Unraveling the Mystery of Electrical Phases

[russ_watters]

And, to re-iterate, if the 2 "halves" were 180 degrees out of phase with each other, they would not combine to form 240 volts; they would cancel each other out to 0 volts (assuming of course that the average value of the sine wave is 0 volts).

You're misunderstanding what is going on. If you tried to combine the two signals on one wire, they'd cancel, but the two signals aren't traveling down the same wire, they are traveling down separate wires. Just draw yourself a graph!

A single phase voltage, by itself, has no "direction" and therefore no phase angle. So, the generators can both supply 230 volts (that is, they each have a potential difference of 230 volts between their individual terminals), but each voltage waveform has no angle. Not until there is some physical connection between the 2 voltages, can you have a phase difference (phase angle) between them.

This is correct...and it is why when you feed both wires into the same device, you are now using each as the reference for the other.

When you measure the voltage between the two wires, you get 240V because when one is +120V, the other is -120V.

 

[Averagesupernova]

I can't tell if you're agreeing with me or arguing with me.



Looking at something from a different perspective doesn't change its direction (or magnitude). A north-bound train goes north, regardless of which side of the tracks you look at it from ... even though it appears to travel from left-to-right when you stand on one side but appears to travel from right-to-left when you stand on the other side.

Likewise, we choose (for voltage selection and load balancing) to look at one half of the secondary winding of a split-phase transformer "from the other side." That doesn't change the waveform; it still rises and falls at the same rate and at the same period in time as the other half. It just changes our perspective of it.

And, to re-iterate, if the 2 "halves" were 180 degrees out of phase with each other, they would not combine to form 240 volts; they would cancel each other out to 0 volts (assuming of course that the average value of the sine wave is 0 volts).

Also, if they were 180 degrees out of phase with each other, we would have 2-phase power and could utilize that phase difference in the starting of electric motors. However, because they are NOT 180 degrees out of phase with each other, we need to use starting capacitors to create a phase difference.


It all comes down to wording...
The 2 halves of the secondary winding of a split-phase transformer are NOT 180 degrees out of phase, but (due to the reference point we choose), they appear to be 180 degrees out of phase with each other.

Tell me this then: Suppose we had two wire-pairs with each pair having 120 VAC on them. They are 180 degrees out of phase with each other by your definintion (whatever that actually is because I cannot tell from your posting). The pairs are electrically isolated from each other using isolation transformers or the method of your choice UNTIL we connect one lead from each pair together. At this point, what will the voltage be between any two of the three nodes?

 

[zgozvrm]

Ref the use of a two phase (180 degree phase diff) for motors. How could that work? ...

Of course, you are correct ... I got ahead of myself a bit there!

For 2-phase to be useful, the phase difference would have to be something other than 0 or 180 degrees; the vectors couldn't be "in line" with each other. The ideal offset would be 90 degrees which would generate the most starting torque in a motor.

In fact 90 degree, 2-phase was the standard in the early 1900's in the U.S. I believe it is still used in a few locations.

 

[zgozvrm]

your two arrows could be both referred to horizon and an angle between them measured, i think;
difference in their angles to horizon is analogous to phase difference ?
i never used a mariner's sextant but i think that's what it does for stars...

No, the phase angle would be the angle between the arrows, regardless of any other chosen reference point. To measure the angle between any 2 vectors (my arrows, for instance), you must connect their tails, then measure the angle between the vectors.

 

[zgozvrm]

you're right on about '...some physical connection...' though not necessarily by a copper conductor
and that was the point of my thought experiment
my observer ties them together at his oscilloscope, or at his clock, or whatever is his measuring device

If you connect your "measuring device" to the terminals of each generator at the same time (in order to make a comparative measurement), you are, in fact, making a physical connection.

 

[zgozvrm]

You're misunderstanding what is going on. If you tried to combine the two signals on one wire, they'd cancel, but the two signals aren't traveling down the same wire, they are traveling down separate wires. Just draw yourself a graph!

There has to be 2 wires to be useful, otherwise you have no circuit.
If we take one wire from one of the hot bus bars in a 120/240 split-phase panel, and another wire from the other hot bus bar, we are taking the full voltage across the SINGLE transformer winding and would have 240 volts. See my next post for a clearer example.

