JP5436984B2 - Induction heating method and melting furnace by induction heating - Google Patents

Description

  The present invention relates to an induction heating method and a melting furnace using induction heating. More specifically, the present invention relates to an induction heating method for continuing induction heating while suppressing changes in the heating parameters, and a melting furnace using induction heating, focusing on changes in the heating parameters due to temperature changes of the metal to be heated.

 As a furnace for melting a metal by using induction heating, for example, there is one described in JP-A-8-82484. This melting furnace is shown in FIG. In the melting furnace, an induction coil 102 is provided around the storage container 101, the high frequency current is supplied from the high frequency oscillator 103 to the induction coil 102 to inductively heat the storage container 101, and the metal to be melted in the storage container 101 is heated by the heat. 104 is heated and melted. As a high-frequency current supplied from the high-frequency oscillator 103 to the induction coil 102, for example, a high-frequency current of 380 kHz is supplied. The molten metal lump is discarded together with the storage container 101.

JP-A-8-82484

  However, when the metal is induction-heated, the heating power P changes depending on the temperature of the metal to be induction-heated. Therefore, induction heating cannot be continued efficiently in the entire induction heating process.

  Further, in the above melting furnace, the molten metal 104 cannot be agitated during induction heating. Therefore, partial temperature variation occurs in the molten metal 104, and melting cannot be performed efficiently.

  An object of the present invention is to provide an induction heating method and an induction heating melting furnace capable of efficiently performing induction heating even when the temperature of the metal to be heated changes when induction heating of the metal is performed. Another object of the present invention is to provide an induction heating method capable of stirring a metal to be heated during induction heating and a melting furnace by induction heating.

  In order to achieve this object, the invention according to claim 1 is an induction heating method in which an AC current for heating is supplied to a heating coil provided outside the container to induce induction heating of the metal to be heated in the container. Inductive heating is performed while changing the frequency of the alternating current for heating stepwise in a direction to suppress the change of the heating parameter {(ρμω) ^ (1/2)} due to the temperature rise of the metal. is there.

  A melting furnace by induction heating according to claim 6 is a container for storing a metal to be heated, and a heating device for induction heating the metal to be heated by supplying an alternating current for heating to a heating coil provided outside the container. And the heating device steps the frequency of the alternating current for heating in a direction to suppress the change amount of the heating parameter {(ρμω) ^ (1/2)} due to the temperature rise of the metal to be heated. Induction heating is performed while changing.

The heating power P in the case of performing induction heating has the relationship of Formula 1.
<Equation 1>
P∝ {(ρμω) ^ (1/2)} × {(NI) ^ 2}
Here, {(ρμω) ^ (1/2)}: heating parameter, ρ: electrical resistivity (= 1 / σ, σ: electrical conductivity), μ: magnetic permeability, ω: angular frequency 2πf (f: frequency) ), N: number of coil turns, I: coil current value.

  When induction heating is performed, it is necessary to maintain the heating power P within an appropriate range of values. When the temperature of the object of induction heating rises by induction heating, its electrical resistivity ρ, electrical conductivity σ, and magnetic permeability μ change, so the heating parameter {(ρμω) ^ (1/2)} changes and heating The electric power P also changes. Here, the object of induction heating refers to the metal to be heated when the container is non-conductive and not induction heated, and both the metal to be heated and the container when the container is induction heated with a conductive one. is there. Considering the factor of the heating power P, first, the number N of turns of the heating coil is not related to the temperature of the object of induction heating. Further, the heating AC current value I is basically adjusted by the voltage. Therefore, the change of the heating power P is suppressed by suppressing the change of the heating parameter.

  The heating parameter increases by increasing the frequency f of the heating AC current, and decreases by decreasing the frequency f. Therefore, when the heating parameter increases in response to a change in the heating parameter in a direction that suppresses the amount of change, that is, due to a rise in the temperature of the target of induction heating, the frequency f is decreased, and when the heating parameter decreases, the frequency f is decreased. increase.

  For example, as in the induction heating method according to claim 4, when the container is non-conductive and the metal to be heated is a non-magnetic material, the frequency of the heating AC current is decreased stepwise. . Further, when the container is non-conductive and the metal to be heated is a magnetic material as in the induction heating method according to claim 5, the frequency of the heating alternating current is increased stepwise.

  The induction heating method according to claim 2 is a method in which a three-phase alternating current for stirring is superimposed on an alternating current for heating and supplied to a heating coil, and induction heating is performed while generating an electromagnetic force for stirring the metal to be heated in the container. Is to do. Further, in the melting furnace by induction heating according to claim 7, the heating device superimposes the three-phase alternating current for stirring on the alternating current for heating and supplies it to the heating coil, and electromagnetic force for stirring the metal to be heated in the container. Is generated. Therefore, the metal in the container can be stirred while induction heating.

  Furthermore, the induction heating method according to claim 3 changes the frequency of the heating alternating current using a resonance circuit. In addition, the melting furnace by induction heating according to claim 8 includes a resonance circuit that increases the frequency of the alternating current for heating. That is, a heating AC current is generated using the resonance circuit. The resonance frequency depends on the capacitance of the capacitor of the resonance circuit, and the resonance frequency is higher than when the resonance circuit is not used. When the resonance circuit is connected to the power supply circuit, the electric heating AC current can be generated without increasing the switching loss by setting the frequency to the resonance frequency.

  In the induction heating method according to claim 1 and the melting furnace by induction heating according to claim 6, even if the temperature of the metal to be heated is changed by induction heating, the change of the heating power P can be suppressed. Induction heating can be continued with the heating power P of the value.

  In addition, in the induction heating method according to claim 2 and the melting furnace by induction heating according to claim 7, the metal to be heated can be heated and melted with stirring, so that the heat is evenly distributed and the partial temperature is increased. Unevenness can be prevented and the melting time can be reduced. Moreover, since the molten metal (metal to be heated) is agitated, the crystal grains of the metal to be heated are refined and the mechanical strength can be improved.

