I have been reading about various arc welders (specifically lincoln
ac-225 and miller xl225) and noticed that they both specify 230v input
that is single phase. My poor brain cannot comprehend how this is
possible. From my understanding to achieve 230v you need to have both
A and B phases.
Additionally, if you can answer the above you can probably answer why
in some cases a device like a hot water heater does not require the
neutral line and just requires 2 hot and a ground.
At the moment my welding experience is limited to a century portable
welder that has two power settings 30A and 70A. Eventually I want to
move up.
As usual my world was relatively simple until I found out that it
wasn't really that simple.
Thanks in advance.
From my understanding to achieve 230v you need to have both
I'm NO expert, but have just brushed up on this.......Service entrance is
usually 3 wires. 1 neutral, and the other 2 are 115V. The transformer is
grounded at the pole, and your service box is grounded. Common ground.
If you connect to either of the 115V circuits, and the neutral, you have
115V power. If you connect across the 2 115V's, you have 230V. Sometimes you
need all of them. (to a range, for example that uses both 115 and 230).
Using 230V, you do not need a neutral. All you need is a *grounding* wire
for protection against a load that shorts out against the frame. That way
the short goes to ground, and not through you! Same for 115V. You use a
neutral, but also a *grounding* wire for protection.
3 Phase power is a different animal. Used for heavy industry and is brought
to the site by the power company. It's used because it's more efficient use
of electricity for large motors and such. No home has it.
NOW, all feel free to correct me. I'm learning too!!
Josh.
You lucky so and so's, green with envy here, but i'm the fool considering
installing a 50kva 3 phase generator in a sound proof breeze block "hut"
down at the bottom of my garden ! Just hope the neighbours aren't reading
this ;o))
J
Of the A, B and C phases traveling the poles outside your home, the
transformer pulls from only one of the phases (unless you truly have three
phase). The transformer provides three wires to you, the two hot wires and
the center tap (neutral wire). In a 110v circuit, current path is from hot
to neutral, using 1/2 of the voltage the transformer is providing you. In a
230v circuit, current path is across the two hots, using all the voltage the
transformer supplies. In this case, the neutral is not needed and is
therefore not used. Regarding your water heater question, there are no
loads other than 230v loads so a neutral is not necessary. In a case where
an appliance uses both 230v and 110v, a clothes dryer comes to mind, the
neutral is necessary for the 110v load.
Do not think of the ground wire as a current carrying conductor. It is
there to bond all the bits and pieces of your electrical system together.
This way, if there were a short circuit in a receptacle box for example,
without the ground the box would be energized and a shock hazard. With the
ground wire, the circuit breaker would trip immediately. It goes deeper
than this but that's the down and dirty.
There have been many very good answers to numerous questions similar to
yours here in the past. If I have left anything out or am incorrect please
let me know.
PS - If you feel comfortable enough doing it, spend a few minutes with a
multimeter at your breaker panel. Measure across every different buss bar,
buss bars to ground, neutral to ground, etc.. It will teach you alot about
your home electrical system.
Shawn
On 28 May 2004 09:44:04 -0700, snipped-for-privacy@basswood.org (Jim) wrote:
It takes two wires to make one phase. In other words, the phase voltage
is the voltage expressed *between* two service conductors. Voltage is the
potential *between* two points, with only a single point, voltage is
*undefined*.
So any 2 wire circuit is automatically *single phase*. Safety ground is not
a phase conductor. It is designed to only carry fault currents, ie current
due to a short to chassis or Earth, so it doesn't enter into this.
When you introduce neutral, things get a bit more confusing. Neutral is
a *live service conductor* connected to the *center tap* of the service
transformer. It is the *same phase* as the two end wires of the winding
(one winding, one magnetic field, one induced voltage vector, one phase),
but the voltage vector *magnitude* expressed between it and either of the
end wires is *one half* the vector magnitude of the total voltage expressed
across the winding. In other words, it has the same phase *angle*, but the
magnitude is different. So you can use one end wire (either one) and the
center tap to get 120 volts from a 240 volt transformer winding. This is still
single phase.
Where the confusion enters is that, in US residential wiring practice, the
neutral wire is connected to Earth at the service entrance panel. What
this means is that if you measure voltage *with respect to Earth*, neutral
reads zero while one end wire reads +120 and the other end wire reads
-120. In other words, it *appears* as if there are two 180 degrees out of
phase voltages. But this is just an artifact of the way you're doing the
measurement. Remember that Earth *cannot* be used as a phase
conductor, so it *should not* be used as the reference for measuring
phases.
