# Additive/Subtractive Polarities



## SteveO.

Additive and subtractive only typically matters when you're paralleling transformers. At all other times it's kind of a non-issue, from what I have learned. Most (larger) transformers are wound subtractively because the winding can be made slightly smaller and you can get a slightly more effective transfer of the magnetic flux between windings. It also helps keep the induced voltage in the core to a minimum. 

On a diagram, you can identify if the transformer is additive by the H1 and X1 terminals being marked opposite of each other while on a subtractive transformer the X1 and H1 terminals will be adjacent to each other. ...Steve


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## Delighted In Yahweh

SteveO. said:


> Additive and subtractive only typically matters when you're paralleling transformers. At all other times it's kind of a non-issue, from what I have learned. Most (larger) transformers are wound subtractively because the winding can be made slightly smaller and you can get a slightly more effective transfer of the magnetic flux between windings. It also helps keep the induced voltage in the core to a minimum.
> 
> On a diagram, you can identify if the transformer is additive by the H1 and X1 terminals being marked opposite of each other while on a subtractive transformer the X1 and H1 terminals will be adjacent to each other. ...Steve


Ok, brain has been tossing this around. So....... parallel transformers. What is it about transformers being run in parallel that makes additive/subtractive polarity matter? Does that mean that both transformers are wound additively?? Would there be an instance where one would be additive and the other subtractive?


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## Salim Elfahim

*Transformers.*

Nice explanation, Steve. Transformers above 200 kVA will usually be wound as subtractive.


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## Delighted In Yahweh

I think I answered my own question, but want to check to make sure. Inductance in parallel is decreased over the whole system. EMF is directly related to the total inductive reactance of the system. Thus the additive polarity aides in overcoming this effect. Therefore transformers in parallel would be wound with additive polarities. 

So am I thinking along proper lines here? And if so, one thing I have learned from being in medicine is that everything in theory is not always reality in practice due to millions of factors. So is it true that in practice all transformers in parallel are wound additively??


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## SteveO.

Delighted In Yahweh said:


> Ok, brain has been tossing this around. So....... parallel transformers. What is it about transformers being run in parallel that makes additive/subtractive polarity matter? Does that mean that both transformers are wound additively?? Would there be an instance where one would be additive and the other subtractive?





Delighted In Yahweh said:


> I think I answered my own question, but want to check to make sure. Inductance in parallel is decreased over the whole system. EMF is directly related to the total inductive reactance of the system. Thus the additive polarity aides in overcoming this effect. Therefore transformers in parallel would be wound with additive polarities.
> 
> So am I thinking along proper lines here? And if so, one thing I have learned from being in medicine is that everything in theory is not always reality in practice due to millions of factors. So is it true that in practice all transformers in parallel are wound additively??


You're thinking too hard and it doesn't have anything to do with inductance, reactance or anything that "deep". The reason they say both transformers need to be the same polarity is so that when you connect them together they're both producing a waveform of the same polarity. You're trying to make sure that the magnetic field is passing through all windings in the same direction and producing an instantaneous voltage of the same polarity. If the two waves were out of phase you'd blow up that final connection. 

That said, you can parallel transformers of different polarity if you know they're different and allow for it when you make your connections between them. When paralleling transformers it's most important to have the same voltage (and tap settings), which is obvious. You also need the same IZ% so they will share the load equally and to make the most of both transformers you would want them to have the same KVA rating or you can't load them both all the way up.


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## chrisfnl

First, think about this DC example.

Take two AA batteries, put them in series, measure the voltage, nominally, 3V (1.5V + 1.5V)

Now, flip one of the batteries so it's polarity is opposite relative to the other one. Measure the voltage, nominally 0V (1.5V + -1.5V).

Now, consider two identical transformers, assume 120V isolation (1:1) transformers.

Connect the primaries in parallel, and the secondaries in series. Measure the voltage across both, you'll get 240V (120V + 120V), as the wave forms are "in phase" and as such add.

Now, reverse the secondary leads on one of the two transformers, again, measure the voltage, total will be zero. 

Polarity in an AC system can be hard to wrap your brain around at first, but in our simple example, when you reversed the leads on one of them, you reversed the polarity of that transformer, such that on the secondary, when the wave form was at a peak in one, it's at a trough in the other, net voltage is zero.

That's the basics of additive and subtractive polarity, or at least as I would define it.

Not entirely sure what Salim Elfahim is referring to, unless they're using autotransformers for their service transformers, in which case, it's still the same, except the primary is wired to the secondary... you can research how that works for yourself.


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## chrisfnl

SteveO. said:


> Additive and subtractive only typically matters when you're paralleling transformers. At all other times it's kind of a non-issue, from what I have learned.


