IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014
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Operating Characteristics of Contactless Power Transfer From HTS Antenna to Copper Receiver With Inserted Resonator Through Large Air Gap Yoon Do Chung, Member, IEEE, Chang Young Lee, Member, IEEE, Dae Wook Kim, Young Soo Yoon, Member, IEEE, Hyun Chul Jo, and Young Jin Hwang
Abstract—With the fast development of various wireless charging applications such as cell phones and electric vehicles, there is substantial interest in contactless power charging across an air gap. The contactless power transfer (CPT) system based on the resonance coupling method, which is composed of separate coils with the same resonance frequency, is feasible for exchanging energy within 2 m. However, generally, because the contactless charging system adopts a normal conducting wire, the size of the antenna is too large to be equipped to deliver a large amount of power promptly. From this point of view, we propose the combination of CPT technology with high-temperature superconducting (HTS) transmitter antenna, which we call the superconducting contactless power transfer (SUCPT). The superconducting transmitter antenna can deliver a mass amount of electric energy in spite of a small-scale antenna. The SUCPT technique is expected as a refined option to transfer a large amount of power and extend the distance. In this study, our research team achieved the improvement of transmission efficiency and extension of transfer distance using an HTS antenna in the inserted resonator coil between the HTS antenna and normal conducting receiver coils. We achieved improved transfer distance and quantity of about 25% compared with the normal conducting antenna under the same power conditions. Index Terms—Contactless power transfer technology, electromagnetic resonance coupling, HTS resonance coil, three-separate resonance coils.
I. I NTRODUCTION
A
TTENTIONS to the technology of transmitting power without wires or connectors have been increasing dramatically in the various industrial applications since it makes possible a convenient power charging system, as well as, creates electrically safe environment for consumer market [1]. The technique of contactless power transfer (CPT) can be realized by inductive and resonance methods. The inductive method utilizes magnetic field around a current carrying wire at very low frequency to wireless couple power to one or more secondary Manuscript received July 14, 2013; accepted September 4, 2013. Date of publication September 11, 2013; date of current version September 27, 2013. Y. D. Chung is with the Department of Electrical Engineering, Suwon Science College, Suwon 445-742, Korea (e-mail:
[email protected]). C. Y. Lee is with Korea Railroad Research Institute, Gyeonggi-do 437-757, Korea. D. W. Kim, H. C. Jo, and Y. J. Hwang are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea. Y. S. Yoon is with the Department of Electrical Engineering, Shin Ansan University, Danwon-Gu, Ansan-si 425-792, Korea. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2013.2281403
systems at good efficiency, however, it works in close-range across a small air gap [2], [3]. On the other hand, the resonance coupling method, which is based on the strongly magnetic resonance coupling, complimented the former problems. Even though the resonance coupling method is a sensitive matching for each resonance, it takes advantages of these features; large air gap, high efficiency and large amount of power transfer [4]. In a practical system, since a new method using a resonator coil, which is based on the physical concept with the same resonance frequency, is inserted to extend the transfer distance and improve the efficiency, such a system has been promisingly studied for various industrial applications such as mobile appliance, biomedicine and transportation, etc. [5]. Practically, the transmitted antenna coil is used from normal conducting wires and thus the size of antenna is too large to be equipped to deliver the large power quickly. It is one of major obstacles to commercialize in the contactless power applications [6]. To overcome such a problem, we proposed the combinations CPT technology with HTS transmitted antenna, it is called as, superconducting contactless power transfer (SUCPT) system [7]. As the superconducting coils can keep an enough current density, the superconducting transmitted antenna can deliver a large quantity of electric energy, as well as, extend the transfer distance in spite of a small scale antenna. From such a viewpoint, as an advanced approach, we design the new SUCPT system with the inserted resonator coil, which is one of creative options to improve the transfer efficiency and spread the delivery distance. In this paper, improved transfer ratio and delivery distance using a normal conducting resonator coil between HTS antenna and normal conducting receiver coils are presented. We obtain improved properties of HTS antenna compared with normal conducting antenna under the same distance variations. In addition, we show the feasibility of impedance matching technique between different materials at 370 kHz range. II. S UPERCONDUCTING C ONTACTLESS P OWER T RANSFER S YSTEM W ITH I NSERTED R ESONATOR C OIL A. Structure and Mechanism The basic principle and structure of a three-separate resonance coupled superconducting contactless power transfer (SUCPT) system with a resonator coil are shown in Fig. 1. It consists of radio frequency (RF) power source (Vs), transmitting antenna coil (Tx), resonator coil (Sx), receiver coil
1051-8223 © 2013 IEEE
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014
where Zij = Zji in a passive and reciprocal system. It is obtained the following expression by the resonant frequency ω0 = 1/ Li Cj as (4) Z11 = Rs , Z22 = R2 , Z33 = R3 + RLoad Z12 = jωk12 L1 L2 , Z13 = 0 Z23 = jωk23 L2 L3 . Fig. 1. Schematic diagram of superconducting contactless power transfer (SUCPT) with three separate resonator coil system.
