What is a Transformer Core?

A transformer core is a structure of thin laminated sheets of ferrous metal (most commonly silicon steel) stacked together, that the primary and secondary windings of the transformer are wrapped around.

Parts of the Core

A transformer core is composed of limbs and yokes that are joined together to form a single structure around which the coils are placed. The manner in which the respective yoke and limbs join together will depend on the type and design of the core.

Limbs

In the above example, the limbs of the core are the vertical sections which the coils are formed around. The limbs can also be located on the exterior of the outermost coils in the case of some core designs. The limbs on a transformer core can also be referred to as legs.

Yoke

The yoke is the horizontal section of the core which joins the limbs together. The yoke and limbs form a pathway for magnetic flux to flow freely.

What does a transformer core do?

The transformer core ensures efficient magnetic coupling between the windings, facilitating the transfer of electrical energy from the primary side to the secondary side.

When you have two coils of wire side by side and you pass an electric current through one of them, an electromagnetic field is induced in the second coil, which can be represented by several symmetrical lines with direction emanating from north to south pole–called lines of flux. With the coils alone, the path of the flux will be unfocused and the density of the flux will be low.

Adding an iron core inside the coils focuses and magnifies the flux to make for a more efficient transfer of energy from primary to secondary. This is because the permeability of iron is much higher than that of air. If we think of electromagnetic flux like a bunch of cars going from one place to another, wrapping a coil around an iron core is like replacing a winding dirt road with an interstate highway. It’s much more efficient.