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.
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