# transformer winding wiring configuration question

I am trying to understand the relationship between how transformers are actually wired, and how they are depicted in schematic diagrams,
with respect to different configurations of multiple windings and the polarity of those windings. I have assumed that the direction of the curls in schematics show the direction of the windings, but I have also seen some conflicts where it seems that is not the case. Perhaps the direction in schematics doesn't mean anything, and the labeled terminals are the only thing to go on?
I'll try to explain what I understand with an example to see where I might be going wrong with this. I'll be using unusual labeling so I can describe what I am doing, rather than refer to some standard way that might be my point of misunderstanding (e.g. so that I am not expressing myself with the very thing I may not be understanding).
Consider a simple transformer with a total of 4 separate windings.
In order to make sure my labels relate precisely to the construction of the transformer, I will assume the windings all turn the same way (let's say clockwise as viewed from the top of the core) and start at the top and end at the bottom. So I will label the windings 1 to 4 and the connections to each with T and B for top or bottom.
I will label the primary connection coming in as PA and PB, and the secondary connection going out as SA and SB (instead of H or X since there may be a standard associated with those I am not aware of).
These diagrams are in ASCII ART and require a fixed proportion font to be viewed correctly. If the diagram is messed up, try changing to the "courier" font.
character * is a connection character + is a non-connect crossover
I could configure this as a step down isolation transformer:
PA -----------------------* *--- SA 240 volts | | 120 volts PB ---* | *-------------------+--- SB | | | | | | | *----------* | | | | | | *-* | *--------+-* | | | | | | | | | |1B 1T| |2B 2T| |3B 3T| |4B 4T| \/\/\/\/ \/\/\/\/ \/\/\/\/ \/\/\/\/ ========================================== The polarity of the output doesn't really matter above since the primary and secondary are isolated. But what if I wire up an autotransformer to get 360 volts. Then it would matter.
PA -----------------------*-* *--- SA 240 volts | | | 360 volts PB ---*-------------------+-+-------------------+--- SB | | | | | | | *----------* | | | | | | *-* * *--------+-* | | | | | | | | | |1B 1T| |2B 2T| |3B 3T| |4B 4T| \/\/\/\/ \/\/\/\/ \/\/\/\/ \/\/\/\/ ========================================== Or even 480 volts:
PA -----------------------*-* *--- SA 240 volts | | | 480 volts PB ---*-------------------+-+-------------------+--- SB | | | | | *-* | | *-* | | | | | | | | | |1B 1T| |2B 2T| |3B 3T| |4B 4T| \/\/\/\/ \/\/\/\/ \/\/\/\/ \/\/\/\/ ========================================== The big question here is the polarity of the windings. The reason I am asking is because I see many wiring configurations for buck-boost transformers that look backwards to me. Surely so many cannot be wrong, so it must be my misunderstanding of something and I suspect it is the polarity issue somewhere. Can someone spot this and explain what is going on?
Here are some real life diagrams I've seen. Could someone explain why it seems backwards to me? I assumed all windings in the same direction would have the same polarity.
http://phil.ipal.org/electric/posted/20050403/autotransformer-diagrams.gif
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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Direction of curls? I wouldn't count on it.
Where the polarity of the transformer is really important, the terminals are marked. For example, power transformers will be marked H1, H2, etc.for higher-voltage terminals and X1, X2...etc. for lower voltage. The polarity and phasing of the terminals is shown on the connection diagram for the transformer.
In the case of instrument transformers (CTs and VTs), a dot is used to indicate polarity - the convention is that current entering the primary at a dot terminal results in current leaving the secondary at its dot terminal. Getting the dots the right way around on a CT installation is good for many hours of checking and discussion....
Bill
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wrote: |
| |> I am trying to understand the relationship between how transformers |> are actually wired, and how they are depicted in schematic diagrams, |> with respect to different configurations of multiple windings and |> the polarity of those windings. I have assumed that the direction |> of the curls in schematics show the direction of the windings, but |> I have also seen some conflicts where it seems that is not the case. |> Perhaps the direction in schematics doesn't mean anything, and the |> labeled terminals are the only thing to go on? |> | | Direction of curls? I wouldn't count on it. | | Where the polarity of the transformer is really important, the terminals | are marked. For example, power transformers will be marked H1, H2, | etc.for higher-voltage terminals and X1, X2...etc. for lower voltage. | The polarity and phasing of the terminals is shown on the connection | diagram for the transformer.
I'm not asking about the markings. But you could say I'm asking how to mark a transformer that isn't marked.
| In the case of instrument transformers (CTs and VTs), a dot is used to | indicate polarity - the convention is that current entering the primary | at a dot terminal results in current leaving the secondary at its dot | terminal. Getting the dots the right way around on a CT installation | is good for many hours of checking and discussion....
And if I move the dot to the other terminal? I don't think it changes just because the marking is changed. My question still remains. Maybe if you look at it as an unmarked transformer it might help.
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snipped-for-privacy@ipal.net wrote:

If you put the dot on the wrong terminal, you marked it wrong. You don't just move symbols around willy-nilly. The dot has a meaning. Let me put it another way: you can draw the symbol for a transformer (using either a dot or marking the terminals) correctly or incorrectly, just as you can draw the symbol for a diode incorrectly (backwards from the way it should be), or correctly. Your question was all about the markings. Maybe you need to ask it differently if we don't understand what you are after.
And it does not matter what direction you - or I - think the schematic means based on the coil orientation on the print - that orientation is irrelevant.
Ed
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On Mon, 04 Apr 2005 04:10:33 GMT snipped-for-privacy@bellatlantic.net wrote:
| If you put the dot on the wrong terminal, you marked it wrong. | You don't just move symbols around willy-nilly. The dot has | a meaning. Let me put it another way: you can draw the symbol | for a transformer (using either a dot or marking the terminals) | correctly or incorrectly, just as you can draw the symbol for | a diode incorrectly (backwards from the way it should be), or | correctly. Your question was all about the markings. Maybe | you need to ask it differently if we don't understand what you | are after.
This is about the fundamentals about how a transformer operates. Does it reverse the polarity? Which terminals would the dots be put on? Put youself in the position of examining an existing transformer. You can see how the windings are actually placed. But there are no markings or dots. So where do they go? Where would they go in the example I described?
| And it does not matter what direction you - or I - think the schematic | means based on the coil orientation on the print - that orientation | is irrelevant.
You can reverse the schematic and it something entirely different. I have seen schematics where they do go to the trouble to twist the connections around to show a specific wireup. So it has to mean something.
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snipped-for-privacy@ipal.net wrote:

The dots show the instantaneous polarity of the transformer terminals, just as the "+" and "-" do for a battery. You connect the windings in series, (or parallel if they are the same voltage), just as you would for a group of battery cells.

