Patent Publication Number: US-2007108192-A1

Title: Induction Heating Apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a Divisional Application of U.S. application Ser. No. 10/521,876, filed Jan. 21, 2005, which is a Section 371 of International Application No. PCT/JP03/09844, filed Aug. 1, 2003, which was published in the Japanese language on Feb. 19, 2004, under International Publication No. WO 2004/016047A1. Each of these parent applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to an induction heating apparatus provided with an electrostatic shield between a matter to be heated and an induction heating coil.  
      Conventionally, as this kind of induction heating apparatus, for example, the apparatus described in Japanese Laid-open Patent Application No. Sho 61-16491 was available.  FIG. 9  is a view showing the equivalent circuit of the induction heating coil and its peripheral portion in this kind of conventional induction heating apparatus.  
      The configuration of the conventional induction heating apparatus will be elucidated below using  FIG. 9 . In  FIG. 9 , numeral  1  designates a top plate; an induction heating coil  2  is provided below the top plate  1 ; and a matter  3  to be heated is placed thereon. Numeral  4  designates an electrostatic shield applied to the bottom face of the top plate  1  and electrically connected to the low-potential portion of an inverter circuit (not shown) for driving the induction heating coil  2  via the electrode  4   a  of the electrostatic shield  4 . As the equivalent circuit of the peripheral portion, the equivalent capacitance C 1  between the induction heating coil  2  and the electrostatic shield  4 , the equivalent capacitance C 2  between the matter  3  to be heated and the electrostatic shield  4 , the equivalent resistance R 1  of a human body at the time when the human body makes contact with the matter  3  to be heated, and the resistance R 2  of the electrostatic shield  4  are shown.  
      In this configuration, when the matter  3  to be heated has low magnetic permeability and is a pan made of aluminum, copper, etc. having a low resistance, the frequency of the current flowing through the induction heating coil  2  is high in comparison with the case when the matter  3  to be heated is an iron pan, that is, a matter having high magnetic permeability and relatively large resistivity and being liable to generate Joule heat; hence, the peak voltage applied to the induction heating coil  2  becomes 1 kV or more.  
      In the case that the electrostatic shield  4  exists and is electrically connected to the low-potential portion as described above, the potential difference between the matter  3  to be heated and the electrostatic shield  4  becomes small, whereby a leak current at the time when a human body makes contact with the matter  3  to be heated is decreased significantly. Hence, safety is ensured even if the human body makes contact with the matter  3  to be heated.  
      When the electrostatic shield  4  is electrically connected to the low-potential portion of the inverter circuit for driving the induction heating coil  2 , a method wherein one end of a lead wire serving as the connection path thereof is connected to the electrode  4   a  of the electrostatic shield  4  applied to the top plate  1  by soldering or by contacting an elastic member, such as a spring, to which the one end of the lead wire is connected, and the other end of the lead wire is connected to the low-potential portion of the inverter circuit has been used generally.  
      However, in the case of the above-mentioned conventional configuration, the strength of the connection between the electrode  4   a  and the lead wire or the stability and reliability of the connection are insufficient; for example, because of some reasons, such as the occurrence of a tension force during a production process, reduction in the strength of solder owing to the heat from the matter  3  to be heated during cooking and the vibration or drop impact of the apparatus, the lead wire may be disconnected from the electrode  4   a , the spring terminal may be oxidized, the contact part between the electrostatic shield and the spring terminal is separated owing to vibration or the like, whereby there is a fear of causing a problem of increasing the contact resistance and preventing the function of the electrostatic shield  4  from performing sufficiently.  
      In order to solve the above-mentioned conventional problem, the present invention is intended to provide an induction heating apparatus capable of ensuring the electrical connection between the electrostatic shield and the low-potential portion of the inverter circuit and allowing the function of the electrostatic shield to perform sufficiently at all times.  
