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I. Overcurrent (OC) Overcurrent is the most frequent type of alarm that occurs in frequency converters. 1.1 Phenomenon (1): It trips as soon as it speeds up during restart. This is a phenomenon of extremely severe overcurrent. The main reasons include: load short circuit, mechanical jamming; damaged inverter modules; insufficient torque of the motor, among other issues. (2) It trips as soon as power is applied; this phenomenon generally cannot be reset. The main reasons are: a faulty module, a faulty drive circuit, or a faulty current detection circuit. (3) The reason why tripping does not occur immediately upon restart but rather during acceleration is mainly due to an excessively short acceleration time, a too low current limit, and a high torque compensation (V/F) setting. 1.2 Example (1): An LG-IS3-4 3.7kW inverter starts up and immediately displays an “OC” error. Analysis and repair: Upon opening the cover, no signs of damage were found. An online measurement of the IGBT (7MBR25NF-120) indicated that it was functioning properly. To further diagnose the issue, the IGBT was removed, and it was confirmed that all 7 high-power transistors operated well both when turned on and off. When measuring the drive circuits of the upper half of the bridge, it was found that one circuit differed significantly from the other two. Upon closer inspection, it was discovered that the output terminal of an optocoupler A3120 was short-circuited to the negative power supply terminal; after replacing it, the three circuits became almost identical. After the module was installed and powered on, everything worked perfectly. (2) A BELTRO-VERT 2.2kW unit trips with an “OC” error as soon as it is powered on, and cannot be reset. Analysis and repair: First, check the inverter module to ensure no issues are found. Next, the drive circuit was checked and no abnormalities were found; it is estimated that the problem does not lie in this area. It is likely to be related to the overcurrent signal processing section. After removing the circuit sensor and powering on the device, everything worked normally, so it was determined that the sensor was damaged. A new sensor was installed, and testing with a load showed normal operation. II. Overvoltage (OU): Overvoltage alarms generally occur during shutdown, and the main causes are too short a deceleration time or problems with the braking resistors and braking units. (1) Instance: The N2 series 3.7kW frequency converters, installed one by one, trigger an “OU” error when shut down. Analysis and repair: Before repairing this machine, it is necessary to first determine the cause of the “OU” alarm. This occurs because when the frequency converter slows down, the speed at which the motor rotor windings cut through the rotating magnetic field increases, resulting in an increase in the rotor’s electromotive force and current. This causes the motor to generate electricity, and the energy generated is directed back to the DC circuit through diodes connected in parallel with the high-power switching transistors in the inverter section, thereby raising the voltage of the DC bus. Therefore, we should focus on checking the braking circuit; the discharge resistors were found to be fine. Upon testing the braking transistor (ET191), it was discovered that it had been damaged. After replacing it, the machine could operate normally when powered on, and rapid stopping also worked without any problems. III. Under-voltage (Uu) Under-voltage is another issue we often encounter in use. The main reason is that the voltage in the main circuit is too low (for 220V systems, it’s below 200V; for 380V systems, it’s below 400V). Possible causes include damage to one of the diode bridges or malfunctioning thyristors, all of which can lead to under-voltage conditions. Another cause is damage to the contactors in the main circuit, which can result in voltage loss across the charging resistors and thus under-voltage. Additionally, faults in the voltage detection circuit can also cause under-voltage issues. 3.1 Example (1): A CT 18.5kW inverter shows a “Uu” error upon power-up. Analysis and repair: Upon inspection, the charging resistors of the rectifier bridge in this inverter were found to be in good condition. However, no sound was heard indicating that the contactor was activating after power was applied; since the charging process in this inverter relies on the contactor’s engagement rather than thyristors, it is assumed that the fault may lie with the contactor, the control circuit, or the power supply. The contactor was removed and powered with 24V DC directly, and it worked properly under these conditions. Next, the 24V DC power supply was checked; upon thorough inspection, it was found that this voltage was output after being regulated by an LM7824 voltage regulator. The regulator was found to be damaged, so a new one was installed, and the system worked properly once powered on again. (2) A DANFOSS VLT5004 inverter – it shows normal operation when powered on, but the error “DC LINK UNDERVOLT” appears once a load is applied. Analysis and repair: This inverter exhibits some unusual behavior, but upon closer analysis the issue isn’t that complicated. This inverter also uses a charging circuit along with contacts to carry out the charging process. No abnormalities were observed when it was powered on; it is likely that the voltage in the DC circuit drops when a load is applied. The voltage in the DC circuit is provided after full-wave rectification by a rectifier