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The following probably provides a fairly good definition of neutral.
http://en.wikipedia.org/wiki/Neutral_wire
What is an electrical circuit? Here is a good definition.
http://www.thefreedictionary.com/circuit
Definition of a circuit does not require a neutral or ground (earth). One closed loop of wire can be considered a circuit. A current can be induced into this wire by putting the wire in a changing magnetic field. For example move a permanent magnet past this loop.
Consider an ungrounded delta three phase supply. Between any two lines you can view this as a single phase voltage source that in some fashion is floating relative to ground (earth). There is no neutral in this system.
Now consider a three phase Y source with none of the four wires grounded. By the above definition this has no neutral. To some extent that makes sense because the potential of all of these lines could be quite different than earth. But in general useage the center of the Y will probably be called neutral whether earthed or not. I doubt you ever find one corner of a Y grounded as the normal configuration.
In a Y balanced system with equal resistive loads, sine wave excitation, whether the load is delta or Y, the current in the center wire of the Y will be zero. Obviously with a delta load on a Y source there is no center wire.
On a Y source with a Y load consisting of rectifiers and capacitor input filters there are very peaked current flows from each leg and these sum to a large current in the center of the Y. For a balanced system this center conductor current is 1.732 * Irms of one leg.
There is a whole long discussion on the 180 deg phase question.
http://forums.mikeholt.com/showthread.php?t=96673
Also see
http://forums.mikeholt.com/showthread.php?t=96673
But you already know about this since you appear there.
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