IEEJ Journal of Industry Applications
Vol.8 No.4 pp.600–607
DOI: 10.1541/ieejjia.8.600

Paper

Development of Three-Phase Wireless Power Transfer Systemwith Reduced Radiation Noise
Keisuke Kusaka∗ Member, Keita Furukawa∗
Jun-ichi Itoh∗a) Senior Member

Student Member(Manuscript received June 12, 2018, revised April 10, 2019)

A three-phase wireless power transfer (WPT) system, which has six primary coils and six secondary coils is proposed in this paper. The three-phase WPT system reduces the radiation noise by canceling the noise using pairs of coils
placed opposite to each other. However, a magnetic interference among the multiple coils decreases the transmission
eﬃciency due to a circulating current among the primary coils, and the secondary coils in WPT systems with multiple
coils. The proposed three-phase WPT system cancels out the eﬀect due to the interference among the six coils on eachside. First, the WPT system with the 12 coils is proposed. Then, a canceling condition of the magnetic interference
among the coils is mathematically introduced from the voltage equation on the coils. Finally, the proposed WPT system is experimentally demonstrated with the 3-kW prototype. The experimental result shows that the radiation noiseat the fundamental frequency is suppressed from 12.2 dBμA to 2.1 dBμA.
Keywords: inductive power transfer, wireless power transfer, three-phase, solenoid coil1.

been proposed (14) (16) (17) . In (16), the radiation noise is reduced
by duplicated transmission coils. The two transmission coilswith diﬀerential current are placed to reduce the leakage radiation noise. However, the duplicated WPT system limits
freedom of coil positions because the magnetic interferenceis caused by the magnetic coupling between the duplicatedcoils (16) . The magnetic interference decreases power factor offundamental wave regarding an inverter output.
Meanwhile, three-phase WPT systems have been proposed
to increase the output power of the WPT system (18)–(21) . The
three-phase WPT systems allow the reduction of the current
on each transmission coil. Thus the three-phase WPT is has apossibility to reduce the copper loss and the iron loss. However, a magnetic interference among the additional coils oneach side occurs in the conventional three-phase WPT system. The magnetic interference does not contribute powertransmission between the primary side and the secondaryside. The magnetic interference causes an interfering induced
voltage on each coils. So, it causes the circulating currents
among the primary coils and the secondary coils.
This paper proposes a three-phase WPT system with 12
solenoid coils. The six pairs of coils placed opposite to eachother contributes reducing the current on the winding without
a byproduct such as a magnetic interference. A contributionof this paper is oﬀering the new transmission coil structure,
which reduces the radiation noise with a small eﬀect on efficiency. In the proposed system, the countercurrent in the
opposite coils cancel out the radiation noise. Moreover, theprimary coils and the secondary coils are placed at an angleof 60 degrees to each other. Consequently, the induced voltage caused by magnetic interference among the 12 coils arecanceled out. First, a system configuration of the proposed
WPT system is described. Next, a design procedure of the
proposed three-phase WPT system with a star-star winding

