The Dot Convention and Dot Notation in a Transformer Phasing

Electrical Technology

Transformer Phasing: The Dot Convention and Dot Notation for Transformer Polarity

The Dot Conversion (also known as the Dot Notation) is a polarity marking in a Transformer Phasing which is used to identify the phase relation between primary and secondary current and voltage in a transformer.

The manufacture and designer put a dot making or alpha numeric character on the both side of primary and secondary windings in a schematic circuit diagram. If both of the dots are on the upper side, it means, the voltage and current are in phase and the direction of primary current is same as the direction of secondary current.

In polarity test of a transformer, they put H1 and H2 or X1 & X1 marking instead of dot marking. It clearly shows that the H1 and H2 marking are used for primary (High Voltage) and H2 for secondary voltage. Same is the case for X1 and X2 making.

The Dot Notation

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

Refer to the 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 (I_S) is opposite from the primary current (I_P) as well.

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

The Dot Conversion as a polarity marking is used to identify both primary and secondary winding terminals of a transformer. Based on this, we may easily than connect the winding terminals of the transformers according to our needs.

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 currents and voltages are in phase. Moreover, the primary and secondary voltages (V_P and V_S) have similar sine waves, also the primary and secondary (I_P and I_S) currents are the same in direction.

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

Now let’s what is the effect of the dot convention in a transformer as follow.

Suppose the N_P and N_S represents the primary and secondary turns where the turns ratio is a = N_P ÷ N_S

Turn Ratio = N_P ÷ N_S = a

Similarly, the E_P and E_S is the primary voltage and secondary induced voltage and I_P and IS is the primary current and secondary respectively. The secondary induced voltage in the transformer assumed randomly for many cases as shown in the fig below.

This clearly shows the turn ratio and the direction of induced voltage and current for different scenarios based on the dot convention of the transformer. That’s why we connect the same dots points in an ideal transformer while referring circuit from one side to another.

Applications of Dot Convention

As transformers are not always in phase (as we assumed based on resistive load) e.g. the relation between voltage and current may be out of phase which depends on the core winding and its terminal directions.

Keep in mind that it may lead to serious damage to the power devices and equipment if we connect the wrong polarity of transformers as compared to the required system design. For this reason, we perform a polarity test (same like polarity of battery) on a transformer to identify the correct winding terminals in accordance. When identified correctly, the dot conventions are used for proper operation of the following systems.

To eliminate the inverse polarity and phase loss
Defeat protective relay operation and inaccurate energy and power measurement caused by reverse polarity of instrument transformers.
Avoid negative power factor reading and measurement.
Cancelation of signals and proper operation of amplifiers and speakers.
To avoid the short circuit in the windings of parallel transformers.

For these reasons, manufacturers print a polarity indication which is a standard now known as the “Dot Convention” or “Dot Notation” on the transformer schematic as they are not transparent and the winding terminals and their direction can’t be seen by naked eye.