circular magnetic structures for ipt.pdf
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3096 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011
Design and Optimization of Circular MagneticStructures for Lumped Inductive Power
Transfer SystemsMickel Budhia, Student Member, IEEE, Grant A. Covic, Senior Member, IEEE, and John T. Boys
AbstractA solution that enables safe, efficient, and convenientovernight recharging of electric vehicles is needed. Inductive powertransfer (IPT) is capable of meeting these needs, however, themain limiting factor is the performance of the magnetic structures(termed power pads) that help transfer power efficiently. Theseshould transfer 25 kW with a large air gap and have good toler-ance to misalignment. Durability, low weight, and cost efficiencyare also critical. 3-D finite-element analysis modeling is used tooptimize circular power pads. This technique is viable, since mea-sured and simulated results differ by 10% at most. A sample of
power pads was considered in this work, and key design param-eters were investigated to determine their influence on coupledpower and operation. A final 2 kW 700-mm-diameterpad was con-structed and tested having a horizontal radial tolerance of 130 mm(equivalent to a circular charging zone of diameter 260 mm) with a200mm airgap. Theleakagemagneticflux of a charging systemwasinvestigated via simulation and measurement. The proposed padsmeet human exposure regulations with measurement techniquesspecific by the Australian Radiation Protection and Nuclear SafetyAgency (ARPANSA) which uses the International Commissionon Non-Ionizing Radiation Protection (ICNIRP) guidelines as afoundation.
Index TermsElectromagnetic compatibility, electromagneticcoupling, inductive power transmission.
I. INTRODUCTION
INDUCTIVE power transfer (IPT) uses a varying magnetic
field to couple power across an air gap, to a load without
physical contact. There are inherent advantages since the com-
ponents are electrically isolated, operation in wet environments
presents no safety risk, and systems are unaffected by such con-
ditions. IPT produces no contaminants and is completelyreliable
and maintenance-free unlike conventional plug-in or brush and
bar contact based methods. Today, it is used in numerous indus-
trial and commercial applications and is continually finding new
applications where safety and convenience are required [1][8].Generally, IPT systems may be grouped into either distributed
or lumped topologies. The former is suited to applications where
continuous power is required, and the latter for cases where
Manuscript received October 7, 2010; revised January 14, 2011; acceptedApril 3, 2011. Date of current version November 18, 2011. Recommended forpublication by Associate Editor C. R. Sullivan.
The authors are with the University of Auckland, 38 Princess Street, Auck-land 1142, New Zealand (e-mail: [email protected]; [email protected]; [email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2011.2143730
power only needs to be transferred at a fixed position. A dis-
tributed system consists of a primary coil laid out in a long loop
forming a track and one or more secondary coils that couple to a
small portion of the track and provide constant power to loads.
A typical application example is a materials handling system
with multiple bogeys where the primary track is in the order of
tens of metres long [7]. A lumped system is based on discrete
primary and secondary coils and power can only be transferred
when the coils are closely aligned and have sufficient mutualcoupling. Lumped systems may be further broken down in to
either closely coupled or loosely coupled types. Closely coupled
lumped systems operate with relatively small air gaps and the
user typically has to plug in the primary, as was the case with
chargepaddles used in an early electric vehicle [9], [10]. Loosely
coupled lumped systems operate with a large air gap and require
no user intervention, and these are the subjects of investigation
in this paper. The work done is in the context of recharging elec-
tric vehicles (EVs) and the loosely coupled lumped topology is
considered more suitable than the distributed type given vehi-
cles are typically parked in known fixed locations, for example,
parking lots, taxi ranks, and garages. Lumped systems vary in
capacity from 0.5 W50 kW and can be used to recharge orpower small electronic devices [1], [4], [8], Automatic guided
vehicles (AGVs) [3], [7], recreational people movers [2] and
electric vehicles (EVs) [5], [6].
The purpose of this research is to investigate the design of a
circular pad structure suitable for battery charging, understand
its operation, using appropriate simulation and experimental
data, and optimize the pad design. A simulation approach us-
ing 3-D finite element analysis (FEA) is used in this work,
therefore measurement versus simulation results are necessary
to ensure computer models are valid. The inherent complex-
ity of the problem is due to field shaping caused by ferrite,
and this means that analytic or precomputed solutions are im-practical [11]. These solutions are generally more practical in
situations where there are no magnetic materials in the vicinity
of the coil as in lower power applications [12]. The work pre-
sented in this paper is in the context of recharging of EVs that
require power levels of 25 kW, over an air gap of 200 mm.
The horizontal tolerance should be sufficiently large to enable a
driver to park without aid from an electronic guidance system to
receive a full power charge. This tolerance was specified to be
within a circular charging zone having a radius between 100
150 mm, although pads with larger tolerance are clearly better.
The ideas presented in this paper are applicable to any lumped
IPT system using circular pads because these are scalable and
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optimized designs result in considerable long-term cost and en-
ergy savings.
The paper is structured as follows: The operation of an IPT
system and the importance of efficient power pads are first ex-
plained. The structure of a prototype power pad is then shown.
Results of an optimization process are then presented together
with performance characteristics of a built pad. This optimiza-
tion process largely describes the design processes as it evolved.
Initially, a small-scale concept prototype padmeasuring 420 mm
in diameter was constructed in the laboratory using known and
available material. This pad was used to validate the output
of the simulator after which optimization could begin. In the
early validation stage, a number of key parameters are deter-
mined including the impact of horizontal and vertical offsets,
coil width and position relative to the ferrite along with the
amount of ferrite required. As this pad was clearly underpow-
ered for the desired operation, a larger pad with a diameter of
600 mm was then considered by way of simulation that enables
other design and optimization features to be considered in keep-
ing with the design objectives. As shown, the pads are scalableand therefore any changes in pad size should include ratiometric
changes to optimized variables to ensure optimal material usage
and efficiency for a given power output. Consequently, desir-
able characteristics of previous designs can be carried over to
the next design. Based on these two optimization stages, a final
700-mm-diameter prototype was built to meet the specifications
(again constructed based on available material). This is again
validated by simulation. Throughout the optimization and devel-
opment process, implementation issues that arise when making
larger diameter power pads are discussed and resolved. Finally,
leakage magnetic fields are measured and simulated in an EV
context with the aim of meeting International Commission onNon-Ionising Radiation Protection (ICNIRP) guidelines. These
have been used as a base for the Australian Radiation Protec-
tion and Nuclear Safety (ARPANSA) standard that recommends
practical approaches for measuring leakage magnetic flux.
