EMC’14/Tokyo
15A-H1
Magnetic Shielding of Wireless Power Transfer
Systems
Tommaso Campi, Silvano Cruciani Mauro Feliziani
Department of Industrial and Information Engineering and Economics
University of L’Aquila
L’Aquila, Italy
ncampi@alice.it , silvano.cruciani@univaq.it , mauro.feliziani@univaq.it
Abstract—This paper deals with the magnetic shielding of the
field generated by a wireless power transfer (WPT) system at the
frequency of 20 kHz. Different shielding techniques are examined
and discussed based on the use of conductive and magnetic
material panels. The performances of the WPT system and the
magnetic field shielding effectiveness (SE) in presence and in
absence of shield panels are calculated and measured.
Keywords— Wireless power transfer (WPT); Magnetic
shielding; Shielding effectiveness; Near magnetic field; Equivalent
circuit; Electromagnetic field numerical computation .
I.
INTRODUCTION
Wireless power transfer (WPT) systems are used to
transfer electrical energy without using wiring systems. The
WPT is based on the inductive coupling between primary and
secondary windings. This coupling increases with the
increasing of the coupling factor k and of the frequency f. So
in the design of WPT systems, k and f should be maximized,
but this is not always possible. Indeed, the coupling factor k
depends on the separation distance between primary and
secondary windings which cannot be reduced too much
depending on the considered application. It is well known that
the goal of the WPT systems is to transfer a considerable
amount of power between windings separated as much as
possible with the highest possible efficiency. The
performances of the WPT systems with small value of k can
be easily improved by introducing capacitors in the equivalent
circuit of the WPT systems to compensate the inductive
behavior of the primary and secondary coils up to obtain
resonance conditions. The working frequency cannot be
arbitrarily increased for two main reasons. First, the use of the
frequency is regulated by national and international rules.
Second, the eddy currents power losses can become relevant
in any conductive material placed in the magnetic field region
generated by the WPT system.
One of the most significant problems in the WPT
systems application is the relevant magnetic field in the
surrounding environment. There are several problems
related to large magnetic fields as:
-
magnetic fields must be compliant with the reference
levels of ElectroMagnetic Field (EMF) safety standards for
human exposure [1];
-
magnetic fields must be compliant with the
electromagnetic compatibility (EMC) regulations to avoid
Copyright 2014 IEICE
disturbances in other electric and electronic apparatuses
and devices;
-
time-harmonic magnetic fields induce eddy currents in
metallic parts of object close to the WPT windings creating
heating and losses, and reducing also the efficiency of the
WPT.
For these reasons the mitigation of the magnetic field is a
crucial problem for a wide application of WPT systems. Since
the WPT system is an intentional source of magnetic field, any
reduction of the magnetic field can dramatically affect the
performances of WPT systems. In this study the magnetic
field mitigation techniques for WPT systems are investigated.
Recently the equivalent circuits of WPT systems in presence
and absence of shields have been defined [2]. Here the
calculated electrical performances of WPT systems
configuration are compared with measurements.
WPT SHIELDING
II.
The equivalent circuit of a WPT systems without
compensation capacitors is shown in Fig. 1, where R1 and R2
are the coil resistances, L1 and L2 are the coil self-inductances,
and M = k(L1L2)1/2 is the mutual inductance. To increase the
performances in the wireless power transfer, the primary and
secondary coils are compensated introducing series or parallel
capacitors.
I1
R1
R2
I2
M
V1
L1
L2
V2
Fig. 1. Equivalent circuit of two coupled coils.
Shielding of magnetic near fields is a relevant problem for
EMC engineers. It is well known that the use of metallic
panels can be inadequate to mitigate the magnetic field at very
low frequency. The magnetic shielding effectiveness (SEH) for
a time-harmonic field is defined as:
422
SEH = 20log10 ( H i H )
(1)
EMC’14/Tokyo
15A-H1
where H and Hi are respectively the magnetic fields in
presence and in absence of the shield at a point in the shielded
region. H and Hi are also known as total and incident fields.
1) The magnetic coupling between primary and secondary
windings is generally increased using magnetic materials that
can be considered as a small part of a magnetic core.
Considering a one-dimensional problem, SEH can be
approximated for conductive shields by:
2) Since the magnetic flux lines follow the path of least
reluctance they are diverted from the region to be shielded into
the magnetic materials producing a shielding effect.