  In addition, in the induction heating method according to claim 3 and the melting furnace by induction heating according to claim 8, there is no switching loss as in the case where the frequency of the alternating current for heating is changed by an inverter, for example, and efficient heating is performed. The frequency of the AC current can be increased.

  Here, when the container is non-conductive and the metal to be heated is a non-magnetic metal as in the induction heating method according to claim 4, the frequency of the alternating current for heating is lowered according to the temperature rise. Thus, it is possible to suppress a decrease in heating power and maintain induction heating.

  Further, as in the induction heating method according to claim 5, when the container is non-conductive and the metal to be heated is a magnetic material, by increasing the frequency of the alternating current for heating according to the temperature rise, A reduction in heating power can be suppressed and induction heating can be maintained.

It is sectional drawing which shows an example of embodiment of the melting furnace of this invention. It is sectional drawing which shows a heating coil and shows the mode of generation | occurrence | production of the magnetic force line in a 1st heating process. It is sectional drawing which shows a heating coil and shows the mode of generation | occurrence | production of the magnetic force line in a 2nd heating process. It is a figure for demonstrating the three-phase alternating current coil which is a heating coil, (A) is a figure which shows the phase difference, (B) is a figure which shows the relative relationship of each phase. It is a block diagram which shows the power supply circuit which supplies the alternating current for heating and the alternating current for electromagnetic stirring to a heating coil. It is a top view which shows other embodiment of the melting furnace of this invention, and shows the heating coil provided in a container bottom part. It is sectional drawing which shows the heating coil provided in a container bottom part, and shows the mode of the generation | occurrence | production of the magnetic force line in a 1st heating process. It is sectional drawing which shows the heating coil provided in a container bottom part, and shows the mode of generation | occurrence | production of the magnetic force line in a 2nd heating process. It is sectional drawing of the conventional melting furnace.

  Hereinafter, the configuration of the present invention will be described in detail based on the form shown in the drawings.

  FIG. 1 shows an example of an embodiment of a melting furnace by induction heating according to the present invention. The melting furnace includes a container 2 for storing the metal 1 to be heated, and a heating device 3 for inductively heating the metal 1 to be heated by supplying an AC current for heating to a heating coil 4 provided outside the container 2. .

  The container (crucible) 2 is formed using a material that can withstand a high temperature for induction heating of the metal 1 to be heated and is not corroded by the metal 1 to be heated. The material of the container 2 is appropriately selected depending on whether or not the metal 1 to be heated is used as a heating element for induction heating. That is, the container 2 may be made of a non-conductive material that is not induction-heated, or may be made of a conductive material that is induction-heated. As a material that is not induction-heated, for example, refractory bricks, ceramics, or the like can be used. In addition, as the material to be induction-heated, for example, a metal that can withstand a high temperature for induction-heating the metal 1 to be heated and is not corroded by the metal 1 to be heated can be used.

  For example, the container 2 is provided with a lid 2 a, and the solid heated metal 1 before melting is put into the container 2 with the lid 2 a opened. However, when the melting furnace exclusively heats the molten metal 1 to be heated, for example, the lid 2a is provided with a supply port for introducing the molten metal 1 to be melted from the supply port. The heated metal 1 may be introduced into the container 2. In this case, it is preferable to provide the lid 2a with an exhaust gas outlet for allowing exhaust gas to escape. By covering the top of the container 2 with the lid 2a, it is possible to prevent heat dissipation, prevent a temperature drop in the container 2, and facilitate temperature management in the container 2. However, it is not always necessary to provide the lid 2a so that it can be opened and closed. For example, when the above-described supply port and exhaust gas port are provided, the opening of the container 2 may be closed with a top plate or the like instead of the lid 2a. . Moreover, when the temperature management in the container 2 is unnecessary, the lid 2a, the top plate, etc. need not be provided.

  A falling nozzle 8 is provided on the bottom plate 2 b of the container 2. The flow-down nozzle 8 is provided with a valve (not shown). The valve can be opened, and the molten metal (molten metal 1 to be heated) can be discharged from the flow nozzle 8.

  The heating device 3 gradually changes the frequency f of the heating AC current in a direction to suppress the change amount of the heating parameter {(ρμω) ^ (1/2)} due to the temperature rise of the metal 1 to be heated. Induction heating is performed while changing. Here, among the heating parameters {(ρμω) ^ (1/2)}, ρ: electrical resistivity of the target of induction heating (= 1 / σ, σ: electrical conductivity), μ: permeability of the target of induction heating Magnetic susceptibility, ω: angular frequency 2πf (f: frequency) of the alternating current for heating.

  The heating device 3 includes a heating coil 4 that generates an electromagnetic field for induction heating an induction heating target, and a power supply circuit 7 that supplies a heating alternating current to the heating coil 4. Here, the object of induction heating refers to the metal 1 to be heated when the container 2 is non-conductive and not induction heated, and the metal 1 to be heated when the container 2 is induction-heated with a conductive one. Container 2.

  The heating coil 4 is provided outside the peripheral wall 2c of the container 2, for example. However, the position where the heating coil 4 is provided is not limited to this, and it may be provided below the bottom plate 2b of the container 2, or may be provided both on the outside of the peripheral wall 2c and below the bottom plate 2b. A gap is provided between the heating coil 4 and the peripheral wall 2 c of the container 2. The heating coil 4 is, for example, an air cooling coil. However, the coil is not limited to the air cooling coil, and may be a coil cooled by a coolant such as water.

  In the present embodiment, the heating device 3 superimposes the three-phase alternating current for stirring on the alternating current for heating and supplies it to the heating coil 4 to generate the electromagnetic force F1 for stirring the metal 1 to be heated in the container 2. I try to let them. That is, the heating coil 4 is also used as electromagnetic force generating means. Therefore, a three-phase AC coil suitable for generating a moving magnetic field is used as the heating coil 4.

