Core Type Transformer – Construction & Working

Core type transformer has a magnetic core, and the primary and the secondary winding around the core. The core type transformer can be categorized according  to the shape of the magnetic core. On the basis of core construction, we classify the transformer as core type and shell type.

We can categorize the transformer into core type and the shell type. In a core type transformer the winding encircles the core whereas in a shell type transformer the core encircles the winding. The winding encircles the core of a transformer. The Single phase transformer has two limbs and three phase transformer has three limbs.

Types of Core Type Transformer

There are two types of core type transformer.

  • Single Phase Transformer
  • Three Phase Transformer

The single phase transformer accepts single phase AC voltage and output single phase AC voltage according to turns ratio of the transformer. Three phase transformer converts three phase AC voltage into three phase AC voltage according to turns ratio. Three phase transformer is most suitable for transmission and distribution purpose.

types of core type transformer

The core of a single phase transformer has a single window. The core of the single phase transformer has two limbs.

Core Type Transformer diagram

Both HV and LV winding is wrapped around  both the  limbs. The LV winding is placed near to the core and then the HV winding is placed after LV on the same limb. This way the  transformer size becomes compact due to less insulation requirement. The transformer cost reduces with this arrangement of placing the LV and HV winding on the same limb.

The main disadvantage of core type transformer is that all the flux produced in the primary does not link to the secondary and some parts of the flux does not link. Thus, the leakage flux in the core type transformer is more as compared to the flux utilization in shell type transformer.

However, the core type arrangement is most suitable for the large rating transformer because of easier access of the winding during maintenance. If there is fault in the inner winding, we can easily remove the the inner winding after removing the outer winding.

Read More: Shell Type Transformer- Its construction & Working

Core & Winding Construction of Core Type Transformer

There are different types of lamination suitable for the core type transformer.

EI types lamination

L-L and U-I stamping lamination when connected together form the requires core shape.

The designer selects the transformer core shapes according to the rating of the transformer. In a small rating transformer, the winding is square or rectangular in the shape therefore, the square or rectangular cross section core is suitable for the core type transformer.

The small rating transformer has lower current carrying capacity conductors and it is easy to wrap the conductor in a square or rectangular shape. The square or rectangular core is economic for the small rating transformer.

For large rating transformer, the thick winding conductor is suitable for carrying the more current. It is difficult to bend the thick conductor in a square or in a rectangular shape. The round cylindrical shaped winding is the best choice for the large rating transformer in view of optimize use of copper conductor.

However, the significant amount of space between the winding and core is unused when the round cylindrical shaped winding on a square cross sectional core limb is used. To reduce this unused space, the stepped cross sectional core is used. The lamination of different shapes are staged to form nearly circular cross sectional core.The cross section of the core may be one stepped, two- stepped or multi stepped.

Application of Core Type Transformer

These type of transformers are best suited for the high voltage applications like power transformer, distribution transformer and auto-transformer.The high voltage  transformer works on high flux density. The thicker core used for the transformer cause lower core loss.

Also, high voltage transformer requires more amount of insulation.It is easier to put insulation in core type transformer.The low voltage winding also provide the insulation between high voltage winding and core. We use  this type of transformer to step up the low voltage to high voltage, and to step down the high voltage to low voltage.

Advantages and Disadvantages

The pros and cons of core type of transformer are as follows:


  • Robust – The symmetrical core protects the cylindrical shaped winding. The core and winding does not affect with vibration caused during transformer charging.
  • Low Iron Loss – The lamination stacking in the core reduces the iron loss.
  • Suitable for High Frequency application-It has low eddy current loss. Therefore, core type transformer is suitable for high frequency application.


  • Not good for outdoors use- The external environmental factors effects the internal parts of the transformer. Therefore, core type transformer is not suitable for outdoors applications, in comparison to dry type transformer.
  • Noisy- It produce the audible noise due to magnetostriction force in the core.

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