Introduction

In this decade, wireless power transfer (WPT) systems forelectrical vehicles (EVs) are actively studied (1)–(11) because
WPT systems will improve the usability of EVs. Standardization of the WPT system for EVs is ongoing for interoperability (12) (13) . The international standard, which will be published
by IEC/ISO will classify the WPT system into five classes
by the maximum input power. The maximum input powerof WPT1 is 3.7 kW or less. The maximum input power of
WPT 2 is larger than 3.7 kW, and 7.7 kW or less. The maximum input power of WPT3 is larger than 7.7 kW, and 11 kW
or less. The maximum input power of WPT4 is larger than
11 kW, and 22 kW or less. The maximum input power of
WPT5 is larger than 22 kW (12) (13) . Currently, the standardization of WPT1 and WPT2 is in progress. However, the standardization of high-power WPT systems such as WPT4 or 5
has not been discussed well despite strong demand for higher
power WPT system for heavy-duty vehicles. One of the difficulties of the high-power WPT system is heat generationof transmission coils because copper loss and iron loss increase (14) . The second is radiation noise. The radiation noisefrom the WPT system must be suppressed to satisfy the regulations, which are published by CISPR or legislated in eachcountry or region. However, the satisfaction of regulations
becomes further diﬃcult because the radiation noise causedby the loop coil is, in principle, increased in proportion to thecurrent on the loop coil (15) .
As a previous study, a radiation noise reduction method bya duplicated 44-kW WPT system for heavy-duty vehicles hasa) Correspondence to: Jun-ichi Itoh. E-mail: itoh@vos.nagaokaut.
ac.jp∗
Nagaoka University of Technology
1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japanc 2019 The Institute of Electrical Engineers of Japan.
600Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）and a delta-delta winding is explained. Then, the cancelingmethod of the magnetic interference among the multiple coils
is mathematically introduced. Finally, the proposed WPTsystem with star-star winding is tested with a 3-kW prototype. Then, the radiation noise is assessed and compared
between the conventional three-phase WPT system and the
proposed three-phase WPT system with 12 coils.
2.

Three-phase WPT System with 12 Coils(a) Star-star winding

In this chapter, the three-phase WPT system is proposed.
The proposed WPT system achieves radiation noise reduction
using 12 coils without magnetic interference among multiple
coils.
2.1 System Configuration
Figure 1 shows the proposed three-phase WPT system with 12 coils. The threephase coils can be connected with not only star-star winding but also delta-delta, star-delta, and delta-star windings.
In this paper, only star-star and delta-delta windings are explained due to the page limitation. In the proposed system,
the power is inductively transmitted through the six primarycoils and the six secondary coils. Each phase has two coils
connected in series. These pairs of coils connected in seriescancel out the radiation noise because the magnetic flux direction is opposite owing to the countercurrent flowing on thecoils (14) .
Besides, the three-phase inverter on the primary side andthe three-phase rectifier in the secondary side are used. The
primary inverter output square-wave voltage in each phase.
The phase diﬀerences of each phase are 120 degrees. Furthermore, resonant capacitors are connected to the output ofthe primary inverter and the input of the rectifier in series.
The series-series compensation technique (22) is applied to the
proposed system in order to cancel out the leakage inductancedue to the weak magnetic coupling. Using three-phase WPTand the series-series compensation technique, copper loss of
the windings can be reduced because the current is shunted
into six coils. Therefore, the proposed WPT system is eﬀective to improve the heat dissipation of the transmission coils.
Figure 2 illustrates the placement of the transmission coils.
In the proposed WPT system, six solenoid coils are used ineach of the primary side and the secondary side. Figure 3
shows the mechanism of the reduction in radiation noise using the pairs of coils. Two transmission coils connected inseries (e.g., Luv1A and Luv1B ) are diﬀerentially connected andare placed in an opposite place. These opposite coils cancelout radiation noise at a measurement point, which is typically
10 m from the WPT system (23) . Other pairs of coils are placedin an angle of 120 degrees to the coils on phase-u each other.
As a drawback of the reduction in radiation noise using opposite coils, magnetic interference occur among the six coilson each the primary side and the secondary side. The position relationship and figure of the coils have to be adjusted
in order to cancel out the magnetic interference. The design
method of the coils will be explained in section 2.3.
2.2 Mathematical Model of Three-phase Coils
Equations (1) and (2) represents the induced voltageof 12 coils on the star-star winding system and the deltadelta winding system, respectively where L1Y s is the selfinductance of the primary coil on the star-star winding, L1Δs
is the self-inductance of the primary coil on the delta-delta(b) Delta-delta winding

Fig. 1. Proposed three-phase IPT system with 12 coils(a) Star-star winding(b) Delta-delta winding

Fig. 2. Placement and connection of coils

Fig. 3. Mechanism of reduction in radiation noise using
solenoid coils

winding.
The symbol M represents the mutual inductances between
the primary coils and the secondary coils, which contributes
to the power transmission, e.g., the mutual inductance between Lu1A and Lu2A . The suﬃx “1” indicates coils on the
primary side, and “2” means coils on the secondary side.
The symbol M a represents the mutual inductance between
the adjacent coils, e.g., the mutual inductance between Lu1A
and Lv1B . The mutual inductance M b represents the mutual
601

