FEATURED TOPIC

Enhanced Functionality of Soft Magnetic Compositesfor High-Performance Axial Gap Motors
Tatsuya SAITO*, Yuta ENOKIZONO, Daichi AZUMA, Tomoyuki ISHIMINE, Tomoyuki UENO,
Yuichi NAKAMURA, and Reiko OKUNO---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------The advancement of electric mobility as well as improvement in the efficiency of home appliance and industrial equipment have
led to a need for higher performance motors. Axial gap motors (AGMs) are attracting attention as a motor that meets the needbecause of their low profile and high torque compared to radial gap motors. We have demonstrated the high torque and highefficiency of AGMs with soft magnetic powder composites (SMPCs) and started mass production of SMPCs for AGMs. In order tocontribute to the further adoption of AGMs, we have developed a low-loss SMPC, pole-shoe teeth core, and thin-insulation-coated
SMPC.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: soft magnetic powder composite, electric motor, axial gap, low profile, high torque

1. Introduction
Against the backdrop of growing calls for environmental friendliness, extensive efforts are being directed atpromoting electric mobility and improving the efficiency ofhome appliances and industrial equipment. Electric motors
are key devices critically affecting the performance of theseapplications. As demand for them has grown, the importance of making them small, light, and efficient has become
ever more apparent. Radial gap motors (RGMs) are a typeof motor in common use today. Their performance has
improved through control method developments and theimproved performance of motor components including
cores, copper windings, and magnets. Meanwhile, RGMs
have their drawbacks including that the torque decreases
with thin shape, which is attributed to their motor structure.
Consequently, axial gap motors (AGMs) have come into
focus. Compared with RGMs, AGMs develop high torquewith a thin-shape design. However, unlike RGMs, AGMs
require a three-dimensionally shaped core, which is difficult
to manufacture using the magnetic steel sheets*1 conventionally used in electric motors. Therefore, AGMs havebeen in use only to a limited extent.
We developed soft magnetic powder composites(SMPCs) with superior magnetic isotropy and a highdegree of geometric freedom. We have been massproducing them for diesel engine fuel injection valves,
boost converter reactors for hybrid vehicle, and ignition
coils.(1)–(3) Drawing on years of experience in these technologies, we developed SMPCs for AGMs and demonstrated
their superiority for motor performance(4) and began massproducing AGM cores in August 2020 (Photo 1). Their use
is expected to expand in the future. For example, AGMs
with the industry’s highest level of power density per unit
weight were recently selected for the mass production of
traction motors for use in hybrid electric vehicles(5). Thus,
expectations are high for the increasing use of AGMs in the
future. This article reports on the features and the latest
development status of our SMPCs and the advantages
gained from applying them to AGMs.

Photo 1. AGM core in the beginning phase of its mass production

2. SMPC: Overview and Features
2-1 Overview of SMPC
SMPCs are compacted using insulation-coated ironpowder and are completed through heat treatment, which isintended to remove strain introduced during compaction
process. Using pure iron or an iron-based alloy with a highsaturation flux density*2 and superb plastic deformation
capacity, our SMPCs ensure a balance between high flux
density and excellent AC magnetic properties through highdensity compacting without breaking the insulating coatinga few tens of nanometers in thickness.
Unlike magnetic steel sheets, SMPCs with a virtually
uniform material structure in every aspect are magneticallyisotropic. Moreover, SMPCs are highly suitable for coresthat require three-dimensional magnetic circuits, such as in
AGMs, since they are net-shape-production*3 by filling amold with a powder suitably meeting various geometricrequirements.
Furthermore, while the measurements obtained fromsingle sheets before lamination are generally listed in catalogs of magnetic steel sheets, their magnetic properties
degrade due to stresses and thermal strains resulting fromstamping, staking, welding, and other product-shaping
processes. This is a cause of the differences between design
values and measurements regarding motor performance.(6),(7)In contrast, with SMPCs, specimens used to acquire
magnetic property data for design purpose are produced

