Sistema polifásico: diferenças entre revisões

[[Image:3 phase AC waveform.svg|thumb|300px|right|One voltage cycle of a three-phase system]]

 

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. Two-phase systems have been replaced with three-phase systems. 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]].

 

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]].

 

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-phase supply completes one full cycle, the magnetic field of a two-pole motor has rotated through 360° in physical space; motors with more pairs of poles require more power supply cycles to complete one physical revolution of the magnetic field and so these motors run slower. [[Nikola Tesla]] and [[Michail Dolivo-Dobrovolsky]] invented the first practical induction motors using a rotating magnetic field - 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 numbers than three have been used. A common practice for [[rectifier]] installations and in [[HVDC]] converters is to provide six phases, with 60 degree phase spacing, to reduce harmonic generation in the AC supply system and to provide smoother direct current. Experimental high-phase-order transmission lines have been built with up to 12 phases. These allow application of [[Extra high tension|Extra High Voltage (EHV)]] design rules at lower voltages and would permit increased power transfer in the same transmission line corridor width.

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* [[Componentes simétricos]]

* [[Corrente alternada]]

* [[Nikola Tesla]]

 

* Thompson, S. P. (1900). [http://books.google.com/books?id=TvwHAAAAMAAJ Correntes polifásicas e motores de corrente alternada]. New York: Spon & Chamberlain.

 

 

[[ca:Sistema polifàsic]]