JP5074535B2 - Induction melting furnace controller - Google Patents

Description

  The present invention relates to a control apparatus for an induction melting furnace that melts a material to be melted stored in a furnace by supplying electric power to a heating coil provided on the outer periphery of the furnace wall through power supply means.

  Conventionally, as a control device for this type of induction melting furnace, as shown in the following Patent Document 1, a load matching circuit having a transformer with a tap connected between an inverter and a heating coil, and a tap of this transformer are used. A matching adjustment mechanism for switching, the matching adjustment mechanism reads the output voltage and output current of the inverter and obtains an appropriate tap, and tap switching for switching the tap so that the tap value of the transformer is the determined appropriate tap. A control device constituted by a circuit is known.

According to such a control device, the output voltage V and the output current I of the inverter are taken in, the ratio of V and I is calculated, and the calculation of the slope K (= I / V) of the load (heated member) characteristic is performed. The impedance matching can be achieved by comparing the obtained K with the inclinations K _{1 to} K _{4 of the} preset load characteristics and the tap values T _{1 to} T 3 and switching to the appropriate tap values.

Japanese Patent Laid-Open No. 10-24184

  However, in the conventional induction melting furnace control device, the number of taps that can be switched is finite, and the switching of taps is accompanied by processing such as turning off the power supply, resulting in a slow response speed. In impedance matching of a changing induction melting furnace, it is difficult to perform impedance matching with high accuracy.

  In view of the above circumstances, an object of the present invention is to provide a control device for an induction melting furnace capable of performing impedance matching with high accuracy according to load impedance.

In order to achieve the above object, the control apparatus for an induction melting furnace according to the first aspect of the present invention is configured to supply power through a power supply means to a heating coil provided on the outer periphery of the furnace wall, so that the object stored in the furnace. A control device for an induction melting furnace for melting a heating material,
Impedance calculation means for calculating a load impedance from the power supply state to the heating coil of the power supply means;
An auxiliary coil connected to the power supply means via a semiconductor switching element connected in reverse parallel and connected in parallel to the heating coil;
Switch control means for performing impedance matching between the load impedance and the power supply impedance of the power supply means by controlling the semiconductor switching element according to the load impedance calculated by the impedance calculation means ;
The switch control means controls the semiconductor switching element to increase the output to the auxiliary coil when the load impedance calculated by the impedance calculation means is smaller than the power supply impedance, and the load impedance is The impedance matching is performed by controlling the semiconductor switching element so as to reduce the output to the auxiliary coil when the impedance is larger than the power supply impedance .

  According to the control apparatus for the induction melting furnace of the first invention, when the load impedance changes with time, the applied voltage applied to the auxiliary coil is varied by controlling the semiconductor switching element in accordance with this, thereby changing the auxiliary coil. The impedance can be changed. Thereby, the combined impedance of the auxiliary coil and the load impedance can be matched with the power source impedance. Therefore, unlike impedance matching by tap switching, impedance matching between the load impedance and the power source impedance can be performed quickly and with high accuracy.

Furthermore, according to the control apparatus for the induction melting furnace of the first invention, from the impedance (R1) of the auxiliary coil and the impedance (R2) of the heating coil, the impedance (R3) of these combined coils is R3 = R2 × R1 / It is expressed as (R1 + R2). The impedance (R3) of the synthetic coil becomes a variable value by changing the impedance (R1) of the auxiliary coil with respect to the impedance (R2) of the heating coil. Therefore, by changing the impedance (R1) of the auxiliary coil via the switching element so as to cancel the change of the load impedance, the impedance (R3) of the combined coil can be matched with the power supply impedance with high accuracy.

The block diagram which shows the whole structure of an induction melting furnace. Explanatory drawing which shows the equivalent circuit of FIG. Explanatory drawing which shows the processing content by a controller.

  With reference to FIG. 1, the control apparatus of the induction melting furnace of this embodiment is demonstrated. The induction melting furnace melts the material to be heated Y accommodated in the melting furnace body X.

  Specifically, the induction melting furnace control device includes a power source 1, a high-voltage power receiving panel 2, a harmonic filter 3, a converter transformer 4, a high-frequency inverter 5, a high-frequency matching device 6, and a thyristor switch 7. A coil 8 and a controller 10.

  The power source 1 is a rated AC power source and is connected to the high voltage power receiving panel 2.

  The high-voltage power receiving panel 2 is a device that energizes / stops the power supply to the induction heating device and shuts off the power supply when a failure occurs. The power fuse 21, the circuit breaker 22, the current transformer 23, and the instrument transformer And a container 24.

  The power fuse 21 is means for interrupting a current in the event of a short circuit accident, and the circuit breaker 22 performs an opening / closing operation associated with energization and stop of the power source.

  The harmonic filter 3 is a device that is connected to the high-voltage power receiving panel 2 and suppresses harmonic current from flowing out from the high-frequency inverter 5 to the power system, and includes a current-limiting reactor 31, a series reactor 32, and a harmonic capacitor. 33.

  The converter transformer 4 is connected to the harmonic filter 3 and adjusts the input voltage to the high-frequency inverter 5 so that the high-frequency inverter 5 outputs a predetermined output voltage.

