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We are developing a small and lightweight direct‐drive system toward the realization of in‐wheel electric vehicles (EVs). To increase the power density of the motor, we developed a direct‐cooling motor that immersed the core, coils, and magnets in cooling oil and improved cooling efficiency. Moreover, we performed measurements of frictional loss, measurement of pressure drop, and a continuous heat‐run test on the test bench. The results demonstrated that direct cooling is effective and continuous operation can be achieved. Additionally, it was found that the coil temperature can be controlled by the flow rate of the cooling oil. In this paper, we report on the concept of direct cooling and the measurement results. [ABSTRACT FROM AUTHOR]

In this study, an asymmetric rotor was developed to reduce the cogging torque and torque ripple regardless of the rotational direction and its driving point. Specifically, the rotor had a linear symmetric structure for bi‐rotation. In addition, a π‐shaped pole rotor was proposed as a new structure to alleviate magnetic saturation at the edge parts of the rotor. Optimal design using the Rosenblock method revealed that the proposed structure can reduce the cogging torque and the torque ripple in a wide range of driving conditions. [ABSTRACT FROM AUTHOR]

This paper formulates the difference in base frequency and maximum torque between distributed and concentrated windings taking into account the air-gap modulation of concentrated-winding permanent magnet (PM) motors. The design superiority is typically confirmed for each winding type during a development period by using time-consuming finite element analysis (FEA). We propose a quantification that enables motor designers to easily judge, at the initial design stage, a superior winding type and to shorten a design period. Throughout our formulation, we found that the distributed-winding PM motors, in principle, have larger q-axis inductance and smaller d-axis inductance than the concentrated-winding motors, which leads to a decrease in the base frequency. On the other hand, the distributed winding inherently has higher winding factor and needs shorter stack length of core than the concentrated winding for identical torque output, which results in an increase in the base frequency. And also, we theoretically quantified the difference in reluctance-torque contribution between the two winding types and found that it was too small to have much impact on practical design. We applied the proposed formulae to a 0.2-kW concentrated-winding Nd-Fe-B magnet motor and were able to quantify the base frequency, the maximum torque, and the minimum stack length of core for a distributed-winding motor with high accuracy without having to rely on FEA. The FEA and measurement results demonstrated the validity of the proposed approach for the fabricated distributed-winding PM motor. [ABSTRACT FROM AUTHOR]

This article proposes the one-package structure of –1 MV direct current (dc) filter circuit and dc measurement system composed of dc voltage dividers and dc current transducers (DCCTs). These apparatuses are becoming more important for the expansion of high-voltage dc (HVDC) power grids. However, at present, they are separately installed keeping a huge distance for air insulation, and an increase in equipment size and footprint has become a major issue. We succeeded in reducing the footprint to 1/100 by storing the aforementioned HVDC vital parts in one package of a pressure vessel. Our developed configuration uses sulfur hexafluoride (SF6) gas not only for insulation purpose but also as a refrigerant to forcibly ventilate vertically in order to allow the dc filter circuit, which is heat generation source, and the dc measuring instruments, which need to minimize the effect of temperature drift, to coexist. We also addressed another issue caused by the limited space in the one-package structure, where the stray capacitance of the dc voltage divider tends to become large and then lowers the measurement accuracy for transient voltage variation. As a countermeasure, the dc voltage divider is composed of much larger capacitance than the stray capacitance and additional damping resistance to avoid LC resonance with wiring inductance. The actual-product tests demonstrated the followings; the dc measurement system has satisfied high measurement accuracy of 0.5% and fast responsivity of 3.3 μs retaining decay rate within −3 dB; the temperature rise of the dc voltage dividers and the DCCTs is reasonably suppressed; the dc measurement system and the dc filter circuit withstood DC −1.2 MV. [ABSTRACT FROM AUTHOR]

A novel criterion for the utility of reluctance torque in permanent magnet (PM) motors is developed. The criterion, reluctance torque ratio α, can be quantified by using the PM flux linkage Ψp of the stator phase windings, the current I, and the direct- and quadrature-axis inductances Ld and Lq, respectively. It has been shown that the conventional salient-pole ratio is not helpful in clearly judging the utility, while the ratio α can be used to make such judgements across various types of motors with different applications and outputs. Through both theoretical consideration and experiments, it has been proved that a motor with large PM flux linkage utilizes little reluctance torque even if the salient-pole ratio Lq/Ld is over 2.0 and that a large Lq/Ld increases iron loss, thus decreasing motor efficiency. We have also shown that the above-mentioned loss increase is caused by the significant superposition of q-axis flux due to the large Lq and have concluded that, in the above-mentioned case, selecting a rotor geometry with less saliency can improve the efficiency. Results are given for two types of four-pole prototype motors with an output power of 0.6 kW, a rated rotation number of 3600 min−1, and Y-connection. [ABSTRACT FROM AUTHOR]

SUMMARY Carrier harmonic losses that consist of iron loss, eddy current loss, and ac copper loss are produced in a permanent magnet machine driven by a PWM inverter. It is already known that a higher motor inductance can lead to lower carrier harmonic losses. This paper investigates the carrier harmonic loss composition of two motors with identical dimensions but different inductances. The results of finite element analysis (FEA) showed that the eddy current loss in the iron core accounts for most of the carrier harmonic loss. It is also shown that the carrier harmonic loss of the iron core is quantifiable using theoretical calculations. [ABSTRACT FROM AUTHOR]

A modified theory and a proposed analytical approach clarified asynchronous torque characteristics of line-starting permanent-magnet (PM) motors, taking into consideration the mutual effect among the fields due to armature current, cage-bar current, and PMs. Theoretical and analyzed results revealed that cage torque had an oscillating component with slip frequency due to magnetic saturation. This component cannot be seen in the conventional theory. It was also found that the coupling effect between PM flux and cage-bar flux caused on magnet torque larger oscillation and more significant negative dc component, compared with the case in the conventional theory. The theoretical and analyzed results were validated on the basis of experimental approaches. Results are given for a two-pole prototype motor with PN = 5 kW, nN = 3000 min−1, VN = 200 V, and Y-connection. [ABSTRACT FROM AUTHOR]

This paper presents two types of prototype line-starting permanent magnet motors for compressor drives; one is a nonsalient-pole machine, and the other is a salient-pole machine. It is proved through both the analyses and experiments that the utility of reluctance torque not only boosts steady-state characteristics but also line-starting capabilities. It was also noteworthy that the salient-pole machine in the steady-state operation could achieve not only better efficiency but also a higher power factor. Finally, the designed motors achieved a 4% to 5% increase in efficiency over all operational points, which prevented a rise in temperature by 30 K, compared with a conventional two-pole induction motor. The results are given for a two-pole prototype motor with PN = 5 kW, nN = 3000 min^-1, VN = 200 V, and Y-connection. [ABSTRACT FROM PUBLISHER]