This paper describes a variable magnetomotive force interior permanent magnet (IPM) machine for use as a traction motor on automobiles in order to reduce total energy consumption during duty cycles and cut costs by using Dy-free magnets. First, the principle of a variable magnetomotive force flux-intensifying IPM (VFI-IPM) machine is explained. A theoretical operating point analysis of the magnets using a simplified model with nonlinear B-H characteristics is presented and the results are confirmed by nonlinear finite element analysis. Four types of magnet layouts were investigated for the magnetic circuit design. It was found that a radial magnetization direction with a single magnet is suitable for the VFI-IPM machine. Magnetization controllability was investigated with respect to the magnet thickness, width and coercive force for the prototype design. The estimated variable motor speed and torque characteristics are presented. Following the explanation of the design process, the experimental results obtained for the prototype machine are described. The torque characteristic and the transient torque behavior during magnetization state (MS) control were confirmed experimentally. Based on the experimental results, the total loss in a typical duty cycle was calculated. Since too frequent manipulation of the magnetization state results in larger loss due to the loss caused by the MS control current, hysteresis control was implemented to reduce the number of MS changes. Using the proposed control algorithm, the total energy consumption was calculated in comparison with that of a conventional motor. It was found that the VFI-IPM motor reduces the total energy consumption significantly.
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