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In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
electric power using a system in which all the
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
s of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...
does not produce a rotating magnetic field; single-phase motors need additional circuits for starting (capacitor start motor), and such motors are uncommon above 10 kW in rating. Because the voltage of a single phase system reaches a peak value twice in each cycle, the instantaneous power is not constant. Standard frequencies of single-phase power systems are either 50 or 60 Hz. Special single-phase
traction power network A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses alternating current (AC) with ...
s may operate at 16.67 Hz or other frequencies to power electric railways.


History

Single phase power transmission took many years to develop. The earliest developments were based on the early
alternator An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.Gor ...
inventions of 19th century Parisian scientist
Hippolyte Pixii Hippolyte Pixii (1808–1835) was an instrument maker from Paris, France. In 1832 he built an early form of alternating current electrical generator, based on the principle of electromagnetic induction discovered by Michael Faraday. Mohamed A. El- ...
, which were later expanded upon by
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy at the University of Glasgow for 53 years, he did important ...
and others in the 1880s. The first full AC power system, based on single phase alternating current, was created by William Stanley with financial support from Westinghouse in 1886. In 1897, experiments began for single phase power transmission.


Applications

In North America, individual residences and small commercial buildings with services up to about 100 kVA (417 amperes at 240 volts) will usually have three-wire single-phase distribution, especially in rural areas where motor loads are small and uncommon. In rural areas where no three-phase supply is available, farmers or households who wish to use three-phase motors may install a
phase converter A phase converter is a device that converts electric power provided as single phase to multiple phase or vice versa. The majority of phase converters are used to produce three-phase electric power from a single-phase source, thus allowing the ope ...
. Larger consumers such as large buildings, shopping centers, factories, office blocks, and multiple-unit apartment blocks will have three-phase service. In densely populated areas of cities, network power distribution is used with many customers and many supply transformers connected to provide hundreds or thousands of kVA, a load concentrated over a few hundred square meters. High-power systems, say hundreds of kVA or larger, are nearly always three-phase. The largest supply normally available as single-phase varies according to the standards of the electrical utility. In the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
a single-phase household supply may be rated 100 A or even 125 A, meaning that there is little need for three-phase in a domestic or small commercial environment. Much of the rest of Europe has traditionally had much smaller limits on the size of single phase supplies resulting in even houses being supplied with three-phase (in urban areas with three-phase supply networks). If heating equipment designed for a 240-volt system is connected to two phases of a 208-volt supply, it will only produce 75% of its rated heating effect. Single-phase motors may have taps to allow their use on either 208-volt or 240-volt supply. A single-phase load may be powered directly from a three-phase distribution transformer in two ways: by connection between one phase and
neutral Neutral or neutrality may refer to: Mathematics and natural science Biology * Neutral organisms, in ecology, those that obey the unified neutral theory of biodiversity Chemistry and physics * Neutralization (chemistry), a chemical reaction in ...
or by connection between two phases. These two give different voltages from a given supply. For example, on a 120/208 three-phase system, which is common in North America, the phase-to-neutral voltage is 120 volts and the phase-to-phase voltage is 208 volts. This allows single-phase lighting to be connected phase-to-neutral. Single-phase power may be used for
electric railways A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electric railways use either electric locomotives (hauling passengers or freight in separate cars), e ...
; the largest single-phase generator in the world, at
Neckarwestheim Nuclear Power Plant Neckarwestheim Nuclear Power Station is a nuclear power plant in Neckarwestheim, Germany, sometimes abbreviated GKN (for german: Gemeinschaftskraftwerk Neckar), operated by EnBW Kernkraft GmbH, a subsidiary of EnBW. GKN 1 Unit I, in service s ...
, supplied a railway system on a dedicated
traction power network A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses alternating current (AC) with ...
.


Grounding

Typically a third conductor, called '' ground'' (or "safety ground") (U.S.) or ''protective earth'' (UK, Europe, IEC), is used as a protection against electric shock, and ordinarily carries significant current only when there is a circuit fault. Several different
earthing system An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affect ...
s are in use. In some extreme rural areas single-wire earth return distribution is used.


Splitting out

Single-phase is sometimes divided in half at the
distribution transformer Single-phase distribution transformer in Canada A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the di ...
on the secondary winding to create
split-phase electric power A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. Its primary advantage is t ...
for household appliances and lighting.Terrell Croft and Wilford Summers (ed), ''American Electricians' Handbook, Eleventh Edition'', McGraw Hill, New York (1987) {{ISBN, 0-07-013932-6, chapter 3, pages 3-10, 3-14 to 3-22.


See also

* Single-wire earth return * Two-phase electric power *
Three-phase electric power Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional neutral ...


References

Electric power distribution Electrical standards AC power