International Journal of Electrical and Computer Engineering (IJECE) Vol.
No.
June 2025, pp.
ISSN: 2088-8708.
DOI: 10.
11591/ijece.
Optimized wireless power transfer for moving electric vehicles by real-time modification of frequency and estimation of coupling coefficient Kazuya Yamaguchi1.
Haruto Terada1.
Ryusei Okamura2.
Kenichi Iida1 Department of Control Engineering.
National Institute of Technology.
Nara College.
Nara.
Japan Major of Business Engineering.
Graduate School of Engineering.
University of Osaka.
Osaka.
Japan
Article Info
ABSTRACT
Article history:
In order to prevent global warming, electric vehicles are increasingly recommended than gasoline-powered vehicles that have been widely used in the past.
However, problems peculiar to electric vehicles exists, and their widespread utilize is not progressing in Japan and other developed countries.
This study performed wireless power transfer assuming that electric vehicles are stationary on a road at some distance from an AC power supply.
Frequency of a power supply has significant influence on efficiency of wireless power transfer, and it is important to adjust this value on any Therefore, an experiment was conducted based on the optimal frequency expression derived in the past to confirm the correctness of the expression, finally it achieved 60% transport efficiency.
Moreover, since the expression includes coupling coefficient between transmission and receiving inductors, its value must be estimated accurately.
In this study, an experiment was conducted to estimate value of coupling coefficient using current and voltage values measured from outside circuits, and it was compared with a theoretical expression obtained from laws on Received Aug 14, 2024 Revised Feb 10, 2025 Accepted Mar 4, 2025 Keywords:
Electric vehicle Estimation of coupling Frequency auto tuning Theoretical approach Wireless power transfer This is an open access article under the CC BY-SA license.
Corresponding Author:
Kazuya Yamaguchi Department of Control Engineering.
National Institute of Technology.
Nara College 22 Yata-cho.
Yamatokoriyama.
Nara.
Japan Email: k-yamaguchi@ctrl.
nara-k.
INTRODUCTION
Recently, along with industrial development, various environmental problems such as global warming and consumption of fossil fuels have been pointed out .
, .
Many methods have been tried around the world to solve these problems.
Among them, electric vehicles (EV.
are noticed as contributing to solving problems without reducing the quality of the modern transportation environment.
In Europe, the U.
, and China.
EVs account for a large percentage of all new vehicles, and made widely in there area globally .
On the other hand, there are area, including Japan, where EVs have not been widely adopted even in developed countries, mainly due to their short cruising range and lack of recharging locations.
As a method to solve those problems, wireless power transfer (WPT) is expected to be utilized.
The basic principle of WPT is based on electromagnetic induction proposed by Faraday and Henry .
Strong resonance by reactance adjustment to realize high power WPTs have greatly advanced WPT field .
The inductance has been evaluated analytically and numerically to supply power to automobiles .
, the rotating magnetic field has been applied to automated guided vehicles (AGV.
by devising a topology .
, and photovoltaic power generation is concomitant used for WPT .
In addition to land-based transports.
Journal homepage: http://ijece.
Int J Elec & Comp Eng
ISSN: 2088-8708
airborne applications are also advancing.
Communication technology optimize UAVs in the power transport space .
, expansion of the charge capacity of UAVs can be realized by reducing current ripple .
, and study .
proposed for methods to improve energy transport efficiency to UAVs by optimizing their Furthermore, its applications are not limited on the earth, antenna design for WPT in space .
and design of focusing lenses for harvesting space sunlight .
are reported.
A load which is charged by WPT must always take into account changes in position.
This means that the coupling coefficient between the transmission and charging circuits is constantly changing, and the frequency must be adjusted to the optimal value each time .
Alternatively, the optimum frequency varies depending on elements such as inductors and capacitors that make up the load.
Currently, a method to calculate coupling coefficient using the parameters on the transmitter and receiver has been proposed .
, although a method to calculate it in real time and to always use the optimal frequency has not been established.
On the other hand, attempts have been made to achieve efficient power transmission by fixing a position of load at the circuit design and devising inductors and Q values .
, .
In this paper, the optimal frequency is calculated and controlled, and the value is required by an inverter on WPT circuit.
The optimal frequency is defined as the value which maximizes efficiency, and efficiency is defined the ratio of transmitting and receiving power.
