Jerry wrote:I have a +/- 15v common ground DC power supply, according to your definition that would make it a two phase DC power supply.
Providing you are conceptually comfortable with a 0 Hz phase, then, yes, I'd happily call it a two phase 0 Hz supply! The fact that it is not '0 and X Volts' [two lines] is what makes it different to a '+X, -X and 0' supply [three lines].
Two phases that are 180 deg apart do, indeed, look like a sinusoid if you tap across them. So do two phase lines of a 3 phase supply! So? Any two lines of a poly-phase system will look like a one phase sinusoid!
The difference between a 230VAC single phase live and neutral, and a 'two phase' 180 deg separation is that the mid-point between those two phased lines is, nominally, always 'earth' potential, so you can send both lines down just two lines, with the centre tapped to earth and then remotely reference each to earth, without either actually being at earth potential. This is exactly the same as 3 phase (that is to say, the sum potential is always averaged to earth), just with one less phase.
Single phase = one line that is AC wrt a neutral [nominally earth] return line.
'Two phases' [but not 'what-is-know-as-two-phase'] = two lines that are AC wrt a neutral return line that are 360/
2 separated in phase.
Three phases = three lines that are AC wrt a neutral return line that are 360/
3 separated in phase.
What-was-called-two-phase = two lines that are AC wrt a neutral return line that are either 90 (or 270 depending on how you choose to look at it) separated.
Four phase would be 4 lines separated by 360/
4 to each other.
etc.