Participated in the 2023 IEEJ National Convention.

Date: Wednesday, March 15 - Friday, March 17, 2023

Venue: Higashiyama Campus, Nagoya University (Furo-cho, Chikusa-ku, Nagoya City)

Presenter: Ren Sato

Lecture Number: 5-109

Title：Investigation of the Effect of Rotor Aspect Ratio on the Winding Length in a Lightweight Motor with CNTs 

Presenter：Qu Yijun　

Presentation Number: 2-064

Title：Investigation of Relationship Between Contact Resistance and Insulating Coatings on Carbon Nanotube Coils 

Presenter：Taichi Ueyanagi

Lecture Number: 2-065

Title: Proposal of Estimation Index for CNT Conductor Fracture Condition Based on Reflective Heat Transfer Model

Investigation of the Effect of Rotor Aspect Ratio on the Winding Length in a Lightweight Motor with CNTs 

In our lab, we're working on using Carbon Nanotubes (CNT) as winding for SPMSMs. CNTs themselves are short, so to create a longer thread-like structure, spun CNT yarn are used. The currently producible CNT yarns don't come close to the electrical conductivity of CNT itself, so longer CNTs are desirable. As of 2021, CNTs with a length of about 14 cm have been produced. Assuming that the technology has advanced and longer lengths are available, this research presentation discussed the lightweighting of SPMSMs. While using CNT instead of CNT thread might provide higher electrical conductivity, it's unrealistic to assume they can be infinitely long. Therefore, the discussion focuses on coil length and mass.

We'll use the longer CNTs as coil windings and discuss the relationship between coil length and motor mass. Previous research [1] pointed out that the aspect ratio of the rotor has a significant impact on winding resistance. Focusing on the rotor's aspect ratio and using it as a variable in our design, we demonstrated the relationship between coil length and motor mass.

Figure 1 shows the motor designed using CNT winding. Figure 2 displays the coil length per phase for each aspect ratio. It was found that the higher the aspect ratio, the shorter the body becomes. Furthermore, Figure 3 shows the weight reduction rate compared to when using copper winding for each aspect ratio. While the lightweighting effect decreases with a higher aspect ratio, it was shown that all designs are lighter than those using copper winding. This indicates that there's a trade-off between lightweighting and body length when considering aspect ratio as a parameter.

We examined the effects on winding length and weight for motors using CNT winding, using aspect ratio as a parameter. We found that both the winding length and weight vary based on the aspect ratio, revealing a trade-off between shortening the winding length and reducing weight. Nonetheless, our results show that, in any case, using CNT winding results in a lighter design compared to using copper winding.

[1] H. Kim, J. Jeong, M. Yoon, J. Moon, and J. Hong : ”Simple Size Determination of Permanent-Magnet Synchronous Ma- chines”, in IEEE Trans Ind Electron, vol. 64, no. 10, pp. 7972- 7983 (2017)

This article investigates the potential of carbon nanotubes (CNTs) as a replacement for metallic wires in weight-critical fields. CNTs offer advantages such as low density and superior electrical conductivity. However, the use of insulating film on CNT coils increases their weight. The study aims to explore the feasibility of making CNT coils directly from CNT yarn without using insulating film, based on the high contact resistance property of CNTs.

The resistance of the CNT yarn was measured in various configurations, such as different diameters of insulating rods and different states of contact and non-contact coils. The resistance values were recorded for different types of CNT yarns, including kg40, mk40, and mk100. The results were analyzed to determine the impact of insulating film and the feasibility of using CNT coils without it.

Based on the experimental results and mathematical calculations, it was concluded that CNT yarns can be used to create coils without the need for an insulating film. The contact resistance of surface-treated and non-surface-treated CNT yarns was compared, and it was found that the contact resistance of CNT yarns without surface treatment was lower. This suggests that CNT coils can be directly made from CNT yarn without the added weight of an insulating film, making them a promising alternative to metallic wires in weight-critical applications.

 Proposal of Estimation Index for CNT Conductor Fracture Condition Based on Reflective Heat Transfer Model

Carbon Nanotubes (CNT: Carbon Nanotube) are gaining attention as potential replacements for copper wires due to their superior properties. Individual CNTs are nano-sized materials, so multiple CNTs are bundled and twisted to form CNT threads for use as conductors. Sekiguchi and others have shown that CNT threads have a negative temperature coefficient of resistance[1]. These temperature characteristics cannot be utilized if the temperature isn't clear. CNT threads have a diameter of several tens of micrometers, making temperature measurement challenging. Therefore, this paper proposes an indicator that can discuss temperature characteristics without directly measuring the temperature. Furthermore, using this indicator, we clarify the conditions under which CNT threads break.

From preliminary experiments, we confirmed that the change in resistance value over time greatly varies depending on the current density. When used as a conductor, it's essential to avoid breaking the CNT thread. To discuss the process leading to the breaking of the CNT thread during electrification without using temperature, we model the heat generation and radiation heat transfer of the CNT thread when electrified. By using the formula for radiation heat transfer during electrification and the formula for CNT thread heat generation, we use the radiation heat per unit time per unit surface area as the evaluation value K. The current density at the time of breaking (Jbreak) is expressed as follows.

The value of K at which the rupture occurs, referred to as Kbreak, was measured, and the breaking current density for the other samples was estimated from Equation (1). For Samples 2 to 4, the current density was increased stepwise from 1 A/mm^2 and electrified until they broke. Figure 1(b) to (d) show the results of the electrification experiments. The horizontal axis represents the current density J, and the vertical axis represents K. Measurement points that did not break are shown in black, and those that did break are shown in red. In Equation (1), 𝜎0 and 𝛼 were determined by electrifying each sample at different current densities and using the least squares method.

From Figures 1(b) to (d), although there are some deviations, it can be generally observed that our estimations align with the actual measurements. From Figure 1, we can also observe changes in the estimated breaking current density values with increasing measurement points when determining the constantsσ0 and 𝛼. For Samples 2 and 3, points that initially exceeded the estimated breaking current density Jbreak (shown by the blue dotted line) based on the first four points, now lie below Jbreak (shown by the green dashed line) estimated from all measurement points. And they ended their measurements without breaking at values below Jbreak. This suggests that the accuracy of estimation improves with an increase in the number of measurement points.

From these findings, it can be concluded that the evaluation indicator K proposed in this paper can accurately estimate the breaking current density without the need for temperature values.

In this research, we aimed for the practical application of CNT threads as conductors. In this paper, we tackled the discussion on temperature characteristics without relying on actual temperature values. We introduced an evaluation index K for temperature and used this index to estimate the current density at which CNT threads would break. The validity of this estimate was confirmed experimentally, demonstrating the effectiveness of our index. Using this index enables us to set an upper limit on the current density in motor design, paving the way for realistic motor designs. Moreover, this index can be applied in situations with temperature constraints, such as when considering the heat resistance temperature of coatings.