Fig. 2. Fig. 1.
Equivalent circuit diagram of SUCPT system with resonator coil of
(Rx), and load. The distance between antenna and resonator coil is denoted by d12 . The distance between other two resonance coils is denoted by d23 , which is the distance that we are interested in for the power transfer quantity and delivery distance. The transfer efficiency is determined by the quality factor Q of each coil. The magnetic resonant coupling component contains producing an LC resonance, and power delivering with electromagnetic coupling method. The proportion of the inductance L to the resistance R of a coil leaves constant for different winding arrangements in the same volume and shape. The quality factor Q, which is an index of delivering rate, is expressed by proportion of L and R. The general definition of the Q, which is based on the ratio of stored power to the dissipated power losses of transmitting and receiving coils, is expressed as: Qi =
ωLi 2πf Li = . Ri Ri
(1)
Fig. 2 shows the simplified equivalent circuit model for magnetically coupled SUCPT with inserted resonator coil. The symbols of R1 , R2 , and R3 mean the loop resistances of each coil and load, while RS is the internal resistance of the source, and RL is the load resistance. The symbol of VS represents the complex amplitude of the ideal voltage source. Coupling coefficient kxy between different coupling coils and mutual inductance MXY of both coils are calculated as [1], [8] kxy
Mxy = Lx Ly
MXY =
|LM easured X − LM easured Y | . 4
(2a) (2b)
In this system, the cross coupling term is neglected (k13 = 0). With the use of clod of element circuit analysis, the current in each coil is derived by a symmetric 3 × 3 inductance matrix as (3): ⎤−1 ⎡ ⎤ ⎡ ⎤ ⎡ Z11 Z12 Z13 VS I1 ⎣ I2 ⎦ = ⎣ Z21 Z22 Z23 ⎦ ⎣ 0 ⎦ (3) I3 Z31 Z32 Z33 0
(4a) (4b) (4c)
The symbols of Z1 , Z2 , and Z3 are impedances of each resonant coil. Based on the known values of VS , resistances, capacitors and quality factor, the input current I1 and transferred 2 1 + k23 Q2 Q3 VS (5) I1 = 2 Q Q + k2 Q Q ) R (1 + k12 1 2 2 3 S √ √ 23 jVS k12 k23 Q1 Q2 Q2 Q3 I3 = (6) 2 2 1 + k12 Q1 Q2 + k23 Q2 Q3 RS (R3 + RLoad ) currents I3 are calculated by (1) through (4) [9]. The system voltage transfer function, VL /VS is expressed as (7) VL I3 (R3 + RLoad ) = VS I1 RS
√ √ k12 k23 Q1 Q2 Q2 Q3 (R3 + RLoad ) = . (7) 2 Q Q 1 + k23 RS 2 3 As the voltage transfer ratio given by (7) is proportional to k23 in the range of 0 < k23 < 1, the efficiency rapidly decreases with an increasing distance d23 . To keep resonance matching, input impedance Zin is one of important design parameters. Here, Zin is can be expressed by (5) Zin =
2 k12 Q1 Q2 Vin (Vs − Rs I1 ) = = Rs 2 Q Q . I1 I1 1 + k23 2 3
(8)
The input impedance Zin should be matched to Rs to achieve the high efficiency. The matching condition for coupling coefficients can be expressed as (9) 2 k12 =
2 Q2 Q3 1 + k23 . Q1 Q2
(9)
When the distance (d12 or d23 ) varies, there is a corresponding change in k23 , the transfer efficiency is varied. Practically, the optimal value of k12 , which is defined by the change of d12 , affects the transfer distance. III. E XPERIMENTAL S ETUP AND R ESULTS A. Experimental Setup To verify extending effects of transmitting waves with HTS antenna, we built two kinds of three-separate resonator CPT system, which is based on strong resonance coupling, corresponding to tape type HTS and copper antennas, respectively as shown in Fig. 3. The size of HTS antenna is about half of copper antenna. The size of receiver coil is fifteen times of HTS antenna. The bulb and LED of 20 W are attached with inserted resonator and receiver coils to confirm transfer quantity practically. The photograph of the fabricated SUCPT system including resonator and experimental performance for HTS and copper antenna are shown in Fig. 4. The specifications of system and RF generator are shown in Tables I and II. In spite of small
CHUNG et al.: CONTACTLESS POWER TRANSFER FROM HTS ANTENNA TO COPPER RECEIVER
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TABLE II S PECFICATIONS OF RF P OWER A MPLIFIER
Fig. 3. Schematic illustration of experimental setup and sequences for SUCPT and CPT systems with inserted resonator coil.
