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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 which ...
electric power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of Power (physics), power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions ...
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 Electrostatics, static electric field, it corresponds to the Work (electrical), w ...
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 Electric machine, 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 Electromagneti ...
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 Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in Hertz (unit), hertz (H ...
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 Railway electrification system, electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses ...
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 Generator (device), 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 stati ...
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 Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude con ...
, which were later expanded upon by
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, Mathematical physics, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy (Glasgow), Professor of Natural Philoso ...
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. 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 Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, Electric power transmission, transmission, and Electric power distribution, distribution. It is a type of polyphase system empl ...
distribution transformer in two ways: by connection between one phase and neutral 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; the largest
single-phase generator Single-phase generator (also known as single-phase alternator) is an alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direc ...
in the world, at
Neckarwestheim Nuclear Power Plant Neckarwestheim Nuclear Power Station is a nuclear power plant A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that ...
, 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 Railway electrification system, electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses ...
.


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 Electrical injury is a physiological reaction caused by electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow ...
, 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 (electricity), ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of eart ...
s are in use. In some extreme rural areas
single-wire earth return Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of altern ...
distribution is used.


Splitting out

Single-phase is sometimes divided in half at the
distribution transformer image:Polemount-singlephase-closeup.jpg, 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, ...
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 th ...
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

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Single-wire earth return Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of altern ...
*
Two-phase electric power Two-phase electrical power was an early 20th-century polyphase alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct ...
*
Three-phase electric power Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, Electric power transmission, transmission, and Electric power distribution, distribution. It is a type of polyphase system empl ...


References

Electric power distribution Electrical standards AC power