Apparatus For Inductors For Induction Heating

Abstract

To braze two platelike elements, in a press, together, particularly to join good heat-conductive metal to stainless steel, for use in cooking utensils, or the like, an induction coil is formed of concentric rings, or of a spiral, and individual coils thereof are placed closer, or farther away from the surface to be heated, by means of adjustment screws, the adjustment of individual coils, or coil portions being carried out in accordance with sensed temperature directly, or differentially sensing temperature along the contiguous surfaces of the plates, by introducing a thermocouple into grooves formed in at least one of the plates and adjusting the height of the coils, or coil portions for minimum temperature difference across the diameter of the plate.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

U.S. Ser. Nos. 54250 and 50274.

The present invention relates to induction heating, and more particularly to methods of, and apparatus for induction heating of flat surfaces, for example, to braze two or more flat metal plates together to obtain a laminated structure.

Induction heaters, to which the present invention relates, are frequently used in the manufacture of cooking utensils, for example, cooking pots, skillets, or the like which have a bottom formed of a laminated structure having one metal layer which is a good conductor of heat, such as copper, or aluminum.

In accordance with the prior art, such inductors are usually hollow and have liquid circulating therein to cool the inductors, They are embedded in a block of plastic, or similar material, which is an integral portion of a brazing press. Thus, upon heating the metal plates to be joined, close to the inductor, and applying compression in the press transmitted by means of a block to the plates of metal to be brazed together, a composite effect of heat and pressure provides for brazed, fused assemblies having a laminated metallic structure.

Inductors used in processes to make such laminated plates are usually in the form of spirals, and particularly screw-type spirals. These spirals are either flat or arranged to be conical, with the tip of the cone centrally located and facing the plates to be brazed. In another form, the inductors are arranged in accordance with a truncated cone.

It has been found that the temperature difference between the maximum and minimum temperatures as measured in the interface is a material factor in the quality of the product to be obtained. At brazing temperature, the temperature differences for a flat inductor, in accordance with the prior art, may be in the order of 70.degree. C.; for a conical inductor, the temperature differences may be in the order of 100.degree. C., the minimum temperature being at the edge of the plate and principally due to the cooling and radiation in the skirt of the cooking utensil. The temperature distribution in the interface with such inductors may also cause a minimum temperature to occur at the middle of the plate, and which may be several tens of degrees less than the maximum temperature which is generally found in the region of a circle having a diameter approximately half of the diameter of the plate to be brazed. Such a temperature distribution gives rise to imperfect fusion of the plates, since in certain regions of the interface close to the minimum, the temperature of fusion may be insufficient; whereas an excessive fusion may occur in the region of the maximum temperature. This difference in fusion results in an eventual laminated structure which is of low-mechanical strength.

It is an object of the present invention to provide a method of operating inductors such that the temperature gradients, or temperature differences between maximum and minimum are low, for example, in the order of .+-.5.degree. C. at the interface; and to provide an apparatus with which the method can be carried out.

SUBJECT MATTER OF THE PRESENT INVENTION

Briefly, a round inductor is provided having either spirals, or concentric loops and, as in the prior art, formed of hollow metallic tubes which are preferably cooled by means of a cooling fluid. In accordance with the invention, the tubes are so arranged that they can be adjusted with respect to a reference plane parallel to that of the plane in which the metals to be brazed are located. In order to adjust the position of the individual loops, or loop portions, perpendicularly with respect to the reference plane, a thermocouple, or other heat-sensitive device is so arranged in the interface between the two plates to be brazed that it can be shifted radially with respect thereto; this can be done, in accordance with an embodiment of the invention, by forming one or more radial grooves in one or the other of the plates to be joined, in which grooves the thermocouple can be displaced, so that maximum and minimum temperatures can be measured; upon such measurement, the height of the individual coil, or spiral portions with respect to the interface plane is then adjusted to make the temperature differences, along a radial dimension, a minimum, so that the temperature distribution across the face of the plate will be essentially even.

In accordance with a feature of the invention, a reference position is selected with a fixed thermocouple; another thermocouple is then movably located, and difference measurements are obtained, for example, by means of an electronic difference amplifier, and the inductor coils are adjusted until the differences are a minimum throughout the diametrical extent of the plate.

In accordance with another feature of the invention, an induction-heating apparatus particularly for use in the manufacture of cooking utensils is provided, in which spiral, or concentric coils are provided which are individually adjustable. After adjustment, the coils, at differential heights, can then be encased and encapsulated in a single block of material so that pressure can be exerted during the brazing step.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIGS. 1a and 1b illustrate, respectively, in schematic form, a fragmentary cross-sectional view of an inductor of the prior art, to fuse a bottom plate on a cooking utensil, and a graph illustrating temperature distribution as a result of the use of the inductor;

FIGS. 2a and 2b illustrate, schematically and in cross section, an inductor arrangement in accordance with the prior art and a temperature distribution graph, respectively, in which the conductor is conical;

FIG. 3 is a schematic cross-sectional view of an inductor in accordance with the present invention;

FIG. 4 is a temperature distribution graph of temperature differences obtained by the inductor in accordance with the present invention;

FIG. 5 is a top view of an inductor useful in accordance with the present invention; and

FIG. 6 is a schematic illustration, to a greatly reduced scale, of an alternative form of inductor coils.

A plurality of coils form a flat inductor 10; the flat inductor is preferably made of copper tubing, the ends 12, 13 of which are coupled to a high frequency generator furnishing heating current; additionally, the interior of the tubes is coupled at 12, 13 to a cooling fluid, such as water, the direction of cooling flow being indicated by the arrows. The entire inductor may be encapsulated in a plastic block which has been left off the drawing for ease of presentation.

A cooking utensil 3, for example, of stainless steel, is to be provided with a heat distribution plate 2 at the bottom; plate 2 is a metal which is a good heat conductor, such as copper, or aluminum. Between plate 2 to be brazed to the bottom of cooking utensil 3, and the inductor 10, a ferromagnetic plate 4, for example, of soft steel may be interposed, this plate 4 being referred to as a distribution plate, or susceptor. The distribution plate 4 functions as a buffer; it limits the temperature close to that of the Curie point of the metal used on the one hand, and further, improves the temperature distribution throughout the extent of the surfaces to be brazed. The interface between plate 2 and the other plate to which it is brazed, that is the bottom of the cooling utensil 2, is usually covered by a layer of metallic alloy facilitating the brazing of the plates together, the metal alloy layer being selected in dependence on the metals to be brazed, is well known in the art.

FIG. 1b illustrates the temperature distribution curve at the interface for an inductor 10 which is spiralled, and flat, applied to heat circular metal plate. The abscissa represents the distance X from the center of the plate 2. The temperature values are shown for aluminum. The ordinate represents the temperature difference .DELTA.T with respect to that at the edge of the plate 2.

Curve 14 shows a dip at the center, two maxima, each approximately half the distance of the radius R of the plate 2. At the circumference of the plate that is at X=R, the curve shows two minima; they are due, primarily, to radiation from the skirt of the utensil 3. The temperature gradient may, as a maximum, approach 50.degree. C./cm.

FIGS. 2a and 2b are generally similar to FIGS. 1a and 1b, respectively, except that in FIG. 2a a conical inductor 11 is shown, in spiral form; and in FIG. 2b curve 15 illustrates the resulting temperature distribution. The temperature differences are even greater than with that of a flat inductor as in FIG. 1a and are indicated on the drawings.

From the foregoing it will be clearly apparent that inductors, as shown in FIGS. 1a and 2a present substantial differences when aluminum is to be joined, by brazing, to a metal of substantially higher melting point such as, for example, stainless steel. If aluminum and stainless steel are to be joined, it is necessary that the temperature differences at the interface of the plates between their maxima and minima be reduced particularly since the fusion temperatures of the flux metal between layers 2 and 3 (and not shown for clarity in the drawing) and of aluminum, or of aluminum alloys, are close together.

A uniform temperature distribution can be obtained, in accordance with the present invention, by an apparatus in accordance with FIG. 3, and by the following method:

A cylindrical housing having a cylindrical wall 21 and a flat plate bottom 20 secured to the cylindrical housing has a central stem 22; the central stem is a rigid material which is also an insulator, such as wood, plasticized paper, ceramic, or the like. The inductor 16, which may be a deformable spiral copper tube, has central turns 17, which are flattened and are held by the stem 22 against bottom 20. Additionally, peripheral spirals 18 are provided, of generally circular cross section and vertically adjustable by means of screw threads formed on vertical stems 24, and adjustable by means of adjusting nuts, with respect to a transverse apertured plate 23, of rigid insulating material. The threaded stems 24 may, for example be brass, soldered at various locations to the turns 18 of the spiral. Plate 23 may, for example, be a laminated composite of fiberglass and silicone resins, secured at its two ends to the cylindrical side wall 21. A cooling grid 60 is arranged below the bottom plate 20, which is preferably formed of copper tubes of rectangular cross section, spirally arranged or laid zigzag backwards and forwards in a serpentine path, and having water circulating therethrough. The copper tubes forming the cooling grid are parallel to each other, and are separated by strips of insulating material.

A plate 5 which is a good heat insulator, that is which is a poor heat conductor, is placed below grid 6. An asbestos cement plate is suitable. A buffer plate, or susceptor 5 of ferromagnetic material, such as soft steel, is located below plate 5 and on top of an aluminum plate 2 to be joined to the bottom of the vessel 3. The aluminum plate 2 has, at its side facing the wall 3, small radial grooves 9, and 9a, just big enough to permit insertion of thermocouples 7, 8, within the groove. FIG. 3 shows the entire arrangement is an exploded view, it being understood that the plate 2 would, in actual operation, be pressed against the bottom 3. Thermocouples 7, 8 sense the temperature at given radial positions across the extent of the plate. A plurality of such grooves may be provided, arranged star-shaped from an origin coinciding with the center of the plate 2, or the grooves may be arranged in a grid network or any other suitable form.

Thermocouple 7 is located in groove 9 such that it is approximately in the middle between the center and the edge of plate 2. Thermocouple 8 is arranged to be moved along the length of the groove 9a, that is from the center towards the end. Output cables of the thermocouples are interconnected to a measuring element 26, known per se. Measuring element 26 may be formed as two separate temperature indicators of identical characteristics and indicating the corresponding temperature at the points where the thermocouples are located; alternatively, a differential amplifier indicating the difference between the temperature from thermocouple 7 and thermocouple 8 may be provided, thermocouple 7 being taken, for example, as a reference.

The bottom of the wall of the vessel to be plated, that is the inside of the vessel is supported by an element of poor heat conductivity, such as a plate of asbestos cement, not shown. Upon application of pressure on wall 21, which will be transmitted to the bottom plate 20, elements (shown exploded in FIG. 3) will compress together against the force of the backup support of plate 3, not shown.

For initial adjustment and arrangement of the inductor, the interface between the aluminum plate 2 and the vessel 3 is not covered with a flux metal. Thermocouple 7 is inserted in groove 9 to be located approximately halfways between the edge and the center of the plate 2, as illustrated in FIG. 3. It supplies a reference voltage corresponding to a reference temperature which is applied to one of the inputs of the measuring element 26, preferably a differential, or center-null reading instrument. Thermocouple 8 is likewise inserted in its groove 9a, and displaced along the length of the groove between the center and the edge of the plate 2 in steps. The output tension from thermocouple 8 is applied to the other input of differential, or null-reading measuring device 26.

As the thermocouple 8 is displaced, the measuring indications derived from meter 26 will be a function of the temperature difference between the reference (thermocouple 7), and the temperature measured by thermocouple 8. If the temperature difference exceeds a certain threshold, then the vertical alignment of the various coils, or coil portions is adjusted by loosening the nuts surrounding the threaded studs 24, and moving the threaded studs up and down, as illustrated in FIG. 3. The sense of displacement of the coils, or coil portions depends on the sign of the temperature difference being indicated on meter 26, that is, if the temperature indicated by thermocouple 8 is higher than that of the reference, the coils are moved farther away, in order to reduce coupling between the coils, or coil portions and the interface. When, however, the temperature drops, the coils portions can be dropped. Of course, the coils are to be moved in such a manner that they are uniformly lifted, or depressed, throughout their diameter, with respect to the axis of the cylindrical wall 21. Adjustment of the coils, or coil portions is carried out in steps, and readjustment after changing a coil portion along the diameter, may be necessary to obtain a maximum temperature difference between the edge, and the center of no more than 10.degree. C., which is desirable for example with certain flux metals.

Plate 2 may have a number of radially, star-shaped arranged grooves 9a into which the thermocouple is consecutively inserted; alternatively, each one of the grooves may be supplied with a separate thermocouple, selectively connected to the difference meter.

Regulation of the temperature may be done at a temperature level which is less than that of the brazing temperature itself, for example, by reducing the power level of the input. The cooling network 6 may then be omitted, or, if present, no cooling fluid need be supplied. The maximum temperature difference measured, and determined by adjustment, must be scaled to the ultimate maximum temperatures which will be encountered. After adjustment, the inductor may be used directly; it is, however, desirable to carefully tighten and secure all adjustment nuts, only the ones on the left-hand side of FIG. 3 having been indicated.

The inductor unit may be encapsulated in a plastic material. If this is desired, the inductor is first adjusted, one with respect to the other, and then the spiral inductor coils are secured, relative to each other, by a resinous material which can harden, such as araldite. Thereafter, the inductor 16 may be separated from rods 24 since their position will be fixed and the encapsulation completed; alternatively, the assembly may be inserted, before or after removal of the rods 24 in an injecting mold for plastic material for injection under vacuum, for example, by means of an epoxy resin or the like.

If the inductor is to be encapsulated in plastic, it is possible that the encapsulation material will deform during the setting or hardening of the plastic material. Before such an inductor is used, therefore, it must be tested for defects. One of the most frequently encountered defects is that the inductor will become slightly oblique with respect to the bottom face 2, or that it will not be exactly centered therewith. In order to test the alignment of the inductor, a thin sheet of a composite of aluminum-polyethylene is applied against the surface of the assembly. High frequency energy is applied for a brief period of time. The high frequency current induced in the aluminum causes the layer of polyethylene to melt at point of maximum induced energy, that is where the aluminum is closest to the inductor coils. Hot spots can thus be determined. The fusion of the polyethylene changes the color of the assembly and the thin sheet will thus give almost a map outline of the heat distribution of the inductor. If the sheet carries an outline of the contour of the inductor block, as cast in plastic, off-center conditions can readily be visualized by the trace made by the melted polyethylene. If the thickness of the trace left by the inductor on the sheet varies, or if the trace disappears entirely in certain places, off-parallel conditions will be indicated between the face of the block and the inductor, as well as the sense of the variation. The inductor, if off-center, may thus be recentered or the inductor, or its covering, may be machined by remachining the face thereof, for example, in a milling machine or the like.

A series of identical inductors may be made by first making a master inductor in accordance with FIG. 3, as above described, and noting the measurement of the respective turns, spaced by predetermined distances from the center, and their distance from plate 23. Once such an inductor has been made, a plastic die, or master can be prepared in which the inductor coils are then laid, the tubes deforming to match the master, so that the windings of the inductor made will be at the same height as those of the first, or master.

The inductor may be of spiral form; or, as seen in FIG. 5, it may consist of a series of circular loop sections, interconnected at an offset, or kink, with next adjacent loop sections. Inductors made of connected circular loops, such as illustrated in FIG. 6, may also be provided, electrical connection, or connection for fluid being indicated schematically; the various turns preferably have their connections offset as seen in the schematic view of FIG. 6. External connections, to power supply and cooling fluid, are obvious and therefore not shown. The individual loops are individually height-adjustable by means of threaded rods, or the like as illustrated in connection with FIG. 3 and not shown specifically in FIG. 6.

Claims

1. Apparatus for manufacturing inductors for induction heating, and particularly for brazing at least two parallel contiguous flat plates together and for said inductors to provide a substantially uniform temperature distribution over the contiguous surfaces of said plates comprising:

two contiguous flat metal plates;

a hollow inductor formed of a plurality of concentric windings each of which is substantially parallel to said plates said inductor having an axis perpendicular to said plates;

means for supplying said inductor with high frequency electric power;

a hollow housing having a flat bottom wall and sidewalls containing said inductor;

a ferromagnetic susceptor plate inserted between said bottom wall and one of said contiguous plates;

plate-shaped means for thermally insulating said bottom wall inserted between said latter and said susceptor plate;

means or measuring the temperature in a plurality of locations between said contiguous plates; and

means for independently adjustably fixing the distance of predetermined portions of said windings within said housing from said bottom wall so as

2. Apparatus as claimed in claim 1, further comprising:

first means for cooling said inductor by circulating a fluid through said winding; and

second means for cooling said housing comprising a platelike element with parallel cooling ducts contiguous with said bottom wall of said housing,

whereby allowing to carry out the adjustment of said winding distances at

3. Apparatus as claimed in claim 1, wherein said adjustable distance fixing means comprise:

an electrically insulating cross plate fixed to said housing sidewalls substantially parallel with said bottom wall;

a plurality of adjustments screws respectively secured to said windings, at one end; and

means for adjustably securing said screws, at their other end, to said cross plate, at least two screws holding opposite points of windings or of

4. Apparatus as claimed in claim 3, wherein said adjustable fixing means further comprise:

a central rod of insulating material, secured with respect to said housing, for supporting the central turns of said inductor in fixed position close

5. Apparatus as claimed in claim 1, wherein said temperature measuring means includes at least one thermocouple movable between said contiguous plates and wherein one of said latter comprises at least one straight groove going from its center to its edge for inserting said thermocouple therein and for the displacement of said latter therealong.