EP2181563B1 - Induction heating method - Google Patents

Abstract

During induction heating of a billet of an electrically conducting material by rotating the billet relative to a magnetic field that is generated by means of at least one direct-current-carrying superconducting winding on an iron core, the reverse-induction voltage can be reduced when a direct current is generated and maintained in the winding at a value that generates in the iron core at least in the region of the winding a magnetic flux density at which the relative permeability of the material of the iron core is less than in a zero-current state of the winding.

Description

The invention relates to a method for inductive heating of a billet of an electrically conductive material by relative movement, in particular generating a rotation between the billet and a magnetic field, which is generated by means of at least one DC-powered superconducting winding on an iron core.

Such a procedure shows the DE 10 2005 061 670.4 , For carrying out the method, for example, a constant-speed cylindrical billet clamped in a rotary-driven jig can be rotated about its cylinder axis in a magnetic field generated by the superconducting winding by means of a constant current. As a result, a largely constant current is induced in the billet. In reality, however, the billet is usually not optimally cylindrical and / or not exactly clamped so that it is not rotated about its cylinder axis. As a result, the magnetic flux through the billet also changes in terms of amount, so that a corresponding amount of non-constant induction current is induced in the billet. The induction current I _ind (t) alternates with the rotational frequency f, ie I _ind (t) = I _ind (t + f ^-1 ). Due to the temporally not constant induction current in the billet, a temporally correspondingly changing magnetic field is generated, which penetrates the superconducting winding and induces a voltage there. This effect is referred to as re-induction and the corresponding voltage as re-induction voltage. Because of this yourself time-varying reverse induction voltage flows through the superconducting winding not temporally constant but a time-variable current, which leads to undesirable losses, so-called reverse induction losses in the superconducting winding.

Similarly, when heating non-cylindrical, rod-shaped billets, e.g. With rectangular or oval cross-section, generated by rotation of the billets a constantly alternating induction current, which causes a correspondingly alternating return induction voltage and thus corresponding reverse induction losses.

Temporally varying return inductive voltages and thus reverse induction losses occur regardless of the shape of the billet, especially at the beginning and at the end of the induction heating when the billet is set in rotation or stopped. Basically, the reverse induction losses occur every time the rotational speed changes.

These return inductance losses must be compensated by a correspondingly powerful current source and increase the cooling power necessary for the superconducting winding.

The US 3,842,243 proposes to heat an electrically conductive billet in an alternating magnetic field. To guide the magnetic flux through the billet sits an AC-powered conductor in a U-shaped yoke. By means of a DC-powered auxiliary coil sitting on a section of the yoke, the section can be driven into magnetic saturation. Therefore, the magnetic flux of the alternating field is no longer complete Billet led, and this is locally heated less in the corresponding area.

The invention has for its object to reduce the back induction losses in the superconducting winding when carrying out the method mentioned in the introduction.

Procedurally, this object is achieved by a method according to claim 1. Advantageous embodiments of the method are specified in the dependent claims 2 to 7. Devices in particular for carrying out the methods are the subject of claim 8. Further developments of the devices are specified in the claims 9 to 15.

In all methods, at least one billet is moved relative to a magnetic field. It does not matter whether the magnetic field is rotated around the billet or vice versa. According to the method of claim 1, a direct current is generated and maintained in the superconducting winding with a magnetic flux density in the iron core at least in the region of the winding at which the relative permeability of the material of the iron core is smaller than in the de-energized state the winding is. As the relative permeability decreases, the back induction and thus the loss in the superconducting winding decreases. At the same time, the magnetic field of the winding leading effect of the iron core is maintained. As a result, the re-induction is reduced.

If two or more billets are simultaneously rotated in a magnetic field generated by the superconducting winding, according to an alternative or optional solution to the problem, the position of the billets relative to one another can be regulated so that subtractively superposing the back induction voltages generated by the alternating induction currents of the billets. Simplified, assuming a homogeneous magnetic field in the area of a billet, the magnetic flux through the billet is approximately proportional to the projection surface of the billet on a plane perpendicular to the field lines. When heating a non-cylindrical billet in the magnetic field, the projection area changes with each angle change. The essence of this solution is to regulate the position of two or more billets to each other so that the summed projection surface of all billets does not or only slightly changes as they move in the magnetic field. Accordingly, the summed magnetic flux through the billets does not change or only minimally, which leads to a minimized re-induction voltage in the winding. It can also be said that the return inductive stresses attributable to the individual billets, that is subtended by their respective changes in the magnetic flux, subtract superficially.