  The heating coil 4 is shown in FIGS. The heating coil 4 is provided so as to surround the peripheral wall 2c. The three-phase AC coil that is the heating coil 4 is a combination of AX, BY, and CZ, and each coil 4a is connected in pairs, and the winding directions of the A, B, and C coils 4a are the same. The winding directions of the X, Y, and Z coils 4a are the same, and the winding directions of the A, B, and C coils 4a and the X, Y, and Z coils 4a are reversed. Here, the number of turns N is determined so as to satisfy the condition of (magnetic field strength) = (number of turns) × (current). The current I flowing through each coil 4a is obtained from (current) = (voltage) / (impedance).

  Each coil 4a is arranged in the order of A → Z → B → X → C → Y → A →... → Y toward the upper side of the container 2 in the axial direction, and the phase difference of each coil 4a is 60 degrees. For example, as shown in FIGS. 4A and 4B, when A is 0 degrees, Z is 60 degrees, B is 120 degrees, X is 180 degrees, C is 240 degrees, and Y is 300 degrees. When a three-phase alternating current for generating electromagnetic force (a three-phase alternating current for stirring) is supplied to the heating coil 4 from the power supply circuit 7, for example, as shown by an arrow in FIG. 3, magnetic lines B1 are formed around the coil 4a. An axially upward moving magnetic field of the container 2 is formed by a change in the current that flows through the peripheral wall 2c of the container 2 and reaches the metal 1 to be heated and flows through each coil 4a.

  By forming an upward moving magnetic field by the heating coil 4, a current flowing in the circumferential direction is generated at a position in the vicinity of the container 2 of the metal 1 to be heated, that is, a position where the magnetic force line B <b> 1 penetrates in the radial direction. For example, a current from the back side to the near side in the figure is generated at the P1 position in FIG. 3, and a current from the near side to the back side in the figure is generated at the P2 position. An upward electromagnetic force F1 is generated from the Fleming's left-hand rule by the moving magnetic field and the current generated in the metal 1 to be heated. The direction of the current generated in the metal 1 to be heated is reversed depending on the location, but the winding directions of the coils 4a of A, B, and C and the coils 4a of X, Y, and Z are also reversed. Electromagnetic force F1 is generated. The electromagnetic force F1 is generated at a position relatively close to the container 2 and drives the metal 1 to be heated at a position relatively close to the container 2 upward.

  In this embodiment, a single-phase alternating current is used for induction heating of the metal 1 to be heated, and a three-phase alternating current is used to generate an electromagnetic force F1 for stirring the heated metal 1 (electromagnetic stirring). Is done. Three-phase alternating current may be used instead of single-phase alternating current for induction heating, but when three-phase alternating current is used, electromagnetic force F1 acts on the solid metal to be heated 1 before melting and moves it. Therefore, in order to prevent the occurrence of such a phenomenon, in this embodiment, a single-phase alternating current is used for induction heating. When both induction heating and electromagnetic stirring are performed simultaneously, the three-phase alternating current for stirring is superimposed on the single-phase alternating current for heating (AC current for heating) and supplied to the heating coil 4.

  A power supply circuit 7 of the heating device 3 is shown in FIG. In FIG. 5, the three-phase alternating current line is omitted as a single line. The alternating current supplied from the alternating current power source 9 is supplied to the heating coil 4 via the inverter 10. The inverter 10 changes the frequency of the alternating current supplied to the heating coil 4. The inverter 10 generates a single-phase heating alternating current and a three-phase stirring alternating current.

  The power supply circuit 7 is provided with a switch mechanism 12. By switching the switch mechanism 12, the inverter 10 and the heating coil 4 are connected to and disconnected from the circuit including the capacitor 11a and the circuit not including the capacitor. When a circuit including the capacitor 11a is connected, the heating coil 4 and the capacitor 11a constitute a series resonance circuit, which will be referred to as the resonance circuit 11. The state in which the resonance circuit 11 is connected is referred to as an on state of the switch mechanism 12, and turning on the switch mechanism 12 is referred to as an on operation. Further, a state where the resonance circuit 11 is not connected is referred to as an off state of the switch mechanism 12, and turning off the switch mechanism 12 is referred to as an off operation.

  Next, the induction heating method of the present invention will be described. In the dielectric heating method, an AC current for heating is supplied to a heating coil 4 provided outside the container 2 to inductively heat the metal 1 to be heated, and the heating parameter { In response to the change of (ρμω) ^ (1/2)}, induction heating is performed while changing the frequency f of the AC current for heating stepwise in a direction to suppress the amount of change. In this embodiment, although the case where the above-mentioned melting furnace is used is demonstrated, you may implement the said induction heating method using things other than this.

The magnetic permeation distance δ of the magnetic field lines generated by the heating coil 4 is obtained by Equation 2.
<Equation 2>
δ = (2 / ωσμ) ^ (1/2)
Here, ω is the angular frequency 2πf (f: frequency) of the alternating current, σ is the electrical conductivity of the object of induction heating, and μ is the magnetic permeability of the object of induction heating.

  That is, when the frequency f of the alternating current for heating to be used is changed, the magnetic permeation distance δ of the magnetic force line B1 generated by the heating coil 4 changes. When the container 2 is non-conductive and not induction-heated, the electromagnetic field reaches the metal 1 to be heated in the container 2 in the first heating process and the second heating process described later. . Further, in the case where the container 2 is conductive and is induction-heated, the lines of magnetic force reach the peripheral wall 2c of the container 2 in the first heating process, and the covering in the container 2 in the second heating process. It reaches the heating metal 1. In the second heating step, the magnetic permeation distance δ is set so that the magnetic field lines B1 can penetrate deeply into the metal 1 to be heated to perform heating efficiently.

