JP6925890B2 - Electromagnetic induction rice cooker - Google Patents

Description

The present invention relates to an electromagnetic induction rice cooker.

In recent years, rice cookers that employ a high-frequency electromagnetic induction device as a heating means for heating the inner pot of the rice cooker have become mainstream. In the case of an electromagnetic induction type rice cooker, an eddy current is generated in the inner pot by a high frequency magnetic field generated by a heating coil, and Joule heat generated by this current is used to generate heat in the inner pot. The heating coil is provided in a flat shape at the bottom of the inner pot. Since the induced magnetic field generated by the heating coil is concentrated immediately above the heating coil, the boiling point is also locally concentrated. When the rice is locally concentrated and boiled, it boils upward from the local part, so that there is a problem that temperature variation occurs throughout the rice and uneven cooking occurs.

In order to solve such a problem, for example, the technique described in Japanese Patent Application Laid-Open No. 2013-252168 (Patent Document 1) has been proposed. Patent Document 1 includes a planar heating coil arranged on the bottom surface of the pot and generating an induced magnetic field for heating the pot, an inverter that applies a high-frequency current to the heating coil, and a control means for controlling the inverter. Disclosed is a configuration in which the inner peripheral portion of the bottom surface of the pot, which does not face the heating coil, is convex toward the inner surface. In Patent Document 1, a path for locally concentrated boiling bubbles to move along a convex shape to the bottom surface of the pot is secured, and the boiling bubbles generated immediately above the heating coil and upward are formed into the convex shape of the bottom surface of the pot. By dispersing it in the path that moves along it, it prevents water from concentrating in a specific place.

Japanese Unexamined Patent Publication No. 2013-252168

In Patent Document 1, the inner peripheral portion of the bottom surface of the pot to which the heating coil does not face is made convex toward the inner surface to disperse the boiling bubbles, but the induced magnetic field generated by the heating coil is that of the heating coil. Since it is concentrated in the vicinity of the inner peripheral portion, the induced magnetic field generated by the heating coil is small on the radial outer side of the bottom surface of the pot, and sufficient heat generation force cannot be obtained. Therefore, it has been difficult to eliminate uneven cooking of rice with the technique described in Patent Document 1.

Therefore, an object of the present invention is to provide an electromagnetic induction rice cooker capable of solving the above-mentioned problems and reducing uneven cooking of rice.

In order to achieve the above object, the features of the present invention are an inner pot detachably housed in the main body, a heating coil arranged below the inner pot, and an inverter that supplies power to the heating coil. and a substrate, the bottom of the kiln to form a non-magnetic metal layer, forming a plurality of magnetic layers having different thicknesses between the inner pot and the non-magnetic metal layer, said hook The bottom is formed in a circular shape, the plurality of magnetic material layers are formed so as to have different thicknesses in the circumferential direction, and the plurality of magnetic material layers are a thick first magnetic material layer and a thin second magnetic material layer. When the heating coil current is low and the heating coil current is low, the first magnetic layer and the second magnetic layer do not have a magnetically saturated region, and when the heating coil current increases, the second magnetic material is included. The layer causes magnetic saturation, the magnetic flux does not pass through, and the magnetic flux interlinks with the first magnetic material layer .

According to the present invention, since the heat generation distribution of the inner pot can be changed, it is possible to provide an electromagnetic induction rice cooker capable of reducing uneven cooking of rice.

It is a structural drawing which divided into four the electromagnetic induction heating rice cooker which concerns on 1st Example of this invention. It is sectional drawing of the bottom surface of the inner pot 1 which concerns on 1st Example of this invention. It is a top view of the bottom part of the inner pot 1 which concerns on 1st Example of this invention. It is a figure which shows the relationship between the thickness of a magnetic material and the saturation magnetic flux. It is a figure which shows the operation waveform of the heating coil current and the magnetic flux which concerns on 1st Example of this invention. It is an operation diagram which shows the magnetic flux path at the time of low current which concerns on 1st Example of this invention. It is an operation diagram which shows the magnetic flux path at the time of the medium current which concerns on 1st Example of this invention. It is an operation diagram which shows the magnetic flux path at the time of a large current which concerns on 1st Example of this invention. It is a rice cooking process diagram which concerns on 1st Example of this invention. It is a figure which shows the relationship between the inverter circuit and the control circuit of the electromagnetic induction rice cooker which concerns on 1st Embodiment of this invention. It is a figure which shows the operation waveform of the IGBT 41 and the IGBT 42 which concerns on the 1st Example of this invention. It is a figure which shows the relationship between the inverter drive frequency and the input power which concerns on 1st Embodiment of this invention. It is sectional drawing of the inner pot structure which concerns on 2nd Example of this invention. It is a top view of the inner pot bottom part which concerns on 3rd Example of this invention. It is sectional drawing of the inner pot which concerns on 4th Example of this invention. It is a figure which shows the relationship between the magnetic field H of ferrite, the magnetic flux density B, and the relative magnetic permeability μ. It is a figure which shows the operation waveform of the heating coil current and the magnetic flux density at the time of medium power which concerns on 4th Embodiment of this invention. It is a figure which shows the operation waveform of the heating coil current and the magnetic flux density at the time of high power which concerns on 4th Example of this invention. It is a figure which shows the relationship between the inverter circuit and the control circuit of the electromagnetic induction rice cooker which concerns on 5th Embodiment of this invention. It is a figure which shows the operation waveform of the IGBT 52 and the heating coil current which concerns on 5th Example of this invention. It is a figure which shows the relationship between the inverter drive frequency and the input power which concerns on 5th Embodiment of this invention.

