EP0744117B1 - Process for operating coreless induction melting and/or holding furnaces and electric switching unit suitable therefor - Google Patents

Abstract

PCT No. PCT/DE95/00175 Sec. 371 Date Oct. 7, 1996 Sec. 102(e) Date Oct. 7, 1996 PCT Filed Feb. 10, 1995 PCT Pub. No. WO95/22238 PCT Pub. Date Aug. 17, 1995In a process for the operation of coreless induction melting and/or holding furnaces-in which there results in the melting operation at a relatively high induction frequency (as compared to the mains frequency) a slight stirring motion in the melt, but a high degree of effectiveness for the melting process, and in the melting and holding operation at a correspondingly lower induction frequency there results a greater stirring motion in the melt, but a lower degree of effectiveness for the melting process-it is provided that at least one capacitor switched in parallel to the induction coil(s) is provided, which capacitor(s), together with the induction coil(s), form a resonant circuit; and that in the transition from the melting operation with slight stirring motion in the melt to the melting or holding operation with greater stirring motion, or in the reverse transition, the capacitor capacitance and/or the inductance present in the resonant circuit is increased, or as the case may be, decreased.

Description

The present invention relates to a method for operating coreless induction melting and / or holding furnaces, in which in the melting mode at an induction frequency a low stirring movement of 100 Hz in the melt and in melting or holding mode with an induction frequency reduced by a maximum of 50%, which is always 50 Hz, a greater stirring movement in the melt results, with at least one induction coil and at least one electrically connected in parallel with the induction coil (s) Capacitor is provided which, together with the induction coil (s), is an electrical one The resonant circuit forms (s), whereby with the transition from melting operation with less agitation in the melt on melting or holding mode with greater stirring movement or in the case of the reverse transition, the capacitor capacitance and / or that in the resonant circuit existing inductance can be increased or decreased.

It also relates to an electrical switching unit suitable for carrying out the method for coreless induction melting and / or holding furnaces with an induction frequency in the range from mains to medium frequency, which is an electrical voltage supply device with a frequency converter or converter as well as a one or more part to one Induction coil arranged around the furnace crucible and at least one inside the electrical one Line path between the induction coil and the frequency converter lying capacitor wherein the capacitor (s) are electrically parallel to the induction coil (s) is (are) connected and, together with this (s), forms an electrical resonant circuit, whose natural frequency is matched to the respective induction frequency, and within of the mentioned path electrical switching elements are provided, by means of which the Capacitance and / or the inductance of the resonant circuit and thus its natural frequency in steps are adjustable.

A coreless induction crucible furnace is known from DE-PS 27 48 136, which operates both at the mains frequency can also be operated at a higher frequency. This will be one Furnace preferably operated at medium frequency, around the solid state in the crucible melted metal quickly. In contrast, should the already melted Metal stirred vigorously and if necessary with slag and alloy treatments the melt are carried out, in such a case a lower one is preferred Frequency, e.g. Mains frequency applied.

A disadvantage of this known induction melting and holding furnace and the process its operation can be seen in the fact that two different power grids must be available, to solve the task, namely the operation in two frequency ranges. The outlay on equipment is consequently considerable, since practically two separate switchgear assemblies are used Need to become.

The solution also requires two utilities with different frequencies according to British Patent No. 508 255 (Fig. 9). In this case, one is made up of two partial coils existing induction coil simultaneously to a supply device of lower frequency and connected to such a higher frequency. To the high effort, as with the object known from the first-mentioned patent must be operated in the case of the British patent specification there is the additional disadvantage that each network is special filters must be protected so that the frequency of a network is not negative the other acts.

From JP-A 060 36865 a solution of the type mentioned is also known the one in the induction furnace when switching to agitation with reduced induction frequency upstream, two-part voice coil from series parallel operation in the Switched parallel operation and in addition to the existing resonant circuit capacitor another capacitor is connected in parallel.

