Input fuses
Low-cost line-side part to check when one phase is missing or unstable.
Lenze i550 Fault LF — What It Means
Fault LF (Line Failure / Phase Loss) on a Lenze i550 drive means the drive has detected that one or more phases of the incoming supply are missing or severely unbalanced. The Lenze i550 is Lenze’s compact, application-optimized inverter for pumps, fans, and conveyor applications. LF fires when the monitored input phase voltages deviate beyond the acceptable imbalance threshold. Operating on a single-phase supply (even briefly) forces the drive to rectify on only two legs, causing abnormal DC bus ripple that stresses the capacitors and power bridge.
Common Causes
- Blown input fuse on one phase — A single-phase fuse failure is the most common cause. The other two phases remain powered, so the drive continues to receive partial power but reports LF.
- Tripped circuit breaker pole — A thermal-magnetic breaker with one tripped pole passes the same partial-power scenario as a blown fuse.
- Loose input terminal — A terminal that was not properly torqued or has vibrated loose creates a high-resistance intermittent connection that reads as phase loss under load.
- Utility supply fault — A broken overhead line, a blown utility transformer fuse, or a distribution fault upstream of the facility can drop one phase to the entire site.
Step-by-Step Fix
- Measure input voltages at L1, L2, L3 — With appropriate PPE, measure all three L-L voltages at the i550 input terminals. A missing or low phase will be immediately apparent.
- Check upstream fuses and breakers — Inspect the protective device feeding the i550. Replace any blown fuse with the type and rating specified in the Lenze i550 installation manual (semiconductor-type fuses are required for drive protection).
- Inspect input terminal tightness — With power off, check the torque of all three input terminals. Retorque to the Nm value specified in the manual for the cable size being used.
- Inspect for heat damage — A loose terminal that has been arcing will show discoloration on the terminal block or cable lug. Replace any damaged components.
- Verify supply at the service entrance — If all drive-level checks pass, have an electrician verify the utility supply is delivering balanced three-phase voltage at the building’s main distribution panel.
- Reset and test — After correcting the supply issue, reset the LF fault from the i550 keypad or via the connected fieldbus, and test the drive under normal operating load.
Parts Often Needed
| Part | Notes |
|---|---|
| Semiconductor input fuses | Use Lenze-specified aR or gR fuse type; do not substitute standard HRC fuses |
| Input terminal block | Replace if arcing damage or severe corrosion is present |
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
All input phase fault diagnosis involves live three-phase voltages. Only qualified electricians wearing appropriate PPE should perform live voltage measurements. If the LF fault points to a utility supply problem, the utility company must be notified — do not attempt to bypass or ignore the fault.