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Ten methods of inverter maintenance (part 1)

Apr 08, 2021

Ten methods of inverter maintenance (part 1)


1. Alarm parameter inspection method


  〖Example 1〗 A certain inverter is faulty, unable to run, and the LED displays “UV” (abbreviation for under voltage). The alarm in the manual is that the DC bus is under voltage. Because the control circuit power of this type of inverter is not taken from the DC bus, but the control power that is separately rectified from the AC input terminal through a transformer. Therefore, it is judged that the alarm should be true. Therefore, start with the power supply and check that the input power supply voltage is correct, and the filter capacitor voltage is 0 volts. Since the short-circuit contactor of the charging resistor does not act, it has nothing to do with the rectifier bridge. The fault range was narrowed to the charging resistor. After the power was cut off, it was detected with a multimeter that the charging resistor was broken. The replacement resistor was repaired immediately.


   [Example 2] After using a Sanken IF 11Kw inverter for more than 3 years, it occasionally displays "AL5" (abbreviation for alarm 5) when it is powered on. The manual says that the CPU is disturbed. After many observations, it was found that it occurred when the charging resistor short-circuited the contactor. Suspected of the interference caused by the contactor, adding a resistance-capacitance filter to the control pin will cause the fault to no longer occur.


   [Example 3] A Fuji E9 series 3.7 kW inverter suddenly appeared OC3 (constant speed medium overcurrent) alarm shutdown during on-site operation, and OC1 (accelerated overcurrent) alarm shutdown occurred when the power was turned on again. I first remove the wires from U, V, and W to the motor, use a multimeter to measure the infinite resistance between U, V, and W. When running at no load, the inverter has no alarm and the output voltage is normal. It can be preliminarily concluded that there is no problem with the inverter. It turned out that there was a joint in the middle of the motor cable, which was covered in the branching groove of the pit with a wooden plate. The insulating tape was aging, and the factory was cleaned and flooded, causing the output to be short-circuited.


   [Example 4] Sanken SVF303, displaying "5", "5" in the manual means DC overvoltage. The voltage value is sampled by the DC bus (about 530V DC) and then separated by the optocoupler. When the voltage exceeds a certain threshold, the optocoupler will act to give the processor a high level. For over-voltage alarm, we can check whether the resistance changes, whether the optocoupler has a short circuit, etc.


  It is not difficult to see from the above cases how important the alarm prompt of the inverter is to deal with the problem, and prompt you to handle the problem in the correct direction.


  2, analogy check method


  This method can be the analogy of the same circuit itself, or the analogy of the faulty board and the known good board. This can help repairers quickly narrow the scope of inspection.


  〖Example 1〗 The Sanken MF15kw inverter was damaged and was sent back for repair. The user could not explain the specific situation. First, use a multimeter to measure the input terminals R, S, and T. Except for a certain resistance between R and T, the resistance between the other terminals is infinite. The input terminals R, S, and T are connected to the positive or negative pole of the rectifier bridge. Diode characteristics. Why are R and T different from the other two groups? The original R and T breakers have control power transformers inside, so they have a certain resistance value. It can be seen from the above that the input part is no problem. Also use a multimeter to check the resistance between U, V, and W, and the three-phase balance. Next, when checking the diode characteristics of the output relative to the DC positive and negative poles, it is found that U is not connected to the positive and negative poles. It is suspected that there is a problem with the U-phase IGBT. Remove it and check that the IGBT is broken. In the driving circuit, the characteristics of the three groups of upper bridge arm control circuit are the same, and the characteristics of the three groups of lower bridge arm control circuit are the same. The comparison method is used to check that Q1 is damaged. After replacement, the trigger pin resistance values are the same for each group, and power on to confirm that the PWM waveform is correct. Reassemble, power on, test and repair.


