Dot Convention In Transformers

Transformers can be found almost everywhere, from large power plants to small SMPS circuits. There are different types of transformers, the exact operation of which depends on the application, but the main operation of the transformer remains the same. If you look at the transformer circuit, you will see “dots”, for example, symbols located at one end of the transformer winding. This symbol is placed in accordance with the rules of points. But what’s wrong? And for what purpose?

What is a points agreement?
A dot rule is a type of transformer winding polarity marking that indicates the end of a winding compared to another winding. Used to display the phase relationship on the transformer diagram and to place the points on the primary and secondary terminals as shown below.

Transformer Spot Rating

Placing a point near the top of the primary and secondary windings, as shown below, indicates that the current polarity of the primary winding voltage is the same as the polarity of the secondary winding. This means that the phase difference between the primary and secondary windings is zero (phase) and that the direction of the secondary current (Is) and primary current (Ip) is the same.

Primary and secondary phase

However, if the point is reversed (for example from primary upwards, secondary downwards or vice versa), as shown below, this means that the primary and secondary currents and voltages from the phase are from 180 degrees and the primary and secondary gives a bell On. Currents (IP and IS) are in the opposite direction.

Primary and secondary are in phase at 180 °

Thanks to this knowledge of the similarity and the polarity of the transformer, the engineer can now control the fate and change the side of the circuit connected to the transformer terminals, thereby determining the phase relationship upside down. In the example of the above-mentioned non-phase transformer, for example, the secondary side is in phase with the primary side, switching the terminal connection method as shown below.

Take a step back

Why is point to point important?
When studying transformers, it is generally assumed that the voltage and current are in phase for the secondary and primary windings (at least for resistive loads). This assumption is usually based on the belief that the secondary and primary windings of the transformer have the same direction. The phase relationship between the primary and secondary currents and voltage depends on how each winding is wound on the core, i.e. the winding is wound on the core in the same direction as shown by -Dessous. Then the voltage and current on both sides must be in phase.

But this assumption is not always true, because the direction of the windings can be changed, as shown in the image above. If the connections are on the same terminal, the voltage (phase) and secondary phase (Is) are opposite to the primary current.

This phase loss and reverse polarity may seem trivial, but they pose serious problems for the protection, measurement and control of the power supply system. Changing the winding of a measuring transformer can, for example, invalidate a protective relay, give inaccurate measurements of power and energy or indicate a negative power factor during the measurement. It can also cause effective short circuits in the transformer windings and parallel signal circuits, which can lead to interference in amplifiers and speakers or cancel the added signal.

Because the transformer is not transparent, it is impossible to know how to connect the circuit to obtain voltage and current in phase (or phase), which reduces the associated risk. Transformer manufacturers that provide reverse polarity connections and phase loss and winding polarity determination methods have developed standards for marking polarity. “The rule of points.”

Transformer alphanumeric label
In addition to the dot line, other polarity marking methods used in transformers are alphanumeric labels, usually consisting of multiple characters that indicate the polarity of the winding with the letters “H” and “X”. The “1” wires (H1 and X1) indicate where the polarity markers are usually located.