Hereinafter, examples of the present invention will be described with reference to the drawings.

The first embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a structural diagram of the electromagnetic induction heating rice cooker according to the first embodiment of the present invention divided into four parts. In the electromagnetic induction rice cooker 10, the inner pot 1 is detachably housed in a bottomed cylindrical body (not shown) having an open upper part, and the main body and the upper part of the inner pot 1 are covered with a lid. A spirally wound heating coil 2 is arranged below the inner pot 1, and a rod-shaped ferrite 3 is radiantly arranged below the heating coil 2. A shield plate 4 is arranged under the ferrite 3. The inverter board 5 that supplies a high frequency current to the heating coil 2 is arranged below the shield plate 4. The radiated magnetic field from the heating coil 2 is shielded by the shield plate 4 to prevent the inverter board 5 from malfunctioning. The inverter board 5 is equipped with semiconductor elements such as a diode bridge and an IGBT, heat radiation fins for cooling the diode bridge and the IGBT, and power supply components for driving the IGBT.

FIG. 2 shows a cross-sectional structural view of the bottom of the inner pot 1 according to the first embodiment of the present invention. The inner pot 1 is made of a metal material. As a metal material, the inner pot 1 is made of, for example, a clad material of an aluminum material and an iron material. The innermost surface layer of the inner pot 1 is formed of an aluminum layer 11, a magnetic material layer 12 is formed on the surface thereof, and a non-magnetic metal layer 13 is formed on the outermost surface. The inner pot 1 is composed of a three-layer structure of an aluminum layer 11, a magnetic material layer 12, and a non-magnetic metal layer 13. A heating coil 2 is arranged below the inner pot 1. Since it is desirable that the inner surface layer of the inner pot 1 is made of a lightweight material having good heat conduction, aluminum is used in this embodiment. Since the magnetic material layer 12 has a high magnetic permeability and a material that generates magnetic saturation is desirable, ferrite is used in this embodiment. The non-magnetic metal layer 13 of the outermost surface layer is a material that also serves as a rust preventive and a heat generating portion of the magnetic material layer 12. A non-magnetic metal such as aluminum is desirable as a material that does not easily rust, but since aluminum has low electrical resistance and is not suitable for electromagnetic induction heating, AlSi in which silicon is added to aluminum is used in this embodiment. AlSi has a higher electrical resistance than aluminum, and induction heating is possible by forming the layer thickness thinner.

In FIG. 2, the magnetic material layer 12 formed between the inner pot 1 and the non-magnetic metal layer 13 is formed on the bottom of the inner pot 1 with different thicknesses t1, t2, and t3. The relationship between the thicknesses of t1, t2, and t3 is t3> t1> t2. Further, the bottom portion of the inner pot 1 of this embodiment is formed in a circular shape, and the magnetic material layer 12 is arranged in the order of t1, t2, t3 from the center of the inner pot toward the outer side in the radial direction. The layer t3 located on the outer side in the radial direction is the thickest, and then the layer t1 on the center side of the inner pot 1 is the thickest. That is, magnetic layers having different thicknesses in the radial direction are formed on the bottom of the inner pot 1. The magnetic layer 12 has the thinnest layer t2 in the intermediate portion located between the outer side in the radial direction and the center of the inner pot 1. The thickness of each layer of the inner pot 1 is about 2 mm for the aluminum layer 11, about 0.1 mm to 0.5 mm for the magnetic material layer 12, and about 0.05 mm to 0.2 mm for the non-magnetic metal layer 13 (AlSi layer). In this embodiment, three layers having different thicknesses are formed, but the number of layers is not limited to three. It is preferable to form at least two (plurality) magnetic layers having different thicknesses.

FIG. 3 shows a plan view of the bottom of the inner pot 1 according to the first embodiment of the present invention. As shown in FIG. 3, different patterns are formed concentrically on the bottom of the inner pot 1 due to the difference in the thickness of the magnetic material layer 12. The pattern of the magnetic material layer 12 has a circular shape, and the first magnetic material layer 12a having a thickness t3, the second magnetic material layer 12b having a thickness t2, and the third magnetic material layer 12c having a thickness t1 from the outside in the radial direction. Are arranged as. At the bottom of the circular inner pot 1, a plurality of magnetic material layers (first magnetic material layer 12a, second magnetic material layer 12b, third magnetic material layer 12c) having different thicknesses in the radial direction are formed. It is formed. In this embodiment, the first magnetic material layer 12a located on the outer side in the radial direction is formed thicker than the second magnetic material layer 12b and the third magnetic material layer 12c located on the inner side in the radial direction. Further, the third magnetic material layer 12c is formed thicker than the second magnetic material layer 12b.

