GB2338778A

GB2338778A - Induction heated cooktops

Info

Publication number: GB2338778A
Application number: GB9813768A
Authority: GB
Inventors: Antoine Dubedout
Original assignee: Electrolux AB
Current assignee: Electrolux AB
Priority date: 1998-06-25
Filing date: 1998-06-25
Publication date: 1999-12-29
Also published as: GB9813768D0

Abstract

In the past, induction heated cooktops were only used with cooking vessels having a conductive bottom. In order to adapt other types of cooking vessel (8) for use with the induction heating principle, an electrically resistive disc (6, 6')is fitted in the top plate (1) in proximity to the induction heating coils (2). The disc (6, 6') heats up because it acts as a load on the induction heating coils, and the heat is carried by conduction to the cooking vessel. The resistive disc (6') may be placed in a depression in the top plate (1). The disc (6') may be removed, ensuring child safety since there will be no heating by accidental turning on.

Description

2338778 1 An induction heating cooking zone.

The invention relates to a cooking zone in a cooktop utilising induction for the transfer of energy to cooking vessels. in induction heating for cooking purposes it is generally required that the cooking vessel has particular magnetic and electrically resistive properties in order that the induction energy provided by the induction coil is attracted into the cooking vessel in order to create eddy currents in the cooking vessel which in turn create the heat required.

The attractiveness of induction heating is that it is very controllable. The only disadvantage, however, is that totally insulating cooking vessels, such as glassware, cannot be used, although it might be desired for esthetic reasons. Similarly, tinned copperware is also excluded, because the resistance is too low to generate heat.

This state of affairs has generally been accepted, although there is in reality a requirement for a construction that would permit free use of cooking vessels and still have the advantage of needing only one type of cooking hob, such as an induction heatina hob.

According to the invention this requirement is fulfilled in that an induction heating zone is provided with an electrically resistive disc wich a suitable resistivity fitted above and essentially concentric thereto, which disc is adapted to carry a cooking vessel and to transmit the heat generated in the disc to the cooking vessel- In this manner, the induction erTergy available at the cooking zone is used with full efficiency to generate heat, and those cooking vessels which do not lend themselves to induction heating can obtain the heat from the heated conductive disc instead. ono particular advantage is related to the fact hat a 1 I,- cooking vessel not adapted to inductive heating would leave a large amount of stray flux (unless there are safety features present which switch off the energy during no load conditions). However, due to the ccnduc- cive disc, the stray field is no ionger dependent on the type of cooking vessel, nor on its position.

In. a preferred embodiment, the disc is removabl placed in a depression in the top surface of the cookrop. This creates several advantages, such as the possiblility to use vessels suitable for induction heating when the disc is removed and other vessels when the disc is in place. One might say that the conductive disc constitutes an adaptor for an induction heating cooktoD. Furthermore, there is improved cleanability when the disc is removed, because there may be provided a smooth transfer between the surround and the depression.

The depression may be deeper than the part of the disc hanging down into it. This creates an insulating layer of air which means that the heat Fenerated in the disc is not conducted into the cooktop itself. The disc may be removed when the cooktop is not in use, thereby preventing inadvertent heat build-up, and thereby improving child safety. The disc may also be given a coat of temperature sensitive material in order to signal that it is hot.

in a further embodiment, the disc top surface i adapted to conform closely to the bottom of a cooking vessel at all temperatures. The advantage of this is that a cooking vessel may be supplied in the sale situation with its individual adaptor in order to provide optimal thermal contact when it is to be used. This means that a cooking vessel of e-g. toughened glass may on one cooking range be used with ligh radiation from below, and on a cooking range according to the present invention it may be used with induction heating.

In a further embodiment, a loose handle is used to 3 remove the disc, said handle conea.:.ning permanent magnets and the disc having a Curie point such that the disc will only be lifted when the temperature is low enough. This prevents handling of the disc which could 5 constitute grave danger.

In a further embodiment, the cooking zone contains coils which are energized in such a way that the disc is lifted by electromagnetic levitation when it is not held in place by a cooking vessel. In this manner a very vivid indication of a non-load condition is obtained.

