Re: AC/voltage basic physics questions, please.

On Jan 28, 10:34 pm, "hhc...@xxxxxxxxx" <hhc...@xxxxxxxxx> wrote:
On Jan 28, 10:40 pm, HC <hboo...@xxxxxxx> wrote:



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?

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?

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). 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?

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

Good questions.

Here is a summary answer. The three wires entering your home as you
summize consists of two "hot lines" and a neutral return. What you may
be missing is the fact that the two "hot" lines are 180-degrees out of
phase with one another. As a result, either side to neutral provides
you with 120-V, but a load placed between the two "hot" lines is
supplied with 240-V, for example an electric stove, clothes dryer or
hot water heater.

Yes, the two hot lines entering your home are 180-degrees out of phase
with one another. A cernter-tapped secondary on your pole transformer
(distribution transformer) accomplishes this.

Your electrical distribution box attempts to put half of the 120-V
circuit breakers on one side of the incoming line, and the balance are
connected to the other side of the line. Hopefully the distribution of
120-V loads is roughly balanced, but that's not a major issue in most
homes.

You are correct that electrical distribution is 3-phase, but the pole
transformer that feeds your home only taps into one of the phases. The
transformer has a primary of typically 8,000-volts, but its output to
homes is the two-phase system that I mentioned above. Transformers
achieve this by using a center-tapped secondary that provides the 3-
wire service to your home at 120-0-120 volts. The center tap on the
secondary winding is the neutral. Take a look at a pole transformer
near your home. The high-voltage enters it though the single, very
large insulator on the top, and the home service feeds exit through
much much smaller insulators.

Now regarding electrical polarity... Alternating current in the US
reverses its direction of flow at a fixed rate of 60 cycles/second, so
it is independent of having a fixed polarity, hence no fixed positive
or negative exists.

Did I miss anything?

Hope this helps.

Harry C.

Harry, thank you for your reply. It is very helpful.

I took a look at the transformer near my house (I'm in a rural area so
there is no trouble being sure that the transformer I was viewing is
connected to my house). You are correct, there is one large insulator
at the top of the transformer and two much smaller ones on the sides
which connect to wires that come to my meter to my house. There are
two wires that come from the poles across the road from my house which
connect to the transformer. The one that is not connected to the
large insulator is connected to a large wire that runs down the pole
to the ground and is also connected to the center terminal on the side
of the transformer. That gives us three terminals on the side of the
transformer: the ones on the left and the right are both connected to
black-insulated wires, the one in the center is connected to a bare
wire. All three are then suspended over to another pole on which my
meter is mounted. In the meter box (which I have personally worked
on) the two black wires connect through the meter, one on the left
side, one on the right side. The white-insulated "neutral" connects
to the bare wire from the transformer.

With what you've stated here and with what I saw I felt I finally had
a direction to travel in to talk to the utility company. I spoke to a
person there who says that, yes, we get a single phase of electricity
from the utility and that the transformer I have is a center-tap
transformer and that there is 120-0-120 as you mention on the
terminals.

Now, that means that, yes, as you say, the two wires would be 180-
degrees out of phase, giving one "hot" at +120v peak while the other,
at that same instant in time, would be -120v peak, fluctuating between
the two peaks (speaking in RMS values).

(*** THIS paragraph below seems unnecessary after reading the page at
howstuffworks.com linked to by Sam Wormley.)
Let me see if I understand this correctly. I understand the low-end
of what a transformer does and more or less how it does it; common/
shared core, primary of some number of turns of wire, secondary of
more, less, or the same number of turns of wire (depending on if it's
a step up, step down, or isolation transformer). So, say the primary
is 14,400 volts (which is what my utility company told me it is) and
the secondary is 240 volts (which it is, or close enough for our
discussion (it might be 250 or so) then the primary on the transformer
should have a 60:1 ratio to the secondary in terms of turns of wire.
Let's say that, for this discussion, that the primary winding has
6,000 turns of wire, the secondary winding has 100 turns of wire.
What you're saying is that one leg of my 240 volts is at one end of
the secondary winding, the other leg, 180 degrees out of phase, is at
the other end of the secondary winding, and in the middle of the
secondary winding, at about turn number 50, there is an electrical
connection to the winding. This electrical connection becomes (is)
the neutral.