They'd only cancel if they were 180 degrees out of phase and of the same magnitude.

In other words, if you combined a 183 volt, 60 Hz signal with a another 183 volt, 60 Hz signal that was 180 degrees out of phase, the result would be 0 volts.

However, if you combined a 183 volt, 60 Hz signal with a 47 volt, 60 Hz signal that was 180 degrees out of phase, the result would be a 136 volt, 60 Hz signal.

If you combine two 183 volt, 60 Hz signals that were 90 degrees out of phase, the result would be 258.8 volts at 60 Hz.

If the two 183 volt signals were exactly in phase with each other, they would combine to form 366 volts.


Just draw yourself a graph!

See http://www.acs.psu.edu/drussell/demos/superposition/superposition.html for a reference if you need.
Also, look up vector addition.

 

[zgozvrm]

Tell me this then: Suppose we had two wire-pairs with each pair having 120 VAC on them. They are 180 degrees out of phase with each other by your definintion (whatever that actually is because I cannot tell from your posting). The pairs are electrically isolated from each other using isolation transformers or the method of your choice UNTIL we connect one lead from each pair together. At this point, what will the voltage be between any two of the three nodes?

First of all, they would only be 180 degrees out of phase depending on your point of view.
They would have to be completely in synch with each other, which (unless they came from the same source) would be difficult without other equipment to adjust for minor offsets in speed.

To answer your question: It would depend on how you connected them...
If you connected them one way, the 2 signals would be 180 degrees out of phase, in which the voltage across the combination would be 0 volts, whereas if you connected them another way, they would be in phase with each other and their voltages would add to 240 volts.


Take for instance 2 D-Cell batteries. Each measures 1.5 volts. When we combine them (as in a flashlight, for instance), the total voltage across the pair is 3 volts. But, if you reverse one battery (making it 180 degrees out of phase with the other), the combined voltage is 0.

If we combine those batteries the "correct" way (where the total voltage across the pair is 3 volts), we can still measure each battery independently: Place the black probe of a voltmeter on the negative terminal of the combined pair and place the red probe at the junction between the 2 batteries ... you'll get 1.5 volts. Next, place the black probe at the junction between the 2 batteries and the red probe at the positive terminal of the combined pair ... you'll read 1.5 volts from the other battery. Now, if you leave the black probe at the junction and move the red probe to the negative terminal of the combined pair, the meter will read NEGATIVE 1.5 volts! All we've done is changed our perspective of that battery (how we look at it), but we have not changed its orientation with respect to the other battery ... they are still in phase.

 

[Averagesupernova]

No, they'd only cancel if they were 180 degrees out of phase and of the same magnitude.

In other words, if you combined a 183 volt, 60 Hz signal with a another 183 volt, 60 Hz signal that was 180 degrees out of phase, the result would be 0 volts.

However, if you combined a 183 volt, 60 Hz signal with a 47 volt, 60 Hz signal that was 180 degrees out of phase, the result would be a 136 volt, 60 Hz signal.

If you combine two 183 volt, 60 Hz signals that were 90 degrees out of phase, the result would be 258.8 volts at 60 Hz.

If the two 183 volt signals were exactly in phase with each other, they would combine to form 366 volts.


Just draw yourself a graph!

See http://www.acs.psu.edu/drussell/demos/superposition/superposition.html for a reference if you need.
Also, look up vector addition.

You are barking up the wrong tree. What you describe can be accomplished through summing. Of course if you sum 2 signals of equal magnitude and opposite phase they will cancel but what is being discussed here about opposite ends of a transformer with a center tap does not apply here.

 

[zgozvrm]

You are barking up the wrong tree. What you describe can be accomplished through summing. Of course if you sum 2 signals of equal magnitude and opposite phase they will cancel but what is being discussed here about opposite ends of a transformer with a center tap does not apply here.

It absolutely DOES apply...
See my example in post #57 which compares the AC situation we are talking about with its DC equivalent.

 

[zgozvrm]

Suppose you had 2 separate, identical single phase transformers, each having a secondary voltage of 120 volts. Now, let's assume that a single 480 volt source is connected to the primary of each transformer (identically connected)...

