IECON2015-Yokohama
November 9-12, 2015

Investigation of Wireless Power Transfer System withSpaced Arranged Primary H-shaped Core Coils forMoving EVs
Chihiro Kato

Kazuma Tsuda

Graduate school of
Science and Engineering,
Saitama University,
Saitama, Japans14mm212@mail.saitama-u.ac.jp

Graduate school of
Science and Engineering,
Saitama University,
Saitama, Japans15mm234@mail.saitama-u.ac.jp

Yasuyoshi Kaneko
Graduate school of
Science and Engineering,
Saitama University,
Saitama, Japan
kaneko@ees.saitama-u.ac.jp

Toshiyuki Fujita
Technova Inc.,Tokyo, Japant_fujita@technova.co.jp

Abstract— As a solution for long charging time and shortrunning range of EVs, research on wireless power transfer
systems for moving EVs have been getting attention. Althoughseveral power transfer systems with single or multiple grounded
coils have been proposed, those systems have difficulties inmaintenance, and require high system implementation andmaintenance cost. In this paper, we evaluate the performance of a
proposed wireless power transfer system for moving EVs,
composed by a row of evenly spaced H-shaped core coils. With
two types of resonance circuit, SP and SS topology, theperformance of the system is evaluated under a constant voltage
operation and a constant current operation. Comparison of theperformance of the system with each resonance circuit in each
operation is presented from the aspect of power output, efficiency
and power factor. Among all experiment cases, power transfer
with average transformer efficiency above 85% is achieved.

Tsuyoshi Matsumura
Graduate school of
Science and Engineering,
Saitama University,
Saitama, Japans13mm233@mail.saitama-u.ac.jp
Tomio Yasuda
Technova Inc.,Tokyo, Japan
yasuda@technova.co.jp

solve these problems. The concept of the system is shown in
Fig.1. This system allows the vehicle’s battery to have lesscapacity, which leads to the lowering of the price of thevehicles and contributes to distribution of these vehicles.
Showa Aircraft Industry Co. Ltd. proposed a system composedof a grounded loop coil and a pick up coil and Bombardier Inc.
proposed a system composed of a primary S-shaped loop coiland a pick up coil[3][4]. However, these loop coil systems are
not able to be expanded to the power transfer system for parked
EVs which have been promoted standardization worldwide,
and this limits the coevolution of two wireless power transfer

I. INTRODUCTION
The development and commercialization of electricvehicles (EVs) and plug-in hybrid EVs (PHVs) are actively
being realized because of environmental concerns andincreasing oil prices. Current PHVs and EVs require aconnection to a power source for battery charging using electric
cables. A wireless power transfer system would have numerous
advantages: it would be convenient, safe, and maintenance-free.
Therefore, wireless power transfer systems are being
investigated worldwide[1].
A wireless power transfer system for EVs must have a large
air gap and good tolerance to misalignment. In addition, it must
be compact and lightweight. Therefore we have developed the
H-shaped core transformer to satisfy these requirements[2].
EVs have the issue of long charging time and short running
range. A wireless power transfer system for moving EVs can

978-1-4799-1762-4/15/$31.00 ©2015 IEEE

005197

Fig. 1. Wireless power transfer system for Moving Vehicle.

yVehicle running direction x
Fig. 2. Outline of an H-shaped solenoid coil.

systems (one for parked EVs/moving EVs). Moreover, all thesesystems with a grounded loop coil have difficulties inmaintenance and extension of the system due to theinseparability of the grounded coil. KERI (The Korea Electro
technology Research Institute) proposed a system composed ofa number of grounded coils arranged to overlap slightly and asecondary pick up coil called receiver. Although it achieved
around 80% of power transfer efficiency, the issue of thesignificantly low efficiency observed at the overlap point isreported[5]. Furthermore, the total cost of the system isconsiderable because the system needs to arrange many coils
without any spaces, which leads to an increase in the number ofthe grounded coils. In response to the need of a system which
is cheaper, easier to extend, and easier to maintain, Technova
Inc. proposed a system with a row of evenly spaced H-shaped
core coils which is relatively small, has a large air gap, andgood tolerance to misalignment[6]. However, study of thissystem in terms of resonance circuit configuration and powersource operation method has not been considered yet. In thisresearch we evaluate the performance of the system with two
types of resonance circuit (SP and SS circuit) along with two
types of power source operation (constant voltage operationand constant current operation) in terms of power factor of
power source output and transformer efficiency throughexperiments. Power transfer with above 85% transformer
efficiency is achieved under those four different cases.

(a) Parallel connection(b) Series connection

II. CIRCUIT OF WIRELESS POWER TRANSFER SYSTEM FOR
MOVING EV

Fig. 3

A. Series connection of primary coils
The outline of an H-shaped solenoid coil is shown in Fig.2.
In proposed system a number of H-shaped core coils need to
be set as the primary coil. However, equipping each primarycoil with a power source and operating many switches
separately in order to activate each coil is undesirable in termsof cost and complexity. Therefore several coils should beconnected to one power source in series or in parallel. Fig.3
shows a series connection and parallel connection of the
primary coils. When several coils are connected to a powersource, the coil which does not form coupling with thesecondary coil shows lower impedance than the one withcoupling. Taking this into account, primary coils areconnected to power source in series. This connection allows
the total impedance of the load of the power source to be thesum of each coil’s impedance. Owing to this, the system canprotect the inverter and coils from high current caused bysmall input impedance of load, as long as there is a primarycoil forming coupling with the secondary coil. The proposal
system is shown in Fig.4. Intervals between primarytransformers is determined in considering the power feeding
range when magnetic gap is 160mm.
B. Circuit constants of the series connection coilState of mutual coupling when connected in series to the nprimary coils is shown in Fig.5. In this case the selfinductance L1 of the entire primary side as viewed from theprimary side of the terminal is expressed as equation (1).

005198Connection method of the primary coilStarting Point
Vehicle running direction x

160mm

600mm
(a) Outline of the proposal system(b) Photograph of the proposal system
Fig. 4. Proposal system.

L1 

n

n

n

 L   Mii 1

1i _ 1 j (i  j )(1)

i 1 j 1

Moreover, the primary coils regarded as interconnected
only between adjacent coils, the equation (1) is simplified asshown in equation (2).
L1 

n 1

n

 L  2 M
ii 1

1i _ 1( i 1)

(2)

i 1

Meanwhile, the mutual inductance M of the primary coils
across the secondary coil is represented by the equation (3).
M

n

M

(3)

2_i

i 1

Therefore, it can be regarded as a plurality of primary coils
connected in series with one coil of the self-inductance L1.
Equivalent self-inductance L1 and mutual inductance M iscapable of directly measured by using a LCR meter in a state
where the primary coils are connected in series.
C. Capacitors’ Effect on Power Factor
Since the wireless power transfer system has a poor
coupling coefficient, resonance capacitors are generally used
to compensate for the leakage inductances. As two main ways
to insert resonance capacitors, our research has proposed SP
and SS topology. Although capacitors on ground side in both
SP and SS topologies has the ability to compensate the power
factor of power source output in the wireless power transfer
system for parked EVs, the system for moving EV need totake variation of the circuit constant into account in order toevaluate appropriate methods from the perspective of thepower factor of power source output. The resonance circuits of
SP and SS topology are shown in Fig.6.

TABLE I: Experimental Conditions.

Capacitors Topology
Frequency of Power Source
Intervals between primary transformers
Magnetic Gap
Number of Turns of Primary Coils
Number of Turns of Secondary Coils
Primary Coil Inductance L1
Secondary Coil Inductance L2
Coupling Coefficient k
Capacitor on Ground Side C1
Capacitor on Secondary Side C2Resistance Load RL

14T
204.6H
6.21H67.0H
0.182
0.185
0.0171F 0.0173F
0.583F 0.0493F
50
10

L11
C12
L12

Capacity of the capacitor on ground side C1 is decided sothat the output of the inverter does not take an advancing statephase within any secondary coil position. Taking this into
account, C1 for SP topology C1SP and C1 for SS topology C1SS
are determined by equation (5). L1 expresses the self
inductance of primary coil, k expresses the coupling
coefficient of the primary coil and the secondary coil.
(5)

L2
C2

L13

(a) SP topology
C11
C2

L11
C12
L2

L12
C13
L13

(b) SS topology
Fig. 6. Experimental circuits.

005199

RL

C13

(4)

1
2
 0 L1
C1SS

4T

C11

Performance of the system with both SP and SS circuits ismeasured. The experimental conditions are shown in Table 1,
and three of the coils in Fig.2 are used as primary coils. In
order to cut off the flux lines which are not shaped towards thecoil in the other side, aluminum shielding (600mm×600mm
thickness 2mm) is placed behind each coil. Capacity of thecapacitor on secondary side C2 is fixed to the value which isresonant with the self inductance of secondary coil L2. C2 isshown by equation (4).

