Inverter overvoltage and undervoltage fault causes and treatment - Database & Sql Blog Articles

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Fault phenomenon 1: Overvoltage (OU): An overvoltage alarm typically occurs during shutdown.

1. The main cause is usually a short deceleration time or a damaged braking resistor.

2. Example: A Taian N2 series 3.7kW inverter displays "OU" when it stops. Diagnosis and repair: Before fixing the unit, it's essential to understand why the "OU" alarm occurs. During deceleration, the motor rotor spins faster, causing an increase in electromotive force and current. The motor enters a power generation state, and the energy from this feedback flows into the DC bus via the diode connected in parallel with the inverter’s high-power switch tube, leading to a rise in DC bus voltage.

3. Troubleshooting: Focus on checking the brake circuit. The discharge resistance was fine, but when testing the brake transistor (ET191), it was found to be broken. After replacing it, the system ran smoothly without any issues during fast stop.

Fault phenomenon 2: Undervoltage (Uu): This is another common issue users face.

1. Main causes include low main circuit voltage (below 200V for 220V series, below 400V for 380V series), a damaged rectifier bridge, improper thyristor operation, or a faulty main circuit contactor. The loss of DC bus voltage can also lead to undervoltage on the charging resistor. Additionally, a faulty voltage detection circuit may trigger the undervoltage alarm.

2. Example: A DANFOSS VLT5004 inverter shows normal power-on display, but after applying a load, it triggers “DC LINK UNDERVOLT.” Diagnosis and repair: Although the issue seems complex at first, careful analysis reveals it's not too difficult. The inverter charges through its charging circuit and contactor. No abnormalities were detected during power-up, so the problem likely stems from a drop in DC link voltage under load. The DC link voltage is rectified by the full-wave rectifier bridge and then filtered by capacitors.

3. Troubleshooting: Inspect the rectifier bridge. After testing, it was found that one bridge arm had an open circuit. Replacing the rectifier bridge resolved the issue.

These examples highlight the importance of systematic troubleshooting and understanding the core components involved in inverter operations. Whether dealing with overvoltage or undervoltage, identifying the root cause quickly can save time and prevent further damage. Always ensure all parts are tested thoroughly before replacement and follow proper safety procedures when working with electrical systems.