JP2020009590A - Induction melting furnace - Google Patents

Abstract

To provide an induction melting furnace capable of reducing dissolution time by optimally controlling output frequency even when a frequency lower limit is set to the output frequency.SOLUTION: In an induction melting furnace, a frequency conversion section 7 comprises a plurality of units of a first unit 7A to an N-th unit 7N, one unit comprising at least one set of inverters 42a, 42b and at least one set of high frequency capacitors 51a, 51b corresponding to the at least one set of inverters. When a calculated load resonance frequency F is equal to or less than a first frequency specified value F1 (NO in a step 11), an inverter is temporarily stopped (STEP 12), and then one unit of the plurality of units is stopped (STEP 13).SELECTED DRAWING: Figure 1

Description

  The present invention relates to an induction melting furnace that melts a material to be heated stored in a furnace by supplying electric power to a heating coil provided on an outer periphery of a furnace wall through a power supply unit.

  Conventionally, as this type of induction heating device, as shown in Patent Document 1 below, a forward converter and an inverse converter include a power conversion unit forming a series resonance type circuit (voltage type circuit). There is known a device including a control device for reducing the output frequency of the unit from a starting frequency set higher than the load resonance frequency of the series resonance type circuit toward the load resonance frequency according to a preset program. (See claim 1 in Patent Document 1 below).

JP-A-3-19082

  Here, in the induction melting furnace, (1) load fluctuations are constantly occurring due to sparks between the materials to be heated (rapid change in frequency due to instantaneous heavy load fluctuations), and (2) rated voltage of the high-frequency capacitor. In order to prevent the output frequency from exceeding, a lower frequency limit is set so that the output frequency is equal to or higher than a certain value with respect to the load resonance frequency.

  However, when a lower frequency limit is set as the output frequency, the load resonance frequency is low in the initial stage of melting, so that the output current is limited by the lower limit of the output frequency, and there is a problem that the output power is greatly reduced.

  In addition, there is a problem that the dissolution rate is reduced due to a decrease in output power, and as a result, the state where the output frequency is further limited to the lower limit value continues for a long time.

  In view of the above circumstances, the present invention provides an induction melting furnace capable of optimally controlling the output frequency even when setting the lower frequency limit for the output frequency and shortening the melting time. Aim.

An induction melting furnace according to a first aspect of the present invention includes a heating coil provided on an outer periphery of a furnace wall, a forward converter, an inverse converter in which first and second switching elements operate alternately, and first and second high-frequency capacitors. Supplies power through a power supply unit including a power conversion unit that forms a resonance type circuit and a control signal generation unit that generates a control signal for the first and second switching elements. An induction melting furnace for melting the stored material to be heated,
A load resonance frequency calculation unit that calculates a load resonance frequency from a supply state of power to the heating coil,
The inverter is configured by a plurality of sets of the first and second switching elements with the first and second switching elements as one set, and the first and second high-frequency capacitors are configured by using the One set of the first and second switching elements is a natural number multiple of the set, and one set of the first and second switching elements and the corresponding first and second sets of the natural multiple of the same are set. It is composed of a plurality of units with a high-frequency capacitor as one unit,
When the load resonance frequency calculated by the load resonance frequency calculation unit is equal to or lower than a predetermined frequency specified value, a part of the plurality of units is stopped.

  According to the induction melting furnace of the first invention, the inventors of the present invention have conducted intensive studies and studied to make the load resonance frequency inversely proportional to the 1/2 power of the product of the inductance L of the heating coil and the capacitance C of the high-frequency capacitor. Focusing on the fact that the capacitance C of the high-frequency capacitor is reduced, it has been found that the load resonance frequency is positively changed.

  That is, one set of the first and second switching elements and the corresponding set of the first and second high-frequency capacitors corresponding to a multiple of a natural number as one unit constitute an inverter with a plurality of units, and the load resonance is performed. When the frequency is equal to or lower than the predetermined frequency specified value, by stopping a part of the plurality of units, the load resonance frequency can be changed so as to be positively increased.

