electromagnetic field exposure feature of a high...

Electromagnetic Field Exposure Feature of a High

Resonant Wireless Power Transfer System in Each Mode

SangWook Park1, ByeongWoo Kim2, BeomJin Choi1

1 EMI/EMC R&D Center, Reliability & Safety R&D Division,

Korea Automotive Technology Institute, Korea

{parksw, bjchoi}@katech.re.kr 2 Department of Electrical Engineering,

University of Ulsan, Korea

[email protected]

Abstract. This paper presents the dosimetry of a high resonant wireless power

transfer (WPT) system under the conditions of a single resonant mode and two

resonant modes: even and odd modes, which occur when the two transmitting

and receiving resonators are very close to each other. The specific absorption

rates (SARs) are calculated with simplified head-size and body-size human

models placed at various distances from the WPT system and in each mode.

Results show that the electric and magnetic fields of the odd mode distributes

stronger than those of the odd mode in the area near to the WPT system, while

the opposite results are found in the far area.

Keywords: dosimetry, specific absorption rate, two resonant modes, wireless

power transfer.

1 Introduction

Nicola Tesla proposed the concept of wireless power transfer (WPT) in the late 19th

century. The idea of wireless power distribution for bulbs was first promoted by him.

As per Tesla, power is delivered through high frequency AC potentials between two

plates or nodes [1]. However, the WPT technique could not be readily adopted for

power distribution at the time, because the technique’s power transfer efficiency

decreased as the distance increased, thus making it infeasible.

A MIT research team proposed a WPT technique based on the highly

electromagnetic resonance phenomenon [2]. The high resonant (HR) WPT technique

is based on the magnetic induction phenomenon. However, the power transfer

efficiency can only be increased by as much as the level of the resonance, i.e., a high

quality factor at the relatively long distance compared to the magnetic induction with

low quality factor. Thus, the technique would need high quality factor coils as

resonators. High quality factor can enable high efficiency. However, power transfer

efficiency, depending on the resonant frequency, is very sensitive because a high

quality factor represents a narrow bandwidth. Thus, for the HR-WPT technique, the

matching condition needs to be carefully considered when aiming to deliver power to

the load with high efficiency. One of the considerations for the technique is that two

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015), pp.158-162

http://dx.doi.org/10.14257/astl.2015.116.32

ISSN: 2287-1233 ASTL Copyright © 2015 SERSC

resonance modes occur at close distance between the two resonant coils [3]. The two

resonance modes represent the two resonant frequencies, i.e., two split resonant

frequencies. This phenomenon should also be considered to maintain high power

transfer efficiency.

The HR-WPT technique has attracted considerable attention in many fields and for

various commercial product categories. Developing mobile electronic products, such

as cell phones and PDAs, that are not dependent on physical power cords would be a

natural progression towards achieving the ultimate mobility of those products. The

WPT technique would be key in this regard. The application of the WPT technique to

electric vehicles (EVs) would also be a convenient advantage, as it would enable

automatic charging of the battery after parking of the vehicle without the need for any

power cord. In addition, the safety advantages from avoiding contact with electrical

components that cause shocks can also be realized. Nevertheless, for EVs, the WPT

technique would need to be capable of providing high electrical power of up to

hundreds of kilowatts and over a large area which implies a wide electromagnetic

field of exposure. Therefore, the application of WPT to EVs requires a comprehensive

analysis to ensure consumer safety.

This paper focuses on the electric and magnetic field exposure hazards of WPT,

especially in single mode and two resonance modes condition. The electric and

magnetic field distribution of a HR-WPT system for each mode are calculated and

compared for compliance to international guidelines [4]-[7]. The dosimetry for the

HR-WPT system with a simplified cylindrical human model is conducted for various

distances between the model and the WPT system in each mode condition.

2 WPT system and mode feature

(a) (b)

Fig. 1. WPT system specification operating in (a) a single mode and (b) two resonance modes

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015)

Copyright © 2015 SERSC 159

The HR-WPT system designed in this work is shown in Fig. 1. The system consists

of two resonant coils and two loops placed inside the coils. The coils have 5 turns and

a pitch of 5 mm and are the high efficiency resonators. The inner loop plays the role

of a matching circuit. The coil radius of the WPT system is designed to be 150 mm,

and the power transfer distance is set at 150 mm. A copper wire with a radius of 2 mm

is used for the system. The coupling coefficient between the resonant coil and the

inner loop changes the input impedance at each port. The matching condition to

obtain maximum power transfer efficiency can be achieved by adjusting the size of

the inner loop, which is related to the coupling coefficient. In the HR-WPT system,

frequency splitting is clearly confirmed as the distance between the two transmitting

and receiving resonant coils decreases. However, for the proper coupling coefficient,

the two splitting resonant frequencies become a single frequency. In this work, by

properly adjusting the size of the inner loop, the HR-WPT system is designed to

contain a single frequency of 13.56 MHz at a loop radius of 107 mm, and two

resonant frequencies of 13.06 MHz and 14.11 MHz at a loop radius of 96 mm, as

shown in Fig. 1 (a) and (b). The two resonant modes at 13.06 MHz and 14.11 MHz

are called “even mode” and “odd mode” in this paper, respectively. The power

transfer efficiencies (|𝑆21|2) for a single mode, even mode, and odd mode are 98.2%,

98.0%, and 96.6%, respectively.