Some schematics represent the actual physical placement of the terminals, as well as indicating the polarity. This may require showing "twisted" connections to get the polarity correct.
The ham license must be a lot simpler now! :-) When I took it, (1945), you'd need to know this sort of thing, especially if you built a "home brew" rig.
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Virg Wall, K6EVE

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wrote:

Folks,
There is an ANSI standard for the marking of power transformers, although the number escapes me at the moment. In general, looking at the high voltage winding the endpoint of the high voltage winding terminal furthest to the right is marked H1. Other taps are marked in sequential order, by voltage measured to H1, until the other endpoint is reached.
Similarly, on the low voltage side, X1 is the terminal with the same polarity as H1 and the other taps are marked accordingly, in ascending order of voltage wrt X1. Note that X1 may be either on the same side as H1 or on the opposite side.
Polarity of a transformer is easily determined by jumpering together one end of the high and low voltage coils and applying an AC voltage to the high voltage windingl. Measuring the voltage between the high and low voltage sides (opposite ends of the windings from the jumper) will yield either the sum (additive polarity) or difference (subractive polarity) of the voltages on the windings. Subractive polarity indicates the two terminals connected to the meter are of the same polarity. Additive polarity indicates that those terminals are opposite polarity. The proper polarity is obvious when one draws the phasor diagram. Note that any AC voltage that won't damage the transformer can be used, but low voltages minimize hazards. Also, be sure to determine the tap terminal layout to avoid a voltage boosting resulting from wiring the AC source to a tap instead of the end points of the high voltage winding.
This is a standard lab exercise in most motor courses aimed at electrical engineering technology college programs. Check with your local community college, if they have a motors course, for more details.
E. Tappert
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On Mon, 04 Apr 2005 18:02:25 GMT Eric Tappert
| There is an ANSI standard for the marking of power transformers, | although the number escapes me at the moment. In general, looking at | the high voltage winding the endpoint of the high voltage winding | terminal furthest to the right is marked H1. Other taps are marked in | sequential order, by voltage measured to H1, until the other endpoint | is reached.
The terminals still have to connect to specific points in the windings. And that's what I want to know. If you have a transformer where you know the exact winding construction, but the terminals are not yet connected to the winding wires themselves, and you have to get this correct the first time (assume you do know exactly how the transformer is constructed with respect to all winding orientations and directions), how do you determine which winding end to connect to which side of the temrinal block?
| Similarly, on the low voltage side, X1 is the terminal with the same | polarity as H1 and the other taps are marked accordingly, in ascending | order of voltage wrt X1. Note that X1 may be either on the same side | as H1 or on the opposite side.
That's voltage polarity?
| Polarity of a transformer is easily determined by jumpering together | one end of the high and low voltage coils and applying an AC voltage | to the high voltage windingl. Measuring the voltage between the high | and low voltage sides (opposite ends of the windings from the jumper) | will yield either the sum (additive polarity) or difference | (subractive polarity) of the voltages on the windings. Subractive | polarity indicates the two terminals connected to the meter are of the | same polarity. Additive polarity indicates that those terminals are | opposite polarity. The proper polarity is obvious when one draws the | phasor diagram. Note that any AC voltage that won't damage the | transformer can be used, but low voltages minimize hazards. Also, be | sure to determine the tap terminal layout to avoid a voltage boosting | resulting from wiring the AC source to a tap instead of the end points | of the high voltage winding. | | This is a standard lab exercise in most motor courses aimed at | electrical engineering technology college programs. Check with your | local community college, if they have a motors course, for more | details.
I really don't want to take a whole course just to get what should be a simple answer. You're getting close here, but the "furthest to the right" reference way above is confusing. It doesn't apply to transformer constructions I am visualising because I'm thinking in terms of the windings from a theoratical perspective, not to a manufactured unit that has a terminal block with letters, numbers, and dots in the right place.
This question isn't about understanding the practicalities of using a manufactured transformer, but rather, about the theory of how one works, especially one I might wind (or unwind) myself. Understanding this then lets me deal with the practicalities in a more fundamental way (e.g. not just memorizing procedures).
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| snipped-for-privacy@ipal.net wrote: | |> This is about the fundamentals about how a transformer operates. |> Does it reverse the polarity? Which terminals would the dots |> be put on? Put youself in the position of examining an existing |> transformer. You can see how the windings are actually placed. |> But there are no markings or dots. So where do they go? Where |> would they go in the example I described? | | The dots show the instantaneous polarity of the transformer terminals, | just as the "+" and "-" do for a battery. You connect the windings in | series, (or parallel if they are the same voltage), just as you would | for a group of battery cells.
Even if connecting them between primary and secondary as in an autotransformer?
Something has to be reversed between primary and secondary. But is it the current or the voltage? I don't have enough understanding of the magnetics to determine imperically which it would be.
|> You can reverse the schematic and it something entirely different. |> I have seen schematics where they do go to the trouble to twist the |> connections around to show a specific wireup. So it has to mean |> something. | | Some schematics represent the actual physical placement of the | terminals, as well as indicating the polarity. This may require showing | "twisted" connections to get the polarity correct. | | The ham license must be a lot simpler now! :-) When I took it, (1945), | you'd need to know this sort of thing, especially if you built a "home | brew" rig.
However, what I'm trying to find out isn't how to use the dots someone might put on, but rather, how to determine where to put the dots on in the right place, or in other cases how to determine if the dots are on in the right place. Assume you build a transformer itself as part of the home brew rig.
If you have 2 identical windings, each beginning at the top of the core, each turning in the same clockwise direction (as viewed from the top of the core), could the dot be put on both terminals that connect at the top of the core? If so, and this were being wired up as an autotransformer where one winding is primary and the other is secondary, but connected in series together, does it still go non-dot to dot?
Or more specifically, with an isolation transformer of the design I just described, when an AC voltage has + on the top terminal and - on the bottom terminal of the primary, will the secondary voltage also be + on the top terminal and - in the bottom terminal (and hence the current in primary and secondary windings be reveresed)?
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wrote:

If you are going to trust some abstract artist (AKA Draughtsman) to draw the dots the way it really is then you are more naive than you have previously demonstrated. As some one else said "The ham license must be a lot simpler now! " You seem to launch off into great statements of "Truth" and then ask the most basic questions. Please try not to mislead the really needy people who come here for knowlegable advice.
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John G

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| If you are going to trust some abstract artist (AKA Draughtsman) to draw | the dots the way it really is then you are more naive than you have | previously demonstrated.
I never said that. Where did you get that idea? Some others seem to be suggesting I do that, but I certainly won't.
| As some one else said "The ham license must be a lot simpler now! " | You seem to launch off into great statements of "Truth" and then ask the | most basic questions.
I'm just trying to get an answer to my question. So far I've been getting answers to other questions.
| Please try not to mislead the really needy people who come here for | knowlegable advice.
No one is being misled by me. Some are obviously trying to drive me away from understanding the theory of transformers, and instead think I should only use already manufactured transformers with dots or other markings already done. In the end, that will be the case. But I still want to know the theory. It's really a simple question, but I guess as is usual in so many newsgroups, either people can't understand such questions, or they are intentionally diverity the answers.
Do _you_ even know the specifics of how a transformer is polarized in terms of its construction?
I very much doubt that transformers end up with random polarization after construction, which would require each one be tested. I very much believe that given a specific consistent construction, there is a specific consistent ... and knowable ... polarization.
But perhaps this is an industry trade secret.
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snipped-for-privacy@ipal.net wrote:

The windings should all be in the same direction, so the start of each winding would be one polarity, and the end would be the opposite polarity. Since a transformer is AC, the polarity markings are stating, "If this lead is positive, these leads are positive at the same time, while the rest are all negative. The dots show the same end of all windings. Go to the library and check out any copy of the ARRL Radio Amateur's Handbook. There are some basics on transformer design that will explain things at a beginners level.
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Former professional electron wrangler.

Michael A. Terrell
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wrote: | snipped-for-privacy@ipal.net wrote: |> |> No one is being misled by me. Some are obviously trying to drive me |> away from understanding the theory of transformers, and instead think |> I should only use already manufactured transformers with dots or other |> markings already done. In the end, that will be the case. But I still |> want to know the theory. It's really a simple question, but I guess |> as is usual in so many newsgroups, either people can't understand such |> questions, or they are intentionally diverity the answers. |> |> Do _you_ even know the specifics of how a transformer is polarized in |> terms of its construction? |> |> I very much doubt that transformers end up with random polarization |> after construction, which would require each one be tested. I very |> much believe that given a specific consistent construction, there is |> a specific consistent ... and knowable ... polarization. |> |> But perhaps this is an industry trade secret. | | | The windings should all be in the same direction, so the start of | each winding would be one polarity, and the end would be the opposite | polarity. Since a transformer is AC, the polarity markings are stating, | "If this lead is positive, these leads are positive at the same time, | while the rest are all negative. The dots show the same end of all | windings. Go to the library and check out any copy of the ARRL Radio | Amateur's Handbook. There are some basics on transformer design that | will explain things at a beginners level.
I've already read that book. It's missing some things that left me with unresolved questions. But maybe you understand more about this enough to fill in the gaps.
When you refer to polarity, are you referring to current polarity or to voltage polarity? If you draw out the transformer circuit, you can see that if the polarity is the same for one (current or voltage) it is the opposite for the other (voltage or current). This can be understood as power going in vs. power going out.
BTW, I'm not really a beginner. But when I was a beginner there were some gaps in what I learned that I am wanting to fill in now. That, and the fact that I am a detail-oriented person, is why I am wanting to know things that others typically think are not important to know. Right now, among the things I still don't fully understand are how magnetic fields build up in coil windings, how they change, and how they induce voltage or current in either the same wire or the opposite wire.
Perhaps I should be asking these questions in a physics forum, instead of an engineering forum?
A long time ago when I was in junior high school, the current lesson in our physical science class was on optics. The teacher was going through all the explanation of how 2 lenses worked to function as a telescope. His explanations were not really any different than what was in the book, but as he was explaining it without reference to the book, I presumed he understood it. But something in both simply didn't make sense. Later I did find out that I didn't "get it" with one aspect that I should have, but neither did the teacher, because when I asked the question, it totally stumped him (whereas today if I were teaching the same lesson and was asked that same question, I'd know where the misunderstanding was). That lecture and book illustrated 2 lenses and draw lines that went from both the top and bottom of the subject, crossed in the middle of the lens, and at a half way point between the two lenses where they were wide apart, suddenly changed direction so as to cross in the middle of the 2nd lens, and emerge coming apart again from that lens. So I simply asked the teacher this question: "I can understand that the light would bend inside the lens, but could you tell me how it is that the light is caused to bend in between the two lenses where there is only air?" He was never able to answer it. But today I know the answer is that these lines do not represent the path of the light, but rather, the end points of image formation. But he didn't know that, or couldn't figure out a way to explain it. The clue should have been that the lines did not bend in the lens. In fact such an illustration is misleading because the image is not formed execpt at specific distances. But if I had that gap of knowledge today, and ask that question, I wonder what kinds of non-answers I would be getting online. Obviously when some piece of knowledge is already wrong (like assuming those lines represented the path of light), even the questions are going to be "wrong". Truly understanding something, IMHO, includes knowing the various ways something could be misunderstood by others, and the wrong conclusions they could make from such a misunderstanding.
I do have another coming question on transformers. But I am hoping that once I understand what I'm trying to ask in this case, that I can figure out the answer to the next question.
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snipped-for-privacy@ipal.net wrote:

What a crock.
Phil, it is simple. Take a simple transformer with two separate windings. The orientation of the curls on the schematic is meaningless. The dots tell you how to wire the coils in series aiding or opposing. Voltage and current polarity is not relevantto dot marking - just forget it completely. After all, you're stuffing AC into the thing and getting AC out, and the so what is + at one moment will be - at another.
To get a series aiding connection, connect a dot to a non-dot. To get a series opposing connection connect a dot to a dot.
You mentioned winding a transformer with 4 coils on the same core, all starting at the top and ending at the bottom, and all wound in the same direction. Every coil should get the dot in the same location - *either* the top or the bottom.
Ed
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On Wed, 06 Apr 2005 05:45:54 GMT snipped-for-privacy@bellatlantic.net wrote: | snipped-for-privacy@ipal.net wrote: |
Not so if they are opposing.
| To get a series aiding connection, connect a dot to a non-dot. | To get a series opposing connection connect a dot to a dot.
I derived one of the answers I wanted from the above, which is that the dots are for _voltage_ polarity.
| You mentioned winding a transformer with 4 coils on the same | core, all starting at the top and ending at the bottom, and all | wound in the same direction. Every coil should get the dot in the | same location - *either* the top or the bottom.
And if 2 of the coils were wound COUNTER-clockwise (while the other 2 remained clockwise), those would have their dots on the opposing end, right? (this is a confirmation because I'm still not 100% sure you understand my question)
And if I used a loop core (square or round), the coils wound on the other end would be reversed? (yeah, another confirmation)
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wrote: <snip long discussion>

