DC Bus Undervoltage

What does DC Undervoltage mean?

The voltage on the internal DC bus has fallen below the minimum detection level (L2-05). This usually indicates a failure of the main power supply, a lost input phase, or an insufficient power supply capacity to meet the drive's demands. This condition causes the drive to shut down or operate erratically.

• Loss of one or more incoming AC power phases (e.g., L1, L2, L3) or an excessive voltage sag in the main power supply.
• Tripped upstream circuit breaker, blown input fuse(s), or loose connections on the drive's input terminals (L1, L2, L3).
• Insufficient incoming AC voltage (e.g., below 380V for a 400V class drive) due to undersized transformer or utility brownout.
• DC bus charge circuit fault (related to ref=72) preventing the bus capacitors from charging adequately.
• Drive's internal rectifiers or DC bus capacitors are failing, leading to inability to maintain voltage.

Click steps to track your progress.

1. 1

   1. Measure the incoming AC line voltage (L1-L2, L2-L3, L3-L1) at the drive's input terminals with a multimeter to ensure all phases are present and within the drive's specified voltage range (e.g., 400-480VAC).

2. 2

   2. Inspect all power wiring connections from the main breaker/disconnect to the drive's input terminals (L1, L2, L3) for tightness, corrosion, or signs of damage.

3. 3

   3. Check all upstream fuses and circuit breakers for continuity or trip status. Replace blown fuses with correct ratings, reset tripped breakers.

4. 4

   4. Monitor the DC bus voltage (U1-03) during power-up to observe its charging behavior. Compare it to the minimum detection level (e.g., parameter L2-05).

5. 5

   5. If the supply voltage is intermittently low, investigate the upstream power supply (e.g., transformer taps, utility voltage stability) to ensure adequate power capacity.

6. 6

   6. For persistent undervoltage with good input power, the internal rectifier or DC bus capacitor bank may be failing, requiring drive repair or replacement.