Electric heating – Inductive heating – Specific inductor configuration
Reexamination Certificate
2000-10-05
2001-11-27
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
Specific inductor configuration
C219S632000, C219S676000, C336S062000, C336S065000, C336S195000
Reexamination Certificate
active
06323469
ABSTRACT:
The present invention relates to the field of the induction heating of workpieces, in particular a shaping and clamping system for a flexible inductor.
Induction heating, of metals in particular, is familiar in many applications, such as the adhesive bonding of bodywork parts, and the curing and tempering of workpieces. Conventional inductors comprise water-cooled, rigid copper tubes which are fixed close to the workpiece in holders. Since the heating of the workpiece depends exponentially on the distance between the inductor and the workpiece surface, a dedicated inductor has to be created for each workpiece shape, to which corrections are impossible or possible only with difficulty.
A flexible induction device is known from RWE Energie “Induktive Erwärmung” [Induction Heating] (1991). In this case, in order to fix the distance from a turbine rotor, the latter is initially provided with an insulating layer, and a flexible cable laid loosely in a tube is wound around the turbine rotor, and the said cable is cooled with water during heating of the rotor by the eddy current which builds up in the cable, in order to protect the inductor from the heat conduction which is present in spite of the insulation. In this case, the distance of the flexible cable is defined by the insulating winding applied to the turbine rotor, the tube wall thickness, which is in the centimeter range here, and the irregular position of the electric conductor in the tube. In the known system, in which the product to be heated forms the inductor shape, the distance between the electric conductor and the surface of the workpiece can be set only in the centimeter range, and therefore the thermal efficiency of induction heating is relatively low.
A further significant disadvantage is that the workpiece to be heated can be heated only when at a standstill, as a result of the geometric coupling with the induction system. This circumstance always leads to extensive coil systems, whose impedance matching is very long-winded.
In addition, the distance between the electric conductor in the tube and the surface of the workpiece varies by up to more than ±1 cm over the circumferential length. Water cooling is carried out only at the surface of the electric conductor, and the remaining water channel permits only low flow rates in its cross section, so that only low current densities in the frequency range up to a maximum of 4 kHz can be used.
DE-U 17 33 800 discloses a cooled electric conductor for the induction heating of workpieces, which can be fixed in contact with the wall of a tube; likewise GB 2 122 057 discloses a (non-flexible) inductor whose individual segments can be fixed in variable positions via holders.
Further prior art which should be mentioned is EP 0 789 438 A2, DE 195 04 742 A1 and EP 0 774 816 A2. In each case, the disadvantage is that defined distances between the inductor and the workpiece, and therefore the targeted input of power, are not possible.
The SU-A1 684 940 ultimately shows a flexible water-cooled conductor, but leaves open (omits) how the conductor is kept free from its wall/partition.
The present invention has therefore been based on the object of providing a flexible inductor in which, although the distances to the surface of the workpiece can be maintained exactly, these can be varied in a very simple way in order, by this means, either to reproduce contours or to produce temperature profiles in the workpiece and also to be able to vary them during the heating, as well as to provide more favorable cooling conditions for the electric conductor, in order to be able to operate at current densities up to 180 A/mm
2
and frequencies up to 200 kHz with one and the same conductor system.
According to the invention, this object is achieved with a liquid-cooled or gas-cooled electric conductor for the induction heating of workpieces, comprising a flexible conductor and an electrically insulating tube which surrounds said conductor at a distance, in that the conductor is fixed in the tube so as to be axially parallel and away from the wall of said tube, and can be fixed in variable positions with respect to the workpiece via holders which are separate from one another.
The fact that the conductor is fixed in position within the tube results in significant advantages. On the one hand, constant volume relationships for the flow of the liquid cooling medium are provided, that is to say the heat transport is constant over the length of the inductor. On the other hand, the tube can be bent, it is therefore possible for contours to be reproduced very easily, the holders defining said contours in relation to the workpiece and at a desired distance, so that prescribed temperatures can be maintained very exactly and can also be varied easily.
The flexible conductor is preferably a stranded copper conductor, and it can be fixed in position in the tube via, for example, broken rings pushed onto the conductor or by pins pushed into the tube and welded or adhesively bonded. The tube particularly preferably has internal profiled sections which hold the line [sic] firmly, either coaxially or else parallel to the sides.
The tube itself can have a round cross section or else, if required, an angular, for example square, cross section.
The tube itself is preferably fabric reinforced, in order to keep the wall thickness (≦3 mm) low even at relatively high coolant pressures, which has a positive effect on the dissipation of the heat radiated back from the workpiece. At the same time, the efficiency is of course also increased by the more effective cooling.
The invention further proposes to use, as the conductor, a tube-like hollow stranded conductor, this providing the possibility of applying coolant to the interior of the conductor as well, so that heat can be dissipated from the channel between the outer of the tube and the stranded conductor to the inner cooling medium as well, in order in this way to increase the electrical efficiency further.
For this purpose, the hollow stranded conductor advantageously has in its interior installed fittings which support the tube profile and which of course likewise have to be flexible, for example comprising independent parts that are separate from one another.
It is also proposed to stiffen the inductor with the aid of the cooling medium (water, gas), for which purpose a pressure of about 3-10 bar is maintained in the interior of the tube, the coolant flowing at a speed of, for example, about 2-10 m/sec.
According to the invention, the distance between the inductor and the workpiece is fixed via clamps which surround the tube and are open toward the workpiece and into which the tube can be inserted at any desired point. The distances between the clamps can in principle be selected freely. The clamps are connected to holders, whose position and length are designed to be variable, in order to be able to shape desired contours of the workpiece or the desired heat profile to be produced.
The fact that the inductor is flexible of course means that the set position of the inductor can itself be varied easily even during heating. This opens up the possibility of controlling the position of the clamps via a temperature measurement, in order to withdraw said clamps, for example after a desired value has been reached, for example in order as a result to keep said temperature constant following rapid heating.
This can also be used for automatic control, the clamps in each case being assigned temperature sensors, whose measured value is used to control the setting of the distance of the inductor with the aid of spindle drives.
A particularly elegant fixing of the current-carrying conductor is achieved with a helix, preferably consisting of a plastic filament, which rests on the inner surface of the tube, the cooling medium being led spirally around the conductor. In this case, the helix can be wound around the stranded conductor, but it can also be produced as an internal profile of the tube or can be pushed into the tube as a separate part before t
Bissdorf Richard J.
Harnisch Werner K.
Foley & Lardner
G.H. Induction Deutschland Induktions-Erwaermungs-Anlagen GmbH
Leung Philip H.
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