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Transformer Construction and Types of Cores

Basic Transformer Construction

A transformer consists of two coils mounted in fixed positions relative to each other. One coil is supplied by an alternating current. As the current alternates, the magnetic field it produces also alternates. This alternating field induces an alternating current in the second coil, which supplies a load. A simple transformer construction consists of two coils wound on a closed iron core. The winding connected to the supply is called the primary winding. The winding connected to the load is called the secondary winding.

If the transformer is used for increasing the potential difference, it is called a step-up transformer. In this kind of transformer, the low-voltage winding is the primary. If the transformer is used for decreasing the potential difference, it is called a step-down transformer. In this case, the high-voltage winding is the primary.

Construction of transformer
Simple Transformer Construction


Standard markings are used for transformer terminals. The terminals of the high-voltage winding are marked H1, H2, and so on. The low-voltage terminals are marked X1, X2, and so on. The number of turns in each winding is abbreviated TH or TX.


Commercial Transformer Construction

The windings of commercial transformers are not placed on separate “legs” of the core. The transformer can be made more efficient by placing both windings on the same leg, or by placing both windings on both legs. Placing the two windings together enables more of the magnetic field of the primary to induce current in the secondary.

There are two ways to combine the primary and secondary windings on a single leg of the core.

  • The windings may be cylindrical and wound one over the other with insulation between them
  • The windings may be shaped like pancakes and stacked alternately on the core
 transformer construction of winding
Windings construction of transformer

Notice the placement of the two windings. The low-voltage transformer winding is wound next to the core. The high-voltage winding is wound over the low-voltage winding.

The construction of transformers varies widely according to their size and use. One variation is the arrangement of the iron core and the windings. There are two basic construction arrangements

  • the core arrangement
  • And the shell arrangement.
    Transformer construction of cores
    Transformer Cores Types

The core-type transformer has a core made in the shape of a hollow square. Half the windings are placed on each leg of the core. The core of the core-type transformer passes through the center of the windings, but does not surround the windings. By contrast, the shell-type transformer has a core that surrounds the windings as well as passing through the center.

Cores Construction of Transformer

Typical transformer cores are made of thin steel sheets, called laminations. The laminations are coated with varnish or with some other insulating material and stacked together to form the core. The number of laminations stacked together determines the thickness of the transformer core. The purpose of this laminated construction is to reduce the eddy currents that waste energy in a transformer.

Another kind of core is made of steel strip or ribbon wound in a spiral on a form. When this type of core is used, power losses are reduced, tighter construction of the transformer is possible, and the magnetic characteristics of the core are improved.

Read more about Transformer losses

Coils Construction of Transformer

Transformer coils are usually made of copper because of its low resistance. They are designed to fit the required number of turns into a minimum amount of space. The cross-sectional area of the conductor must be large enough so that the conductor can carry the maximum required current without overheating. The coil must also be designed to leave enough space for insulation and for cooling ducts.

Round copper wire is used in transformer coils that carry low current. Round wire larger than number 10 is generally not used because of the large amount of wasted space between wires. Either square or rectangular wire ribbon is used instead.

The wire is insulated with cotton, enamel, paper, or a combination of materials. The insulation must have the proper dielectric strength for the potential difference between it and other parts of the transformer.

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