Core Type and Shell Type Transformers

Depending on the type of construction used, the transformers are classified into two categories.

  1. Core type Transformer
  2. Shell type Transformer

Difference between Power Transformer and Distribution Transformer

Core Type Transformer

In core type construction, as shown in the figure, the coils are wound around the two limbs of a rectangular magnetic core.
Core Type Transfomer
Core Type Transfomer
Each limb carries one half of the primary winding and one half of the secondary winding so as to reduce the leakage reactance to the minimum possible. 
The LV (low voltage) winding is wound on the inside nearer to the core while the HV (high  voltage) winding is wound over the LV winding away from the core in order to reduce the amount  of insulation materials required.
Small transformers may have cores of rectangular or square cross section with rectangular or circular coils but it is wasteful in case of large capacity transformers.

In case of large sized transformers stepped cruciform core with circular cylindrical coils is employed, as illustrated in figure below

Stepped cruciform core employs laminations of different sizes.

Though the cost of manufacturing of such a cruciform core is much greater, but the circular coils that are used are easier to wind and provide more mechanical strength, especially when short-circuit occurs.

Other advantages of using cruciform core are,high space factor and reduced mean length of turns resulting in reduced copper loss. 

Shell type Transformer

In shell type construction, the coils are wound on the central limb of a three limb core. The entire flux passes through the central limb and divides into two parts going to side limbs, as shown in figure.
Consequently, the cross-sectional area (and hence width) of the central limb is twice of that of each of the side limbs. 

Sandwich type winding is used in such a construction. 

Comparison of Core & Shell Type Transformers

Core TypeShell Type
Easy in design and construction.Comparatively complex
Has low mechanical strength due to non bracing of windingsHigh mechanical strength.
Reduction of leakage reactance is not easily possible.Reduction of leakage reactance is highly possible
The assembly can be easily dismantled for repair work.It cannot be easily dismantled for repair work.
Better heat dissipation from windings.Heat is not easily dissipated from windings since it is surrounded by core.
Has longer mean length of core and shorter mean length of coil turn. Hence best suited for EHV (Extra High Voltage) requirements.It is not suitable for EHV (Extra High Voltage) requirements.

Application of Core & Shell Type Transformers

In shell type transformers advantage is gained through the core being used to protect the windings from mechanical damage. 
The shell type construction gives better support against electromagnetic forces between current carrying conductors. These forces are of considerable magnitude under short-circuit conditions. 
The shell type construction is commonly used for small transformers where a square or rectangular core cross-section is suitable for economic consideration. 
The shell type construction needs more specialized fabrication facilities than core type,while the latter offers an additional advantage of permitting visual inspection of coils in the-case of a fault and ease of repair it substation site. 

For these reasons, the present practice is to use core type transformers in large high voltage installations.

Why Core Type in HV and Shell Type in LV?

Core type transformers are popular in High voltage applications like Distribution transformers, Power transformers, and obviously auto transformers. 

Reasons are, High voltage corresponds to high flux. So, for keeping iron loss down we have to use thicker core. So core type is better choice. At high voltage we require heavy insulation. In core type winding, putting insulation is easier. In fact LV(low voltage) winding it self acts as an insulation between HV (high voltage) winding and core.

Where as, Shell type transformers are popular in Low voltage applications like transformers used in electronic circuits and power electronic converters etc. 

Reasons are, at low voltage, comparatively you require more volume for the copper wires than that of iron core. So the windows cut on the laminated sheets have to be of bigger proportion with respect to the whole size of the transformer. So, shell type is a better choice.

In shell type we don’t care about the insulation much and insulation is thin and light. So we can put the winding anyway we want in the shell.

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