JP3713058B2 - High frequency induction heating device - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a high-frequency induction heating apparatus that supplies high-frequency power to a heating coil by switching a pulsating flow obtained by full-wave rectification of an AC power supply, and more specifically, resonance of a series resonance circuit. The present invention relates to a high-frequency induction heating apparatus that automatically searches for a frequency.
[0002]
[Prior art]
As a high-frequency induction heating device capable of reducing the size of a power supply, a technique disclosed in Japanese Patent Laid-Open No. 3-216989 has been proposed. This prior art is configured to switch a pulsating flow obtained by full-wave rectification of an AC power supply at a predetermined frequency. That is, the smoothing reactor and the capacitor are omitted, and the volume of the power source is reduced. And the high frequency electric power obtained by switching is supplied to the series resonance circuit which consists of a heating coil and a capacitor | condenser through a pulse transformer. For this reason, the current flowing through the series resonance circuit repeatedly increases and decreases corresponding to the waveform shape of the pulsating flow for each cycle of the pulsating flow, and the minimum value thereof is zero.
[0003]
For this reason, when the current flowing through the series resonant circuit decreases in each cycle of the pulsating flow, the phase synchronization based on the current flowing through the series resonant circuit becomes unstable. Therefore, switching is performed using a signal that is phase-synchronized with a reference signal that is a frequency approximate to the resonance frequency of the series resonance circuit during a period in which the current value of the series resonance circuit is a fraction of the maximum. In a period in which the current value is sufficient, switching is performed using a signal that is phase-synchronized with the current waveform flowing through the series resonance circuit. For this reason, in a period in which the current is sufficient, the frequency of the high-frequency power supplied to the series resonance circuit exactly matches the resonance frequency of the series resonance circuit, and as a result, high heating efficiency is obtained.
[0004]
[Problems to be solved by the invention]
However, when the above configuration is applied to a high-frequency centrifugal caster, the following problems occur. In other words, the shape of the mold used in the high-frequency centrifugal caster is a shape corresponding to the shape of the product to be cast. There is also. For this reason, the heating coil needs to have various sizes corresponding to the shape of the mold.
[0005]
On the other hand, the impedance of the heating coil varies with the size. For this reason, the frequency of the series resonance circuit which consists of a heating coil and a capacitor turns into a different frequency for every heating coil. This means that the frequency of the high frequency power supplied to the series resonance circuit needs to be a frequency corresponding to each of the heating coils. That is, in order to be able to replace heating coils having different shapes, it is necessary to change the frequency of the high-frequency power according to the heating coil. In the case of such a configuration, it is necessary to set a frequency according to each heating coil, which increases the labor during the heating operation. Further, when the setting is wrong, inefficient heating is performed, and the burden on the switching circuit is increased, which causes a problem that the possibility of element destruction increases.
[0006]
Also, providing the heating coil with protrusions and transmission holes for identifying the resonance frequency and providing identification means for identifying the positions of the protrusions and the transmission holes on the main body side causes a problem of incurring structural complexity.
[0007]
The present invention has been devised to solve the above-mentioned problems, and the object of the present invention is to increase the current flowing through the series resonant circuit when the frequency of the series resonant circuit and the frequency of the high frequency power approach each other. Paying attention to this, every time the frequency of the reference signal is changed, the current flowing through the series resonant circuit is judged, so even when a series resonant circuit with a different resonant frequency is connected, high-frequency power with the optimum frequency is automatically supplied. An object of the present invention is to provide a high-frequency induction heating device that can do this.
[0008]
Also, the determination of the current flowing through the series resonant circuit, by a determined locking signal of whether delivered at a predetermined timing, high-frequency induction heating device which can be made simple the structure of the resonant determination unit Is to provide.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the high frequency induction heating apparatus according to the first aspect of the present invention switches a pulsating flow obtained by full-wave rectification of an AC power source, and converts the high frequency power obtained by the switching into a series resonance circuit. A switching circuit to be supplied, a reference signal source that generates a signal of a specified frequency as a reference signal, a current value flowing through a series resonance circuit and a predetermined set value are compared, and the current value is the set value When the current value exceeds the set value, a current determination unit that outputs a lock signal and a phase-synchronized output sent to the switching circuit as a signal indicating the switching timing When the unlock signal is output, phase synchronization is performed with the reference signal, and when the lock signal is output, the signal flows to the series resonance circuit. A PLL circuit that performs phase synchronization with the signal, a resonance determination unit that detects a current value of high-frequency power flowing in the series resonance circuit, and sends a frequency approximation signal when the detected current value exceeds a predetermined reference value; and a reference Each time the frequency of the reference signal generated by the signal source is specified, it is determined whether or not a frequency approximate signal is sent from the resonance determination unit. When the frequency approximate signal is sent, the frequency of the reference signal at this time is determined. Is configured to include a frequency determination unit that determines a frequency corresponding to the series resonance circuit.
