Core type and Shell type Transformers

Constructionally, the transformers are of two general types, distinguished from each other merely by the manner in which the primary and secondary coils are placed around the laminated core.

The two types are:-

• Core type
•  Shell type

Another recent development is spiral core or wound core type, the trade name being spirakore transformer.

Core type transformer:-

In the core type transformer, the windings surround a considerable part of the core. In the simplified diagram of the core type transformer, the primary and secondary winding are shown located on the opposite limbs of the core, but in actual construction they are interleaved to reduce leakage flux. As in the figure, half the primary and half the secondary winding have been placed side by side or concentrically on each limb, not primary on one limb and the secondary on the other.

The coil used are form-wound and are of the cylindrical type. In small size core type transformers, a simple rectangular core is used with cylindrical coils which are either circular or rectangular form. But for large size core type transformers, round or circular cylindrical coils are used which are so wound as to fit over a cruciform core section as shown in the figure.

The circular cylindrical coils are used in most of the core type transformer because of their mechanical strength.

Since the LV winding is easiest to insulate, it is placed nearest to the core.

Shell type transformer:-

In these case also, the coils are multi-layered disc type usually wound in the form of packets. The different layers are insulated from each other by paper. The complete winding consists of stacked discs with insulation space between the coils- the spaces forming horizontal cooling and insulationg ducts.
The cores and the coils of thge transformers must be provided with rigid mechanical bracing in order to prevent movement and possible insulation damage. Good bracing reduces vibration and the objectionable noise-a humming sound during operation.