  As a result, even when the lower limit of the output frequency is set, when the material to be heated is a cold material, the resonance frequency is low in the initial stage of melting, the output current is limited by the lower limit of the output frequency, and the output power is greatly increased. Can be prevented. As a result, it is possible to solve the problem that the dissolution rate decreases due to the decrease in the output power, and as a result, the state in which the output frequency is further limited to the lower limit continues for a long time.

  As described above, according to the induction melting furnace of the first aspect, even when the lower limit value of the output frequency is set, the output frequency can be optimally controlled to shorten the melting time.

An induction melting furnace according to a second aspect of the present invention is the induction melting furnace according to the first aspect,
When the load resonance frequency calculated by the load resonance frequency calculation unit is higher than a predetermined frequency specified value, a part of the plurality of units that have been stopped is started.

  According to the induction melting furnace of the second aspect, in the first aspect, even when a part of the plurality of units is stopped at a load resonance frequency equal to or lower than a predetermined frequency specified value, the load resonance frequency is increased with the progress of melting. When the number of units has risen, a part of the plurality of stopped units can be activated.

  As described above, according to the induction melting furnace of the second invention, even when the lower limit value is set for the output frequency, the output frequency is more optimally controlled according to the melting state, thereby shortening the melting time. Can be.

1 is an overall configuration diagram illustrating a configuration of an induction melting furnace according to an embodiment. FIG. 2 is an explanatory diagram showing details of a part of a circuit of the induction melting furnace in FIG. 1. 2 is a flowchart showing the contents of control processing in the induction melting furnace of FIG. 1.

  The induction melting furnace of the present embodiment will be described with reference to FIG. The induction melting furnace melts the material to be heated X stored in the melting furnace.

  Specifically, the control device of the induction melting furnace includes a power supply 1, a high-voltage power receiving panel 2, a transformer for a conversion device 3, a power conversion device 4, a high-frequency matching device 5, an induction heating device 6, a control circuit, And a control device (controller) 100.

  Note that the power conversion device 4 and the high-frequency matching device 5 correspond to a power conversion unit of the present invention.

  The power supply 1 is a rated AC power supply and is connected to the high-voltage power receiving board 2.

  The high-voltage power receiving panel 2 is a device for turning on / off the power to the induction heating device and shutting down the power when a failure occurs, and includes a power fuse 2a and a circuit breaker 2b. The power fuse 2a is a means for interrupting a current in the event of a short circuit, and the circuit breaker 2b performs an opening / closing operation when power is supplied and stopped.

  The converter transformer 3 is connected to the high-voltage power receiving board 2 and adjusts the input voltage to the power converter 4 to a predetermined value.

  The power conversion device 4 is connected to the conversion device transformer 3 and is a device for generating an arbitrary high-frequency current from a 50 Hz or 60 Hz commercial power supply, and is a forward converter 41 a or 41 b that is an AC / DC converter. And at least one set of inverse converters 42a and 42b, which are DC / AC converters, and are controlled by an output control signal from the control circuit 10.

  Specifically, the power converter 4 includes diode-type forward converters 41a and 41b on the input side, IGBT-type inverters 42a and 42b on the output side, and smoothes in series with the forward converters 41a and 41b, respectively. Reactors 43a and 43b are connected, and smoothing capacitors 44a and 44b are connected in parallel with the forward converters 41a and 41b.

  The IGBT type inverters 42a and 42b correspond to the first and second switching elements of the present invention.

  Further, the power converter 4 detects a DC voltage on the output side of the forward converters 41a and 41b and outputs a DC voltage signal (a), and a DC voltage detector 45 that detects a DC current and outputs a DC current signal ( b), and output values of the DC voltage detector 45 and the DC current detector 46 are output to the control circuit 10.

  The high-frequency matching device 5 is provided between the power converter 4 and the induction heating device 6, and improves the load power factor because the induction heating device 6 has a low power factor.

  Specifically, the high-frequency matching device 5 detects at least one set of the first and second high-frequency capacitors 51a and 51b corresponding to one set of the inverters 42a and 42b, and detects and outputs the output current of the high-frequency matching device 5. It comprises a current detector 52 for outputting a current signal (d) and a voltage detector 53 for detecting an output voltage and outputting an output voltage signal (e).

