Phase versus Polarity

Status
Not open for further replies.

dbuckley

Senior Member
Following from the (now closed) Battle of the Phases thread, in context of the difference between polarity and phase, I made the throw-away comment:

We did all this stuff in the audio world eons ago, I thought everyone knew this stuff...

After Bob closed the thread, I got a number of PMs saying basically that mains power and audio were different, and sine waves are different to non-repititive waveforms such as pink noise. Rather than shuttle PMs back and forth, I thought I'd post a considered response here.

Lets start with a picture, which I hope will be non-controversial(!)

mhf_pvp_1.jpg


We have a perfectly ordinary transformer, the sort that feeds many residential installations, with a MV primary, and a 120/240 secondary. I've illustrated where the oscilloscope probes go, and the assumption is that we are going to feed this transformer with one complete cycle of power from the 60Hz MV source. The oscilloscopes are set to display one complete cycle starting from when the primary starts its ascent.

Now, one school of thought states that the difference between waveforms A and B is that they are phase shifted 180 degrees, and another says that they are polarity inverted without phase difference.

Lets look more closely.

The underlying question is "what transform can I apply to waveform A to get waveform B?".

The polarity school says that you take the value of any point in waveform A, change it's sign, and plot it and you then get waveform B. The phase shift school says you need to apply, well, phase shift. "Phase shift" is another word that means time, so if you delay waveform A by 180 degrees, you get waveform B. 180 degrees is an odd thing to delay something by, but doing some simple maths (the only kind I can do), at 60Hz, each full cycle of the waveform (which is 360 degrees) takes 16.666 milliseconds. So delaying by 180 degrees is half that, so delay waveform A by 8.333 milliseconds and you get waveform B.

Which is almost true, lets look at this pictorially.

mhf_pvp_2.jpg


Whats wrong with this picture? Well, with the phase shift way, you lose the first half cycle, and the second half cycle isn't seen on the scope at all, becasue as noted at the top, the scope only displays one full cycle.

Can this really be what is happening.

I say not, and to those that say I'm wrong, consider this:

mhf_pvp_3.jpg


What we have here is nine transformers, each wired with swapover wires, and as there is an odd number of transformers, the whole thing will end up inverting the incoming power.

Now if each transformer introduces 180 degrees of phase shift, or a delay of 8.333 milliseconds, then nine of the things gives (180 x 9) 1620 degrees of phase shift. In time, thats 9 x 8.333ms, or about 75ms. If that is the case then the two bulbs will come on 75ms apart, and thus it will be visible that electricity is being "delayed" through those transformers.

Just for completeness, you cant argue that the phase shift clock resets every 360 degrees, as they either (a) time goes backwards, or (b) the transformers know they are undoing a previous transformers work.

Nah.

Polarity inversion is polarity inversion, and phase shift is phase shift.

Now all the above is real world, based on transformers, oscilloscopes and other stuff electrical people deal with. In the world of maths then maybe there are scenarios where there is no difference between polarity and phase shift, and you can substitue a polarity inversion with a 180 degree phase shift. What that tells us is that the math model is not dealing with the full reality, but for some sorts of problems that may not matter. But, in the real world, polarity inversion and phase shift are never the same.

(Still editing this post for spelling mistakes!)
 
Last edited:

quogueelectric

Senior Member
Location
new york
I couldnt agree with you more. The only comment I would like to point out is that there will probably be some phase shift from the primary input to the xformer and the secondary output at 240 I dont think that they will stay perfectly in phase I believe the secondary will lag the primary somewhat. Nice Presentation.
 

mivey

Senior Member
time shift does not equal polarity inversion

time shift does not equal polarity inversion

I agree as well.

Before, speaking for myself, the term "phase shift" was being used to indicate the observed difference in waveforms caused by a polarity change.

It may have been less confusing to call this observation a "phase difference".

A "phase shift" as described here, is a time shift and can never, ever, even if the waveforms in steady-state act exactly the same for a million years, be the same as a polarity shift.

In my case, it was a difference in terminology that caused the initial confusion. Thanks to David for clearing this up.
 

rattus

Senior Member
Already knew that:

Already knew that:

mivey said:
I agree as well.

