JP6211175B2 - Induction heating cooker - Google Patents

Description

  The present invention relates to an induction heating cooker.

Some conventional induction heating cookers determine the temperature of an object to be heated based on the input current or control amount of an inverter.
For example, it has a control means for controlling the inverter so that the input current of the inverter becomes constant, and when the control amount changes more than a predetermined amount within a predetermined time, it is determined that the temperature change of the object to be heated is large. An induction heating cooker that suppresses the output of an inverter has been proposed (see, for example, Patent Document 1).

JP 2008-181892 A (page 3 to page 5, FIG. 1)

  In the induction heating cooker described in Patent Document 1, the drive frequency of the inverter is controlled so that the input power is constant, and the temperature change of the object to be heated is determined by this control amount change (Δf). However, depending on the material of the object to be heated, the control amount change (Δf) of the drive frequency becomes minute, and there is a problem that the temperature change of the object to be heated cannot be detected.

  On the other hand, when the temperature of the object to be heated is detected using the temperature detecting means, there is a problem that the accuracy of temperature detection may be lowered. For example, in a non-contact type temperature detecting means such as an infrared sensor, there is a problem that the accuracy of temperature detection is low when the temperature of the object to be heated is low (for example, 100 ° C. or less). Also, in contact type temperature detection means such as a thermistor, when a heated object (pan) with a warped bottom is heated, if a gap occurs between the heated object and the top plate, the accuracy of temperature detection is improved. There was a problem of being lowered.

  In addition, when performing constant temperature control that keeps the temperature of the object to be heated almost constant at any timing during the heating, the object is heated regardless of the shape, material, and temperature range of the object to be heated. It is desired to keep the temperature of an object constant with high accuracy.

  The present invention has been made to solve the above-described problems, and provides an induction heating cooker that can detect a temperature change of an object to be heated by an inexpensive method regardless of the material of the object to be heated. Is. Moreover, the induction heating cooking appliance which can implement | achieve highly accurate constant temperature control without using a temperature detection means is obtained.

An induction heating cooker according to the present invention includes a heating coil that induction-heats an object to be heated, a resonance capacitor that is connected to the heating coil and forms a resonance circuit, an inverter circuit that supplies high-frequency power to the heating coil, By controlling a drive signal of the inverter circuit, a control unit that controls high-frequency power supplied to the heating coil, an operation unit that receives a selection operation of a predetermined operation mode, and a coil current that flows through the heating coil Coil current detecting means for detecting the voltage and voltage detecting means for detecting an applied voltage applied to the resonance circuit, and the predetermined operation mode is a constant temperature operation for keeping the temperature of the object to be heated constant. The control unit, when the constant temperature operation mode is selected as the predetermined operation mode, the inverter circuit In a state of fixing the dynamic frequency such that said ratio of the coil current and the applied voltage is constant, the by varying the duty ratio of the drive signal of the inverter circuit, which constant impedance of the resonant circuit It is.

An induction heating cooker according to the present invention includes a heating coil that induction-heats an object to be heated, a resonance capacitor that is connected to the heating coil and forms a resonance circuit, an inverter circuit that supplies high-frequency power to the heating coil, By controlling the drive signal of the inverter circuit, a control unit for controlling the high frequency power supplied to the heating coil, coil current detection means for detecting a coil current flowing in the heating coil, and an input to the inverter circuit comprising an input current detecting means for detecting a current, and a primary voltage detection means for detecting the primary voltage applied to the inverter circuit, an operation unit for accepting an operation of selecting the operation mode predetermined, said predetermined is operation mode includes a constant temperature operation mode to maintain the temperature of the heated object constant, the control unit, the constant-temperature driving mode When selected, the inverter circuit has a constant ratio of a product of the input current and the primary voltage and a value obtained by squaring the coil current in a state where the drive frequency of the inverter circuit is fixed. The duty ratio of the drive signal is varied to make the impedance of the resonance circuit constant .

  This invention can detect the temperature change of the object to be heated by an inexpensive method regardless of the material of the object to be heated. In addition, high-precision constant temperature control can be realized without using temperature detection means.