  The details of the frequency converter 7 including the inverters 42a and 42b and at least one set of the first and second high-frequency capacitors 51a and 51b corresponding thereto will be described later with reference to FIG.

  The induction heating device 6 generates an eddy current in the material X to be heated stored in the melting furnace main body by applying a high-frequency current supplied from the power conversion device 4 and the high-frequency matching device 5 to the heating coil 61. The material to be heated X is heated and melted by Joule heat generated between the metal materials due to the eddy current.

  The control circuit 10 controls the operation and stop of the induction melting furnace, adjusts the output, etc., and controls the power factor detection unit that detects the output power factor as the control device of the induction melting furnace, and controls the IGBT type inverters 42a and 42b. And a function as a control signal generation unit for performing

  Specifically, the control circuit 10 includes, for example, a power factor detection unit and a PLL control unit, and the power factor detection unit generates an output current signal (d) of the high-frequency matching device 5 which is an output value of the current detector 52. The output power factor of the AC current / voltage output from the high-frequency matching device 5 is calculated from the output voltage signal (e) of the high-frequency matching device 5 which is the output value of the voltage detector 53. Further, the PLL control unit generates a control signal for controlling the IGBT type inverters 42a and 42b based on the output power factor calculated by the power factor detection unit.

  The control device 100 mainly performs operation / stop of the induction furnace, adjustment of power supply to the material X to be heated, and protection for preventing breakage / failure of component devices.

Specifically, the control device 100 has a function as a load impedance calculating unit. The load impedance [% R] is calculated in the control device 100 from the output power Po and the output voltage Vo of the power conversion device 4 (% R = Vo ² / Po).

  Further, the control device 100 has a function as a load resonance frequency calculation unit. The load resonance frequency F is calculated as a resonance frequency in a resonance circuit formed between the matching device 5 and the heating coil 61 {F = 1 / [2π√ (LC)]｝.

  Then, the control device 100 repeatedly executes the load impedance calculation process and the load resonance circuit calculation process at a constant processing cycle.

  Next, with reference to FIG. 2, the details of the frequency conversion unit 7 whose description has been deferred will be described.

  The frequency conversion unit 7 includes, as one unit, a unit including at least one set of the inverters 42a and 42b and at least one set of the first and second high-frequency capacitors 51a and 51b corresponding thereto. It is composed of a plurality of units of the N-th unit 7N.

  Here, the first unit is constituted by a set of high-frequency capacitors 51a1 and 51b1 each of which is a natural number corresponding to one set of the inverters 42a1 and 42b1. Pairs are shown).

  Similarly, the second unit is constituted by a set of high-frequency capacitors 51a2 and 51b2, each of which is a natural number multiple corresponding to one set of inverters 42a2 and 42b2. Is shown).

  Hereinafter, similarly, the N-th unit is constituted by a set of natural number multiples in which one set of high-frequency capacitors 51aN and 51bN corresponds to one set of inverters 42aN and 42bN. The case of 1-time set is shown).

  A plurality of units of the first unit 7A to the N-th unit 7N are connected in series to the heating coil 17, and control signals to the inverters 42a1, 42b1 to 42aN, 42bN of each unit ( c1) to (cN) are output from the control circuit 10, so that the start and stop of each unit can be controlled.

  Next, the contents of the control processing in the induction melting furnace of the present embodiment will be described with reference to FIG.

  First, when melting is started via the control device 100 and the control circuit 10 (STEP 10), the control device 100 sets the load resonance frequency F to a resonance circuit formed between the matching device 5 and the heating coil 61. {F = 1 / [2π} (LC)]} (STEP 11).

  Here, when the calculated load resonance frequency F is larger than the first frequency specified value F1 (YES in STEP11), the melting is continued as it is.

  On the other hand, when the calculated load resonance frequency F is equal to or lower than the first frequency specified value F1 (NO in STEP11), the inverter is temporarily stopped (STEP12), and one of the plurality of units is stopped (STEP13).

  For example, in the case of the six-unit configuration of the first to sixth units, when the resonance frequency F becomes lower than the first frequency prescribed value F1, one of the six units is stopped.

