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Three-phase electric power

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Three-phase electric power

Three-phase electric power (abbreviated ) is the most widely used form of alternating current (AC) for electricity generation, transmission, and distribution. It is a type of polyphase system that uses three wires (or four, if a neutral return is included) and is the standard method by which electrical grids deliver power around the world.

In a three-phase system, each of the three voltages is offset by 120 degrees of phase shift relative to the others. This arrangement produces a more constant flow of power compared with single-phase systems, making it especially efficient for transmitting electricity over long distances and for powering heavy loads such as industrial machinery. Because it is an AC system, voltages can be easily increased or decreased with transformers, allowing high-voltage transmission and low-voltage distribution with minimal loss.

Three-phase circuits are also more economical: a three-wire system can transmit more power than a two-wire single-phase system of the same phase-to-phase voltage while using less conductor material. Beyond transmission, three-phase power is commonly used to run large induction motors, other electric motors, and heavy industrial loads, while smaller devices and household equipment often rely on single-phase circuits derived from the same network.

Three-phase electrical power was first developed in the 1880s by several inventors and has remained the backbone of modern electrical systems ever since.

The conductors between a voltage source and a load are called lines, and the voltage between any two lines is called line voltage. The voltage measured between any line and neutral is called phase voltage. For example, in countries with nominal 230 V power, the line voltage is 400 V and the phase voltage is 230 V. For a 208/120 V service, the line voltage is 208 V and the phase voltage is 120 V.

Polyphase power systems were independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky, Jonas Wenström, John Hopkinson, William Stanley Jr., and Nikola Tesla in the late 1880s.

Three phase power evolved out of electric motor development. In 1885, Galileo Ferraris was doing research on rotating magnetic fields. Ferraris experimented with different types of asynchronous electric motors. The research and his studies resulted in the development of an alternator, which may be thought of as an alternating-current motor operating in reverse, so as to convert mechanical (rotating) power into electric power (as alternating current). On 11 March 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Two months later Nikola Tesla gained U.S. patent 381,968 for a three-phase electric motor design, application filed October 12, 1887. Figure 13 of this patent shows that Tesla envisaged his three-phase motor being powered from the generator via six wires.

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