It's not called that because it is ONE phase coming from the powerco and is
split to make two 120Vac lines into the residence. Thus the proper term is
"split phase", not 2 phase. Although I hear 2 phase a lot, I know what
they'e talking about so it's NBD to me.
Nah, it's just a bunch of egos here wanting to show how much they know and
hoping their guesses are right for the most part. This is a useless thread
with no useful information due to the interest in egoes rather than fact.
It's typical of this newsgroup for the last year or so in fact and does no
one any good. Post after post is filled with guesses and by gollies from
those who feel the need to confuse, not assist anyone.
Perhaps like your "guesses and by gollies" about power factor correction
capacitors, using an oven neutral for a ground and class 2 power sources
which were confused and completely wrong.
This thread is excessively about semantics.
How about this one:
"A pair of dots indicates like polarity.
Typically, the transformer will come with some kind of schematic
diagram labeling the wire leads for primary and secondary windings. On
the diagram will be a pair of dots similar to what is seen above.
Sometimes dots will be omitted, but when H and X labels are used
to label transformer winding wires, the subscript numbers are supposed
to represent winding polarity. The 1 wires (H1 and X1) represent
where the polarity-marking dots would normally be placed.
The similar placement of these dots next to the top ends of the
primary and secondary windings tells us that whatever instantaneous
voltage polarity seen across the primary winding will be the same as
that across the secondary winding. In other words, the phase shift
from primary to secondary will be zero degrees.
On the other hand, if the dots on each winding of the transformer do
not match up, the phase shift will be 180o between primary and
secondary, like this: (Figure below) "
Continue on in the above reference to the next section where the
transformer that has two secondary windings, and keep the above
discussion of phase in mind. They may not come right out and say it,
but clearly you can have transformer outputs that are out of phase
with each other, and hence, two distinct phases exist.
On Jan 23, 1:09 pm, firstname.lastname@example.org wrote:
You can try to side step with cute posts all you want, but won't
the simple questions I posed that go to the core of the issue:
Suppose I take a black box that consists of various linear circuit
and is powered by a 120V AC outlet. Inside that box, I have a common
I ask students in a first year electrical engineering course lab
graph the voltages at circuit points A, B, and C relative to the
reference point. I have the circuit designed so that the waveform
at point B
lags the one at A by 30 degrees and the waveform at point C lags the
one at A
by 180 degrees. I ask thefollowing questions:
What is the phase relationship between waveforms A and B?
What is the phase realtionship between waveforms A and C?
How many different voltage phases are there in the black box
at points A, B, and C?
What is your answer? Is it that there are 3 phases or is that there
can be only one, because it's originating from an outlet
that has only one phase?
Do I need to know exactly how the voltages were generated, whether
it came from a wall outlet, battery/inverter, transformer etc to
answer any of those questions?
If your answer is that there are 3 phases present, then continue to
the next part. I have another black box that merely consists
of the 3 wire 240V service. The common reference
point is the neutral, point A is one hot, point B, the other hot.
What is the phase relationship between waveforms A and B?
How many phases are present?
Note the usual disclaimer. I did not just say, nor have I said
that the 240V service is commonly called a two phase service.
I have no problem with:
additive/subtractive polarity or connection
positive or negative polarity
To interconnect separate windings you have to know if the connection is
additive or subtractive.
I have no problem with "two distinct phases" with regard to the current
in the start and run windings for a single phase motor - there is a
non-trivial phase angle that can vary over a wide range.
I have a problem with "two distinct phases exist" for a single phase
transformer - the voltages on the distinct phases are always exactly
opposite polarity. The "two distinct phases" are handled with trivial
plus and minus signs (or dots). "Two phases" out of a single phase power
transformer is guaranteed to cause arguments. "Two distinct phases"
covers any phase angle and can lead to miscommunication.
David N's post that said:
"but the 120+120$0 system we've been discussing actually is a 2-phase
system, even though it's not really called that. One side is 180Ì out of
phase with the other side, so by definition you have a 2-phase system."
It is, as I think almost everyone agrees now, not a "2-phase system".
(David more recently writes in another newsgroup it is "truly two phase
David also wrote:
"Again: the output of a center-tapped transformer, whatever its use, is
in fact 2 distinct and separate phases. But for some reason, it's not
I agree - it is not called that. And I elaborated on why it is not
called that. Like that in calculations you don't use 180 degrees out of
phase. You use trivial plus and minus signs.
