The paper presents a methodology for determination of harmonic iron losses in laminated iron cores under both sinusoidal and sinusoidal pulse width modulated voltage excitations (SPWM) excitation for various frequencies. The method is based on measurements performed by using a convenient Epstein device supplied by a sinusoidal and a SPWM source, which are stored and processed in a PC by using an appropriate data acquisition card. It is shown that the switching frequency does not considerably affect iron losses under SPWM supply and vary non-linearly with flux density with respect to the ones measured under sinusoidal excitation for the same fundamental frequency.