Drive cooling fan
Common heat-related VFD and servo-drive failure point.
Danfoss VFD Fault UL — What It Means
Fault UL (Underload) on a Danfoss FC series drive (FC102, FC202, FC302) indicates that the motor is drawing significantly less current than expected for the commanded speed and load. This is a process protection fault — the drive’s underload curve detected that the mechanical load has dropped below the configured threshold, which typically signals a process problem: a broken belt on a fan or pump, pump cavitation, a conveyor chain that has snapped, or an empty hopper on a mixer.
Common Causes
- Broken or slipped drive belt — A fan or pump driven by a V-belt loses its mechanical connection; the motor spins freely at low torque while the driven equipment does not move.
- Pump cavitation — A pump that has lost prime or is running against a closed valve cavitates — the impeller spins in vapor rather than liquid, dramatically reducing torque requirement.
- Broken coupling or shaft — A failed flexible coupling or sheared shaft key on the motor decouples it from the driven load.
- Underload parameters set too aggressively — Parameters 37-01 (Underload function) and 37-04 (Minimum motor current) configured with thresholds too close to the actual loaded operating point cause nuisance UL faults.
Step-by-Step Fix
- Inspect the driven equipment — Before touching the drive, physically inspect the driven load. Check the drive belt for breakage or slippage. Check the coupling. Confirm the pump/fan is mechanically connected to the motor.
- Check pump prime — For centrifugal pump applications, confirm the pump has prime and that all isolation valves are open. Reprime the pump if cavitation is suspected.
- Review the underload parameters — Access the Danfoss parameter set (par 37-01 through 37-06). If the underload function is enabled, verify the trip delay (37-02) and the warning/trip threshold (37-04) are appropriate for the application. Increase the warning delay to filter transient low-load events.
- Compare actual vs. expected current — Monitor the drive’s output current display (par 16-14) during normal operation. Compare to the motor’s rated current at the operating speed. Underload is indicated by current more than 20–30% below the expected loaded value.
- Clear the fault and restart — Once the mechanical cause is corrected (belt replaced, pump primed), reset the UL fault via the LCP display (Reset key) or digital input and restart.
Parts Often Needed
| Part | Notes |
|---|---|
| Drive belt (V-belt or synchronous) | Match pitch and length to OEM specification |
| Flexible coupling insert | Replace spider/element if coupling is used |
| Pump foot valve or check valve | If pump repeatedly loses prime |
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 UL trips after belt replacement or pump priming and no mechanical fault is visible, have a process engineer review the system design. Recurring cavitation indicates undersized pump, incorrect speed setting, or system curve mismatch.