Re: AC/voltage basic physics questions, please.

On Jan 28, 9:53 pm, "Rosco" <nos...@xxxxxxxxxx> wrote:
"HC" <hboo...@xxxxxxx> wrote in message

news:33c04e46-f6f7-430a-b52b-f475b1ff2de0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



Folks, this is probably so basic as to be silly but it's something I
have not found a concise answer to yet, so please bear with me. I
think this is the right group to ask because what I'm wanting to know
is how electricity works, in my particular questions, at a low level;
I'm thinking along the lines of actual electron flow. Your help is
appreciated.

I live in the US and what's bugging me is trying to understand what is
going on with 120v AC that we have from our standard household
outlets. I understand some basic stuff about electricity (like that
electrons move through conductors, the atomic layout of materials
(protons/neutrons in the nucleus, electrons in a cloud around that))
and so forth. Really basic stuff. What's still eluding my
understanding is what's going on at a low-level with 120v AC? I mean,
I replaced the breaker box in my home myself; I wired my shop with
both 120v and 240v AC and put in outlets, lights, and the breaker box,
too, so I can work with the stuff, but that got me a close look at
some things I don't understand:

First, the power that comes in from the utility transformer comes
across three wires; two "hots" and a "ground" or "neutral". In my
breaker box I have tied the neutral to the ground; the neutral bar is
connected to the "neutral" from the utility and to the "ground" in the
box and the ground from the box is actually connected to a copper-clad
iron rod I have driven in the ground. For the 120v breakers they
connect one of the "hots" through the load to "ground" or "neutral".
It all works great. What I'm not understanding is this; since
electricity requires a complete circuit, in order for power to travel
from the "hot" to "neutral", that means current must flow across the
neutral/ground. But, since it's AC, the current switches directions,
so doesn't that mean that there is a positive voltage then from the
"neutral/ground" to the "hot" leg during part of the cycle?

yes, then it goes negitive, then positive,



Second, is the alternating current based on two terminals reversing
polarity (say, terminal A positive and terminal B negative/ground,
then reversing and terminal A becoming negative/ground and terminal B
becoming positive) or is it that one terminal is actually at a
positive voltage and one terminal is at a similar but negative voltage
(say, for DC, that terminal A is +5 and terminal B is -5 as opposed to
terminal A being +5 and terminal B being simply ground?) and then that
reverses?

the neutral/ground is at 0 volts, the hot line swings between + 130 to -130
and back to + 130 at a 60 hz rate.



Third, it's my loose belief from my reading that all major electrical
generation is done in three phases (at least for our utilities here in
the US).

yes.

If the power coming into my property here is 240v and comes
across two "hot" wires, it would seem they would have to be 180
degrees apart...but if the electricity is generated in three-phases
then there are no phases that are 180 degrees apart. How are we
getting this 180 degrees out OR is it just that they are two phases
and they are not 180 degrees apart which accounts for why we need
start capacitors and such on our 240v AC motors whereas three-phase
motors supposedly do not need them?

nope, you get two of the phases not all three, some houses only get one,
some building get all three.
say you have phase A and phase B and not the phase C.
the phases are 120 degrees apart, and the voltage between A and B is 240.

if you google for it, you may find a diagram that looks like an equalaterial
triangle and shows the three phases, what the voltages are across each legs
and to ground.

Then there are delta three phase wiring, and Y three phase wiring. And
transformers can convert delta into Y

Good luck, seems like you are on the right path.



Anyway, I'm not sure I've done a good job of describing my questions
and I'm sorry for that. I feel like this stuff should be basic but I
still don't understand it and that bugs me. Thanks for any help you
can offer.

--HC

Rosco, thank you for your reply. Your information helps. What Harry
C. has posted about center-tapped transformers helped me in
understanding your information, too. I had not even thought of a
transformer it terms of having different taps (although I knew that
some did, I didn't think of it for use in this manner). So, there is
part of the cycle where the neutral is positive compared to the
"hots"; how come we don't get shocked handling it (or supposedly do
not; I'm not one for testing this stuff by trial and error)? It would
seem (but I don't know this to be right) that for one leg to be at a
negative voltage then there must be a positive force on the other
leg. That is, if terminal A is at +130v at one instant in time and
terminal B is at 0 volts (neutral) then there is electricity that is
under pressure (considering conventional current) to go from the "hot"
wire, terminal A, to the neutral wire, terminal B. When terminal A
reverses polarity and becomes -130v then, by comparison, the "neutral"
terminal B should have higher potential and therefore positive voltage
to terminal A and that would seem to make the neutral "hot" and
dangerous. Is that correct?

I was doing some looking at delta and star (Y) alternators (actually,
I have been very sporadically for a couple of months as I've been
reading about home-brew wind power (http://www.otherpower.com/
turbineplans.html and other sites). It's my interest in that, and my
reading about the two different wiring schemes (and my lack of
comprehension of them) that has focused me on trying to understand
these basic principles I'm asking about now. I hate not understanding
and knowing stuff and a bunch of reading through wiki-pedia and other
sites hasn't clarified it yet. I'll get there, little by little.

Thanks again.

--HC
.

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