DC Bus Undervoltage

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The DC bus voltage fell below its trip point (≤190Vdc for 208-240Vac Drives; ≤380Vdc for 480Vac Drives). This is often due to low input voltage at the R/L1, S/L2, and T/L3 terminals, an acceleration time set too short, or excessive input voltage fluctuations, risking drive instability or shutdown.

• Input AC line voltage at R/L1, S/L2, T/L3 terminals is consistently below the drive's minimum operating voltage.
• Momentary power sag or brownout condition on the incoming AC line causing voltage dip.
• Loose or corroded connections at the drive's input terminals (R/L1, S/L2, T/L3) or upstream power distribution points.
• Excessive voltage drop across a long input power cable or undersized wiring for the motor current.
• Aggressive acceleration (C1-01) drawing high current, causing voltage sag on a weak power supply.

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1. 1

   1. Using a multimeter, measure the AC input voltage at R/L1, S/L2, T/L3 terminals and verify it is within the drive's specified range.

2. 2

   2. Measure the DC bus voltage across the P(+) and N(-) terminals during operation, especially during acceleration.

3. 3

   3. Confirm tightness of all power wiring connections at the drive's R/L1, S/L2, T/L3 terminals and upstream disconnects.

4. 4

   4. Increase the acceleration time (e.g., parameter C1-01 or C1-03) to reduce peak current demand during startup.

5. 5

   5. Check the rating of upstream transformers, power supply, and wiring for adequacy to support the drive's load.