Resonant heating

Abstract

It is described an electric heating device (1) comprising at least one resonant circuit (2; 2-1, 2-2, 2-3) including arranged in parallel a capacitor means (C) and an induction means (L) -specifically an induction coil or a wired induction lattice- to be magnetically coupled to at least one electrically conducting magnetic object (3; 3-1, 3-2, 3-3) for developing heat in the object ascribed to heat producing losses due to a combination of eddy currents, hysteresis losses and associated non-linear effects, and a power supply (4) connected to the at least one resonant circuit (2; 2-2, 2-2, 2-3) for providing an output signal (Vin, Iin) thereto, whereby the resonant circuit (2) is configured to be brought in parallel resonance by the output signal at a resonance frequency creating a parallel resonant current (ILR) in the induction coil (L) for inducing the heat producing losses in the object (3).

Description

[0001]RESONANT HEATING The present invention relates to an electric heating device and an electric induction method which uses the 5 electric heating device. Such an electric heating device is generally known. In particular it is known to electrically heat an induction coil core by means of a power supply for providing an AC 10 output signal to the induction coil which transfers its developed core heat to an object to be heated, such as a cooking ring or a hot plate. The AC output signal may create hysteresis and eddy currents in the coil core which also results in the wanted heat generated in the object. 15 It is a disadvantage of the known electric heating device that the amount of heat generated and actually transferred to the object to be heated is limited due to a low efficiency which makes the device suitable for a 20 limited amount of practical applications. It is an object of the present invention to provide an electric heating device and an associated heating method showing an improved heat generating efficiency. 25 Thereto the electric heating device has the features that it comprises: - at least one resonant circuit including arranged in parallel a capacitor means C and an induction means L, 30 specifically an induction coil or a wired induction lattice, to be magnetically coupled to at least one electrically conducting magnetic object for developing heat in the object ascribed to heat producing losses due to a possible combination of eddy currents, hysteresis losses and / or associated non-linear effects, and - a power supply connected to the at least one resonant circuit for providing an output signal thereto, whereby the resonant circuit is configured to be 5 brought in parallel resonance by the output signal at a resonance frequency creating a parallel resonant current in the induction means for inducing the heat producing losses in the object. 10 In the electric heating device according to the invention use is made of a parallel resonant LC circuit wherein upswing current oscillations exited by the output signal of the power supply are used to generate a magnified resonant current through the induction means L, 15 specifically an induction coil or a wired induction lattice. In working condition of the device according to the invention the mentioned induction means L is magnetically coupled to the electrically conducting magnetic object to be heated. Therefore hysteresis, eddy 20 currents and / or non-linear heating effects are developed in the object resulting in a magnified boost of the amount of heat created therein. As in general the quality factor Q of the resonant parallel LC circuit will be higher than 1 during resonance the resonant coil current will be Q -being 25 the quality factor- times larger than the resonant input current flowing from the power supply to the LC circuit. This means that more resonant / magnified current dependent heat will develop in the object and that the maximum magnetic induction B in the object material can be reached 30 at a lower resonant input power of the power supply taken from the mains. Or if that maximum of B is not yet reached it can be reached during resonance at a lower device input power and associated lower power costs. A further advantage of the device according to the invention in case the resonant condition is being maintained is that the amount of heat generated, in the capacitor means C and -more general- in the induction means L particularly an induction coil or a wired induction lattice as well as in the 5 connections or connecting cables to these parallel LC means, is minimal. This is due to the maximum value of the so called dynamic resistance Rdyn at resonance which minimizes the current to be input. Meanwhile the maximum blind power is being exchanged optimally between the 10 parallel connected L and C every cycle of the resonance period. Consequently it is an advantage of the electric heating device according to the invention that the 15 efficiency at resonance will be improved, which also widens the application and commercialization possibilities of the device according to the invention. Advantageous concerning the magnetic coupling between 20 the induction coil / lattice and the magnetic object is that the dimensions of the induction coil / lattice can be adjusted and their outline can be made conform to the practical applications at hand. Of further importance is that the magnetic coupling does not require any fixtures to 25 the object to be heated. Furthermore it is possible but not required for a good magnetic coupling between the coil lattice and the object to insert core material between its windings, as holding its winding(s) in the vicinity of the magnetic object already provides a sufficient magnetic 30 coupling. This is among other practical applications important for example if rails or switches embedded in rail transport systems which are already provided with signaling and safety systems have to be deiced or heated to melt clinging snow or ice. These systems are therefore not influenced and do not require any modification if the electric heating device according to the invention is being used simultaneously with such systems. 5 In an embodiment of the electric heating device according to the invention the device is characterized in that the output signal provided by the power supply has at least one harmonic having a frequency capable of exciting the resonance in the at least one resonant circuit. In a 10 further embodiment of the electric heating device according to the invention the output signal provided by the power supply has further harmonics for exciting additional resonance(s) on at least one higher resonance frequency in the at least one resonant circuit. 15 If one harmonic in the frequency spectrum of the power supply output signal excites the LC circuit at or near its resonance frequency it will start to resonate at that – basic- frequency creating a possible combination of 20 hysteresis, eddy currents and / or associated non-linear heat developing effects. However higher harmonics may be able to simultaneously excite resonance at the frequency of such higher harmonics as well for creating more types of possibly partial resonances leading to still more heat 25 developed in the magnetic object. Such multi resonant action by the resonant LC circuit is possible because any mentioned induction means shows capacitor properties at higher frequencies and any capacitor means shows inductive properties at higher frequencies too. These –so called 30 parasitic or distributed- inductances and capacitances can interact at such higher frequencies and simultaneously form additional series and / or parallel resonances superposed on the basic resonance frequency thus creating extra high frequency effects and wanted heat in the object. A further embodiment of the heating device according to the invention is characterized in that the power supply is provided with at least one transformer having a core 5 comprising ferrite, which at least one transformer is a voltage transformer having a primary to secondary winding ratio N:1 where N is a number larger than 1, whereby the secondary winding(s) provide(s) the output signal of the power supply to be input to the at least one resonant 10 circuit. Power supplies of the type having a transformer in general share the advantage that their input and output are isolated which is often wanted or even required in 15 practice. For the operation of a power supply its energy is stored in the magnetic field of the transformer. The power supply input energy may be transferred directly from its input via the transformer to its output. That the heating device according to the invention has a power supply with a 20 ferrite core transformer is a choice made to allow higher operating frequencies up to several tenths to hundred or more kilohertz in the power supply which has the associated advantages of –to summarize- high efficiency, low transformer losses in particular at high frequencies and 25 reduction in sizes and material costs. And for the resonant circuit this means that at such higher frequencies resonance can be secured with smaller values and sizes of coils and capacitors while handling the same power level required for the induction coil to effectively heat its 30 magnetic object. But more importantly the ferrite core transformer chosen is a voltage transformer having a primary to secondary winding ratio N:1 where N is larger than 1. Consequently the operating voltage in the power supply is reduced to a lower output voltage and its output current which is input to the resonant circuit is increased. Furthermore this increased input current is magnified Q times due to the resonance by the resonant circuit, which 5 even further increases the induction coil current and heat developed in the core / object by the induction coil. That the transformer is a voltage transformer having a primary to secondary winding ratio which is larger than 1 reduces the power supply output voltage to a safer lower 10 level and secures that some minimum level of up-transformed current to the resonant LC circuit can easily be provided by a moderate power supply against minimum costs. It is further established by the inventor that the voltage transformer secures a more stable and effective 15 resonance of the LC circuit, as its excitation is more purely sinusoidal having less higher harmonics. Furthermore the resonance is less dependent on the kind of power supply and especially on the type of output stage and the spike output waveforms provided by the power supply. 20 A still further embodiment of the heating device according to the invention, which has the advantage that a variety of power supplies is applicable, is characterized in that the power supply (4) is an AC or DC power supply, 25 for example a SMPT (Switched Mode Power Supply), such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) power supply or an IGBT (Insulated-Gate Bipolar Transistor) power supply, in particular a DC pulsed IGBT power supply. 30 Another embodiment of the heating device according to the invention is characterized in that the IGBT power supply is a DC pulsed power supply which generates a DC pulsed output signal having a duty cycle adjustment range whereby adjustment of the duty cycle, which influences its respective harmonic output frequencies and their amplitudes, excites the resonance frequency of the resonant circuit. The choice of a possible application dependent fine 5 tuning capability of the value of the resonance frequency which is to a large extend determined by the capacitor means and the inductive means can easily be matched to the mentioned exciting output frequencies of the power supply by adjusting the duty cycle accordingly. This is an 10 advantage of the device according to the invention as the choice of for example higher operating frequencies for transformers normally applied in for example SMPS –Switched Mode Power Supplies- allow the application of lighter transformer cores, lesser windings and less voluminous 15 transformers therein. At the other hand the lower the operating frequency for a given number of primary transformer windings is the lower the transformer hysteresis losses are and the larger the output energy is which creates, by