Before, speaking for myself, the term "phase shift" was being used to indicate the observed difference in waveforms caused by a polarity change.

It may have been less confusing to call this observation a "phase difference".

A "phase shift" as described here, is a time shift and can never, ever, even if the waveforms in steady-state act exactly the same for a million years, be the same as a polarity shift.

In my case, it was a difference in terminology that caused the initial confusion. Thanks to David for clearing this up.

Agreed. Technically, it is not a phase shift, and I have already said as much, and have been using the term "phase difference". Still, in a steady state situation one could not tell the difference. I hope we agree that an inversion is tantamount to a 180 degree phase difference. The math says so.
 

mivey

Senior Member
time difference

time difference

And agree that the "observed" time difference seen by looking at the two waveforms in the polarity change case is not the same as a time shift.

[edit: typo]
 

rattus

Senior Member
mivey said:
And agree that the "observed" time difference seen by looking at the two waveforms in the polarity change case is not the same as a time shift.

[edit: typo]

Yep, but after a half cycle you can't tell the difference anyway. So it is much ado about not very much. The phasors would still be written with a 180 degree phase difference. Steady state is assumed with phasors, so we assume these waves have been waving since day one.
 

dbuckley

Senior Member
rattus said:
I hope we agree that an inversion is tantamount to a 180 degree phase difference.

tantamount : equivalent in value, significance, or effect

I absolutely disagree, which is the misunderstanding I'm trying to correct, which is why I bothered to write that lot up.

rattus said:
The math says so.

No it doesn't. The math doesn't say what happens; the observations in the real world say what happens. If maths tells you there is a phase difference, then, as Rod Tidwell would have said to Jerry Maguire, "show me the time baby" ;)

The math version of the world may be good enough for a mathematical understanding in the mathematical world, and indeed the numbers may come out the same. But the maths world and the real world should not be confused; in the real world polarity inversion and phase difference are separate and distinct phenomena.
 

mivey

Senior Member
dbuckley said:
the real world polarity inversion and phase difference are separate and distinct phenomena.

Not quite.

When phase difference is defined as what is observed when you have a polarity inversion (which is what has been said), then they are they same.

I am using the term "phase shift" to go along with "time shift".

I am using the term "phase difference" to describe what is observed with a polarity inversion.
 

mivey

Senior Member
phase difference

phase difference

phase difference as referenced in this text:

From pg 90 of "Techniques of Circuit Analysis" 1972 Carter/Richardson where they were talking about forming polyphase sources by using voltage sources separated by phase differences:

"...two voltage phasors in opposition-that is, with a phase difference of 180 degrees; a single-phase transformer with a center-tapped secondary winding would be such a source."
 

iwire

Moderator
Staff member
Location
Massachusetts
rattus said:
you can't tell the difference anyway. So it is much ado about not very much.

But it has been YOU that has made this whole issue a big deal here at the forum. :grin:

I am not saying that is a bad thing, just ain't gonna let you say it is 'much about nothing' without commenting that you have always been at the root of the discussions. :smile:
 
Last edited:

Rick Christopherson

Senior Member
dbuckley said:
Lets start with a picture..
I just wanted to commend you on an excellent presentation and writeup. This was very well thought out.

The one thing that seems to get lost in a lot of these discussions is that A and B are actually the same, and the only thing that has changed is that we inverted the voltmeter. A and B are not inverted. A and B are not out of phase. We have just chosen to view one of them with the polarity of our voltmeter inverted. This is the part that is a personal preference for some, but not necessarily all. This is appropriate when measuring voltages at a load center, but as Jim Dungar pointed out in a previous thread, when completing a circuit analysis, it will lead to having your current flow opposite the polarity of a voltage source.
iwire said:
But it has been YOU that has made this whole issue a big deal here at the forum. :grin:

I am not saying that is a bad thing, just ain't gonna let you say it is 'much about nothing' without commenting that you have always been at the root of the discussions. :smile:
Well Put!
 

rattus

Senior Member
Not really:

Not really:

iwire said:
But it has been YOU that has made this whole issue a big deal here at the forum. :grin:

I am not saying that is a bad thing, just ain't gonna let you say it is 'much about nothing' without commenting that you have always been at the root of the discussions. :smile:

This statement was in response to dbuckley's claim that there is a difference in phase shift and phase difference which is technically correct, but I feel is not really pertinent to the main discussion.

