Low-frequency induction heater

Abstract: In a low-frequency induction heater comprising a primary side coil having a core and a secondary side conductive hollow cylindrical member surrounding the primary side coil, it is sought to improve the Joule heat generation efficiency and solve problems peculiar to composite materials such as thermal deformation, electrolytic corrosion and difficulty of manufacture by forming the conductive hollow cylindrical member using a sole stainless steel material having a thickness ranging from 2 mm to 6 mm. In a preferred embodiment, a coil 2 is wound around a rod-like core 1, which is in turn surrounded by a conductive hollow cylindrical member 3 made of a sole stainless steel material having a thickness ranging from 2 mm to 6 mm. When an AC current passes through the coil 2, an alternating magnetic field is set up in the axial direction of the coil 2, causing an inducted current in the conductive hollow cylindrical member 3. Joule heat is thus generated in the member 3 due to the electric resistance thereof.

Description: This invention relates to a low-frequency induction heater comprising a primary winding coil made of aluminium wire having a core and a secondary winding conductive hollow cylindrical member surrounding said primary winding coil, wherein said conductive hollow cylindrical member is constituted of stainless steel. The heater utilizes a one-turn transformer as an electromagnetic induction heat generator. Such a heater is known from EP-A-383 272. [DESCRIPTION OF THE PRIOR ART] Heretofore, an electric fryer has been proposed which comprises an oil container, a pipe-like portion formed substantially in a central portion of the oil container, and a induction heater inserted in the pipe-like portion with a gap provided by means of positioning ridges, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 39,525/1983. Meanwhile, the inventor of the present invention has earlier proposed a low-frequency electromagnetic induction heater, which comprises an induction coil wound on a core, and a single metal pipe or two or more different metal pipes combined into an integrated structure around the induction coil, the gap between the induction coil and the pipe or pipes being filled with a resin molding, as disclosed in EP 0 383 272 A1. However, in the former heater, i.e., the electric fryer, heat generated from the induction heater is transferred to the pipe-like portion which constitutes a part of the oil container through the air gap between the induction heater and the pipe-like portion, thus causing the problem of low heat transfer efficiency. Therefore, when oil in the container is heated to a cooking temperature necessary for producing fries, tempuras or the like, the induction heater is elevated in temperature to a considerably high temperature, thus having an adverse effect on the coil and the core constituting the induction heater. Particularly, the temperature of the induction heater is liable to exceed the permissible temperature limit of the coil insulator. In the latter heater, i.e., the low-frequency electromagnetic induction heater, the secondary winding constitutes a part of the container. Thus, Joule heat is generated by electromagnetic induction in the container. This acheives the advantage that a satisfactory energy transfer efficiency can be obtained to avoid an excessive temperature rise of the coil and the core. However, where the secondary winding utilizes a combination of copper having low electric resistivity and stainless steel having good durability, i.e., where copper pipe and stainless steel pipe (a part of the vessel) are combined into an integrated structure, the heater has the following demerits. (1) When the overall pipe structure is heated and elevated in temperature, the difference in the coefficient of thermal expansion between the two metals causes circumferential elongation of the copper pipe relative to the stainless steel pipe, that is, a portion of the circumference of the copper pipe expands inwardly to produce an air gap between the inner copper pipe and the outer stainless steel pipe. In the portion where the air gap is produced, the heat transfer efficiency deteriorates and localizes the temperature rise, thus causing oxidation of the copper. Figures 11(a) and 11(b) are sectional views showing the state of the gap formation. Before the temperature rise, the copper pipe 21 and the stainless steel pipe 22 are perfectly integrated (Figure 11(a)). After the temperature rise, however, an air gap 23 is formed between the two pipes 21 and 22 (Figure 11(b)). (2) If a leakage current, e.g., a grounding current, is caused in the heater while there is water attached to contact portions of the copper and stainless steel pipes, an electrolytic corrosion is brought about which deteriorates the life of the secondary winding. (3) For combining the copper pipe and stainless steel pipe into an integrated structure, high dimensional accuracy is required for the shapes of both the pipes, and this inevitably leads to an increased cost of manufacture. (4) In case the copper pipe and stainless steel