EP1678985B1 - Heating element for electric hot plates and method for producing such a heating element - Google Patents

Description

The invention relates to a heating element for electric heating plates, in particular for glass ceramic plates, which is formed as a band-shaped elongated strip with a length L _R , a height h _R and a thickness s _R of electrically conductive material with a specific resistance ρ _R and standing upright in the position of use is mounted on a thermally insulating pad, wherein the strip on the pad side facing tabs having a height h _s and a width b _s , which interlock via an engagement depth e of a portion of the height h _s in the pad, between which Tabs gaps with a height h _s and a width b _{F are} formed, which form over the distance height h _s minus engagement depth e ventilation gaps.

The invention further relates to a method for producing a heating element for electric heating plates, in particular glass ceramic plates, wherein a strip having a length L _R and a height h _{R is} cut out of a strip-shaped electrically conductive material having a specific resistance ρ _R and a thickness S _R , which has on one side tabs with a height h _s and a width b _s and therebetween gaps having a height h _s and a width b _F.

The specified heating elements are used in particular for kitchen appliances, wherein the band-shaped elongated strip is arranged spirally or meandering edgewise on a base, wherein in addition to the spiral or meandering arrangement, a corrugation of the strip is common to the corresponding length of the strip on the smallest possible To accommodate surface to optimally distribute the heat over the heating surface. By connecting the heating element to the supply voltage, a current flow is forced through the strip, whereby it begins to glow with the emission of heat. Above the heating element is preferably a transparent shield, such as a glass ceramic plate, arranged on which the respective cooking vessel can be placed.

For example, US 3 991 298 A describes a heating element which consists of a fabric-like heating element.

Usually, the strip at the facing the pad Side tabs and between the tabs ventilation gaps on. Through this ventilation, a minimization of the heat extraction is achieved by the support of the strip on the substrate. For example, US 5,477,031 A describes such a heating element with a plurality of spaced apart mounting tabs.

According to the desired rated power of the heating element, the strip must have a corresponding resistance. This resistance depends on the specific resistance of the used electrically conductive material of the length, which is usually determined by the size of the hearth and is dependent on the cross section of the strip. Thus, with a fixed length of the strip and the specific resistance ρ _R determined by the material of the strip, the cross-section of the strip required for a given nominal power and thus for a given thickness of the strip, the height thereof can be determined and such a strip produced accordingly. Due to manufacturing tolerances and tolerances of the thickness of the strip, the resulting electrical resistance does not necessarily coincide with the desired electrical resistance, whereby deviations from the desired heating power occur.

The aim of the present invention is to provide a heating element of the type specified above, which also has the most accurate tolerances of the material from which the strip is produced, resulting in a nominal resistance as accurate as possible in a desired nominal power, even in the reality occurring. The heating element should be as simple and inexpensive to produce.

Another object of the present invention is to provide an above-mentioned method for producing a heating element with which the most accurate nominal resistance of the heating element and thus the highest possible nominal power can be achieved. The method should be as simple and inexpensive to carry out.

The first object according to the invention is achieved in that the height h _{R of} the strip is determined according to the relationship h _R = ρ _R * L _m / (s _m * R _T ) * ε in accordance with the resistance R _T and / or relative to a defined measuring length L _m . or is dimensioned according to the mean thickness s _{m of} the strip to obtain a predetermined resistance R _N corresponding to a desired rated power P _{N of} the heating element, where ε is the resistance reduction factor taking into account the tabs. In the specification according to narrow width tolerances of the strip material from which If the strips are produced, it is also possible to recalculate the material thickness via the resistance measurement. This can be dispensed with a material thickness measurement, without exceeding predetermined error limits. The heating element according to the invention is thus characterized by a height of the strip, which is dependent on the actual conditions of the material from which the strip is made, which is the actual resistance, measured over a defined measuring length and the measured average thickness of the strip dependent becomes. A fine adjustment of the desired resistance of the strip over changing the total length of the strip is not necessary. This would require a high additional manufacturing costs. In the heating element according to the invention, the relevant parameters for the total resistance are adapted to the real conditions, whereby manufacturing tolerances have no effect on the total resistance and thus the desired rated power of the heating element more.