SEH (ω ) = A (ω ) + R (ω ) + M (ω )
(2)
where A represents the absorption loss of the wave as it
proceeds through the shield barrier, R represents the reflection
loss caused by field reflection on the shield surface, and M
represents the additional effects of multiple reflections and
transmissions. For magnetic near field sources the most
significant term in (2) is the absorption loss A = 20log10(et/δ),
being t the shield thickness and δ = (πfμσ)-1/2 the penetration
depth which is reported in Fig.1 copper and aluminum
materials. The shielding performances of copper and
aluminum are good enough (e.g., δ ≈ 0.5 mm at f = 20 kHz),
but there is another very relevant problem. Any conductive
panels embedded in the magnetic field produced by the WPT
system can be considered to be another windings of the same
WPT system in short-circuited conditions [2]. It means that
the eddy currents produced in the shield panels create a
magnetic field opposite to the incident one. As consequence
the total magnetic field is reduced and therefore also the WPT
performances are degraded due to the relevant power losses in
the conductive shield. In conclusion the shielding
performances of conductive shields can be adequate to
mitigate the magnetic field produced by the WPT system, but
the reflected field can affect the performances of the power
transfer and requires a further capacitance compensation to get
again the resonance condition.
Better performances can be obtained using magnetic
shields. The magnetic shielding consists in the use of high
magnetic permeability material panels that provide a
preferential path for the magnetic flux lines. The incident
magnetic field is not blocked as in conductive shielding, but it
is diverted into the magnetic material. Closed topology shields
surrounding the volume to be shielded are the preferred
configuration since the magnetic flux lines form closed loops,
but this topology is not compatible with a WPT system due to
the presence of the air gap between primary and secondary
windings. Nevertheless the use of magnetic panels can
improve the quality of the WPT system for two separate
reasons:
Penetration depth δ [mm]
3
Copper
Aluminum
2
1
0
1
10
Frequency [kHz]
100
Fig. 2. Frequency behaviour of the penetration depth for copper and
aluminium.
Copyright 2014 IEICE
As described above the use of magnetic materials modifies
the values of self and mutual inductances in the equivalent
circuit. So the compensation capacitors of the WPT system
without shield must be varied in presence of the magnetic
shields to obtain again resonance.
III.
APPLICATIONS
A simple configuration of a WPT system has been realized
at the EMC laboratory of the University of L’Aquila to
evaluate experimentally the shielding performances of simple
shield panels used to mitigate the magnetic field generated by
the WPT system. The considered WPT system is composed by
the feeding circuit, the coupled coils, the compensation
capacitors and the load. The supply is given by an electronic
circuit fed in DC with an inverter that generates a square wave
voltage source Vs at the nominal frequency f0 =20 kHz. The
inductive coupling is produced by two identical coaxial
stacked planar coils with 5 turns, internal coil radius Rint=70
mm, external coil radius Rest=83.5 mm, enameled copper wire
radius D=2.5 mm, distance between the stacked coils dc = 40
mm. The primary coil is fed by a variable voltage source Vs
whose values are adequately chosen in order to transfer the
power P2=30 W on the resistive load (RL=5 Ω) of the
secondary coil. The inductive primary circuit is compensated
by a series capacitor C1 while the secondary circuit is
compensated by a parallel capacitor C2.
The WPT configuration is also analyzed by a numerical
software tool (COMSOL) based on the solution of the quasi
static magnetic field equations. The circuital parameters of the
system composed by the coupled coils are extracted by the
software tool and are also measured using the Wayne Kerr
4265 LCR meter. The coil resistances measured at f0=20 kHz
are: R1=R2=0.038 Ω. Assuming the coils in air without any
shield the measured inductances are: L1=L2=7.85 μH and
M=2.05 μH, and the extracted values are: L1=L2=7.44 μH and
M=2.02 μH. This last value M is obtained by connecting the
two coils in series in phase and antiphase configurations as
M=(Xph-Xaph)/4ω, being ω the angular frequency, Xph the
measured reactance in phase configuration and Xaph the
measured reactance in antiphase configuration. Using a seriesparallel compensation, the values for the capacitors are
obtained as C1=1/(ω02(L1−M122/L2)) and C2=1/(ω02L2) being
ω0=2πf0 [17]. The calculated and the measured efficiencies of
the considered WPT system in air with C1=9.20 μF and
C2=8.52 μF are reported in Fig. 3.
Several configurations of planar square shields with side
length ls=20 cm, different thickness t and different materials
(copper, aluminum and ferrite) are analyzed. The geometrical
configuration of the coil systems together with the planar
shield is depicted in Fig. 4. The shielding panel is placed
parallel to the secondary coil at a variable distance ds from the
secondary coil. The examined shield characteristics are
423
EMC’14/Tokyo
15A-H1
TABLE II.
reported in Table I. The measured and calculated self and
mutual inductances are reported in Table II.
Test
case #
The electrical performances are evaluated fixing the power
P2 = 30 W and the frequency f0 = 20 kHz required by the load.
Using the measured circuit parameters in Table II the WPT
efficiency η, the real power on the primary circuit P1 and the
peak value of the voltage source Vs are reported in Table III.
