why core of a transformer is laminated

The reason we laminate the iron cores in transformers is because we want to limit what are called eddy currents. Transformers are basically two coils of wire wrapped around a core of iron. They work by induction. Induction occurs when current flows in one conductor (or one set of windings in the transformer) and the magnetic field that forms around that conductor (that set of windings) sweeps the other conductor (the other set of windings) and induces a voltage. In order to increase the effectiveness of the transformer, we need to improve the way the magnetic fields are coupled from one set of windings to the other set. Iron conducts magnetic lines of force well, so we use that to help conduct the magnetic lines of force from coil A to coil B. Problem is, iron is also a conductor, and it's being swept by the magnetic field as well. If we didn't use laminations, the iron core would provide a place for the magnetic lines to produce (induce) current, and that current flowing in the core would heat the core up really fast and waste energy. By laminating the cores, we break up the current paths within that core and limit eddy currents.