Museum of Plugs and Sockets logo, small 3-phase AC generator
basic principle
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Animation showing the principle of three-phase synchronous generation by Bo Krantz Simonsen.


3-phase generator animation


At the power station, an electrical generator converts mechanical power into a set of three alternating electric currents (AC), one from each coil of the generator. The magnet (red = north pole; blue = south pole) represents the rotor of the generator. The coils are arranged such that the currents vary sinusoidally at the same frequency. Because of the positioning of the coils around the rotor the three complementary currents (L1, L2 and L3) show a phase separation of one-third cycle (120, see graph). The generator frequency is typically 50 Hz or 60 Hz, varying by country.
Nul = Neutral.

The electricity grid brings three-phase current to homes. There it is split out at the main distribution board to single phase, by using either L1, L2 or L3, and Neutral. 
For a balanced and efficient use of the network L1, L2 and L3 are chosen for an equal number of houses fed by the same local substation. Only occasionally three-phase is used in homes for electric stoves, dryers etc.


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