JP2895078B2 - Electromagnetic cooker - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention supplies a high-frequency current to a heating coil, and generates an eddy current in an object to be heated by a magnetic flux from the heating coil. The present invention relates to an electromagnetic cooker that heats an object to be heated with Joule heat generated by an electric current.

(Prior art) An electromagnetic cooker that heats an object to be heated by an electromagnetic induction action is safe because it does not generate a flame and is clean because the top plate for mounting the object to be heated can be made of crystallized glass. In addition, various electromagnetic cookers have been developed, having advantages such as high heat efficiency.

The conventional electromagnetic cooker shown in FIG. 5 supplies a predetermined DC voltage from a DC power supply circuit 101 to an inverter circuit 103. When the drive circuit 115 turns on and off the transistor 113, the heating coil 107 and the resonance capacitor 109 are set in a series resonance state, and the magnetic flux generated from the heating coil 107 causes an electromagnetic induction effect to vortex the object to be heated such as a pan (not shown). An electric current is generated and heated.

Pulse width modulation circuit (PULSE WIDTH MO) with built-in oscillator
DULATION) 119 corrects the oscillation cycle of the oscillation pulse of the oscillator based on the timing pulse based on the resonance voltage from the inverter circuit 103. The pulse width modulation circuit 119 modulates the pulse width of the oscillation pulse based on the signal voltage Von from the on-time setting circuit 123. Therefore, when the driving circuit 115 receives the pulse signal from the pulse width modulation circuit 119, the driving circuit 115 outputs the transistor for a time corresponding to the pulse width of the pulse signal.
Turn on 113. The on-time setting circuit 123 is a comparison circuit 125
The comparison circuit 125 is connected to the input current detection circuit 129.

An input current detection circuit 129 detects an input current Iin from the AC power supply based on a detection signal from a current transformer CT provided in the AC power supply, and compares a signal voltage Vin corresponding to the input current Iin with a comparison circuit 125. Of the load detection circuit 143.

When the input setting relating to the heating power for heating the object to be heated is performed by operating the input setting operation section 131, the input setting value Vs is input via the input power setting circuit 133 and the input current setting circuit 135.
et is supplied to the inverting input terminal of the comparison circuit 125. Therefore, the comparison circuit 125 compares the input set value Vset with the signal voltage Vin, and operates the on-time setting circuit 123 according to the comparison result. For example, for the input set value Vset, the signal voltage Vin
Is larger, the transistor Tr1 is turned on and the value of the signal voltage Von input to the pulse width modulation circuit 119 is set smaller.

The load detection circuit 143 monitors the load state by comparing the signal voltage Von from the on-time setting circuit 123 with the signal voltage Vin from the input current detection circuit 129, that is, the signal voltage corresponding to the input current Iin. For example, when an iron pan is placed on the heating coil 107, since an appropriate input current Iin flows, it is determined that the load is appropriate and the pulse width modulation circuit is determined.
Continue the operation of 119. Conversely, in the no-load state or when the aluminum pan is placed on the heating coil 107, the input current Iin becomes small, it is determined that the load is inappropriate, and the stop signal Vstp is output to the inverter oscillation stop circuit. By outputting to 145, the operation of the pulse width modulation circuit 119 is stopped and the heating operation is prohibited.

Such a load state monitoring operation by the load detection circuit 143 is performed when a power switch (not shown) is turned on or when the inverter circuit 103 starts an oscillation operation.

By the way, in the conventional electromagnetic cooking device shown in FIG. 5, the value of the input power set value set by the input power setting circuit 133 by operating the input power setting operation unit 131 is a predetermined threshold level,
For example, when the power is less than 1 kW, the value of the input current Iin from the AC power supply unit PW is controlled to be constant, and the on / off operation of the inverter circuit 103 is controlled according to the input power set value.

More specifically, when the value of the input power setting value set by the input power setting circuit 133 is less than 1 kW, the input current setting circuit 135 always applies the predetermined voltage Vl to the inverting input terminal of the comparison circuit 125. Output. Thus, the input current Iin input from the AC power supply is set to a predetermined constant value, for example, 10A. At this time, the inverter on / off control circuit 153 operates based on the signal 133b from the input power setting circuit 133, and controls the on / off operation of the inverter circuit 103 via the pulse width modulation circuit 119 and the drive circuit 115. That is, a heating period in which the transistor 113 serving as the switching means repeats the on / off operation and a pause period in which the on / off operation of the transistor 113 is stopped after the heating period is set, and a ratio of the length of the heating period to the pause period Is controlled in accordance with the above-described input power set value signal 133b. For example, when the input power setting value set by the input power setting circuit 133 is 500 W, the inverter circuit 103 repeats the on / off operation every 20 ms.

