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The effects of supplying-voltage background distortion on the behavior of high power pulse-width modulation (PWM) adjustable-speed drives (ASD) are systematically analyzed by means of a new and very fast approach essentially based on the modulation function theory. The proposed approach is fully described with reference to both dc-side and ac-side voltages and currents and by taking into account both characteristic and noncharacteristic harmonic distortion on supplying voltages. Starting from the results of an analysis performed with reference to a PWM ASD case-study, some considerations on the converter behavior in presence of interharmonic components on the supplying voltages are developed. It can be summarized that at certain frequencies depending on the dc-side L, C filter parameter values, also small amplitude supplying voltage interharmonic components can cause the presence of incredibly strong interharmonic current components on both dc-side and ac-side; furthermore, they appear to be more relevant than those caused by the already documented motor-frequency modulation. Finally, the accuracy and the usefulness of the proposed simple model are tested and confirmed by running several simulations using the well known EMTP software package.