GB2389767A

GB2389767A - Apparatus for energy transfer by induction

Info

Publication number: GB2389767A
Application number: GB0226892A
Authority: GB
Inventors: Ron Shu-Yuen Hui
Original assignee: City University of Hong Kong CityU
Current assignee: City University of Hong Kong CityU
Priority date: 2002-06-10
Filing date: 2002-11-18
Publication date: 2003-12-17
Anticipated expiration: 2022-11-18
Also published as: GB2389767B; GB0226892D0; GB0213375D0

Abstract

The apparatus includes a primary induction unit including a planar surface for receiving a secondary element to which energy is to be transferred. The primary induction unit comprises an array of primary transformer windings to provide a substantially uniform magnetic flux distribution over said planar surface. The winding connections of the array may be serial, parallel or both and multiple overlapping layers may be used. The individual spiral windings may be circular, square or hexagonal.

Description

. 2389767

APPARATUS FOR ORGY TRANSFER BY SUCTION

s FIELD OF THE VliNTION

This invention relates to an apparatus for He Prefer of energy by electncal induction, and in particular to art induction heating apparatus, for example an induction cooking apparatus.

10 BACKGROUND OF THE INVENTIO19

Induction heating equipment such as an induction cooker usually uses a single spiral winding as the primary transformer winding. When excited by a high-fiequency ac voltage generated by a power electronic circuit, a high-hequency ac magnetic nux is created in the spiral winding. This magnetic flux points in a direction perpendicular to the plane of the spiral winding.

15 Protected by a layer of electrical insulator, the ac excited spiral winding provides an induction surface. When a secondary element such as a metallic cooking pot is placed on top of the electrically insulated surface of the spiral primary winding, curTent will be induced in the secondary metallic natenal by induction due to the ac magnetic flux created in the spiral primary winding.

The conduction loss due to she induced current in the secondary element causes heating of the do secondary element.

PRIOR ART

Similar induction heating equipment using a single spiral winding for drying printing maten:tls has boon proposed by bilges ot al in US patent 6,34g,671. Ulrich at al proposed an 25 induction heating system using a flexible coil in US patent 6,346,690 and US patent 6,22g,126. The flexible coil is wound in a concentric manner and is fed by a popover supply. There are limitations in using a single spiral winding as the inductive element. First, the magnetic field distribution is not

unutorm, The magnetic flux (A is proportional to Tic product of Tic current (i) in and the number of tunes (N) of the winding. For a spiral winding, the number of turns is highest in the cerise of the spiral winding and lowest in the edge of the spiral winding. This mews that Me magnitude of tile magnetic field is C8t in the c= - "d decrees Boo the centre. Fig. 1() shows a Wick will

5 spiral winding. If excited with an ac power source at high frequency (say over lOlz), an as magnetic flux is generated. Fig.1(b3 shows the measured magnitude plot of the magnetic Reid distribution of the single spiral winding in P-igl(a) It can be seen that the magnitude of the magnetic field is highest in the centre of the spiral winding and this magnitude decreases with

increasing distance Tom the centre.

10 If the secondary element Is not placed in the central area of the inductive surface, the magnetic flits it experiences is small and derefore the current induced in it is also small.

Consequently, the heating effect is far fi-om satisfactory This nonuniform magnetic field

distribution and its associated non-unifonn heating problem limit the performance of many commercial induction heating systems.

SUMMARY OF THE INVENTION

According to the present invention there is provided apparatus for the transfer of energy by electrical induction comprising a primary induction unit Coined win a planar surface for receiving at least one secondary element to which energy is to be transferred, wherein said primary induction 20 unit comprises an alTay of primary transformer windings so as to provide a substantially uniform magnetic flux distribution over said planar surface.

In a preferred ambodin t, the primary winding are c^Tmected in groups define a plurality of encrgy transfer areas whereby energy may be transferred to a plurality of secondary elements provided simultaneously on said planar surface. The pnmary windings may be corrected 25 In series and/or in parallel. The windings may be circular, rectangular, square or polygonal spirals.

Preferably multiple layers of such arrays may be provided, win the array in one layer befog offset relative to the as y another layer such Mat regions of one layer diet generate maximum flux coincide with regions of another later that generate minimum flux.

The present inrcation is particularly well suited in a preforTod embodiment for realization as S an induction heating apparatus, and the primary induction Wit may be an induction cooker, with the secondary element(s) being one or more metallic cooking utensils.

Viewed from another broad aspect the present invention provides apparatus for the transfer of enormity by electrical induction conpridug a prudery induction unit fanned with a planar surface for receiving at least one secondary element to which energy is To be transferred, wherein said at 10 least one secondary element may be located anywhere on said planar surface and energy is only transferred from said primary induction unit in Me area of said surface where the said at least secondary element is located.

Viewed from another broad aspect the present invention provides apparatus for generating substantially uniform magnetic flux over a surface, composing at least tuto layers each being I 5 fanned with an array of pumas electrical windings, wherein the array of a first said layer is offset relative to the array of a second said layer such that regions of said first layer that generate maximum nagnetic flux coincide with regions of said second layer that generate minimum magnetic flux.

20 BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described by way of example and with rofercnoc to the accompanying drawings, in whióh: Figs.l(a) and (b) show (a) a conventional single spiral winding, and (b) the resulting magnetic field distribution,

25 Fi.2 illustrates a typical inductive cooking apparatus,

Fig.3 illumines an array of primary windings according to an embodiment of the invention, Fig.4 shows the scte of a primary inductive unit according to an embodiment of the invention}n (a) exploded view and (b) perspective view, P.igs,5(a) and (1) show;m array of primary windings according to an embodiment of the 5 invention, and (b) the magnetic field generated thereby,

Figs 6(a) and (b) show (a) an embodiment of the invention having groups of windings, and (b) an equivalent circuit, Fig.7 shows first layer of a IS winding array for use in a multilayeit embodiment, Fig.8 shows a second layer of a 3x4 Grinding array for use in conjunction with the layer of 10 Fig.7 in a multi-layer embodiment, Fig.9 shows the layers of Fig.7 and Fig.8 in the twolayer structure, Fig.10 is simplified version of Fig..

Fig,l 1 is a simplified version of Fig.8, Fig.12 is a simplified version of Pig.9, I Fig. 13 is a plot showing the smnoing effect of the two-layer structure, Fig.14 shows a hexagonal spiral windings Fig. IS is e. simplified forth of Fig. 14, Fig. 16 shows a single-layer of hexagonal spiral windings, Fig.17 shows two adjacent hexagonal spiral windings, 20 Fig.18 shows the mmf distribution ofthe adjacent windings of Fig.17, Fin 19 shows three adjacent hexagonal spiral winding and the peaks and minima of the flux distribution, Fig.20 shoves two overlapped layers of hexagonal spiral windings, Fig.21 shows the location ofthe peak flux in the stnctwe of Fig.20, 25 Fig.22 corresponds to Fig.21 but also shoves the location of the flux minima,

Fig,23 shows an embodiment of the invention romped with Free overlapped layers, Fig.24 corresponds to Fig.23 but shows the Cation of the flux peaks, Fig.25 is a plot showing the mifomity of Tic flux distribution along a line, Fig.26 shows a square spiral rindug, 5 Fig.27 is a simplified version of Fig.26, Fig 28 shows a first layer of square spiral windings, Fig.29 corresponds to Fig.28 but shows t}= location of the flux maxima and minima' Fig.30 shows two overlapped layom of square spiral windings including the location of the flux maxima and ninima, 10 Fig.3l shows three overlapped layers of square spiral windings including the location of Me flux maxima, and minima, and Fig.32 Shows four overlapped layer" of square spiral windings including the location of Me flux maxima and minutia.

I 5 DETAIN: DESCRIPTION OF PREFERRET) EMBODIMENTS

This invention relates to a new induction system for the transfer of energy, and in particular to induction heaters and induction cooicore. As shown in Fig.2, the induction system consists of two units, namely (1) a power delivenog inductive circuit that contains the primary circuit of an array of planar isolation transformer windings and (2) a secondary element that couples energy from the 20 prunary inductive circuit. In the context of an induction cooker, the secondary element may be a metallic pan 1, while the first element is the indtian heating surface 2 on which the pan is to be placed. In a prepared embodiment of the invention, a plurality of primary transformer wudugs are located in die primary inductive circuit, and preferably in the form of an array as shown in Pig. 3 US The windings may be circular, rectangular, hexagonal, square or any polygonal shape. The

windings may be connected iD senes, or in parallel or in a combination of both. By providing an array of windings a more uniform magnetic flux distribution can be provided, and the possibility of localized charging zone mechanism is enabled as will be discussed furcr below. It will be understood that only the Lea of Be}nduction surface covered by tho secondary eloquent will be 5 activated, while most of Be non-covered area radiates virtually no energy.

