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