DE112013007556B4 - induction cooker - Google Patents

Abstract

An induction cooker comprising:a heating coil (11a) adapted to heat an object (5) to be heated by induction;a drive circuit (50) adapted to supply high-frequency power to said heating coil (11a);a load determination unit (32), adapted to perform a load determination process for determining a load of the object (5);wherein the process includes determining the material of the object (5) as being in a material category of at least two of a magnetic material, a high resistance non-magnetic material and a non-magnetic material low resistance falling through major classification,a controller (45) adapted to control an operation of the operating circuit (50) to control the high-frequency power supplied to the heating coil (11a);an input current detection unit (25a) adapted to detect an input current to the operating circuit (50); anda coil current detection unit (25b) configured to detect a coil current flowing through the heating coil (11a),wherein the load determination unit (32) is configured to based on a current change amount (11) in at least one of the input current or the coil current, in a period from the start of power supply of the heating coil (11a) to the lapse of a first heating period (t1),determining, by a subordinate classification, the material of the object (5) determined by the main classification as one of a plurality of materials, wherein the plurality of materials fall into a category determined by the main classification, wherein the controller (45) is adapted to control operation of the operation circuit (50) in accordance with a determination result by the load determination unit (32).

Description

technical field

The present invention relates to an induction cooker.

Technical background

In some related art induction cookers, a material of an object to be heated is determined based on an output current flowing through a heating coil (see Patent Literature 1, for example).

An induction cooker of Patent Literature 1 determines a material of the object to be heated from aluminum, stainless steel, and iron based on the output current of the heating coil.

Furthermore, in related art induction cookers, a temperature of the object to be heated is determined based on an input current to or a control amount for an inverter (see, for example, Patent Literatures 2 and 3).

An induction cooker of Patent Literature 2 includes control means for controlling the inverter so that the input current to the inverter becomes constant. When the control amount changes by a predetermined amount or more in a predetermined period of time, the induction cooker determines that a change in a temperature of the object to be heated is large and suppresses an output of the inverter. It is also disclosed that when the change in a control amount becomes equal to or smaller than the predetermined amount in the predetermined time period, the induction cooker judges that water boiling is completed and reduces an operation frequency to reduce the output of the inverter.

In Patent Literature 3, an induction cooker is proposed that includes input current change amount detection means for detecting the change amount in input current and temperature determination processing means for determining a temperature of the object to be heated based on the change amount in input current detected by the input current change amount detection unit. It is disclosed that when it is determined by the temperature determination processing means that the temperature of the object to be heated becomes equal to a boil-over temperature, a stop signal is output to stop the heating.

quote list

patent literature

• Patent Literature 1: JP S63 - 2284 A (page 2 to page 4)
• Patent Literature 2: JP 2008 - 181892 A (page 3 to page 5 and 1 )
• Patent Literature 3:. JP H05 62773 A (page 2 to page 3 and 1 )

The U.S. 2012/0 061 381 A1 discloses an induction cooking apparatus used in a general household kitchen or the like for performing temperature control of a cooking vessel by means of an infrared sensor.

The JP 2013 - 54 873 A discloses an induction cooking appliance configured to operate an electrical circuit according to a control mode suitable for a cooking vessel material.

Summary of the Invention

Technical problem

In the induction cooker described in Patent Literature 1, the object to be heated can be roughly classified and determined as any of aluminum, stainless steel, and iron. In particular, a rough determination can be made as to whether the object to be heated consists of a magnetic material or a non-magnetic material, for example based on properties of the material. However, the disadvantage is that the determined material of the object to be heated cannot be further classified. For example, the problem lies in that an object to be heated made of aluminum and an object to be heated made of copper are both low-resistance non-magnetic materials, and therefore not for a more detailed determination of the materials can be further classified.

In the induction cooker described in Patent Literature 2, an operation frequency of the inverter is controlled so that an input current becomes constant, and a change in temperature of the object to be heated is determined based on a change (Δf) in a control amount thereof. However, for some materials of the object to be heated, there is a problem that the change (Δf) in the amount of control of the operating frequency becomes so extremely small as to cause an error in detecting the temperature change of the object to be heated.

The temperature detection device for the induction cooker described in Patent Literature 3 has a problem that when an object made of different material is placed during heating, there is a risk that the input current becomes excessive depending on an operation frequency of the inverter , whereby a temperature of the inverter is increased, resulting in breakage of the inverter.

The present invention has been made to solve at least one of the problems described above, and provides an induction cooker capable of determining a material of an object to be heated with high accuracy. The present invention also provides an induction cooker capable of detecting a temperature change of the object to be heated regardless of the material of the object to be heated.

Troubleshooting

The invention is defined in claim 1 appended hereto. According to an embodiment of the present invention, there is provided an induction cooker, specifically comprising: a heating coil configured to inductively heat an object to be heated; an operating circuit configured to supply high-frequency power to the heating coil; a load determination unit configured to perform a load determination process of determining a load on the object; a controller configured to control an operation of the operation circuit to control high-frequency power supplied to the heating coil; an input current detection unit configured to detect an input current to the operation circuit; and a coil current detection unit configured to detect a coil current flowing through the heating coil, wherein the load determination unit is configured to obtain a current change amount of at least one of the input current and the coil current in a period from the start of power supply of the heating coil to the end a first heating period, and determining a material of the object based on the current change amount, wherein the controller is configured to control operation of the operation circuit in accordance with a determination result from the load determination unit.

Advantageous Effects of the Invention

According to one embodiment of the present invention, the material of the object to be heated can be determined with high accuracy.

character list

• [ 1 ] 1 14 is an exploded perspective view showing an induction cooker according to Embodiment 1.
• [ 2 ] 2 13 is a diagram showing an operating circuit of the induction cooker according to Embodiment 1.
• [ 3 ] 3 12 is a functional block diagram showing an example of control of the induction cooker according to Embodiment 1.
• [ 4 ] 4 14 is a characteristic diagram for determining load for an object to be heated based on a relationship between a heating coil current and an input current of the induction cooker according to Embodiment 1.
• [ 5 ] 5 14 is a relational diagram of input current with respect to an operating frequency of the induction cooker according to Embodiment 1 when a temperature of the object to be heated changes.
• [ 6 ] 6 is a graph of a proportion in an enlarged manner, which in 5 surrounded by the dashed line.
• [ 7 ] 7 are curves showing relationships between time and each, an operating frequency, a temperature and currents of the induction cooker according to Embodiment 1.
• [ 8th ] 8th 14 is a flowchart showing an example of operation of the induction cooker according to Embodiment 1 in a water heating mode.
• [ 9 ] 9 14 is a characteristic diagram for load determination for an object to be heated based on a relationship between the heating coil current and the input current of the induction cooker according to Embodiment 1.
• [ 10 ] 10 are curves showing relationships between time and each, operating frequency, temperature and currents of the induction cooker according to Embodiment 1.
• [ 11 ] 11 are curves showing relationships between time and each, operating frequency, temperature and currents in the induction cooker according to Embodiment 1.
• [ 12 ] 12 13 is a diagram showing another operating circuit of the induction cooker according to Embodiment 1.
• [ 13 ] 13 13 is a diagram showing part of an operation circuit of an induction cooker according to Embodiment 2.
• [ 14 ] 14 12 are graphs showing an example of an operation signal of a half-bridge circuit according to Embodiment 2.
• [ 15 ] 15 Fig. 12 is a diagram showing part of an operation circuit of an induction cooker according to Embodiment 3.
• [ 16 ] 16 12 are graphs showing an example of an operation signal of a full bridge circuit according to Embodiment 3.

Description of the embodiments

Embodiment 1

(Configuration)

1 13 is an exploded perspective view of an induction cooker according to embodiment 1.