For this purpose, for example, two identical rectangular billets with square cross-section, rotated at the same angular velocity in each case about their longitudinal axes and aligned with these longitudinal axes at least approximately orthogonal to the field lines of the magnetic field generated by the current-carrying coil, the position of the billets is controlled to each other so that the two billets are mutually rotated by 45 ° about their parallel longitudinal axes, for then the magnetic flux through one of the two billets increases to the same extent as it decreases by the other billet. If the river has reached its maximum through one billet, it subsequently decreases again, with the flow through the other ticket increasing to the same extent. The summed magnetic flux through the billets is ideally constant. Then, the reinduction stresses attributable to the individual billets are at least partially canceled by subtractive superposition. The same effect, though not so pronounced, is achieved when, for example, two parallelepiped billets with non-congruent cross-sectional areas are heated simultaneously. This is especially true for cuboid billets with pronounced rectangular cross-section.

According to a further alternative or optional solution, with simultaneous inductive heating of two or more billets by rotating in a magnetic field generated by a dc superconducting winding, the relative movement of the billets relative to each other can be controlled so that the reinduction stresses generated by the time varying induction currents of the billets subtract super subtractively (Claim 2). As with the methods described in the two preceding paragraphs, this solution is also concerned with rotating the billets in a magnetic field in such a way that their summed projection area is at least largely constant. Moreover, by regulating the movement of the billets relative to one another, alternatively or optionally, the time-related change in the magnetic flux through the billets, summed up due to changing rotational speeds of the individual billets relative to the magnetic field, can be minimized. For example, two preferably identical, for example, cylindrical billets rotated about their respective longitudinal axis can be turned in opposite directions and preferably with the same angular velocity in terms of absolute value (claim 3). As a result, the individual indices to be assigned to the individual billets at the start and at the end of the heating, ie when starting or when stopping the Drehbwegung, different signs, so that in the ideal case when starting and stopping an extinction of the effective Rückinduktionsspannung in the winding by subtractive superposition of the individual billets attributable Rückinduktionsspannungen occurs.

Of course, the process can also be carried out with the simultaneous heating of different billets. If the cross sections of the billets have symmetries, these can be exploited in a targeted manner. For example, one may replace a first of the cylindrical billets of the above example with a square-shaped bar and replace the second cylindrical billet with a regular octahedral cross-section billet. Now, the first billet is rotated at double the angular velocity as the second and opposite to this. Regardless of the shape, the billets are preferably aligned against each other prior to the start of the rotation so that the magnetic flux either increases initially or decreases initially with the start of the rotational movement through both billets. Preferably, at the start of the rotational movement, the projection surfaces of the two billets on a plane perpendicular to the magnetic flux are both maximum or both minimum. If the two billets are rotated in the same direction (with unchanged ratio of the angular velocities to each other), the billets are aligned before starting so that with the start of Drehbwegung the magnetic flux decreases by one of the billets initially and increases by the other initially. In this case, at the start of the rotational movement, the projection area of a billet is preferably maximum and the projection area of the other billet is minimal. In both cases, the magnetic changes Flow through the two billets in opposite directions, so that the individual billets assigned return induction voltages have different signs and subtractive superponieren.

As a superconducting winding, for example, a band-shaped high-temperature superconductor (HTSC) can be used. As HTSC, for example, cuprate superconductors are referred to, ie rare earth copper oxides, such as YBa ₂ Cu ₃ O _7-x .

The value of the direct current can be kept at least substantially constant by a regulated current source connected to the winding. Due to the low re-induction, this constant current source can have a smaller control range and thus be less expensive than when carrying out the method according to the prior art.

The device, in particular for carrying out any of the above-described methods, has a superconducting winding on an iron core, a DC source for generating a DC current in the winding, at least one chuck for a billet of an electrically conductive material and a rotary drive for generating a relative movement between the coil and the clamping device. In one embodiment, the value of the DC current generated in the winding by the DC power source is set so that the relative permeability of the iron core is reduced at least in the region of the winding relative to the currentless state of the winding (claim 8).

