Cooling fan
Common VFD, drive, UPS, and cabinet overheating failure point.
Schneider Altivar Fault ULF — What It Means
ULF on a Schneider Electric Altivar drive indicates a motor underload fault. The drive detected that the motor is drawing significantly less current than expected for its speed and load — typically below 20–30% of the rated current threshold set in the underload parameters. An underload condition usually means the driven load has been lost or disconnected: a broken belt, a cavitating pump, or a coupling failure. The drive trips to alert the operator that the process is no longer functioning correctly.
Common Causes
- Broken belt or coupling — The mechanical link between the motor and the driven load has failed. The motor spins freely with no load, drawing very little current.
- Cavitating or air-locked pump — A pump running without sufficient fluid (dry run, low tank level, or vapor lock) draws significantly less current than normal operating load.
- Conveyor or process jam resolved but load disconnected — After a jam clears, if a conveyor section disconnects or a chain breaks, the motor loses its load.
- ULF threshold set too high — The underload detection threshold in the Altivar parameters is configured so sensitively that normal light-load conditions trigger a false ULF fault.
Step-by-Step Fix
- Inspect the driven equipment for mechanical failure — Check belts, couplings, chains, and shafts between the motor and the load. A visually broken or slipping belt is the most common cause of ULF.
- Check pump suction conditions — If the driven load is a pump, verify that suction piping is not air-locked, that the inlet valve is open, and that the tank or source has adequate fluid level.
- Review the underload parameters — Access the underload detection parameters on the Altivar (typically LUL — Underload current threshold, and tUL — Underload time delay). Verify the threshold is set appropriately for your application. Increase the time delay to prevent nuisance trips during brief low-load transients.
- Confirm motor current under normal conditions — If the load is intact and the process is operating normally but ULF still trips, measure actual motor current with a clamp meter and compare to the ULF threshold setting. Adjust the threshold downward to accommodate normal operating current range.
- Repair or replace the mechanical failure — Replace a broken belt, coupling, or chain. Prime the pump if air-locked.
- Disable ULF if not required — For applications where underload protection is not needed, ULF detection can be disabled in the Altivar parameters. Consult your application requirements before doing this.
- Reset the fault — After resolving the load issue or adjusting parameters, reset the ULF fault via keypad and confirm the drive runs without re-tripping.
Parts Often Needed
| Part | Notes |
|---|---|
| Drive belt (V-belt or synchronous) | Replace if worn, cracked, or broken |
| Flexible coupling insert | Replace if rubber insert or spider has failed |
| Pump foot valve / check valve | Replace if pump is losing prime repeatedly |
Need Replacement Parts?
Before ordering, match the drive part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
If the driven load appears mechanically intact but ULF continues to trip, a drive technician should review the full parameter set and perform a load current analysis to correctly configure the underload detection thresholds for the specific application.