EP1818638B1 - Method for monitoring an induction oven and induction oven - Google Patents

Abstract

Monitoring an induction furnace, comprising an induction coil (4) surrounded by spring-loaded yokes, with insulation between the coil and yokes, comprises independently determining the insulation status of each yoke in relation to the coil and/or earth and locating any existing or incipient defects in the insulation. An independent claim is also included for an induction furnace as above where each yoke (5) is equipped with a sensor (10) for detecting a change in the resistance of the coil-yoke insulation (9) in the vicinity of each individual yoke and/or the leakage current between each individual yoke and earth.

Description

The invention relates to a method for monitoring an induction furnace according to the features of the preamble of patent claim 1 and to an induction furnace with the features of the preamble of claim 4.

In such induction furnaces, which are used for the molten metal, the induction coil consists of a hollow profile made of copper, which is traversed for the purpose of cooling water, in order to meet the high thermal load of the induction coil. The coil voltage is up to 3000 V in today's induction furnaces, which corresponds to a winding voltage of about 125 V. The frequency of the supply voltage varies depending on the application and size of the furnace, it is about 200 Hz for applications for melting iron materials with furnace sizes in the range of 10 t. Thus, for the high-performance furnaces, a power density of 1 MW / t is installed, accordingly at a 10 t Ovens provided a capacity of 10 MW.

For the safe and trouble-free operation of such an induction furnace, the electrical insulation of the induction coil must be designed accordingly. On the one hand, the coil turns against each other and on the other hand, the coil must be sufficiently insulated from the ground potential, which rests in the known designs on the furnace frame and the yokes. in this connection Problems occur due to the electromagnetic principle of action of the induction furnaces with the parameters described above, because it comes to extreme loads on the electrical insulation between the coil and the yoke yokes by vibration. This insulation consists of the coil insulation, by means of which the coil turns are insulated from each other, and of a multilayer insulation structure between the induction coil and each yoke.

Also, the operational thermal loads as well as the mechanical stresses due to thermal expansion of the crucible material lead on the one hand and by breaking or delivery of the crucible to the other to further stresses of the coil yoke insulation. In addition, moisture present promotes faster aging of the coil yoke insulation, which penetrates into the insulation, especially during new deliveries of the furnace. Ultimately, there is an electrical breakdown of the coil yoke insulation, thereby creating a ground fault of the coil, which is detected by the default earth fault monitoring in the known induction furnaces.

However, the earth fault monitoring does not indicate over which path and at which point the induction coil is earthed. On the one hand, the earth fault can be initiated via the melt which reaches the coil. On the other hand, the ground fault can be present over the worn insulation layers towards the grounded yoke.

It is a considerable effort to find the way and the location of the ground fault in such induction furnaces. In addition, there is the loss due to the production failure of the induction furnace. Even if the ground fault is identified as the end of the coil via the defective coil yoke insulation to the yokes, it is not clear over which of the yokes the inductor is grounded. A common induction furnace with a 10 ton crucible has about 10 to 12 yokes, which must be gradually removed from the induction coil until it reaches the yoke, after its distancing from the coil interrupted the earth fault and thus the point of ground fault is determined. The loosening of the fittings of the strained against the induction coil yokes alone is a time-consuming affair.

The invention is therefore an object of the invention to provide a monitoring system for an induction furnace of the type in question in order to locate a fault in the coil yoke insulation easier and faster. This object is achieved with the method features of claim 1 and with the subject features of claim 4.

For the invention it is essential that one is oriented with regard to the error detection of the individual, spaced-apart yokes of the induction furnace. Error detection can be done in two ways.

On the one hand, the yoke construction is allowed to remain, in which case the yokes remain grounded. This has the advantage that the most accessible arranged, not touch-protected yokes can not lead a dangerous electrical potential. So constructive changes to the induction furnace are not required. In the coil yoke insulation, namely in the insulation structure adjacent to each of the yokes, an indicator in the form of a wire electrode is inserted, which may be designed as a comb-shaped electrode or a meandering electrode. With a multi-layered structure of the insulation between the coil insulation and each yoke, the wire electrode can be positioned between two layers, such as between two layers of mica insulation. The wire thickness of the indicator is so low that the electromagnetic field can not heat the sensor wire. Due to the areal arrangement of the indicator wire, the entire area of a yoke is monitored, and it is also possible to monitor partial areas of a yoke by providing a plurality of such indicators spatially separate from one another.

