1. Introduction Over the past decade or so, with the penetration of power electronics, microelectronics, and modern control theory into the field of AC electric drives, variable frequency AC speed control has gradually replaced the old slip-ring speed control, pole-changing speed control, and DC speed control systems. It can almost be said that wherever there are AC motors, there are frequency converters in use. Their most important characteristics are high-efficiency drive performance and good control characteristics. A typical general-purpose frequency converter generally includes the following components: rectifier bridge, inverter bridge, intermediate DC circuit, pre-charge circuit, control circuit, and drive circuit. The drive circuit plays a crucial role in the quality of a frequency converter. This article will discuss common problems with drive circuits and their solutions. "Drive circuit" is a general term. With the continuous development of technology, drive circuits themselves have evolved from pin-type drive circuits to optocoupler drive circuits, then to thick-film drive circuits, and the newer integrated drive circuits. The latter three types of drive circuits mentioned above are still frequently encountered in maintenance. 2. Repair methods for several drive circuits (1) Causes and inspection of drive circuit damage There are various causes of drive circuit damage. Generally speaking, the problems are nothing more than no output of the three phases U, V, and W, or unbalanced output, or balanced output but jittering at low frequency, as well as start-up alarms, etc. When the fast fuse after the large capacitor of a frequency converter is open, or the IGBT inverter module is damaged, the drive circuit is basically impossible to be intact. Do not replace it with a good fast fuse or IGBT inverter module, as this can easily cause the newly replaced good device to be damaged again. At this point, you should focus on checking for arcing marks on the drive circuit. You can first disconnect the drive pins of the IGBT inverter module and use a multimeter in resistance mode to measure whether the resistance values ​​of the six drive circuits are the same (however, in a few cases, the resistance values ​​of the six drive circuits are not the same: such as Mitsubishi and Fuji inverters). If the resistance values ​​of the six circuits are basically the same, it cannot completely prove that the drive circuit is intact. Next, you need to use an oscilloscope to measure whether the voltage on the six drive circuits is the same and whether the waveforms of the six drive circuits are consistent when a start signal is given. If you do not have an oscilloscope, you can also try using a digital multimeter to measure the DC voltage of the six drive circuits. Generally speaking, the DC voltage of each drive circuit is about 10V when not started and about 2-3V after starting. If the measurement results are all normal, it can be basically determined that the drive circuit of this inverter is good. Next, connect the IGBT inverter module to the drive circuit. However, remember that if you are not 100% sure, the safest method is to disconnect the P of the IGBT inverter module from the DC bus and connect a series of light bulbs or a resistor with a higher power in between. This will protect the IGBT inverter module from being burned by the discharge current of the large capacitor when a large current occurs in the circuit. Here are a few examples related to the drive circuit when repairing the inverter: (2) Yaskawa 616G5, 3.7kW inverter. The fault phenomenon of the Yaskawa 616G5, 3.7kW inverter is that the three-phase output is normal, but the motor shakes at low speed and cannot operate normally. First, it is estimated that most of the problems are due to damage to the inverter drive circuit. The correct solution is to open the inverter after confirming the fault phenomenon, remove the IGBT inverter module from the printed circuit board, and use an electronic oscilloscope to observe whether the waveforms of the six drive circuits are consistent when they are turned on. Find the drive circuit that is inconsistent and replace the optocoupler on that drive circuit. Generally, it is PC923 or PC929. If the inverter has been used for more than 3 years, it is recommended to replace all the electrolytic capacitors in the drive circuit. Then observe with an oscilloscope. After the six waveforms are consistent, install the IGBT inverter module and perform a load test. The jitter phenomenon is eliminated. (3) Fuji G9 inverter. The fault of the Fuji G9 inverter is that there is no display when it is powered on. It is estimated that the inverter switching power supply is damaged. Open the inverter and check the switching power supply line. However, after checking, there is no damage to the switching power supply components and lines. There is no display of DC voltage at the DC positive and negative terminals. At this time, it is estimated that there may be a problem with the drive. Remove all the capacitors in the drive circuit. It was found that some capacitors were leaking. Replace them with new electrolytic capacitors. After powering on again, it works normally. (4) Delta inverter. The fault phenomenon is arcing at the output of the inverter. After disassembly and inspection, it was found that the IGBT inverter module was shorted and the printed circuit board of the drive circuit was severely damaged. The correct solution is to first remove the damaged IGBT inverter module. When disassembling, the main thing is to protect the printed circuit board from secondary damage caused by human error. Replace the damaged electronic components on the drive circuit one by one and connect the open circuits on the printed circuit board with wires (note that the burnt parts should be scraped clean to prevent arcing again). Then, under the condition that the resistance and voltage of the six drive circuits are the same, use an oscilloscope to measure the waveform. However, as soon as the inverter is turned on, it reports an OCC fault (the Delta inverter will alarm when it is turned on without the IGBT inverter module). Use a light bulb to connect the P1 of the module to the printed circuit board, and connect the others with wires. It still trips OCC when restarted. It is determined that there is still a problem with the drive circuit. Replace the optocouplers one by one. It was found that the optocouplers of the drive circuit have a detection function. One of the optocouplers has a damaged detection function. After replacing it with a new one, the start is normal. 3. Conclusion With the continuous development of frequency converters, the technology of frequency converter drive circuits is also advancing rapidly. What has been covered here is only a small part. It is hoped that this will be helpful to the majority of technicians and frequency converter enthusiasts. It is also hoped that frequency converter practitioners can exchange ideas more often so that everyone's technology can be further improved.