Difference between Core Type and Shell Type Transformer

Differences Digital Electronics Electronics & Electrical

A transformer is a static electrical machine that is used to increase or decrease the voltage and current in an AC (Alternating Current) system. Based on construction, the transformers are broadly classified into following two types

Core Type Transformer
Shell Type Transformer

In this tutorial, we will highlight all the major differences between a core type transformer and a shell type transformer. But before discussing the differences, let's understand the basics of core type and shell type transformer, so that it becomes easier to understand the differences between them.

What is a Core Type Transformer?

In the core type transformer, the magnetic circuit of the transformer consists of two sections namely two vertical section called limbs and two horizontal sections called yokes. The half of each winding (primary and secondary windings) is placed on each limb of the core, so that the leakage flux can be minimized.

In the core type transformer, the low voltage winding is always placed next to the core and the high voltage winding is placed around the low voltage winding. It is because, the placement of low voltage winding near to the core reduces the need of insulating material required.

The major advantage of a core type transformer is that it is easier to dismantle for repair and maintenance. Also, the natural cooling in a core type transformer is efficient. However, the core type transformers need higher magnetizing current.

The core type transformers are mainly used in high voltage applications such as distribution and power transformers.

What is a Shell Type Transformer?

A shell type transformer consists of one central limb and two outer limbs as shown in the figure of shell type transformer. In the shell type transformer, both primary and secondary windings are placed on the central limb. The function of the two outer limbs is to complete the path of low reluctance for magnetic flux.

In the shell type transformer, each winding is divided into subsections, where the low voltage winding and high voltage winding subsections are alternatively placed on the central limb in the form of a sandwich. For this reason, this winding is called sandwich or disc winding.

The major advantage of the shell type transformer is that it gives better support against the electromagnetic forces between the current carrying conductors. Also, the shell type transformer provides a shorter magnetic path, thus it requires smaller magnetizing current. However, the major disadvantage of the shell type transformer is that it has poor natural cooling. Consequently, the shell type transformers are preferred in low voltage applications such low power circuits and electronic circuits.

Differences between Core Type and Shell Type Transformers

The following table highlights the key differences between a core type transformer and a shell type transformer-

Basis of Difference	Core Type Transformer	Shell Type Transformer
Definition	A type of transformer in which the magnetic circuit consists of two vertical sections called limbs and two horizontal sections called yokes and the windings placed on the limbs is called core type transformer.	A type of transformer in which the magnetic circuit consists of one central limb and two outer limbs, and both primary and secondary windings are placed on the central limb is called a shell type transformer.
Surrounding type	In a core type transformer, the windings surround the core of transformer	In a shell type transformer, the magnetic core surrounds the windings of the transformer.
Shape of core laminations	The U and I shaped laminations are used for constructing the core of the core type transformer.	The core of the shell type transformer is made up of either U and T shaped laminations or E and I shaped laminations.
Cross section of core	The cross section of the core of a core type transformer may be square, cruciform two-stepped and threestepped.	The cross section of the core of a shell type transformer is rectangular.
Number of limbs and yokes	A core type transformer consists of two limbs and two yokes.	A shell type transformer consists of three limbs and two yokes.
Number of magnetic circuits	Core type transformer consists of a single magnetic circuit.	A shell type transformer consists of two magnetic circuits.
Type of winding	A core type transformer has concentric winding (or cylindrical winding).	A shell type transformer has sandwich winding (or interleaved winding or disc winding).
Placement of winding	In a core type transformer, the windings are placed on two separate limbs. Where, low voltage winding is placed next to the core and the high voltage winding around the low voltage winding.	In a shell type transformer, both primary and secondary windings are placed on the central limb.
Conductor material	Core type transformer requires more conductor material for windings.	Shell type transformer requires less winding conductor material.
Iron for core construction	Core type transformer requires less iron for core construction.	Shell type transformer needs comparatively more iron for core construction.
Core loss	The core loss is more in case of core type transformer because the total magnetic flux flows through the entire core.	The core losses is comparatively less in a shell type transformer because half of the total flux flows in the entire core.
Copper loss	Core transformer has more copper loss.	The copper loss in a shell type transformer is comparatively less.
Natural cooling	In a core type transformer, the natural cooling is relatively more effective due to distributed windings.	Shell type transformer has poor natural cooling because the core surrounds the windings.
Repair and maintenance	Core type transformer can be easily dismantle for repair and maintenance.	The dismantling of a shell type transformer for maintenance is relatively difficult.
Average winding length	The average length of winding is shorter in a core type transformer which results in lower percentage impedance.	In a shell type transformer, the average length of winding is longer which results in higher percentage impedance.
Average core length	In a core type transformer, the average length of magnetic core is longer.	In a shell type transformer, the average length of magnetic core is shorter.
Problem of tank heating	In a 3-phase core type transformer, there is no path for the flow of zero sequence flux as a result it flows through the air gap and causes heating of the tank.	The shell type transformer provides the path for zero sequence current through the lateral lags. Consequently, there is no problem of tank heating in the shell type transformer.
Applications	The core type transformers are used for high voltage and high power transformers.	The shell type transformers are preferred for low voltage and low power applications.

Conclusion

The most significant difference is that the core type transformers are suitable for high-voltage and high-power applications, whereas the shell type transformers are most suitable for low-voltage and low-power applications.l