The detection of an insulation defect between the induction coil and each individual yoke is carried out by measuring the insulation resistance, ie the electrical resistance between the coil and the indicator and / or the indicator and the yoke. Since these measurements are made on each individual yoke, a defect of the coil yoke insulation can be localized, and it can be on the yoke in question or on one Part of this yoke be limited. Due to the exposed arrangement of the indicator not only an early error detection but also the timely perception of an impending error is possible. Thus, the induction coil with the indicator may already be in contact before it comes to ground fault as a result of complete destruction of the coil yoke insulation. Instead of the resistance measurement for detecting the insulation defect, a voltage measurement between the indicator and the coil or the grounded yoke can take place, it can also be a current flow from the coil to the indicator and / or a current flow between the indicator and the yoke detected and evaluated. Furthermore, by measuring the individual values and by evaluating the absolute values of the measured values, the measuring method makes it possible to determine whether there is a ground fault due to moisture or a saturated ground fault. By varying the frequency of the measuring voltage, it is certainly possible to distinguish a moisture lock from a metallic short.

On the other hand can be provided according to the invention to cancel the usual grounding of the yokes. This happens, for example, by the installation of a yoke insulation between each yoke and the furnace frame, especially in the region of the clamping devices. The earthing of each individual yoke is in this case via a single grounding line to the grounded oven rack out. Any current flow, ie a leakage current, through this grounding line is individually monitored for each yoke. Especially for this purpose, a measuring method via shunts or the use of current clamps. If a ground fault is indicated by the default ground fault monitoring, the damaged area can now be limited to the earth by evaluating the leakage current at each individual yoke, which is located in the area of the relevant yoke. This evaluation of the leakage currents is recorded, and by comparing the individual leakage currents of the yokes and by evaluating the level of the leakage can be further identified whether it is leakage due to moisture, as they are unavoidable in the re-delivery, or as a result a massive ground fault.

The invention makes it possible to carry out targeted repairs as a result of the localization of the breakdown of the coil yoke insulation, namely at the localized defect. This results in savings not only due to a reduced repair costs, but also by a shorter repair time, which shortens the production loss.

Fig. 1

eine schematische Darstellung einer Induktionsofen-Anlage mit einem im Vertikalschnitt dargestellten Induktionsofen,

Fig. 2

in vergrößerter Darstellung einen Vertikalschnitt durch einen Abschnitt des Randbereichs des Induktionsofens nach Fig. 1 in vergrößerter Wiedergabe,

Fig. 3

einen Vertikalschnitt durch den äußeren Randbereich des Induktionsofens einschließlich einem Teil des Ofengestells in erster Ausführung,

Fig. 4

eine stark vergrößerte Einzelheit des Grenzbereichs zwischen einer der Windungen der Induktionsspule und einem der Joche des Induktionsofens in der Fig. 3 entsprechenden Ausführung,

Fig. 5

eine schematische Draufsicht auf den Induktionsofen in der den Fig. 3 und 4 entsprechenden Ausführung,

Fig. 6

eine Ansicht eines der Indikatoren, die gemäß Fig. 4 in die Spulen-Joch-Isolation eingesetzt sind,

Fig. 7

eine Ansicht eines solchen Indikators in zweiter Ausführung,

Fig. 8

einen Vertikalschnitt durch den äußeren Randbereich des Induktionsofens gemäß Fig. 1 in zweiter Ausführung und

Fig. 9

eine schematische Draufsicht auf den Induktionsofen in der Ausführung gemäß Fig. 8.