[0010]
The high-frequency induction heating device outputs a lock signal at a timing later than the lock timing, with the resonance determination unit taking the timing at which the lock signal is output when the current value exceeds the set value as the lock timing. The frequency approximate signal is transmitted when it is output.
[0011]
[Action]
The operation of the invention of claim 1 will be described below.
[0012]
When the frequency of the high frequency power transmitted from the switching circuit is separated from the resonance frequency of the series resonance circuit, the impedance of the series resonance circuit is high and the current value of the flowing high frequency power is small. Therefore, an unlock signal is only sent from the current determination unit. As a result, since the PLL circuit sends out an output that is phase-synchronized with the reference signal, the switching circuit performs switching at the frequency of the reference signal.
[0013]
That is, when the frequency specified by the frequency determination unit is separated from the resonance frequency of the series resonance circuit, the switching circuit performs switching according to the reference signal. The resonance determination unit does not generate a frequency approximation signal because the current value of the high-frequency power flowing through the series resonance circuit at this time is small. For this reason, the frequency determination unit determines that the specified frequency is not the resonance frequency of the series resonance circuit connected to the switching circuit, and specifies a new frequency for the reference signal source.
[0014]
On the other hand, when the difference between the frequency specified by the frequency determination unit and the resonance frequency of the series resonance circuit is within a predetermined range, the impedance of the series resonance circuit is low and the current value of the high-frequency power is large. For this reason, a lock signal is sent from the current determination unit, and an output that is phase-synchronized with the signal flowing through the series resonance circuit is sent from the PLL circuit. As a result, the current value of the high frequency power flowing through the series resonance circuit further increases, and a frequency approximation signal is sent from the resonance determination unit. Therefore, the frequency determination unit determines that the specified frequency is a frequency corresponding to the resonance frequency of the series resonance circuit.
[0016]
The lock signal is transmitted when the current value of the series resonance circuit that changes in response to the peak value of the pulsating flow exceeds the set value. The lock timing is determined by the relationship between the difference between the frequency of the reference signal and the resonance frequency of the series resonance circuit and the peak value of the pulsating flow. This means that when the frequency of the series resonant circuit and the frequency of the reference signal are different within a predetermined range, the lock timing is a timing within a known range.
[0017]
For this reason, at the timing delayed from the known range, when the frequency of the series resonance circuit and the frequency of the reference signal are in a predetermined range, the lock signal has already been output. That is, it is determined that the lock signal is output at a timing delayed from the lock timing indicates that the frequency specified by the frequency determination unit is a frequency corresponding to the resonance frequency of the series resonance circuit.
[0018]
This also means that a comparison circuit that compares the current value of the series resonance circuit with the set value is not required in the resonance determination unit.
[0019]
【Example】
An embodiment of the present invention will be described below with reference to the drawings.
[0020]
FIG. 1 is a block diagram showing an electrical configuration of an embodiment of the high-frequency induction heating apparatus of the present invention.
[0021]
In the figure, the rectifier circuit 11 is constituted by a diode bridge, and is a block that supplies a pulsating current 32 obtained by performing full-wave rectification of a single-phase 200 V AC power supply 31 to the switching circuit 16.
[0022]
The switching circuit 16 has a configuration using four switching elements, and a power MOSFET is used as the switching element. Then, the pulsating flow 32 given from the rectifier circuit 11 is subjected to switching that alternately repeats non-inversion and inversion of polarity, and the high frequency power obtained by the switching is supplied to the series resonance circuit via the pulse transformer PT. 21.
[0023]
The heating coil L is a heating coil used for a high-frequency centrifugal casting machine. For this reason, heating coils L of various sizes created corresponding to the shape of the mold are exchanged and connected to the pulse transformer PT. For this reason, the resonance frequency of the series resonance circuit 21 including the heating coil L and the capacitor C varies depending on the shape of the heating coil L to be connected. The lowest resonance frequency is 300 KHz and the highest resonance frequency is 750 KHz. And the resonance frequency when the other heating coil L is connected is in this range.
