single-phasing
The abnormal operation of a three-phase machine when its supply is changed by the accidental opening of one conductor.
single-phasing: when one phase of three goes missing
Single-phasing occurs when a three-phase motor or machine loses electrical supply on one of its three conductors while still receiving power on the other two. This happens most often through an accidental break in the line, a tripped breaker on one phase only, or a loose connection at a distribution point. The motor does not stop immediately; instead, it continues to run but operates in a severely degraded and dangerous condition.
A three-phase induction motor produces its rotating magnetic field through balanced current distributed equally across all three phases, each offset by 120 degrees. When one phase is lost, this balance collapses. The motor must rely on the two remaining phases to maintain rotation, but the resulting magnetic field becomes highly asymmetrical. The motor will draw much higher current through the two remaining conductors, often 1.5 to 2 times the normal full-load current, while delivering only a fraction of its rated torque.
Why this damage matters
Prolonged single-phasing causes rapid overheating of the stator windings because current concentrates unevenly through the coils. The motor frame temperature can rise by 20 to 40 degrees Celsius within minutes. If the machine cannot be stopped quickly, the winding insulation fails, leading to complete burnout and replacement of the motor. Many industrial motors are destroyed this way before the loss of one phase is even noticed, especially if the motor is lightly loaded and a protection relay is not in place.
Single-phasing is distinct from phase imbalance, where all three phases are present but carry unequal voltages. Detection requires either direct electrical testing with a multimeter or clamp meter on each phase, or installation of a three-phase monitor relay that will trip the motor contactor if any phase drops below a threshold voltage. Standard overload relays do not always catch single-phasing quickly enough because the motor does not draw full load current immediately in light-duty applications.
The condition occurs most commonly at the service entrance or in poorly maintained distribution panels where vibration has loosened lugs or where corrosion has built up at termination points. It can also follow a deliberate opening of a disconnect switch on only one phase during troubleshooting work. Prevention requires proper maintenance of electrical connections, correct protective relaying, and disciplined lockout procedures that isolate all three phases simultaneously.