The aluminum layer 11, the magnetic material layer 12, and the non-magnetic metal layer 13 are formed by surface treatment such as plating or thermal spraying. When forming the magnetic layer 12 having different thicknesses, the spraying position and the spraying time are changed, or masking or the like is used to form the magnetic layer 12.

FIG. 4 shows the relationship between the thickness of the magnetic material and the saturation magnetic flux. Ferromagnets such as iron have a large saturation magnetic flux, and even if the thickness is reduced, the saturation magnetic flux is large. On the other hand, in ferrite or the like of a magnetic device, the saturation magnetic flux changes greatly depending on the thickness, so the magnetic saturation region can be controlled by changing the thickness of the bottom of the inner pot 1.

FIG. 5 shows the operating waveforms of the heating coil current and the magnetic flux according to the first embodiment of the present invention. FIG. 5 shows changes in the heating coil current and the magnetic flux density at an inverter drive frequency of 25 kHz. In the figure, the saturation magnetic flux (t1) indicates the saturation magnetic flux at the position of t1 of the magnetic material layer 12 in FIG. 2, and the saturation magnetic flux (t2) is t2 of the magnetic material layer 12. It shows the saturation magnetic flux at the position of. As the magnetic material layer 12, ferrite whose saturation magnetic flux changes depending on the thickness is used. An alternating current of up to about 10 A is flowing through the coil. The magnetic flux shown by the broken line changes in proportion to the magnitude of the coil current.

FIG. 6 shows a magnetic flux path at a low current according to the first embodiment of the present invention. In the low current region, since it is equal to or less than the saturation magnetic flux (t2) in FIG. 5, all the magnetic material layers 12 are not magnetically saturated, and the magnetic flux passes through the magnetic material layer 12 of the entire pot bottom, so that the entire inner pot surface is covered. Heat is generated in the entire AlSi layer.

FIG. 7 shows the magnetic flux path at the time of medium current according to the first embodiment of the present invention. In the medium current region shown in FIG. 5, the magnetic flux density becomes larger than the saturation magnetic flux (t2), so that the t2 region of the magnetic material layer 12 undergoes magnetic saturation and the magnetic flux density drops sharply. As a result, as shown in FIG. 7, magnetic saturation occurs in the t2 region of the thinnest magnetic material layer 12 at medium power, and magnetic flux does not pass through. Therefore, the heat generation in the AlSi layer on the surface is reduced, and the heat is generated in the AlSi layer on the surface of the t1 and t3 regions of the magnetic material layer 12.

FIG. 8 shows the magnetic flux path at the time of a large current according to the first embodiment of the present invention. At the time of a large current in FIG. 8, since the saturation magnetic flux (t1) is exceeded, the t1 and t2 regions of the magnetic material layer 12 are magnetically saturated, and the magnetic flux passes only through the t3 region of the magnetic material layer 12. Therefore, the surface AlSi layer in the t3 region of the magnetic material layer 12 generates heat. By controlling the electromagnetic induction inverter circuit 30 by the control circuit 45 in this way (FIG. 10) and changing the magnitude of the heating coil current, the heat generation distribution can be changed and the heat generation region can be dispersed, so that uneven cooking of rice can be suppressed. can do.

Next, an example in which this embodiment is applied to the rice cooking process will be described with reference to FIG. FIG. 9 is a rice cooking process diagram according to the first embodiment of the present invention. The rice cooking process is divided into a water absorption process, a heating process, and a steaming process. Rice starch is clogged from the outside to the cells in the center of the rice grain, and when heated without absorbing water, it changes from the surface side of the rice grain to rice, so the inside remains hard. For this reason, it is desirable to soak the rice in water in advance so that the starch in the center is sufficiently absorbed with water. Normally, the ideal time for rice to absorb water is 30 minutes in the summer and 60 minutes in the winter. However, depending on the situation, it may be necessary to cook rice in a short time. Therefore, the rice cooker is provided with a water absorption process that allows rice to absorb water in a short time. In order to allow rice to absorb water in a short time, it is desirable to carry out the process at a high temperature not exceeding the gelatinization temperature (about 60 ° C.) of starch, and a medium current is supplied to the heating coil 2 to heat the rice with medium power. After the water absorption process is completed, the process proceeds to the heating process. In the heating step, water is rapidly heated to a boiling state, and a large current is supplied to the heating coil 2 to promote the formation of starch into a lake and the starch is cooked at once, and the starch is heated with a large amount of electric power. At this time, in the inner pot 1, the region t3 (the region of the magnetic material layer 12a in FIG. 3), that is, the outer portion of the bottom surface of the inner pot 1 is heated. Heating at high power is local, but in the outer part of the inner pot 1, rice rises along the side wall of the inner pot 1 together with boiling bubbles, reaches the upper part of the inner pot 1, and then the center of the inner pot 1. Move in the direction. By repeating this operation, convection of rice is generated in the inner pot 1, the whole rice is heated, and uneven cooking of rice can be suppressed. After the heating process is completed, the process proceeds to the steaming process. Only in the heating process, a large amount of water remains on the surface of the rice, and it is not possible to penetrate the rice firmly into the inside. Therefore, in order to allow the moisture on the surface to permeate into the inside by the steaming step, it is heated with low electric power. At this time, since the entire bottom surface of the inner pot 1 is heated, the water distribution of the rice can be made uniform. According to this embodiment, the heat generation distribution can be changed by the heating power (heating coil current), and it becomes possible to realize heating suitable for the rice cooking process.