IEEJ Journal IA, Vol.8, No.4, 2019

Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）⎛⎞ ⎛⎜⎜⎜ vu1A ⎟⎟⎟ ⎜⎜⎜L1Y s⎜⎜⎜ v ⎟⎟⎟ ⎜⎜⎜ M⎜⎜⎜ u1B ⎟⎟⎟ ⎜⎜⎜ c1⎜⎜⎜⎜ vv1A ⎟⎟⎟⎟ ⎜⎜⎜⎜ Mb1⎜⎜⎜ v ⎟⎟⎟ ⎜⎜⎜ M⎜⎜⎜ v1B ⎟⎟⎟ ⎜⎜⎜ a1⎜⎜⎜vw1A ⎟⎟⎟ ⎜⎜⎜ Mb1⎜⎜⎜⎟ ⎜⎜⎜⎜vw1B ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ Ma1⎜⎜⎜⎟=⎜⎜⎜⎜ vu2A ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ M⎜⎜⎜ vu2B ⎟⎟⎟ ⎜⎜⎜ 0⎜⎜⎜⎟ ⎜⎜⎜⎜ vv2A ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ 0⎜⎜⎜⎟ ⎜⎜⎜⎜ vv2B ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ 0⎜⎜⎜v ⎟⎟⎟ ⎜⎜⎜ 0⎜⎝ w2A ⎟⎠ ⎜⎝
vw2B
0
⎞ ⎛
⎛
⎜⎜⎜ vuv1A ⎟⎟⎟ ⎜⎜⎜L1Δs⎜⎜⎜⎜ vuv1B ⎟⎟⎟⎟ ⎜⎜⎜⎜ Mc1⎟ ⎜⎜⎜⎜⎜⎜⎜vvw1A ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ Mb1⎟ ⎜⎜⎜⎜v⎜⎜⎜ vw1B ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ Ma1⎜⎜⎜vwu1A ⎟⎟⎟ ⎜⎜⎜ Mb1⎟ ⎜⎜⎜⎜⎜⎜⎜vwu1B ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ Ma1⎟=⎜⎜⎜⎜⎜⎜⎜ vuv2A ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ M⎜⎜⎜ vuv2B ⎟⎟⎟ ⎜⎜⎜ 0⎟ ⎜⎜⎜⎜⎜⎜⎜vvw2A ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ 0⎟ ⎜⎜⎜⎜⎜⎜⎜vvw2B ⎟⎟⎟⎟⎟ ⎜⎜⎜⎜⎜ 0⎟ ⎜⎜⎜⎜v⎜⎝ wu2A ⎟⎟⎟⎠ ⎜⎜⎜⎝ 0
vwu2B
0