In addition, the heating power P for induction heating has the relationship of Equation 3.
<Equation 3>
P∝ {(ρμω) ^ (1/2)} {(NI) ^ 2}
Here, ρ: electrical resistivity of the target of induction heating (= 1 / σ, σ: electrical conductivity of the target of induction heating), μ: permeability of the target of induction heating, ω: angular frequency of AC current for heating 2πf (f: frequency), N: number of turns of the heating coil 4, and I: heating AC current value.

  Since the required value of the heating power P varies depending on the volume of the induction heating target, the value of the heating power P is appropriately set according to the volume of the induction heating target. Further, the number N of turns of the heating coil 4, the alternating current value I for heating, and the like are appropriately set according to the volume of the target for induction heating.

The heating parameter {(ρμω) ^ (1/2)} is set to, for example, the order of 10 ^ (− 4) or a value close thereto. Table 1 shows typical metal heating parameters.
For example, in the case of copper, when the temperature is 20 ° C., the frequency f of the alternating current for heating is 60 kHz, the heating parameter is 8.96 × 10 ^ (− 5), and when 1200 ° C., the frequency f is 20 kHz and the heating parameter is 1. Set to 83 × 10 ^ (− 4). In the case of iron, when the temperature is 20 ° C., the frequency f of the heating AC current is 1 kHz and the heating parameter is 3.99 × 10 ^ (− 4). When the temperature is 1536 ° C., the frequency f is 20 kHz and the heating parameter is 4. 68 × 10 ^ (− 4).

  When induction heating is performed, it is necessary to maintain the heating power P within an appropriate range of values. When the temperature of the object of induction heating rises by induction heating, its electrical resistivity ρ, electrical conductivity σ, and magnetic permeability μ change, so the heating parameter {(ρμω) ^ (1/2)} changes and heating The electric power P also changes. Since the number of turns N of the heating coil 4 and the alternating current value I for heating do not change even if the temperature of the object of induction heating changes, by suppressing the change of the heating parameter {(ρμω) ^ (1/2)} The change of the heating power P can be suppressed.

  The heating parameter {(ρμω) ^ (1/2)} increases from the angular frequency ω = 2πf by increasing the frequency f of the alternating current for heating, and decreases by decreasing the frequency f. Therefore, when the heating parameter increases in the direction of suppressing the amount of change with respect to the change in the heating parameter due to the temperature increase, that is, when the heating parameter increases due to the temperature increase of the object of induction heating, the heating parameter decreases. By increasing the frequency f, the change of the heating power P can be suppressed, and the value can be prevented from deviating from an appropriate range according to the induction heating target.

  For example, the frequency f of the alternating current for heating is changed in two stages. That is, the frequency f of the alternating current for heating is changed between the first heating process until the solid metal 1 to be heated is melted and the second heating process after the melting.

(Example 1 of subject of induction heating)
When the container 2 is non-conductive and is not induction heated (when the container 2 does not become a heating element), that is, when the object of induction heating is the metal 1 to be heated, when the metal 1 to be heated is a non-magnetic material, The frequency f of the second heating process is set lower than that of the first heating process. Non-magnetic metals have different electrical conductivities σ of about one order of magnitude between low and high temperatures, that is, in the solid state and in the molten state. That is, the low temperature is higher. By dividing the induction heating process into the following two stages from the viewpoint of suppressing the change in the heating power P, efficient heating becomes possible. Examples of nonmagnetic metals include copper and aluminum, but are not limited thereto.

First heating step: induction heating is performed using a heating alternating current having a relatively high frequency f (for example, 60 kHz).
Second heating step: Induction heating is performed using a heating alternating current having a relatively low frequency f (for example, 20 kHz).

  Thus, by changing the frequency f of the alternating current for heating in two steps, the heating parameter can be set to 10 ^ (-4) order or a value close to this in each step.

  The magnetic permeability μ also changes with the temperature change of the metal 1 to be heated. However, in the case of a nonmagnetic metal, the influence of the change of the magnetic permeability μ on the change of the heating parameter is the electric conductivity σ (electrical resistivity ρ). Is sufficiently smaller than the influence of the change in the heating parameter on the change in the heating parameter. Therefore, in the case of a nonmagnetic metal, the frequency f of the alternating current for heating is adjusted for the change in the electrical conductivity σ (electrical resistivity ρ).

  For example, the AC power supply 9 of the power supply circuit 7 has a commercial frequency of 50 Hz, and the inverter 10 generates a single-phase AC (heating AC current) having a frequency of 20 kHz used in the second heating step. When a single-phase alternating current (heating alternating current) having a frequency of 60 kHz used in the first heating process is made, the switch mechanism 12 is turned on to connect the resonance circuit 11 and the inverter 10 makes the 20 kHz The frequency of single-phase alternating current is tripled to 60 kHz. Here, the capacitor 11a of the resonance circuit 11 has a capacity to resonate three times.

  That is, in the first heating step, the switch mechanism 12 is turned on to connect the resonance circuit 11, and the single-phase alternating current having a frequency of 20 kHz produced by the inverter 10 is resonated three times to obtain a heating alternating current having a frequency of 60 kHz. To the heating coil 4. In the second heating step, the switch mechanism 12 is turned off to disconnect the resonance circuit 11, and the single-phase alternating current having a frequency of 20 kHz generated by the inverter 10 is supplied as it is to the heating coil 4 as an alternating current for heating.

  The operation of the switch mechanism 12 may be manually operated by an operator or may be automatically operated by a computer, for example. As an automatic operation by a computer, for example, the temperature of the metal 1 to be heated is detected by a temperature sensor, and the switch mechanism 12 is switched from an on state to an off state when the detected temperature reaches the melting point of the metal 1 to be heated. Conceivable. However, the present invention is not limited to this.

  In this embodiment, since the resonance circuit 11 is used to switch the frequency f of the heating AC current, switching loss does not occur, for example, when the frequency is changed by an inverter, and the heating AC current is efficiently generated. The frequency f can be increased. In particular, when the frequency of the heating AC current is high as in the first heating step of the present combination 1, the switching loss of the inverter becomes significant, so that the effect of using the resonance circuit 11 is great.