As in 1 As shown, an induction cooker 100 includes at its top a top plate 4 on which an object 5 to be heated (which may be simply referred to as “object 5” hereinafter) such as a pot is placed. The induction cooker 100 includes in the top plate 4 a first heating panel 1, a second heating panel 2 and a third heating panel 3 as heating panels designed to heat the object 5 by induction, and further includes a first heating unit 11, a second heating unit 12 and a third heating unit 13 corresponding to the respective heating panels. The object 5 to be heated can be placed on any of the heating panels to be heated by induction.

In Embodiment 1, the first heating unit 11 and the second heating unit 12 are arranged to be provided on the left and right on a front side of a main body, and the third heating unit 13 is provided substantially in the center of a rear side of the main body.

The arrangement of the heating panels is not limited to this. For example, the three heating panels can be arranged side by side in a substantially linear fashion. Moreover, an arrangement can be made in which a center of the first heating unit 11 and a center of the second heating unit 12 are at different positions in a depth direction.

The whole of the top plate 4 is entirely formed of a material that is transparent to infrared rays, such as a heat-resistant tempered glass or crystal glass, and is fixed to an outer periphery of a top opening of the main body of the induction cooker 100 via a rubber boot or a sealing material in a waterproof state. Circular pot position indicators are formed on the top plate 4 by painting, printing or the like, indicating the general placing positions of pots corresponding to the heating areas (heating fields) of the first heating unit 11, the second heating unit 12 and the third heating unit 13.

On the front of the top plate 4, an operation unit 40a, an operation unit 40b, and an operation unit 40c (hereinafter sometimes collectively referred to as "operation unit 40") are provided as input devices used to set heating power and cooking menus (water heating mode, frying mode, and other modes ) for heating the object 5 by the first heating unit 11, the second heating unit 12 and the third heating unit 13 are formed. In addition, in the vicinity of the operation unit 40 are a display unit 41a, a display unit 41b, and a display unit 41c (hereinafter sometimes collectively referred to as “display unit 41”) for displaying an operation state of the induction cooker 100, input and operation details of the operation unit 40, and the like are designed as a notification unit 42 is provided. The present invention is not particularly limited to the case where the operation units 40a to 40c and the display units 41a to 41c are provided for the respective heating panels, or a case where the operation unit 40 and the display unit 41 are provided commonly for the heating panels.

Below the top plate 4 and inside the main body, the first heating unit 11, the second heating unit 12 and the third heating unit 13 are provided, and each of the heating units includes a heating coil (not shown).

Provided within the main body of the induction cooker 100 are an operation circuit 50 capable of supplying high-frequency power to the heating coil of the first heating unit 11, the second heating unit 12 and the third heating unit 13, and a controller 45 capable of controlling an operation of the entire induction cooker 100 including the operating circuit 50 is designed.

The heating coils have a substantially circular planar shape, and are constructed by winding in a circumferential direction a conductive wire made of an insulation-coated metal (e.g., copper, aluminum or other metals). An induction heating operation is performed by supplying the high-frequency power of the operation circuit 50 to each of the heating coils.

2 12 is a diagram showing the operation circuit of the induction cooker according to Embodiment 1. The operation circuit 50 is provided for each of the heating units. Circuit layouts of the heating units can be the same or can be different from each other. In 2 only the operating circuit 50 is shown. As in 2 As shown, the operating circuit 50 includes a DC power supply circuit 22, an inverter circuit 23, and a resonance capacitor 24a.

An input current detection unit 25a detects a current supplied from an AC power supply (conventional power supply) 21 to the DC power supply circuit 22 and outputs a voltage signal corresponding to an input current value to the controller 45 .

The DC power supply circuit 22 includes a diode bridge 22a, a reactor 22b and a smoothing capacitor 22c, and converts an AC voltage input from the AC power supply 21 into a DC voltage to supply the DC voltage to the inverter circuit 23 to spend

The inverter circuit 23 is a so-called half-bridge type inverter in which IGBTs 23a and 23b serving as switching elements are connected in series to the output of the DC power supply circuit 22, and diodes 23c and 23d are connected in parallel to the IGBTs 23a and 23b, respectively are connected as freewheeling diodes. The inverter circuit 23 is designed to convert DC power, which is output from the DC power supply circuit 22, into high-frequency AC power of about 20 kHz to about 50 kHz, and to supply the high-frequency AC power to a resonance circuit that includes a heating coil 11a and the resonance capacitor 24a. The resonance capacitor 24a is connected in series to the heating coil 11a, and the resonance circuit has a resonance frequency according to an inductance of the heating coil 11a, a capacitance of the resonance capacitor 24a, and the like. The inductance of the heating coil 11a changes in accordance with characteristics of the object 5 (metal load) when the metal load is magnetically coupled, and the resonance frequency of the resonance circuit changes in accordance with the inductance change.

With the above configuration, a high-frequency current of about several tens of amperes flows through the heating coil 11a. With the high-frequency magnetic flux generated by the flow of high-frequency current, the object 5 placed immediately above the heating coil 11a on the head plate 4 is heated by induction. The IGBTs 23a and 23b, which are switching elements, are made of silicon-based semiconductors, for example, but may be formed using wide-bandgap semiconductors made of silicon carbide, a gallium nitride-based material, or other wide-bandgap materials.

The wide bandgap semiconductors can be used for the switching elements to reduce injection losses in the switching elements. In addition, even when a switching frequency (operating frequency) is set to a high frequency (high speed), the operating circuit satisfactorily radiates heat, with the result that a cooling fin for the operating circuit can be made small, and reductions in size and cost of the operating circuit can be realized.

A coil current detection unit 25b is connected between the heating coil 11a and the resonance capacitor 24a. The coil current detection unit 25b detects a current flowing through the heater coil 11a, for example, and outputs a voltage signal corresponding to a heater coil current value to the controller 45.

A temperature detection unit 30 is formed of a thermistor, for example, and detects a temperature based on heat transmitted from the object 5 to the top plate 4 . The temperature detection unit 30 is not limited to the thermistor, and any sensor such as an infrared sensor can be used.

3 FIG. 12 is a functional block diagram showing an example of the controller of the induction cooker according to Embodiment 1. The controller 45 is explained with reference to FIG 3 described.

The controller 45, which is constructed by a microcomputer, a digital signal processor (DSP) or the like, is designed to control the operation of the induction cooker 100, and includes an operation control unit 31, a load determination unit 32, an operation frequency setting unit 33, a current change detection unit 34, a power adjustment unit 35 and an input/output control unit 36 .

The operation control unit 31 outputs operation signals DS to the IGBTs 23a and 23b of the inverter circuit 23 to cause the IGBTs 23a and 23b to perform a switching process and thereby the inverter circuit 23 to operate. Then, the operation control unit 31 controls the high-frequency power supplied to the heating coil 11 a to control heating of the object 5 . Each of the operating signals DS is, for example, a signal having a predetermined operating frequency of about 20 kHz to about 50 kHz with a predetermined ON duty ratio (e.g., 0.5).

The load determination unit 32 is configured to perform a load determination process on the object 5 and determine a material of the object 5 as a load. Details of the load determination process are described below.

The operation frequency setting unit 33 is configured to set an operation frequency f of the operation signals DS output to the inverter circuit 23 to supply power from the inverter circuit 23 to the heating coil 11a. Specifically, the operation frequency setting unit 33 has a function of automatically setting the operation frequency f in accordance with a determination result of the load determination unit 32. More specifically, the operation frequency setting unit 33 stores, for example, a table for determining the operation frequency f in accordance with the material of the object 5 and the set heating power. When inputted with a result of the load determination and the set heating power, the operation frequency setting unit 33 refers to the table to determine a value fd of the operation frequency f. The operating frequency setting unit value 33 sets a frequency higher than the resonance frequency of the resonance circuit so that the input current does not become too large.

In this way, the operating frequency setting unit 33 operates the inverter circuit 23 at the operating frequency f that corresponds to the material of the object 5 based on the load determination result, with the result that an input current increase can be suppressed, and thus the temperature increase of the inverter circuit 23 can be suppressed to increase reliability.