means of the inductive coil / lattice, the 20 wanted heat in the magnetic object. At present the heating device according to the invention will be elucidated further, together with their additional advantages, while reference is being made to the 25 appended drawings. In the drawings: Fig. 1 shows a schematic view of an electric heating device according to the present invention; Fig. 2 shows an example of a general B-H magnetisation curve; 30 Fig. 3 shows an illustration an example of a B-H magnetisation curve for a steel core containing magnetic object to be heated; Fig. 4 shows a parallel arrangement of transformers which separately feed respective resonant circuits meant for a variety of applications with several electric heating devices according to the invention, and Fig. 5 shows a lattice type of windings of an induction coil applied in a resonant circuit of the electric heating device according to fig. 1. Fig. 1 shows an electric heating device 1 which comprises a resonant circuit 2 including arranged in parallel a capacitor means C and an induction means L, specifically in the form of an induction coil or a wired induction lattice. In use the induction means L –here shown as a coil L- is magnetically coupled to an electrically conducting magnetic object 3 for developing heat in the object 3. The device 1 further comprises a power supply 4 connected to the resonant circuit 2 for providing a signal on the output 5 to the input 6 (both indicated schematically) of the parallel resonant circuit 2. By assigning proper values to both the capacity of the means C and the self inductance of the induction means L the resonance frequency fres and its angular frequency ωres = 2πfres = (LC)-1 / 2are well known determined in the ideal case of neglected ohms losses in the LC circuit 2. As in practice the induction and capacitor means L and C respectively are not ideal, for later reference a resistor R in series with the means L is drawn in Fig. 1. In this practical model of the parallel RLC resonant circuit 2 as shown the resistor R stands for ohms losses given this practical case also occurring in electric connections and connecting cables, as well as in the capacitor means C and particularly in the one or more windings of the induction means L. Basic calculations of the electric properties of the resulting parallel RLC resonant circuit 2 reveal that at resonance when the input impedance Zinof the circuit 2 seen by the power supply 4 has its maximum and is equal to the so called dynamic ohms resistance Rdyn = L / RC, that setting the imaginary part of Zin equal to zero learns in the practical case that: ωres = {(1 / LC)-(R / L)2}1 / 2If the second term on the right can be neglected relative to the first term then this leads in the simplified case to the above mentioned well known practical formula: ω2resLC = 1 The parallel resonant circuit 2 is known to have the property that virtually at the resonance frequency –in fact slightly above fres- the input impedance Zin has its maximum value Rdyn. At the resonance frequency the input current Iin to the circuit 2 which then is in phase with the input voltage Vin has its minimum. Returning to the practical case mentioned above and defining a quality factor of the circuit 2 by Q = 2π times the maximum energy stored in the circuit divided by the total energy lost in the resistance R per cycle it follows that: Q = {(L / R2C) – 1}1 / 2which in the simplified case leads to the cancelling of the factor minus 1. In a practical embodiment of the device 1 wherein: L = 1,25 µH, C = 2 µF , R = 0.1 Ω and fres= 100 KHz this results in a Q of approximately eight. This means that for this embodiment the current ILR which is the current through the parallel branch of the series arrangement of L and R is approximately eight times larger than the input current Iin. In the parallel resonant circuit 2 current magnification in particular of the current through the coil L takes place if Q > 1 which is due to the mutual exchange of energies between the capacitor means C and the inductive coil L at resonance. These effective energies are CV2in and LI2LR. So at a moderate input current Iinthe resonance magnifies the current ILRthrough the induction coil L in this case eight times which leads to more efficient heat producing losses in the electrically conducting magnetic object 3, which is magnetically coupled with the coil L. 5 These losses in the material of the object 3 are due to a combination of possibly multi resonant frequency eddy currents, hysteresis losses and / or associated non-linear effects arising in the so-called magnetization curve of the object material, which will be elucidated hereinafter. 10 In the practical case it follows for the bandwidth BW defined as fres / Q that BW = R / 2πL. So if the output signal provided by the power supply 4 has at least one harmonic preferably having a frequency somewhere around fres then this frequency -will while in itself being suppressed by 15 the resonating circuit 2- excites a first harmonic resonance therein. The same is true if the output signal provided by the power supply 4 has further harmonics which may be able to excite additional resonance(s) on at least one higher resonance frequency in the resonant circuit 2. 20 For example if the power supply provides a positive DC output block signal having a certain duty cycle then a duty cycle of 50% will according to Fourier analyses, apart from the DC component and the first harmonic frequency having a magnitude of 64% of the amplitude of the block 25 signal, have higher uneven harmonics with respective magnitudes of 21% (third harmonic) and 13% (fifth harmonic) of the block signal amplitude. These higher harmonics could trigger multi resonant higher frequency resonances. The same is true for AC power supplies which could 30 also trigger a multi resonant action in the resonant circuit. Such actions are possible because any induction coil shows capacitor properties at higher frequencies and any capacitor means shows inductive properties at higher frequencies. These –so called parasitic or distributed- inductances and capacitances can interact at such higher frequencies and form additional resonances superposed on the basic resonance frequency thus creating extra high frequency effects and heat in the object (3). 