As shown in Fig.4, the primary transformer circuit unit transmits electrical energy at high frequency through a flat inductive surface that contains an array of primary transformer windings.

The inductive surface can be of circular, square, rectangular hexagonal or any polygonal shape depending on the requirement of the applications. In Fig.47 a rectangular inductive surface is used 10 as an example. The primary transposer windings can be of circular, square, rectangular, hexagonal or any polygonal shape. If utilization of space is needed, these windings can be wound into hexagonal shape. These windings can be wound in a concentric or spiral manner. For low power applications, they can be printed on a circuit board.

An important aspect of the present invention the primary transponder windings are arranged t 5 in the form of an array. These windings can be connected in series or in parallel or a combination of both. The advantage of using an array of primary trarsfonner windings is to generate a more uniform magnetic field than a single winding. This unifonn magnetic field dietributian is illustrated

in a practical example of a 4x4 winding array as shown in Fig.5(a). When connected In series (of the same polarity) and excited with a highfrequency ac voltage, this 4xA winding array generates a 90 magnetic field that is almost uniform as shown in Fig.5(b). Each winding generates its own

magnetic field. When arranged in an array mater, the overall magnetic field is much more unifonn

than the field generated by a single winding. This can be clearly seen by comparing the magnitude

plots of Fig.l(b) and Fig.5(b).

Besides using Be series connection, the primary transformer windings can be connected in 25 parallel or a combination of series and parallel. Fig. 6(a) shows an example with an array otpumary

windings forming four groups in few zones A,B,C and D. This means that the proposed induction has more than one primal transfonner winding connected in parallel to the power inverter of the primary induction It the secondary element is placed on Zones A and B. the equivalent circuit of He proposed system is shown in Fig.6(b). As the secondary olomont is close to the primary windings in Zones A and B (but not C and D), only the primary windings A and B sense the secondary load. Primary windings C and D have virtually no load and therefore their secondary circuits are essentially open circuits. This means that only the overlapped area between the induction surface and the secondary element has energy transfer. This localized activation principle is an important feantre of this invention.

10 In a preferred embodiment, He present invention features a primal induction circuit that has (1) a switched mode popover electronic circuit, (2) a transformer that consists of a primary winding or a group of primary windings connected in series or in parallel or a combination of both, (3) an optional EMI shield (which may be a combination of a ferrite sheet and a copper sheet) and (4) a flat interface surface on which the secondary element or elements can be placed and can 15 couple energy fiom the primary induction circuit. The schematic of the primary induction system is shown in Figs4(a) and (b).

The induction system can be powered by AC or DC power sources. If the power supply is the AC mains, the Arched mode power electronic circuit should perform a low-frequency (50 or 60Hz) AC to DC power conversion and then DC to high-frequency (typically in the range from 20 20kHz to lDMHz) AC power conversion. This high-frequency AC voltage will feed the primary winding or windings of the primary induction circuit. If the power supply is a DC voltage source (e.; battery), the switched mode power supply should perfonn a DC to hgh-frequency AC power conversion. The hithhequency voltage is fed to the primary winding or windings of the transformer.

The induction system is able to induce cutrent in the secondary eleracut or elements. For induction heating systems, the heat distribution should be as uniform as possible. In addition, only the overlapped area of the primary induction surface and the secondary element provides energy trulscr. order to achieve thee finctions, the AC magnetic flux experienced by the ssóonday 5 element should be as even as possible A standard spiral winding as shown in Fig.l(a) is not suitable to meet this requirement because its flux distribution is not unifonn as shown in Fig. l(b) when Me winding is excited by an AC power source. The reason for such non-uniform magnetic flux distribution is that the number of bents in Me central region of the single spiral winding is largest. As the magnetic flux or magnetomotive force (mmf) is proportional to the product of the 10 number of turn and the current in the winding, the magnitude ofthe magnetic flux or mmf is highest in the centre of the winding.

An improved method proposed in this invention is to ensure that the magnetic flux distribution experienced by secondary element is as uniform as possible. This method can be realized by using a "distributed" primary transfonner winding array structure as shown in Fig.3.