The magnetic flux induction maps calculated in a plane at a
distance of 10 centimeters from the secondary coil with and
without shielding are reported in Fig. 5.
1
2
3
4
5
6
7
8
9
Efficiency η
0.8
L1
[μH]
7.02
7.00
7.04
7.00
7.88
7.10
7.07
7.11
7.07
TABLE III.
0.6
0.4
0
10
15
20
Frequency [kHz]
25
L2
[μH]
3.85
3.87
3.83
3.81
10.77
4.71
4.75
4.70
4.66
COMSOL
M12
[μH]
0.64
0.63
0.67
0.64
2.88
0.85
0.86
0.87
0.85
L1
[μH]
6.92
6.92
6.91
6.92
7.86
7.00
7.00
7.00
7.00
L2
[μH]
3.82
3.81
3.76
3.80
11.23
4.59
4.59
4.56
4.59
M12
[μH]
0.70
0.69
0.69
0.70
2.99
0.92
0.91
0.90
0.91
MEASURED AND CALCULATED ELECTRICAL QUANTITIES
Test
case #
Calculated
Vs [V]
Calculated
P1 [W]
Calculated
ηnum
Measured
ηmeas
1
2
3
4
5
6
7
8
9
5.57
5.58
5.60
5.51
5.41
5.53
5.34
5.34
5.31
87.54
89.16
90.84
85.36
35.88
66.01
66.57
67.24
65.08
0.34
0.33
0.33
0.42
0.83
0.45
0.45
0.45
0.46
0.33
0.32
0.32
0.35
0.80
0.45
0.45
0.45
0.46
Calculated
Measured
0.2
MEASURED AND CALCULATED INDUCTANCES
LCR Meter
30
Fig. 3. Efficiency of the WPT system in air without shielding.
-0.5
y [m]
150
(a)
100
0
50
0.5
-1
-0.5
0
x [m]
0.5
1
(a)
-0.5
y [m]
15
10
0
(b)
Fig. 4. Geometrical configuration: (a) 3D model; (b) 2D cross section.
5
0.5
-1
TABLE I.
SHIELD CONFIGURATIONS
σ
0
x [m]
0.5
1
(b)
-0.5
1
ds
[mm]
7.5
t
[mm]
1
3.7⋅107
1
7.5
2
3.7⋅107
1
7.5
6
15
7
1
7.5
0.7
10
2400
7.5
6
3.7⋅107
1
11.5
1
Aluminum
3.7⋅10
7
1
11.5
2
(c)
Aluminum
3.7⋅107
1
11.5
6
Copper
5.9⋅107
1
11.5
0.7
Fig.5. B-map [μT] at a distance of 0.1 m from the secondary coil. (a) Without
shielding. (b) With aluminum shielding (test case # 3). (c) With magnetic
shielding (test case #5).
Material
1
Aluminum
[S/m]
3.7⋅107
2
Aluminum
3
Aluminum
4
Copper
5.9⋅10
5
Ferrite
0
6
Aluminum
7
8
9
μr
35
30
y [m]
Test case #
Copyright 2014 IEICE
-0.5
25
20
0
5
0.5
-1
424
-0.5
0
x [m]
0.5
1
EMC’14/Tokyo
15A-H1
Shielding Effectiveness [dB]
The maximum and average shielding effectiveness (SE)
obtained by simulation for all the considered test cases in a
cylindrical region (radius 0.3 m, height h=0.2 m) just behind
the shield are reported in Fig. 6. The calculated and measured
magnetic flux induction B at a distance of 10 cm from the
secondary coil for the test cases #1 and #5 are shown in
Figs.7(a) and 7(b). The magnetic field has been measured by
the magnetic field meter Wandel&Golterman EFA3.
Max
100
Finally, the shielding effectiveness cannot be considered a
significant parameter to evaluate the shielding performances
of a WPT system since the presence of the shield modifies the
incident magnetic field behavior.
REFERENCES
Average
[1]
80
60
[2]
40
20
[3]
0
#1
#2
#3
#4
#5
#6
Test case
#7
#8
#9
[4]
Fig. 6. Maximum and average SE calculated under the shield.
Magnetic flux density B [μT]
least for the shielded configurations examined in this work.
On the contrary, a magnetic shield can improve the efficiency
of the WPT system and can also mitigate the field if it is
adequately shaped.
20
[5]
Measurement
Simulation
15
[6]
10
5
0
[7]
0
0.05
0.1
0.15
0.2
x [m]
0.25
0.3
0.35
0.4
[8]
Magnetic flux density B [μT]
(a)
40
[9]
Measurement
Simulation
30
[10]
20
10
0
[11]
0
0.05
0.1
0.15
0.2
x [m]
0.25
0.3
0.35
0.4
(b)
[12]
Fig. 7. Magnetic flux induction B (peak value) along the x-axis at a distance
of 0.1 m from the secondary coil: conductive shield (test case #1) (a);
magnetic shield (test case #5) (b).
