Transformer Dot Convention 

Created: 5/20/2020Last Updated: 07/06/2023


The dot convention is a notation used to indicate the phase relationships between voltage and current in primary and secondary windings of a transformer. Depending on how the windings are wound around the transformer core, the phase relationship can be either in-phase or 180 degrees out of phase.

Rule of Thumb

Detailed Explanation

 In a transformer, current flows into the primary winding to generate a magnetic flux. This flux is then coupled and "extracted" by the secondary winding to deliver power to the load. The secondary winding extracts the magnetic flux by opposing any change in flux and generating a current in the opposite direction, following Lenz's Law.

The dot convention signifies the placement of the dot end for each winding terminal based on this principle. When currents flow into the dot end of both the primary and secondary windings, the resultant flux inside the transformer adds up, indicating that the individually generated fluxes flow in the same direction.

It is important to note that the actual operational current direction of a transformer follows Lenz's Law, not the dot convention. The dot convention serves as a visual aid for engineers to quickly understand the voltage and current phase relationship.

Reference and Further Reading: