ニデック株式会社ロゴ
NIDEC CORPORATION inquiry
  • Home
  • Lecture 17:The Superconductor Induction Motor

Lecture 17:The Superconductor Induction Motor

2025/7
NIDEC Technical Adviser 
Takashi Kenjo

Motors exist in a wide variety of types, as I dwell in this series of topics on mysteries about motors. This is why we constantly discuss how to select, drive, and control the right motor for applications. Selecting the right motor, though, is not so easy. Take motors to drive electric vehicles, for example. Selecting such motors has been an issue for the past 50 years or more, but the present world of motor science does not seem to have identified the best motor for that purpose yet. The same holds true for electric airplanes. Identifying the right motor to drive them is a classic yet new topic.

What if we adopt superconductivity, a state-of-the-art technology? To answer this question, I set an interview with Professor Taketsune Nakamura at Kyoto University, who is one of the experts in this area.

Nakamura:As Program-specific Professor, I’m in charge of the laboratory donated by Nidec Corporation. In this lab launched on April 01, 2017, I work on a variety of topics, including clarifying the basic electromechanical energy conversion process, optimization design, control technology, and systemization, mainly for rotary machines. For these researches, we use various magnets, magnetic materials, and conductive materials including superconducting ones.

Kenjo: I imagine that superconductivity can be applied to a motor in various ways. Especially interesting is the use of superconductor materials for the squirrel-cage rotor of an induction motor.

Nakamura:  First of all, a superconducting motor must be cooled to a cryogenic temperature to operate it. Therefore, it is important to minimize cooling costs, and this is why R&D was focused on high-temperature superconducting (HTS hereafter) materials that achieve a superconducting state at relatively high temperatures has been mainstream. So far, HTS materials have been considered for use in almost all types of motors. And, among present HTS motors, the mainstream is for the synchronous motor equipped with field windings. There were almost no studies applying HTS materials to a squirrel-cage induction motor until the early 2000s, but the first solid experimental research results came from a paper in 2003 [1] by a group from Soonchunhyang University of the Republic of Korea, which showed the possibility of synchronous rotation. Their results caught the attention of Professor Itsuya Muta, who was my supervisor at the time and currently Professor Emeritus at Kyoto University

He started studying the HTS induction motors, but the Korean group suspended its R&D activities thereafter. At our lab, in the meantime, Professor Muta and an excellent master's course student worked hard to produce a basic theory to a certain extent. When the professor retired and the student graduated to enter a company, I wondered whether I should continue the research or stop. I recalled when I was studying the physical properties of the HTS materials as a student at Kyushu University. I was then fascinated by the depth of this physics; this recollection prompted my decision to continue the study.ただし，当時は高温超電導誘導モータの研究者が世界的にも皆無に近かったうえに，もともと駆動原理上すべり（ ）を伴う損失の大きなモータを高温超電導化することに反論が強く，また抵抗が低い高温超電導材料を籠型巻線に適用すれば制動効果の減少に伴って不安定系になり，そもそも駆動できるはずがないという批判を学会などで多く受けました。

そこで，向けられた批判に対して，一つ一つ地道に理論検討と実験検証を積み重ねました。そして，国内外の大学・研究所・企業の研究者・技術者から評価・応援をいただけるようになり，高温超電導誘導モータは少しずつ市民権を得るようになっています。これまでに，基礎回転理論，設計・制御技術を確立し，さらには自動車・航空機等のシステム応用技術を開発中です。既に100 kW級程度までならば大学で試作ができる状況ですし，複数の企業様と共にそれ以上の出力機の開発も予定しています。現在では，まずは液体水素ポンプにおいて，世界で最も実用化が近い超電導回転機技術と考えられます [2] 。

Kenjo:  　In the 11th installment of this series, I discussed non-Lorentz forces. A symbolic motor that utilizes this force is the hysteresis motor. The torque equation for this motor was derived by B.R. Teare as part of his doctoral thesis in 1937. He then presented a coherent theory in the AIEE journal in 1940.

