1 Fault causes and preventive measures of the inverter
The frequency converter is composed of main circuit, power circuit, IPM drive and protection circuit, cooling fan and so on. Its structure is mostly in a modular or modular form. Due to incorrect use or unreasonable setting environment, the inverter may malfunction or malfunction, or it may not meet the expected operation results. In order to prevent problems before they occur, it is especially important to carefully analyze the cause of the failure in advance.
1.1 Analysis of common faults in the main circuit
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 an electrolytic capacitor is mainly determined by the DC voltage and internal temperature applied to both ends. The type of capacitor has been selected in the loop design, so the internal temperature determines the life of the electrolytic capacitor. Electrolytic capacitors directly affect the service life of the inverter. For every 10 °C rise in temperature, the life is halved. 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. An AC or DC reactor with improved power factor can reduce the ripple current and extend the life of the electrolytic capacitor.
In the maintenance of the capacitor, the deterioration of the electrolytic capacitor is usually judged by the electrostatic capacity which is relatively easy to measure. When the electrostatic capacity is less than 80% of the rated value and the insulation resistance is 5 MΩ or less, the electrolytic capacitor should be replaced.
1.2 Typical failure analysis of the main circuit
Symptom: The inverter trips overcurrent during acceleration, deceleration or normal operation.
The first thing to distinguish is due to the load or the cause of the inverter. If it is the fault of the inverter, the history can be used to check the current at the time of trip, which exceeds the rated current of the inverter or the set value of the electronic thermal relay. If the three-phase voltage and current are balanced, then it should be considered whether there is overload. Or sudden changes, such as motor stalls. When the load inertia is large, the acceleration time can be extended appropriately, and the process is not damaged by 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 part has failed. First, it is possible to determine whether the IPM module is damaged by measuring the positive and negative resistances 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 has failed. If the IPM module is over-current or the inverter trips to ground during deceleration, it is generally the module of the upper half of the inverter or its drive circuit is faulty; and when the IPM module is over-current during acceleration, it is the module of the lower half of the bridge or its drive Part of the circuit is faulty. The cause of these faults is mostly caused by external dust entering the inverter or the environment is humid.
1.3 Control loop failure analysis
The control loop affects the life of the inverter. It is the power supply part. It is the smoothing capacitor and the snubber capacitor in the IPM board. The principle is the same as the above, but the pulsating current passing through the capacitor here is the value that is basically not affected by the main loop load. Therefore, its life is mainly determined by temperature and power-on time. Since the capacitors are soldered on the circuit board, it is difficult to judge the deterioration by measuring the electrostatic capacity. Generally, it is estimated whether the service life is close to the service life based on the ambient temperature of the capacitor and the use time.
The power supply board supplies power to the control loop, the IPM drive circuit, the surface operation display panel, and the fan. These power supplies are generally obtained by rectifying the DC voltage output from the main circuit through the switching power supply. Therefore, if a certain power supply is short-circuited, in addition to the damage of the rectifier circuit of this circuit, it may affect other parts of the power supply. For example, the control power supply is short-circuited with the common ground due to misoperation, resulting in partial damage of the switching power supply on the power supply circuit board. A short circuit in the power supply causes other power supplies to be powered down, etc. It is usually easier to find by observing the power board.
The logic control circuit board is the core of the inverter. It concentrates on large-scale integrated circuits such as CPU, MPU, RAM, EEPROM, etc. It has high reliability, and the probability of failure itself is small, but sometimes it is completely controlled by booting. The terminals are closed at the same time, causing an EEPROM fault in the inverter, as long as the EEPROM is reset again.
The IPM board contains drive and buffer circuits, as well as overvoltage and missing protection circuits. The PWM signal from the logic board is used to input the voltage drive signal into the IPM module through optical coupling. Therefore, the optocoupler on the IPM module should be measured while detecting the mode fast.
1.4 Cooling system
The cooling system mainly includes a heat sink and a cooling fan. The cooling fan has a short life. When the service life is approaching, the fan generates vibration, and the noise increases and finally stops. The inverter has IPM overheat trip. The life of the cooling fan is trapped in the bearing, which is about 10,000 to 35,000 h. When the inverter is running continuously, it takes 2 to 3 years to replace the fan or bearing. In order to extend the life of the fan, fans of some products only operate when the drive is running, not when the power is turned on.
