Dot Convention on Electrical Transformers is an important concept that should be understood by every one in power industry. In this article Electrical Engineering XYZ shares what is meaning of dot convention on Electrical Transformers.
In order to denote phase relationships, transformer manufacturers have come up with a sort of polarity marking standard. It is called the dot convention, and is nothing more than a dot placed next to each corresponding leg of a transformer winding:
Similar Placement of Dots
The similar placement of these dots next to the top ends of the primary and secondary windings tells us that whatever instantaneous voltage polarity seen across the primary winding will be the same as that across the secondary winding. In other words, the phase shift from primary to secondary will be zero degrees.
Different Placement of Dots
On the other hand, if the dots on each winding of the transformer do not match up (as shown above), the phase shift will be 180o between primary and secondary, like the waves show above.
Typically, the transformer will come with some kind of schematic diagram labeling the wire leads for primary and secondary windings. On the diagram will be a pair of dots. Sometimes dots will be omitted, but when “H” and “X” labels are used to label transformer winding wires, the subscript numbers are supposed to represent winding polarity. The “1” wires (H1and X1) represent where the polarity-marking dots would normally be placed.
Further Reading/Handbooks on Transformers:
- The Ultimate Guide to Transformers
- The Transformer Polarity Handbook
- Electrical Transformers Handbook
- What does 3000:1 indicates on Current Transformer
- Current Transformer
- Rating Factor of Current Transformers
- Transformer’s Power and Current Capacity
- Instrument Transformer Connections Handbook PDF Download
Text extracted from: Tony R Kuphaldt’s Electrical Transformers Handbook under CC BY 4.0 License