When input electric power is supplied to the primary of transformer , some portion of this electric power is used to compensate core losses in transformer i .e . Hysteresis loss in transformer as well as Eddy Current loss in transformer core as well as some part of the input power is lost as I2R loss and dissipated as heat in the primary and secondary winding , as because these windings have some internal resistance in them . The 1st one is referred to as core loss or iron loss in transformer and the later is referred to as ohmic loss or copper loss in transformer . Another loss occurs in transformer , referred to as Stray Loss , due to Stray fluxes link with the mechanical structure and winding conductors .