Nakamura:  My HTS induction motor has this characteristic, too. In a hysteresis motor, its semi-hard magnetic steel * itself controls the magnetic flux’s state. In an HTS induction motor, on the other hand, it is realized by the HTS squirrel-cage winding consisting of HTS rotor bars embedded in a silicon steel core and HTS end rings to control the magnetic flux’s state. It is important to note that the HTS induction motor not only realizes the high output and high efficiency characteristics that the hysteresis motor couldn’t [3] , but also enables an intelligent feature such as excellent acceleration [4] )

Kenjo:  　　By the way, as I understand, the BCS theory by John Bardeen and others cannot explain the high-temperature superconductivity…

Nakamura: The temperature where a superconductor reaches the superconducting state is called “critical temperature.” The BCS theory claimed that the critical temperature could reach around 40 K (= -233.15ºC) at the highest. The critical temperature’s upper limit is called “BCS’s wall.”. However, the BCS theory cannot explain the critical temperature of a copper-oxide high-temperature superconductor, discovered in 1986, whose critical temperature far exceeds 40 K and even 77 K (-196°C = liquid nitrogen’s atmospheric boiling point). Researches are still ongoing with the aim of establishing a new theory to replace the BCS theory, but no reliable theory has yet been realized.

Simple, BCS theory-based explanation of the superconductivity theory:

Groups of small fish swimming in the sea do not crash into each other. Electrons in metals should move in large groups to avoid crash into atomic nuclei. To do so, arrangement will be necessary among electrons. The theory to explain the arrangement is called the “BCS theory” after the names of its three advocates (Bardeen, Cooper, and Schrieffer). Using technical terms, the BSC theory can be explained as follows:

個When free-moving individual electrons form a large group, they follow the Fermi statistics, while when two electrons form a pair as shown in Illustration 17-1 via a crystal lattice at extremely low temperatures, they follow the Bose statistics, thereby causing the electrons to flow orderly 

The above explanation is a theory in an area known as quantum mechanics or wave mechanics that deals with elementary particles.

導体に流れる電流は電子の集団的な運動であり，原子核との衝突等によって熱が発生するものと解釈される。それが常電導の電気抵抗の源である。しかし，物体によっては，ある温度以下になると2個の電子が対を成してするすると流れて電気抵抗がゼロになる。
Illustration 17-1 Electric current flowing inside a conductor is a collective movement of electrons, interpreted as generating heat through crashes with atomic nuclei, etc. That is the source of the electric resistance in normal conduction. However, in some materials, two electrons form a pair and flow smoothly below a certain temperature, to eliminate electric resistance.

コラム 天才の交差 Bardeen と Teare

超電導のBSC理論とトランジスタの発明でノーベル物理学賞を２回受賞したのがJohn Bardeenであることを知る読者は少なくないと思います。
第11回は非ローレンツ力の不思議を書きBenjamin Teareによるヒステリシスモータのトルク理論を論じました。彼のトルク式の解釈を拡大することによって誘導子モータを説明できることも示しました。BardeenとTeareはウィスコンシン大学の同級生でした。1928年，学部から修士に進学するときTeareは物理から電気工学に，Bardeenは電気から物理に逆転向したのです。Bardeenは事象の本質を解明しようとする好奇心に対して，Teareは実用的な結果がすぐに出ることの洞察力が数学力によって裏打ちされていました。その資質は学生のときから発揮され，彼はGEにスカウトされました。
後に彼らは1941年にNaval Ordinance Labに同時期に勤務したことがあるようです。

Superconductor induction motor: An outlook

Now, let’s see the structure of the HTS induction motor that Professor Nakamura is working on. Illustration 17-2 (a) shows a conceptual diagram of the motor and (b) shows conductor bars composed of an HTS material and a normal conducting material. Illustration 15-1 of the 15 th lecture shows typical structure of a squirrel-cage induction motor, and the photo in Illustration 14-2 of the 14 th lecture shows a squirrel-cage structure in which conductor bars and end rings are combined into a single unit.

