EP0482635A2 - Electromagnetic inductor with ferrite core for heating electric conducting material - Google Patents

Abstract

The present induction-heating device (10) serves for heating electrically conducting materials up to temperatures exceeding 300 DEG C. The device (10) comprises an open core (12) made of a ferric material. A Litz wire coil (15) is wound around the core (12). A power source (17) is connected to the ends (16) of the coil (15) so as to produce an excitation current in the latter, inside a frequency range varying from 12 to 25 kHz, in such a way as to generate a magnetic field when magnetised. Magnetic flux concentrator tubes (18) made of an electrically conducting material are arranged around the coil (15) and near the core (12), and are embedded in a material (19) which conducts heat but does not conduct electricity, for the purpose of maximising the useful flux. A cooling fluid circulates through the concentrator tubes (18) so as to cool the tubes (18), the core (12) and the coil (15). An induction zone is defined by the magnetic field generated between the opposing poles (13, 13') of the core (12) and penetrating the surface of the part to be heated. The part is heated by the eddy currents generated by the variable magnetic field on the surface. …<IMAGE>…

Description

The present invention relates to an induction heater that uses an open core made of ferric material with a coil of Litz wire through which an excitation current flows to produce a variable magnetic field which is concentrated between the open core poles by means of magnetic flux concentrators made of electrically conductive tubes in close contact with a heat conductive but non-electrically conductive material in order to drain the heat generated in the coil and the core, while a coolant is circulated in the concentrator tubes.

Several types of high frequency induction heating devices have been proposed in the prior art. The US patent 4,359,620 represents a good summary of the prior art by describing that one of the problems encountered with many induction heating stations using magnetic cores is that of the high heat losses in their core. This is especially true if the intensity and frequency of the fluctuating magnetic field generated is increased enough to be suitable for welding metal, for example. However, this causes the problem of increasing the core temperature, and the core begins to melt. Cores made of laminated magnetic materials which are used in most transformers have large losses due to eddy currents as well as the resulting film effect at frequencies exceeding 20 KHz. In addition, the conductive nature of the core sheets presents a real danger of electric shock when used in workstations. induction heating having a large amount of power supplied to their excitation coils.

In an attempt to reduce this problem, US Patent 2,785,263 discloses the use of cores made of ferrite. Such a material has a relatively high magnetic permeability and low conductivity and has been found to be an ideal material for use in induction heating stations. However, other problems have arisen as a result of the use of such cores and, more particularly, in order to saturate the poles so that they contribute as much as possible to the density of flux generated in a part arranged between them, it it is necessary to substantially saturate the entire core, this being very inefficient and resulting at high frequencies, in huge heat losses. US Patent 4,359,620 attempts to solve this new problem by using a core construction which concentrates a magnetic field with high flux density between its two ends which are separated very little and tapered. A periodic voltage is supplied to the core and a capacitor is connected to the excitation core so as to form a resonant circuit which is used for controlling the frequency and the phase of the periodic voltage supplied to the circuit in order to keep it in resonance. Again, this patent does not deal with the high heat losses in the core and the problem of the core and the coil which are subjected to high temperatures, thus limiting the magnitude of the intensity of the flux density of the magnetic field generated, which limits the use of the induction heating station due to its low resistance to heat and its lack of constancy in its heating.

The object of the present invention is to develop an improved induction heating device for heating ferromagnetic materials at temperatures rising at least up to 300 ° C., this device overcoming the disadvantages of the prior art mentioned above. -above.

Another object of the present invention is to develop an improved induction heating device for heating ferromagnetic materials to temperatures of at least up to 300 ° C, in which the core is made of ferric material and uses a coil of Litz wire, and in which the improvement resides in magnetic flux concentrator tubes which are positioned around the coil at a short distance from the core while a coolant circulates inside the tubes in order to cool the core and the coil. This allows excitation currents to be applied to the coil in a frequency range of 12 to 25 kHz so that the eddy currents in the magnetic field produced can generate 4 to 20 kW of heat in a conductive surface. electricity and mainly ferromagnetic positioned in the field. The temperature, frequency and power values are only illustrative and in no way limitative.

The present invention also aims to develop an improved induction heating device as described above and, in addition, in which the core and the coil are positioned in a material which conducts heat but which does not conduct electricity. which is a composite material made of epoxy and powdered copper or aluminum.