II. IPT SYSTEMS
A typical IPT system comprises three main components, the
power supply, the magnetic coupling structure, and the pick-up
(PU) controller, as shown in Fig. 1. Thepower supply produces a
sinusoidal current in the VLF (1040 kHz) frequency range that
excites an inductive transmitter pad. A parallel compensation
capacitor (C1) is chosen so that its impedance is matched to thatof the transmitter padinductanceL1at the operational frequency.
This allows transmitter pad current,I1 , to resonate and the large
reactive current inL1creates a greater flux density in the vicinity
of the transmitter pad. This minimizes the VA rating of the
power supply for a given load, as the switches within the supply
only need to pass real power [13]. The transmitter and receiver
pads act as a loosely coupled transformer that enables power
transfer over relatively large air gaps. The IPT PU consists of
receiver pad inductanceL2 and a switched mode controller. The
leakage inductance of L2 is compensated using C2 , which is
also selected to have a matched impedance with the receiver
pad at the operational frequency, forming a parallel resonant
Fig. 1. Components of an IPT system.
tank. The voltage across C2 is then rectified and a switched
mode controller enables the resonant tank to operate at a defined
quality factor (Q) to boost power transfer and provide a usable
dc output [14].
The power output of an IPT system (Pout) is quantified by the
open circuit voltage (Voc ) and short circuit current (Isc ) of the
receiver pad as well as the quality factor, as shown in (1) [13].
Pou t =PsuQ = Voc IscQ = MI1MI1
L2Q = I21
M2
L2Q. (1)
Here Psu is the uncompensated power and is equal to the product
ofVoc and Isc , is the angular frequency of the transmitter pad
currentI1 ,Mis the mutual inductance between the pads andL2is the inductance of the receiver pad with the transmitter pad
open-circuited. As shown in (1), the output power is dependent
on the power supply (I12), magnetic coupling (M2/L2) and
PU controller (Q). The operational frequency and current of thepower supply are limited by those switching devices presently
available, and both have to be balanced against switching and
copper losses. In practical applications, Qis constrained to 46
due to component VA ratings and tolerances [13], [15]. If
and I1 are constant, Psu can be used for making comparisons
between different pad designs. It is essential that the power
pads have the highest M2/L2 to ensure the overall feasibility,
cost effectiveness, and efficiency of the complete system. Mis
highly dependent on the separation between the pads and the
distance between pad centers, whereas L2 is fixed by the pad
parameters such as size and number of turns in the coil. In
practice whileL2s position relative toL1 does have some smalleffect on both inductances, such variation is minimized due to
the inherently large air gaps required in EV charging.
III. POWERPADS
The circular magnetic structures considered in this work,
which are used to couple flux between the primary transmit-
ter and secondary receiver, are referred to as power pads and
these are nominally identical. Each has six main components,
as shown in the exploded view of Fig. 2. The aluminium ring
and backing plate shield the chassis of the EV and surround-
ing area from stray magnetic fields, which is discussed later
in Section V. The power pads can be made to be lower cost,
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3098 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011
Fig. 2. Exploded view of a power pad.
more robust, and lighter than commonly usedinductive couplers.
Conventional techniques use pot cores [5], [16][18], U-shaped
cores [1], [19], ferrite discs or plates [6], [12], [17], [20], [21], or
E-cores [1]. These designs are comparatively fragile and expen-
sive due to the geometry of the large pieces of ferrite required
to achieve the desired flux path. In addition, designs using potor E-cores are necessarily thick and, therefore, compromise the
ground clearance of the EV or require extensive chassis modifi-
cation. Coupling is highly dependent on the area through which
mutual flux passes and, given the large horizontal tolerance
(300 mm) required by an unguided EV, small coupler designsare infeasible. Also, small couplers usually operate with a small
air gap to ensure the necessary coupling; however, this can also
result in highly constrained and sensitive systems, as discussed
in Section V.
Arrays of coils with ferrite backing have been successfully
used to allow power transfer over large areas in low power
systems as discussed [4]; however, this technique is not costeffective for higher power EV charging systems. As shown
in [22], each row or column of coils needs to be switched to
prevent field cancellation in the center of the multilayer array
to increase power transfer. This is impractical with high-power
systems given the large currents required. Coreless coils, as
shown in [12], [23], [24], are generally not suitable for high-
power applications where ferrous materials are in close prox-
imity to the system due to eddy and hysteresis losses. Field
shaping with ferrite constrains flux to desired paths improving
coupling and consequently preventing excessive energy loss in
surrounding materials due to leakage magnetic field, which also
needs to be considered for safety reasons [7]. As shown in (1),
the mutual inductance between couplers and the primary cur-rent have the greatest influence on power transferred, while the
self-inductance of the coil is ideally constant independent of
the position of the receiver to ensure that it can be easily tuned
to resonance. In order to compensate for lower coupling and
maintain good efficiency, coreless systems are typically oper-
ated at higher frequencies in the range of hundreds of kHz to
MHz [25], [26]. Efficient operation at such high frequencies is
not possible with high-power systems due to performance lim-
itations of available semiconductor devices. Note that systems
with relatively large air gaps in relation to the pad size, as shown
in [26], are generally only suited to small-scale systems where
efficiency is not a major issue.
The power pads, shown here, overcome several physical limi-
tations of common couplers by using multiple smaller bars held
in place by a shock absorbing coil former with further protec-
tion provided by the aluminium and plastic case. These power
pads are relatively thin compared to standard core topologies,
and they are lighter than conventional circular coupler designs
that use solid ferrite discs.
