Why can any two phases be connected together?


by foo
Tags: connected, phases
Phrak
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#37
Jan18-10, 08:04 PM
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Quote Quote by zgozvrm View Post
Also, if you have 2 equal voltages from different phases, you still have a potential difference, and therefore you will have current flow. For instance, say you have one leg measuring 100V at 0 degrees, and another leg measuring 100V at 120 degrees, there is a potential difference of 100V at 60 degrees. This can be shown using vector addition (see attachment).

Note that the only time the voltages between any 2 phases of a 3-phase system coincide is at plus or minus 60V.
You folks need to be a little more careful. This is incorrect. The potential difference is not the result of summing phase vectors but taking differences. The resultant, rereferenced phase has a magnitude of 120 * Sqrt(3) =~ 208 V and a phase angle of either 150 or -30, not 120 degrees, depending on which leg is referenced ground.
zgozvrm
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#38
Jan18-10, 09:10 PM
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Quote Quote by Phrak View Post
You folks need to be a little more careful. This is incorrect. The potential difference is not the result of summing phase vectors but taking differences. The resultant, rereferenced phase has a magnitude of 120 * Sqrt(3) =~ 208 V and a phase angle of either 150 or -30, not 120 degrees, depending on which leg is referenced ground.
You are absolutely correct ... I got in a hurry with that post.
zgozvrm
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#39
Jan18-10, 09:13 PM
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Quote Quote by Phrak View Post
'2-phase' is a misnomer. The correct designation is split phase
It's only a misnomer when used to describe the standard 240/120V systems we use in the U.S. (like we are talking about in this thread).

There is such a thing as 2-phase (2 voltages that are 90 degrees out of phase from each other), but let's not get into that here.
Averagesupernova
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#40
Jan18-10, 09:22 PM
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Quote Quote by zgozvrm View Post
It's only a misnomer when used to describe the standard 240/120V systems we use in the U.S. (like we are talking about in this thread).

There is such a thing as 2-phase (2 voltages that are 90 degrees out of phase from each other), but let's not get into that here.
Oh come one! Are you sure you don't want to further confuse anyone watching this? It's not in the U.S. I believe. An island somewhere isn't it?
Phrak
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#41
Jan18-10, 09:24 PM
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Quote Quote by zgozvrm View Post
It's only a misnomer when used to describe the standard 240/120V systems we use in the U.S. (like we are talking about in this thread).

There is such a thing as 2-phase (2 voltages that are 90 degrees out of phase from each other), but let's not get into that here.
I'm aware of that. So called 2-phase systems are an historical curiosity. However, the context to which this misnomer is commonly applied is split phase. It may be technically accurate to identify split phase as "2-phase", and I'm not greatly upset by it for this reason, but the usage in communcation is still "split phase".
Averagesupernova
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#42
Jan18-10, 09:29 PM
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I hardly ever hear anyone refer to it as split phase. Split phase to me is a type of electric motor that has a high resistance starting winding that is taken out of circuit by a centrifugal switch.
foo
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#43
Jan18-10, 09:36 PM
P: 27
Anyone know where I can find a diagram of how the load completes the 3-phase circuit? On the secondary side, nothing is going to happen until a load draws current right? But how does this complete the loop?
Averagesupernova
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#44
Jan18-10, 09:53 PM
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I thought the link in post #17 did a pretty good job. Just imagine that each coil peaks in voltage at a different time. Current is drawn by each coil at a different time also.
foo
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#45
Jan18-10, 10:00 PM
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So the coils here prevent a direct short?
Phrak
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#46
Jan18-10, 10:15 PM
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Quote Quote by Averagesupernova View Post
I hardly ever hear anyone refer to it as split phase. Split phase to me is a type of electric motor that has a high resistance starting winding that is taken out of circuit by a centrifugal switch.
I don't know who you are talking to. Never the less, the common technical designation for two phases power, 180 degrees apart, is "split phase".
Phrak
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#47
Jan18-10, 10:17 PM
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zgozvrm, I don't know how to draw diagrams as you do. Your phase diagram was very well rendered. How do you do it?
zgozvrm
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#48
Jan18-10, 11:11 PM
P: 754
Quote Quote by Averagesupernova View Post
If you search, you will find that I have argued many times that what some people call 2-phase is more correctly just single phase.
Agreed. There does exist a thing called 2-phase power, but it doesn't apply here.

Quote Quote by Averagesupernova View Post
I don't see how you say that when referencing the scope ground to the center tap of a transformer and measuring each end with seperate probes on a dual channel scope that the observed voltages are not 180 degrees out of phase.
What I said was that if you placed the ground clips of 2 channels both on the center tap and the probes to each end, you would, in effect, be reversing the orientation of one of the channels in relation to the other and, therefore the waveforms would appear to be 180 degrees out of phase.