Mc1
L1Y s
Ma1
Mb1
Ma1
Mb1
0
M
0
0
0
0

Mb1
Ma1
L1Y s
Mc1
Mb1
Ma1
0
0
M
0
0
0

Ma1
Mb1
Mc1
L1Y s
Ma1
Mb1
0
0
0
M
0
0

Mb1
Ma1
Mb1
Ma1
L1Y s
Mc1
0
0
0
0
M
0

Ma1
Mb1
Ma1
Mb1
Mc1
L1Y s
0
0
0
0
0
M

M
0
0
0
0
0
L2Y s
Mc2
Mb2
Ma2
Mb2
Ma2

0
M
0
0
0
0
Mc2
L2Y s
Ma2
Mb2
Ma2
Mb2

0
0
M
0
0
0
Mb2
Ma2
L2Y s
Mc2
Mb2
Ma2

0
0
0
M
0
0
Ma2
Mb2
Mc2
L2Y s
Ma2
Mb2

Mc1
L1Δs
Ma1
Mb1
Ma1
Mb1
0
M
0
0
0
0

Mb1
Ma1
L1Δs
Mc1
Mb1
Ma1
0
0
M
0
0
0

Ma1
Mb1
Mc1
L1Δs
Ma1
Mb1
0
0
0
M
0
0

Mb1
Ma1
Mb1
Ma1
L1Δs
Mc1
0
0
0
0
M
0

Ma1
Mb1
Ma1
Mb1
Mc1
L1Δs
0
0
0
0
0
M

M
0
0
0
0
0
L2Δs
Mc2
Mb2
Ma2
Mb2
Ma2

0
M
0
0
0
0
Mc2
L2Δs
Ma2
Mb2
Ma2
Mb2

0
0
M
0
0
0
Mb2
Ma2
L2Δs
Mc2
Mb2
Ma2

0
0
0
M
0
0
Ma2
Mb2
Mc2
L2Δs
Ma2
Mb2

⎞ ⎛
⎞
0 ⎟⎟ ⎜⎜ iu1 ⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
0 ⎟⎟ ⎜⎜ −iu1 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
0 ⎟⎟ ⎜⎜ iv1 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
0 ⎟⎟⎟ ⎜⎜⎜ −iv1 ⎟⎟⎟⎟⎟⎟⎜
0 ⎟⎟⎟⎟ ⎜⎜⎜⎜ iw1 ⎟⎟⎟⎟⎟
M ⎟⎟⎟⎟⎟ d ⎜⎜⎜⎜⎜−iw1 ⎟⎟⎟⎟⎟⎟ ⎜⎟ · · · · · · · · · · · · · · (1)Ma2 ⎟⎟⎟⎟ dt ⎜⎜⎜⎜ iu2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
Mb2 ⎟⎟ ⎜⎜ −iu2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
Ma2 ⎟⎟ ⎜⎜ iv2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
Mb2 ⎟⎟ ⎜⎜ −iv2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
Mc2 ⎟⎟⎠ ⎜⎜⎝ iw2 ⎟⎟⎟⎟⎠
L2Y s
−iw2
⎞ ⎛
⎞
0
0 ⎟⎟ ⎜⎜ iuv1 ⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
00 ⎟⎟ ⎜⎜ −iuv1 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
00 ⎟⎟ ⎜⎜ ivw1 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
00 ⎟⎟⎟ ⎜⎜⎜−ivw1 ⎟⎟⎟⎟⎟⎟⎜
M
0 ⎟⎟⎟⎟ ⎜⎜⎜⎜ iwu1 ⎟⎟⎟⎟⎟
0M ⎟⎟⎟⎟⎟ d ⎜⎜⎜⎜⎜−iwu1 ⎟⎟⎟⎟⎟⎟ ⎜⎟ · · · · · · · · · · · · · (2)Mb2 Ma2 ⎟⎟⎟⎟ dt ⎜⎜⎜⎜ iuv2 ⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟⎟
Ma2 Mb2 ⎟⎟ ⎜⎜ −iuv2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
Mb2 Ma2 ⎟⎟ ⎜⎜ ivw2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
Ma2 Mb2 ⎟⎟ ⎜⎜−ivw2 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟
L2Δs Mc2 ⎟⎟⎠ ⎜⎜⎝ iwu2 ⎟⎟⎟⎟⎠
Mc2 L2Δs
−iwu2

0
0
0
0
M
0
Mb2
Ma2
Mb2
Ma2
L2Y s
Mc2

zero, the relationship between the primary coil L1Y s and thesecondary coil L2Y s is seen as the same as a transmission with
one-by-one.
Assuming the three-phase equilibrium, Eq. (3) is transformed as

inductance between the coils, which are placed apart from
120 degrees, e.g., the mutual inductance between Lu1A and
Lw1A . The mutual inductance M c represents the mutual inductance between the opposite coils, e.g., the mutual inductance
between Lu1A and Lu1B . Note that other unwanted mutual inductances, e.g., the mutual inductance between Lu1A and Lu2B
are ignored in Eq. (1).
It is clear from Eq. (1) that the unwanted mutual inductances cause an unwanted induce voltage on each of the transmission coils. This induced voltage decreases eﬃciency due
to a circulating current. The canceling method of these interfering induced voltages will be explained in the next section.
2.3 Cancellation of Magnetic Interference Coupling
In this section, the canceling method of unwanted induced
voltage caused by magnetic interference is explained. The
cancellation method of the magnetic interference coupling is
explained with the star-star winding. However, the cancellation technique can be used for the delta-delta winding.
The induced voltage on Lu1 is derived from the first columnof Eq. (1) as

vu1A = L1Y s

diu1
diu2
+M
dt
dt


2
− ωIm Mc1 cos ωt+(Ma1 − Mb1 ) cos ωt− π3
4(Ma1 − Mb1 ) cos ωt − π , · · · · · (4)3where I m is the maximum value of the primary current iu1 ,which flows in each of the primary coils.
From Eq. (4), it is found that the unwanted induced voltage
does not occur when Eq. (5) is satisfied.
Ma1 = Mb1 + Mc1 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (5)Since the self-inductance of the primary coils L1Y s is equal
to each other, the canceling condition of the interfering induced voltage is

diu2
diu1
diu1
+M
− Mc1
dt
dt
dt
div1 diw1
div1 diw1
− Ma1
+
+ Mb1
+
.
dt
dt
dt
dt
· · · · · · · · · · · · · · · · · · · · · (3)vu1A = L1Y s

ka1 = kb1 + kc1 .· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (6)Therefore, designing the magnetic coupling among the primary coils according to Eq. (6) cancel out the eﬀect of unwanted coupling on the primary side.
Similarly, the unwanted coupling on the secondary side canbe canceled out by designing the magnetic coupling according to Eq. (7).

The first term of the right side in Eq. (3) is the induced
voltage by the self-inductance, the second term is the mutual
inductance caused by the coil on the secondary side. The
second term contributes to power transmission. The third to
fifth terms are the induced voltage caused by the interference
coupling. If the sum of the third to fifth terms of Eq. (3) is

ka2 = kb2 + kc2 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (7)602

IEEJ Journal IA, Vol.8, No.4, 2019

Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）power factor of the fundamental wave will degrade when thesum of the interference coupling kb and kc does not equal toka . However, the power factor of fundamental wave remains
at high even when the interference couplings increase as longas Eq. (6) is satisfied. In other words, these results show thatthe magnetic interferences among the transmission coils do
not aﬀect the power factor of the fundamental wave when therelation of the interference coupling fits on Eq. (6).
3.

Design of Three-phase WPT System

In this chapter, the design method of the proposed threephase WPT system with the star-tar winding and the deltadelta winding is described. In this design procedure, the induced voltage generated by the magnetic interference among
the multiple coils can be ignored with assuming Eqs. (6), (7).
3.1 Star-star Winding
The design method of the
proposed three-phase WPT system with the star-star winding
shown in Fig. 1(a) in this section.
1) Equivalent AC resistance
The equivalent AC resistance is expressed using the rated
output voltage V DC2 , and rated output power P. The Eq. (8)can be introduced by expanding a derivation of an equivalent
AC resistance of a single-phase rectifier to the three-phaserectifier (24) . The equivalent AC resistance for a phase is expressed by(a) kb = 0

Req =

2
6 VDC28 (VDC2 /2)2=.· · · · · · · · · · · · · · · · · · (8)
P/3
π2
π2 P

2) Secondary inductance
Maximizing a transmission eﬃciency, it has been proposedthat an impedance of the secondary inductance is equal to theimpedance of the equivalent AC resistance (25) . The secondary
self-inductance for a certain phase should be (9) by ignoringa winding resistance and assuming the following resonancecondition where ω is the transmission angular frequency, k is
the coupling coeﬃcient.

