Analysis and Maintenance of Common Faults of General Inverter

1 Introduction With the rapid development of electronic technology, the technology of frequency converter has gradually become mature and perfect, and has been successfully applied to various fields. However, if the inverter is used improperly and the environment is harsh, various faults or poor operating conditions may still occur, shortening the service life of the equipment. The author will make a general analysis of the problems encountered during maintenance. Although there are many types of frequency converters, 1 Introduction With the rapid development of electronic technology, the technology of frequency converters has gradually become mature and perfect, and has been successfully applied to various fields. However, if the inverter is used improperly and the environment is harsh, various faults or poor operating conditions may still occur, shortening the service life of the equipment. The author will make a general analysis of the problems encountered during maintenance. Although there are many types of frequency converters, and the circuits are constantly updated, the faults that occur are nothing more than faults such as overcurrent, overvoltage, undervoltage, overload, overheating, unbalanced output, and no display. 2 Overcurrent Overcurrent is the most frequent occurrence of inverter alarms. 2.1 Phenomenon (1) When restarting, it trips as soon as the speed increases. This is a very serious phenomenon of overcurrent. The main reasons are: short-circuit of the load, jamming of mechanical parts; damage of the inverter module; too small torque of the motor. (2) It jumps after power-on. Generally, this phenomenon cannot be reset. The main reasons are: bad module, bad drive circuit, and bad current detection circuit. (3) It does not trip immediately when restarting but when accelerating. 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 Example (1) An RNB3003 3.7kW inverter jumps 'OC' as soon as it is started. Analysis and maintenance: Open the cover and find no signs of burnout. The online measurement of the IGBT basically determines that there is no problem. In order to further determine the problem, remove the IGBT. After measuring the 7 units of high-power transistors, the turn-on and turn-off are very good. When measuring the driving circuit of the upper half bridge, it was found that there was a clear difference between one channel and the other two channels. After careful inspection, it was found that the output pin of an optocoupler A3120 was short-circuited with the negative pole of the power supply. After replacement, the three channels were basically the same. Everything is fine when the module is installed and powered on. (2) A RNB3002 2.2kW frequency conversion will jump 'OC' and cannot be reset. Analysis and maintenance: First check the inverter module and find no problems. Secondly, check the drive circuit and there is no abnormal phenomenon. It is estimated that the problem is not in this part. It may be in the part of over-current signal processing. After removing the circuit sensor and powering it on, it shows that everything is normal. Therefore, it is considered that the sensor is broken. Find a new one. After loading the test, everything is normal. 3 Overvoltage and overvoltage alarms generally appear when the machine is stopped. The main reason is that the deceleration time is too short or the braking resistor and braking unit have problems. (1) Example An RNB3003 3.7kW inverter jumps 'OU' when it stops. Analysis and maintenance: Before repairing this machine, we must first understand the cause of the 'OU' alarm. This is because when the frequency converter is decelerating, the speed of the motor rotor winding cutting rotating magnetic field increases, and the electromotive force and current of the rotor increase. The motor is in the state of generating electricity. The feedback energy flows to the DC link through the diode connected in parallel with the high-power switch tube in the inverter link, which causes the DC bus voltage to rise. Therefore, we should focus on checking the brake circuit and measuring the discharge resistance. When the brake tube was measured, it was found to have broken down. After replacement, it was powered on and it was stopped quickly. 4 Undervoltage Undervoltage is also a problem we often encounter in use. The main reason is that the main circuit voltage is too low (380V series is lower than 400V). The main reason is that one of the rectifier bridges is damaged or the thyristor three circuits are not working properly, which may cause the occurrence of undervoltage faults. Second, the main circuit contacts The damage to the DC bus may cause undervoltage on the charging resistor. The voltage detection circuit fails and the undervoltage problem occurs. 4.1 For example, a RNB3018 18.5kW inverter jumps to 'LU2' when it is powered on. ●Analysis and maintenance: After checking that the charging resistance of the rectifier bridge of this inverter is good, but no contactor action is heard after power-on, because the charging circuit of this inverter does not use thyristor but depends on the contactor To complete the charging process, it is believed that the fault may lie in the contactor or the control circuit and the power supply. The contactor is removed and the 24V DC contactor is working normally. Then check the 24V DC power supply. After careful inspection, the voltage is output after the LM7824 zener tube is stabilized. It is measured that the zener tube is damaged. Find a new product to replace it and power on and work normally. 5 Overheating Overheating is also a relatively common fault. The main reasons are: the surrounding temperature is too high, the fan is blocked, the temperature sensor is not performing well, and the motor is overheated. 5.1 For example, a customer of RNB3022 22kW inverter reported that it jumped 'OH' after running for about half an hour. ●Analysis and maintenance: Because the fault occurs after a period of operation, the temperature sensor is unlikely to be broken. Maybe the temperature of the inverter is indeed too high. After power on, it is found that the fan rotates slowly and the protective cover is full of cotton wool. (Because the inverter is used in the textile industry), the fan is running well after being cleaned, and the fault does not jump again after several hours of operation. Therefore, it is necessary to improve the requirements from various aspects, and also to consider the economic benefits to ensure that the goals are achieved.