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 of the failure 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 this unit, it's essential to understand why the “OU” alarm appears. When the inverter decelerates, the motor rotor spins faster, causing an increase in electromotive force and current. This makes the motor act like a generator, sending energy back to the DC bus through the diode connected in parallel with the inverter’s high-power switch tube, thus raising the DC bus voltage.

3. Troubleshooting: Focus on checking the brake circuit. After measuring the discharge resistance and finding no issues, the brake transistor (ET191) was found to be damaged. Replacing it resolved the problem, and the machine now stops smoothly without any issues.

Fault phenomenon 2: Undervoltage (Uu): This is another common issue encountered during operation.

1. Main causes include low main circuit voltage (below 200V for 220V systems, below 400V for 380V systems), a faulty rectifier bridge, or improperly functioning thyristor channels. It could also be due to a damaged main circuit contactor or a fault in the voltage detection circuit.

2. Example: A DANFOSS VLT5004 inverter shows a normal power-on display but triggers a “DC LINK UNDERVOLT” alarm when a load is applied. Diagnosis and repair: Although the symptoms seem complex, a careful analysis reveals that the inverter charges through its charging circuit and contactor. No issues were found during power-up, but the DC link voltage dropped when the load was applied. The rectifier bridge performs full-wave rectification, and the capacitor smooths the voltage before supplying it to the system.

3. Troubleshooting: Check the rectifier bridge. After testing, one of the bridge arms was found open, and replacing the component fixed the problem.