[0024]
The current determination unit 23 is a block that flows through the series resonance circuit 21 and compares the current value detected by the current transformer 22 with a predetermined set value. More specifically, since the current waveform detected by the current transformer 22 is a waveform in which the pulsating current 32 is switched at a frequency within the range of 300 to 800 KHz, the direct current that detects the envelope from the output of the current transformer 22 is used. A coupling envelope detection circuit is provided. The waveform detected by the envelope detection circuit is a waveform that approximates the waveform of the pulsating flow 32 (see reference numeral 36 in FIG. 2). An unlock signal (L level) is output to the output 37 during a period when the waveform output from the envelope detection circuit does not exceed the set value. Further, a lock signal (H level) is output to the output 37 during a period in which the output waveform exceeds the set value.
[0025]
Although various configurations can be used for the envelope detection circuit described above, in this embodiment, in order to simplify the circuit configuration, a half-wave rectifier circuit and a frequency component of 300 KHz or more are removed. An envelope detection circuit comprising a DC coupling integrating circuit is used.
[0026]
The phase shift circuit 24 shifts the detected waveform detected by the current transformer 22 at an angle corresponding to the set value of the variable resistor provided therein, and the phase-shifted detected waveform is transferred to the PLL circuit 19. This is a block to be sent out, and improves the power factor of the AC power supply 31.
[0027]
The reference signal source 20 is a block that generates a signal having a specified frequency at intervals of 10 KHz within a range of 300 to 800 KHz corresponding to the range of the resonance frequency of the series resonant circuit 21. The generated signal is sent to the PLL circuit 19 as a reference signal. More specifically, a frequency synthesizing circuit (as a conventional technique) that is configured by a reference signal generation circuit such as 10 MHz, a PLL circuit, and a frequency dividing circuit that can set the frequency dividing ratio from the outside and that can specify the frequency from the outside. Known).
[0028]
The PLL circuit 19 includes a phase synchronization unit 17 known as a conventional technique including a phase comparator, a loop filter, a charge pump, and a VCO, and a switch for switching between a reference signal output from the reference signal source 20 and an output of the phase shift circuit 24. 18. When the unlock signal is output from the current determination unit 23, the reference signal is guided to the phase synchronization unit 17 and phase-synchronized with the reference signal. When the lock signal is output, the output of the phase shift circuit 24 is guided to the phase synchronization unit 17 and phase-synchronized with the signal of the series resonance circuit 21. The phase-synchronized output is sent to the switching circuit 16 as a signal indicating the switching timing.
[0029]
The zero cross detection circuit 12 detects the timing when the voltage of the AC power supply 31 becomes 0 V, and generates a sequence of zero cross pulses indicating the detected timing with a rising edge, as indicated by reference numeral 33 in FIG. Then, the generated pulse is sent to the start circuit 13, the resonance determination unit 14, and the frequency determination unit 15.
[0030]
When a start signal 38 instructing the start of heating is given from a control unit composed mainly of a CPU or the like, the start circuit 13 delays the start signal 38 until the time when the AC power supply 31 crosses zero, thereby switching the circuit. This is a block for soft-starting 16 operations. Then, the delayed start signal 35 is sent to the switching circuit 16 and the frequency determination unit 15.
[0031]
The resonance determination unit 14 is a block that detects the current value of the high-frequency power flowing through the series resonance circuit, and sends a frequency approximation signal when the detected current value exceeds a predetermined set value .
[0032]
Specifically, when the resonance frequency of the series resonance circuit 21 and the frequency of the reference signal are within a predetermined range, the timing at which the lock signal is output is defined as the lock timing (time T7 in FIG. 2 ), and the period from the lock timing T7. If the timing delayed by t2 is called the detection timing, a pulse train (see reference numeral 39) indicating the detection timing is generated by giving a delay of the period t1 to the zero cross pulse. Then, at the detection timings T2, T4, T5, T8,... Indicated by the edges of the pulse train 39, it is determined whether or not the current value of the series resonance circuit 21 exceeds the set value .
[0033]
More specifically, the reference value serving as a determination reference is set to a value equal to the set value (indicated by V1) in the current determination unit 23. And, at the detection timings T2, T4, T5, T8,..., When the level of the output 37 is H level indicating that the lock signal is transmitted, the current value of the series resonant circuit 21 is equal to or higher than the set value. It is determined that Further, at the detection timings T2, T4, T5, T8,..., When the level of the output 37 is L level indicating that the unlock signal is output, the current value of the series resonance circuit 21 is equal to or less than the set value. It is determined that
[0034]
The frequency determination unit 15 is a block that determines whether or not a frequency approximate signal is transmitted from the resonance determination unit 14 after designating the frequency of the reference signal generated by the reference signal source 20. When the frequency approximate signal is transmitted, the frequency of the reference signal at this time is determined as the frequency corresponding to the series resonance circuit 21.
[0035]
The operation of one embodiment of the present invention will be described below.
