WO1997020451A1

WO1997020451A1 - An induction heating hotplate

Info

Publication number: WO1997020451A1
Application number: PCT/DK1996/000494
Authority: WO
Inventors: Laurent Jeanneteau; Christian Eskildsen
Original assignee: Aktiebolaget Electrolux
Priority date: 1995-11-27
Filing date: 1996-11-27
Publication date: 1997-06-05
Also published as: AU7621896A

Abstract

In an induction hotplace the surface available for high frequency currents must be high in relation to the cross section. This is traditionally obtained by means of Litz wire which is expensive because each individual wire has to be lacquer insulated. According to the invention sufficient surface area is obtained by using a strip of conductive material which in a horizontal pancake coil sits vertically. By supporting the strip without enclosing it, cooling air may be circulated. Alternatively, molten metal may be cast into the core material, ensuring an intimate thermal contact.

Description

An induction heating hotplate

The invention concerns an induction heating hotplate comprising a coil and a core structure in a magnetic material .

Hotplates for induction heating of food usually consists of a disc shaped coil which is disposed in conjunction with magnetic material so as to shape the magnetic field to engage a vessel to heated. In WO94/05137 it is described how such a coil is embedded in a concrete-like material with the core structure made of a magnetic concrete.

The coils for such hotplates are habitually manufactured according to the Litz principle, i.e. the conductor consists of a number of individually insulated conductors which are joined at the ends and run in parallel but scrambled like in a telephone cable rather than in a uniform rope fashion. This is due to the skin effect which is considerable at the ultrasonic frequencies involved and which means that the centre of a conductor does not contribute to conduction of these frequencies. Litz wire in great lenghts is expensive because its manufacture comprises a large number of steps. Manufacturers of induction hotplates hence resort to local manufacture of the shorter pieces needed for the coils, but this is a complication requiring a large number of bobbins which have to be reloaded before each new piece. Also, the individual connection of each and every wire to the circuit puts a special demand on workmanship which is difficult to automate.

It is the purpose of the invention to improve the structure of coils for induction heating hotplates avoiding the use of a Litz configuration. This is obtained in a hotplate according to the invention which is particular in that the coil is a single conductor spiral coil which maximizes the surface with respect to the cross section of the conductor material.

An advantageous embodiment of the invention uses a strip shaped conductor with its flat side perpendicular to the cooking surface. This is an efficient use of the conductor and one which has good cooling properties.

A further advantageous embodiment of the invention is particular in that the conductor is hollow. In this way the use of conducting material not contributing to conduction is avoided to a large measure.

A further advantageous embodiment is particular in that the hollow conductor is in the form of an extruded tube. This provides the particular advantage that the coil assembly may cool down quickly.

A further advantageous embodiment supports the conductor material by means of a structure which is open to air circulation. This brings the further advantage that convective cooling may be used efficiently to permit a higher thermal loading of the conductive material .

A process for the manufacture of a coil according to the invention is particular in that molten conductor material is cast in situ in a spiral shaped channel in an electrical insulator.

The advantage of the configuration lies in that its insulation is much simpler than in the known case and that the use of an insulating support will permit casting in situ. A rough surface would even be tolerated in the insulating support which may hence itself be cast in an insulating material before pouring the conductor.

A further process for the manufacture of a coil according to the invention is particular in that the channel is formed partly of heat tolerant insulating material and partly of removable pieces of mould.

Aluminium presents itself as having particular advantages, because it iε both more environmentally friendly and less expensive for equivalent current carrying capacity than copper. This material is alεo simple to cast and to extrude.

The invention will be described in greater detail in the following with reference to the drawing, in which Fig. 1 shows the lay-out of a coil according to the invention,

Fig. 2 shows cross sections of conductors which are useful in the invention, In Fig. la is shown an electrical hotplate of the induction type constructed according to the invention. A spiral shaped conductor strip 1 is supported by electrically insulating struts 2 which project upwards from the bottom part 3 of the well into which the coil is fitted. The bottom part is made of magnetically conductive material, εuch as a magnetic concrete. At the edges of the well, the magnetically conductive material rises skirt-like around the coil. This construction permits air-cooling of the coil 1 due to the provision of holes 4 in the bottom part. At the centre C and at the edge E there are thicker struts made of the same conductor material to which electrical connections are made.

In Fig. lb is shown a vertical section through a coil 1 which has been manufactured by casting in situ in electrically insulating material 5 which has a magnetically conductive material 6 surrounding it.

In Fig. 2 are shown various cross-sections of conductor 1 which perform according to the invention when formed into a spiral. A typical coil has 20 turns of increasing diameter and a lenght of conductor of ca. 9 .

Claims

P A T E N T C L A I M S

1. An induction heating hotplate comprising a coil and a core structure in a magnetic material c h a r a c t e r i s t i c i n that the coil is a single conductor spiral coil which maximizes the surface with respect to the cross section of the conductor material.

2. A hotplate according to claim 1, c h a r a c t e r i s t i c i n that a strip shaped conductor is used with its flat side perpendicular to the cooking surface.

3. A hotplate according to claim 1, c h a r a c t e r i s t i c i n that the conductor is hollow.

4. A hotplate according to claim 1, c h a r a c t e r i s t i c i n that that the hollow conductor is in the form of an extruded tube.

5. A hotplate according to claim 1, c h a r a c t e r i s t i c i n that the conductor material is supported by means of a structure which is open to air circulation.

6. A procdure for the manufacture of a hotplate according to claim 2, c h a r a c t e r i s t i c i n that molten conductor material is cast in situ in a spiral εhaped channel in an electrical insulator.

7. A procedure according to claim 6 for the manufacture of a hotplate according to claim 5, c h a r a c t e r i s t i c i n that the channel is formed partly of heat tolerant insulating material and partly of removable pieces of mould.