     BRIEF SUMMARY OF THE INVENTION  
      In order to solve the above-mentioned conventional problem, an induction heating apparatus in accordance with the present invention has a configuration wherein a stationary plate having electrical insulation is provided between a top plate and an induction heating coil, and the stationary plate is provided with an electrostatic shield and connection portions for connecting the electrostatic shield to a low-potential portion. Hence, production is facilitated and the influence of the high temperature of a matter to be heated on the electrostatic shield is relieved in comparison with the conventional method wherein the top plate is provided with the electrostatic shield. In addition, since, unlike the top plate, the stationary plate is not a component constituting the outer shell, it has freedom in the selection of shape and matter, and the connection between the electrostatic shield and the connection portions can have a stable configuration, whereby it is possible to ensure inexpensively connection with high reliability less susceptible to the influences of vibration of the apparatus, drop impact of the apparatus, etc. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
      In the drawings:  
       FIG. 1  is a cross-sectional view showing the configuration of the main section of an induction heating apparatus in accordance with Embodiment 1 of the present invention;  
      Part (a) of  FIG. 2  is a perspective view showing the cover for stationary plate of the induction heating apparatus in accordance with Embodiment 1 of the present invention, and part (b) of  FIG. 2  is a perspective view showing the stationary plate of the induction heating apparatus;  
      Part (a) of  FIG. 3  is a cross-sectional view (a cross-sectional view from the front) showing the main section of a configuration wherein a connection terminal of the induction heating apparatus in accordance with Embodiment 1 of the present invention is installed to the induction heating coil, and part (b) of  FIG. 3  is a cross-sectional view showing the main section from the right;  
       FIG. 4  is a cross-sectional view showing the insulation configuration of the connection terminal of the induction heating apparatus in accordance with Embodiment 1 of the present invention;  
       FIG. 5  is a cross-sectional view showing the connection between the electrostatic shield and a lead wire of the induction heating apparatus in accordance with Embodiment 1 of the present invention;  
       FIG. 6  is a magnified perspective view showing the main section in the vicinity of the connection portion of the connection terminal, viewed from the side of the top plate, in an induction heating apparatus in accordance with Embodiment 2 of the present invention;  
       FIG. 7  is a magnified perspective view showing the main section in the vicinity of the connection portion of the connection terminal, viewed from the side of the induction heating coil, in the induction heating apparatus in accordance with Embodiment 2 of the present invention;  
       FIG. 8  is a cross-sectional view showing the main section in the vicinity of the connection portion of the connection terminal in the induction heating apparatus in accordance with Embodiment 2 of the present invention; and  
       FIG. 9  is a cross-sectional view showing the configuration the conventional induction heating apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An induction heating apparatus in accordance with an aspect of the present invention comprises an induction heating coil for induction heating a matter to be heated, a top plate provided between the above-mentioned matter to be heated and the above-mentioned heating coil, driving means for supplying a high-frequency current to the above-mentioned induction heating coil, and an electrostatic shield having conductivity, provided between the above-mentioned top plate and the above-mentioned induction heating coil and connected to the low-potential portion of the above-mentioned driving means directly or via an impedance, wherein a stationary plate having electrical insulation is provided between the above-mentioned top plate and the above-mentioned induction heating coil, the above-mentioned stationary plate is provided with the above-mentioned electrostatic shield and connection portions connected to the above-mentioned electrostatic shield, and the above-mentioned electrostatic shield is connected to the above-mentioned low-potential portion via the above-mentioned connection portions.  
      With this configuration, since the electrostatic shield having conductivity, provided between the top plate and the induction heating coil and connected to the low-potential portion (a portion having a potential lower than that of the high-potential portion of the heating coil, for example, a power supply voltage to be input, a DC voltage after the rectification thereof or a potential close thereto) is provided, the electrostatic coupling between the high-voltage portion generated in the heating coil and the matter to be heated is decreased, and the high-frequency high voltage generated in the heating coil is applied to the body of the user via stray capacitance between the heating coil and the matter to be heated, whereby leak current flowing in the body of the user can be suppressed.  