  The high frequency inverter 5 is connected to the converter transformer 4 and generates an arbitrary high frequency current from a commercial power supply of 50 Hz or 60 Hz, and includes a forward converter 51 that is an AC / DC converter, a direct current It is composed of an inverse converter 52 which is an AC converter, and is controlled by an operation / stop / output control signal from the controller 10.

  Further, the high frequency inverter 5 has a built-in DC reactor 53, which removes ripple components superimposed on the DC voltage generated by the forward converter 51, and prevents excessive damage caused when a load such as the reverse converter 52 or the coil 8 is damaged. Suppresses sudden outflow of current.

  Further, the high-frequency inverter 5 outputs a signal indicating a failure state to the controller 10 at the time of failure and electrical data during operation such as voltage, current, power, and frequency to the controller 10.

  The high-frequency matching device 6 is a device that is connected to the high-frequency inverter 5 and that performs impedance matching between the high-frequency inverter 5 and a heating coil 8B described later. The high-frequency matching device 6 includes a high-frequency matching transformer 61 for impedance matching, and a power factor adjusting capacitor 62 that compensates the delay power factor of the heating coil 8B with the advance power factor.

  The thyristor switch 7 includes semiconductor switching elements 81 and 82 connected in reverse parallel, that is, a positive half-cycle thyristor switch 81 and a negative half-cycle thyristor switch 82. A desired voltage is applied to an auxiliary coil 8A, which will be described later, by turning the power ON / OFF at an arbitrary electrical angle.

  In addition, these components 1-7 are equivalent to the electric power supply means of this invention.

  The coil 8 has an auxiliary coil 8A and a heating coil 8B. The auxiliary coil 8A is connected to the high-frequency inverter 5 side via the thyristor switch 7, and the synthesis coil of the auxiliary coil 8A and the heating coil 8B is changed by changing the synthesis coil with the heating coil 8B according to the applied voltage of the thyristor switch 7. Impedance, that is, load impedance viewed from the power source (high frequency inverter 5 side) is changed.

  The heating coil 8B generates an eddy current in the material to be heated Y such as iron housed in the melting furnace body X by the high frequency power supplied from the high frequency inverter 5, and a joule generated between the metal materials by the eddy current. The material Y to be heated is heated to melt.

  The controller 10 has a function as an impedance calculation means as a control device for the induction melting furnace and a switch control means for controlling the thyristor switch 7 as well as controlling the operation / stopping of the induction melting furnace and the output adjustment.

  Specifically, the controller 10 constantly monitors the electrical data signal output from the high-frequency inverter 5 and calculates the load impedance R at a constant processing cycle. The method for calculating the load impedance R is described in a prior patent document (for example, Japanese Utility Model Publication No. 1-158097) by the applicant of the present application, and thus a detailed description thereof is omitted. Various methods for calculating the load impedance from the high-frequency power supplied to the coil 8B can be adopted.

  Further, the controller 10 operates the thyristor switch 7 based on the calculated load impedance R, and as described above, from the combined impedance of the auxiliary coil 8A and the heating coil 8B, that is, from the power source (high frequency inverter 5 side). Change the viewed load impedance.

  The controller 10 functions as an arithmetic unit (sequencer) for executing the control process by storing and holding a program for executing the control process in a memory (not shown) and reading the necessary program from the memory.

  Further, each means constituting the controller 10 is configured by hardware such as a CPU, a ROM, and a RAM. However, each of these means may be configured by common hardware. A part or all may be comprised by different hardware.

  Next, operation control of the thyristor switch 7 by the controller 10 will be described with reference to FIGS. 2 and 3.

  First, as shown in an equivalent circuit in FIG. 2, in the induction melting furnace of FIG. 1, an auxiliary coil 8A and a heating coil 8B are connected in parallel to an AC power source. A thyristor 7 is provided for adjusting the voltage applied to the auxiliary coil 8A.

In such a configuration, assuming that the impedance of the auxiliary coil 8A is R1 and the impedance of the heating coil 8B is R2, the impedance R3 of the combined coil when these are one coil is:
R3 = R2 × R1 / (R1 + R2) (Equation 1)
It is expressed as

  Here, the impedance R2 of the heating coil 8B corresponds to the load impedance of the induction melting furnace, and this load impedance changes with time as shown in FIG.

FIG. 3 shows the time variation of each impedance with the horizontal axis representing time and the vertical axis representing impedance (% R). Here, the impedance (% R) on the vertical axis is set so that the load impedance at the rated power and the rated voltage is 100% R (= V ² / P).

Impedance R2 of the heating coil 8B starts the dissolution at time t _0, the low impedance state, increases as the temperature increase of the material to be heated Y, when the elapsed time t _1, a peak value becomes a high impedance state After descending. Then, the impedance R2 of the heating coil 8B, when elapsed time t _2, the becomes low impedance state again, eventually, approaches 100% R, dissolved melted material to be heated Y all is completed at time t _3.