SUMITOMO ELECTRIC TECHNICAL REVIEW · NUMBER 92 · APRIL 2021 · 47

through a similar process to shaped products. Consequently,
one advantage of SMPCs is small differences between
design values and measurements in products.
2-2 SMPC material properties
Motor cores are required to have a high flux density
contributing to high torque and a low energy loss in thecore contributing to efficiency (hereinafter, “iron loss”).
Table 1 presents the DC-BH and iron loss characteristics of
our SMPCs and magnetic steel sheets for motors. The
SMPC HB2 exhibits a higher flux density in the highmagnetic-field region than the magnetic steel sheets do,enabling it to avoid decreases in torque resulting from themagnetic saturation of the core. This comes from the highdensity compacting of our SMPCs using an iron powder
with a higher saturation flux density than magnetic steel
sheets.
Compared with magnetic steel sheets 0.35 mm inthickness used in general electric motors, the HB2 exhibits
an advantage in iron loss even in the low-frequency range
often used by motors. The advantage increases in higherflux and higher-frequency ranges. Moreover, the HX3developed with the aim of making the core further low-loss
exhibits properties comparable to those of the low-loss
Table 1. Magnetic properties of SMPCs and magnetic steel sheets(8)Magnetic Flux density (T)Material

SMPC

B2kA/mB20kA/m

B50kA/m

HB2

1.02

1.76

HX3

0.66

0.35 mm-thick(JIS 35A360)

Magnetic
0.30 mm-thicksteel(30HX1600)
sheet
0.20 mm-thick(20HX1200)

Iron loss (kW/m3)
Bm = 1.0 T

Bm = 1.7 T

400 Hz

1 kHz

400 Hz

1 kHz

2.04

221

696

513

1640

1.66

1.81

146

413

311

912

1.45
(1.60)

1.90(1.98)

2.00(2.09)

227
(153)

1006
(650)

628

2760

-

-

-

178
(109)

734
(433)

474

1996

-

-

-

128
(82)

487
(304)

344

1335

grade magnetic steel sheets 0.20 mm in thickness.
Moreover, because the SMPC is a low-iron-loss material in the high-frequency range, the material is expected tosuppress energy losses increasing due to high harmonics
generated in an inverter-powered motor. We in practiceevaluated the iron-loss characteristics of inverter-powered
motors. The SMPC showed diminished high-harmoniccomponents superimposed on its hysteresis loop*4, asshown in Fig. 1. Consequently, while the magnetic steel
sheets exhibited 10% or more increases in loss resulting
from imposed high harmonics, the SMPC suppressed theincrease to approximately 3%.

3. Application to Axial Gap Motors
3-1 Overview of axial gap motor
SMPCs are superior to magnetic steel sheets in termsof performance, as described above. To be a competitive
motor even with costs taken into account, it is preferable toapply an SMPC to a motor, like an AGM, that benefits
from the extra degree of geometric freedom of SMPC.
An AGM is a motor that develops high torque from athin-shape design. It is structured as schematically shownin Fig. 2. While conventionally predominant RGMs have astructure in which the stator and rotor are arranged cylindrically, AGMs are structured so that the stator and rotor
are overlaid in the direction of the axis of rotation. In athin-shape design, RGMs develop reduced torque due to a
reduced facing area between the rotor and stator. In
contrast, AGMs maintain high torque because the facingarea between rotor and stator remains unchanged.

