EP2027753A1

EP2027753A1 - Electric heating device with temperature detection through dielectric layer

Info

Publication number: EP2027753A1
Application number: EP07747463A
Authority: EP
Inventors: Simon Kaastra
Original assignee: Ferro Techniek Holding BV
Current assignee: Ferro Techniek Holding BV
Priority date: 2006-05-23
Filing date: 2007-05-23
Publication date: 2009-02-25
Also published as: WO2007136268A1; NL2000081C2; CN200950672Y

Abstract

The invention relates to an electric heating device for heating material such as liquid, wherein the heating device comprises a substantially flat carrier of material with good thermal conduction, on a first side of which material for heating can be placed, an electrically insulating layer arranged on the second side of the carrier, heating conductors arranged on the electrically insulating layer, connecting means for connecting the heating conductors to a source of electrical energy and a control circuit connected between the connecting means and the heating conductors, wherein at least one detector electrode is arranged which is in contact with the electrically insulating layer and connected to the control circuit, wherein the control circuit is adapted to measure the detector current flowing through the detector electrode and wherein the control circuit is adapted to give a signal when the detector current exceeds a predetermined value.

Description

Electric heating device with temperature detection through dielectric layer

The present invention relates to an electric heating device which is provided with a substantially flat carrier of material with good thermal conduction, on a first side of which material for heating can be arranged, an electrically insulating layer arranged on the second side of the carrier, heating conductors arranged on the thermally insulating layer, connecting means for connecting the heating conductors to a source of electrical energy, and a control circuit connected between the connecting means and the heating conductors.

Such heating devices are generally known. They are used for instance as heating element in household appliances for heating water, such as kettles, coffee-making machines and washing machines.

In order to prevent boiling dry and the associated damage and fire hazard, such heating devices are generally provided with temperature-dependent switches, such as switches provided with a bimetal, or with detection means for measuring the ohmic resistance of heating conductors for the purpose of determining the temperature therefrom.

It has been found that there is a need for this purpose of temperature sensors operating on a different mechanism.

The present invention provides for this purpose such a heating device which is provided with at least one detector electrode, which is in contact with the electrically insulating layer and connected to the control circuit, wherein the control circuit is adapted to measure the detector current flowing through the detector electrode and wherein the control circuit is adapted to give a signal when the detector current exceeds a predetermined value. Use is herein made of the leakage current between the heating conductors and the detector electrode. In many cases this leakage current is greatly dependent on temperature. The invention makes use of this dependence to determine the temperature.

According to a first preferred embodiment, the electrically insulating layer is formed at least partly by a dielectric layer, and the detector electrode is separated from the heating conductor by the dielectric layer. Use is made here of the dielectric properties of the layer. Such dielectric materials display this temperature-dependent behaviour, making them particularly suitable for this application. They moreover have the advantage that the relevant currents flowing through the dielectric, and thus through the detector electrode, are small compared to the current flowing through the heating conductor, but not so small that they are difficult to measure.

An attractive construction results when the detector electrode is embedded in the electrically insulating layer. This prevents the detector electrode being connected to the heating conductors via a surface. Dirt and moisture, which could greatly disrupt the temperature measurement, could after all accumulate on this surface.

According to a subsequent preferred embodiment, the electrically insulating layer comprises a first partial layer, the detector electrode is arranged on the first part layer, and the electrically insulating layer comprises a second partial layer in which the detector electrode is embedded. A certain optimization is hereby possible; the first partial layer serves for the thermal conduction to the carrier and for electrical insulation, while the second partial layer can be optimized for an optimal temperature-dependent behaviour.

These advantages emerge more strongly when the second electrical partial layer is formed by dielectric material.

The dielectric layer is preferably manufactured from a material, the characteristic of which has an inflection point between temperature and relative dielectric constant. This inflection point results in an easily detectable change in the detector current.

A subsequent preferred embodiment provides the measure that the control device is adapted to reduce the power supplied to the heating conductor when the detector current flowing through the detector electrode exceeds a predetermined value. This prevents an early response of a thermal switch-off safety assumed to be present in application of this measure. The control according to this measure will after all already respond at a lower temperature, whereby the temperature at which the thermal switch-off safety responds is only reached later and a better temperature control takes place. The control can moreover be set such that the power is increased to the original value when the thus reduced power is insufficient to maintain the temperature at which this control responds, and a better temperature control is thus obtained.