As the pads are intended for stationary charging systems for
EVs parked in residential spaces, some assumptions have been
made. The vehicle is assumed to be constrained in the forward
direction by wheel chocks or some other barrier, therefore a
horizontal tolerance of 100150 mm in each lateral direction
is desired; this allows a 200300-mm-wide charging zone but
larger tolerances are clearly better for ease of parking. Also, the
EV is assumed to require a ground clearance of up to 200 mm.
Note the pads are circular and, therefore, not directional.
The power supply that is intended for use with the power
pads (as indicated in Fig. 1), operates from single-phase and is
ideally low cost and has near unity power-factor, as described
in [27], [28]. This supply uses an inductor-capacitor-inductor(LCL) impedance-converting network that converts the voltage-
sourced inverter into a current source that is suitable for driving
the primary pad inductance. It also filters the square wave output
of the bridge minimizing RF interference. The power supply
achieves cost efficiency through the removal of the large dc
bus capacitance and by using the leakage inductance of the
output-isolating transformer to form thefirst inductor of theLCL
network. The second inductor is formed by the inductance of the
transmitter pad and connecting wires; although in practice this is
often larger than desired so that a series capacitance is added to
achieve the desired value. The reduced component count makes
the powersupply extremely light andcompact. Although the lowbus capacitance increases the safety of the power supply, which
is especially important in a domestic setting, this results in a
100 Hz modulation on the dc bus. This modulation combined
with the 38.4 kHz modulated output of the inverter bridge results
in peak currents that are twice as high as the RMS current,
consequent care must be taken in the design to ensure the ferrite
in the primary pad does not saturate. As the focus of this paper is
the magnetic designand optimization, the operationof the power
supply will not be discussed further, except that it is assumed to
produce 23 Arms sinusoidal currents in the transmitter at either
38.4 kHz or 50 kHz.
An important objective in any pad design is to ensure that
the native quality factor of both, the transmitter and receiverpad inductances (QL ), are high. This ensures low loss and high
efficiency because losses in the pad are equal to the VA across
each its terminals divided by the QL . In the presented design
that follows, QL (equal to the reactance of the pad divided by
its native resistance) was found by measurement to be around
250. This highQL is a function of the design structure in Fig. 2,
which produces a flux pattern directed toward the receiver. The
ferrite strips guide the majority of the flux out of the front of
the pads while the aluminum adds rigidity to the structure and
helps remove any heat due to loss.
For the transmitter pad, the driving VA is essentially con-
stant given the power supply drives it with a constant controlled
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BUDHIA et al.: DESIGN AND OPTIMIZATION OF CIRCULAR MAGNETIC STRUCTURES 3099
current (23 A) during operation. As shown in Section V, its self-
inductancedoes not change significantly even with displacement
of the receiver pad due to the large air-gap. The largest pad built
requires 36 kVA to deliver 2 kW. The worst-case loss in this pad
is therefore only 140 W; this is spread over the surface of the
pad and there is little or no measurable temperature rise during
operation.
The losses on the receiver side are much lower than the trans-
mitter pad, given this native pad QL is much higher than the
largest operational circuit Q (governed by the controller to be
6), as described in Section II. The charging zone of a paral-lel tuned receiver (as shown in Fig. 1) is determined by ener-
gizing the transmitter at rated maximum primary current and
selecting the number of turns on the receiver to design Isc , so
that at worst-case separation it equals the required load current
and ensures power can be delivered. As discussed in many pa-
pers [2], [13], [14], the voltage on the load is naturally fixed
by either the battery or a voltage controller. As such, the oper-
ationalQ is fixed, based on the position of the receiver, as the
output voltage across the load is proportional to Voc Q. Thus a2 kW pad has a VA at worst displacement ofQPou t =12 kVAgiving a pad loss of 48 W. A 5 kW system would have 120 W
loss; however, this is spread over a larger surface area.
With pads aligned at the center of the charge zone the pad
Voc and Isc are both higher than at the edge of the charging
zone where full power can still be delivered, however, the
electronic controller on the secondary (as discussed in [13])
simply operates so that the average power delivered to the load
is controlled by lowering the operating Q and limiting the cur-
rent. Consequently, pad losses are reduced in this operating
region. Alternatively, the primary current can be reduced by
the supply to lower Voc and Isc within this region to achievethe same result. When the receiver pad is offset outside the
specified charge zone, both Voc andIsc drop, and if operation
is continued, the secondary Q will rise to compensate for the
drop in Voc to achieve the required output voltage. However,
Isc cannot be increased and, because it is below the required
load current, full power cannot be delivered. Thus, the pad loss
stays approximately constant in this region as its output VA is
approximately constant, but if the power transfer becomes too
low the secondary controller or primary power supply will shut
OFF.
A. Modelling the Power PadsThe pads of Fig. 2 have been modeled with a 3-D FEA pack-
age called JMAG. The dimensions of the pad and, therefore,
model are shown in Fig. 3, along with the excitation conditions.
The measured and simulated profiles are shown in Fig. 4. In
these results, the horizontal offset describes the distance be-
tween the pad centers, while the vertical offset describes the
separation between the plastic covers, each of which is 5-mm
thick. The EV battery requires an input power of 2 kW, and this
is the assumed maximum power rating of a typical household
mains socket.
As shown, there is a small error between the measured and
simulated results. The simulation results are slightly conserva-
Fig. 3. Pad layout and dimensions.
Fig. 4. Measured and simulated Psu against horizontal offset at specifiedvertical offsets. Operational Q of a 2 kW system with a 40 mm air gap is alsoshown based on measured results.
tive and the error may be due to simplifications in the model.
A compromise between the size of the surrounding air region
and elements in the model has been made to achieve the highest
accuracy to simulation time ratio. The manufacturing tolerances
(+/3 mm) result in coil and ferrite positions that are not as pre-cisely positioned as in the simulation. The peak flux density is
less than 150 mT; therefore, nonlinearity in the ferrite has been
ignored and it is considered to be an isotropic material with a
relative permeability of 2300. Overall, the results are in excel-
lent agreement and enable further pad designs to be explored
with confidence.
