Downsizing of Three-Phase Wireless Power Transfer
System with 12 coils by Reducing Magnetic
Interference
Rintaro Kusui
Dept. of Electrical, Electronics, andInformation Engineering
Nagaoka University of Technology
Nagaoka, Japankusui@stn.nagaokaut.ac.jp

Keisuke Kusaka
Dept. of Electrical, Electronics, andInformation Engineering
Nagaoka University of Technology
Nagaoka, Japan
kusaka@vos.nagaokaut.ac.jp

Jun-ichi Itoh
Dept. of Electrical, Electronics, andInformation Engineering
Nagaoka University of Technology
Nagaoka, Japan
itoh@vos.nagaokaut.ac.jp

Shuichi Obayashi
Corporate Research & Development Center
Toshiba Corporation
Kawasaki, Japanshuichi.obayashi@toshiba.co.jp

Tetsu Shijo
Corporate Research & Development Center
Toshiba Corporation
Kawasaki, Japan
testu.shijo@toshiba.co.jp

Masaaki Ishida
Corporate Research & Development Center
Toshiba Corporation
Kawasaki, Japanmasaaki1.ishida@toshiba.co.jp

Abstract—In this paper, the placement of 12 coils for a threephase WPT system with low-radiation noise is proposed for a
reduction of a coil area. Increasing the charging power of awireless power transfer (WPT) system is required in order toimprove user convenience. One of the problems caused by thetransmission of large power in a WPT system is radiation noisefrom the transmission coils. The proposed three-phase WPTsystem with the 12 coils suppresses radiation emission and cancels
out the magnetic interference caused by multiple coils. However,
it is necessary to increase the radius of the 12 coils, which are
circularly placed, to satisfy the cancellation conditions of themagnetic interference. The proposed placement of the 12 coils
reduces a radius by rotating each pair of the primary coils and thesecondary coils. Simultaneously, the magnetic interference iscanceled out and the radiation noise is suppressed. First, theprinciple of the cancellation of magnetic interference andradiation noise reduction is explained with an electromagnetic
field analysis. Then, the emission from the 22-kW prototype has
been measured according to the CISPR11 in an anechoic chamber.
The magnetic interference is canceled out and the radiation noise
is suppressed as same as the conventional placement with areduced area by 74.5% in comparison with the conventional
method.
Keywords— wireless power transfer, three-phase transmission,
leakage magnetic flux

I. INTRODUCTION
The number of electric vehicles (EVs) has been increasing
for reducing the emission of NOx gas. The travel distance of
EVs is typically shorter than conventional vehicles, so users
need to more frequently charge a battery. Because of thisinconvenience, EVs are not still a substitute for conventional
vehicles with gasoline or diesel engines. Wireless power transfer(WPT) systems are actively studied [1-6] from the expectation
to improve the convenience of EV users. WPT transmits power
without a cable, thus the user will start charging without aconnection of a charging cable.

XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE

Focusing on the wired chargers, the charging power has been
increasing due to the increase in on-board battery capacity.
Therefore, wireless chargers are also required to increasecharging power. However, one of the problems for increasing
charging power is radiation noise. The radiation noise interfereswith electronic equipment or radio communication, and may
adversely affect human health. Therefore, the WPT system mustcomply with the regulations established in each region or
country based on CISPR and ICNIRP. The leakage magnetic
flux has been shielded using metals or magnetic materials [7-10].
In addition, a reactive shield that reduces EMF emission byadditional short-circuit coils is proposed [11-13]. However, an
eddy current or short current is generated on the shield or
additional windings causing large losses. In [14], the reduction
method of radiation noise using the spectrum spread has been
proposed. By randomly changing the transmission frequency,
the peak of radiation noise is suppressed. However, this method
will not comply with the international standard of WPT for EVs,
which will be published by ISO/IEC. In [15], a two-channel
WPT system has been proposed. The two-channel WPT system
has two primary coils and two secondary coils placed in order tocancel out the radiation noise. Nevertheless, the flux whichtransmits the power has interfered between two channels when
the radiation emission is canceled. In [16-17], a three-phase
WPT system with 12 coils has been proposed. The three-phase
WPT reduces the radiation noise without magnetic interference
between the multiple coils. The six solenoid coils circularly
placed on the primary and secondary side to suppress theradiation noise and cancel out the magnetic interference. The
magnetic interference is canceled out by adjusting the magnetic
coupling between the each primary and secondary side coils.
However, the radius is increased in order to satisfy theinterference cancellation condition at the conventional opposite
placement.
In this paper, the placement to satisfy the cancellation
condition with a smaller radius of 12 coils is proposed. In the
proposed placement, each pair of the primary and secondarycoils are rotated in order to reduce the magnetic coupling among
the each the primary and secondary coils because the directionof the flux causing the interference is changed by the rotation of
coils. Therefore, the cancellation condition is satisfied with the
reduced area of coils. In the rest of the paper, first, the magnetic
interference cancellation condition of the three-phase WPTsystem is shown and the placement reducing 12 coils area is
proposed. Next, magnetic interference and magnetic fieldemission are analyzed. Then, the radiation noise of a 22-kW
prototype is measured in the anechoic chamber complied withCISPR 11.

iu1
Cu1

u1
v1

w1

VD C1

Figure 2 shows the connection of the transmission coils andthe resonant capacitors. The pairs of coils on the common phaseare connected in series in a differential connection and placed
opposite. Due to the differential connection, the magnetic flux,
which is generated by two coils connected in series, is canceledout at the measurement point far from the coils. The coils of eachphase are placed every 120 degrees. However, the multiple coilscause magnetic interference such as Man, Mbn, and Mcn among
the primary side and secondary side, where n represents theprimary side as 1 and the secondary side as 2. The magnetic
interference causes power factor deterioration, due tounnecessary induced voltage that does not contribute to powertransmission.
B. Cancellation of Magnetic Interference
The unnecessary induced voltage is canceled out by
adjusting magnetic coupling among the coils on the primary sideand the secondary side.
The induced voltage on 12 coils is expressed as

Cu2

Lu1A

Lu2A

Lu1B

Lu2B
Lv2B

Lv1B
Cw 1 Lv1A

Lw 2B
Lw 1A
Lw 1B Lw 2A

u2

Cv2

R

v2

Lv2A Cw 2

w2 VD C2

Fig. 1. Three-phase WPT system with 12 coils.
Cv1

II. THREE-PHASE WPT SYSTEM WITH 12 COILS
A. Circuit Configuration
Figure 1 shows the circuit configuration of the three-phase
WPT system with 12 coils. The WPT system is based on a threephase WPT system with the coils, which are connected in starstar connection on the primary side and the secondary side,
respectively. The resonant capacitors are connected to the outputof the inverter and the input of the rectifier on each phase in theseries.

Cv1

vuv1

iu2

M

Cv2
Ma1

Cu1

Cw1

Lw1B

Lv1A

Mb1

Lu1A
Lv1B

Lu1B
Mc1
Lw1A

Lv2A

Lw2B
Mb2

Lu2B
Mc2
Lw2A

Ma2
Cu2

Lu2A
Lv2B
Cw2

Fig. 2. Conection of the 12-coils and resonant capacitors.

Focusing on the first line of (1), the induced voltage on thecoil Lu1A is expressed as

vu1 A = L1Ys

diu1
di
di
+ M u 2 − M c1 u1
dt
dt
dt
di
di
di 


 di
− M a1  v1 + w1  + M b1  v1 + w1 
dt 
dt 
 dt
 dt

(2).

