Analysis of common causes and preventive measures of inverter failures - Fuji Electric

1. Analysis of common faults in the main circuit

The main circuit is mainly composed of three-phase or single-phase rectifier bridges, smoothing capacitors, filter capacitors, IPM inverter bridges, current limiting resistors, contactors and other components. Many of these common faults* are caused by electrolytic capacitors. The life of an electrolytic capacitor is mainly determined by the DC voltage and internal temperature applied to both ends, and the type of capacitor has been selected in the circuit design, so the internal temperature plays a decisive role in the life of the electrolytic capacitor. Electrolytic capacitors have a direct impact on the service life of the inverter, which is halved for every 10°C increase in temperature. Therefore, on the one hand, the appropriate ambient temperature should be taken into account during installation, and on the other hand, measures can be taken to reduce the pulsating current. The use of AC or DC reactors with improved power factor can reduce the ripple current, thereby extending the life of the electrolytic capacitor.

When the capacitance is less than 80% of the rated value and the insulation resistance is less than 5 MΩ, the electrolytic capacitor should be replaced.

2. Typical fault analysis of the main circuit

Fault phenomenon: The inverter trips with overcurrent during acceleration, deceleration or normal operation.

First of all, it is necessary to distinguish whether it is caused by the load or by the inverter. If it is the fault of the inverter, the current at the time of tripping can be queried through the historical record, which exceeds the rated current of the inverter or the set value of the electronic thermal relay, and the three-phase voltage and current are balanced, then whether there is overload or sudden change, such as motor stall rotor, etc. When the load inertia is large, the acceleration time can be appropriately extended, and this process does not damage 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 judged that the IPM module or related parts are faulty. First of all, you can judge whether the IPM module is damaged by measuring the forward and reverse resistance between the output terminals U, V and W of the main circuit of the inverter and the P and N terminals on the DC side. If the module is not damaged, the drive circuit is faulty. If the IPM module is overcurrent or the inverter trips in a short circuit to the ground during deceleration, it is generally the module of the upper half of the inverter or its driving circuit that is faulty; The overcurrent of the IPM module during acceleration is a partial fault of the module or its driving circuit in the lower half of the bridge, and the cause of these failures is mostly caused by external dust entering the inverter or the environment is wet.

3. Control loop failure analysis

The control loop affects the life of the inverter is the power supply part, which is the smoothing capacitor and the buffer capacitor in the IPM circuit board, the principle is the same as the above, but the pulsating current passing through the capacitor here is basically a fixed value that is not affected by the load of the main circuit, so its life is mainly determined by the temperature and power-on time. Since capacitors are soldered to circuit boards, it is difficult to determine the deterioration by measuring the capacitance, and it is common to estimate whether the capacitor is approaching its useful life based on the ambient temperature and usage time of the capacitor.

The power supply circuit board provides power to the control loop, IPM drive circuit, surface operation display board, and fan, etc., and these power supplies are generally obtained by rectifying the DC voltage output from the main circuit through the switching power supply. Therefore, a short circuit of a power supply, in addition to the damage of the rectifier circuit of the circuit, may also affect the power supply of other parts, such as the control power supply and the common ground short circuit due to misoperation, resulting in the damage of the switching power supply part on the power supply circuit board, and the short circuit of the fan power supply causes other power supplies to be powered off. Generally, it is easy to find by looking at the power supply circuit board.

The logic control circuit board is the core of the inverter, which concentrates CPU, MPU, RAM, EEPROM and other large-scale integrated circuits, has high reliability, and the probability of failure itself is very small, but sometimes all the control terminals are closed at the same time due to start-up, resulting in EEPROM failure of the inverter, which only needs to reset the EEPROM.

The IPM circuit board contains driver and snubber circuits, as well as protection circuits such as overvoltage and phase loss. The PWM signal from the logic control board feeds the voltage drive signal into the IPM module through optocoupler, so the optocoupler on the IPM module should be measured at the same time as the mode speed is detected.

4. Cooling system

The cooling system mainly consists of heat sinks and cooling fans. Among them, the life of the cooling fan is short, and when the service life is approaching, the fan vibrates, the noise increases and then stops, and the inverter overheats and trips IPM. The life of the cooling fan is trapped by the bearing, which is about 10000-35000h. When the inverter runs continuously, the fan or bearing needs to be replaced every 2-3 years. In order to extend the life of the fan, some products only run the fan when the drive is running and not when the power is on.

