Undervoltage
Schneider Electric · Altivar Machine ATV320 Variable Speed Drive
What does USF mean?
An undervoltage condition detected. This means the incoming AC supply voltage or the internal DC bus voltage has dropped below safe operating limits. This can cause erratic drive behavior, loss of motor torque, and can indicate issues with the power supply. The drive will trip to protect itself.
Common Causes
- Input AC supply voltage dropped below the drive's minimum operating threshold (e.g., below 340V for a 400V drive).
- Momentary power sag or brownout event on the incoming AC line.
- Blown fuse or tripped circuit breaker on one or more incoming power phases (L1, L2, L3).
- Loose connection or high resistance at the drive's input power terminals (L1, L2, L3).
Repair Steps & Checklist
Click steps to track your progress.
- 1
1. Measure the incoming AC line voltage (L1-L2, L2-L3, L3-L1) at the drive's input terminals using a multimeter.
- 2
2. Check all fuses and circuit breakers in the supply line to the drive for continuity or tripped status.
- 3
3. Inspect the drive's input power terminals (L1, L2, L3) for secure connections and signs of overheating.
- 4
4. Monitor the DC bus voltage (e.g., display parameter d0-03) and compare it to the expected operating range (e.g., 560VDC for 400VAC).
- 5
5. Check the upstream power distribution system (e.g., transformer taps, utility supply) for stable voltage delivery.