Phil, if you are talking about how to determine the proper 'polarity', there are a couple of ways. One way, used by electricians all the time was mentioned before, tieing one end of primary to one end of a secondary, applying a low voltage to primary terminals and measuring the voltage between the primary terminal and secondary terminal that aren't connected.
But reading your followup posts, you seem to be looking for the 'other' method. Such as how do manufacturers figure out just how to mark the connections in the first place when they wind them.
Well, the answer goes back to the theory. Remember that when an isolated load is connected to any secondary of an energized transformer, the magneto-motive force created by the secondary current is opposing the MMF of the primary current.
So pick a hand, either hand (I like 'right hand'). Now, wrap your figures of the 'chosen' hand, in the direction of current flow (for a half-cycle) through the primary windings around the core. Note which way along the core your thumb is pointing. Now, take your opposite hand (I hope you still have two). Point the thumb of that hand (for me, it would be the 'left hand') in the same direction along the core. Now your fingers of that hand wrap around the core in the direction of current flow in the secondaries. Regardless how many secondaries there are, and which way they are wrapped, the current in*all* secondaries will go around in the direction of your second hand's fingers. You're done.
If your primary and secondary windings make just one 'pass' down the length of the core, and they wind around the core in the same direction, then the 'dot marked lead' of the secondary is at the same end of the windings as the 'dot marked lead' of the primary. You can pick either end for the 'primary dot', just have to make sure you put the 'secondary dot' at the same end. If the secondary winding twists around the core in the opposite direction, then the 'secondary dot marked lead' is at opposite end from the 'primary dot marked' lead.
But be careful when talking about 'ends'. After all, if the primary has many turns, it may wind clockwise down from the 'top', get to the bottom, and continue to make windings in a second layer, working back up to the 'top'. So both ends of the winding can come out at the same 'end', one at the innermost layer, the other at the outermost layer. It is more important to note which direction *around* the core the current flows when on a particular half-cycle (i.e. which way do your fingers 'wrap').
Hope this helps...
daestrom
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wrote:
| Phil, if you are talking about how to determine the proper 'polarity', there | are a couple of ways. One way, used by electricians all the time was | mentioned before, tieing one end of primary to one end of a secondary, | applying a low voltage to primary terminals and measuring the voltage | between the primary terminal and secondary terminal that aren't connected.
Yes, but not in the context of having a physical transformer present, rather, in the context of a theoratical construction. The test you describe would work to let someone hook up a transformer. But I'm looking to understand the orientation of transformer design with 100% confidence (it's not 100% yet ... and things that in the past were 95% turned out to be wrong about 5% of the time).
| But reading your followup posts, you seem to be looking for the 'other' | method. Such as how do manufacturers figure out just how to mark the | connections in the first place when they wind them.
I'm sure they can figure it out by the first method after a sample has been made. I wouldn't blame an engineer for testing it that way to be sure.
| Well, the answer goes back to the theory. Remember that when an isolated | load is connected to any secondary of an energized transformer, the | magneto-motive force created by the secondary current is opposing the MMF of | the primary current.
This I know. But there is some ambiguity to this. You said force and then current. The problem I see is that I don't know which it really is that determines the orientation. Given 2 identical windings, if the _voltage_ ends up being the same on the same ends, then the 2 _currents_ are flowing in opposite directions. This is explained by the fact that power is going in on the primary and power is going out on the secondary. Understanding the "opposing force" would help. But due to the loose usage of terms when many people speak about theory, especially "force" vs. "current", I just can't be sure what is going on there.
| So pick a hand, either hand (I like 'right hand'). Now, wrap your figures | of the 'chosen' hand, in the direction of current flow (for a half-cycle) | through the primary windings around the core. Note which way along the core | your thumb is pointing. Now, take your opposite hand (I hope you still have | two). Point the thumb of that hand (for me, it would be the 'left hand') in | the same direction along the core. Now your fingers of that hand wrap | around the core in the direction of current flow in the secondaries. | Regardless how many secondaries there are, and which way they are wrapped, | the current in*all* secondaries will go around in the direction of your | second hand's fingers. You're done.
I'm sure all the secondaries are like any other secondary. I'm sure all the primaries are like any other primary. It's the relationship between the primary and the secondary that I haven't pinned down.
Looking at the core from one end, I believe the direction of wire wrap, e.g. clockwise vs. counter-clockwise, is the issue. Whether the wire starts at the bottom and ends up at the top, or starts at the top and ends up at the bottom, is not. Or a wire could wind CCW going from top to bottom and then continue winding CCW in a new layer going back to the top, repeating until the needed number of windings are done. It would still be CCW from the referenced view.
For consistency, I would bundle all the windings together as a "cable" and wind this cable as described. While that may be a lousy way to construct a real-life transformer, I think it clearly shows the idea of everything in the same orientation.
When I energize the transformer, the primary current is opposed by the field. Whether that is an actually a current opposition or a voltage opposition would not matter (yet) since it affects the same winding. But when we look at the 2nd winding as a secondary, there would be a voltage potential there, but not being connected to a load, no current. Now if a load is connected, is the _current_ on the 2 windings going to be in the same direction? If I have described my scenario clearly enough, someone who thoroughly understands this should be able to say yes or no. If the currents are the same then the voltage on the secondary will be opposite because power is being taken out instead of being put in.
But is that so, that both primary and secondary currents go in parallel when power is drawn from the secondary? It seems that can't work because it would increase the field strength, and something I read suggest the secondary has to be tapping into the field strength, effectively lowering it, for power to be taken out.
So the other supposition is that the current in the primary and the current in the secondary will be going in opposite direction, cancelling each other out. This would then give identical voltage polarities.
While I'm still sitting on the fence, the "opposite current" scenario seems more plausible because it would have to be in order to correctly describe how 2 wires in a cable supplying power to some load will cancel each other's magnetic field. But I've yet to see clear, detailed, unambiguous wording that confirms this.
| If your primary and secondary windings make just one 'pass' down the length | of the core, and they wind around the core in the same direction, then the | 'dot marked lead' of the secondary is at the same end of the windings as the | 'dot marked lead' of the primary. You can pick either end for the 'primary | dot', just have to make sure you put the 'secondary dot' at the same end. | If the secondary winding twists around the core in the opposite direction, | then the 'secondary dot marked lead' is at opposite end from the 'primary | dot marked' lead. | | But be careful when talking about 'ends'. After all, if the primary has | many turns, it may wind clockwise down from the 'top', get to the bottom, | and continue to make windings in a second layer, working back up to the | 'top'. So both ends of the winding can come out at the same 'end', one at | the innermost layer, the other at the outermost layer. It is more important | to note which direction *around* the core the current flows when on a | particular half-cycle (i.e. which way do your fingers 'wrap').
OK, I think I can conclude, as I mentioned earlier, that the ends don't really matter. It's strictly the direction around the core.
BUT ...
If we are dealing with a core in the form of a loop, with a primary on one side of the loop, and a secondary on the other, then we have to reverse things because the field itself has been turned around.
Here is a more complex scenario which is one of those things that has given me the want to find out precisely about this. Suppose I have an "E" core transformer, which has 3 vertical bars crossing between a top bar and a bottom bar. This is the typical core used for a 3-phase transformer. Label the three vertical core segments A, B, and C. If I wind a primary around core A and energize it, the field will loop around through cores B and C. If I wind a secondary around core C, now what happens? Will the secondary around core C be able to get full power out, despite B being present? Or will the B core reduce the available power in some way?
Suppose I wind a tertiary winding around core B, and monitor the voltage by drawing a trivial few microamps. Will this voltage change as more and more power is drawn from the winding around core C?
What happens if I put a big load on B, or even solidy short it? How will that affect the power I can get from C? What if I put a resonant circuit on the B core, peaked at a high impedance at the power frequency?
If I had 3 windings around a single core, I can better visualize what might be happening, despite a few lingering doubts or ambiguities about how it all works. But with the E-core, things are "stranger" here.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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wrote: <snip>

