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The pulse width modulation (PWM) inverter has been used for the control of a motor since the rotating speed can be changed efficiently and easily. In order to estimate the iron loss in an iron core with sufficient accuracy under a PWM inverter excitation, it is necessary to understand correctly the influence of operating condition and circuit parameter on iron losses. In this paper, we measured magnetic properties of the nonoriented electrical steel sheet excited by a single-phase full-bridge PWM inverter using a ring specimen that corresponds to a magnetic circuit of an actual motor core. The effect of the carrier frequency and circuit resistance on the iron loss and the behavior of minor loops was examined. It is shown that the iron loss is increased when the circuit resistance is increased due to the generation of minor loops. Therefore, the circuit resistance should be decreased in an actual motor system.