The primaries are obviously in phase with each other since they are supplied by the same voltage source (any signal is in phase with itself).
The signal on the secondary of any transformer is in phase with the signal on its primary (due to the way transformers work; how voltage is induced).
Therefore the voltages on the secondaries of each transformer are in phase with each other.
Agreed?


If we connect the X2 terminals of each transformer together, one will be reversed with respect to the other (their secondary voltages will be 180 degrees out of phase) such that the sum total of the voltage across the combination (as measured from the X1 terminals) will cancel out to 0 volts.
But, if we connect X2 of xfmr #1 to X1 of xfmr #2, the total voltage across the 2 transformers (as measured from X1 of xfmr #1 to X2 of xfmr #2), we would see 240 volts. That is because the signals add, and they are in phase.
Now if we attach a wire to the connection between the 2 transformers, we have in effect, created a center tap. Note that the secondary voltages are still 120 volts each and they are still in phase with each other.

The center tap does not change the phase (or polarity) of any part of the transformer's windings. It merely gives you a point to tap off a lesser voltage. Granted, in reference to the center tap, the 2 signals look to be 180 degrees out of phase. In fact, they are not. The presence of that tap didn't change the orientation of one half of the winding!

 

[Averagesupernova]

Hmmmmm. So you ARE admitting that it is about perspective. Interesting. Let me ask you this then:

All we've done is changed our perspective of that battery (how we look at it), but we have not changed its orientation with respect to the other battery ... they are still in phase.

Concerning the above quote, what have you used to determine that they are in phase? What perspective are you using? You cannot just say 'it is because I say so.' No one can. There is no absolute phase. It is relative and in this case it is relative to the center tap/node so in this case the opposite ends of the transformer/dry cells are 180 degrees out of phase.
-
Edit: Looks like you posted before I was done. I'm still standing by what I say. There is no absolute phase.

 

[zgozvrm]

Hmmmmm. So you ARE admitting that it is about perspective. Interesting.

I've always maintained that it's a matter of perspective. The difference is that I'm saying that the perspective doesn't change the direction of the vector, it just make the vector appear differently; it's still the same, unaltered vector.
Once again... if I look at a train from one side of the tracks, it goes left-to-right. But if I look at that same train from the other side of the tracks, it goes right-to-left. The train has not reversed directions, I just changed my perspective, so it seems like it's going the other way.

Concerning the above quote, what have you used to determine that they are in phase? What perspective are you using? You cannot just say 'it is because I say so.' No one can. There is no absolute phase. It is relative and in this case it is relative to the center tap/node so in this case the opposite ends of the transformer/dry cells are 180 degrees out of phase.
-
Edit: Looks like you posted before I was done. I'm still standing by what I say. There is no absolute phase.

It's an example. If you open your mind just a little, I think you can see what I'm talking about...
If you have 2 batteries laying in front of you on the table with their negative terminals both to your left and their positive terminals both to your right, they would be "in phase." If you turn one battery around, they would be "out of phase."

Yes, I know there is no "phase" per se, it was merely an example to illustrate my point. If you can't see and/or understand that, I don't think we have anything more to discuss.


EDIT:
Anything with a magnitude and direction can be represented by a vector.
The voltage of a battery can be considered its magnitude.
The polarity of a battery represents its direction.
Therefore, batteries can be represented by vectors.
We can use vectors to determine the total voltage of several batteries randomly connected end-to-end.
Batteries can only be connected 2 ways: negative-to-positive, or positive to negative. This only allows for phase angles of 0 and 180 degrees to make sense.

 

[zgozvrm]

Just so you know... I work with signals and signal measurement like this all the time (and have been doing so for over 20 years), so I do know what I'm talking about in this regard.

Granted, what we're talking about is a minor technicality to the "casual" electrician but there IS a difference, and it's not that difficult to understand. I'm merely trying to show the correct way to look at a split-phase system. Yes, it may be "easier" for some to think of it as being two out of phase signals, but that doesn't make it right.

 

[sophiecentaur]

I've always maintained that it's a matter of perspective. The difference is that I'm saying that the perspective doesn't change the direction of the vector, it just make the vector appear differently; it's still the same, unaltered vector.
Once again... if I look at a train from one side of the tracks, it goes left-to-right. But if I look at that same train from the other side of the tracks, it goes right-to-left. The train has not reversed directions, I just changed my perspective, so it seems like it's going the other way.