IV.
CONCLUSIONS
A numerical and experimental analysis has been carried out
to evaluate the shielding performances of conductive shield
panels to mitigate the magnetic field produced by a WPT
system. The high conductivity of the metal panels in copper or
aluminum produces relevant eddy current losses reducing
dramatically the efficiency of the WPT system. The use of
high permeability, low losses materials improves the
performances of a WPT system.
The obtained results demonstrate that the metallic shields
are good enough to mitigate magnetic fields at 20 kHz, but the
efficiency of the WPT system can be strongly degraded, at
Copyright 2014 IEICE
[13]
[14]
[15]
[16]
[17]
425
ICNIRP Guidelines, “Guidelines for Limiting Exposure to TimeVarying Electric and Magnetic Fields (1 Hz - 100 kHz),” Health Physics
vol. 99, no. 6, pp. 818-836, Dec. 2010.
S. Cruciani and M. Feliziani, “Mitigation of the magnetic field generated
by a wireless power transfer (WPT) system without reducing the WPT
efficiency”, EMC Europe –Int. Symposium on EMC, Bruges, Belgium,
Sept. 2-6, 2013.
C.-S. Wang, O.H. Stielau, and G.A. Covic, “Design considerations for a
contactless electric vehicle battery charger,” IEEE Trans. Ind.
Electronics, vol. 52, no. 5, pp. 1308 - 1314, Oct. 2005.
C. Zhu, C. Yu, K. Liu, and R. Ma, “Research on the topology of wireless
energy transfer device,” in Proc. IEEE 2008 Vehicle Power and
Propulsion Conference, 3-5 Sept. 2008, Harbin, China.
A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M.
Soljacic, “Wireless power transfer via strongly coupled magnetic
resonances,” Science, vol. 317, no. 5834, pp. 83-86, Jul. 2007.
R. B. Schultz, V.C. Plantz and D.E. Brush, “Shielding theory and
practice,” IEEE Trans. Electromag. Compat., vol. 30, no.3, pp. 187-201,
Aug. 1988.
Seungyoung Ahn and Joungho Kim, “Magnetic field design for high
efficient and low EMF Wireless Power Transfer in On-Line Electric
Vehicle,”, EUCAP 2011, Rome, Italy, 11-15 April 2011.
R.G. Olsen, “Some observations about shielding extremely lowfrequency magnetic fields by finite width shields”, IEEE Trans.
Electromagn. Compat., vol. 38, no. 3, pp. 460-468, August 1996.
W. M. Frix and G. G. Karaday, “A circuital approach to estimate the
magnetic field reduction of nonferrous metal shields,” IEEE Trans.
Electromag. Compat, vol. 39, no. 1, pp. 24-32, Feb. 1997 .
C. Buccella, M. Feliziani, and V. Fuina, “ELF magnetic field mitigation
by active shielding,” IEEE Int. Symp. Ind. Electronics, vol. 3, pp. 994998, Nov. 2002.
C. Caruso, M. Feliziani, and F. Maradei, “ELF magnetic field produced
by the ac electrification in a railway carriage”, in Biological Effects of
Electromagnetic Fields, P. Stavroulakis Ed., Springer Verlag 2003,
Berlin, Gemany.
G. G. Karady, C. V. Núñez and R. Raghavan, “The feasibility of
magnetic field reduction by phase relationship optimization in cable
systems”, IEEE Trans. Power Delivery, vol. 13, no. 2, pp. 647-654, Apr.
1998.
A. S. Farag, M. M. Dawoud and I. O. Habiballah, “Implementation of
shielding principles for magnetic field management of power cables”,
Electric Power System Research, 48, pp. 193–209, 1999.
Yaping Du, T. C. Cheng and A. S. Farag, “Principles of powerfrequency magnetic field shielding with flat sheets in a source of long
conductors”, IEEE Trans. Electromag. Compat., vol. 38, no. 3, pp. 450459, Aug. 1996.
S. Asakura, N. Kikuma, H. Hiyayama, and K. Sakakibara, “Shielding
Effect of Magnetic Resonant Wireless Power Transfer,” 2011 IEICE
General Conference, B-4-69, Mar. 2011.
Jiseong Kim et al., “Coil design and shielding methods for a magnetic
resonant Wireless Power Transfer System, Proc. IEEE, vol. 101, no. 6,
pp. 1332-1341, Jun. 2013.
C.-S. Wang, G.A. Covic, O.H. Stielau, “Power transfer capability and
bifurcation phenomena of loosely coupled inductive power transfer
systems,” IEEE Trans. Ind. Electronics, vol.51, no.1, pp.148-157, Feb.
2004.