INDUCTION HEATING ELEMENT

Abstract: A heating element for an inductive cooking hob comprises a flat spirally-wound coil (30) consisting of a plurality of parallel insulated conductors. The coil is wound in a spirally-extending groove (32) disposed on the upper surface of a plastics former (31). A circular base (33) of highly magnetically permeable material is disposed on the underside of the former (31). The base (31) focuses the magnetic field generated by the coil into a heating area above the coil, and also prevents any surrounding metalwork from inadvertently being heated.

Description: Induction Heating Element This invention relates to an induction heating element. Cooking hobs are known which comprise a large induction heating coil, on which pans having a conductive base can be stood. In use, a high frequency electrical signal (in excess of 20 KHz) is applied to the coil, which generates a magnetic field that induces secondary currents in the pan base. The base of the pan is not an ideal conductor, thus as the current flows electrical energy is dissipated as heat in the base of the pan. This type of heating is able to deliver up to 5K of heating power directly into the base of the pan. Thus, cooking hobs of this type are extremely fast and efficient. Generally the hob comprises a flat spirally wound coil connected at its opposite ends to a high frequency current supply. The magnetic field around the coil induces a current in any conductive objects which come within the field. Thus, the coil has to be spaced apart from metal components in the housing of the hob, in order to avoid any undesirable heating of these. The efficiency of space utilisation in induction cooking hobs is therefore poor, owing to the large space isolation required around the coils. Thus in accordance with this invention as seen from a first aspect, there is provided an induction heating element comprising a coil and an element of magnetically permeable material disposed adjacent at least an external part of said coil. In use, the lines of magnetic flux around the coil flow through the magnetically permeable element rather than through the surrounding casing of the hob, which is less permeable. Preferably one side of the coil is exposed so that the magnetic field is projected outwardly from that side to heat conductive cooking utensils inductively coupled with the coil. Preferably said magnetically permeable element comprises a circular base on which the coil is mounted. Additionally or instead, said permeable element encircles the coil. The circular base shields any surrounding metalwork from the magnetic field and also focuses the field upwardly towards the heating area. Preferably the coil is flat. Preferably the coil comprises a spirally-wound conductor. The conductor is preferably disposed on the surface of a flat former. Preferably the former comprises a spirally-extending groove in its upper surface for receiving the conductor. Preferably the former comprises a plastics material, preferably having a high thermal conductivity. The material of the former enables any heat in the coil to be rapidly and evenly transferred. Preferably the circular base is disposed on the underside of the former. It will be appreciated that the former keeps the surface of the coil flat. Uneven electromagnetic coupling with the pan will occur if the surface of the coil is uneven, and thus hot spots will occur owing to the uneven energy transfer. A flat surface to the coil minimises hot spots, due to each turn being exactly the same distance from the pan. Preferably the element comprises ferrite material, which raises the inductive reactance of the coil. Induction heating coils are generally driven by a high- frequency supply current. At these high frequencies current tends to flow only in a thin layer adjacent the surface of the conductor, rather than in its full cross sectional area. This so-called skin effect causes increased I 2 R losses, owing to the increased current density flowing through the surface layer of the conductor. Thus, in accordance with this invention as seen from a second aspect, there is provided an induction heating element having a flat spirally-wound coil which comprises a plurality of parallel connected conductors. Thus, at high frequencies the skin effect is minimised by using a plurality of conductors to pass the current. The combined surface area of these conductors is much greater than the effective surface area of a conventional conductor, thus the I 2 R losses are minimised. Preferably the conductor comprises a plurality of conductive strips or circular-section conductors connected in parallel. Preferably the conductors are insulated from each other. Preferably the conductors form a winding, which is also preferably insulated. A large proportion of the supply current to inductive cooking hobs is dissipated as heat in the centre of the coil. It has been proposed to remove the centre of the coil in order to alleviate this heating problem, however this leads to uneven heating of the pan. We have now devised an induction heating element which alleviates this problem. In accordance with this invention as seen from a third aspect, there - is provided an induction heating element comprising a flat spirally wound coil disposed around a core of magnetically permeable material. Thus, any uneven heating of the pan is avoided by utilising a magnetically permeable core which directs the magnetic field towards the centre of the coil. Heating of the centre of the coil is alleviated since the centre has been replaced by the magnetically permeable core. Preferably the core comprises ferrite. Embodiments of this invention will now be described by way of examples only, and with reference to the accompanying drawings, in which: FIGURE 1 is a sectional view through an embodiment of induction heating element in accordance with this invention; FIGURE 2 is a sectional view of the induction heating element of Figure 1 showing the generated magnetic lines of flux; and FIGURE 3 is a sectional view through a second embodiment of induction heating element in accordance with this invention. Referring to Figure 1 of the drawings, there is shown an induction heating element comprising a flat spirally-wound coil 10. The coil 10 is would around a central cylindrical core 11 of highly magnetically permeable material such as ferrite. The coil is mounted inside a housing 12 having a circular base 13 and an upstanding peripheral wall 14. The housing 13 is also formed from highly magnetically permeable material such as ferrite, and the core 11 may be formed integrally with its base 13. The coil 10 comprises a single winding of a rectangular or square-section conductor 20. The conductor 20 comprises a plurality of conductive strips 21 coated with an insulating material (not shown) , contained within an outer insulating coating 22. Insulated supply terminals 23 are connected to opposite ends of the conductor 20 and project through the base of the housing 12. In use, when a high frequency supply is connected between the terminals 23, the coil generates a magnetic field as shown in Figure 2. The lines of magnetic flux extend through the base 13 and peripheral wall 14 of the magnetically permeable housing and through the core 11. The upper surface of the heating element is open, and thus the lines of magnetic flux extend upwardly into free space. Conductive cooking utensils placed on the upper surface of the heating element inductively couple with the coil 10, and secondary alternating currents are developed in the utensils. These currents develop I 2 R losses owing to the resistance of the material from which the utensil is formed: this generated heat may be used for cooking. Referring to Figure 3 of the drawings, there is shown an alternative embodiment of induction heating element. The heating element comprises a flat spirally-wound coil 30 disposed on a circular former 31 formed from a plastics material having a high thermal conductivity. The former 31 comprises a spirally-extending groove 32 in its upper surface, in which the conductor of the coil is wound. A flat circular base 33 of a highly magnetically permeable material, such as ferrite, is recessed into the underside of the former 31. The coil 30 comprises a single circular-section winding which consists of a plurality of circular-section conductors coated with an insulating material. The winding is twisted from end-to-end, such that each conductor extends in a helical fashion. This arrangement gives a shallow coil which is less susceptible to skin effects, and thus runs at a lower temperature than conventional coils. The coil winding is sufficiently flexible to enable the coil to be wound almost to the centre, so that uneven heating of the pan is avoided.