The invention will be explained in more detail with reference to the drawings of embodiments. Showing:

Fig. 1

a schematic representation of an induction furnace system with an induction furnace shown in vertical section,

Fig. 2

in an enlarged view a vertical section through a section of the edge region of the induction furnace Fig. 1 in enlarged reproduction,

Fig. 3

a vertical section through the outer edge region of the induction furnace including a part of the furnace frame in the first embodiment,

Fig. 4

a greatly enlarged detail of the boundary between one of the turns of the induction coil and one of the yokes of the induction furnace in the Fig. 3 corresponding design,

Fig. 5

a schematic plan view of the induction furnace in the Fig. 3 and 4 corresponding design,

Fig. 6

a view of one of the indicators according to Fig. 4 are inserted in the coil yoke insulation,

Fig. 7

a view of such an indicator in the second embodiment,

Fig. 8

a vertical section through the outer edge region of the induction furnace according to Fig. 1 in the second embodiment and

Fig. 9

a schematic plan view of the induction furnace in the execution according to Fig. 8 ,

The induction furnace according to Fig. 1 has a crucible 2 for receiving a molten metal 1, which consists of a refractory material. On the outside, the crucible 2 surrounds a so-called coil guard 3, on which an induction coil 4 is arranged coaxially to the crucible 2 on the outside.

Details of the induction coil 4 shows Fig. 2 in which one recognizes one of the coil turns 4.1 in full cross-sectional representation. The coil turns 4.1 consist of a hollow copper profile, so that in the interior of each coil turn 4.1, a cooling channel 4.2 is formed, is passed through the cooling water.

Out Fig. 1 goes on show that on the outside of the induction coil 4 yokes 5 are arranged, which are aligned axially parallel to the induction coil 4 and the - like the Figures 5 and 9 show - are distributed in the circumferential direction around the induction coil 4 around evenly. The individual yokes 5 have distances from each other and are bent so that they conform to the induction coil 4 in the circumferential direction.

The yokes 5 are braced against the induction coil 4, this is especially the Figures 3 and 8th , Serve with clamping devices 6 with clamping screws 6.1, which are supported to the outside on the outer steel furnace frame 6.2. The clamping screws 6.1 act on the associated yoke 5 in the radial direction of the induction coil 4. In the execution of the induction furnace according to Fig. 3 There is a metal connection between each yoke 5 and the furnace frame 6.2 via the clamping screws 6.1, which is grounded in the usual way.

Ultimately, you take out Fig. 1 nor the electrical supply device 7 for operating the induction coil 4, to which further a Erdschlußwächter 8 is connected, which responds in the event of a ground fault of the coil 4. Regardless of this earth fault monitor 8, the induction furnace according to the invention is equipped with a monitoring system, which will be discussed in more detail below.

Fig. 2 and Fig. 4 in a further enlarged view illustrate the structure of the electrical insulation between the induction coil 4 and each of the yokes 5. For this purpose, a coil yoke insulation 9 is provided, which includes a coil insulation 9.1, by means of the coil turns 4.1 are insulated from each other. Between the induction coil 4 and each grounded yoke 5 is in operation a voltage in the order of 3,000 volts. An insulation structure 9.2 is designed thereon, which consists of several layers, in particular of mica layers.

With increasing wear, the electrical resistance of the entire coil yoke insulation, which, like it, decreases 4 and FIG. 5 monitored by means of a sensor device 10 to detect the change in resistance. Between two of the layers of the insulation structure 9.2, at least one indicator 11 is inserted, which consists of an electrical conductor. Fig. 6 shows such an indicator in the form of a meandering conductor 11.1 and Fig. 7 in the form of a comb-shaped conductor 11.2. The meandering conductor 11.1 as an indicator allows a self-diagnosis in terms of an interruption. Because of the existing two ends to which can be connected, an electrical continuity test is possible.

As Fig. 4 Furthermore, a voltage, a current or a resistance is measured between each indicator 11 and the yoke 5 on the one hand and one of the coil windings 4.1 on the other hand, corresponding display devices 10.1 are provided. The indicators 11 and the display devices 10.1 are each part of a sensor device 10, and each one of these sensor devices 10 is associated with each of the yokes 5, such as Fig. 5 shows. Becomes a ground fault of the induction coil 4 or is such a ground fault already, this is signaled by the relevant sensor device 10, and one thus obtains the immediate indication in the yoke 5 which threatens a breakdown of the coil yoke insulation 9 and can take the necessary countermeasures at the relevant point.

