Editor: Faults and repairs of frequency converters

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Sharing of experience in inverter repair: Recently, Lingkun Electric received a call from a customer who needed repairs for a Mitsubishi inverter. The Mitsubishi E540-0.75KW to 3.7KW inverter was experiencing the “E7” fault; according to the manual, it was due to a damaged CPU board. The customer wanted to purchase a new CPU board, but in fact the problem lay in the communication circuit within the module. Since this is an integrated module, it cannot be repaired – the entire module must be replaced or a new inverter purchased.

For replacing the module in Mitsubishi A540-7.5/5.5KW inverters, a dummy load can be used for wiring purposes. Since these inverters don’t have fast fuses, using a dummy load makes wiring more complicated. Our approach is as follows: First, secure the 7MBI50-120 module firmly. Draw a wire from the P terminal, then cover that terminal with two or three layers of electrical tape to isolate it from the circuit board. Next, install the drive board; at this point, all components except the P terminal should be secured with screws. The dummy load is then connected between this wire and the P1 terminal of the inverter. Start the inverter at 5HZ, check that the output voltage is balanced, then turn off the power supply to allow the filter capacitors to discharge. Loosen the screws holding the drive board in place, pull out the wire forcefully (without removing the drive board), remove the electrical tape from the P terminal, and finally secure everything with screws. Inverters of this model cannot be started without a module installed, and it’s not possible to observe the waveform of the drive circuit before installing the module. Failing to do so can easily lead to damage to the module!

A few additional notes on inverter repair:
1. Inverters can only reduce voltage; they cannot increase it. 2. The inverter itself is not an energy saver; its energy-saving effect arises from the waste of electrical energy that occurred due to the inability to adjust the speed previously. 3. The inverter is an electromagnetic interference source. 　　4. The IGBT modules for frequency converters and the main control boards cannot be manufactured domestically on a large scale, resulting in high prices. 　　5. The cost of inverters varies greatly depending on whether shoddy materials are used, and their service life also differs significantly. 　　6. The inverter requires a high-quality power supply. 　　7. The lifespan of the inverter is not infinite; the fan and electrolytic capacitors are the first to wear out. 　　8. Inverters are a combination of high-voltage and low-voltage components; their main circuitry is highly precise, and poor working conditions or inadequate maintenance lead to a high failure rate. 　　There are several points to note when using frequency converters for fans: 1) The deceleration time should not be too short; generally, it should be 3-5 minutes. 　　2) Do not use the “free parking” and “automatic reset” functions, unless you have enabled the “speed tracking” function. 　　3) If the “speed tracking” function is not enabled, it is not possible to start the frequency converter while the fan is still rotating due to inertia. 　　4) The input voltage requires even greater stability. 5) The three-phase currents of the motor should be relatively balanced, and there should be no issues with the motor bearings. 　　Recently, we repaired a Mitsubishi A540-55K inverter. It was brought to us by a novice technician who wasn’t able to fix it on his own. The problem with this unit was a faulty module; after replacing the module, the novice tried to check whether the drive was functioning properly by disconnecting the trigger wires of the module. As soon as power was applied, the inverter tripped. Upon further inspection, it was found that another module had been damaged! He couldn’t figure out why this happened for a long time. It turns out that when the trigger wires of an IGBT module are disconnected, a small amount of voltage may remain, causing the module to be in a semi-conductive state. This leads to short circuits and damage once power is applied. GTR modules do not have this characteristic, which is why they can be tested in this way. We have repaired many Mitsubishi A240-22K inverters, and in all cases the problem was faulty modules. The reasons for this are poor maintenance, such as dust clogging the radiator, dirty circuit boards, or ineffective heat dissipation grease. The output module of this inverter (PM100CSM120) is an integrated module; even if just one channel fails, the entire module must be replaced, resulting in high repair costs. Good replacement modules are also hard to find. If your inverter isn’t damaged yet, you need to take extra care in its maintenance—especially during these hot days. Recently, Lingkun Electric repaired an Yaskawa 616G5-55KW inverter that was severely damaged. There were fast fuses in place (one for each phase), but the electrician, likely due to lack of experience, didn’t check whether there were any problems with the modules. Since he couldn’t find the fast fuses right away, he used a copper wire as a substitute. When the inverter was turned on, there was a loud noise, and two modules exploded. The feedback circuit was damaged, and the control board could not be repaired either. Replacing the board caused significant losses. Based on our experience, if fast fuses are blown, it’s likely that there’s a problem with the modules, but it’s not certain that the fast fuses will actually blow. We’ve seen this practice of using copper wires as substitutes for fast fuses many times. Lingkun Electric, an inverter repair company, has encountered cases where careless electricians failed to remove the shorting clips when wiring the auxiliary power supply (R1, T1) of Mitsubishi A540 inverters. As a result, the inverter got damaged, and they couldn’t understand why. When the shorting clips weren’t removed, R and R1, as well as T and T1, were connected inside the inverter. The electrician thought that connecting two wires from R and T wouldn’t cause any problems, but in reality, connecting R to S1 and T to R1 caused inter-phase short circuits. Since the connections between R and R1, and T and T1 went through the middle layer of the power board, this led to the power board being damaged and splitting into two parts. In general, it’s not necessary to connect the auxiliary power supply (R1, T1). Some novice technicians don’t know how to use a dummy load when repairing inverters. Whenever there’s a problem with the drive and a module gets damaged, they assume that the quality of the module is poor. A dummy load can be created using a resistor with a resistance of a few hundred ohms (a light bulb can also be used). This resistor is connected in series with the main circuit. If there are fast fuses, they should be removed first, and then the resistor is installed. If there are no fast fuses, the resistor can be connected anywhere in the main circuit. This resistor acts as a current limiter, preventing the module from being damaged in case of a short circuit. Only after confirming that the inverter’s output is normal after powering it on should the dummy load be removed. 　In many factories, the power supply is provided by generators. When these generators fail, the high voltage generated can damage inverters and electronic equipment! This is a common occurrence – last year, a wire drawing factory in Shenzhen had over twenty 30KW inverters damaged at once, resulting in a shutdown of more than ten days and significant losses. The factory implemented various protection measures for the generators, but with little effect. Later, we came up with a passive protection method: we installed varistors across the air switches at the input terminals of the inverters or equipment (821K for 380V systems and 471K for 220V systems). When high voltage appears, the varistors short-circuit, causing the air switch to trip and thus protecting the inverters. This reduced the failure rate of the inverters. Varistors are inexpensive, making this a cost-effective solution. There is no strict requirement regarding the wattage of varistors used in parallel connections (triangular connection for three-phase systems); those connected to higher input currents should use varistors of larger capacity (or several varistors in parallel). When a varistor activates, it creates a complete short circuit. It’s also important that the air switch used is of good quality and reacts quickly, with an appropriate protection current level. The varistors should be installed at the output terminal of the air switch.

