JP2016076455A - Induction heating cooker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce overvoltage and reduce oscillation, which occur at a resonance circuit when a non-magnetic metal load is heated.SOLUTION: An induction heating cooker comprises: variable voltage DC power supply means having power factor improvement means arranged in a previous stage of inverter means; load determination means for determining depending on a load material whether a load is a high-impedance load which is mainly a magnetic metal load or a low-impedance load which is mainly a non-magnetic metal load; and a mechanism for changing a resonance frequency determined by a heating coil and a resonance capacitor between the high-impedance load and the low-load impedance load. The power factor improvement means has operation modes of a power factor priority mode which prioritizes a power factor and a voltage stability priority mode which has voltage stability higher than that in the power factor priority mode. In the induction heating cooker, the variable voltage DC power supply means is controlled in the power factor priority mode when determined as the high-impedance load and in the voltage stability priority mode when determined as the low-impedance load depending on the determination result of the load determination means, and the resonance frequency is changed to the high-impedance load or the low-impedance load and an inverter operation is performed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an induction heating cooker.

  The induction heating cooker can control the electric power generated in the metal load (pan) by controlling the current flowing through the heating coil. In addition, since the heating efficiency is high, the temperature of the metal load can be quickly raised, cooking can be done in a short time when boiling or stir-fry, and there is no open flame, and overheating by the temperature sensor. High safety such as protection.

  In recent years, not only a magnetic metal pan made of iron or magnetic stainless steel that can be used conventionally, but also a technique that can heat a pan made of non-magnetic metal such as aluminum or copper has been put into practical use.

  In such an induction heating cooker that can heat a non-magnetic metal, a DC stabilized power source is disposed in front of the inverter means to stabilize the DC voltage applied to the inverter means. This is to reduce the vibration due to the repulsive force generated between the heating coil and the load generated by the vibration component of the commercial power supply frequency.

  As a DC voltage stabilization method, a smoothing capacitor having a sufficiently large capacity is inserted after rectification of the commercial power supply voltage (Patent Document 1), or a DC power supply circuit including a booster circuit having a power factor improving function is used. (Patent Document 2).

Japanese Patent No. 3884664 Japanese Patent No. 4900288

  In general, electrical appliances are required to suppress the electric current of the distribution line as much as possible and to take out the electric power effectively, and this is largely synonymous with increasing the power factor. In a load with a low power factor, reactive power other than active power increases, and the current flowing through the power cable increases, causing a failure due to harmonics. Needless to say, if such a load is connected in parallel, the influence is further increased. In particular, when a capacitor having a large capacity as viewed from the commercial power supply is connected, the power factor is greatly reduced.

  In recent years, many devices having a large capacitance in a power supply circuit of an electric appliance have a power factor improving circuit arranged in front of a load circuit so as to obtain a power factor close to 100%.

  In an induction heating cooker for heating a conventional magnetic metal load (pan), a power circuit arranged in front of the inverter means connected to the heating coil is composed of a choke coil and a capacitor as a rectifier circuit for rectifying commercial power. It consists of a filter circuit. However, the time constant of the coil and capacitor is small for the power of the load, and the power factor is close to 100%.

  However, as described above, when a nonmagnetic metal load is heated, a repulsive force is generated due to a vibration component of the commercial power supply frequency. Therefore, it is desirable to apply a DC voltage that is as smooth as possible to the inverter means. However, in the magnetic metal load, since an attractive force due to the magnetic flux generated from the heating coil is generated between the load and the coil, these are canceled out. Therefore, when heating a nonmagnetic metal load, a smoothness of a DC voltage is required as compared with heating a magnetic metal load, and a so-called nonmagnetic metal (aluminum or copper) load can be heated. In an all-metal induction heating cooker, at least when heating a nonmagnetic metal load, a power factor correction circuit is arranged to stabilize and supply a voltage to be applied to the inverter means.