A null occurs in the power profile at a horizontal offset of
160 mm regardless of separation, as shown in Fig. 4. This isconsistent between measured and simulated results and arises
at the point where mutual coupling between the coils reduces to
zero. This is due to flux cancellation, as shown in Fig. 5, where
magnetic flux density vectors in a cross-section through the
centers of both pads are plotted. In this position, the directions
of flux from opposite sides of the transmitter pad coil, as shown
by idealised paths, oppose each other and, therefore, effectively
cancel resulting in almost no induced voltage in the receiver pad
coil.
Although power transfer at the null is extremely small and
practical operation in its vicinity is not possible, its existence
is responsible for the fundamental horizontal tolerance limit of
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Fig. 6. Varying spread of the coil on centered pads at 40 mm and 80 mmseparations,I1 = 23 A at 50 kHz.
and ferrites. It may appear that more power can be transferred
if the coil is moved outward by 4 mm and the ferrites moved in
by 4 mm since the maxima of both curves are symmetric about
the y-axis, however; moving the coil out by 4 mm means the
ferrite is effectively moved in by 4 mm relative to the coil and
moving both components to their local optimum positions will
actually decrease power transfer. Best performance is achieved
if the ferrites are slightly offset toward center, a ferrite central
diameter of 230 mm should be used for a 420-mm-diameter
pad. This approach is preferable to increasing the coil central
diameter, as doing so would require extra copper and increase
losses and cost. The coil central diameter, as shown in Fig. 3,
should be approximately 57% of the pad diameter. The position
of the power null, as shown in Fig. 4, is affected by the coildiameter but using a larger coil to shift the null is not possible
since the coupled power drops significantly as the coil is made
larger.
B. Improving Coupling by Adding Ferrite
Coupling will improve if more ferrite is added, and this may
be done by adding more bars, making the bars longer, wider
or thicker, or by adding feet that extend the portion of ferrite
uncovered by the coil. To determine which variable should be
maximized, various simulations were undertaken while keeping
the pads at a fixed separation and vertically aligned (without
any horizontal offset) for ideal power transfer. An efficiencycomparison in terms of ferrite utilization was then made using
VA/m3 of ferrite as a metric. Adding ferrite that is not utilized
efficiently will make the pad unnecessarily heavy and expensive.
The volumetric comparison focussed on changes to the original
design of Fig. 3.The impact oncoupling (via Psu ) was studied by
investigating changes in each variable of interest. The length,
width, thickness, the addition of feet. and number of ferrite
bars were varied by simulation. The variables were swept from
their minimum to maximum possible value given geometrical
constraints. The notation in the legend shows the limits of the
variable, while the increment used is preceded by a comma.
Results are shown in Fig. 8 below.
Fig. 7. (a)(d) Various pad designs where: (a) Original, (b) varying ferrite,(c) varying coil and ferrite, and (d) varying coil position, (e)(f) Psu withI1 =23 A at 50 kHz and pad separation at 40 mm and 80 mm, respectively.
As expected, four of the curves intersect at a point equal
to the volume of the initial pad. The curve for the pad with
feet has a small offset at this point because additional ferrite
is present (compared to the original design) to make the feet.
The gradient of the curve relating to increasing ferrite length is
highest, indicating that this gives the best ferrite utilization. This
is expected since longer blocks permit the highest flux paths in
air above each bar. The performance of the pad is influenced
least by the thickness of the ferrite bars, consequently, if pad
weight is a critical designparameter,thinner blocksmay be used.
However. doing so makes saturation more likely and increases
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3102 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011
Fig. 8. Volumetric comparison of changes in pad parameters at a separationof 80 mm.I1 =23 ARMS at 50 kHz.
hysteresis loss. The peak flux density in a 6-mm-thick bar was
found under theconditions simulatedhere to be 176mT, whereas
it is 102 mT in a 10-mm-thick bar, it is possible to operate with
thinner blocks.
C. Relating Coupled Power to Pad Size
The 420 mm pads do not achieve sufficient power transfer forpractical EV charging even with bars that are made as long as
possible. Larger pads are required for greater power transfer and
improved horizontal tolerance. The position of a null in a power
profile for a given pad occurs when the horizontal offset is 40%
of the pad diameter and this null can only be shifted further from
the origin by using larger pads. This also has the desirable effect
of smoothing the profile for a given offset. As circular pads by
their nature are scalable, the conclusions reached from the inves-
tigation in part B have been applied to a variable size pad model.
The number of turns of 4-mm diameter wire is adjusted to cover
40% of the ferrite bar length, the eight 30-mm-wide ferrite bars
are made as long as practically possible and were centered on
the coil. The coils central diameter (D in Fig. 4) has been setto 57% of the pad diameter. Optimal ferrite coverage can be
achieved by winding coils with a larger pitch or by increasing
the number of turns. The former approach results in impracti-
cal designs as coils with excessive pitches allow flux leakage
between turns reducing the flux path height, and hence coupled
power. The flux path height is shown by the idealized flux lines
in Fig. 5(a), and this height is the main reason why ferrite length
has the greatest effect on Psu for a given volumetric increase.
The graph in Fig. 9(a) shows vertical profiles for various pad
sizes, this profile is formed as the separation is increased be-
tween horizontally aligned pads. The simulation frequency has
been changed to 38.4 kHz as this optimization work is being
Fig. 9. (a) Vertical power profile for 300800 mm diameter pads with anexcitation current of 23 ARMS at 38.4 kHz, (b) layout of a 600 mm pad.
done in parallel with power supply development and presently
available switches are more suited a lower frequency. Perfor-
mance at 50 kHz can be easily determined by linear scaling.
The structure of a 600 mm pad is shown in Fig. 9(b)
The coupled power increases substantially with vertical off-
sets that are relatively low for given pads as exemplified. A
500 mm pad has aPsu of 1 kVA at 100 mm and it is 3 kVA for a
600 mm pad, the power coupled triples while the pad diameteronly increases by 20%. This large variation makes matching
a particular pad for an application challenging, as the high-
est power pad is not necessarily suitable when tolerance is
considered.
As discussed in Section III-A, using a 420 mm pad to couple
2 kW resulted in designs that were extremely sensitive to
changes in both the horizontal and vertical position, relative
to normal operating position of 40 mm (similar behaviour is
shown for a 400 mm pad in Fig. 9 above). Extreme sensitivity
to changes in vertical offset is common to all pads and is clearly
illustrated by the initial steepness of all the respective curves.
The high initialPsu for pads with close relative vertical spacing
combined with the fixed position of the null results in designsthat are necessarily intolerant of horizontal offset. For these
reasons designs that operate in the shaded region of Fig. 9(a)
are preferable and considered more practical in an EV context
where insensitivity to positioning error is of major importance.
The relationship between pad size and coupled power becomes
more apparent, as the pads become larger; the useful operation
zone appears to increase steadily and is roughly shown to be
linear by the gradient of the shaded area. Clearly, larger pads
are less sensitive to vertical separation, and this is indicated by
the divergence of the shaded area toward the right.
The fundamentalPsu limit of the modeled circular pads with
the layout shown in Fig. 9(b) is 2 kVA at 220 mm and an
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3104 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011
Fig. 12. Comparison of different ferrite arrangements.
current of 23 ARM S at 38.4 kHz. The ferrite is already being
utilized relatively efficiently, but more needs to be added in
order to improve coupling. There are numerous possibilities,
however, designs that require simple ferrite cutting operations
are preferred. The most optimal of the different pad designs are
ranked in order of power and are shown Fig. 12. In all cases,it is assumed that the ferrite bars used in construction have the
following dimensions: 118 mmL 30 mmW 10 mmT, asthese are readily available as I parts of EI cores. Each pad
is adequately described by Psu at different ideal separations,
the number of ferrite bars required and the ferrite utilization
efficiency, which has been calculated at a separation of 100 mm.
Profiles of the various pad topologies have been compared, and
it has been noted that regardless of the maximum Psu that results
when the receiver pad is positioned such that it is centered on
the transmitter, all receiver pads experience a null when offset
horizontally by approximately 40% of the pad diameter. The
slope of power profile is determined by the position of the null
and the Psu with the pads centered. The Psu for pads with a
100 mm air gap of type (e) in Fig. 12 is 2.5 kVA and is 3.8 kVA
for pad(a). Both pads have a null in their profileat approximately
240 mm, thus the slope for type (a) is greater. As shown in
Section IV-A, it is not advisable to increase the diameter of the
coilwiththe aim of shiftingthe null as Psu will drop substantially
increasing sensitivity to misalignment.
Designs with the best ferrite utilization have long or narrow
ferrite blocks shown in Fig. 12(e) and (d), respectively. The
former provides favourable magnetic paths that increase flux
density above the pad, while the latter enables flux around the
coil to be guided more effectively. The desirable features of
both have been combined to create a model of an optimal pad,as shown in Fig. 13. This pad weighs 15.6 kg and is capable of a
transferring 5 kW across an air gap of 150 mm with a horizontal
tolerance of 90 mm and, therefore, a full power charging zone
180 mm in diameter (using a maximum operational Qof 6). Its
Psu profile is shown in Fig. 14 along with the required Q at a
separation of 150 mm. This pad requires 31.5 standard ferrite
bars and has a ferrite utilization value of 3.80 VA/cm3 and is
within a few percent of the most optimal utilization, as shown in
Fig. 12(e), which is 3.89 VA/cm3 of ferrite. Assuming the EV
is constrained in the forward direction, as would typically be
case when parking in a garage, a 180-mm-wide charging zone
is possible with an air gap of 150 mm, and this is considered
Fig. 13. Optimal pad layout.
Fig. 14. Simulated profile of an optimized pad with 200 mm separation.Q
curves shown for 5 kW output at 100 mm and 150 mm.
within the ability of a driver if appropriate markings are made
on the ground.
A 200 mm ground clearance requirement, as desired, may
be satisfied if the transmitter is elevated by 50 mm, which is a
completely practical solution. Furthermore, the installation cost
within a garage of sucha systemis minimal for the user, since the
floor needs little modification. An appropriately marked trans-
mitter pad is unlikely to become a tripping hazard, especially if
a low-gradient circular ramp is placed around it.
The above optimization process was done assuming that the
desired ferrite sizes are available or larger bars can be cut to
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BUDHIA et al.: DESIGN AND OPTIMIZATION OF CIRCULAR MAGNETIC STRUCTURES 3105
Fig. 15. 700 mm pad layout.
size. In cases where ideal ferrite is not available, preferable
compromises are to make the ferrite blocks as long as possible
or as narrow as possible. The layout shown in Fig. 12(c) requires
readily available ferrite bars, although ferrite utilization is low,
good performance is achieved since a relatively large number
of bars are used. Conversely, the pad in Fig. 12(e) also uses
unmodified bars and has the highest ferrite utilization, although
it has only 18 bars resulting in a lowerPsu .
V. IMPLEMENTATION OF ANIPT SYSTEM
A 2 kW charging system operating off a standard single-phase
power socket with an air gap of approximately 200 mm is more
desirable than the previously optimized 150 mm 5 kW system.
The length and number of the ferrite bars is the most important
factor relating to power transfer at a given separation and a
700-mm diameter pad model was constructed, and is shown in
Fig. 15. The dimensions of the pad were largely dictated by
the availability of ferrite and machining capability. Each of the
12-ferrite spokes is made up of three standard ferrite I bars
(93 mmL 28 mmW 16 mmT) from EPCOS. The extraone and a half bar pieces, as shown in Fig. 12(b), were notplaced in the gaps in order to minimize weight and to avoid
cutting ferrite. The coil comprises 26 turns of 4-mm diameter
Litz wire. The operational frequency was reduced to 20 kHz as
the switches used for the power control were best suited to a
slightly lower frequency. The measured and simulated results
are shown in Fig. 16. Notably, a power null occurs when the
secondary is offset from the primary at approximately 40% of
the pad diameter.
