Transformer Dot Convention

Created: 5/20/2020Last Updated: 5/20/2020

Definition: A dot convention indicates voltage and current phase relationships between primary and secondary winding. Depending on the how the winding is wrapped around the transformer core, the phase relationship could be in phase or 180 out of phase.

Rule of thumbs

    1. Current goes into the primary coil dot end and comes out of secondary coil dot end.
    2. Voltage polarity are equal at the dot end point of each winding.

Detailed Explanation

    • Current goes into the primary winding to generate a magnetic flux, and then that flux coupled and "extracted" by the secondary winding to deliver power to the rest of the load.
    • The way the secondary winding "extracts" magnet flux is by opposing the change in magnetic flux by generating a current in the direction that would produce magnetic flux is the opposite direction (Lenz's Law).
    • The dot convention indicates the placement of dot end for each winding terminal based on this principle:
      • When currents flow into the dot end for both primary and secondary winding, the resultant flux inside the transformer must adds up meaning the each individually generated flux flows in the same direction.
      • Note: The actual operation current direction of a transformer obey's Lenz's law not by dot convention. Dot convention provides a quick visual for engineers to understand voltage and current phase relationship.

Reference and Further Reading:

"Dot Convention in transformers",