5 The power supply can further be of the SMPT -Switched Mode Power Supply- type, such as a MOSFET –Metal Oxide Semiconductor Field Effect Transistor- power supply or an IGBT -Insulated-Gate Bipolar Transistor- power supply, in particular a DC pulsed IGBT power supply. The power supply 10 4 is provided with one or more –generally power output- transformers 7 each having a core 8 comprising ferrite, which at least one transformer is a voltage transformer 7 having a primary to secondary winding ratio N:1 where N is larger than 1. Consequently the secondary transformer 15 winding(s) provide(s) the more purely sinusoidal low voltage high current output signal of the power supply 4 for input to the resonant circuit 2. Use is made in the ferrite transformers of the favorable electric and magnetic properties of ferrite at higher frequencies allowing the 20 use of smaller inductors, capacitors, transformers and a low number of windings while handling the same output power level. This in turn reduces the size and material costs in the device 1 and results in low losses at high frequencies. Thus the resonance frequencies of the resonant circuit 2 25 may lie between 10–125 KHz, in particular between 25–100 KHz, more particularly between 30-75 KHz. Furthermore the N times higher input current Iin provided by the voltage transformer 7 to the circuit 2 is able to excite a more stable resonance at under circumstances considerable 30 levels, which is useful for some practical applications. Excitation outside the resonance frequency or close to the boundaries of the bandwidth around that frequency should be avoided as that will lead to more heat developed in the means L and C and in the cables and connections. It may be necessary for acquiring more theoretical accuracy in the calculations of total inductance, wanted resonance frequency, developed heat and power(s) to include a measurable or calculable self induction value and / or a mutual induction value of the ferrite core voltage transformer 7 therein. The various sources of the in the object 3 generated wanted losses are due to a possible combination of eddy currents, hysteresis losses and / or associated non-linear effects as outlined above. For the eddy currents to arise the object 3 will be magnetic and electrically conducting, whereby the power Pe of the eddy currents induced in the material of the object is: Pe= KeB2maxf2resτ2with: Kebeing the eddy current coefficient whose value depends on electrical properties –resistivity- of the material of the object 3 to be heated, Bmaxis proportional to the maximum magnetic induction in the object 3 created by a respective resonant current in the induction coil at fres and –if applicable- at higher harmonics of fres, fres is a respective resonance frequency whereon the parallel resonant circuit 2 of the device 1 resonates, and τ depends on the sizes of the object 3 to be heated. As the conducting magnetic object 3 is subjected to a changing inner magnetic induction B originating from a changing outer field strength H due to the resonance current through the induction coil L a voltage is induced in the material of the object 3 and according to Faraday’s law circulating currents are induced therein which are known as eddy currents. These eddy currents cause ohms losses which heat up the material of the object and the developed power Pethereof is given by the above formula. It is clear from this formula that the maximum magnetic induction Bmax and the resonance frequency fres are both squared and consequently, given some material dependent Bmax, a higher value of fres raises the developed heat in the object quadratically; also above the value of Bmax. Apart from the various hardware (ohmic) limitations in the device 1 one other limiting factor of the applicable resonance frequency is the penetration depth of the eddy currents which is due to the so called skin effect which is proportional to 1 / fres1 / 2. This effect forces the eddy currents to the surface of object, but at the other hand allows depth adjustment of the developed heat in dependence on fres. The relation between the magnetic induction B and the field strength H is given by the so called B-H magnetization curve; a general example whereof being given in fig. 2. According to the formula B = µH = µ0µrH the curve represents the magnetic behavior of the material of the object. At zero current and H field the curve starts linearly and then curves to the point where µr that is the relative permeability is equal to 1, and µ0 = 4π 10-7[H / m] being the permeability of vacuum. From thereon the magnetic induction B saturates at Bmax. In general µr turns out to be a non linear multi-valued direction coefficient in this magnetization curve. The time average power Ph of the hysteresis loss induced in the object is given by Steinmetz’s empirical equation having temperature dependent Steinmetz coefficients ƞ, a and b, which is: Ph = ƞ Bbmax fares V with: ƞ being the hysteresis coefficient –generally found empirically from a B-H hysteresis curve fitting whose value depends on the nature of the material of the object to be heated, Bbmax is proportional to the maximum magnetic induction in the object created by a respective resonant sinusoidal current in the induction coil at fres and –if applicable- at higher harmonics of fres, with the value of the superscript b for present magnetic materials close to 2, fares is the respective resonance frequency whereon the resonant circuit resonates, with the value of superscript a generally close to 1, and V is the volume of the material of the object to be heated. The power Ph given by the above formula results from completion of a full turn in the B-H curve; in this case as shown in fig. 2 magnetization in one direction followed by magnetization in opposite direction. In the material of the object 3 this cycle of magnetization involves directing the so called Weiss areas