Many people replace faulty modules in inverters, only for them to fail again within a few days. They bring these units to us without understanding the reason, only to find that some screws weren’t tightened properly. It may seem like a minor issue, but it can be fatal for inverters. We’ve found that in inverters installed on vibrating equipment such as industrial washing machines or machine tools, the screws connecting the main circuit and those securing the modules tend to loosen over time. In such cases, the modules are usually the first to get damaged. If other screws aren’t tightened after replacing the module, it will fail quickly, leading people to blame the quality of the module. It’s important to avoid installing inverters on vibrating equipment, as even the best inverters can be damaged quickly under such conditions.

We often see experienced repair technicians who are too confident and skip using a dummy load when repairing inverters, thinking it’s too troublesome. Yet, this still increases the risk of damaging the modules. Using a dummy load virtually eliminates this risk, unless you’re using fake modules!

Many people aren’t familiar with the structure of the rectifier module CVM40CD120 in Fuji’s G9-5.5KW inverters. Here’s a brief overview:
Rectifier section: R, S, T, A (+), N- (-)
Charging thyristors: A, P1, Gth (trigger)
Braking tubes: DB, N-, G7 (trigger); DB, B+ are the freewheeling diodes
Power switch transistors: D8, S8, G8
Thermistors: Th1, Th2

The Yamaken MF series has a common problem: it sometimes displays an “Erc” error. In such cases, follow these steps: Set parameter 90 to “7831”; the inverter will then display “PASS”. Next, enter the inverter’s capacity value, and then restore the parameters to their default settings (parameter 36 = 1).
Inverter capacity values: 2.2KW – 23, 3.7KW – 24, 7.5KW – 26, 15KW – 28, 22KW – 30, 30KW – 316I8, 45KW – 33, 75KW – 35, 110KW – 37. Other power levels can be determined similarly.

