Analysis of the 8 Major Causes of Faults in Inverters and Preventive Measures

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The inverter consists of several components, including the main circuit, power supply circuit, IPM drive and protection circuit, and cooling fan. Its structure is mostly in a unitized or modular form. Due to incorrect usage methods or inappropriate setup conditions, it is easy for the frequency converter to malfunction or experience faults, or fail to achieve the desired operating performance. To prevent problems before they occur, it is particularly important to conduct a thorough analysis of the causes of failures in advance. 1. Analysis of common faults in the main circuit The main circuit mainly consists of components such as three-phase or single-phase rectifier bridges, smoothing capacitors, filter capacitors, IPM inverter bridges, current-limiting resistors, and contactors. Many of these common failures are caused by electrolytic capacitors. The lifespan of an electrolytic capacitor is primarily determined by the DC voltage applied across it and its internal temperature. Since the capacitor model has been selected during circuit design, the internal temperature plays a decisive role in the lifespan of the electrolytic capacitor. Electrolytic capacitors have a direct impact on the service life of inverters; generally, for every 10°C increase in temperature, the service life is reduced by half. Therefore, on the one hand, an appropriate ambient temperature should be considered during installation, and on the other hand, measures can be taken to reduce pulsating current. The use of AC or DC reactors to improve the power factor can reduce pulsating current, thereby extending the lifespan of electrolytic capacitors. During capacitor maintenance, the electrostatic capacitance, which is relatively easy to measure, is usually used to assess the degradation of electrolytic capacitors. When the electrostatic capacitance falls below 80% of the rated value and the insulation impedance is below 5 MΩ, it is necessary to consider replacing the electrolytic capacitor. 2. Analysis of typical faults in the main circuit – Fault phenomenon: The inverter trips due to overcurrent during acceleration, deceleration, or normal operation. First, it is necessary to determine whether it is caused by the load or by the inverter. If it is a fault with the frequency converter, the current at the time of tripping can be checked using the historical records; if this current exceeds the rated current of the frequency converter or the set value of the electronic thermal relay, and the three-phase voltage and current are balanced, then it is necessary to consider whether there is an overload or some sudden change, such as a motor sticking. When the load inertia is high, the acceleration time can be appropriately increased; this process does no damage to the inverter itself. If the current at the time of tripping is within the rated current of the inverter or within the set range of the electronic thermal relay, it can be determined that there is a fault in the IPM module or related components. Firstly, it is possible to determine whether the IPM module is damaged by measuring the forward and reverse resistances between the output terminals U, V, and W of the inverter’s main circuit and the P and N terminals on the DC side, respectively. If the module is not damaged, then there is a fault in the drive circuit. If an IPM module experiences overcurrent during deceleration, or if the inverter trips due to a short circuit to ground, it is usually a fault with the modules in the upper half of the inverter’s bridge circuit or its drive circuits. If overcurrent occurs in the IPM modules during acceleration, then it is a fault with the modules in the lower half of the bridge circuit or its drive circuits. The causes of such faults are often external dust entering the inverter or high humidity in the environment. 3. Fault analysis of the control circuit: What affects the lifespan of an inverter in terms of the control circuit is the power supply section, namely the smoothing capacitors and the buffer capacitors in the IPM circuit board. The principle is the same as mentioned earlier; however, the pulsating current flowing through these capacitors remains constant and is not affected by the load in the main circuit. Therefore, their lifespan is primarily determined by temperature and the duration of operation. Since the capacitors are all soldered on the circuit board, it is difficult to determine their degree of degradation by measuring their electrostatic capacitance; generally, it is estimated whether they are approaching the end of their useful life based on the ambient temperature of the capacitors and the time they have been in use. The power supply circuit board provides power to the control circuit, the IPM drive circuit, the surface operation display panel, as well as fans, etc. This power is generally DC voltage output from the main circuit, which is then rectified separately using switching power supplies. Therefore, when a short circuit occurs in one power supply line, apart from damaging the rectifier circuit of that particular line, it may also affect other parts of the power supply system. For example, an accidental connection between the control power supply and the common ground can damage the switching power supply section on the power circuit board; a short circuit in the fan power supply can cause other power supplies to lose power, etc. It is generally easy to detect by observing the power supply circuit board. The logic control circuit board is the core of the inverter; it integrates large-scale integrated circuits such as CPUs, MPUs, RAM, and EEPROM. It boasts high reliability, with a low probability of malfunctioning. However, sometimes the activation of the device can cause all the control terminals to close simultaneously, leading to an EEPROM failure in the inverter. This issue can be resolved by resetting the EEPROM. The IPM circuit board includes drive and buffer circuits, as well as overvoltage and under-voltage protection circuits. The PWM signal from the logic control board drives the voltage signal to the IPM module via an optocoupler; therefore, while testing the module, it is also necessary to measure the optocoupler on the IPM module. 