Quote Quote by Averagesupernova View Post
Do you feel that two totally different secondary windings are required in order to be considered 180 degrees out of phase? Just exactly what do you consider a requirement before you can say two signals are 180 degrees out of phase?
Yes. Let's start with the simplest of transformers having a turns ratio of, say 2:1. This xfmr will have a high voltage primary coil with 2 leads (one at each end of the coil), labeled H1 and H2 and a low voltage secondary coil, also with 2 leads (one at each end of the coil), labeled X1 and X2. If you apply a standard (sinusoidal) AC voltage to the primary coil, a voltage of 1/2 the value will be induced on the secondary coil. When the primary voltage rises, so will the induced voltage (and vice-versa), so you can see that the induced secondary voltage will be in phase with the primary voltage (but at 1/2 the amplitude). Now, if you were to reverse the leads measuring that voltage, it would appear to be 180 degrees out of phase with the primary. Agreed?

Now, let's take a 2nd transformer that is identical to the 1st one, except that it has a "center-tapped" secondary (let's label this lead as X0). This is a single coil with a lead attached to each end (X1 and X2) and one attached to the center of the coil (X0). There would be 1/2 the number of turns in the secondary coil as there are in the primary (just as in the 1st xfmr). The center tap will have half that number of turns (or 1/4 the number of turns in the primary) on either side (between X0 and X1 and between X0 and X2). If you were to apply the same AC voltage to the primary coil and measure the voltage from one of the end leads, say X1 to the center tap X0 (ignoring X2 for now), you would in effect have a xfmr with a turns ratio of 4:1. Again, the induced secondary voltage will rise when the primary voltage rises (and vice-versa), so it, too, is in phase with the primary. Agreed?

No matter how many times a single secondary coil is tapped, a voltage measured from any 2 leads connected to different points on the coil will rise and fall with the inducing primary voltage.

This supports why we correctly refer to 240/120V systems as "split phase" rather than "2-phase" ... there are not 2 different phases, but rather a single phase that has been split in two parts.

To repeat an earlier example, I can make a D-cell battery look as though it supplies negative 1.5 volts merely by reversing the leads of my voltmeter. This is what we're doing with the scope. So, in effect, what we end up with is a single primary coil and 2 secondary coils connected end-to-end. Imagine then there are actually 2 separate secondary coils, each with leads connected at both ends (only). The secondary induced voltages will both rise and fall in time with the source (primary) voltage. These voltages are both in phase with the primary voltage and, therefore in phase with each other.

Using vectors to illustrate: We know that the two 120V voltages of split-phase 240/120V add up to 240V. So if we take the voltage between X0 and X1, and assume its angle to be 0 degrees (it's not in reference to anything, so we can choose any angle), we would have a vector of length 120 pointing directly to the right. Now, take the voltage between X0 and X2 and let's assume that it is 180 degrees out of phase from the first voltage. We would then have a vector of length 120 pointing directly to the left. I you add these 2 vectors, you can see that they would cancel each other out. Alternatively, if we have 2 vectors both of length 120 pointing in the same direction and add them together, we would have a resultant vector of length 240 pointing in the same direction as the original 2 vectors.

Remember, too, that split-phase power (3-wire 240/120V power) is considered single phase, not 2-phase (you said it, too).


Quote Quote by Averagesupernova View Post
Question for you: Suppose I had 3-phase delta 240 volts with a center tapped transformer for the neutral to provide the 120 volt circuits coming into a room (all 4 wires). Lets call this power source A. Suppose I also have a standard 3-wire 240 volt (typical residential in the U.S.) coming into the same room. Lets call this power source B. I then 'manufacture' a new signal from power source B. Never mind the method I use to do it. This new signals phase and voltage are adjusted relative to the two 'hot' wires from power source B to form the third leg of a 'new 3-phase system'. I now run out of this room power source A, and power source B along with power source B's newly 'manufactured' signal. I just keep them separate with no indication which is which. Could you tell the difference? And if so, why?
I'm not really sure what the point of this question is. Especially since you don't disclose how you derived 3-phases from the standard 3-wire 240 volt source.
zgozvrm
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#49
Jan18-10, 11:13 PM
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Quote Quote by Phrak View Post
zgozvrm, I don't know how to draw diagrams as you do. Your phase diagram was very well rendered. How do you do it?
I use AutoCAD (but could use any drawing program). Then, I copy and paste the image into Microsoft Photo Editor and save the image as a PNG file.
zgozvrm
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#50
Jan18-10, 11:24 PM
P: 754
Quote Quote by Averagesupernova View Post
Oh come one! Are you sure you don't want to further confuse anyone watching this? It's not in the U.S. I believe. An island somewhere isn't it?
I don't quite understand what you are asking/stating here. I was trying to be clear about the type of power source we're talking about ... the type we use in the United States. In another discussion I had several months ago, someone from Australia made a big stink about the difference between their standard power and ours (the U.S.), which didn't even apply to the point of the discussion (as it doesn't here, either). I was simply trying to avoid that mess, but in doing so, it seems that I created it anyway!
zgozvrm
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#51
Jan18-10, 11:54 PM
P: 754
Quote Quote by Averagesupernova View Post
I like to explain things in terms that makes it easy to visualize. 3-phase delta is a pretty easy one to answer. There are three transformer windings (secondaries) that are hooked in a series. Drawn out they appear as a triangle, hence the reason we call it delta. Each 'phase' comes off of a node from two windings. So, grab any two phases and you can see they are directly across a transformer winding. I don't see how you could not see that you can source power from any two phases. Maybe I missed the point?
Speaking of misnomers ... The word, "phase" is often incorrectly used.

