor runs with a lighter sound. Before and after repair, when running a motor and load of the same power, the three-phase current of the latter motor is significantly lower. 4. Conclusion Three-phase imbalance at the inverter output is a typical and common fault, but in practice, various complex problems may be encountered. We hope to exchange ideas and better serve our customers. [b][align=center]For details, please click: A Brief Discussion on Unbalanced Output of Frequency Converters[/align][/b]