The inverter consists of the main circuit, power circuit, IPM drive and protection circuit, cooling fan and other parts. Its structure is mostly unitized or modular. Incorrect use or unreasonable setting environment will easily cause the inverter to malfunction and fail, or fail to meet the expected operating effect. In order to prevent problems before they happen, it is particularly important to carefully analyze the cause of the failure in advance.
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 common faults are caused by electrolytic capacitors. The life of electrolytic capacitors is mainly determined by the DC voltage applied to both ends and the internal temperature. The capacitor model has been selected during the circuit design, so the internal temperature plays a decisive role in the life of the electrolytic capacitor. Electrolytic capacitors will directly affect the service life of the inverter. Generally, the life is halved for every 10°C increase in temperature. Therefore, on the one hand, the appropriate ambient temperature should be considered during installation, and on the other hand, measures can be taken to reduce the pulsating current. The use of AC or DC reactors that improve the power factor can reduce the pulsating current, thereby extending the life of the electrolytic capacitor.
When maintaining the capacitor, the electrostatic capacitance, which is relatively easy to measure, is usually used to judge the deterioration of the electrolytic capacitor. When the electrostatic capacitance is less than 80% of the rated value and the insulation impedance is below 5MΩ, the electrolytic capacitor should be considered for replacement.
Fault phenomenon: The inverter trips due to overcurrent during acceleration, deceleration or normal operation.
First, it should be distinguished whether it is caused by load or inverter. If it is a fault of the inverter, the current at the time of tripping can be checked through historical records. If it 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, it should be considered whether there is overload or mutation, such as motor stalling. When the load inertia is large, the acceleration time can be appropriately extended. 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 setting range of the electronic thermal relay, it can be judged that the IPM module or related parts have failed. First, the IPM module can be judged whether it is damaged by measuring the forward and reverse resistance between the main circuit output terminals U, V, and W 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 overcurrent or the inverter short-circuit to ground and trips during deceleration, it is generally due to a fault in the module of the upper half bridge of the inverter or its drive circuit; and if the IPM module overcurrent during acceleration, it is due to a partial fault in the module of the lower half bridge or its drive circuit. The causes of these faults are mostly due to external dust entering the inverter or the humid environment.
The control circuit affects the life of the inverter in the power supply part, which is the smoothing capacitor and the buffer capacitor in the IPM circuit board. The principle is the same as above, but the pulsating current passing through the capacitor here is a fixed value that is basically not affected by the main circuit load, so its life is mainly determined by temperature and power-on time. Since the capacitors are all welded on the circuit board, it is difficult to judge the deterioration by measuring the electrostatic capacitance. Generally, it is estimated whether it is close to its service life based on the ambient temperature and usage time of the capacitor.
The power circuit board provides power to the control circuit, IPM drive circuit, surface operation display panel, and fan, etc. These power supplies are generally obtained by rectifying the DC voltage output from the main circuit through the switching power supply. Therefore, if a power supply is short-circuited, in addition to the damage to the rectification circuit of this circuit, it may also affect the power supply of other parts. For example, due to misoperation, the control power supply is short-circuited with the common ground, resulting in damage to 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 cut off. Generally, it is easier to find 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 CPU, MPU, RAM, EEPROM, etc. It has high reliability and a low probability of failure. However, sometimes all control terminals are closed at the same time when the inverter is turned on, resulting in EEPROM failure. This can be solved by resetting the EEPROM.
The IPM circuit board contains drive and buffer circuits, as well as overvoltage and phase loss protection circuits. The PWM signal from the logic control board inputs the voltage drive signal into the IPM module through optical coupling. Therefore, when detecting the module, the optical coupler on the IPM module should also be measured.
The cooling system mainly includes heat sink and cooling fan. The cooling fan has a short life. When it is close to the end of its life, the fan will vibrate, the noise will increase and finally stop, and the inverter will trip due to IPM overheating. The life of the cooling fan is limited by the bearing, which is about 10,000 to 35,000 hours. When the inverter is running continuously, the fan or bearing needs to be replaced every 2 to 3 years. In order to extend the life of the fan, the fan of some products only runs when the inverter is running instead of when the power is turned on.