誘導モータのロータの籠型導体に超電導材を利用する仕組み。
Illustration 17-2 Mechanism to use an HTS material for a squirrel-cage rotor of an induction motor.  

Illustration 17-3 shows an example of how the above structure is implemented. Illustration 17-4 shows an assembly of the conductor bars and end rings. For a basic experiment to check its principle, this motor has simple end rings in place. If it is required to drive this motor even when the HTS is in a normal conducting state, the normal conducting bars will also require separate end rings made of a normal conductor.

定格出力1.5kWの汎用誘導機のロータを造り変えた実験機。Nidec製品研究所の実験台で分解して構造を見る。ステータは36スロットに４極巻線を設置。組み立てられたモータは液体窒素に浸漬（しんし，しんせき）される。
Illustration 17-3 A test model with a rotor modified from an original one for a general-purpose 1.5kW rated induction machine. Disassembled at a Nidec lab for a structural examination. The stator has four-pole windings on 36 slots. The assembled motor is immersed into liquid nitrogen.
常電導材と超電導材の組み合わせによる導体棒（conductor bar）と端絡環の形成；ロータ鉄心からはみ出した超電導棒にテープ状の超電導を巻きつけてハンダ付け。
Illustration 17-4 Conductor bars and end rings formed by combining HTS and normal conducting materials. The HTS tapes are wrapped around the conductor bars protruding from the rotor slots, and they are soldered.

Illustration 17-5 shows a cross-sectional conceptual diagram of a rotor’s structure. (a) shows the cross-section of the rotor core. Its lamination is made of a 0.5mm-thick silicon steel plate, with bars installed in 26 closed slots. The lamination is laser-welded and laminated at 13 points. (b) is a conceptual diagram of an end ring’s welding installation.

ロータ構造の概念図
Illustration 17-5 Conceptual diagram of rotor’s structure.

誘導機と同期機特性　同期・非同期 両立性 (Synchronous-Asynchronous availability)

以上はモータの構造に関する事柄でした。次は，この構造と超電導の関係からどのような特性のモータが実現できるのか，それを取りあげます。

まず，超電導現象としては直流を想定します。 図17-4 のロータ構造において各導体に流れる電流は時間的に一定であると想定します。導体毎に電流の向きと大きさは異なっていても，それぞれの位置では臨界電流以下の電流であるとします。臨界電流とは何か？それを説明するのが 図17-6 の電圧と電流の関係図です。臨界電流以下であれば，導体には電圧が現れません。この場合にはロータは永久磁石のように向きが変化しない磁界を発生しているので，この状態でモータは同期モータの性質をもちます。つまりすべり ＝0です。しかし，この運転法で負荷を大きくしていくと，ロータが同期速度より少し低くなります。するとロータの巻線には電磁誘導による電圧が発生して電流が低い周波数の交流になり， 図17-7 に示すように臨界電流を超えます。この状態は常電導状態で大きな電流が流れるようなもので，導体棒に流れる電流は直流ではなく，１Hzほどのゆっくりと変化する交流になります。第15回に論じた誘導モータの2次抵抗が低い状態に似てきます。

これは同期運転から非同期運転に滑らかに推移することを意味します。強い永久磁石を使う同期機ではこの推移ができません。同期脱出と呼ばれるのですが，ガックという感じでロータの回転が止まってしまいます。では弱い永久磁石を使うとどうなるでしょうか？それがヒステリシス同期モータであって，同期脱出しても滑らかに回ります。これが 図17-8 に示す同期・非同期 両立性です。超電導モ―タは，負荷によって自動的に誘導機にも同期機にもなり，ヒステリシス同期モータに似ているのですが，実際のヒステリシスモータよりはずっと大きな同期脱出トルクが得られます。