  Further, when a high frequency alternating current such as a frequency of 60 kHz is supplied to the heating coil 4, the skin effect in which the current concentrates on the coil surface becomes remarkable, and the soundness such as cooling of the heating coil 4 and maintenance of insulation is ensured. Becomes difficult. In the present embodiment, the frequency f of the alternating current for heating can be lowered in the second heating step in which the metal 1 to be heated is at a higher temperature, which is advantageous from the viewpoint of ensuring the soundness of the heating coil 4.

(Example 2 of induction heating target)
When the container 2 is non-conductive and not induction-heated (when the container 2 does not become a heating element), that is, when the object of induction heating is the metal 1 to be heated and the metal 1 to be heated is a magnetic body, The frequency f of the second heating process is set higher than that of the first heating process. The magnetic metal has a magnetic permeability μ that differs by about two orders of magnitude between the low temperature and high temperature, that is, the solid state and the molten state. That is, the low temperature is higher. By dividing the induction heating process into the following two stages from the viewpoint of suppressing the change in the electric power P, efficient heating becomes possible. Examples of the magnetic metal include, but are not limited to, iron, nickel, cobalt, and the like.

First heating step: induction heating is performed using a heating alternating current having a relatively low frequency f (for example, 1 kHz).
Second heating step: Induction heating is performed using a heating alternating current having a relatively high frequency f (for example, 20 kHz).

  Thus, by changing the frequency f of the alternating current for heating in two steps, the heating parameter is set to the order of 10 ^ (-4) or a value close to this in each process without extremely reducing the magnetic field penetration distance δ. can do.

  The electrical conductivity σ (electric resistivity ρ) also changes with the temperature change of the metal 1 to be heated. However, in the case of a magnetic metal, the change in the electrical conductivity σ (electric resistivity ρ) changes the heating parameter. The influence given is sufficiently smaller than the influence that the change of the magnetic permeability μ has on the change of the heating parameter. Therefore, in the case of a magnetic metal, the frequency f of the heating alternating current is adjusted with respect to the change in the magnetic permeability μ.

  For example, the AC power supply 7 of the power supply circuit 7 uses a commercial frequency 50 Hz, and the inverter 10 generates a single-phase AC (heating AC current) having a frequency of 1 kHz used in the first heating process. When a single phase alternating current (heating alternating current) having a frequency of 20 kHz used in the second heating process is made, the switch mechanism 12 is turned on to connect the resonance circuit 11 and the 1 kHz made by the inverter 10. The frequency of the single-phase alternating current is increased 20 times to 20 kHz. Here, the capacitor 11a of the resonance circuit 11 has a capacity for resonating 20 times.

  That is, in the first heating step, the switch mechanism 12 is turned off to disconnect the resonance circuit 11, and a single-phase alternating current having a frequency of 1 kHz generated by the inverter 10 is supplied as it is to the heating coil 4 as a heating alternating current. In the second heating step, the switch mechanism 12 is turned on to connect the resonance circuit 11, and the single-phase alternating current having a frequency of 1 kHz produced by the inverter 10 is resonated 20 times to obtain a heating alternating current having a frequency of 20 kHz. To the heating coil 4.

  The operation of the switch mechanism 12 may be manually operated by an operator or may be automatically operated by a computer, for example. As the automatic operation by the computer, for example, the temperature of the metal to be heated 1 is detected by a temperature sensor, and the switch mechanism 12 is switched from the off state to the on state when the detected temperature reaches the melting point of the metal 1 to be heated. Conceivable. However, the present invention is not limited to this.

  In this embodiment, since the resonance circuit 11 is used for switching the frequency f of the alternating current for heating, for example, switching loss as in the case of increasing the frequency f by an inverter does not occur, and the heating circuit is efficiently used. The frequency f of the alternating current can be increased.

(Example 3 of induction heating target)
When the container 2 is conductive and induction-heated (when the container 2 becomes a heating element for heating the metal 1 to be heated), for example, in the first heating step, the magnetic penetration distance δ of the electromagnetic field reaches the container 2. In the second heating step, the magnetic permeation distance δ of the electromagnetic field penetrates the container 2 and reaches the metal 1 to be heated. Thus, both the container 2 and the metal 1 to be heated are induction-heated. The switching from the first heating process to the second heating process is performed, for example, at a timing at which the metal 1 to be heated is melted. That is, only the container 2 is induction-heated in the first heating process before melting, and the container 2 and the metal 1 to be heated are induction-heated in the second heating process after melting. As described above, the magnetic permeation distance δ of the electromagnetic field varies depending on the frequency f of the alternating current for heating. Therefore, by switching the frequency f of the alternating current for heating between the first heating step and the second heating step, the magnetism of the electromagnetic field is changed. The permeation distance δ is changed, and the target of induction heating is changed.

  Here, in the second heating step in which the electromagnetic field reaches the metal 1 to be heated, it is necessary to consider whether the metal 1 to be heated is a non-magnetic material or a magnetic material. The frequency f of the AC current for heating is determined in consideration of the electric conductivity σ or the magnetic permeability μ.

  That is, when the container 2 is conductive and induction-heated, (1) the magnetic permeation distance δ of the electromagnetic field, and (2) the electrical conductivity σ or the permeability of the container 2 or the container 2 and the metal 1 to be heated. The frequency of the alternating current for heating in the first heating step and the second heating step so as to suppress the change of the heating parameter accompanying the temperature rise of the heated metal 1 and the container 2 in consideration of both the magnetic permeability μ. Switch f. That is, (1) in the second heating process, the electromagnetic field passes through the container 2 and reaches the metal 1 to be heated, and (2) in the first heating process, the heating parameter of the container 2 is the second heating process. Then, it is necessary that the heating parameters of the metal to be heated 1 are in an appropriate range (for example, the order of 10 ^ (-4) or a value close thereto).

