Soft iron core is used in transformer because the soft iron core has good magnetic permeability that concentrates magnetic lines of force and reduce energy loss.
The core of a transformer is made of soft iron because it has high permeability so it provides complete linkage of magnetic flux of the primary coil to the secondary coil. Therefore it has high coercivity and low retentivity.The simplest motion producer is a solenoid, which consists of a coil with a soft iron core (or armature) which slips easily in and out of the coil. Soft iron is used because it does not retain its magnetism when the current is switched off; in other words, it does not become permanently magnetized.
The transformer core provides a magnetic path to channel flux. The use of highly permeable material (which describes the material's ability to carry flux), as well as better core construction techniques, helps provide a desirable, low reluctance flux path and confine lines of flux to the core. Iron Core Coils have low stray magnetic fields, which can eliminate a number of problems. When stray magnetic fields are high, it can interfere with nearby power systems and produce eddy currents in steel.
A transformer core is a static device that transmits power from one source to another through electromagnetic induction. These are pieces of magnetic material with a high magnetic permeability, which is used to guide magnetic fields in transformers.The core of a transformer is made of soft iron because it has high permeability so it provides complete linkage of magnetic flux of the primary coil to the secondary coil. Therefore ithas high coercivity and low retentivity.Soft iorn provides the best material for the core of a transformer as its permeability (μ) is very high. Its hysteresis curve is of small area and its coercivity is very low.