Danfoss VLT drives (HVAC Drive FC 102, AutomationDrive FC 302, and the earlier VLT 2800/5000 families) are workhorses of European building services and machinery. When one trips, the alarm code on the LCP tells you most of the story — if you know how to read it. Here are the alarms we see most on units arriving at our bench, what they usually mean in the field, and which ones signal a drive at end of life. Alarm numbering varies slightly by series and firmware — confirm in the design guide for your exact model.
Supply-side alarms
- Mains phase loss. One input phase is missing or badly imbalanced — check fuses, terminals, and upstream contactor before condemning the drive. Prolonged running on two phases stresses the DC-link capacitors.
- DC-link overvoltage. Classic on decelerating high-inertia loads: the motor regenerates into the DC bus. Extend ramp-down time, or fit a brake resistor/chopper. Repeated overvoltage trips age the bus capacitors.
- DC-link undervoltage. Sagging supply or failing soft-charge circuit. If it appears at switch-on with a healthy supply, the pre-charge circuit is suspect — a repairable fault, but one that takes the drive off the wall.
Load-side alarms
- Overcurrent / short circuit. Trips instantly? Megger the motor and cable first — an insulation fault downstream is far more common than a failed drive stage. Trips only under load? Check for mechanical binding and correct motor data in the parameter set.
- Earth (ground) fault. The drive detects leakage to earth — usually motor winding insulation or a damp cable gland, occasionally a failed output stage inside the drive.
- Motor thermal / ETR trip. The electronic thermal relay model says the motor is overheating: verify actual load, motor data parameters, and cooling before raising limits.
Drive-internal alarms — the replacement signals
Heatsink over-temperature with clean fins and a spinning fan points to a failing fan bearing or degraded thermal interface — fans are the number-one wear item in any VFD. Internal fault codes (control board, current measurement, charge circuit) that survive a full power cycle and parameter reset are hardware failures. On drives 10–15+ years old, board-level repair rarely beats a tested replacement on cost once labour and risk are counted.
Repair vs refurbished replacement — the honest maths
Electrolytic DC-link capacitors and cooling fans define a drive’s lifespan — both age with temperature and time regardless of duty. When an older VLT starts producing internal or thermal alarms, the economical sequence is: fans first (cheap, fast), then judge. If alarms persist, a bench-tested refurbished drive of the same power size and options — with the parameter file cloned across via the LCP — restores the system in an afternoon at a fraction of new list price, and sidesteps the multi-week lead times current on some new drive sizes.
PLC North stocks tested Danfoss VLT drives across the common HVAC and machinery ratings. Send us your type code from the nameplate and your alarm history — we will tell you frankly whether it is worth repairing or replacing.