  Thus, when the load impedance, which is the impedance R2 of the heating coil 8B, is in a low impedance state or a high impedance state, it is difficult to apply the rated power of the power source to the heating coil 8B. That is, in the low impedance state, the current easily flows, and the current limit functions to control the inverter output voltage so that the rated current is not exceeded. On the other hand, in a high impedance state, current hardly flows and the voltage rises. In either of the low impedance state and the high impedance state, the load impedance does not match the power source impedance, and the maximum applied power cannot be applied to the heating coil 8B.

  Therefore, the controller 10 changes the applied voltage of the auxiliary coil 8A via the thyristor switch 7 with respect to the time change of the impedance R2 of the heating coil 8B as described above, and the impedance R1 of the auxiliary coil 8A is changed as shown in FIG. As shown. That is, the impedance R1 of the auxiliary coil 8A is changed so as to have an opposite phase to the time change of the impedance R2 of the heating coil 8B with reference to the R1 target value. Thereby, the impedance R3 of the synthetic coil can be set to 100% R from the initial stage of melting as shown in FIG.

  Next, a specific method for adjusting the combined coil impedance R3 will be described.

First, when the voltage applied to the auxiliary coil 8A is reduced to 0 by the thyristor switch 7, the combined coil impedance R3 becomes the impedance R2 of the heating coil 8B, and therefore R3 = R2. On the other hand, when the applied voltage of the auxiliary coil 8A is reduced to 33% of the mark of the heating coil 8B by the thyristor switch 7, the impedance R1 of the auxiliary coil is
R1 = R2 × (1 / 0.33) ² ≈ 9R2
It becomes. Substituting this into the above equation 1, the inductance R3 of the composite coil is
R3 = R2 × 9R2 (9R2 + R2) = 0.9R2
It becomes. This is the second state.

Further, when the applied voltage of the auxiliary coil 8A is reduced to 50% of the mark of the heating coil 8B by the thyristor switch 7, the impedance R1 of the auxiliary coil is
R1 = R2 × (1 / 0.5) ² ≈4R2
It becomes. Substituting this into the above equation 1, the inductance R3 of the composite coil is
R3 = R2 × 4R2 (4R2 + R2) = 0.8R2
It becomes. This is the third state.

  Here, the characteristics when the load impedance (impedance R2 of the heating coil 8B) changes using the second state as the reference R1 target value will be described.

  First, when the load impedance is in a low impedance state, for example, 90% R, the controller 10 changes the voltage applied to the auxiliary coil 8A by the thyristor switch 7 from the second state to the first state. At this time, the inductance R3 of the composite coil changes from 0.9R2 to 1.0R2. That is, R3 changes to 1.0 / 0.9 times≈1.1 times.

  As a result, even if the load impedance is low, the load impedance of the combined coil can be 90% R × 1.1≈100% R so as to compensate for this.

  On the other hand, when the load impedance is in a high impedance state, for example, 110% R, the controller 10 changes the voltage applied to the auxiliary coil 8A by the thyristor switch 7 from the second state to the third state. At this time, the inductance R3 of the composite coil changes from 0.9R2 to 0.8R2. That is, R3 changes to 0.8 / 0.9 times≈0.9 times.

  Thereby, even if the load impedance is high impedance, the load impedance as the combined coil can be set to 110% R × 0.9≈100% R so as to cancel out this.

In this way, by changing the applied voltage of the auxiliary coil 8A by the thyristor switch 7 according to the state of the load impedance, it is not necessary to switch taps, and the combined impedance of the auxiliary coil 8A and the heating coil 8B, that is, the power source ( The load impedance viewed from the high-frequency inverter 5 side) can be changed to achieve impedance matching with the power supply impedance with high accuracy. Thus, it is possible to heat dissolved material to be heated Y at the maximum applied power, the dissolution completion time t ₃ can be greatly shortened.

DESCRIPTION OF SYMBOLS 1 ... Power supply, 2 ... High voltage receiving board, 3 ... Harmonic filter, 4 ... Voltage device for converters, 5 ... High frequency inverter, 6 ... High frequency matching device, 7 ... Thyristor switch (semiconductor switching element), 8 ... Coil, 8A ... auxiliary coil, 8B ... heating coil, 10 ... controller (impedance calculation means, switch control means), X ... melting furnace body, Y ... heated material.

Claims (1)

A control device for an induction melting furnace that melts a material to be heated stored in the furnace by supplying electric power to the heating coil provided on the outer periphery of the furnace wall through power supply means,
Impedance calculation means for calculating a load impedance from the power supply state to the heating coil of the power supply means;
An auxiliary coil connected to the power supply means via a semiconductor switching element connected in reverse parallel and connected in parallel to the heating coil;
Switch control means for performing impedance matching between the load impedance and the power supply impedance of the power supply means by controlling the semiconductor switching element according to the load impedance calculated by the impedance calculation means ;
The switch control means controls the semiconductor switching element to increase the output to the auxiliary coil when the load impedance calculated by the impedance calculation means is smaller than the power supply impedance, and the load impedance is The induction melting furnace control apparatus , wherein the impedance matching is performed by controlling the semiconductor switching element so as to reduce the output to the auxiliary coil when the power impedance is larger than the power source impedance .

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