Undervoltage in the DC bus

What does LU mean?

The DC bus voltage is lower than the value specified in C0173. This indicates insufficient input power to the drive, potentially due to issues with the mains voltage or supply cabling, preventing proper drive operation.

• Input mains voltage sag below specified minimum (e.g., <340VAC for a 400V drive) during heavy load transients.
• Blown or undersized input fuses (e.g., F1, F2, F3) or tripped input circuit breaker (Q1) interrupting one or more phases.
• Loose or high-resistance connections in the main power circuit (e.g., at terminals L1, L2, L3 on the drive input).
• Faulty input rectifier bridge module (e.g., measuring open circuit diodes with a multimeter).
• Incorrectly configured voltage monitoring threshold in Parameter C0173 (e.g., set too high for the actual supply).

Click steps to track your progress.

1. 1

   Measure the incoming line-to-line voltage at terminals L1, L2, L3 of the drive with a multimeter and verify it is within +/-10% of the nominal supply voltage.

2. 2

   Inspect input fuses (e.g., F1, F2, F3) for continuity with an ohmmeter and replace any open circuit fuses with correctly rated replacements.

3. 3

   Torque all main power connections at the drive input (L1, L2, L3) and output (U, V, W) to the manufacturer's specified value (e.g., 5Nm).

4. 4

   Monitor the DC bus voltage (e.g., via digital display or oscilloscope at DC+ and DC- terminals) during operation to identify transient sags.

5. 5

   Verify Parameter C0173 (DC bus undervoltage threshold) is set appropriately for the nominal input voltage and drive model, adjusting if necessary.