      Since the stationary plate having electrical insulation is provided between the top plate and the induction heating coil and the stationary plate is provided with the electrostatic shield and the connection portions for connecting the electrostatic shield to the low-potential portion, production is facilitated and also the influence of the high temperature of the matter to be heated on the electrostatic shield is relieved in comparison with the conventional method wherein the electrostatic shield and the connection portions are formed on the rear face of the top plate. In addition, since, unlike the top plate, the stationary plate is not a component constituting the outer shell, it has freedom in the selection of shape and matter, and the connection between the electrostatic shield and the connection portions can have a stable configuration, whereby it is possible to ensure highly reliable connection which is less susceptible to the influences of vibration of the apparatus, drop impact of the apparatus, etc. Since the connection portions are separate from the top plate, the assembly work of the apparatus is facilitated. In addition, the connection portions can have various forms in accordance with circumstances, for example, a form using a method wherein a connector is connected to one end of each lead wire and the other end is connected to the electrostatic shield, and a form using a method wherein connection terminals are directly connected to the electrostatic shield.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since the connection portions are firmly secured and electrically connected to the electrostatic shield in a state wherein the connection points are made stationary by soldering, bonding, pressure welding, etc., the electrical connection between the electrostatic shield and the connection portions is strengthened and stabilized.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since the connection portion is integrated with a connection terminal that can be connected to and disconnected from a connection wire and the above-mentioned connection terminal is configured so as to be secured to the stationary plate, the configuration for playing two roles, that is, the reliable electrical connection to the electrostatic shield and the facilitation of the connection to and disconnection from the electrostatic shield, is simplified or attained so as to have less space, whereby the handling of the stationary plate and the connection terminal is facilitated. Furthermore, since the connection terminal itself is secured to the stationary plate, work for connection to and disconnection from the connection terminal and work for routing or securing wires can be carried out easily.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since a cover for stationary plate having electrical insulation and covering the electrostatic shield is provided on the opposite side of the stationary plate, the exposed parts of the electrostatic shield can be reduced; in the case that the electrostatic shield is connected to a live part directly or via an impedance, electric shock owing to inadvertent contact therewith when the top plate is broken or under repair can be prevented, or in the case that other conductive components are disposed therearound, dielectric breakdown between these components and the electrostatic shield hardly occurs.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since the cover for stationary plate is firmly secured to the stationary plate so as to cover part or whole of the electrostatic shield and the connection portions, the portions, securely connected to the electrostatic shield, in the connection portions are held between the stationary plate and the cover for stationary plate, whereby the secure connection or the securing of the electrostatic shield is reinforced further firmly, and breakage and peeling owing to bending, vibration, etc. hardly occur. Furthermore, since the stationary plate and the cover for stationary are integrated, the handling is facilitated.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since an insulator in a half-cured state is used for at least one of the stationary plate and the cover for stationary plate, and heated and cured after assembly to attain integration, the stationary plate and the cover for stationary plate, between which the connection portions are held, can be integrated easily by pressure application while being heated, and the effect of reinforcing the electrostatic shield or the secure connection portions can be enhanced.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since raw mica is used for at least one of the stationary plate and the cover for stationary plate, and heated after assembly to attain integration, the cover for stationary and the stationary plate can be integrated easily, and the heat resistance of the cover for stationary can be raised. Furthermore, their thicknesses can be reduced.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since inorganic fiber containing an adhesive is used for at least one of the stationary plate and the cover for stationary plate, and heated after assembly to attain integration, the cover for stationary and the stationary plate can be integrated easily, and their heat resistance can be raised.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since the connection portion is provided close to the winding on the low-potential side, instead of the winding on the high-potential side, of the induction heating coil, with reference to the potential to which the electrostatic shield is connected, even in the case that the connection portion has an exposed part or is covered with an insulator but the covering is damaged, dielectric breakdown, such as sparking, owing to a high potential difference from the potential of the induction heating coil positioned close thereto, hardly occurs, whereby malfunctions and the like in the drive circuit is prevented from occurring and reliability is enhanced.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since part of the connection terminal is bent and the connection terminal is secured to the stationary plate, the connection terminal can be installed stably without taking much space.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since the connection terminal is securely connected to the electrostatic shield using a conductive adhesive, the electrical connection between the connection portion and the connection terminal can be made stable.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since the connection terminal is retained in the induction heating coil base for supporting the induction heating coil, the connection terminal can be installed stably.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since a slit portion is provided at least at one position of the stationary plate from its external circumference, the stationary plate or the cover for stationary plate can be prevented from being deformed by the heat received from the matter to be heated or the induction heating coil.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, since a slit portion is provided at least at one position of the cover for stationary plate from its external circumference, the stationary plate or the cover for stationary plate can be prevented from being deformed by the heat received from the matter to be heated or the induction heating coil.  
      In the above-mentioned induction heating apparatus in accordance with another aspect of the present invention, the connection terminal is secured to the stationary plate with a crimping member and electrically connected to the electrostatic shield. The thermal expansion coefficient of the stationary plate or the electrostatic shield generally differs from the thermal expansion coefficient of the connection terminal. When the stationary plate and/or the electrostatic shield expand and contract repeatedly owing to temperature change during a long-term use, cracks may occur at the connection portion between the electrostatic shield and the connection terminal or improper conduction may be caused at the connection portion, owing to mechanical stress. In the induction heating apparatus in accordance with the present invention, since the connection between the electrostatic shield and the connection terminal is secured with the crimping member (for example, a grommet), the crimping member maintains proper conduction therebetween while playing a role of relieving mechanical stress, thereby preventing occurrence of cracks of improper conduction.  
