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The interior permanent-magnet (IPM) motor is widely used in industrial applications due to its high power density. Despite this efficiency, the average torque increase achieved by the reluctance torque leads to high torque ripple, the major cause of motor noise and vibration. Therefore, many studies related to the size optimization have been performed to reduce this ripple effect. This paper suggests an optimal stator shape design for the IPM motor satisfying both improved torque performance and geometrical simplification for manufacturing. To control the geometrical complexity, the concepts of the phase-field model and the level-set method are employed. The optimization problem is formulated to minimize two values: the first is the torque ripple done by adjusting the torque values at any rotor position to constant target torque, and the second is the fictitious interface energy of the phase-field model as it narrows the interfacial region. The proposed method is applied to the structural stator design of a 12-pole 18-slot IPM motor developed as a power source for hybrid electric vehicles.