Undervoltage

What does USF mean?

This fault indicates that the input voltage supplied to the drive controller is too low, possibly due to a transient voltage dip, a persistent low line voltage, or a damaged internal component like a load resistor. Undervoltage can cause erratic drive behavior, loss of motor control, and potential component stress.

• Incoming AC line voltage consistently below the drive's minimum rated input voltage (e.g., below 340V for a 400V drive).
• Transient voltage sag or brownout on the incoming AC power supply.
• Loose or corroded connections at the drive's input terminals (L1, L2, L3) or upstream power distribution.
• Blown input fuse on one or more phases leading to phase imbalance and low DC bus.
• Damage to the drive's internal rectifier bridge causing insufficient DC bus charging.

Click steps to track your progress.

1. 1

   1. Measure incoming AC line voltage (L1-L2, L2-L3, L3-L1 terminals) with a true RMS multimeter and compare to drive's minimum rated input voltage.

2. 2

   2. Inspect all input power connections (L1, L2, L3) for tightness and signs of corrosion.

3. 3

   3. Check continuity of all incoming line fuses (if present) and replace any blown fuses.

4. 4

   4. Monitor DC bus voltage (drive display parameter P0-08 or similar) during operation to observe voltage dips.

5. 5

   5. Confirm the mains transformer (if used) is correctly sized for the load and has adequate voltage regulation.