Abstract This paper introduces common faults and maintenance methods of frequency converters, and briefly describes the causes of these faults and their prevention and control measures. Keywords: Frequency converter; Fault analysis; Maintenance 0 Introduction With the continuous development of industrial automation, frequency converters have been widely used in various industries. Frequency converter technology has become increasingly sophisticated and mature, with more powerful functions and improved reliability. However, improper use, incorrect operation, or untimely maintenance can still lead to faults or shutdowns, thus shortening the equipment's lifespan. Typically, after 5-8 years of normal use, frequency converters enter a period of high failure incidence, frequently experiencing component burnout, frequent protection circuit activation, and other faults that seriously affect their normal operation. Therefore, daily maintenance and repair are particularly important. 1. Common Inverter Faults and Repair Solutions 1.1 Inverter Rectifier Module Damage Damage to the inverter rectifier module is one of the common faults of inverters. Early inverter rectifier modules used diodes, but currently, most rectifier modules use thyristors. Medium and high power ordinary inverter rectifier modules are generally three-phase full-wave rectifiers. Rectifier devices are prone to overheating and breakdown. When they fail, the fast-acting fuse blows, and the entire machine shuts down. When replacing the rectifier module, a layer of thermally conductive silicone grease should be evenly applied to the surface in contact with the heat sink before tightening the mounting screws. If the same model of rectifier module is unavailable, other types of rectifier modules of the same capacity can be used as a substitute. For example, the Fuji G7S uses a rectifier module with thyristor protection. The difference between it and ordinary rectifier modules is that it uses thyristors instead of the main circuit contactor, improving the reliability of the inverter. The Fuji G9S low-power inverter rectifier module integrates the thyristor and the switching transistor into one unit. Damage to the rectifier module is often closely related to the external power supply of the machine. Therefore, when the rectifier module fails, do not blindly power on the machine; check the peripheral equipment first. 1.2 Inverter Charging Circuit Faults General inverters are typically voltage-type inverters, using an AC-DC-AC operating mode. Due to the large capacitance of the smoothing capacitor on the DC side, the charging current is very large at the moment the inverter is connected to the power supply, which may cause the power switch to trip. Therefore, a starting resistor is set in the charging circuit to limit the charging current. After charging is complete, the control circuit short-circuits the resistor through the contactor contacts or thyristor. Charging circuit faults generally manifest as a burnt-out starting resistor, with the inverter alarm displaying a DC bus voltage fault. Frequent switching on and off of the inverter's AC input power supply, poor contact of the short-circuit contactor, or an increased on-resistance of the thyristor can all cause the starting resistor to burn out. In this case, a resistor of the same specification can be purchased for replacement. At the same time, the cause of the burnt resistor must be found. If the fault is caused by the frequent switching of the input power supply, this phenomenon must be eliminated. If the fault is caused by the short circuit of the contactor contacts or the short circuit of the thyristor, these components must be replaced before the inverter can be put into use again. 1.3 Inverter displays overcurrent 1) Overcurrent occurs in the system during operation. The reasons are roughly as follows: (1) The motor current suddenly increases when the motor encounters an impact load or the transmission mechanism is "stuck". (2) The output side of the inverter is short-circuited, such as the connection line between the output end and the motor being short-circuited, or the motor being short-circuited internally. (3) The inverter itself is not working properly, such as the upper and lower devices of the same bridge arm in the inverter bridge being "straight-through", which makes the positive and negative poles of the DC voltage in a short-circuit state. 2) The load has a large inertia, and the acceleration time is set too short. The speed of the motor rotor cannot keep up due to the large load inertia, resulting in a large acceleration current. 3) When the load inertia is large and the deceleration time is set too short, the motor rotor will maintain a high speed due to the large load inertia, resulting in excessive speed at which the rotor windings cut the magnetic lines of force, causing overcurrent. To address the above fault symptoms, check the following aspects: 1) Is the working machinery jammed? 2) Use a megohmmeter to check for short circuits on the load side. 3) Is the inverter power module damaged? 4) Is the motor's starting torque too low, preventing the drive system from starting? 5) Is the acceleration time set too short? 6) Is the deceleration time set too short? 7) Is the torque compensation (V/F ratio) set too high, causing excessive no-load current at low frequencies? 8) Is the electronic thermal relay setting improper, with the operating current set too low, causing the inverter to malfunction? If these issues are not the cause, disconnect the current transformer on the output side and the Hall current sensor on the DC side, reset them, and run the system to see if the overcurrent phenomenon still occurs. This is because Hall sensors that detect current are easily affected by environmental factors such as temperature and humidity, causing their operating point to drift and leading to overcurrent. If the problem persists, it is very likely that the 1PM module is faulty. The 1PM module contains protection functions for overvoltage, overcurrent, undervoltage, overload, overheating, phase loss, and short circuit. Replacing it with the same model should resolve the issue. 