1.5 External electromagnetic induction interference
If there is interference source around the inverter, they will invade the inverter through the radiation or power line, causing the control circuit to malfunction, causing abnormal operation or shutdown, and even damage the inverter in severe cases. The specific methods to reduce noise interference are: all the relays around the inverter and the control coil of the contactor, and the absorption device for preventing the surge voltage, such as the RC surge absorber, the wiring can not exceed 20 cm; try to shorten the control loop The line distance is separated from the main circuit; the wiring distance of the inverter control circuit should be more than 15 mm, and the distance from the main circuit should be more than 10 cm; when the inverter is far away from the motor (more than 100 m), At this time, the cross-sectional area of ​​the wire can be increased to ensure that the line voltage drop is within 2%, and the inverter output reactor should be installed to compensate the charging current of the distributed capacitance generated by the long-distance wire. The grounding terminal of the inverter should be grounded according to the regulations. It must be grounded reliably at the special grounding point. It can not be mixed with electric welding and power grounding. The radio noise filter is installed at the input end of the inverter to reduce the input high harmonics, thus reducing the power line to The noise effect of the electronic equipment; a radio noise filter is also installed at the output of the frequency converter to reduce the line noise at its output.
1.6 Installation Environment
The frequency converter is an electronic device device and has detailed requirements for the installation environment in its manual. In special cases, if these requirements are not met, the corresponding suppression measures must be adopted as much as possible: vibration is the main cause of mechanical damage to electronic devices. For vibration shocks, vibration and other vibration-proof measures should be used; moisture and corrosion Gas and dust will cause rust, poor contact, and low insulation of the electronic device to form a short circuit. As a precautionary measure, the control board should be treated with anti-corrosion and dust-proof treatment, and a closed structure; temperature is an important factor affecting the life and reliability of electronic devices. Factors, especially semiconductor devices, should be installed in accordance with the environmental conditions required by the device or to avoid direct sunlight.
In addition to the above points, it is also necessary to check the air filter and cooling fan of the inverter regularly. For special high-cold occasions, in order to prevent the microprocessor from working properly due to low temperature, necessary measures such as setting an air heater should be taken.
1.7 Power Supply Abnormal
Power supply anomalies are roughly divided into the following three types, namely, phase loss, low voltage, power failure, and sometimes their mixed form. The main cause of these anomalies is mostly caused by wind, snow, and lightning strikes, and sometimes due to short-circuit to ground and phase-to-phase short circuit in the same power supply system. Lightning strikes vary greatly by region and season. In addition to voltage fluctuations, some power grids or units that generate electricity by themselves will also experience frequency fluctuations, and these phenomena sometimes appear repeatedly in a short period of time. In order to ensure the normal operation of the equipment, the power supply of the inverter is also required.
If there is a motor and induction cooker directly activated in the vicinity, in order to prevent the voltage drop caused by the input of these devices, the power supply should be separated from the power supply of the inverter to reduce the mutual influence.
For equipment that can continue to operate after an instantaneous power failure, in addition to selecting the inverter at the appropriate price, the speed reduction ratio of the motor load should also be considered in advance. When both the inverter and the external control loop adopt the instantaneous power failure compensation mode, after the loss of pressure is restored, the overspeed of the acceleration is prevented by the speed measurement of the speed measuring motor.
For equipment that requires continuous operation, an automatic uninterruptible power supply unit with automatic switching should be installed. Like a frequency converter with a diode input and a single-phase control power supply, although it is in a phase loss state, it can continue to work, but the individual device current in the rectifier is too large, and the pulse current of the capacitor is too large. The longevity and reliability of the device have an adverse effect and should be checked and processed as soon as possible.
1.8 Lightning strike, induction lightning
The surge voltage formed by lightning strikes or induced lightning strikes sometimes causes damage to the inverter. In addition, when the power system has a vacuum circuit breaker on the primary side, short circuit opening and closing will generate a high surge voltage. In order to prevent overvoltage damage caused by the surge voltage, it is usually necessary to apply a absorbing device such as a varistor at the input end of the inverter. The HV surge absorber should be added to the vacuum circuit breaker. 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.
2 Fault self-diagnosis and prevention function of the inverter itself
The old type of transistor inverter mainly has the following disadvantages: easy to trip, not easy to restart, and low overload capability. Due to the rapid development of IGBT and CPU, the inverter has improved the self-diagnosis and fault prevention functions, which greatly improves the reliability of the inverter.
If the "all-area automatic torque compensation function" in the vector control inverter is used, the cause of the failure such as "insufficient starting torque" and "decreased output due to environmental conditions" 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 time, and quickly corrects and compensates the output voltage to offset the change in the output torque of the inverter caused by changes in external conditions.
In addition, since the software development of the inverter is more perfect, various fault prevention measures can be set in the inverter in advance, and after the fault is resolved, the operation can be continued, for example, the motor in the free parking process is restarted; The internal fault is automatically reset and maintained continuously; when the load torque is too large, the operating curve can be automatically adjusted to detect the abnormal torque of the mechanical system.
There are many reasons for the failure of the inverter. Only in practice, constantly groping and summarizing can eliminate various faults in time.
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