The overcurrent fault trip of the frequency converter is one of the most common and complicated faults. When the fault occurs, the inverter protection will immediately act and stop, and the fault code or fault type will be displayed. In most cases, the cause of the fault can be quickly found and corrected according to the displayed fault code, but there are also some reasons for the fault, which are not single, but include over current, short circuit, under voltage, grounding, overheating, Various factors such as harmonic interference that may cause a trip. In order to find the cause of the fault and troubleshoot it, it can be done according to the fault diagnosis flowchart shown in Figure 1.
In particular, inverters using IPM modules contain protection functions such as overcurrent, short circuit, undervoltage, grounding, and overheating, and these fault signals are introduced to the controller through the fault output terminal Fn of the control pin of the module. When the controller receives the fault information, the controller immediately blocks the pulse output and displays the fault information on the control panel. However, once the fault occurs inside the module, it is difficult to find the cause of the fault. Therefore, when troubleshooting such faults, it is first necessary to distinguish whether the trip is caused by the external load or by the internal cause of the inverter; the inverter is tripping during the acceleration process, the deceleration process or the constant speed process. After the distinction, the scope of the fault finding can be narrowed to facilitate quick troubleshooting.
1. General troubleshooting steps
1) If there is no obvious fault trace on the appearance, you can first remove the cable connected to the motor from the inverter and test the inverter and motor separately. If the inverter is also connected to an external control signal circuit, it is also best to disconnect it. This allows the inverter to be tested manually. If it is normal, the inverter has no problem or damage.
2) Further check if there is any change in the setting value, it is best to re-set it again.
3) Then use a test control signal or potentiometer to connect to the external control terminal to test the external signal control performance of the inverter. If it is normal, the inverter is intact and the external signal and motor can be further checked.
4) The external control signal is generally the output signal of various sensors, or from the controller, it should be tested according to the test method of the sensor or controller, preferably by the on-site signal calibrator.
5) For the inspection of the motor, first check the insulation with a multimeter and a megohmmeter. If the contactor is installed on the output side of the inverter, check whether the contacts of the contactor are normal.
6) If all of the above are normal, if the conditions permit, it is best to use the commercial frequency power supply for the starter motor test, and let it run for a period of time to observe whether there is an abnormality.
Through the above work, under normal circumstances, the cause of the accident will be narrowed down to a certain extent. In the above-mentioned inspection process, at which point the abnormality is found, at which point the inspection is further examined.
2. Using the operation record to analyze the cause of the fault If the fault history of the inverter is used, it can be inquired that the current at the time of the trip exceeds the rated current of the general-purpose inverter, or exceeds the setting value of the electronic thermal relay, and the three-phase voltage and current If it is balanced, it should consider whether it is the possibility of motor overload or sudden change of load, such as motor stall, sudden load shedding of the motor (suddenly disconnecting the load during normal operation of the inverter), etc., the former generally occurs when the motor is connected to the machine. The mechanical cause of the part, or the motor bearing has a problem, the latter generally occurs when the external control signal is lost. If the three-phase current is unbalanced, it may be the power supply side phase failure, the motor terminal or the internal winding disconnection. If the current at the time of tripping is within the rated current of the general-purpose inverter or the setting range of the electronic thermal relay, it can be determined that the inverter module or related part inside the inverter has failed. Firstly, it can be judged whether the inverter module is damaged by measuring the positive and negative resistance between the main circuit output terminals U, V, W of the common inverter and the P and N terminals of the DC side. If the module is not damaged, it is driven. The circuit has failed, and this situation is generally rare.
3. Analysis of the cause of the fault during the shifting process If the inverter module overcurrents during deceleration or the inverter trips to the ground, it is generally the fault of the upper half of the inverter bridge or its drive circuit, and when accelerating If the inverter module overcurrent is faulty in the lower half of the bridge or its drive circuit, after checking, confirm that the inverter bridge arm is damaged, and the inverter should work normally after replacement.
If the inverter is found to be hot after tripping, the carrier frequency may be adjusted too high. If the above-mentioned possible failure occurs, and the inverter has not newly replaced the hardware, after restarting the system. The carrier frequency should be appropriately reduced.
4. Analysis of the cause of the fault caused by the peripheral device In the overcurrent fault that occurs during the operation of the inverter, the cause of the fault caused by the external inverter is often overlooked. Now, the situation of the overcurrent protection action caused by the external cause is analyzed.
1) A sudden change in the motor load causes a large inrush current to cause an overcurrent protection action.
2) The insulation inside the motor or the motor connection cable is broken, causing short circuit between the turns or phases and ground, resulting in overcurrent protection. For short-circuit to ground faults, if the general-purpose inverter has grounding protection, the grounding protection also acts.
3) When the speed control encoder is installed in the inverter control system, the speed feedback signal will be over-current if it is lost or abnormal.
4) External control signal line disconnection or sensor failure can also cause overcurrent, resulting in overcurrent protection action. Some inverters now have a feedback signal disconnection protection function, which can be prevented by setting.
5) If the contactor is installed on the output side of the inverter, the instantaneous contact and damage of the contacts of the contactor is also a common cause of overcurrent protection, because the contact surface will be oxidized during the long-term operation of the contactor. A layer of membrane resistors leads to poor contact, resulting in phase loss and is not easy to find, so you should use a multimeter to check if the contacts are normal.
5. Conclusion This article is based on the experience of the maintenance of the inverter, the author of some analysis and discussion of the cause of the overcurrent fault of the inverter, because the faults in the operation of the inverter in various forms, just like the maintenance of other electrical appliances, there are Many are unexpected, which not only requires us to understand the working principle of the specific inverter and the principle of protection circuit design, but also requires us to conduct in-depth and serious analysis of the fault, so as to quickly and accurately deal with the fault problem and reduce the downtime. To ensure smooth production.
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