An interior permanent magnet synchronous motor (IPMSM) is characterized by high efficiency in a wide range of applications. However, when operating at high speeds, torque ripple and rotor loss occur because of the spatial harmonics generated by fluctuations in the airgap permeance. This study demonstrates the suppression of torque ripple and harmonic loss by magnetic wedges using finite element analysis (FEA). First, the authors examined the torque and loss characteristics by varying the slot opening width and relative permeability of the magnetic wedge. Results indicate that the torque ripple can be suppressed even if the opening width is widened to half the slot pitch. However, this improvement effect saturates when the relative permeability of the magnetic wedge becomes larger than r = 10. Furthermore, the authors confirmed the effect of the magnetic wedge by visualizing the harmonic components of the eddy current loss. The findings reveal that the eddy current loss represents a trade-off between the stator and rotor when using the magnetic wedge.
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.
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