Energy and Power Engineering, 2013, 5, 1072-1076
doi:10.4236/epe.2013.54B204 Published Online July 2013 (http://www.scirp.org/journal/epe)
Research of Magnetic Co upler Array for IPT System
Chao Hu, Yue Sun, Zhi-hui Wang, Shi-jie Zhou
School of Automation, Chongqing University, Chongqing, China
Received March, 2013
ABSTRACT
The advance of Inducive Power Transfer (IPT) system is capable to transfer large power across an air gap of sufficient
distance, but the power level and charging area of receiver are limited by the magnetic coupler of IPT system. This pa-
per analyses the correlative factors which effect maximum output power (Pm), it reveals Pm is inversely proportional
with magnetic flux of power receiving coils. New ferrite array structure is proposed as the basic part of magnetic cou-
pler that focusing on enhancing the equilibrium of magnetic flux distribution at charging area and increasing power
transfer distance. The method of winding on ferrite array is quite flexible and the power transmission distance can be
increased by changing the mode of ferrite array windings while magnetic field uniform is reduced, users can chose the
suitable mode of winding for different IPT system. Finally the validity of theory analysis is tested by a 3D finite ele-
ment modeling tool.
Keywords: Inducive Power Transfer; Ferrite Array; Magnetic Coupler; Finite Element Modeling
1. Introduction
Inductive Power Transfer (IPT) technology is a new
power transmission technology which is based on elec-
tromagnetic induction principle, wireless power trans-
mission between the power supply by using modern
power electronics technology and high frequency con-
verter technology, in addition applying modern control
theory [1-3]. IPT technology is much safer, reliable,
flexible comparing to traditional mechanical contact
power supply, as a result, a research upsurge was set off
around the world in recent years [4].
Magnetic coupler is the key part of IPT system,
whether the magnetic circuit design is reasonable or not
directly affects the power, efficiency and other important
parameters of the whole IPT system [5-7]. The receiving
coil and transmission coil realize the non-contact trans-
mission via electromagnetic field, electromagnetic con-
version efficiency is lower while the transmission dis-
tance increases, and IPT technology is generally applied
to the wireless power transmission [8].
IPT system can have one or more receivers, the main
circuit of IPT system with a plurality of receiver is as
shown in Figure 1. In order to ensure IPT system can
work normally, the magnetic coupler of transmitter can
be composed of a large area resonant coil, or a plurality
of small area resonant coil array. For IPT system with
multiple receiving coils, the single large resonant coil at
the transmitter is very difficult to achieve equilibrium
magnetic circuit in a plane, while multiple resonant coils
produce a relatively one.
Transmission coil array generated IPT system mag-
netic circuit is appropriate for single receiver, it can
flexibly move within the magnetic coupler array of
transmitter. Compared with single transmitter resonant
coil, a plurality of transmitter resonant coil magnetic
coupler array reduces the electromagnetic energy trans-
mission distance, however, relatively gains equilibrium
magnetic field distributions.
Paper 9 described a three layers and hexagonal mag-
netic array structure, the structure of magnetic coupler
array is designed by air core, spatial distribution of mag-
netic field in the space field more uniform, but the three
layer structure of magnetic coupler structure of energy
Figure 1. The main circuit of IPT system with several
pick-ups.

C. HU ET AL. 1073
transmission distance restrictions. In this paper single
magnetic array is proposed, taking the ferrite with higher
permeability as the core, increasing the energy transmis-
sion distance of the magnetic array.
2. Output Power and Efficiency of IPT
System
2.1. The Maximum Output Power of IPT System
Output power and efficiency of the resonance mechanism
is essential for the selection of resonance coil parameters
when designing the IPT system.
The maximum output power Pm of system is:
2
2
mocscs 0p
s
M
P=VIQ= IQ
L

s
(1)
where Voc is the open-circuit voltage of receiver, Isc is
the short-circuit current of transmitter, Qs is the quality
factor of receiver circuit tuning , ω0 is the resonant fre-
quency, M is mutual inductance, Ls represents the induc-
tance of transmission coil.
The resonant circuit can be divided into four resonant
networks: SS, SP, PS, PP (S represents series resonant
and P stands for parallel resonance) according to the
compensation system [10]. For high power IPT system,
transmitter needs to be set to S type resonant network, so
this paper will analyze the SS, SP resonant network of
IPT system, in this condition, the current of transmission
coil (Ip) is described as:
()
P
dc r
I
4V2 pZ (2)
where Vdc is the DC input voltage of IPT system, Zr is the
reflection impedance. When receiver resonance network
are S and P network respectively, Qs and Zr are as fol-
lows:
(S)
(P)
22
0L
r22
Ls
M/R
Re Z=MR/L






