Two-phase electric power
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A simplified diagram of a two-phase alternatorTwo-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by 90 degrees. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. Some early two-phase generators had two complete rotor and field assemblies, with windings physically offset by 90 electrical degrees to provide two-phase power. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines.
The advantage of two-phase electrical power was that it allowed for simple, self-starting electric motors. In the early days of electrical engineering, it was easier to analyze and design two-phase systems where the phases were completely separated.  It was not until the invention of the method of symmetrical components in 1918 that polyphase power systems had a convenient mathematical tool for describing unbalanced load cases. The revolving magnetic field produced with a two-phase system allowed electric motors to provide torque from zero motor speed, which was not possible with a single-phase induction motor (without extra starting means). Induction motors designed for two-phase operation use the same winding configuration as capacitor start single-phase motors.
Three-phase electric power requires less conductor mass for the same voltage and overall amount of power, compared with a two-phase four-wire circuit of the same carrying capacity.  It has all but replaced two-phase power for commercial distribution of electrical energy, but two-phase circuits are still found in certain control systems.
Two-phase circuits typically use two separate pairs of current-carrying conductors. Alternatively, three wires may be used, but the common conductor carries the vector sum of the phase currents, which requires a larger conductor. Three phase can share conductors so that the three phases can be carried on three conductors of the same size. In electrical power distribution, a requirement of only three conductors rather than four represented a considerable distribution-wire cost savings due to the expense of conductors and installation.
Two-phase power can be derived from a three-phase source using two transformers in a Scott connection. One transformer primary is connected across two phases of the supply. The second transformer is connected to a center-tap of the first transformer, and is wound for 86.6% of the phase-to-phase voltage on the 3-phase system. The secondaries of the transformers will have two phases 90 degrees apart in time, and a balanced two-phase load will be evenly balanced over the three supply phases.
Three-wire, 120/240 volt single phase power used in the USA and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase