US2856498A - High frequency electric induction heating systems - Google Patents

Description

@ch 14, 1958' D. s. JONES HIGH FREQUENCY ELECTRIC INDUCTION HEATING SYSTEMS Filad Oct. 24. 1955 ZSheets-Sheet 1 lNvEN-row.

DAv lu GRIFFITHS Jones .BYW W ATTORNEYS Oct. 14, 1958 v D. e. JONES 2,855,493

HIGH FREQUENCY ELECTRIC mnucnon HEATING SYSTEMS Filed Oct. 24, 1955 2 Sheets-Sheet 2 \NVENTOR DAym Gamrrrus Jauss A! ATTORNEYS iijnite States Patent HIGH FREQUENCY ELECTRIC INDUCTION HEATlNG YSTEMS David Griffiths Jones, Cheltenham, England, assignor to Deiapena dz Son Limited, Cheitenham, England Application October 24, 1955, Serial No. 542,395

Claims priority, application Great Britain November 30, 1950 7 Claims. (Cl. 21910.75)

This invention relates to high frequency electric inducticn heating systems and has for one of its objects to provide a. high frequency induction heating system the output of which can be varied by means which are controlled manually or automatically and having the advantage that when controlled automatically there is no appreciable delay in the desired variation of the output.

Another object of the invention is to provide a high frequency induction heating system in which variation of the output is effected by varying the direct current flowing through a control coil mounted on the magnetic core of a transformer of the system, which variation alters the effective permeability of the magnetic core and thereby varies the flux linkage between primary and sec ondary windings of the transformer, increase in the supply of direct current to the control coil lowering the permeability of the magnetic core and in consequence decreasing the output of the induction heating system and vice versa.

By means, therefore, of a single transformer incorporated in the high frequency induction system the output of the latter, which is fed to a work coil, can be varied to suit the requirements of a workpiece being heated by the work coil or maintained at a substantially constant level. This application is a continuation-inpart of my application Serial No. 258,969, filed November 29, 1951, now abandoned.

The supply of direct current for the control coil may be obtained either from a circuit to which the transformer input or output is connected, the required direct current being supplied through a rectifier, or alternatively an independent direct current source may be provided.

In operation, when no direct current is supplied to the control coil the primary and secondary windings of the transformer are closely linked by the flux in the magnetic core so that the transformer output is as high as possible in relation to the input. As direct current is passed through the control coil the effective permeability of the magnetic core is reduced and consequently the flux linkage between the windings is reduced with the result that the value of the transformer output is reduced.

The output from the induction heating system is, therefore, dependent on the value of the direct current in the control coil and can be increased or decreased according to, respectively, a decrease or increase of the direct current supplied to the control coil.

Due to the rapidity with which the output of the induction heating system can be controlled treatment of articles requiring only extremely short heat treatment, for example one or two seconds, can be satisfactorily effected. Thus the means employed for controlling the output current embodied in the present invention have a considerable advantage over the conventional means for varying the current in induction heating systems as such means involve the use of mechanically moving 2,856,498 Patented Oct. 14, 1958 devices which cannot be operated with sufficient rapidity to effect control during short periods of heating.

A still further object of the invention is to provide a high frequency induction heating system which enables adjustment of the output current fed to the work coil to be made to suit the particular requirements of the article being heated whilst maintaining the output current of the generator constant, enables compensation for changes in supply mains voltage to be made, or the maintenance of a high level of output power from the generator to be effected when the physical characteristics of the material being heated alter with temperature.

An important example of the last-mentioned application is in heating magnetic materials such as steel, where the power absorbed from the work coil by the material, when the latter is heated above the Curie temperature, is very much less than it is when such material is below the Curie temperature. If the output current is increased as the temperature of the work rises above the Curie temperature then the power absorbed by the material being heated may be kept at a high level.

Other objects and advantages of the present invention will become apparent from the following detailed de scription of two specific embodiments thereof, when taken in conjunction with the accompanying diagrammatic drawings, in which:

Figure 1 illustrates a high frequency induction heating system including a generator in which an oscillator valve is incorporated, a transformer having a control coil fed with direct current from the source supplying the anode of the oscillator valve, and a work coil fed from the secondary windings of the transformer,

Figure 2 illustrates an alternative arrangement in which the control coil is fed from a rectifier connected across the secondary windings of the transformer,

Figure 3 shows the winding arrangements of the transformer illustrated in Figures 1 and 2, and

Figure 4 is a cross-sectional view of one of the windings of the transformer.

Referring to Figures 1 and 3 of the drawings, a generator for high frequency current is indicated generally at 27, such generator including an oscillator valve 28. A high tension rectifier 29 fed with current from the mains 30 supplies positive direct current to the anode 31 of the oscillator valve 28 through a lead 32. High frequency current from the generator 27 is fed to a transformer, separately illustrated in Figure 3, having a core 10 which in the example shown is of laminated construction and is built up from thin strip magnetic material, although the core may be otherwise constructed from magnetic core material suitable for use at radio frequencies. Such core is of rectangular shape to provide a closed iron circuit and is provided, as shown in Figure 3, with three parallel limbs of which two 11, 12 are constituted by the opposite end members of the rectangle while the third 13 is positioned centrally of the end members. The outer limbs 11, 12 form the transverse portions of U-shaped members 14, 15, the limbs 14a, 15a of such members extending inwardly towards each other so that their free ends abut the opposite ends of the central limb 13 of the core. Thus the three parallel limbs 11, 12, 13 of the core are connected together at opposite ends by parallel side members formed by the limbs 14a, 15a of the U-shaped members.

The primary windings of the transformer consist of two similar coils 16, 17 mounted respectively on the outer limbs 11, 12 of the core, the coils being wound in opposite directions upon the respective limbs and connected in series as shown. A similar arrangement is provided for the secondary winding which comprises two coils 18, 19 arranged in series and mounted respectively on the outer limbs 11, 12. Thus the windings are arranged on the core 10 with a primary and secondary coil on each of the outer limbs thereof. .A control .coil 20 is wound around the third or central limb 13 of the core and is adapted to be connected to a suitable source of direct current.

In the embodiment shown in Figure l 'the primary windings 16, 17, which together with parallel condensers form the tank circuit inductance of the valve oscilla'tor, are fed with current from the generator .27 whilst the output from the secondary windings 18, 19 is fed to a work coil 33. By means of a lead 3% direct current from the cathode .35 of the oscillator valve 28 is fed to the control .coil 20 of the transformer through a filter circuit 36. The opposite end of the control coil 20 is connected by a lead 37 to the negative side 29a of the rectifier .29.

During operation of the induction heating system positive high tension current flows from the rectifier '29 through the lead :32 to the anode 31, from the latter to the cathode 35 and thence through the lead 34 and control coil 20 to the negative side 2% of the rectifier. When absorption of power by the article being heated by the work coil 33 from the secondary windings 18, 19 of the transformer is reduced, with consequential reduction in the current flowing through the primary windings 16, 17 and from the anode 31 to the cathode 35 of the oscillator valve 28, the direct current flowing through the control coil 20 is also reduced with the result that the output of the secondarywindings 18, 19 of the transformer is increased. In consequence of this arrangement, as the rectifier current decreases when the absorption of power fro-m the output is reduced, for example when steel being heated rises above the Curie point, the output current fed 'to the work coil 33 by the secondary windings 18, 19

'is increased, and soincreases the effective output power of the generator. A'variable resistance 38 is arranged in parallel "with the control-coil -20 to enable adjustment of theamount of current flowing through the latter.