(3)
(S)
(P)
0s L
s
L0s
L/R
Q= R/( L)




 (4)
Equation (2), (3), (4) are substituted in Equation (1),
the maximum output power of SP, SS system can be ex-
pressed as:
(SS)
(SP)
222 22
Ldc 0
m22 22
sdc L
8RV/ (pM)
P
8L V/(MpR)






(5)
According to Equation (5), when the resonance fre-
quency and input voltage are constants, there are two
factors for Pm of SS IPT system will be influenced by RL
and M, two factors for Pm of SP IPT system will be in-
fluenced by RL, M and Ls. The design of M, Ls is closely
tion of different structure of coils and inductance, mutual
inductance can be calculated by using magnetic field
simulation software when the index of maximum output
power and system operation at rated load are given.
related to the resonance coil, the magnetic field distribu-
2.2. The Efficiency of IPT System
er is shown as Coupled mode equation for the receiv
Equation (6) [11]
ds
d
s
ws esp
ia aa a
t
a

 
(6)
where ap and as are energy amplitudes of transmitter and
receiver respectively. The power of receiver got from the
transmitter is dependent on
κ ap, where κ is coupling
coefficient. Γw is power decay rate of load and Γe is other
power attenuation rate (mainly the attenuation rate of
electromagnetic radiation). Then the power amplitudes of
transmitter and receiver can be expressed as:
,
ikt
pp
s
Ae (7)
(8)
where Ap and As are power coefficie
).
(
ew
i
ss
sAe


)t
nt of transmitter and
receiver respectively. The energy consumption of re-
ceiver is mainly in the load and other energy, namely sp
= ss, then
//(
spew
AA k

 (9)
The load power consumption Pw can be
)
Total power Pt of re
(11)
where Γp is th
expressed as
Pw=2Γw|As|2, (10
ceiver can be expressed as
Pt=2Γp |Ap|2+2(Γw +Γe)|As|2
e energy decay rate of transmitter, then
efficiency of system can be described as:

2
2
2
22
wws
P|A|
22
(1 )(1 )
2
tppwe s
w
eep
2
ww
eep e
==
P
A
++A
k
=k
+++






 
(12)
According to Equation (12), the system will achieve
m

aximum efficiency when 2 /1/
we ep
k. If
κ > Γe, κ > Γp, then effect orom f energy transmission f
3)
w
transmitter to receiver is the best, which means the cou-
pling structure is in the state of strong coupling status.
The value of κ can be expressed as Equation (1
hen the state of the resonant mechanism is resonance.


i/2
p
s) kM LL (13

C. HU ET AL.
1074
According to Equation (6) and Eq
ciency of IPT system is related to the
w
upler Array
ter under
netic en-
hich can be defined as:
uation (13), the effi-
value of M, in other
ords, it is related to the magnetic induction intensity
produced by transmitting coil.
3. Design of Magnetic Co
From the analysis of energy efficiency, transmit
the excitation source will emission electromag
ergy to space, receivers get electromagnetic energy.
When the resonant parameters, mutual inductance of
transmitter and receiver is reasonable, magnetic coupler
can realize high efficiency, high power energy transmis-
sion. At this time, the energy emission ability of trans-
mitter will determine the wireless transmission parame-
ters of the power efficiency.
Electromagnetic energy in the plane is related to the
decay rate of magnetic flux, w
ddddSB
BS BS
dtdtdt dt

 
(14)
where B is the magnetic induction intensity
of magnetic flux, θ stands for angle between receiver and
, S is the area
transmitter. Receiver in IPT system is invariant under
normal circumstances, therefore, the value of d/dt

