DC Bus Undervoltage

What does LU mean?

The DC bus voltage has dropped below acceptable limits, typically due to low incoming mains voltage or an issue with the power supply module. Incorrect controller wiring to the mains (e.g., 400V controller on 240V mains) can also trigger this. Persistent undervoltage can cause intermittent operation or prevent the drive from starting.

• Incoming mains voltage has dropped below the minimum specified operating voltage (e.g., below 340V for a 400V drive).
• Faulty or undersized mains power supply transformer or upstream breaker/fuse.
• Incorrect drive wiring to mains (e.g., 400V drive connected to 230V mains without proper configuration).
• Momentary power sag on the mains supply during heavy load transients or switching events.
• Internal power supply module failure within the drive (e.g., rectifier bridge, DC link capacitor degradation).

Click steps to track your progress.

1. 1

   1. Measure the incoming mains voltage (L1-L2, L2-L3, L3-L1 for 3-phase or L-N for 1-phase) at the drive's input terminals.

2. 2

   2. Verify that the measured mains voltage falls within the specified operating range of the drive (e.g., 380-480VAC).

3. 3

   3. Check the mains wiring configuration against the drive's rated voltage and the available mains supply.

4. 4

   4. Inspect the connections of the mains power cables at the drive's input terminals for looseness or corrosion.

5. 5

   5. Monitor the mains voltage for transient sags using a power quality analyzer during operation.