Undervoltage

What does -USF mean?

This fault signifies that the input voltage to the drive controller has dropped below its minimum operating threshold or experienced a significant transient dip. A damaged pre-charge resistor or internal power supply issue could also prevent the DC bus from charging correctly. Operating with undervoltage can cause erratic drive behavior, prevent it from starting, or damage internal components over time.

• Input line voltage (L1, L2, L3) dropping below the drive's minimum operating voltage (e.g., <340V for a 400V drive).
• Momentary power sag, brownout, or complete power loss on the utility supply affecting the input voltage.
• Loose or corroded connections on the input power circuit (e.g., main breaker, contactor, drive input terminals) causing voltage drop.
• Blown pre-charge resistor (if applicable) within the drive, preventing proper DC bus charging at startup.
• Incorrectly configured undervoltage trip level (Parameter P5-01) set too high for the actual line voltage.

Click steps to track your progress.

1. 1

   Measure input line voltage (L1-L2, L2-L3, L3-L1) at the drive input terminals (R, S, T) during operation or when the fault occurs.

2. 2

   Inspect all input power connections (breakers, contactors, fuses, drive terminals) for tightness, corrosion, and proper seating.

3. 3

   Check for external power supply issues (sags, brownouts) using a power quality analyzer over a period of time.

4. 4

   If accessible and safe, measure the DC bus voltage (+DC, -DC) on the drive. Compare to drive specifications (power OFF first).

5. 5

   Verify undervoltage trip level (Parameter P5-01) is at its default setting or appropriate for the supply voltage (if configurable).