Next, the configuration of the electromagnetic induction inverter circuit 30 for realizing these will be described. FIG. 10 is a diagram showing the relationship between the inverter circuit and the control circuit of the electromagnetic induction rice cooker according to the first embodiment of the present invention.

The rectifier circuit 32 rectifies the alternating current supplied from the commercial power supply 31 and outputs it as direct current to the current resonance inverter unit 34 via the filter circuit 33. The filter circuit 33 smoothes the direct current rectified by the rectifier circuit 32. A current resonance inverter unit 34 is connected between the node p of the positive electrode terminal and the node n of the negative electrode terminal, which are the outputs of the filter circuit 33. The current resonance inverter unit 34, which is a high-frequency inverter, is configured by connecting the heating coil 35 of the resonance circuit 36 to a half-bridge circuit in which the IGBT 41 and the IGBT 42 are connected in series.

A diode D1 (first diode) is connected in antiparallel to the IGBT 41 (first semiconductor switching element) of the upper arm, and a snubber capacitor 43 (first snubber capacitor) is connected in parallel.

A diode D2 (second diode) is connected in antiparallel to the IGBT 42 (second semiconductor switching element) of the lower arm, and a snubber capacitor 44 (second snubber capacitor) is connected in parallel. Here, let the connection point of the IGBTs 41 and 42 be the node a.

The snubber capacitors 43 and 44 are charged and discharged by the breaking current at the turn-off of the IGBTs 41 and 42. The capacitance of the snubber capacitors 43 and 44 is sufficiently larger than the output capacitance between the collector and the emitter of the IGBTs 41 and 42. Therefore, the change in the voltage applied to the IGBTs 41 and 42 at the time of turn-off is reduced, and the turn-off loss is suppressed. In the current resonance inverter unit 34, a direct current is supplied from the rectifier circuit 32 via the filter circuit 33, converted into a high-frequency alternating current and output, and drives the heating coil 35.

The resonance circuit 36 includes resonance capacitors 38 and 39 (first and second resonance capacitors) and a heating coil 35. A series circuit of resonance capacitors 38 and 39 is connected between the node p and the node n. A heating coil 35 is connected between the node b connecting the resonance capacitors 38 and 39 and the node a connecting the IGBTs 41 and 42. The direction of the resonance current IL of the heating coil 35 is positive in the direction from the node a to the node b (the direction of the arrow in FIG. 16). Further, a current detector 37 for detecting the current of the heating coil 35 is provided between the node a and the node b.

The control circuit 45 includes a drive circuit 47-1 for driving the IGBT 41, a drive circuit 47-2 for driving the IGBT 42, and a control means 48 for controlling the coil current. The drive circuits 47-1 and 47-2 are both controlled by the drive signal generation circuit 46. The control circuit 45 controls the current resonance inverter unit 34.

Next, a normal heating operation will be described with reference to FIG. FIG. 11 is a diagram showing operating waveforms of the IGBT 41 and the IGBT 42 according to the first embodiment of the present invention. In mode M11, the upper arm IGBT 41 is off and the lower arm IGBT 42 is on. The collector voltage of the IGBT 41 of the upper arm keeps a predetermined value. The collector voltage of the lower arm IGBT 42 is approximately 0V.

At this time, a current flows from the filter capacitor C0 to the path of the resonance capacitor 38, the heating coil 35, and the IGBT 42, and a current flows from the resonance capacitor 39 to the path of the heating coil 35 and the IGBT 42. Since the IGBT 41 of the upper arm is off, no current flows. When the control circuit 45 turns off the IGBT 42 in the mode M11, the mode M12 is entered.

In the mode M12, the IGBT 41 of the upper arm and the IGBT 42 of the lower arm are off. Due to the energy stored in the heating coil 35, a current flows from the snubber capacitor 43 to the path of the resonance capacitor 38 and the heating coil 35, and a current flows from the heating coil 35 to the path of the snubber capacitor 44 and the resonance capacitor 39. At this time, the collector voltage of the IGBT 42 of the lower arm gradually rises to zero voltage switching, and the switching loss becomes small. When the collector voltage of the IGBT 42 of the lower arm (voltage of the node a) exceeds the voltage of the node p of the positive electrode terminal, the mode M13 is entered.

