電気学会論文誌●（●●●●●●●部門誌）
IEEJ Transactions on ●●●●●●●●●●●●●●●
Vol.●● No.● pp.●-●●
DOI: ●.●●/ieejeiss.●●.●

LetterReduction in Leakage Magnetic Fluxof Wireless Power Transfer Systems with Halbach Coils
Keisuke Kusaka＊a) Member,Kazuki Yamagata＊ Non-member
Jin Katsuya＊＊ Non-member,
Tetsu Sato＊＊ Non-member(Manuscript received Jan. 00, 20XX, revised May 00, 20XX)
This paper proposes wireless power transfer (WPT) system with Halbach coils to reduce leakage magnetic flux. The transmission
coil using the Halbach coils generates magnetic flux in the vertical direction and coils that generate a magnetic flux in the horizontal
direction. The WPT systems with Halbach and spiral coils are developed and experimentally tested to verify the effect of reducing
the leakage magnetic flux density. The experimental results show that the magnetic flux density decreases by 35.6% at maximum.
Keywords : wireless power transfer, Halbach, magnetic flux leakage

1.

Introduction

400 mm
300 mmAluminum plate

In a wireless power transfer (WPT) system with magneticinduction, it is impossible to prevent leakage of magnetic flux dueto its operating principle. From the perspective of human protection,
International Commission on Non-Ionizing Radiation Protection(ICNIRP) has published the guidelines for leakage magnetic flux[1]. In order to reduce the leakage magnetic flux, active and passive
shielding methods with additional windings have been proposed [2].
However, the additional windings cause the cross-coupling between
a main winding and a canceling winding [3].
This letter proposes a wireless power transfer system with
Halbach coils which reduce the leakage magnetic flux withoutcross-coupling. In the Halbach winding, part of the transmission
coil also contributes to canceling out the leakage magnetic field.

2.

45 mm
5 mm
N15 mm45mm

FerriteAluminum plate(a) Spiral coils.
400 mm
300 mmAluminum plate
45 mm
5 mm

WPT system with Halbach coils

Nc2 Nh2

Gap
100 mm
5 mm
45mm

Figure 1 shows the cross-sections of the WPT coils. Fig. 1 (a) isthe conventional spiral windings. The spiral winding has a windingon the ferrite plate, which generates magnetic flux in only thevertical direction. There is no capability to reduce the leakage
magnetic flux. Thus spiral coil needs additional windings forpassive or active shielding.
Fig. 1 (b) is the Halbach coils. The Halbach coil replicates theHalbach array [4], which concentrates magnetic flux in one
direction through the arrangement of magnets, using coil windings.
The Halbach winding has two types of windings, one generates
magnetic flux in the vertical direction, and the other generates
magnetic flux in the horizontal direction. The horizontal magnetic
flux strengthens the vertical magnetic flux between the primary coiland secondary coils. On the other hand, the magnetic flux iscanceled out at the top and bottom of the coils. These coils are

Nc1 Nh1Ferrite
No2No1FerriteAluminum plate(b) Halbach coils.
Fig. 1. Transmission coils for WPT.
arranged vertically with ferrite plates sandwiched between themand connected in series. Thus, there is no cross-coupling among theHalbach coils.

3. Experimental evaluation
3.1 Evaluation points
The evaluation points for leakage
magnetic flux density are determined regarding the origin O,
located 25 mm above the surface of the buried primary coil, asshown in Fig. 2. The measurement points for the leakage magnetic
flux are determined considering the stationary and dynamic
charging of EVs. In this study, measurement points 1A and 1B aredetermined for the protection of occupants in vehicles as a reference
to the origin point O. The points 2A and 2B are for the protectionof pedestrians standing near the vehicle during stationary charging.

a) Correspondence to: Keisuke Kusaka.
E-mail: kusaka@vos.nagaokaut.ac.jp＊
Nagaoka University of Technology.
1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japan＊＊Honda R&D Co., Ltd.
4630 Shimotakanezawa, Haga-machi, Hagagun, Tochigi 3213321, Japan© 200● The Institute of Electrical Engineers of Japan.