Assuming an operational Q of 6 is allowed, this 700 mm
pad allows a charging zone with a full power diameter of
260 mm with an air gap of 210 mm. This is better than our
initial specification of a minimum charge radius of 200 mm. Al-though there are large sections of aluminium that do not appear
to be shielded from the coil by ferrite, the loss in the aluminium
backing pate is low and this is reflected by the highQL value of
the pad (as discussed in Section III),which is measured to be 250
with a precision LCR meter. This low loss is partly attributed
to the low resistivity of aluminium (minimizing I2R loss), the
physical distance between it and the coil and because it only
has to shield leakage flux. The ferrite strips guide a significant
proportion of the flux generated by the transmitter coil away
from the backing plate and upward to the receiver. The pads
were operated with 2 kW being transferred for several hours
without thermal issues.
Fig. 16. Simulated and measured horizontal profiles at 210 mm separationwith a primary current of 23 ARMS at 20 kHz and operational Q for 2 kWoutput.
Since the pads operate with a relatively large air gap unlike
those used in [6], [17], [20], transmitter and receiver pad induc-
tance variations are minimal; therefore, there is little effect on
tuning in both the power supply and PU during operation as a
result of misalignment. If smaller air gaps are chosen, the cou-
pling is improved, but the output power will be made sensitive
to any misalignment. Consequently, such variations will need
to be handled in the design of the system, as discussed in [27].
The graphs in Fig. 17(a) and (b) show the measured variation
of the transmitter and mutual inductances against separation
and horizontal offset, respectively. The 26 turn pads are identi-cal and, therefore, have essentially the same self-inductance of
540 H. This large inductance means that 1.6 kV is required
across the terminals of the transmitter to get a current of 23
A. This presents a potential safety hazard and requires careful
terminal insulation. Compensation capacitors can, however, be
placed in series with the pad winding and internal to the pad
structure to effectively reduce the inductance, and hence the
voltage at the external terminals. If required, the capacitors can
be distributed throughout the winding in order to meet voltage
limits. However, these approaches inherently reduce pad relia-
bility due to additional internal connections and ideally should
be avoided. Alternatively, the pad can be bifilar wound to lower
the inductance and, hence, driving voltage while keeping the NIproduct and, therefore, the generated flux constant.
The second approach was chosen noting a bifilar wound pad
needs to be driven with 46 A. The inductance of each 13-turn
coil was 130 H and 131 H for the inner and outer winding,
respectively. The inductance of the outer winding with the inner
shorted was 5.0H, and it was 5.1 H for the opposite set of
measurements. This corresponds to a total mutual inductance of
256 H. The inductance of the bifilar pad is 130 H, whichgives a terminal voltage of 780 V. This is below the general
upper safety limit of 1 kV. By constraining the voltage with a
bifilar winding, suitable components can be selected with VA
ratings that guarantee operation without fault.
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3106 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011
Fig. 17. Transmitterpad andmutual inductances against (a)vertical offset and(b) horizontal offset (at 200 mm separation).
A. Practical Issues Meeting Field Leakage Regulations
To enable application on an EV, the pads should ideally com-ply with the International Commission on Non-Ionising Radi-
ation Protection guidelines (ICNIRP). These regulations have
been established to limit human exposure to time-varying EMF
with the aim of preventing adverse health effects. ICNIRP stip-
ulates that the general public should not be exposed to body av-
erage RMS flux densities greater than 6.25T in the frequency
range of 0.8 to 150 kHz. The limit is raised for occupational
exposure and is frequency dependent, between 0.8 to 65 kHz
the limit is 30.7 T and between 65 to 150 kHz the exposure
level in T is defined by 2.0/f,wherefis the frequency in MHz;
this corresponds to 13.3 T at 150 kHz [30]. ICNIRP does
not explicitly describe measurement techniques for determining
whether systems meet the guidelines making design develop-ment difficult. Measurement techniques have been addressed by
the Australian Radiation Protection and Nuclear Safety Agency
(ARPANSA), based on ICNIRP guidelines formulating hu-
man exposure standards covering frequencies from 3 kHz to
300 GHz [31]. As such, the ARPANSA standard exposure lev-
els are also frequency dependent, the body average reference
level for general public exposure to magnetic fields is 6.1 T
between 10 to 150 kHz. The occupational exposure level be-
tween 65 to 100 kHz is defined by 1.63/fAm1 , at 100 kHz itis 20.5 T. Between 100 and 150 kHz the level is defined by
9.16/f 0.25 Am1 (fin MHz), corresponding to 18.5 T at the
upper limit.
Fig. 18. 2 kW pads with an air gap of 200 mm.
Fig. 19. Measuredand simulatedfluxdensityalong a contour midwaybetweenaligned and offset pads beginning at the center.
The measurement techniques described in the standard also
include spot limits, body average, and temporal averaging. Spot
limits may be up to a factor of20greater than the exposure
level at a given frequency; consequently, the maximum exposure
level for the general public at a given spot is 27.3 T in the
frequency range of 10 to 65 kHz. Temporal averaging allows
exposure measurements to be taken over a time of 100 s, as
such this may be 1 or 10 cycles. Spatial averaging is applicable
to frequencies lower that 100 MHz and involves taking the
average exposure level at four points on the human body, the
head, chest, groin, and knees. As long as the average value is at
or below the exposure level and no spots exceed the applicable
spot maximum, the system is considered to be in compliance.For systems presented in this paper, this means the general
public should not be exposed to an average flux density greater
than 6.1T and with no spots exceeding 27.3 T.
