This paper presents an effective procedure for analyzing the operation of the medium frequency heating system for ferromagnetic pieces. Given the non-sinusoidal shape of the output voltage of the inverter, it was taken into account the harmonics of capacities within the system, selecting the most important ones. At the working frequency of the installation, the depth of penetration of the electromagnetic field is very low compared to the other geometric dimensions and therefore it is suggested the adoption of a uniform environment for this superficial area. The thermal diffusion problem has been solved analytically, using the separation of the solution in spatial eigenfunctions

Keywords: Surface heating, Periodic non-sinusoidal regime, Inverter-inductor circuit