Three-phase electric power (abbreviated 3ϕ) 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; not counting any protective conductor) and is the standard method by which electrical grids deliver power around the world. In a three-phase system, each of the three phases 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.
Terminology The conductors between a voltage source and a load, excluding those serving neutral, mid-point and protective functions, are called line conductors. The voltage between any two line conductors is called line-to-line voltage, while the voltage measured between any line conductor and a neutral conductor is called line-to-neutral voltage, per section 195 of IEC 60050. Other sections define variations of these terms, with section 601 having phase to phase voltage and phase to neutral voltage, and section 141 having polyphase line-to-line voltage and polyphase line-to-neutral voltage. Some electrical textbooks contain the terms line voltage (equivalent to line-to-line) and phase voltage (equivalent to line-to-neutral). System voltage is often expressed in the form X/Y, as found in IEC 60038, with X being line-to-neutral voltage, and Y being line-to-line voltage. For example, in countries with nominal 230 V power (line-to-neutral), where line-to-line voltage is 400 V, this may be expressed as 230/400 V. Transformer windings consist of either delta (often symbolised as 'Δ') or star (often symbolised as 'Y') configurations, per IEC 60050 terminology. The star arrangement is more commonly known as wye in the US.
History 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. These alternators operated by creating systems of alternating currents displaced from one another in phase by definite amounts, and depended on rotating magnetic fields for their operation. The resulting source of polyphase power soon found widespread acceptance. The invention of the polyphase alternator is key in the history of electrification, as is the power transformer. These inventions enabled power to be transmitted by wires economically over considerable distances. Polyphase power enabled the use of water-power (via hydroelectric generating plants in large dams) in remote places, thereby allowing the mechanical energy of the falling water to be converted to electricity, which then could be fed to an electric motor at any location where mechanical work needed to be done. This versatility sparked the growth of power-transmission network grids on continents around the globe. Mikhail Dolivo-Dobrovolsky developed a three-phase electrical generator and a three-phase electric motor in 1888 and studied star and delta connections. His three-phase three-wire transmission system was displayed in 1891 in Germany at the International Electrotechnical Exhibition, where Dolivo-Dobrovolsky used the system to transmit electric power at the distance of 176 km (110 miles) with 75% efficiency. In 1891 he also created a three-phase transformer and short-circuited (squirrel-cage) induction motor. He designed the world's first three-phase hydroelectric power plant in 1891. Inventor Jonas Wenström received in 1890 a Swedish patent on a similar three-phase system. The possibility of transferring electrical power from a waterfall at a distance was explored at the Grängesberg mine. A 45 m fall at Hällsjön, Smedjebackens kommun, where a small iron work had been located, was selected. In 1893, a three-phase 9.5 kV system was used to transfer 400 horsepower (300 kW) a distance of 15 km (10 miles), becoming the first commercial application.
Principle
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