Common faults and solutions of inverter drive circuit?

In recent ten years, with the penetration of power electronics technology, microelectronics technology and modern control theory into the field of AC electric drive, variable frequency AC speed regulation has gradually replaced the past speed regulation systems such as slip speed regulation, pole-changing speed regulation and DC speed regulation. It can almost be said that frequency converters are used where there are AC motors. Its main characteristics are efficient driving performance and good control characteristics.

At present, the general frequency converter generally includes the following parts: rectifier bridge, inverter bridge, intermediate DC circuit, precharge circuit, control circuit, drive circuit and so on. The driving circuit plays an important role in the quality of frequency converter. Now let's talk about the common problems and solutions of the driving circuit.

The driving circuit is just a general term. With the continuous development of technology, the driving circuit itself has gone through from the pin element driving circuit to the optocoupler driving circuit, to the thick film driving circuit, and a relatively new integrated driving circuit. At present, the last three driving circuits mentioned above are often encountered in maintenance.

2 Maintenance methods of several driving circuits

Cause and inspection of (1) drive circuit damage

There are many reasons for driving injuries. Generally speaking, the problem is that there is no output in U, V and W phases, or the output is unbalanced, or the output is balanced but the low frequency jitter, and the alarm is started. When the fast-melting circuit of large-capacity inverter is open, or the IGBT inverter module is damaged, the driving circuit is basically not intact, and the good fast-melting circuit or IGBT inverter module must not be replaced, which will easily cause the newly replaced good device to be damaged again. At this time, it is important to check whether there are spark marks on the drive circuit. Here, you can unplug the driving pin connection of IGBT inverter module first, and measure whether the resistance values of the six driving circuits are all the same with a multimeter (but a few inverter driving circuits are not all the same, such as Mitsubishi and Fuji inverters). If the six resistance values are basically the same, it cannot be completely proved that the driving circuit is intact. Then we need to use an electronic oscilloscope to measure whether the voltages on the six driving circuits are the same, and whether the waveforms of the six driving circuits are the same when a start signal is given. If we don't have an electronic oscilloscope, we can also try to use a digital electronic multimeter to measure the DC voltages of six driving circuits. Generally speaking, the DC voltage of each driving circuit is about 10V when it is not started, and about 2-3V after it is started. If the measurement results are all normal, we can basically judge this. Then connect the IGBT inverter module to the driving circuit, but remember that the safest way is to disconnect the P of the IGBT inverter module from the DC bus, and indirectly connect a string of light bulbs or a resistor with higher power, so as to protect the IGBT inverter module from being burned by the discharge current of a large capacitor when the circuit has a large current. The following are some examples related to the drive circuit when maintaining the frequency converter:

(2) An Chuan 6 16G5, 3.7kW frequency converter.

An Chuan 6 16G5, 3.7kW frequency converter, the fault phenomenon is that the three-phase output is normal, but the motor is dithered at low speed and cannot run normally. First of all, it is estimated that most of the drive circuits of the inverter are damaged. The correct solution should be to turn on the inverter after determining the fault phenomenon, remove the IGBT inverter module from the printed circuit board, observe whether the waveforms of the six-way drive circuits are consistent when they are connected with an electronic oscilloscope, find out the inconsistent drive circuits, and replace the optocoupler on the drive circuits, usually PC923 or PC929. If the inverter has been used for more than 3 years, it is recommended to replace all electrolytic capacitors in the drive circuit and then observe it with an oscilloscope. After the six waveforms are consistent, install IGBT inverter module to carry out load experiment to eliminate jitter.

(3) Fuji G9 frequency converter

Fuji G9 inverter, the fault is not displayed when it is powered on.

It is estimated that the switching power supply of the inverter may be damaged. Turn on the frequency converter to check the switching power supply circuit. After checking, the circuit of the switching power supply device is not damaged, and the DC voltage is not displayed at the positive and negative parts of DC. At this time, it may be a driving problem. At the beginning, all the capacitors in the drive circuit were removed, and some capacitors were found to be leaking. The electrolytic capacitors were replaced with new ones, and they worked normally after being re-energized.

(4) Delta frequency converter

Delta frequency converter, the fault phenomenon is that the output end of the frequency converter is on fire. After disassembly and inspection, it was found that the IGBT inverter module was broken down and the printed circuit board of the driving circuit was seriously damaged. The correct solution is to remove the damaged IGBT inverter module first, and try to protect the printed circuit board from human secondary damage when removing it. Replace the damaged electronic components on the drive circuits one by one and connect them with wires to the open circuit on the printed circuit board (pay attention to scrape the burnt parts here to prevent the ignition from happening again). Then, measure the waveform with an oscilloscope under the condition that the six drive circuits have the same resistance and voltage, but when the inverter is turned on, it will report the OCC fault (Delta inverter has no IGBT inverter module to give an alarm), and use a light bulb to connect the P 1 of the module with the printed circuit board. Everything else is wired, and OCC will jump when it is restarted. Make sure there is something wrong with the drive circuit. After replacing the optocouplers one by one, it is found that the optocoupler detection function of the driving circuit is damaged. After replacing it with a new one, it will start normally.

3 Conclusion

With the continuous development of frequency converter, the driving circuit technology of frequency converter is changing with each passing day, and what can be involved here is only rare. I hope it can help technicians and inverter enthusiasts, and I also hope that inverter practitioners can communicate more so that everyone's technology can go up a storey still higher.

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