pipe combined into an integrated structure is used as a secondary winding, the number of layers of a primary winding increases from 4 layers to 6 layers. This means that heat dissipation from the inside of the primary winding is difficult, finally causing overheating in the primary winding. SUMMERY OF THE INVENTION It is an object of this invention to provide a low-frequency induction heater with a high efficiency of energy transfer to the cooking material. It is yet another object of this invention to provide a low-frequency induction heater the rate of temperature rise thereof is rapid. It is yet another object of this invention to provide a low-frequency induction heater which can prevent a localized temperature rise. It is yet another object of this invention to provide a low-frequency induction heater which can prevent strain, deformation or electrolytic corrosion therof to have satisfactory life and durability. It is yet another object of this invention to provide a low-frequency induction heater which can permit cooking under a stabilized temperature condition. It is yet another object of this invention to provide a low-frequency induction heater which can prevent deterioration of the cooking oil and extend the use period thereof. The present invention has been developed in order to accomplish the above objects. A low-frequency induction heater according to the invention has a structure as defined in claim 1. Preferred embodiments are given in claims 2 to 13. It is preferable in the above aspect of the invention that a temperature sensor is provided inside said conductive hollow cylindrical member. It is preferable in the above aspect of the invention that the number of layers of the primary winding coil is in a range of from 1 layer to 2 layers. It is preferable in the above aspect of the invention that the core is made of silicon steel plate. It is preferable in the above aspect of the invention that the outer diameter of the core is in a range of 10 mm to 200 mm. It is preferable in the above aspect of the invention that the length of the core is in a range of 100 mm to 2,000 mm. It is preferable in the above aspect of the invention that the diameter of the wire comprised in the primary winding coil is in a range of 2 mm to 8 mm. It is preferable in the above aspect of the invention that the number of turns of the wire in the first layer comprised in the primary winding coil is in a range of 50 turns to 200 turns. It is preferable in the above aspect of the invention that the number of turns of the wire in the second layer comprised in the primary winding coil is in a range of 10 turns to 70 turns. It is preferable in the above aspect of the invention that the length of the secondary winding conductive hollow cylindrical member is in a range of 100 mm to 2,000 mm. It is preferable in the above aspect of the invention that the outer diameter of the secondary winding conductive hollow cylindrical member is in a range of 30 mm to 300 mm. It is preferable in the above aspect of the invention that the temperature sensor is a thermocouple. It is preferable in the above aspect of the invention that the temperature sensor is provided inside an upper portion of the secondary winding conductive hollow cylindrical member. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary perspective view showing an embodiment of the low-frequency induction heater according to the invention. Figure 2 is a sectional view showing the same embodiment of the low-frequency induction heater. Figure 3 is a sectional view showing a low-frequency induction heater according to the invention, which has a temperature sensor buried inside a conductive hollow cylindrical member. Figure 4 is a schematic representation of an example of temperature control circuit. Figures 5(a) and 5(b) show an example of a low-frequency induction heating cooking device using the low-frequency induction heater according to the invention, with Figure 5(a) being a plane view and Figure 5(b) being a front view. Figure 6 is a perspective view showing the same low-frequency induction heating cooking device using the low-frequency induction heater according to the invention. Figure 7 is an exploded perspective view showing a magnetic circuit comprising cores and coils, used for the low-frequency induction heating cooking device using the low-frequency induction heater according to the invention. Figures 8(a) to 8(c) are electric connection diagrams, with Figure 8(a) being a diagram in case of passing a single-phase AC current through a single coil, Figure 8(b) being a diagram in case of passing a three-phase AC current through three coils in Y-connection, and Figure 8(c) being a diagram in case of passing a three-phase AC current through three coils in delta-connection. Figure 9 is a sectional view showing the low-frequency induction heating cooking device using the low-frequency induction heater according to the invention in a state of heating a contained liquid such as water or oil. Figures 10(a) and 10(b) are examples of graphs showing temperature variations when temperature control of the low-frequency