In this case, the adjustment of the height h _{R of} the strip can be effected in that the height h _R 'of the strip above individual or in groups arranged gaps between the tabs is smaller than the height h _{R of} the remaining strip. This will change the resulting resistance of the strip. For example, every second gap can be executed with a correspondingly smaller height h _R '.

A change in the height of the strip may also be achieved by having the strip on the opposite side of the tabs with incisions of a different height than the remaining height h _{R of} the strip.

These incisions may, for example, have a wavy shape. Apart from the precise adjustment of the resistance of the strip by these cuts, a changed annealing behavior is achieved by different current densities at the upper edge of the strip.

According to a further feature of the invention it is provided that the tabs and gaps have rounded corners. The radius of curvature in turn has an influence on the total resistance and can therefore also be changed to compensate for manufacturing tolerances.

Preferably, the strip is formed of a metal alloy.

With regard to the method, the object of the invention is achieved by measuring the resistance R _T related to a measuring length L _m and / or the average thickness S _{m of} the strip during the cutting process of the strip and, depending on this, the height h _{R of} the strip Achieving a desired resistance R _N corresponding to a desired rated power P _{N of} the heating element is changed. Thus, during the production of the heating element, so to speak, "in-line" with the cutting takes into account the real conditions and the manufacturing tolerances and makes a corresponding adjustment of the height of the strip.

In this case, the fine trimming of the heating element can be achieved to the desired rated power by changing the height h _{s of} the tabs during the cutting process of the strip, whereby the effective height of the resistance of the strip is changed.

A change in the resulting resistance of the strip can also be achieved by changing the width b _{F of} the gaps as a function of the resistance related to a measuring length L _m and the mean thickness of the strip during the cutting process of the strip.

Furthermore, it is also occasionally possible to arrange gaps with greater height h _s ' than the height h _{s of} the remaining gaps during the stripping process of the strip.

Furthermore, the width b _{s of} the tabs can also be changed during the strip cutting process.

Finally, trimming of the resulting resistor can also be accomplished by cutting it on the opposite side of the tabs during the strip removal process. These cuts alter the effective height of the resulting resistance of the strip.

The cuts can be made wave-shaped.

If, according to a further feature of the invention, two or more identical strips are simultaneously cut out of a band-shaped material, with the tabs of one strip being complementary to the gaps of the adjacent strip, two or more heating elements can be made of a band-shaped material without substantial material waste be made quickly and inexpensively. Another advantage of multiple use of a ribbon material is the elimination of ribbon guide tolerances and bandwidth tolerances in the cutting process, which can be achieved by severing a narrow strip of waste at the outer edges of the ribbon. The adjustment of the cutting position thus gives the entire tolerance width for the strip. Thus, a very narrow width tolerance can be achieved, which in turn can be dispensed with a preceding material thickness measurement to achieve a certain resistance R _N.

Suitable methods for the production are the application of a plasma jet, water jet, laser beam or an electro-erosion process.

Of course, combinations of the above-mentioned possibilities of changing the strip parameters as a function of the resistance R _T related to a measuring length L _m and the average thickness s _{m of} the strip are possible.

The present invention will be explained in more detail with reference to the accompanying drawings.

• Fig. 1 einen Teilabschnitt eines bandförmigen länglichen Streifens zur Bildung eines Heizelements für elektrische Heizplatten;
• Fig. 2 einen Ausschnitt eines Streifens gemäß einer weiteren Ausführungsform;
• Fig. 3 ein Ausschnitt eines Streifens gemäß einer weiteren Ausführungsform der Erfindung;
• Fig. 4 eine schematische Darstellung eines Herstellungsverfahrens, bei dem zwei identische Streifen aus einem bandförmigen Material ausgeschnitten werden;
• Fig. 5a die relative Querschnittsänderung in Abhängigkeit der fertigungsbedingten Materialdickenabweichung des Streifens;
• Fig. 5b die relative Widerstandsänderung in Abhängigkeit der Materialstärkenabweichung des Streifens; und
• Fig. 5c die relative Heizleistungsänderung in Abhängigkeit der Materialstärkenabweichung eines Streifens.

