On Sun, 23 Jan 2011 05:09:04 -0800 (PST), wrote:


Nobody I know would call 120/240 2-phase. You wouldn't buy a single
You can walk up the stairs, or you can walk down - and two people can
walk up and down the stairs - does not make it 2 stairways.

On Jan 23, 1:09*pm, wrote:
On Sun, 23 Jan 2011 05:09:04 -0800 (PST), wrote:

Nobody I know would call 120/240 2-phase. You wouldn't buy a single

*You can walk up the stairs, or you can walk down - and two people can
walk up and down the stairs - does not make it 2 stairways.

You can try to side step with cute posts all you want, but won't
answer
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
components
and is powered by a 120V AC outlet. Inside that box, I have a common
reference point.
I ask students in a first year electrical engineering course lab
experiment to
graph the voltages at circuit points A, B, and C relative to the
common
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.

wrote:
On Jan 19, 1:32 pm, bud-- wrote:
David Nebenzahl wrote:
On 1/18/2011 9:49 AM bud-- spake thus:
wrote:
On Jan 17, 12:51 pm, bud-- wrote:
wrote:
I'm a degreed Electrical Engineer, and it isn't BS to me. If you
take a signal centered around zero volts, and another one in the
same system that is also centered around zero volts, but
is 180 deg out of phase and they share that common zero volt
referrence, then: One leads the other by 180deg One lags the other
by 180deg One is the opposite of the other One is the negative of
the other. You have two phases
Nobody I know would call 120/240 2-phase. You wouldn't buy a single
core transformer and specify whether it is in-phase or 180
degrees out of phase. You don't get multiple phases out of a single
transformer. If you ask for a 2-phase transformer you will
completely confuse the transformer rep.
Analysis of real multiphase electricity commonly uses phasor
analysis, using SQR(-1).
A simple 120/240V system is single phase with the math handled
with *trivial* plus and minus signs. "2-phases" confuses trivial
math.
Whether 2 phases confuses anyone or not has no bearing on the fact
that there are two phases. I could describe many physical
processes by either very simple terms or varying degrees of
complexity. When looking at electrical waveforms, that trivial
plus and minus sign can equate to being described as 180deg out of
phase.
I noticed you didn't specifically refute any of the statements:
One leads the other by 180deg One lags the other by 180deg One
is the opposite of the other One is the negative of the other. You
have two phases
When the "phases" come from a single phase source (the utility
transformer), and one of the "phases" is the negative of the other,
calling them 2 phases makes no particular sense.
When I connect my 120-to-120V isolation transformer (for repairing
equipment) to one of the "phases" is the secondary the "A" phase or
the "B" phase?
I guess I'd have to call that question a red herring.
It is a minor illustration that "2 phases" is not useful.

It makes no sense to say you get 2 phases out of what is obviously a
single-phase utility transformer.



In the case of a transformer such as you describe, presumably with no
center tap, then yes, there's only one phase. Only one set of conductors.
We're talking about something different: a center-tapped transformer,
such as the utility company uses to deliver what's typically called
"split-phase" power (i.e., 120-0-120).
There, you *do* have two phases.
From the wikipedia articlehttp://en.wikipedia.org/wiki/Split_phase
"it is sometimes incorrectly referred to as 'two phase'."
(The article also suggests split phase is not the best name because of
confusion with split-phase motors - which do start on 2-phases. I have
never heard "split-phase" used for a 120/240V service.)

You can invent your own language. Where is any reasonable source that
says a single phase transformer has 2 phases.


How about this one:

http://www.allaboutcircuits.com/vol_2/chpt_9/4.html

"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 €œH€� and €œX€� labels are used
to label transformer winding wires, the subscript numbers are supposed
to represent winding polarity. The €œ1€� 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.


I have no problem with:
in-phase/out-of-phase
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=240 system we've been discussing actually is a 2-phase
system, even though it's not really called that. One side is 180ÌŠ 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
power".)

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
called that."

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.

--
bud--

bud-- wrote:

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).


Actually, I have no objection to saying the "A-leg" is 180 degrees out
of phase with the "B-leg" either (though I would state it differently).

I have a problem with making them different "phases".