A magnetic core and windings are two basic parts of a transformer. The windings are wound over the core and the core encircles the windings. Based on the way the core and the winding are made, transformers are classified into two types: Core type and shell type transformers. Both of them refers to the arrangement of core with reference to the windings. In this article, let us compare both types.
Core type transformer
Core type transformers have a rectangular core made of laminated ferromagnetic steel. The windings are wound around the limbs of the core. The low voltage winding is made nearer to the core the high voltage winding surrounds it. The LV and HV winding are insulated from each other and the core itself.
Shell type transformer
In shell type transformers, the windings are wound around the middle limb as shown in the figure above and the core surrounds the winding. The primary and the secondary winding are divided into a number of coils and are arranged alternatively.
Difference between core type and shell type transformers
The difference between the core type and shell type transformers are illustrated in the above figures.
As discussed earlier, in core type transformers, the winding surrounds the core whereas in shell type transformers the winding surrounds the middle limb of the core and side limbs surrounds the windings. The core type transformers are easier to dismantle and repair.
Short circuit withstand ability
The force between the primary and secondary windings is directly proportional to the current flowing through them. When the secondary is short circuited with the primary energized, since the direction of current is these winding are in opposite direction, there exist a force of repulsion between them.
In core-type transformers, as the LV winding surrounds the HV windings, the inner winding suffers a force crushing it towards the core and the outer winding is pushed outwards. Hence the windings are susceptible to damage under short circuit conditions. On the other hand, the shell type transformers have a greater capacity to withstand the short circuit conditions.
The leakage reactance of a shell type transformer is lesser than that of a core type transformer.
Ease of maintenance
The windings in a core type transformers can be accessed easily. Therefore it is easy to dismantle for maintenance. But when it comes to shell-type transformers, since the coil is surrounded by the core, inspection and repairing of coils becomes difficult.
In core-type transformers the winding is more exposed. Hence cooling is better in the winding than in the core.
Whereas in the shell type transformers the core is more exposed than the windings. Therefore, cooling is better in the core than in the winding.