What term describes a three-phase motor operating with one phase absent, effectively running on two phases?

What term describes a three-phase motor operating with one phase absent, effectively running on two phases?

• A. Balanced operation
• B. Single phasing
• C. Open delta
• D. Two-phase operation

Answer: (B)

Single phasing describes a three-phase motor running with one supply line missing. In a healthy three-phase system the three windings are fed with voltages 120 degrees apart, creating a smooth rotating magnetic field. When one phase is absent, only two phases energize the stator, so the magnetic field becomes unbalanced and the motor produces far less torque. The currents in the remaining windings rise, causing overheating and a higher risk of damage; the torque also pulsates and the motor can stall under load. Protective devices or driven equipment typically trip to prevent damage. Open delta is a transformer configuration, not the situation described here, and balanced operation or two-phase operation don’t fit this scenario.

Single phasing describes a three-phase motor running with one supply line missing. In a healthy three-phase system the three windings are fed with voltages 120 degrees apart, creating a smooth rotating magnetic field. When one phase is absent, only two phases energize the stator, so the magnetic field becomes unbalanced and the motor produces far less torque. The currents in the remaining windings rise, causing overheating and a higher risk of damage; the torque also pulsates and the motor can stall under load. Protective devices or driven equipment typically trip to prevent damage. Open delta is a transformer configuration, not the situation described here, and balanced operation or two-phase operation don’t fit this scenario.