※Measurements of toroidal specimens※Figures in parentheses: values from single-sheet testingFig. 2. Different motor structures

Fig. 1. Differences in hysteresis loop (a) and iron loss (b) with high harmonics
imposed and not imposed

For RGMs comprised of two-dimensional magnetic
circuits, using magnetic steel sheets, it is easy to construct acore. In contrast, AGMs require three-dimensional magnetic
circuits to be formed. To use magnetic steel sheets toconstruct them, required techniques include lamination of
sheets varying in width, joining steel sheets to change the48 · Enhanced Functionality of Soft Magnetic Composites for High-Performance Axial Gap Motors

direction of lamination, and wire winding round a woundcore. These requirements lead to greater manufacturing
difficulty and higher cost. In contrast, using SMPCs withmagnetic isotropy and a high degree of geometric freedom,
it is easy to construct cores for AGMs. Consequently, on
top of the aforementioned performance benefits, SMPCs
are expected to facilitate the commercialization of AGMs.
3-2 Properties of SMPCs and motor efficiency
improvement
Using the parts and configurations illustrated inFig. 2, we have compared and evaluated motor performance
under the preconditions of identically sized and flat-shaped
RGMs and AGMs. We have proved, through analysis anduse of actual motors, that AGMs develop about 1.6 times thetorque of RGMs with a 1% higher maximum efficiency.(4) In
this verification, the SMPC used in the AGM was the HB2.
Additionally, the newly developed material HX3 withfurther reduced iron loss, as shown in Table 1, was used toevaluate motor performance using it as the stator core in
the AGM. The same design specifications, including thegeometry of the stator core, as those of the conventional
AGM were used (Table 2).
Table 2. Design specifications for the tested motor
Volume of motor

0.392 ℓ (Φ110 × 41.3 t)

Rotor

StatorNo. of pole

10Magnet weight89.2 gMagnet material

S5B-17ME

No. of slot

12Core materials

SMPC: HB2, HX3Space factor of coil40%

Maximum rotational speed

6000 rpm

Operating temp. (CAE)

80℃

Figure 3 presents efficiency maps for each motor.
Using the HX3, the motor efficiency improved by 1% to
2% across almost the entire operating region in comparison
with the use of the HB2. Consequently, the HX3 is effective for improving the motor efficiency, as intended.
Meanwhile, Fig. 4 plots the torque characteristics of the
AGM incorporating the HX3, which were largely similar to
those of using the HB2. Comparisons of the DC-BH char-

1.3

Stator core HB2

1.1

94%

0.9
0.7
0.5

95.6%

94.1%

90%

0.3

acteristics of the HB2 and HX3, presented in Table 1, showthat the HX3 had lower magnetic permeability*5 than the
HB2. Despite this general disadvantage in terms of torquecharacteristics, the HX3 exhibited a contradictory result.
One reason for this result is that, in the structure of theAGM under development by us, the extent of the stator
core’s influence on the overall magnetic resistance*6 of the
motor is not dominant. Our AGM has an air gap of 1.0 mm
between the rotor and stator, which is wider than those ofgeneral RGMs—including our RGM built for comparison
purposes—designed to approximately 0.5 mm. Moreover,in our AGM, the number of air gaps per magnetic circuit is
twice greater than those of RGMs. The magnetic resistanceof the AGM used for the current verification was calculated(Fig. 5). Even if the magnetic permeability of the stator
core decreases to half that of the HB2, the overall magnetic
resistance of the motor increases only by 1.9% and thetorque decreases by no more than 1.9%. In addition, otheranalysis results show that, even with a similar iron loss tothat of the HB2, magnetic permeability decreasing to onefifth leads to approximately 1% improvement in motor
efficiency in the light load and high rotational speed
regions.(8) These findings are contradictory to the conventional requirement that the motor core material should havehigh magnetic permeability. Yet this result suggests a novel
material development option, that is to reduce iron loss
even sacrificing magnetic permeability, depending on the
motor configuration—not limited to AGMs—and in which
operation region to improve motor efficiency. Moreover, inline with these development guidelines, we have developed
a material with another 40% iron loss reduction from theStator core HX3

