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 https://www.mrosupply.com/popular_products/10-hp-215tc-motors/. 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.