The 'force' I spoke of is Magneto-Motive Force, not voltage. MMF is measured in ampere-turns. It is directly proportional to the *current* (not voltage) through a winding.
Rule # 1: The MMF generated by any current flowing in the secondary is *always* opposite the MMF generated by current flowing in the primary. From this simple rule, you can figure out all your answers.

This is true. Did I say something to make you think otherwise??

No. See Rule # 1
The varying magnetic field (created by varying MMF of primary) induces a voltage in primary that opposes the applied voltage. Since the secondary is wound right along side the primary (in your 'cable' example), the induced voltage in the secondary would have the same polarity at any instant as the applied voltage on the primary. If a load is connected to the secondary, the induced voltage in the secondary creates a current flow to the load. The MMF *must* of the secondary *must* be opposite the MMF of the primary (see Rule # 1). Since you have them wound in the same direction, the only way to get opposite MMF is to have opposite direction of current flow.
So for some instant in the cycle when the electrons are traveling CW around the core in the primary winding, the electrons in the secondary winding are traveling CCW.

No. The currents would be traveling in opposite directions (see Rule # 1).

The currents are not 'traveling in the same direction', so the voltage on the secondary is the *same* polarity as the primary. (in your winding example)

Congratulations, you've talked yourself out of your mistake. The currents do *not* travel in the same direction around the core, so the field does not increase.

How much clearer do you want me to word this? The *current* in the secondary travels in the opposite direction around the core as the primary *current*.

No, you have to mentally 'cut the core' and unwrap it. Once you open up a toridal core and straighten it out, or 'slide' the secondary winding over to the same side as the primary, you have a simple solution.

Well first of all, you don't have a three-phase transformer with just one primary winding and one secondary winding. Your example is a single-phase transformer on a three-phase core. Bad, bad, very bad.
But the answer is still there. When you have current flowing through the primary around the 'A' leg, suppose that on some particular half-cycle, the MMF of the primary is such that your 'thumb' points upward. So 'lines of flux' will rise through the iron from the top of the coil, turn with the iron and 'flow' downward through the iron in legs B and C. When a load is connected to your secondary winding, its MMF *must* oppose this 'flow of lines of flux'.
Looking down from the top, you can think of the 'lines of flux' rising up toward you in the 'A' leg, turning and going down away from you in the 'C' leg. So if the current in the primary is flowing CW at a particular instant (as viewed from above), then the current in the secondary will also be flowing CW at the same instant (as viewed from above). The reason it *seems* to be the same direction this time is because the magnetic field did a u-turn in the iron. So we still have an opposition (rule #1 still applies).
BTW, with no load on the secondary, the voltage output of the secondary is *not* going to be what you'd expect from the turns ratio. If the 'split' of the magnetic flux between the B and C legs were perfect, you would get half the voltage that the turns ratio alone would predict. Can you see why?

The 'B' core will greatly reduce the amount of power available on the 'C' winding. This is because when the secondary current begins to flow through a connected load, the secondary's MMF (which remember opposes the primary's) will simply 'divert' much of the magnetic flux of the 'A' leg that was originally 'flowing' into the C leg of iron, over to the B leg of iron. This results in much less magnetic flux in the C leg and correspondingly less induced voltages. (i.e. the voltage output of the C winding will drop drastically as it is loaded)

Yes. The exact 'split' of magnetic field from the primary A winding will vary drastically with load. So a heavy load on the 'C' winding will shift the 'split' such that the field strength in C is reduced and the field strength in B is increased. This is why you almost *never* make a transformer like this. Three-phase transformers are *not* wound as you're describing.

Now, you can have all sorts of fun. A shorted turn on B would effectively reduce the amount of primary flux that goes through the B leg, forcing more through the C leg. Or, you can 'notch' the iron in the B leg so small amounts of flux pass as before, but 'large' amounts saturate the iron in the area of the notch. So limiting the amount of power that can be drawn through that leg.
A ferro-resonant transformer with a tuned tertiary winding is just such a 'critter'. With a high-reluctance path (an air-gap) to the winding with a tuned resonant circuit (a series circuit tuned for *low* impedance at power frequency) draws little power in the tertiary because of this air-gap, leaving most of the power for the secondary. But a slight increase in primary MMF (as caused by a rise in supply voltage) will increase the power drawn by the tertiary and thus the power supplied to the secondary remains fairly constant. Such things are also known as constant-voltage transformers, or by the brand name Solo-Transformers.