in the material and the rearranging and possible shifting of the boundaries of these areas in one and the other direction. These actions cost energy which is equal to the area of the enclosed magnetization curve and creates heat in the material of the object. In this formula the developed heat progresses linearly with fres but its increase in turn increases the developed heat in the object too. For the B-H hysteresis curve fitting, for example use can be made of –mainly software implemented- nonlinear regression techniques. Concerning Bmax in both above formulae for inter alia a martensitic type of cast steel as shown in fig. 3 µrvaries between 120 and 1800, while B is between 1.8 at saturation and 0.7 [Wb / m2] and H is between 12,000 and 300 [A / m] respectively approximately. Basically all natural substances and materials have magnetic properties, but ferromagnetic materials show higher values of µrand thus higher values of the saturated magnetic Bmaxinduction and as such result in higher powers Pe and Ph. A further heat developing effect in the object 3 is due to the power Pnl of various harmonics components created in the material of the object during the passing through and non-linear relative permeability behavior of the B-H hysteresis curve of the material of the object 3 as shown in the example of figs. 2 and 3. Parseval’s theorem known in Fourier analysis quantifies this power Pnl.During the course of the non-linear B-H curve a sine resonant current and H wave result in a possible frequency doubled B field if µr shows a –possibly partly- quadratic B-H curve behavior. Clearly µr shows an even more advanced behavior leading to multiple higher harmonics resulting in eddy like currents having various frequencies and running on various depth with and against (for negative Fourier harmonics) each other and thus creating several heat sources in the material object 3 while running around the B-H curve with the speed of the resonance frequency / frequencies and passed one saturated B magnetization to the opposite magnetization. When it comes to practical applications of the electric heating device 1 the electrically conducting magnetic object 3 can be or can be part of a melting plot, a hot plate, a melting furnace, a radiator such as for a central heating, a heat exchanger, a defroster or deicer apparatus, a tubing or reservoir for heating a liquid medium or for heating some solid material for example a rails for transport such as for trains, trams or switches. Some higher coil current applications require the use of Litz wires to reduce negative consequences of the skin effect arising at higher frequencies. The rail and switches application will be elucidated hereafter in greater detail. A side or section of for example an I shaped steel rail 3, which is considered to be the object to be heated, is magnetically coupled (shown schematically in fig. 1 with dotted lines) to the induction means L, here being the coil. The coil may be fixed to one 5 or more of the bottom side 3-1, the upper side 3-2, or one or both of the standing sides 3-3 of the rail. The rail may also be movable in transverse direction and then be the tongue part of a switch whereto the coil is fixed. The resonant heat outlined above and developed in the 10 respective coil or respective coils on the one or more sides propagates through the steel material and clears the rails by melting frozen clinging ice or snow. The rail 3 will generally be kept given local weather conditions at a temperature well above freezing point to prevent any 15 freezing. This requires control of the various resonance determining parameters outlined above. Malfunctioning frozen switches can this way be kept or made operative again. In that case the induction coil L or each induction coil L may be given one winding only 20 which is provided with an isolating coating, which winding may be several meters long. The relative permeability of the steel rail 3 adds to a generally limited extend – dependent on the internal structure of the steel molecules- to the self-inductance of the coil L to also achieve 25 resonance at a wanted resonance frequency. Of course the resonance frequency can also be influenced by choosing the appropriate value of the dimensions and number of windings of the coil and the capacity of the capacitor means C. Also in this exemplified application of the device 1 at least 30 part of the shape of the object rail and the outline of windings of the induction coil are chosen conform in shape to optimize the magnetic coupling. In another example the device 1 is used to help electrically heat a conventional gas heated central heating system. This is an example of an application where the induction coil L is removably fixed to a metal convector object 3 of the heating system. The metal of the convector having magnetic properties is magnetically coupled to the 5 coil L which will influence the coils self-inductance which in turn will influence the resonance in line with the formulae given hereinbefore. In this example the coil L may have several windings which are for example elliptical, circular or longitudinal in shape or outline -which shape 10 may be two dimensional or three dimensionally curved (see figs. 4 and 5)- which shortens, centralizes and optimizes magnetic coupling with the applied object concerned. In general the shape of the coil L is as much as possible adapted to the application concerned and to the demands of 15 the wanted magnetic field line pattern. It is also possible to make use of several resonant circuits 2-1, 2-2, 2-3. For example three of these circuits are shown in fig. 4, which are by way of example separately supplied with electrical power through parallel arranged 20 transformers 7-1, 7-2, 7-3 which are shown to be connected to oscillator terminals A and B in possibly one and the same power supply 4. At wish these three transformers could be replaced by one transformer having three separated secondary coils and one primary coil all on the same 25 kernel / yoke. Further advantageously each of the resonant circuits 2-1, 2-2, 2-3 may be individually fine