Inverter lifespan and maintenance

With the development of factory automation technology, frequency converters, with their advantages of convenient speed regulation, reliable performance, and significant energy savings, are increasingly becoming important drive and control equipment, widely used in industries such as textiles, petrochemicals, metallurgy, hoisting, wire drawing, brewing and beverage, and building control. Because the electronic components used in frequency converters have a limited lifespan, and factors such as the installation environment and mains voltage also affect their lifespan, routine maintenance is crucial to ensuring reliable and continuous operation and extending their service life.

I. Basic Structure of Frequency Converter:  

The image above shows the structure of a frequency converter, which mainly consists of the following four parts:

1. Rectifier

The function of a rectifier is to rectify the input three-phase alternating current (AC) into direct current (DC). There are two basic types of rectifiers: controlled and uncontrolled. Uncontrolled rectifiers consist of diodes, while controlled rectifiers contain oxygen.

2. Inverter

An inverter is the output stage of a frequency converter. An inverter can convert direct current into alternating current of any frequency. Currently, almost all inverters are composed of insulated gate field-effect transistors (IGBTs), while early inverters were mostly composed of high-power transistors.

3. Intermediate circuit

The intermediate circuit functions as a filter and energy storage circuit. It is mainly composed of high-voltage, high-capacity electrolytic capacitors and reactors. The capacitors are approximately ideal voltage sources, providing input power to the inverter.

4. Control Circuit

The control circuit is responsible for all control tasks, including acquiring main circuit signals, receiving control commands, controlling the gate triggering of the inverter IGBTs, and providing a human-machine interface.

II. Main factors affecting the service life of frequency converters:

      Based on the inverter's structure, the main internal components include resistors, capacitors, inductors, semiconductor devices, PCB boards, and cooling fans. Properly designed inductors, resistors, and PCB boards have a long service life; however, the environment still significantly impacts the lifespan of components such as resistors and PCB boards. The average lifespan of semiconductor devices is up to 10,000 years (the lifespan of transistor packaging and wire bonding is much shorter), so active failure of semiconductor devices is generally not discussed. Damage to high-power transistors and diodes in the main circuit is usually caused by three extreme factors: overvoltage, overcurrent, and overheating.

Cooling fans have a limited lifespan, primarily due to the limited lifespan of their bearings. Currently, the best models can achieve 50,000 hours, roughly equivalent to 5 years of continuous operation at full speed. Therefore, Danfoss employs intelligent control for its cooling fans. When the heatsink temperature is below 40°C, the cooling fan remains off; then, as the heatsink temperature increases, the fan speed gradually increases until it reaches full speed. This maximizes the fan's lifespan.

Large-capacity aluminum electrolytic capacitors have a limited lifespan, which depends on product selection, ambient temperature, and the effective value of the charging current. Danfoss frequency converters use electrolytic capacitors with the highest operating temperature and longest lifespan. Furthermore, the main circuit employs LC filtering, which significantly suppresses the effective value of the charging current. Therefore, the lifespan of the electrolytic capacitors in Danfoss frequency converters is comparable to that of the entire unit.

III. The main reasons why inverters are easily damaged or have their service life shortened during use:

1. Poor operating environment

Unfavorable environmental conditions include excessive humidity, corrosive gases, excessive dirt or oil, and high operating temperatures. Excessive humidity can reduce the internal insulation of the inverter, causing discharge breakdown, leading to PCB board mold growth, wire breakage, or poor connector contact. It also accelerates the oxidation of copper busbars and metal structural components, resulting in poor contact in the main circuit or power terminal blocks. Corrosive gases and excessive dirt or oil in the environment cause significant damage to the PCB board and small electronic components, significantly reducing the inverter's lifespan. Danfoss addresses environmental corrosion by applying an enhanced coating to all PCBs; however, many inverters on the market do not employ any PCB board protection measures. Excessive dirt can also disrupt the dynamic balance of the fan impeller, causing additional fan vibration, which greatly shortens the lifespan of the fan bearings. High operating temperatures affect the lifespan of many components, including resistors, semiconductor devices, electrolytic capacitors, and fans. The lifespan of these components is typically exponentially related to ambient temperature; for example, for every 10°C decrease in operating temperature, the lifespan of electrolytic capacitors doubles. Therefore, maintaining the heat dissipation channels of the frequency converter and ensuring the reliable and normal operation of the cooling fan are of great significance for extending the service life of the frequency converter.

1. Inverter output short circuit or short circuit to ground

Short circuits can be caused by phase-to-phase short circuits in the motor, short circuits to ground in the motor, or short circuits to ground caused by poor insulation or worn sheaths in the motor cables. The IGBTs in the inverter module of a frequency converter have a very short withstand time for short-circuit overcurrent; typically, short-circuit time is not allowed to exceed 10μS. Therefore, frequency converters have very fast overcurrent detection and protection circuits. However, despite this, the current value during a short circuit is very large (approximately 10 times the rated value), which puts enormous stress on the semiconductor devices. Semiconductor manufacturers generally believe that short-circuit overcurrents cannot be periodically repeated, and short-circuit overcurrent protection has a probabilistic characteristic, meaning that short-circuit overcurrent protection is not 100% successful. Frequent output short circuits, especially output-to-ground short circuits, can cause fatal damage to the frequency converter. Therefore, daily inspections of the insulation of the motor and motor cables are necessary to ensure the normal and reliable operation of the equipment.

1. The input grid surge voltage is too high.

The main causes of input voltage surges include direct lightning strikes to the power grid, disconnection of large inductive loads in the power grid, opening and closing of large power factor compensators, large DC motor loads in the same power grid segment, load short circuits causing power supply tripping, and arcing due to poor insulation of power lines. Input voltage surges can cause fatal damage to the inverter's input rectifier module. Since the power grid voltage surges originate from external sources, the inverter itself cannot provide active protection, only passive protection. The main protection methods are to install metal oxide varistors and high-frequency absorption capacitors on the inverter's power input side, and to install absorption capacitors or electrostatic discharge tubes on the rectifier module. The inverter's input voltage surge protection is usually designed according to a fixed standard, which can meet the needs of most applications, but is not suitable for a few extreme application environments. If damage to the inverter's varistor is found during routine inspections, or if an investigation reveals a previous tripping of the upstream automatic switch, it is necessary to consider installing external surge absorbers and fast-acting fuses.

IV. Conclusion: The key to extending the service life of frequency converters lies in routine maintenance.

In conclusion, to ensure the reliable and continuous operation of frequency converters and extend their service life, routine maintenance is crucial. Furthermore, problems identified during maintenance should be addressed promptly and accurately to mitigate adverse effects on practical applications, taking into account external operating environment and power grid quality.