# transformer winding wiring configuration question

• posted

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.

• posted

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

• posted

| | 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.

• posted

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

• posted

| 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.

• posted

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.

• posted

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

• posted

On Mon, 04 Apr 2005 16:06:23 GMT VWWall wrote: | 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)?

• posted

| 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).

• posted

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.

• posted

| 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.

• posted

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.

• posted

On Tue, 05 Apr 2005 17:42:11 GMT Michael A. Terrell 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.

• posted

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

• posted

On Wed, 06 Apr 2005 05:45:54 GMT snipped-for-privacy@bellatlantic.net wrote: | snipped-for-privacy@ipal.net wrote: | |>On Tue, 5 Apr 2005 19:45:45 +1000 John G 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. |>

|>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. |>

|> |>

| 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.

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)

• posted

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

• posted

You miss the point. The ouput of each coil will be AC, whether connected opposing or aiding. The net effect of connecting the two AC signals could be 0, but that does not mean that there is no AC in the coil. The energy in the magnetic field *has* to go somewhere as the field varies - and it goes to electrical current in the coil.

The polarity is for aiding or opposing, not for plus or minus, not for current or voltage. It is AC The voltage will be + at one moment and - at another moment. The direction of current flow will likewise change. It is AC.

Yes. I'll "draw" 2 schematics - the coils are on the same core:

---dot /////--dot/////--- thats's two coils series aiding

---dot/////---/////dot--- that's two coild series opposing In the first case, all the windings go the same direction In the second case one set of turns is clockwise, the other is counterclockwise.

I don't understand your question. Here's two coils series aiding, the first wound on a ferrite rod, the second wound on a toroid core,

---dot /////-----dot/////---

---dot /////-----dot/////---

Note that the "schematics" are identical.

Instead, why not wind up a few transformers with 2 or more secondary coils? If you experiment with this, you'll develop an understanding that is likely to top whatever understanding you gain from reading the answers here.

Ed

• posted

| 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.

• posted

|>Not so if they are opposing. |> |>

| You miss the point. The ouput of each coil will be | AC, whether connected opposing or aiding. The | net effect of connecting the two AC signals could | be 0, but that does not mean that there is no AC | in the coil. The energy in the magnetic field *has* | to go somewhere as the field varies - and it goes to | electrical current in the coil.

I'm not expecting DC.

|>I derived one of the answers I wanted from the above, which is that the |>dots are for _voltage_ polarity. |>

| The polarity is for aiding or opposing, not for plus | or minus, not for current or voltage. It is AC | The voltage will be + at one moment and - at | another moment. The direction of current flow | will likewise change. It is AC.

Pick a moment. It will be 0 or it will be + on one end and - on the other end.

|>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) |> |>

| Yes. I'll "draw" 2 schematics - the coils are on the same core: | ---dot /////--dot/////--- thats's two coils series aiding | ---dot/////---/////dot--- that's two coild series opposing | In the first case, all the windings go the same direction | In the second case one set of turns is clockwise, the other | is counterclockwise.

So how does the electricity know where you put the dots?

Seriously, I'm not asking about dots. I'm asking about the relationship between construction and how to wire it (for which dots is a customary marking standard).

|>And if I used a loop core (square or round), the coils wound on the |>other end would be reversed? (yeah, another confirmation) |> |>

| I don't understand your question. Here's two coils series aiding, the first | wound on a ferrite rod, the second wound on a toroid core, | ---dot /////-----dot/////--- | ---dot /////-----dot/////---

Some transformers have both windings around a common center. Then the core needs to loop back around, usually made on 2 sides for some reason I don't know, yet, and complete a circle.

Other transformers have a single square loop with one winding on one side, and another winding on the other side. The sense of one is clearly revered.

| Note that the "schematics" are identical.

And hence, a point of confusion.

If schematics do not have dots, which is the correct assumption:

1. As if both dots are on the same end.
2. As if one dot on one end if primary, and other end of secondary.

|> |>

| Instead, why not wind up a few transformers with 2 or more | secondary coils? If you experiment with this, you'll develop | an understanding that is likely to top whatever understanding | you gain from reading the answers here.

I prefer to get the complete theoretical understanding first, then go play with toys. If I understood it correctly, things won't blow up.

• posted

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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