Two phase
Two-phase electrical power was used in some early 20th century factories and the distribution systems that served them. It utilized two phases, 90° apart, and was usually supplied using four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor.
Induction motors designed for two-phase operation use the same winding configuration as capacitor start single-phase motors.
Three-phase electric power provides smoother operation and requires smaller conductors for the same voltage and overall amount of power. It has all but replaced two-phase power.
Two-phase power can be derived from a three-phase source using two transformers in a Scott connection.
Three-wire, 120/240 volt single phase power used in the USA is sometimes incorrectly called "two-phase" by laypersons. We refer to this here by the British name, split phase.
See also