Simulations and measurements have been undertaken to in-
vestigate leakage fields around a system transferring 2 kW
across an air gap of 200 mm with the pads centered. Measure-
ments were taken with a Narda ELT-400 three-axis exposure
level tester that has a probe area of 100 cm2 . The leakage flux
density is greatest along the line midway between the pads, as
shown in Fig. 18. The measured and simulated results along
this line are shown in Fig. 19, with appropriate exposure limits
plotted.
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BUDHIA et al.: DESIGN AND OPTIMIZATION OF CIRCULAR MAGNETIC STRUCTURES 3107
Fig. 20. Measured flux density applying spatial averaging across a femalestanding 170 mm from the edge of a power pad system transferring 2 kW.
The graphed contour illustrates the worst leakage flux from
the pads; leakage directly above and below the pads is lower
due to shielding provided by the aluminium case and ring. The
spot flux density is 27.3 T at a distance of 500 mm from the
center of the pad or 150 mm from the pad edge. Leakage is
marginally worse with misaligned pads where the 27.3 T limit
is reached 550 mm from center. Spatial averaging is applied for
the worst-case scenario, where a 1.5-m-tall female is standing
170 mm from the pad edge; the results are shown in Fig. 20.
The system will easily comply with ARPANSA regulations.
VI. CONCLUSION
In order to ensure that IPT systems are as efficient, cost effec-
tive, and light as possible, it is critical that the desired coupling
between the power pads is achieved with a minimum amount
of ferrite. The difference between finite-element models and
measured results shown here is 10% at most, meaning that pads
optimized using the simulator will perform as expected in prac-
tice. The fundamental horizontal tolerance limit for chargingpads that are circular is shown to be approximately 40% of the
pad diameter. Narrow, evenly spaced ferrite bars give the most
effective performance to weight result.
A 2 kW IPT system was also built and tested using a
700-mm diameter power pad. Existing ferrite bars were used
for convenience and offer a better practical solution since a bar
made up of individual ferrite pieces is less likely to fracture than
a single long solid bar. Leakage fields have been investigated
via simulation and measurement and show that the pads meet
ICNIRP guidelines according to ARPANSA regulations. The
quantitative results presented in this paper form a basis for the
proper design of power pads for IPT systems.
REFERENCES
[1] K. Chang-Gyun, S. Dong-Hyun, Y. Jung-Sik, P. Jong-Hu, and B. H. Cho,Design of a contactless battery charger for cellular phone, IEEE Trans.
Ind. Electron., vol. 48, no. 6, pp. 12381247, Dec. 2001.[2] G. A. Covic, G. Elliott, O. H. Stielau, R. M. Green, and J. T. Boys, The
design of a contact-less energy transfer system for a people mover system,inProc. PowerCon 2000, vol. 1, pp. 7984.
[3] T. Hata and T. Ohmae, Position detection method using induced voltagefor battery charge on autonomous electric power supply system for vehi-cles, in Proc. The 8th IEEE Int. Workshop Adv. Motion Control, 2004,Kawasaki, Japan, pp. 187191.
[4] S. Y. R. Hui and W. W. C. Ho, A new generation of universal contactlessBattery Charging platform for portable Consumer Electronic equipment,
IEEE Trans. Power Electron., vol. 20, no. 3, pp. 620627, May 2005.
[5] R. Laouamer, M. Brunello, J. P. Ferrieux, O. Normand, and N. Buchheit,A multi-resonant converter for non-contact charging with electromag-netic coupling, in Proc. 23rd Int. Conf. Ind. Electron. Control Instrum.,1997, vol. 2, pp. 792797.
[6] F. Nakao,Y.Matsuo,M. Kitaoka, andH. Sakamoto,Ferrite core couplersfor inductive chargers, in Proc. Power Convers. Conf., 2002, Osaka,
Japan, vol. 2, pp. 850854.[7] P. Sergeant and A. Van Den Bossche, Inductive coupler for contactless
power transmission, IET Electr. Power Appl., vol. 2, no. 1, pp. 17,2008.
[8] F. F. A. Van Der Pijl, J. A. Ferreira, P. Bauer, and H. Polinder, Designof an inductive contactless power system for multiple users, inProc. 41st
Annu. Ind. Appl. Conf., 2006. Tampa, FL, pp. 18761883.[9] K. W. Klontz, D. M. Divan, and D. W. Novotny, An actively cooled
120 kW coaxial winding transformer for fast charging electric vehicles,IEEE Trans. Ind. Appl., vol. 31, no. 6, pp. 12571263, Nov./Dec. 995.
[10] R. Severns, E. Yeow, G. Woody, J. Hall, and J. Hayes, An ultra-compacttransformer for a 100 W to 120 kW inductive coupler for electric vehi-cle battery charging, in Proc. 11th Annu. Appl. Power Electron. Conf.
Exposition, 1996, vol. 1, pp. 3238.[11] H. F. Blanchette and K. Al-Haddad, Solving EMI-related problems for
reliable high-power converters design with precomputed electromagneticmodels, IEEE Trans. Power Electron., vol. 25, no. 1, pp. 219227, Jan.2010.
[12] L. Xun and S. Y. Hui, Optimal design of a hybrid winding structure
for planar contactless battery charging platform, IEEE Trans. PowerElectron., vol. 23, no. 1, pp. 455463, Jan. 2008.[13] J. T. Boys, G. A. Covic, and A. W. Green, Stability and control of
inductively coupled power transfer systems, in Proc. IEE Electr. PowerAppl., Jan., 2000, vol. 147, no. 1, pp. 3743.
[14] W. Chwei-Sen, O. H. Stielau, and G. A. Covic, Design considerations fora contactless electric vehicle battery charger,IEEE Trans. Ind. Electron.,vol. 52, no. 5, pp. 13081314, Oct. 2005.
[15] O. H. Stielau andG. A. Covic, Design of loosely coupled inductive powertransfer systems, in Proc. Power Syst. Technol. Int. Conf., 2000, vol. 1,pp. 8590.