It's an example. If you open your mind just a little, I think you can see what I'm talking about...
If you have 2 batteries laying in front of you on the table with their negative terminals both to your left and their positive terminals both to your right, they would be "in phase." If you turn one battery around, they would be "out of phase."

Yes, I know there is no "phase" per se, it was merely an example to illustrate my point. If you can't see and/or understand that, I don't think we have anything more to discuss.


EDIT:
Anything with a magnitude and direction can be represented by a vector.
The voltage of a battery can be considered its magnitude.
The polarity of a battery represents its direction.
Therefore, batteries can be represented by vectors.
We can use vectors to determine the total voltage of several batteries randomly connected end-to-end.
Batteries can only be connected 2 ways: negative-to-positive, or positive to negative. This only allows for phase angles of 0 and 180 degrees to make sense.

I think you are confused by what is meant by 'phase' and using it to describe Potential difference. Your use of the word "perspective" is unusual and doesn't introduce anything more useful than the word 'reference', which is what is used normally in this context. I think it implies that you are avoiding accepted vocabulary in attempt to prove yourself right in a matter where you appear to be shaky. The Maths of vectors speaks for itself.
Using batteries in order to 'explain' phase is a non starter because there is only 'polarity' with DC and not the continuum of phase values which exist with AC. Yes- it's true that you can get zero volts from a pair of batteries if you connect them 'the wrong way round'. This is because V-V=0.
If you connect them the 'right way round' (i.e. negative terminal of one to positive terminal of the other) then the Potential difference across the two will be V-(-V), which equals 2V.

If you take the instantaneous voltage values of the PDs across the two halves of a centre tapped secondary, you will, again get twice the voltage that appears across a single half. This is because, as with the batteries, you are getting V-(-V) across the 'far ends' of the winding. There is no meaning to any statement about the 'phase' across the whole secondary because there is no reference. The only way you can introduce 'phase' is by considering the PDs across the two windings (having committed yourself to which end of each secondary is your reference). You can then talk in terms of the two PDs referenced to a common 'ground' and subtract them to get the resultant PD.

 

[Averagesupernova]

Zgo, you don't need to simplify it to the point of dry cells for me. It is a simplified way to look at it and not without value. I understand it quite well. It seems to me that sophie is correct in saying that it appears you are avoiding certain things simply so that you can be correct.
-
What is really funny about this is that yesterday I found the old thread I referred to earlier in this thread and the one who I was arguing with over this is the same guy I am arguing with now. And I recall coming to the same conclusion that sophie did in that you simply wanted to be correct at any cost. Unfortunately I cannot find it again but I am sure someone more savy than I am probably can.

 

[zgozvrm]

super: Yes, I remember that. I don't recall the discussion, just the argument. And the same could be said about you (both then and now) ... that you simply want to be correct at any cost. That's just another way of saying, "I'm right and you're wrong ... you just won't listen to me." Or as a 3rd grader might say (as he's covering his ears), "Na, na, na. I'm not listening!"

You seem hell-bent on trying NOT to understand what I’m saying.

No … I’m not confused about the term, "phase." And, no, I’m not using it to describe potential difference. What I keep saying is that a voltage (be it AC or DC) can be represented by a vector and that if you have 2 voltages, there can be a comparison between the 2 vectors that represent them. Whenever you have 2 vectors (of similar types), they can be added together, and you can measure the angle between them (the phase difference, or phase angle). If the angle is 0 degrees, the vectors are said to be "in phase."

Yes, the mathematics of vectors speaks for itself, but apparently, you’re not listening.

As for my use of the word, "perspective" (by the way, I also used the word, "reference"), I never meant for it to be "anything more useful." It’s called a synonym – it’s another way of saying the same thing. I could just as well have said, "point of view" … it all means the same thing. Besides, if you tell someone something in a certain way, and they fail to understand you, do you tell them again in exactly the same words? No. Hence, my use of synonyms.

As for my use of batteries in explaining myself, there absolutely can be a phase angle between 2 voltages: 0 or 180 degrees. Now, if you want to get even MORE technical, there cannot be a phase SHIFT in DC voltages, but there can be a phase ANGLE. I'm thinking that this is where your difficulty in understanding me lies ... I think you're confusing "phase shift" with "phase angle."