The current change detection unit 34 detects a current change amount ΔI of at least one of the input current and the coil current per predetermined time when the inverter circuit 23 is operated at the operation frequency f=fd set by the operation frequency setting unit 33 . The predetermined time may be a preset time period, or may be a time period changeable by a user's operation on the operation unit 40 .

The power adjustment unit 35 is configured to adjust an adjustment amount for the operation signal DS when the current change amount ΔI detected by the current change detection unit 34 becomes equal to or smaller than a threshold value. Specifically, the power adjustment unit 35 determines a preset increase amount Δf in an operating frequency as the adjustment amount. Then, the operation control unit 31 releases the setting of the operation frequency f, and increases the operation frequency f by the adjustment amount Δf (f=fd+Δf) to operate the inverter circuit 23.

The operation control unit 31 reduces the electric power supplied to the heating coil 11a in accordance with the adjustment amount set by the power adjustment unit 35 . The operation control unit 31 releases the setting of the operation frequency f=fd, and increases the operation frequency f by the increase amount Δf (f=fd+Δf) to operate the inverter circuit 23 .

(Operation)

Next, an example of operation of the induction cooker 100 according to Embodiment 1 will be described.

First, an operation in a case where the object 5 placed on the heating field of the top plate 4 is heated by induction with heating power set by the operation unit 40 will be described.

The object 5 to be heated is placed on the heating panel by the user, and a user's instruction is inputted by operating the operation unit 40 to start heating (inputting heating power). Then, the load determination unit 32 of the controller 45 executes the load determination process. The load determination process executed by the load determination unit 32 includes a “load determination process at the heating start” and a “load determination process during a heating operation”.

(Load determination process at heating start)

The material of the object 5 (pot) serving as a load is classified by a main classification (by which the object is roughly classified) as a magnetic material, non-magnetic High resistance material, or a low resistance non-magnetic material determined based on magnetic characteristics and impedance characteristics. The magnetic material includes, for example, iron and ferrite-based stainless steel (for example, 18Cr: JIS SUS430). The high resistance non-magnetic material includes, for example, austenitic stainless steel (for example, 18Cr-8Ni: JIS SUS304). The low resistance non-magnetic material includes aluminum or copper.

The load determination unit 32 classifies and roughly determines the material of the object 5 into any category corresponding to the magnetic material, the high-resistance nonmagnetic material, and the low-resistance nonmagnetic material based on a relationship between the input current and the coil current in the load determination process at the heating start. The load determination unit 32 also determines whether or not there is a no-load state, including a state in which the object 5 is not placed on the top plate 4 and a state in which the object 5 is not placed on the top plate 4, based on the relationship between the input current and the coil current which the object 5 is not placed in a sufficient size.

4 14 is a characteristic diagram for load determination for the object to be heated based on a relationship between the heating coil current and the input current in the induction cooker according to Embodiment 1.

As in 4 As shown, the relationship between the coil current and the input current differs depending on the material of the object 5 (pot) placed on the top plate 4. The load determination unit 32 stores a load determination table expressing the relationship between the coil current and the input current shown here in tabular form in advance 4 . By storing the load determination table internally, the load determination unit 32 can be designed in an inexpensive configuration.

In the load determination process at heating start, the controller 45 operates the inverter circuit 23 at a preset operation frequency for load determination for a predetermined determination period to detect the input current from an output signal output from the input current determination unit 25a. At this time, the controller 45 also detects the coil current from an output signal output from the coil current detection unit 25b. The controller 45 determines the category of the material of the placed object (pot) 5 to be heated from the detected coil current and input current and from the load determination table of FIG 4 , which shows the ratio. In this way, the load determination unit 32 roughly classifies and determines the material of the object 5 placed above the heating coil 11a based on a relationship between the input current and the coil current.

After the load determination process is performed at the heating start, the controller 45 performs a control operation based on a load determination result.

When the load determination result indicates that there is no load, the controller 45 controls the notification unit 42 to notify that heating cannot be performed, thereby prompting the user to place a pot. At this time, no high-frequency power is supplied from the drive circuit 50 to the heating coil 11a.

When the load determination result indicates any of the magnetic material, the high-resistance nonmagnetic material, and the low-resistance nonmagnetic material, the pots are made of materials that can be heated by the induction cooker 100 of Embodiment 1. Therefore, the controller 45 determines the operating frequency in accordance with the particular material. The operating frequency is set to a frequency higher than the resonance frequency so that the input current does not become excessively high. The operating frequency can be determined by referring to a frequency table, for example, in accordance with the material of the object 5 and the set heating power.

The controller 45 operates the inverter circuit 23 in a state where the specified operating frequency is fixed and starts an induction heating operation. In the state where the operating frequency is fixed, ON operation (ON/OFF ratio) of the switching elements of the inverter circuit 23 is also fixed.

In this way, the material of the object 5 placed above the heating coil 11a is detected and determined by the main classification. In accordance with the material of the object 5, the operating frequency of the inverter circuit 23 is determined. The inverter circuit 23 operates at the operating frequency. Therefore, the inverter circuit 23 can be operated in a fixed manner at the operating frequency in accordance with the material of the object 5, resulting in an increase in input current can suppress. Therefore, a temperature rise of the inverter circuit 23 can be suppressed, enabling reliability improvement.

(Load determination process during a heating operation)

5 14 is a relational diagram of the current with respect to the operating frequency of the induction cooker according to Embodiment 1 with a temperature change of the object to be heated.

In 5 the thin line represents a characteristic at a time when a temperature of the object 5 (pot) is low, while the thick line represents a characteristic when the temperature of the object 5 is high.

As in 5 shown, the reason why the characteristic changes depending on the temperature of the object 5 is that magnetic coupling between the heating coil 11a and the object 5 changes due to an increase in electrical resistance and a reduction in magnetic permeability of the object 5 caused by cause a rise in temperature.

6 is a graph of a proportion in an enlarged manner, which in 5 surrounded by the dashed line.

The controller 25 supplies the electric power with a fixed operating frequency of the inverter circuit 23 to the heating coil 11a, thereby implementing the heating of the object 5 . In this case, changes as in 6 shown when the temperature of the object 5 changes from low to high, a current value (operating point) when the operating frequency changes from point A to point B. Along with an increase in temperature of the object 5, the current gradually reduces.

The current change amount mentioned above differs depending on the material type of the object to be heated. Some materials have a large amount of current change, and the other materials have a small amount of current change.

As in 4 shown have approximately the same values for object 5 (pot with a "large" current change) drawn as a black diamond sign and object 5 (pot with a "small" current change) drawn as a black square sign the input current and the output current at a low temperature in an early stage of heating. In the above-mentioned load determination process at the heating start, the materials of the objects to be heated 5 described above are both determined as the magnetic material.

When the temperatures of the objects to be heated 5 rise due to the heating operation, a current for the pot with a “large” current change, drawn as a sketched rhombus, decreases. On the other hand, compared to the pot with a "large" current change, the current magnitude decrease for the pot with a "small" current change, plotted as a square sign sketched, is small.

Based on the above-mentioned fact, in the load determination process during the heating operation, the load determination unit 32 obtains a current change amount I1 of each of the input current and the coil current in a range from the start of power supply to the heating coil 11a to the lapse of a first heating period. Then, based on the current change amount I1, the material of the object 5 is determined by a sub classification (finer than the main classification). Specifically, the material is determined from a plurality of materials that fall into the category that falls into the main classification in the above-mentioned load determination process at the heating start.

For example, the load determination unit 32 stores in advance a relationship between the current change amount 11 and the material type associated with the value of the input current and the value of the coil current based on experimental data or the like.

Then, the load determination unit 32 determines the material type of the object 5 by the subordination classification by referring to the pre-stored information of the relationship between the current change amount 11 and the material type, based on at least one of the input current detected by the input current detection unit 25a or the coil current that is detected by the coil current detection unit 25b.

In this way, the material of the object 5 can be determined by the subclassification as, for example, a material of iron, ferrite-based stainless steel, and the like, which falls under the category of magnetic material.