The first term of (2) on the right side shows the induced
voltage by self-inductance, and the second term shows theinduced voltage by magnetic coupling between the primary coiland the secondary coil, which transmits power. By contrast, thethird to fifth terms show unnecessary induced voltage bymagnetic interference, which does not contribute to powertransmission.
Assuming that the three-phase currents flowing through thecoils are balanced, (2) is simplified as

diu1
di+ M u2
dt
dt
2 


− ω I m1  M c1 cos ωt + ( M a1 − M b1 ) cos  ωt − π 
3 


4 

+ ( M a1 − M b1 ) cos  ωt − π  
3 


=
vu1 A L1Ys
 vu1 A   L1Ys  vu1B   M c1
 v  M
 v1 A   b1
 vv1B   M a1
 
 vw1 A   M b1
 vw1B   M a1=
 vu 2 A   M
v   0
 u2B  
 vv 2 A   0  vv 2 B   0
v   0
 w2 A  v   0
 w2 B  

M c1
L1Ys
M a1
M a1
M a1
M b1
0
M

M b1
M a1
L1Ys
M c1
M b1
M a1
0
0

M a1
M b1
M c1
L1Ys
M a1
M b1
0
0

M b1
M a1
M b1
M a1
L1Ys
M c1
0
0

M a1
M b1
M a1
M a1
M c1
L1Ys
0
0

0
0
0
0
0
L2Ys
M c2

0
M
0
0
0
0
M c2
L2Ys

0
0
0
0

M
0
0
0

0
M
0
0

0
0
M
0

0
0
0
M

M b2
M a1
M b2
M a1

M a1
M b2
M a1
M b2

M

0

0

0

0
M
0
0
0
M b2
M a1

0
0
M
0
0
M a1
M b2

0
0
0
M
0
M b2
M a1

L2Ys
M c2
M b2
M a1

M c2
L2Ys
M a1
M b2

M b2
M a1
L2Ys
M c2

0   iu1  
0   −iu1 
0   iv1  
0   −iv1  
0   iw1 M  d  −iw1  
M a1  dt  iu 2 
M b 2   −iu 2 
M a1   iv 2  
M b 2   −iv 2 


M c2
i 
  w2 
L2Ys   −iw 2 

(1),where x represents A or B. The LnYs is the self-inductance ofeach coil, and M is the mutual inductance of the magnetic
coupling that transmits power.

(3),
where Im1 is the amplitude of the primary current.
The unnecessary induced voltage by the magnetic
interference shown in (3) is canceled out when the relationshipamong the magnetic couplings Ma1, Mb1, and Mc1 among the
primary coils satisfy (4).

M a1 − M b1 − M c1 =0

(4)

The condition for cancellation magnetic interference isexpressed as (5) using the coupling coefficient among the
primary coils when the self-inductance of all primary coils is thesame.

ka1 − kb1 − kc1 =
0

Lv1A

ka1

kb1

(5)

The cancellation condition of the magnetic interference on
the secondary side is derived as (6) in the same manner.

k a 2 − kb 2 − k c 2 =
0

Lw1B

r
Lu1A

Lu1B

kc1

Lw1A

Lv1B

(6)

By satisfying both (5) and (6), the unnecessary induced
voltage due to the magnetic interference is canceled out. In otherwords, the three-phase WPT system with 12-coil is as same as
the six independent single-phase WPT systems with two coils
for each.
C. Proposed Placement of 12 coils
The cancellation conditions calculated by the equation (5)and (6) must be satisfied however the magnetic coupling ka
between adjacent coils is greatly larger than the magnetic
coupling kb and kc because the distance between the adjacentcoils is shorter than the other coils.
Figure 3 (a) shows top view of the conventional placement.
In the conventional placement, the coils connected in series areplaced in the face of each other. Therefore, in the conventional
placement, the magnetic coupling between adjacent coils must
be reduced by increasing the radius r of the 12 coils in order tosatisfy (5) and (6).
Figure 3 (b) shows top view of the proposed 12 coils
placement. In the proposed placement, the coils circularly
placed are rotated around the center of each coil. Due to therotation of each coil, the direction of the magnetic flux thatinterlinks across the same side coils and causes interference ischanged. Thus, the unnecessary induced voltage is decreased. In
other words, the magnetic coupling among the common sidecoils is reduced. Therefore (5) and (6) are satisfied even at
smaller radius r.
The main magnetic coupling between the primary coil andthe secondary coil that transmit the power does not significantly
change because the coils are rotated for each pair of primarycoils and secondary coils. Furthermore, the connection of thecoils is not changed. Consequently, the transmission power andthe reduction performance of radiation magnetic field is not
significantly affected, when the coils are miniaturized by the
proposed placement.
III. ELECTROMAGNETIC ANALYSIS
A. Analytical ModelIn this section, the magnetic interference and radiation noiseof a three-phase WPT system with the 12 coils are analyzed. The
transmission coil cores of the analysis model have a length l of
500 mm, a width w of 250 mm, and a thickness h of 10 mm. The
radius r indicates the distance from the center of the 12 coils to
the center of each coil, and the coil rotation angle θ indicates theangle of rotation with the center of each coil. Table I shows(a) Conventional placement
Radiation noise
Analysis point