That you've chosen the wrong reference should be obvious since the
sum of 120 and -120 is ZERO, and not the 240 volts that is actually
present between the two end conductors (normally called "hots").
In vector math terms, when you use the center tap as the reference
you're trying to measure one half of a vector from tail to head (proper)
while measuring the other half of the vector head to tail (backwards).
When you do that, the vector sum comes out wrong. Doing it right,
you measure each half vector tail to head, and the sum comes out
correctly. That's what phase is all about.
If you properly do the measurement *between* the phase conductors,
starting at one end winding, at a particular snapshot instant of the AC
cycle what you get is 0, +120, +240. At another snapshot instant you'd
get 0, -120, -240. Etc. In other words, *one phase* across the entire
winding, or *any part* of the winding.
To have 2 or 3 phase power, you need a supply transformer, or
transformers, with 2 or 3 *separate* secondary windings, and 2
or 3 separate input windings, properly driven by the distribution
network with the proper phase relationships. Each winding
develops *one* phase voltage across it. So for 2 phase power,
you need 4 phase conductors, one from each end of each
separate winding. The phase angle between the two phases is
90 degrees, also called phase quadrature. This is almost *never*
seen in the US today, though there are a few old industrial areas
around Niagara Falls and Philadelphia which still use it.
Now 3 phase has a special property, because each phase is separated
by 120 degrees from the next, and because 3x12060, you can carry
3 phase on just 3 wires by tying the adjacent end wires of each of the
three windings together at the transformers (they're always at the same
potential at the same time). In other words, you start with 6 wires, but
because of the special properties of 3 phase, you can pair them and
only use 3 wires to carry the currents from the transformers. Note that
these 3 wires form 3 *pairs* with each other L1-L2, L2-L3, and L3-L1,
so you can have 3 phases.
You can't do that with 2 phase because 90+90 only equals 180, which
is only half of an AC cycle (360), so the adjacent end conductors are
*not* the same voltage at the same time, in fact they are always different
(one is at zero when the other is at maximum), and you'd create a nasty
imbalance situation if you tried to pair them together the way you can with
3 phase (ie the paired conductor would have to be sized twice the size
of the other two, and the fault current potential would be twice the phase
current potential, meaning sizing breakers gets really complicated).
This special property of 3 phase means the utility can save a bundle on
wire, since it can use 3 equal sized wires instead of 4 equal size (or 3
with one twice as large as the others) in the distribution system to carry
the same peak currents. (Breakers are simpler too.) That's why 3 phase
is chosen instead of 2 phase.
Note that Earth ground never enters into any of this. Earth ground is
for electrical *safety* only, and should never be used to carry service
currents, and should *never* be used as a reference for phase voltage
measurements. That way lies madness.
Gary
Thank you Gary, that was one of the most understandable, well written,
and interesting notes I've ever read on the subject. You ought to be a
technical writer, or at least write a book.
Jamie Norwood
Gary Coffman wrote:
In a situation where there is 208v 3 phase, how does 208v single phase work
off that panel? In other words, the same panel has 3 breakers ganged
together for a 3 phase load and 2 breakers ganged together for a single
phase load. I understand how this works in my home panel (2 breakers ganged
vs. one breaker) but there is no phase angle difference. Is it the 120
degree phase angle difference that yields 208v?
Measuring a single hot gives 120v, and at home this would give 240v when the
two hots are used, but in the 208v panel you get just that across two hots,
208v.
Thanks,
Shawn
(Jim) wrote:
On Mon, 31 May 2004 23:27:26 -0400, "Shawn" <shawn_75ATcomcastDOTnet> wrote:
Ok, 208 is Wye connected. That means there are 3 hots and a neutral.
One end of each of the three windings (120 volt windings) of the supply
transformer(s) are tied together to form the neutral connection. The other
3 ends of the windings form the 3 phase conductors. From any phase to
neutral you get 120 volts, phase to phase you get 208 (2080*tan(120)).
That's just the way the vector math works out for a 120 degree difference
between phases of 3 120 volt windings.
Now the original example I gave would be delta connected. There is
no neutral in a 220 delta 3 phase system, so you can't get 120 volts
from it without using an extra stepdown transformer connected across
any 2 legs. You get 220 volt 1 ph between any 2 of the 3 phase
conductors. So you need a 220 to 120 stepdown transformer (1.83:1
ratio).
There are other 3 phase schemes too.
