In a three-phase system, how are the voltages phased relative to each other?

• A. They are in phase with each other.
• B. The voltages are 120 degrees apart in phase.
• C. The voltages alternate at 90 degrees relative phase.
• D. The voltages are 60 degrees apart.

Answer: (B)

In a balanced three-phase system, the voltages are 120 degrees apart in phase. This means each phase reaches its peak at a different moment, and the three phasors are spaced evenly around the circle: one at 0°, another at 120°, and the third at 240° (or equivalent with sign choices). This 120-degree separation keeps the instantaneous sum of the voltages nearly constant, which gives smooth, continuous power delivery and a rotating magnetic field in motors. If the phases were in phase, their peaks would coincide and current and torque would fluctuate; if they were separated by 60° or 90°, the system wouldn’t produce the same balanced, constant-power performance.