15 This planar winding array consists of many spiral windings. These spiral vvindings can have hexagonal, circular, square or rectangular patterns. These transformer windings can be connected in series, in parallel or a combination of both to the high-frequency AC voltage generated in the power supply in the primary charger circuit.

Pig.5(a) shows a practical example with the transformer winding array connected in series 20 so that all the fluxes created in the windings point to the same direction. Fig.5(b) shows the measured flux distribution of one planar transformer when the windings in the transfonncr array are connected in genes. This mearomont confirms the uniform magnetic flux distribution of the alTay structure. Comparison of Fig. l(b) and Fig.5(b) confirms the improvement of the uniform magnetic field distnution using the transformer array structure In addition the transformer array structure

enables the possibility of creating a multiple primary transformer windings structure vldch allows for a new localized energy transfer mechanism as explained below.

The primary transfonner windings can also have a combination of sends and parallel connections if desired. Such an arrangement allows the induction surface to provide various 5 induction regions to eater for difforsot sizes of Me secondary olemcuts. Fig,6(a) illustrates this localized induction zone principle. Assume Mat the transformer array is divided into 4 zones (A, B. C and D). The transformer windings within each zone are connected in series to fond one primary winding group with distributed magnetic flux feature. There will be four primary windings in the equivalent circuit as shown in Fig.6(b). If the secondary element is placed on Zones A and B. Be 10 equivalent electrical circuit is shown in Fig.6(b). Only the parallel primary transformer winding groups for Zones A and B are loaded because they can sense a nearby secondary element that can couple energy from the primary windings. Therefore, they will generate magnetic flux in Zones A and B. Primary transformer windings (: and D are not loaded because they have no secondary element close to them. Their equivalent secondary circuit arc simply an opcnircuit (Fig.6Cb)). As 15 a result, power transfer between the primary induction circuit and the secondary element takes place basically through the coupled regions (areas) of the induction interface surface covered by the secondary element. The non-covered area of the induction surface will transfer very limited energy.

This special design avoids unnecessary or excessive electromagnetic interference.

In the present invendon, at least in preferred fonns, there is proposed a new induction 20 system that overcomes the non uniform magnetic flux problem arising from using a single spiral wudug. More importantly, the proposed induction system, based on a winding array structure, can generate a near-uniform magnetic field over Be needed areas on the inductive surface and thus

pro-vice a near-uniform heating effect in the secondary element. Based on a multiple primary transformer winding concepts, the invention also operates on a new localized energized zone 25 principle so that energy is coupled to the secondary element primarily through the overlapped area

lo of the inductive and the secondly element. That is to say, energy is only banefened when a secondary winding or element is placed over one or more of tho primary windlags, and energy is only transferred Dom those primary windings that couple to the secondary Ending or element, mainly the plenary winding or windings directly beneath the secondary winding or element, and 5 possibly also (though to a lesser extent) any nearby primary windings that are close enough to couple to the secondary winding or clement to a significant extent.

In the embodiments described above a single layer of transfonner arrays is provided.

However, in order to generate a more Deform magnetic field distribution, multi-layer transformer

arrays can be used. The following embodiments dosonbe how multiple layers of transfomcr arrays 10 may be used that can provide a very uniform magrretic held distribution on the energy transfer surface. Fig7 shows a 4xS primal planar transfonner winding array which consists of squarespiral winding patterns. This can be fabricated on one layer of a printed circuit board structure for low power applications, though for high power applications it should be an actual winding. It should be 15 noted that, for an individual winding patted in the array, the magnitude of the magnetic flux is highest in the center of the spiral winding. The magnitude of the magnetic flux is smallest in the sTnall gap between adjacent winding patterns.

A second layer with a 3x4 transformer winding array is shown in Fig.8. The individual winding patters in both layers arc identical, As shown in Fig.s, by having the vo layers of arrays 20 arranged in such a manner that the center (region of maximum magnetic flux magnitude) of a winding pattern on one layer is placed on the gap (region of minimum magnetic flux magnitude) between adjacent wludil' patterns on the other layer, the variation of Me magnetic field magnitude

can be minimized and the magnetic flux magnitude can therefore be made as even as possible over the overlapped surface. The essence of the mult{layer transformer arrays is to have a displacement 25 between the individual winding patterns of the Tao layers so that the regions of the maximum

magnetic field magnidc of one layer is "evened out" by Me regions of the Sum magnetic

field magnitude.