The invention will be described in detail in the following with reference to the drawing, in which Fig. 1 shows an induction heating assembly having concentric hole in the top plate for a resistive disc, and Fig. 2 shows an induction heating assembly having concentric depression in the top plate into which fits resistive disc.

In Fig. 1. is shown a cooktop i in a glass-ceramic material providing a heat tolerant and mechanically resistant surface. Below the cooktop an induction coil assembly 2 is fitted, consisting of a core 3 and a pancake shaped coil 4. Above the induction coil assembly there is created a hole 5 through the cocktop, into which is fitted a disc shaped element 6. The disc has a suitable resistivity below that of pure copper. The disc 6 creates the secondary winding of a transformer for ELF energy created together with the induction coil assemblY, and when the ELF energy (20-100 kHz) is applied, the disc 6 heats up. Stray radiation may be controlled by extensions 7 to the disc. The heat is conducted into a cooking vessel 8 which is in intimate thermal contact with the disc 6. Such cooking vessels can be of any kind which would not otherwise be heacable by ELF energy.

In Fig. 2, the same elements may be found, but in scead of a through hole 5 there is provided a depression 4 S' which has an edge forming part of an inverted conical surface, corresponding tn a bevelled edge on the plate 61. This means that:he plate is actually removeable. This occurs predominantly in three situations: i) for cleaning, 2) when it is desired to use the cooktop with suitable vessels for induction heating of same, and 3) when preventing accidental burning by touching the discs which become hot when ELF energy is supplied. Ordinary safety measures for induction cooktops which prevent the supply of ELF energ-.v in a no-load condicion would not regard the fitting of a disc 6 without a cooking vessel 8 as a no-load situation. The bevelled edge of the disc 6' means that when it expands upon heal,-ng, it will lift itself very slightly in the depression, but the cooktop will be completely tree of thermal stresses due to this expansion.

However, there is provided a temperature sensor 9 which sits at the bottom of the depression 5', and this may monitor either the temperature of:he disc 61 in case the disc goes all the way down to the bottom of the depression, or the temperature of the induction coil assembly in case there is provided an insulating airgap between the bottom surface of the disc 61 and the bottom of the depression 5J. In order to maintain a high efficiency it is undesirable for the induction coil assembly no reach elevated temperatures.

The disc 61 may be provided with a ribbed lower surface no function as a shallow skillet or roasting surface when inverted; in this case there would be two opposing bevelled edges. This means that this type of Cooking may be performed directly on the cookcop.

l P A T E N T C L A 1 M S 1. k cooking zone in a cooktop (1) utilising induction for the transfer of energy to cooking vessels (9), c h a r a c t e r i z e d i n that an induction heating zone (2) is provided with an electrically resistive disc (6, 6') with a suitable resistivity fitted above and essentially concentric thereto, which disc is adapted to carry a cooking vessel and to transmit by conduction the heat generated in the disc (6, 61) to the cooking vessel (8).

2. A cooking zone according to 1 claim 1, i n that the disc (61) is removably placed in a depression (51) in the top surface of the cooktop (1).

3. A cooking zone according to claim 1, c h a r a c t e r i z e d i n chat the disc top surface (6, 61) is adapted to conform closely co the bottom of a cooking vessel (8) at all temperatures 4. A cooking zone according to claim 1, c h a r a c t e r i z e d i n that a loose handle is used to remove the disc (61), said handle containing permanent magnets and the disc (61) having a Curie point such that the disc will only be lifted when the temperature is low enough.

S. A cooking zone according to claim 1, c h a r a c t e r i z e d i n that the cooking zone contains coils which are energized in such a way that the disc (61) is lifted by electromagnetic levitation when it is not held in place by a cooking vessel.

6. A cooking zone according to claim 1, c h a r a c t e. r i z e d i ri that the disc (6') is provided with a coat made in a temperature sensitive material which changes colour when nhe temperature is 3-9 above a pre-determined value.

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