tuned to the shape and in particular the power demands dictated by the appliance object 3 or for example as shown in figs. 1 or 4, by the object sides numerated 3-1, 3-2, 3-3. Given 30 the resonance frequency concerned, e.g. around 40 KHz, or harmonic frequencies the combination of values of capacity C and induction –in particular of the coil L, together with its transformer induction concerned- can be chosen in the circuits 2-1, 2-2, 2-3, possibly together with wanted individual winding ratios here called N1, N2 and N3 for the three transformers 7-1, 7-2, 7-3. This provides freedom of choice and flexibility in particular for fine tuning the power demands of the respective objects. A further example 5 given here is the application of the device 1 on switches in e.g. train rail transport means to be heated where there are fixed rails, and moving rail parts, called tongues, and so called bolts and clamps each having different dimensions, shapes, masses and each requiring different 10 amounts of resonant heat power to be developed therein. And all this with one and the same power supply 4. Fig. 5 shows the induction coil L having at least one winding in the form or a netting, lattice or mesh type of outline here curved in the third dimension. The lattice L 15 has longitudinal and transverse wires being electrically conducting and the wires are mutually electrically connected at nodes K. Said curved outline has to be such that a uniform impedance / resistance is maintained in order to secure a uniform current distribution over the parallel 20 wires of the lattice. Examples of three dimensionally curved outlines are e.g. cylindrical or toroidal. In the outline of fig. 5 the current ILR like in the embodiments described before develops a magnetic field, but now this field emanates from the lattice L mesh windings and the 25 outline of the lattice. The material of these windings may have magnetic properties and / or the shape of the lattice may result in a surrounding magnetic field. But there is more. If the current ILR through the lattice and divided over its wires has a high frequency such as over and in 30 this case between 75-125 KHz than apart from said normal magnetic field around the wires an additional magnetic behavior appears which is due to the fact that the current in each wire begin to experience a skin effect which is accompanied by a magnetic field which lags an electric field in time. For very good conductor wires this means that the magnetic field is very large compared to the electric field and lags in phase by almost 45º. Therefore the field energy in the lattice L is almost entirely 5 magnetic in nature. And combined with the capacitor means C this secures resonance. Ultimately the total field is identifiable with a value for L. The lattice is here also magnetically to be coupled to an electrically conducting magnetic object 3, as 10 exemplified above in relation to the mentioned practical applications of the electric heating device 1. Like outlined before this object has the magnetic properties that it develops therein the mentioned possible combination of heat producing losses. However the lattice windings can 15 also be made of an ohms resistance material. The total resistance value of this lattice L is indicated R in fig. 1. The value of R should be such low –as can be inferred from the calculations above- that resonance can still be established in the resonance circuit 3 concerned. The ohms 20 heat producing losses ILR2R in the lattice L will be transported to the object concerned, and in this embodiment the lattice windings are –apart from magnetically- also thermally coupled to said object to transport the ohms heat thereto. This leads to an alternatively developed heat in 25 the object which heat is now also ascribed to ohms heat producing losses in said lattice windings. The total heat developed is now the sum of the here described ohms heating plus the heat due the heat producing combined losses described earlier. However the quality factor Q will be 30 smaller due to the effects of an possibly deliberately introduced higher value of R. The application of this embodiment may be seen in the heating of for example containers or bottles, or in drying processes. A further embodiment of the application of the lattice mesh of fig. 5 can be found in the use of the resonant heating produced by the lattice L, at wish combined with ohmic heating, where it is for example buried in soil of e.g. sports fields to allow players despite snowy or icy 5 soil conditions to play on an unfrozen underground. The lattice L may be included under for example tiles, or applied in asphalt such as used in runways or ramps, or in walls, ceilings for heating buildings, et cetera. Also soil, plaster, tiles, asphalt and the like are to be 10 considered as objects being to a certain extend electrically conducting and having magnetic properties to substantially allow for example eddy current heating therein. In embodiments concerned, such as also described thus 15 far, the object 3 can also be moist, water or some other liquid, substance or medium, such as snow, ice et cetera wherein the mentioned combination of eddy currents, hysteresis losses and / or associated non-linear effects is capable of producing heat therein directly, and / or 20 indirectly via heat developed in for example the rails. In that case the medium concerned has to allow at least eddy currents therein to flow safely. In the examples of the electric heating device 1 given in this description it is preferred to connect the 25 capacitor C close to the secondary windings of the transformer 4, so that if necessary from thereon cables, which are shown dotted in the resonator circuit 2 of fig.1, may be connected to the inductor means L. The upon resonance magnified current carrying cables may contain 30 theft sensitive copper wires or the cheaper aluminium wires, although the latter has a higher specific resistance which may result in more heat developed in such a cable and will show a lower quality factor of the resonance(s), however its skin depth is larger than that of copper.