It is a disassembled perspective view which shows the induction heating cooking appliance which concerns on Embodiment 1. FIG. It is a figure which shows the drive circuit of the induction heating cooking appliance which concerns on Embodiment 1. FIG. It is a load discrimination | determination characteristic view of the to-be-heated object based on the relationship between the coil current and the input current in the induction heating cooking appliance which concerns on Embodiment 1. FIG. It is a correlation diagram of resistance of a heating coil and inductance at the time of the temperature change of the to-be-heated object of the induction heating cooking appliance which concerns on Embodiment 1. FIG. It is a figure which shows a part of drive circuit of the induction heating cooking appliance which concerns on Embodiment 1. FIG. 6 is a diagram illustrating an example of a drive signal for a half-bridge circuit according to Embodiment 1. FIG. It is a figure which shows the input power characteristic with respect to the duty ratio of the half bridge circuit which concerns on Embodiment 1. FIG. It is a figure which shows the relationship between the drive frequency, temperature, duty ratio, input current, and time of the induction heating cooking appliance which concerns on Embodiment 1. FIG. It is a figure which shows another drive circuit of the induction heating cooking appliance which concerns on Embodiment 1. FIG.

Embodiment 1 FIG.
(Constitution)
1 is an exploded perspective view showing an induction heating cooker according to Embodiment 1. FIG.
As shown in FIG. 1, an induction heating cooker 100 has a top plate 4 on which an object to be heated 5 such as a pan is placed. The top plate 4 includes a first heating port 1, a second heating port 2, and a third heating port 3 as heating ports for inductively heating the object to be heated 5, and corresponds to each heating port. The first heating unit 11, the second heating unit 12, and the third heating unit 13 are provided, and the object to be heated 5 can be placed on each heating port to perform induction heating. Is.
In the first embodiment, the first heating means 11 and the second heating means 12 are provided side by side on the front side of the main body, and the third heating means 13 is provided at substantially the center on the back side of the main body. .
In addition, arrangement | positioning of each heating port is not restricted to this. For example, three heating ports may be arranged side by side in a substantially straight line. Moreover, you may arrange | position so that the position of the depth direction of the center of the 1st heating means 11 and the center of the 2nd heating means 12 may differ.

  The top plate 4 is entirely made of a material that transmits infrared rays, such as heat-resistant tempered glass or crystallized glass, and a rubber packing or sealing material is interposed between the upper surface and the outer periphery of the upper surface of the induction heating cooker 100 main body. Fixed in a watertight state. The top plate 4 has a circular shape indicating a rough placement position of the pan corresponding to the heating range (heating port) of the first heating means 11, the second heating means 12, and the third heating means 13. The pan position indication is formed by applying paint or printing.

  On the front side of the top plate 4, there is a heating power and cooking menu (boiling mode, fried food mode when heating the article 5 to be heated by the first heating means 11, the second heating means 12, and the third heating means 13. Etc.) are provided as an input device for setting the operation unit 40a, the operation unit 40b, and the operation unit 40c (hereinafter may be collectively referred to as the operation unit 40). Further, in the vicinity of the operation unit 40, as the notification unit 42, a display unit 41a, a display unit 41b, and a display unit 41c that display the operation state of the induction heating cooker 100, the input from the operation unit 40, the operation content, and the like. Is provided. Note that the operation units 40a to 40c and the display units 41a to 41c are not particularly limited, for example, when the operation units 40a and 41c are provided for each heating port, or when the operation unit 40 and the display unit 41 are provided collectively.

  A first heating means 11, a second heating means 12, and a third heating means 13 are provided below the top plate 4 and inside the main body, and each heating means is a heating coil (not shown). Z).

  Inside the main body of the induction heating cooker 100, there are a drive circuit 50 for supplying high frequency power to the heating coils of the first heating means 11, the second heating means 12, and the third heating means 13, and the drive circuit 50. And a control unit 45 for controlling the operation of the whole induction heating cooker.

  The heating coil has a substantially circular planar shape, and is configured by winding a conductive wire made of an arbitrary metal (for example, copper, aluminum, etc.) coated with an insulating film in the circumferential direction. An induction heating operation is performed by being supplied to the heating coil.

  FIG. 2 is a diagram illustrating a drive circuit of the induction heating cooker according to the first embodiment. In addition, although the drive circuit 50 is provided for every heating means, the circuit structure may be the same and may be changed for every heating means. In FIG. 2, only one drive circuit 50 is shown. As shown in FIG. 2, the drive circuit 50 includes a DC power supply circuit 22, an inverter circuit 23, and a resonance circuit 30.