(Problems to be Solved by the Invention) However, the conventional electromagnetic cooker has an input power setting circuit.
When the value of the input power set value set by 133 is equal to or less than a predetermined value, the inverter circuit 103 is turned on and off, and the oscillation frequency of the transistor 113 that performs a switching operation during the heating period of the inverter circuit 103 is changed. Depending on the material of the object to be heated, it varies with a predetermined band width, and there has been a problem that accompanying noise is generated over a wide band.

More specifically, FIG. 6 is a characteristic diagram showing the frequency characteristics with respect to the ON time of the transistor 113 as a switching element for each material of the object to be heated.
Is a frequency characteristic curve for an iron pan, FIG. 6 (B) is a frequency characteristic curve for a non-magnetic stainless steel pan, and FIG. 6 (C).
Is the frequency characteristic curve for a stainless steel pan with magnetism,
FIG. 6 (D) is a frequency characteristic curve in the case of a so-called three-layer pot formed of three layers of stainless steel-iron-stainless steel.

FIG. 7 is a characteristic diagram showing the frequency characteristics of the transistor 113 with respect to the oscillation cycle of the inverter circuit 103 for each material of the pan, and FIG. 7A is a frequency characteristic diagram in the case of an iron pan, and FIG. B) is a frequency characteristic diagram of a non-magnetic stainless steel pan, FIG. 7 (C) is a frequency characteristic diagram of a magnetic stainless steel pan, and FIG. 7 (D) is an inverter circuit 103.
FIG. 3 is an explanatory diagram showing an oscillation cycle of the first embodiment.

As shown in FIG. 7, when the value of the input power set value set by the input power setting circuit 133 is, for example, 1 KW or less, the inverter circuit 103 is controlled to be turned on and off. Every time is started, the load detection operation is performed only for a predetermined time T1, for example, 10 ms. That is, the input power setting circuit 133
If the value of the input power set value set by the above is 1 KW or less, the on-time setting timer 127 operates based on the signal 133a from the input power setting circuit 133, and the transistor T
r3 and Tr4 are turned on only for a predetermined time T1, that is, for 10 ms. As a result, the on-time setting circuit 123 outputs a predetermined signal voltage Von, and sets the on-time of the transistor 113 to a constant value, for example, 14 μs. Therefore, during a certain period T1 for detecting such a load state, the transistor 11
3 oscillates at a frequency corresponding to an on-time of 14 μs. That is, in the case of the iron pan shown in FIG. 6 (A), oscillation occurs at 26.6 KHz, and in the case of the non-magnetic stainless steel pan shown in FIG. 6 (B), oscillation occurs at 31 KHz. In the case of a stainless steel pot having the magnetic properties shown in Fig. 7, the oscillation occurs at 24 KHz.

When the predetermined period T1 for such load detection has elapsed, in the next period T2, the inverting input terminal of the comparison circuit 125 sets the set value V1
Input current Ii from the AC power supply PW
The value of n is controlled to a constant value, for example, 10A. Therefore, as shown in FIG. 7, in the case of a non-magnetic stainless steel pot, the oscillation frequency of the transistor 113 changes from 31 KHz to 33.4 KHz.
The oscillation frequency of 113 changes from 24KHz to 22.7KHz. Thus, as shown in FIG. 7 (B), the non-magnetic stainless steel pot oscillates with a bandwidth of 2.4 KHz every 20 msec, and in the case of a magnetic stainless steel pot, FIG. 7 (C) As shown in the figure, it oscillates with a bandwidth of 1.3 KHz every 20 ms.

Thus, depending on the material of the object to be heated, the transistor 11
The oscillation frequency of 3 changes with the bandwidth, and the accompanying noise occurs over a wide band.