induction heating cooking device filled with oil using the low-frequency induction heater according to the invention. Figures 11(a) and 11(b) are sectional views showing the state of formation of an air gap between a copper pipe and a stainless steep pipe, with Figure 11(a) showing the pipes before temperature rise and Figure 11(b) showing the pipes after the temperature rise. Figure 12 is a perspective view showing a core which constitutes the low-frequency induction heater according to the invention. Figure 13 is a sectional view showing a primary winding coil around the core shown in Figure 12. DETAILED DESCRIPTION OF THE INVENTION According to the above aspects of the invention, the conductive hollow cylindrical member as the secondary winding constitutes a part of the container, and Joule heat is generated directly in the container by electromagnetic induction. Thus, satisfactory efficiency of energy transfer to cooking materials in the vessel can be obtained, and also the temperature rise of the coil and the core can be suppressed. Further, since the conductive hollow cylindrical member is constituted of a sole stainless steel material, an uniform coefficient of thermal expansion can be achieved to preclude strain or deformation due to the temperature rise. Further, when a leakage current occurs with water or the like attached to the conductive hollow cylindrical member, the member is never electrolytically corroded because it is made of a single material. Compared to the secondary winding structure according to prior art obtained by integrating a copper pipe with a thickness of 0.5 mm to 1 mm and a stainless steep pipe with a thickness of 1 mm, according to the invention it is possible to prevent the efficiency of Joule heat generation by electromagnetic induction from decreasing because of a structure of the conductive hollow cylindrical member as the secondary winding made of stainless steel and having a large thickness and a large sectional area, thus offering a low electric resistance. Particularly, in case of connecting the primary side of the heater to a commercial power source, in which case input voltage or the primary side is fixed to predetermined votage, e.g., 100 V or 200 V in Japan, with electric resistance of the secondary side increasing, induction current of the secondary side tends to be reduced, and the power factor of the primary side tends to be reduced to increase reactive power. In order to increase Joule heat generation in the secondary side, it may be thought to (1) reduce electric resistance of the secondary side, (2) reduce the number of turns of the primary side, or (3) combine above (1) with above (2). However, excessively increasing the thickness of the conductive hollow cylindrical member leads to demerits in view of the manufacture, cost and weight of the low-frequency induction heater. Therefore, from the standpoints of the Joule heat generation, cost of the product and so on, the thickness of the conductive hollow cylindrical member is suitably in a range of 2 mm to 6 mm. Further, since the commercial power supply frequency (i.e., 50 or 60 Hz) is used, the skin effect that is observed in high-frequency induction heating does not have substantial influence, and Joule heat is generated uniformly over the entire cross section no matter how large the thickness of conductive hollow cylindrical member. Further, with the thickness of the conductive hollow cylindrical member increasing, it is possible to bury a temperature sensor or the like in the member for detecting the temperature thereof. It is further possible to hold a constant heating temperature of the conductive hollow cylindrical member through control of the primary winding current or voltage by comparing the temperature sensor output to a predetermined reference level. Further, as the secondary winding, a standard stainless steel pipe circulating in quantities in the market may be used directly. Since it is available inexpensively, the cost of the product can be greatly reduced. Further, with a low-frequency induction heater, which utilizes such a conductive hollow cylindrical member directly as a part of the cooking vessel, satisfactory energy transfer efficiency can be obtained. Because it also has a large contact area with water or oil in the container, quick heating can be obtained while preventing a localized temperature rise. Accordingly, it is possible to suppress oxidation of oil and generation of oil mist due to high temperature and also reduce the time interval from the start of energization until it is ready to cook. Further, by using stainless steel for the entire cooking vessel including the conductive hollow cylindrical member, the cooking vessel is less corroded by cooking materials containing salt, acid or alkali. Further, because the number or layers of the primary winding coil is from 1 layer to 2 layers, it is possible to suppress the temperature rise of the primary winding owing to heat generated in the inside of the primary winding effectively dissipating. Thus, it is possible to prevent an accident caused by defective insulation of a insulator