A transformer is constructed with a primary winding, a secondary winding and a magnetic core. Those windings are mounted on each side of the core. This magnetic core carries the useful magnetic flux or in other words this core provides a low reluctance path for the magnetic circuit. A transformer core effectively confines the magnetic flux and in this way it helps to bound those fluxes in a definite path. The core also provides strong mechanical strength for the transformer windings. This is the basic construction of any transformer. The details transformer construction is elaboarted in the following articles.
Transformer core construction
Transformer cores are build up with several thin metallic sheets, and these sheets are termed as laminations. Generally, very high grade silicon steel is used for this purpose. A collection of several thin laminations are tightly clamed and forms the complete magnetic core. Actually, a good transformer design approaches to profitable operation of transformer. So, apart from providing necessary flux carring path, a well designed laminated silicon steel core effectively reduces the hyteresis and eddy current loss of a transformer.
In the magnetic core, laminations are insulated from each other by a heat resistant enamel insulation. As the primary winding and secondary winding both are mounted on each side of this core, so there must be some specific method of mounting them. In transformer, the magnetic core is constructed with the a combination of vetical legs and horizontal sections. The vertical legs are called limbs and the horizontal sections are called the yokes. You can see in the picture how a core of a core type transformer is constructed with limb and yoke. As per the construction, transformers are classified in two categories: namely core type transformer and shell type transformer. The details of these two types of transformer constrution are described in the following articles.
Core type transformer construction
We know that, every transformer has two sides, namely HIgh Voltage Side (h.v. side) and Low Voltage Side (l.v. side). They are nothing but high voltage winding and low voltage winding. Actually, the arrangement of these coils is quite different in core type construction and shell type construction Mainly ‘U’ and ‘I’ type laminations are used in core type transformer. These alphabets are actually used to represent the geometyrical structure of the lamination pieces. In the picture, you can see how the ‘U’ shaped and ‘I’ shaped laminations are stacked together with each other and forms a complete piece of lamination sheet. In core type transformer construction, the magnetic core is constructed with two vertical legs and two horizontal sections. That means, this construction has two limbs and two yokes.
Now we will discuss how the h.v. winding and l.v. winding are placed into the core. Initially, the part of prewound low voltage winding is placed around the transformer core and thereafter a strong insulation is provided around the winding. Then the part of prewound high voltage winding is placed around the low voltage winding. This arrangement effectively reduces the required insulation material. We can tell that, both high voltage winding and low voltage winding are placed in concentric coil arrangement. So, the winding style which is used in core type construction is termed as concentric winding. In the diagram, the green winding is the high voltage winding and the grey winding is low voltage winding and you can see that, equal part of both high voltage and low voltage windings are placed around two limbs. Therefore, a uniform magnetic flux path is offered by the core itself and the entire magnetic flux φ is distributed over the core.
- The advantage of core type construction is it is very easy to reassemble in the time of repairing.
Shell type transformer construction
Now we will know about the shell type transformer construction. In this construction, two yoke(horizontal section) and three limbs (vertical legs) are used. The middle limb is termed as the central limb. The core which is used in shell type transformer construction is basically made of either ‘U’ and ‘T’ or ‘E’ and ‘I’ shaped laminations. These two types of arrangement are described in the diagram.
Basically, Both high voltage and low voltage winding are alternately wound only on the central limb. Initialy, the prewound low voltage winding is placed around the central limb and thereafter the high voltage winding is placed around the low voltage winding. In this way, a sandwich structure is formed. So, the style of winding which is used in shell type transformer is termed as sandwich winding. The effective magnetic flux φ is equally divided due to the presence of the additional central limb. The total amout of flux φ is shared as φ / 2 among the two sides of the central limb. This type of construction is very economical because here the required conductor material( copper winding) is less and this leads to a effective reduction in transformer cost.
- There are some advantages of using this shell type construction as compared to the core type construction. The shell type construction provides superior support against electromagnetic forces between the current carring conductors. But as the shell type construction offers shorter path for flux flow, so the magnetizing current is comparatively lesser than the core type construction. In this way, the construction of transformer is discussed and hopefully you understood all the technical details of transformer construction.