1 Introduction In the 80s and 90s of the last century, the inverter has just entered the Chinese market and is now widely accepted, and is used in metallurgy, textile, printing and dyeing, printing, hood production lines, and buildings, water supply and other fields. Because the frequency converter does have many advantages in terms of electric drive speed regulation than traditional speed regulation, such as energy saving, equipment miniaturization, and improved comfort, such as elevators, trams, etc., can achieve automatic control to improve control accuracy and improve product quality. In order to improve production efficiency and product qualification rate, inverters will be widely used in China in the future.
Because the frequency converter is an electronic product, it has a design life in theory. In actual applications, it sometimes has to be alarmed. The probability of failure is shown in Figure 1. 
Figure 1 Fault diagram of the inverter In actual application, the fault rate of the inverter is also related to the correct use, maintenance and use environment. It is not difficult to see the prospects of the inverter repair field from Figure 1. The unbalanced output of the inverter is a typical fault of the inverter. It is discussed here for the majority of peers.
2 Inverter basic working principle of inverter Inverter three-phase (uvw) AC output frequency waveform quality and voltage balance directly affect the state of asynchronous motor speed control and motor life, more importantly, affect the life of the inverter, After the maintenance of the inverter, the waveforms of the U, V, W three-phase AC output meet the requirements and the voltage balance is the most basic.
Usually, the inverter mainly has: main circuit IGBT, or GTO and other power switching devices constitute an inverter to provide voltage regulation and frequency modulation power to the asynchronous motor. The voltage, current and frequency of the power supply output are controlled by the control command of the control circuit, and the control is performed. The command is obtained by an external run command operation. For applications requiring precise control speed or fast response, the calculation should also include the signals detected by the drive main circuit and the drive system for closed-loop control. The protection circuit should be constructed in addition to the overvoltage protection of the main circuit of the inverter, overcurrent protection, and faults caused by the overheating protection of the main circuit. It should also protect the asynchronous motor and the transmission system, etc., thus directly affecting U, V, W. Inverter failure of the output main circuit is critical.
The inverter is opposite to the rectifier. The inverter converts the DC power to the AC power of the desired frequency, and turns on and off the six power switching devices of the upper bridge and the lower bridge for a determined time. as shown in picture 2. 
Fig. 2 Schematic diagram of the inverter In Fig. 2, S1~S6 form a bridge inverter circuit. When these 6 switches are alternately turned on and off, the phase difference between the three ends of the U, V and W outputs can be obtained. 3Ï€ three-phase AC voltage. It can be seen that the voltage waveforms of S1~S6 in the driving circuit are the same as the voltage waveform, which is especially important for output voltage balance. Figure 3 is a typical IGBT gate drive circuit encountered in the maintenance of the inverter. 
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 on-state losses, while also limiting the short-circuit current and the power stress it imposes. When the gate voltage is at zero, the IGBT is in the off state in order to ensure that the IGBT remains off when the collector-emitter voltage exhibits dv/dt noise, and a turn-off bias is applied to the gate. Bias can also reduce turn-off losses. The H series IGBT reverse bias voltage is in the range of -5V~15V.
3 Inverter output imbalance and countermeasures In the actual maintenance, U, V, W output imbalance can be divided into three cases:
(1) The inverter display shows: (MISSMG MOTO PHASE) output phase loss. If the detection circuit is faulty, the IGBT module and the drive circuit are directly checked. The conclusion is that the IGBT module is damaged and the drive circuit is also problematic. By replacing the IGBT module and components on the drive circuit such as optocoupler, PNP, NPN, a pair of drive transistors, electrolytic capacitors, voltage regulators, etc. can basically solve the problem.
(2) The inverter output U, V, W differs by about 100V. (Output 380V is an example.) A drive circuit in the middle of S1~S6 in the drive circuit has no drive voltage and drive signal waveform, and the output terminals U and V are measured. Between W and P.
(3) The DC voltage between U, V and W-N can be found that the driving voltage of this circuit is abnormal or there is no driving signal waveform, which causes the phase difference caused by one of U, V and W not working properly.
The solution is to check whether the voltage of the drive circuit is normal, whether the optocoupler is broken, whether the electrolytic capacitor leaks, etc. The 6-way waveform measured by the oscilloscope meets the technical requirements, and the problem can be solved.
There is another phenomenon that the phase difference between the three-phase output AC voltage of the inverter U, V, W is greater than 3%. Although it can be used, it cannot be used for a long time and a large load. This is mainly due to the main device asymmetry between the driving circuits S1~S6, such as the technical parameters of the transistor, the parameters of the Zener diode, the liquid dryness of the capacitor, the leakage and leakage, etc., and the loss of the device on the 6-channel driving circuit makes it There is a certain difference in the parameters, resulting in a slight potential difference between the inverter output U, V, W. Although the above situation can be used, it is technically unacceptable. Our company pursues excellence to screen and aging various devices, such as transistor technical parameters and voltage regulator tube technical parameters are consistent, pairing, etc., to ensure that the drive signal in the drive circuit meets the technical requirements, to ensure that the IGBT module is saturated, and the on-time is consistent. On the quality assurance, the repaired frequency converter is doing load test, the motor sounds light during the motor running, with the same power motor and the same power load before and after repair, the latter three-phase motor is relatively much smaller .
4 Conclusion Inverter output three-phase unbalance is a typical fault of the inverter, but in practice, you may encounter various complicated problems. I hope everyone can communicate together, and we also hope to better serve our customers.
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