In addition, a new problem arises when cooking is performed using a so-called three-layer pot with a conventional electromagnetic cooker. That is, as shown in FIG. 8, at the timing when the transistor 113 is turned on, the voltage across the resonance capacitor 109 does not completely fall to OV, so that a short-circuit current Is flows through the transistor 113 and power loss increases. This short-circuit current Is varies depending on the material of the object to be heated. As shown in FIG. 9 (B), the short-circuit current is larger in the case of the three-layer pan than in the case of the iron pan shown in FIG. 9 (A). Is flowing. Therefore, the input power setting circuit
When the value of the input power set value set by 133 is 1 KW or less, the value of the input current Iin from the AC power supply unit is set to 10 A
The on / off operation of the inverter circuit 103 is controlled in the state set as described above. In such a control method, when a three-layer pot is used, the power loss increases, and the amount of heat generated thereby increases. For this reason, it is necessary to take a countermeasure for dissipating the heat generated by the power loss, which raises a problem that the cost increases.

The present invention has been made in view of the above problem, and even when the input power set value is set to a low value, induction heating is performed at the frequency at the time of load detection, thereby narrowing the noise bandwidth and switching means. It is an object of the present invention to provide an electromagnetic cooker that reduces noise caused by fluctuations in the oscillation frequency of the device and reduces the amount of heat generated by power loss to further reduce costs.

[Means for Solving the Problems] In order to achieve the above object, an electromagnetic cooker provided by the present invention is provided with a switching unit that repeats an on / off operation, and is heated according to an on time of the switching unit. Heating means for inductively heating an object, input setting means for performing an input setting for heating the object to be heated, and heating wherein the switching means repeats an on / off operation when a value of the input setting is equal to or less than a predetermined value. A first period for setting a period and a pause period for suspending the on / off operation of the switching means after the elapse of the heating period, and controlling a ratio of the length of the heating period to the pause period according to the value of the input setting. A control unit, a load detection unit that detects a power supply current and detects a load state by comparing with a value of the input setting, and when the value of the input setting is equal to or less than a value at the time of load detection. Was constructed and a second control means for controlling the on-time of said switching means to a value corresponding to the frequency at the load sensing.

(Effect) The electromagnetic cooker according to the present invention has a heating means for heating an object to be heated by generating an eddy current in the object to be heated by a magnetic flux generated by an electromagnetic induction action. The object to be heated is heated according to the ON time. The apparatus further includes input setting means for performing input setting for heating the object to be heated, and when the value of the input setting is equal to or less than a predetermined value, while controlling the ON time of the switching means to a predetermined constant value. A heating period in which the switching means repeats the on / off operation, and a pause period in which the on / off operation of the switching means is suspended after the heating period elapses, and the ratio of the length of the heating period to the pause period is set as an input setting. Control according to the value. The load detecting means detects a power supply current and detects a load state by comparing the detected power supply current with a value of an input setting.

Therefore, when the value of the input setting is equal to or less than the value at the time of load detection, the heating power of the object to be heated is controlled with the ON time of the switching means adjusted to a value corresponding to the frequency at the time of load detection. Even when the value of the input setting is a low value, the oscillation frequency of the switching means at this time can be set to a value corresponding to the frequency at the time of load detection, so that the noise bandwidth is reduced and the noise is reduced. Can be achieved. Further, since the oscillation frequency of the switching means is set to a value corresponding to the frequency at the time of load detection, power loss can be reduced.

(Example) Hereinafter, an example according to the present invention will be described in detail with reference to the drawings.

First, the configuration will be described with reference to FIG. 1. A commercial power supply PW, which is an AC power supply, is connected to the DC power supply circuit 1. The DC power supply circuit 1 includes four diodes D connected in a bridge.
It is composed of 1, D2, D3, D4 and a smoothing capacitor C1, rectifies and smoothes AC power from the commercial power supply PW, and converts it into predetermined DC power. The DC power supply circuit 1 is connected to the inverter circuit 3 and supplies a predetermined DC power to the inverter circuit 3.

In the inverter circuit 3, the heating coil 7 and the capacitor 9 for resonance are connected in series, and the transistor 13 is connected in parallel with the capacitor 9 for resonance. A free-wheeling diode 11 is connected in parallel to the resonance capacitor 9. The drive circuit is the base of transistor 13.
The heating coil 7 and the capacitor 9 for resonance are set to a series resonance state by the transistor 13 being turned on and off based on a signal from the drive circuit 15, and the magnetic flux generated by the heating coil 7 An eddy current is generated in the object to be heated such as a pan (not shown) by the electromagnetic induction action to heat the object to be heated.