  Ρ (= 1 / σ) and μ that determine the heating parameters {(ρμω) ^ (1/2)} of the container 2 and the metal 1 to be heated are different depending on the material, temperature, etc. The frequency f of the alternating current is appropriately set according to the material and temperature of the container 2 and the metal 1 to be heated. That is, although it cannot be determined uniformly, for example, when the container 2 is a non-magnetic metal, the following two-stage heating is performed.

First heating step: induction heating is performed using a heating alternating current having a relatively high frequency f (for example, 60 kHz).
Second heating step: Induction heating is performed using a heating alternating current having a relatively low frequency f (for example, on the order of 1 kHz).

  Thus, by changing the frequency f of the alternating current for heating in two steps, the heating parameter can be set to 10 ^ (-4) order or a value close to this in each step. In the second heating step, not only the temperature of the container 2 is raised but also the metal 1 to be heated is heated, so that efficient heating is possible.

  For example, in the case of container 2: made of SUS304 having a thickness of 5 mm and metal to be heated 1: aluminum, the frequency f of the alternating current for heating in the second heating step is set to 1 kHz, so that the metal 1 to be heated and the container 2 are heated. Is 800 ° C., the magnetic penetration distance δ of the electromagnetic field of the container 2 made of SUS304 is 17.6 mm, and the heating parameter of the metal 1 (aluminum) to be heated is 4.6 × 10 ^ (− 4). . Since the heating parameter is generally in the order of 10 ^ (-4) or a value close thereto, in this example, induction heating of the metal 1 to be heated can be sufficiently performed.

  For example, the AC power supply 9 of the power supply circuit 7 uses a commercial frequency of 50 Hz, and the inverter 10 generates a single-phase AC (heating AC current) having a frequency of 1 kHz used in the second heating step. When a single-phase alternating current (heating alternating current) with a frequency of 60 kHz used in the first heating process is made, the switch mechanism 12 is turned on to connect the resonance circuit 11 and the 1 kHz made by the inverter 10 The frequency of the single-phase alternating current is increased 60 times to 60 kHz. Here, the capacitor 11a of the resonance circuit 11 has a capacity to resonate 60 times.

  That is, in the first heating step, the switch mechanism 12 is turned on to connect the resonance circuit 11, and the single-phase alternating current with a frequency of 1 kHz produced by the inverter 10 is resonated 60 times to obtain an alternating current for heating with a frequency of 60 kHz. To the heating coil 4. In the second heating step, the switch mechanism 12 is turned off to disconnect the resonance circuit 11, and the single-phase alternating current having a frequency of 1 kHz generated by the inverter 10 is supplied as it is to the heating coil 4 as the alternating current for heating.

  The operation of the switch mechanism 12 may be manually operated by an operator or may be automatically operated by a computer, for example. As an automatic operation by a computer, for example, the temperature of the metal 1 to be heated is detected by a temperature sensor, and the switch mechanism 12 is switched from an on state to an off state when the detected temperature reaches the melting point of the metal 1 to be heated. Conceivable. However, the present invention is not limited to this.

  In this embodiment, since the resonance circuit 11 is used for switching the frequency f of the heating AC current, there is no switching loss as in the case of increasing the frequency by an inverter, for example, and the heating AC is efficiently performed. The frequency f of the current can be increased.

  Further, when a high frequency alternating current such as a frequency of 60 kHz is supplied to the heating coil 4, the skin effect in which the current concentrates on the coil surface becomes remarkable, and the soundness such as cooling of the heating coil 4 and maintenance of insulation is ensured. Becomes difficult. In the present embodiment, the frequency f of the alternating current for heating can be lowered in the second heating step in which the metal 1 to be heated is at a higher temperature, which is advantageous from the viewpoint of ensuring the soundness of the heating coil 4.

(Example 4 of subject of induction heating)
When the container 2 is a magnetic metal, for example, the following two-stage heating is performed.

First heating step: induction heating is performed using a heating alternating current having a relatively low frequency f (for example, on the order of 1 kHz).
Second heating step: Induction heating is performed using a heating alternating current having a relatively high frequency f (for example, on the order of 20 kHz). However, induction heating may be performed at a relatively low frequency f (for example, on the order of 1 kHz). That is, two-stage heating may be performed at a relatively low frequency f.

  For example, in the case of container 2: iron having a thickness of 5 mm (magnetic material at room temperature), the magnetic field penetration distance δ is extremely reduced by setting the frequency f of the alternating current for heating in the first heating step to 1 kHz. It is possible to set the heating parameter to the order of 10 ^ (-4). In the second heating step, for example, if the iron container temperature is 800 ° C., the heating parameter can be set to the order of 10 ^ (− 4) at a frequency of 20 kHz. However, when the frequency f of the heating AC current is 1 kHz, the heating parameter is slightly reduced, but the magnetic field penetration distance δ is about 20 mm. Since heating is performed, efficient heating is possible.

  For example, the AC power supply 7 of the power supply circuit 7 uses a commercial frequency 50 Hz, and the inverter 10 generates a single-phase AC (heating AC current) having a frequency of 1 kHz used in the first heating process. When a single phase alternating current (heating alternating current) having a frequency of 20 kHz used in the second heating process is made, the switch mechanism 12 is turned on to connect the resonance circuit 11 and the 1 kHz made by the inverter 10. The frequency of the single-phase alternating current is increased 20 times to 20 kHz. Here, the capacitor 11a of the resonance circuit 11 has a capacity for resonating 20 times.

  That is, in the first heating step, the switch mechanism 12 is turned off to disconnect the resonance circuit 11, and a single-phase alternating current having a frequency of 1 kHz generated by the inverter 10 is supplied as it is to the heating coil 4 as a heating alternating current. In the second heating step, the switch mechanism 12 is turned on to connect the resonance circuit 11, and the single-phase alternating current having a frequency of 1 kHz produced by the inverter 10 is resonated 20 times to obtain a heating alternating current having a frequency of 20 kHz. To the heating coil 4.