One uses 240 volt windings on the transformers in delta configuration,
but one of the windings is center tapped. So you can get 120 between
either one of the two phase conductors connected to the winding with
the center tap, and the center tap (neutral). You can also get 240 1 ph
between any two of the phase conductors, or 240 3 ph between all 3
conductors. Do *not* try to connect between the third leg (called the
high leg) and the neutral, that voltage will be much higher than 120 volts.
There are even more ways 3 ph can be wired, but I'll not go into
those unless people have specific questions about them.
Gary
Hey Gary,
Since you are so good at explaining this stuff where even *I* can
understand it, could you give me a quick explaination on this?: How
does the power company in say Austin Tx start with the 220 single phase
service from 1 transformer on the pole running to the building with 2
hots and a bare neutral (connected to ground on the pole), make it into
3 phase by adding a small transformer and running 1 more wire into the
building? Is it 220 3 phase or 208 3 phase? (Now that I think about it
that was a stupid question, of course it is 220 3ph....uhhh right?) I
still got (I'm not in that building anymore) 110 from 1 hot and a
neutral, and 220 single phase from the original 2 hots. I actually knew
the answer to this many years ago when my brain was functioning better,
but I seem to have lost track of that memory.
BTW, I am building the Hanrahan phase converter (I moved to the
'country' with only single phase service) that is on Metalwebnews. I am
having a master electrician come by and check my wiring on it. Pretty
neat project, huh?
Thanks,
Jamie Norwood
Gary Coffman wrote:
You mean the city power company - I suspect.
They are giving 3 phases, not 3 phase. Talk to a professional .
Phase angle should be three 120 degrees - sounds like you have two at 180 and
another perhaps at 90 from them.
Now if the transformer is a multi-phase type and the inverted V is running on
the lines -
e.g. two of the three phase lines as is done in the country - and the third is
generated
on-site - as done at many a farm site or dairy.... You might. Likely do.
240 and 208 are three phase. But the phase angle is the important for 3-phase.
220 is two legs of 180 degree - standard household 220 voltage / dual 120.
Is it Pertanallies ? - likely inverted V two of the three legs.
Martin
Jamie Norwood wrote:
--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
Actually, it is 3 phase. Likely 240V delta. The reason for the odd
looking voltage pattern is that the grounded point is a center tap of
one of the three 240V sides. This gives, referenced to this point,
120V, -120V (120V at a 180 degree phase angle), and 208V at 90
degrees.
Measureing the voltages between the phase legs will show 240V between
all three pairings, with phase angles of 0, 120 degrees, and 240
degrees.
This setup used to be very common, but is less so now in a lot of
areas, because a) it removed the need for another transformer to get
120V appliance power and ws therefore cheap, b) it provided redundancy
in case of a failed transformer (if on of the three goes out, the
other two will still provide all three phases, at about 58% capacity),
c) it could be run using only two HV phase leads to the transformer
bank, using why-wred primary, though the neutral then caried
significant power even with balanced loading.
Many power co's would just use the earth for the neutral. Not safe,
not real reliable, but legal for them, though not for you or me. The
power co can basicly do what it wants- it is exempt from NEC,
inspectors, etc, in most areas.
That setup is called open delta. It is normal delta, but one transformer
is missing. Because the phases of the two transformers are 120 degrees
and 120 degrees, the third phase is just what's left over, ie 360 -120 -120120 degrees, which is what you need to close the phase circle. The
voltages are 240 (center tapped), 240, and because of the geometry,
closing the circle makes the final phase voltage 240 too.
Because there is no winding supporting the third phase, the voltage
regulation isn't as good as with ordinary delta, but it works well enough
when the building loads are mainly 1 phase, with only a small amount
of 3 ph needed to run motors (normally just air handlers). You see this
a lot in office buildings and strip malls.
Gary
It isn't really two phases, but a single phase w/ a center tap formng the
grounded neutral, thpiugh the two hot leads are often refered to as phase A
and B etc.
"Three phase is either 3 or 4 wires (depending if a neutral is present
(usually wye wired) or not (usually delta wired) , and may provide 120/208V
240V, 277/480V, 480V or other/higher). The 240V of a 120/240V single phase,
3 wire service enterance will provide the voltage the welder requires (run
two hots and a ground, no neutral, from a two pole breaker or pair of
cartridge fuses).
One phase of a 3 phase 240V system, will also work but you are much more
likely to encounter 120/208V 3 phase systems commercially and the welder may
not be happy at only 208V from one of the three phases.
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