      As described above, the present invention can achieve an induction heating apparatus capable of improving the reliability in the electrical connection between the electrostatic shield and the low-potential portion of the inverter circuit thereof and securely suppressing leak current at the time when the user makes contact with a matter to be heated.  
      Embodiments specifically indicating best modes for embodying the present invention will be elucidated referring to the drawings.  
     Embodiment 1  
      The general outline of an induction heating apparatus in accordance with Embodiment 1 of the present invention will be elucidated. In  FIG. 1 , numeral  11  designates a top plate provided in the upper portion of the main body (not shown) forming an outer shell, and numeral  12  designates an induction heating coil placed on an induction heating coil base  13 . Numeral  14  designates a matter to be heated, such as a pan, which is heated by induction heating, and numeral  15  designates a stationary plate made of an inorganic insulator, such as mica. Numeral  16  designates an electrostatic shield made of a mixture of an adhesive and conductive paint containing carbon or the like and applied onto the stationary plate  15 , the electrostatic shield comprising a conductive pattern which covers the entire area of the induction heating coil  12  so that the high voltage of the induction heating coil  12  is not induced in the matter  14  to be heated, and connection portions  16   a  provided at both ends of this conductive pattern.  
      Numeral  17  designates a connection terminal made of brass, and the connection portion  17   a  at its end is opposed to the connection portion  16   a  of the electrostatic shield  16  and securely connected thereto with a conductive adhesive or the like. Numeral  18  designates a cover for stationary plate made of an inorganic insulator, such as mica, covering the electrostatic shield  16 , its connection portions  16   a  and the connection portions  17   a  of the connection terminals  17 .  
      Numeral  19  designates a drive circuit, such as an inverter circuit, provided below the induction heating coil  12  to supply a high-frequency current to the induction heating coil  12 . The electrostatic shield  16  is connected to a predetermined potential, which is relatively low and at which an electrostatic shielding effect is produced by the connection of the electrostatic shield, for example, a DC power supply potential input from this drive circuit  19  via lead wires  20  or a potential lower than that of the high potential portion of the induction heating coil. The connection is carried out directly or via an appropriate impedance element, such as a capacitor or a resistor, depending on the circumstances. The stationary plate  15  and the cover for stationary plate  18  are installed on the bosses  21  of the induction heating coil base  13  using screws.  
      Next, the configurations of the stationary plate  15 , the electrostatic shield  16 , the connection terminals  17  and the cover for stationary plate  18  will be elucidated using  FIG. 2 . Part (a) of  FIG. 2  is a perspective view showing the shape of the cover for stationary plate  18 , and part (b) of  FIG. 2  is a perspective view showing the configurations of the stationary plate  15 , the electrostatic shield  16 , the connection terminals  17  and the cover for stationary plate  18 .  
      As shown in part (a) of  FIG. 2 , the cover for stationary plate  18  has an opening  22  in its central portion so that a temperature sensor (not shown) making contact with the bottom face of the top plate  11  and detecting its temperature is disposed. In addition, the cover for stationary plate  18  is provided with installation holes  23  for installation on the main body and a slit portion  24 . Furthermore, the cover for stationary plate  18  is provided with protrusion portions  25  for covering the upper faces of the connection terminals.  
      The stationary plate  15  is provided with an opening  26  and installation holes  27 , respectively corresponding to those of the cover for stationary plate  18 , and the electrostatic shield  16  is provided between this opening  26  and these installation holes  27 . This electrostatic shield  16  has a planar C-shape, covers the entire area of the induction heating coil  12  and is provided with the connection portions  16   a  at both ends of the C-shape. The connection portions  16   a  are connected to the connection portions  17   a  of the connection terminals  17 . The connection between the connection portion  16   a  and the connection portion  17   a  is carried out using a conductive adhesive. By this connection, the connection terminals  17  are firmly secured to the stationary plate  15 .  
      At this time, the use of a mixture of an adhesive and conductive powder, such as carbon, the same matter as that of the electrostatic shield  16 , is effective in improvement of electrical connection and productivity.  
