The magnetic cores of electric motors, which are stators and rotors, with rare exceptions, are constructed from thin metal sheets (low carbon steel plates) with a thickness of less than 1 mm, grouped in sheet metal packages. Magnetic nuclei surrounded by coils (windings), where alternating currents circulate, cause a magnetic flux also alternating and for that reason, these nuclei are subject to the action of parasitic currents. These currents are known as Foucault currents, which are responsible for appreciable loss of power in these nuclei.
Some higher performance motors are designed with silicon steel plates, with a percentage of approximately 3% silicon. The total process for making these cores consists essentially of lamination, stamping, a treatment for electrical insulation, packaging and fixing. With regard to low carbon steel plates, the process for insulation is based on a heat treatment, where the packages of plates are placed in kilns for a certain time, there being oxidation of the surface of the plates, and as a consequence, the formation of an insulating layer of iron oxide between the adjacent sheets. Some types of silicon steel sheets are provided by manufacturers with a paint based oxide on one of the surfaces.