The pulse width modulation circuit 19 is connected to the drive circuit 15 and to the voltage setting circuit 23. When the voltage Von for setting the ON time from the voltage setting circuit 23 is input, the pulse width modulation circuit 19 receives the pulse width corresponding to the voltage Von. Is output to the drive circuit 15. When the drive circuit 15 receives the pulse signal from the pulse width modulation circuit 19, it turns on the transistor 13 for a time corresponding to the pulse width of the pulse signal. Therefore, according to the value of the voltage Von from the voltage setting circuit 23, the transistor 13
On time changes. That is, when the voltage Von changes, the pulse width of the pulse signal from the pulse width modulation circuit 19 changes, and the heating output, that is, the heating power by the inverter circuit 3 is changed by changing the ON time of the transistor 13. A connection point between the heating coil 7 and the resonance capacitor 9 is connected in a feedback manner to a feedback input terminal of the pulse width modulation circuit 19, and the resonance voltage of the heating coil 7 and the capacitor 9 is supplied to the pulse width modulation circuit 19.

 Next, the internal configuration of the voltage setting circuit 23 will be described.

The resistors R1 and R2 are connected in series, a predetermined DC voltage Vcc is applied to one end of the resistor R1, and one end of the resistor R2 is connected to the ground. Capacitor C3 in parallel with this resistor R2
Is connected. The connection point between the resistors R1 and R2 is connected to the input terminal P1 of the pulse width modulation circuit 19. The input terminal P1 is connected to the collector of the transistor Tr1 via the resistor R3, and the base of the transistor Tr1 is connected to the output terminal of the comparison circuit 25 via the resistor R4. Therefore, these circuit units operate based on the comparison output from the comparison circuit 25, and set the voltage Von for adjusting the on-time of the transistor 13, which is the switching means. The terminal P1 of the pulse width modulation circuit 19 is connected to the transistor Tr via the resistor R6.
3 and the collector of the transistor Tr4 via the resistor R7. A predetermined DC voltage Vcc is applied to the emitter of the transistor Tr3, and the emitter of the transistor Tr4 is connected to the ground. The base of the transistor Tr3 is connected to the on-time setting circuit 27 via the resistor R5. Similarly, the base of the transistor Tr4 is connected to the on-time setting circuit 27 via the resistor R8. Therefore, the voltage setting circuit 23 sets the transistor Tr3 based on the signal from the on-time setting circuit 27.
And turning on the transistor Tr4 to set the voltage Von to a predetermined value.

Next, the input current detection circuit 29 and its peripheral devices will be described.
A current transformer CT is provided on a power supply line between the AC power supply PW and the DC power supply circuit 1, and an input current Ii from the AC power supply PW is provided.
Outputs a detection signal with a value proportional to n. The current transformer CT is connected to a rectifier circuit formed by bridge-connecting four diodes D11, D12, D13, and D14. A resistor R11 and a capacitor C11 are connected in parallel to this rectifier circuit. The anodes of diodes D12 and D14 are resistors
It is connected to a predetermined DC voltage Vcc via R9 and to ground via a resistor R10. The cathodes of the diodes D11 and D13 on the output side of the input current detection circuit 29 are connected to the non-inverting input terminal of the comparison circuit 25 and the load detection circuit 43, respectively. The input current detection circuit 29 outputs a signal voltage Vin corresponding to the input current Iin from the AC current PW to the non-inverting input terminal of the comparison circuit 25 and the load detection circuit 43.

Next, an input setting means for making an input setting relating to a heating power for heating an object to be heated (not shown) will be described.