The above stated switch-off safety can be formed by a separate control, but it can also be obtained by making use of the same- configuration as used for the temperature control which responds at a lower temperature. For this purpose a subsequent embodiment provides the measure that the device comprises a first and a second detector electrode which are each embedded in a different dielectric. This measure can be implemented by selecting the dielectrics such that the detector current comprises a discontinuity at a different temperature.

Yet another preferred embodiment provides the measure that the control device is adapted to reduce the power supplied to the heating conductor when the detector current through the first detector electrode exceeds a predetermined value, and to interrupt the power supplied to the heating conductor when the detector current through the second detector electrode exceeds a predetermined value. With this measure a separate temperature control is wholly unnecessary.

According to a specific preferred embodiment, the detector electrode is adapted to conduct electric current in the vicinity of a region in which a part of the heating conductor with a high potential is located close to a heating conductor with a low potential.

The primary object of the detector electrode is to conduct leakage current from the heating conductor to the control circuit. With a suitable geometry the detector electrode can however also have a second function, i.e. to provide a type of temperature- dependent leakage current between two parts of the heating conductor having a preferably greatly differing potential. Such a leakage current from a part of the heating conductor with a low potential, through the dielectric and to the detector electrode, through the detector electrode and from the detector electrode to a part of the heating conductor with a high potential, can reach considerable values when the distances through the dielectric are not too great and when the temperature reaches high values. This current can become so great that the parts of the heating conductor through which this combined current passes reach a temperature such that the conductor melts or burns there. The current is hereby interrupted so that an intrinsic safety is obtained. This effect can be reinforced by arranging a layer of material with good conduction parallel to the detector electrode or incorporated in the detector electrode at those locations where a great potential difference prevails or can prevail between nearby parts of the heating conductor.

More than two electrodes may be applied; this allows to obtain more switch points at different temperatures. The variation in switch points may be obtained through adequate choice of materials, that is such that each of the electrodes is surrounded by material having its inflection points at different temperatures. Other possibilities, like combinations of layers of different materials are not excluded.

The present invention will now be elucidated with reference to the accompanying figures, in which:

Figure 1 is a schematic cross-sectional view of a first embodiment; Figure 2 is a schematic cross-sectional view of a second embodiment; and Figure 3 shows a graph for elucidation of the invention.

Figure 1 shows a carrier 1 which functions for instance as bottom of a container which can be filled with liquid for heating. The top side of carrier 1 is adapted to come into contact with the liquid for heating. In order to make the heat transfer as effective as possible, the carrier is manufactured from material with good thermal conduction, such as steel. An electrically insulating layer 2 is arranged on the underside of carrier 1. Because the thermal energy must be propagated through this insulating layer 2, a material is preferably selected for this layer which combines a high electrical resistance with a low thermal resistance. A detector electrode 3 is formed on this layer 2. This detector electrode 3 is for instance circular. A dielectric layer 4 is placed on insulating layer 2 and over detector electrode 3. This dielectric layer 4 of course has electrically insulating properties and preferably thermally conducting properties. Because this dielectric layer 4 fulfils a function in the detection system according to the invention, the layer has dielectric properties. Arranged on this dielectric layer is a heating conductor 5 with a resistance value such that it can convert the required power from electrical energy to thermal energy. As a result of the dielectric properties of dielectric layer 4 a leakage current will start to flow through the dielectric layer, which will necessarily also flow through the detector electrode, in the case of a potential difference between the detector electrode and the heating conductor.

The heating device according to the invention further comprises a thermal safety circuit 6 which is for instance provided with a bimetal. Other principles can also be applied. Arranged between safety circuit 6 and the heating conductor is a control circuit 7 which is connected to detector electrode 3. Control circuit 7 is adapted to measure the current flowing through detector electrode 3 and to give a warning signal when the current flowing through the detector electrode exceeds a predetermined value. This warning signal can be used to warn the user of the heating device, but can also be used to initiate a function in the appliance of which the heating device forms part. In the present case the signal is used to reduce the power supplied to the heating conductor.

The behaviour of the dielectric layer is elucidated with reference to figure 3. Figure 3 shows a graph of the relation between temperature and leakage current of a layer of dielectric material at a determined potential difference between the electrodes arranged on either side of the layer. This shows that, when a dielectric material of a determined composition is used, the layer has the behaviour that at a determined temperature the leakage current through the material layer suddenly increases strongly in the manner of a discontinuity. This phenomenon is used in the present invention. A continuous line here shows the behaviour of a first composition of the dielectric layer, and a broken line shows the behaviour of a second composition. The discontinuity in question, in the form of an inflection point, occurs at different temperatures.

Finally, figure 2 shows a cross-section of a second embodiment of the invention. This embodiment differs from the first embodiment due to the presence of two detector electrodes and two different dielectric layers manufactured from different materials.

This is shown by the presence of a first dielectric layer 4a with a first dielectric material, which extends over only a part of the surface of the carrier and covers a first detector electrode 3a, and of a second dielectric layer 4b with a second dielectric material, which extends over another part of the surface of the carrier and covers a second detector electrode 3b. The first dielectric material is herein chosen so that it has a characteristic as according to the continuous line in figure 3 and the second dielectric material is chosen so that it has a characteristic corresponding to the broken line in figure 3.

Both detector electrodes are connected to a control circuit 8 which is adapted to reduce the power supplied to the heating conductor when the leakage current carried through the first detector electrode exceeds a predetermined value, and to switch off the heating conductor when the leakage current carried through the second detector electrode exceeds another predetermined value.

A particularly suitable enamel composition for application in a dielectric layer of the heating element, preferably the first dielectric layer, comprises between 0 and 10% by mass OfV₂O₅, between 0 and 10% by mass of PbO, between 5 and 13% by mass of B2O₃, between 33 and 53% by mass of Sid, between 5 and 15% by mass of AI₂O₃, between 0-10% by mass of ZrO₂ and between 20 and 30% by mass of CaO. If desired, the preferred composition also comprises between 0 and 10% by mass OfBi₂O₃. Such a composition results in an enamel layer with an improved durability when used in heating elements. The enamel composition can be melted relatively easily and herein has a favourable viscosity, whereby it can be applied easily to different types of surface. The enamel composition adheres particularly well to metals, in particular to steel, more particularly to ferritic chromium steel, and still more particularly to ferritic chromium steel with numbers 444 and/or 436 according to the American AISI norm. The maximum compressive stress of the enamel layer which can be obtained from the enamel composition lies in the range between 200 - 250 MPa for the new composition. For known enamel compositions the maximum compressive stress generally lies in the range of 70 - 170 MPa. The preferred enamel composition furthermore has a high temperature resistance so that prolonged exposure to temperatures up to about 53O°C, with peak loads up to 700⁰C, does not cause problems. A first dielectric layer on the basis of the preferred enamel composition therefore has little risk of breakdown, in other words is less susceptible to degeneration owing to prolonged load at a high voltage than known enamel compositions. The properties of the enamel composition are furthermore such that the chance of crack formation in a dielectric layer manufactured therefrom is reduced in the case of temperature changes. The preferred enamel composition has the additional advantage that dielectric layers with the desired properties can be applied to the surface for heating in small layer thicknesses. This enhances the heat conduction.

A particular preferred embodiment comprises a dielectric in which at least the lithium and/or sodium and/or potassium content of the first and the second dielectric layers differ from each other. It is advantageous herein if the enamel composition of the first dielectric layer is substantially free of lithium and/or sodium ions. In a preferred composition according to the invention the second dielectric layer comprises at least lithium and/or sodium ions.

In a preferred embodiment the enamel composition comprises between 0.1 and 6% by weight of potassium. Owing to the addition of potassium the load-bearing capacity of the adhesion of the enamel composition to a substrate surface, for instance the surface for heating, is less critical. In an assembly of such an enamel composition with a substrate surface there occurs less deformation at increased temperatures, in particular in the case of overheating. This is particularly advantageous when the enamel composition is burnt into a heating element. The compressive stress is reduced but is still high enough to prevent the undesired formation of hair cracks. At percentages of potassium higher than 6% by weight the chance of hair crack formation has however been found to increase. In combination with the absence of other alkali metal ions, in particular lithium and sodium, a low leakage current at increased temperatures also remains ensured.

The surface for heating, on which the dielectric is arranged, can be manufactured from any heat-conducting material. The surface for heating is preferably manufactured substantially from metal, for instance steel and/or aluminium. Particularly advantageous is ferritic chromium steel, preferably with a chromium content of at least 10% by weight

It is advantageous if the coefficient of expansion of the material from which the surface for heating is manufactured does not differ too much from the coefficient of expansion of the first dielectric layer, for instance no more than 20 to 45%, for instance relative to steel, more preferably no more than 20 to 35%. The coefficient of expansion of the second layer preferably does not differ any more than 0 to 25% relative to that of the first layer. A heating element is thus obtained which has been found to be very well able to withstand temperature changes. Particularly the formation of hair cracks in both the dielectric enamel layers according to the invention has been found to be hereby much less. It has been found that the chance of hair cracks increases again at a difference in coefficient of expansion of lower than 20%. It will be apparent that the coefficient of expansion of an enamel composition can be readily adapted to the coefficient of expansion of the surface for heating by for instance adjusting the alkali metal content Adjusting the potassium content in the enamel composition is recommended here, since the leakage current is hardly influenced hereby at increased temperature. Conversely, it is also possible to choose another material for the surface for heating.

Claims

1. Electric heating device for heating material such as liquid, wherein the heating device comprises: - a substantially flat carrier of material with good thermal conduction, on a first side of which material for heating can be placed;

- an electrically insulating layer arranged on the second side of the carrier;

- heating conductors arranged on the thermally insulating layer;

- connecting means for connecting the heating conductors to a source of electrical energy;

- a control circuit connected between the connecting means and the heating conductors, characterized by at least one detector electrode which is in contact with the electrically insulating layer and connected to the control circuit, wherein the control circuit is adapted to measure the detector current flowing through the detector electrode and wherein the control circuit is adapted to give a signal when the detector current exceeds a predetermined value.

2. Electric heating device as claimed in claim 1, characterized in that the electrically insulating layer is at least partly a dielectric layer, and that the detector electrode is separated from the heating conductor by the dielectric layer.

3. Electric heating device as claimed in claim 1 or 2, characterized in that the detector electrode is embedded in the electrically insulating layer.

4. Electric heating device as claimed in claim 3, characterized in that the electrically insulating layer comprises a first partial layer, that the detector electrode is arranged on the first part layer, and that the electrically insulating layer comprises a second partial layer in which the detector electrode is embedded.

5. Electric heating device as claimed in claim 4, characterized in that the second electrical partial layer is formed by the dielectric layer.

6. Electric heating device as claimed in claim 5, characterized in that the dielectric layer is manufactured from a material, the characteristic of which has an inflection point between temperature and relative dielectric constant.

7. Electric heating device as claimed in any of the foregoing claims, characterized in that the control device is adapted to reduce the power supplied to the heating conductor when the detector current flowing through the detector electrode exceeds a predetermined value.

8. Electric heating device as claimed in claim 6 or 7, characterized in that the device comprises a first detector electrode which is embedded in a first dielectric layer and that the device comprises at least a second detector electrode which is embedded in a second dielectric layer.

9. Electric heating device as claimed in claim 8, characterized in that the first dielectric layer is manufactured from a first material and that the second dielectric layer is manufactured from a second material.

10. Electric heating device as claimed in claim 9, characterized in that the first material has a characteristic between temperature and relative dielectric constant with an inflection point at a first temperature and the second material has a characteristic between temperature and relative dielectric constant with an inflection point at a second temperature.

11. Electric heating device as claimed in claim 10, characterized in that the control device is adapted to reduce the power supplied to the heating conductor when the detector current through the first detector electrode exceeds a predetermined value, and to interrupt the power supplied to the heating conductor when the detector current through the second detector electrode exceeds a predetermined value.

12. Electric heating device as claimed in claim 8, characterized in that the device comprises a first detector electrode which is embedded in a first dielectric layer, a second detector electrode which is embedded in a second dielectric layer and at least a third layer imbedded in a third layer.

13. Electric heating device as claimed in claim 12, characterized in that the first dielectric layer is manufactured from a first material having a characteristic between temperature and relative dielectric constant with an inflection point at a first temperature, that the second dielectric layer is manufactured from a second material, having a characteristic between temperature and relative dielectric constant with an inflection point at a second temperature, and that the third dielectric layer is manufactured from a third material having a characteristic between temperature and relative dielectric constant with an inflection point at a third temperature.

14. Electric heating device as claimed in claim 13, characterized in that the control device is adapted to reduce the power supplied to the heating conductor when the detector current through the first detector electrode exceeds a predetermined first value, to reduce the power supplied to the heating conductor further when the detector current through the second detector electrode exceeds a predetermined second value and to interrupt the power supplied to the heating conductor when the detector current through the third detector electrode exceeds a predetermined third value.

15. Electric heating device as claimed in any of the foregoing claims, characterized in that the detector electrode is adapted to conduct electric current in the vicinity of a region in which a part of the heating conductor with a high potential is located close to a heating conductor with a low potential.