What are eddy currents ? Why and where are they undesirable & How they are minimised?

Whenever a conductor or a part of it is moved in a magnetic field cutting magnetic field lines or placed in a changing magnetic field, the free electrons in the bulk of the metal start circulating in closed paths equivalent to current-carrying loops. These loop currents resemble eddies in a fluid stream and are hence called eddy or Foucault currents after Jean Bernard Leon Foucault who first detected them.

Eddy currents result in generation of heat amounting to energy loss in the cores of transformers, motors, induction coils, etc.

To minimize the eddy currents, instead of a solid metal block, cores are made of thin insulated metal strips.