The present invention also aims to develop an improved induction heat device as described above and in which the core takes the form of an E which forms two opposite poles and a central pole between which a magnetic field is generated, around the central pole, the coil being wound with concentrator tubes being positioned around the coil and near the opposite poles in order to increase the magnetic flux generated between the poles, outside on the surface to be heated .

According to the foregoing objects, an advantageous embodiment of the invention provides an induction heating device for heating an electrically conductive and mainly ferromagnetic material at temperatures ranging at least up to 300 ° C. The device includes an open core made of ferric material, a coil of Litz wire wrapped around the core, a power source connected to the coil to produce an excitation current in the coil within a range frequency varying between 12 and 25 kHz in order to generate a magnetic field when magnetized. Magnetic flux concentrator tubes made of an electrically conductive material are positioned around the coil and near the core in a heat conductive but non-electrically conductive material. A coolant circulates in the concentrator tubes to cool the core and the coil. An induction zone is defined by the magnetic field generated between the opposite poles of the core and orientable near an electrically conductive surface in order to heat this surface electromagnetically by means of eddy currents generated between the opposite poles of the core and concentrated therebetween by the concentrator tubes.

• Les figures 1 et 1A sont des vues en section illustrant la réalisation d'un dispositif de chauffage par induction suivant la présente invention;
• la figure 2 est une vue en perspective illustrant la configuration du dispositif de chauffage par induction de la figure 1;
• la figure 3 est une vue en perspective illustrant l'utilisation du dispositif de chauffage par induction de la présente invention et, sur cette figure, plusieurs de ces dispositifs sont positionnés de façon rapprochée le long d'un rouleau chauffeur de calandrage tel qu'utilisé dans une machine à papier afin de sécher une feuille de papier continue;
• la figure 4 est une vue en bout de la figure 3, et
• la figure 5 est une vue en plan illustrant le positionnement des inducteurs le long du cylindre de chauffage.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings in which:

• Figures 1 and 1A are sectional views illustrating the embodiment of an induction heating device according to the present invention;
• Figure 2 is a perspective view illustrating the configuration of the induction heater of Figure 1;
• FIG. 3 is a perspective view illustrating the use of the induction heating device of the present invention and, in this figure, several of these devices are positioned close together along a calendering heating roller as used in a paper machine to dry a continuous sheet of paper;
• FIG. 4 is an end view of FIG. 3, and
• Figure 5 is a plan view illustrating the positioning of the inductors along the heating cylinder.

Referring now to these drawings, and more particularly to FIG. 1, there is generally shown at 10 an induction heating device according to the present invention which is shown here as being spaced a little apart from the surface of a calendering roller 11 d 'a paper machine so as to heat the ferromagnetic material positioned on the outer surface of the calendering roller. The heater comprises a ferrite core 12 which has the shape of an E providing opposite arms 13 and 13 'and a central leg 14 around which a coil 15 of Litz wire is wound. The coil 15 has terminals 16 to which a controllable power source 17 (see Figure 2) is connected so as to supply an excitation current to the coil in a frequency range from 12 to 25 kHz.

The improvement of the induction heating device according to the present invention lies in the contribution of magnetic flux concentrator tubes 18 which are positioned around the coil 15 very close to the core 12. The concentrator tubes 18 are positioned in a conductive material heat but not electrically conductive 19 and are spaced from the core and the coil. One end of the tubes 18 is electrically isolated from the side plates 22a or 22b illustrated in FIG. 1A. The material 19 is a composite of an epoxy or a synthetic resin generally, and of copper or aluminum powder which is positioned in the enclosure 20. The enclosure 20, as illustrated in FIG. 2, is a rectangular enclosure formed from a powdered ceramic material and fiberglass. A layer of aluminum paint 21 is applied to the induction surface of the enclosure which is positioned a short distance from the electromagnetic surface to be heated so as to reduce the heat transfer by external radiation with return to the surface d induction 21 of the enclosure 20. A metal shield 22, 22a, and 22b is also positioned in the enclosure 20 and, as illustrated here, against the upper wall and the two side walls of the latter in order to electromagnetically shield the inductor.

As illustrated in Figure 2, a pressurized water supply 23 is used for the circulation of cooling water through the magnetic flux concentrator tubes 18 so as to cool the core and the coil in the enclosure 20 heated by Joule effect on the surface of the tubes and inside the coil, and heat from the surface of the workpiece. This cooling effect allows the application of an excitation current in a high frequency range varying between 12 and 25 kHz, from where the induction heater 10 can generate approximately between 4 and 25 kW of power while the coolant keeps the internal temperature of the enclosure below 60 ° C, these values being non-limiting. The concentrator tubes 18 also concentrate the magnetic field produced between the poles 24 and 14. The inductance of the core also varies between 40 and 125 μH depending on the dimensions of the core used and the frequency of the selected source, these values being nonlimiting.