      For the configuration of installing the connection terminals  17  to the stationary plate  15 , the above-mentioned adhesive may be used with mechanical connection. For example, such a configuration as shown in part (b) of  FIG. 2  may be used for securing, wherein the stationary plate  15  is provided with holding portions  28  each having a width corresponding to that of the connection terminal  17 , and the connection terminals  17  are each provided with pawl portions  29  adapted to the holding portion  28 ; furthermore, the connection terminals  17  are bonded to the stationary plate  15  and the pawl portions  29  are bent to embrace the holding portions  28  at the same time.  
      In this embodiment, the connection terminals  17  are each provided with recess portions  30  so that they can be secured to the induction heating coil base  13 . Furthermore, numeral  31  designates a slit portion provided in the stationary plate  15  and provided between both the connection terminals  17 .  
      After the connection terminals  17  are installed to the stationary plate  15 , the stationary plate  15  and the cover for stationary plate  18  are overlaid so that the installation holes  27  of the stationary plate  15  are aligned with the installation holes  23  of the cover for stationary plate  18 , and then they are heated, whereby they are firmly secured and integrated. This integration is attained by pressurizing and heating an adhesive component included in the electrostatic shield  16  and an adhesive applied to part of at least one of the stationary plate  15  and the cover for stationary plate  18 .  
      At this time, the cover for stationary plate  18  is bonded to the stationary plate  15  while the protrusion portions  25  of the cover for stationary plate  18  cover the connection portions  17   a  of the connection terminals  17 , whereby the bonding strength and insulation between the connection terminals  17  and the connection portions  16   a  of the electrostatic shield  16  can be improved.  
      In this embodiment, the internal side of the induction heating coil  12  is a high-potential side, and the external side is a low-potential side; hence, the distance between the high-potential winding portion of the induction heating coil  12  and the connection terminals  17  is made larger so that dielectric breakdown, such as sparking, hardly occurs, whereby malfunctions and the like in the drive circuit  19  are prevented from occurring and reliability can be enhanced further.  
      As the bonding method being used at the time when the cover for stationary plate  18  is installed to the stationary plate  15 , a method wherein raw mica in which a silicone-based adhesive remains is used for the stationary plate  15  and the cover for stationary plate  18 , and the adhesive remaining in the raw mica reacts and is cured to integrate the stationary plate  15  and the cover for stationary plate  18  is also used.  
      At this time, if an adhesive similar to that used for the bonding of the raw mica is used as the adhesive to connect the connection portions  16   a  of the electrostatic shield  16  to the connection portions  17   a  of the connection terminals  17 , the adhesives are blended in well because of their similarity, whereby not only the bonding between the connection portions  16   a  and the connection portions  17   a  but also the bonding between the stationary plate  15  and the connection terminals  17  becomes stronger.  
      The case wherein raw mica is used for the stationary plate  15  and the cover for stationary plate  18  is described above; however, the bonding method is not limited to this, even if raw mica is used for one of the stationary plate  15  and the cover for stationary plate  18 , practically problem-free bonding can be attained although the force of bonding is low in comparison with the case wherein the raw mica is used for both. This method wherein the raw mica is used for at least one of the stationary plate  15  and the cover for stationary plate  18  as described above has a practical effect of omitting an adhesive application process.  
      In addition to the methods described above, another method is used wherein an insulator in a half-cured state is used for one of the stationary plate  15  and the cover for stationary plate  18 , and heated and cured after their overlaying so that they can be integrated. As this kind of insulator, inorganic fiber or inorganic powder formed of silicate or the like or heat-resistant fiber formed of polyamide-imide or the like, impregnated with a glass-based adhesive or a silicone-based adhesive, half-cured and being in a film or sheet state is available.  
      The stationary plate  15  and the cover for stationary plate  18  tend to be thermally expanded or deformed by the heat received from the matter  14  to be heated, such as a pan, and the induction heating coil  12 ; however, the thermal expansion is absorbed by the slit portion  24  provided in the cover for stationary plate  18  and by the slit portion  31  provided in the stationary plate  15 , whereby the deformation can be suppressed.  
      Since the slit portion  24  is provided between both the connection terminals  17 , the electrostatic shield  16  can be maintained in the C-shape. Furthermore, the case wherein such a slit portion is provided in both the stationary plate  15  and the cover for stationary plate  18  is shown in  FIG. 2 ; however, it may be provided in one of them.  
