In-Reply-To: <[EMAIL PROTECTED]> Organization: http://www.cosmicpenguin.com/911
The most common industrial means of obtaining motive power from electricity is the induction motor; these can be built very simply and cheaply if powered by three-phase AC, since an array of 3 (or 6, 9, etc.) stator coils produces the necessary rotating magnetic field (which turns the unpowered -- thus simple and cheap -- rotor). Single-phase induction motors require a whole extra starter system, including relays, capacitors, and additional stator coils, or (on smaller motors) pole-shading rings, etc., all of which produce a weaker starting magnetic field than the simple stator coils on the three-phase motors, which also produce their running field. On Sat, Apr 30, 2005 at 08:13:56AM -0700, Jones Beene wrote: >... >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 >
