Phase loss is one of the fastest ways to burn out a three-phase motor. If a phase is lost while the motor is running, the rotor keeps turning and everything may look normal from outside; but because the same shaft power now has to be drawn through two phases, the current in those phases rises well above the rated value. In a delta-connected motor, the current sharing between windings is also upset, so some windings heat up far more than others.

If a phase is missing while the motor is stationary, no rotating field is created: the motor hums, cannot start and draws a current close to locked-rotor current. Serious heating starts within seconds.

Common causes: a single blown fuse, a burnt contactor contact, a loose terminal or cable lug, a damaged extension cable. To protect against it:

  • Use a motor protection circuit breaker or thermal overload relay that is sensitive to phase loss.
  • In critical plants, add a phase sequence and phase loss relay as well.
  • On fused circuits, arrange for the motor supply to be disconnected completely when a single fuse blows.

For selecting protection devices, see our article on thermal relay and fuse selection.