The input setting means includes an input power setting operation unit 31, an input power setting circuit 33, and an input current setting circuit 35. The input power setting operation unit 31 includes an operation switch and the like related to a heating power for heating the object to be heated.
Output to The input power setting circuit 33 is an input power setting operation section
When 31 is operated, a signal 33 to start load detection
a is output to the setting timer 41. Input power setting circuit 33
Is, for example, an input power value of 1 KW as the reference heating power, and is a proper load when the value of the input power operated by the input power setting operation unit 31 is equal to or less than the reference value, that is, 1 KW or less. In this case, the signal 33b is output to the inverter on / off signal generator 51. Input power setting circuit
33 outputs a signal corresponding to the input power to the input current setting circuit 35 when the value of the input power operated by the input power setting operation section 31 exceeds 1 KW described above. The input current setting circuit 35 outputs an input set value Vset corresponding to the input power to the inverting input terminal of the comparison circuit 25. Therefore, the comparison circuit 25 compares the voltage Vin from the input current detection circuit 29 with the input set value Vset set by the input setting means, and operates the voltage setting circuit 23 according to the comparison result.

The setting timer 41 performs a predetermined period T1 for performing load detection.
When the signal 33a from the input power setting circuit 33 is input, the timer information regarding this period T1 is output to the on-time setting circuit 27. Accordingly, the on-time setting circuit 27 operates the transistor Tr3 and the transistor Tr4 only for a predetermined period T1, and sets the voltage Von to a predetermined value. At this time, the ON time of the transistor 13 is set to, for example, 10 μsec by the voltage Von. The terminal P1 of the pulse width modulation circuit 19 is connected to the load detection circuit 43, and the voltage Von output from the voltage setting circuit 23 is supplied to the load detection circuit 43. The load detection circuit 43 is supplied with the voltage Vin from the input current detection circuit 29, and the load detection circuit 43 receives the input voltage Von and the voltage Vin.
That is, the load state is monitored by comparing the input current Iin with the corresponding signal voltage. For example, when an iron pot is placed on the heating coil 7, an appropriate input current Iin flows, so that it is determined that the load is appropriate. On the other hand, when no load is applied or when the aluminum pan is placed on the heating coil 7, the input current Iin becomes small, and it is determined that the load is improper. Output to the circuit 45. Thereby, the inverter oscillation stop circuit 45 stops the operation of the pulse width modulation circuit 19 and prohibits the heating operation. Such a load detection operation is repeatedly performed based on the signal 43a output from the load detection circuit 43. For example, as shown in FIG.
When the oscillating operation is started, the load detecting operation is repeatedly performed for a predetermined period T1.

Inverter ON / OFF signal generator 1 is an input power setting circuit
When the signal 33b is input from the input power setting circuit 33, the oscillation cycle of the inverter circuit 3 as shown in FIG. 3D is set. That is, the heating period Ton in which the transistor 13 as the switching means repeats the on-off operation, and the transistor after the elapse of the heating period Ton.
A pause period Toff is set to suspend the on / off operation of the switch 13. Further, the inverter on / off signal generator 51 controls the ratio of the length of the heating period Ton to the length of the pause period Toff according to the value of the signal 33b which is the input set value. The inverter ON / OFF signal generator 51 is connected to each of the ON time setting circuit 27 and the inverter ON / OFF control circuit 53, and outputs a signal 51a relating to the oscillation cycle of the inverter circuit to the ON time setting circuit 2.
7 and the inverter ON / OFF control circuit 53. The inverter on / off control circuit 53 is connected to the pulse width modulation circuit 19, and the inverter on / off signal generator
When a signal 51a from 51 is input, the pulse width modulation circuit 19 is controlled based on the signal 51a to control the oscillation cycle of the inverter circuit 3. When the ON time setting circuit 27 receives the signal 51a from the inverter ON / OFF signal generator 51, the ON time setting circuit 27 sets the transistor T for a predetermined period T2 based on the signal 51a.
Activate r3 and transistor Tr4. That is, as shown in FIG. 3 (D), the on-time setting circuit 27 continues the voltage Vo only for the predetermined period T2 after the predetermined period T1 has elapsed.
Set n to a predetermined value. Thus, the ON time of the transistor 13 is set to a constant value, for example, 14 μs during the heating period Ton.

 Next, the operation will be described.

For example, an operation will be described in which a load such as an iron pan is placed on the heating coil 7 and input power as a heating power for heating the iron pan is set to a value exceeding, for example, 1 KW.

