In reference to your Q:
[Are you describing on any given time line ( what we draw as 0 for sine waves) what can (or cannot)exist simutaniously???]
Not too sure exactly what you're describing here [my fault, not yours!! my mind is a little tiny thing
], so let me take a stab at an interpretation:
The current in one coil of the generating device [or xformer] will travel in one direction, stop at zero, then reverse direction, once again stoping at zero. Zero refers to the point on the generator when the rotor's field is at a point where there's no magnetic induction imposed on the coil, so current flow stops.
Best way to visualize the current flowing in a coil is to completely drop the wave thought [don't look at it as a wave], and instead just visualize the current flowing back and forth in a coil.
This will help describe phase displacements and why the currents of a coil are of the same origin.
Use the idea of one coil, place it horizontal [on the "X" axis in nerd talk], then mentally "copy and paste" a copy of that coil in a 120 degree rotation from the X axis.
Copy/paste another copy of the newly placed coil and make it 120 degrees in rotation from the 2nd coil [use a mental protractor for this one
You now have the basis of a Delta 3 phase [polyphase] setup.
Now insert a rotating magnet in the center of the delta - make the rotating magnet have a + pole on 1/2 the rotation, and a - pole on the other 1/2 - so they are facing the coils.
Spin the magnet and visualize the way it will cut magnetic fields across the coils.
We now have 3 independent phases [and looking at it in the wave format - even though it's a no-no right now, there's 3 wave forms with a 120 degree offset].
If we remove one coil and rotate one of the remaining 2 coils so that it's 90 degrees from the other coil, we now have a 2 phase system. We will now have 2 independent phases [and waves
] with a 90 degree time lag between them.
If we remove one more coil, that leaves only one coil, and we now have a 1 phase system. Since there's no other phase coils except this one, it's no longer a polyphase system - so there's only one independent phase [and wave
Hopefully all this baloney is somewhat in the area of what you are asking.
In reference to your Q:
[May I ask you to describe, if you know (and it appears that it should be knowable, see below), with respect to the input waveform, is one 90 degrees ahead and one 90 degrees behind, or what?
For this scenario, I have my oscilloscope at the transformer and I am displaying all three waveforms in parallel.]
From your description, it appears that you are describing a 3 phase wave form [from the 3 waves on the scope's display].
To have 3 waves and do something with all 3 of them, they could not be 90 degrees offset. One would fall into another when they become 180 degrees offset [one at + peak, another at - peak]. Here, the peak falls into the valley, which cancels the amplitudes of the effected waves.
The one at 90 degrees will still be working, so in this case we would have 2 coils that produce nothing between them selves [each other], but will produce to the coil at 90 degrees.
That would be in fact a 2 phase system, since only 2 phases can produce wave forms [only 2 phases can produce an output current].
This arrangement needs to have a 120 degree offset, so all 3 coils can produce waves [currents].
Take a brief browse over the above text I refered to Steve [Sparky], in this message and see if that also covers your Q.
Once again, I'm not exactly sure if I have addressed your question in the manner that you indended [did I understand it correctly??], so I hope to have explained things here with as much respect to your questions.
Let me know if I'm missing everything here.
Scott SET - the micro brain / tiny mind guy