With this solution, only one supply network is required, but it always requires too much effort in components that are not used in the melting shop.

The object of the present invention is therefore to provide a method and a switching unit Operation of a generic oven to specify, with only a little additional technical effort Operation with multiple frequencies is made possible, with these additional expenses are inexpensive and can be implemented in a small space.

immer mindestens eine Induktionsspule und mindestens zwei im Schmelzbetrieb bei erhöhter Induktionsfrequenz in Reihe geschaltete Kondensatoren vorgesehen sind, die zur Erhöhung der Kondensatorkapazität durch Umschaltung parallel geschaltet werden und/oder

daß immer mindestens ein Kondensator und mindestens zwei im Schmelzbetrieb bei erhöhter Induktionsfrequenz parallel geschaltete, getrennte oder durch Anzapfung einer Induktionsspule hergestellte Teilspulen vorgesehen sind, die zur Erhöhung der Induktivität durch Umschaltung in Reihe oder teilweise spannungslos geschaltet werden.

This object is achieved in the method according to the invention in that

always at least one induction coil and at least two capacitors connected in series in melting mode at an increased induction frequency are provided, which are connected in parallel to increase the capacitor capacity by switching and / or

that at least one capacitor and at least two sub-coils connected in parallel in melting mode at increased induction frequency, separated or produced by tapping an induction coil are provided, which are switched to series or partially de-energized by switching over to increase the inductance.

The sub-claims 2 and 3 contain particularly advantageous procedures according to the invention. In particular, it has proven to be particularly economical that only the upper part - especially the upper half - the induction coil for intensive mixing of the molten metal is switched on near the bathroom mirror and the existing one for the entire oven coil designed capacitor capacity is connected to this coil section.

A circuit of the type mentioned at the outset which is suitable for carrying out the method is according to the invention constructed so that in the case of a one-piece induction coil at least two Capacitors are provided and the switching elements are designed such that at their Actuation first capacitors connected in series to each other connected in parallel to each other and / or induction coils connected in parallel in series or can be partially de-energized and vice versa.

Due to these measures of low technical effort, it becomes possible to use the induction furnace to apply different frequencies despite only one power supply device. The solution according to the invention has the further advantage that it is inexpensive and can be realized in a space-saving manner.

Either individual capacitors or induction coils can be taken in by themselves are switched over or a switchover for capacitors and induction coils can be made simultaneously. When switching two Capacitors of the same size from series to parallel connection can therefore the natural frequency of the resonant circuit can be increased or decreased by a factor of 2. This frequency change can be further increased by the fact that either several capacitors or induction coils are used or both modules are switched simultaneously.

The switching unit according to the invention can further provide that at least two switching elements can be locked against each other by means of an electrical or electromechanical lock.

Such locking is required when the capacitors and / or induction coils make switching by several switching elements necessary, but not all Switching elements may be operated simultaneously.

The switching unit according to the invention can also be designed such that the induction coil (s) is (are) divided by means of at least one tap.

This version is an alternative to two separate induction coils.

A particularly advantageous embodiment of the switching unit according to the invention can provide that only the part of the turns of the induction coil (s) lying near the melt pool surface is supplied with the supply voltage.

This operating mode is particularly useful when stirring and holding the induction furnace because the efficiency of the power transmission to the melt in the area of the melt pool area is highest.

Instead of dividing the induction coil by means of a tap, the invention can Switching unit can also be designed so that at least two electrically separate induction coil parts are provided and that at least the upper induction coil part with the entire Capacitance of the capacitors is applied.

When switching from melting operation with higher frequency, i.e. with little stirring in the melt, on melting or holding mode with a larger stirring motion here the capacitor capacitance above that to achieve an optimal efficiency required measure increased. The resonance circuit of the induction furnace is changed, i.e. the Vibration frequency of the resonant circuit is reduced. Overall, this can be done in a simple manner the frequency of the feed current of the furnace is reduced as desired and the stirring movement be increased in the weld pool.

