What is the definition of phase loss (single-phasing)?

• A. When a three-phase motor loses power on one of its three phases due to a blown fuse, open wire, or failed contact
• B. When one phase becomes unavailable and the motor continues to run on the remaining two phases
• C. When a three-phase motor loses power on two phases leaving only one
• D. When a motor is started on a single phase with a capacitor

Answer: (B)

Phase loss, or single-phasing, occurs when one of the three supply phases to a motor becomes unavailable. In that case the motor is still energized by the two remaining phases and continues to run, but with an unbalanced magnetic field. That imbalance reduces the available torque, causes pulsating torque and higher currents in the remaining windings, and can lead to overheating or damage if it persists. This matches the idea that one phase is missing and the motor keeps running on the other two phases. The other scenarios describe causes or setups that are not defining phase loss in a three‑phase motor (for example, a single‑phase capacitor startup arrangement is a different situation).