Inductor For Internal Heating

Abstract

An inductor for internal induction heating wherein the outer working surface only is made of a high electrically conductive material, and the remainder thereof defining a water-cooling passage is made of a less conductive material so that high-frequency electrical current must flow in the working surface which is closely adjacent to the surface to be heated. A conduit having one side made of copper and all other sides made of plastic is employed so that high-frequency electrical current is concentrated in the copper side which functions as the working surface.

Description

The present invention relates to heating, and, more particularly, to induction heating.

The present invention is applicable to heating the inner surfaces of small holes in metal workpieces, especially steel plates, and will be described with particular reference thereto; although, those skilled in the art will immediately recognize that the invention has considerably broader applications. Thus, the present invention is also useful for heating interiors of metal tubes.

The present invention is primarily concerned with liquid-cooled solenoid-type coils, both single turn and multiturn coils which are inserted in openings the surfaces of which must be heated. However, under special conditions and circumstances the invention also has use in other types of coils.

It is a well-known physical fact that electric current will take the path of least resistance. For example, in a laminated structure of two conductors, that conductor which provides the least amount of resistance will carry most of the electrical current that is passed through and along the laminated structure. In unitary structures of homogeneous composition, electric current will take the shortest path since the total resistance will be less for the shortest path. In solenoid coils this phenomenon manifests itself in that electrical current concentrations are greater in the interior of the coil than on the exterior, since the helical path about the interior diameter is shorter than the helical path of the exterior diameter.

Current concentration at the interior of solenoid coils is actually beneficial when a workpiece to be inductively heated is placed within the solenoid coil. Furthermore, when the diameter of the solenoid coil is substantially larger compared to the diameter or dimensions of the conductor forming the solenoid coil, the effects of current concentration are minimized. When designing internal coils (coils that are placed within the workpiece), however, the current distribution or concentration in the conductor has important ramifications. This is particularly true for internal coils of small dimensions in which the conductive path offered by the outer surface of the coil can be twice that of the inner surface. In such instances, the resistance offered by the outer surface can be twice that of the inner surface. The great differences in resistance of the two surfaces manifests itself by concentrating current on the inner surface producing inefficient internal heating.

Recognizing the effects of current concentration in internal coils, it has been suggested that the design of internal coils be altered to minimize losses associated therewith. For example, it has been suggested that rather than employing water-cooled conductors of circular cross section for inductors, that conductors with oblong cross sections be employed with the major axis being oriented parallel to the longitudinal axis of the induction coil. This suggestion merely minimizes the problem by reducing the relative differences between the inside and outside diameters of the internal induction coil. Although attempts were made to overcome the foregoing difficulties, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

The present invention contemplates new and improved apparatus with a method for the operation thereof, which overcomes all the foregoing problems and others, and provides a composite internal inductor which is more efficient in operation.

In accordance with the present invention, there is provided an improved inductor which comprises a tubular member composite conductor having at least two surfaces, one of which surfaces is a working surface and adapted to be in close-spaced relationship to the surface to be heated. The working surface is made of a material of high electrical conductivity while the other surface is made of a material of much lower and preferably no electrical conductivity so that electrical current is concentrated in the working surface.

In accordance with a more limited aspect of the present invention, there is provided induction-heating apparatus including a high-frequency generator and tubular composite inductor electrically connected to the generator, the composite inductor having a working surface made of a highly electrically conductive material while the remainder of the conductor is made of a much less electrically conductive material, whereby electrical current is concentrated in the working surface so that induction heating of a metal workpiece placed near the working surface is more efficient.

There is also provided in accordance with the present invention a method for inductively heating metal workpieces. The method comprises providing a source of high-frequency alternating electric current, and passing the electrical current from the source through a composite conductor which has at least two surfaces, each made of a different material, with one material being more conductive than the other so that electric current is concentrated in a more conductive surface. A metal workpiece is placed adjacent to the more conductive surface with a small airgap therebetween so that the inductor and the metal workpiece are inductively coupled whereby the workpiece is efficiently heated by induction.

The principal object of the present invention is to provide an improved composite conductor to increase the efficiency of induction heating.

Another object of the present invention is to provide an improved composite internal coil for inductively heating internal openings in metal workpieces.

A further object of the present invention is to provide an improved inductor for uniformly heating small recesses within metal workpieces by induction heating.

Yet another object of the present invention is the provision of a method for induction heating interior spaces within metal workpieces.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a pictorial drawing of an induction generator employing a composite coil in accordance with the present invention to heat the interior of a recess in a metal workpiece;

FIG. 2 is a perspective of a preferred embodiment of the present invention; and,

FIG. 3 is a cross-sectional view of the inductor shown in FIG. 2 and taken along the line 3--3.

Referring now to the drawings wherein the showings are for the purpose of illustrating preferred embodiments of the present invention only, and not for the purpose of limiting same, FIG. 1, a pictorial view of the present invention, shows a high-frequency generator 10 with a composite inductor 12, in accordance with the present invention, electrically connected thereto at 14 and 16, and a metal workpiece 18 having a circular opening 20, the walls of which it is desired to inductively heat.

High-frequency generator 10 can take the form of any commercially available high-frequency generator. Thus, high-frequency generators of the motor-generator type, spark gap oscillator type or vacuum tube oscillator type can be employed. Of course, if a highly specialized heat treatment requires the use of ultrahigh-frequency current, or necessitates high-power transfer efficiencies, one type of generator may be selected over the other to suit the particular circumstances.

The leads to the inductor 12 and the inductor itself are in the form of rectangular tubing or conduit so that a coolant can be circulated to cool the inductor and its leads. Provisions for cooling are especially important so as to remove the heat generated by the large currents which flow in the inductor.

Workpiece 18 can be any metal which can be inductively heated. Thus, the workpiece can be steel, iron, stainless steel, aluminum, brass, copper and most other metals. As shown in FIG. 1, the workpiece is depicted as being rectangular, but it will be appreciated by those skilled in the art that it can be of any size or shape. For example, workpiece 18 can be a tube in which it is desired to harden the inner surface while the exterior surface remains substantially unaffected. Moreover, the shape of opening 20 does not have to be circular, as the inductor, in accordance with the present invention, can be varied in shape to suit specific needs.

FIGS. 2 and 3 show a preferred embodiment of the present invention. Inductor 50 is a single loop 52 formed from a rectangular conduit 54, as best seen in FIG. 3. The diameter of loop 52 is selected to fit the internal dimensions of the workpiece being heated, and not out of merely coil design considerations. As best seen in FIG. 3, the conduit has three sides 56 made of a nonconductive material, such as plastic or nylon, and a fourth flat side 58 made of a high conductive material, such as copper. Side 58 functions as a working surface of the inductor, and its location on the inductor is selected so that it can be placed adjacent to the workpiece being inductively heated. Inductor 50 is connected to a source of high-frequency electrical energy, not shown in the drawings by lead 60, and a coolant, such as water, is circulated through conduit 54 to cool inductor 50 during operation.

Composite inductors in accordance with the present invention can be made in any suitable manner. When the inductor is hollow for cooling purposes, the electrically conductive material can be joined with the electrically nonconductive material by any means that insures a leak-proof structure. Thus, a tongue and groove can be employed, or the conductive surface can be affixed to the nonconductive portion by a curable adhesive. Most generally, copper will be employed as the conductive portion of the composite conductor. As noted hereinbefore, plastic, glass and high-resistivity metals can be employed as the nonconductive portion of the composite inductor. Glasses and plastics which can be employed should have sufficient strength so as to resist the stresses caused by unequal expansion of the two different materials as the heat and cool is used. Examples of glasses that can be employed include crown and flint glasses. Plastics which can be employed, although the invention is not limited thereto, including nylons, fluoroplastics, acrylics, polyolefins, and vinyl polymers.

In operation, high-frequency electrical energy is applied to a composite conductor having a conductive working surface and a high resistivity or nonconductive, nonworking portion, whereby electrical current flows primarily or only in the working surface. The working surface is placed adjacent, but not in contact, with the area of the workpiece to be heated so that the inductor and workpiece are inductively coupled to inductively heat the workpiece.

Although the present invention has been described in conjunction with the preferred embodiments, it is to be understood that modifications and variations may be restored to without departure from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered within the purview and scope of the invention and appended claims.

Claims

Having thus described my invention I claim:

1. In an inductor for inductively heating the inner cylindrical surface of an opening in a workpiece, said inductor comprising two leads adapted to be connected to a spaced high-frequency power source, and a single loop connected between said leads and having an outer surface with a diameter slightly less than the diameter of said inner cylindrical surface, said leads and said loop having internal coolant passageways, the improvement comprising, said loop formed from an electrical insulated annular channel terminating generally at said outer surface and an electrically conductive ribbon over said channel and forming said outer surface whereby said coolant passageway of said loop is formed by said channel and said ribbon.