Frank and Richard,
Your "a cord of three strands is not quickly broken" quote is interesting. Could this have referred to a braid or plait I wonder.
Most likely. And an anthropologist might suspect that somewhere back in prehistory, women taught men that plaiting animal or plant fibers, like they did with their own hair, would make for a stronger rope... or is that sexist?
Anyway, the thought occurred that perhaps there is also something "special" but not easy to quantify, along the lines of Frank's "third component" in three-phase AC, which makes it the standard over other possibilities. With only two polarities, one might ask why have three-phase at all, or else... if there is any advantage to using multiple phases, why not have four or six, etc? (which actually you do have in transformers). There is information on the net about this, but none of it seems to have the complete answer - other than **cost** or should I say, "duh, it all gets back to cost."
In fact, three-phase is more economical than any other number of phases, it seems, in that it uses less tonnage of a conductor to get the same amount of power from point A to point B. But for applications like rectifiers and synchronous converters where DC is produced, it is most efficient to use six-phase AC input, which is easily produced from three-phase in a transformer.
The experts say that if you are transmitting a certain amount of power single-phase, adding one more conductor operated at the same line voltage and current and using three-phase will increase the power transmitted by 72% with only a 50% increase in the amount of copper and losses. That advantage is obvious, but is there more to the story than cost and why is that the case anyway ? Terry probably knows the answer... or maybe it is part of the mystery of a spiral helix... ;-)
Jones