Simulations and experiments are conducted around the optimal frequency, and it can be confirmed that the power transmission efficiency is maximum near the optimal frequency in both cases.
However, the expression for the optimal frequency includes the coupling coefficient between inductors in addition to the circuit constants, and this value must be accurately determined.
The coupling coefficient can be expressed using the magnetic flux obtained from Biot-Savart's law, although the expression includes parameters related to a shape of the inductor, such as radius and cross-sectional area, and there may not be confirmed from outside a circuit.
Hence this study attempts a method to calculate the value from externally measured currents and voltages, and compares it with the expression obtained from theory.
AUTOMATIC FREQUENCY TUNING FOR IMPROVEMENT TRANSMITTION EFFICIENCY
Transmission circuit and efficiency The circuit of this study and parameters are shown in Figure 1 and Table 1.
This circuit is largely composed of an inverter section and a mutual induction section.
The inverter section is consisted of four MOSFETs, which are controlled by the switching IC TL494.
Furthermore, the TL494 is controlled by a microcontroller through the digital potentiometer AD8400, and the microcontroller controls the frequency of the inverter section consequently.
Figure 1.
The circuit of WPT with frequency auto tuning Table 1.
Parameters on the circuit Parameters PULSE 1 ycI1, ycI2 , ycI3 Values Oe5 to 5 V 1 nF Parameters ycI4 , ycI5 ya2 , ya3 ya1, ya2 Values 047 F For highly efficient WPT, it is important to determine the frequency of the inverter section.
In this study, the frequency is adopted such that the ratio of load power to transmission power is maximized.
If point A in Figure 1 is the transmission voltage, the frequency is calculated as .
Optimized wireless power transfer for moving electric vehicles by real-time A (Kazuya Yamaguch.
A yce= ISSN: 2088-8708 ycI ycI ya Oo ycI ya .
cI ya 2 ycI yco 2ya ya ) 2yuU Then yco is coupling coefficient between ya1 and ya2 .
Moreover, since yco is obtained by the positional relationship between ya1 and ya2 as shown in .
, and this positional relationship determines the optimal frequency .
yco= yuNycI1 ycI2 2yuUOoya1 ya2 .
c1 yccycu 2 yccyc 2 yccyc 2 )2 .
Then yuN is the magnetic permeability, ycI1 and ycI2 are the total area of each inductor, yc1 is the radius of ya1 , and yccycu , yccyc , yccyc are the center position of ya2 with ya1 as the origin.
Based on the above information, this study constructed a system in which a microcontroller performs optimal frequency control in real time when the yccyc position is changed.
Figure 2 shows the experiment on our laboratory, and Figures 3.
show the calculated and measured efficiency - frequency relationships each other at yccyc = 10 mm.
The dashed lines in Figure 3 shows the optimum frequency at yccyc = 10 mm obtained from .
Figure 2.
Experiments on automatic frequency tuning according to distance .
Figure 3.
Variation of efficiency versus frequency and optimum frequency .
calculated values and .
measured values Discussion From .
, the optimum frequency is calculated to be 145 kHz at yccyc = 10 mm.
Figure 3.
shows that efficiency at the frequency is 60.
9%, which is the highest value compared to other frequencies.
From above result, it is concluded that .
correctly achieves high-efficiency WPT.
On the other hand, the result based on the calculated values shown in Figure 3.
indicate that there are two extreme values of efficiency.
Int J Elec & Comp Eng.
Vol.
No.
June 2025: 2706-2712
Int J Elec & Comp Eng
ISSN: 2088-8708
This is due to the fact that .
is derived by a method that finds extreme values of efficiency, and the number of the values increases with the number of inductor-capacitor pairs .
ACCURATE ESTIMATION OF COUPLING COEFFICIENT FOR CORRECT CALCULATION
OF FREQUENCY
In the previous section, it is argued that frequency should be adjusted according to the positional relationship between the inductors.
On the other hand, the formula for correct frequency calculation includes coupling coefficient.
Therefore, if this value cannot be accurately measured, the optimal frequency cannot be This section measures the voltage at each element externally and measures the coupling coefficient from the results.
In addition, it is compared with the coupling coefficient derived from the theory described in the previous section.
A circuit to measure mutual inductance A circuit to measure coupling coefficient correctly is shown in Figure 4.
From Figure 4, the following circuit .
can be obtained.
[ .