in the primary winding. Now, embodiments of the low-frequency induction heater according to the invention will be described with reference to the drawings. Figure 1 is a fragmentary perspective view showing an embodiment of the low-frequency induction heater according to the invention, and Figure 2 is a schematic sectional view of the same. A coil 2 is wound around a core 1, and a conductive hollow cylindrical member 3 made of sole stainless steel material is placed around the coil 2. In case, for instance, an AC current of 10 A (rms) with a voltage of 100 V (rms) at a frequency of 50 or 60 Hz passing through the coil 2 which has 100 turns as the primary winding of the transformer, an alternating magnetic field occurs in the axial direction of the coil 2, and a magnetic circuit is formed in the core 1 made of a high magnetic permeability material. The conductive hollow cylindrical member 3 surrounding the coil 2 functions as the secondary side of the transformer, and an induction current is generated in the member 3 in accordance with the time differential of the alternating magnetic field. Supposing in the absence of loss peculiar to the transformer, an induction current of 1,000 A (rms) at a voltage of 1 V (rms) flows in the secondary winding with a turn ratio of the primary to the secondary of 100 to 1. This induction current is converted by the electric resistance of the conductive hollow cylindrical member 3 into Joule heat, thus heating the member 3. In thermal contact with an object and the heated member 3, the object can receive heat transferred from the member 3 and be heated. The energy transfer between the coil 2 and the conductive hollow cylindrical member 3 is mostly effected by the alternating magnetic field, and therefore an air gap may be present between the coil 2 and member 3. Particularly, when heating oil or like object up to a high temperature, the permissible temperature of the insulation of the coil 2 is liable to be exceeded due to transfer of heat from the conductive hollow cylindrical member 3 to the core 1 and coil 2, and therefore it is suitable that an air gap is provided between the coil 2 and member 3. Where there are losses peculiar to the transformer, typically the hysteresis loss and eddy current loss in the core 1 and the copper loss due to the resistance of the coil 2, the core 1 and the coil 2 are liable to be elevated to a considerably high temperature due to heat generation. In such case, they are suitably air-cooled by supplying air to the gap. The conductive hollow cylindrical member 3, as noted above, is made of a sole stainless steel material and a thickness thereof in a range of 2 mm to 6 mm. In this case, by reducing the number of turns of the primary side coil, the amount of the generated Joule heat may be increased with the secondary side induction current increasing. In addition, the reduction of the turns number of the coil can bring about reduction of the price of the low-frequency induction heater. Figure 12 is a perspective view showing a core which constitutes the low-frequency induction heater according to the invention. The core 1 may be manufactured as follows. A high magnetic permeability material plate such as silicon steel plate is laminated by forming a shape of coil, and fixed by filling adhesive such as resin among each layer to be formed to a cylindrical shape as a whole, and then a slit is made along the axial direction. The slit prevents induction current loss due to magnetic flux passing inside the core along the axial direction. The shape of the core 1 may be determined under consideration for matters in design such as inner diameter and length of the conductive hollow cylindrical member 3, turn number and shape of the coil 2, quantity of magnetic flux passing inside core, consumption power, etc. Concretely, the outer diameter of the core 1 is preferably in a range of 10 mm to 200 mm, especially, most preferably in a range of 55 mm to 70 mm. The inner diameter of the core 1 is preferably 50 mm or less, especially, most preferably 20 mm or less. The width of the slit of the core 1 is preferably in a range of 0.5 mm to 10 mm, especially, most preferably in a range of 1 mm to 5 mm. The length of the core 1 is preferably in a range of 100 mm to 2,000 mm, especially, most preferably in a range of 350 mm to 500 mm. Figure 13 is a sectional view showing a primary winding coil around the core shown in Figure 12. A wire 30 comprised in the coil 2 is made of aluminium wire (ALO) having a low electric resistance and a high permissible temperature, the diameter therof is preferably in a range of 2 mm to 8 mm, especially, most preferably in a range of 4 mm to 6 mm. The number of layers of the coil 2 is in a range of 1 layer to 2 layers, and it is also preferable that winding density varies between the first layer and the second layer and/or winding density varies partly in each layer. The wire in the first layer of the coil 2 is wound densely around the side face of the core 1, the number of turns is preferably in a range of 50 turns to 200 turns, especially, most preferably in a range of 80 turns to 120 