As the temperature of the object 5 rises, both the input current and the coil current decrease. Therefore, in the load determination process during the heating operation, the material of the object 5 can be determined using the current change amount I1 in one, the input current and the coil current. If input current and coil current are both used, only one current change amount I1=sqrt{(change amount in input current) ² +(change amount in output current) ² } needs to be used. Using both the input current and the coil current, determination accuracy of the material of the object 5 can be further improved.

The first heating period may be a preset period, or may be determined based on the heating power or the cooking mode set by a user's operation of the operation unit 40 .

After the load determination process, while the heating process is being performed, the controller 45 performs a control operation based on the load determination result. For example, the operating frequency can be corrected in accordance with the material type of the object 5 determined by the subordination classification. Alternatively, an upper limit of an input heating power may be set in accordance with the material type of the object 5, for example.

As described above, based on the amount of current change in the period from the start of power supply to the heating coil 11a to the elapse of the first heating period, the material of the object 5 is determined by the subordination classification as one of the plurality of materials falling within the category defined by the Set is determined by the subordination classification. Therefore, the material of the object 5 can be determined with high accuracy.

(detection of a temperature change)

In a state where the operating frequency is fixed, the currents (input current and coil current) decrease in accordance with the temperature of the object 5. As in 6 As shown, when the temperature of the object 5 changes from low to high, the current value (operating point) changes from the point A to the point B with the operating frequency changes. Therefore, the currents decrease as the temperature of the object 5 increases.

Because of the above fact, the controller 25 obtains a current change amount (input current or coil current) per predetermined time (current change amount ΔI) at the fixed operating frequency of the inverter circuit 23, and detects a temperature change of the object 5 based on the current change amount ΔI.

As described above, the temperature change of the object 5 can be detected based on the current change. Therefore, compared with a temperature sensor or the like, the temperature change of the object can be detected at a higher speed.

(water boiling mode)

Next, an operation performed when the water boiling mode for performing a water boiling operation for water poured into the object 5 is selected from the operation unit 40 as the cooking menu (operation mode) will be described.

7 are curves showing relationships between time and each, operating frequency, temperature and currents of the induction cooker according to Embodiment 1.

In 7 elapsed time and characteristic changes during operation in the water boiling mode after water is poured into the object 5 are shown. 7(a) is a curve of the operating frequency, 7(b) is a curve of temperature (water temperature), and 7(c) is a curve of currents (input current and coil current). Note that information such as a black square character in 7(c) Drawings like the black square sign in 4 are equivalent to.

As in 7(a) As shown, the inverter circuit 23 is controlled at the fixed operating frequency. Then rises as in 7(b) shown, the temperature (water temperature) of the object 5 gradually increases to boiling. After cooking, the temperature becomes constant at around 100 °C. At that time take as in 7(c) shown, in accordance with the temperature rise of the object 5, the currents gradually decrease. After the water boils and the temperature becomes constant, the input current also becomes constant. In particular, the constant current means the water is boiling to complete the water boiling.

As in 7(c) shown, regardless of whether the object 5 is the pot with a “large” current change or the pot with a “small” current change, the current gradually decreases. After the water boils and the temperature becomes constant, the input current also becomes constant.

From the above fact, the controller 25 of Embodiment 1 obtains the current change amount per time (current change amount ΔI) at the fixed operating frequency of the inverter circuit 23. When the current change amount ΔI becomes equal to or smaller than the threshold value, it is determined that the water boiling is completed.

Details of the water boiling mode described above are made with reference to FIG 8th described.

8th 14 is a flowchart showing an operation example of the induction cooker according to Embodiment 1 in the water boiling mode.

First, after the object 5 is placed on the heating field of the top plate 4 by the user, the operation unit 40 receives a user's operation to start heating (inputting heating power). Then, the load determination unit 32 performs the load determination process at the heating start. In the load determination process at the heating start, the category of the material of the object to be heated (pot) 5 is determined by the main classification using the load determination table indicating the relationship between the input current and the coil current (step ST1). When determining that there is no load, the controller 45 controls the notification unit 42 to notify that there is no load, controlling so that high-frequency power is not supplied from the operation circuit 50 to the heating coil 11a.

Next, the operation frequency setting unit 33 determines the value fd of the operation frequency f in accordance with the category of the material determined based on the result of the load determination process at the heating start (step ST2). At this time, the operating frequency f is set to the operating frequency f=fd, which is higher than the resonance frequency of the resonance circuit, so that the input current does not become excessively large.

In step ST2, the operation frequency setting unit 33 may determine the value fd of the operation frequency f such that the high-frequency power supplied to the heating coil 11a has a maximum value in accordance with the category determined from the main classification by the load determination unit 32. For example, when the material of the object 5 is a magnetic material, the value fd of the operating frequency f is determined so that the electric power supplied to the heating coil 11a becomes 3 kW. Further, for example, when the material of the object 5 is a low-resistance nonmagnetic material, the value fd of the operating frequency f is determined so that the electric power supplied to the heating coil 11a becomes 1 kW.

As described above, by setting the high-frequency power supplied to the heating coil 11a to the maximum value, a temperature change rate of the object 5 increases to increase the current change amount I1 in the load determination process during the heating operation. Therefore, the determination accuracy of the object 5 can be further improved.

Thereafter, the operation control unit 31 controls the inverter circuit 23 with the operation frequency f fixed to fd, thereby starting the induction heating operation (step ST3). The load determination unit 32 starts measuring a first heating period t1 along with the start of the induction heating operation.

After the heating with the fixed operating frequency starts, the temperature (water temperature) of the object 5 gradually rises to boiling ( 7(b) ). During the control with the fixed operating frequency, the input current value (operating point) changes from the point A to the point B at the operating frequency, as in FIG 6 shown. Along with the rise in temperature of the object 5, the current gradually decreases.

During the induction heating operation, the current change detection unit 34 calculates the current change amount ΔI at predetermined sampling intervals (step ST4).

Next, the load determination unit 32 determines whether or not the first heating period t1 has elapsed from the heating start (step ST5). When the first heating period t1 has elapsed from the heating start, the operation returns to step ST4.

When the first heating period t1 has elapsed from the heating start, the load determination unit 32 determines the material of the object 5 as one of a plurality of materials falling into the category determined by the main classification through the subordinate classification in the above-mentioned load determination process during the heating operation (Step ST6).

When the first heating period has elapsed, the controller 45 may also control the inverter circuit 23 at the present operation frequency for load determination for the predetermined determination period in step ST6. In particular, in a case where power is started to be supplied to the heating coil 11a and the first heating period t1 has elapsed, the controller 45 control the inverter circuit 23 at the preset operating frequency for load determination for the predetermined determination period. The load determination unit 32 can determine the material of the object 5 based on the amount of change I1 generated between the current at the start of power supply to the heating coil 11a and the current after the first heating period t1 has elapsed.

Next, the controller 45 sets a threshold (Iref) of the current change amount ΔI in accordance with the material of the object 5 determined by the load determination unit 32 in step ST6 (steps ST7). An initial value of the threshold (Iref) may be preset, or may be inputtable via the operation unit 40 or the like. Alternatively, the initial value of the threshold value can be determined from the result of the load determination process at the heating start in step ST1.

The threshold value (Iref) is set such that the larger the current change amount I1 during the first heating period t1 in the material of the object 5 determined by the load determination unit 32, the larger the threshold value (Iref) is set. In particular, when the material of the object 5 has a small current change amount I1, a cooking determination condition is set narrow (the threshold value is set small), so that, for example, the current change amount ΔI per predetermined time is obscured by noise generated during detection of the current becomes. Furthermore, when the material of the object 5 has the large current change amount I1, the boiling determination condition is set wide (the threshold value is set high). In this way, boiling in accordance with the material of the object 5 can be recognized with high accuracy.

After step ST7, the controller 45 determines whether or not the current change amount ΔI is equal to or smaller than the threshold value (Iref) set in accordance with the material of the object 5 (step ST8).

