In this article, the electromagnetic force and vibration characteristics of 10-pole 12-slot and 14-pole 12-slot permanent magnet synchronous motors (PMSMs) are examined considering the tooth modulation effect. In contrast to the conventional electromagnetic force analysis process which considers only the radial force of the lowest spatial harmonic order, the radial and tangential forces include low and high spatial harmonic orders which can be modulated with a lower spatial harmonic order, are analyzed. Then, a structural finite-element (FE) model is developed, and vibration analysis is performed by applying a modulated force and moment. In the analysis, the amplitude and phase of the modulated radial force, tangential force, and moment are considered. The vibration analysis result shows that the 10-pole 12-slot PMSM is disadvantageous with respect to vibration when compared to the 14-pole 12-slot PMSM owing to the large modulated force. Also, it is confirmed that the effect of modulated tangential force and moment cannot be neglected, and the phase difference between modulated radial, tangential force and moment is also an important factor in vibration. Finally, the two motors are fabricated, and the validity of the FE analysis results is verified using the test results.

This paper proposes the design process of an ultra-high-speed surface-mounted permanent magnet synchronous motor for a fuel-cell electric vehicle air compressor. Proposed design process enables ultra-high-speed motor design while considering mechanical stresses of the rotor materials according to the temperature. In the rotor design stage, the worst temperature condition of the permanent magnet and retaining sleeve on mechanical stress is investigated and reflected using analytical method. Rotor dimension such as permanent magnet and retaining sleeve thickness is determined considering both electromagnetic performance and mechanical characteristics. Then, from the initially designed model, the eddy current loss according to the shape ratio (SR) and torque density (TD) is analytically derived using proportional equation and finite-element analysis result. Then, SR and TD to reduce the eddy current loss and maximize the efficiency are determined while considering the first bending critical speed. Finally, to verify the proposed design process, the prototype is fabricated and tested. Test results show good agreement with the finite-element analysis results, and it is confirmed that the rotor is operated at the highest rotational speed without mechanical failure.

This paper proposes a method to suppress the torque ripple caused by misalignment of the gearbox. Misalignment is a common fault caused by the manufacturing tolerances, and generates torque ripple. Therefore, even if a constant electromagnetic torque is transmitted from the motor to the gearbox, a large torque ripple may be transmitted to the load because of misalignment. In the proposed method, a harmonic current injection method, in which harmonic current is injected into the armature current, is used to suppress the torque ripple produced by misalignment. The amplitude and phase of the harmonic current are calculated using the misalignment torque ripple, and a method for estimating the misalignment torque ripple, which does not require a torque sensor, has been presented. Using the estimation method and harmonic current injection method, the misalignment torque ripple can be suppressed effectively. The phenomenon of torque ripple production by misalignment is analyzed, and a current control algorithm based on harmonic current injection method is proposed. The effectiveness of the proposed method is verified through experiments at various load torque. Using this method, a high degree of torque control can be achieved with even a low quality gearbox.

Step-skew is a common technique for eliminating the cogging torque of a target harmonic order in permanent magnet synchronous motors (PMSMs). However, when step-skew is applied to the rotor, the cogging torque of the target harmonic order is not completely eliminated due to 3-D leakage flux. Therefore, the 3-D leakage flux should be considered in designing a PMSM with step-skew for cogging torque reduction. The most accurate way to consider the 3-D leakage flux is to perform 3-D finite element analysis (FEA), but it has the disadvantage of high computation time. To resolve this challenge, this paper proposes a design method that utilizes transfer learning to reduce the time for 3-D FEA while maintaining accuracy. Through the proposed method, a large amount of 2-D FEA-based data and a small amount of 3-D FEA-based data are used instead of a large amount of 3-D FEA-based data, with similar accuracy as using a large amount of 3-D FEA-based data, and the computational time is highly reduced. Finally, a prototype is fabricated and tested to verify the validity of the proposed design method for cogging torque reduction.

Surface-mounted permanent magnet synchronous motors (SPMSMs) usually adopt a retaining sleeve for ultrahigh-speed (UHS) applications. The retaining sleeve usually has high electrical conductivity and cannot be segmented. Accordingly, the rotor eddy current loss is high. Moreover, the higher the eddy current loss, the higher is the rotor temperature. Therefore, the rotor eddy current loss should be considered in the design stage. However, the eddy current loss is generally calculated through three-dimensional finite-element analysis (3-D FEA) and considered cumbersome because of the high computation time. This article proposes a simple estimation method for the rotor eddy current loss of a UHS SPMSM according to the motor size. In this study, the mechanically stable cross-sectional structure of the rotor was fixed, and a formula for predicting the eddy current loss of the rotor based on motor size was devised. In this method, the basic electromagnetic formula for eddy current and the frozen permeability method were utilized. The proposed method was validated via comparisons with the results of 3-D FEA.