We have no disagreement on the physics, just the terminology. You can
certainly use "180 degrees out of phase" or "different phases" if you
want. IMHO it is excessively complicated, not useful, and can lead to
confusion, miscommunication, and error (as by David).
The people who have objected mostly work with power systems, including
multi-phase power systems. You will not likely find anyone in the power
industry who considers a trivial 180 degree fixed shift a different phase.
On 1/17/2011 6:26 AM email@example.com spake thus:
Thanks; I feel vindicated.
To throw out another example, I mentioned push-pull amplifiers in my
earlier reply. These are almost always preceded by a stage called a
phase splitter or phase inverter, which takes a signal and splits it
into two phases, one 180° from the other. Every electronics engineer in
the world would agree that this stage produces two distinct phases from
a single phase. Which is exactly what our center-tapped transformer does.
Again, just to make it crystal-clear, the electrical industry uses the
term "2-phase power" in a very specific way that does *not* include this
way of generating two phases. Nonetheless, it does generate two phases,
so technically speaking it is two-phase power.
Even if you would get laughed at by the power company for asking for a
Comment on quaint Usenet customs, from Usenet:
To me, the *plonk...* reminds me of the old man at the public hearing
I haven't seen one.
(Weren't the original Westinghouse/Tesla AC generators at Niagara falls
I have seen Scott (or T connected) small 3 phase transformers that
essentially convert 3-phase to 2-phase to 3-phase (2 transformers for
480/277 to 308/277).
Three phase probably uses significantly less copper to convey a given
amount of power.
Three phase motors are probably cheaper than single phase starting at
somewhere less than 1 HP.
You combine a 120V transformer winding with another 120V transformer
winding that is in-phase to get 240V. In fact, as everyone knows, it is
a single winding with a center tap.
You won't find an electrical engineer for power systems who will say
120/240V is not single phase. You are not likely to find an electrician
that deals with 3-phase who says 120/240V is not single phase. Wikipedia
is not likely to say 120/240 is 2-phase.
For a 3-phase transformer, you use 2 transformers in a Scott connection
for the primary, and with a Scott connection on the secondaries. The
primary is 3-phase. The secondary is also 3-phase. The voltage in the
transformers is at 90 degrees - 2-phase. A disadvantage is the
transformer currents are not in phase with the voltage so the
transformers can't be used at their full rating. It is practical for
small 3-phase transformers.
I am too lazy to look up prices (which also requires matching quality).
My notes say over a 1/2 HP motor is cheaper in 3-phase. You don't need a
winding that is only used to start the motor. And you don't need the
start switch paraphernalia and often a capacitor. Motor control is
likely more expensive.
Fans of "2-phase" could ask for a 120/240 2-phase service from their
Simpler, I agree. Whether or not the volume (of single-phase) motors exceeds
the difference in complexity is the question. Also, I suppose, it depends on
who's buying (in what quantity - inventory costs as well as manufacturing).
On Sun, 16 Jan 2011 11:31:08 -0600, " firstname.lastname@example.org"
Well, all I know is I can buy 2HP 3 phase motors for considerably
lower cost than 2HP single phase here in Waterloo and they are
generally smaller as well. When you get to 5HP and higher, the
difference REALLY becomes obvious.
Not sure how 1/2 HP compares.
Also, lots of decent used 3 phase motors are available CHEAP, while
good used single phase are less common (because 3 phase only burn out
or need bearings, while single phase can also have starter problems,
bad caps, etc - and are also more prone to burning out when starter
On Sun, 16 Jan 2011 15:30:48 -0600, " email@example.com"
When a 3 phase motor ends up in the scrapyard because it outlasted
the machine it was on, you only need, at worst, a pair of bearings.
When a single phase motor ends up in the scrapyard you likely need a
$16 starting cap as well as the bearings - and may also need to
cleen/repair the starting switch.
The big problem is you need a 3 phase supply to run the 3 phase motor
- and outside of industrial plants 3 phase is "relatively" rare.
Would you please guide me out of this morass these folks have
constructed for themselves. You know how the dry transformers that we
install all the time have output voltage selection taps to compensate
for the variations in utility input voltage so we still end up with
120 volts for the general purpose receptacles? What do you say we wire
one with a conductor off of each voltage selection tap. Do we know
have six phase or eight phase. Dam I cant keep my tongue that far out
in my cheek without it starting to hurt.
I just thought of another question for the two phase crowd. How many
phases on the output of a high leg center tapped phase delta
transformer? By there logic it must be four. Dam my cheek is
starting to hurt again!
"This alternating current stuff is just a fad. It is much too
dangerous for general use." Thomas Alva Edison
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