ϕ

3m

Lw1B

Lv1A

ka1
kb1
kc1

Lu1A

θ

r

Lu1B
l

Lw1A

Lv1B

w

(b) Proposed placement
Fig. 3. Top view of 12 coils placement.
TABLE I.

SPECIFICATION OF PROTOTYPE.

Prameters

SymbolRated power

P

22

kW

Primary voltage

VDC1

650

V

Secondary voltage

VDC2

200~400

V

Value

Transmission frequency

f

85

kHz

Air gapg

90mm

Core width

w

250

mm

Core length

l

500

mm

Core thickness

h

10

mm

Primary

N1

8

turn

Secondary

N2

5

turn

Primary

L1

31.5

µH

Secondary

L2

12.8

µH

Number of turns
Self inductanceCore

PC95,TDK

Litz wire

2UEWLZ 7×130(φ0.1)

MOSFETs

BSM300A12P2E001,
ROHM

DiodesIDW40G120C5BFKSA1,Infineon Technologies

Litz wire

TD

500

ns

parameters of the coils in the magnetic interference and radiationnoise analysis.

Figure 5 shows a vector diagram of the magnetic flux density
when a current flows through only one coil. Fig. 5 (a) shows themagnetic flux without rotation of the coil. The magnetic flux isinterlinked across the adjacent coil in the direction to generate
the unnecessary induced voltage. The magnitude of theinterlinkage flux that causes the induced voltage means thestrength of the coupling. Therefore, when the coils are notrotated, the cancellation condition cannot be satisfied due to thestrong coupling among the adjacent coils. On the other hand,from Fig. 5 (b), the magnetic flux interlinks across the adjacentcoil from the side and flows to right and left when the coil isrotated by 40deg. For this reason, the magnetic coupling ka
between the adjacent coils is equivalently reduced, and themagnetic interference cancellation condition (5) and (6) issatisfied even when the radius is small. At other angles, the flux
linkage branched to the left and right does not match, andmagnetic coupling ka is larger than when the rotation angle θ is40deg. so that the magnetic interference is not sufficiently
canceled out.
C. Radiation Noise Analysis
Figure 6 shows the electromagnetic analysis results of thefundamental wave component of the radiation noise. The
radiation noise is analyzed at the radius r of 850 and 550 mm,
and the coils rotation angle θ of 0 and 40 deg. The zero-rotation
angle is the conventional placement and 40 deg. is the proposed
placement. The radiation noise should be evaluated at 10 m fromthe system for a compatibility test of CISPR. However, thisanalysis is performed at 3 m from the center of the 12 coils inorder to shorten the analysis time. The value θ indicates theangle at which the radiated magnetic field is analyzed. From Fig.
6, the average of the radiation noise is suppressed by 4.2
dBµA/m, by reducing the radius of the 12 coils. The distance
between the same phase coils that connected differentially is
shortened by reducing the radius. When the distance between thecoils is short, the radiated noise is further reduced. As a result,

