Why can any two phases be connected together?

[Phrak]

Two different phases of a standard three-phase system are never in phase with each other.

Need to be a bit careful how this is worded.

No, zgozvrm's quote is practically a totalogy. Different phases are different phases. A phase consists of a reference zero, a magnitude and a (relative, gaugable) direction.

By the way '2-phase' is a misnomer. The correct designation is split phase

 

[Phrak]

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]

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]

'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]

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]

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]

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]

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]

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]

So the coils here prevent a direct short?

 

[Phrak]

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]

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]

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.

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 separate 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.

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).

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]

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]

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]

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]

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.edu.au/jw/electricmotors.html#three"

Edit: Oops. I thought you were asking about foos images.

 

[zgozvrm]

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]

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 separate 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 separate 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.

 

[zgozvrm]

Well, Averagesupernova, it sounds like we're arguing about the same thing ... it's all about point of view/point of reference. My point is (and always has been) that there is only one voltage phase being generated on the secondary side of a single phase transformer. When we choose to split it and look at the 2 halves differently, (let me be clear about this) it appears to look like 2 different phases. In actuality, it's 2 parts of the same phase looked at differently so that they appear to be 180 degrees out of phase. And yes, in reference to the center tap, they are 180 degrees out of phase.

As for your question about "manufacturing" 3-phase from a single phase source, my answer is, "Of course not." If the 3 voltages measured between each of the 3 legs of power source A are all at the same amplitude and at the same angular displacement from each other, and the same holds true for power source B, I obviously could not tell the difference between the two.

I still don't understand the point of the question, though (or how it relates to the discussion of the thread).

 

[Averagesupernova]

Well, Averagesupernova, it sounds like we're arguing about the same thing ... it's all about point of view/point of reference.

Maybe I didn't explain it well enough in post #26, but that WAS the whole point of the post was to show that depending on your point of reference, some things in a 3 phase system can be quite confusing concerning phase angle. Sorry for any confusion.

As for your question about "manufacturing" 3-phase from a single phase source, my answer is, "Of course not." If the 3 voltages measured between each of the 3 legs of power source A are all at the same amplitude and at the same angular displacement from each other, and the same holds true for power source B, I obviously could not tell the difference between the two.

I still don't understand the point of the question, though (or how it relates to the discussion of the thread).

I'm not talking about 'manufacturing' all three phases from a split-phase source. I'm talking about using the existing split-phase source for 2 of the legs and manufacturing just the third one. Just so we are talking about the same thing. I don't see how it cannot relate to this thread.

 

[zgozvrm]

I'm talking about using the existing split-phase source for 2 of the legs and manufacturing just the third one. Just so we are talking about the same thing. I don't see how it cannot relate to this thread.

Yes, you are right; this CAN be done. It could be done using a Scott-T transformation (you don't even need to "out of phase" voltages in order to do it). This is a special case, and there are substantial losses involved with this type of set-up. It could also be done with electronics (VFD's do this all the time). Neither of these is a good solution for a true power source, though.

Now, looking back, I guess I could tell one 3-phase voltage source from the other, if you told me that one was delivered using a 3-phase transformer, and the other with a Scott-T transformer (and their associated ratings). By measuring voltage, I would not be able to tell, but by loading the voltage sources down and measuring amps I could tell you one from the other (or loading them down until one failed - the Scott-T, being less efficient). Or, if you allowed me to have the power turned off, some resistance measurements would tell me which one was which.

BTW - none of this really relates to the OP's original question which had to do with trying to understand how two legs of a 3-phase power source could possible complete a circuit with a load connected between them ... he seemed to think that a neutral wire was necessary to "return the current."

 

[Averagesupernova]

I didn't ask if it could be done, I asked if you could tell. And no, it was assumed your only tools would be a scope and voltmeter without turning the power off and no loading. Sorry for not being more specific. Thank you.

 

[zgozvrm]

It could be done using a Scott-T transformation (you don't even need to "out of phase" voltages in order to do it).

Actually, this is not true: Looking closer at the Scott-T set-up, it would require two voltage sources that are 90 degrees out of phase in order to create 3-phase power in which the voltages are 120 degrees out of phase.

If you know of a way to do this (without electronics), I'd like to see it. I'm not saying that it can't be done, I just don't know how.

 

[Averagesupernova]

I am not specifically familiar with Scott-T. I have heard of it. What I had in mind was electronic. Actually, the reason I said

Never mind the method I use to do it.

was to avoid opening another can of worms in this thread.

 

[foo]

Here is another way to illustrate my point (hopefully the pictures will help)

That ROCKS!