turns. The wire in the second layer of the coil 2 is wound sparsely in parts on an insulating sheet 31 such as mica foil or the like around the side face of the first layer, the number of turns is preferably in a range of 10 turns to 70 turns. especially, most preferably in a range of 20 turns to 40 turns. Thus, because the number of layers of the primary winding coil is from 1 layer to 2 layers, it is possible to suppress the temperature rise of the primary winding owing to heat generated in the inside of the primary winding effectively dissipating. In case, for instance, water is going to be boiled continuously for 12 hours with the low-frequency induction heater according to the invention, temperature of the inside of the primary winding coil reaches only 185 °C. Meanwhile, in case water is going to be boiled contonuously with another low-frequency induction heater wherein the number of layers of the primary winding coil is 5 layers, temperature of the inside of the primary winding coil reaches 499 °C near the melting point of the aluminium wire for 2 hours from the beginning of energizing. The core 1 with the primary winding obtained as noted above, is positioned about the center of the conductive hollow cylindrical member 3 shown in Figure 1. The shape of the conductive hollow cylindrical member 3 may be determined under consideration for matters in design such as electric resistance, calorific power, consumption power, the shape of heating cooking device, etc. Concretely, it is preferable that the conductive hollow cylindrical member 3 as a part of the cooking vessel, as noted above, is made of a sole stainless steel material such as SUS316, SUS304, etc ( Japanese Industrial Standard G 4303 ∼ 4316 ) and the thickness thereof in a range of 2 mm to 6 mm, preferably in 2.5 mm to 4 mm. The length of the conductive hollow cylindrical member 3 is preferrably in a range of 100 mm to 2,000 mm, especially, most preferably in a range of 400 mm to 500 mm. The outer diameter of the conductive hollow cylindrical member 3 is preferably in a range of 30 mm to 300 mm, especially, most preferably in a range of 80 mm to 120 mm. Figure 3 is a sectional view showing a low-frequency induction heater according to the invention, which has a temperature sensor buried inside a conductive hollow cylindrical member. A temperature sensor 4 such as a thermocouple is inserted and secured in an elongated bore formed in a part of the conductive hollow cylindrical member 3. The temperature sensor 4 detects the temperature of the conductive hollow cylindrical member 3 and outputs, for instance, a voltage signal proportional to the detected temperature. Conventionally, because the temperature sensor 4 was disposed outside the conductive hollow cylindrical member 3 and inside the vessel (e.g., in heated oil in electric frier), the sensor was liable to be broken during the cooking operation. Meanwhile, according to the invention, the temperature sensor 4, which is inserted inside the conductive hollow cylindrical member 3, never obstructs the cooking operation or cleaning operation, and it can prevent the operator from damaging the temperature sensor 4 by his mistake. The position of the temperature sensor 4 inside the conductive hollow cylindrical member 3 is preferably in an upper portion of the member 3. This is so because the operator can burnish the outer side of an upper portion of the conductive hollow cylindrical member 3 clean whenever the operator removes scales or stains attached to the member 3. This means that it is possible to avoid erroneous operation of temperature control due to attached scales. Figure 4 is a schematic representation of an example of temperature control circuit. The output of the temperature sensor 4 is amplified to a predetermined level by an amplifier (not shown) and then coupled to an input terminal 12, and thence to a comparator 13. Meanwhile, a signal from a reference signal generator 11, in which a reference level corresponding to a predetermined temperature, is coupled to the comparator 13 for comparison of the two input signals. Power supplied from a power supply terminal 14 to the low-frequency induction heater 10 is on-off controlled by a switching element 15. The power supplied to the low-frequency induction heater 10 is turned off when the temperature of the conductive hollow cylindrical member 3 exceeds the reference temperature, and is turned on when the former temperature becomes lower than the latter tempeature. In this way, the heating temperature of the conductive hollow cylindrical member can be stabilized to the neighborhood of the reference temperature. When the primary side input power is high, the on-off switching is suitably effected at the zero crossing point of the voltage or current in order to prevent noise or surges. The above temperature control circuit used for the low-frequency induction heater according to the invention is by no means limitative, and it is possible to adopt temperature control circuits well-known to skilled persons. Now, a low-frequency induction heating cooking device incorporating the low-frequency