Dry Transformer Core Design
There are two main types of transformer core; these are the shell type and core type. The most common transformer type is the core type transformer, whilst shell type transformers are often used when space is restricted; trains are often fitted with shell type transformers due to height and width restrictions. The type of transformer shown in this 3D model is a core type transformer.
Core Type Transformer - Most common type of transformer core. The windings wrap around the core.
Shell Type Transformer – Less common of the two types of transformer cores. The core encloses the windings.
The core consists of yokes, limbs and windows. The yokes are the top and bottom sections of the core onto which the limbs are connected; the coupling of the yokes to the limbs connects them magnetically. The rectangular gaps between the limbs and yokes are called windows. Normally the limbs are vertically orientated and thus the whole transformer is vertically orientated. The yoke and limbs are connected together at a 45º angle; this gives a large contact surface area between the two parts. The core shown in this 3D model is of the three limbed three phase type. As the core yoke and limbs are connected with each other, the core must be able to carry the entire flux throughout all parts.
It is possible to construct a five limb three phase transformer in order to reduce the total height of the core; this height reduction is made possible due to the increase in flux return paths (two additional limbs). The outer limbs of a five limb core are typically only half as thick in diameter as the inner/main limbs; this is because they are required to carry less flux than the main limbs. Despite the advantages in height reduction, five limb cores require more material and are thus more expensive than their three limb counterpart; they are thus only used when space is limited or a reduced transport profile is required.
Three Limb Core – Flux distributed evenly throughout all limbs. More common than five limb cores and cheaper to construct.
Five Limb Core – Flux not distributed evenly throughout the limbs. More expensive than three limbs cores, but allow the construction of a transformer with a lower height, this may be necessary if height restrictions apply during transport or installation.
It is typical to construct the lower yoke and limbs, then insert the windings into the windows and construct the upper yoke to close the window.
Core Designs and Applications
The design of a core type transformer depends upon its application. Most systems require voltage transformation across all three phases and these phases will be encased as part of a single transformer. There are other instances though where it is not desirable to encase all three phases within a single unit; this is particularly relevant for large transformers where failure of a single phase would render the entire transformer inoperable e.g. a large power station with a single step-up transformer. In these cases, it is advantageous to have one transformer per phase and a spare transformer held on site (the single phase spare is smaller and less expensive than its three phase counterpart).