Single-phase electric power

In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages 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 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 networks may operate at 16.67 Hz or other frequencies to power electric railways. Single phase power transmission took many years to develop. The earliest developments were based on the early alternator inventions of 19th century Parisian scientist Hippolyte Pixii, which were later expanded upon by Lord Kelvin 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. 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.