The way I understood it and that made sense when connecting two phases together was that one of the phases was in the exact opposite range(polarity).

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So if one phase is at +120 but the other is nearing zero and not -120, how does current return?

 

Plot Sin(x) + Sin(x + 120) and you will see that it produces a sine wave displaced 60 degrees from your reference phase with the same amplitude as your reference phase. Similarly plot the sum of any of the two phases. Try adding all phases and see what you get.

 

Connect the phases together and you hava a short. Not good.

 

See more of this discussion here:

http://www.physicsforums.com/showthread.php?t=370366

 

The way you understood it was wrong. Two phases are never in direct oposition but only 120 degrees apart.

 

Connect the phases together and you hava a short. Not good.

Nonsense. An alternator is dependent on wiring the 3 phases together, either in a delta configuration or a "Y" (star) configuration.

 

It sounds like you are thinking polarity in an AC environment. You have two phases oscillating. On the same wire they are just two voltages on a wire. You are just pushing electrons around until you give them somewhere to turn into heat.

 

Nonsense. An alternator is dependent on wiring the 3 phases together, either in a delta configuration or a "Y" (star) configuration.

There is often confusion about the use of the word "phase." It has become acceptable for electricians to refer to one of the legs of a 3-phase power source as a phase. Unfortunately, this creates more confusion then not.

You are right, each of the 3 coils of a 3-phase generator (or alternator) are connected together in either wye or delta configuration. I believe what the OP was referring to is connecting a load to "2 phases" meaning "2 legs" of a 3-phase power source.

 

If you connect between 2 of the phases of a 3 phase supply, the result will still be a sine wave of the same amplitude as any of the single phases. There will just be a 180 degree phase shift relative to the reference phase.

 

The amplitude across any two phases will be somewhat less than double the voltage of a single phase to neutral.

When one phase is at it's peak, the other connected one is 120 degrees away not 180 degrees away. For example, 240V single phase = 415V three phase not 480V.

 

Nonsense. An alternator is dependent on wiring the 3 phases together, either in a delta configuration or a "Y" (star) configuration.

Only if you think there is no voltage difference between the two phases.

 

If you connect between 2 of the phases of a 3 phase supply, the result will still be a sine wave of the same amplitude as any of the single phases. There will just be a 180 degree phase shift relative to the reference phase.

this is also dubious, and the language is confusing.

In a WYE connection, the three phases are connected together and reading line to line will read a sine wave of 1.73 times the magnitude of any single phase. (phase as being line to neutral)
WYE connections typically have the neutral connected to ground, and connecting the two phases together will result in a lot of arcing and sparks.
In delta it is the same voltage. line to line is the same as phase voltage.
Also, the phase(s) will be 120degrees lead or lag from the reference waveform

I don't consider the waveform at the other end of a split supply single phase 120/240 volt transformer to be 180 degrees out of phase, because that implies that there are two phases. A proper 2 phase system has two phases 90 degrees apart.

 

correct but what a lot of people don't understand is that the phases (loops) in a wye wound generator are only connected at one end
hence 3 generated phases 120 degrees apart.
the question is misleading to the point of output connection.
if an output connection is connected to another (without a sufficient load) there will be catastrophic damage due to short circuit
and half a sine wave is not a phase so to speak but an asynchronous inversion of a split phase(center tapped single phase)
split phase can drastically short and trip out breakers easily but will not sustain an arc blast.
while three phase, six, and 12 phase can sustain the arc flash with devastating results.
there should be some clarification as to the term phase because too many people misunderstand and come to the wrong conclusions

 

Necropost -- You are answering a thread that has been inactive for nearly six years. If you believe further discussion is warranted, post a new thread and link to this one.