This is where the confusion starts for most people. Although it has become accepted, it is confusing to call the 3 leads coming off a 3-phase transformer, "phases" (forget neutral and/or ground connections, for now). It is generally clearer to call those wires "legs": Leg A, leg B, & leg C. Measuring between 2 legs, you will see different phases. For example, you might find that the voltage measured from leg A to leg B is 240 volts. You will find that the voltage measured from leg B to leg C is also 240 volts, but that it lags the first voltage by 120 degrees (i.e. it is out of phase by 120 degrees). And, you will also find that a 3rd voltage can be seen by measuring from leg C to leg A and that it lags the 2nd voltage by another 120 degrees (240 degrees from the first voltage).

Remember that it takes two reference points to have a voltage; you can't say that a single wire coming off a transformer has a certain voltage (voltage is the difference in electrical pressure between 2 points). It must be measured in reference to another point. Also, it takes 2 or more voltage measurements to be able to find a phase angle between them. (So it is generally incorrect to give a single voltage measurement a phase angle). For simplicity, we usually we label the first of 2 or more measurement taken to be at 0 degrees. (In fact, we could use any number.)

So, if we call any (or all) of the three secondary wires coming off a 3-phase transformer (whether delta or wye) a "phase," what exactly does that mean? That single wire, by itself is useless and has no voltage, so it cannot be compared to a voltage and, therefore it has no phase angle. When you measure the voltage between any 2 legs, you are measuring across one of 3 coils, each giving a voltage at a different phase angle in relation to the other two.
dlgoff
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#52
Jan19-10, 07:31 AM
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Quote Quote by Phrak View Post
zgozvrm, I don't know how to draw diagrams as you do. Your phase diagram was very well rendered. How do you do it?
It's from the link I provided.
http://www.animations.physics.unsw.e...ors.html#three

Edit: Oops. I thought you were asking about foos images.
zgozvrm
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#53
Jan19-10, 11:22 AM
P: 754
Here is another way to illustrate my point (hopefully the pictures will help):

If I have four D-Cell batteries connected in series like this:

and measure them with a voltmeter, I get 6 volts.
As you can see the batteries are all "in phase" with each other. That is, the direction of their polarity is consistent.

Now, if I reverse the meter leads like this:

the meter reads -6 volts. Did the voltage of the batteries change? No! I'm just looking at them differently; they are all still "in phase" with each other.

Now,if I measure from one end of the series-connected batteries to the middle connection (the center-tap) like this:

I now get a reading of 3 volts.

And, if I measure from the middle connection to the other end of the series-connected batteries like this:

I still get a reading of 3 volts.

However, if I now move my red (positive) probe back to the other end of the series-connected batteries, leaving the black (negative) probe at the middle connection like this:

I get a reading of -3 volts.

This is similar to how most people view a 240/120V 3-wire single-phase connection. The phasing (or, in the case of the DC batteries, the polarity) of any part of the system never reverses, only the way we choose to look at it does.

Just as the upper 2 batteries in my diagrams are always "in phase" with the lower 2 batteries, one half of a coil is always in phase with its other half.
Averagesupernova
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#54
Jan19-10, 12:33 PM
P: 2,449
zgozvrm, I get it. I've always 'gotten it'. I just don't understand why you insist that a center tapped winding cannot be considered to have each end of the coil 180 degrees out of phase with each other when the voltage is referenced to the center tap. But somehow, you claim that having two completely seperate windings that it is ok to call them 180 degrees out of phase with each other when the scope is hooked accordingly. So then do you feel that you can no longer call them 180 degrees out of phase if you were to hook the seperate windings together end to end? This essentially would become a single center tapped winding. Many power transformers are configured like this. To me it looks like this: I stand facing North. I can say for fact that the sun gets up to my right and sets to my left. Then I stand facing South and I say that now the sun gets up to my left and sets to my right. You argue that I cannot say that because I'm just facing the wrong way. I no longer wish to argue this specific point. However, I would still like your opinion to the question I asked about manufacturing a new 'phase' or 'leg' or whatever you prefer to call it.


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