Why can any two phases be connected together?

[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!