circular magnetic structures for ipt.pdf

8/14/2019 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

0885-8993/$26.00 2011 IEEE


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BUDHIA et al.: DESIGN AND OPTIMIZATION OF CIRCULAR MAGNETIC STRUCTURES 3097

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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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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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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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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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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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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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.


12/13


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.


13/13


[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.

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