Two-phase electric power

This article is about electric systems with 90° phase difference. For systems with two opposite (180°) live wires, see split-phase electric power.
A simplified diagram of a two-phase alternator[1]

Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. 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 to provide two-phase power. The generators at Niagara Falls installed in 1895 were the largest generators in the world at that time, and were two-phase machines. Three-phase systems eventually replaced the original two-phase power systems for power transmission and utilization. Active two-phase distribution systems remain in Center City Philadelphia, where many commercial buildings are permanently wired for two-phase,[2] and in Hartford, Connecticut.[3]

  1. ^ Figure 1253 from the 1917 Hawkins Electrical Guide
  2. ^ Company advertising services for two-phase: http://www.phillyfacility.com/two_phase.htm
  3. ^ Williams, Al (2018-03-15). "A Tale Of Two Phases And Tech Inertia". Hackaday. Retrieved 2023-02-04.