[0036]
When high frequency power is supplied to the series resonance circuit 21, the power factor of the AC power supply 31 is deteriorated when the frequency of the high frequency power is lower than the resonance frequency of the series resonance circuit 21. In order to prevent the power factor from deteriorating, assuming that an operation start instruction by the start signal 35 is given at time T1, the frequency determination unit 15 uses the instruction for setting the frequency of the reference signal as the maximum frequency of 800 KHz as a reference. The signal source 20 is provided. The switching circuit 16 starts a switching operation in accordance with the start signal 35 at time T1. Therefore, high-frequency power of 800 KHz is supplied to the series resonance circuit 21. In this state, the frequency determination unit 15 waits for a frequency approximate signal to be sent from the resonance determination unit 14 before the pulse indicating the time T3 is given.
[0037]
The resonance frequency of the series resonance circuit 21 is 400 KHz, which is far away from 800 KHz, so that the value of current flowing through the series resonance circuit 21 is small. For this reason, even when time T2 is reached, the current value of the series resonance circuit 21 remains below the set value V1, and the current determination unit 23 maintains the level of the output 37 at the L level indicating the unlock signal. . As a result, a frequency approximation signal is not sent from the resonance determination unit 14.
[0038]
When this state continues and time T3 is reached, the frequency determination unit 15 gives an instruction for setting the frequency of the reference signal to 790 KHz to the reference signal source 20, and the frequency determination signal is received from the resonance determination unit 14 in the same manner as described above. Wait for it to be sent out. Since the frequency approximate signal is not transmitted at this time as well, the frequency determination unit 15 thereafter decreases the frequency instruction value to be given to the reference signal source 20 by 10 KHz every time the zero cross pulse is introduced. Every time an instruction is given, the operation of waiting for a frequency approximation signal to be sent from the resonance determining unit 14 is repeated.
[0039]
The reference value of the frequency of the reference signal given to the reference signal source 20 by the frequency determination unit 15 at time T9 is 450 KHz. Since the frequency of 450 kHz is close to 400 kHz of the resonance frequency of the series resonance circuit 21, the current value of the series resonance circuit 21 shows an increase. At time T6, the current value exceeds the set value V1, and the current determination unit 23 sends an H level indicating a lock signal to the output 37. However, the time T6 is a time delayed from the detection timing T5. Since the output 37 is at the L level at the time T5, the resonance determining unit 14 does not send out the frequency approximation signal. Therefore, the frequency determination unit 15 gives an instruction to the reference signal source 20 to set the frequency of the reference signal to 440 KHz at time T10.
[0040]
After time T10, high-frequency power of 440 kHz is applied to the series resonance circuit 21. For this reason, the current value of the series resonance circuit 21 increases, and the current value exceeds the set value V1 at the lock timing T7. That is, at the detection timing T8, the output 37 becomes H level indicating that the lock signal is being transmitted. For this reason, an H level that is a frequency approximation signal is sent to the output 34 of the resonance determination unit 14.
[0041]
As a result, the frequency determination unit 15 determines that 440 KHz is a frequency corresponding to the connected heating coil L, and thereafter fixes the frequency instructed to the reference signal source 20 to 440 KHz. Therefore, the PLL circuit 19 sends an output of 440 KHz to the switching circuit 16 during the period in which the current determination unit 23 outputs the unlock signal. Further, during the period in which the lock signal is output, an output that is phase-synchronized with the signal flowing through the series resonance circuit 21 is sent to the switching circuit 16.
[0042]
In the heating by the above-described operation, a period in which the frequency of the high-frequency power matches the resonance frequency of the series resonance circuit 21 (a period in which the current value of the series resonance circuit 21 is maximum and a lock signal transmission period), The ratio with the unit period of the cycle is substantially the maximum value. Therefore, the heating efficiency is maximized.
[0043]
From this, regarding the interval t2 between the lock timing T7 and the detection timing T8, even when the transmission timing of the zero cross pulse varies and the lock timing T7 varies due to the influence of noise or the like, the time T8 is later than the time T7. The heating efficiency improves as the value is set to a smaller value within a range in which it is guaranteed that the time is reached. Therefore, also in this embodiment, a small value is adopted for the period t2.
[0044]
When the resonance determination unit 14 is configured to transmit a frequency approximate signal when a lock signal is output, the indicated value of the frequency given to the reference signal source 20 at time T9 is a reference in subsequent heating. It becomes the frequency of the signal. This is heating by the current waveform shown in the period from time T9 to time T10, and the period during which high-frequency power matching the resonance frequency of the series resonance circuit 21 is supplied is short, resulting in inefficient heating.
