A brief discussion on how to troubleshoot VFD (Variable-frequency Drive) soft faults

Core Tips:

1. Overcurrent

Overcurrent is the most frequent alarm phenomenon of VFD (Variable-frequency Drive).

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; motor torque is too small, etc.

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

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

1.2 Examples

(1) An LG-IS3-4 3.7kW VFD (Variable-frequency Drive) tripped "OC" as soon as it was started

Analysis and repair: No signs of burnout were found when the cover was opened. Online measurement of the IGBT (7MBR25NF-120) basically determined that there was 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. It was found that one circuit was significantly different from the other two circuits when measuring the drive circuit of the upper half bridge. After careful inspection, it was found that the output pin of an optocoupler A3120 was short-circuited to 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 worked well.

(2) A BELTRO-VERT 2.2kW variable frequency drive tripped "OC" when it was powered on and could not be reset.

Analysis and repair: 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, 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 is normal.

2. Overvoltage

Overvoltage alarm usually appears 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 VFD (Variable-frequency Drive) jumps "OU" when it is shut down.

Analysis and repair: Before repairing this machine, we must first understand the reason for the "OU" alarm. This is because when the VFD (Variable-frequency Drive) 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 power generation state. The feedback energy flows to the DC link through the diode 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 with 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 running, and there was no problem with rapid parking.

3. Undervoltage

Undervoltage is also a problem we often encounter during use. The main reason is that 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 one of the three thyristor circuits may lead to undervoltage faults. Secondly, damage to the main circuit contactor may cause DC bus voltage loss on the charging resistor, which may lead to undervoltage. There is also a voltage detection circuit failure and undervoltage problem.

3.1 Example

(1) A CT18.5kW VFD (Variable-frequency Drive) jumps "Uu" when powered on.

Analysis and maintenance: After inspection, the rectifier bridge charging resistors of this VFD (Variable-frequency Drive) are all good, but no contactor action is heard after power-on. Because the charging circuit of this VFD (Variable-frequency Drive) does not use thyristors 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. Find a new one to replace it and it works normally after powering on.

(2) A DANFOSS VLT5004 VFD (Variable-frequency Drive). The display is normal when powered on, but after adding load, it jumps to "DC LINK UNDER VOLT" (DC circuit voltage is low).

Analysis and repair: This VFD (Variable-frequency Drive) is quite special in terms of phenomenon, but if you analyze the problem carefully, it is not that complicated. This VFD (Variable-frequency Drive) also completes the charging process through the charging circuit and contactor. No abnormal phenomenon was found when powered on. It is estimated that it is caused by the voltage drop of the DC circuit when adding load. The voltage of the DC circuit is provided by full-wave rectification of the rectifier bridge and then smoothed by the 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 has poor performance, and the motor is overheated.

4.1 Example

A customer of an ABB ACS500 22kW VFD (Variable-frequency Drive) reported that the "OH" signal jumped after running for about half an hour.

Analysis and maintenance: Because the fault occurred after running for a period of time, it is unlikely that the temperature sensor is broken. It may be that the temperature of the VFD (Variable-frequency Drive) is indeed too high. After powering on, it was found that the fan rotated slowly and the protective cover was filled with a lot of cotton wool (because the VFD (Variable-frequency Drive) is used in the textile industry). After cleaning, the fan ran well when it was turned on. After running for several hours, this fault did not jump again.