3 Dosimetry

(a) (b)

Fig. 2. Simplified cylindrical human model position with respect to the WPT system: (a) head-

size cylindrical model, (b) body-size cylindrical model.

Fig. 2 shows the cylindrical model position with respect to the WPT system. The

specific absorption rates (SARs) are calculated for each simplified head- and body-

size human models at various distances (d) between the WPT system and the

simplified human model. The sphere model is more appropriate compared to a

cylindrical shape for the human head. However, to compare two simplified human

models at the same distance and exposure shape, the cylindrical shape is chosen for

the head-size model. The dielectric properties of the cylindrical model were set to be


160 Copyright © 2015 SERSC

2/3 of that of muscle tissue, which represents the average dielectric properties of the

human body. The electrical properties of the muscle tissue are taken from Gabriel’s

Cole–Cole models [8]. The ratio of odd mode field intensity to even mode field

intensity is shown in Fig. 3. The results show that the field intensity of the odd mode

is stronger than that of the even mode in the area very near to the WPT system while

the contrary result is observed in the area far from the WPT system. Thus, the SARs

of the even mode are larger than those of the odd mode in the area near to the WPT,

while contrary results are observed in the area far from the WPT. The maximum

allowable powers (MAPs) referring to guideline limits can be calculated from the

SARs of 1 W input power. The MAPs for the head-size and body-size human models

are shown in Fig. 4. As shown in Fig. 4 (b), MAP results for body-size human model

indicate that the single mode and the odd mode have advantages in near and far area

from the WPT, respectively. The lowest MAP, i.e., the worst exposure, depends on

the mode and distance between the WPT system and the human body. This result

suggests that we should consider both localized SAR and whole-body SAR.

(a) (b)

Fig. 3. Ratio of even model field intensity to odd mode field intensity for (a) electric field and

(b) magnetic field

(a) (b)

Fig. 4. Maximum allowable powers at various distances between the WPT and the human

model for (a) head-size model and (b) body-size model


Copyright © 2015 SERSC 161

4 Conclusion

The dosimetry was conducted for the HR-WPT system when operating in the single

mode and two resonant modes. The SARs were calculated using simplified head-size

and body-size human models at various distances between the WPT system and the

human model. The field intensity of odd mode was stronger than that of the even

mode in the area near to the WPT, while contrary results were observed in the area far

from the system. The worst exposure scenario was found at the localized SAR of odd

mode in the near area and the whole-body SAR of even mode in the far area from the

WPT system. The MAP results suggested that we should consider both the localized

SAR and the whole-body SAR. In future work, the dosimetry will be conducted with

a precise whole-body voxel human model based on anatomical structures.

Acknowledgments. This work was supported by a grant “Development of

Induction/magnetic resonance type 6.6kW, 90% EV Wireless Charger (No.

10052912)” from the Ministry of Trade, Industry and Energy.

References

1. N. Tesla.: Apparatus for transmitting electrical energy. US patent number 1,119,732, issued

in December 1914.

2. A. Kurs, A. Karalis, R. Moffatt, J. D. Joannpoulos, P. Fisher, and M. Soljacic.: Wireless

power transfer via strongly coupled magnetic resonances. Science, 317, 83--86 (2007)

3. A. P. Sample, D. A. Meyer, and J. R. Smith.: Analysis, experimental results, and range

adaptation of magnetically coupled resonators for wireless power transfer. IEEE Trans. Ind.

Electron., 58(2), 544--554 (2011)

4. ICNIRP.: Guidelines for limiting exposure to time-varying electric, magnetic, and

electromagnetic fields (up to 300 GHz). Health Phys., 74, 494--522 (1998)

5. ICNIRP.: Guidelines for limiting exposure to time-varying electric and magnetic fields (1

Hz to 100 kHz). Health Phys., 99, 818--836 (2010)

6. IEEE Standard for Safety Levels With Respect to Human Exposure to Electromagnetic

Fields, 0–3 kHz, IEEE Standard C95.6 (2002)

7. IEEE Standard for Safety Levels With Respect to Human Exposure to Radiofrequency

Electromagnetic Fields, 3 kHz to 300 GHz, IEEE Standard C95.1 (2005)

8. C. Gabriel and S. Gabriel.: Compilation of the dielectric properties of body tissues at RF and

microwave frequencies. Brooks AFB, San Antonio, TX, USA (2006) [Online] Available:

http://www.brooks.af.mil/AFRL/HED/hedr/reports/dielectric/home.html


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