Fig. 4. Photograph of experimental performances for Cases I and II including (a) experimental setup and (b) HTS and copper coils. TABLE I S PECFICATIONS OF SUCPT S YSTEM
size of HTS coil, HTS coils contain the higher impedance and inductance than copper materials (tape and tube types). Fortunately, it helps to keep the strong resonance coupling with copper coils. In addition, in this experiment, we achieved that the reflection ratio is zero between Tx and Sx coils at 10 cm and 7 cm for Cases I and II, respectively, using variable L-C components, which are attached with Rx coil. In order to measure power transfer proportion, voltage and current probes are installed in the each coil, respectively. Experimentally, we acquired the extension of delivery distance due to HTS antenna in the inserted resonator CPT system over 30 cm as shown in Fig. 4.
Fig. 5. Experimental results of voltage and current of Sx coil in the Case I at d23 = 40, 70, 100 cm, respectively, fixed d12 = 10 cm.
B. Experimental Results The current and voltage values for Tx, Sx and Rx coils within 1 m of d23 are measured to investigate wide spread effects of SUCPT under the perfect matching impedance condition of d12 . The moving positions of d23 are changed within 100 cm at fixing d12 = 10 and 7 cm, respectively. Fig. 5 shows experimental results of induced voltage and current distributions in the Sx coil with HTS antenna. In this system, the HTS antenna is supplied by RF power generator of 60 W. Apparently, even though the d23 is enhanced, the variations of voltage and current waves was a little, relatively. That means strong impedance matching between Tx and Sx coils is formed. But, the reason of variations is enhancement of distance d23 . That means the transmitting wave of Rx coil is reflected back when the impedance matching keeps loose due to enhancing distance. And thus, the induced waves of Sx coil is added to reflected ones. The peak values of induced voltage and current at d23 = 40, 70, 100 cm are 398, 405, 413 V and 1.6, 1.8, 1.9 A, respectively. Fig. 6 shows measured results of Rx coil. As surely seen, the transmitted waves is gradually decreased since the impedance matching keeps loose due to varying d23 . The peak values of transferred voltage and current at d23 = 40, 70, 100 cm are 138, 107, 39 V and 0.48, 0.26, 0.15 A, respectively. Fig. 7 shows measured results of the phase angle comparisons between Sx and Rx coils at d23 = 100 cm. Obviously, it can maintain constant phase angle for each coil across d23 = 100 cm. That means it efficiently fulfill resonance coupling for each other. Fig. 8 shows measured waveform of Sx coil by copper antenna under the same RF power conditions of Fig. 5. Compared with waveform by HTS antenna, induced peak values are surely reduced in spite of larger size. Especially, the magnitude
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014
Fig. 6. Experimental results of voltage and current of Rx coil in the Case I at d23 = 40, 70, 100 cm, respectively, fixed d12 = 10 cm.
Fig. 9. Experimental results of voltage and current of Rx coil in the Case II at d23 = 40, 70, 100 cm, respectively, fixed d12 = 7 cm.
Fig. 7. Phase angle comparison of measured voltage and current of Sx and Rx coils in the Case I at d12 = 10 cm and d23 = 100 cm.
Fig. 10. Phase angle comparison of measured voltage and current of Sx and Rx coils in the Case I at d12 = 10 cm and d23 = 100 cm.