  The input current detection unit 25 a detects a current input from the AC power supply (commercial power supply) 21 to the DC power supply circuit 22 and outputs a voltage signal corresponding to the input current value to the control unit 45.

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

  The inverter circuit 23 is a so-called half-bridge type inverter in which IGBTs 23a and 23b as switching elements are connected in series to the output of the DC power supply circuit 22, and diodes 23c and 23d are parallel to the IGBTs 23a and 23b as flywheel diodes, respectively. It is connected to the. The inverter circuit 23 converts the DC power output from the DC power supply circuit 22 into a high-frequency AC power of about 20 kHz to 50 kHz, and supplies the AC power to the resonance circuit 30 including the heating coil 11a and the resonance capacitor 24a.

  The resonant capacitor 24a is connected in series with the heating coil 11a. The resonance circuit 30 includes a heating coil 11a and a resonance capacitor 24a. The resonance circuit 30 has a resonance frequency corresponding to the inductance of the heating coil 11a, the capacity of the resonance capacitor 24a, and the like. The inductance of the heating coil 11a changes according to the characteristics of the metal load when the object to be heated 5 (metal load) is magnetically coupled. The resonance frequency of the resonance circuit 30 changes according to the change in inductance.

  With this configuration, a high-frequency current of about several tens of A flows through the heating coil 11a, and the object to be heated placed on the top plate 4 directly above the heating coil 11a by the high-frequency magnetic flux generated by the flowing high-frequency current. 5 is induction heated. The IGBTs 23a and 23b, which are switching elements, are composed of, for example, a silicon-based semiconductor, but may be configured using a wide band gap semiconductor such as silicon carbide or a gallium nitride-based material.

  By using a wide band gap semiconductor for the switching element, the conduction loss of the switching element can be reduced, and the heat radiation of the driving circuit 50 is good even when the switching frequency (driving frequency) is high (high speed). The heat dissipating fins of the circuit 50 can be reduced in size, and the drive circuit 50 can be reduced in size and cost.

  The coil current detection means 25b is connected to a resonance circuit 30 composed of the heating coil 11a and the resonance capacitor 24a. For example, the coil current detection unit 25b detects a current (coil current) flowing through the heating coil 11a and outputs a voltage signal corresponding to the detected current value to the control unit 45.

  The voltage detection unit 26a detects a voltage (applied voltage) applied to the resonance circuit 30 including the heating coil 11a and the resonance capacitor 24a, and outputs a voltage signal corresponding to the detected voltage value to the control unit 45.

  The primary voltage detection unit 26 b detects a DC voltage (primary voltage) generated by the DC power supply circuit 22, and outputs a voltage signal corresponding to the detected voltage value to the control unit 45. That is, the primary voltage detection unit 26 b detects the primary voltage applied to the inverter circuit 23. In FIG. 2, the primary voltage detection means 26b is provided at both ends on the output side of the smoothing capacitor 22c, but may be provided on the output side of the diode bridge 22a (between the diode bridge 22a and the reactor 22b).

  The control part 45 consists of a microcomputer or DSP (digital signal processor) etc., and controls operation | movement of the induction heating cooking appliance 100. FIG.

(Operation)
Next, operation | movement of the induction heating cooking appliance 100 which concerns on Embodiment 1 is demonstrated.
First, the operation in the case where the object to be heated 5 placed on the heating port of the top plate 4 is induction-heated by the thermal power set by the operation unit 40 will be described.

  When the user places the object to be heated 5 on the heating port and instructs the operation unit 40 to start heating (heating power input), the control unit 45 (load determination unit) performs a load determination process.

FIG. 3 is a load discrimination characteristic diagram of an object to be heated based on the relationship between the coil current and the input current in the induction heating cooker according to the first embodiment.
Here, the material of the heated object 5 (pan) serving as a load is largely divided into a magnetic material such as iron or SUS430, a high resistance nonmagnetic material such as SUS304, and a low resistance nonmagnetic material such as aluminum or copper. Separated.

  As shown in FIG. 3, the relationship between the coil current and the input current differs depending on the material of the pan load placed on the top plate 4. The control unit 45 stores therein in advance a load determination table in which the relationship between the coil current and the input current shown in FIG. 3 is tabulated. By storing the load determination table therein, the load determination means can be configured with an inexpensive configuration.