If the device has at least one further rotationally driven clamping device, then the clamping devices can optionally or alternatively be driven in opposite directions and preferably with approximately the same angular velocity in terms of absolute value (claim 9). For example, the clamping devices may have correspondingly controlled drive motors. Alternatively, at least two jigs can be driven by a common motor. A gearbox with drives running in opposite directions and with the same angular velocity in absolute terms transmits the engine power to the clamping devices.

Alternatively or additionally, the apparatus may have means for determining the respective re-induction voltages produced by the time-varying induction currents in the billets. By a control, which evaluates the previously determined return inductive voltages, the rotary drives of the jigs are controlled so that subtractively superposing the respectively induced by the billets Rückinduktionsspannungen (claim 10). For example, the position of the billets to each other and / or the relative movement of the billets to each other can be controlled by the controller.

The iron core used can be a rod in the simplest case. At both ends of the rod, a billet can be moved, in particular rotated, relative to the magnetic field emerging from the rod. The magnetic inference takes place through the free space.

Better the iron core used can be an at least approximately C-shaped yoke. An at least approximately C-shaped yoke has an air gap between two Pole shoes of the otherwise annular cross-sectionally closed yoke in which the billet can be rotated. Such an iron core allows good guidance of the magnetic flux through a billet to be heated. Compared to the rod, the magnetic inference is also due to the iron core.

According to a preferred embodiment, the iron core is an approximately E-shaped yoke, each with an air gap between the center leg and the respective end leg for receiving a billet. The winding is preferably arranged on the middle leg. Such an iron core makes it possible to heat two billets simultaneously with only one winding and also to guide the magnetic return flow through the iron core. For this purpose, in each of the air gaps per a billet is moved relative to the magnetic field, preferably rotated in the air gap.

Preferably, the iron core consists at least partially of layered sheets. This reduces possible eddy currents in the iron core. Correspondingly, the eddy current dissipation power that heats the iron core decreases and the measures for cooling the iron core can be smaller. At the same time, the possible heat input from the iron core to the superconducting winding is reduced.

Particularly preferably, the sheets are at least partially layered approximately orthogonal to the plane in which the current induced in the billet flows for the most part. This allows a good guidance of the magnetic field with low eddy current losses.

The cross section in the region of the winding is preferably chosen to be smaller than outside the winding. This further reduces the reinduction.

Fig. 1

die Ansicht eines Induktionsheizers,

Fig. 2a

ein Magnetsystem eines Induktionsheizers mit einem stabförmigen Eisenkern,

Fig. 2b

eine Seitenansicht des Magnetsystems aus Fig. 2a,

Fig. 3a

ein Magnetsystem mit einem C-förmigen Joch als Eisenkern,

Fig. 3b

das Magnetsystem aus Fig. 3a in der Frontansicht,

Fig. 4a

ein Magnetsystem mit einem E-förmigen Joch als Eisenkern,

Fig. 4b

das Magnetsystem aus Fig. 4a in der Frontansicht und

Fig. 5

ein Beispiel der Rückinduktionsspannung als Funktion des Wicklungsstroms.

Reference to the drawings, the invention will be further explained. It shows in each case schematically simplified and by way of example:

Fig. 1

the view of an induction heater,

Fig. 2a

a magnet system of an induction heater with a rod-shaped iron core,

Fig. 2b

a side view of the magnet system Fig. 2a .

Fig. 3a

a magnet system with a C-shaped yoke as the iron core,

Fig. 3b

the magnet system off Fig. 3a in the front view,

Fig. 4a

a magnet system with an E-shaped yoke as iron core,

Fig. 4b

the magnet system off Fig. 4a in front view and

Fig. 5

an example of the return induction voltage as a function of the winding current.

The induction heater in Fig. 1 serves to heat a billet 10 by rotating the billet 10 in a magnetic field generated by a magnetic system 50. For this purpose, the billet 10 is clamped between a right and a left pressure element 2a and 2b of a clamping device and by a motor 1 rotatably driven. A gear 3 connects the motor shaft with the shaft of the movable in the direction of the two-sided arrows jig 2a.