Ryoke
Rteeth

95%

Rpm
Rgap

94%
93%
92%
91%
90%

R = 2Rcore + 2Rpm + 4Rgap*Rcore = Ryoke + 2Rteeth

1.9%UP

Rgap

93%
92%
91%

Fig. 4. Torque characteristics of newly developed AGMs

Half

Rteeth

r

HB2

HX3
Ryoke

0.1
1000 2000 3000 4000 5000 6000[

1000 2000 3000 4000 5000 6000[

Fig. 3. Efficiency maps of newly developed AGMsr

Fig. 5. Magnetic resistance of AGM vs. magnetic permeability of core

SUMITOMO ELECTRIC TECHNICAL REVIEW · NUMBER 92 · APRIL 2021 · 49

HX3 and are evaluating it on actual motors. While it is noteasy to alter the magnetic permeability of magnetic steel
sheets, SMPCs enable magnetic permeability to vary throughchanges to the thickness of the coating, iron powder particle
size, and compacting density. Consequently, a feature of
SMPCs is that the material gives another degree of freedom
in motor design.
3-3 Motor downsizing by AGM design
While the foregoing discussion described that, given
identical size, AGMs develop higher torque than RGMs,
practical applications require downsized motors withoutcompromising performance. To meet this challenge, we
studied how much commercially available RGMs can be
downsized by turning them into AGMs. Table 3 provides
an example comparison. We designed an AGM using thesame radial dimensions and magnet material as a commercially available RGM to develop comparable torque andefficiency. The AGM demonstrated a 58% motor height
reduction and a 50% motor weight reduction. Furthermore,
the cost of this motor was calculated based on the price perunit weight of the key components, that is, magnet, coil,and core. It was found that the cost could be cut by approximately 15% at the maximum, partly owing to reduced
usage of parts made possible by downsizing.
Table 3. Example of motor downsizing by AGM design
AGM

Commercially available RGM

20.5 mm( 49%)

40 mm

83 mm

35 mm
( 58%)

casing))0.95 kg (▲44%)

1.7 kg

1.6 kg (▲50%)

3.2 kg85～96%

100%

0.65 Nm
90.6%

90.9%

Meanwhile, a lineup of RGMs differing in performance may be offered, using the same stamping dies, yetchanging the number of laminated sheets. With AGMs
incorporating SMPCs, it is also possible to vary the coreheight using the same mold. Consequently, as with RGMs,
an expanded lineup of AGMs varying in performance canbe offered while reducing the mold cost (Table 4).

4. Enhanced Functionality of SMPCs
The foregoing discussion described that an AGM
design incorporating SMPCs can make motors smaller andimprove their efficiency. To augment this advantage
further, we are exploring core modeling and working on
the development of peripheral technologies, in addition to
improving the material characteristics, as described below.
4-1 Molding technology for pole-shoe teeth cores
In general, RGMs have a stator core provided withpole shoes at the teeth ends for improved motor torque andefficiency, reduced motor-originated noise and vibration,and enhanced reliability. Providing pole shoes is also often
considered for AGMs because similar effects are observed.
To build such a core from an SMPC, it was conventionallyimpossible to fabricate an integral part consisting of poleshoes, teeth, and a yoke because, after compression
molding, the molded part could not be released from themold. Consequently, pole shoes or a yoke were separately
constructed and they were subsequently joined to build acore with pole shoes. This construction method, however,has a few drawbacks: that the need for more than one typeof mold is disadvantageous in terms of cost; that clearances
at joints reduce torque and efficiency; and that assembled
cores show poor accuracy in height, although this accuracy
is critical for determining the size of the air gap, which
affects motor performance.
We explored technical solutions for these drawbacks,
apart from the conventional compacting method, anddeveloped a technique of forming an integral partconsisting of pole shoes, teeth, and a yoke, as illustrated inTable 5. This technique requires only one type of mold.
Moreover, it has proved itself to enable the compacting
accuracy in the direction of core height to improve 10
times or more as compared with the conventional method.
Thus, it is likely to lead to improved accuracy in the size ofthe air gap. Although yoke pieces are joined to each other,
AGMs described in Chapter 3 involve similar divisions,and it has been demonstrated through analysis and on an
actual machine that this way of division does not affectmotor performance. This technology is thought to improvethe degree of freedom of AGM design, thereby helping
achieve additional performance improvements.
Table 5. Features of the newly developed axial core with pole shoes
With pole shoes