Claims

CLAIMS 1. An electric heating device (1) comprising: - at least one resonant circuit (2; 2-1, 2-2, 2-3) including arranged in parallel a capacitor means (C) and an induction means (L), specifically an induction coil or a wired induction lattice, to be magnetically coupled to at least one electrically conducting magnetic object (3; 3-1, 3-2, 3-3) for developing heat in the object ascribed to heat producing losses due to a possible combination of eddy currents, hysteresis losses and / or associated non-linear effects, and - a power supply (4) connected to the at least one resonant circuit (2; 2-2, 2-2, 2-3) for providing an output signal (Vin, Iin) thereto, whereby the resonant circuit (2) is configured to be brought in parallel resonance by the output signal at a resonance frequency creating a parallel resonant current (ILR) in the induction means (L) for inducing the heat producing losses in the object (3; 3-1, 3-2, 3-3).

2. The electric heating device (1) according to claim 1, characterized in that the output signal (Vin, Iin) provided by the power supply (4) has at least one harmonic having a frequency capable of exciting the parallel resonance in the at least one resonant circuit (2; 2-2, 2-2, 2-3).

3. The electric heating device (1) according to claim 1 or 2, characterized in that the output signal (Vin, Iin) provided by the power supply (4) has further harmonics for exciting additional resonance(s) on at least one higher resonance frequency in the at least one resonant circuit (2; 2-2, 2-2, 2-3).

4. The electric heating device (1) according to any of the claims 1-3, characterized in that the power supply (4) is provided with at least one transformer (7; 7-1, 7-2, 7-3) having a core (8) comprising ferrite, which at least one 5 transformer is a voltage transformer having a primary to secondary winding ratio N:1 where N is a number larger than 1, whereby the secondary winding(s) provide(s) the output signal (Vin, Iin) of the power supply (4) to be input to the at least one resonant circuit (2; 2-2, 2-2, 2-3). 0 5. The electric heating device (1) according to claim 4, characterized in that the at least one ferrite core transformer (7; 7-1, 7-2, 7-3) shows a self induction and / or a mutual induction which together with the induction5 of the induction coil (L) contribute(s) to the resonance(s) and the resonance frequency of the at least one resonant circuit (2; 2-2, 2-2, 2-3).

6. The electric heating device (1) according to claim 4 or0 5, characterized in that the power supply (4) is provided with one or more than one ferrite core voltage transformer (7-1, 7-2, 7-3), whereby each of the transformer(s) secondary winding(s) is connected to its conducting magnetic object (3; 3-1, 3-2, 3-3) via its respective 5 resonant circuit (2; 2-1, 2-2, 2-3).

