The Function of the Iron Core in a Transformer

If we wound two facing coils in air, only a tiny fraction of the flux would transfer between them. The iron core is what makes the transformer practical: a high-speed path for the magnetic flux that links the two windings with high efficiency.

The Basic Function

The iron core provides a low-reluctance magnetic path (high permeability) for the flux generated by the primary winding, directing it to link the turns of the secondary winding with as little leakage as possible. Without the core, the flux lines would scatter into the air and the efficiency of energy transfer would collapse.

Why Silicon Steel?

• High magnetic permeability: it conducts flux far more easily than air.
• Adding silicon raises the electrical resistance of the iron, reducing eddy currents and lowering hysteresis loss.
• It is made in the form of thin insulated laminations rather than a solid block — and there's a whole story behind this related to losses.

The Core and Transformer Losses

The core is responsible for two types of losses collectively called iron losses: hysteresis loss (remagnetizing the core every half cycle) and eddy current loss. These losses appear as soon as the transformer is connected to the grid, even without load, which is why they are also called no-load losses. Read more in Iron Losses vs Copper Losses.