In modern society, energy is in short supply and the price of energy has risen sharply. All industries advocate energy conservation. In particular, the power industry, all equipment is energy-intensive. The use of frequency converters can greatly reduce energy consumption. Therefore, frequency converters are used in equipment of all sizes. Frequency converters are used throughout the power plant. The safe operation of frequency converters has become a key link in the power plant. Once a frequency converter has a problem, it will lead to damage or even explosion of large equipment, endangering the safe operation of the power plant and causing incalculable economic losses to the power plant. It is evident that it is imperative for power industry personnel, especially power plant staff, to master some knowledge of frequency converter faults and fault analysis, so as to be able to detect the operating status of frequency converters in the first place. The following are ten knowledge of frequency converter fault phenomena and fault analysis, which I hope will be helpful to you. I. Overcurrent (OC) Overcurrent is the most frequent phenomenon of frequency converter alarm. 1.1 Phenomenon (1) When restarting, it trips as soon as the speed is increased. This is a very serious overcurrent phenomenon. The main reasons are: load short circuit, mechanical parts are stuck; inverter module is damaged; motor torque is too small, etc. (2) Tripping immediately upon power-on. This phenomenon is generally unrecoverable. The main reasons are: module failure, drive circuit failure, and current detection circuit failure. (3) Tripping not immediately upon restart but during acceleration. The main reasons are: acceleration time setting is too short, current upper limit setting is too small, and torque compensation (V/F) setting is too high. 1.2 Case Study An LG-IS3-4 3.7kW frequency converter trips "OC" immediately upon startup. Analysis and repair: No signs of burnt-out were found after opening the cover. Online measurement of IGBT (7MBR25NF-120) basically indicates no problem. To further determine the problem, the IGBT was removed and the high-power transistors of the 7 units were measured. The switching on and off states were good. When measuring the drive circuit of the upper half-bridge, one circuit was found to be significantly different from the other two. After careful inspection, it was found that the output pin of an optocoupler A3120 was short-circuited to the negative terminal of the power supply. After replacement, the three circuits were basically the same. The module was installed and powered on, and everything ran smoothly. II. Overvoltage (OU) Overvoltage alarms generally occur during shutdown, primarily due to insufficient deceleration time or a damaged braking resistor. 2.1 Example A Taian N2 series 3.7kW frequency converter tripped "OU" during shutdown. Analysis and Repair: Before repairing this machine, it's crucial to understand the cause of the "OU" alarm. This is because during deceleration, the motor rotor windings cut the rotating magnetic field at an increased speed, increasing the rotor's electromotive force and current, putting the motor in a generating state. The feedback energy flows to the DC link through the diode connected in parallel with the high-power switching transistor in the inverter stage, causing the DC bus voltage to rise. Therefore, we should focus on checking the braking circuit. The discharge resistor was found to be normal. However, the braking transistor (ET191) was found to be short-circuited. After replacement, the machine was powered on and ran without problems, even with rapid shutdown. III. Undervoltage (Uu) Undervoltage is also a common problem encountered during use. The main reason is that the main circuit voltage is too low (below 200V for the 220V series and below 400V for the 380V series). The main causes are: damage to one circuit of the rectifier bridge or malfunction of any of the three thyristor circuits, which can lead to undervoltage faults. Secondly, a damaged main circuit contactor can cause DC bus voltage loss across the charging resistor, potentially leading to undervoltage. Another cause is a fault in the voltage detection circuit. 3.1 Example A Danfoss VLT5004 frequency converter displays normal operation upon power-on, but trips with a "DC LINK UNDERVOLT" error after applying a load. Analysis and Repair: This inverter appears unusual at first glance, but a closer analysis reveals the problem is not so complex. This inverter also uses a charging circuit and contactor to complete the charging process. No abnormalities were observed upon power-up. It is estimated that the issue is caused by a voltage drop in the DC circuit when a load is applied. The DC circuit voltage is provided by full-wave rectification via a rectifier bridge, followed by smoothing by a capacitor. Therefore, the rectifier bridge should be the primary focus. Measurement revealed an open circuit in one arm of the rectifier bridge. Replacing it with a new one resolved the problem. IV. Overheating (OH) Overheating is also a relatively common fault. Main causes include: excessively high ambient temperature, fan stall, malfunctioning temperature sensor, and motor overheating. 