(b) kb = 0.1

L2Y = Lu2 = Lv2 = Lw2 =

6
π2 kω

2
VDC2
· · · · · · · · · · · · · (9)P

The secondary inductances have to be half because twotransmission coils are connected in each phase in the proposed WPT system. Thus, inductance for a certain coil is
L2Y s = L2Y /2.
3) Primary inductance
From the voltage ratio between the input DC voltage V DC1
and the output DC voltage V DC2 under the resonance condition, a primary self-inductance for obtaining the desired rated
voltage is expressed by

2
2
VDC1
6 VDC1,L1Y = Lu1 = Lv1 = Lw1 = L2Y= 2
VDC2
π kω P· · · · · · · · · · · · · · · · · · · (10)(c) kb = 0.2

Fig. 4. Eﬀect of magnetic interference coupling on
power factor

Figure 4 shows the simulated relationship between the inverter output power factor and the magnetic interference couplings ka , kb , and kc . The dotted line on the graph represents
Eq. (6). Figures 4(a), (b), and (c) show the relation when themagnetic interference coupling kb is zero, 0.1 and 0.2, respectively. When the magnetic interference coupling is smallenough to be negligible, the power factor closes to unity. The

where the primary inverter is operated with a square-wave operation. In the proposed WPT system, the transmission coils
are divided and connected in series. The inductances represented by Eq. (10) have to be half. Thus, the inductance ofthe primary coil is L1Y s = L1Y / 2.
4) Resonance capacitors
The resonance capacitors are connected to the output of603

IEEJ Journal IA, Vol.8, No.4, 2019

Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）the primary inverter and the input of the rectifier in series.
The resonance capacitors are designed to be resonated withthe self-inductance at the transmission angular frequency ω.
Thus, the resonant capacitors can be designed as
Cu1Y = Cv1Y = Cw1Y =Cu2Y = Cv2Y = Cw2Y =1
ω2 L1Y

4.

Figure 5 and Table 1 show the prototype and its specifications. In this paper, the proposed WPT system with thestar-star winding is experimentally demonstrated. The magnetic interference couplings are ka = 0.048, kb = 0.011, andkc = 0.003. Each transmission coils have ferrite plates of
245 × 215 × 10 mm. The ferrite plates are put into boxes
made from acrylic. As windings for the transmission coils, a
litz-wire is used.
Figure 6 shows the operation waveforms with the 3-kW
prototype. Figure 6(a) shows the inverter output voltage
vuv1 , output current iu1 , rectifier input current iu2 and output, · · · · · · · · · · · · · · · · · (11)1
. · · · · · · · · · · · · · · · · · (12)
ω2 L2Y

Note that, the operating frequency is slightly adjusted toachieve a zero-voltage switching of the MOSFETs in the inverter.
3.2 Delta-delta Winding
This section describes thedesign method of the proposed three-phase WPT system withthe delta-delta winding shown in Fig. 1(b).
The equivalent resistance for the star connection is as sameas the star-star winding.
1) Secondary inductance
The secondary inductance L2Δ should be equal to theimpedance of the equivalent AC resistance. Thus the secondary inductance with a star winding is expressed by
L2Y =Experimental Verification

2Req
6 VDC2= 2· · · · · · · · · · · · · · · · · · · · · · · (13)kω
π kω P

with assuming small winding resistance under the resonancecondition.
The secondary inductance expressed by Eq. (13) have to betransformed into the inductance for the delta-delta winding.
The secondary inductance is expressed by(a) Three-phase inverter

2
18 VDC2.
π2 kω P· · · · · · · · · · · · · · · · · · · (14)L2Δ = Luv2 = Lvw2 = Lwu2 = 3L2Y =The inductance expressed in Eq. (14) have to be half in the
proposed WPT system because the transmission coils are divided and connected in series.
2) Primary inductance
The primary inductance is calculated by Eq. (15) from thevoltage ratio between the primary DC voltage and the secondary DC voltage when the primary inverter is operated insquare-wave operation mode.

