IndustrialDriveDataDrive Board & Diagnostic Database
Danfoss fault record

Alarm 4: Mains Phase Loss

The VLT reports mains phase loss, trips at power-up, or loses ready state when plant load changes.

Meaning of the indication

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

Checking order

  1. Measure L1-L2-L3 at the drive input terminals, not only at the upstream panel.
  2. Check input fuses, disconnect, contactor poles and loose terminals.
  3. Compare phase-to-phase voltages under no-load and loaded plant conditions.
  4. Inspect rectifier/DC-link evidence only after input evidence is credible.
  5. Record whether the alarm follows line sag, recent wiring work or a blown fuse.

Suspect areas

Input phaseFuseContactorLoose terminalRectifierDC-link ripple
Fault-specific caution

A missing phase can still leave dangerous voltage present inside the drive.

Diagnostic workflow

Workflow
Danfoss VLT Alarm 4 Mains Phase Loss Workflow

Danfoss VLT reports alarm 4 or shows input phase instability.

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