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All electric transformers regardless of their size, have three fundamentals parts:
- High voltage winding:
It is an insulated coil of copper wire, with a low cross section built to receive or deliver the highest voltage of the electric transformer.
- Low voltage winding:
It is an insulated coil of copper wire, with a bigger cross-section built to receive or deliver the lowest voltage of the electric transformer.
When the electric transformer is installed, the winding that is connected to the source is called primary winding and the winding that is connected to the load is called secondary winding.
- The Core:
The core is built with magnetic plates with a high proportion of silicon (4%); the core is also grain-oriented and has very low hysteresis losses. These plates have on one side an insulation that was impregnated in the metallurgical process.
There are two general configurations:
a - The core-type configuration.
The windings are wrapped on each leg (primary and secondary). The magnetic flux flows through the legs.
b - The shell-type configuration.
The center leg is twice the cross-section of either the two outer legs. The magnetic flux is divided equally into two returning path (the two outer core legs).
 
The rectangular legs are used in small transformers (up to 50KVA).
In more powerful electric transformers a staggered core section is used to increase the cooling surface. There are some special section core arrangement in order to avoid the overheating (look at the picture)
The picture right below shows the outer part of a transformer. On the picture there are the names of the different visible parts.
Internally, it is possible to see the core, the primary and secondary windings with their respective terminals. All of this is submerged in a dielectric (electrical non-conductive) oil that serves as a heat sink.
 
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