Hello everyone btw. One day a co-worker and I were talking and he asked why do they call it single phase when there are two hot wires. I though about it for a minute and answered because it actually IS only one phase. It comes from your power supplier on only one line, It goes through a transformer the secondary coil is connected between the two outer taps, and the middle of the coil is center-tapped to get a mid point. Therefore it is single phase 240 with a grounded mid point.
When I asked my apprenticeship teacher he started drawing a picture of two sine waves 180? apart. And insisted it really was two distinct phases.
Who's right?
I think you understand clearly. The
model you are using is the one I use. Both 120V sources point the same direction and are joined nose to tail at the neutral.
It's important to understand the concept of a "model". We can't see the electrons or even measure them directly. We can't see the electric field. So we develop a "model", a collection of equations, diagrams, formula, that describe how the system works. There are often several models that will work for any given system. You need to pick out the one that best fits what you are trying to do. In this case, the one you picked out works the best for me to describe a single phase 120/240V house services.
The 200+ thread Jim mentioned, is interesting, not because any of the participants really had anything to say about
the truth of single phase, but rather because each was adamant their model was
The Truth. I'm reading along thinking, "Who cares? Either will work - as long as one defines the model."
However, as you have noted with your instructor - some are rabid in their choice/selection of the model.
And, also as you have noticed, it is pretty hard for the apprentice to tell the instructor he is screwed up. Other than the mis-application of the idomatic term "2phase", he isn't too screw up.
cf