Inverter is composed of main loop, power loop, IPM drive and protection loop, cooling fan and so on. Its structure is mostly unitary or modular form. Due to the incorrect use method or unreasonable setting environment, it is easy to cause the misoperation and failure of the frequency converter, or fail to meet the expected operation effect. It is important to analyze the cause of failure carefully in advance to prevent it from happening.

1. Analysis of common faults of the main loop

The main circuit is mainly composed of three-phase or single-phase rectifier bridge, smoothing capacitor, filter capacitor, IPM inverter bridge, current limiting resistor, contactor and other components. Many of these common faults are caused by electrolytic capacitors. The life of electrolytic capacitor is mainly determined by the DC voltage and internal temperature added at both ends. The type of capacitor has been selected in the circuit design, so the internal temperature plays a decisive role in the life of electrolytic capacitor. The electrolytic capacitor will directly affect the service life of the frequency converter. Generally, the service life will be halved every time the temperature rises by 10℃. Therefore, on the one hand, proper ambient temperature should be considered during installation, and on the other hand, measures can be taken to reduce pulsating current. Using ac or DC reactors with improved power factor can reduce the pulsating current and thus prolong the life of electrolytic capacitors.

During capacitor maintenance, the deterioration of the electrolytic capacitor is usually judged by the electrostatic capacity that is easy to measure. When the electrostatic capacity is less than 80% of the rated value and the insulation impedance is less than 5 M ω, the electrolytic capacitor should be replaced.

2. Typical fault analysis of main loop

Fault phenomenon: frequency converter in acceleration, deceleration or normal operation of overcurrent trip.

First of all, it should be distinguished whether it is caused by load or frequency converter. If it is the fault of the frequency converter, the current at the time of tripping can be queried through the historical record. It 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. It should be considered whether there is overload or mutation, such as motor blocking. When the load inertia is large, the acceleration time can be appropriately extended, and this process does not damage the converter itself. If the tripping current is within the rated current of the frequency converter or within the setting range of the electronic thermal relay, it can be judged that the IPM module or related parts have failed. Firstly, the positive and negative resistors between the output terminals U, V and W of the main loop of the frequency converter and the P and N terminals of the DC side can be measured to determine whether the IPM module is damaged. If the module is not damaged, the drive circuit is faulty. If IPM module overcurrent or inverter short circuit trip to the ground when decelerating, it is generally the inverter's upper half bridge module or its drive circuit fault; When the IPM module overflows during acceleration, it is the module of the lower half bridge or part of its drive circuit failure. The reasons for these failures are mostly caused by the external dust entering the inverter or the humidity in the environment.

3. Control loop fault analysis

It is the power supply part of the control circuit that affects the life of the inverter, which is the buffer capacitor in the smooth capacitor and IPM circuit board. The principle is the same as the above, but the pulsating current passing through the capacitor here is a constant value that is basically not affected by the load of the main circuit, so its life is mainly determined by temperature and power time. Because the capacitors are welded on the circuit board, it is difficult to judge the deterioration situation by measuring the electrostatic capacity. Generally, according to the ambient temperature and service time of the capacitor, it is estimated whether it is close to its service life.

The power supply circuit board provides power to the control circuit, IPM drive circuit, surface operation display board and fan, etc. These power supplies are generally from the main circuit output DC voltage, through the switching power supply and then rectification respectively. Therefore, a short circuit of a power supply may affect other parts of the power supply in addition to the damage of the rectifier circuit. For example, due to misoperation, the control power supply is short-connected to the public ground, resulting in partial damage to the switching power supply on the power circuit board, and the short circuit of the fan power supply leads to other power supplies. Generally by observing the power circuit board is relatively easy to find.

The logic control circuit board is the core of the inverter, which integrates the CPU, MPU, RAM, EEPROM and other large-scale integrated circuits. It has high reliability, and the probability of failure is very small, but sometimes all the control terminals will be closed at the same time due to the startup, resulting in EEPROM failure of the inverter. This is done by simply resetting the EEPROM.

IPM circuit board contains drive and buffer circuit, as well as over voltage, lack of equal protection circuit. The PWM signal from the logic control board is input to the IPM module through optical coupling, so the optical coupling on the IPM module should be measured while the mode speed is detected.