12
 0 L1 (1  k 2 ),
C1SP

SS
85kHz
600mm
160mm
14T

Fig. 5. Mutual coupling of series connection coilIII. EVALUATION TROUGH POWER TRANSFER EXPERIMENT12 0 L2
C2

SP

RL

Furthermore, in order to avoid over-voltage, C1 is split into
three capacitors, C11 ～ C13, which have three times morecapacity, and connected alternately with each primary coil.
The center point of the primary coil in the middle is defined as
the starting point. In this experiment, load power, power factorof the power source output and transformer efficiency areobserved when a secondary coil is moved in the running
direction of the vehicle from 0mm to 1200mm at intervals of
50mm.
A. Comparison of Power Source Operation
Fig.7 shows the load power PL and transformer efficiency
ηTR at each secondary coil position with two types ofresonance circuits along with two types of power source
operation. Under the constant voltage operation, data was
unable to be sampled over 800mm for the SP circuit, and over
700mm for the SS circuit due to a critical high current fromthe power source. The transformer efficiency is relatively highat the point where the secondary coil face one of the primarycoils (0mm or 600mm) in both topologies, and the variation
range of the value is small when the secondary coil is over thearea between two primary coils (0mm~600mm). In addition,
there are few differences between the values from the two
types of operations. Incidentally, load power plots show
different characteristics between the two power source
operations. While data from a constant current operation plots
the highest value at the point where the secondary coil faces
one of the primary coils (0mm or 600mm), data from a
constant voltage operation plots the highest value at the point
where secondary coil is located at the middle of the two
primary coils (300mm). Moreover, transformer efficiency at
the highest load power point takes the highest value in aconstant current operation and the lowest value in a constantvoltage operation. Therefore a constant current operation
contributes to reducing the power source capacity.
B. Comparison of SS, SP Topology
Fig.8 shows the power factor of the power source output at
each secondary coil position. The power source is driven witha constant current operation. Under both topologies the power
factor is relatively high at the point where the secondary coilfaces one of the primary coils (0mm or 600mm), and it is low
at the point where the secondary coil located in the middle oftwo primary coils (300mm). The variation range of the value
is small when the secondary coil is over the area between two
primary coils (0mm~600mm). In addition, the system under
SP topology keeps a higher value whilst the secondary coil isincreasingly displaced from primary coils.
In the SP topology, coupling coefficient k is included in
the equation of primary side capacitor C1. In constant, it is notinclude in the equation of C1 in the SS topology. k varies
depending on the position of the secondary coil. However, theresult of the previous section, SS system has a larger power
factor variation than the SP system. Fig.9 shows the absolutevalue |Zin|, the real part Re (Zin) and the imaginary part Im(Zin).
Zin represents the impedance of the behind the power source.
Im(Zin) in the SP topology is negative value when thesecondary coil position is 500mm and 600mm. Therefore,(a) SP topology

Fig. 7.

Fig. 8(b) SS topology
Measured load power and ηTR.

Measured power factor of power source output.

absolute values of those points are shown by a broken line.
Im(Zin) is determined by the accuracy of the capacitor on
primary side C1. In the SP topology, it is varied in response to
the primary coil position. However, the variation in the SS
topology is small. In the equation (5), it has appeared
difference of including a coupling coefficient. It is also
possible to reduce Im(Zin) by more accurately designing C1.
Meanwhile, Re(Zin) changes according to the secondary coilposition in both topologies. This is because influence of load
resistance RL changes according to factor of k. In the SP
topology, a fluctuations in the power factor is small because
Re(Zin) and Im(Zin) change equally. In the SS topology, Im(Zin)
is fixation approximately, but Re(Zin) is to fluctuate, so
fluctuation of power factor occurs.

005200IV. CONCLUSION
In this paper, we evaluate the performance of proposed
power transfer system for moving EV with two resonancecircuits along with two different power source operations.
Under all of the cases we tested, this system with spaced lined
primary coils achieved power transfer with transformer
efficiency above 85% on average when the secondary coil isover the area between two primary coils (0~600mm). The
system has potential to lower the implementation andmaintenance cost, and alleviates the difficulties ofmaintenance and extension of the system. The efficiency andthe supplied load power need to be improved, and the intervals
between the primary coils need to be wider for practical use.
Structure of the primary coil and winding number ratio of thetransformer need to be considered for further sophistication.

(a) SP topology

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005201(b) SS topology
Fig. 9Measured impedance Zin