Both Additive and Subtractive Transformer terms are related to polarity concept.
Transformer polarity is very important in the electric power industry, and so terms have been coined for different polarity orientations of transformer windings. If polarity dots for primary and secondary windings lie on the same physical side of the transformer it means the primary and secondary windings are wrapped the same direction around the core, and this is called a subtractive transformer. If polarity dots lie on opposite sides of the transformer it means the primary and secondary windings are wrapped in opposite directions, and this is called an additive transformer. The terms “additive” and “subtractive” have more meaning when we view the effects of each configuration in a grounded AC power system. The following examples show how voltages may either add or subtract depending on the phase relationships of primary and secondary transformer windings:
Transformers operating at high voltages are typically designed with subtractive winding30 orientations, simply to minimize the dielectric stress placing on winding insulation from inter-winding voltages. Instrument transformers (PTs and CTs) by convention are always subtractive.
Article extracted from Transformer Polarity Handbook from Tony R Kuphaldt’s Lessons in Electrical Circuits.
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Also see:
- Why PT Secondary Winding should never be Short-Circuited
- Why CT secondary winding should never be open-circuited
- Basics of Potential Transformers
- Basics of Current Transformers
- What is Basic Difference between Resistors and Resistivity