First, a load detection operation described later is performed based on a signal 33a from the input power setting circuit 33, and the input current setting circuit 35 outputs an input set value Vset corresponding to the input power to the inverting input terminal of the comparison circuit 25. This allows the comparison circuit 25
Operates the voltage setting circuit 23 to control the value of the voltage Von. The pulse width of the pulse signal from the pulse width modulation circuit 19 changes according to the value of the voltage Von, and changes the on-time of the transistor 13. That is, the drive circuit 15 sets the input set value
By turning on / off the transistor 13 based on a pulse signal from the pulse width modulation circuit 19 output according to Vset, the heating coil 7 and the resonance capacitor 9 are set to a series resonance state. In such a resonance state, when the transistor 13 is turned on, a saw-toothed collector current ic as shown in FIG.
Is turned off, a collector voltage Vce as shown in FIG. 2 (B) is applied. When the heating coil 7 and the resonance capacitor 9 are set in the series resonance state as described above, an eddy current is generated at the bottom of the pan (not shown) by the electromagnetic induction effect of the magnetic flux generated from the heating coil 7 to heat the pan.

Next, an operation in the case where the input power is set to 1 kW or less by operating the input power setting operation unit 31 when performing heat retention or the like will be described.

First, a load detection operation is started based on a signal 33a output from the input power setting circuit 33. More specifically, the setting timer 41 operates the on-time setting circuit 27 for a predetermined period T1 based on the signal 33a. As a result, the on-time setting circuit 27 sets the transistor Tr3 and the transistor Tr4
Is turned on to set the voltage Von to a predetermined value. As a result, the ON time of the transistor 13 is set to a predetermined value, for example, 14 μs. At this time, the load setting circuit 43
The value of the voltage Von from 23 and the voltage V from the input current detection circuit 29
The value of “in” is compared to determine whether the load placed on the heating coil 7 is proper or inappropriate.

On the other hand, the inverter on / off signal generator 51 sets the oscillation cycle of the inverter circuit 3 as shown in FIG. 3 (D) based on the signal 33b from the input power setting circuit 33. If the load detection circuit 43 determines that the load is improper, for example, it outputs a signal 43a to the setting timer 41 after a lapse of a predetermined time, and outputs a signal 43c to the inverter on / off signal generator 51. Thus, the above-described load detection operation is repeatedly executed. When the load detection circuit 43 determines that the load is appropriate, the heating operation is continued. That is, inverter ON / OFF signal generator 51 outputs signal 51a to ON time setting circuit 27. As a result, the on-time setting circuit 27 performs the predetermined period T2 after the predetermined period T1 for performing the load detection has elapsed as shown in FIG.
During this period, the transistor Tr3 and the transistor Tr4 are operated to set the voltage Von to a predetermined value. As a result, the ON time of the transistor 13 is set to 14 μs. Therefore, during the heating period Ton, the ON time of the transistor 13 is set to a constant value, that is, 14 μs, so that the oscillation frequency of the transistor 13 is set to a constant value.

Such a load detection operation and the subsequent heating operation are repeatedly executed at predetermined intervals. That is, as shown in FIG. 3 (D), in the heating period Ton, the load detection operation is performed with the ON time of the transistor 13 set to the predetermined value in the predetermined load detection period T1. In addition, the heating operation is performed with the ON time of the transistor 13 set to a predetermined time, that is, 14 msec, during a predetermined period T2 subsequent to the load detection period T1. When such a predetermined heating period Ton elapses, the operation of the inverter circuit 3 stops during the suspension period Toff, and the heating operation is prohibited. Hereinafter, the inverter circuit 3 is similarly operated at predetermined intervals to heat the object to be heated. Therefore, as shown in FIG. 3 (A), during the heating period Ton, the transistor 1
3 is a frequency corresponding to an on-time of 14 μs, that is, 26.6 KHz
Oscillates at

Next, a non-magnetic stainless steel pan is placed on the heating coil 7, and the input power as a heating power for heating the non-magnetic stainless steel pan is controlled by operating the input power setting operation section 31 to 1 KW or less. The operation when the value is set will be described.

As described above, the inverter on / off signal generator 51 sets the oscillation cycle of the inverter circuit 3 as shown in FIG. 3D based on the signal 33b from the input power setting circuit 33. In the period of the heating period Ton, a load detection period T1 and a predetermined period T2 following the load detection period T1 are set in the same manner as described above, and in the period T1, the ON time of the transistor 13 is set to 14 μs, and in the period T2, Also, the ON time of the transistor 13 is set to 14 μs. Thereby, FIG. 3 (B)
As shown in the figure, during the heating period Ton, the transistor 13 performs an oscillating operation at a constant frequency corresponding to the ON time of 14 μs, that is, 31 KHz.