  However, even if the power factor correction circuit is arranged, the output voltage is not completely smoothed and includes some ripples. Furthermore, when heating a non-magnetic metal load, it is necessary to drive at a higher frequency than when heating a magnetic metal load, and at the same time, it has a large equivalent inductance and a low equivalent resistance. The generated resonance voltage may be 10 times or more that when the magnetic metal load is heated. Therefore, the higher the ripple component, the higher the peak voltage of the resonance voltage, especially when heating a nonmagnetic metal load, and it is necessary to pay attention to the surrounding parts and the case with sufficient insulation distance. Depending on the condition, abnormal discharge may occur, causing a circuit failure and making it unusable. Conventionally, the resonance voltage itself has been kept low by adjusting the number of turns of the heating coil and the distance of the load (pan) or by setting the inverter operating frequency lower.

  The present invention is for solving the above-mentioned problems, and can heat at least a nonmagnetic metal load such as aluminum or copper, and when heating a nonmagnetic metal load in an induction heating cooker having a power factor improving means, The smoothness of the output voltage in the power factor improving means is set higher than the smoothness when heating the magnetic metal load. This means that when heating a non-magnetic metal load, the smoothness of the output voltage is increased by lowering the power factor slightly, and the peak current value and the current value for the commercial power supply also increase. Thus, the maximum value of the heating power of the non-magnetic metal load is reduced by more than the amount corresponding to the power factor reduction with respect to the heating power maximum value of the magnetic metal load.

  According to the present invention, when heating a nonmagnetic metal load, the ripple component of the voltage applied to the inverter means can be reduced, the peak value of the resonance voltage generated in the resonance circuit can be suppressed, and the resonance can be reduced. The vibration sound caused by the vibration of the envelope of the current is reduced, and it can be suppressed that the cooking environment becomes uncomfortable for the user. In addition, the current value of the commercial power source caused by the power factor drop that occurs when heating a non-magnetic metal load does not exceed the current capacity of the outlet or power cable, eliminating the cause of fire due to abnormal heating etc. can do.

Main block diagram of induction heating cooker, Configuration example of variable voltage DC power supply means Example of operation waveform of variable voltage DC power supply means of FIG. Configuration example of power control means Configuration example of mode switching method Configuration example of other variable voltage DC power supply means Configuration example of other power control means Example of operation waveform of variable voltage DC power supply Example of impedance depending on load material Configuration example of inverter means Example of inverter current and resonance voltage

  An embodiment of the present invention will be described below with reference to FIGS.

  In FIG. 1, the electric power supplied from the commercial power source 1 is converted into direct current by the rectifier circuit 2 and the voltage is stabilized by the direct-current variable voltage power source means 3 having power factor improving means. The output voltage is connected to the inverter means 5, and an eddy current is generated in the metal load (pan) by flowing a high-frequency current through the heating coil 6 to cause self-heating.

  Main control means 10 receives power supply voltage VAC, power supply current IAC and inverter current IINV, calculates input power from VAC and IAC, and detects the state of the load from IAC and IINV. Depending on these states, the drive signal INV-GATE for the switching element in the inverter means 5, the MODE for designating the operation mode of the power supply control means 20 for controlling the operation of the DC variable voltage power supply means 3, and the designation of the output voltage The VSET signals for performing are output.

  The power control means 20 inputs the rectified power supply voltage VIN, the power supply current VIN, and the output voltage VOUT, and outputs a PWM signal so that the output voltage becomes a predetermined voltage while performing the power factor improving operation by the MODE signal and the VSET signal. Output to the DC variable voltage power supply means 3.

  FIG. 2 is an example of commonly used boost type power factor correction means, and FIG. 3 is an example of waveforms of input voltage, current and output voltage and inverter current flowing in the heating coil.

  In FIG. 2, the positive output of the rectifying means 2 forms a closed circuit to the negative output by the choke coil 301 and the switch means 302, and is connected to the output capacitor 304 via the diode 303 at the connection portion of the choke coil 301 and the switch means 302. To do. By applying an appropriate pulse pattern to the switch means 302 to perform a switch operation, a boost type power factor correction operation using the back electromotive force of the choke coil 301 can be obtained.

  FIG. 3 shows respective waveforms when the power factor correction operation is performed, and the input voltage VIN and the input current IIN are substantially similar. If they are completely similar, the power factor is 100%, and the power factor decreases as the similarity is shifted.

  The output voltage VOUT includes ripple voltage fluctuations of the upper VRIPP and the lower VRIPM with respect to the average voltage VAVR. Due to this fluctuation, the inverter current flows a peak current that has an envelope similar to the waveform of the average voltage and ripple voltage. Usually, when the power factor is 100%, a certain amount of ripple voltage remains, but in order to reduce this, it is necessary to change the input voltage waveform VIN and the input current waveform IIN from similar states.

  FIG. 4 is a configuration example of the power supply control means 20. The power source control means 20 is a power factor priority operation by changing a control value as shown in FIG. 5 with respect to a control means used for power factor improvement called a general current continuous mode. It is possible to switch whether to give priority to the stability of the output voltage. However, there are other methods such as a current discontinuous mode and a current critical mode as power factor improvement methods, and both can be switched similarly by the method shown in FIG. In recent years, it has become possible to configure these controls by software, but the configuration as the control is the same.

  In FIG. 4, VOUT is divided to a suitable voltage by resistors 211 and 212 and smoothed by a capacitor 213. If necessary, the VSET signal is synthesized from the main control means 10 to be VOUT ′ and input to the operational amplifier 202 having the reference potential 201 (Vref) and the gain G1. The output, the input IIN, and the multiplier 203 are input, and the output and VIN are input to the operational amplifier 204 having the gain G2. A reference oscillator 205 serving as a reference for the output and the operation timing of the switch element 302 is input to the comparator 206, and the output is input to the switch element 302 as a PWM signal.

  According to this power supply control means, the VIN waveform and the IIN waveform are similar, and the output voltage is controlled so that the voltage obtained by dividing VOUT is substantially the same as Vref. However, as described above, in the circuit configuration of FIG. 2, the output voltage is higher than the input voltage and cannot be stepped down, but any voltage output higher than the input voltage is possible according to the VSET signal of the main control means 10. It becomes.

  FIG. 5 is an example of a configuration for switching between the power factor priority mode and the voltage stability priority mode in the configuration of the power supply control unit 20 of FIG. The gain of the operational amplifier 202 to which VOUT ′ and VREF are input is switched and set to G1L and G1H by the MODE signal (L / H), respectively. At this time, G1L <G1H. If the gain is set to G1L and the power factor is set to the maximum, when the MODE signal is set to H, the output for the difference between VOUT ′ and VREF increases, the power factor decreases, but the stability with respect to the output voltage VOUT Increase. Therefore, the power factor priority mode and the voltage stability priority mode can be switched, and a configuration in which the mode can be switched by the control signal from the main control means 10 can be realized.

  FIG. 6 shows a configuration example of another power factor improving means, which is different from the configuration of FIG. A step-down means constituted by a switch element 311, a choke coil 313 and a freewheeling diode 312 is connected to the positive output of the rectifying means 2, and a step-up means constituted by a switch element 314, a backflow prevention diode 315 and a smoothing capacitor 316 is arranged. At this time, the step-down means and the step-up means share the choke coil 313. Since the output of the rectifying means 2 is directly turned on and off, the noise component is easily applied to the commercial power supply, and therefore measures such as inserting an LC filter before rectification or immediately after rectification are required (not shown). )

  If the switch elements 311 and 314 are controlled by a suitable drive pulse train, the output voltage can be arbitrarily set while improving the power factor. For example, when a voltage higher than the input voltage is required, the configuration shown in FIG. 2 is equivalent to driving the switch element 314 with a suitable pulse train while the switch element 311 is turned on. If a voltage lower than the input voltage is required, the switch element 311 is driven with a suitable pulse train when the input voltage is higher than the target voltage, and the switch element 311 is turned on when the input voltage is lower than the target voltage. The switch element 314 may be driven with a suitable pulse train while maintaining the above.