Oeyui0 ycoya1 ya2
Oeyui0 ycoya1 ya2
OeycI2
][ 1 ]
When yui0 satisfies the following .
yui0 = Ooya1 ya1 Ooya2 ya2 From the above conditions, the coupling coefficient yco is obtained as .
yco= ycO1 AOoycO12 Oe4ycI1 ya2 .
cI2 ya2 ycO2 ) .
2yui0 Ooya1 ya2 ya2 In .
, since a designer can determine the value of ycO1N , yco can be estimated by measuring the current ya2N and voltage ycO2N of ycIL .
Also .
has two solutions, but takes positive values when yco is large and negative values when yco is small .
In this section, .
which is obtained from the measured values is compared with .
, which is obtained from the inductor configurations and distance between inductors.
The configurations of inductors used in this experiment is shown in Figure 5 and Table 2.
This inductor is wound horizontally and consists of two vertical layers.
Therefore the value of the radius varies from place to place, and as a result, the area in a loop changes each place .
yco Figure 4.
A circuit for estimating coupling coefficient based on experimental values Table 2.
Configuration of inductor Inner radius 10 mm Turn number Self-inductance 1 H Experimental and theorical values of the variation of yco with distance between inductors are shown in Figure 6.
Considering the size of the inductors, the range of distances between inductors is 0 to 20 mm.
The values for each element are shown in the Table 3.
Optimized wireless power transfer for moving electric vehicles by real-time A (Kazuya Yamaguch.
A ISSN: 2088-8708 Figure 5.
Inductor used on experiment Coupling coefficient yco Experiment Eq.
Estimation Eq.
Distance between inductors .
Figure 6.
Comparison of experimental and estimated values of coupling coefficients Table 3.
Values of elements on an experimental circuit Parameters ycO1 Values 39 nF Parameters ya2 ycIya Values 39 nF Discussion From coupling coefficient estimation, it is found that the coupling coefficient decreases gradually as the distance between inductors increases, as shown in the Figure 6 for both experimental and estimated However large errors can be observed, especially in the range of large distance.
In a situation where the inductors are almost separated, an error of over 200% is observed.
This is due to the fact that the cube of the position is included in .
, and the values of current and voltage used in the calculation in .
are small.
When WPT is applied to EVs, the distance between a power supply and a load is often large, thus it is necessary to consider the validity of the approximation accuracy and to increase size of an inductor on power supply circuit .
Moreover, in this study, an ultrasonic sensor was used to measure the distance, but if speed of a target is fast, feedback takes time and the optimum frequency adjustment may not be made in Therefore a more immediate response sensor is required .
, .
, or a sensor should be installed in a location where the EVs are moving at a sufficiently low speed.
CONCLUSION
In this paper, an inverter circuit and a mutual induction circuit that constitute WPT system were The inverter circuit was controlled by a microcontroller, and frequency was programmed to always adopt the optimum value.
As a result, the power transport efficiency was greater when the optimal frequency was adopted than at other frequencies.
The frequency autotuning method successfully improved efficiency, but errors between experimental and theoretical values were confirmed in the previous section.
To improve the error, an expression for coupling coefficient was calculated from circuit equation, and its value was estimated from experimental values.
The estimation results show that the error tends to be large, especially in the range where the distance between inductors is large.
Int J Elec & Comp Eng.
Vol.
No.
June 2025: 2706-2712
Int J Elec & Comp Eng
ISSN: 2088-8708
FUNDING INFORMATION
Authors state no funding involved.
AUTHOR CONTRIBUTIONS STATEMENT
This journal uses the Contributor Roles Taxonomy (CRediT) to recognize individual author contributions, reduce authorship disputes, and facilitate collaboration.
Name of Author Kazuya Yamaguchi Haruto Terada Ryusei Okamura Kenichi Iida C : Conceptualization M : Methodology So : Software Va : Validation Fo : Formal analysis ue ue ue ue ue ue ue ue ue ue ue ue ue ue ue ue I : Investigation R : Resources D : Data Curation O : Writing - Original Draft E : Writing - Review & Editing ue ue ue ue ue ue ue ue ue ue ue ue ue ue Vi : Visualization Su : Supervision P : Project administration Fu : Funding acquisition CONFLICT OF INTEREST STATEMENT Authors state no conflict of interest.
DATA AVAILABILITY
The evidences of some data on this study can be submitted by HT and RO if those are required.
REFERENCES