Special Issue

Superconductivity
Contribution to the achievement of SDGs
by Furukawa Electric Group

Development of Elemental Technologyfor Superconducting Motor
Kairi Onyu*, Yusuke Yamada*, Yoshikazu Okuno*, Hiroshi Orito*, Tomoaki Toratani*
The superconducting phenomenon was discovered in 1911 and the research ofmaking superconducting wire has been advancing since. The superconducting
wire was put to practical use and commercial use as the wire to transmit a large current with alow loss. The idea of a superconducting motor using the superconducting wire has been discussed for a long time. And even though a large number of prototypes were produced, therewere still a lot of problems for practical use on a commercial level mainly in the technology ofcryogenic refrigeration and insulation to maintain the cryogenic state. To solve this problem, weproposed the vacuum insulation structure which was lighter in weight and easier to handle incomparison with the present technology. Further, we produced a prototype of the superconducting motor reflecting the structure and evaluated it. The prototype of the superconducting motor
produced this time demonstrated that the superconducting state came into effect easily eventhough its structure was constructed with a lightweight plastic one.

ABSTRACT1.

INTRODUCTION

The superconducting phenomenon is the one in whichelectric resistance becomes zero under the cryogenic
temperature and has been researched for a long timesince its discovery in 1911 until today. The superconducting wire which was made by making the superconducting
material into wire shape is broadly classified into the lowtemperature superconducting wire composed of metallic
materials and the high temperature superconducting wire
composed of oxide substances. We, Furukawa ElectricGroup are the rare company in the world to work on bothtypes of low temperature superconducting materials andhigh temperature ones.
It is important to improve the efficiency of a motor torealize carbon neutrality for which we have to reduce theemissions of greenhouse gases to total zero by 2050.
Approximately more than 50% of domestic power consumption is consumed in motor use. Assuming that theefficiency of all the motors in operation is improved by 1%,
15.5 billion kWh of power consumption will be reduced,
and it is equivalent to the reduction of 6,900 thousand
tons of CO2 emission.
The research of the superconducting motor has been
carried out for a long time and a large number of prototypes were produced in the research of universities and in
the academic-industrial collaboration projects. They
aimed at the reduction of the Joule loss which is one of
the main factors of motor loss and the improvement of*Electronics Laboratory, Research & Development Division

the power density by high magnetic force. The superconducting material is required to be applied to accomplish ahigh power density that has never been realized by any ofthe present copper wire motors. However, the coolingtechnology under –196°C is required even for the hightemperature superconducting wire, which is called a hightemperature one in the superconducting industry and also
in addition advanced insulation technology is required.
Therefore, those problems have demerits such as upsizing, reduced total efficiency, and increased cost by thoseincidental facilities for cooling and insulation are working
against the practical use at a commercial level.
In this paper, we will report on the invented vacuum
insulated structure aiming at the practical use of thesuperconducting motor and the result of the prototype
that reflects the structure.

2.

THE PRESENT INSULATION TECHNOLOGY

In the field of superconducting technology, the vacuum
insulating container was traditionally applied as the insulation technology. Its basic structure is composed of an
airtight container containing a refrigerant called a cryostat
contained in a vacuum container, even though its shapeand accessories are varied for the application. Glass fiber
reinforced plastic is applied to the material for the container for its use where the occurrence of an eddy current
loss in a high magnetic field becomes a problem.
However, it is often made of stainless steel from the viewpoint of easier manufacturing processing and lower cost.
Further, the vacuum insulated part includes techniques toFurukawa Electric Review, No. 55 2024