  In the load determination process, the control unit 45 drives the inverter circuit 23 with a specific drive signal for load determination, and detects the input current from the output signal of the input current detection means 25a. At the same time, the control unit 45 detects the coil current from the output signal of the coil current detection means 25b. The control part 45 determines the material of the to-be-heated material (pan) 5 mounted from the detected coil current and input current, and the load determination table showing the relationship of FIG. Thus, the control part 45 (load determination means) determines the material of the article 5 to be heated placed on the heating coil 11a based on the correlation between the input current and the coil current.

After performing the above load determination processing, the control unit 45 performs a control operation based on the load determination result.
When the load determination result is no load, the control unit 45 causes the notifying unit 42 to leave it unheatable and prompts the user to place the pan. At this time, high frequency power is not supplied from the drive circuit 50 to the heating coil 11a.

When the load determination result is a magnetic material, a high-resistance nonmagnetic material, or a low-resistance nonmagnetic material, these pans are materials that can be heated by the induction heating cooker 100 of the first embodiment. Therefore, the control part 45 determines the drive frequency according to the determined pot material. This drive frequency is set to a frequency higher than the resonance frequency so that the input current does not become excessive. The drive frequency can be determined by referring to a frequency table or the like corresponding to the material of the article 5 to be heated and the set heating power, for example.
The controller 45 drives the inverter circuit 23 with the determined drive frequency fixed, and starts an induction heating operation. In the state where the drive frequency is fixed, the input power (output power of the inverter circuit 23) becomes constant by fixing the on-duty ratio (on / off ratio) of the switching element of the inverter circuit 23.

  FIG. 4 is a correlation diagram of resistance and inductance of the heating coil when the temperature of the object to be heated of the induction heating cooker according to Embodiment 1 changes. 4A is a characteristic diagram of the resistance R of the heating coil with respect to the temperature of the object to be heated, and FIG. 4B is a characteristic diagram of the inductance L of the heating coil 11a with respect to the temperature of the object to be heated 5, both having a predetermined frequency. It is a characteristic under certain conditions.

  As shown in FIG. 4, when the temperature of the object to be heated 5 increases, the resistance R and the inductance L of the heating coil 11a both increase. If the frequency is constant, the temperature of the object to be heated 5 is increased. The resistance R and the inductance L are predetermined values. That is, it means that the temperature of the object to be heated 5 can be kept constant by making the values of the resistance R and the inductance L of the heating coil 11a constant at a predetermined frequency.

FIG. 5 is a diagram illustrating a part of the drive circuit of the induction heating cooker according to the first embodiment. In FIG. 5, only a part of the configuration of the drive circuit 50 shown in FIG. 2 is illustrated.
As shown in FIG. 5, the inverter circuit 23 includes two switching elements (IGBTs 23 a and 23 b) connected in series between the positive and negative buses, and diodes 23 c and 23 d connected to the switching elements in antiparallel. One arm is provided.

The IGBT 23 a and the IGBT 23 b are driven on and off by a drive signal output from the control unit 45.
The control unit 45 turns off the IGBT 23b while turning on the IGBT 23a, turns on the IGBT 23b while turning off the IGBT 23a, and outputs a drive signal that turns on and off alternately.
Thereby, the IGBT 23a and the IGBT 23b constitute a half-bridge inverter circuit that drives the heating coil 11a.

  Next, the control operation of the input power (thermal power) based on the drive frequency and the duty ratio of the inverter circuit 23 will be described.

FIG. 6 is a diagram illustrating an example of a drive signal of the half bridge circuit according to the first embodiment. FIG. 6A shows an example of the drive signal for each switch in the high thermal power state, and FIG. 6B shows an example of the drive signal for each switch in the low thermal power state.
The control unit 45 outputs a high-frequency drive signal higher than the resonance frequency of the load circuit to the IGBT 23 a and the IGBT 23 b of the inverter circuit 23. By varying the duty ratio with the frequency of the drive signal fixed, the output of the inverter circuit 23 is increased or decreased.

FIG. 7 is a diagram showing input power characteristics with respect to the duty ratio of the half-bridge circuit according to the first embodiment. FIG. 7 shows the input power when the duty ratio of the drive signal is changed with the drive frequency of the inverter circuit 23 fixed.
As shown in FIG. 7, when the drive frequency is fixed (constant), the input power (output power of the inverter circuit 23) can be increased or decreased by changing the duty ratio. In this case, generally, the input power is controlled by using any one of the duty ratio from 0 to 0.5 or from 0.5 to 1. When the duty ratio is changed from 0 to 0.5, the input power can be increased by increasing the duty ratio.

