Transformer Phasing - The Dot Notation

  • Last Post 08 October 2017
Chris posted this 03 October 2017

The Dot Notation

Generally, when we study about Transformers, we assume that the primary and secondary voltage and currents are in phase. But, such is not always the case. In Transformer, The phase relation between primary and secondary currents and voltages depends on how each winding is wrapped around the core.

Refer to fig (1) and (2), you may see that the primary sides of both transformers are identical i.e. primary windings of both transformers wrapped in the same direction around the core.

But in fig (2) you may notice that the secondary winding is wound around the core in the opposite direction from the secondary winding in fig (1).

Consequently, the voltage induced in the Secondary winding in fig (2) is 180° out of phase as compared with the induced voltage in secondary in fig (1) and the direction of secondary current (IS) is opposite from the primary current (IP)

So we see that

  • The primary and secondary voltage and current are in phase in fig (1)
  • The primary and secondary voltage and current are 180° out of phase in fig (2)


Dot Convention

To eliminate any confusion in the phase relation between primary and secondary voltage and current, a dot convention has been adopted for transformer schematic diagrams. Dots are placed on the top of primary and secondary terminals as shown in fig (3) and (4)

In fig (3), we see that dots are placed at the top in both primary and secondary terminals. It shows that the primary and secondary current and voltages are in phase. Moreover, the primary and secondary voltages (VP and VS) have similar sine wave, also the primary and secondary (IP and IS) currents are same in direction.

The story is opposite in fig (4). We can see that one dot is positioned at the top in primary terminal and the other one (dot) is placed at bottom of secondary terminal. It shows that the primary and secondary current and voltages are 180° out of phase. In addition, the primary and secondary voltages (VP and VS) sine waves are opposite to each other. Also the primary and secondary currents (IPand IS) are opposite in direction.

Please note: This article is in reference to:


If Voltage and Currents are not in phase

For example, in an Autotransformer, we have Voltage in one polarity and Current in another. I want to quote Wikipedia:

If two mutually coupled inductors are in series, the dot convention can be used in the same manner as in the case of autotransformers. Relative polarity in autotransformer drawings is usually quite obvious by physical placement of the windings in circuit drawings.


 So we see that the Mutual Inductance is the determining factor but layout is important to take into account.


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Vasile posted this 08 October 2017

I like this post because you try to explain something simple and this is what we all need to understand.Simple things.But I would rather focus on the magnetic interaction of the 2 scenarios you presented,that happen in the core.Why?Because regardless if u mantain the primarys direction of winding and change the secondarys direction of winding,the primarys field and the secondarys field will always oppose in the time of field rise.In my opinion it this this field opposition that makes the power in mirror the power out and we do not want that.


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Atti posted this 08 October 2017

Hi everybody!

I can approve the transformer experiments in the topic only. The flux change of the load is with an effect onto the original arrangement.See it here:  

Excuse me if cannot be understood,but I use web translation.

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Chris posted this 08 October 2017

Hi Vasile, Electromagnetic Induction, Mutual, Self Induction or any other type of Induction, is always Equal and Opposite.

This never changes and the article above is not clear on that. When the article says: "Primary and Secondary Voltage and Current are 180° out of phase" after saying: "Primary and Secondary Voltage and Current are in phase" is confusing at best.

This is however in the context of Winding Direction. Not Induction.

I try to think of the Dot Notation in terms of Voltage and Current Direction, but also in terms of how it is Induced or Applied. Each being opposite. An applied Voltage, and there for Current, will be opposite to an Induced Voltage and Current. This is Lenz's Law, equal and Opposite.


Chris posted this 08 October 2017

Hi Atti - An excellent demonstration! Thank you for sharing this with us!

This does show a lot, a huge amount of data when studied!

Your work reminds me of the excellent work done by: Melvin Cobb

Yes you're right, each Induced Current is equal and Opposite, showing how a single System can invoke Electromagnetic Induction more than once!

There is a Magnetic Field equilibrium, each Field will try to equalise, and a single System does not have to be Symmetrical, we want an Asymmetrical System, in your case you have Two Outputs for the price of One!

Excellent Work! Thanks again for sharing!