4.1 Example A customer reported that an ABB ACS500 22kW inverter tripped with an "OH" signal after approximately half an hour of operation. Analysis and Repair: Since the fault occurred after a period of operation, the temperature sensor was unlikely to be faulty. It's more likely that the inverter temperature was indeed too high. After powering on, the fan was found to rotate slowly, and the protective cover was clogged with a lot of lint (as this inverter is used in the textile industry). After cleaning, the fan ran well, and the fault did not recur after several hours of operation. V. Output Imbalance Output imbalance generally manifests as motor vibration and unstable speed. Main causes include: faulty module, faulty drive circuit, faulty reactor, etc. 5.1 Example A Fuji G9S 11KW inverter had an output voltage difference of approximately 100V. Analysis and Repair: Initial online inspection of the inverter module (6MBI50N-120) revealed no problems. Measurement of the 6-channel drive circuit also showed no faults. Upon removing the module, it was found that one of the high-power transistors on the bridge could not conduct and shut down properly; this module was damaged. After confirming the drive circuit was fault-free, replacing it with a new one resolved the issue. VI. Overload Overload is also one of the most frequent inverter faults. When encountering overload, we should first analyze whether it is the motor or the inverter itself that is overloaded. Generally speaking, motors have a strong overload capacity, and as long as the motor parameters in the inverter's parameter table are set correctly, motor overload is unlikely to occur. However, inverters themselves have a poor overload capacity and are prone to overload alarms. We can check the inverter's output voltage. VII. Power Supply Failure This is the most common fault in many inverters, usually caused by a short circuit in the load of the switching power supply. Danfoss inverters use the new pulse width integrated controller UC2844 to adjust the output of the switching power supply. The UC2844 also has current detection and voltage feedback functions. When there is no display, no voltage at the control terminals, or the DC 12V and 24V fans not running, we should first consider whether the switching power supply is damaged. VIII. SC Fault SC fault is a common fault in Yaskawa inverters. Damage to the IGBT module is one of the causes of SC fault alarms. In addition, damage to the drive circuit can also easily lead to SC fault alarms. In its drive circuit design, Yaskawa uses the PC923 drive optocoupler for the upper bridge. This is an optocoupler with an amplifier circuit specifically designed for driving IGBT modules. Yaskawa's lower bridge drive circuit uses the PC929 optocoupler, which has an internal amplifier and detection circuit. Furthermore, motor vibration, three-phase current and voltage imbalance, and frequency display without voltage output are all possible signs of IGBT module damage. There are several reasons for IGBT module damage. First, external load failures can damage the IGBT module, such as short circuits or stalling. Second, aging of the drive circuit can lead to distorted drive waveforms or excessive drive voltage fluctuations, causing IGBT damage and resulting in an SC fault alarm. IX. GF—Grounding Fault Grounding faults are also common. Besides motor grounding issues, the most likely cause is the Hall sensor. Hall sensors are easily affected by environmental factors such as temperature and humidity, causing their operating point to drift and triggering a GF alarm. X. Current Limiting Operation During normal operation, we may encounter the inverter indicating a current limit. When a typical frequency converter experiences a current-limiting alarm, it cannot operate smoothly. The voltage (frequency) first drops until the current falls below the allowable range. Once the current is below the allowable value, the voltage (frequency) will rise again, leading to system instability. Danfoss frequency converters employ internal slope control to find the operating point without exceeding the predetermined current limit and control the motor to run smoothly at the operating point. A warning signal is then fed back to the customer, who then checks the load and motor for problems based on the warning information.