7. The electric heating device (1) according to any of the claims 1-6, characterized in that the power supply (4) is an AC or DC power supply, for example a SMPT (Switched Mode0 Power Supply), such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) power supply or an IGBT (Insulated-Gate Bipolar Transistor) power supply, in particular a DC pulsed IGBT power supply.

8. The electric heating device (1) according to claim 7, characterized in that the IGBT power supply (4) is a DC pulsed power supply which generates a DC pulsed output signal (Vin, Iin) having a duty cycle adjustment range 5 whereby adjustment of the duty cycle, which influences its respective harmonic output frequencies and their amplitudes, excites at least one the resonance frequencies of the respective resonant circuit (2; 2-2, 2-2, 2-3). 0 9. The electric heating device (1) according to any of the claims 1-8, characterized in that the electrically conducting magnetic object (3) is part of a melting plot (3), a hot plate (3), a melting furnace (3), a radiator convector (3) such as for a central heating, a heat 5 exchanger (3), a defroster or deicer apparatus (3), a tubing (3), a container (3) or a reservoir (3) for heating a liquid medium, or for heating some solid material (3) for example a rails (3) for transport such as for trains, trams or switches wherein the heat is used to prevent the rails0 or switches from getting icebound or to clear the material by indirectly melting frozen clinging ice or snow, or whereby the object (3) is moist, water, ice, snow or some other liquid or medium which is possibly also directly heated due to the mentioned heat producing losses and / or5 ohmic losses developed therein.

10. The electric heating device (1) according to any one of the claims 1-9, characterized in that at least part of the shape of the object (3; 3-1, 3-2, 3-3) and the outline of0 windings of the induction coil (L) are conform and brought in close proximity, such that the magnetic field created by the resonant current (ILR) in the windings of the induction coil (L) and the electrically conducting magnetic object (3; 3-1, 3-2, 3-3) magnetically couple.

11. The electric heating device (1) according to any one of the claims 1-10, characterized in that the induction coil (L) has at least one winding having a lattice outline, an elliptical outline, a circular outline or a longitudinal outline, and which at least one winding may be two dimensionally or three dimensionally outlined.

12. The electric heating device (1) according to claim 11, characterized in that the induction coil (L) with at least one winding having a lattice outline has its winding made of an ohms resistance material for additionally developing heat in the object (3; 3-1, 3-2, 3-3) ascribed to ohms heat producing losses in said winding, which winding is also coupled to said object to transport the ohms heat thereto.

13. An electric induction method which uses an electric heating device (1) according to any of the claims 1-12, whereby a parallel resonant current (ILR) flowing in its induction coil (L) induces heat producing losses in an object (3) comprising a conducting magnetic material, which heat producing losses are ascribed to a combination of powers Pe, Ph and Pnl respectively due to eddy currents, hysteresis losses and associated non-linear relative permeability effects, such that: - the power Pe of the eddy currents induced in the material of the object (3) is: Pe = Ke B2max f2res τ2with: Ke being the eddy current coefficient whose value depends on electrical properties of the material of the object (3) to be heated, Bmax is proportional to the maximum magnetic induction in the object (3) created by a respective resonant current(ILR) in the induction coil at fresand –if applicable- at higher harmonics of fres, fres is a respective resonance frequency whereon the parallel resonant circuit (2) of the device (1) resonates, and τ depends on the sizes of the object (3) to be heated; - the time average power Ph of the hysteresis loss induced in the object (3) is given by Steinmetz’s empirical equation having Steinmetz coefficients ƞ, a and b, is: Ph = ƞ Bbmax fares V with: ƞ being the hysteresis coefficient –generally found empirically from a B-H hysteresis curve fitting whose value depends on the nature of the material of the object (3) to be heated, Bbmaxis proportional to the maximum magnetic induction in the object (3) created by a respective resonant current (ILR) in the induction coil at fres and –if applicable- at higher harmonics of fres, with the value of the superscript b for the present magnetic materials close to 2, fares is the respective resonance frequency whereon the resonant circuit (2) resonates, with the value of superscript a generally close to 1, and V is the volume of the material of the object (3) to be heated; and - the power Pnl of higher harmonics is created in the material of the object (3) during the passing through of the generally non-linear relative permeability behavior in the B-H hysteresis curve of the material of the object(3).

14. The electric induction method according to claim 13, characterized in that the resonance frequency of the resonant circuit (2) is between 10–125 KHz, between 25–100 KHz, particularly between 30-75 KHz.

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