The magnet system 50 may, as in Fig. 2a and 2b shown greatly simplified, a DC powered superconducting winding 60 on a rod-shaped iron core 55.2 include. Between the winding 60 and the iron core 55.2 is an insulation element 61, for example a vacuumized cavity, which reduces the heat input into the winding 60 (only Fig. 2b ). The rod-shaped iron core 55.2 carries the magnetic field (not shown) generated by the DC-powered winding 60, which exits the two end faces 56.2, 57.2 of the iron core 55.2 as if from a lens and enters the billets 10 located there via an air gap. When the billets 10 are moved in the magnetic field, eg rotated, the magnetic flux changes relative to the billet 10 and an induction current is induced in the billet 10. The induction current in the billets 10 in turn generates a further magnetic field which superimposes with the magnetic field generated by the winding and induces a voltage in the winding 60 back. In order to operate the superconducting winding 60 with optimum efficiency, the time variation of the current flowing through the winding 60 is preferably zero, ie, w _wi ( t ) = 0. However, due to the time-invariant back-induced voltage, İ _wi ( t ) ≠ 0. The back induction can be reduced by feeding the winding 60 with a direct current which preferably lowers the relative permeability to shortly before the saturation region. Then, when the magnetic field generated by the induction current superimposed additively with the magnetic field generated by the winding 60, the additional field strength due The low relative permeability of the iron core 55.2 is not or only poorly guided by the iron core 55.2 to the winding 60, but extends substantially "unguided" from. Correspondingly smaller is the change in the magnetic flux through the winding 60 and thus the back-induced voltage.

In another embodiment, the magnet system 50 may consist essentially of a C-shaped iron core 55.3 with a preferably HTSC winding 60 (FIG. Fig. 3a and 3b ).

The winding 60 is fed by a regulated DC power source 80. The iron core carries the magnetic field thus generated, which is symbolized by the black arrows (only Fig. 3b ). Unlike the embodiment according to Fig. 2 the magnetic inference is not due to the free space but through the leg 57.3 ( Fig. 3b ). At least one billet 10 to be heated is located between the two legs 56.3, 57.3 of the iron core 55. Differently than illustrated, the billet 10 to be heated is generally not exactly cylindrical and is usually not rotated exactly around its cylinder axis. Accordingly, the surface of the billet 10 penetrated by the magnetic flux and thus the back induction varies, as a result of which the current through the superconducting winding also varies. As previously described, the back induction is reduced by appropriate selection of the value of the DC current with which the winding 60 is fed. The sectional area of the iron core 55.3 at right angles to the magnetic field symbolized by the black arrows is reduced in the area of the winding 60 in comparison to the corresponding areas of the legs 56.3, 57.3. Evident is the reduced thickness d _{wi of} the iron core in the area of the winding compared to the thickness d _{f of} the free legs. This further reduces the relative permeability of the iron core in the region of the winding. Alternatively, the iron core 55.4, as in Fig. 4a and Fig. 4b shown, also be E-shaped. Between the free legs 71 and 72 or 72 and 73 is ever a bag into which a billet 10 is introduced. On the middle free leg 72 sits a coil with a HTSC winding 60, of a only in Fig. 4b is shown fed regulated DC power source 80. Essentially, the iron core 55.4 is comprised of laminated sheets 58 stacked orthogonal to the plane in which the current induced in the billets 10 flows.

Fig. 5 shows the calculated reinduction voltage U _ind in volts as a function of the winding current I _wi from 120 kW heating power by rotation of a billet in the field of a winding on an iron core having 3000 turns, with a uniform change in the rotational frequency of the billet relative to the winding within 1s 8 Hz. For small currents (eg, I _wi ≈ 50 A), the return induction voltage has its maximum value of about 220 V. With increasing current I _wi , the reverse induction initially decreases greatly in magnitude. An increase in the current I _wi for example, about 15 A to I _wi ≈ 65 A lowers the back-induction voltage U _ind magnitude to approximately 100 V.