When the temperature of the object 5 changes from low to high, the current change amount ΔI decreases ( 7(c) ). After the water boils and the temperature becomes constant, the current also becomes constant ( 7(c) ). Based on the constant current, the controller 45 determines that the current change amount ΔI of the current has become equal to or smaller than the threshold value (Iref) in the first heating period t1.

When the current change amount ΔI becomes equal to or smaller than the threshold value, the operation control unit 31 of the controller 45 releases the determination of the operation frequency and increases the operation frequency f of the inverter circuit 23 by the increase amount Δf (f=fd+fΔ) to increase the high-frequency power (heating power). which is supplied to the heating coil 11a (step ST9). In particular, during heat maintenance for the object 5, the heating power high enough to raise the temperature is not required. Therefore, an amount of heating from the heating coil 11a to the object 5 is reduced.

The controller 45 uses the notification unit 42 in FIG. 12 to notify the completion of water boiling (step ST10). The notification unit 42 notifies the cooking completion on the display unit 41, or gives the user an audio notification using a speaker (not shown), and therefore a method therefor is not particularly limited.

As described above, in the water boiling mode for setting the water boiling operation, the change amount ΔI of the current per predetermined time is obtained in the state where the operating frequency of the inverter circuit 23 is fixed. When the amount of change per predetermined time is equal to or smaller than the predetermined threshold value, the completion of water boiling is notified by the notification unit 42 .

Therefore, the conclusion of water boiling can be communicated quickly. Accordingly, the induction cooker that is easy to use can be achieved.

Furthermore, the controller 45 does not need a highly accurate microcomputer to calculate the current change amount ΔI of the input current. Therefore, the induction cooker capable of detecting water boiling by an inexpensive method can be achieved.

Furthermore, the material of the object 5 is determined by the main classification from the load determination unit 32, so that the threshold value (Iref) is set in accordance with the material of the object 5. Therefore, the kettle identification can be performed with high accuracy in accordance with the material of the object 5 .

(Variants)

Next, variants of the load determination process during the heating operation will be described.

(Determination considering temperature in early period of heating)

The current change amount I1 in the period from the heating start to the lapse of the first heating period t1 depends on the temperature change amount of the object 5, and is therefore influenced by the temperature of the object 5 in the early period of heating. Therefore, by determining the material considering the temperature of the object 5 at the start of heating, the material can be determined with increased accuracy.

9 12 is a characteristic diagram for a load determination for the object to be heated based on the relationship between the heating coil current and the input current of the induction cooker according to Embodiment 1.

In 9 a case is shown where object 5 drawn as a black diamond sign (pot with a "large" current change) and object 5 drawn as a black square sign (pot with a "small" current change) in have an intermediate temperature in the early part of the heating.

Compared to the case mentioned above, which occurs in 4 is shown where the temperature is low in the early part of heating, the drawn positions are different. In addition, the current change amounts are reduced until the temperature becomes high.

Because of the above fact, the load determination unit 32 obtains the temperature of the object 5 detected by the temperature detection unit 30 at the heating start. Then, in the load determination process during the heating operation, the load determination unit 32 determines, by the subordinate classification, the material of the object 5 as a material from the plurality of materials that falls into the category determined by the main classification based on the temperature of the object 5, which is determined by the temperature detection unit 30 is detected at the heating start, and the current change amount I1 in the period from the heating start to the lapse of the first heating period t1.

For example, a relationship between the current change amount I1 and the kind of material obtained by experimental data or the like is stored in advance in association with the temperature in the early period of heating. Then, the load determination unit 32 refers to the pre-stored information of the relationship between the current change amount I1 and the material type based on the temperature of the object 5 at the heating start and the current change amount I1 to determine the material type of the object 5 by subordination classification.

In this way, the material of the object 5 can be determined with increased accuracy.

(Determination considering radio frequency power)

The temperature change rate of the object 5 depends on the high-frequency power (heating power) supplied to the heating coil 11a, and the current change amount I1 in the period from the heating start to the elapse of the first heating period t1 is affected by the high-frequency power. Therefore, by determining the material considering the high-frequency power (operating frequency) in the period from the heating start to the elapse of the first heating period t1, the material can be determined with increased accuracy.

10 are curves showing relationships between time and each, operating frequency, temperature and currents of the induction cooker according to Embodiment 1.

In 10 shows the elapsed time and the characteristic changes in the case where the operating frequency is compared with the example shown in FIG 7 shown is set higher (the high frequency power is set lower). 10(a) is a curve of the operating frequency, 10(b) is a curve of temperature (water temperature), and 10(c) is a curve of currents (input current and coil current).

When the high-frequency power fed to the heating coil 11a is as small as in 10(b) shown, the temperature rises slowly to boiling. Furthermore, as in 10(c) shown, regardless of whether the object 5 is a pot with a "big" current change, or the pot with a "small" current change, the current decreases slowly. As a result, the current change amount I1 in the first heating period t1 becomes smaller compared with that of the example shown in FIG 7 is shown.

Based on the above-mentioned fact, in the load determination process during the heating operation, the load determination unit 32 determines, by the subordinate classification, the material of the object 5 as one of the plurality of materials that falls into the category defined by the main classification based on the high-frequency power (operating frequency) of the Operation circuit 50 and the current change amount I1 in the period from the heating start to the lapse of the first heating period t1.

For example, a relationship between the current change amount I1 and the kind of material is stored in advance in association with the high-frequency power through experimental data or the like. Then, the load determination unit 32 refers to the pre-stored information of the relationship between the current change amount I1 and the material type based on the high-frequency power and the current change amount I1 in the first heating period t1 to determine the material type of the object 5 by subordination classification.

In this way, the material of the object 5 can be determined with increased accuracy.

(Setting of first heating period t1)

In the above-mentioned load determination process in the water boiling mode, the load determination is performed before the detection of boiling after the heating start. In particular, it is desired to set the first heating period t1 before a cooking time.

For the reason mentioned above, in the load determination process during the heating operation, the load determination unit 32 sets the first heating period t1 in accordance with a current change amount I2 of at least one of the input current and the coil current, in a range from the heating start to the lapse of a second heating period t2, which is shorter than the first heating period t1.

11 12 are graphs showing relationships between time and each of operating frequency, temperature, and currents in the induction cooker according to Embodiment 1. FIG.

In 11 shows the elapsed time and the characteristic changes when a water amount in the object 5 compared to the example shown in FIG 7 is shown is smaller. 11(a) is a graph of the operating frequency, 11(b) is a graph of temperature (water temperature) and 11(c) is a graph of the currents (input current and coil current).

As in 11(b) shown, when the amount of water in the object to be heated 5 is small, the heating time until boiling is shortened. Furthermore, as in 11(c) shown, regardless of whether the object 5 is the pot with a "large" current change or the pot with a "small" current change, the current suddenly decreases.

Therefore, when the current change amount I2 is in the range from the heating start to the lapse of the second heating period t2, the load determination unit 32 sets the first heating period t1 short.

In contrast, when the current change amount I2 is small as in a case when the amount of water in the object 5 is large or the high-frequency power is small, the first heating period t1 is set long.

For example, a relationship between the current change amount I2 and the first heating period t1 is obtained through experimental data or the like that is stored in advance. Then, the load determination unit 32 refers to the pre-stored information based on the current change amount I2 in the second heating period t2 to set the first heating period t1.

In this way, the material of the object 5 can be determined with increased accuracy.

The first heating period t1 can also be set in accordance with a rate of change of at least one of the input current and the coil current in the period from the heating start to the lapse of the second heating period t2. For example, when the rate of change is large, the current drops suddenly. Therefore, the first heating period t1 is set short. When the rate of change is small, the current change is slow. Therefore, the first heating period t1 is set long.

The setting operation of the first heating period t1 may be periodically performed for a plurality of times.

(combination of variants)

The above-mentioned variants of the load determination process during the heating operation can also be arbitrarily combined. For example, in the load determination process during the heating operation, the load determination unit 32 can determine, by a subordinate classification, the object 5 as a material from the plurality of materials that falls into the category determined by the main classification based on the temperature of the object 5, which is determined by the Temperature detection unit 30 at which the heating start is detected, and the operation frequency of the operation circuit 50 and the current change amount I1 in the period from the heating start to the lapse of the first heating period t1.

