With the rapid development of electronic technology, the technology of frequency converters has gradually matured and improved, and has been successfully applied to various fields. However, if the inverter is used in an improper environment, it may still cause various faults or poor operating conditions and shorten the service life of the equipment. The author makes a general analysis of the problems encountered in the maintenance.
Although the variety of inverters, the circuit is constantly updated, but its failures are nothing more than overcurrent, overvoltage, undervoltage, overload, overheating, output imbalance, no display and other faults.
2 Overcurrent and overcurrent are the most frequent alarms of the inverter.
2.1 phenomenon
(1) When restarting, it will trip at a speed. This is a very serious overcurrent phenomenon. The main reasons are: load short circuit, mechanical parts are stuck; inverter module is damaged; motor torque is too small and so on.
(2) Jumping on power-on, this phenomenon can't be reset generally. The main reasons are: bad module, bad drive circuit, and bad current detection circuit.
(3) When restarting, it does not trip immediately but during acceleration. The main reasons are: the acceleration time setting is too short, the current upper limit setting is too small, and the torque compensation (V/F) setting is high.
2.2 Examples
(1) One LG-IS3-4 3.7kW inverter jumps "OC" as soon as it starts up
Analysis and maintenance: Open the cover and found no signs of burnout. The online measurement IGBT (7MBR25NF-120) basically judges that there is no problem. To further judge the problem, the IGBT is removed and the 7-cell high-power transistor is turned on and off. well. When measuring the driving circuit of the upper half bridge, it was found that there was a clear difference between the other two paths. After careful inspection, it was found that the output pin of one optocoupler A3120 was short-circuited with the negative pole of the power supply, and the three paths were basically the same after replacement. The module is fully powered and installed.
(2) A BELTRO-VERT 2.2kW variable frequency power-on jumps "OC" and cannot be reset.
Analysis and repair: First check the inverter module and found no problems. Secondly, there is no abnormality in the inspection of the drive circuit. It is estimated that the problem is not in this block. It may be in the over-current signal processing part. After the circuit sensor is removed and powered on, it shows that everything is normal. Therefore, the sensor is considered to be bad. After the load test, everything is normal.
3 Overvoltage and overvoltage alarms generally occur when the machine is stopped. The main reason is that the deceleration time is too short or there is a problem with the braking resistor and the brake unit.
(1) Example One Taian N2 series 3.7kW inverter jumps “OU†when it stops.
Analysis and maintenance: Before repairing this machine, we must first understand the reason of the "OU" alarm. This is because when the inverter is decelerating, the speed of the rotor winding of the motor is faster, and the electromotive force and current of the rotor increase. The motor is in the power generation state, and the feedback energy flows to the DC link through the diode connected in parallel with the high-power switch tube in the inverter link, so that the DC bus voltage is increased, so we should focus on checking the brake circuit and measuring the discharge resistance without problems. When the brake pipe (ET191) was measured, it was found to have broken down. After the replacement, the power was turned on and there was no problem with the quick stop.
4 Undervoltage and undervoltage are also problems we often encounter in use. Mainly because the main circuit voltage is too low (220V series is lower than 200V, 380V series is lower than 400V), the main reason: a certain damage of the rectifier bridge or abnormal operation of the thyristor three-way may cause undervoltage faults. When the second main circuit contactor is damaged, the DC bus voltage loss may cause undervoltage on the charging resistor. There is also a problem that the voltage detection circuit is faulty and undervoltage occurs.
4.1 Examples
(1) A CT 18.5kW inverter powers up “Uuâ€.
â— Analysis and maintenance: After checking the rectifier bridge charging resistance of this inverter is good, but the contactor is not heard after power-on, because the charging circuit of this inverter is not using thyristor but by contactor The pull-in is used to complete the charging process, so it is considered that the fault may occur in the contactor or control loop and the power supply part. The contactor is separately removed and the 24V DC contactor works normally. Then check the 24V DC power supply, after carefully checking that the voltage is output after being regulated by the LM7824 regulator tube, measure the voltage regulator tube is damaged, find a new product to replace the power supply and work normally.
(2) One DANFOSS VLT5004 inverter, the power-on display is normal, but after loading, jump “DC LINK UNDERVOLT†(low DC link voltage).
â— Analysis and maintenance: This inverter is special in terms of phenomena, but if you analyze the problem carefully, it is not so complicated. The inverter also completes the charging process through the charging circuit and the contactor. It is estimated that any abnormal phenomenon is caused by the voltage drop of the DC loop when the load is applied, and the voltage of the DC loop is rectified by the full-wave rectification of the rectifier bridge, and then provided by the capacitor after the flat wave, so the rectifier bridge should be inspected and measured. It was found that the rectifier bridge had a bridge arm open circuit, and the problem was solved after replacing the new product.
5 Overheating and overheating is also a common fault. The main reasons are: the surrounding temperature is too high, the fan is blocked, the temperature sensor performance is poor, and the motor is overheated.
5.1 For example, an ABB ACS500 22kW inverter customer reflects “OH†when it is running for about half an hour.
â— Analysis and maintenance: Because it is faulty after running for a period of time, the temperature sensor is unlikely to be bad. The temperature of the inverter may be too high. After the power is turned on, the fan is slow to rotate. The protective cover is covered with a lot of cotton wool. (Because the inverter is used in the textile industry), the cleaning fan runs well after cleaning, and does not jump again after running for several hours.
6 Output unbalanced output imbalance is generally characterized by motor jitter and unstable speed. The main reasons are: bad module, bad drive circuit, and bad reactor.
6.1 For example, a Fuji G9S 11KW inverter has an output voltage difference of about 100V.
â— Analysis and maintenance: Open the machine and initially check the inverter module (6MBI50N-120). No problems were found. The 6-way drive circuit was not found to be faulty. The module was removed and the bridge was found to have a high-power transistor that could not be normally turned on. Closed, the module has been damaged, and it is normal after replacing the new product after confirming that the drive circuit is fault free.
7 Overload overload is also one of the frequent faults of the inverter. When we see the overload phenomenon, we should first analyze whether the motor is overloaded or the inverter itself is overloaded. Generally speaking, the motor has strong overload capability, as long as the inverter parameter list The motor parameters are properly set, generally no motor overload occurs. The inverter itself is prone to overload alarm due to poor overload capability. We can detect the inverter output voltage, current detection circuit, and other fault-prone points to eliminate faults. .
7.1 For example, an LG IH 55KW inverter often jumps "OL" during operation.
â— Analysis and maintenance: According to the customer, this machine was originally used on a 37kw motor, and is now used on a 55kw motor. The parameters have not been reset, so the problem may be on the parameters. After checking the inverter current limit setting, the rated current of the motor is 37kw. After the parameters are reset, the load is normal.
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