[16] S. Valtchev, B. Borges, K. Brandisky, and J. B. Klaassens, Resonantcontactless energy transfer withimproved efficiency, IEEE Trans. Power
Electron., vol. 24, no. 3, pp. 685699, Mar. 2009.[17] H. Sakamoto, K. Harada, S. Washimiya, K. Takehara, Y. Matsuo, and
F. Nakao, Large air-gap coupler for inductive charger [for electric vehi-cles], IEEE Trans. Magn., vol. 35, no. 5, pp. 35263528, Sep. 1999.
[18] J. Hirai, K. Tae-Woong, and A. Kawamura, Study on intelligent batterycharging using inductive transmission of power and information, IEEETrans. Power Electron., vol. 15, no. 2, pp. 335345, Mar. 2000.
[19] D. A. G. Pedder, A. D. Brown, and J. A. Skinner, A contactless electricalenergy transmission system, IEEE Trans. Ind. Electron., vol. 46, no. 1,pp. 2330, Feb. 1999.
[20] M. Dockhorn, D. Kurschner, and R. Mecke, Contactless power trans-mission with new secondary converter topology, in Proc. 13th Power
Electron. Motion Control Conf. 2008. Poznan, Poland, pp. 17341739.[21] Y. Matsuo,O. M. Kondoh,and F. Nakao, Controllingnew diemechanisms
for magnetic characteristics of super-large ferrite cores, IEEE Trans.Magn., vol. 36, no. 5, pp. 34113414, Sep. 2000.
[22] L. Xun and S. Y. Hui, Simulation study and experimental verification ofa universal contactless battery charging platform with localized chargingfeatures, IEEE Trans. Power Electron., vol. 22, no. 6, pp. 22022210,Nov. 2007.
[23] J. L. Villa, J. Sallan, A. Llombart, and J. F. Sanz, Design of a highfrequency inductively coupled power transfer system for electric vehiclebattery charge, Appl. Energy, vol. 86, no. 3, pp. 355363, 2009.
[24] S. Judek andK. Karwowski, Supply of electric vehiclesvia magneticallycoupled air coils, in Proc. 13th Power Electron. Motion Control Conf.2008. Poznan, Poland, pp. 14971504.
[25] L. Xun, W. M. Ng, C. K. Lee, and S. Y. Hui, Optimal operation ofcontactless transformers with resonance in secondary circuits, in Proc.23rd Annu. IEEEAppl. PowerElectron. Conf. Exposition, 2008. Austin,TX, pp. 645650.
[26] B. L. Cannon, J. F. Hoburg, D. D. Stancil, and S. C. Goldstein, Magneticresonant coupling as a potential means for wireless power transfer tomultiple small receivers, IEEE Trans. Power Electron., vol. 24, no. 7,pp. 18191825, Jul. 2009.
[27] H. Chang-Yu, J. T. Boys, G. A. Covic, and M. Budhia, Practical consid-erations for designingIPT systemfor EV battery charging, in Proc. IEEEVehicle Power Propulsion Conf., 2009. Dearborn, MI, pp. 402407.
-
8/14/2019 Circular Magnetic structures for IPT.pdf
13/13
3108 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011
[28] J. T. Boys, C. Y. Huang, and G. A. Covic, Single-phase unity power-factor inductive power transfer system, in Proc. IEEE Power Electron.Specialists Conf., 2008. Rhodes, Greece, pp. 37013706.
[29] G. A. Covic, J. T. Boys, M. L. G. Kissin, and H. G. Lu, A three-phaseinductive power transfer system for roadway-powered vehicles, IEEETrans. Ind. Electron., vol. 54, no. 6, pp. 33703378, Dec. 2007.
[30] Guidelines for limiting exposure to time-varying electric, magnetic, andelectromagnetic fields (up to 300 GHz), Health Phys., vol. 74, no. 4,pp. 494522, 1998.
[31] Maximum exposure levels to radiofrequency fields 3 kHz to 300 GHz,AustralianRadiation Protectionand Nuclear Safety Agency (ARPANSA),2002.
Mickel Budhia (S05) received the B.E. degree inelectrical and electronic engineering from the Uni-versity of Auckland, Auckland, New Zealand, in2008,wherehe is currentlyworkingtowardthe Ph.D.degree.
His research interest includes analyzing and de-signing magnetic couplers used in inductive power
transfer systems for electric vehicle charging.
Grant A. Covic (S88-M89-SM04) received theB.E. (Hons.) and Ph.D. degrees in electrical and elec-tronic engineering from the University of Auckland(UoA), New Zealand, in 1986 and 1993, respectively.
He wasappointed as a full time Lecturer in 1992, aSenior Lecturer in 2000, and in 2007 as an AssociateProfessor in the Electrical and Computer Engineer-ing Department at the UoA, New Zealand. Currently,he jointly heads power electronics research with
Prof. John Boys at the UoA and is cofounder andChief Research Engineer of a new global start-up
company HaloIPT focusing on electric vehicle charging infrastructure. Heholds a number of US patents with many more pending in the area of inductive(contactless) power transfer (IPT). His current research and consulting interestsinclude power electronics, electric vehicle battery charging, and IPT from whichhe has published more than 100 refereed papers in international journals andconferences.
John T. Boysreceived the B.E., M.E., and Ph.D. de-greesin electricaland electronicengineeringfrom theUniversity of Auckland, Auckland, New Zealand, in1963, 1965, and 1968, respectively.
After completing hisPh.D.he waswith SPSTech-nologiesfor fiveyears before returningto academiaasa Lecturer at the University of Canterbury. He movedto Auckland in 1977, where he developed his work inpowerelectronics. He is currentlya Professor of elec-tronics at the University of Auckland, New Zealand,in the Department of Electrical and Computer Engi-
neering and co-founder of HaloIPT. He has published more than 100 papers ininternational journals and is the holder of more than 20 US patents from whichlicenses in specialized application areas have been granted around the world.His specialist research areas are power electronics and inductive power transferwhere he and Prof. G. A. Covic jointly head power electronics research. He is aFellow of the Royal Society of New Zealand and a Distinguished Fellow of theInstitution of Professional Engineers, New Zealand.