Three Phase Motors: Single Phasing

Single phasing is a condition that occurs when one of the three phases that is supplying voltage to a three-phase motor is lost. When this occurs the current across the remaining two legs can go to 1.73 times (173%) the normal FLA (please refer to the figure below).

During this condition the excess current flowing through the other windings will cause those windings to overheat. This could permanently damage the winding insulation and possibly cause a fire inside the motor. Caution should be implemented to ensure that overloads on the motor are rated as to prevent this condition.