トランジスタ技術SPECIAL [4] にも記したのですが，超電導誘導モータの始動時にはすべり回転を経由して同期回転に収束します。このすべり回転は定常状態でも実現可能です。 つまり，同期回転とともに両立します。実施した負荷試験結果では，産業用3.7 kWの定格出力のモータのステータ鉄心を用いたものでも20 kWの最大同期出力が実現しています。さらに過大な負荷を印加したところ，最大同期出力の2倍以上(41.3 kW；回転数：1760 rpm，60 Hzでのすべり ＝0.022)の出力が達成されました。つまり，もとの出力の10倍以上の出力が容易に実現されたのです。通常回転時は，効率の高い同期回転で運転し，過大な負荷が印加されると自律的にすべり運転に移行して，損失を伴いながらも運転を継続することができて，過負荷耐量の性能を持たせることが可能です。

図17-4 の試作機は，定格1.5kWのものを超電導化したものであり，初期の実験ではその3倍の4.5kWの出力を確認しています。多くのテキストでは，モータに発生するトルク（回転力）をローレンツ力で説明しています。しかし本当にローレンツ力を使うモータはコアレスDCモータなどの少数です。経済ベースでは99％以上が非ローレンツ力を利用してモータは回っています。非ローレンツ力を説明したのが，コラムに記したようにTeareの非線形理論でした。非線形の典型である磁気ヒステリシスによって発生するトルク(回転力)と鉄損の関係を整然と説明した理論です。それについては，インダクター（誘導子）によるトルクの原理と合わせて，第11回に詳しく記しました。トルクの発生に関してそれに類似であるばかりか,さらに意味深い機能が超電導に宿されるのが不思議です。

横軸を電流，縦軸を電圧として表す臨界電流特性。常温の銅やアルミでは電圧と電流が比例関係（線形）にあるが，超電導では赤線のような非線形になる。これによって安定な高い加速性が保たれる。
Illustration 17-6 Critical current characteristics, with current on the horizontal axis, and the voltage on the vertical axis. While voltage and current are proportional to each other (linear) in copper and aluminum at room temperature, it becomes non-linear (the red curve) in superconductivity, maintaining a stable, high-level acceleration.

Stable acceleration

The HTS is characterized by extremely low loss, which achieves high efficiency and torque characteristics of motors. On the other hand, low resistance means that the motor cannot be controlled, raising concerns that the motor can easily start vibrating unstably upon a sudden acceleration and deceleration. However, an HTS induction motor can maintain stable rotation, due to its non-linear resistance shown in Illustration 17-6 . A case of 20 kW-class motor is shown in reference [6] . It describes that, even with a load 1.5 times the rated output, the motor reached steady-state speed in acceleration time of 0.4s without vibrations. For the prototype shown in Illustration 17-3 , we used an inverter * , not a commercial 60Hz power source, to conduct an experiment at 38Hz. As a result, we confirmed a 4.5kW output, which is three times the rated output of 1.5kW.
There is an electric power regeneration mechanism deeply related to the improvement of energy efficiency. When an electric vehicle goes down a slope, gravitational potential energy is converted into electricity, and the battery is charged. Reference [7] describes the excellent features of an HTS induction motor in this technology. * Inverter: an electronic circuit that uses transistors, FETs, etc. to convert direct current into three-phase alternating currents of required voltage and frequency.

エネルギー効率の向上に深く関係する電力回生の仕組みがあります。電気自動車なら坂道を下るとき，重力によるポテンシャルエネルギーを電力に変えながらバッテリーを充電する仕組みです。参考資料 [4] ではこの面でも超電導誘導・同期機の優れた特徴が示されています。

交流が発生するときの臨界電流の作用

図17-7 　交流が発生するときの臨界電流の作用
電流がロータの導体棒の臨界電流以下になると，鎖交していた磁束が捕捉される。ロータの回路方程式では超電導状態になった時に

より ＝時間的に変化のない一定の鎖交磁束となる。つまり磁束が捕捉されると考えられる。ロータはあたかも永久磁石のように振る舞い，モータとしては同期機になる。

サンプル画像
図17-8 　誘導モータ，ヒステリシスモータ及び超電導誘導・同期機のT-N特性。ヒステリシスモータは同期・非同期両立性を備える。超電導モータにもこの特性が現れる。この不思議こそ意味深長である。