Now, the question of phase difference between L1 and L2 is important. It is a search for the TRVTH, and this question should be settled as soon as Rick provides the rest of his reference.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I _like_ the term 'phase difference' rather than 'phase shift' in this case. Because as has been noted, the B leg is _not_ generated by a time delay, but if you were to evaluate the phase of the B leg with respect to the primary source, the B output is clearly 180 degrees out of phase.

I will repeat myself that in the pure sinusoidal 60Hz case, the inverted output would be equivalent and indistinguishable from a 180 degree phase shifted output. This is not to say that a time delay is happening in the transformer coil, just that you could not tell the pure 60 Hz inverted output from the pure 60 Hz output delayed for 1/2 cycle. If the transformer were replaced with a motor-generator set, it would be possible for leg B to truly be time delayed from leg A; but in so far as the output is a perfect sinusoid, such output would be indistinguishable from that of a center tapped transformer.

Phase shift is closely associated with a time delay. But phase differences can clearly be produced without any sort of time delay. In the three phase world, a delta-wye transformer produces a 30 degree phase difference on its output, without any sort of time delay elements. This is somewhat different from the single phase case, in that we have time differences available at the input.

-Jon

P.S. As long as the discussion remains civil, I appreciate these pot stirring threads.
 

ronaldrc

Senior Member
Location
Tennessee
I think that was a great presentation and explained to even someone like myself can understand.

I never thought of showing that sine with the first half the cycle missing if you understand anything about basic electricity that proves it is a polarity change and not a phase shift since all this takes place at the same time
and both start at the same time.

I realize when we are trouble shooting in electrical work we do use the neutral as a common place to check our voltage from, the neutral to see if it is correct.

But we are only checking for 120 VAC. and like I told Rattus if he would reverse either set of those tester leads he would be reading two 120 volt sources swinging from positive to negative at exactly at the same time.
It is simply a polarity reversal reading and only because of the way he is measuring it.

I think the main confusion here is in Electronics they do change positive to negative for sideband conversion and TV color information modulation and demodulation but they also separate these two signals and feed them to to different amplifiers and they are processed as out of phase, but they are in phase.But never the less when studying Electronics they say these are out of Phase.

Again a great illustration

Ronald :)
 
Last edited:

coulter

Senior Member
Gentlemen -
We have a few that are fond of saying, "Well the math may say that, but it doesn't happen that way in the real world." (paraphrased)

I think the whole point of a math model is to predict how the real world will react. We don't build a full scale model every time we design something. This is true of bridges, highways, chemical plants, transmission lines ...

So if the math doesn't match the real world, that tells me:

1. We don't understand the physical phenomena well enough to accurately model it.

2. We don't understand the model well eough to apply accurate boundry conditions.

3. Our measurement equipment is not accurate enough, or mis-applied to test the corellation

My inclination is math models do not give 100% correlation with the real world. But they let me get close enough to predict, within the accuracy of the instrumentation, the system behavior.

carl
 

Smart $

Esteemed Member
Location
Ohio
dbuckley said:
...Which is almost true, lets look at this pictorially.

mhf_pvp_2.jpg


Whats wrong with this picture? Well, with the phase shift way, you lose the first half cycle, and the second half cycle isn't seen on the scope at all, becasue as noted at the top, the scope only displays one full cycle...
I'd just like to note that this representation is inaccurate. I don't know of any commonly used split- or multi-phase generation system that does not have any output prior to the delay time. The delay or lag or shift is only the reference point in the cycle, but the waveform exists at t = 0.
 
Last edited:

mivey

Senior Member
nit-pick

nit-pick

Smart $ said:
I'd just like to note that this representation is inaccurate.

That is being nit-picky and has nothing to do with the point dbuckley was making.

[edit] I mean this in a non-inflammatory way. It just did not help clarify dbuckley's point
 

mivey

Senior Member
pre-existing signal

pre-existing signal

Smart $ said:
I don't know of any commonly used split- or multi-phase generation system

Outside of switching transients, what about a transfer switch?
 
Status
Not open for further replies.
Top