In this way, the material of the object 5 can be determined with increased accuracy.

(Configuration example of the other operation circuit)

An example using another operating circuit will be described below.

12 FIG. 14 is a diagram showing the other operation circuit of the induction cooker according to Embodiment 1. FIG.

The operating circuit 50, which is 12 1 additionally includes a resonant capacitor 24b in the configuration shown in FIG 2 is shown. The rest of the configuration is the same as that of 2 , and the same parts are denoted by the same reference numerals.

As described above, the heating coil 11a and the resonance capacitors constitute the resonance circuit. Therefore, the capacitances of the resonance capacitors are determined by the maximum heating power (maximum input power) required for the induction cooker. In the operating circuit 50 shown in 10 1, the resonant capacitors 24a and 24b are connected in parallel to allow a capacitance of each capacitor to be halved. Therefore, even if two resonance capacitors are used, an inexpensive control circuit can be obtained.

Furthermore, by arranging the coil current detection unit 25b on the resonance capacitor 24a side of the resonance capacitors connected in parallel with each other, the current flowing through the coil current detection unit 25b becomes half the current flowing on the heating coil 11a side. Therefore, the coil current detection unit 25b having a small size and a small capacity can be used, a small and inexpensive control circuit can be obtained, and an inexpensive induction cooker can be obtained.

Although the half-bridge type inverter circuit 23 was described in Embodiment 1, a configuration using a full-bridge type or single-switch voltage resonance type inverter or other inverters may also be used.

Furthermore, although the method of using the ratio between the coil current and the primary current in the load determination process at the heating start performed by the load determination unit 32 has been described, a detection method for resonance voltages across both buses of the resonance capacitor can also be used for performing the load determination process, and therefore any load determination method can be used.

In the above description, the method in which the operating frequency is changed to control the high-frequency power (heating power) has been described, but a method in which turning ON (ON/OFF ratio) the switching elements of the inverter circuit 23 is changed to control the heating power.

Embodiment 2

In Embodiment 2, the above operation circuit 50 according to Embodiment 1 will be described in detail.

13 12 is a diagram showing part of the operation circuit of the induction cooker according to embodiment 2. 13 12 is an illustration of only a configuration of part of the operation circuit 50 according to Embodiment 1 described above.

As in 13 shown, the inverter circuit 23 includes a set of arms including two switching elements (IGBTs 23a and 23b) connected in series between positive and negative buses, and diodes 23c and 23d connected in reverse parallel to the switching elements, respectively .

The IGBTs 23a and the IGBT 23b are controlled to be turned on and off with operation signals from the controller 45. FIG.

The controller 45 outputs the operation signals for turning on and off the IGBT 23a and the IGBT 23b alternately so that the IGBT 23b is set in an OFF state while the IGBT 23a is ON and the IGBT 23b is set in an ON state is offset while the IGBT 23a is OFF.

In this way, the IGBT 23a and the IGBT 23b form a half-bridge inverter designed to drive the heating coil 11a.

The IGBT 23a and the IGBT 23b constitute a “half-bridge inverter circuit” according to the present invention.

The controller 45 inputs the operation signals with the high frequency to the IGBT 23a and the IGBT 23b depending on the input electric power (heating power) to adjust a heating output. The operating signals, which at the IGBT 23a and the IGBT 23b are varied in a range of operating frequency higher than the resonance frequency of a load circuit including the heating coil 11a and the resonance capacitor 24a by a current flowing through the load circuit in a delayed phase fluently compared to a voltage applied to the load circuit.

Next, the operation of controlling the input electric power (heating power) with the operating frequency and the ON duty ratio of the inverter circuit 23 will be described.

14 12 are graphs showing an example of operation signals of a half-bridge circuit according to Embodiment 2. 14(a) Fig. 12 is an example of the operating signals of the respective switches in a high heater power state. 14(b) 12 is an example of the operation signals of the respective switches in a low heater power state.

The controller 45 outputs the operation signals with the high frequency, which is higher than the resonant frequency of the load circuit, to the IGBT 23a and the IGBT 23b of the inverter circuit 23.

The frequency of each of the operation signals is varied to increase or decrease the output of the inverter circuit 23 .

For example, as in 14(a) 1, when the operating frequency is reduced, the frequency of the high-frequency current supplied to the heating coil 11a approaches the resonance frequency of the load circuit, with the result that the electric power supplied to the heating coil 11a is increased.

On the other hand, as in 14(b) 1, when the operating frequency is increased, the frequency of the high-frequency current supplied to the heating coil 11a deviates from the resonant frequency of the load circuit, with the result that the electric power supplied to the heating coil 11a is reduced.

Furthermore, the controller 45 varies the operating frequency to control the input electric power as described above, and can also vary the ON ratio of the IGBT 23a and the IGBT 23b of the inverter circuit 23 to control a period in which the output voltage of the inverter circuit 23 is applied, and thus to control the electric power fed to the heating coil 11a.

In a case of increasing the heating power, a ratio (ON duty ratio) of an ON time of the IGBT 23a (OFF time of the IGBT 22b) in a range of operation signals is increased to increase a voltage application time width in a period.

On the other hand, in a case of reducing the heating power, the ratio (ON duty ratio) of the ON time of the IGBT 23a (OFF time of the IGBT 23b) in a span of the operation signals is reduced to reduce the voltage application time width in a period.

In an example from 14(a) Illustrated is a case where ratios of an ON time T11a of the IGBT 23a (OFF time of the IGBT 23b) and an OFF time T11b of the IGBT 23a (ON time of the IGBT 23b) in a period T11 of operation signals are the same (ON duty ratio of 50%).

On the other hand, in an example of 14(b) a case of a case where ratios of an ON time T12a of the IGBT 23a (OFF time of the IGBT 23b) and an OFF time T12b of the IGBT 23a (ON time of the IGBT 23b) in a period T12 of Operating signals are the same (ON duty ratio of 50%).

The controller 45 holds the ON duty ratio of the IGBT 23a and the IGBT 23b of the inverter circuit 23 in the state in which the operating frequency of the inverter circuit 23 is fixed when determining the current change amount ΔI in the input current (or the coil current) per predetermined time , as described in Embodiment 1 above.

In this way, the current can be determined from the amount of change ΔI in the input current (or the coil current) per predetermined time in a state where the electric power fed to the heating coil 11a is fixed.

Embodiment 3

In Embodiment 3, the inverter circuit 23 using a full bridge circuit is described.

15 12 is a diagram showing part of an operation circuit of an induction cooker according to embodiment 3. In 15 only differences from the operation circuit 50 in Embodiment 1 described above are illustrated.

In Embodiment 3, two heating coils are provided for one heating panel. The two heating coils have correspondingly different diameters and are arranged concentrically, for example. Hereinafter, the smaller-diameter heating coil is referred to as “inner coil 11b”, and the larger-diameter heating coil is referred to as “outer coil 11c”.

The number and arrangement of the heating coils are not limited to this. For example, a configuration can be adopted in which a plurality of heating coils are arranged around a heating coil located at the center of the heating panel.

The inverter circuit 23 includes three sets of arms each including two switching elements (IGBTs) connected in series between positive and negative buses and diodes connected in reverse parallel to the switching elements, respectively. Hereinafter, of the three sets of arms, one set will be referred to as "common arm", and the other two sets of arms as "inner coil arm" and "outer coil arm", respectively.

The common arm is an arm connected to the inner coil 11b and the outer coil 11c, and includes an IGBT 232a, an IGBT 232b, a diode 232c, and a diode 232d.

The inner coil arm is connected to the inner coil 11b, and includes an IGBT 231a, an IGBT 231b, a diode 231c, and a diode 231d.

The outer coil arm is connected to the outer coil 11c, and includes an IGBT 233a, an IGBT 233b, a diode 233c, and a diode 233d.