In the mode M13, the diode D1 is turned on, a current flows through the paths of the diode D1, the resonance capacitor 38, and the heating coil 35, and a current flows from the diode D1 through the paths of the filter capacitor C0, the resonance capacitor 39, and the heating coil 35. The control circuit 45 turns on the gate of the IGBT 41 of the upper arm during the energization period of the diode D1. The current resonance inverter unit 34 shifts to the mode M14 when no current flows through the diode D1.

Since the IGBT 41 is already turned on in the mode M14, zero voltage switching is performed and no switching loss occurs. Due to the energy stored in the resonance capacitor 39, a current flows from the resonance capacitor 38 to the path of the IGBT 41 and the heating coil 35, and a current flows from the filter capacitor C0 to the path of the IGBT 41, the heating coil 35 and the resonance capacitor 39. As a result, energy is stored in the heating coil 35. When the control circuit 45 turns off the IGBT 41 in the mode M14, the mode transitions to the mode M15.

In mode M15, the upper arm IGBT 41 and the lower arm IGBT 42 are off. Due to the energy stored in the heating coil 35, a current flows from the filter capacitor C0 to the path of the snubber capacitor 43, the heating coil 35, and the resonance capacitor 39, and a current flows from the snubber capacitor 44 to the path of the heating coil 35 and the resonance capacitor 39. At this time, the collector voltage of the IGBT 41 gradually rises, resulting in zero voltage switching.

Next, when the snubber capacitor 43 is charged to the potential of the node p and the snubber capacitor 44 is discharged, the diode D2 is turned on and the mode M16 is entered.

In the mode M16, when the diode D2 is turned on, a current flows through the paths of the diode D2, the heating coil 35, and the resonance capacitor 39, and a current flows through the paths of the resonance capacitor 38, the filter capacitor C0, the diode D2, and the heating coil 35. The control circuit 45 turns on the IGBT 42 during the energization period of the diode D2. When no current flows through the diode D2, the mode M11 is entered again.

By repeating the above six types of operations M11 to M16, a high-frequency resonance current IL flows through the heating coil 35 to heat the inner pot 1 placed on the upper side of the heating coil 35.

FIG. 12 is a diagram showing the relationship between the inverter drive frequency and the input power according to the first embodiment of the present invention. FIG. 12 shows the characteristics of the series resonance method in the operation of the circuit method described above. Normally, when adjusting the electric power, the inverter drive frequency is changed to operate on the resonance curve to control the electric power. In this embodiment, by providing a plurality of magnetic material layers having different thicknesses, the resonance frequency changes depending on the heating power. As a result, the frequency operates on the resonance curve 1 at low power, the resonance curve 2 at medium power, and the resonance curve 3 at high power. When the inner pot 1 is provided with a magnetic metal layer such as iron, it operates on one line such as the resonance curve 1 regardless of the magnitude of the electric power. Therefore, when power is controlled, the operating frequency range becomes wide. Moreover, since the maximum value of the resonance curve is low, it is difficult to increase the power consumption. On the other hand, in this embodiment, since the resonance curve changes due to the heating power, it operates on the operation curve shown in FIG. 12, the inverter drive frequency range can be narrowed, the maximum value becomes large, and a larger thermal power can be realized than before. ..

As described above, according to the present embodiment, by providing a plurality of magnetic material layers having different thicknesses, the heated portion of the inner pot 1 can be changed only by changing the magnitude of the current flowing through the heating coil 2. Therefore, control suitable for cooking rice can be easily performed. Then, it is possible to suppress uneven cooking of rice at the time of cooking rice, and it is possible to improve the taste of rice.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 13 is a cross-sectional view of the inner pot structure according to the second embodiment of the present invention. The difference from the first embodiment is that an inclined portion is formed in a portion where the thickness changes so that the thickness of the magnetic material layer 24 is continuously formed. That is, the magnetic material layer 24 is formed so that the thickness differs along the radial direction. The innermost surface layer of the inner pot 1 is formed of an aluminum layer 11, a magnetic material layer 24 is formed on the surface thereof, and a non-magnetic metal layer 13 is formed on the outermost surface. The inner pot 1 is composed of a three-layer structure of an aluminum layer 11, a magnetic material layer 24, and a non-magnetic metal layer 13.

Ferrite is used for the magnetic material layer 24. The non-magnetic metal layer 13 of the outermost surface layer uses AlSi obtained by adding silicon to aluminum.