      The case wherein the slit portion is provided at one position is shown in  FIG. 2 ; however, its position is not limited to this, it may be provided at a plurality of positions as a matter of course. In this case, however, if the slit portions are formed across the range from the external circumference to the opening, the stationary plate  15  or the cover for stationary plate  18  is divided into a plurality of pieces by slit portions; this is not desirable. Since actual deformation is significant at the external circumference, the slit portions should only be provided in the vicinities of the external circumference, and for practical purposes, the slit portions extended to an intermediate position between the external circumference and the opening are sufficient in length.  
      The electrostatic shield  16  is formed into a C-shape, the connection terminals  17  are provided in the vicinities of both ends thereof, and the resistance between the connection terminals  17  is measured, whereby the judgment about electrical characteristics, for example, whether or not wires are disconnected and whether or not the electrostatic shield  16  is a normal product having a proper resistance value, can be made easily.  
       FIGS. 3   a  and  3   b  are cross-sectional views showing in respective directions the main section of a configuration wherein the connection terminal  17  is installed to the induction heating coil base  13 ; the installation is done by fitting the ribs  32  provided on the induction heating coil base  13  into the recess portions  30  of the connection terminal  17 . By this fitting of the connection terminal  17  to the ribs  32 , when a Faston terminal is used for the connection between the connection terminal  17  and the lead wire  20  and when the Faston terminal is inserted/removed, the force acting between the connection terminal  17  and the connection portion  16   a  is received by the ribs  32 , and the connection between the connection terminal  17  and the connection portion  16   a  is not detached, whereby the reliability of the electrical connection can be improved.  
      In the connection between the electrostatic shield  16  and each of the lead wires  20 , even if the lead wire  20  is directly connected at the connection portion  16   a  using an conductive adhesive and this connection part is held between the stationary plate  15  and the cover for stationary plate  18  as shown in  FIG. 5 , it is possible to obtain an effect almost similar to that in the case that the above-mentioned connection terminal is used.  
      When a lower cover for stationary plate  33  is bonded to the rear of the connection terminal  17  as shown in  FIG. 4 , the live parts of the pawl portions  29  of the connection portion  17   a  can be prevented from being exposed, whereby the insulation of the connection terminal  17  can be improved.  
      In this embodiment, the electrostatic shield  16  is formed into a C-shape, and the connection terminals  17  are provided in the vicinities of both ends thereof; however, the number of the connection terminals  17  may be one or more, and briefly speaking, the number does not matter so long as the electrostatic shield  16  of the stationary plate  15  and the drive circuit  19  can be electrically connected via the connection terminals  17 .  
      It is described that the electrostatic shield  16  is made of a material mainly consisting of carbon; however, other conductive materials, such as tin oxide, may also be used.  
      As described above, in this embodiment, the stationary plate  15  having electrical insulation is provided between the top plate  11  and the induction heating coil  12 , and the stationary plate  15  is provided with the electrostatic shield  16  and the connection portions  17   a  for connecting the connection wires (in the case that the connection is carried out via capacitors, the connection wires from the capacitors) from the low-potential portion of the drive circuit  19  to the electrostatic shield  16  (including the connection portions  16   a ); hence, with this configuration, production is facilitated and the effect of the high temperature of the matter  14  to be heated on the electrostatic shield  16  is relieved, in comparison with the conventional method wherein the electrostatic shield and the connection portions are formed on the rear face of the top plate  11 . In addition, work for electrically connecting both securely is also facilitated. Furthermore, since the connection portions  17   a  are not integrated with the top plate  11 , work for assembling the main body of the apparatus is also facilitated.  
      Since the connection portion  17   a  is integrated (electrically connected) with the connection terminal  17  being used for connection and disconnection of the connection wire, work for the connection and disconnection between the electrostatic shield  16  and the low-potential portion of the drive circuit  19  can be carried out via the connection terminals  17  easily and securely.  
      In this embodiment, as the connection portion, the connection portion  17   a  is integrated with the connection terminal  17  and securely connected to the electrostatic shield  16  using an adhesive; however, as another example, as shown in  FIG. 5 , a configuration wherein the connection portion  17   a  is used as one end of the lead wire  20 , made contact with the electrostatic shield  16  (integrated with the connection portion  16   a ) and mechanically pressure-welded, held and secured to attain electrical connection, and the other end of the lead wire is provided with a connection terminal (not shown) can also produce a similar effect. Even in this case, the reliability of the connection is raised further by including a conductive adhesive at the contact part.  