  Here, an example of a circuit configuration is shown. The capacitance of the high-frequency capacitor 51a (51b) having a rating of 3000 kvar-3300 V-300 Hz is 146 uF. Also, if the inverter output current is Io = 8600A, assuming that 16 high frequency capacitors are mounted, the capacitor current per one high frequency capacitor is Ic = 537.5A.

At this time, from Vc = I / (2πF × C) [V], even with the same capacitor current Ic = 537.5 A, the potential across the high-frequency capacitor fluctuates according to the inverter oscillation frequency f, which is the output frequency. Since Vc "= 3255 V at 180 Hz, the rated voltage of the high-frequency capacitor 51a (51b) exceeds 3300 V at F2 ≦ 177.5 Hz. (Additionally, in order to prevent such excess, 60% F of the rated frequency is used. Is set to the lower limit of the inverter oscillation frequency f.)
Here, the first frequency regulation value is, for example, 60% F (for example, 180 Hz) with respect to the rated frequency, and the resonance frequency F corresponds to the lower limit of the inverter oscillation frequency f.

  In this case, by stopping one of the six units, the resonance frequency F ′ increases by 9.5% from the relation of {F ′ = F × {(6/5)}, and becomes F ′ = 197 Hz. Become.

  By increasing the resonance frequency F to F ', the voltage of the high-frequency capacitor 51a (51b) can be reduced from Vc = 3255V (180Hz) to Vc' = 2974V (197Hz), and further close to the inverter oscillation frequency f. Thus, a decrease in inverter output power can be minimized.

  By dissolving with one unit stopped (STEP 14), the output frequency can be optimally controlled and the dissolution time can be shortened even when setting the lower frequency limit for the output frequency.

  Next, when the calculated load resonance frequency F becomes larger than the second frequency regulation value F2 (YES in STEP21), the control device 100 temporarily stops the inverter (STEP22) and then stops. One unit is activated (STEP 23).

  Here, similarly to the first frequency specified value, the second frequency specified value F2 may be 60% F (for example, 180 Hz) with respect to the rated frequency, but may be different from the first frequency specified value F1. .

If the potential Vc at both ends of the high-frequency capacitor [= Ic / (2πf × C)] is 80% V or less than the high-frequency capacitor rated voltage Vc ′, the first frequency specified value F1 (second frequency specified value F2) can be arbitrarily set. It may be changed. Here, Ic is a high-frequency capacitor current, f is an inverter oscillation frequency, and C is a high-frequency capacitor capacitance.

Then, by restarting the melting after returning one unit (STEP 24), when the load resonance frequency rises with the progress of the melting, the output frequency is more optimally controlled according to the melting state, The dissolution time can be shortened. On the other hand, when the calculated load resonance frequency F is equal to or lower than the second frequency specified value F2 (NO in STEP21), melting is continued with the inverter temporarily stopped.

Until the dissolution is completed (NO in STEP 30), the processes in STEP 11 to STEP 24 are repeatedly executed, and when the dissolution is completed (YES in STEP 30), tapping is performed (STEP 40).

The above is the details of the control processing in the induction melting furnace of the present embodiment. According to such an induction melting furnace, even when the frequency lower limit is set for the output frequency, the output frequency is optimally controlled and the melting time is controlled. Can be reduced. More specifically, as a first effect, when the resonance frequency F is changed by the conventional circuit connection method, the switching high-frequency capacitor 51a (51b) always requires an opening / closing mechanism. However, the frequency conversion unit 7 is defined as a unit composed of at least one set of inverters 42a and 42b and at least one set of high-frequency capacitors 51a and 51b corresponding to the set. By configuring and functioning with a plurality of 7N units, the capacitance of the high-frequency capacitors 51a1, 51b1 to 51aN, 51bN can be varied by stopping / operating the inverters 42a1, 42b1 to 42aN, 42bN. As a result, the load resonance frequency F can be variably controlled. Accordingly, it is not necessary to set the lower limit of the inverter oscillation frequency f low.

