Transformers - continued.

Main Winding Types

This figure illustrates how windings are wrapped around the core to form a transformer.

Each of the windings, as shown, consists of two windings: primary and secondary one wound over the other. The portions of the figure that show more than one winding indicate windings for each phase. In general, ac systems are either one-phase of three-phases. Thus, in the illustration where only one winding is shown - it is really representing a single-phase transformer - primary and secondary windings wound one over the other.

Similarly when three windings are shown - it is actually three individual phases - each winding consisting of the primary and secondary windings - one wound over the other.

The graphic also shows two main core designs: Core and Shell. The differences are illustrated in the graphic. Some additional differentiating comments are noted below:

Core Type Transformer Characteristics

Core has two limbs.
The core has only one magnetic circuit.
The coil has limited mechanical protection
Better cooling due to exposure of large surface area.
Natural cooling is always there.
Maintenance of such transformers is difficult due to access problems.
The winding surrounds a considerable part of core.

Shell Type Transformer Characteristics

It has two magnetic circuits.
Core has three limbs.
Better mechanical provided to the core.
Forced cooling is not very effective.
Not possible to provide natural cooling to all current carrying coils.
This transformer is not easy to repair.
Core surrounds a large part of the windings.