Some people try to increase the motor’s torque by setting the inverter’s torque enhancement parameter (or minimum output voltage) very high. This results in high starting currents, frequent “overcurrent” errors, and increased risk of module damage. Torque enhancement should be done gradually, with regular monitoring of current levels. For heavy loads, “vector control” is recommended, as it allows the inverter to deliver maximum torque automatically. The inverter needs to be “tuned” (more details available upon request), but not all inverters have this feature. It’s also not advisable to lower the basic frequency. The standard basic frequency for motors in China is 50Hz. While lowering this frequency can increase torque, it also causes a sharp rise in current, which can damage both the inverter and the motor.

Some people don’t install air switches at the inverter’s power input terminals. When a module fails, it can cause severe damage to the circuit board, sometimes rendering it unrepairable. This is especially true for brands of inverters that don’t come equipped with fuses. The current rating of the fuses should also be appropriate, and they should be of good quality.

Fuji’s G9 inverters in the 3.7KW–7.5KW range have one common issue: their cooling fans have high power and fast speeds, so their lifespan is reduced in dusty working environments. When the fan fails, the inverter doesn’t immediately trigger an “overheat” protection mechanism (possibly because the protection temperature setting is too high). As a result, the internal temperature of the inverter rises, causing the small capacitors in the drive circuit and power circuit to age prematurely. Usually, the switching power supply stops working first, and the inverter stops functioning. At this point, replacing the fan and the two small capacitors in the power circuit can restore normal operation. It’s also advisable to replace the capacitors in the drive circuit.

Since inverters are relatively expensive devices, with significant price differences between brands, and given their high failure rate, many people struggle when choosing an inverter. We believe that if the proper functioning of the inverter is crucial for your production processes, if your equipment is located far away, or if you don’t want to constantly trouble maintenance staff, it’s better to choose a high-quality, branded inverter. However, not all branded inverters are suitable for everyone. Some are sensitive to moisture and dust, requiring a favorable environment to function properly. If your motor operates smoothly, without frequent stops, with light loads, stable power supply voltage, and a good operating environment for the inverter, and if a failure doesn’t disrupt production, then you might consider buying a cheaper inverter from a reputable brand to save costs.

Some people don’t take into account the inverter’s requirements when tuning it. They set the acceleration and deceleration times to less than 1 second based solely on production needs. This leads to frequent inverter failures. When acceleration is too rapid, the motor current increases. High-quality inverters will automatically limit the output current and extend the acceleration time, while lower-quality inverters will experience reduced lifespan due to the high current. It’s advisable to set the acceleration time to at least 2 seconds. When deceleration is too rapid, the inverter is subjected to the back EMF of the motor during shutdown, which can easily damage the modules! It is advisable to use a braking unit for sudden stops of the motor; otherwise, extend the deceleration time or opt for a free-stop method. This is especially true for large fans with high inertia, where the deceleration time usually takes several minutes. Recently, two factories experienced failures in their 75KW inverters, with one module failing in each case. The price of one of those modules was only 1300 yuan (with a total of 6 modules in the inverter), while the module for the other unit cost 23,000 yuan (an integrated module). Therefore, when purchasing an inverter, you must consider future maintenance issues!

It is common to find that some people, when repairing inverters by replacing modules themselves, fail to apply thermal silicone on the bottom side of the modules. As a result, the heat generated by the modules cannot be effectively dissipated to the radiator, leading to overheating and damage to the modules. It is also incorrect to use latex sealant (as some people do), as its effect is the opposite.

Many people replace modules in inverters, only for them to fail again within a few days. Without understanding the reason, they bring the inverters to us, only to find that some screws were not tightened properly. Although this may seem like a minor issue, it can be fatal for the inverter. We have found that many inverters installed on vibrating equipment such as industrial washing machines or machine tools tend to have their connection screws and module fastening screws loosen over time. In such cases, the modules are usually the first to get damaged. If other screws are not tightened after replacing the module, the module will fail quickly, and people then blame the poor quality of the module. It is important to avoid installing inverters on vibrating equipment, as even the best inverters can be damaged quickly in such conditions.

Some people try to increase the torque of the motor by setting the torque enhancement parameters (or minimum output voltage) of the inverter very high. This results in high starting currents, frequent “overcurrent” errors, and damage to the modules. Torque enhancement should be done carefully, with gradual adjustments while monitoring current levels. For heavy loads, “vector control” is the best option, as it allows the inverter to automatically deliver maximum torque. The inverter needs to be “tuned” (self-learning*), but not many inverters have this function. It is also not advisable to lower the basic frequency. The standard basic frequency for motors in China is 50Hz. While lowering this frequency can increase torque, it also causes a sharp rise in current, which can damage both the inverter and the motor.

