Induction heating coil

Sept. 27, 1966

3,275,784

J. MERRETT

INDUCTION HEATING COIL Filed July 12, 1963

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3,275,784

Patented Sept. 27, 1966

United States Patent Office

3,275,784 INDUCTION HEATING COIL

John Merrett, Gerrards Cross, England, assignor to The Hoover Company, North Canton, Ohio, a corporation of Ohio

Filed July 12,1963, Ser. No. 294,628 Claims priority, application Great Britain, Aug. 9,1962, 30,614/62

Claims. (Cl. 219—10.79)

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. The invention is concerned with cases in which the coil may be required to heat surfaces of different sizes, for example pots of different diameters.

According to the present invention a coupling coil for inducing eddy currents in surfaces, such for example as the bottom walls of pots of different sizes, to heat them, includes two or more sections covering different areas of the surface to be heated and electrically connected in parallel.

Conveniently one section surrounds another, for example, in the case of circular coils the two sections may be concentric. Thus one arrangement of a coupling coil includes two sections wound spirally in the same direction one within the other, with their inner ends connected to the other terminal. In another arrangement, also including two sections wound spirally in the same direction one within the other, the inner end of the inner section and the outer end of the outer section are connected to one terminal of the source of current, while the outer end of the inner section and the inner end of the outer section are connected to the other terminal. In a further arrangement, two sections are wound spirally in opposite directions, one within the other, the inner end of the inner section and the outer end of the outer section are connected to one terminal of the current supply, while the outer end of the inner section and the inner end of the outer section are connected to the other terminal.

The sections may have approximately equal surface heating areas.

The invention is primarily concerned with a boiling ring adapted to operate with pots of different sizes. In order to obtain maixmum heating power with the largest pot it is desirable that the coupling coil should have a diameter roughly equal to that of the largest pot. The current flowing in the coil will depend upon the fact that the coil is magnetically coupled to the bottom of the pot which functions as a short-circuited transformer winding.

If, therefore, a pot is placed on a coupling coil comprising a single winding and the bottom wall of the pot is smaller in area than the coil, the outer portion of the coil will not be coupled to the bottom of the pot and will present an increased inductance thereby cutting down the current not only in this outer portion of the coil but in the inner portion as well. Hence, when a small pot is placed on a large coil of conventional design the heating power will generally be reduced to a negligible amount.

It is an object of the invention to overcome the foregoing difficulty by means of a coupling coil constructed in sections which are connected in parallel. Thus when a small pot is placed on the coil it will cover the inner section which will carry substantially the same current as when a large pot is placed on the coil. The current in the outer section is reduced to a negligible value but this does not affect the current in the inner section.

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

FIGURES 1 and 2 are respectively diagrammatic views of two forms of coupling coils with the sections connected in different ways, and

FIGURE 3 is a sectional view of FIGURE 1 to which has been added a top plate for the coupling coil and on which the pots to be heated are supported.

In the arrangement shown in FIGURE 1 the coil 10 comprises two sections 11 and 12 each of flat annular form wound from narrow conducting tape. The ends 13, 14, of the inner section 11 and the ends 15 and 16 of the outer section 12 are brought out and connected to each other so that the two sections are connected in parallel.

In the arrangement shown, the inner end 13 of the inner section 11 and the inner end 15 of the outer section 12 are connected to a lead 17 while the outer end 14 of the inner section 11 and the outer end 16 of the outer section 12 are connected to a lead 18. The leads 17 and 18 are connected by conductors 19 and 20 respectively to a high frequency generator 21 connected to a source of current 22, and the generator 21 supplies current at a frequency up to 20,000 cycles per second. The top surface of the coil 10 is suitably covered with a thin plate 23 of stainless steel on which the pot to be heated is supported.

In a modified arrangement in order to avoid conductors in close proximity at substantially different voltages, the outer end 14 of the inner section 11 and the inner end 15 of the outer section 12 are connected to one line conductor 19 while the inner end 13 of the inner section 11 and the outer end 16 of the outer section 12 are connected to the other line conductor 20.

In the arrangement shown in FIGURE 1 current flows in the same direction (clockwise or counterclockwise at a given moment) in both sections, so that when the pot is removed from the coil 10 the inductance rises. This has the advantage that when a pot is removed the current is automatically reduced. Moreover with certain types of generators at supersonic frequency it may have the feature, which may be an advantage, that when the pot is removed the frequency drops into the audible range so that a whistling note is heard as a reminder that the current is left switched on with no pot in position.

The arrangement of FIGURE 2 is similar to that of FIGURE 1 except that the coil 24 has its inner section 25 wound counterclockwise while the outer section 26 is wound clockwise, the inner end 27 of the inner section 25 and the outer end 28 of the outer section 26 being connected to a lead 29, while the outer end 30' of the inner section 25 and the neighboring inner end 31 of the outer section 26 are connected together at 32 and joined to a lead 33. Conductors 34 and 35 connect the leads 29 and 33 to a suitable high frequency generator 36 which is connected to a source of current 37. The coil 24 is also supplied with an unshown top plate 23 to support the pots to be heated.. This arrangement avoids conductors in close proximity at substantially different voltages but also has the characteristic that the inductance rises when the pot is removed.

In practice the impedances of the two coil sections may be made approximately equal, and this condition is generally satisfied when the coils have approximately equal surface areas, for example the inner section could have a diameter of five inches and the outer section an external diameter of seven inches. With the sections connected in parallel and a large pot in position on the top plate 23 for the coils 10 or 24 the currents in the two sections will then be approximately equal. If any pot large enough to cover the outer sections is placed in position both sections of each coil 10 and 24 will contribute power and the full output will be produced. With pots that cover the inner sections 11 or 25 but not the outer sections 12 and 26 the current in the outer sections will be negligible and the power output will be reduced to about half its maximum. Accordingly a single type of coil may be employed for

3,275,784 all pots on a cooking stove and it will be unnecessary to provide different sized coils for different sized pots, thus, simplifying manufacture and increasing flexibility for the user.

It will be appreciated that the coupling coils may be divided into more than two sections. Where a number of sections are employed it is possible by selecting a suitable combination of sections wound or connected for current flow in the same direction, and sections wound or connected for current flow in opposite directions, to ensure that no change of inductance occurs when the pot is removed.

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.