EP1062841B1

EP1062841B1 - A cooktop

Info

Publication number: EP1062841B1
Application number: EP99902514A
Authority: EP
Inventors: Christian Eskildsen; Pelle ÖSTLUND
Original assignee: Electrolux AB
Current assignee: Electrolux AB
Priority date: 1998-02-10
Filing date: 1999-02-10
Publication date: 2006-06-21
Anticipated expiration: 2019-02-10
Also published as: AU2266099A; DE69932039D1; WO1999041949A3; WO1999041949A2; DE69932039T2; EP1062841A2; ATE331419T1

Abstract

A replaceable cooktop for use with induction heating of cooking vessels is provided by separating the coil structures from the top surface. The top surface is a slab of an insulating material, such as wood, marble, or plastic provided with cavities on the lower side which correspond closely to the coil structures projecting from the mechanical structure below. The top is perfectly flat and may be used as a general kitchen worktable.

Description

The invention relates to a cooktop using induction at elevated frequencies for the transfer of energy to cooking vessels placed on said cooktop.
Three main advantages are known in respect of the use of induction heating for cooktops, 1) the surface may be completelty flat and hence easy to clean, 2) the surface does not become hot due to the primary transfer of energy, 3) the cooking vessel does not need to have a plane bottom surface. A further advantage is: the thermal time constant of heating is basically only dependent on the cooking vessel..
In latter years vanous attempts have been made to integrate the cooktop in a kitchen environment by suitable choice of colours and other styling features. Even natural materials, such as marble or wood have been suggested for cooktops, using the induction principle, such as in DK 0279/97. In this case, magnetic core material is cast in situ in the natural material, either visible from the top or invisibly, and the effect is that a kitchen designer is able to install what looks like an ordinary work surface, but certain areas are able to provide heat to cooking vessels placed thereon. In this construction, the coil-and-core structure is integrated into the kitchen table, and when this is worn, the whole unit must be changed which is very similar to changing an old-fashioned installation.
It has been determined that such known constructions do not provide the functionality desired in a modem kitchen, in particular that there is a discrepancy between a source of inductive power potentially having a very long service life, and a surface, in particular in wood, which gets worn through use. It is the purpose of the invention to provide such greater functionality, and this purpose has been achieved in a cooktop of the kind referred to above which has obtained the characterizing features indicated in claim 1. This in effect means that a strong wooden slab may be milled from behind so that the slab fits over the projecting coil structures, and the thin bottom of the recesses or cavities will be supported directly by the projecting coil structures. For reasons of concentration of magnetic flux it is desirable to make the remaining thickness of wood fairly small. The slab of wood may be chosen to fit in with the remaining wooden structures in the kitchen, and indeed food may be prepared directly on the kitchen table, the wood forming a suitable basis for knives, etc.
In an advantageous embodiment of the invention, the top of the projecting coil structures are slightly upwardly curved, the height of crown being in the interval 0.2-1.5 mm. This has the advantage that the support of the thin remaining layer of wood is increased, and the wood may hence be almost veneer-like in character and still retain its strenght.
In a further advantageous embodiment of the invention, the radius of curvature of the curved top is larger in the plane of the direction of the grain of the wood than across the grain. The top surface of the projecting coil structures thereby become ellipsoidal in character. This embodiment takes into account the difference in flexibility of the veneer along and across the grain.
The invention will be described in greater detail in the following with reference to the drawing, in which

Fig. 1 shows a set of induction coil structures without any cooktop,
Fig. 2 shows a cooktop with recesses or cavities corresponding to the placement of the induction coil structures from below, and
Fig. 3 in exagerration shows the cupola shape of an induction coil according to an embodiment of the invention.

In Fig. 1 is shown four induction coil structures 1, 2, 3, and 4 which comprise a core of ferrite or magnetically conducting concrete and a coil, energised by a source of ELF power which is not shown. The induction coil structures are disposed below a protective layer 5 in one piece, although they protrude like low cylinders or truncated cones above the plane of the support 6. The protective layer 5 is thin, as it is completely supported by the coil-and-core structures and the support plane. It does not have to be heat resistant, because the only heating it will experience are losses in the coil and possible conduction of heat from the cooking vessels above. The protective layer may be vacuum molded in a thermoplastic material.
In Fig. 2 is shown a slab 7 of wood or other natural material or a plastic material. Cavities or recesses 8, 9, 10, and 11 are formed so that the slab may be lowered on top of the support plane with the induction coil structures in such a way that each induction coil structure 1, 2, 3, and 4 is completely enclosed in the cavity, with the top of each coil structure supporting the thin layer 12, 13, 14, and 15 of material remaining after shaping the cavities. The material for the slab only needs to fulfil the conditions of thermal and electrical insulation, and that the strenght of the remaining thin layer is sufficient not to tear when a kitchen utensil slides across it. The top is perfectly flat and may be used as a general kitchen worktable.
In Fig. 3 is shown an embodiment of the coil structure which ensures that the thin layer 12, 13, 14, and 15 is fully supported. This is obtained by a very small stretching of the thin layer due to a very small curvature of the top of the induction coil structures 1, 2, 3, and 4. Due to the difference in elasticity in the two directions in wood, viz. along and across the grain it has been determined to be advantageous to provide a difference in the radius of curvature in the two directions. The radius of curvature is in practice in the order of 1 to 5 m. In case the protective layer 5 is a thin sheet of e.g. polyester, the slight curvature is provided in the induction coil, and tightening the sheet transfers this curvature to the top supporting the thin layer 12, 13, 14, and 15.

Claims

A cooktop using induction at elevated frequencies for the transfer of energy to cooking vessels placed on said cooktop,
characterised in that it comprises a support (6) for a number of induction heating coil structure (1,2,3,4) projecting from the support (6) and covered by a protective surface (5) and a corresponding table in wood or other insulating material (7) having recesses (8,9,10,11) on the lower side corresponding to the projecting coil structures (1,2,3,4), the thin bottom of the recesses (12,13,14,15) being supported by the projecting coil structures (1,2,3,4).
A cooktop according to claim 1 in particular for use with a wooden material, characterised in that the top of the projecting coil structures (1,2,3,4) are slightly upwardly curved, the height of crown being in the interval 0.2-1.5 mm.
A cooktop according to claim 2 made in a wooden material, characterised in that the radius of curvature of the curved top is larger in the plane of the direction of the grain of the wood than across the grain.