Show:

• Figure 1 is a partial section of a band-shaped elongated strip to form a heating element for electric heating plates.
• FIG. 2 shows a detail of a strip according to a further embodiment; FIG.
• 3 shows a section of a strip according to a further embodiment of the invention;
• Fig. 4 is a schematic representation of a manufacturing process in which two identical strips are cut out of a band-shaped material;
• 5a shows the relative change in cross section as a function of the production-related material thickness deviation of the strip;
• Fig. 5b shows the relative change in resistance as a function of the material thickness deviation of the strip; and
• Fig. 5c, the relative Heizleistungsänderung depending on the material thickness deviation of a strip.

Die Querschnittsfläche und Länge des Heizleiters werden entsprechend der festgelegten Windungsform des Streifens 1 und dem vorgegebenen elektrischen Widerstand R_N ausgelegt. Nachdem der Widerstand R_N durch den Zusammenhang $$R_N={\mathit{\rho }}_R\frac{L_N}{A_R}$$

gekennzeichnet ist, wobei ρ_R der spezifische Widerstand, L_R die Länge des Streifens 1 und A_R der Querschnitt des Streifens 1 ist, und A_R=h_R.S_R, wobei h_R die Höhe des Streifens und s_R die Dicke des Streifens 1 angibt. Bei einer festgelegten Länge L_R=4,7 m und einer Dicke s_R von 0,038 mm ergibt sich gemäß dem vorliegenden Beispiel eine Querschnittsfläche A_R=0,133 mm² bzw. eine Höhe h_R des Streifens 1 von 3,507 mm.Fig. 1 shows a section of a band-shaped elongated strip 1, as it is used with a certain length L _R for the production of a heating element with a predetermined nominal power P _N. The elongated strip consists of one area 2 with a height h _R and tabs 3 arranged thereon with a height h _s and a width b _s , between which tabs 3 gaps 4 having a height h _s and a width b _{F are} formed, the engagement depth e being the proportion of the height h _s , with which the tabs can interlock in a thermally insulating pad (not shown). Depending on the thickness of the strip 1 and the material used and thus the respective specific resistance ρ _R , the desired nominal resistance R _N can be calculated for a desired nominal power P _N for a given nominal voltage U _N. For example, the nominal resistance R _{N is} at a desired rated power of 1200 W and a rated voltage of 230 V according to the formula $$R_N=\frac{U_N}{I_N}=\frac{U_N^2}{P_N}=44.083\mathit{\Omega }$$

The cross-sectional area and length of the heating conductor are designed according to the predetermined winding shape of the strip 1 and the predetermined electrical resistance R _N. After the resistance R _N by the context $$R_N={\mathit{\rho }}_R\frac{L_N}{A_R}$$

where ρ _{R is} the resistivity, L _{R is} the length of the strip 1 and A _{R is} the cross section of the strip 1, and A _R = h _R .S _R , where h _{R is} the height of the strip and s _{R is} the thickness of the strip Indicates strip 1. For a given length L _R = 4.7 m and a thickness s _R of 0.038 mm results according to the present example, a cross-sectional area A _R = 0.133 mm ² and a height h _{R of} the strip 1 of 3,507 mm.

Due to the manufacturing tolerances of the height h _R and the tolerances of the thickness s _{R of} the strip 1 in the delivery state thus varies the cross-sectional area A _{R of} the strip 1. According to these cross-sectional deviations, the electrical resistance R _{N of} the strip and thus changes under the assumption of constant supply voltage U _N achieved heating power. Examples of the relative change in cross section, the relative change in resistance and the relative change in heating power as a function of the material thickness deviation Δs _R are shown in FIGS. 5a-5c.