Without pole shoes

18%)
88.9%

91.5% (

1.6%)

Developed

Integrallycompacted

Table 4. Example of motor performance variation enabled by varying

Motor height24.5 mm35.6 mm
46.7 mm(Core: 10.1 mm×2) (Core: 15.6 mm×2) (Core: 21.1 mm×2)
Torque constantNm/(Arms/mm2)Max. eﬃciency

0.062

0.114

0.178

90.4%

92.6%

94.5%

1

50 · Enhanced Functionality of Soft Magnetic Composites for High-Performance Axial Gap Motors

2

1(High-

4-2 Improved insulating coating technology
In the core winding process, insulating paper or aresin bobbin provides insulation between the core and coil.
The conventional insulating paper and resin bobbin are 500µm or more in thickness and take up some space, necessitating a reduced number of turns of the coil for this space.
Meanwhile, the coil is one of the parts that become hottest
in the motor. By providing the above-mentioned part, alarge thermal resistance is produced between the coil andcore, degrading the coil’s heat dissipation capacity andimposing design restrictions on the motor due to temperature rise. As a solution to this challenge, we have workedon the development of a technology of providing thesurfaces of the core with a resin coating a few tens of
micrometers in thickness that offers high dielectricstrength.(4) Through coating thickness reduction andproductivity improvement efforts, a thin and high-insulating coating (0.1 kV/µm or more) has been developed, as
shown in Fig. 6. Table 6 presents the verification results forthe heat-dissipation merits made possible by this technology. One sample core had windings with insulating
paper between the core and the windings. The other sample

core was provided with windings and the newly developed
coating. These samples were compared with respect to the
maximum temperature reached during electric current
application. The use of the newly developed coating
contributed to the maximum reachable temperature being
44°C lower than that observed with the use of insulating
paper. Thus it becomes possible to apply a higher electric
current commensurate with this temperature difference,
enabling the motor to be further reduced in size.
Furthermore, the reduced number of parts is expected tomake motor assembly simpler.

5. Conclusion
We improved the performance of SMPCs, demonstrated the material’s use in axial gap motor (AGM) designfor motor downsizing and efficiency improvements, andestablished novel development guidelines for motor cores
toward a low-carbon society. We also demonstrated an
AGM that halves both volume and weight with the same
performance for commercially available radial gap motor,
and showed that it can contribute to cost reduction.
Furthermore, it developed a pole-shoe teeth core withsuperb productivity and an insulation coating technology
for thin and high-insulating coatings to enhance the advantages of AGMs. In addition to these, we are working on thedevelopment of peripheral technologies such as for fixing
SMPCs(10) and an innovatively structured AGM(11) to help
AGMs come into wider use.

6. Acknowledgements

Fig. 6. Insulation voltage of the newly developed coating

Table 6. Heat dissipation comparison of coated core and paper-insulated core

We are deeply grateful to Professor Takemoto and Dr.
Tsunata at the Graduate School of Natural Science andTechnology, Okayama University for their invaluable guidance and support for our electromagnetic analysis of
motors and evaluation of actual motors.

＊1

＊2

＊3

＊4

Technical Terms

Magnetic steel sheet core: Thin sheets of iron-silicon
alloy, laminated to form soft magnetic cores.
Generally, their iron loss characteristics improve withdecreasing sheet thickness; however, this is costly
due to declining productivity.
Saturation flux density: The maximum flux density(the amount of magnetic flux per unit area) reachable
by a magnetic material. Higher saturation flux density
enables making cores smaller.
Net-shape production: A powder molding processused to obtain a final product shape simply throughpressure molding using a mold without the need for a
post-process such as machining.
Hysteresis loop: The flux density of a magnetic
material changes differently in response to anincrease or decrease in the field strength, forming a
curve known as a hysteresis loop.

