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Design of inverters for use with induction motors requires a knowledge of the impedances of the machine to harmonics of the supply frequency. This paper presents an harmonic equivalent circuit and describes how its parameters may be obtained. It shows that, when the harmonic current magnitudes and relative phase angles are such as to produce phase modulation of the rotating field, the harmonic magnetizing inductance is equal to the fundamental saturated magnetizing inductance. When the current harmonics produce amplitude modulation of the rotating field, the harmonic magnetizing inductance is equal to the incremental slope of the fundamental magnetization curve at the point of operation. Measurements made with phase currents having controllable harmonic content are presented to corroborate the theory.