The present invention refers to an induction cooking plate or hob as defined in the preamble of the appending claim 1.

In a cooking plate for a range or a hob where the heat generation is effec¬ ted by inductive heating by means of an induction coil the useful magnetic flux is closed through the cooking-utensil which is to be heated. Beside the useful flux, at the same time also leak fluxes appear which form a magnetic leak field that surrounds the cooking plate. As this leak field is generated by an electric current of high frequency (of the magnitude of 25 kHz) the field may cause interference, in particular upon reception of AM radio broadcastings. in order to reduce the leak field it has been proposed in connection with ceramic induction hobs to provide annular conducting coatings on the supporting surface below the cooking-utensil. According to other proposals conducting rings or cylinders have been provided to surround the coil. An especially good effect has been achieved by pieces of ferrite being disposed below the coil to collect the field in that area.

By use of the field-screening arrangements described it has been possible to restrict the leak field from the coil in the direction away from the cooking- utensil and sideways, respectively. However, between the cooking-utensil suppor¬ ting surface and the upper side of the coil there is an area where a leak field can propagate. Measurings have shown that as long as this leak field remains uninfluenced the improvements achieved by the various screening arrangements will only be marginal. In addition, the various screening arrangements can involve practical difficulties.

The object of the invention is to eliminate or at least considerably reduce the leak field without any appreciable reduction of the useful field. The object will be achieved in an induction cooking plate or hob having the characterizing features of claim 1. Preferred embodiments appear from the appending sub-claims.

The invention will now be described with reference to the enclosed drawings.

Fig. 1 schematically shows a ceramic cooking hob.

Figs. 2-5 show four different arrangements of induction coils.

Figs. 6 and 7 show two different constructive embodiments of coils with coil cores.

Fig. 8 is a section along the line VIII - VIII in Fig. 6, and

Fig. 9, finally, is a section along the line IX - IX in Fig. 7. In Fig. 1 there is shown a conventional cooking hob 10 of the ceramic type which is provided with a supporting surface 11 for a cooking-utensil 12. The heating is performed inductively by means of an induction heating unit 13 disposed below the supporting surface and mounted on a holder 14, schematically shown. The heating unit comprises an induction coil 15 generating a magnetic field which is closed through the cooking -utensil 12 to heat said utensil. Below the coil 15 an annular ferrite disc 16 is provided to collect the field lines direc¬ ted downwards and by that to restrict the leak field in that direction.

Even if in the conventional hob according to Fig. 1 screening arrangements are provided around the coil, the useful field which is directed upwards has to be left uninfluenced. Accordingly, field lines can come out in the area bet¬ ween the coil and the bottom of the cooking-utensil. As the heating effect required involves a demand for heavy current in the coil, the outcoming leak field will be of a correspondingly large strength causing annoying interference.

Now, in accordance with the invention it is suggested to replace the single induction coil by two - or more coils which are symmetrically disposed with respect to a vertical line through the supporting surface and the center of the cooking-utensil. In each of these coils a current of the same magnitude is to flow, preferably the same current. However, the said currents are to cause different directions of the magnetic fields created so that in each field point, at a distance from the cooking hob the field contributions from the various coils essentially cancel out each other.

In Fig. 2 an embodiment is shown having four coils 17 - 20 which are sym¬ metrically disposed. The coils form a square where along the sides of the square two adjacent coils have opposite directions of winding while diagonally opposite coils have the same direction of winding. The coils, being shown from above, have been winded on cores 21 made of ferrite and indicated in the center of each coil.

In Fig. 3 two plane coils 22, 23 have been coaxially disposed and here the cores have been designated 24 and 25. As shown by the arrows, the currents in the coils have opposite directions of flow.

In Fig. 4 two identical coils 26, 27 have been symmetrically arranged, one straight below the other. The currents in the coils are equal but have oppo-

site directions. Here, a ferrite disc 28 is required which is disposed between the coils to close the upward field from the lower coil 27. Otherwise, the fields from the coils 26, 27 would counteract in the cooking-utensil area.

In Fig. 5 there is indicated the possibility of arranging several coils in matrix form. If the coils have a common winding as shown, the directions of the fields will be those indicated by plus and minus signs. I na field point far from the cooking hob the field contributions will essentially cancel out each other.

In Figs. 6 and 7 and in Figs. 8 and 9, respectively, examples have been given on how to realize in practice the embodiments of Figs. 2 and 3. With re¬ ference to Figs. 6 and 8 the coils 17 - 20 are incorporated in a coil holder 39 having a whole base plate 29 with an upward edge 30 as well as core parts 31 , 32, 33, 34 on which the coils have been pushed. The coils have the shape of a qua¬ drant of a circle but they may have a circular shape as well. The holder is made of ferrite resulting in a good screening against the leak field in a downward direction as well as laterally.

In Figs. 7 and 9 there is shown how in the same way as shown with referen¬ ce to Figs. 6 and 8 the embodiment of Fig. 3 can be realized. A similar coil holder has a base plate 35, a peripheral edge 36 as well as a central core portion 37 and an annular core portion 38.

Calculations of the resulting fields in a field point have been made for the embodiments of Figs. 2 - 4. These calculations show that the strength of the leak field is reduced to less than one per cent of the strength of the cor¬ responding field from a single coil. In the embodiment of Fig. 2, then, each coil stands for one quarter of the heating effect supplied.

Measurings have also been carried out as to the efficiency of an arrange¬ ment according to Fig. 2. These measurings have shown that a magnetic coupling to the cooking-utensil per unit length of the wire in the coil equals the magnetic coupling in the single coil case according to Fig. 1. Here, the magnetic coupling is defined as the inductively transferred power per Ampere of current and per meter of wire and the magnetic coupling is also a measure of the efficiency.