27

Development of Elemental Technology for Superconducting Motor

HEAT TRANSFER MECHANISM OF
INSULATION MATERIAL

In the market, mainly we have fiber insulation materials
represented by glass wool, foam insulation materials suchas urethane foam and polyethylene foam, and particulate
insulation materials such as silica aerogel.
The heat transfer phenomena in insulation materials areclassified into three mechanisms, those are the solid heattransfer in which heat is transferred through the solid heat
conduction, the convection heat transfer in which heat istransferred accompanying the convection movement ofgas molecules, and the radiation heat transfer in whichheat is transferred through the movement of radiation
heat. And the heat moves along with the apparent thermal conductivity which is the total of each mechanism of
the heat transfer. The solid heat transfer depends on thedensity of the material and the lower the density of thematerial becomes, the less the heat is transferred in general. The convection heat transfer is almost free from theinfluence of the material density and contributes less tothe heat transfer along with the longer mean free path.
Therefore, its influence on the apparent thermal conductivity is almost zero in the area where the degree of vacuum is higher than a certain level (Figure 1).
The apparent thermal conductivity of insulation materials in a vacuum strongly depends on the properties andconditions. The convection heat transfer largely contributes to that of the fiber insulation materials. The apparent
thermal conductivity is low in a high vacuum, but it
increases rapidly when the degree of vacuum becomes
lower. The foam insulation materials are less influenced
by the convection heat transfer and have a stable thermal

Total
Radiation

SolidConvection

Material density
Figure 1

Apparent thermal conductivity of insulating materials.

Thermal conductivity [mW/mK]

3.

conductivity free from the degree of vacuum, but theirfundamental solid thermal conductivity tends to be higher
than that of the fiber insulation materials. Silica aerogel
which is applied to a core material of vacuum insulation
panels because the particulate insulation materials areporous materials with more than 90% of porosity per its
volume to have a small material density. Further, because
either particle size or porous size is nano level, both thesolid thermal conduction among particles and the convection heat transfer in a porous are restrained and the surface area of radiation and scattering is increased to have ahigh insulation effect. In addition, it is known that theincrease of its thermal conductivity caused by the lowering degree of vacuum is slower than that of fiber insulationmaterials and foam insulation materials (Figure 2).

Apparent thermal conductivity

extremely reduce the heat flow from the outside, such as
keeping the high vacuum of 10 –3 Pa or higher at least tocontrol the convection heat transfer caused by gas molecules and in addition to equip with a reflective material tocontrol the heat transfer caused by radiation. When
applying a cryocooler as a substitute for a refrigerant, thestructure becomes simpler because no airtight structureis necessary, however, the vacuum container is indispensable for the purpose of insulation. Considering thevacuum container, at least several hours are necessary
for the exhaustion depending on the performance of thepump and the size of the container, and further, it results
in increasing the weight of the total system because thecontainer is made of metallic material. Although not muchinfluence caused by the weight is harming the fixed
equipment such as a generator, it is worried that the meritof the superconducting may be lost in its application inmoving bodies such as electric aircrafts and vehicles towhich the superconducting motor is expected to perform.
Therefore, in this research, we have examined the insulation structure which was easier to handle and more capable of weight reduction in comparison with the presenttechnology.

Nitrogen pressure [Pa]Figure 24.

Effects of atmospheric pressure on thermal
1)
conductivity of insulation materials .

NEW CONCEPT OF INSULATIONSTRUCTURE

We have focused on the fact that the thermal conductivityof the particulate insulation materials was not so dependent on the degree of vacuum even in the area of the low
degree of vacuum around 10 2 Pa. And applying the particulate insulation materials in the low degree of vacuum,
we have examined an easy gain in the insulation perforFurukawa Electric Review, No. 55 2024

28

Development of Elemental Technology for Superconducting Motormance which is equivalent to that of the present insulation structure in the high degree of vacuum (Figure 3). The
following advantages are expected as the required
degree of vacuum for the insulation is reduced to thelevel of a low degree of vacuum.
(1) The process of vacuum is simplified. (The exhausttime is remarkably reduced because only low to
medium vacuum pumps are required for the vacuum
exhaust equipment.)(2) Organic materials such as plastic can be positivelyapplied because the insulation performance is notsensitive to outgases.
(3) Restrictions on the materials and the processing
accuracy can be relaxed.
The present(high vacuum insulation)

The concept(low vacuum insulation)

High vacuum

Low vacuum

Objectforcooling

Objectforcooling
Insulation material

Metal housing
Figure 3

Plastic housing

Conceptual diagram of insulation structure.

