A problem caused by inverter filter capacitor

A few days ago, an ACS800 inverter experienced current and frequency fluctuations just after it was turned on, and the amplitude of the fluctuations was relatively large, completely exceeding the amplitude of the fluctuations caused by load changes. The first feeling was that there was a problem with the inverter, so it was quickly shut down.

In order to verify that the problem is not caused by the load, the load is removed and the machine is restarted. The frequency still fluctuates, which indicates that the problem is undoubtedly with the inverter.

When the machine was running idle, the bus voltage also fluctuated. After it stopped, the bus voltage did not change much, which basically ruled out the impact of the rectifier part. To be on the safe side, the machine was disassembled and the rectifier was measured, and the result was consistent with the judgment.

Then, the key to the problem lies in the capacitor. Of course, it may also be a problem with the inverter or drive, but when there is a problem with the inverter switch, the phenomenon is often not the case. The probability of failure in the drive circuit is not high, so the capacitor is the focus of the inspection.

As it turned out, it was indeed a capacitor problem. The capacity of five out of six capacitors had decreased. After replacing all six capacitors, the problem improved when the computer was restarted.

In fact, it is quite strange that the capacitor of ABB's inverter has been used for less than seven years and it has already started to break down.

The role of capacitors in the inverter has been mentioned in the previous section, which is to smooth the wave in order to provide a good operating environment for the IGBT.

The capacitors in the inverter are all electrolytic capacitors, which can be made relatively large. In order to increase its capacitance, several groups of capacitors are often connected in parallel into one group, a total of two groups. It is difficult to match these two groups of capacitors equally, so a voltage-equalizing resistor is often connected in parallel to each other to make the voltage across the two groups of capacitors tend to be stable.

Electrolytic capacitors are greatly affected by temperature, followed by their withstand voltage level. When capacitance decreases, temperature is most likely the culprit.

In fact, now think about it, this inverter is installed outdoors and is surrounded by a closed environment. It has just passed a hot summer that has not been seen in many years. It is understandable that the capacitor has such problems.