Electromagnetic Field Exposure Feature of a High Resonant Wireless Power Transfer System in Each Mode

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1 , pp Electromagnetic Field Exposure Feature of a High Resonant Wireless Power Transfer System in Each Mode SangWook Park 1, ByeongWoo Kim 2, BeomJin Choi 1 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 bywokim@ulsan.ac.kr 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 19 th 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 ISSN: ASTL Copyright 2015 SERSC

2 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 Fig. 1. WPT system specification operating in a single mode and two resonance modes Copyright 2015 SERSC 159

3 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 MHz at a loop radius of 107 mm, and two resonant frequencies of MHz and MHz at a loop radius of 96 mm, as shown in Fig. 1 and. The two resonant modes at MHz and MHz are called even mode and odd mode in this paper, respectively. The power transfer efficiencies ( S 21 2 ) for a single mode, even mode, and odd mode are 98.2%, 98.0%, and 96.6%, respectively. 3 Dosimetry Fig. 2. Simplified cylindrical human model position with respect to the WPT system: headsize cylindrical model, 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 bodysize 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

4 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, 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. Fig. 3. Ratio of even model field intensity to odd mode field intensity for electric field and magnetic field Fig. 4. Maximum allowable powers at various distances between the WPT and the human model for head-size model and body-size model Copyright 2015 SERSC 161

5 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 ) 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 A. Kurs, A. Karalis, R. Moffatt, J. D. Joannpoulos, P. Fisher, and M. Soljacic.: Wireless power transfer via strongly coupled magnetic resonances. Science, 317, (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), (2011) 4. ICNIRP.: Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Phys., 74, (1998) 5. ICNIRP.: Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 khz). Health Phys., 99, (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: Copyright 2015 SERSC