Abstract:
An induction heating device and method are disclosed. The device may include a support plate and a coil assembly and a plurality of ferrite bars or the like interposed between the coil assembly and the support plate. Additionally, a central polymeric element may be directly in contact with and interposed between the coil assembly and the support plate. The central element may have a plurality of radial seats configured for engagement with at least one end of the ferrite bars, the seats are configured to locate the bars in a predetermined position.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an induction heating device comprising a support plate and a coil assembly, between the coil assembly and the support plate being interposed a plurality of ferrite bars. With the term “ferrite bars” we mean any kind of elongated magnetic field concentrator located beneath the coil assembly. 
       BACKGROUND 
       [0002]    The above heating devices are used to heat and cook food thanks to heat energy generated in a ferromagnetic container placed above the coil assembly and supported on a ceramic glass or the like. 
         [0003]    The known induction heating devices consist of a series of layers dedicated to different functions. All of them are composed of different materials and geometries. In some solutions layers are connected with glue. All the parts (with the exception of the support plate, usually made of aluminum) have a hole in the center which is designed to allow the insertion of a sensor holder for a thermal sensor contacting the ceramic glass. On the top of the last layer, i.e. the coil assembly, there is a thermal insulating layer for instance of rock wool. The sequence of the layers, starting from the top is: rock wool&gt;glue&gt;copper coil&gt;glue&gt;mica&gt;glue&gt;ferrite&gt;glue&gt;aluminum base. Mica and copper coil are usually supplied as a single assembly. 
         [0004]    Other heating induction devices, for instance as described in EP0713350 and EP1560462, comprise a disk-shaped plastic support interposed between the support plate and the coil assembly and having a plurality of housings for containing the ferrite bars. Also in these solutions glue is used for holding together the different layers. 
         [0005]    All the above known solutions present drawbacks, either in view of the extensive use of glue which makes the assembly process quite complex and unreliable in terms of controlled final dimensions, or in view of the increase of cost due to the use of quite complex and large components as the disk-shaped plastic supports. Another problem is linked to the need of having, in an automated assembly process, a reliable and constant thickness of the flat induction heating device, particularly because the quantity of glue cannot always being dosed in a constant manner. 
       SUMMARY 
       [0006]    It is an object of the present invention to provide an induction heating device which does not present the above drawbacks and which is easy and economical to be produced. 
         [0007]    According to the invention, the above object is reached thanks to the features listed in the appended claims. 
         [0008]    The technical solution according to the present invention comprises a small and simple plastic central component that exploits the symmetric geometry of the coil assembly in order to design a core that can also preferably integrate the sensor holder and preferably connects the different layers with a quick mechanical snap-engagement fastening system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Further advantages and features according to the present invention will be clear from the following detailed description, provided as a not limiting example, with reference to the attached drawings, in which: 
           [0010]      FIG. 1  is a perspective view of a key component of the induction heating device according to the invention; 
           [0011]      FIG. 2  is an exploded view, partially cross sectioned, of an induction heating device according to the invention, which includes component of  FIG. 1 ; 
           [0012]      FIG. 3  is a perspective view from the bottom of the induction heating device of  FIG. 2 , in an assembled configuration; 
           [0013]      FIG. 4  is an enlarged perspective view of a central portion of the device of  FIG. 2 , in an assembled configuration, 
           [0014]      FIG. 5  is a schematic top view of how the key component of  FIG. 1  is used for fitting the ferrite bars; and 
           [0015]      FIGS. 6 and 7  are schematic views similar to  FIG. 5  showing different solutions for different layouts for ferrite bars position according to different coil sizes. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    With reference to the drawings, the induction heating device comprises a central component K composed of a co-injection of two different materials. With reference to  FIG. 1 , the component K comprises a plastic body  2  of thermoplastic material, on which is co-injected a central part of rubber, the sensor holder  1 . The plastic body  2  is made of thermoplastic or thermosetting polymer having a high Young&#39;s modulus. 
         [0017]    The shape of the component K recalls that of a snowflake, since it comprises a central portion  10  shaped as a regular polygon, for instance an hexagon (where the rubber or similar elastomeric material is centrally co-injected), on whose apexes  10   a  are integrally formed other regular auxiliary polygons  12 , in the shown example triangles. 