You learned right, a transformer by itself has no "polarity", it can only have polarity relative to another transformer.


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## SteveO.

chrisfnl said:


> You learned right, a transformer by itself has no "polarity", it can only have polarity relative to another transformer.


That's not exactly true though. The primary and the secondary are additive or subtractive compared to each other as well.


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## Delighted In Yahweh

chrisfnl said:


> Now, consider two identical transformers, assume 120V isolation (1:1) transformers.
> 
> Connect the primaries in parallel, and the secondaries in series. Measure the voltage across both, you'll get 240V (120V + 120V), as the wave forms are "in phase" and as such add.
> 
> Now, reverse the secondary leads on one of the two transformers, again, measure the voltage, total will be zero.




I can see that pretty clearly. In what scenario then would it be more beneficial to utilize one transformer in additive and one transformer in subtractive as Steve suggested is or can be done??

And you can be assured I will be digging up autotransformers :tank:


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## chrisfnl

SteveO. said:


> That's not exactly true though. The primary and the secondary are additive or subtractive compared to each other as well.


Fair point, a more accurate phrasing is a coil can only have polarity relative to another coil.

A transformer, being composed of two or coils, would have a relative polarity between those coils (The coils being fixed in location relative to each other).

Whether it's additive or subtractive is a matter of how those coils are wired together relative to the polarity.

Auto-transformers aside, if you're talking about a conventional (as in isolated, magnetically linked, not electrically linked) transformer, while the coils would have polarity relative to each other, they're not additive or subtractive, it only becomes additive or subtractive when wired in series or parallel with another coil.

Also, if you know of one, I'm in need of a synonym for "relative".




Delighted In Yahweh said:


> I can see that pretty clearly. In what scenario then would it be more beneficial to utilize one transformer in additive and one transformer in subtractive as Steve suggested is or can be done??
> 
> And you can be assured I will be digging up autotransformers


In what scenario? Whatever one gives you the voltage you need. 

Need a step up transformer? Secondary is additive relative to the primary. Need a step down? Secondary is subtrative relative to the primary.

Are you referring to multiple transformers in a system, some additive, some subtractive? Probably not a practical application, but sure it can be done...

I'd offer you the title of the reference I used when I did a course on transformers, but I don't have it with me at the moment, and to be honest, I didn't find it that useful, the descriptions were good, but the math was "weird" (In that it didn't use vector notation for phases... used needlessly long formulas).

There's all kinds of different transformers, potential transformers, current transformers, high impedence transformers, auto-transformers, centre tap transformers,and more, but they're all just transformers, and they all work the same way, they're just linked coils...


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## SteveO.

Delighted In Yahweh said:


> I can see that pretty clearly. In what scenario then would it be more beneficial to utilize one transformer in additive and one transformer in subtractive as Steve suggested is or can be done??
> 
> And you can be assured I will be digging up autotransformers :tank:


There's no benefit to using one of each, I just meant it could be done if needed. If you're paralleling transformers in the field it's best to use two of the exact same transformers to ensure they're as close to identical as possible. The percent IZ is important to make sure they share the load equally, as is the voltage.


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## SteveO.

I think we're both saying the same thing in our own terms but I'll add this anyways:



chrisfnl said:


> Whether it's additive or subtractive is a matter of how those coils are wired together relative to the polarity.


The polarity of the voltage is determined by the direction the magnetic field passes across the windings, instantaneously and the voltage will reflect that, regardless of whether it's connected to anything else or not. A scope on the primary and secondary will show whether the waves are in phase or 180 degrees out of phase. 

Not arguing, just trying to get the last word. :laughing:


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## Delighted In Yahweh

chrisfnl said:


> Also, if you know of one, I'm in need of a synonym for "relative".


One to make you sound really sophisticated and upity.......analogous!!!


Simple and to the point.......dependent or related.

All things aside thank you both Chris and Steve. Tapping your brains recesses has been quite exhilarating and informative. Many more things to 'look' in to. HEH HEH:thumbup:


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## Delighted In Yahweh

SteveO. said:


> There's no benefit to using one of each, I just meant it could be done if needed. If you're paralleling transformers in the field it's best to use two of the exact same transformers to ensure they're as close to identical as possible. The percent IZ is important to make sure they share the load equally, as is the voltage.


I can certainly see how the current impedance percentage variation between transformers is most necessary. Thanks Steve for your insight.


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## chrisfnl

SteveO. said:


> Not arguing, just trying to get the last word.


Nope, last word is all mine. ALLLL MINE!

If I let you get the last word, then legally, we're married.


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## Alamandorious

Just getting ready to write my IP, this thread is a great refresher for me. Thanks guys!


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