Single phasing is a power supply-related fault, wherein the supply to one of the phases is cut-off. The motor will continue to run in 2 phases with vibrations at reduced speed. As the windings connected to 2 phases draw a very high current, this will damage the windings if the protection circuit does not work.
Single phase winding failure is a result of an open in one phase of power supply to the motor. The open is usually caused by a blown fuse, an open contactor, a broken power line or bad connections. This can be prevented with properly set Over-load Relay, Thermistors and Single Phase Preventors.
Illustration:
In the image below, due to single phasing, notice that one set of copper windings retain their original color, while two other sets of Windings turned black due to overloading.
Manufacturers of AC Motor don’t accept this as a manufacturing defect as it is caused due to faulty power supply.