Why the world's electricity travels in three waves offset by a third of a cycle
Almost every power grid on Earth sends electricity as three alternating currents, each peaking a third of a cycle after the last. That arrangement, worked out by several inventors in the late 1880s, delivers steadier power over less copper than single-phase wiring and makes big motors spin by themselves.
Three-phase power is the dominant form of alternating current for generating, transmitting and distributing electricity. Three conductors carry currents of the same frequency and size, each shifted by 120 degrees from the others, so the combined power flow stays nearly constant rather than pulsing. With a balanced load the three currents add to zero at every instant, meaning each wire serves as the return path for the other two. As a result, a three-wire system carries more power than a two-wire single-phase one at the same voltage while using less conductor material.
Why three? Constant power is possible with any number of phases above one, but two phases lack the neutral-current cancellation and can make motors produce a lumpier torque, while more than three add needless complexity. A fourth, neutral wire is common in low-voltage distribution, letting the network hand out separate single-phase supplies to homes and small devices. Voltages are often written as a pair, such as 230/400 V, the first figure measured from line to neutral and the second between two lines.
The idea emerged from motor research. In 1885 Galileo Ferraris studied rotating magnetic fields and asynchronous motors. Nikola Tesla received a US patent for a three-phase motor, filed on October 12, 1887. Mikhail Dolivo-Dobrovolsky built a three-phase generator and motor in 1888, and at the 1891 International Electrotechnical Exhibition in Germany sent power 176 km with 75 per cent efficiency. Jonas Wenström, John Hopkinson and William Stanley Jr. are also credited with independent inventions of polyphase systems.
Getting the order right matters in practice. Wiring colours differ by country and voltage, and phases must be connected in the correct sequence or three-phase motors run backwards, which is no good for pumps and fans. Joining two different phases directly creates a short circuit.
Source: Three-phase electric power