If there are interference sources around the inverter, they will invade the inverter through radiation or power lines, causing the control circuit to malfunction, resulting in abnormal operation or shutdown, and even damage to the inverter in severe cases. Specific methods to reduce noise interference are: install absorption devices to prevent impact voltage, such as RC surge absorbers, on the control coils of all relays and contactors around the inverter, and the wiring distance of the control circuit cannot exceed 20cm; try to shorten the wiring distance of the control circuit and separate it from the main circuit; the twisted node distance of the inverter control circuit wiring should be more than 15mm, and keep a distance of more than 10cm from the main circuit; 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 line voltage drop is within 2%, and at the same time, the inverter output reactor should be installed to compensate for the charging current of the distributed capacitance generated by the long-distance wire. The inverter grounding terminal should be grounded as required and must be reliably grounded at a dedicated grounding point. It cannot be mixed with welding or power grounding. A radio noise filter should be installed at the inverter input to reduce input high-order harmonics, thereby reducing the noise impact from the power line to the electronic equipment. At the same time, a radio noise filter should also be installed at the inverter output to reduce the line noise at its output.
The inverter is an electronic device, and has strict requirements for the installation environment. The manual contains detailed requirements for the installation and use environment. In special cases, 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. For occasions with large vibration impact, vibration-proof measures such as rubber should be adopted; moisture, corrosive gases and dust will cause electronic devices to rust, poor contact, reduced insulation and form short circuits. As a preventive measure, the control board should be treated with anti-corrosion and dust prevention, 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 or direct sunlight should be avoided according to the environmental conditions required by the device.
In addition to the above points, it is also very necessary to regularly check the air filter and cooling fan of the inverter. For special high-cold occasions, in order to prevent the microprocessor from not working properly due to low temperature, necessary measures such as setting up an air heater should be taken.
Power supply abnormality can be roughly divided into the following three types, namely phase loss, low voltage, power outage, and sometimes a combination of them. The main causes of these abnormal phenomena are mostly caused by wind, snow, and lightning strikes on the transmission lines, and sometimes by ground short circuits and phase-to-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 experience frequency fluctuations, and these phenomena sometimes occur repeatedly in a short period of time. In order to ensure the normal operation of the equipment, corresponding requirements are also put forward for the power supply of the inverter.
If there are direct-starting motors and induction cookers nearby, in order to prevent the voltage drop caused by the input of these devices, their power supply should be separated from the power supply of the inverter to reduce mutual influence.
For equipment that requires continued operation after a momentary power outage, in addition to selecting a suitable price inverter, the speed reduction ratio of the motor load should also be considered in advance. When both the inverter and the external control circuit adopt the instantaneous power outage compensation method, after the loss of pressure is restored, the speed of the speed measuring motor is measured to prevent overcurrent during acceleration.
For equipment that must run continuously, the inverter should be equipped with an automatic switching uninterruptible power supply device. For example, an inverter with diode input and using a single-phase control power supply can continue to work even in a phase-loss state, but the current of some components in the rectifier is too large, and the pulse current of the capacitor is too large. If it runs for a long time, it will have an adverse effect on the life and reliability of the inverter, and it should be checked and handled as soon as possible.
The impulse voltage caused by lightning strike or induced lightning strike may sometimes damage the inverter. In addition, when the primary side of the power system is equipped with a vacuum circuit breaker, short-circuit switching will generate a higher impulse voltage. In order to prevent overvoltage damage caused by impulse voltage, it is usually necessary to add an absorption device such as a varistor to the input end of the inverter. The vacuum circuit breaker should be equipped with an RC surge absorber. If there is a vacuum circuit breaker on the primary side of the transformer, the control sequence should be ensured to disconnect the inverter before the vacuum circuit breaker is activated.
The old transistor inverter has the following main disadvantages: easy to trip, difficult to restart, and low overload capacity. Due to the rapid development of IGBT and CPU, the inverter has added a complete self-diagnosis and fault prevention function, which greatly improves the reliability of the inverter.
If the "full-range automatic torque compensation function" in the vector control inverter is used, the fault causes such as "insufficient starting torque" and "output reduction caused by environmental conditions change" will be well overcome. This function uses the high-speed calculation of the microcomputer inside the inverter to calculate the torque required at the current moment, and quickly correct and compensate the output voltage to offset the change in the inverter output torque caused by changes in external conditions.
In addition, due to the more complete software development of the inverter, various fault prevention measures can be set in advance inside the inverter, and after the fault is resolved, it can still continue to run, for example: restart the motor during free parking; automatically reset the internal fault and maintain continuous operation; when the load torque is too large, it can automatically adjust the operation curve and detect abnormal torque of the mechanical system.
There are many reasons for inverter failure. Only by constantly exploring and summarizing in practice can we eliminate various failures in time.