Our modules undergo strict testing before being sold. However, there are always some dishonest people who try to return the modules after damaging them, which we cannot accept. Our return policy requires that returns be made within one month before installation. If the modules are to be covered under a warranty, the inverter must be sent to us for repair, and reasonable fees will be charged.

If you have many inverters in the same area of your factory, or if you work in a brewery, beverage factory (where the environment is humid), chemical plant, or ceramic factory (where there is a lot of dust), or in a boiler room (where temperatures are high), it is advisable to install the inverters in rooms with air conditioning. This can significantly reduce the failure rate of the inverters and extend their lifespan.

After repairing numerous inverters, we’ve noticed a common characteristic: if the power supply for the inverter’s switching power supply does not come directly from the filter capacitors in the main circuit, but rather from an independent source at the input stage, it’s better to convert 380V to 220V (through rectification) before supplying power to the switching power supply. Although this makes the inverter more complex, it reduces its failure rate. After all, many inverter failures are related to the switching power supply. When the inverter is running, the DC voltage in the main circuit is often unstable. If the switching power supply relies on the filter capacitors in the main circuit for power, it is prone to failure. We hope that inverter designers will take this issue into consideration.

Factory grounding wires rarely break, but when they do, it can still cause the inverter to be damaged, even though no one gets electrocuted. In one brewery, more than a dozen Danfoss inverters were damaged due to arcing at the motherboard terminals, which caused the motherboards to burn out. Upon on-site investigation, it was found that a motor was leaking electricity, and the factory’s ground wire had rusted and broken; as a result, high-voltage current flowed back into the inverter’s main board through the inverter’s ground wire! The ground wire is also important for lightning protection; if the electrician is available, it would be advisable to have him check whether the ground wire is about to break. Several points need to be considered regarding the wiring of the load connected to the inverter: 1) It should be connected between the capacitor and the module, not between the rectifier and the capacitor, as the discharge from the capacitor alone can be enough to damage the module. 　　2) When the power supply for the switching power supply comes through a fast fuse (such as Fuji G9-11KW), it is not possible to connect a dummy load to that fast fuse; otherwise, the light bulb will turn on once power is supplied, and the switching power supply may stop working.
3) The dummy load should also be connected after the DC voltage detection point. In this way, when the light bulb lights up due to abnormal output from the inverter, the inverter will not trigger a “low voltage” alarm, allowing you to determine which output circuit is faulty. Many people call us saying they want to learn how to repair inverters here, but we believe that learning this skill cannot be achieved overnight. As long as you grasp a few key points and practice over time by coming here frequently, you can definitely learn a lot! We will also share what we know with everyone here.
Recently, it has been difficult to find good used modules, while there are many fake ones (or those that have been repaired), especially high-priced Mitsubishi and Fuji modules!
Quality issues with integrated inverters: Some brands of low-power inverters feature an integrated design, with the output module, power supply, and drive circuit all combined into one unit. As a result, even minor faults in these modules make repair difficult, and replacing them is expensive (almost as much as the cost of the entire unit), so such units often end up being discarded. It’s common to see factories’ repair shops filled with such inverters. Therefore, we hope that inverter manufacturers will pay more attention to quality issues when producing integrated inverters, taking into account various abnormal situations that may occur during use, and increasing the safety factors for those parts that tend to fail frequently. They should also provide distributors with sufficient spare parts so that damaged inverters can be repaired promptly. Lingkun Electrical is a professional service provider for circuit board chip-level repairs; we have our own repair center dedicated to fixing various complex circuit boards. Our services include: circuit board repair, frequency converter repair, DC speed controller repair, programmable logic controller (PLC) repair, human-machine interfaces and touch screen repair, servo drive repair, instrument repair, industrial power supply repair, CNC system repair, medical equipment repair, and more. Through our meticulous services, we establish a strong communication platform with our clients. Address: Room 601, Building A, Nature New World, No. 106 Majiabao East Road, Fengtai District, Beijing, Postal Code: 100068. Contact person: Mr. Zhong, 13901070560. There are several points to note when using frequency converters for fans: 1) The deceleration time should not be too short; it generally needs to be 3-5 minutes. 　　2) Do not use the “free parking” and “automatic reset” functions, unless you have enabled the “speed tracking” function. 　　