The windings are the only electrical path within a transformer: one winding receives energy from the source, another delivers it to the load, and between them there is nothing but magnetic flux. Here we explain the function of each winding and why they are usually made of copper.
The Primary and Secondary Windings
- Primary Winding: The terminal connected to the supply source, which draws power from the grid and produces the magnetic flux.
- Secondary Winding: The terminal on which the induced voltage appears and which feeds the load.
- The naming follows the supply side, not the physical position of the winding; the same transformer may have its role reversed if the supply direction is reversed.
Why Copper?
The winding is usually made of copper because of its high conductivity, which reduces resistance and therefore the copper loss I²R and the resulting heat. Aluminum is used in some designs for economic or weight reasons, with a larger cross-section to compensate for its lower conductivity.
What Threatens the Windings?
| Threat | Effect | Detection Method |
|---|---|---|
| Excessive heat | Degradation of conductor insulation and inter-turn short circuits | Temperature monitoring + DGA analysis |
| Moisture | Weakened insulation and earth leakage | Insulation resistance test |
| Poor connection or damaged conductor | Increased resistance and localized heating | Winding resistance test |
| Mechanical short-circuit forces | Deformation and displacement of the turns | Specialized tests and comparison with reference values |
Sample answer: The primary is the winding connected to the supply source, and the secondary is the one connected to the load; the naming follows the direction of power flow, not the voltage level. In a step-up transformer, the primary is the lower-voltage side, while in a step-down transformer it is the higher-voltage side, so the primary is not necessarily the high-voltage side.
Assuming the primary winding is always the high-voltage side. The naming is functional, based on the supply side, and a step-up transformer is a direct example that contradicts this assumption.
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