If a three-phase motor is started on a single phase, it will:

If a three-phase motor is started on a single phase, it will:

• A. Draw power from the two remaining phases
• B. Draw power from only one phase
• C. Always short-cycles
• D. Runs cooler

Answer: (B)

A three-phase motor needs three phase-shifted currents to create a rotating magnetic field that produces starting torque. If you energize only one phase, only one winding is powered, so there isn’t a rotating field. In that situation, the electrical power available to the motor comes from the single energized phase, and the other two phases do not supply power because they’re not energized relative to the supply. The result is poor or no starting torque and a tendency for high current in that one winding, which can cause overheating or voltage problems if continued. So the best description is that the motor would draw power from only one phase.

A three-phase motor needs three phase-shifted currents to create a rotating magnetic field that produces starting torque. If you energize only one phase, only one winding is powered, so there isn’t a rotating field. In that situation, the electrical power available to the motor comes from the single energized phase, and the other two phases do not supply power because they’re not energized relative to the supply. The result is poor or no starting torque and a tendency for high current in that one winding, which can cause overheating or voltage problems if continued. So the best description is that the motor would draw power from only one phase.