SUMITOMO ELECTRIC TECHNICAL REVIEW · NUMBER 92 · APRIL 2021 · 51

＊5

＊6

 agnetic permeability: The factor of proportionality
M
determined when expressing the relationship between
the field strength H and the flux density B by: B =μH. Higher flux density can be achieved from higher
magnetic permeability at a lower coil current.
Magnetic resistance: A measure of how strong
opposition the magnetic flux encounters in a magnetic
circuit.

Contributors The lead author is indicated by an asterisk (*).

T. SAITO*• ‌Dr. Eng.
Assistant Manager, Advanced & Materials
Laboratories

Y. ENOKIZONO• ‌Advanced & Materials Laboratories

References(1)	
Y. Shimada at el., “Development of High-Performance P/M Soft
Magnetic Material,” J-Jpn. Soc. Powder Powder Metallurgy, Vol.53,
No.8, pp.686-695 (August 2006)(2)	N. Igarashi at el., “Pure Iron Based Magnetic Composite Core That
Enables Downsizing Automotive Reactors,” SEI Technical Review, No.
80, pp.98-103 (April 2015)(3)	T. Ueno at el., “Development of a Soft Magnetic Powder Core with
Distinct Magnetic Characteristics, Examples of Its Practical
Applications, and Future Outlook,” SEI Technical Review, No.82,
pp.1-7 (April 2016)(4)	A. Watanabe et al., “Thin and High-Torque Axial Gap Motor Using
Soft Magnetic Powder Cores,” SEI Technical Review, No. 86, pp.106112 (April 2018)(5)	“Ferrari selects YASA electric motor for SF90 Stradale, the company’s
first hybrid production series super car.” U.K., YASA Limited
homepage, https://www.yasa.com/news/ferrari-selects-yasa-for-sf90stradale/(6)	M. Yabumoto et al., “Electrical Steel Sheet for Traction Motors ofHybrid/Electric Vehicles,” NIPPON STEEL TECHNICAL REPORTNo. 87 pp.57-61 (July 2003)(7)	T. Wakisaka at el., “Electrical Steel Sheet for Traction Motor of Hybrid/
Electrical Vehicles,” NIPPON STEEL TECHNICAL REPORT, No.103,
pp.116-120 (May 2013)(8)	
NIPPON STEEL & SUMITOMO METAL, NON-ORIENTED
ELECTRICAL STEEL SHEETS catalog, pp.25(9)	R. Tsunata et al., “Investigation of development policy of SMC material
for high efficiency over a wide operating range in axial gap motors
having coreless rotor structure,” MAGDA conference, No.28, pp.34-41(2019)(10)	Y. Enokizono et al., “Magnetic Properties of Electromagnetic Steel
Sheets and Soft Magnetic Powder Core under Compressive Stress,”
IEEJ, The Annual Meeting record I.E.E. Japan 2019, G209‐Cl(11)	R. Tsunata et al., “A Proposal of Ultra-Flat Axial Gap Motor Employing
C-type SMC Core,” IECON 2019-45th Annual Conference of the IEEE
industrial Electronics Society Lisbon, portugal, pp.847-879 (2019)

D. AZUMA• ‌Dr. Eng
Assistant General Manager, Advanced & Materials
Laboratories

T. ISHIMINE• ‌Group Manager, Advanced & Materials Laboratories

T. UENO• ‌Dr. Eng
General Manager, Advanced & Materials
Laboratories

Y. NAKAMURA• ‌Group Manager, Analysis Technology ResearchCenter, R&D Laboratories

R. OKUNO• ‌Assistant General Manager, Sumitomo Electric
Sintered Alloy, LTD.

52 · Enhanced Functionality of Soft Magnetic Composites for High-Performance Axial Gap Motors