Masatsugu Takemoto

This article proposes a novel transverse- and axial-flux permanent magnet machine (T-AFPM) using a C-type stator core for reducing system size via an ultra-flat shape. With an axial length of only 19.7 mm, this ultra-flat shape contributes markedly to reducing system size in industrial applications such as water pumps. In general, AFPMs are suitable for a flat shape because of their high torque density with a short axial length. However, it is difficult to use conventional AFPMs to achieve an ultra-flat shape because of structural problems and insufficient performance. By contrast, the proposed T-AFPM achieves a highly manufacturable structure, high efficiency, and the required output power despite its extremely short axial length. Herein, the T-AFPM is compared with conventional AFPMs with various configurations by means of three-dimensional finite-element analysis, and experiments on a T-AFPM prototype are reported. From the simulation and experimental results, the proposed T-AFPM shows high efficiency (IE5 class), the required output power, and suitable structural properties for an ultra-flat shape.

This article proposes a time-efficient and practical design method for determining appropriate skew structures for permanent magnet synchronous motors (PMSMs). Various PMSMs use skew to suppress torque ripple, but 3-D finite element analysis (3-D-FEA) is required in order to accurately determine an appropriate structure for skewed PMSMs, resulting in a long analysis time. Therefore, this article constructs a hybrid analysis method that combines numerical calculations and minimal 3-D-FEA. The aim of this method is to be practical and easy to use, even for novice designers, and to accurately and quickly design skewed PMSMs. In this article, the effectiveness of the proposed method is clarified through several case studies, and then, a skewed PMSM designed using the proposed method is verified experimentally. It is also revealed that suppression of voltage harmonics contributes to improving the performance of PMSMs in experiments.

This paper investigates the impact of soft magnetic composite (SMC) properties on an axial flux permanent magnet machine (AFPM) employing ferrite permanent magnet (PM) in traction applications. In general, the efficiency of an AFPM increases as the iron loss of the SMC decreases. However, the torque and output power of the AFPM also decrease at higher speed above the base speed due to the decrease in magnetic permeability because, typically, when the iron loss of an SMC decreases, the permeability also decreases. In this paper, many virtual SMC materials with different iron loss and permeability are used for finite element analysis of the proposed AFPM in order to clarify the sensitivity to SMC characteristics. First, the impact of the permeability on the torque and output power is investigated because the output power is very important in traction applications. Additionally, the total energy loss of AFPMs employing various SMCs is evaluated using the WLTC driving cycle. Furthermore, accuracy of simulation is evaluated using experiments of downscaled and actual size prototypes employing some SMC materials. Finally, this paper shows the newly developed SMC materials and discusses suitable SMC properties from the perspective of efficiency and output power in traction applications.

This paper gives a comprehensive overview of the current research trends in the skewing technique for permanent magnet synchronous motors (PMSMs). The skewing technique has been widely used in many applications to reduce the cogging torque and torque ripple in PMSMs. There are many ways to implement the skew, and new techniques are continually being developed. First, this paper summarizes the types of skew structures and presents a survey of existing techniques. Specific emphasis is placed on what kind of skew structure is selected depending on the PMSM configuration. Second, the optimal value of the skew angle for each structure is comprehensively explained, and the discrepancy between theory and finite element analysis is discussed. The definition of skew angle varies across the literature, and one of the purposes of this paper is to organize the definition in an easy-to-understand manner. Additionally, this paper offers 3D-FEA results of various PMSMs employing the skew for quantitative comparison. Then, this paper discusses the properties of PMSMs using the skew by structure and the latest trends, and finally describes future prospects.

This paper evaluates the harmonic current caused by a pulse width modulation (PWM) inverter and how it affects the efficiency of a novel axial-flux permanent-magnet machine using a ferrite permanent magnet (AF-FePM) in traction applications. First, differences between the finite element analysis (FEA) and experimental results are discussed using a prototype of the proposed AF-FePM. Second, the AF-FePM is compared with a commercially available radial-flux permanent-magnet machine using a Nd-sintered magnet (RF-NdPM). For both machines, the efficiency and loss are calculated using FEA when applying the sinusoidal and harmonic currents. Additionally, we present the superior efficiency of the AF-FePM under the PWM harmonic current during a WLTC driving cycle because the designed model employs the ferrite magnet and a round copper wire, unlike the RF-NdPM. Finally, motor and inverter losses at different switching frequencies are also evaluated. This paper eventually shows that the proposed AF-FePM would be one of the suitable candidates to enhance high efficiency under PWM harmonic current condition based on comprehensive discussion.