5. External electromagnetic induction interference

If there are interference sources around the inverter, they will invade the inside of the inverter through radiation or power lines, causing the control loop to malfunction, causing abnormal operation or shutdown, and even damage to the inverter in severe cases. The specific methods to reduce noise interference are: on the control coils of all relays and contactors around the inverter, install absorption devices to prevent impulse voltage, such as RC surge absorbers, and the wiring of which cannot exceed 20cm; Minimize the wiring distance of the control loop and separate it from the main circuit; The distance between the wiring and stranding joints of the inverter control loop should be more than 15mm, and the distance between the inverter control loop and the main circuit should be more than 10cm; When the inverter is far away from the motor (more than 100m), on the one hand, the cross-sectional area of the wire can be increased to ensure that the voltage drop of the line is within 2%, and the output reactor of the inverter should be installed at the same time to compensate for the charging current of the distributed capacitance generated by the long-distance wire. The grounding terminal of the inverter shall be grounded according to the regulations, and must be reliably grounded at the special grounding point, and cannot be mixed with electric welding and power grounding; A radio noise filter is installed at the input of the inverter to reduce the input of higher harmonics, thereby reducing the noise impact from the power line to the electronic equipment; At the same time, a radio noise filter is also installed at the output end of the inverter to reduce the line noise at the output end.

6. Installation environment

The inverter is an electronic device device, and there are detailed requirements for the installation and use environment in its manual. In special circumstances, if these requirements cannot be met, corresponding suppression measures must be adopted as much as possible: vibration is the main cause of mechanical damage to electronic devices, and for occasions with large vibration impact, rubber and other vibration protection measures should be used; Moisture, corrosive gases and dust will cause electronic devices to corrode, poor contact, insulation reduction and form a short circuit, as a preventive measure, the control board should be treated with anti-corrosion and dust-proof, and a closed structure should be adopted; Temperature is an important factor affecting the life and reliability of electronic devices, especially semiconductor devices, air conditioning should be installed according to the environmental conditions required by the device or avoid direct sunlight.

In addition to the above points, it is also necessary to regularly check the air filter and cooling fan of the inverter. For special alpine occasions, in order to prevent the microprocessor from working normally due to low temperature, necessary measures such as setting up air heaters should be taken.

7. The power supply is abnormal

There are three types of power supply abnormalities, namely phase loss, low voltage, and power outage, and sometimes a mixture of them. The main causes of these anomalies are mostly caused by wind, snow and lightning strikes on transmission lines, and sometimes also due to short circuits to the ground and interphase short circuits in the same power supply system. Lightning strikes, on the other hand, vary greatly depending on the region and season. In addition to voltage fluctuations, some power grids or self-generating units, there will also be frequency fluctuations, and these phenomena are sometimes repeated in a short period of time, in order to ensure the normal operation of the equipment, the inverter power supply also puts forward corresponding requirements.

If there are directly started motors and induction cookers nearby, in order to prevent the voltage drop caused by the commissioning of these equipment, the power supply should be separated from the power supply of the inverter to reduce the mutual influence.

For equipment that requires continuous operation after a momentary power outage, in addition to choosing a frequency converter at the right price, the proportion of motor load should be considered in advance. When both the inverter and the external control loop adopt the instantaneous power failure compensation method, after the loss of voltage is restored, the speed measurement motor is used to prevent overcurrent during acceleration.

For equipment that requires continuous operation, the inverter should be equipped with an uninterruptible power supply device with automatic switching. For example, the inverter with diode input and single-phase control power supply can continue to work even though it is in a state of phase loss, but the current of individual devices in the rectifier is too large, and the pulse current of the capacitor is too large, if the long-term operation will have a negative impact on the life and reliability of the inverter, it should be checked and dealt with as soon as possible.

8. Lightning strikes, induction lightning

The impulse voltage generated by lightning strikes or induced lightning strikes can sometimes cause damage to the inverter. In addition, when there is a vacuum circuit breaker on the primary side of the power supply system, the short-circuit opening and closing will produce a high impulse voltage. In order to prevent overvoltage damage due to impulse voltage, it is usually necessary to add an absorber such as a varistor at the input end of the inverter. RC surge absorbers should be added to vacuum circuit breakers. If there is a vacuum circuit breaker on the primary side of the transformer, the inverter should be disconnected before the vacuum circuit breaker is operated in the control sequence.

Analysis of common causes and preventive measures of inverter failures - Fuji Electric

Analysis of common causes and preventive measures of inverter failures - Fuji Electric

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