What are Eddy Currents | Why Machine Cores are Laminated ?

Let the situation be as shown below. A block of metal is lying perpendicular to an increasing magnetic field, pointing upwards. Due to this changing magnetic field, currents are induced in the block in the clockwise direction in planes perpendicular to the magnetic field, when seen from above the block.

When the magnetic flux linking the block changes an emf is induced because the free electrons of the metal experience force. As per Lenz’s law, the direction of this emf and hence current should be such that it opposes the cause of its production. Here the cause is the increase in magnetic field. Therefore, the currents flow in clockwise direction  in circles in planes perpendicular to the field so that they can produce magnetic  flux in opposite direction thereby opposing the increasing main flux. These circulating currents are called eddy currents.

These currents can be large enough even for small induced voltages, because metal block has very low resistance. Therefore, eddy currents can produce considerable heating. In many situations eddy currents pose problems. For example in transformer core, armature of a dc motor and dc generator. This results in wastage of energy and rise of temperature of the machine. Therefore must be kept to as low value as possible.

why machine cores are  Laminationed ?
To reduce eddy currents, the armature and the core in case of transformer is sliced into thin sheets (planes parallel to the magnetic field) called laminations as shown above.

Each lamination is insulated from the other by a thin layer of varnish. Now, what happens is that the currents follow longer paths and also area of cross-section for current path is also  reduced. Both these factors increase the resistance of the current path and hence eddy current is reduced. 

Eddy current Applications :- The eddy current concept is the underlying principle of an induction cooker.