Because E-core are not used for simple, single phase transformers. When an E core is used for a three-phase transformer, *two* windings are wound around each leg. A primary, and the same phase's secondary. The three primaries (one on each leg) are connected similar to how three single-phase transformers might be connected (either wye or delta). And the three secondaries are also connected in much the same way (either wye or delta).
Putting a single phase primary on one leg, and two different secondaries on the other legs is *not* something I've run across in my many years (except the ferro-resonant transformer I described earlier).
daestrom
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wrote: |
| wrote: | <snip> |> |> | Well, the answer goes back to the theory. Remember that when an |> isolated |> | load is connected to any secondary of an energized transformer, the |> | magneto-motive force created by the secondary current is opposing the |> MMF of |> | the primary current. |> |> This I know. But there is some ambiguity to this. You said force and |> then |> current. The problem I see is that I don't know which it really is that |> determines the orientation. Given 2 identical windings, if the _voltage_ |> ends up being the same on the same ends, then the 2 _currents_ are flowing |> in opposite directions. This is explained by the fact that power is going |> in on the primary and power is going out on the secondary. Understanding |> the "opposing force" would help. But due to the loose usage of terms when |> many people speak about theory, especially "force" vs. "current", I just |> can't be sure what is going on there. | | The 'force' I spoke of is Magneto-Motive Force, not voltage. MMF is | measured in ampere-turns. It is directly proportional to the *current* (not | voltage) through a winding.
Is this a flux density of the magnetic field?
| Rule # 1: The MMF generated by any current flowing in the secondary is | *always* opposite the MMF generated by current flowing in the primary. From | this simple rule, you can figure out all your answers.
So how can that be? How do the electrons ... and whatever it is the magenetic field is made of ... know which is primary and which is secondary?
|> | two). Point the thumb of that hand (for me, it would be the 'left |> hand') in |> | the same direction along the core. Now your fingers of that hand wrap |> | around the core in the direction of current flow in the secondaries. |> | Regardless how many secondaries there are, and which way they are |> wrapped, |> | the current in*all* secondaries will go around in the direction of your |> | second hand's fingers. You're done. |> |> I'm sure all the secondaries are like any other secondary. I'm sure all |> the |> primaries are like any other primary. It's the relationship between the |> primary and the secondary that I haven't pinned down. |> |> Looking at the core from one end, I believe the direction of wire wrap, |> e.g. |> clockwise vs. counter-clockwise, is the issue. Whether the wire starts at |> the bottom and ends up at the top, or starts at the top and ends up at the |> bottom, is not. Or a wire could wind CCW going from top to bottom and |> then |> continue winding CCW in a new layer going back to the top, repeating until |> the needed number of windings are done. It would still be CCW from the |> referenced view. | | This is true. Did I say something to make you think otherwise??
I'm paraphrasing it to look at it in a different perspective to be sure my understanding of it is consistent.
|> For consistency, I would bundle all the windings together as a "cable" and |> wind this cable as described. While that may be a lousy way to construct |> a real-life transformer, I think it clearly shows the idea of everything |> in |> the same orientation. |> |> When I energize the transformer, the primary current is opposed by the |> field. |> Whether that is an actually a current opposition or a voltage opposition |> would not matter (yet) since it affects the same winding. But when we |> look |> at the 2nd winding as a secondary, there would be a voltage potential |> there, |> but not being connected to a load, no current. Now if a load is |> connected, |> is the _current_ on the 2 windings going to be in the same direction? | | No. See Rule # 1 | | The varying magnetic field (created by varying MMF of primary) induces a | voltage in primary that opposes the applied voltage. Since the secondary is | wound right along side the primary (in your 'cable' example), the induced | voltage in the secondary would have the same polarity at any instant as the | applied voltage on the primary. If a load is connected to the secondary,
It would seem to me that the induced voltage on the secondary would have the opposite polarity as the applied voltage. The reason I say this is the magnetic field is going to induce a voltage on one wire, then it will induce a voltage on another wire running along with it in exactly the same polarity. Since that polarity is the opposite of the applied voltage, it would be opposite in the secondary as well.
It gets back to the same ambiguity. How can a magnetic field that is inducing voltage in two wire do it differently on one vs. the other just on the basis that one is a primary and one is a secondary.
| the induced voltage in the secondary creates a current flow to the load. | The MMF *must* of the secondary *must* be opposite the MMF of the primary | (see Rule # 1). Since you have them wound in the same direction, the only | way to get opposite MMF is to have opposite direction of current flow. | | So for some instant in the cycle when the electrons are traveling CW around | the core in the primary winding, the electrons in the secondary winding are | traveling CCW.
Maybe I'm misunderstanding your description of induced voltage. This makes sense about the current because it is consistent with wiring practice. That is, we run 2 wires along side each other to power a load, and the magnetic fields between them cancel out nearly to zero.
| No. The currents would be traveling in opposite directions (see Rule # 1).
I can see that. I just can't see it from Rule #1 yet.
|> If the currents are the same then the |> voltage on the secondary will be opposite because power is being taken out |> instead of being put in. | | The currents are not 'traveling in the same direction', so the voltage on | the secondary is the *same* polarity as the primary. (in your winding | example)
Then I must be misunderstanding because I got a different meaning than this from a previous statement.
|> But is that so, that both primary and secondary currents go in parallel |> when |> power is drawn from the secondary? It seems that can't work because it |> would |> increase the field strength, and something I read suggest the secondary |> has |> to be tapping into the field strength, effectively lowering it, for power |> to |> be taken out. | | Congratulations, you've talked yourself out of your mistake. The currents | do *not* travel in the same direction around the core, so the field does not | increase.
What mistake?
A lot of this posting is paraphrasing what others post. If what I say seems wrong, then one of:
1. What they posted was wrong. 2. What they posted was poorly written. 3. I just didn't understand what they posted. 4. I understood what they posted by paraphrased it poorly. 5. My paraphrasing was misunderstood.
I've always believed, and your post here is confirming, that the currents will be in the opposite direction. Thus if I wire 2 windings together the same way, such as I could do by using a 2-wire cable, with the winding only going from top to bottom (for clarity, not for practical construction) then when at some instant the current is going from top to bottom on the energized winding (primary), it is going from bottom to top on the secondary.
That also tells me if the voltage is - at the top on one where the power is coming in, it will be - on the top of the other where the power is heading out.
| How much clearer do you want me to word this? The *current* in the | secondary travels in the opposite direction around the core as the primary | *current*.
