A brief discussion on how to eliminate soft faults in frequency converters

1. Overcurrent

Overcurrent is the most frequent inverter alarm phenomenon.

1.1 Phenomenon

(1) When restarting, the circuit breaker trips as soon as the speed increases. This is a very serious phenomenon of overcurrent. The main reasons are: load short circuit, mechanical parts are stuck, inverter module is damaged, the torque of the motor is too small, etc.

(2) The circuit trips when powered on. This phenomenon cannot be reset. The main reasons are: the module is broken, the drive circuit is broken, and the current detection circuit is broken.

(3) When restarting, the circuit breaker does not trip immediately but during acceleration. The main reasons are: the acceleration time is set too short, the current upper limit is set too small, and the torque compensation (V/F) is set too high.

1.2 Examples

(1) An LG-IS3-4 3.7kW inverter trips "OC" when it is started

Analysis and repair: No signs of burnout were found after opening the cover. The online measurement of IGBT (7MBR25NF-120) basically showed no problem. To further determine the problem, the IGBT was removed and the 7-unit high-power transistor was measured to be turned on and off. All of them were fine. When measuring the drive circuit of the upper half bridge, one circuit was found to be significantly different from the other two. 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 circuits were basically the same. The module was installed and powered on and everything was running well.

(2) A BELTRO-VERT 2.2kW inverter trips “OC” when powered on and cannot be reset.

Analysis and repair: First, check the inverter module and find no problem. Then check the drive circuit and find no abnormality. It is estimated that the problem is not in this part, but may be in the overcurrent signal processing part. After removing the circuit sensor and powering on, it shows that everything is normal, so it is believed that the sensor is broken. After replacing it with a new one, the load test shows that everything is normal.

2. Overvoltage

The overvoltage alarm usually occurs when the machine is shut down. The main reason is that the deceleration time is too short or there is a problem with the brake resistor and brake unit.

(1) Example

A Taian N2 series 3.7kW inverter tripped "OU" when shutting down.

Analysis and repair: Before repairing this machine, we must first find out the reason for the "OU" alarm. This is because when the inverter is decelerating, the speed at which the motor rotor winding cuts the rotating magnetic field increases, the rotor's electromotive force and current increase, and the motor is in a generating state. The feedback energy flows to the DC link through the diode connected in parallel with the high-power switch tube in the inverter link, causing the DC bus voltage to increase. Therefore, we should focus on checking the brake circuit. There is no problem in measuring the discharge resistance. When measuring the brake tube (ET191), it was found to have been broken down. After replacing it, it was powered on and ran, and there was no problem with quick stopping.

3. Undervoltage

Undervoltage is also a problem we often encounter during use. It is mainly because the main circuit voltage is too low (220V series is lower than 200V, 380V series is lower than 400V). The main reasons are: damage to one of the rectifier bridges or abnormal operation of the three thyristors, which may lead to undervoltage faults. Secondly, damage to the main circuit contactor may cause the DC bus voltage loss on the charging resistor, which may lead to undervoltage. There is also a problem of undervoltage caused by a fault in the voltage detection circuit.

3.1 Examples

(1) A CT18.5kW inverter trips “Uu” when powered on.

Analysis and repair: After checking, the rectifier bridge charging resistors of this inverter are all good, but no contactor action is heard after power-on. Because the charging circuit of this inverter does not use thyristor but relies on the contactor to complete the charging process, it is believed that the fault may be in the contactor or control circuit and power supply. After removing the contactor and adding 24V DC alone, the contactor works normally. Then check the 24V DC power supply. After careful inspection, the voltage is output after being stabilized by the LM7824 voltage regulator. The voltage regulator is damaged. After replacing it with a new one, it works normally after power-on. (2) A DANFOSSVLT5004 inverter, the power-on display is normal, but after adding load, it jumps to "DCLINKUNDERVOLT" (low DC circuit voltage).

Analysis and repair: This inverter looks rather special in terms of its phenomenon, but if you analyze the problem carefully, it is not that complicated. This inverter also completes the charging process through a charging circuit and a contactor. No abnormal phenomenon was found when it was powered on. It is estimated that it was caused by a voltage drop in the DC circuit when the load was added. The voltage in the DC circuit is full-wave rectified by a rectifier bridge and then smoothed by a capacitor. Therefore, the rectifier bridge should be checked in particular. After measurement, it was found that one arm of the rectifier bridge was open. The problem was solved after replacing it with a new one.

4. Overheating

Overheating is also a common fault. The main reasons are: the ambient temperature is too high, the fan is blocked, the temperature sensor performance is poor, and the motor is overheated.

4.1 Examples

A customer of an ABB ACS50022kW inverter reported that the "OH" signal occurred after running for about half an hour.

Analysis and repair: Because the fault occurred after running for a period of time, the temperature sensor is unlikely to be broken. The temperature of the inverter may be too high. After power-on, it was found that the fan was rotating slowly and the protective cover was filled with a lot of cotton wool (because the inverter is used in the textile industry). After cleaning, the fan ran well when it was turned on, and the fault did not occur again after running for several hours.

5. Output imbalance

Output imbalance generally manifests itself as motor shaking and unstable speed. The main reasons are: bad module, bad drive circuit, bad reactor, etc.

5.1 Examples

A Fuji G9S11KW inverter has an output voltage difference of about 100V.

Analysis and repair: After opening the machine and performing a preliminary online inspection of the inverter module (6MBI50N-120), no problems were found. The 6-way drive circuit was also measured and no faults were found. The module was removed and measured and it was found that one of the upper bridge high-power transistors could not be turned on and off normally. The module was damaged. After confirming that the drive circuit was fault-free and replacing it with a new one, everything was normal.

6. Overload

Overload is also one of the faults that the inverter frequently beats. When we see the overload phenomenon, we should first analyze whether it is the motor overload or the inverter itself overload. Generally speaking, the motor has a strong overload capacity. As long as the motor parameters in the inverter parameter table are properly set, the motor overload will not occur. However, the inverter itself is prone to overload alarm due to its poor overload capacity. We can detect the inverter output voltage, current detection circuit, and other fault-prone points to eliminate the faults one by one.

6.1 Examples

A LGIH55KW inverter often jumps to "OL" during operation.

Analysis and repair: According to the customer, this machine was originally used on a 37kw motor, but now it is used on a 55kw motor. The parameters have not been reset, so the problem may be with the parameters. After checking, the variable frequency current limit is set to the rated current of the 37kw motor. After the parameters are reset, everything is normal with load.