If it is desired .to'maintain the output current constant, the control current may be ob'tained by rectifying 'aismall current obtained'from aheoutput voltage. -An increase'in the output current will then supply an increased control current through the control coil 20, which will reduce the flux linkage between input and output circuits, i. e. between the primary and secondary windings 16, 17; 18, 19, and tend to keep the output current constant. Such an arrangement is i-sh'own in Figure 2 which illustrates a circuit in which, as in theprevious embodiment, high frequency current is supplied to the primary windings 16, 17 'by a generator 27. A full wave rectifier 39 is connected .by leads'40, 41 across the output of the secondary windings 18, .19 mounted on the core 10 of 'the transformer. Direct current from the output side of the rectifier 39 is fed through leads 42, 43 to the control coil 20, a variable resistance 44 being incorporated in the lead 43 to control the'proportion of the output current of the transformer which is fed through the control coil. An increase in the output current from the secondary windings 18, .19'williresult in an increase in the current passing through the'rectifier-39 and control coil 20 which will reduce the flux linkagebetween the primary and secondary windings 16, 1-7; 18, 19 and tend to keep the output fed to the work coil 33 constant.

Alternatively, for manual control the value of the control current may be determined by a variable resistance 21 inserted in one of the two leads 22, 23 by which direct current is supplied to the control coil 20 from any suitable source of electrical energy. If it is not desired to utilise the direct current of the generatorof the induction heating apparatus the leads'22, 23 may, for example,

'be connected to the output terminals of a small rectifier.

In order to minimise the pick-up of radio or high -frequency voltage in the control coil "20 the core of' the transformer and the primary and secondary 'windings'1'6,

17; 18, 19 thereof should be constructed as symmetrically as possible. It will be appreciated that, as the primary windings 16, 17 are wound about the respective limbs 14-, 15 in opposite directions, the instantaneous magnetic fluxes induced in the central limb 13 by each primary winding flow through such limb simultaneously in opposite directions so that these magnetic fluxes cancel each other 'and do not induce alternating currents in the control coil 20. To ensure additional stability in the direct current supply to the control coil, an inductance capacity filter 24 is included in the leads 22, 23 to the coil 20 so as to prevent any induced radio or high frequency voltage being fed back into the control circuit.

It has been found in practice that, when the core 10 is of laminated construction, for radio frequency induction heating systems a suitable thickness for the laminae is .002 in., whilst for normal high frequency systems a suitable thickness is .007 in. Where butt joints are formed in the core, such as those formed at 25 by the abutting ends and sides of the U-shaped members 14, 15 and the central limb 13, the abutting faces of the core should be ground to provide smooth mating surfaces and care must be exercised during such grinding operation to avoid burring the ends of the laminae.

in spite of the fact that the materials from which the core is constructed have a low loss factor and that the laminae are formed from thin material, a considerable amount of heat is generated in the transformer and 'has to bedispersed. Losses'due to such heat generation,

as -well'as the size and consequent cost of the transformer,

which contains oil in which the transformer and its "windings-are submerged, the leads to such windings pass- 'ing through the sides of the tank. As shown in Figure *4, either the primary winding 16, 17 or the secondary winding 18, 19, or both of such windings, are constructed of-copper tube through which water is passed for cooling purposes. In circuits in which the primary and secondary windings have a common end connection the water circuits through the windings may be connected in series. Heat generated in the windings is removed directly therefrom by the water the temperature of which rises on'ly slightlyduring its passage through the wind- 'ings, whilst heat generated in the laminated core through hysteresis and eddy current is transmitted to the-oil in the 'tank 26. As soon as the temperature of the oil exceeds that of the water passing through the windings of the trarisformerheat is transferred from the oil through the copper tubes forming the windings to the water and is conducted away thereby. To ensure adequate circulation of-the' oil around the entire surface of the core 10 :the windings should'be well spaced therefrom but this is not disadvantageous since the high voltages normally used demand adequate spacing 'betweenthe windings and the'corc.

I'claim:

1. An induction heating system including a generator for high frequency current, at least one oscillator valve for said generator, aH. T. rectifier which feeds the anode of said oscillator valve, a transformer having a closed iron circuit and provided with three limbs, two primary windings wound in opposite directions and arranged one on each outer limb,two secondary windings wound in opposite directions and -arranged one on each outer limb, a single control coil wound about the central limb of the transformer'toreceive direct current, andrneans for feeding current from said H. T. rectifier to said control coil, whereby as the current of the generator decreases with decrease in the absorption of power from its output the current flowing through the control coil of the transformer also decreases and the output thereof is increased with consequential increase in the effective output power of the generator.

2. A high frequency electric induction heating system comprising in combination a generator for high frequency electric current, a single transformer comprising a magnetic core, primary and secondary windings arranged on said core, a single control coil mounted on said core to receive a controlling direct current, a work coil to which high frequency current is fed by said secondary windings of the transformer the primary windings of which are energised by said generator, and means for varying the supply of direct current fed to said control coil which variation alters the effective permeability of the magnetic core and thereby varies the flux linkage between the primary and secondary windings, increase in the supply of direct current lowering the permeability of the magnetic core and in consequence decreasing the output of the induction heating system and in the current flowing through said work coil and vice versa.

3. A high frequency electric induction heating system comprising in combination a generator for high frequency electric current, a single transformer comprising a magnetic core structure, a primary winding arranged on said core structure and energised by said generator, a secondary winding also arranged on the core structure, a single control coil mounted on the core structure to receive a controlling direct current, a work coil to which high frequency current is fed by said secondary winding, and means, comprising electric circuits which include a rectifier, for variably supplying a portion of the generator output to said control coil, such portion being converted to direct current by said rectifier.

4. A high frequency electric induction heating system comprising in combination a generator for high frequency current, an oscillator valve for said generator, 8. high tension rectifier which feeds the anode of said oscillator valve, a single transformer comprising a magnetic core, primary windings arranged on said magnetic core and fed with current from said oscillator valve, secondary windings arranged on said magnetic core, a work coil to which current is supplied by said secondary windings, a single control coil for said transformer arranged on said magnetic core, and means for feeding current from said high tension rectifier to said control coil, whereby as the current of said generator decreases with decrease in the absorption of power from its output the current flowing through the control coil of said transformer also decreases and the output of the transformer is increased with consequential increase in the efiective output power of the induction heating system and in the current fed to the work coil.

5. A high frequency induction heating system comprising a single transformer having a closed iron circuit and provided with three limbs, two primary windings arranged one on each outer limb of the transformer, two secondary windings arranged one on each outer limb of the transformer, a single control coil wound about the central limb of the transformer, means for variably feeding direct current to the control coil, a valve oscillator the tank circuit inductance of which comprises said primary windings, and a work coil which is fed by said secondary windings, the inductive coupling between said primary and secondary windings, and therefore the current fed to the work coil, being varied by varying the direct current flowing through said control coil.

6. In a high frequency induction heating system, a single transformer having an iron circuit comprising a core of substantially rectangular shape provided with three limbs two of which are constituted by the opposite end members of the rectangle while the third limb is positioned centrally of and parallel to the end members, and parallel side members which connect the three limbs of the core together at opposite ends, two primary windings wound in opposite directions and arranged one on each of said end members, two secondary windings wound in opposite directions and arranged one on each of said end members, a single control coil wound on said central limb to receive direct current, means for variably feeding direct current to the control coil to vary the inductive coupling between said primary and secondary windings and therefore the current fed to the control coil, a valve oscillator the tank circuit inductance of which comprises said primary Windings, and a work coil fed from said secondary windings.

7. A high frequency induction heating system according to claim 5 and further comprising a vessel by which the 0 transformer and its windings is surrounded and adapted to contain oil in which the transformer is immersed, at least one of the windings of the transformer being constructed of tubing through which a liquid cooling medium flows to cool such winding, the heat generated in the transformer being transferred to the oil which in turn is cooled by transference of heat therefrom to the liquid cooling medium flowing through the winding.

References Cited in the file of this patent UNITED STATES PATENTS 1,797,268 Lee Mar. 24, 1931 1,987,730 Cravath Jan. 15, 1935 2,481,644 Callaway Sept. 13, 1949 2,678,419 Bennett May 11, 1954 2,734,164 Knowlton Feb. 7, 1956 2,748,241 Mohr May 29, 1956 2,748,356 Kaehni May 29, 1956

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