In the drive signal shown in FIG. 6, in the example of FIG. 6A, the ratio of the on time T11a of the IGBT 23a (the off time of the IGBT 23b) and the off time T11b of the IGBT 23a (the on time of the IGBT 23b) in one cycle of the drive signal is The same case, that is, the case where the duty ratio is 50% is illustrated.
In the example of FIG. 6B, the IGBT 23a OFF time T11d (IGBT 23b ON time) is longer (duty ratio 40) than the IGBT 23a ON time T11c (IGBT 23b OFF time) in one cycle of the drive signal. %)).
Here, since the time T11 of one cycle of the drive signals in FIGS. 6A and 6B is the same, that is, the drive frequency of the drive signals is the same, the drive signal of FIG. It can be seen that the input power is higher and the thermal power state is higher.

(Constant temperature operation mode 1)
Next, an operation when a constant temperature operation mode in which the current temperature of the article to be heated 5 is kept constant at any timing during heating is selected as a cooking menu (operation mode) from the user by the operation unit 40. Will be described.

As described above, when a heating start operation is performed from the user by the operation unit 40, the control unit 45 performs a load determination process, determines a driving frequency according to the determined pan material, and fixes the driving frequency. In this state, the inverter circuit 23 is driven to perform an induction heating operation.
When the constant temperature operation mode is set according to an instruction from the user in the heating operation state, the control unit 45 performs constant temperature control for keeping the current temperature of the article to be heated 5 constant. Here, the elapsed time and the change of each characteristic when performing the constant temperature control will be described with reference to FIG.

FIG. 8 is a diagram showing the relationship between the drive frequency, temperature, duty ratio, input current and time of the induction heating cooker according to the first embodiment. FIG. 8 shows the elapsed time and changes in each characteristic when water is introduced into the object to be heated 5 and the constant temperature operation mode is set during heating. FIG. 8 (b) shows the temperature (water temperature), FIG. 8 (c) shows the duty ratio of the drive signal of the inverter circuit 23, and FIG. 8 (d) shows the coil current.
Note that the coil current shown in FIG. 8 (d) is not an instantaneous value but shows a changing tendency such as an average value or an effective value, and the time axis of FIG. 8 is sufficiently long compared to the period of the drive signal. .

As shown in FIG. 8A, the inverter circuit 23 is controlled in a state where the driving frequency is fixed at f1 in the initial stage of heating. Further, as shown in FIG. 8C, the duty ratio of the drive signal is also fixed. As shown in FIG.8 (b), the temperature (water temperature) of the to-be-heated material 5 rises gradually. As shown in FIG. 8D, the coil current gradually decreases as the temperature of the article 5 to be heated increases.
By the user's operation, the constant temperature operation mode is set at time t1 (timing indicated by a broken line in FIG. 8).

  As shown in FIG. 8A, the control unit 45 increases the drive frequency of the inverter circuit 23 from f1 to f2 and decreases the input power (thermal power) at the timing of the time t1 set in the constant temperature operation mode. . Further, as shown in FIG. 8C, the control unit 45 reduces the duty ratio of the drive signal of the inverter circuit 23 to reduce the input power (thermal power).

If the induction heating operation is continued with the heating power (input power) before time t1, the temperature of the object to be heated 5 rises, so the heating power (input power) is changed at the timing (time t1) when the constant temperature operation mode is set. Reduce.
Here, as shown in FIG. 8A, the input power is reduced by increasing the drive frequency from f1 to f2, but the present invention is not limited to this. The drive frequency of the inverter circuit 23 may be kept constant at f1, and the input power may be reduced by greatly reducing only the duty ratio of the drive signal of the inverter circuit 23 as shown in FIG. 8C.

  After the time t1 when the constant temperature operation mode is set, the controller 45 drives the inverter circuit 23 based on the applied voltage detected by the voltage detection means 26a and the coil current detected by the coil current detection means 25b. The constant temperature control is performed by changing the duty ratio of the drive signal in a state where is fixed.

  Here, the impedance Z of the resonance circuit 30 including the heating coil 11a and the resonance capacitor 24a is expressed by the following equation (R) where R is the resistance of the heating coil 11a, L is the inductance, C is the capacitance of the resonance capacitor 24a, and f is the frequency. 1).