2
2
VDC1
18 VDC1
L1Δ = Luv1 = Lvw1 = Lwu1 = L2Δ= 2
VDC2
π kω P· · · · · · · · · · · · · · · · · · · (15)(b) Transmission coils

Fig. 5. 3-kW prototype
Table 1. Specifications of the prototype

The primary inductances also have to be half.
3) Resonance capacitors
The resonance capacitors are connected in series to the output of the inverter and the input of the rectifier. The resonancecapacitors are designed to resonate with the primary and thesecondary inductance at the transmission angular frequency
ω.
Cu1Δ = Cv1Δ = Cw1Δ =Cu2Δ = Cv2Δ = Cw2Δ =3
ω2 L1Δ· · · · · · · · · · · · · · · · · · (16)3
· · · · · · · · · · · · · · · · · · (17)
ω2 L2Δ

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Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）(a) For conventional three-phase IPT system(a) Inverter output voltage, output current, rectifier input current and output
voltage(b) For proposed three-phase IPT system

Fig. 8. Measurement of radiation noise

eﬃciency is expected to be improved in an actual application.
Figure 8 shows the measuring point of the radiation noise.
The radiation noise is measured at 2 m from the coil edge.
The radiation noise is measured in a shielded room. As
the conventional system, the three-phase WPT system withthree coils is tested for the comparison on the radiation noise.
When the conventional method is tested, the six coils (coil B
in Fig. 2) are removed. The transmission coils of phase-v andphase-w are placed in an angle of 120 degrees to the coils onphase-u.
Moreover, the radiation noise is compared under the same
current condition because in principle the radiation noisefrom the loop coil is proportional to the current on the coils.
By adjusting the current on the coils, the fair comparison canbe achieved except the coil change. In order to remove theeﬀect of the radiation noise from the inverter, the inverter isplaced out of the shield room.
Figure 9 shows the radiation noise. Figure 9(a) is the radiation noise generated by the conventional three-phase WPTsystem with six coils where the coils B in Fig. 2 are removed.
Figure 9(b) is the radiation noise generated by the proposed
WPT system with 12 coils. The measurement distance andconditions are same as those in Fig. 9(a). In both the experiments, the common transmission coils are used. In order tohave a fair comparison, the conduction currents on each coil
are adjusted to be 6.0 A by adjusting the input DC voltage.
The input DC voltage for the proposed 12 coils system is393 V. On the other hand, the input DC voltage is reduced
to 202 V for the conventional WPT system with six coils.
Note that the inverter and the rectifier are put outside of theshielded room in order to evaluate only the radiation noisefrom the transmission coils.
The radiation noise at the fundamental frequency is suppressed from 12.2 dBμA to 2.1 dBμA by the proposed WPTsystem. It is confirmed that the proposed three-phase WPT(b) Inverter output voltage and output currents on the primary side

Fig. 6. Operation waveforms

Fig. 7. Eﬃciency characteristic

voltage V DC2 . Note that the line voltage vuv1 as the inverter
output voltage is observed. Thus, the inverter output current lags 30 degrees with respect to the inverter output linevolage. It means that the power factor of fundamental wave
can be corrected when the relationship among the magnetic
interference couplings ka , kb , and kc satisfy Eqs. (6), (7).
Thus, the eﬀect of the magnetic interference coupling on the
power factor is canceled out. Moreover, it is confirmed thatpower is transmitted from the primary side to the secondaryside.
Figure 7 shows the measured eﬃciency characteristic fromthe primary DC voltage to the secondary DC voltage. In fact,
the eﬃciency takes into account the power loss in the inverter
and the rectifier. The maximum eﬃciency reaches 91%. The
reason why the eﬃciency is not so high because the transmission coils of the prototype are a scaled-down model. Thus,
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Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）References
(1)

(2)

(3)

(4)

(5)
(a) Conventional three-phase IPT system with six coils
(6)

(7)

(8)

(9)

( 10 )( 11 )(b) Proposed three-phase IPT system with 12 coils

Fig. 9. Radiation noise at 2 m from the coil edge†( 12 )

system is eﬀective to reduce the fundamental radiation noise.
However, the radiation noise on the third-order harmonics ofthe proposed system slightly increases.
5.