In the following part of the description, the method according to the invention and the method according to the invention Switching unit based on exemplary embodiments shown in figures explained.

Fig. 1 einen Induktionstiegelofen mit einer Schalteinheit gemäß der Erfindung, mittels der eine Absenkung der Induktionsfrequenz durch Erhöhung der Kondensatorkapazität für die gesamte Spule erreichbar ist;

Fig. 2 eine Fig. 1 entsprechende Darstellung, bei der eine Absenkung der Induktionsfrequenz durch Erhöhung der Kondensatorkapazität und der Induktivität der Ofenspule erreicht wird;

Fig. 3 eine Fig. 1 bis 2 entsprechende Darstellung, bei der eine Absenkung der Induktionsfrequenz durch Erhöhung der Induktivität der Spule erreicht wird sowie

Fig. 4 eine Fig. 1 bis 3 entsprechende Darstellung, bei der eine Absenkung der Induktionsfrequenz durch Erhöhung der Induktivität bei einer zweigeteilten Spule erreicht wird.

In detail show:

1 shows an induction crucible furnace with a switching unit according to the invention, by means of which a lowering of the induction frequency can be achieved by increasing the capacitor capacity for the entire coil;

FIG. 2 shows a representation corresponding to FIG. 1, in which the induction frequency is reduced by increasing the capacitor capacitance and the inductance of the furnace coil;

Fig. 3 is a Fig. 1 to 2 corresponding representation, in which a reduction in the induction frequency is achieved by increasing the inductance of the coil and

Fig. 4 is a Fig. 1 to 3 corresponding representation, in which a lowering of the induction frequency is achieved by increasing the inductance in a two-part coil.

1 shows schematically a coreless induction crucible furnace 1 with a furnace crucible 2 surrounding it Induction coil 3 shown. The induction coil 3 is via lines 4.5 and over a frequency converter or oscillating circuit converter 6 connected to a three-phase network 7.

In addition, capacitors are 8.9. provided that with the help of a switch 10 to each other in Row - and in parallel to the induction coil 3 - can be connected to the lines 4,5. In order to increase the capacitance of the capacitors 8.9 fourfold and thus the frequency the capacitors 8.9 can also be reduced by half using switches 11, 12 and lines 13, 14 are connected in such a way that the capacitors 8, 9 in each case electrically parallel to one another, but overall also electrically parallel to the induction coil 3 lie, the switch 10 then being opened again.

In the application example according to FIG. 2, the induction coil 3 also has a center arranged tap 16. With the help of a switch 23, the induction coil 3 can Lines 4,5 are connected to the resonant circuit converter 6. The capacitors 8.9 are connected in series between the lines 4,5. When the switch 23 and the further switches 24, 25 and 29, the switches 23 and 29 being opened via a lock must be, the capacitors 8.9 in parallel with the top half connected to the induction coil 3. In this case, both the capacitance of the capacitor as well as the inductance of the induction coil increases.

3 shows an application example of the invention in which the inductance is doubled in the case of a two-part induction coil 3a, 3b. In normal melting operation, the switch 23 is closed and the two coil parts 3a, 3b are thus supplied with the supply voltage. In the state shown, that is to say when switch 23 is not closed, only coil part 3a is switched on. In this case, the operating frequency by the factor 2nd reduced.

FIG. 4 shows a coreless induction crucible furnace in which partial coils 3a, 3b are provided Connected to the three-phase network 7 via lines 4.5 and via the resonant circuit converter 6 are. A capacitor 15 is connected in parallel with the partial coils 3a, 3b. By Closing the switches 27 and 28 can both partial coils 3a, 3b in parallel from Resonant circuit converter 6 are applied. To the frequency of the coil parts 3a, 3b to reduce the applied voltage by approx. 50%, the partial coils 3a, 3b are switched 26 connected in series to the oscillating circuit converter 6. Here too, the switch 26 is on the one hand and on the other hand, the switches 27, 28 are interlocked in such a way that the switches 27, 28 can only be closed when the switch 26 is open.

