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The demand for high-speed servomotors is increasing, and minimal losses in both high-speed and high-torque regions are required. Copper loss reduction in permanent magnet motors can be achieved by configuring concentrated winding, but there are more spatial harmonics compared with distributed winding. At high-speed rotation, the eddy current loss of the rotor increases, and efficiency tends to decrease. Therefore, we propose a motor in which a composite ring made from resin material mixed with magnetic powder is mounted on the stator to suppress spatial harmonics. This paper describes three characteristic motor types, namely, open-slot motors, composite-ring motors, and closed-slot motors. Spatial harmonics are reduced significantly in composite-ring motors, and rotor eddy current loss is reduced by more than 50% compared with open-slot motors. Thermal analysis suggests that the saturation temperature rise value is reduced by more than 30 K. The use of a composite ring is effective in reducing magnet eddy current loss during high-speed rotation. Conversely, the torque characteristics in the closed-slot motor are greatly reduced as well as the efficiency. Magnetic circuits and simulations show that on electrical steel sheets with high relative permeability, the ring significantly reduces the torque flux passing through the stator, thus reducing the torque constant. To achieve reduced eddy current loss during high-speed rotation while ensuring torque characteristics with the composite ring, it is necessary to set the relative permeability and thickness of the composite ring according to motor specifications.

Variable magnetic flux motors have been proposed to achieve high-efficiency operation over a wide drive range ranging from high torque to high-speed. This paper clarifies the high efficiency area expansion effect of a variable flux motor with a magnetic composite material inserted in the rotor. To achieve the effect of enlarging the maximum efficiency range, this motor uses magnetic composite materials with much lower saturation magnetic flux density and iron loss than electromagnetic steel plate. This paper reports the magnetic and mechanical properties of the magnetic composite material using Fe-Si-Al alloy, considering low iron loss. The simulation results clarified the variable magnetic flux characteristics for reducing loss at high-speed rotation by actively utilizing the magnetic saturation region of the magnetic composite material. The efficiency improvement in the high-speed region is attributed to the suppression of spatial harmonics without significantly reducing the q-axis inductance. Furthermore, the proposed motor achieves both the expansion of the high-efficiency range and relaxation of mechanical stress by increasing the thickness of the bridge part adjacent to the permanent magnet.

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