The common-arm IGBT 232a and IGBT 232b, the inner-coil-arm IGBT 231a and IGBT 231b, and the outer-coil-arm IGBT 233a and IGBT 233b are operated with the operation signals output from the controller 45 to and get turned off.

The controller 45 outputs operation signals for turning on and off the IGBT 232a and the common-arm IGBT 232b alternately so that the IGBT 232b is brought into an OFF state while the IGBT 232a is ON and the IGBT 232b is brought into an ON state state while the IGBT 232a is OFF.

Similarly, the controller 45 outputs operation signals for alternately turning on and off the inner coil arm IGBT 231a and IGBT 231b, and the outer coil arm IGBT 233a and IGBT 233b.

In this way, the common arm and the inner coil arm form a full-bridge inverter designed to drive the inner coil 11b. Furthermore, the common arm and the outer coil arm form a full-bridge inverter designed to drive the outer coil 11c.

The common arm and the inner coil arm form a “full bridge inverter circuit” according to the present invention. Furthermore, according to the present invention, the common arm and the outer coil arm form a “full bridge inverter circuit”.

A load circuit including the inner coil 11b and a resonance capacitor 24c is connected between an output point (node of the IGBT 232a and the IGBT 232b) of the common arm and an output point (node of the IGBT 231a and the IGBT 231b) of the inner coil arm .

A load circuit including the outer coil 11c and a resonance capacitor 24d is connected between the output point of the common arm and an output point (node of the IGBT 233a and the IGBT 233b) of the outer coil arm.

The inner coil 11b is a heating coil wound in a substantially circular shape and has a small outer shape, and the outer coil 11c is arranged in the vicinity of the inner coil 11b.

A coil current flowing through the inner coil 11b is detected by a coil current detection unit 25c. The coil current detection unit 25c detects a peak of a current flowing through the inner coil 11b, for example, and outputs a voltage signal corresponding to a peak value of a heater coil current to the controller 45.

A coil current flowing through the outer coil 11c is detected by a coil current detection unit 25d. For example, the coil current detection unit 25d detects a peak of a current flowing through the outer coil 11c and outputs a voltage signal corresponding to a peak value of a heating coil current to the controller 45 .

The controller 45 inputs the high frequency operation signals to the switching elements (IGBTs) of each arm depending on the input electric power (heating power) to adjust the heating output.

The operation signals, which are output to the switching elements of the common arm and the inner coil arm, are varied in a range of operation frequency higher than a resonance frequency of the load circuit including the inner coil 11b and the resonance capacitor 24c by a current that flowing through the load circuit is to be fluently controlled in a delayed phase compared to a voltage applied to the load circuit.

Similarly, the operation signals output to the switching elements of the common arm and the outer coil arm are varied in a range of operation frequency higher than a resonance frequency of a load circuit including the outer coil 11c and the resonance capacitor 24d by one To control current flowing through the load circuit to flow in a delayed phase compared to a voltage applied to the load circuit.

Next, a control operation of the input electric power (heating power) with a phase difference between the arms of the inverter circuit 23 will be described.

16 12 are graphs showing an example of operation signals from the full bridge circuit according to Embodiment 3.

16 (a) Fig. 12 is an example of the operation signals of the respective switches and a supply timing of each of the heating coils in the heating high power state.

16 (b) 12 is an example of the operation signals of the respective switches and a supply timing of each of the heating coils in the low heating power state.

The supply timings that are in 16(a) and 16(b) are related to a potential difference of the output points (nodes of pairs of IGBTs) of the respective arms, and a state in which the output point of the common arm is smaller than the output point of the inner coil arm, and the output point of the outer coil arm is denoted by “ ON". On the other hand, a state in which the common arm output point is higher than the inner coil arm output point and the outer coil arm output point, and an equal potential state are indicated as “OFF”.

As in 16 As shown, the controller 45 outputs operation signals to the IGBT 232a and the common-arm IGBT 232b at a high frequency higher than the resonant frequency of the load circuit.

In addition, the controller 45 outputs operation signals to the inner-coil-arm IGBT 231a and IGBT 231b and the outer-coil-arm IGBT 233a and IGBT 233b which have progressed in phase relative to the common-arm operation signals. Frequencies of the operating signals of the respective arms are the same frequency, and ON ratios thereof are also the same.

At the output point (node of a pair of IGBTs) of each arm, depending on the ON/OFF state of the pair of IGBTs, a positive bus potential or a negative bus potential, which is an output from the DC power supply circuit during it is switched at the high frequency. In this way, the potential difference between the common arm output point and the inner coil arm output point is applied to the inner coil 11b. Similarly, the potential difference between the output point of the common arm and the output point of the outer coil arm is applied to the outer coil 11c.

Therefore, the phase difference between the common arm operation signals and the inner coil arm and outer coil arm operation signals can be increased or decreased to adjust high-frequency voltages applied to the inner coil 11b and the outer coil 11c and to control high-frequency output currents and the input currents , which flow through the inner coil 11b and the outer coil 11c.

In the case of increasing the heating power, a phase α between the arms is increased to increase the voltage application time width in a span. An upper limit of the phase α between the arms is a case of reverse phase (phase difference of 180°), and an output voltage waveform at this time is a substantially rectangular wave.

In the example of 16(a) a case is shown where the phase α between the arms is 180°. In addition, a case where the ON duty ratio of the operation signals of each arm is 50%, that is, a case where ratios of an ON time T13a and an OFF time T13b are the same in a period T13.

In this case, a supply ON time width T14a and a supply OFF time width T14b of the inner coil 11b and the outer coil 11c have the same relationship in a span T14 of the operation signals.

In the case of reducing the heating power, the phase α between the arms is reduced compared to the high heating power state to reduce the voltage application time width in a margin. A low limit of the phase α between the arms is set, for example, to such a level that an overcurrent flowing through the switching elements and destroying them is avoided in relation to the phase of the current flowing through, for example, at the time of turn-on flows through the load circuit.

In the example of 16(b) a case is presented where the phase α between the arms is compared to 16(a) is reduced. The frequency and ON duty ratio of the operating signals from each arm are the same as in 16(a) .

In this case, the supply ON time width T14a of the inner coil 11b and the outer coil 11c in a span T14 of the operation signals is a period corresponding to the phase α between the arms.

In this way, the electric power (heating power) fed to the inner coil 11b and the outer coil 11c can be controlled with the phase difference between the arms.

In the above description, the case was described where both the inner coil 11b and the outer coil 11c perform the heating operation, but the operation of the inner coil arm or the outer coil arm can be stopped so that only one of the inner coil 11b and of the outer coil 11c can perform the heating operation.

The controller 45 sets each of the phases α between the arms and the ON ratio of the switching elements of each arm in a fixed state, in the state in which the operating frequency of the inverter circuit 23 in determining the current change amount ΔI of the input current (or the coil current) per predetermined time as described in Embodiment 1 above. The other operations are similar to Embodiment 1 described above.

In this way, the current change amount ΔI in the input current (or the coil current) per predetermined time can be determined in a state where the electric power inputs to the inner coil 11b and the outer coil 11c are fixed.

In Embodiment 3, the coil current flowing through the inner coil 11b and the coil current flowing through the outer coil 11c are respectively detected by the coil current detection unit 25c and the coil current detection unit 25d.

Therefore, in the case where both the inner coil 11b and the outer coil 11c perform the heating operation, and even in a case where one of the coil current detection unit 25c and the coil current detection unit 25d cannot detect the coil current value due to an error, for example the current change amount ΔI of the coil current per predetermined time can be detected based on a value detected from the other.

In addition, the controller 45 may determine each current change amount ΔI in the coil current per predetermined time detected by the coil current detection unit 25c and use the current change amount ΔI in the coil current per predetermined time detected by the coil current detection unit 25d and the larger of the current change amounts ΔI to perform each of the determination processes described in Embodiment 1 above. In addition, an average of the current change amounts ΔI can be used to perform each of the determination processes described in Embodiment 1 above.