4. Cooling system

The cooling system mainly includes heat sink and cooling fan. Among them, the cooling fan has a short life. When it is near the service life, the fan vibrates and stops after the noise increases. IPM overheating and tripping of the inverter occurs. The life of the cooling fan is trapped by the bearing, about 10000 ~ 35,000 h. When the frequency converter runs continuously, it is necessary to replace the fan or bearing every 2 to 3 years. To extend the life of fans, some products have fans that only run when the converter is running, not when the power is on.

5. External electromagnetic induction interference

If there are interference sources around the frequency converter, they will invade the internal of the frequency converter through radiation or power line, cause the control loop misaction, resulting in abnormal work or shutdown, and even damage the frequency converter in serious cases. Specific methods to reduce noise interference include: all relays and contactor control coils around the frequency converter should be equipped with absorption devices to prevent impulse voltage, such as RC surge absorber, whose wiring should not exceed 20 cm; Shorten the wiring distance of the control loop as far as possible, and separate it from the main loop; The distance between the wiring hinge of the inverter control loop should be above 15 mm, and the distance between it and the main loop should be above 10 cm; When the frequency converter is far away from the motor (more than 100 m), on the one hand, the cross sectional area of the wire can be increased to ensure that the line voltage drop is within 2%. At the same time, the output reactor of the frequency converter should be installed to compensate for the charging current of the distributed capacitance generated by the long distance wire. The grounding terminal of the frequency converter shall be grounded according to the provisions, and shall be reliably grounded at the special ground point. It cannot be used together with electric welding and power grounding. The frequency converter input is installed with a radio noise filter to reduce the input of high harmonics, so as to reduce the impact of noise from power line to electronic equipment; At the same time, a radio noise filter is installed at the output end of the converter to reduce the line noise at the output end.

6. Installation environment

Frequency converter belongs to electronic device, in its manual has detailed installation environment requirements. In special cases, if these requirements can not be met, the corresponding suppression measures should be adopted as far as possible: vibration is the main cause of mechanical damage to electronic devices, for the occasion of large vibration impact, rubber and other vibration avoidance measures should be used; Damp, corrosive gas and dust will cause corrosion of electronic devices, poor contact, insulation reduction and short circuit, as a preventive measure, should be anti-corrosion and dustproof treatment of the control board, and the use of closed structure; Temperature is an important factor affecting the service life and reliability of electronic devices, especially semiconductor devices. Air conditioners 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 frequency converter. For special cold occasion, in order to prevent the microprocessor from working properly due to low temperature, necessary measures such as setting air heater should be taken.

7. The power supply is abnormal

Power supply anomalies can be divided into the following three kinds, namely, phase loss, low voltage, power failure, and sometimes their mixed forms. These anomalies are mostly caused by wind, snow and lightning strikes on transmission lines, and sometimes by ground and phase short circuits in the same power supply system. Lightning strikes vary greatly by region and season. In addition to voltage fluctuations, some power grids or self-generating units will also have frequency fluctuations, and these phenomena sometimes occur repeatedly in a short period of time. In order to ensure the normal operation of equipment, corresponding requirements are also put forward for the power supply of frequency converter.

If there are directly started motor and induction cooker equipment nearby, in order to prevent the voltage reduction caused by the input of these equipment, the power supply should be separated from the power supply of the frequency converter to reduce the mutual influence.

For the equipment that requires instantaneous power failure to continue to operate, in addition to the selection of a suitable price of frequency converter, the speed reduction ratio of the motor load should also be considered in advance. When the inverter and the external control loop adopt the instantaneous power outage compensation mode, after the loss of voltage recovery, the speed measuring motor is used to prevent the over-current in acceleration.

For equipment requiring continuous operation, the frequency converter should be equipped with an automatic switching power supply device. For example, the inverter with diode input and the use of single-phase control power supply, although in the state of phase loss, but also can continue to work, 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 cause adverse effects on the life and reliability of the inverter, should be checked and treated as soon as possible.

8. Lightning strike, inductive lightning

The impulse voltage formed by lightning strike or induced lightning strike will sometimes cause the damage of the converter. In addition, when the primary side of the power supply system has a vacuum circuit breaker, the short circuit will produce a higher impulse voltage. In order to prevent overvoltage damage caused by impulse voltage, it is usually necessary to pressurize sensitive resistance and other absorption devices at the input end of the frequency converter. RC surge absorber shall be added to vacuum circuit breaker. If there is a vacuum circuit breaker at the primary side of the transformer, the inverter should be disconnected before the vacuum circuit breaker is operated in the control sequence.