Influence of magnetic interference
ka-kb-kc

B. Magnetic Interference Analysis
Figure 4 shows the analysis results of magnetic interference
when the rotation angle θ is changed with a radius of 850 and
550 mm. The equation (5), (6) must be satisfied to cancel out themagnetic interference. The equation (5), (6) must be satisfied tocancel out the magnetic interference. When the value of ka − kb
− kc is not zero, it is not canceled out. Then the value of ka − kb
− kc means the infulence of magnetic interference. When therotation angle θ is zero, the remaining interference of the 550mm radius is three times larger than the 500-mm radius. This is
the limitation for reducing the area of coils. From Fig. 4, it isshown that the remaining magnetic interference ka − kb − kc is
reduced by rotating the coils by 40deg., i.e., the effect ofmagnetic interference is reduced. However, when the rotation
angle is 40deg. or more, the effect of magnetic interference isincreased. From this result, the radius is reduced from 850 to 550
mm with the same magnetic interference. Therefore, the area ofthe coils is reduced by 74.5%, with a rotation angle θ of 40deg
and a smaller radius of 300 mm. Note that the minimum radiusr is 500 mm because the core of the coil is 250 mm wide.
However, in practice, there is an acrylic case that holds the cores
and a winding. Due to this, in this paper, a radius of 550 mm isused in order to have a margin of 50 mm.

0.020

r=550 mm0.015

Magnetic interferance iscanceled out.

0.010
r=850 mm0.005
0Unnecessary induced voltage is 0-0.005

0

10

20
30
40
50
60
Coil rotation angle θ [deg]Fig. 4. Magnetic interference with changing rotation angle of coils.

(a) Conventional placement (θ = 0deg.) (b) Proposed placement (θ = 40deg.)
Fig. 5. Vector diagram of magnetic flux density.

ϕ
0
330

116

30

112

r=550 mm
θ=0 deg300

r=850 mm
θ=0 deg60

108r=850 mm
θ=40 deg

104

Hrad[dBµA/m]
r=550 mm
θ=40 deg

270

90

120

240

150

210
180

Fig. 6. Radiation noise analyzed by electromagnetic analysis.

the reduction effect is slightly improved when the radius r is
reduced.
IV. EVALUATION OF 22-KW PROTOTYPE
A. System configuration
Figure 7 shows the configuration of a 22-kW prototype for atest. The input of the system is connected to a variabletransformer so that the primary DC voltage is adjusted. The
power is circulated from the output to the input of the WPTsystem by the bi-directional buck-boost chopper. Besides, thebi-directional buck-boost chopper adjusts the secondary DC

iu1
Cu1

+

Cv1
Cw 1

vuv1

iu2
Lu1A

Lu1B
Lv1B
Lv1A

Cu1

Lu2A
Lw 1B

Lw 1A Lw 2A

Lw 2B

Lu2B
Lv2B

Cv1
Lv2A

Cw 1

VDC1

+

+

+

VDC2

Fig. 7. System configuration of the 22-kW prototype.

