US3256417A

US3256417A - Induction heating coils

Info

Publication number: US3256417A
Application number: US294470A
Authority: US
Inventors: Merrett John
Original assignee: Hoover Co
Current assignee: Hoover Co
Priority date: 1962-08-09
Filing date: 1963-07-12
Publication date: 1966-06-14
Anticipated expiration: 1983-06-14
Also published as: NL296248A; GB1048082A

Description

June 14, 1966 J. MERRETT INDUCTION HEATING OOILS 2 Sheets-Sheet 1 Filed July 12, 1963 June 14, 1966 J. MERRETT 3,

INDUCTION HEATING COILS Filed July 12, 1965 2 Sheets-Sheet 2 Fig.4

5-EEEZEWE2JEZZZZZZ2I 6-mmmm'2m 7 mmmm azzlzzzzzzm Fig. 6

United States Patent O 3,256,417- INDUCTION HEATING COILS John Merrett, Gerrards Cross, England, assignorto The Hoover Company, North Canton, Ohio, a corporation of Ohio vFiled July 12, 1963, Ser. No. 294,470 Claims priority, application Great Britain, Aug. 9, 1962, 30,615/ 62 4 Claims. (Cl. 21910.79)

The present invention relates to induction heating and is concerned with the construction of a coupling coil for inducing eddy currents in a surface such, for example, as the bottom wall of a pot, to heat it for cooking.

According to the present invention a coupling coil is wound so that in any section through the axis there are conductors at different distances from the top surface of the coil and comprises a number of separate current paths electrically connected in parallel, each current path having parts of its length nearest to the top surface of the coil and parts farther from it.

In one form of the invention the coil is wound from a strip of braid formed from a number of insulated conductors. The individual insulated conductors may be formed of anodized aluminum wire.

. In an alternative form of the invention a flat coil for heating a flat surface may comprise two or more axially superposed layers each including a number of sections one within the other, and each electrical path may include a section of each layer. Preferably, all' the sections in anyone layer are of substantially equal impedance.

Conveniently the sections arewound from flat strip or tape of conducting material in which case the Width of the tape is preferably not substantially greater than the skin depth at the frequency to be employed.

For heating a pot up to say eight inches in diameter a simple coupling coil wound in a single spiral-from fiat tape conductor might have a thickness between /8 and inch in order to produce suitable heating power.-

When a pot is placed on such a coil energized at a frequency of say 20,000 cycles per second, the current will be concentrated in the top layer of the spiral and in the lower layer of the pot. The restriction of current flow in the pot is an advantage as it increases its effective electrical resistance and hence the power that can be dissipated in it with a given current. On the other hand, the restriction of current flow in the spiral coil is a disadvantage as it considerably increases the power dissipated in the coil and is liable to cause overheating of the coil as well as reducing the efiiciency of generation of heat in the pot Thus the ratio 'of heating power developed in the pot to heating power developed in the coil is proportional to the square root of the ratio of resistivity of the pot to resistivity of the coil, and as it is difficult in practice to make these resistivities very different without introducing costly complexities, it is difficult to ensure that much more than half the heating power is developed in the pot.

An object of the invention is to force the current to fiow uniformly through the depth of the spiral coil, with a considerable reduction in its effective resistance and a considerable increase of overall efliciency.

The invention may be performed in various ways but certain specific embodiments will be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a top view of one form of coil;

FIGURE 2 is a side view of the coil shown in FIG- URE 1;

FIGURE 3 is an enlarged view indicating the braiding of the conductor of FIGURE 2;

FIGURE 4 is a fragmentary view of a coil divided into sections;

Patented June 14, 1966 "ice FIGURE 5 is a diagram of a coil such as that of FIG- URE 4 showing one way of connecting the sections electrically and to which a top plate has been added to support pots to be heated, and

FIGURE 6 is a diagram similar to FIGURE 5 showing another way of electrically connecting the; sections.

In the arrangement shown in FIGURES 1 to 3 the coil 10 is of fiat annular form wound in a spiral from a strip of braid formed of anodized aluminum wire having

ends

11 and 12. The wire may, or example, be .010 inch diameter, of which twenty-four groups, each of two wires, are braided in a conventional fashion. That is to say, as indicated diagrammatically in FIGURE 3, twelve groups of wires 13 are wound in a coarse pitch right hand helix, while the other twelve groups of wires 14 are wound in a coarse pitch left hand helix, each group passing alternately above and below successive pairs of groups of the opposite hand. After forming, such a tubular braid is flattened and forms a strip of thickness about .045 inch and width about .150 inch. This is wound to formthe coil 10. The

ends

11 and 12 of the coil are connected to a high frequency generator 13 connected to a source of alternating current 14, and the generator supplies current at a frequency up to 20,000 cycles per second. The top surface of the coil 10 is suitably covered by a plate 9 of stainless steel on which the pots to be heated are supported.

In the embodiment shown in FIGURE 4 the coil 15 is wound from solid conducting strip. The coil comprises a number of sections each of flat annular form wound from narrow conducting tape. The sections are in four layers identified as 1, 2, 3 and 4 and each layer com-- prises four concentric annular sections A, B, C and D. The sections are electrically connected in parallel in groups of four so as to provide four parallel current paths. As indicated in FIGURE 5 the four groups comprise, respectively, sections A1, B2, C3 and D4 connected in series by leads 16, sections A2, B3, C4 and D1 connected in series by conductors 17, sections A3, B4, C1 and D2 joined by conductors 18 and sections A4, B1, C2 and D3 connected by leads 19. Each of the sections is connected in parallel by

leads

21 and 22 to a high frequency generator 23 connected to a source of current 24. The generator 23 supplies current at a frequency up to 20,000 cycles per second. A top plate 25 of stainless steel for supporting the pots to be heated is shown in FIGURE 5 above the coil 15.

The radial width of each annular portion AB-C and D becomes progressively smaller so that each portion will have approximately equal surface areas for the pots to be heated and also provide substantially equal impedance in each section.

Each section is wound with tape or wire having a width or diameter less than the skin depth at the frequency employed. The skin depth is given in centimeters in free space where p is the resistivity, u is the permeability and f is the frequency. Alternatively the skin depth is given in inches by 6 skin depth would be considerably less than the value given by the above formula according to the method of winding the coil.

The several sections in each layer have approximately the same impedance when coupled to the bottom of a pot placed upon the top plate 25, and the total impedance of each current path will be the same, as the whole coil is symmetrical. This condition is satisfied since each section has the same projected area of winding, assuming the coils are close wound. In these circumstances as all the current paths are connected in parallel across the current supply 24, the input current will divide equally. Provided that the total thickness of the stack of coils is not too great (less than about inch) so that all coils are satisfactorily coupled to the pot, the overall power transfer efficiency can be increased, possibly to something of the order of 60% or even 80%..

Similarly in the case of the braided Winding of FIG- URES 1 to 3 the anodized surface provides insulation between adjacent wires and each Wire meanders from a position closer to the surface than any other, to a position farther from the surface than any other. Thus the shape and position of each wire is practically similar to that of any other, so that the inductances of all the current paths are equal, and equal currents will flow through them all.

In the coil 29 shown in FIGURE 6 each current path is afforded by turns or sections lying between the same radial limits. The paths are afforded by a number of concentric coils, E, F, G and H, each having a section or a number of turns close to the surface of a pot to be heated and

others

6, 7 and 8 farther away but near enough to contribute useful heating. The

layers

5, 6, 7 and 8 of the coil sections E, F, G and H are connected respectively in series by

leads

30, 31, 32 and 33. The sections E, F, G and H are connected in parallel by

conductors

34 and 35 to a suitable high frequency generator 36 which is connected to a source of alternating current 37. The coil 29 is also supplied with a suitable top plate 38 of stainless steel to support the different sized pots to be heated.

The irnpedances of the paths are matched so that when all coil sections are covered by a pot on the plate 38 the currents in them will be substantially equal, and if some only are covered by a pot the currents in them will remain substantially unaltered although the current in the uncovered paths will fall to a negligible value.

While I have shown and described several embodiments of my invention, it is to be understood that those embodiments are to be taken as illustrative only and not in a limiting sense. I do not wish to be limited to the particular structure shown and described but to include all equivalent variations except as limited by the scope of the claims.

I claim:

1. A flat coupling coil suitable for inducing eddy currents in a flat surface such as the metal bottom of a pot to heat it, in which the coil is wound with conductors so that in any section through the axis of the coil the conductors are arranged at different distances from the surface being heated, and including a number of separate current path electrically connected in parallel, each path having parts of its length nearest tothe surface being heated and parts farther from it, and said coil comprising a plurality of axially superposed layers each layer including a plurality of conductor sections one within the other and each current path including a section of each layen.

2. A coil as described in claim 1 in which each layer includes a section in each of a number of annular zones, and all the sections of each such zone are included in the same current path.

3. A coil as described in claim 1 in which all the sections in any one layer are of substantially equal impedance.

4. A coil as described in calim 1 in which the sections are wound from flat strip or tape of conducting material and the width of the tape is not substantially greater than the skin depth at the frequency to be employed.

References Cited by the Examiner UNITED STATES PATENTS 1,342,209 6/1920 Giblin 336-490 1,698,650 1/1929 Parks 336 FOREIGN PATENTS 826,849 4/1938 France. 507,556 9/1930 Germany.

12,923 5/1911 Great Britain.

ANTHONY BARTIS, Acting Primary Examiner.

RICHARD M. WOOD, Examiner.

BENDER, Assistant Examiner,

Claims

1. A FLAT COUPLING COIL SUITABLE FOR INDUCING EDDY CURRENTS IN A FLAT SURFACE SUCH AS THE METAL BOTTOM OF A POT TO HEAT IT, IN WHICH THE COIL IS WOUND WITH CONDUCTORS SO THAT IN ANY SECTION THROUGH THE AXIS OF THE COIL THE CONDUCTORS ARE ARRANGED AT DIFFERNET DISTANCE FROM THE SURFACE BEING HEATED, AND INCLUDING A NUMBER OF SEPARATE CURRENT PATH ELECTRICALLY CONNECTED IN PARALLEL, EACH PATH HAVING PARTS OF ITS LENGTH NEAREST TO THE SURFACE BEING HEATED AND PARTS FARTHER FROM IT, AND SAID COIL COMPRISING A PLURALITY OF AXIALLY SUPERPOSED LAYERS EACH LAYER INCLUDING A PLURALITY OF CONDUCTOR SECTIONS ONE WITHIN THE OTHER AND EACH CURRENT PATH INCLUDING A SECTION OF EACH LAYER.