IndustrialDriveDataDrive Board & Diagnostic Database
Diagnostic workflow

Danfoss VLT Alarm 4 Mains Phase Loss Workflow

Entry symptom: Danfoss VLT reports alarm 4 or shows input phase instability.

Safety controls before proceeding
  • Input terminals remain hazardous.
  • Use qualified measurement procedure and PPE.

Investigation sequence

1

Measure at drive

Measure L1-L2, L2-L3 and L1-L3 at the drive input terminals.

2

Check protection path

Inspect fuses, disconnect, contactor poles and loose terminals.

3

Measure under load

Compare voltage condition when plant loads start or when the drive tries to run.

4

Inspect internal boundary

If input evidence is normal, review rectifier/DC-link sensing after safe isolation.

Stop conditions

  • Qualified live measurement is unavailable
  • One phase is missing
  • Contactor or fuse is overheating

Linked records

Alarm 4
Mains Phase Loss

One or more input phases are missing, low or unstable, or the drive input sensing/DC-link ripple indicates a phase problem.

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Danfoss VLT Input Phase and DC-Link Path

Routes alarm 4 and deceleration overvoltage through line phases, fuses, contactor, rectifier, DC-link ripple, high line voltage and braking energy paths.

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