,
d/Sdt is zero. The decay rate of the magnetic flux is
related to d/Bdt, which means Pm is inversely -
ith magnetic flux of power receiving coils when
the frequenIp is fixed.
Magnetic coupler structure of IPT system is as shown
in Figure 2:
propor
tional w
cy and
windings by ferrite array, the adjacent coil
w
ge magnetic induction intensity in
th
Magnetic coupler is composed of a plurality of inde-
pendent coil
indings are connected by series connection, and the
current of windings are same. In addition, each magnetic
core coil has the same winding turns, core size and all of
the other parameters.
In short-distance wireless power transmission, each
core coil generates lar
e upper space, as the core reducing the reverse the
weakened degree around magnetic field. In long-distance
wireless power transmission, magnetic field of all coil
windings gathers to at the upper center of the magnetic
coupler. Within certain height, the equilibrium of mag-
netic is well, assuming the area of magnetic coupling
Figure 2. The structure of magnetic coupler array.
mechanism is large enough, then the magnetic field area
with good equilibrium will also be enough for providing
energy for a plurality of pick-up structure.
4. Simulation of Magnetic Coupler Array
A 9 ferrite array is taken as the example for analyzing
magnetic coupler array, the model is built by 3D finite
elem ag-
.
ore
ort
ent modeling tool. Figure 3 and Figure 4 are m
netic induction intensity distribution of short-distance,
long-distance respectively, short-distance plane is at the
height of 2cm from the magnetic coupler and
long-distance plane is at the height of 15cm
As shown in Figure 3, magnetic field of adjacent c
coils exist interaction reverse weakened on the sh
–distance plane, which result the magnetic induction in-
tensity near the ferrite winding is relatively small com-
paring to other regions. Center ferrite winding and other
ferrite windings are adjacent in magnetic coupler array,
thus the magnetic induction intensity above the central
region is relatively small compared with the other ferrite
windings.
Long-distance wireless power transmission is more
extensively used, and then magnetic induction intensity
area is concentrated in the center area. The magnetic in-
duction intensity in the region determines whether the
receiving coil can obtain sufficient electromagnetic pow-
er,
Figure 3. Magnetic induction intensity distribution on the
short-distance plane.
Figure 4. Magnetic induction intensity distribution of on
long- distance plane.

C. HU ET AL. 1075
equilibrium of magnetic in central region determines how
much energy area can be obtained in the receiving coil,
namely the operating range of receiver side.
Magnetic coupler in the paper is designed to be a
symmetrical structure, if a length of 20cm line above the
center area for 15cm exists, magnetic induction intensity
distribution along the line is as shown in Figure 5. The
magnetic induction intensity within the circular area with
a diameter of 20cm achieve between 718 μT to 764 μT,
the magnetic induction intensity within circular area with
a diameter of 15cm is 748 μT~764 μT. Accordingly, the
structure in this paper can provide an effective field a
ium, and longer transmission dis-
shows dis-
tri
rea
for long-distance wireless power transmission with larger
area, better equilibr
tance.
Suppose a size of 20 cm × 20 cm plane which is lo-
cated above the coils about 15cm. Figure 6
bution of
φ above ferrite windings array and air
windings array. When the abscissa is 0, the plane is lo-
cated right above the winding array.
According to Figure 6, it is obvious that φ above the
ferrite windings array is bigger than air windings array
which means the magnetic coupler can effectively in-
crease the transmission power and distance.
Figure 5. Magnetic induction intensity distribution curve on
the long-distance plane.
Figure 7. Magnetic induction intensity distribution curve on
the long- distance plane when winding turns increased.
magnetic coupler array center ferrite
has greatly affected the magnetic induction intensity at
specified height, winding turns of central magnetic core
coil was increased double, other parameters remain the
same. Magnetic induction intensity distribution of a line
which is above the coil for 15cm with a length of 20cm is
as shown in Figure 7.
According to Figure 6 and Figure 7, when excitation
current of each coil is unchanged, magnetic induction
intensity above the center coin is obviously increased by
increasing the turns of winding at center ferrite of the
magnetic coupler. In the proposed model, the magnetic
de
owledgements
The author would like to thank the National Natural Sci-
Under long-distance wireless power transmission,
winding number of
induction intensity within diameter of 20cm region is 870
μT~1060 μT, the maximum magnetic induction intensity
increased by 39%, but the equilibrium of magnetic is is
creased.
If the charging area is not so important in practical ap-
plication, winding turns of the center ferrite can in-
creased to achieve longer distance for wireless power
transmission, in other hand, if charging area is pursued,
the symmetric parameter of magnetic coupler can be
chosen.
5. Conclusions
This paper proposed a new magnetic coupler which is
composed of a plurality of ferrite winding array. The
kind of magnetic coupler can produce magnetic field
with good equilibrium, provides higher power transmis-
sion, and increases the transmission distance. In long-
distance power transmission, longer power transmission
distance was achieved by doubling the winding turns of
the center ferrite, while the charging area was reduced.
Parameters of the magnetic coupler array can be de-
signed according to the actual application of receivers,
the flexible modes of windings for magnetic coupler pro-
vided reference for the design of IPT system.
6. Ackn
Figure 6. Distribution of φ above ferrite windings array and
air windings array.

C. HU ET AL.
1076
China (Grant No. 51277192) f
ed Magnetic
17, No.5834, 2007, pp
.1143254
ence Foundation ofor fi-
nancial support.
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