Two Types of Transformer in Common Use.
When the magnetic circuit takes the form of a single ring encircled by two or more groups of primary and secondary windings distributed around the periphery of the ring, the transformer is termed a core-type transformer.
When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic material distributed around its periphery, the transformer is termed a shell-type transformer.
Actually, core-type (or “core-form”) in U.S. Power-transformer engineering usage means that the coils are cylindrical and concentric (the outer winding over the inner) whereas shell-type (or “form”) denotes large pancake coils that are stacked or interleaved to make primary-secondary (P-S) groups.
Except for certain extremes of current rating, the choice between the core- and shell-type construction is largely a matter of manufacturing facilities and of individual preference.
Core-form transformer characteristic features are a long mean length of magnetic circuit and a short mean length of windings. The three-leg (one active leg) and four-leg (two active) construction of single-phase cores and the five-leg (three active) construction of 3-phase cores are used to reduce overall height.
In these cases, the core encloses the cylindrical windings in a similar fashion to the shell-form construction. The simple concentric primary (inside) and secondary (outside) winding arrangement is common for all small- and medium-power transformers.
However, large MVA transformers frequently have some degree of interleaving of windings, such as secondary-primary-secondary (S-P-S). The core-form construction can be used throughout the full size range of power transformers.
Shell-form transformer characteristic features are short mean length of magnetic circuit and long mean length of windings. This results in the shell-form transformer having a larger area of core and smaller number of winding turns than the core form of same output and performance.
Also, the shell form would typically have a greater ratio by weight of steel to copper.