Claims: CLAIMS 1. An induction heating element comprising a coil and an element of magnetically permeable material disposed adjacent at least an external part of said coil. 2. An induction heating element as claimed in claim 1, in which one side of the coil is exposed. 3. An induction heating element as claimed in claims 1 or 2, in which the magnetically permeable material comprises a circular base on which the coil is mounted. 4. An induction heating element as claimed in claim 3 in which the coil is flat. 5. An induction heating element as claimed in claim 4, in which the coil comprises a spirally wound conductor. 6. An induction heating element as claimed in claim 5, in which the coil is disposed on the upper surface of a flat former. 7. An induction heating element as claimed in claim 6, in which the base is disposed on the under surface of the former. 8. An induction heating element as claimed in claims 6 or 7, in which the upper surface of the former comprises a spirally-extending groove which receives the conductor of the coil. 9. An induction heating element as claimed in any of claims 6 to 8, in which the former comprises a plastics material. 10. An induction heating element as claimed in claim 9, in which the plastics material has a high thermal conductivity. 11. An induction heating element according to any preceding claim, in which the magnetically permeable material encircles the coil. 12. An induction heating element having a flat spirally- wound coil which comprises a plurality of parallel connected conductors. 13. An induction heating element as claimed in claim 12, in which the conductors form a winding. 14. An induction heating element as claimed in claim 13, in which the winding is circular section. 15. An induction heating element as claimed in claims 13 or 14, in which the conductors are circular section. 16. An induction heating element as claimed in claim 13, in which the conductors comprise strips of conductive material. 17. An induction heating element according to any of claims 12 to 16, in which each conductor is insulated. 18. An induction heating element as claimed in claim 13, in which the winding is insulated. 19. An induction heating element comprising a flat spirally-wound coil disposed around a core of magnetically permeable material. 20. An induction heating element as claimed in claim 19, in which the cofcl is disposed on a base of magnetically permeable material. 21. An induction heating element as claimed in claims 19 or 20, in which the coil is encircled by a peripheral wall of magnetically permeable material.

Inventors: SNOWBALL MALCOLM ROBERT, SNOWBALL, MALCOLM, ROBERT
Applicants: INDUCED ENERGY LTD, SNOWBALL MALCOLM ROBERT, INDUCED ENERGY LTD., SNOWBALL, MALCOLM, ROBERT
Application year: 1995
Main classification: H05B6
IPC codes: H05B6