The magnetic material layer 24 is formed on the bottom of the inner pot 1 with different thicknesses t1, t2, and t3. The relationship between the thicknesses of t1 to t3 is t3> t1> t2. Further, the magnetic material layer 24 is arranged in the order of t1, t2, t3 from the center of the inner pot 1 toward the outer side in the radial direction. The thickness of t1 is inclined so as to decrease from the center of the inner pot 1 toward t2 which is radially outward. The thickness of t2 is inclined so that the thickness increases from t2 toward t3 which is radially outward. The aluminum layer 11, the magnetic material layer 24, and the non-magnetic metal layer 13 are formed by a metal film treatment such as thermal spraying. When forming the magnetic layer 24 having different thicknesses, the spraying position and the spraying time are changed.

In this embodiment, since the thickness of t2 is changed, the portion that generates heat changes when a low current, a medium current, or a large current is applied to the heating coil 2. Since the operation overlaps with the contents described with reference to FIGS. 6, 7, and 8, the description thereof will be omitted here.

According to the second embodiment of the present invention, since the magnetic material layer 24 is inclined so that the thickness of the magnetic material layer 24 changes continuously, it is continuous in the radial direction according to the heating coil current. The heat generation distribution of the inner pot 1 can be changed, and uneven cooking of rice can be suppressed.

Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 14 is a plan view of the bottom of the inner hook 1 according to the third embodiment of the present invention. The difference from the first embodiment and the second embodiment is that the thickness of the magnetic layer is formed to be different in the circumferential direction. In FIG. 10, magnetic material layers 24 and 25 are formed in a fan shape at the bottom of the inner pot 1. These magnetic material layers have a structure in which the magnetic material layer 24 is thick and the magnetic material layer 25 is thin. Then, the magnetic material layers 24 and the magnetic material layers 25 having different thicknesses of the magnetic material layers are alternately arranged in the circumferential direction. With such a structure, the heat generation distribution can be changed by the heating coil current. When the heating coil current is low, there is no magnetic saturation region, magnetic flux is interlinked on the entire surface of the bottom of the pot, and heat is generated in the non-magnetic metal layer 13 (AlSi layer) on the surface. When the heating coil current increases, the magnetic material layer 25 undergoes magnetic saturation, magnetic flux does not pass through, magnetic flux interlinks only with the magnetic material layer 24, and the non-magnetic metal layer 13 (AlSi layer) on the surface generates heat. In this way, the heat generation distribution in the inner pot 1 changes depending on the magnitude of the heating coil current, and uniform heating becomes possible.

According to the third embodiment of the present invention, since the thickness of the magnetic layer is formed to be different in the circumferential direction, the heat generation distribution can be changed in the circumferential direction according to the heating coil current, and the rice in the circumferential direction can be changed. It can promote convection and suppress uneven cooking of rice.

Next, a fourth embodiment of the present invention will be described with reference to FIGS. 15 to 18. FIG. 15 is a cross-sectional view of the inner pot according to the fourth embodiment of the present invention.

What is different from the first to third embodiments is the structure formed without changing the thickness of the magnetic layer 27. The inner pot 1 is composed of a three-layer structure of an aluminum layer 11, a magnetic material layer 27, and a non-magnetic metal layer 13. Ferrite is used for the magnetic material layer 27. In this configuration, the relationship between the magnetic field H of ferrite, the magnetic flux density B, and the relative magnetic permeability μ will be described.

FIG. 16 is a diagram showing the relationship between the magnetic field H of ferrite, the magnetic flux density B, and the relative magnetic permeability μ. In FIG. 16, in the relationship between the magnetic field H and the magnetic flux density B, when the magnetic field H is applied from the outside, the magnetic flux density B increases along the initial magnetization curve due to the magnetic field in the ferrite. The region in which the magnetic field H is increased until the magnetic flux density B does not increase any more is called the saturated magnetic flux density Bs, and represents the maximum magnetic flux density B that the magnetic material can flow.

On the other hand, in the relationship between the magnetic field H and the specific magnetic permeability μ, in the region where the magnetic flux density B increases with respect to the magnetic field H, the specific magnetic permeability μ increases depending on the inclination of the magnetic flux density B. When the magnetic flux density B approaches the saturation region, the slope of the magnetic flux density B becomes small, and the relative magnetic permeability μ decreases. When the magnetic field H is further increased, magnetic saturation occurs and magnetism is lost, and the relative magnetic permeability μ decreases to the magnetic permeability of vacuum, that is, 1.

The magnetic flux generated from the heating coil 2 is a characteristic proportional to the heating coil current. When the heating coil current is small, the magnetic material layer 27 is not magnetically saturated, magnetic flux passes through the entire magnetic material at the bottom of the inner pot 1, and the non-magnetic metal layer 13A (AlSi) on the surface generates heat.

17 and 18 show changes in the magnetic flux density when the heating coil current is increased. FIG. 17 is a diagram showing operating waveforms of the heating coil current and the magnetic flux density at the time of medium power according to the fourth embodiment of the present invention. FIG. 18 is a diagram showing operating waveforms of the heating coil current and the magnetic flux density at the time of high power according to the fourth embodiment of the present invention.