[0045]
If the supplementary explanation is given below, the frequency determination unit 15 is configured to instruct a frequency value that is sequentially decreased by 10 KHz with 800 KHz as the start frequency, and therefore, the worst time is required for reaching the predetermined frequency. In some cases, the time is 50 cycles of the pulsating flow 32. This required time is about 0.4 seconds in the 60 Hz region and only 0.5 seconds in the 50 Hz region, and is 0.2 to 0.3 seconds in the average value of the various heating coils L. Therefore, in spite of the configuration in which the frequency search is performed every time an instruction to start heating is given, the delay in starting heating is practically a value that does not hinder.
[0046]
In addition, since the indicated value of the reference signal is changed in the direction of decreasing from the higher frequency side, the frequency of the reference signal at the time of heating is higher than the resonance frequency of the series resonance circuit 21. The effect is that it is possible to prevent power factor deterioration.
[0047]
A high-frequency induction heating device according to claim 1 is a switching circuit that supplies high-frequency power obtained by switching a pulsating flow to a series resonance circuit, and a reference signal that generates a signal having a specified frequency as a reference signal. A current value flowing through a series resonance circuit and a set value, and a current determination unit that outputs an unlock signal when the current value does not exceed the set value, and outputs a lock signal when the current value exceeds the set value; The output indicates the switching timing, and when the unlock signal is output, the reference signal is detected, and when the lock signal is output, the PLL circuit that is phase-synchronized with the signal flowing through the series resonance circuit and the current value of the series resonance circuit are detected and detected. a resonance determination unit for transmitting a frequency approximation signal when exceeding a preset value the current value predetermined for each to specify the frequency of the reference signal, the frequency approximation signal delivered Determining whether, when the frequency approximation signal is sent out, and a frequency and determining the frequency determining unit to the corresponding value specified for this time series resonant circuit. Therefore, the frequency of the reference signal that is optimal for the resonance frequency of the connected series resonance circuit is automatically searched, and even when series resonance circuits with different resonance frequencies are connected, high-frequency power of the optimum frequency is automatically supplied. It is possible to do.
[0048]
The high-frequency induction heating device uses the resonance determination unit to output a lock signal at a timing delayed from the lock timing, with the lock signal being output when the current value exceeds the set value. The frequency approximate signal is transmitted when it is output. Therefore, a comparison circuit between the current value of the series resonance circuit and the set value is not required, and the configuration of the resonance determination unit can be simplified.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an electrical configuration of an embodiment of a high-frequency induction heating apparatus according to the present invention.
FIG. 2 is a timing chart showing main signals of an embodiment of the present invention.
[Explanation of symbols]
14 resonance determination unit 15 frequency determination unit 16 switching circuit 19 PLL circuit 20 reference signal source 21 series resonance circuit 23 current determination unit 31 AC power supply 32 pulsating flow L heating coil

Claims (1)

A switching circuit that switches a pulsating flow obtained by full-wave rectification of an AC power supply and supplies high-frequency power obtained by switching to a series resonance circuit;
A reference signal source that generates a signal of a specified frequency as a reference signal;
The current value flowing through the series resonant circuit is compared with a predetermined set value, and when the current value does not exceed the set value, an unlock signal is output, and the current value exceeds the set value. In this case, a current determination unit that outputs a lock signal;
A phase-synchronized output is sent to the switching circuit as a signal indicating switching timing. When the unlock signal is output, phase synchronization is performed with the reference signal, and when the lock signal is output, the phase-synchronized output flows to the series resonant circuit. A PLL circuit that performs phase synchronization with the signal;
The current value of the high-frequency power flowing through the series resonance circuit is detected, and the timing at which the lock signal is output when the current value exceeds the set value is set as a lock timing, and at a timing delayed from the lock timing, a resonance determination unit for sending the frequency approximation signal when the lock signal is determined whether the output is outputted,
Every time the frequency of the reference signal generated by the reference signal source is designated, it is determined whether or not the frequency approximate signal is transmitted from the resonance determining unit, and when this frequency approximate signal is transmitted, A frequency determination unit that determines a frequency of a reference signal as a frequency corresponding to the series resonant circuit;
The voltage of the AC power supply performs detection timing becomes 0V, generates a zero cross pulse indicating the detected timing, Bei example a zero crossing detector circuit for transmitting the generated zero-cross pulse in the resonator determination unit and a frequency determination section,
The resonance determination unit determines whether the lock signal is output at a detection timing delayed from the zero cross pulse, and the frequency determination unit indicates a frequency to the reference signal source every time the zero cross pulse is introduced. the given reference signal source frequency induction heating apparatus characterized that you generate a signal of a specified frequency.

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