In order to examine the 'unifonn magnetic field magnitude' feature of Me proposed!

overlapped multi-layer transponder arrays, this 'magnitude smoothing' concept is illustrated in 5 simplified diagrams in Fig.lO to 12. Fig.tO is a siruplified version of Fig.7. Bach solid square ire Fig.10 represents a squarespiral winding pattern in the first layer (Fig.7). Fig. ll is a simplified version of the Fight. Bach dotted square represents a square-spiral winding pattern in the second layer (Fig.g). The simplified version of the nulti-layer array structure is shown in Fiz.12. From Fig. 12, it can be seen that the overlapped array structure (with correct displacomoslt between He i 10 So layers) divides each squarespira1 winding pattern into four smaller sub-regons. The important feature is that the four sub-regions arc identical in teens of winding structure. Despite that fact that He distribution of the magnetic field magnitude on the surface of each individual square-spiral

winding is not uniform, the distribution of the resultant magnitude field magnitude on the surface of

each Buregion Is more or less identical because of the overlapped multilayer winding stare i 1 S The concept of the generating uniform magnetic field magnitude over the energy trensicr surface is

illustrated in Fig.13.

In this example, a multi-layer transfo'ner winding array structure that can provide a ifom magnetic field magnitude distribution is described. This example is based on quare-6piral

winding patterns. In principle, winding patterns of other shapes can also be applied as long as the 20 resultant magnetic field magnitude distribution is as unifonn as possible.

The use of two layers of transformer arrays can reduce the variation in the magnetic flux over the energy transfer surface. However, there may still be some variations and the use of a Tree or four layer structure may provide a still more uniform flux distribution as described in the following embodiments.;

The following embodiment is a structure comprising three layers of planar waning arrays. l This winding structure can generate magnetomotive force (mew of substantially even magnitude over the energy transfer surface. Each shindig away consists of a plurality spiral windings each of which are of an hexagonal shape. A spiral winding arranged in hexagonal shape is shown in 5 Fig.14. For simplicity, it will be represented as a hexa, gon as shown in Fig.15. A plurality of hexagonal spiral windings can be arranged as an array as shown in Fig.16. These windings can be connected in parallel, in series or a combination of both to the electronic driving circuit If a cement passes through each spiral winding pattern, a Tnagnetomotive force (mmr0, which is equal to the product of die number of turns (N) and current (1) (i.e. Ah, is generated. Fig.11 shows To spiral! 10 winding patterns adjacent to each otter and the per-unit mniplot over the distance (dotted line) can be linearized as shown ire Fig. 18. It can be seen that me mmf distribution over the distance is not uniforms. The maximum mmf occurs in the center of the hexagonal pattern and the minimum mm occurs in the edge of the pattern.

Figl9 shows three adjacent windings. Thc maximum mmiregion is labeled by a symbol 'P' i 15 (which stands for Peak mmfl. The minimum mmf region at the junction of 2 patterns is labeled as V' (which stands for Valley of the miff distribution). In order to generate a uniform mmf distribution over the planar energy transfer surface, two more layers of winding arrays should be added. This principle is explained firstly by adding a second layer of winding array to the first one as ShDWO in Fig.20. The second layer is placed on the first one an such a way that the peak mm 20 positions (P) of line patterns of one layer are placed directly over the valley positions (V) of the patterns in the other layer. Fig.2I highlights the peak positions of the patterns that are directly over the valley positions of the other layer for the two overlapped layers in Fig. 20.

It can be observed from Fig.ZI, however, that the use of two layers of winding arrays, while presenting an improvement over a single layer, does not offer the optimal solution for generating i 25 uniform mmf over the inductive energy transfer surface. For each hexagonal pattern in the twotayer

s1:ruculre, the peak positions occupy me central position and thme (out of six) vertices of each hexagon. The remaking decree vertices are valley positions (V) Mat need to be filled by third layer of winding arrays. These valley positions me shown in Fig.22 as empty squares.! Careful examination of Fig22 shows mat mere are six peak positiom (P) sulTousding each 5 vellum position. Therefore, third layer of a hexagonal winding array can be used to M1 up all these remaining valley positions. By placing We central positions (peak mnipositions) of me hexagonal winding patterns of Me third layer of tle winding array over the remaining valley positions of the two-layer structure, an optimal threlayer structure is formed as shown in Fig 23. Fig.24 highlight' the peak mmf positions of the three-layer stature. It can bc obeyed mat all central positions and! 10 Fences of all hexagonal patterns have peals mm/.