Similarly, when a stainless steel pan requiring magnetism is placed on the heating coil 7 and the input power is set to a value of 1 KW or less, as shown in FIG. Since the ON time of the transistor 13 is set to 14 μs, the transistor 13 performs an oscillating operation at a constant frequency of 24 KHz.

Next, the case of heating using a three-layer pot will be described. That is, when the three-layer pot is placed on the heating coil 7 and the input power as the heating power for heating the three-layer pot is set to 1 kW by operating the input power setting operation unit 31, As described above, during the heating period Ton, the on time of the transistor 13 is set to 14 μs, so that the transistor 13 performs an oscillating operation at a constant frequency. At this time, the short-circuit current flowing through the collector of the transistor is about 66 A, which can be reduced to the same short-circuit current as in the case of an iron pot. Therefore, power loss due to this short-circuit current can be greatly reduced.

Next, another embodiment according to the present invention will be described with reference to FIG.

In the present embodiment, the heating coil 7 of the inverter circuit 3 and the capacitor 10 are connected in parallel, and the heating coil 7 and the capacitor 10 are set in a parallel resonance state by the on / off operation of the transistor 13, and the heating coil 7 generates the heat. It is characterized in that an object to be heated is heated by generating an eddy current by an electromagnetic induction effect by a magnetic flux.

In the embodiment shown in FIG. 4, similarly to the embodiment shown in FIG. 1, when the input power value set by operating the input power setting operation unit 31 is lower than a predetermined reference value, Since the on time of transistor 13 forming inverter circuit 3 is set to a predetermined constant value, transistor 13 performs an oscillating operation at a constant frequency. Therefore, even when the input power is set to a low value, it is possible to reduce the generation of noise accompanying this.

[Effects of the Invention] As described above, according to the present invention, when the value of the input setting for heating the object to be heated is equal to or less than the value at the time of load detection, the on-time of the switching means is reduced at the time of load detection. Since the heating operation is controlled in a state adjusted to the value corresponding to the frequency, the bandwidth of the noise is reduced,
Generation of noise can be significantly reduced.

Further, the short-circuit current of the switching means can be suppressed low, and the power loss accompanying the short-circuit current can be greatly reduced. In addition, since the power loss accompanying this can be greatly reduced, the photothermal structure can be simplified and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment according to the present invention, FIG. 2 is a signal waveform diagram of FIG. 1, and FIG. 3 is a diagram showing the oscillation frequency of the switching means when the embodiment of FIG. FIG. 4 is a circuit diagram showing another embodiment of the present invention, FIG. 5 is a circuit diagram showing a conventional example, FIG.
FIG. 7 is a characteristic diagram showing the frequency characteristics with respect to the ON time of the conventional switching means for each material of the object to be heated. FIG. 7 is a characteristic diagram showing the frequency characteristics of the conventional switching means for each material of the object to be heated. FIG. 8 is an explanatory diagram showing a short-circuit current flowing through a conventional transistor, and FIG. 9 is a characteristic diagram showing a short-circuit current flowing through a collector of the transistor with respect to input power in a conventional example. 3 Inverter circuit 7 Heating coil 13 Transistor 23 Voltage setting circuit 27 On-time setting circuit 31 Input power setting operation unit 33 Input power setting circuit 35 Input current setting circuit 51 Inverter on / off signal generator 53 Inverter on / off control circuit

Claims (1)

(57) [Claims] 1. Switching means for repeating on / off operation, heating means for inductively heating an object to be heated in accordance with the on time of the switching means, and input setting means for performing input setting for heating the object to be heated. And, when the value of the input setting is equal to or less than a predetermined value, a heating period in which the switching means repeats on / off operation,
A first control unit that sets a pause period for suspending the on / off operation of the switching unit after the elapse of the heating period, and controls a ratio of the length of the heating period to the pause period according to a value of an input setting; Load detection means for detecting a power supply current and detecting a load state by comparing the input setting value with the input setting value; and when the input setting value is equal to or less than the value at the time of load detection, the on-time of the switching means is set to a load. An electromagnetic cooker comprising: a second control unit that controls a value corresponding to a frequency at the time of detection.