Above about 80 A causes a further increase in the current only a relatively small reduction of the return induction voltage U _ind . For example, increasing the current I _wi from about 80 A to about 100 A merely reduces the reinduction voltage by about 20 V. The optimum operating range for the induction heater is between about 60 A (-180,000 ampere turns) and about 80 A (~ 240,000 ampere turns), especially at about 70 A (~ 210,000 ampere turns), because then the relative permeability of the iron core has a value only allows a relatively low re-induction, but at the same time still sufficient so that the iron core leads the magnetic field generated by the superconducting winding to the billet.

Claims (15)

1. A method for inductive heating of a billet (10) made of an electrically conductive material by rotating the billet (10) relative to a magnetic field which is generated by means of at least one superconducting winding (60) which is supplied with direct current and disposed on an iron core (55.2, 55.3, 55.4), characterized in that the winding (60) is supplied with a direct current which has a value which generates a magnetic flux density in the iron core (55.2, 55.3, 55.4) at least in the region of the winding (60) in which the relatively permeability of the material of the iron core (55.2, 55.3, 55.4) is lower than in the current-free state of the winding (60).
2. A method according to claim 1, in which at least two electrically conductive billets (10) are heated by rotation of the billets (10) relative to the magnetic field which is generated by at least superconducting winding (60) on an iron core (55.4), which winding is supplied with direct current, with a temporally varying induction current being generated in each billet (10) which causes one back-induction voltage each in the winding (60), characterized in that the movement of the billets (10) relative to one another is controlled in such a way that back-induction voltages superpose in a subtractive manner.
3. A method according to claim 2, characterized in that the billets (10) are rotated in opposite directions with respect to each other.
4. A method according to claim 2, characterized in that the position of the billets (10) relative to one another is controlled in such a way that the back-induction voltages superpose in a subtractive manner.
5. A method according to claim 2, claim 3 or claim 4, characterized in that the billets (10) are rotated with an angular velocity which is approximately the same according to the amount.
6. A method according to one of the claims 1 to 5, characterized in that the value of the direct current is controlled by the winding (60) to a substantially constant value.
7. A method according to one of the claims 1 to 6, characterized in that the cross section of the iron core (55.2, 55.3, 55.4) is chosen smaller in the region of the winding (60) than outside of the winding (60).
8. An apparatus for the inductive heating of at least one billet (10) made of an electrically conductive material, comprising at least one superconducting winding (60) on an iron core (55.2, 55.3, 55.4), a direct-current source (80) for generating a direct current in the winding (60) and at least one clamping apparatus for the billet (10) which is rotatably driven relative to the winding (60), characterized in that the value of the direct current generated in the winding (60) by the direct current source (80) is set in such a way that the relative permeability of the iron core (55.2, 55.3, 55.4) is reduced at least in the region of the winding (60) relative to the current-free state of winding (60).
9. An apparatus according to claim a for the inductive heating of at least two billets (10) made of an electrically conductive material, comprising at last two clamping apparatuses which are rotatably driven relative to the winding (60), in which one of the billets (10) can be clamped, characterized in that the clamping apparatuses are driven in opposite directions.
10. An apparatus according to claim 8 or 9 for the inductive heating of at least two billets (10) made of an electrically conductive material, comprising at last two clamping apparatuses which are rotatably driven relative to the winding (60), in which one of the billets (10) can be clamped, characterized in that the apparatus comprises means for determining the back-induction voltages each caused by temporally varying induction currents in the billets (10), and the apparatus comprises a control device which controls the rotary drives of the clamping apparatuses in such a way that the respectively caused back-induction voltages are superposed in a subtractive manner.
11. An apparatus according to claim 9 or 10, characterized in that the clamping apparatuses are rotated with an angular velocity which is approximately the same according to the amount.
12. An apparatus according to one of the claims 8 to 11, characterized in that the iron core (55.3) is an approximately C-shaped yoke.
13. An apparatus according to one of the claims 8 to 12, characterized in that the iron core (55.4) is an approximately E-shaped yoke with an air gap each for receiving one billet each between the middle leg and the respective end leg.
14. An apparatus according to one of the claims 8 to 13, characterized in that the iron core (55.4) consists at least partly of layered sheets (58).
15. An apparatus according to one of the claims 8 to 14, characterized in that the iron core (55.3) has a smaller cross section in the region of the winding (60) than outside of the winding (60).

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