정인성
(In-Sung Jeong)
1
최효상
(Hyo-Sang Choi)
2†
-
(Dept. of Electrical Engineering, Chosun University, Korea)
-
(Dept. of Electrical Engineering, Chosun University, Korea)
Copyright © The Korean Institute of Electrical Engineers(KIEE)
Key words
Wireless Power Transfer (WPT), Superconducting coil, Resonance frequency, Selectivity of frequency
1. Introduction
The interest in electric vehicles (EVs) has been increasing rapidly due to the impending
exhaustion of fossil fuels and the environmental pollution caused by toxic emissions.
As EVs are driven by electric motors, they do not need fossil fuel, which can also
solve the problem of gas emissions. Therefore, the use of EVs is recommended worldwide.
To succeed in commercializing EVs, however, the issue concerning the battery and charging
method should first be solved. EVs can currently travel up to 300 km with a single
battery charge, but EVs cannot be commercialized unless and until the issue concerning
the battery charging system is addressed. The present EVs depend on the wired charging
method, but insufficient charging stations have been installed due to the inconvenience
associated with the use of cables, the potential electric shock hazards, and the restrictions
involved in the installation of charging stations. The desire to address these concerns
has increased the interest in the wireless charging method. As such method does not
directly use cables, it can address the issues of charging inconvenience and electric
shock hazards. The problem of space limitation can be solved as well because such
charging system can be installed and used in public parking lots. A high-efficiency
wireless charging system (WCS) using superconducting coils was proposed. As superconducting
coils have zero resistance at a critical temperature, they can increase the overall
charging efficiency of the WCS by minimizing the loss caused by the conventional coils
used in the general WCS(1-5). This highly efficient superconducting WCS was applied to EV. The same resonance
frequency produced by Tx and Rx coils was applied to the ground and the EV respectively.
At this point, depending on the type of EV, the level of elevation of vehicles is
different. In this case, the mutual inductance changes between the Tx and Rx coils.
The resonance frequency is determined by the inductance (L) and capacitor (C). Interchanges
between the Tx and Rx coil will alter the value of the L and the resonance frequency.
When the impedance miss matching occurs to the electric device, wireless power transmission
efficiency is drastically reduced.
In this paper, the variable capacitor was applied to the wireless charging system
to address these problems. The variation of inductance and the mutual inductance was
analyzed according to distance between the coils and EV. To maintain constant resonance
frequency, C value was varied according to variation of L value. With these results,
the characteristics of superconducting wireless power transfer were analyzed.
2. Theory
2.1 Resonance frequency
When two objects share the same frequency, energy is transferred between them. This
frequency is called “resonance frequency.” The transmitter and receiver coils of WCS
should be designed to share the same frequency to facilitate energy exchanges. Therefore,
the resonance frequency is the key to wireless charging. Equ. (1) represents the resonance frequency. Such frequency consists of the inductance (L)
and capacitance (C). The values of L and C can be adjusted to create selectivity
at specific frequencies [5].
2.2 Experiment design
Fig. 1 (a) shows a WCS that adopted high-frequency structure simulation (HFSS). Fig 1 (b) represents a superconducting wireless filled system on EV vehicles. The Tx coil was
applied to the ground and the Rx coil was applied to the underside of the vehicle.
The spiral type coil was applied for downsize the volume to place in the underside
of the vehicle.
Fig. 1. (a) The modeling of the transmitter and receiver coils (b) The WCS modeling
applying in EV
Two transmitter and receiver coils made of a super- conducting material were designed.
The selected resonance frequency was 350 kHz, within the frequency band range recommended
by WPC (325-450kHz), which allows the avoidance of interference between high-frequency
communication bands and reduces the adverse effect on the human body. Table 1 shows the coil design parameters. The diameter and pitch of the coils were selected
considering the specifications of an actual EV. The transmitter and receiver coils
were made with the same shape. As the superconducting coil has zero resistance below
the critical temperature, the impedance matching of the transmitter and receiver coils
was made 0 Ω. In WCS, L stands for the wireless charging coil. First, the wireless
charging coil was designed, and the L value was measured through HFSS. Then the C
value capable of resonating at 350 kHz was calculated using Equ. (1).
Table 1. Parameters of Superconducting coil
|
Parameters
|
Value
|
|
Material
|
Superconductor
|
|
Diameter
|
20 [cm]
|
|
Pitch
|
1 [cm]
|
|
Height
|
1.2 [cm]
|
|
Inductance (L)
|
16.73 [$\mu$H]
|
|
Capacitance (C)
|
12.36 [nF]
|
|
Impedance matching
|
0 [$\Omega$]
|
3. Experimental
3.1 Mutual inductance
Fig. 2 shows the changing S-parameter of the WCS when the transmitter and receiver coils
were placed at 10, 20, 30, and 40 cm, respectively. When applying a wireless charging
system to the EV and the ground, the distance variation according to the lower height
of the EV is unavoidable. Therefore, it is essential to analyze the characteristics
of the wireless charging system according to the distance variation. Fig. 2(a) is a graph of the reflection coefficienct(S11). At the 20cm interval, it was resonated
at 350kHz, thereby confirming that it was about –27.1dB. The reflection coefficients,
however, were -0.5, -0.3, and -3 dB, respectively, at the 350 kHz frequency when the
distance was 10, 30, and 40 cm, confirming that the signals of the transmitter coil
were not transmitted properly but bounced back. Fig. 2(b) is a graph of the transmitting coefficient (S21) when the distance was changed at
the same intervals as in Fig. 2(a). The transmitting coefficients for the different distances were -22, -1.87, -12,
and –31dB, respectively. As with the case of S11, the majority of the signals were
not transmitted to the receiver coil.
Fig. 2. S-parameters according to the changing distance of the wireless charging system
(a) S11 (b) S21
Resonance could occur in the other frequency band. When the power frequency applied
to WCS is 350 kHz, a significant amount of efficiency loss is expected at distances
other than 20 cm. It was confirmed that the frequency is divided according to the
changing distance between the transmitter and the receiver.
The frequency selection changes according to the distance between the transmitter
and receiver coils due to the mutual inductance. The inductance of each coil occurring
between the two coils affects the resonance frequency. Equ. (2) is a resonance frequency equation that considers the mutual inductance.
The clearance of the vehicle from the ground depends on the vehicle type. As such,
the distance between the transmitter and receiver coils of WCS also changes. Therefore,
the mutual inductance of the two coils should be considered when applying WCS to an
EV.
3.2 Variable wireless charging system
Fig. 3 shows the proposed variable WCS. First, a WCS that resonates at 350 kHz was designed.
To change the capacitor value according to the change in the mutual inductance, capacitors
were connected in parallel using a switching system. The spacing between the Tx and
the Rx coils varies from 20, 30 and 40 cm. The reason is that the height of the lower
part varies depending on the size of the EV. Variation, Medium-sized car, Sports utility
vehicle were selected for the (simulation, experiment).
Table 2 shows the mutual inductance according to the distance, which was calculated using
the data in Fig. 2. The mutual inductance was calculated using Equ. (2). The 20 cm standard, which demonstrates the resonance characteristics at 350 kHz,
was adopted. Considering the L and M values, the capacitance at 350 kHz is the figure
with a 350 kHz resonance frequency.
Fig. 3. Superconducting coils schematic with a variable capacitor
Table 2. Mutual-inductance & Capacitance according distance
|
Distance
|
20 [cm]
|
30 [cm]
|
40 [cm]
|
|
Wireless charging
Frequency
|
350 [kHz]
|
362 [kHz]
|
371 [kHz]
|
|
Inductance &
Capacitance
|
16.73 [uH] & 12.36 [nF]
|
|
Mutual Inductance
|
0 [uH]
|
-1.09 [uH]
|
-1.94 [uH]
|
|
Capacitance at
350kHz
|
12.36 [nF]
|
13.22 [nF]
|
13.89 [nF]
|
Fig. 4. Smith chart according to the distance of the variable wireless charging system
(a) 20cm (b) 30cm (c) 40cm
3.3 Experimental results
Fig. 4 is a Smith chart according to the distance of the variable WCS. The characteristic
impedance can change according to matching of L and C values. If the L and C values
is not matched, then WCS efficiency will decrease dramatically. Therefore, impedance
matching is essential. When the impedance of Tx and Rx is 0 Ω, impedance matching
is also 0 Ω. The impedance values at resonance point on the smith chart is 1.00075+0.0000173i,
1.0075- 0.0000569i, 1.0075-0.00425i when the distance between Tx and Rx is 20, 30,
40 cm respectively. The reactance values at resonance point was close to 0Ω. So, it
could be confirmed that the impedance matching is well operated. Fig. 5 shows the S-parameters of the WCS using variable capacitors. Fig. 5(a) shows the reflection and transmitting coefficients when the distance between the
transmitter and receiver coils is 20 cm.