  As shown in FIG. 4, it can be seen that the resistance R and the inductance L of the heating coil 11 a change depending on the temperature of the article to be heated 5. Further, the capacitance C of the resonant capacitor 24a is a constant value regardless of the temperature of the article 5 to be heated. Therefore, as shown in Expression (1), the temperature of the object to be heated 5 is controlled by controlling the impedance Z to be constant while the frequency f (here, the drive frequency of the inverter circuit 23) is fixed. Can be made constant.

  The impedance Z of the resonance circuit 30 can be expressed by the following equation (2), where V is the applied voltage detected by the voltage detection means 26a and Ic is the coil current detected by the coil current detection means 25b. That is, the impedance Z is determined by the ratio (V / Ic) between the coil current Ic and the applied voltage V.

[Equation 2]
Z = V / Ic (2)

For this reason, the control unit 45 determines the duty ratio of the drive signal of the inverter circuit 23 so that the ratio between the applied voltage V detected by the voltage detection unit 26a and the coil current Ic detected by the coil current detection unit 25b is constant. By controlling, the impedance Z can be made constant, and the temperature of the article to be heated 5 can be made constant.
That is, the control unit 45 controls the duty ratio of the drive signal 5 according to the applied voltage V detected by the voltage detection unit 26a and the coil current Ic detected by the coil current detection unit 25b. It is possible to realize a constant temperature operation mode in which the temperature, that is, the water temperature is controlled to be constant.

As shown in FIG. 8D, the control unit 45 controls the duty ratio as shown in FIG. 8C according to the detection result of the coil current after the time t1. That is, when the coil current Ic increases and the ratio (V / Ic) between the coil current Ic and the applied voltage V decreases, the duty ratio of the drive signal is increased. When the coil current Ic decreases and the ratio (V / Ic) between the coil current Ic and the applied voltage V increases, the duty ratio of the drive signal is decreased.
By performing such constant temperature control, the temperature (water temperature) can be kept constant as shown in FIG. In addition, the dotted line of FIG.8 (b) has shown the temperature rise when not performing constant temperature control.

As described above, in the present embodiment, the duty ratio of the drive signal of the inverter circuit 23 is set so that the ratio between the coil current Ic and the applied voltage V is constant while the drive frequency of the inverter circuit 23 is fixed. Variable. That is, the duty ratio of the drive signal of the inverter circuit 23 is varied so that the impedance Z of the resonance circuit 30 is constant.
For this reason, constant temperature control is realizable without using temperature detection means, such as a thermistor or an infrared sensor. Moreover, the temperature of the to-be-heated object 5 can be accurately controlled to be constant regardless of the shape and material of the to-be-heated object 5 and the temperature zone of the to-be-heated object 5.

  Further, since the duty ratio of the drive signal is controlled according to the applied voltage V detected by the voltage detection means 26a and the coil current Ic detected by the coil current detection means 25b, constant temperature control can be realized with an inexpensive configuration. it can.

  Moreover, since the water temperature in the to-be-heated material 5 can be kept constant at a low temperature, for example, a boiling temperature (100 ° C.) or less, the temperature of the to-be-heated material 5 is kept constant in a low temperature range such as hot water cooking. The constant temperature control to keep can be realized, and an easy-to-use induction heating cooker can be obtained.

  In the above description of the constant temperature operation mode, the operation in the case where the constant temperature operation mode is selected at an arbitrary timing during heating has been described, but the present invention is not limited to this. For example, at the start of heating, the user may set a desired temperature for performing constant temperature control, and the constant temperature operation mode may be executed. For example, the control unit 45 starts heating with a predetermined heating power and changes the duty ratio of the drive signal of the inverter circuit 23 so that the temperature of the heated object 5 becomes constant when the set temperature is reached. Control may be started.

(Constant temperature operation mode 2)
Another control operation when the constant temperature operation mode is selected by the operation unit 40 will be described.
After time t1 when the constant temperature operation mode is set, the control unit 45 detects the input current detected by the input current detection means 25a, the coil current detected by the coil current detection means 25b, and the primary voltage detection means 26b. Based on the primary voltage, constant temperature control is performed by changing the duty ratio of the drive signal while the drive frequency of the inverter circuit 23 is fixed.

  Here, the input power Pin input to the inverter circuit 23 is expressed by the following equation (3), where V1 is the primary voltage detected by the primary voltage detection means 26b and Iin is the input current detected by the input current detection means 25a. Can show.