bridge, followed by smoothing by capacitors. Therefore, the rectifier bridge should be checked carefully. Measurements revealed that one of the bridge arms in this rectifier bridge was open circuit, and replacing it with a new one resolved the problem. IV. Overheating (OH) Overheating is also a relatively common fault. The main causes include high ambient temperatures, fan stalling, poor performance of temperature sensors, and motor overheating. For example, a customer reported that an ABB ACS500 22kW inverter would display an “OH” error after operating for about half an hour. Analysis and repair: Since the fault occurred after the unit had been in operation for some time, it is unlikely that the temperature sensor is damaged. It’s possible that the temperature of the inverter was indeed too high. After powering it on, it was observed that the fan rotated slowly, and the protective cover was filled with a lot of cotton fluff (as this inverter is used in the textile industry). After cleaning it, the fan operated properly when started up, and no such faults occurred again after several hours of operation. V. Unbalanced output: Unbalanced output is typically manifested as motor vibration and unstable rotation speed. The main causes include a faulty module, a defective drive circuit, or a damaged reactor. 5.1 Example: A Fuji G9S 11KW inverter has an output voltage difference of around 100V. Analysis and repair: The machine was turned on for a preliminary online inspection, and no issues were found with the inverter module (6MBI50N-120). Testing of the 6 drive circuits also revealed no faults. Upon removing the module and conducting further tests, it was discovered that one of the high-power bridge transistors could not conduct or turn off properly; this module was damaged. After confirming that there were no faults in the drive circuits, a new module was installed, and everything worked normally again. VI. Overload: Overload is also one of the common faults that cause frequent fluctuations in frequency converters. When we encounter an overload situation, we should first determine whether it is the motor that is overloaded or the frequency converter itself. Generally speaking, motors have a high capacity to handle overload; as long as the motor parameters set in the frequency converter’s settings are appropriate, motor overload is unlikely to occur. On the other hand, frequency converters have a lower capacity to handle overload, so they are prone to generating overload alarms. We can check the output voltage of the frequency converter to detect this issue. VII. Damaged switching power supply: This is the most common fault in many frequency converters, usually caused by a short circuit in the load of the switching power supply. Danfoss frequency converters use a new type of pulse width modulation integrated controller, the UC2844, to regulate the output of the switching power supply; this controller also has functions such as current detection and voltage feedback. When phenomena such as no display, no voltage at the control terminals, and non-functioning DC12V/24V fans occur, we should first consider whether the switching power supply is damaged. VIII. SC fault: The SC fault is a relatively common issue with Yaskawa frequency converters. The IGBT module is damaged, which is one of the reasons for the SC fault alarm. Furthermore, damaged drive circuits can also easily cause SC fault alarms. In the design of its drive circuits, Yaskawa uses the drive optocoupler PC923 for the upper bridge; this is an optocoupler equipped with an amplification circuit and designed specifically for driving IGBT modules. For the lower bridge drive circuits, Yaskawa employs the optocoupler PC929, which features an internal amplification circuit as well as a detection circuit. Furthermore, motor vibration, unbalanced three-phase current and voltage, as well as the presence of a frequency display without any voltage output, are all possible signs of damaged IGBT modules. There are various reasons for the damage of IGBT modules. Firstly, failures in the external load can lead to damage to the IGBT module, such as short circuits or stall conditions in the load. Secondly, aging of the drive circuit may also cause distortion in the drive waveform, or excessive fluctuations in the drive voltage that can damage the IGBT, thereby triggering an SC fault alarm. IX. GF – Ground Fault: Ground faults are also common types of faults that occur from time to time. Apart from addressing issues related to motor grounding, the most likely source of such faults is the Hall sensor. Due to the influence of environmental factors such as temperature and humidity, the operating point of the Hall sensor can easily shift, resulting in a GF alarm. X. Operation with current limitation: During normal operation, we may encounter a message from the inverter indicating that the current limit has been reached. For ordinary frequency converters, they cannot operate smoothly when a current-limiting alarm is triggered; the voltage (frequency) must first be reduced until the current drops to an acceptable level. Once the current falls below this acceptable value, the voltage (frequency) rises again, thereby causing instability in the system. Danfoss inverters use internal slope control to find the operating point without exceeding the predetermined current limit, ensuring that the motor operates smoothly at that point. They also send warning signals to the customer; based on these warnings, we can check whether there are any issues with the load or the motor.