This paper proposes a novel axial-flux permanent magnet machine (AFPM) employing ferrite permanent magnets (PMs) and round copper wire. The proposed AFPM adopts a novel rotor structure and uses tooth-tips with a suitable trapezoidal shape. These structures compensate for the low magnetomotive force of the round copper wire and ferrite PMs, achieving high performance at low cost. Additionally, compared with an off-the-shelf radial-flux permanent magnet machine (RFPM) using Nd-sintered PMs and rectangular copper wire, the proposed AFPM achieves the same output power and higher efficiency, despite using ferrite PMs and the round copper wire. Finally, a prototype of the proposed AFPM was manufactured and evaluated experimentally. The prototype achieved a high efficiency of over 95% across a wide operating area while maintaining required maximum torque, suggesting its potential for traction applications.

This paper proposes a novel airgap structure for magnetic bearing (MB) that realizes 70% smaller lower side coil space area than that of a conventional MB for turbomachinery system. The proposed MB contributes to reducing the total axial length of a 3-axis MB by 17.9%. This means that the critical rotation speed can be increased, improving a serious problem for turbomachinery system that requires high-speed rotational speed. This paper shows comparison of suspension force between a conventional and the proposed MB, which is analyzed by 3D-FEA. Finally, superiority of the proposed MB is shown from various perspective.

This study aims to improve the torque performance and manufacturability of axial-flux permanent magnet (AFPM) machines. Hence, we propose a novel AFPM machine that employs a soft magnetic composite (SMC) core with tooth-tips constructed by a one-pressing process and die. In this paper, the proposed AFPM machine is compared to two conventional AFPM machines using an SMC core. One of them has open-slot structure without tooth-tips. Another model employs an SMC core with tooth-tips pressed by a conventional pressing process that requires multiple operations and dies. As a result of the comparison, the proposed AFPM machine realizes a much higher torque than the two conventional machines. Additionally, the manufacturability of an SMC core with tooth-tips pressed by the proposed method is superior to the conventional one because the proposed structure can be realized by the one-pressing process and die. Furthermore, two prototypes of the proposed AFPM machine and the conventional one with an open-slot structure are fabricated, and then, they are compared by experiments. Consequently, the proposed AFPM machine achieves a 15.7% higher torque than that of the conventional machine using an open-slot structure. Finally, this paper presents an improved design of an AFPM machine with SMC cores using the proposed pressing process. As a result, the proposed AFPM realizes a 20% larger torque than that of a conventional model employing an open-slot structure.

Variable flux motors with adjustable magnetic flux have been gaining attention because of their capability to simultaneously achieve a high torque density and high efficiency. In addition, the output power characteristic, which is related to acceleration performance, in high-speed areas is important in traction applications. However, typical traction motors have lower output power in high-speed areas. In this paper, a Hybrid-Excitation Variable Flux Memory Motor (HE-VFMM) is therefore proposed to enhance output power characteristics under six-step operation mode in high-speed area. The proposed HE-VFMM can perform magnetic flux adjustment with two components: field winding and variable flux permanent magnet (VPM), thus dramatically increasing flux adjustment range. The simulation results show the proposed HE-VFMM achieves 23.7% higher output power at 17,000 rpm than that of an existing traction motor in Prius 4th generation that has the same size while maintaining high efficiency in the frequently used operating area. Additionally, it was found that variable magnetic flux is very effective in enhancing the output power, especially in the high-speed region because the magnetic saturation in the stator core is mitigated by field-weakening control. Consequently, as the rotational speed increases, an increase ratio of the output power caused by the adjustable magnetic flux becomes higher. This paper shows that the proposed HE-VFMM is an effective method for improving the problem of low output power in high-speed regions in traction motors.

This paper proposes an axial-flux permanent magnet machine (AFPM) that uses a neodymium-bonded permanent magnet (Nd-bonded PM) and a coreless rotor structure and assesses how it performs in industrial applications requiring high efficiency and low cost. The AFPM is able to achieve high efficiency in high-speed regions because its Nd-bonded PM can restrain eddy current loss. Additionally, the AFPM can continuously function at the rated operating point without a cooling system, so the expenses for one can be eliminated. Hence, the AFPM can simultaneously achieve high efficiency and low-cost mass production. In this paper, a comprehensive comparison including the thermal characteristics of the proposed AFPM and a conventional radial-flux permanent magnet machine (RFPM) is shown. Furthermore, this paper also focuses on how the aspect ratio influences the thermal characteristics of both machines. Finally, the effectiveness of the proposed AFPM in industrial applications is assessed using 3D-FEA and experimental results.