Thus, due to the cross-sectional deviations ΔA _R , deviations of the rated power occur $$\mathrm{\Delta }\begin{array}{}\end{array}\mathit{P}\left({\mathrm{\Delta }\begin{array}{}\end{array}\mathit{s}}_R,,{\mathrm{\Delta }\begin{array}{}\end{array}\mathit{H}}_R\right)=\frac{U_N^2}{\left(R_N+\mathrm{\Delta }\begin{array}{}\end{array}\mathit{R}\left({\mathrm{\Delta }\begin{array}{}\end{array}\mathit{s}}_R,,{\mathrm{\Delta }\begin{array}{}\end{array}\mathit{H}}_R\right)\right)}-P_N$$

$$h_T=\frac{{\mathit{A}}_T}{s_m},$$

wobei A_T der tatsächliche Querschnitt des Streifens 1, L_m die definierte Messlänge, R_T der gemessene Widerstand über die Messlänge und s_m die gemessene mittlere Dicke des Streifens 1 darstellen. Die Trimmung auf den Sollquerschnitt A_R des Streifens erfolgt durch Änderung der Höhe h_R des Streifens 1. Die entsprechende Höhe h_RS errechnet sich aus dem Querschnitt A_R und der Materialstärke s_m gemäß $$h_{\mathit{RS}}=\frac{{\mathit{A}}_R}{s_m},$$

wobei h_RS die Sollhöhe des Streifens 1, A_R der Sollquerschnitt des Streifens 1 und s_m die gemessene Dicke des Streifens 1 ist. Die Laschen 3 und dazwischen angeordneten Lücken 4 beeinflussen den Gesamtwiderstand des Streifens 1, was durch einen Abminderungsfaktor des Gesamtwiderstands, der experimentell bzw. rechnerisch ermittelt werden kann, berücksichtigt wird. Der Querschnitt des Streifens unter Berücksichtigung der Laschen 3 rechnet sich entsprechend $$A_{\mathit{RF}}=L_R\frac{{\rho }_R}{\left(\frac{R_N}{\mathrm{ϵ}}\right)}\mathrm{und}{h}_{\mathit{RF}}=\frac{{\mathit{A}}_{\mathit{RF}}}{s_R},$$

wobei A_RF der Sollquerschnitt des Streifens 1, h_RF die Sollhöhe des Streifens 1 und ε der Widerstandsabminderungsfaktor aufgrund der Laschen 3 ist.To maintain the required heating power P _N despite the material and production-related deviations of the cross-sectional area .DELTA.A _R strip 1 must be trimmed accordingly. This could be done by changing the total length L _{R of} the strip 1, but this requires a high additional manufacturing cost. According to the present invention, this Nachtrimmung done by changing the height h _{R of} the strip 1. Such a change in the height h _{R of} the strip 1, both the width tolerance and the material thickness tolerance of the strip 1 can be compensated. For this purpose, the resistance R _T and the average material thickness S _m over the measured length L _{m of} the strip 1 are determined over a defined measuring length L _m and from this the actual cross-sectional area A _T or the effective shape deviation or the effective height h _{T of} the strip 1 correspondingly the following formulas are recalculated: $$A_T=L_m\frac{{\mathit{\rho }}_R}{R_T}$$

$$H_T=\frac{{\mathit{A}}_T}{s_m}.$$

where A _{T represents} the actual cross section of the strip 1, L _m the defined measuring length, R _T the measured resistance over the measuring length and s _m the measured average thickness of the strip 1. The trim to the nominal cross section A _{R of} the strip is effected by changing the height h _{R of} the strip 1. The corresponding height h _{RS is} calculated from the cross section A _R and the material thickness s _m according to $$H_{\mathit{RS}}=\frac{{\mathit{A}}_R}{s_m}.$$

where h _{RS is} the nominal height of the strip 1, A _{R is} the nominal cross section of the strip 1 and s _{m is} the measured thickness of the strip 1. The tabs 3 and intervening gaps 4 affect the total resistance of the strip 1, which is taken into account by a reduction factor of the total resistance, which can be determined experimentally or computationally. The cross section of the strip taking into account the tabs 3 pays off accordingly $$A_{\mathit{RF}}=L_R\frac{{\rho }_R}{\left(\frac{R_N}{\mathrm{\epsilon }}\right)}\mathrm{and}{H}_{\mathit{RF}}=\frac{{\mathit{A}}_{\mathit{RF}}}{s_R}.$$

wherein A _RF of the target cross-section of the strip 1, h _RF is the nominal height of the strip 1 and the ε Widerstandsabminderungsfaktor due to the tabs. 3