S11 is about -27.1 dB, and S21 is about -1.87 dB. The resonance frequency is 350 kHz.
Fig. 5(b) shows the S11 and S21 values when the distance between the transmitter and receiver
coils is 30 cm. The values are about -24.8 and -2 dB, respectively. Fig. 5(c) shows the S-parameters when the distance between the two coils is 40 cm. The reflection
and transmitting coefficients are -26 and -1.91 dB, respectively.
Table 3. Comparison of experimental results
|
|
Normal
Capacitor at 350 [kHz]
|
Variable
capacitor at 350
[kHz]
|
|
20[cm]
|
S11 [dB]
|
-27.1
|
-27.1
|
|
S21 [dB]
|
-1.87
|
-1.87
|
|
30[cm]
|
S11 [dB]
|
-0.3
|
-24.8
|
|
S21 [dB]
|
-12
|
-2
|
|
40[cm]
|
S11 [dB]
|
-3
|
-26
|
|
S21 [dB]
|
-31
|
-1.91
|
Fig. 5. S-parameters according to the distance of the variable wireless charging system
(a) 20cm (b) 30cm (c) 40cm
Table 3 shows the S-parameter values according to the availability of a variable capacitor
in WCS. If the height of the EV vehicle is not taken into consideration, the efficiency
of the wireless charging system will decrease drastically because of discordance of
resonance frequency. However, using a wireless charging system with a variable capacitor,
the resonance frequency can be harmonized with the level of elevation of the EV and
the ground, and wireless charging can be achieved a high efficiency.
4. Conclusion
With the increase in the number of EVs being used, the interest in wireless charging
is rapidly increasing. Wireless charging is expected to play a decisive role in the
commercialization of EVs because they can eliminate the inconveniences that drivers
experience with the conventional wired charging system. In this study, a variable
WCS using superconducting coils was proposed. In the said WCS, the transmitter and
receiver coils affect the mutual inductance level. The size of the mutual inductance
varies depending on the distance between the transmitter and receiver coils. In this
case, the selectivity of the resonance frequency is influenced, thereby decreasing
the efficiency of wireless charging. The bottom clearance of an EV differs from one
vehicle to another. It causes a difference in the frequency selection of the WCS from
one system to another. Therefore, inevitably, it changes the charging efficiency in
the same WCS. The variable WCS can maintain constant resonance frequency characteristics
by changing the capacitor value according to the mutual inductance value of the transmitter
and receiver coils. As a result, it was confirmed that the frequency selectivity was
kept constant even when the vehicle’s bottom clearance was changed.
Furthermore, high efficiency was maintained because superconductors were applied to
the WCS. It is believed that stable and highly efficient wireless charging would be
feasible even in vehicles of various clearances and sizes if sufficient studies would
be made on a variable WCS using superconducting coils.
감사의 글
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and
Technology (NRF-2015R1D1A1A01059489)
This work was supported by the National Research Foundation of Korea(NRF) grant funded
by the Korea government(MSIT) (No.2018R1A2B2004242)
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저자소개
1987년 12월 11일 생
2012년 조선대 전기공학과 졸업(학사)
2014년 동 대학원 졸업(공학석사)
2018년 동 대학원 졸업(공학박사)
Tel : 062-230-7054
E-mail : no21park@hanmail.net
1966년 2월 21일 생
1989년 전북대 전기공학과 졸업(학사)
1994년 동 대학원 전기공학과 졸업(공학석사)
2000년 동 대학원 전기공학과 졸업(공학박사)
현재 조선대 전기공학과 교수, 시민 르네상스 평생교육원 원장, 미래사회융합대학 학장
Tel : 062-230-7025
E-mail : hyosang@chosun.ac.kr