Such control can be performed to more accurately determine the current change amount ΔI of the coil current per predetermined time even in a case where one of the coil current detection unit 25c and the coil current detection unit 25d has low detection accuracy.

Although an IH (Induction Heat) heating cooker has been described in the above Embodiments 1 to 3 as an example of the induction cooker of the present invention, the present invention is not limited thereto. The present invention can be applied to any induction cooker using the induction heating method, such as a rice cooker for induction heating cooking.

Reference List

1

first heating field

2

second heating field

3

third heating field

4

headstock

5

object to be heated

11

first heating unit

11a

heating coil

11b

inner coil

11c

outer coil

12

second heating unit

13

third heating unit

21

AC power supply

22

DC power supply circuit

22a

diode bridge

22b

choke coil

22c

smoothing capacitor

23

inverter circuit

23a, 23b

IGBT

23c, 23d

diode

24a-24d

resonant capacitor

25a

input current detection unit

25b-25d

coil current detection unit

30

temperature detection unit

31

operational control unit

33

load determination unit

33

operating frequency setting unit

34

current change detection unit

35

power adjustment unit

36

input/output controller

40a-40c

operating unit

41a-41c

display unit

42

notification unit

45

control unit

50

operating circuit

100

induction cooker

231a, 231b IGBTs 232a, 232b

IGBT

233a, 233b

IGBT

231c, 231d

diode

232c, 232d

diode

233c, 233d

diode

Claims (15)

An induction cooker comprising: a heating coil (11a) adapted to heat an object (5) to be heated by induction; a drive circuit (50) adapted to supply high-frequency power to said heating coil (11a); a load determination unit (32) configured to execute a load determination process for determining a load of the object (5); the process of determining the material of the object (5) as falling into a material category of at least two of a magnetic material, a high resistance non-magnetic material and a low resistance non-magnetic material by main classification includes a controller (45) arranged to control a operation of the operating circuit (50) is adapted to control the high-frequency power supplied to the heating coil (11a); an input current detection unit (25a) configured to detect an input current to the operating circuit (50); and a coil current detection unit (25b) configured to detect a coil current flowing through the heating coil (11a), wherein the load determination unit (32) is configured to based on an amount of current change (11) in at least one of the input current and the coil current , in a period from the start of power supply of the heating coil (11a) to the lapse of a first heating period (t1), determining, by a subordinate classification, the material of the object (5) determined by the main classification as one of a plurality of materials wherein the plurality of materials fall into a category determined by the major classification, wherein the controller (45) is adapted to operate the operating circuit (50) in accordance with a determination result by the load determination unit (32). induction cooker after claim 1 wherein the controller (45) is adapted to control an operating frequency of the operating circuit (50) during the period from the start of power supply to the heating coil (11a) to the lapse of the first heating period (t1). induction cooker after claim 1 or 2 wherein the load determination unit (32) is adapted to determine the material of the object (5) by a main classification as falling into a material category of at least two of a magnetic material, a non-magnetic high resistance material and a non-magnetic low resistance material based on a relationship between the input current and the coil current. induction cooker after claim 1 , in which the controller (45) is designed to operate the operating circuit (50) so that the high-frequency power supplied to the heating coil (11a) assumes a maximum value in accordance with the one category defined by the main classification from the load determination unit (32) is determined; and setting the operating frequency of the operating circuit (50) during the period from the start of power supply to the heating coil (11a) until the lapse of the first heating period (t1). Induction cooker according to one of Claims 1 until 3 , further comprising a temperature detection unit (30) which is designed to detect a temperature of the object (5), wherein the load detection unit (32) is designed to determine the object (5) as a material from the plurality of materials by a subordinate classification, which in the one category which is determined by the main classification falls based on the temperature of the object (5), the temperature being detected by the temperature detection unit (30) at the start of power supply to the heating coil (11a); and the current change amount (11) in the period from the start of power supply to the heating coil (11a) to the lapse of the first heating period (t1). Induction cooker according to one of claims 1 until 5 wherein the controller (45) is adapted to set an operation frequency of the operation circuit (50) during the period from the start of power supply to the heating coil (11a) until the lapse of the first heating period (t1); and the load determining unit (32) is adapted to determine, by a subordinate classification, the object (5) as a material from the plurality of materials falling into the one category determined by the main classification based on the operating frequency of the operating circuit (50) and the current change amount (11) in the period from the start of power supply to the heating coil (11a) to the lapse of the first heating period (t1). induction cooker after claim 1 , further comprising a temperature determining unit (30) configured to detect a temperature of the object (5), the load determining unit (32) being configured to classify the material of the object (5) into a material category of at least two out of a main classification determine falling magnetic material, a high resistance nonmagnetic material, and a low resistance nonmagnetic material based on a relationship between the input current and the coil current; the controller (45) is adapted to control the operation circuit (50) in accordance with the one category determined from the main classification by the load determination unit (32) to set an operation frequency of the operation circuit (50); and the load determining unit (32) is adapted to determine, by a subordinate classification, the object (5) as one of a plurality of materials falling into the one category determined by the main classification based on the temperature of the object (5) , the temperature detected by the temperature detection unit (30) at the start of power supply to the heating coil (11a), and the operating frequency of the operating circuit (50) and the current change amount (11) in the period from the start of power supply to the heating coil (11a ) until the end of the first heating period (t1). Induction cooker according to one of claims 1 until 7 wherein the controller (45) is adapted to control the operating circuit (50) at a present operating frequency for load determination for a determination period when power supply to the heating coil (11a) is to be started and after the first heating period (t1) elapses ; and the load determination unit (32) is designed to determine the material of the object using a difference between a current at the start of power supply to the heating coil (11a) and a current after the first heating period has elapsed (t1) as the current change amount (11). Induction cooker according to one of claims 1 until 8th , in which the load determination unit (32) is designed to determine the first heating period (t1) in accordance with a change amount or a change ratio of at least one of the input current or the coil current, in a period from the start of the power supply of the heating coil (11a) to set to the end of a second heating period (t1), which is shorter than the first heating period (t1). Induction cooker according to one of claims 1 until 9 wherein the controller (45) is adapted to when a change amount (ΔI) per time of at least one of the input current or the coil current in a state in which an operating frequency of the operating circuit (50) is fixed becomes equal to or smaller than a threshold which is set according to the material of the object (5), controlling the operation of the drive circuit (50) to vary the high-frequency power supplied to the heating coil (11a). Induction cooker according to one of claims 1 until 10 , further comprising: an operation unit (40) configured to receive an operation of selecting an operation mode; and a notification unit (42), wherein when a water boiling mode for setting a water boiling operation for water is selected as the operation mode, the controller (45) is adapted to operate the operation circuit (50); and when an amount of change (ΔI) per predetermined time in at least one of the input current or the coil current in a state in which an operating frequency of the control circuit is fixed becomes equal to or smaller than a threshold value set according to the material of the object, control the notification unit (42) for notifying completion of water boiling. induction cooker after claim 10 or 11 wherein the controller (45) is adapted to set the threshold to increase as the current change amount (11) of the material of the object (5) increases, the material being determined by the load determination unit (32). Induction cooker according to one of claims 1 until 12 wherein the controller (45) is adapted to set an ON duty ratio of switching elements of the operation circuit (50) in a state in which an operation frequency of the operation circuit (50) is set. Induction cooker according to one of claims 1 until 12 wherein the operating circuit (50) comprises a full-bridge inverter circuit including at least two arms each including two switching elements connected in series; and the controller (45) is adapted to set an operating frequency of the switching elements of the full-bridge inverter circuit, an operating phase difference of the switching elements between the at least two arms, and an ON duty ratio of the switching elements. Induction cooker according to one of claims 1 until 12 wherein the operating circuit (50) comprises a half-bridge inverter circuit including an arm including two switching elements connected in series; and the controller (45) is adapted to set an ON-duty ratio of the switching elements in a state in which an operating frequency of the switching elements of the half-bridge inverter circuit is fixed.

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