FIG. 2 shows a variant of the strip 1 according to FIG. 1, with some gaps having a higher height h _s ' than the height h _{s of} the remaining gaps 4, whereby the effective resistance of the strip 1 can be influenced. In addition, the correspondingly higher gaps 5 provide better ventilation of the heating element, since more air can flow through.

In the embodiment according to FIG. 3, incisions 6 are arranged in a wave form on the side of the strip 1 opposite the tabs 3, whereby the height h _{R of} the flexible region 2 of the strip 1 can be influenced. In addition, 6 changes in the current density caused by the wave-shaped incisions, which lead to a change in the annealing behavior and thus the luminous behavior of the band.

wobei b_B die Breite des Bandes 7 und s_R die Dicke des Bandmaterials sind. Der Widerstand des Bandes 7 bezogen auf die gewünschte Länge L_R der Streifen 1, 1' beträgt $$R_{\mathit{BR}}={\mathit{\rho }}_R\frac{L_R}{A_B},$$

und der Bandwiderstand bezogen auf die Messlänge $$R_{\mathit{BM}}={\mathit{\rho }}_R\frac{L_m}{A_B}\mathrm{.}$$

Fig. 4 shows schematically the production of two strips 1, 1 'of a single band 7, wherein the tabs 3 and the gaps 4 of the bands 1, 1' are designed correspondingly complementary. The width b _{BT of} the band 7 corresponds to twice the height h _{R of} the strips 1, 1 'and the height h _{s of} the tabs 3. The cross-sectional area of the band 7 is $$A_B=b_B\cdot s_R.$$

where b _{B is} the width of the belt 7 and s _{R is} the thickness of the belt material. The resistance of the belt 7 relative to the desired Length L _{R of} the strips 1, 1 'is $$R_{\mathit{BR}}={\mathit{\rho }}_R\frac{L_R}{A_B}.$$

and the band resistance with respect to the gauge length $$R_{\mathit{BM}}={\mathit{\rho }}_R\frac{L_m}{A_B}\mathrm{,}$$

wobei A_RF der Sollquerschnitt des Bandes und s_BM die mittlere Stärke des Bandes ist. Rückgerechnet betragen der Widerstand des Bandes und die Heizleistung bezogen auf die gesamte Länge L_R gleich $$R={\mathit{\rho }}_{\mathit{R}}\frac{L_{\mathit{R}}}{\left(h_{\mathit{Rerf}}\cdot s_{\mathit{BM}}\right)}\cdot \mathrm{ϵ}\mathrm{.}$$

The desired height h _R or h _{s of} the strips 1, 1 'can be determined from the resistances measured during production over the measuring length and the measured mean strip thickness s _BM . The required height h _Rerf the strip 1, 1 'is $$H_{\mathit{fert}}=\frac{A_{\mathit{RF}}}{s_{\mathit{BM}}}.$$

where A _{RF is} the nominal cross section of the belt and s _{BM is} the average thickness of the belt. Calculated back, the resistance of the band and the heating power based on the entire length L _R equal $$R={\mathit{\rho }}_{\mathit{R}}\frac{L_{\mathit{R}}}{\left(H_{\mathit{fert}}\cdot s_{\mathit{BM}}\right)}\cdot \mathrm{\epsilon }\mathrm{,}$$

The present invention shows an efficient method for adapting the resistance of heating elements for electric heating plates, in particular for glass ceramic plates, to the desired rated power taking into account manufacturing tolerances.