Amount of heat ﬂow

We have analyzed the heat transfer with simulation toverify our tentative theory. As an analysis model, we havedesigned such a structure which had a cooler of liquid
nitrogen and a motor with a vacuum container containing
the cooler, and we have calculated the dependence of thevacuum container on the degree of vacuum against theheat flow from the outside to the low temperature part
under the condition of normal outside temperature andnormal outside pressure. The result is shown in Figure 4.

5.

PROTOTYPE OF THE
SUPERCONDUCTING MOTOR

We have designed and made a prototype of the superconducting motor based on the result of our analysis of
the conceptual structure to examine the insulation structure of low vacuum conditions. It was the basic idea
behind the design of prototype models of the superconducting motor produced in the past to apply a stronger
magnet force excited by the superconducting coil replacing that of a permanent magnet in order to improve thepower density. And in its cooling structure, a coolant wascirculated in the shaft of a rotor to cool the superconducting coil of the field magnet. We estimated that the coolingstructure mentioned above was adopted because its
structure could be simpler in comparison with the structure in which the armature coil of a stator was cooled witha coolant.
We have designed a superconducting motor with asmall power which emphasized small size and lightweight, because the purpose of our research was to
examine our insulation concept in the low vacuum. Our
3-D design model is shown in Figure 5 and the specifications are shown in Table 1.
To make the motor smaller in size and lighter in weight,
we adopted a rotor with a magnetic field exited by a permanent magnet and applied the superconducting coil tothe armature coil of a stator. The superconducting coil
was manufactured with the REBCO high temperature
superconducting wire made by SuperPower Inc., a mem-New conceptof insulation

Present insulation

10 -4 10 -3 10 -2 10 -1 10 0

the same level of heat flow as the high vacuum area ofthe present technology was gained. This result is notapplicable to all cases because it depends on the influence of the volume and shape of the container, and physical properties and condition of the insulation materials.
However, it suggests that good insulation can be realizedeven in the low degree of vacuum when applying thedesign with a proper structure and proper materials
based on our heat transfer analysis.

10 1

10 2

10 3

10 4

10 5

Degree of vacuum / Pa
Figure 4

Heat flow into insulated structures.

In the present technology, the heat flow is increasingalong with the lowering of the degree of vacuum. On theother hand, it was verified that in the conceptual model inwhich an insulation material was adopted, the heat flow
was reduced even in the low vacuum area, and almost

Figure 53-D design model of our superconducting motor.

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Development of Elemental Technology for Superconducting Motor

Superconducting motor specifications.

Table 1ItemsComposition

Type of motor

DC brushless inner rotor type

Field magnet source

Permanent magnet

Armature

Superconducting coil

Superconducting wire

REBCO

Number of poles

4 polesSlots

3 slots

Full external dimensions

L 287 x W 180 x H 324 mm

Weight13 kg

Method of coolingSuperconducting coils immersed inliquid nitrogen

Maximum torque

3.0 N･mMaximum rotation speed

1770 rpm

Power supply voltage

24 VMaximum current

30 A

Control method

PWM control with inverterber of our group. Further, we have applied a stator coremade of laminated electromagnetic steel sheets because
the motor did not require a high magnetic force for its
rather small power. With a mind based on our analysis ofelectromagnetic field not to create magnetic saturation in
the stator core and to reduce the magnetic field to which
the superconducting coil is exposed as much as possible, we have designed dimensions, shape, magnet, coil,and structure of the motor to achieve the required torqueand rotational speed. Further, applying the advantage ofthe insulation structure against the low vacuum, we haveadopted a plastic material for the vacuum container. An
external appearance of our prototype of the superconducting motor is shown in Figure 6.
We cooled the prototype motor with liquid nitrogen tofind that the liquid nitrogen could be stored in the internal
low temperature container without frost sticking on thesurface of the vacuum container and confirmed that oursuperconducting coil could rotate as a motor in thesuperconducting state.

DCPowerSuppryInverter
PWM
ControlPowerAnalyzer
Figure 7

Figure 66.

Prototype superconducting motor.