In FIG. 17, magnetic saturation occurs near the peak of the heating coil current, and the magnetic flux density drops sharply. After that, after passing the peak point, the magnetic saturation is eliminated as the heating coil current decreases, and the magnetic flux density rises sharply. Further, as the heating coil current decreases, the magnetic flux density also decreases. Then, a large magnetic flux change occurs four times in the heating coil current cycle at the time of medium power. In induction heating, an eddy current flows through the metal due to a change in the magnetic flux in the metal, and heat is generated. Therefore, the amount of heat generated increases as the change in the magnetic flux increases.

FIG. 18 shows the change in magnetic flux density when the heating coil current is further increased. As shown in FIG. 18, it can be seen that the magnetic flux density of the medium current portion of the heating coil current drops sharply. After the magnetic flux density decreases, the magnetic saturation is eliminated, so that the magnetic flux density increases sharply. After the magnetic flux density increases, the heating coil current also gradually increases, the magnetic flux density also gradually decreases, and the magnetic flux density gradually increases as the heating coil current decreases. Then, after the magnetic flux density rises, magnetic saturation occurs, and the magnetic flux density drops sharply. After that, the magnetic saturation is eliminated and the magnetic flux density increases. After the magnetic flux density increases, the magnetic flux density also decreases as the heating coil current decreases. In the heating coil current cycle at high power, a large magnetic flux change occurs eight times. This is because, from the relationship between the magnetic field and the specific magnetic permeability in FIG. 16, as the magnetic field increases, the specific magnetic permeability decreases, so that the magnetic flux density decreases and becomes equal to or less than the saturated magnetic flux density. As a result of the occurrence of a plurality of magnetic field changes within the heating coil current cycle in this way, the calorific value of the inner pot 1 increases due to the increase in the eddy current loss as described above.

According to this embodiment, ferrite is used as the magnetic material layer 27 formed substantially uniformly on the entire bottom of the inner pot 1, and the heating coil current is controlled in consideration of the magnetic field H and the magnetic flux density B of the ferrite to control the heating coil. since a plurality of magnetic field change in current period is possible to generate, it is possible to increase the amount of heat generated by eddy current loss. As a result, the entire bottom of the inner pot 1 can be heated almost uniformly, and uneven cooking of rice can be suppressed.

Next, the fifth embodiment will be described with reference to FIGS. 19 to 21. The fifth embodiment is a modification of the inverter means and the control means in FIG. FIG. 19 is a diagram showing the relationship between the inverter circuit and the control circuit of the electromagnetic induction rice cooker according to the fifth embodiment of the present invention.

In FIG. 19, the electromagnetic induction inverter circuit 40 is composed of a commercial power supply 31, a rectifier circuit 32, a filter circuit 33, a voltage resonance inverter unit 50, and a control circuit 54.

Next, the connection relationship of each configuration will be described.
The commercial power supply 31 is connected to the filter circuit 33 via the rectifier circuit 32. A voltage resonance inverter unit 50 is connected to the filter circuit 33. The filter circuit 33 is composed of an inductor L0 and a filter capacitor C0. In the voltage resonance inverter section 50, the resonance circuit 51 and the IGBT 52 are connected in series. A diode D3 is connected to the IGBT 52 in antiparallel. In the resonance circuit 51, the heating coil 35 and the resonance capacitor 53 are connected in parallel. The control circuit 54 includes a control means 48, a drive signal generation circuit 46, a drive circuit 55, and a comparison circuit 56. The control circuit 54 controls the voltage resonance inverter unit 50.

Next, a normal heating operation will be described with reference to FIG. FIG. 20 is a diagram showing operating waveforms of the IGBT 52 and the heating coil current according to the fifth embodiment of the present invention. Here, the direction of the current of the heating coil 35 is positive in the direction of the arrow (the flow direction from the heating coil 35 toward the IGBT 52).

The mode M21 is a period from when the IGBT 52 is turned off to the peak of the collector voltage of the IGBT 52.

When the IGBT 52 is turned off in the mode M21, the current flowing through the IGBT 52 is cut off, and the energy stored in the heating coil 35 causes the current to flow in the path between the heating coil 35 and the resonance capacitor 53. At this time, the collector voltage of the IGBT 52 rises in a sinusoidal manner, resulting in zero voltage switching (ZVS).

The mode M22 is a period from the peak to 0V when the collector voltage of the IGBT 52 reaches 0V. When the collector voltage of the IGBT 52 reaches its peak, the current of the heating coil 35 switches from positive to negative, the direction of the current is reversed, and the current flows through the paths of the resonance capacitor 53 and the heating coil 35.

The mode M23 is the energization period of the diode D3. In the mode M22, when the resonance capacitor 53 is discharged and the collector voltage of the IGBT 52 becomes 0V, the diode D3 is turned on, and a current flows through the paths of the heating coil 35, the filter capacitor C0, and the diode D3. The control circuit 54 turns on the gate of the IGBT 52 within the energization period of the diode D3. In the mode M23, when the energy of the heating coil 35 is exhausted, the heating coil current switches from negative to positive.