4. Cooling system: The cooling system mainly consists of heat sinks and cooling fans. Among them, the cooling fans have a relatively short lifespan. As they approach the end of their service life, they begin to vibrate, generate more noise, and eventually stop rotating; this causes the inverter to trip due to IPM overheating. The lifespan of the cooling fan, limited by the bearings, is approximately 10,000 to 35,000 hours. When the inverter runs continuously, the fan or bearings need to be replaced every 2 to 3 years. To extend the fan’s lifespan, the fans in some products operate only when the inverter is running, rather than when the power supply is turned on. 5. External electromagnetic induction interference: If there are interference sources around the inverter, they can penetrate into the inverter through radiation or power lines, causing errors in the control circuit and leading to abnormal operation or shutdown; in severe cases, it may even damage the inverter. Specific methods to reduce noise interference include: installing surge-absorbing devices, such as RC surge arresters, on the control coils of all relays and contactors surrounding the inverter, with wiring lengths not exceeding 20 cm; minimizing the length of the wiring in the control circuit and keeping it separate from the main circuit; ensuring that the spacing between the twisted pairs in the inverter’s control circuit is at least 15 mm, with a distance of at least 10 cm from the main circuit; when the inverter is located far away from the motor (more than 100 m away), it is possible to increase the cross-sectional area of the wires to keep the voltage drop within 2%, and it is also necessary to install an inverter output reactor to compensate for the charging current resulting from the distributed capacitance caused by long-distance wiring. The grounding terminal of the inverter should be grounded in accordance with regulations; it must be connected to a dedicated grounding point reliably, and it cannot be shared with welding or power grounding connections. A radio noise filter is installed at the input side of the inverter to reduce high-order harmonics in the input signal, thereby minimizing the noise impact from the power lines on the electronic equipment. Meanwhile, a radio noise filter is also installed at the output side of the inverter to reduce line noise at that end as well. 6. Installation environment: The frequency converter is an electronic device, and its manual specifies the detailed requirements for the installation and operating environment. In special cases, where it is truly impossible to meet these requirements, appropriate suppression measures must be taken as much as possible: vibration is the main cause of mechanical damage to electronic devices; in environments with severe vibration, shock-absorbing measures such as rubber should be used. Moisture, corrosive gases, and dust can cause electronic devices to rust, lead to poor contacts, and reduce insulation resulting in short circuits. As preventive measures, control panels should be treated to resist corrosion and dust, and a sealed structure should be employed. Temperature is an important factor affecting the lifespan and reliability of electronic devices, especially semiconductor devices; air conditioning should be installed or direct sunlight should be avoided according to the environmental conditions required by the device. In addition to the points mentioned above, it is also very necessary to regularly check the air filter and cooling fan of the frequency converter. For special high-cold environments, to prevent microprocessors from failing to function properly due to low temperatures, necessary measures such as installing air heaters should be taken. 7. Power supply abnormalities are generally divided into the following 3 types: phase loss, low voltage, and power outage; sometimes mixed forms of these also occur. The main causes of these abnormal phenomena are usually damage to the transmission lines caused by wind, snow, and lightning strikes; sometimes they are also due to ground faults and inter-phase short circuits within the same power supply system. Thunderstorms vary greatly depending on region and season. In addition to voltage fluctuations, some power grids or self-generated power systems also experience frequency fluctuations, and these phenomena sometimes occur repeatedly in short periods of time. To ensure the proper operation of equipment, corresponding requirements are also placed on the power supply for inverters. If there are devices such as directly-starting motors and induction cookers in the vicinity, to prevent voltage drops caused by their activation, their power supplies should be separated from that of the frequency converter in order to reduce mutual interference. For equipment that needs to continue operating after an instantaneous power outage, in addition to selecting an inverter at an appropriate price, the speed reduction ratio of the motor load should also be considered in advance. When both the inverter and the external control circuit employ instantaneous power loss compensation, after voltage is restored, speed measurement via a tachometer is used to prevent overcurrent during acceleration. For equipment that must operate continuously, an automatic switching uninterruptible power supply should be installed alongside the frequency converter. Inverters with diode inputs and single-phase control power supplies can continue to operate even in the presence of a missing phase, but excessive current in certain components of the rectifier as well as high pulse currents in the capacitors can have an adverse effect on the inverter’s lifespan and reliability if such conditions persist over time; therefore, it is necessary to conduct inspections and take corrective actions promptly. 8. Lightning strikes and induced lightning: The surge voltages generated by lightning strikes or induced lightning can sometimes also cause damage to frequency converters. Furthermore, when a vacuum circuit breaker is present on the primary side of the power supply system, short-circuit switching generates high impulse voltages. To prevent overvoltage damage caused by surge voltages, it is usually necessary to install absorption devices such as varistors at the input of the inverter. RC surge arresters should be added to vacuum circuit breakers. If there is a vacuum circuit breaker on the primary side of the transformer, the control sequence should ensure that the frequency converter is disconnected before the vacuum circuit breaker operates. Original title: Analysis of the 8 Major Causes of Inverter Failures and Preventive Measures