  The operation of the switch mechanism 12 may be manually operated by an operator or may be automatically operated by a computer, for example. As an automatic operation by a computer, for example, the temperature of the metal 1 to be heated is detected by a temperature sensor, and the switch mechanism 12 is switched from an on state to an off state when the detected temperature reaches the melting point of the metal 1 to be heated. Conceivable. However, the present invention is not limited to this.

  In this embodiment, since the resonance circuit 11 is used for switching the frequency f of the heating AC current, there is no switching loss as in the case of increasing the frequency by an inverter, for example, and the heating AC is efficiently performed. The frequency f of the current can be increased.

  In addition, when performing two-step heating at a relatively low frequency f, the frequency f may be switched by the inverter 10 without using the resonance circuit 11.

  Although the combination of the container 2 and the metal 1 to be heated as described above can be considered, in any case, the heating parameter increases as the temperature of the target of induction heating increases, and an appropriate heating power P can be obtained. If it is out of range or is likely to go out of range, the frequency f of the heating alternating current is lowered to suppress the increase in the heating parameter, and conversely, the heating parameter decreases as the temperature of the object of induction heating increases, When the heating power P is out of the range where the appropriate heating power P can be obtained or when it is about to fall, the frequency f of the alternating current for heating is increased to suppress the reduction of the heating parameter.

  In the present embodiment, the heated metal 1 is induction-heated while generating the electromagnetic force F1 that stirs the heated metal 1 in the heated metal 1 by the heating coil 4 in the second heating step. Yes. That is, a single-phase alternating current for induction heating is supplied to the heating coil 4 and at the same time, a three-phase alternating current for stirring is superposed and supplied. For example, a low-frequency three-phase alternating current of 20 Hz to 100 Hz is supplied as a three-phase alternating current for stirring. However, the frequency of the three-phase alternating current for stirring is not necessarily limited to this range, and may be any frequency that can generate the electromagnetic force F1 for stirring the metal 1 to be heated in the metal 1 to be heated. By stirring the molten metal (metal 1 to be heated), more efficient heating becomes possible, and the crystal grains of the metal are refined, so that the mechanical strength can be improved.

  The heating coil 4 on the outer periphery of the container 2 generates an electromagnetic force F1 that drives the metal 1 to be heated at a position relatively close to the peripheral wall 2c of the container 2 upward. The to-be-heated metal 1 driven upward by the electromagnetic force F1 is reversed near the liquid surface and descends near the center of the container 2, and then is reversed near the bottom 3b and rises along the peripheral wall 2c. Thus, the metal 1 to be heated is agitated.

  In any of the above combinations 1 to 4, electromagnetic stirring can be performed while dielectrically heating the metal 1 to be heated in the second heating step. For example, in the case of the above combination 1, the inverter 10 generates a stirring frequency, for example, a three-phase alternating current of 20 Hz and a single-phase alternating current (heating alternating current) of 20 kHz in the second heating step, and these are superimposed. And supplied to the heating coil 4.

  After sufficiently heating the metal to be heated 1 with electromagnetic stirring, when the valve of the falling nozzle 8 is opened, the molten metal (the metal 1 to be heated) in the container 2 is discharged from the flowing nozzle 8.

  The above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the scope of the present invention. For example, in the above description, the heating coil 4 is provided on the outer periphery of the container 2, but it may be provided on the bottom in addition to the outer periphery or instead of the outer periphery.

  The heating coil 4 provided at the bottom of the container 2 (hereinafter, when distinguished from the heating coil 4 on the outer periphery of the container 2, the outer one is referred to as a heating coil 4A and the bottom one is referred to as a heating coil 4B). 6 to FIG. In the present embodiment, a heating coil 4B is provided at the bottom of the container 2 in addition to the heating coil 4A on the outer periphery of the container 2. The heating coil 4 </ b> B is composed of an AC coil, and is provided so as to face the bottom surface of the bottom plate 2 b of the container 2 and to be arranged along the circumferential direction so as to surround the falling nozzle 8. In the present embodiment, the heating coil 4B at the bottom of the container 2 performs induction heating as well as the heating coil 4A at the outer periphery of the container 2 and generates an electromagnetic force F2 to drive the molten metal (heated metal 1) (electromagnetic stirring). To do.

  In the present embodiment, three-phase AC coils 4b, 4c, and 4d are used as the heating coil 4B. However, AC coils other than the three-phase AC coils 4b, 4c, and 4d may be used. Four sets of three-phase AC coils 4b, 4c and 4d are provided. However, the number of sets of the three-phase AC coils 4b, 4c, and 4d is not limited to four. The coils 4b, 4c, and 4d for each phase are arranged so as to be shifted by 30 degrees, for example. In the present embodiment, since the three-phase AC coils 4b, 4c, 4d are arranged at the bottom of the container 2, each coil 4b, 4c, 4d is shaped like a fan and spaced from each adjacent coil. 4b, 4c and 4d are arranged. The phase difference between the coils 4b, 4c and 4d is 120 degrees.

  In FIG. 8, a symbol with “•” in a circle means that a current flows from the back side toward the near side in the drawing. Moreover, the symbol which described x in (circle) means that the electric current is flowing toward the back | inner side from the near side with respect to drawing. Here, the number N of turns of each of the coils 4b, 4c, and 4d is determined so as to satisfy the condition of (magnetic field strength) = (number of turns) × (current). Further, the current value I flowing through each of the coils 4b, 4c, 4d is obtained from (current) = (voltage) / (impedance).