FerriteN2Gap100 mm

1

WPT with Halbach coils reducing leakage magnetic flux (Keisuke Kusaka et al.)Table I. Specifications of the prototype.
Number of turn
[turn]
Self-inductance [mH]

Coupling coefficient k
Primary current I1 [A]Primary current I2 [A]Transmission frequency[kHz]

Spiral
N1, N2: 25
L1: 520
L2: 530

HalbachNc1, Nc2: 15
No1, No2: 10
Nh1, Nh2: 10
L1: 470
L2: 480

0.34
2.44
2.75

0.19
2.44
2.75

96

86

Magnetic flux density [mT]

1000

6050

Spiral Halbach
1004030

1020
10

101A

2B

153

3B

1B

2B

3B

Magnetic flux density at each measurement
point in Fig. 2.

300

frequency is also adjusted to maintain the resonance condition.
3.3
Experimental results
The leakage magnetic flux
density of the WPT systems with the conventional spiral coils andproposed Halbach coils are measured and compared. The magnetic
flux density is measured with ELT-400.
Figure 4 shows the measured magnetic flux density at eachmeasurement point in Fig .3. In all of the measurement points, themagnetic flux density is reduced. In particular, the effect ismaximum at measurement point 1A above the transmission coil,and the magnetic flux density is reduced by 35.6%. The point where
the reduction effect is weakest is 2A-I, and the reduction rate is8.2%. This is because the additional windings for the Halbach coil
Nh1 and Nh2 produce magnetic flux in the horizontal direction.

125
1A
2A-II

153

3A-II

Secondary coil
150

150

175
O

2A-I 2A-II 3A-I 3A-II

Measurement pointFig. 4.
1B

Reduction rate [%]The points 3A and 3B are for the protection of pedestrians or riders
standing slightly away from the vehicle for dynamic charging. Notethat the distance from the origin is similarly scaled down, as shownin Fig. 2. The measurement points in the width direction are scaled
based on an assumed vehicle width and the width of the aluminum
plate used in the experiment. Similarly, in the height direction, themeasurement distance is scaled based on the transmission distancefor the assumed vehicle and prototype.
3.2PrototypeFigure 3 shows the test environment
with a prototype with a rated power of 500 W. The specifications of
the prototype are shown in Table I. In the experiment, the numberof turns is determined so that the primary and secondary
inductances are almost the same for the spiral and Halbach coils forcomparison. Each winding is on the laminated PCBs with a size of300 × 200 × 1.6 mm, with ten layers. The spiral and Halbach coils
are configured by changing the wire on the PCBs. Due to thedifference in the winding structure, there is a difference in themutual inductance, nevertheless the same transmission distance.
The primary voltage and load resistance are adjusted to ensure thatthe primary and secondary currents are equal for each winding
under the resonance condition. For this reason, the transmission

200

2A-I

153

3A-I
[mm]

Primary coilFig. 2. Evaluation points of leakage magnetic flux forthe prototype.

4. Conclusion
The WPT system with Halbach coils is proposed to reduce theleakage magnetic flux density in this letter. The magnetic flux
density is reduced by 35.6% at maximum compared to theconventional spiral coils.
(a) PCB for Halbach coil

References

Exposure Level tester
ELT-400(1)(2)

Aluminum shield
Sec. coil(3)

Pri. coil

Aluminum shield(b) Measurement of leakage magnetic flux density
Fig. 3. WPT system with Halbach coils.

(4)2International Commission on Non-Ionizing Radiation Protection (ICNIRP),“ICNIRPGUIDELINESFOR
LIMITINGEXPOSURE
TO
ELECTROMAGNETIC FIELDS (100 kHz to 300 GHz),” (2020)
S. Cruciani, T. Campi, F. Maradei and M. Feliziani, “Wireless Charging inElectric Vehicles: EMI/EMC Risk Mitigation in Pacemakers by Active Coils,”
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K. Furukawa, K. Kusaka and Jun-ichi Itoh, “EMF Reduction withCompensated Current RMS Based on Short Canceling Coils for WirelessPower Transfer Systems,” IEEJ Transactions on Industry Applications, Vol.
141, No. 5, pp. 405-415 (2021)
K. Halbach, “Design of Permanent Multipole Magnets with Oriented RareEarth Cobalt Material,” Nuclear Instruments & Methods, Vol. 19, No. 1, pp.
1-10 (1979)IEEJ Trans. ●●, Vol.●●, No.●, ●●●