Oct 19, 2020
Talking about ten necessary skills for inverter maintenance
There are many ways to learn inverter maintenance, but the direction is not right and the hard work is in vain, so it is important to grasp the direction. In order to let everyone master inverter maintenance knowledge faster, today I will share with you the 10 essential skills for inverter maintenance:
After taking over a faulty inverter, remove the inverter shell and clean it up. First, carefully observe the appearance, shape and integrity of the relevant components on the circuit board with your eyes according to the type of inverter fault. This method is very important for the preliminary judgment of the quality of charging resistors, filter electrolytic capacitors, IGBT/rectifier bridge modules and even PCB circuit boards. As shown in Figure 1, there is sometimes no value displayed on the display screen during the use of a certain brand of 75KW inverter; the main board output DC10V power supply and there is an intermittent failure. After disassembling the machine, the author found the cause of the fault at a glance-the rectifier diode used for the switching power supply to the main board +15V power supply branch was fired due to welding, which caused serious ablation of the PCB board!
This method is mainly aimed at three aspects of the inverter: firstly, it is to judge whether the DC bus charging relay/contactor is normal or not, which is very important for troubleshooting the undervoltage fault during the operation of the inverter. In addition, listen carefully for abnormal noise of the transformer used in the switching power supply, which is also very effective for preliminary determination of whether there is an overload phenomenon in the switching power supply. The last is to monitor the operation of the inverter cooling fan.
After the inverter fails and the power is turned off, quickly disassemble the machine and do protective work. Use your finger to quickly touch the relevant electronic components and IC integrated blocks on the internal circuit board of the inverter. Once it is found that some components have a significant year-on-year temperature rise, the fault must be on it or in the surrounding circuits! Figure 2 shows a 35KW inverter of a certain brand. The DC bus over-voltage protection is caused by the deterioration of the resistance of the DC bus voltage monitoring resistor. From the figure, you can feel how high the heat emitted by the resistor was during the incident!
When the frequency converter is working, if there is a phenomenon of good and bad working time, it may be caused by open welding failure of some components in the internal circuit. For some chip ICs with finely arranged pins and numerous chip ICs, it is not easy to find out if there are any problems by visual observation. At this moment, we might as well use insulated plastic rods/wood rods (conducting metal objects are strictly prohibited), and press the suspected components with appropriate force when the power is on. This method is very effective for eliminating small SMD components, especially SMD ICs, but it is necessary to prevent electric shock and short circuit during operation.
This method is a supplement to the fourth detection method. After all, the fourth method can only be effective for small SMD components, and it is not convenient for some high-power electronic components or circuit parts with high voltage danger. operating. For this, we can change the pressure to knocking, and use an insulating tool to knock with proper force near the suspected fault point. In most cases, the fault object can be quickly locked. Figure 3 shows the false solder joints detected by this method that caused a small-capacity inverter to intermittently report overvoltage faults!
This method mainly relies on multimeter detection, and nowadays, digital multimeter is mostly used. Regarding the maintenance of various faults of the inverter, about 65% of them are solved with a multimeter. Regarding how to use a multimeter to measure, I believe that the majority of electricians can be proficient in the application. I only emphasize one point: because there are many high-voltage energy storage components inside the inverter, remember to discharge before performing the measurement operation after the power is off, otherwise the multimeter will not be guaranteed Yeah! The appearance and color of the model shown in Figure 4 look the same. The nominal resistance value of the 1/4W color ring resistance is 15KΩ. The measured value with a multimeter has become infinite (due to the deterioration of the resistance, a certain brand of 22KW inverter Report "output current unbalance" fault)!
After talking about using a multimeter to measure, let's talk about testing again-it refers to testing with an oscilloscope that can visually display the waveform. As far as inverter maintenance is concerned, the use of an oscilloscope is generally aimed at the six inverter pulse signals of the inverter (the drive signal of the brake power tube/module is a switch value, and there is no need to use an oscilloscope to detect). When using an oscilloscope to detect, pay attention to whether the signal waveform is normal or not to meet the working requirements; whether the driving signal amplitude and frequency range meet the requirements for driving and other information. This maintenance method is indispensable for the maintenance of the inverter power tube/module after it is burned out!
This method is talking about short-circuiting. In the process of inverter repair, especially when the IGBT/IPM is removed due to damage, and the pulse drive circuit is separately energized to repair the pulse drive circuit, if the drive optocoupler model is A316J and this type of chip containing the IGBT/IPM fault detection function, the module is damaged Or removal often fails to make the optocoupler open normally. At this time, it is necessary to use wires to short-circuit the components for IGBT/IPM fault detection (mostly high back-voltage diode anodes) and the negative end of the inverter's DC bus (some marked N or GND) to deceive the inverter main control Let it think that the power module is intact and then achieve the purpose of normal transmission of the drive pulse signal. Figure 5 shows a brief introduction to the detection of IGBT modules in a certain brand of 55KW inverter and pulse signal drive circuit.
Break-disconnect also. Everyone knows that the internal circuit of the inverter contains many protection functions for itself or the load. When these protection functions have problems, we can use the open/disconnect method to determine and repair.
For example: All inverters have output overcurrent monitoring and protection functions. But some products are very unscientific in designing this function-when a failure occurs, it is impossible to clearly indicate which phase has the problem, which is very troublesome for the repairer. For the maintenance of this situation, we can take the method of disconnecting the current transformer/current detection subunit used for each phase detection from the subsequent comparison circuit one by one, and the fault point will be clear at a glance when the input signal failure of the disconnected channel disappears (some models Manual reset is required to clear the fault display). Of course, this method is also suitable for overhauling protection circuits such as temperature.
Release-discharge. The frequency converter contains electrolytic capacitors of various specifications and capacities. These capacitors have a relatively high probability of failure of the frequency converter due to the reduction in capacity. Regarding the detection of these capacitors, the general maintenance personnel usually use the method of observing the shape and measuring with a capacitance meter for maintenance, but these two methods have certain limitations. For this reason, I use incandescent bulbs/small bulbs and conduct a discharge comparative test on the measured object after charging. This method can intuitively compare whether the capacity of the measured object meets the requirements. According to my summary, the method has The efficiency is above 80%. The electrolytic capacitor with a nominal 50V 220uF actual capacity in Figure 6 was identified by the discharge detection method.
The above ten methods of overhauling the inverter, hope to help everyone.