OK, that's clear. But what I read from you about the voltage seems to be the opposite of what I expect. I would expect the voltage polarity to be the same at the same instant, when the current is reversed. But you have it stated the other way.
At least if we agree about the current, I'm half way to understanding this.
| No, you have to mentally 'cut the core' and unwrap it. Once you open up a | toridal core and straighten it out, or 'slide' the secondary winding over to | the same side as the primary, you have a simple solution.
I sort of did that. I just did it the other way around, starting with a straight core and bending it around.
|> Here is a more complex scenario which is one of those things that has |> given |> me the want to find out precisely about this. Suppose I have an "E" core |> transformer, which has 3 vertical bars crossing between a top bar and a |> bottom bar. This is the typical core used for a 3-phase transformer. |> Label |> the three vertical core segments A, B, and C. If I wind a primary around |> core A and energize it, the field will loop around through cores B and C. |> If I wind a secondary around core C, now what happens? | | Well first of all, you don't have a three-phase transformer with just one | primary winding and one secondary winding. Your example is a single-phase | transformer on a three-phase core. Bad, bad, very bad.
Certaing it is not a three phase transformer; it's just the core from one and other windings added for single phase.
| But the answer is still there. When you have current flowing through the | primary around the 'A' leg, suppose that on some particular half-cycle, the | MMF of the primary is such that your 'thumb' points upward. So 'lines of | flux' will rise through the iron from the top of the coil, turn with the | iron and 'flow' downward through the iron in legs B and C. When a load is | connected to your secondary winding, its MMF *must* oppose this 'flow of | lines of flux'. | | Looking down from the top, you can think of the 'lines of flux' rising up | toward you in the 'A' leg, turning and going down away from you in the 'C' | leg. So if the current in the primary is flowing CW at a particular instant | (as viewed from above), then the current in the secondary will also be | flowing CW at the same instant (as viewed from above). The reason it | *seems* to be the same direction this time is because the magnetic field did | a u-turn in the iron. So we still have an opposition (rule #1 still | applies).
I can see that from the reversal of the orientation because the field is turned around in C with respect to its origin in A.
| BTW, with no load on the secondary, the voltage output of the secondary is | *not* going to be what you'd expect from the turns ratio. If the 'split' of | the magnetic flux between the B and C legs were perfect, you would get half | the voltage that the turns ratio alone would predict. Can you see why?
No, I don't see why. In this scenario I see ambiguity. That means I just don't know what would happen. Or why.
|> Will the secondary |> around core C be able to get full power out, despite B being present? Or |> will the B core reduce the available power in some way? | | The 'B' core will greatly reduce the amount of power available on the 'C' | winding. This is because when the secondary current begins to flow through | a connected load, the secondary's MMF (which remember opposes the primary's) | will simply 'divert' much of the magnetic flux of the 'A' leg that was | originally 'flowing' into the C leg of iron, over to the B leg of iron. | This results in much less magnetic flux in the C leg and correspondingly | less induced voltages. (i.e. the voltage output of the C winding will drop | drastically as it is loaded) | |> |> Suppose I wind a tertiary winding around core B, and monitor the voltage |> by drawing a trivial few microamps. Will this voltage change as more and |> more power is drawn from the winding around core C? |> | Yes. The exact 'split' of magnetic field from the primary A winding will | vary drastically with load. So a heavy load on the 'C' winding will shift | the 'split' such that the field strength in C is reduced and the field | strength in B is increased. This is why you almost *never* make a | transformer like this. Three-phase transformers are *not* wound as you're | describing.
I would not expect them to be wound that way.
|> What happens if I put a big load on B, or even solidy short it? How will |> that affect the power I can get from C? What if I put a resonant circuit |> on the B core, peaked at a high impedance at the power frequency? | | Now, you can have all sorts of fun. A shorted turn on B would effectively | reduce the amount of primary flux that goes through the B leg, forcing more | through the C leg. Or, you can 'notch' the iron in the B leg so small | amounts of flux pass as before, but 'large' amounts saturate the iron in the | area of the notch. So limiting the amount of power that can be drawn | through that leg. | | A ferro-resonant transformer with a tuned tertiary winding is just such a | 'critter'. With a high-reluctance path (an air-gap) to the winding with a | tuned resonant circuit (a series circuit tuned for *low* impedance at power | frequency) draws little power in the tertiary because of this air-gap, | leaving most of the power for the secondary. But a slight increase in | primary MMF (as caused by a rise in supply voltage) will increase the power | drawn by the tertiary and thus the power supplied to the secondary remains | fairly constant. Such things are also known as constant-voltage | transformers, or by the brand name Solo-Transformers.
OK, this does sound like fun, though I was more interested in playing with harmonics.
|> If I had 3 windings around a single core, I can better visualize what |> might |> be happening, despite a few lingering doubts or ambiguities about how it |> all works. But with the E-core, things are "stranger" here. | | Because E-core are not used for simple, single phase transformers. When an | E core is used for a three-phase transformer, *two* windings are wound | around each leg. A primary, and the same phase's secondary. The three | primaries (one on each leg) are connected similar to how three single-phase | transformers might be connected (either wye or delta). And the three | secondaries are also connected in much the same way (either wye or delta). | | Putting a single phase primary on one leg, and two different secondaries on | the other legs is *not* something I've run across in my many years (except | the ferro-resonant transformer I described earlier).
Well, let me take the normal three phase transformer and vary it just a bit. Instead of varying it by construction (let's leave it at normal), let's vary it by placing a solid short on ONE of the secondary windings. Would this then cause an _increase_ in the MMF of the other two phase cores, thus _raising_ their secondary voltages?
This would be consistent with your statement from above, repeating:
| Yes. The exact 'split' of magnetic field from the primary A winding will | vary drastically with load. So a heavy load on the 'C' winding will shift | the 'split' such that the field strength in C is reduced and the field | strength in B is increased. This is why you almost *never* make a | transformer like this. Three-phase transformers are *not* wound as you're | describing.
I have seen a scenario where, during a storm, I saw the voltage rise quite a bit for a couple seconds, from around 120 to an estimated 140 to 150. A 60 watt light bulb got brighter to about the level somewhere between a 75 watt and a 100 watt bulb. A few more seconds after this I heard from a distance a loud boom like some primary distribution had shorted. My speculation is that a different phase shorted, and the transformer that was supplying power on all these phase raised the voltage on the others due to the high current of the one that was shorted. Plausible?
I'm going to save this whole post, because it must be me who is simply misunderstading an earlier induced voltage reversal description. This I'll have to re-read a few more times to be sure.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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wrote:

