SINGLE PHASING
What does single phasing mean:
Loads using three-phase power sources are subject to loss of one of the three phases from the power distribution system. This condition is known as "single-phasing." The loss of a single phase on a three-phase line may be due to a downed line or a blown pole top fuse on the utility system. Loss of a single phase may also result from a single-phase overload condition causing one fuse to blow, or an equipment failure within the end-user's facility. About 14% of the motor failures are due to single phasing.
Single phasing may occur due to many reason and one that’s reason are rupturing of any one phase in the three phase supply line, open circuit in any the fuses, breaking in the contact of any one phase, due to all such reasons the motor continues to rotate even on that two phases with increased load, approximately around 57% of the load gets increased which are drawn by the other two phases and current increases which trends to damage the motor winding.
The loss of one phase, or "leg," of a three-phase line causes serious problems for induction motors. The motor windings overheat due primarily to the flow of negative-sequence current, a condition that exists anytime there is a phase voltage imbalance. The loss of a phase also inhibits the motor's ability to operate at its rated horsepower. If single-phasing occurs when a motor is rotating, the torque produced by the remaining two positively rotating fields continues to rotate the motor and develop the torque demanded by the load.
The negatively rotating field, the field associated with the lost phase, produces currents in inductive loads resulting in voltages in the faulted leg of the three-phase supply. These voltages may be nearly equal to the phase voltage that was lost. Therefore, detecting a single-phasing condition by measuring the voltages at the motor terminals is usually unproductive.
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