The mode M24 is the energization period of the IGBT 52. At this time, since the gate of the IGBT 52 is already on, current starts to flow. As a result, the voltage resonance inverter unit 50 realizes zero voltage switching in which no switching loss occurs. The current flows through the path of the filter capacitor C0, the heating coil 35, and the IGBT 52 and the path of the commercial power supply 31, the rectifier circuit 32, the inductor L0, the heating coil 35, the IGBT 52, and the rectifier circuit 32.
By repeatedly operating the modes M21 to the above modes M24, a high-frequency alternating current flows through the heating coil 35 to heat the pot.

FIG. 21 shows the relationship between the inverter drive frequency and the input power according to the fifth embodiment of the present invention. FIG. 21 shows the characteristics of the parallel resonance method in the circuit method and operation described above. Normally, when adjusting the electric power, the inverter drive frequency is changed to operate on the resonance curve to control the electric power. In this embodiment, since a plurality of layers having different thicknesses of the magnetic material layer are provided, the resonance curve changes depending on the heating power. In FIG. 21, the resonance curve 1 changes at low power, the resonance curve 2 changes at medium power, and the resonance curve 3 changes at high power. When the inner pot 1 is provided with a magnetic metal layer such as iron, it operates on one line such as the resonance curve 1 regardless of the magnitude of the electric power. Therefore, when power is controlled, the operating frequency range becomes wide. Moreover, since the maximum value of the resonance curve is low, it is difficult to increase the power consumption. On the other hand, in this embodiment, since the resonance curve changes due to the heating power, it operates on the operation curve shown in FIG. 21, the inverter drive frequency range can be narrowed, the maximum value becomes large, and a larger thermal power can be realized than before. ..

According to this embodiment, since a plurality of magnetic material layers having different thicknesses are provided, the resonance curve can be changed and a large thermal power can be realized.

The present invention is not limited to the above-described examples, and includes various modifications. For example, the above-described examples have been described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the described configurations.

1 Inner pot 2, 35 Heating coil 3 Ferrite 4 Shield plate 5 Inverter board 10 Electromagnetic induction rice cooker 11 Aluminum layer 12, 24, 25, 27 Magnetic material layer 13 Non-magnetic metal layer 30, 40 Electromagnetic induction inverter circuit 31 Commercial power supply 32 Rectifier circuit 33 Filter circuit 34 Current resonance Inverter section 36, 51 Resonance circuit 37 Current detector 38, 39, 53 Resonance capacitors 41, 42, 52 IGBT
43,44 Snubber capacitor 45, 54 Control circuit 46 Drive signal generation circuit 47-1,47-2, 55 Drive circuit 48 Control means 50 Voltage resonance inverter part 56 Comparison circuit

Claims (5)

An inner pot detachably housed in the main body, a heating coil arranged below the inner pot, and an inverter board for supplying electric power to the heating coil are provided, and a non-magnetic metal layer is provided on the bottom of the inner pot. , And a plurality of magnetic material layers having different thicknesses are formed between the inner pot and the non-magnetic metal layer.
The bottom of the inner pot is formed in a circular shape, and the plurality of magnetic material layers are formed so as to have different thicknesses in the circumferential direction.
The plurality of magnetic material layers include a thick first magnetic material layer and a thin second magnetic material layer.
When the heating coil current is low, the first magnetic material layer and the second magnetic material layer do not have a magnetic saturation region, and when the heating coil current increases, the second magnetic material layer causes magnetic saturation. , An electromagnetic induction rice cooker characterized in that magnetic flux does not pass through and magnetic flux is interlinked with the first magnetic material layer. In claim 1,
An electromagnetic induction rice cooker characterized in that the plurality of magnetic material layers are ferrite. In claim 1,
An electromagnetic induction rice cooker characterized in that the non-magnetic metal layer is AlSi obtained by adding silicon to aluminum. Oite to claim 1,
It has a rectifier circuit that converts alternating current to direct current, and a current resonance inverter unit that converts direct current connected to the rectifier circuit to alternating current.
The current resonance inverter unit
A resonance circuit composed of the heating coil and a resonance capacitor,
It is composed of an upper and lower arm composed of a first semiconductor switching element and a second semiconductor switching element connected in series with the resonance circuit.
An electromagnetic induction rice cooker characterized in that the resonance frequency of the resonance circuit is changed according to the heating power supplied to the heating coil. In claim 1,
It has a rectifier circuit that converts alternating current to direct current, and a voltage resonance inverter unit that converts direct current connected to the rectifier circuit to alternating current.
The voltage resonance inverter unit
A resonance circuit composed of the heating coil and a resonance capacitor,
It is composed of a first semiconductor switching element connected in series with the resonance circuit.
An electromagnetic induction rice cooker characterized in that the resonance frequency of the resonance circuit is changed according to the heating power supplied to the heating coil.

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