Before fitting a replacement for a motor that burnt out from phase loss, find out in the panel which phase dropped out and why. If the cause of the interruption is still there, the new motor will end up on two phases in the same way. The checks and the interpretation of their results are a job for qualified electrical staff; the list below is to make clear what to ask them for.

DRG-200L1-2 30 kW 3000 rpm cast iron frame DRG three-phase motor – motor burnout from phase loss

Why does a phase drop out?

A three-phase motor that loses one phase may keep running, or if it is stopped it will not start at all; either way the remaining windings carry excess current and overheat. A phase can be lost on the grid side, but more often the cause is within the site: a single blown fuse, a worn contactor contact, a loose terminal or a damaged cable.

Panel checklist

  1. Fuses: Which phase's fuse has blown, and whether the fuse type and holder are sound.
  2. Contactor: Burning, blackening or pitting on the contacts; one pole making weaker contact than the others.
  3. Connection points: Looseness or signs of heating on busbars, switches and terminals.
  4. Cable: Crushing, rodent damage or insulation damage on the run between panel and motor.
  5. Protection scheme: Whether the existing motor protection device responds to phase loss; reassessment of the scheme for the new motor. Settings depend on the nameplate and the staff's decision.
  6. Grid: Voltage balance between phases at the site intake.

How to recognise phase loss in the winding

In a star-connected motor two phase coils, and in a delta-connected motor one phase coil, may look clearly more darkened than the rest. A rewind shop can confirm this pattern. If the finding points to phase loss, fitting a bigger motor or choosing a more powerful model is not the answer.

Warranty aspects

Failures caused by a missing phase or phase imbalance are not covered by the warranty. Keeping a short record showing that the panel was checked before the new motor was fitted also makes it easier to assess any problem that may arise later.

The general logic for selecting panel protective devices is explained on the overload relay, fuse and contactor selection page; once the panel is ready, you can choose a model of the same power in the three-phase motor shop.