Question:
Why won’t a three-phase motor run as a Two-phase motor if one phase is lost?
Answer:
The motor normally runs because of the rotating magnetic field created by three voltages at different phase angles, coming off lines A, B, and C. The voltages are between A and B, B and C, C and A. If one line is open- say, the B connection- the two voltages A-B and B-C disappear, leaving only the one C-A. The field doesn’t rotate. If the motor is stopped, it can’t start, if its running, it will soon overheat or stall.