1.4 Inverter Overvoltage and Undervoltage Protection Activation Overvoltage and undervoltage protection activation in the inverter is mostly caused by fluctuations in the mains voltage. In the inverter power supply circuit, if a large-load motor is directly started or stopped, it will cause a large instantaneous fluctuation in the mains voltage, leading to the inverter's overvoltage and undervoltage protection activation, preventing normal operation. This situation usually does not last long; normal operation will resume after the mains voltage fluctuation subsides. This situation can only be avoided by increasing the capacity of the power supply transformer and improving the grid quality. In addition, overvoltage faults in frequency converters can also occur due to the converter driving a large inertia load. In this case, the converter's deceleration and stopping is regenerative braking. During the stopping process, the converter's output frequency decreases linearly, while the load motor's frequency is higher than the converter's output frequency. The load motor is in a generating state, converting mechanical energy into electrical energy, which is absorbed by the smoothing capacitor on the DC side of the frequency converter. When this energy is large enough, the voltage on the DC side of the frequency converter will exceed the overvoltage protection setting of the DC bus and trip. For this type of fault, one solution is to set a longer deceleration time parameter, increase the braking resistor, or add a braking unit; another is to set the frequency converter's stopping mode to free stop. Another possibility is that the frequency converter's rectifier section or detection circuit is damaged, causing a fault alarm. Voltage detection generally involves sampling the DC bus voltage, comparing it with the overvoltage protection setting, and then transmitting the difference to the microcontroller. If any component in the rectifier bridge, filter capacitor, sampling circuit, or comparator circuit malfunctions, this alarm will occur. For example, a Danfoss VLT5004 frequency inverter displays "DC LINK UNDERVOLT" (low DC circuit voltage) after power-on, but displays "DC LINK UNDERVOLT" after applying a load. While this seems unusual, careful analysis reveals the problem is not so complex. This inverter also uses a charging circuit and contactors to complete the charging process. No abnormalities were observed upon power-on, suggesting the fault was caused by a voltage drop in the DC circuit under load. Since the DC circuit voltage is provided by full-wave rectification via a rectifier bridge and then smoothed by capacitors, the rectifier bridge should be the primary focus of inspection. Measurement revealed an open circuit in one arm of the rectifier bridge; replacing it resolved the issue. 1.5 Drive Circuit Faults The inverter drive circuit of the frequency inverter is also prone to failure. There are usually obvious signs of damage, such as cracked, discolored, or broken components (capacitors, resistors, diodes, and printed circuit boards), but the entire drive circuit is unlikely to be damaged. The general approach is to follow the schematic diagram and locate the fault point step-by-step for each drive circuit. The first step in troubleshooting is to clean the entire circuit board thoroughly. If any broken wires are found, they should be repaired. Damaged components should be identified and replaced. Based on my practical experience, suspected components should be assessed through measurement, comparison, and substitution. Some components require offline testing. After repairing the drive circuit, use an oscilloscope to observe the output waveforms of each drive circuit signal. If the three-phase pulse magnitudes and phases are unequal, the drive circuit still has abnormalities (mismatched parameters of replaced components can also cause this phenomenon), and the inspection and repair should be repeated. Damage to the high-power transistor drive circuit is also one of the causes of overcurrent protection activation. The most common symptoms of drive circuit damage are phase loss, unequal three-phase output voltages, and unbalanced three-phase currents. For example, a Danfoss VLT5062 frequency converter showed normal operation upon power-on, but the measured three-phase output voltages were unbalanced. When a motor was connected, it alarmed "MISSING MOT, PHASE W" (motor W phase missing). After inspection, it was found that the inverter drive trigger circuit of the W phase had no output waveform. After repairing the W phase drive trigger circuit, the frequency converter worked normally. 1.6 Motor Overheating and Inverter Overload Display: If this fault occurs with an inverter already in operation, the load condition must be checked. For newly installed inverters, this fault is likely due to improper V/F curve settings or incorrect motor parameter settings. For example, a newly installed inverter drives a variable frequency motor with rated parameters of 220 V/50 Hz, while the inverter's factory settings are 380 V/50 Hz. Because the installer did not correctly set the inverter's V/F parameters, the rotor experiences magnetic saturation after a period of operation, causing the motor speed to decrease, leading to overload and overheating. Therefore, the corresponding parameters must be set correctly before using a new inverter. Additionally, when using the inverter's sensorless vector control mode, incorrect settings of the load motor's rated voltage, current, capacity, etc., can also lead to motor overload and overheating. Another scenario is that setting the inverter's carrier frequency too high can also cause motor overload and overheating. The last scenario is when the inverter frequently operates at low frequencies, causing the motor to operate at low frequencies for extended periods. Poor motor cooling leads to overheating after a period of operation. In this case, a cooling system needs to be installed. For example, a customer reported that an ABB ACS500 22 kW inverter displayed "OH" (overheating) after about half an hour of operation. Since the fault occurred after a period of operation, analysis suggested that a faulty temperature sensor was unlikely; the inverter's temperature was likely simply too high. Upon reconnecting the power, it was found that the fan was rotating slowly and the protective cover was clogged with lint (as the inverter is used in the textile industry). After cleaning and restarting, the fan ran well, and the inverter did not exhibit the fault again after several hours. 2. Conclusion During the use of inverters, various problems will inevitably arise. However, as long as the operation and maintenance personnel understand the basic working principles of inverters and continuously summarize their experience in practical operation, these problems can always be solved. References: 1. Siemens Frequency Inverter Handbook; 2. Fundamentals of Motors and Drives, edited by Li Fahai, Tsinghua University Press. Author Biographies: Feng Yongmin, male, college diploma, born in 1977, assistant engineer, many years of experience in electrical maintenance, currently the company's electrical maintenance team leader. Yang Wentao, male, bachelor's degree, born in 1975, engineer, many years of experience in electrical control and computer control automation programming and maintenance, currently the company's production technology department head. Author's Affiliation: Xinshun Industrial Co., Ltd., Shangqiu City, Henan Province. Mailing Address: Production Department, Xinshun Industrial Co., Ltd., No. 55, Longhai South Road, Shangqiu City, Henan Province. Postcode: 476000. Tel: 0370-2331079. Email: [email protected]