The main elements of a transformer are ; two coils and a laminated steel core. The two coils are insulated from each other as well as from the steel core. In order to keep the core loss to a minimum, the core of a transformer is built up of thin laminations of highly permeable ferromagnetic material such as silicon sheet steel. Silicon steel is used to reduce hysteresis losses. The lamination’s thickness varies from 0.014 inch to 0.024 inch. A thin coating of varnish is applied to both sides of the lamination in order to provide high inter lamination resistance.
Basically two types of construction are in common use for the transformers: core type and shell type.
Core Type Transformers
In a simple core type transformer the magnetic core is built up of laminations to form a rectangular frame. The laminations are cut in the form of L-shape strips. In a core-type transformer, each winding may be evenly split and wound on both legs of the rectangular core. While placing these windings, an insulation layer is provided between core and lower winding and between the two windings. To reduce the insulation, low voltage winding is always placed nearer the core.
Core type transformers are used for High voltage applications like distribution transformer, power transformer etc.
Shell Type Transformers
In case of shell type transformer, each laminations is cut in the form of long strips of E’s and I’s. In order to avoid high reluctance at the joints where the laminations are butted against each other, alternate layers are stacked differently to eliminate continuous joints.
In such type of transformer, the core has three legs or limbs. The winding is placed on the central limb so that, central limb carries whole of the flux, whereas the central limbs carry half of the flux. The width of central limb is about double of the outer limbs. Similarly, core type transformer, low voltage winding also placed nearer the core and high voltage winding is placed outside the low voltage.
Shell type transformers are used in Low voltage applications like transformers used in electronic circuits and power electronic converters etc.