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Two-phase electric power
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. As of 21st century, two-phase power was superseded with three phases and is not used in the industry. There remains, however, a two-phase commercial distribution system in Philadelphia, Pennsylvania; many buildings in Center City are permanently wired for two-phase and PECO (the local electric utility company) has continued the service.
==Comparison with single-phase power==
The advantage of two-phase electrical power over single-phase 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.〔Thomas J. Blalock "The first polyphase system: a look back at two-phase power for AC distribution", in ''IEEE Power and Energy Magazine'', March–April 2004, ISSN 1540-7977 p. 63〕 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 an additional starting means.) Induction motors designed for two-phase operation use a similar winding configuration as capacitor start single-phase motors (however, in a two-phase induction motor, the impedances of the two windings are identical, whereas in a single-phase induction motor, the impedances can be, and usually are, quite different, to reduce cost without sacrificing starting performance; indeed, some single-phase capacitor start/capacitor run induction motors have superior starting characteristics when compared and contrasted to two- or three-phase induction motors.)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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