   [Example 2] There is an inverter, the phenomenon is that the panel display is normal, the digital setting frequency and operation are normal, but the terminal control fails. Use a multimeter to check that there is no 10V voltage on the terminal. Starting from the switching power supply, all groups of power supplies are normal, and it seems that the problem lies in the connecting wires. However, it takes some time to find 10V among the 32 flat cables without drawings. There is just a good 22KW one, so first write down the voltage of each pin of the 22KW flat cable connected to the ground, and then compare the 37KW one. The voltage of each pin to ground will quickly find the difference. The pins of the original socket were soldered, and the inverter used oxidation after a period of time to make it completely non-conducting, so re-soldered and repaired.


   [Example 3] There is a carding machine in a wool spinning mill, using Siemens 440 frequency converter, two 5.5KW and one 7.5KW to realize synchronous operation. F0011 or A0511 often shut down after two years of operation of one of the 5.5KW units. Both of these alarms indicate that the motor is overloaded. Remove the motor belt and turn the motor and equipment by hand. There is no abnormally heavy phenomenon. Exchange the two 5.5KW motors and find that the original inverter alarms. There is a problem with the inverter. The analogy method can be used not only to check the internal circuits of the machine, but also to judge on-site problems.


  3, spare board replacement inspection method


  Using a spare circuit board or a circuit board of the same type to confirm the fault, narrowing the scope of inspection is a very effective method. If there is a problem with the control board, there is often no other choice but to replace it, because most users hardly get the schematic and layout drawings, which makes it difficult to make chip-level repairs. Circuit boards other than the control board, such as the power supply board and the drive board, can be repaired, and other chapters will further introduce it. Here we mainly introduce the replacement of the control board.


  4, isolation inspection method


  Some faults are often difficult to determine in which area, and the isolation method can simplify complex problems and find out the cause of the fault faster.


   [Example] Repairing an Intech inverter, the phenomenon is that there is no display after power-on, accompanied by a beep sound. Based on experience, it can be concluded that the switching power supply is overloaded, the feedback protection works and shuts off the output of the switching power supply, and the beep generated by turning off again and turning off again. First remove the control panel, power on and find that it is still the same, then disconnect the diodes of each group of power supply one by one, and finally find that the 15V used by the fan is faulty. However, the fan does not have a signal to run. It shouldn't be a problem with the fan itself, it seems to be a problem with the front end of the fan. Finally, it was found that the characteristics of the 15V filter capacitor were wrong, and the filter capacitor was removed for measurement, and it was really aging. Replace it with a new capacitor and fix it.


   5. Visual inspection method


   is to use the sensory organs of human hands, eyes, ears, and nose to find the cause of the malfunction. This method is commonly used and used first. The maintenance principle of "outside then inside" requires that the maintenance personnel should first adopt the methods of looking, smelling, asking, and touching when encountering a failure, and inspect them one by one from the outside to the inside. Some failures can quickly find the cause using this intuitive method, otherwise a lot of time will be wasted, or even no way to start. Use vision to check whether the connection of line components is loose, whether the disconnected contactor is ablated by electric shock, whether the pressure is frequent, whether the heating element is overheated and discolored, whether the electrolytic capacitor is swelled and deformed, and whether the pressure component has an obvious breakdown point. Smell if there is a burnt smell after power-on, and check if the heating element is hot. It is very important to ask, asking the user about the process of the fault, which helps to analyze the cause of the problem, and it is convenient to directly hit the critical point. Sometimes asking colleagues is also a shortcut.


   [Example] A Sanken IP 55KW inverter is damaged during the warranty period and there is no display when it is powered on. Open the cover of the machine and carefully observe all parts. It is found that the charging resistor is burned out, the contactor coil is burned out and the shell is burnt. After questioning, it turns out that the user's power supply voltage is low, and the inverter often stops due to undervoltage, so a booster is specially equipped for the inverter. However, the user did not notice that the voltage would return to normal at night, and as a result, the contactor was burned out first and then the charging resistor was burned out. The rectifier bridge and electrolytic capacitors survived the relatively high withstand voltage. Replace damaged components to repair.