      Since the connection terminal  17  is configured so as to be secured to the stationary plate  15 , the configuration for playing two roles, that is, the stable connection of the connection terminal  17  to the electrostatic shield  16  and the facilitation of the connection and disconnection between the connection terminal  17  and the driving means  20 , is simplified or attained so as to have less space, whereby the handling of the stationary plate  15  and the connection terminal  17  is facilitated. Furthermore, since the connection terminal  17  itself is secured to the stationary plate  15 , work for connection to and disconnection from the driving means  20  and work for routing or securing wires can be carried out easily.  
      Since the cover for stationary plate  18  having electrical insulation and covering the electrostatic shield  16  is provided on the opposite side (the upper side in this case) of the stationary plate  15 , the exposed parts of the electrostatic shield  16  can be reduced; in the case that the electrostatic shield  16  is connected to the live part of the drive circuit  19  directly or via an impedance, electric shock owing to inadvertent contact therewith when the top plate  11  is broken or under repair can be prevented, or in the case that other conductive components are disposed therearound, dielectric breakdown between these components and the electrostatic shield can be prevented.  
      Since it is configured that at least the secure connection parts for securely connecting the electrostatic shield  16  to parts (the connection portions  17   a ) of the connection terminals  17  and the electrostatic shield in the vicinities thereof are covered and firmly secured to the stationary plate, the portions (the connection portions  17   a ) of the connection terminals  17 , securely connected to the electrostatic shield, are held between the stationary plate  15  and the cover for stationary plate  18 , whereby the secure connection or the securing of the electrostatic shield  16  is reinforced further firmly, and breakage and peeling owing to bending, vibration, etc. hardly occur. Furthermore, since the stationary plate  15  and the cover for stationary plate  18  are integrated, the handling is facilitated.  
      Since an insulator in a half-cured state is used for at least one of the stationary plate  15  and the cover for stationary plate  18 , and heated and cured after assembly to attain integration, the stationary plate  15  and the cover for stationary plate  18 , between which the connection parts are held, can be integrated easily by pressure application while being heated, and the effect of reinforcing the electrostatic shield  16  or the connection portions  17   a  can be enhanced.  
      Since raw mica is used for at least one of the stationary plate  15  and the cover for stationary plate  18 , and heated after assembly to attain integration, the cover for stationary plate  18  and the stationary plate  15  can be integrated easily, and the heat resistance of these can be raised. Furthermore, their thicknesses can be reduced.  
      Since inorganic fiber containing an adhesive is used for at least one of the stationary plate  15  and the cover for stationary plate  18 , and heated after assembly to attain integration, the cover for stationary plate  18  and the stationary plate  15  can be integrated easily, and their heat resistance can be raised.  
      Since the connection portion  17   a  or the connection terminal  17  is provided close to the winding on the low-potential side, instead of the winding on the high-potential side of the induction heating coil  12 , with reference to the potential to which the electrostatic shield  16  is connected, even in the case that the connection portion  17   a  or the connection terminal  17  has an exposed part or is covered with an insulator but the covering is damaged, dielectric breakdown, such as sparking, owing to a high potential difference from the potential of the induction heating coil  12  positioned close thereto, hardly occurs, whereby malfunctions and the like in the drive circuit  19  are prevented from occurring and reliability is enhanced.  
      Since part of the connection terminal  17  is bent and the connection terminal  17  is secured to the stationary plate  15 , the connection terminal  17  can be installed stably without taking much space.  
      Since the connection terminal  17  is securely connected to the electrostatic shield  16  using a conductive adhesive, the electrical connection between the connection portion  16   a  of the electrostatic shield  16  and the connection portion  17   a  of the connection terminal  17  can be made stable.  
      Since the connection terminal  17  is retained in the induction heating coil base  13  for supporting the induction heating coil  12 , the connection terminal  17  can be installed stably.  
      Since the slit portion is provided at least at one position on the stationary plate  15  from its external circumference, the stationary plate  15  or the cover for stationary plate  18  can be prevented from being deformed by the heat received from the matter  14  to be heated or the induction heating coil  12 .  
      Since the slit portion  24  is provided at least at one position on the cover for stationary plate  18  from its external circumference, the stationary plate  15  or the cover for stationary plate  18  can be prevented from being deformed by the heat received from the matter  14  to be heated or the induction heating coil  12 .  