( 13 )

Conclusion( 14 )

This paper proposed three-phase wireless power transfer
system. The proposed system has six primary coils and six
secondary coils. The two coils on a phase are connected inseries and these coils are diﬀerentially connected. The twocoils reduce the radiation noise because the magnetic flux
direction is opposite. Moreover, the six coils on each side
allow to cancel out the magnetic interference among the multiple coils. Due to the cancel of the magnetic interference,
the WPT system can be operated with an unity power factor
of fundamental wave regarding the inverter output.
In this paper, the eﬀect of the magnetic interference and its
cancellation method is mathematically introduced. Then, theproposed scaled-mode was experimentally tested with the 3kW prototype. The experimental results shows that the maximum DC-to-DC eﬃciency is 91%. Finally, the radiationnoise is evaluated and compared to the conventional threephase WPT system with six coils. The proposed method suppresses the radiation noise, which is measured at 2 m from thecoil edge, from 12.2 dBμA to 2.1 dBμA on the fundamental
frequency.

( 15 )( 16 )( 17 )( 18 )( 19 )( 20 )( 21 )† It should be noted that the measurement method in this paper does notcomply with the regulations published by CISPR.

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Three-Phase Wireless Power Transfer with Reduced Radiation Noise（Keisuke Kusaka et al.）( 22 )( 23 )( 24 )( 25 )

Jun-ichi Itoh (Senior Member) was born in Tokyo, Japan, in 1972. Hereceived his M.S. and Ph.D. degree in electrical andelectronic systems engineering from Nagaoka University of Technology, Niigata, Japan in 1996, 2000,
respectively. From 1996 to 2004, he was with Fuji
Electric Corporate Research and Development Ltd.,Tokyo, Japan. He was with Nagaoka University ofTechnology, Niigata, Japan as an associate professor.
Since 2017, he has been a professor. His research interests are matrix converters, dc/dc converters, power
factor correction techniques, energy storage system and adjustable speed
drive systems. He received IEEJ Academic Promotion Award (IEEJ Technical Development Award) in 2007. In addition, he also received Isao Takahashi Power Electronics Award in IPEC-Sapporo 2010 from IEEJ, 58th
OHM Technology Award from The Foundation for Electrical Science andEngineering, November, 2011, Intelligent Cosmos Award from Intelligent
Cosmos Foundation for the Promotion of Science, May, 2012, and Third
prize award from Energy Conversion Congress and Exposition-Asia, June,
2013. Prizes for Science and Technology (Development Category) from theCommendation for Science and Technology by the Minister of Education,
Culture, Sports, Science and Technology, April 2017. Dr. Itoh is a senior
member of the Institute of Electrical Engineers of Japan, the Society of Automotive Engineers of Japan and the IEEE.

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Canales: “Modeling and eta-alpha-Pareto Optimization of Inductive Power
Transfer Coils for Electric Vehicles”, IEEE Journal of Emerging and Selected
Topics in Power Electronics, Vol.3, No.1, pp.50–64 (2015)

Keisuke Kusaka (Member) received his B.S. and M.S. degrees from
Nagaoka University of Technology, Niigata, Japan in
2011, 2013, respectively. From 2015 to 2016, he waswith Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland as a trainee. In 2016,
he received his Ph.D. degree in energy and environment science from Nagaoka University of Technology. From 2016 to 2018, he was with Nagaoka University of Technology as a researcher. He is currently
an assistant professor at Nagaoka University of Technology. His current research interests include an inductive power transfer
system and high-frequency converters. He received the second prize paper
award in IPEC-Niigata 2018.

Keita Furukawa (Student Member) received his B.S. and M.S. degrees in electrical, electronics and information engineering from Nagaoka University of Technology, Niigata, Japan in 2016 and 2018, respectively. He iscurrently a Ph.D. candidate at Nagaoka University ofTechnology, Niigata, Japan. His research interests include an inductive power transfer. He is a member of
the Institute of Electrical Engineers of Japan and theInstitute of Electrical and Electronics Engineers.

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