Claims (18)

1. A heating element for electric heating plates, in particular for glass ceramic plates, which is designed as tape-shaped longitudinal strip (1) having a length L_R, a height h_R and a thickness s_R made of electrically conductive material with a specific resistance ρ_R and which, when being used, is provided in an upend position on a thermically isolating base, wherein the strip (1) has lugs (3) with a height h_s and a width b_s on the side facing the base, which lugs (3) mesh with the base over an engagement depth e of a portion of the height h_s, gaps (4) with a height h_s and a width b_F being formed between said lugs (3), said gaps (4) forming aerating gaps over the distance height h_s minus engagement depth e, characterised in that, according to the relation $h_{\mathit{R}}={\mathit{\rho }}_R\frac{L_m}{\left(s_m{R}_T\right)}\mathit{ϵ},$

   

   the height h_R of the strip (1) is dimensioned in correspondence with the resistance R_T which is based on a defined measuring length L_m, and/or in correspondence with the mean thickness s_m of the strip (1) for obtaining a predetermined resistance R_N according to a desired nominal power P_N of the heating element, wherein ε is the resistance-reducing factor that considers the lugs (3).
2. The heating element according to claim 1, characterised in that, between the lugs (3), the height h_R' of the strip (1) above single gaps (5) or gaps (5) arranged in groups is smaller than the height h_R of the remaining strip (1).
3. The heating element according to claim 1 or 2, characterised in that the strip (1) has recesses (6) provided on the side opposite the lugs (3), said recesses having a different height than the remaining height h_R.
4. The heating element according to claim 3, characterised in that the recesses (6) are designed to be wavy.
5. The heating element according to any one of claims 1 to 4, characterised in that the lugs (3) and the gaps (4) have rounded edges.
6. The heating element according to claim 1, characterised in that the strip (1) is made of a metal alloy.
7. A method for producing a heating element for electric heating plates, in particular glass ceramic plates, wherein from a tape-shaped electrically conductive material with a specific resistance ρ_R and a thickness s_R a strip (1) with a length L_R and a height h_R is cut out, said strip, on one side, having lugs (3) with a height h_s and a width b_s and gaps (4) provided therebetween with a height h_s and a width b_F, characterised in that during the cutting process of the strip (1) the resistance R_T, which is based on a measuring length L_m, and/or the mean thickness s_m of the strip (1) is measured and, as a function thereof, the height h_R of the strip (1) is altered in correspondence with a desired nominal output P_N of the heating element for obtaining a desired resistance R_N.
8. The method according to claim 7, characterised in that the height h_s of the lugs (3) is altered when cutting out the strip (1).
9. The method according to claim 7 or 8, characterised in that the width b_F of the gaps (4) is altered when cutting out the strip (1).
10. The method according to any one of claims 7 to 9, characterised in that, when cutting out the strip (1), individual gaps (5) having a larger height h_s' than the height H_s of the remaining gaps (4) are arranged.
11. The method according to any one of claims 7 to 10, characterised in that, when cutting out the strip (1), the width b_s of the lugs (3) is altered.
12. The method according to any one of claims 7 to 11, characterised in that, when cutting out the strip (1), said strip is cut into on the side opposite the lugs (3).
13. The method according to claim 12, characterised in that, when cutting out the strip (1), said strip is cut into along a wave line.
14. The method according to any one of claims 7 to 13, characterised in that two or more identical strips (1, 1') are simultaneously cut out of a tape-shaped material, wherein the lugs (3) of a strip (1, 1') are designed to be complementary to the gaps (4) of the strip (1, 1') arranged beside it.
15. The method according to any one of claims 7 to 14, characterised in that the strip (1) is cut out by means of a plasma jet.
16. The method according to any one of claims 7 to 14, characterised in that the strip (1) is cut out by means of a water jet.
17. The method according to any one of claims 7 to 14, characterised in that the strip (1) is cut out by means of a laser jet.
18. The method according to any one of claims 7 to 14, characterised in that the strip (1) is cut out by means of an electro-erosion method.

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