[Equation 3]
Pin = V1 × Iin (3)

  The input power Pin is converted into high frequency power by the inverter circuit 23. If the conversion efficiency into high frequency power by the inverter circuit 23 is η, the output power Pout of the inverter circuit 23 can be expressed by the following equation (4).

[Equation 4]
Pout = η × Pin (4)

  Further, the output power Pout can be expressed by the following equation (4) using the impedance Z of the resonance circuit 30 and the coil current Ic.

[Equation 5]
Pout = Z × Ic ² (5)

  Using equations (3) to (5), the impedance Z of the resonance circuit 30 can be expressed by the following equation (6).

[Equation 6]
Z = η × Pin / Ic ²
= Η × (V1 × Iin) / Ic ² (6)

  Since the change in the conversion efficiency η of the inverter circuit 23 is small, the impedance Z of the resonance circuit 30 is obtained from the input current Iin, the primary voltage V1, and the coil current Ic.

  For this reason, after the time t1 when the constant temperature operation mode is set, the control unit 45 fixes the drive frequency of the inverter circuit 23 based on the input current Iin, the primary voltage V1, and the coil current Ic. The constant temperature control is performed by changing the duty ratio of the drive signal.

  That is, the control unit 45 controls the duty ratio of the drive signal of the inverter circuit 23 so that the ratio between the product of the input current Iin and the primary voltage V1 and the value obtained by squaring the coil current Ic is constant. Thus, the impedance Z can be made constant, and the temperature of the object to be heated 5 can be made constant.

  Even in the operation as described above, constant temperature control can be realized without using the temperature detecting means. Moreover, the temperature of the to-be-heated object 5 can be accurately controlled to be constant regardless of the shape and material of the to-be-heated object 5 and the temperature zone of the to-be-heated object 5.

  In addition, since the applied voltage is not used in this operation, the voltage detecting means 26a can be omitted. When the voltage detection means 26a detects the voltage applied to the resonance circuit 30, the detection circuit may be complicated due to factors such as the applied voltage becoming a high frequency voltage. In this operation, the voltage detection means 26a can be omitted, so that the constant temperature control can be performed with a simple detection circuit.

  In this operation, a detection error occurs due to a change in the value of the conversion efficiency η. However, in the constant temperature operation mode of the present embodiment, since the duty ratio is controlled under the condition that the drive frequency of the inverter circuit 23 is constant at f2, the loss in the inverter circuit 23, that is, the change in the conversion efficiency η is small. Even if this operation is used, constant temperature control can be realized.

In addition, in the constant temperature operation mode 1 and 2 mentioned above, although the case where the water in the to-be-heated material 5 (pan) was heated was demonstrated to the example, this invention is not limited to this.
For example, oil is introduced into the object to be heated 5 and constant temperature control for keeping the temperature of the oil constant can be performed by performing the above-described constant temperature operation modes 1 and 2 also in cooking fried food such as tempura. .

In the case of deep-fried food cooking, the oil temperature is generally higher than that in the case of water, so that the oil temperature may be lowered by adding the ingredients of the deep-fried food.
In the present embodiment, by performing the above-described constant temperature control, the temperature of the object to be heated 5 can be controlled to be constant even when the food is added to the oil. Therefore, an easy-to-use induction heating cooker without fried food cooking failure can be obtained.

(Configuration example of another drive circuit)
Next, an example using another drive circuit will be described. FIG. 9 is a diagram showing another drive circuit of the induction heating cooker according to the first embodiment. The drive circuit 50 shown in FIG. 9 is obtained by adding a resonance capacitor 24b to the configuration shown in FIG. Other configurations are the same as those in FIG. 2, and the same parts are denoted by the same reference numerals.

  As described above, since the resonance circuit 30 is configured by the heating coil 11a and the resonance capacitor, the capacity of the resonance capacitor is determined by the maximum heating power (maximum input power) required for the induction heating cooker. In the drive circuit 50 shown in FIG. 9, by connecting the resonance capacitors 24a and 24b in parallel, the respective capacities can be halved, and an inexpensive control circuit can be obtained even when two resonance capacitors are used. .