Am I getting that right?

That would mean that as the electromagnetic field forms and collapses
around the wire the voltage is in one direction through the secondary
and then reverses through the secondary. Continuing, at one *moment*
in time, at the peak voltage in one "direction" (polarity) there would
be +240v measured between the beginning and the end of the secondary,
and, at the complete opposite condition, when the "direction" has
changed (polarity reversed), for one moment it time, the voltage would
be -240v. I'm using the idea of positive and negative voltage to
clarify that the polarity is reversing; of course an AC measurement
would simply show 240 volts. These peak values would only exist for
an instant, twice each cycle (one time at maximum voltage of one
polarity, one time at maximum voltage of the other polarity) and the
rest of the time the voltage would be somewhere between the two
extremes (peaks). Now, with the polarity flipping across the
secondary winding let's look at it frozen in time when the maximum
voltage exists (240v) in one polarity, say, the left terminal is at
maximum positive voltage and the right terminal is at zero. With
three terminals you'd have: +240-+120-0 for that brief instant in time
of peak voltage of one polarity. That should mean that if you have a
light bulb designed for 120vac operation you could connect it between
the left terminal and the middle and get the left terminal with 120
volts positive potential to the middle terminal (call this Leg 1: 120
volts positive). Or you could connect it to the right terminal and
the middle terminal and get a 120 volts negative potential (call this
Leg 2: 120 volts negative). Scroll forward through time and the
polarity reverses to the opposite maximum and you'd have: 0-+120-
+240. Leg 1 would now have: -120 volts and Leg 2 would have +120
volts. Of course this is going on cyclically but it helps me to think
of it in small pieces and to visualize what is occurring at each
finite step (well, at least the ends (maximum voltage in one polarity
then the maximum in the other polarity)). Again, to be sure I'm
clear, I understand that the AC is fluctuating at 60 cycles per second
so the idea of any "positive" or "negative" voltage is probably not
really keeping things clear but I lack the technical terms to describe
it accurately. What I mean is that there are peaks of maximum voltage
of one polarity and then peaks of voltage of maximum opposite polarity
and I wanted to discuss them both so, with out a better way to
describe them I used "positive" and "negative".

That description feels different than my first two postulates (I'm not
sure I'm using that term correctly but it "feels" right).
Furthermore, before actually posting what I've written here, I read
the rest of the current replies I have, and checked out this page that
Sam Wormley left (http://science.howstuffworks.com/power9.htm) which,
at the bottom, has a diagram of a center-tapped transformer which
seems to be exactly what we're talking about here.

In the end, to summarize what I think know: from what you've said,
what I've read on howstuffworks.com, what my utility company said, and
what I've seen at my transformer pole: power comes onto my property in
a single phase with a ground-wire which the utility company
technically calls a "neutral" (but which IS bonded (if I'm using the
term correctly) to ground in a variety of locations including at each
and every pole (according to the guy I spoke with at the utility
company). Across the street, where the power lines are run, there are
four wires: three phases plus this neutral (I can see the four wires,
the utility company man tells me the top three are the three phases).
The phase I get my feed from enters my transformer which steps it down
from 14,400vac to 240vac; one wire from the utility enters the
transformer through a large insulator, the other wire, neutral, is
bonded to the transformer case, the middle terminal on the side of the
transformer and is what I use as a neutral and a ground in my circuit
breaker boxes, and connected to ground/earth, and is the center-tap.
The other two terminals on the side of the transformer go to the meter
as 240vac. To get 120vac devices are connected to one "hot" and the
neutral. The neutral is, at the peaks, either 120 volts higher
potential or 120 volts lower potential than one "hot" and the opposite
for the second "hot"; this cycles 60 times per second. The two "hots"
are considered 180-degrees out of phase and therefore provide 240vac
between them.

Thank you again for your post; it was the biggest boost to my
understanding I've gotten in a long time.

--HC
.

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