The location of a damaged area can be further limited by providing two or more of the indicators 11 in the region of one of the yokes 5, and the sensor device 10 is equipped such that each of the indicators 11 is detected.

8 and 9 show an arrangement with which the same purpose is otherwise fulfilled. Here, the yokes are 5 electrically insulated against the ground potential leading furnace rack 6.2 that a ground fault can not be done via the clamping screws 6.1 of the clamping device 6. Therefore, an electrical insulation 12 in the form of pressure and heat-resistant insulating pieces is provided between each of the yokes 5 and the furnace frame 6.2, which are arranged between the clamping screws 6.1 and the associated yoke 5. Each yoke 5 is connected to the oven rack 6.2 by means of its own ground line 13, and each of these ground lines 13 is in this case part of the sensor device 10, by means of a ground fault can be detected at the respective yoke 5. For this purpose, the sensor device 10 on an ammunition A, which is inserted either in the ground line 13 of each yoke 5 or at least connectable thereto. This depends on whether you are either appropriate Fig. 9 on each of the ground lines 13 permanently a sensor device 10 provides or in the case of one of Erdschlusswächter 8 ( Fig. 1 ) earth fault successively checked the individual ground lines 13 to a flowing over it leakage current successively. For this purpose, current clamps are suitable, and depending on the type of training of the sensor device 10, it may be necessary to insert a shunt in each of the earth lines 13.

Claims (7)

1. Method for monitoring an induction furnace which comprises an induction coil (4), against the outer side of which yokes (5) are arranged which are spaced apart from each other around the periphery of the coil, said yokes (5) being pressed against the coil by means of clamping devices resting on the outer furnace frame, wherein there is an electrical coil / yoke insulation (9) between each yoke and the coil,
   characterised in that
   a change in the electrical insulation state in relation to the induction coil and / or the earth potential is detected on each individual yoke independently of the other yokes and then the point of impending or actual damage is located in the coil / yoke insulation.
2. Method according to claim 1,
   characterised in that
   a resistance, voltage and / or current measurement is carried out between each yoke and the induction coil and / or the earth potential.
3. Method according to claim 1 or 2,
   characterised in that
   the flow of a leakage current is monitored between each yoke and the earth potential, for which purpose an electrical insulation is provided between each yoke and the furnace frame.
4. Induction furnace with an induction coil (4), against the outer side of which yokes (5) are arranged which are spaced apart from each other around the periphery of the coil, said yokes (5) being pressed against the coil (4) by means of clamping devices (6) resting on the outer furnace frame (6.2), wherein there is an electrical coil / yoke insulation between each yoke (5) and the coil (4),
   characterised in that
   a sensor device (10) is allocated to each of the individual yokes (5), with which either a resistance change in the coil / yoke insulation (9) in the region of each individual yoke (5) and / or the flow of a leakage current between each individual yoke (5) and the earth potential is / are detected.
5. Induction furnace according to claim 4,
   characterised in that
   the sensor device (10) comprises an indicator (11) which consists of an electrical conductor arranged in the coil / yoke insulation, by means of which electrical conductor a resistance-dependent, voltage-dependent and / or current-dependent signal is detected which is evaluated in the sensor device (1).
6. Induction furnace according to claim 5,
   characterised in that
   the conductor arranged as an indicator (11) in the coil / yoke insulation (9) is formed as a meandering conductor (11.1) or as a comb-like conductor (11.2).
7. Induction furnace according to claim 4,
   characterised in that
   the furnace frame (6.2) is earthed and an electrical insulation is provided between each yoke (5) and the furnace frame (6.2), whereby each yoke is connected individually by means of an earth cable (13) to the earth potential and at least one ammeter (A) is provided which can be connected to this earth cable (13) or is inserted therein.

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