Claims (8)

1. Method for operating coreless induction smelting and/or holding furnaces in which, in the smelting mode, a gentle stirring motion in the melt results at an induction frequency of 100 Hz and, in the smelting or holding mode, a stronger stirring motion in the melt results at an induction frequency which is reduced by a maximum of 50% in comparison with this and is always 50 Hz, in which method at least one induction coil and at least one capacitor are provided, which capacitor or capacitors is or are connected electrically in parallel with the induction coil or coils and forms or form an electrical tuned circuit together with the induction coil or coils, the capacitor capacitance and/or the inductance present in the tuned circuit being increased or reduced, respectively, during the transition from the smelting mode with a relatively gentle stirring motion in the melt to the smelting or holding mode with a stronger stirring motion and, respectively, during the reverse transition, characterized in that at least one induction coil and at least two capacitors are always provided, which are connected in series at the increased induction frequency in the smelting mode and are connected in parallel by switching in order to increase the capacitor capacitance, and/or in that at least one capacitor and at least two coil elements are always provided, which coil elements are connected in parallel or are separated at the increased induction frequency in the smelting mode or are produced by a tapping on an induction coil and are connected in series or are partially deactivated by switching in order to increase the inductance.
2. Method according to Claim 1, characterized in that in the case of a full furnace, or a furnace crucible which is filled almost entirely with metal melt, only some of the coil turns, in particular those close to the liquid bath surface, are connected to the electrical voltage supply device of the induction furnace.
3. Method according to one of the preceding claims, characterized in that, if there are a plurality of separate induction coil elements, only the upper coil element or elements is or are switched on.
4. Electrical switching unit for coreless induction smelting and/or holding furnaces having an induction frequency in the region of the mains to mid-frequency, which electrical switching unit has an electrical voltage supply device with a frequency changer or converter (6) as well as an integral or multi-element induction coil (3;3a,3b), which is arranged around a furnace crucible, and at least one capacitor (8,9;15) which is located within the electrical conduction path (4,5,13,14) between the induction coil (3;3a,3b) and the frequency changer (6), the capacitor or capacitors (8,9,15) being electrically connected in parallel with the induction coil or coils (3,3a,3b) and, together with it or them, forming an electrical tuned circuit whose natural frequency is tuned to the respective induction frequency, and electrical switching elements (10,11,12;23,24, 25,29;26,27,28) being provided within the said conduction path (4,5,13,14), by means of which the capacitance and/or the inductance of the tuned circuit, and thus its natural frequency, can be adjusted in steps in order to carry out the method according to one of Claims 1 to 3, characterized in that, in the case of an integral induction coil (3), at least two capacitors (8,9) are provided and the switching elements (10,11,12;23,24,25,29;26,27,28) are designed in such a manner that, when they are operated, capacitors (8,9) which are initially connected in series with one another can be connected in parallel with one another and/or induction coils (3a,3b) which are initially connected in parallel can be connected in series or can be partially deactivated, and visa versa.
5. Switching unit according to Claim 4, characterized in that at least two switching elements (10,11,12;23,24,25,29;26,27,28) can be locked with respect to one another by means of an electrical or electromechanical interlock.
6. Switching unit according to one of Claims 4 or 5, characterized in that the induction coil or coils (3,3a,3b) is or are split by means of at least one tap.
7. Switching unit according to one of Claims 4 to 6, characterized in that the portion of the turns of the induction coil or coils (3,3a,3b) which is located in the vicinity of the liquid bath surface has the supply voltage applied to it.
8. Switching unit according to one of Claims 4 to 7, characterized in that at least two electrically isolated induction coil elements (3a,3b) are provided.

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