Core type Transformer and Shell type Transformer

Transformers have mainly three parts—

1. Core,
2. Primary winding ,
3. Secondary winding

First, we describe core of the transformer.-

Core

The core is the vital part of the transformer which provides a continuous magnetic path a minimum of air-gap. The core is construction with laminated steel sheet which is high silicon content, high permeability, and low hysteresis loss.
For minimizing the eddy current loss, the steel sheet is laminated each other by a light coat of core-plate varnish or by an oxide layer on the surface. The thickness of lamination varies from 0.35 mm for a frequency of 50 Hz to 0.5 mm for a frequency of 25 Hz.
The core lamination joined in the form of a strip which is shown infig.-2. For avoiding narrow gaps in between layer, the laminated layers are staggered in order. And these staggered joints are said ‘imbrieated’ joint. Constructionally , the transformer are two types. The two types are—

1. Core type.
2. Shell type.

In both core is shell-type transformer, the laminations are in the following types. i.e L, E, &I types which are shown in fig. To avoid high reluctance at joints, the laminations are butted each other which is shown in fig.

Core-type Transformer

In this Core type transformer, the windings surround a considerable part of the core. Generally, the core of the core-type transformer is a rectangular shape and the coils are both circular or rectangular inform and the windings are located on the opposite limbs of the core which is shown in fig. In most of the large-size core –type transformer, round or circular cylindrical coils are used because the mechanical strength of circular cylindrical coils is high.

And these cylindrical coils are wound in a helical layer with different layers insulated from each other by paper, cloth, micarta board or cooling ducts. For reducing leakage flux, and H.V & L.V winding are placed one after another separating with high insulation cylinder on fuller board which is shown in fig.

Shell type Transformer

The shell type transformer is a simple rectangular form and the core surrounds the considerable portion of the windings which is shown in fig. Both the primary & secondary windings are placed in the one limb. And the coils are wound in from of multi-layer disc type. The different layers of the multi-layer disc are insulated from each other by paper.