View source for Two-phase electric power

{{Short description|Electric power distribution system}}
{{about|electric systems with 90° phase difference|systems with two opposite (180°) live wires|split-phase electric power}}

[[Image:Elementary Two Phase Alternator.jpg|thumb|400px|A simplified diagram of a two-phase alternator<ref>Figure 1253 from the 1917 Hawkins Electrical Guide</ref>]]

'''Two-phase electrical power''' was an early 20th-century [[polyphase system|polyphase]] [[alternating current]] electric power distribution system. Two circuits were used, with voltage [[phase (waves)|phase]]s 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 [[electric generator|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 electric power|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|Center City]] [[Philadelphia]], where many commercial buildings are permanently wired for two-phase,<ref>Company advertising services for two-phase: http://www.phillyfacility.com/two_phase.htm</ref> and in [[Hartford, Connecticut]].<ref>{{Cite web |last=Williams |first=Al |date=2018-03-15 |title=A Tale Of Two Phases And Tech Inertia |url=https://hackaday.com/2018/03/15/a-tale-of-two-phases-and-tech-inertia/ |access-date=2023-02-04 |website=Hackaday |language=en-US}}</ref>
==Comparison with single-phase power==
The advantage of two-phase electrical power over [[single-phase electric power|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.<ref name="blalock">{{cite journal |last1=Blalock |first1=T.J. |title=The first polyphase system a look back at two-phase power for AC distribution |journal=IEEE Power and Energy Magazine |date=March 2004 |volume=2 |issue=2 |pages=63–66 |doi=10.1109/MPAE.2004.1269626}}</ref> 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 [[Electric motor#Single-phase AC induction motors|capacitor start]] single-phase motors. However, in a two-phase induction motor, the impedances of the two windings are identical. 

Two-phase circuits also have the advantage of constant combined power into an ideal load, whereas power in a single-phase circuit pulsates at twice the line frequency due to the zero crossings of voltage and current.

==Comparison with three-phase power==
[[Three-phase electric power]] requires less conductor mass for the same voltage and overall power, compared with a two-phase four-wire circuit of the same carrying capacity.<ref>Terrell Croft and Wilford Summers (ed), ''American Electricans' Handbook'', Eleventh Edition, McGraw Hill, New York (1987) {{ISBN|0-07-013932-6}} page 3–10, figure 3–23</ref> It has 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. The vector sum of balanced three-phase currents, however, is zero, allowing for the neutral wires to be eliminated. 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.

While both two-phase and three-phase circuits have a constant combined power for an ideal load, practical devices such as motors can suffer from power pulsations in two-phase systems.<ref name="blalock"/> These power pulsations tend to cause increased mechanical noise in transformer and motor laminations due to [[magnetostriction]] and torsional vibration in generator and motor drive shafts.

Two-phase power can be derived from a three-phase source using two [[transformer]]s 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 three-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.

==See also==
{{div col}}
*[[Polyphase system]]
*[[Rotary converter]]
*[[Single-phase electric power]]
*[[Split-phase electric power]]
* [[Three-phase electric power]]
{{div col end}}

==References==
; Notes:
{{Reflist|group=note|liststyle=lower-roman}}

; Specific references:
{{Reflist}}
; General references:
{{refbegin}}
*[[Donald G. Fink]] and H. Wayne Beaty, ''Standard Handbook for Electrical Engineers'', Eleventh Edition, McGraw-Hill, New York, 1978, {{ISBN|0-07-020974-X}}
* Edwin J. Houston and Arthur Kennelly, ''Recent Types of Dynamo-Electric Machinery'', copyright American Technical Book Company 1897, published by P. F. Collier and Sons New York, 1902
{{refend}}

{{Electric machines}}

[[Category:Electric power]]
[[Category:AC power]]