EVALUATION OF THE
SUPERCONDUCTING MOTOR

In order to calculate the efficiency of our superconducting
motor, we put a counterforce by regeneration of a dynamometer against a rotational force of the superconducting motor and calculated the efficiency from the torqueand the rotation speed in the state based on the equation(1). Further, we measured the efficiency of a copper wire
motor which had an equivalent power. Although we could
not compare them precisely because the magnet and thecoil structure of the copper wire motor were different fromthose of the superconducting motor, the comparison
could be instructive as a reference when the size of electromagnetic steel sheets from which the power could be
estimated was similar. The conceptual diagram of theevaluation of motor efficiency is shown in Figure 7 andthe evaluation results of the motor efficiency are shown inFigure 8.

Superconducting
Motor

Torquemeter

Regenerative
motor

InverterElectronic
loadTorque data
Rotation number data

Conceptual diagram of motor efficiency evaluation.

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Development of Elemental Technology for Superconducting Motor

Superconducting M 2.0 N・m

Copper wire M 2.0 N・m

Superconducting M 1.5 N・m

Copper wire M 1.5 N・m

Superconducting M 1.0 N・m
Superconducting M 0.5 N・m

Copper wire M 1.0 N・m
Copper wire M 0.5 N・m

100

Motor eﬃciency / %

9080
7060
50

0200

400

600

800

1000Rotation speed / rpm
Figure 8

Motor efficiency evaluation results.

In general, as the efficiency of a motor fluctuates
depending on its operation speed, it is optimized to havea high efficiency at its rated operation speed. In such a
motor as for an automobile, which is required to have awide operation area, the ratio of Joule loss increases
either in a low speed area and in a high speed area tocause a lowering of the efficiency. Joule loss in an armature coil is zero in the superconducting motor and wehave confirmed that the efficiency of the superconducting
motor was higher at each measured operation speed.
In the operation of our superconducting motor, thedegree of vacuum reached to the required level in 15
minutes or less of the vacuum exhaust time. We havesteeply reduced the process time until the start of coolingin comparison with that of the present technology.
Further, it is an advantage of the operation in the low vacuum that maintenance and leakage repair are easier withthe use of general-purpose sealing material or a glue.
Furthermore, the weight of our prototype motor isapproximately 13 kg and that with a vacuum container
made of present stainless steel is 35 kg or more according to our tentative calculation. As a result, we have realized a great weight reduction.

7.

CONCLUSION

112 years have passed since the discovery of the superconducting phenomenon. We, Furukawa Electric havecontinued to research it for many years, however, applied
products that have been put to practical use were limited
to MRI magnets and the instruments used such as inaccelerator fields and fusion fields where none of thepresent normal conducting materials could be applied,
and the worldwide scale of the business has been still
small. The main factors of the limited practical use mentioned above are expensive superconducting materials, ahigh hurdle against cooling and insulating to utilize thesuperconducting phenomenon and that the superconducting technology itself is still in the research phase andmany parts of it are still unresolved. A lot of research on
the superconducting material itself has been carried out,
however, elemental technologies to make use of thesuperconducting technology have not matured yet.
In this research, we have worked on developing theinsulation technology, one of the elemental technologies
for the superconducting motor. The fruits of this researchwere that we have successfully developed a superconducting motor that was lightweight with a short start-up
time based on a vacuum container making use of theadvantages of low-vacuum insulation structure and foundthat it operated at a higher efficiency at each of various
rotation speed in comparison with that of a normal temperature conducting motor. Applying the fruits of thisresearch to the experiment and the evaluation related tothe superconducting field, we are contributing to theresearch and development to make the superconducting
technology familiar and easier to use.

ACKNOWLEDGEMENT

The authors wish to express thanks to Professor
Takashi Kodama, Kyushu Institute of Technology andAssociate Professor Ikuya Kinefuchi, the University ofTokyo for their support and advice for this research.

REFERENCES(1)η : Motor efficiency
P in : Supply power to the motor
Pout : Power from the motor
T : Power torque (N·m)
N : Power rotation speed (rpm)1) J. Fricke et al: Journal of Thermal Envelope and Building Science,
28 (2005), 319.

Furukawa Electric Review, No. 55 202431