UNDERVOLTAGE

What does USF mean?

This fault indicates an insufficient incoming line supply voltage, a transient voltage dip, or a potential issue with the drive's internal precharge resistor. If the voltage drops below the drive's operational threshold, it will be unable to operate reliably or at all, causing unexpected shutdowns and potential component stress. Sustained undervoltage can prevent motor start-up or lead to erratic performance.

• Incoming line supply voltage (L1, L2, L3) dropping below the drive's minimum operating threshold (e.g., < 300V for a 400V class drive).
• Transient voltage dip or sag on the utility grid, often caused by large loads starting nearby or distribution network issues.
• Blown fuse or tripped circuit breaker in the drive's main input power supply or control power circuit.
• Faulty internal pre-charge resistor or rectifier section in the drive, preventing the DC bus from charging correctly.
• Loose or high-resistance connections in the main incoming power circuit to the drive.

Click steps to track your progress.

1. 1

   1. Measure the incoming line voltage (L1-L2, L2-L3, L3-L1) at the drive input terminals using a true-RMS multimeter and compare to the drive's minimum input voltage specification.

2. 2

   2. Monitor the DC bus voltage (Udc, typically r0.1, r1.1, or similar) using drive diagnostics; observe if it drops below the undervoltage trip threshold (e.g., 380VDC for a 400V drive).

3. 3

   3. Inspect all fuses in the main input power circuit and control power circuit for continuity; replace any blown fuses with specified ratings.

4. 4

   4. Tighten all input power terminal screws (L1, L2, L3) on the drive and at upstream circuit breakers or contactors.

5. 5

   5. Check the utility grid quality or consult with the facility's electrician to identify and mitigate external sources of voltage sags or brownouts.

6. 6

   6. If equipped, verify the resistance of the internal pre-charge resistor (requires professional service and discharge of DC bus) if the DC bus fails to charge on power-up.