Fig. 8. Experimental results of voltage and current of Sx coil in the Case II at d23 = 40, 70, 100 cm, respectively, fixed d12 = 7 cm.
of induced current waveform is remarkably reduced. The induced peak values of transferred voltage and current at d23 = 40, 70, 100 cm are 314, 331, 351 V and 1.34, 1.47, 1.53 A, respectively. Fig. 9 shows measured waveform of Rx coil by copper antenna. Apparently, the transmitted waves are suddenly decreased over d23 = 100 cm. The peak values of transferred voltage and current at d23 = 40, 70, 100 cm are 99.8, 96.3, 48.7 V and 0.45, 0.37, 0.19 A, respectively. The phase angle are compared with Sx and Rx coils across 100 cm as shown in Fig. 10. Compared with results of Fig. 7, the phase difference for each coil is evidently caused. That means transmitted power are rapidly decreased. The measured transferred power in the Rx coil for Cases I and II are calculated within d23 = 100 cm as shown in Fig. 11. It is investigated that the transmitted power in the Case I are rightly improved compared with Case II. Especially, in the Case II, the phase shifting between Sx and Rx obviously
Fig. 11. Measured results of transfer power according to Cases I and II within 100 cm of d23 .
caused. From this reason, average of transferred power is reduced about 25% even though the size of HTS coil is about half of copper coil. IV. C ONCLUSION This study demonstrated the efficient effects for HTS antenna in the CPT technology with inserted resonator coil. In addition, it is expected as a creative option to improve the transfer efficiency and extend the delivery distance. The characteristics and relations for coupled resonance phenomenon between varying distance and delivery efficiency were successfully examined. Especially, we fulfilled the possibility for impedance matching between different materials at 370 KHz resonant frequency. Thus, we achieved that the average transmitted power is improved over 25% compared with copper antenna. It is confirmed
CHUNG et al.: CONTACTLESS POWER TRANSFER FROM HTS ANTENNA TO COPPER RECEIVER
that The HTS coil has advantage of strong resonance coupling method with copper coil since it keep larger impedance. From this reason, it is concluded that the HTS coil is more suitable antenna than receiver. However, to utilize to practical use for HTS coil, the study of cooling vessel design should be demonstrated to reduce the cost in the practical applications in the next studies. Our research target is to evaluate such a transfer system with efficient transfer properties by HTS coil and is technically feasible with a superconducting contactless energy charging and storage system. R EFERENCES [1] J. R. James and P. S. Hall, Handbook of Microstrip Antenna. London, U.K.: Peter Peregrinus, 1989. [2] M. G. Egan, D. L. O’Sullivan, J. G. Hayes, M. J. Willers, and C. P. Henze, “Power-factor-corrected single-stage inductive charger for electric vehicle batteries,” IEEE Trans. Ind. Electron., vol. 54, no. 2, pp. 1217–1226, Apr. 2007.
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[3] W. C. Brown, “The history of power transmission by radio waves,” IEEE Trans. Microw. Theory Tech., vol. 32, no. 9, pp. 1230–1242, Sep. 1984. [4] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljaˇci´c, “Wireless power transfer via strongly coupled magnetic resonances,” Science, vol. 317, no. 5834, pp. 83–86, Jul. 2007. [5] B. L. Cannon, J. F. Hoburg, D. D. Stancil, and S. C. Goldstein, “Magnetic resonant coupling as a potential means for wireless power transfer to multiple small receivers,” IEEE Trans. Power Electron., vol. 24, no. 7, pp. 1819–1925, Jul. 2009. [6] E. W. Schmidt and T. Staring, “Limitation of inductive power transfer for consumer application,” in Proc. Eur. Conf. Power Electron. Appl., Sep. 2009, pp. 1–10. [7] D. W. Kim, Y. D. Chung, H. K. Kang, Y. S. Yoon, H. M. Kim, and T. K. Ko, “Effects and properties of contactless power transfer for HTS receiver coils via electromagnetic resonance coupling,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, Jun. 2013. [8] T. Imura and Y. Hori, “Maximizing air gap and efficiency of magnetic resonant coupling for wireless power transfer using equivalent circuit and Neumann formula,” IEEE Trans. Ind. Electron., vol. 58, no. 10, pp. 4746– 4752, Oct. 2011. [9] T. P. Duong and J. W. Lee, “Experimental results of high-efficiency resonant coupling wireless power transfer using a variable coupling method,” IEEE Microw. Wireless Compon. Lett., vol. 21, no. 8, pp. 442–444, Aug. 2011.