3) If the “speed tracking” function is not enabled, it is not possible to start the frequency converter while the fan is still rotating due to inertia. 　　4) The input voltage requires even greater stability. 　　5) The three-phase currents of the motor should be relatively balanced, and there should be no issues with the motor bearings. A Brief Discussion on Common Faults and Repair Strategies for ABB Inverters. Here, we will discuss some of the common faults associated with ABB inverters with users everywhere: Repair and fault handling of the ACS300 inverter. One of the faults we often encounter is damage to the switching power supply. The switching power supply in the ACS300 inverter uses a waveform generator integrated circuit called LT1244, which has functions similar to those of the UC3844 chip. Damage to this integrated circuit can occur due to sudden changes in operating voltage or damage to the load connected to the switching power supply. As a result of years of use, the electrolytic capacitors also reach the end of their useful life, and these capacitors used for filtering become the direct cause of damage to the switching power supply. During maintenance, we find that the rectifier bridges in ACS300 frequency converters often get damaged. Perhaps for economic reasons, a compact three-phase full-bridge rectifier from an international rectifier manufacturer was chosen; it has a small size and low current-carrying capacity, as well as poor heat dissipation, which is why it gets damaged after being used for some time. The ACS300 main control board also experiences a fairly high rate of failures; communication issues between the control panel and the main board, as well as CPU failures on the main board, occur from time to time. Such failures are usually difficult to resolve. The ACS300 uses Mitsubishi’s IPM modules, which have a relatively low failure rate. If a module is damaged, it must be replaced; however, it is essential to ensure that the drive circuit is fully functional before making such a replacement. For the repair and troubleshooting of ACS500 frequency converters, one of the most common faults is damage to the drive thick film. This drive thick film includes not only the drive circuit but also functions such as short-circuit detection, IGBT module monitoring, and overcurrent detection. Thanks to its robust protection features, the high-power modules in the ACS500 rarely get damaged. During repairs, if the drive thick-film component is damaged and no replacement parts are available, we can only repair the thick film itself. Since these thick-film components are soldered onto ceramic substrates, heat dissipation occurs quite rapidly; it is important to be careful not to damage the components by keeping the soldering iron applied to them for too long. Due to limitations related to the amount of time it is used, the cooling fan of the ACS500 can also fail. A common symptom is that only a \"buzzing\" sound is heard after power is applied, but the fan does not rotate. Since it is an axial flow fan, the fan coils and bearings are usually in good condition; upon inspection, it was found that the deflection capacitor was faulty, and normal operation was restored after replacing it. When it comes to the repair and troubleshooting of ACS600 frequency converters, their performance and quality are quite reliable. However, factors such as the influence of the surrounding environment, improper parameter settings, and incorrect operation can all cause damage to the frequency converter. Of course, natural wear and tear is also an inevitable issue for frequency converters of all brands. Unlike previous ABB inverters, the ACS600 inverter uses fiber optic communication, **which increases the communication time between the CPU board and the I/O board. However, this may also lead to faults such as “LINK OR HWC” and “PPCC LINK”; the occurrence of such faults is not necessarily due to damage to the fiber optics. “ The “PPCC LINK” fault is a common issue with ACS600 frequency converters; damage to the CPU board NINT-4X or the I/O board NIOC-0X can both lead to this fault. Damaged switching power supplies are also a problem in ACS600 frequency converters; the faults mainly occur in the switching transistors. Short circuits in these transistors often cause the power resistor used for current limiting to burn out as well. “The “SHORT CIRCUIT” output short-circuit fault is the most common type of fault we encounter. It results from damage to the trigger board NGDR-0X of the inverter module. The ACS600 uses intelligent modules, and faults in the load or various issues that arise during operation can all lead to damage to these modules. Damage to the modules often results in damage to the drive board as well. Since spare parts are relatively expensive, the cost of repairing inverters is also high; therefore, higher standards are required of technicians when it comes to performing board-level repairs. Regarding the newly introduced ACS550, ACS510, ACS150, ACS350, ACS50, ACS-M1, and ACS800 frequency converters, since they have been on the market for only a short time, they are now in a stable phase without any significant or typical faults. In the case of a 2340 short circuit in the ACS800, it is necessary to replace the IGBT+ABDR components; ABB’s service policy is quite user-friendly. Free replacement for low-power units (except in cases where the interface board is damaged). To ensure the stability of the inverters after maintenance, ABB replaces the circuit boards with brand new ones instead of repairing the existing ones. We feel more at ease using it.