SSC JE Electrical Engineering 2013 Paper

Inductors with ferromagnetic cores have additional energy losses due to hysteresis and eddy currents in the core, which increase with frequency. At high currents, iron core inductors also so gradual departure from idea behaviour due to nonlinearity caused by magnetic saturation of the core.
If the current through a ferromagnetic core will is high enough that the magnetic core saturates the inductance will not remain constant but will change with the current during each half of the period to provide the maximum flux density therefore the inductor with ferromagnetic core and up having non sinusoidal excitation current.