induction heater according to the invention will be described. Figures 5(a) and 5(b) show an example of the low-frequency induction heating cooking device using the low-frequency induction heater according to the invention, with Figure 5(a) being a plane view and Figure 5(b) being a front view. Figure 6 is a perspective view showing the cooking device. As shown, the cooking device 5 has three spaced-apart conductive hollow cylindrical members 3 disposed inside and integrated therewith. A core 1 and a coil 2 shown in Figure 7 are inserted inside each of the conductive hollow cylindrical members 3. The individual cores 1 have their opposite ends coupled together by cores or yokes 6 and 6′ to form a magnetic circuit. Where the cooking device 5 has a small volume, only a single conductive hollow cylindrical member 3 may be sufficient. Where the device 5 has a large volume, four or more conductive hollow cylindrical members may be provided to preclude temperature distribution fluctuations of water or oil in the cooking device. In general, the greater the diameter and the number of the conductive hollow cylindrical members 3, the greater is the heat transfer surface area of the members 3, and thus the heat transfer efficiency is the more satisfactory, thus permitting prevention of the oxidation of oil due to a localized temperature rise. Figures 8(a) to 8(c) show examples of electric connection of a coil or coils 2. Figure 8(a) is a connection diagram in case of a single-phase AC current passing through the coil 2. Figure 8(b) is a connection diagram in case of a three-phase AC current passing through the three coils 2 in Y-connection. Figure 8(c) is a connection diagram in case of passing a three-phase AC current through the three coils 2 in delta-connection. Where the low-frequency induction heater according to the invention is energized with a three-phase AC current, the input capacity of the primary side in passing a three-phase AC current is preferably in a range of 1 kw to 100 kW per three coils. Figure 9 is a sectional view showing the low-frequency induction heating cooking device using the low-frequency induction heater according to the invention in a state of heating a contained liquid such as water or oil. And the numeral 9 is a valve. The conductive hollow cylindrical members 3 are provided in an intermediate portion of the cooking device 5 in the height direction thereof. The conductive hollow cylindrical members 3 are heated by Joule heat generated by induced current, and transfers heat to the surrounding liquid 7 such as water or oil. As the liquid 7 is heated, its specific gravity is reduced. Thus, the heated liquid is moved upward, causing the liquid 7 before heating to be brought to the neighborhood of the conductive hollow cylindrical members 3. With this phenomenon of convection, the liquid 7 is heated efficiently. To effect heating in conformity to a predetermined cooking content, the current passed through the coils is controlled to sustain a constant temperature by detecting the temperature with the temperature sensor provided at a predetermined position and comparing the detected temperature with a preset temperature. A holding member for holding the cooking material, for instance, a metal net or rack, may be disposed between the conductive hollow cylindrical members 3 and the liquid surface, where the cooking material such as fries or the like is supported. Alternatively, noodles or like cooking material may be put into a metal basket or vessel, which may be set as a whole in the liquid 7 for cooking. The liquid 7 below the conductive hollow cylindrical members 3 does not substantially participate in the phenomenon of convection by heating and tends to keep still at a lower temperature than the liquid 7 above the conductive hollow cylindrical members 3. Accordingly, cooking residues 8 or foreign liquid produced during the cooking is not drawn into the phenomenon of convection, but is collected on the bottom of the cooking vessel 5. Thus, it is hardly attached to the cooking material, and the cooking can be finished satisfactorily. Figure 10 shows graphs of temperature change in case the temperature control is performed in the low-frequency induction heating cooking device full of oil, using the low-frequency induction heater according to the invention. Figure 10(a) is a graph in case the output of the temperature sensor positioned inside the conductive hollow cylindrical member is used as an input signal for the temperature control. Figure 10(b) is a graph in case the output of a temperature sensor disposed in the neighborhood of a place, in which the cooking material is supported, is used as an input signal for the temperature control. With the oil temperature detection system shown in Figure 10(b), the temperature change of the conductive hollow cylindrical member is in a range of about 50 °C, and the temperature change of oil is in a range of about 5 °C. In contrast, with the conductive hollow cylindrical member temperature detection system shown in Figure 10(a), the temperature change of the conductive hollow cylindrical member is suppressed