Causes and effects of single-phasing induction motors

It is well known that a three-phase induction motor will continue to operate when a disturbance of some sort causes the voltages supplied to the motor to become single phased. Single phasing can occur as a result of a fuse blowing or a protective device opening on one phase of the motor. Other possibilities include feeder or step-down transformer fuses blowing. Even though the motor will continue to operate in this condition, the motor will heat up very quickly, and it is essential that the motor be removed from service by the opening of a motor circuit breaker or some other type of protective device. This paper will describe three different ways in which an induction motor will operate in a single-phase condition. For purposes of this paper, "single phase" will include any condition in which the three line-to-line voltage phasors appear on the same line.