GB2048025A

GB2048025A - Induction coil for an induction cooking appliance

Info

Publication number: GB2048025A
Application number: GB8011370A
Authority: GB
Original assignee: Sachs Systemtechnik GmbH
Current assignee: Sachs Systemtechnik GmbH
Priority date: 1979-04-25
Filing date: 1980-04-03
Publication date: 1980-12-03
Also published as: IT1128772B; FR2455418A1; GB2048025B; JPS6342398B2; FR2455418B1; IT8053165V0; DE2916779A1; JPS55154092A; IT8067656A0

Abstract

An induction coil is provided for an induction cooking appliance, having a pancake coil the winding diameter D of which is greater than its axial winding height H. The number of turns of the coil per radial unit of length decreases from an inner zone (15) placed in the middle of the coil to a radially adjoining intermediate zone (17) and then increases again from the intermediate zone (17) to a radially adjoining outer zone (19). In this manner it is possible to generate a homogeneous distribution of heat in the metal bottom of a cooking vessel 11. <IMAGE>

Description

Induction coil for an induction cooking appliance The invention relates to an induction coil for an induction cooking appliance, having a pancake coil the winding diameter of which is greater than its axial winding height.

Induction cooking appliances are known. Their manner of operation is based on the fact that by means of the induction coil, eddy currents are generated in the metal bottom of the cooking vessel deposited above the induction coil and heat the bottom of the cooking vessel. Induction cooking appliances are used predominantly in the kitchen, but can also be used for the heating of pot-type articles of a general nature, for example in workshops etc.

The induction coils of known induction cooking appliances are single-layer or multi-layer disc-type pancake coils. The pancake coils are flat and closely wound from high-frequency strands. They are fitted close beneath a plate of glass ceramic material unaffected by changes of temperature, so that the plane of the coil and the bottom of the pot are parallel with one another.

The known pancake coils of disc form generate a non-homogeneous heating profile in the bottom, especially when high grade steel cooking vessels are used. A nearly cold zone remains in the centre of the cooking vessel bottom, which is adjoined between the centre and the edge of the bottom by an annular zone of high heating. The temperature differences then occurring in the bottom can lead to deformation of the cooking vessel. Furthermore the foods are cooked with different intensities in the zones of different heat.

If cooking vessels are used the bottom area of which is smaller than the diameter of the annular zone of high temperature, then they must be shifted to specific points of the cooking zone since otherwise they do not become hot.

It is the problem of the invention to produce an induction coil for an induction cooking appliance which generates a homogeneous distribution of heat in the metal bottom of the cooking vessel.

This problem is solved according to the invention in that the number of turns per radial unit of length decreases from an inner zone placed in the middle of the coil to a radially adjoining intermediate zone and then increases again from the intermediate zone to an outer zone radially adjoining the intermediate zone.

The number of turns per radial unit of length is preferable at the maximum in the inner zone. Here more than half of the turns of the pancake coil are expediently arranged. The inner zone greatly concentrates the magnetic flux and thus reduces the zone free from eddy currents and thus inductively not heatable to a few square centimetres. The turns of the inner zone are dimensioned so that the annular zone of high temperature generated in conventional induction coils is shifted as far as possible radially inward towards the centre. The turns of the inner zone should generate an approximately trapeziumshaped temperature profile in which the central zone is also heated by thermal conduction. The turns of the intermediate zone and of the outer zone serve for the homogenisation of the temperature distribution in the remaining part of the cooking vessel bottom.

In a preferred form of embodiment the pancake coil is of multi-layer winding in the inner and outer zones and wound with a smaller number of layers, preferably a single layer, in the intermediate zone.

At least a part of the layers of turns of the inner zone and/or of the outer zone can have different radial layer widths with constant pitch of the turns, from layer to layer. In this way it is possible to achieve gradual transitions of the temperature profiles generated in the individual zones. In place of layers of different widths it is also possible for at least one part of the layers of turns of the inner zone and/or of the outer zone to be wound with a distance between turns increasing towards the intermediate zone. In this way likewise the winding density can be modified continuously.

The temperature profile can also be influenced by the distance of the uppermost layerof turns in each case from the metal bottom of the cooking vessel. The eddy currents induced in the bottom are the greater, the closer the coil is arranged to the bottom. A certain minimum distance, which is given by the temperature stability of the insulation of the turns, must admittedly not be understepped. Preferabiy the turn layers of the inner and outer zones placed on the upper side are placed substantially in one plane. The uppermost layer of turns of the intermediate zone is here situated beneath this plane. The radial temperature distribution of the bottom of the vessel can be sufficiently well linearised by adjustment of the distance in the intermediate zone.The distance of the uppermost layer of turns of the intermediate zdne from the plane of the inner and outer zones can be variously dimensioned in the radial direction; it preferably becomes smaller in the radially outward direction.

In a preferred form of embodiment the lowermost layer of turns of the three zones is a single-layer pancake coil extending commonly over all zones, on to which further layers are wound upwards in the inner and outer zones. The common lowermost layer of turns serves as carrier of the layers thereabove, so that it is especially also possible to apply layers with varying turn spacings. The common lowermost layer of turns is preferably bulged out downwards in dished form towards the inner zone. The winding of the inner zone is contained in the outward bulge.

The pancake coil can however on the other hand also consist of three component windings for the individual zones which are separate but connected in series in the same direction. It is an advantage of this form of embodiment that the component windings can be separated and thus wound more simply.

Optimum results can be achieved if the inner zone is wound in four or five layers and has a diameter of about 40 to 50% of the total diameter of the pancake coil. The outer zone can be wound in two layers and have a radial width of about 6 to 10% of the total diameter of the pancake coil.

Examples of embodiment of the invention are to be explained in greater detaii hereinafter with reference to drawings, wherein: FIGURE 1 shows an induction coil, built up from separate component coils, for an induction cooking appliance and FIGURE 2 shows a second form of embodiment of an induction coil.

Figure 1 shows the induction coil of a cooking appliance, formed as rotationally symmetrical pancake coil 1, which is connected through supply leads 3, 5 to a high-frequency generator 7 of the cooking appliance. The diameter D of the pancake coil 1 is greater than its axial height H. The pancake coil 1 is fitted at a distance d beneath a horizontal, non-conductive support plate 9 on to which a cooking vessel 11 is placed with a metal bottom 1 3 consisting especially of high-grade steel. The high-frequency magnetic field generated by the pancake coil 1 induces eddy currents in the metal bottom 13 which heat the metal bottom 13 and thus the contents (not illustrated further) of the cooking vessel 11. The distance dprevents the electrical insulation of the turns of the pancake coil 1 from being damaged by the heat radiation issuing from the metal bottom 13.

The pancake coil 1 consists of three component windings 15, 1 7 and 1 9 connected in series with one another with the same direction of winding.

The component windings 1 5 to 1 9 are each wound from high-frequency strands with turns lying closely against one another and are so dimensioned that a uniform temperature distribution results in the radial direction within the metal bottom 1 3. The component winding 1 5 is wound in a plurality of layers, with six layers in the example of embodiment, and extends from the centre of the pancake coil over about 40 to 50% of the diameter D. The component winding 1 5 alone generates in the metal bottom 13 a temperature profile of approximately trapezium form as illustrated diagrammatically at 21 in Figure 1.The magnetic flux generated by the component winding 1 5 is so great that the zone in the centre not heated by eddy currents is of negligible size and is heated by thermal conduction to approximately the temperature of the radial surroundings. The radially outer marginal zone of the metal bottom 13 is heated by the component winding 19. The component winding 19 is likewise of multi-layer formation and has three layers in the example of embodiment according to Figure 1. The uppermost layer is arranged in the plane of the uppermost layer of the component winding 1 5. The temperature profile generated in the metal bottom 1 3 by the component winding 19 alone is illustrated at 23.

The component winding 1 7 is arranged in the intermediate zone between the inner and the outer zones. It is of single-iayer formation and extends, calculated from above downwards, from the fourth layer of the component winding 1 5 to the third layer of the component winding 19. By reason of the greater distance from the metal bottom 1 3 and the lower number of layers the influence of the component winding 1 7 upon the temperature profile of the metal bottom 1 3 is at the minimum. The component winding 17 is intended to equalise the temperature break between the temperature profiles 21 and 23 of the component windings 15 and 19, so that as a whole a uniform temperature distribution occurs in the radial direction of the metal bottom 13, as illustrated at 25.

An induction coil with a diameter of about 20 cm. can be wound from high-frequency copper strands with about 4 sq. mm. cross-section and dimensioned as follows:- The component winding 1 5 can have four to five layers with a diameter of 8 to 10 cm. The component winding 1 9 can be wound in three layers and have a radial width of 1.2 to 2 cm. The component winding 1 7 can fill out the intervening zone with one layer, the radially outer half of the layer extending at the level of the lowermost layer of the component winding 19 and the radially inner half being bent over to the fourth layer of the component winding 1 5.

Figure 2 shows another example of embodiment of a pancake coil 31 usable as induction coil of an induction cooking appliance, in which a fine correction of the temperature profile is achieved by graduation of the diameters of the layers of turns of an inner component winding 33 and by modification of the turn interval of an outer component winding 35. The pancake coil 31 comprises a rotationally symmetrical winding layer 37 drawn in towards the middle in dish form, which extends from the centre to the outer edge.

The component winding 33 is of multi-layer formation; its further layers are arranged in the trough formed by the winding layer 37. The uppermost layer 39 of the component winding 33 has a smaller diameter than the two layers lying therebeneath of the component winding 33, which has a total of four layers in combination with the winding layer 37. The operation and manner of action of the component winding 33 correspond otherwise to those of the component winding 1 5 in the example of embodiment according to Figure 1.

Beside the winding layer 37 in common with the component winding 33 the component winding 35 comprises an additional winding layer 41 in which a part of the inwardly placed turns 43 is wound with radially inwardly increasing interval.

The component winding 35 corresponds to the component winding 19 of the example of embodiment according to Figure 1.

The common winding layer 37 forms in the region of the trough edge a component winding 45 which corresponds to the component winding 1 7 according to Figure 1. The depth of the trough is dimensioned so that the uppermost layer 39 of the component winding 33 and the layer 41 of the component winding 35 lie in one plane parallel with the bottom of the vessel. The diameter of the uppermost layer 39 of the component winding 33 and the interval of the turns 43 of the layer 41 are dimensioned so that a uniformly linear temperature profile results in the radial direction of the bottom of the vessel.

Claims

1. An induction coil for an induction cooking appliance, having a pancake coil the winding diameter of which is greater than its axial winding height, wherein the number of turns per radial unit of length decreases from an inner zone placed in the middle of the coil to a radially adjoining intermediate zone and then increases again from.

the intermediate zone to a radially adjoining outer zone.

2. An induction coil as claimed in claim 1, wherein the number of turns per radial unit of length is at the maximum in the inner zone.

3. An induction coil as claimed in claim 2, wherein more than half of the turns of the pancake coil are arranged in the inner zone.

4. An induction coil as claimed in claim 1, wherein the pancake coil is wound with a plurality of layers in the inner zone and the outer zone and with a smaller number of layers in the intermediate zone than in the inner and outer zones.

5. An induction coil as claimed in claim 4, wherein the pancake coil is wound with a single layer in the intermediate zone.

6. An induction coil as claimed in claim 4, wherein at least some of the layers of turns of the inner zone and/or of the outer zone have different radial layer widths from layer to layer, with constant pitch of the turns.

7. An induction coil as claimed in claim 1, wherein at least some of the layers of turns of the inner zone and/or of the outer zone are wound with an interval of the turns increasing towards the intermediate zone.

8. An induction coil as claimed in claim 1, wherein the turn layers of the inner zone and of the outer zone which are placed on the upper side lie substantially in one plane and in that the uppermost layer of turns of the intermediate zone is arranged beneath this plane.

9. An induction coil as claimed in claim 8, wherein the distance of the uppermost layer of turns of the intermediate zone from the plane of the inner zone and of the outer zone is of smaller dimensions in the radially outward direction.

10. An induction coil as claimed in claim 1, wherein the lowermost winding layer of the three zones is a single-layer pancake coil extending in common over all the zones, on to which further layers are wound in the upward direction in the inner zone and in the outer zone.

11. An induction coil as claimed in claim 1, wherein the pancake coil consists of three separate component windings for the individual zones, which are connected in series in the same direction.

12. An induction coil as claimed in claim 1, wherein the inner zone is wound in four to six layers and has a diameter of about 40 to 50% of the total diameter of the pancake coil.

13. An induction coil as claimed in claim 1, wherein the outer zone is wound in two layers and has a radial width substantially of about 6 to 10% of the total diameter of the pancake coil.

14. An induction coil for an induction cooking appliance substantially as described herein with reference to the accompanying drawings.