in a range of about 5 °C and the temperature change of oil is controlled in a range of about 1 °C. Thus, a very high accuracy temperature control can be realized. As has been described in the foregoing, by using the low-frequency induction heater according to the invention, it is possible to obtain a satisfactory efficiency of energy transfer from the conductive hollow cylindrical member to the liquid in the container. It is thus possible to improve the rate of temperature rise and reduce the time from the start of energization till the start of cooking. In addition, power supplied to the primary side can be used efficiently for the heating of the liquid in the container. It is thus possible to prevent a localized temperature rise and obtain an effect of saving energy. Further, since the conductive hollow cylindrical member is made of a single material, its strain or deformation due to temperature rise or its electrolytic corrosion due to leakage current can be prevented, and thus it is possible to provide a heater having satisfactory life and durability. Further, by forming the conductive hollow cylindrical member by using stainless steel such that its thickness is in a range of 2 mm to 6 mm, it is possible to prevent reduction of the Joule heat generation efficiency. In addition, it is possible to improve the mechanical strength, thus preventing deformation or strain during cooking or cleaning of the cooking device. Further, with a temperature sensor or the like buried inside the conductive hollow cylindrical member, it is possible to hold a constant heating temperature of the conductive hollow cylindrical member, thus permitting cooking under a stabilized temperature condition. Further, because the number of layers of the primary winding coil is in a range of from 1 layer to 2 layers, it is possible to suppress the temperature rise of the primary winding and prevent an accident caused by defective insulation of an insulator in the primary winding. Thus, the reliability of the product can be improved. Further, by using the low-frequency induction heating cooking device using the low-frequency induction heater according to the invention, quick heating can be obtained while preventing a localized temperature rise of water or oil in the cooking vessel. Particularly, it is possible to prevent deterioration of the cooking oil and extend the use period thereof.

Claims: A low-frequency induction heater (10) comprising a primary winding coil (2) made of aluminium wire having a core (1) and a secondary winding conductive hollow cylindrical member (3) surrounding that primary winding coil (2), said conductive hollow cylindrical member (3) being constituted of stainless steel material, characterized in that , said core (1) has a coil shape and is made of laminated, high magnetic permeability material plate and has a slit in the axial direction, and in that said conductive hollow cylindrical member (3) has a thickness ranging from 2 mm to 6 mm. The low-frequency induction heater (10) according to claim 1, wherein a temperature sensor (4) is inserted and secured in an elongated hole formed in a part of said conductive hollow cylindrical member (3). The low-frequency induction heater (10) according to claim 1, wherein the number of layers of said primary winding coil (2) is in a range of 1 layer to 2 layers. The low-frequency induction heater (10) according to claim 1, wherein said core (1) is made of silicon steel plate. The low-frequency induction heater (10) according to claim 1, wherein the outer diameter of said core (1) ranges from 10 mm to 200 mm. The low-frequency induction heater (10) according to claim 1, wherein the length of said core (1) ranges from 100 mm to 2,000 mm. The low-frequency induction heater (10) according to claims 1 or 3, wherein the diameter of said wire comprised in said primary winding coil (2) is in a range of 2 mm to 8 mm. The low-frequency induction heater (10) according to claim 3, wherein the number of turns of said wire in a first layer in said primary winding coil (2) is in a range of 50 turns to 200 turns. The low-frequency induction heater (10) according to claim 3, wherein the number of turns of said wire in a second layer in said primary winding coil (2) is in a range of 10 turns to 70 turns. The low-frequency induction heater (10) according to claim 1, wherein the length of said secondary winding conductive hollow cylindrical member (3) ranges from 100 mm to 2,000 mm. The low-frequency induction heater (10) according to claim 1, wherein the outer diameter of said secondary winding conductive hollow cylindrical member (3) ranges from 30 mm to 300 mm. The low-frequency induction heater (10) according to claim 2, wherein said temperature sensor (4) is a thermocouple. The low-frequency induction heater (10) according to claim 2 or 12 wherein said temperature sensor (4) is provided inside an upper portion of said secondary winding conductive hollow cylindrical member (3).

Inventors: IGUCHI ATSUSHI, IGUCHI, ATSUSHI
Applicants: HIDEC CORP LTD, HIDEC CORPORATION LTD.
Application year: 1991
Main classification: H05B6
IPC codes: H05B6