Editing Polyphase system

{{Short description|Means of distributing alternating-current electrical power}}
{{About|electrical power distribution|the signal processing concept with the same name|Polyphase matrix}}
[[Image:3 phase AC waveform.svg|thumb|284px|One voltage cycle of a three-phase system]]

A '''polyphase system''' (the term coined by [[Silvanus Thompson]]) is a means of [[Electric power distribution|distributing]] [[Alternating current|alternating-current]] (AC) [[electric power|electrical power]] that utilizes more than one '''AC phase''', which refers to the [[phase offset]] value (in degrees) between AC in multiple conducting wires; ''phases'' may also refer to the corresponding terminals and conductors, as in [[Electrical wiring#Colour codes|color codes]]. Polyphase systems have two or more energized [[electrical conductor]]s carrying alternating currents with a defined phase between the voltage waves in each conductor. Early systems used 4 wire [[Two-phase electric power|two-phase]] with a 90° phase angle,<ref>The first polyphase system: a look back at two-phase power for AC distribution, IEEE Power and Energy Magazine (Volume: 2, Issue: 2, Mar-Apr 2004) [https://ieeexplore.ieee.org/document/1269626]</ref> but modern systems almost universally use [[Three-phase electric power|three-phase voltage]], with a phase angle of 120° (or 2π/3 radians).  

Polyphase systems are particularly useful for transmitting power to [[electric motor]]s which rely on alternating current to rotate. Three-phase power is used for industrial applications and for [[power transmission]]. Compared to a single-phase, two-wire system, a three-phase three-wire system transmits three times as much power for the same conductor size and voltage, using only 1.5 times as many conductors, making it twice as efficient in conductor utilization.

Systems with more than three phases are often used for [[rectifier]] and power conversion systems, and have been studied for power transmission.

==Number of phases==
In the very early days of commercial electric power, some installations used [[Two-phase electric power|two-phase]] four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor and, by using a four-wire system, conceptually the phases were independent and easy to analyse with mathematical tools available at the time.<ref>Terrell Croft, ''American Electricians' Handbook, Sixth Edition'', McGraw Hill, 1948, pp. 54–57</ref>

Two-phase systems can also be implemented using three wires (two "hot" plus a common neutral). However this introduces asymmetry; the voltage drop in the neutral makes the phases not exactly 90 degrees apart.

Two-phase systems have been replaced with three-phase systems. The move from two to three phases was originally motivated by making a more ideal rotating field for AC motors: [[Mikhail Dolivo-Dobrovolsky]] calculated that, for simple winding configurations of the time, the magnetic field fluctuation can be reduced from 40% to 15%{{cn|date=April 2024}}. This is less important in modern machines that create a nearly ideal rotating field using [[Coil_winding_technology#Characterization_of_distributed_winding|sinusoidally distributed windings]], but three-phase systems retain other advantages.

A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a [[Scott-T transformer|Scott-connected transformer]], which can also produce three-phase power from a two-phase input.

A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order. A 3-wire system with two phase conductors 180 degrees apart is still only single phase. Such systems are sometimes described as [[Split-phase electric power|split-phase]].

==Motors==
[[File:3phase-rmf-noadd-60f-airopt.gif|thumbnail|right|Three-phase electric machine with rotating magnetic fields]]

Polyphase power is particularly useful in [[Alternating current|AC]] motors, such as the [[induction motor]], where it generates a [[rotating magnetic field]]. When a three-or-more-phase supply completes one full cycle, the magnetic field of a two-poles-per-phase motor has rotated through 360° in physical space; motors with more than two poles per phase require more power supply cycles to complete one physical revolution of the magnetic field and so these motors run more slowly. Induction motors using a rotating magnetic field were independently invented by [[Galileo Ferraris]] and [[Nikola Tesla]] and developed in a three-phase form by [[Mikhail Dolivo-Dobrovolsky]] in 1889.<ref>[https://books.google.com/books?id=thOPkFjrj5MC&q=tesla Ion Boldea, Syed Abu Nasar, The Induction Machine Handbook  - CRC Press, 2002, page 2]</ref> Previously all commercial motors were DC, with expensive [[commutator (electric)|commutators]], high-maintenance brushes and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, are self-starting and have little vibration compared with single-phase motors.

==Higher phase order==
Once polyphase power is available, it may be converted to any desired number of phases with a suitable arrangement of transformers.  Thus, the need for more than three phases is unusual, but higher phase numbers than three have been used.