Now with additional reference to Figures 3 to 5, there is shown a typical application of an electromagnetic induction heating device according to the present invention. As illustrated here, a plurality of heaters 10 are alternately positioned, offset and side by side along a heating calendering roller 30 of a paper machine (not shown). The heaters 10 are spaced apart from the roller 30 as illustrated in FIG. 4 and are stationary with respect to the roller 30. Their specific spacing and their mutual relationship make it possible to obtain a controlled temperature along the width of the roller. These heaters 10 can also be supplied with electrical power or parallel power in series alignment or individually. We also contemplate the installation of temperature sensors (not shown) to detect the temperature along the surface of the roller 30 and their use for the individual control of power sources so as to vary the excitation current of their respective coil. in order to individually control the heat generated by these inductors, so as to obtain the required temperature arrangement along the calendering roller.

Although FIGS. 3 to 5 relate to an application in the manufacture of paper, it is pointed out that these induction heating stations have a multitude of other applications and they could, for example, be used in other industries for laminating or glazing a sheet-like material. The efficiency of this heating device has also been calculated as being in the order of 95% when calculated by the proportion of useful heat generated relative to the electric power used. For example, in the application of the calendering roller, the heating devices according to the present invention can generate approximately 250 kW of heat per meter of length of the electrically conductive material used in the construction of the calendering roller.

It is understood that the present invention is in no way limited to the embodiments described above and that any obvious modifications made to them remain within the scope of the invention, provided that these modifications fall within the scope of the appended claims.

Claims (11)

An induction heater for heating an electrically conductive and mainly ferromagnetic material to temperatures of at least up to 300 ° C, said device comprising an open core made of ferric material, a coil of coiled Litz wire around said core, a power source connected to said coil to produce an excitation current in said coil within a frequency range varying between 12 and 25 kHz in order to generate a magnetic field when magnetized, tubes magnetic flux concentrators made of an electrically conductive material are positioned around said coil and near said core in a heat conductive but non-electrically conductive material, a cooling fluid circulating in said concentrator tubes in order to cool said core and said coil, an induction zone is defined by said magnetic field generated between the opposite poles of said core and orientable near an electrically conductive surface in order to heat said surface electromagnetically by means of eddy currents generated between said opposite poles of said core and concentrated therebetween by said concentrator tubes. An induction heater according to claim 1 wherein said core is an E-shaped core and made of ferric material having high magnetic permeability, said core having opposite arms the ends of which constitute said opposite poles and a central leg around which said spool of Litz wire is wound. An induction heater according to claim 1 wherein said core is supported in isolation in an enclosure constructed of an electrically non-conductive material, said concentrator tubes also cooling the interior of said enclosure. An induction heater according to claim 3 wherein the material of said enclosure is a composite of moldable ceramic and fiberglass, said enclosure having an aluminum paint (non-conductive) on at least one induction surface to to reduce the heat transfer by radiation with return to said induction surface. An induction heater according to claim 4 wherein said enclosure is a rectangular enclosure having a lower induction surface formed according to the geometry of the surface of the heated workpiece, and a metal shield in at least one upper wall and two side walls of said enclosure for the electromagnetic shielding of said inductor. An induction heater according to claim 1 wherein said heat conductive but non-conductive material Electricity is a composite material comprising synthetic resins and copper powder. An induction heater according to claim 1 wherein said heat conductive but non-electrically conductive material is a composite material comprising synthetic resins and aluminum powder. An induction heater according to claim 1 wherein said concentrator tubes are connected to a liquid circulation system for cooling said Joule-heated heater on the surface of said tubes inside said coil, and to which the external environment contributes. A heating system for heating a movable surface made of an electrically conductive material, said system comprising a plurality of induction heaters according to claim 1, said heaters being positioned across the direction of movement of said electrically conductive material electricity from its opposite edges. A heating system according to claim 9 wherein said movable surface is an outer surface of a heating roller used in the heat treatment of film-like materials. An induction heater according to claim 1 wherein said heaters each have a rectangular shaped induction surface, said induction surfaces of said plural heaters being alternately positioned, offset and side by side along said heater roller.

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