Science Fair Project Encyclopedia
Single-phase electric power
The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120°. These three conductors are commonly housed in a single conduit (e.g. a metal pipe), but they are also commonly separately housed, or spaced in open-air, such as along utility poles. Standard frequencies are either 50 or 60 Hz. The voltage across a pair of these conductors, or between a single conductor and a neutral conductor is single phase electric power.
Both methods of splitting out single phase from a three phase system are in common use. Where three phase distribution is in use at low voltages (below 500V), circuits for normal appliances are nearly always split out phase-neutral. However in North America it is common to split out power line-line for appliances that would normally be wired HOT-HOT in a split single phase system and such appliances are usually designed to cope with the lower voltage this gives (208V rather than 240V, see Y-delta transform)
On higher voltage systems (kilovolts) where a single phase transformer is in use to supply a low voltage system the method of splitting seems to vary by country. In North America the primary of the step-down transformer is wired across a single high voltage feed wire and ground, at least for smaller supplies (see photo of transformer on right). In Britain the step-down primary is wired phase-phase.
Where 3 phase stops and single phase begins
High power systems are nearly always three phase. But the largest supply normally available as single phase varies considerably by country and in some cases region. In the UK it is often as high as 100A or even 125A meaning that there is little need for 3 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 3 phase.
North America has a rather unusual setup due to the lower voltage of their standard electrical appliances. They use split phase (often incorrectly referred to as two phase) for most low voltage distribution and generally have a transformer per building. They only use three phase for large buildings (shopping centres, factories, office blocks, blocks of flats and similar).
Such split phase systems are occasionally seen in the UK where large loads are needed off only two high voltage phases but are rare.
Due to the greater availability of single phase power compared to three phase there has been demand for powerful motors that can work off it and therefore the designs of induction motors for single-phase power incorporate special features to attain starting torque that would not otherwise be possible with only single phase power.
Typically a third conductor, called a ground or earth is also used for safety, and ordinarily only carries significant current when there is a circuit fault. The ground may be the conduit or armouring in which insulated conductors are run. This can however be at risk. Exact details on whether it is allowed to use this for earthing do vary by country. In the USA older style BX cable used the metal jacket as the ground but modern single phase BX cable includes a ground conductor made of copper which is not insulated from the metal jacket to provide a more reliable ground connection. In the UK steel wire armoured cables don't generally have a dedicated ground core however it is common practice to use a cable with one more core than needed to give a better ground path.
Note that two phase power, meaning the simultaneous provision of sine wave and cosine wave electricity (i.e. 90 degrees out of phase) is no longer widely used. But some people incorrectly describe split single phase services as "two phase", when in fact such services are really still single phase power.
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