Single phase
|
|
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. Single phase power is what is commonly available to residential and light-commercial consumers, especially in areas where there is only one high voltage conductor leading into the community (e.g. many streets only have access to one phase on the utility poles, as shown for the rural stepdown transformer at right). The design of induction motors for single-phase power incorporates special design 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 current when there is a circuit fault. The ground may be the conduit in which insulated conductors are run. For example, older style BX cable uses the metal jacket as the ground, but this sometimes creates a safety hazard if it corrodes. Therefore modern single phase BX cable includes a third ground conductor made of copper, to provide a more reliable ground connection. The ground conductor is often not insulated from the conduit or jacket.
When there are two "hot" conductors, such as for a residential 240 volt "line to line" stove, dryer, or the like, the power is still single phase. For example, a 240 volt center tapped (120 volt plus 120 volt, 180 degrees out of phase) service is still single phase.
Sometimes single phase electrical appliances are wired line-to-line in a three phase service. For example, a single phase residential stove is often connected line-to-line in a 208V/120Y service, so that it receives 208 volts rather than 240 volts. Most residential 240 volt appliances are designed so that they can also operate from 208 volts. Many downtown condominiums feature such line-to-line services at the 208 volt level that arises from the use of two out of the three legs of a three phase service.
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.