In order to confimn that the mmf over the surface has aiforrn mmf distribution' any distance between any two adjacent pelvic mmf positions can be considered as illustrated in Fig.25. If the winding pattems are excited in the same manner and polarity so that the mmf generated by each layer of the winding array are always ir: the saline direction at any moment, the resultant mmf is i I 5 simply the sum of the mmf generated by each layer. Fig.25 shows that the resultant mmf over the distance between any two adjacent peak positions in Pig.25 is equal to 1.0 per unit. This confirms that the proposed three-layer winding array structure can be used to generate highly unifonn mmf over the inductive energy trfor Europe When used a contactless, inductive energy transfer surface, this uniform mmf distribution feature ensures that, for a given airgap, a secondary coupling 20 winding can always couple the same amount of magnetic flux regardless of the position of the secondary element.

In another embodiment, the ree-layer winding array stmcturc can be constructed as a four-

layer, with one of the four layers accommodating the return paths of the spiral windings to the electronic driving circuit. i

A firmer embodiment is based again on square spiral winding pattems. this embodiment four laborers of square spiral wielding arrays are used to generate highly form numb over Me surface. As in the hexagonal embodiment described above, for convenience of illustration each square-spiral winding pattem (Fig.26) is simplified as a square snbol (Fig.27) in *be following S dewnption.

Fig.8 shows the first layer of the squarespirl winding anay. The mmlin the central region of each square pattern is highest. These regions are highlighted as 'Peak' or (P) in Fig.29The regions of the ninimum numb (ice. Me valleys) occurs along the edges of the squaw pattems. These regions are highlighted with dot. (a) in Fig.2.

I O In order to reduce the mmiripples on the surface, the peals (P) positions of a second layer of square-spiral winding array can placed over some of the valley positions (a) as shown in Fig.30.

When a third layer of square-spiral winding array is added to the structure in Fig.30, the resultant layout is shown in Fig.3 I. It can now be observed that one more layer of the square-spiral windings is needed to fill up all the valleys with peaks as shown in Fig.32.

t,

Claims

ICES 1. Apparatus for Me transfer of energy by electrical induction

composing a primary induction unit formed with a planar surface for receiving at least one secondary element to which energy is to be transferred, wherem said primary induction unut composes an array of primary transformer windings so ns to provide a substantially uniform magnetic f1WC distribution over said planer surface,
2. Apparatus as claimed Ln claim 1 comprising at least two planar arrays of primary winding: wherein one said planar array overlies the other said away,
3. Apparatus as claimed in claim 2 wherein a first said array is offset relative to a second said array such that regions of said first Gray that generate maxims magnetic flux coincide with melons of said second array that generate nininum magnetic flux.

5
4. Apparatus as claimed in claim 3 composing three layers of hexagonal windings.
5. Apparatus as claimed in claim 3 comprising four layers of square windings
6. Apparatus as claimed in claim 1 wherein said primary windings are connected in groups to 2.0 define a plurality of energy trmefer areas whereby energy may be transferred to a plurality of secondary elements provided simultaneously on said planar surface.
7. Apparatus as claimed in claim 1 wherein said primary windings are connected in series andlor in parallel.
S. Apparatus as claimed in claim I wherein said windings are hexagonal, circular, rectangular, square or polygonal spirals,
9. Apparatus as claimed in claim 1 wherein said apparatus is an induction heating apparatus.
10. Apparatus as claimed in claim 9 wherein said primary induction unit is an induction cooker, and wherein said secondary element(s) is/are a metallic cooking utensil.
11. Apparatus for Me transfer of energy by electncal induction conpasing a primary induction 10 unit romped with a planar surface for receiving at least one secondary element to which energy is to be transferred, wherein said at least one secondary element may be located anywhere on said planar surface and energy is only transcended Mom said purnary induction unit in the area of said surface Where the said at least secondary element is located.

15
12. Apparatus for generating substantially unifonn magnetic flux over a surface, composing at least two layers each being formed with an array of primary electrical windings, wherein the array of a first said layer is offset relative to the alTay of second said layer such tlut regions of said first layer that generate maximum magnetic flux coincide with ret ions of said second layer that generate minimum magnetic nux.

13, Apparatus as claimed in claim 12 comprising three layers of hexagonal windings.

l4. Apparatus as claimed in claim 13 comprising four layers of square wudings.