  Further, by arranging the coil current detection means 25b on the resonance capacitor 24a side of the resonance capacitors connected in parallel, the current flowing through the coil current detection means 25b is half of the current flowing through the heating coil 11a. The capacity coil current detection means 25b can be used, a small and inexpensive control circuit can be obtained, and an inexpensive induction heating cooker can be obtained.

  In the above invention, the method of controlling the drive frequency at f1 in the initial stage of heating (before t1 in FIG. 8) has been described. However, in the initial stage of heating, the drive frequency is controlled such that the input power is controlled to be constant. You may use the system which changes.

  In the above-described invention, the configuration in which both the voltage detection means 26a and the primary voltage detection means 26b are provided has been described, but a configuration in which either one is provided may be employed.

  In the first embodiment, the half-bridge inverter circuit 23 has been described. However, a configuration using a full-bridge inverter or a single-voltage resonance inverter may be used.

  Further, the method of using the relationship between the coil current and the primary current in the load determination processing in the load determination means has been described, but a method of performing the load determination processing by detecting the resonance voltage at both ends of the resonance capacitor may be used. The determination method is not particularly limited.

  DESCRIPTION OF SYMBOLS 1 1st heating port, 2nd heating port, 3rd heating port, 4 Top plate, 5 To-be-heated object, 11 1st heating means, 11a Heating coil, 12 2nd heating means, 13 1st heating port Three heating means, 21 AC power supply, 22 DC power supply circuit, 22a diode bridge, 22b reactor, 22c smoothing capacitor, 23 inverter circuit, 23a, 23b IGBT, 23c, 23d diode, 24a, 24b resonance capacitor, 25a input current detection means , 25b Coil current detection means, 26a Voltage detection means, 26b Primary voltage detection means, 30 Resonance circuit, 40a to 40c Operation part, 41a to 41c Display part, 42 Notification means, 45 Control part, 50 Drive circuit, 100 Induction heating cooking vessel.

Claims (5)

A heating coil for inductively heating an object to be heated;
A resonant capacitor connected to the heating coil and constituting a resonant circuit;
An inverter circuit for supplying high-frequency power to the heating coil;
A control unit for controlling high-frequency power supplied to the heating coil by controlling a drive signal of the inverter circuit;
An operation unit that receives a selection operation of a predetermined operation mode;
Coil current detection means for detecting a coil current flowing in the heating coil;
Voltage detecting means for detecting an applied voltage applied to the resonant circuit;
With
The predetermined operation mode includes a constant temperature operation mode for keeping the temperature of the object to be heated constant,
The controller is
When the constant temperature operation mode is selected,
In a state where the drive frequency of the inverter circuit is fixed, the duty ratio of the drive signal of the inverter circuit is varied so that the ratio between the coil current and the applied voltage is constant, and the impedance of the resonance circuit is set. Induction heating cooker to make constant . A heating coil for inductively heating an object to be heated;
A resonant capacitor connected to the heating coil and constituting a resonant circuit;
An inverter circuit for supplying high-frequency power to the heating coil;
A control unit for controlling high-frequency power supplied to the heating coil by controlling a drive signal of the inverter circuit;
Coil current detection means for detecting a coil current flowing in the heating coil;
Input current detecting means for detecting an input current to the inverter circuit;
Primary voltage detection means for detecting a primary voltage applied to the inverter circuit;
An operation unit that receives a selection operation of a predetermined operation mode;
With
The predetermined operation mode includes a constant temperature operation mode for keeping the temperature of the object to be heated constant,
The controller is
When the constant temperature operation mode is selected,
In a state where the drive frequency of the inverter circuit is fixed, the ratio of the product of the input current and the primary voltage and the value obtained by squaring the coil current is constant. An induction heating cooker that varies the duty ratio to make the impedance of the resonance circuit constant . The controller is
When the constant temperature operation mode is selected,
The induction heating cooker according to claim 1 or 2 , wherein the drive frequency of the inverter circuit is fixed after increasing the drive frequency of the inverter circuit. The inverter circuit is
It is composed of a half-bridge inverter circuit having an arm that directly connects two switching elements,
The controller is
The induction heating cooking appliance as described in any one of Claims 1-3 which varies the duty ratio of the said switching element in the state which fixed the drive frequency of the said switching element of the said half-bridge inverter circuit. The load determination means which performs the load determination process of the said to-be-heated material based on the correlation of the input current to the said inverter circuit, and the coil current which flows into the said heating coil, The any one of Claims 1-4 further provided. The induction heating cooker according to item.

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