     Embodiment 2  
      An induction heating apparatus in accordance with Embodiment 2 of the present invention will be elucidated using FIGS.  6  to  8 . In the induction heating apparatus in accordance with Embodiment 2, the method for securing the connection terminal  17  to the stationary plate  15  and the electrostatic shield  16  differs from that of Embodiment 1. In other points, the induction heating apparatus in accordance with Embodiment 2 is almost the same as that in accordance with Embodiment 1. The method for securing the connection terminal  17  to the stationary plate  15  and the electrostatic shield  16  in the induction heating apparatus in accordance with Embodiment  2  will be elucidated. In the induction heating apparatus in accordance with Embodiment 2,  FIG. 6  is a magnified perspective view showing the main section in the vicinity of the connection portion of the connection terminal  17 , viewed from the side of the top plate  11 .  FIG. 7  is a magnified perspective view showing the main section in the vicinity of the connection portion of the connection terminal  17 , viewed from the side of the induction heating coil  12 .  FIG. 8  is a cross-sectional view showing the main section in the vicinity of the connection portion of the connection terminal  17 .  
      Two cut portions  41  are provided in the stationary plate  15 , whereby a holding portion  28  is formed between the cut portions  41 . A grommet  40  (a securing member) holds the connection terminal  17 , the stationary plate  15  and the electrostatic shield  16  to secure them integrally and to maintain electrical conduction between the connection terminal  17  and the electrostatic shield  16 .  
      The connection terminal  17  has pawl portions  29  and wing portions  17   b  and  17   c . The pawl portions  29  are bent to embrace the holding portion  28 . When a force for bending the connection terminal  17  inward is applied, the wing portion  17   b  is used for support on both sides of the cut portions  41  so that the connection terminal  17  and the holding portion  28  are not bent. When a force for bending the connection terminal  17  outward is applied, the wing portion  17   c  is used for support so that the connection terminal  17  and the holding portion  28  are not bent easily. The wing portion  17   c  makes contact with the electrostatic shield  16  over a wide area and ensures secure electrical conduction between the connection terminal  17  and the electrostatic shield  16 .  
      The thermal expansion coefficient of the stationary plate  15  or the electrostatic shield  16  generally differs from the thermal expansion coefficient of the connection terminal  17 . When the stationary plate  15  and/or the electrostatic shield  16  expand and contract repeatedly owing to temperature change during a long-term use, cracks may occur at the connection part between the electrostatic shield  16  and the connection terminal  17  or improper conduction may be caused at the connection portion, owing to mechanical stress. In Embodiment 2, the grommet  40  is used to secure the stationary plate  15 , the electrostatic shield  16  and the connection terminal  17 . Although the grommet  40  firmly secures the stationary plate  15  and the connection terminal  17  in the thickness direction of the stationary plate  15 , slight sliding is allowed among the stationary plate  15 , the electrostatic shield  16  and the connection terminal  17  in a direction parallel to the surface of the stationary plate  15 . The grommet  40  prevents cracks and improper conduction from occurring while playing a role of relieving mechanical stress owing to temperature change. With this configuration, proper conduction between the electrostatic shield  16  and the connection terminal  17  is ensured for a long period of time.  
      Even if cracks occur in the electrostatic shield  16  in the vicinity of the grommet  40 , the grommet  40  holds down the cracks, and the grommet  40  and the wing portion  17   c  ensure conduction between the connection terminal  17  and the electrostatic shield  16 , whereby no problem occurs in the operation of the induction heating apparatus.  
      When the lower cover for stationary plate  33  is bonded to the rear of the connection terminal  17 , the live parts of the grommet  40  and the pawl portions  29  of the connection portion  17   a  can be prevented from being exposed, whereby the insulation of the connection terminal  17  can be improved.  
      Securing between the stationary plate  15  and the connection terminal  17  may be carried out using a crimping member other than the grommet.  
      Since the holding portion  28  is not protruded from the stationary plate  15  in Embodiment 2, the cover for stationary plate  18  has no protrusion portions  25 .  
      As in Embodiment 1, the connection terminal  17  is retained to the induction heating coil base  13 .  
      As described above, the present invention can achieve an induction heating apparatus capable of improving reliability in the electrical connection between the electrostatic shield and the low-potential portion of the inverter circuit thereof and securely suppressing leak current at the time when the user makes contact with a matter to be heated.  
      The induction heating apparatus in accordance with the present invention is useful for cooking apparatuses and the like.  
      It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.