Not quite. Think of MMF as 'voltage' or force that is trying to create a magnetic field. Just how much magnetic field is *actually* created is a function of the MMF and the permeability of the flux path. A given MMF applied to a core with low permeability creates a lower flux density (magnetic field strength) than the same MMF applied to a core with high permiability.

They don't. And they don't have to. The magnetic field acts on the electrons in both coils in exactly the same manner. If the current in the primary is flowing CW, the magnetic field generates forces on the electrons trying to push them CCW (*opposite* the direction they are actually moving). In the primary, this force opposes the externally applied voltage, but is not powerful enough to overcome the external voltage. So the current (electron flow) still flows against the magnetic force. In the secondary, there is no externally applied voltage, so the magnetic force on the electrons has nothing to counter-act its effect.
<snip>

That is correct, but you're getting tangled up in semantics about the word 'opposite'.
Looking at your simple, 'cable' winding going CW around a core from the 'top' to the 'bottom' in one pass. When the applied voltage to the primary is negative on the wire connected to the top, electrons flow from the top, CW around the core to the bottom and return to the voltage source. As the current increases, the changing magnetic field applies a force on the electrons trying to push them CCW back 'up' the winding. This electro-motive force (i.e. 'Voltage') is trying to push electrons out the top connection, so we would say the EMF is such that the top of the coil is negative with respect to the bottom of the coil. This EMF 'opposes' the applied external voltage and limits the current flow. One *could* say it is the 'opposite' polarity of the applied source, but that can be a bit misleading. Afterall, if you look at the polarity at the 'top' of the winding, it is negative, just like the applied voltage. So is it really 'opposite polarity'??
The increasing magnetic field that is developing a force on the electrons in the primary, trying to push them CCW back 'up' the winding, is doing the exact same thing to the electrons in the secondary winding. They too are being 'pushed' CCW to the 'top' of the winding. But there is no external voltage source applied to the secondary that is stronger than the EMF induced by the magnetic field. If an external load is connected, the EMF created by the increasing magnetic field will push electrons right out the top of the secondary winding and through the load. From a macroscopic standpoint, we say the top of the winding is 'negative' since it is the 'source' terminal of electrons flowing to the load. (while the bottom terminal is 'positive' since it is the 'sink' terminal for electrons returning from the load.

It doesn't. That seems to be your biggest misconception. It works on the two wires *exactly* the same way. But in the case of the primary, it is *not* strong enough to actually determine the direction of current flow. In the primary, it merely retards the magnitude of current that would otherwise flow due to the applied voltage.
Consider for a moment two batteries, a 12VDC battery and a 11VDC battery. Connect the positive of the 12V to the positive of the 11V using a 1 ohm resistor. Similarly, connect the negative of the 12V to the negative of the 11V. This is similar to what you might have for the primary circuit at one particular instant in the sin wave. The lower voltage of the 11V EMF in the primary winding opposes current flow, but the 12V applied source 'wins' and electrons flow from the negative side of the applied source, *into* the negative terminal of the primary winding (against the magnetically induced EMF).
When the secondary is unloaded, the varying magnetic flux of the core induces the maximum amount into the primary. But this is still less than the applied voltage. But this situation results in very little current flowing in the primary (because the induced EMF is high, and opposes the applied voltage). As load is applied to the secondary, the secondary current creates a MMF that opposes the MMF created by the primary. This results in a slight drop in the EMF induced into the primary winding. Now, with the same applied voltage, a smaller EMF in the primary winding (and other factors the same), the primary current will increase.
In fact, as secondary current creates stronger and stronger MMF that opposes the primary's MMF, the current in the primary will increase so that its MMF counters the secondary's MMF to maintain a balance.

Remember that in a 'load', the electrons travel from the negative terminal of the load, thru the load, to the positive terminal. This is the case in the primary. But in any 'source', internally, the electrons travel from the positive terminal back to the negative terminal. This is the case in the secondary. So while the 'top' terminal of both the primary and secondary may be negative, the current flow at the two terminals is 'opposite'.
<snip>

With an 'E' core with the primary on A leg and secondary on C leg, the magnetic flux generated by the primary in the 'A' leg, is twice the flux density 'flowing' through the 'B' and 'C' legs (the 'lines of flux' split and return to the 'A' leg by two paths, the 'B' leg and the 'C' leg). With only half the flux density in the C leg as that in the A leg, you will get about half the induced voltage in the C winding that you would expect from the turns ratio alone.
<snip>

As with so many things, 'it depends'. Are you still energizing the primary of that leg? If so, the magnetic flux in the leg with a shorted secondary will be low and the primary current will be very high (no emf induced in the primary to oppose primary current). If we *assume* that your shorted secondary has zero resistance, then the magnetic flux in that leg will be very low. The applied voltage will have little opposition (just the resistance of the winding) and current flow in the primary for that phase will be extreme.
But the voltage induced in the other secondaries (of a proper three-phase transformer) is a function of the flux in their legs, not the shorted leg. So their voltage will not be significantly affected (the shorted leg has little affect on the flux density in the other legs).

No, because 'my statement from above' was for a different situation. One with a primary on one leg, a secondary on a different leg, and the third leg being the experimental one. In *that* situation the flux density in the leg holding the secondary *is* affected by changes to the experimental leg. Not the same thing at all.

While the voltage surge happened (so it *must* be possible), it most likely wasn't a voltage increase caused by any 'magic' inside a three phase transformer. What does happen often, is a fault on one phase will cause a shift in the neutral point. Instead of the neutral being in the 'center' of the three phases so that the phase voltages of a wye connected system are balanced, the fault 'pulls' the neutral voltage closer to that phase. This 'stretches' the distance from other phases to neutral, increasing the phase-neutral voltage for those phases. (forgive me the 'pull'/'stretch' terms, but that's what it looks like when you see it on a phasor diagram ;-)
daetrom
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