High-phase-order (HPO) power transmission has been frequently proposed as a way to increase transmission capacity within a limited-width [[Right-of-way (property access)|right of way]].<ref name=TDWorld>{{Cite news |url=http://tdworld.com/overhead-transmission/high-phase-what |title=High-Phase What? |date=July 1, 2011 |first=Vito |last=Longo |journal=Transmission & Distribution World |archive-url=https://web.archive.org/web/20160728074923/http://tdworld.com/overhead-transmission/high-phase-what|archive-date=28 July 2016}}</ref>  Transmitted power is proportional to the square of the phase-to-ground voltage drop, but transmission lines require conductors spaced adequately distant to prevent both phase-to-[[electrical ground|ground]] and phase-to-phase [[electrical arc]]s.  For three-phase power, the phase-to-phase voltage, which is {{math|{{sqrt|3}}≈1.7}} times the phase-to-ground voltage, dominates.  Higher-phase systems at the same phase-to-ground voltage have less voltage difference between adjacent phases, allowing a tighter conductor spacing.  For six- and higher-phase power systems, the dominant effect becomes the phase-to-ground voltage instead.<ref>{{cite conference|doi=10.1109/STIER.1990.324628|title=High phase order transmission|first=T.&nbsp;F.|last=Dorazio|publisher=IEEE|pages=31–32|conference=Technical Conference on the Southern Tier|date=25 April 1990|location=Binghamton, NY}}</ref>  

Six-phase operation thus lets an existing [[double-circuit transmission line]] carry more power without requiring additional conductor cable.  However, it requires the capital expense and impedance losses of new phase-converting [[power transformer|transformer]]s to interface with the conventional three-phase grid.<ref name=TDWorld/>  They are particularly economical when the alternative is upgrading an existing [[extra high voltage]] (EHV, more than 345&nbsp;kV phase-to-phase) transmission line to ultra-high voltage (UHV, more than 800&nbsp;kV) standards.<!--A number of promising references can be found by web searching for "high phase order power transmission", all locked up behind the IEEE paywall-->

Between 1992 and 1995, New York State Electric & Gas operated a 1.5 mile 93kV 6-phase transmission line converted from a double-circuit 3-phase 115kV transmission line.  The primary result was that it is economically favorable to operate an existing double-circuit 115kV 3-phase line as a 6-phase line for distances greater than 23–28 miles.<ref name=NYTrial>{{cite web|title=High Phase Order Transmission Demonstration|url=http://www.cerc-reactors.com/articles/miso16sep2004seminar/6-PhaseOTLs/eseerco.pdf|website=CERC-Reactors.com|publisher=NY State Electric & Gas}}</ref>{{rp|pp=xvii-xviii}}  

Three-phase power lines rely on [[transposition (transmission lines)|transposition]] to equalize across all phases transmission losses due to slight deviations from ideal geometry.  This is not possible with higher-phase lines, because a transposition can only swap adjacent phases, and the [[dihedral group]] on {{mvar|n}} elements coincides with the full [[symmetric group]] only for {{math|''n''&leq;3}}.  Full application of even that limited transposition scheme is necessary to [[arc suppression|properly protect]] against ground faults.<ref name=NYTrial/>{{rp|45-52}}  

Multi-phase power generation designs with 5, 7, 9, 12, and 15 phases in conjunction with multi-phase [[induction generator]]s (MPIGs) driven by wind turbines have been proposed.  An induction generator produces electrical power when its rotor is turned faster than the ''[[synchronous speed]]''.  A multi-phase induction generator has more poles, and therefore a lower synchronous speed.  Since the rotation speed of a wind turbine may be too slow for a substantial portion of its operation to generate single-phase or even three-phase AC power, higher phase orders allow the system to capture a larger portion of the rotational energy as electric power.{{dubious|date=June 2024}}{{citation needed|date=December 2018}}

== See also ==
* [[Single-phase electric power]]
* [[Three-phase electric power]]
* [[Delta-wye transformer]]
* [[Phase converter]]
* [[Polyphase coil]]
* [[Y-Δ transform]]
* method of [[Symmetrical components]]

== References ==
{{Reflist}}

==Further reading==
* Thompson, S. P. (1900). [https://books.google.com/books?id=TvwHAAAAMAAJ Polyphase electric currents and alternate-current motors]. New York: Spon & Chamberlain.

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[[Category:AC power]]

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