GB2325134A

GB2325134A - An induction heating oven cavity

Info

Publication number: GB2325134A
Application number: GB9805307A
Authority: GB
Inventors: Christian Eskildsen
Original assignee: Electrolux AB
Current assignee: Electrolux AB
Priority date: 1997-03-13
Filing date: 1998-03-12
Publication date: 1998-11-11
Also published as: GB9805307D0

Abstract

An oven cavity is energized by means of eddy currents induced inside the cavity walls 4a, in baking plates or in a suitable baking tin 3 by means of externally installed induction heating units. The tin comprise ferromagnetic material, and may have pole pieces to increase the flux through the tin. The unit may comprise a laminated ferromagnetic material mantle 3 and a coil 1 or a series of coils and associated mantle pieces.

Description

An induction heating oven cavity.

The invention relates to an oven cavity comprising side walls, a top, a bottom, and a door. Such cavities are energized by various well-know means, such as resistance heating of the walls and bottom, using radiation as the transport mechanism for heat energy, by circulation of heated air, by steam, and by microwaves.

Dependent on the source of heat the food behaves differently during processing. A low thermal time constant is obtained when as little as possible outside the food is heated, and the heating of the whole cavity assembly requires thermal insulation of the whole cavity in order not to radiate too much heat to the outside.

Microwaves are inherently efficient in that the energy is predominantly generated in the food itself, however microwave radiation outside the cavity has to be avoided, and this requires complex sealing at the door.

Harmful microwave radiation may be present outside the cavity, even though it is not noticeable as a loss in heating efficiency inside the cavity.

There is hence a need to obtain a functionality similar to that of microwaves without the risk of harmful radiation. Induction heating has made its mark in cooktops and are so safe that they are used without more shielding than that required for efficient operation. In this case ELF (20kHz-lOOkz) is used.

However, as the heat is generated in the cooking vessel, or indeed in any ferromagnetic plate with a suitable resistance to Foucault currents, simple transfer of induction heating elements to the outside of the plates of an oven cavity would only mean that now induction heating rather than galvanically connected resistance heating were used. In effect this would just be a different type of radiant heat oven cavity.

It is the purpose of the invention to provide an oven cavity with induction heating without the disadvantages mentioned. This is obtained in that an induction field of ELF is established inside the cavity of the oven.

According to one embodiment of the invention, such a field is established by means of a mantle of ferromagnetic material surrounding the cavity and provided with an alternating magnetic field by means of a coil supplied with ELF energy. Such a mantle simutaneously acts as a shield against radiation of ELF energy outside the cavity. The mantle may comprise ferrite material which has the highest known relative permeability which is above 2000.

In another embodiment, the mantle is used as pole pieces for creating a non-uniform induction field inside the cavity, at least 80W of the total field being present in a cross section comparable in dimension to a baking tin or similar cooking vessel disposed in the cavity.

In an advantageous embodiment, the mantle is built as a laminate of very thin laminations of directional permeability material of varying lenghts, the maximum thickness of which being present inside the energizing coil. Such laminations have a very high permeability, and the mantle has to be annealed after shaping.

Preferably, in this case the mantle is integrated with the cavity for mutual mechanical reinforcement. The field lines will exit at the ends of each lamination which in essence forms a minute, narrow but long pole.

In a further advantageous embodiment, the coil and mantle are subdivided into a number of cooperating parallel coils and mantle pieces. This means that a localized field may be obtained by selective energization and by interference between mantle pieces.

In a further embodiment, removeable pole pieces are disposed inside the cavity to shape the field to the cooking vessel to be used. This has the advantage that several types of cooking vessel can be used optimally.

In a further advantageous embodiment, the cooking vessel itself carries pole pieces for attracting and receiving field lines.

The invention will be described in greater detail with reference to the drawing which shows a cavity with a mantle and coil according to the invention.

The drawing shows a coil 1 which is supplied with ELF energy (in the range 20 kHz to 100 kHz) and which creates magnetic field lines 2 in a magnetic core structured in a mantle shape 3. The mantle 3 does not completely surround the cavity 4 which is in a nonconductive material and consists of top 4a, bottom 4b and side walls 4c, 4d. Inside the cavity 4 there is disposed a baking tin 5 with up-turned edges 6. The structure of the core 3 is such that the field lines 2 are forced to exit, partly because the cross section is reduced above and below the cavity, and partly because there is a gap 7 in the core. It does not detract from the idea of the invention to provide the coil on top, having the gap 7 below the cavity or any other orientation commensurate with the need to build the oven into a cabinet.

Claims

PATENT CLAIMS

1. An oven cavity comprising side walls, a top, a bottom, and a door, c h a r a c t e r i s e d i n that an induction field of ELF is established inside the cavity of the oven.

2. An oven cavity according to claim 1, c h a r a c t e r i s e d i n that the induction field is established by means of a mantle of ferromagnetic material surrounding the cavity and provided with an alternating magnetic field by means of a coil supplied with ELF energy.

3. An oven cavity according to claim 2, c h a r a c t e r i s e d i n that the mantle is used as pole pieces for creating a non-uniform induction field inside the cavity, at least 80% of the total field being present in a cross section comparable in dimension to a baking tin or similar cooking vessel disposed in the cavity.

4. An oven cavity according to claim 2, c h a r a c t e r i s e d i n that the coil and mantle are subdivided into a number of cooperating parallel coils and mantle pieces.

5. An oven cavity according to any of the claims 2-4, c h a r a c t e r i s e d i n that the mantle is integrated with the cavity for mutual mechanical reinforcement.

6. An oven cavity according to any of the claims 2-5, c h a r a c t e r i s e d i n that the mantle comprises ferrite material.

7. An oven cavity according to any of the claims 2-5, c h a r a c t e r i s e d i n that the mantle is built as a laminate of very thin laminations of directional permeability material of varying lenghts, the maximum thickness of which being present inside the energizing coil.

8. An oven cavity according to any of the above claims, c h a r a c t e r i s e d i n that removeable pole pieces are disposed inside the cavity to shape the field to the cooking vessel to be used.

9. An oven cavity according to any of the above claims, c h a r a c t e r i s e d i n that the cooking vessel itself carries pole pieces for attracting and receiving field lines.