  In the second heating step, the power supply circuit 7 supplies a heating AC current and a stirring three-phase AC current to the heating coil 4A and the heating coil 4B. When a three-phase alternating current for stirring is supplied to the heating coil 4B from the power supply circuit 7, for example, as indicated by an arrow B2 in FIG. 8, the magnetic force lines B2 that pass through the bottom plate 2b of the container 2 and circulate through the inside and outside Occur. A magnetic field line B2 is generated for each coil, but a rotating magnetic field directed toward one side in the circumferential direction of the container 2 is formed by the phase difference of each coil, the direction of the current flowing through each coil, and its change.

  By forming a rotating magnetic field in the circumferential direction by the heating coil 4B, a current flowing in the radial direction along the bottom plate 2b is generated at a position near the bottom plate 2b, that is, a position where the magnetic lines of force B2 penetrate in the radial direction. For example, at the P3 position in FIG. 8, a current from the back side to the near side in the figure is generated, and at the P4 position, a current from the near side to the back side in the figure is generated. Due to the rotating magnetic field and the current generated in the metal 1 to be heated, an electromagnetic force F2 is generated in the circumferential direction along the bottom of the container 2 from Fleming's left-hand rule. Although the direction of the current generated in the metal 1 to be heated is reversed depending on the location, the direction of the magnetic field line B2 is also reversed, so that the electromagnetic force F2 in the same circumferential direction is always generated. This electromagnetic force F2 is generated in the metal 1 to be heated, and the metal 1 to be heated is driven in the circumferential direction and stirred.

  When the heating coil 4 is provided on both the outer periphery and the bottom of the container 2 as described above, the molten metal (the metal to be heated 1) can be agitated by generating different flows depending on the electromagnetic forces F1 and F2. Stirring can be performed satisfactorily.

  In the above description, the electromagnetic forces F1 and F2 for stirring the metal 1 to be heated are generated in the second heating step. However, the electromagnetic forces F1 and F2 for stirring the metal 1 to be heated are not generated. May be. That is, only induction heating may be performed. In addition, it is not always necessary to generate the electromagnetic forces F1 and F2 for stirring the heated metal 1 in all of the second heating process. For example, the electromagnetic forces F1 and F2 are generated only in the latter stage of the second heating process. May be.

  In the above description, the first heating process and the second heating process are switched before and after the metal to be heated 1 is melted. However, the timing of the switching is not limited to this. Switching may be performed based on the temperature, the elapsed time from the start of induction heating, or the like.

  In the above description, the frequency f of the heating alternating current is switched to two stages, but the number of switching stages is not limited to two stages, for example, three stages, four stages, or five stages or more. Also good. Here, when the frequency f of the alternating current for heating is switched using the resonance circuit 11, the resonance circuits 11 having different capacities of the capacitors 11 a are arranged in parallel according to the number of switching stages, and are alternatively selected by the switch mechanism 12. It is preferable to switch to.

  For example, the switching of the frequency f of the alternating current for heating is switched to three stages of the first heating process, the second heating process, and the third heating process. In the first heating process, the frequency is changed by the inverter 10. The heating AC current having the frequency A is used, the heating AC current having a frequency n1 times that of A is used in the second heating step, and the heating AC current having a frequency n2 times that of A is used in the third heating step. Consider the case of using current. In this case, a resonance circuit 11 (hereinafter, referred to as a resonance circuit 11 for the second heating process) having a capacitor 11a capable of resonating n1 times and a capacitor 11a having a capacity capable of resonating n2 times is provided. The resonance circuit 11 (hereinafter, referred to as the resonance circuit 11 for the third heating step) is connected in parallel with the resonance circuit 11 shown in FIG. That is, the two resonance circuits 11 are arranged in parallel, and this is arranged in series with the heating coil 4. Then, the switch mechanism 12 is operated, and the two resonance circuits 11 are separated in the first heating step. In the second heating step, the switch mechanism 12 is operated to selectively connect the resonance circuit 11 for the second heating step. Further, in the third heating step, the switch mechanism 12 is operated to selectively connect the resonance circuit 11 for the third heating step.

  By increasing the number of switching steps of the frequency f of the heating alternating current, it is possible to suppress changes in the heating parameters more finely.

DESCRIPTION OF SYMBOLS 1 Heated metal 2 Container 4 Heating coil 3 Heating apparatus 11 Resonance circuit F1, F2 Electromagnetic force which stirs the heated metal 1

Claims (8)

  In an induction heating method in which an alternating current for heating is supplied to a heating coil provided outside a container to induce induction heating of the metal to be heated in the container, a heating parameter {(ρμω) ^ (1 / 2)}, the induction heating method is characterized in that induction heating is performed while gradually changing the frequency of the heating AC current in a direction to suppress the amount of change.   A three-phase alternating current for stirring is superimposed on the alternating current for heating and supplied to the heating coil, and induction heating is performed while generating an electromagnetic force for stirring the metal to be heated in the container. Item 2. The induction heating method according to Item 1.   The induction heating method according to claim 1, wherein the frequency of the heating AC current is changed using a resonance circuit.   2. The induction heating method according to claim 1, wherein when the container is non-conductive and the metal to be heated is a non-magnetic material, the frequency of the heating alternating current is lowered stepwise.   2. The induction heating method according to claim 1, wherein when the container is non-conductive and the metal to be heated is a magnetic material, the frequency of the heating alternating current is increased stepwise.   A container for storing the metal to be heated, and a heating device for inductively heating the metal to be heated by supplying an alternating current for heating to a heating coil provided outside the container, the heating device comprising the metal to be heated Inductive heating is performed while changing the frequency of the heating AC current stepwise in a direction to suppress the change amount of the heating parameter {(ρμω) ^ (1/2)} due to the temperature rise of A melting furnace that uses induction heating.   The heating device superimposes a three-phase alternating current for stirring on the alternating current for heating, supplies the heating coil to the heating coil, and generates an electromagnetic force for stirring the metal to be heated in the container. Item 7. A melting furnace by induction heating according to Item 6.   The melting furnace by induction heating according to claim 6, wherein the heating device includes a resonance circuit that increases a frequency of the AC current for heating.

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