voltage VDC2 from 200 to 400 V. The bi-directional buck-boost
chopper emulates a constant voltage load such as a battery.
Figure 8 shows the transmission coil of the prototype. Fig. 8.
(a) shows the conventional placement with a radius r = 850 mm
and an angle θ = 0 deg., and (b) shows the proposed placementwith a radius r = 550 mm and an angle θ = 40 degrees. The
transmission coils have a ferrite core plate of 500 × 250 × 10
mm. The ferrite cores are put into an acrylic case. The windingused was a litz wire.
B. Operation Waveform
Figure 9 shows the operation waveforms of the three-phase
WPT system when the 12 coils are placed with a radius r = 550
mm and rotation angle θ = 40deg. The primary DC voltage VDC1
is 650 V, the secondary DC voltage VDC2 is 400 V, and the output
power is 22 kW. The secondary DC voltage emulates theconstant voltage load by the bi-direction buck-boost chopper.
From Fig. 7, it is shown that the inverter output voltage isoperating under the resonance condition because the rising andfalling of the inverter output line-voltage are synchronized withthe zero-cross of the inverter output current. Therefore, it is seenthat the unnecessary induced voltage, that deteriorates the power
factor, does not occur. In other words, the three-phase WPTsystem sufficiently canceled out the magnetic interference when
the proposed placement is applied.
C. Radiation Noise Evaluation
Figure 10 shows the radiation noise measured according to
CISPR 11, class A, group 2 in the anechoic chamber in Niigata,Japan. Note that the broken line from 150 kHz to 30 MHz in Fig.
10 indicates the regulation in the magnetic emission regulated
by CISPR 11, class A, group 2. The regulation below 150 kHz
has not been regulated in CISPR 11 edition 6.2, so far. Fig. 10(a) shows the result for the conventional placement in which theradius r is 850 mm and the coil is not rotated. Fig. 10 (b) shows
the result for the proposed placement in which the radius r is 550
mm and the rotation angle θ is 40 deg. From Fig. 10, it is shownthat the three-phase WPT system with 12 coils sufficiently
satisfies the regulation regulated from 150 kHz or more in boththe conventional placement and the proposed placement.
Focusing on the fundamental frequency component, theradiation noise is suppressed from 74.9 to 73.5 dBµA/m by the
proposed placement. The proposed configuration of coils
achieves the reduction of the coil area by 74.5% without
degradation of the performance of radiation noise reduction.

(a) conventional placement (r = 850 mm θ = 0°)(b) proposed placement (r = 550 mm θ =40°)
Fig. 8. Transmission coils for 22-kW prototype.
0
Inverter output voltage vuv1(1kV/div)
Inverter output current iu1(50A/div)0
Rectifier input current iu2(50A/div)0
Output voltage VDC2(500V/div)
040µs/div

Fig. 9. Operation waveform.

V. CONCLUSION
The placement for miniaturizing a three-phase WPT systemwith 12 coils is proposed in this paper. The system has six

74.9 dBµA/m

80
CISPR11 ClassA Group2

40

0

-40
0.01

Floor noise

ACKNOWLEDGMENT
This research is financially supported by the Ministry ofInternal Affairs and Communications of Japan.
REFERENCES

[3]
[4]

40

0
Floor noise

-40
1
10
30
0.01
0.10
1
10
frequency f [MHz]frequency f [MHz](b) proposed placement (r =550 mm θ =40°)(a) conventional placement (r =850 mm θ =0°)
Fig. 10. Radiation noise at 10 m from coil edge in anechoic chamber.

In future work, the system efficiency and the suppression ofradiation noise will be improved by optimizing the design ofcoils.

[2]

73.5 dBµA/m

CISPR11 ClassA Group2

0.10

primary coils and six secondary coils placed circularly. Due tothis, the magnetic interference is canceled out and the radiationnoise is suppressed at the measurement point of the emission.
The cancellation condition must satisfy kan − kbn − kcn = 0 for thecancellation of the magnetic interference among the coils. The
proposed placement satisfies the cancelation condition with asmaller radius of coils because the magnetic coupling between
the adjacent coils is reduced by the rotation of each coil. The
remaining magnetic interference kan − kbn − kcn and the magnetic
flux density at 3 m from the center of 12 colils are analyzed byelectromagnetic analysis. As the analysis result, the interferenceis canceled out when the coil rotation angle is 40deg. Bycanceling the interference, the area of the coils is reduced by
74.5%. Then, the emission of the 22-kW prototype with orwithout the proposed configuration of the coils is evaluated withcomplying with CISPR 11. The radiation noise is suppressedfrom 74.9 to 73.5 dBµA/m at the fundamental frequency and themagnetic interference is canceled out. The results show that the
proposed configuration achieves both the reduction in the sizeof coils and radiation noise at the fundamental frequency.

[1]

radiation noise [dBµA/m]

radiation noise [dBµA/m]

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