The point I'm trying to make is a minor one and if you choose not to understand what I'm saying, it really will make no difference. The electrons will continue to flow.

Let me ask you this, though: Did you read my post #60? If so, did you agree with it? If you didn't agree, why not?

I really would like to continue the debate, at least to the point that you fully understand what I am saying. And not just an, "Okay, I understand" just to put an end to the discussion. (Remember that all along, I have agreed that the 2 halves of the secondary winding produce 120 volt sine waves that are, indeed, 180 degrees out of phase in reference to the center tap.)

 

[zgozvrm]

Oh, and what "certain things" am I supposedly avoiding so that I can simply be correct?
If am ignoring something, I would like to know what it is. I thought I had been pretty thorough.

 

[sophiecentaur]

@zgozvrm
aamof, I don't agree with that post (60) when you say that the two voltages are obviously in phase. If you happen to use, as a reference, the 'same ends' of the two windings and if they are wound in the same sense, then the voltages at their other ends are in phase. If you look at the PD between these two connections, then it will be zero. If you happen to have chosen different ends for your reference then the PD between them will be twice V.
In the case of a centre tapped secondary, it is the second case that applies and, in that case, the two connections are in antiphase, referred to the centre tap (grounded).
If you were to take another voltage reference - and this would need to be produced from another AC signal of the same frequency and at some other relative phase and referenced, in some way, to some point on our secondary winding then you could indeed say that there was a different phase angle between these two voltages. Are you suggesting a third winding should be involved? Otherwise there would be a totally unspecified situation as it would be floating with no specific PD relative to any reference.
This hardly seems worthwhile even considering so we are left with two windings which can either be connected 'in phase' or 'in antiphase' - producing either Zero volts or Double the volts of a single winding. Unless you are considering a special case of bi-filar windings for specifically cancelling out any induced voltage then we are dealing with two anti-phase windings giving a useful (2V) output.

As the phase of a signal refers to the time / (angle) difference (as in Cos(ωt+Φ)), I can't see where DC could usefully be included - as the value is the same at all times. DC Polarity and AC Phase are not synonymous although there are similar aspects to them.

 

[zgozvrm]

sophie: I don't think you followed what I was saying. I will make some diagrams to clarify.

... and supernova? What about you?I will have to continue this later...

 

[Averagesupernova]

Yes I have read post #60 quite a few times. And like spohie I cannot agree with it although I am trying my best to see your point. I ask again how you are determining that they are in phase? What is the reference? You seem to imply that there is an absolute phase and there quite simply isn't. Since we know and agree that reversing the leads on our scope or phase meter will cause an apparent 180 degree phase shift between 2 signals doesn't it stand to reason that the negative (for the lack of a better word) lead be placed on the same node for both channels? Maybe you don't think so but I would say that is common practice at the very least.
-
Take a differential amplifier chain such as that used in oscilloscopes. When the signal gets to the plates on the CRT (old school stuff), one plate goes more positive while the other goes more negative referenced to ground when there is a signal being injected into the scope. It is the same thing with the center tapped transformer. Outputs from differential amplifiers are by definition 180 degrees out of phase with each other referenced to ground. Are you saying the for some reason this would not apply to transformers?
-
Check the links out for farther proof:
http://www.tpub.com/neets/book8/32b.htm
http://web.engr.oregonstate.edu/~traylor/ece112/lectures/diff_amp.pdf

 

[sophiecentaur]

I wonder if we considered a three phase system, that we could resolve this. The only reason that the two 'ends' of a WYE (star) system have a PD between them is because there is a phase difference (ref neutral and ref anywhere else, aamof). If they were in phase there would be no PD between them. Likewise, the two ends of the secondary of a 'split phase' are in antiphase. If they were in phase (ref to anywhere) and of equal magnitude, there would be no PD between them.
How can there be anything 'special' about the volts at the two ends of a centre tapped secondary that would make you say the phases (and amplitudes) were the same but have a PD between them? I guess the only thing you could say is that the magnetic flux through the core, for each half of the winding is actually in phase.

 

[Averagesupernova]

We went less than two hours yesterday from the time I posted in #65 until post #69 when Zgo said:

... and supernova? What about you?

And here we are nearly a whole day later with no reply from Zgo. I would also like to continue this debate.