  FIG. 7 is a configuration example of the power supply control means 20 that performs the power factor improving operation in the configuration of FIG. The configuration is basically the same as that of FIG. 4, but the final stage comparator 206 is set to 206A and 206B in order to output the drive pulse trains (PWM1, PWM2) of the step-down switch element 311 and the step-up switch element 314, respectively. In this configuration, the output of the reference oscillation means 205 is compared as it is, and the output is compared with the output obtained by adding VOFS. As a result, the output of the operational amplifier 204 becomes a source signal for performing a power factor improving operation, and also becomes a source signal for generating a pulse train for step-down / boost operation. Further, the switching configuration between the power factor priority mode and the voltage stability priority mode shown in FIG. 5 can be applied as it is.

  FIG. 8 shows an operation example of the variable voltage power supply means 3 of this embodiment. When VIN is a sine wave full-wave rectification waveform and power factor priority mode, input current waveform IIN1 and output voltage waveform VOUT1 are obtained. When voltage stability priority mode is selected, input current waveform IIN2 and output voltage waveform VOUT2 are obtained.

  Since the voltage applied to the inverter means 5 is the output of the variable voltage power supply means 3, as described above, the inverter current flowing through the heating coil has a peak current proportional to the envelope of the voltage.

  Here, an example of an inverter load (an equivalent inductance and an equivalent resistance when the heating coil 6 and a metal load are combined) and an operating frequency of an all-metal induction heating cooker that is put into practical use is shown.

  FIG. 9 shows a combination of equivalent inductance, equivalent resistance, and inverter operating frequency when an iron enamel pan is used as the magnetic metal load and an aluminum pan is used as the non-magnetic metal load. As shown in FIG. 9, the values of the equivalent inductance and equivalent resistance that are part of the resonance circuit are greatly different. A magnetic metal load has a high impedance, and a nonmagnetic metal load has a low impedance. In particular, since the equivalent resistance tends to be low for nonmagnetic metal loads, a high inverter operating frequency is required.

  FIG. 10 shows an example of inverter means that can cope with such various loads. In FIG. 10, between the positive power supply terminal and the negative power supply terminal, there are an arm A (501 and 502) and an arm B (503 and 504) in which two switch elements are connected in series. Is connected to the freewheeling diode. The heating coil 6 is connected to the middle point of the arm B, and the resonance capacitor 507 for nonmagnetic metal load is connected to the other end. A switch 505 is connected to the connection point between the heating coil 6 and the resonance capacitor 507, and is connected to the midpoint of the arm A through the magnetic metal resonance capacitor 506.

  When heating the magnetic metal load, the switch 505 is short-circuited, and a series resonance body of the resonance capacitor 506 and the heating coil 6 is connected between the arm A and the arm B, and the switch elements in the arm A and the arm B are connected. A high frequency current is caused to flow through the series resonators by driving each other.

  When heating the non-magnetic metal load, the switch 505 is opened, the arm A is in a quiescent state, and only the arm B is operated as a so-called SEPP circuit, and a high frequency current is applied to the series resonator of the resonance capacitor 507 and the heating coil 6. Shed.

  As shown in FIG. 9, since the equivalent resistance is sufficiently large even at a low inverter operating frequency in the magnetic metal load, the resonance frequency of the series resonator must be set low, and conversely, it must be set high in the nonmagnetic metal load. However, this is realized by switching operation of the switch 505.

  This inverter means 5 selects either a magnetic metal load or a non-magnetic metal load at the start of heating, determines the state from the power source current IAC, the inverter current IINV, etc. generated after driving the switch element, If the above configuration is suitable, the heating operation is continued, and if not suitable, the configuration is changed so that the heating start operation is started again. Determine if the load is heating.

Assuming that the input power (power supplied from the commercial power source) is W and the conversion efficiency is 100%, if the inverter current (resonant current) is I, the equivalent inductance is L, and the equivalent resistance is R, the following equation is established. To do.

