The major difference between a core-type and shell-type transformer is that a core-type transformer has two limbs and two yokes, and a shell-type transformer has three limbs and two yokes. The core-type transformer is used for high voltage. Whereas, a shell-type transformer is used for low voltage.
A transformer is a static piece of equipment that steps up or steps down the voltage as per the requirements. The transmission & distribution of electrical power is not possible without the use of a transformer. Thus, transformers are the backbone of an electrical power network.
We can classify the transformers into two categories.
- Core Type Transformer
- Shell Type Transformer
In this article, we will first understand what is the core type & shell type transformers and, the major differences between them. Now let’s discuss the basics of core type and shell type transformers.
What is a Core Type Transformer?
The magnetic core of the transformer has laminations cut in the L-shape strips. The transformer has two horizontal sections called yokes and two vertical sections called limbs. The winding is placed on the limb. 50 % of the primary and secondary winding is placed on each limb of the core. Thus, the interleaving of the primary and secondary winding reduces the leakage flux.
In the core-type transformer, the low-voltage winding is placed near the core and the high-voltage winding is placed around the low-voltage winding. With this arrangement, the insulation requirement greatly reduces lowering the cost of the transformer.
The core-type transformer can be easily dismantled for repairs and maintenance. Also, the core-type transformer has better natural cooling. However, the core-type transformer draws a large magnetizing current compared to the shell-type transformer.
The core type transformers are widely used in high-voltage applications like power and distribution transformers.
What is a Shell Type Transformer?
A shell-type transformer has two outer limbs and one central limb.
In a shell-type transformer, both primary and secondary winding are placed on the central limb and the outer limbs do not have any winding. The outer limb completes the path of the magnetic circuit. Thus, the central limb carries the entire flux, and the side limbs carry half of the flux. This is the reason that a central limb has two times the area to that of the outer limbs.
In a shell-type transformer, the low-voltage and high-voltage winding are alternatively placed on the central limb. The winding is called sandwich or disc winding because of the placement of winding in the sandwich pattern.
The shell-type transformer provides better support against the electromagnetic forces produced between the current-carrying conductors. Also, the transformer draws very less magnetizing current because of the shorter magnetic path. However, the major drawback of the shell-type transformer is that it has poor natural cooling. and therefore they are suitable for low-voltage applications such as 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 | The magnetic circuit of core type transformer has two vertical sections called limbs, and two horizontal sections called yokes. The winding placed on the limbs is called a core type of transformer. | The magnetic circuit has one central limb and two outer limbs. Both primary and secondary windings are placed on the central limb is called a shell-type transformer. |
| Surrounding type | The winding surrounds the core | The magnetic core surrounds the windings |
| Shape of core laminations | The U and I shaped laminations | Either U and T shaped laminations or E and I shaped laminations. |
| Cross section of core | The cross-section of the core may be square, cruciform two-stepped, or three-stepped. | The cross-section of the core is rectangular. |
| Number of limbs and yokes | two limbs and two yokes. | three limbs and two yokes. |
| Number of magnetic circuits | A single magnetic circuit. | Two magnetic circuits. |
| Type of winding | concentric winding (or cylindrical winding). | sandwich winding (or interleaved winding or disc winding). |
| Placement of winding | The windings are placed on two separate limbs. | Both primary and secondary windings are placed on the central limb. |
| Conductor material | More conductor material for windings. | Less winding conductor material. |
| Iron for core construction | Requires less iron for core construction. | More iron for core construction. |
| Core loss | The total magnetic flux flows through the entire core therefore core loss is more. | Half of the total flux flows in the entire core therefore core loss is less. |
| Copper loss | More copper loss. | Less copper loss |
| Natural cooling | Good natural cooling | Poor natural cooling |
| Repair and maintenance | Repair and maintenance are easy because the transformer can be dismantled easily. | It is difficult to dismantle the transformer. |
| Average winding length | The average length of winding is shorter. | The average length of winding is longer. |
| Average core length | In a core-type transformer, the average length of the magnetic core is longer. | In a shell-type transformer, the average length of the magnetic core is shorter. |
| % Impedance | Lower percentage Impedance | Higher percentage Impedance |
| Problem of tank heating | The zero sequence flux flows through the air gap and links to the tank. The circulating current causes the heating of the tank. | There is no problem with tank heating in the shell-type transformer because it provides the path for zero sequence current. |
| Applications | High voltage and high power | Low voltage and low power |
Conclusion
The major difference between core and shell-type transformers is that the core type transformers are most suitable for high-voltage and high-power applications, on the other hand, the shell-type transformers are most suitable for low-voltage and low-power applications.