Constant instantaneous power delivery
Three-phase systems use three sinusoidal voltages separated by 120 electrical degrees to maintain nearly constant instantaneous power flow. While single-phase AC power oscillates between zero and a peak twice per cycle, the three phases overlap such that their sum remains relatively flat. This smooth delivery eliminates the mechanical vibration that plagues single-phase generators and motors.
Voltage relationships and phase rotation
The three voltages sum to zero at any instant, a property that allows neutral wires to carry negligible current in balanced systems. Phase rotation (the order in which phases reach their peak) can be clockwise or counterclockwise, affecting motor direction without changing the supply. Standard industrial practice uses phase A, B, C at 0, 120, and 240 degrees respectively.
Why the grid runs on three-phase
Three-phase transmission and distribution reduce copper losses, enable more compact generator and motor designs, and deliver power more efficiently than single-phase at the same voltage. The constant power flow permits smaller, lighter equipment to handle the same load. Most industrial equipment and utility scale generation relies on three-phase for this reason.