W = RI ² (Equation 1)
I = √ (W / R) (Equation 2)

At this time, the voltage (resonance voltage) V generated in the equivalent inductance L is f if the inverter operating frequency is f.

V = 2πfLI (Equation 3)

Therefore, when the input power is 3 kW under the conditions of FIG. 9, the inverter current and the resonance voltage have the values shown in FIG.

  Here, if the output voltage of the variable voltage power supply means 3 has a ripple component of 20%, peak vibration of that component occurs in both the inverter current and the resonance voltage. This is not a problem because the absolute value of the generated voltage is relatively low in the case of a magnetic metal load. large. In this example, the average of 4237V is about 5084V at the peak.

  Therefore, as shown in this embodiment, the output voltage stability of the variable voltage power supply means is required particularly when heating a nonmagnetic metal load.

  However, as already mentioned, the power factor decreases when the stability of the output voltage is increased. Naturally, when trying to obtain the same maximum power with a magnetic metal load and a non-magnetic metal load, the reactive power is added to the current supplied from the commercial power source with the non-magnetic metal load, so a large current flows through the power cable. It will be.

  Even in products such as so-called built-in type and stationary type that have multiple induction heating cooking units from a single outlet, current for the number of outlets will flow if used for cooking at the same time. The current capacity may be exceeded.

  Therefore, in the induction heating cooker of the present embodiment, when heating is performed using the output voltage stability priority mode with respect to the power factor priority mode, control is performed to suppress the power source current in consideration of the power factor decrease. In this case, at the maximum power that can be set by the user, the latter power is equivalent to reducing the power factor decrease in the output voltage stability priority mode with respect to the power factor priority mode.

  For example, when the output voltage stability priority mode is reduced to 95% with respect to the power factor of 100% with respect to the power factor priority mode, the input current is suppressed to 95% or less at the maximum.

By carrying out this control, it is possible to prevent overcurrent from exceeding the current capacity of the power cable and outlet due to overcurrent, and to suppress overvoltage that occurs when heating a nonmagnetic metal load, leading to high safety induction A cooker can be provided.

  1 commercial power supply, 2 rectifier circuit, 3 DC variable voltage power supply means, 5 inverter means, 6 heating coil, 10 main control means, 20 power supply control means, 202 operational amplifier, 203 multiplier, 204 operational amplifier, 205 reference oscillator, 205 reference oscillation Means, 206 comparator, 211, 212 resistor, 213 capacitor, 301 choke coil, 302 switch means, 303 diode, 304 output capacitor, 311 switch element, 312 freewheeling diode, 313 choke coil, 314 switch element, 315 backflow prevention diode, 316 Smoothing capacitor, 501, 502 Arm A, 503, 504 Arm B, 505 Switch, 506 Resonance capacitor for magnetic metal, 507 Resonance capacitor

Claims (2)

Inverter means for changing the inverter operation mode according to the load material;
Variable voltage direct current power supply means having power factor improving means arranged in the preceding stage of the inverter means;
Load discriminating means for discriminating between a high impedance load that is mainly a magnetic metal load and a low impedance load that is mainly a non-magnetic metal load, depending on the load material;
A mechanism for changing a resonance frequency determined by a heating coil and a resonance capacitor in the high impedance load and the low impedance load, and
The power factor improving means has a power factor priority mode that prioritizes the power factor and an operation mode of a voltage stability priority mode that is higher in voltage stability than the power factor priority mode.
According to the determination result of the load determination means,
For high impedance loads, power factor priority mode,
In low impedance load, voltage stability priority mode,
In addition to controlling the variable voltage DC power supply means,
An induction heating cooker characterized by performing an inverter operation by changing a resonance frequency to a high impedance load or a low impedance load.   The induction heating cooker according to claim 1, wherein the maximum power of the inverter means in the voltage stability priority mode is lower than the maximum power in the power factor priority mode.