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A simplified diagram of a two-phase alternator [1] Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. Usually circuits used four wires, two for each phase.
However, 130/225 V, three-wire, two-phase electric power discontinued systems called B1 are used to run old installations in small groups of houses when only two of the three-phase high-voltage conductors are used. The phase shift in Europe is 120°, as is the case with three-phase current. That is why we calculate 130 V × √ 3 = 225 V.
Polyphase systems have two or more energized electrical conductors carrying alternating currents with a defined phase between the voltage waves in each conductor. Early systems used 4 wire two-phase with a 90° phase angle, [1] but modern systems almost universally use three-phase voltage, with a phase angle of 120° (or 2π/3 radians).
These operate at voltages of between 110 and 260 volts (phase-to-earth) depending upon national standards. A few decades ago small dwellings would be fed a single phase using a dedicated two-core service cable (one core for the active phase and one core for the neutral return).
As an example, to adjust the flow of AC power on a hypothetical line between Seattle and Boston would require adjustment of the relative phase of the two regional electrical grids. This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the grid.
The most common three-phase system will have three hot legs, 208 V to each other and 120 V each to the neutral. An older, but still widely used, high-leg delta system uses three phases with 240 volts phase-to-phase for motor loads, and 120 volts for lighting loads by use of a center-tapped transformer; two of the phases are 120 volts to neutral ...
The return path for the current in any phase conductor is the other two phase conductors. Constant power transfer is possible with any number of phases greater than one. However, two-phase systems do not have neutral-current cancellation and thus use conductors less efficiently, and more than three phases complicates infrastructure unnecessarily.
Electrical sockets for industrial, commercial and domestic purposes generally provide two or more current carrying (live [10] [11]) connections to the supply conductors. These connections are classified as poles. A pole will be either a neutral [12] connection or a line [13]