Future prospects

Kenjo: have so far described mysteries surrounding motors in 17 lectures of this series. When discussing motors, the three main pillars are important:  1) the mechanical structure itself,  2) the electric and electronic circuits to drive the motor, and  3) control items such as rotating angle, speed, and torque.  In this series, I focused mainly on the structure and the physical characteristics of the materials used. Professor Nakamura, you are implementing researches into using the HTS materials in the structure of induction motors, a typical type of motor that does not use permanent magnets. Will you comment on future prospects?

Nakamura:  　Approximately half of electricity generated around the world is used for motors. As stated in this series, while many types and applications of motors exist, industrial squirrel-cage motors account for a significant portion of the electricity consumption. This is one of the reasons that I chose the application of superconductivity to this kind of motor for my research theme. The range of the outcomes of this research is wide, in my opinion.

Kenjo:  Can your technology be applied to rotate airplane propellers? I apologize for talking about myself, but when Nidec Motor Engineering Research Laboratory was established in 2005, one of the themes was the creation of a small, high-power motor. In order to have a clear goal, we chose the F5B racing glider. The key was to design a motor that could lift the aircraft 150 meters above the ground in two seconds. We were successful, and a young pilot from Italy won the F5B 2010 World Championship, using our motor. Based on the data from that time I recently tried to design a motor for a human-carrying eVTOL, but found that heat in the stator windings can be extremely high as long as normal copper wire is used. Professor Nakamura, can we use your HTS technology to resolve this issue?

Nakamura: In the area of transportation, the construction of the linear maglev Chuo Shinkansen is underway, but it seems to take several more years before its realization. Other possible transportation equipment include cars, ships, and airplanes. Particularly active development of HTS motors aimed for electric aircraft is ongoing world widely. However, from my perspective as a rotating machinery expert, at least on a published basis, a structure that guarantees performance close to practical use has not yet been made clear. In particular, there seems to be not many research results that clarify the loaded characteristics as experimental data, so it is essential to first build a solid track record as a HTS rotating machine. Automotive drive motors typically have a maximum output of over 100 kW and an average output in several tens of kW range. When operating them while cooling with a separate refrigeration system, it becomes difficult to find advantages in terms of efficiency and cost. However, hydrogen engine vehicles equipped with liquid hydrogen tanks (the boiling point of liquid hydrogen is -253°C) are being developed for racing purposes, so if we can utilize such cold environments, we may be able to leverage the advantages of HTS motors. In fact, we at Kyoto University, in collaboration with Torishima Pump Mfg. Co., Ltd., have successfully conducted pump combination tests on the liquid hydrogen pump. We are working on as part of the NEDO project, achieving a world maximum flow rate of 30.5 m3/h and a world highest pressure of 1.6 MPa for machines in the tens of kW range at a speed of 5,000 rpm, making practical application in sight

Kenjo:  My last question: What are the major challenges if we further advance and make the stator windings also utilize superconducting materials?

Nakamura:  A motor in which not only the rotor's (field) winding but also the stator's three-phase windings for alternating currents are made of HTS materials is called a fully-superconducting motor, and this is considered the ultimate form of a high-efficiency superconducting rotating machine. However, even in no-load tests, there are few successful examples [8] , and regarding clear successful examples of loaded rotation tests or variable speed tests published, my group is currently the only one in the world, making it a rotating machine with generally high hurdles [9] . Furthermore, when the superconducting 3-phase stator windings are excited with a 3-phase voltage source, there is a risk of unbalanced currents and issues with alternating current losses; i.e. when an alternating electromagnetic field is applied to superconducting materials, losses occur due to a mechanism similar to the hysteresis loss. So, we must be cautious.

Kenjo:  Thank you very much Professor Nakamura.

References, etc.: