open access

https://doi.org/10.14943/doctoral.k14628

https://hdl.handle.net/2115/82503

Study of Enhancing Added Value of Magnetic Bearings in High-Speed Rotation Machine System

日本語

magnet-less

2021-06-30

Hokkaido University

103p

Conventional bearings are in a difficult condition to be applied to high-speed rotation because of the mechanical friction and wear out occur during high-speed rotation. Therefore, instead of the conventional bearings, magnetic bearings that can generate shaft bearing capacity while being non-contact with the rotor are widely adopted. In recent years, in addition to demanding higher speed and smaller size of the entire rotating machine system, low loss and low price of magnetic bearings become issues. What’s more, in applications such as industrial turbo molecular pumps, there remains a problem that the attractive force of a permanent magnet deteriorates the workability of inserting and removing the rotor. Therefore, there are practical problems such as making magnetic bearings magnet-less and improving the assembleability of the entire system, and increasing the added value of magnetic bearings is attracting attention. There are two types of general magnetic bearings, heteropolar type and homopolar type. Conventional heteropolar magnetic bearings are inexpensive because they have simple structures and do not have magnets. However, it has the disadvantages that it is difficult to cool down because there is a lot of iron loss in the rotor. The reason of the iron loss is there is an alternating magnetic flux generated in the gap. On the other hand, the conventional homopolar magnetic bearing is easy to cool down because it has less iron loss in the rotor, but the disadvantage is that it uses a large amount of magnets and its structure is complicated. In addition, since it is necessary to maintain a constant slot area, the width between the suspension poles is wide, the magnetic flux density distribution of the gap is uneven, and it is thought to be the reason of the iron loss in the rotor during high-speed rotation. Therefore, our research team proposed a new homopolar magnetic bearing using four C-shaped cores. However, although the magnetic bearing with the proposed new structure can narrow the width between the suspension poles, because of the stator adopts a straight tooth shape in order to concentrate the magnetic flux in the gap, the magnetic flux density of the rotor core facing the stator teeth is high, while the magnetic flux density is low between the suspension poles, the change of magnetic flux density in the gap causes loss during high-speed rotation. As a result of further suppressing the width between the suspension poles in order to suppress the iron loss, there is a possibility that cause the leakage flux between the poles, which leads to a significant decrease in the suspension force characteristics. Therefore, in this study, we pay attention to the above problems and adopt a fully closed slot shape, which is not generally used, to further reduce the iron loss generated in the rotor. In addition, the proposed structure shows that the suspension force does not decrease and the assembling property does not deteriorate, and it is possible to solve problems such as the high cost and complicated structure of the conventional homopolar magnetic bearing, and further improve the performance. On the other hand, in general, suspension system that can actively controls 5 degrees of freedom, a disk-shaped rotor is used for the thrust magnetic bearing (TMB) and two radial magnetic bearings(RMB) for the rotor support part. However, by this structure not only the cost is high due to the large number of units in the entire system, but also the critical speed of revolution is lowered by increasing the shaft length as the size increase. In addition, since the disk in rotor is used for TMB, the assembability of the entire system is reduced, and it is difficult to balance the rotating rotor. Therefore, in order to improve the assemblability of the entire system and reduce number of units, our research team used a cylindrical rotor instead of a disk-shaped rotor for axial displacement. We are proposing a T+RMB type magnetic bearing that can actively control 3 degrees of freedoms. The system using T+RMB has the excellent advantage of miniaturization of the system while being able to actively control 5 degrees of freedom by combining with a bearingless motor (BelM). In addition, the rotor length can be expected to be shortened by reducing the number of units of the system, which is a countermeasure for issues such as passing critical speeds and balancing of the rotor. However, in applications such as industrial turbo molecular pumps, it is necessary to disassemble and clean it regularly, but the attractive force of the permanent magnet in T+RMB that provides the bias magnetic flux make it hard to insert and remove the rotor. Therefore, in this study, we will examine the magnetless T+RMB for the purpose of improving the assembleability of industrial turbo molecular pumps. We propose two types of structures depending on the method of adding the bias function to the T+RMB, then compare and examine the suspension force characteristics and loss characteristics of each structure using 3D-FEM, and verify the validity and effectiveness of the proposed structure. Finally, we examined the value added of magnetic bearings from the viewpoint of low loss, low cost, and magnet-less, and then summarizes the future task in terms of high added value, low cost, and resource saving of the magnetic beaarings.

2021-06-30

甲第14628号

博士(工学)

(主査) 教授 小笠原 悟司, 教授 五十嵐 一, 教授 近野 敦, 教授 竹本 真紹(岡山大学 大学院 自然科学研究科)

情報科学研究科(システム情報科学専攻)

博士論文

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