Will a Three-Phase Motor Burn Out During Phase Loss?

When a Three-Phase Motor Loses Phases, Will the Windings Always Burn Out?

Some people have raised questions about single-phase loss and two-phase loss. There is a common belief that when the power supply loses one phase, two windings will burn out; and when two phases are lost, one winding will burn out.

From the perspective of input voltage balance, phase loss is the extreme condition of voltage unbalance. Whether one phase or two phases are missing, the motor will not be able to operate normally, and in some cases the windings may even be burned.

We have previously analyzed the case of single-phase loss. For motors connected in delta, it is possible that one winding burns out while the other two remain basically intact. For motors connected in star (wye), when one phase is missing, the typical fault pattern is that two windings are burned while the remaining one is largely undamaged.

Today, assuming the motor itself is in good condition, we focus on the situation where two phases of the input power are lost. In circuits equipped with comprehensive protection devices, severe current or voltage unbalance will trigger the protection system, effectively safeguarding the motor. However, if there is no protection device, or if it fails to operate properly, winding faults caused by phase loss may occur.

For both delta and star connections, when two supply phases are disconnected—so-called two-phase loss—none of the windings has both a voltage supply and a closed circuit path. As a result, no current flows in any winding, and therefore no winding burnout will occur under this condition.

However, if the phase-loss event starts with the loss of one phase and then progresses to the loss of two phases, the final effect may be equivalent to that of a single-phase disconnection. In practice, electrical faults in motor windings rarely occur in isolation. One problem often leads to another, making analysis more complicated. Without sufficient experience, it is difficult to reach an objective and accurate conclusion.