         [0018]    Such shape is due to the main technical purpose of the component K, i.e. to constrain the ends of ferrite bars  4  in the position required to channel or concentrate the electromagnetic field. Between each side  10   b  of the hexagon defining the central portion  10  of the component K and two facing sides  12   a  of two adjacent auxiliary triangles  12  it is defined a quadrangular seat  14  (open on top and bottom) for an end  4   a  of a ferrite bar  4 . In the example shown in  FIGS. 1 ,  2  and  4 , the central component K defines six seats  14  for the ferrite bars, but of course it can be formed with a different number of seats (as shown in  FIGS. 6 and 7 ). The thickness of the portion  10  corresponds to the thickness of the ferrite bars  4 , so that all such components can be sandwiched between other components of the induction heater (as it will be clear from the following description) in a very precise way in terms of final thickness. Moreover, the dimension of the seats  14  (which allows the assembly of the ferrite bars  4  with a predetermined degree of interference), together with the stiffness of the thermoplastic or thermosetting material allows a very stable position of the ferrite bars  4 , despite the forces acting on them during normal operation. 
         [0019]    The sensor holder  1  is a co-injection of rubber, with a total height slightly higher than the thickness of the central portion  10 , because the holder  1  needs to generate a spring effect to keep a sensor  8  ( FIGS. 2 and 4 ) pressed on the bottom face of the glass (not shown) of the induction cooktop. 
         [0020]    The central portion  10  of the component K is also provided with a plurality of elastic hooks  16  and  18  which are oriented parallel to the central symmetry axis of the component K. A first crown of upper hooks  16  can be seen in  FIGS. 1 and 2 , while a second crown of lower hooks  18  can be seen in  FIG. 3 . 
         [0021]      FIG. 2  shows the entire induction heater assembly with the central component K. A copper coil  6  is stuck on a mica layer  5  with glue and they form a single component. The coil is assembled in the following sequence: the central component K is connected to an aluminum base or plate  3  through the lower crown of hooks  18  with a light pressure in order to allow a snap-engaging action of the hooks  18  on a circular edge  22   a  of a central hole  22  of the base  3  ( FIG. 3 ). The central hole  22  with its edge  22   a  can be replaced by small slots (not shown) which can be snap-engaged by the lower crowns of hooks  18  for reaching the same technical result. The dimension of the elastic hooks  18  is selected in order to fix in a stable way the component K on the aluminum base  3  without the use of any glue. At this point of the assembly process, the ferrite bars  4  are collocated with interference in the defined seats  14 , obtaining a configuration (shown in  FIG. 5 ) in which the ferrite bars  4  are radially centered on the component K as rays of a star. Then the mica layer  5  and the copper coil  6  assembly is positioned on the central component K and with a little pressure (similarly to what already done for the base  3 ), connected with the upper hooks  16  so that they snap-engage with a circular edge  24   a  of a central hole  24  of the coil assembly, visible in  FIG. 4 . The component K is interposed and in contact with the aluminum base  3  and with the coil assembly, particularly with the mica layer  5  thereof. 
         [0022]    A thermal insulation layer  7 , for instance of rock wool, is then placed on the copper coil  6  and a temperature sensor  8 , for instance a NTC sensor, is joined to the central component K through its insertion in corresponding joints  26  ( FIG. 4 ) on the sensor holder  1  of the central component K, so that a mechanical constraint of the rock wool  7  is obtained. The rock wool layer  7  is preferably provided with a central hole  7   a  so that the sensor  8  is interposed between the sensor holder  1  and the ceramic glass of the induction cooktop. 
         [0023]    The embodiment shown in  FIG. 6  differs from the above in the different number of ferrite bars (eight instead of six) in order to match a different size of the coil and in the different shape of the central component K′ (octagonal and not hexagonal). Moreover the ferrite bars  4  and  40  have two different dimensions, with the longer bars  4  closer to the sensor holder  1 . 
         [0024]    In the embodiment shown in  FIG. 7  (for a coil having a large size) the central component K″ has an overall pentagon shape and it is adapted to house ten ferrite bars  4  and  42 , also in this case of different length. 
         [0025]    The solution according to the invention, independently on which embodiment is used, has many benefits in terms of cost reduction and improved assembly procedure. 
         [0026]    First of all, it is possible to get rid of the glue required, in the known solution, to position the ferrite bars on the aluminum base. Accordingly, there is also a decrease of the assembly time of the induction heating device and of the ferrite bars, by replacing the glue with snap-engaging fastener as hooks integral with a central simple component. There is a more accurate positioning of the ferrite bars, with a reduction in position variability caused by the unreliable quantity (and thickness) of the glue, and therefore a better control of the electromagnetic field in the working conditions of the induction heating device. It is also possible to avoid the use of glue required to position the mica layer on the ferrite, with a decrease of connection time of the mica layer by replacing the glue with snap-engaging fasteners. 
         [0027]    Last but not least it is possible to easily integrate the temperature sensor holder with a decrease of the number of components, increasing the stability of the sensor holder because it is no longer connected with fasteners, but it is part of a single body.