  For example, the resonance frequency F decreases at the initial stage of the melting of the cold material, which is a problem in the conventional method, and the inverter output current Io is limited by being limited to the lower limit of the inverter oscillation frequency f, and the inverter output power Po is limited. Can be resolved. As a result, the inverter output power Po can be applied for a long time with the high setting, so that the melting ability is improved.

  Also, as the second effect, the switch current or disconnector used for the switching mechanism is expensive at about 1,000,000 yen per unit because the capacitor current Ic is large, and the device has a large external shape. There was no way to prevent the size from increasing. On the other hand, in the induction melting furnace of the present embodiment, since the inverters 42a1, 42b1 to 42aN, and 42bN themselves are used as the switching mechanism, there is no need to add a switch or disconnector, and the price and the outer dimensions of the apparatus are reduced. Can be suppressed.

  Further, as a third effect, in the case of the industrial induction heating apparatus, when the resonance frequency F differs from the design value on site, for example, one unit is removed or added as a standard. On the other hand, the conventional IGBT-type inverter and the high-frequency capacitor unit are connected by a large copper bus bar, and changing the high-frequency capacitor requires a great deal of labor. However, the induction melting furnace of the present embodiment makes it possible to easily change the resonance frequency.

  In the present embodiment, when the calculated load resonance frequency F is equal to or lower than the first frequency specified value F1 (NO in STEP11), the inverter is temporarily stopped (STEP12), and one of the plurality of units is removed. Although the operation is stopped (STEP 13), the present invention is not limited to this. A plurality of first frequency regulation values F1 are provided in a stepwise manner, and two or three units are provided in a stepwise manner in accordance with a decrease in the load resonance frequency F. May be stopped. Alternatively, two or three units may be stopped at one time according to the decrease in the load resonance frequency F.

  Similarly, in the present embodiment, when the calculated load resonance frequency F becomes larger than the second frequency regulation value F2 (YES in STEP21), the inverter is temporarily stopped (STEP22) and then stopped. Although one unit is activated (STEP 23), the present invention is not limited to this. A plurality of second frequency specified values F2 are provided in a stepwise manner, and the unit is stopped in response to an increase in the load resonance frequency F. The unit or three units may be activated in stages. May be performed. Alternatively, two or three units that have been stopped at once may be activated according to the load resonance frequency F.

REFERENCE SIGNS LIST 1 power supply 2 high-voltage power receiving panel 3 transformer for conversion device 4 power conversion device 5 high-frequency matching device 6 induction heating device 7 frequency conversion unit 10 control circuit (control signal generation , Γ control circuit (phase difference detection unit), 41a, 41b ... diode type forward converter, 42a, 42b (42a1, 42b1 to 42aN, 42bN) ... IGBT type inverse converter, 51a, 51b (51a1, 51a1) 51b1 to 51aN, 51bN) High-frequency capacitor, 61: heating coil, 100: control device, X: material to be heated.

Claims (2)

A heating coil provided on the outer periphery of the furnace wall includes a forward converter, an inverter in which first and second switching elements operate alternately, and a first and a second high-frequency capacitor, which constitute a resonance type circuit. By supplying power through a power supply unit including a conversion unit and a control signal generation unit that generates a control signal for the first and second switching elements, the material to be heated stored in the furnace is melted. An induction melting furnace,
A load resonance frequency calculation unit that calculates a load resonance frequency from a supply state of power to the heating coil,
The inverter is configured by a plurality of sets of the first and second switching elements with the first and second switching elements as one set, and the first and second high-frequency capacitors are configured by using the One set of the first and second switching elements is a natural number multiple of the set, and one set of the first and second switching elements and the corresponding first and second sets of the natural multiple of the same are set. It is composed of a plurality of units with a high-frequency capacitor as one unit,
An induction melting furnace, wherein a part of the plurality of units is stopped when the load resonance frequency calculated by the load resonance frequency calculation unit is equal to or less than a predetermined frequency specified value. The induction melting furnace according to claim 1,
An induction melting furnace, wherein when the load resonance frequency calculated by the load resonance frequency calculation unit is higher than a predetermined frequency specified value, a part of the plurality of units that have been stopped is started.