Transformer Construction | Core type and Shell type Transformer

A transformer is  constructed in such a way that transformer  construction cost and operational losses are minimum. Transformer construction can be divided into four parts as :-

1.Type :- There are two types of transformers, the core type and shell type transformer as shown. The vertical portions are called limbs and top and bottom portions are called yokes. 
Core type has 2 limbs while shell type has 3.  In the core type, the windings surround a considerable part of steel core. In the shell type, the steel core surrounds a major part of the windings.

2.Transformer core :- The magnetic core used is a stack of thin laminations electrically insulated from one another by thin layers of varnish. The laminations are made as thin as possible to reduce eddy current loss as  eddy current loss is directly proportional to the square of thickness (t) of lamina assuming all other factors constant.

eddy current loss,  P_e = K_e f² B_m² t²

Further, K_e  is a constant depending on the resistivity of the core material, therefore core material with good electrical conductivity must be used.

This also explains why cores are laminated in transformer ?

The core is assembled such that the butt joints in the adjacent layers are staggered as shown.

This staggering of butt joints avoids continuous air gap as the flux can take path from neighbouring layer at the joints and at the same time staggering maintains mechanical strength of the core. Air gap also avoids magnetic saturation of the core.

For reducing hysteresis loss, core material with low core loss i.e small B-H curve area and high permeability , like silicon steel or cold rolled grain oriented steel is used.

3.Windings :- Flux linking only one winding is called leakage flux. A reduction in this leakage flux is desired as it improves the efficiency considerably. Consequently, an effort is made to reduce it. 

In core type transformer, this is achieved by placing half of each windings (Low voltage winding and high voltage winding) on each limb. Whereas in shell type transformer, the Low voltage and high voltage windings are sandwitched.

The windings material must have low resistivity to have low copper losses. In large power transformers, cores have circular cross-section as, for a given area circle has smallest perimeter, thereby reducing amount of copper required and hence transformer construction cost is reduced.

4.Cooling :- Cooling is a big problem especially when working with high power as it may damage insulation. Low power transformer are air cooled whereas high power transformers are oil cooled. 
why oil is used in transformers and why not water ? !! 
Its because oil has superior properties as :-

1.Greater specific heat capacity so that it can absorb more quantity of    heat per degree rise in temperature.

2.Greater conductivity thus, heat is transferred out quickly.

3.Breakdown strength of oil used is about six times that of water           therefore, provides very good insulation and ensures increased      reliability at high voltages.

As you can see its single phase transformer construction but the underlying principle is same for three phase transformer. 



Related concepts :-
       (1) Transformer construction images
       (2) Open Circuit and Short Circuit test on transformer
          (3)  Transformer working Principle
          (4)  Voltage regulation of transformer
          (5)  Efficiency of a Transformer