Danfoss FC 302 Alarm 4: what to check for mains phase loss?
My Danfoss FC 302 drive is throwing an Alarm 4 (Mains Phase Loss) while running under load. I've checked the supply and all three phases are present, but the alarm persists. What should I check next?
Alarm 4 on a Danfoss VLT FC 302 indicates a missing phase on the supply side or excessive mains voltage imbalance. This alarm can also appear due to a fault in the input rectifier. It typically occurs while the drive is running under load, after a fuse or breaker opens, or on a weak or unbalanced supply. This alarm points to issues on the input side of the drive.
Likely causes and checks
1Blown input fuse or open breaker pole
- Points to it
- A sudden onset of Alarm 4, especially if one phase reads near zero at the drive input, strongly suggests a blown input fuse or an open breaker pole on one of the supply phases.
- Check
- Measure all three phase-to-phase voltages at the drive input terminals. With power locked out, check all input fuses for continuity.
2Loose or burnt input connection or contactor contact
- Points to it
- Intermittent trips or visible heat discoloration on a terminal or contactor pole indicate a poor connection or a burnt contact, leading to a phase loss condition.
- Check
- Lock out power, then inspect and re-torque all input terminals. Also, check the contacts of any input contactor for wear or damage.
3Supply voltage imbalance
- Points to it
- If all phases are present but unequal, or if other drives on the same supply are also showing warnings, the issue could be an unbalanced supply voltage.
- Check
- Measure the supply voltages and the supply currents to the drive under load, following Danfoss's recommended procedures.
4Input rectifier fault
- Points to it
- If the supply is balanced and complete, and the alarm still persists, the problem may be an internal fault within the drive's input rectifier.
- Check
- This requires specialized service. Refer to qualified service personnel, as the rectifier is an internal component of the drive.
Next check: Since you've confirmed all three phases are present, the next step is to measure the phase-to-phase voltages at the drive input terminals to ensure they are balanced and within specification. Also, check for any signs of heat discoloration on the input terminals or contactor contacts, and verify their tightness.
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