EP1602262A1

EP1602262A1 - Electric heater having a contact hot plate

Info

Publication number: EP1602262A1
Application number: EP04716592A
Authority: EP
Inventors: Wolfgang Thimm
Original assignee: EGO Elektro Geratebau GmbH
Current assignee: EGO Elektro Geratebau GmbH
Priority date: 2003-03-05
Filing date: 2004-03-03
Publication date: 2005-12-07
Also published as: ZA200505471B; CN1757265A; DE10310255A1; WO2004080127A1

Abstract

In one embodiment of the invention, an electric heater (11) can be provided with a contact hot plate (12), on which wire-shaped heating filaments (14) are arranged. At least one of the heating filaments (14b) is made of a material having a PTC effect, e.g. an iron-base alloy. This retains, during all operating states of the heating filament, a crystal structure that is centered in a cubically inward manner, and simultaneously has the PTC effect. In addition to its heating function, a heating element (14) of this type also comprises an independent temperature control for preventing excess temperature whereby enabling the elimination of separate temperature limiters with switching contacts.

Description

Description Electric heater with a contact hotplate

Application and state of the art ^*

The invention relates to an electric heater with a contact hotplate with the features of the preamble of claim 1.

It is known, for example, to provide cast hotplates on their underside with heaters in which heating coils surrounded by insulating material work as a heater. To prevent overheating, these cast iron hotplates have a built-in temperature limiter. This is usually embedded in the middle of the underside and interrupts the energy supply to the cast-iron hotplate when a certain specified temperature is exceeded, for which purpose it has separating electrical contacts. It is considered disadvantageous to always arrange a temperature limiter as a separate component on the hotplate.

Task and solution

The invention has for its object to provide a heater mentioned at the outset with which the disadvantages of the prior art can be avoided and in particular an additional temperature limiter with opening electrical contacts can be dispensed with.

This object is achieved by a heater with the features of claim 1. Advantageous and preferred embodiments of the invention are specified in the further claims and are explained in more detail below. The wording of the claims is made the content of the description by express reference.

BESTATIGUNGSKOPIE According to the invention, the heater with the contact hotplate has at least one wire-shaped or wire-like heater for heating the contact hotplate. The heater consists of a metallic material with PTC properties.

The advantage of a heater made of PTC material for a contact hotplate is that above a certain temperature, which roughly corresponds to a limit temperature that should not be exceeded in normal operation, the electrical resistance of the heater increases sharply. This reduces the current flow accordingly and the heating output drops. The lowering of the heating power in turn causes a return to the operating temperature range. The use of a heater made of such a PTC material has the advantage that it is not just a pure securing element. The heater is self-limiting in addition to its property as a heating element, so it combines two properties.

By providing a PTC heater, it is therefore possible to dispense with the usual temperature limiters, which above all have mechanically operated and electrically opening contacts. Such temperature limiters are expensive, complex to assemble and difficult to arrange on some contact hotplates, especially if their outer surfaces, which are available for attachment, are not flat.

The heater can either consist of a so-called iron-based alloy, which maintains a cubic, inner-centered crystal structure during all operating states of the heater. All operating states of the heater are understood to mean different temperatures at which the heater is operated as intended. In the case of contact hotplates, these are usually temperatures up to a few hundred degrees, for example up to 300 ° C or 400 ° C. The maintaining The special crystal structure has the advantage that a phase transition from α-iron to γ-iron is avoided during normal operation. This way the heater has a better durability. Above all, however, it is possible to obtain PTC properties in addition to raising a Curie temperature of the material, which can be set relatively precisely in a desired range. Such a material is described for example in EP 1213540 A2, to which reference is expressly made here.

Furthermore, the heater can consist of a cobalt-iron alloy as a metallic material with PTC properties, which contains, for example, approximately 70% cobalt-iron. Such a material is available, for example, under the name CF25 from Vacuumschmelze.

The heater can also be made of a tungsten alloy or a molybdenum alloy. Such alloys are known for use in halogen lamps or light bulbs.

A temperature-resistance coefficient greater than 6 is preferably provided, so that the temperature rise is relatively strong from the desired temperature limit range. Larger values above 8 can also be selected, for example 10.

Furthermore, an iron-based alloy can be characterized in that it contains vanadium. The vanadium content can be 1 to 3 or 4% by weight, for example approx. 1.5% by weight. Other alloys can contain a small proportion of molybdenum, for example up to 4% by weight. It can also contain 1 to 4% by weight of titanium.

The arrangement of the heater on the contact hotplate can be such that it is arranged on its underside for heating it. A con- tact should be as flat as possible or ensure the best possible heat transfer.

For example, it is possible to arrange a heater according to the invention in a tube, for example in a coiled form. It should be covered with insulation, for example quartz sand, ceramic or the like. This tube in turn can be placed on the underside of the contact hotplate, for which purpose it is advantageously flattened to increase the contact area.

Alternatively, the heater may be surrounded by insulation in an airtight manner. Such insulation can be based on glass or ceramic and should accordingly be temperature-stable. It can also be polymer ceramics. The insulation is arranged here with the heater on the underside of the contact hotplate. It is also conceivable to provide an evacuated insulation within which the heater runs.

A heater or a contact hotplate can have several wire-shaped heaters, for example two or three. At least one of the heaters, in particular exactly one heater, advantageously consists of an iron-based alloy mentioned above. So part of the available heating power is self-regulating, which can either have a direct effect on the other heaters or over the entire heat input onto the contact hotplate.

In an exemplary embodiment of the invention, it is possible to connect a heater made of the aforementioned iron-based alloy with at least one conventional heater, which can also consist of wire made of conventional resistance material, in series. Overheating the heater with the PTC effect then causes the entire branch to be throttled or switched off, and thus also the conventional one Heater. A conventional heater or the like can be, for example, an FeCrAI alloy. exhibit. Such a heating conductor is described in EP 542128 A1, to which reference is expressly made here.

In another exemplary embodiment of the invention, there can be a parallel connection of a heater with an aforementioned iron-based alloy and a conventional heater. In this case, the heater regulates itself or only its own branch. Furthermore, it is of course also possible to provide mixtures of parallel and serial circuits.

The heaters, in particular in the aforementioned possible wirings, can be provided in several separate branches. A heater made of an iron base alloy mentioned above should be provided in at least one of the branches. It is possible to apply electrical energy to all branches together. The individual branches can run in so-called defined surface areas, which can surround and adjoin one another, but essentially do not overlap or penetrate. The surface areas are advantageously adapted to the shape of the contact hotplate, for example round and concentric with one another.

Any switching device which can produce different circuits can be used to connect such branches to several heaters. These can be serial and / or parallel, wherein several branches can be interconnected in completely free wiring and supplied with electrical energy. Different branches can be interconnected in different ways. Depending on the type of wiring, they can be supplied with different amounts of energy. This is especially true when such a contact plate according to the invention is similar to a conventional hotplate with several heating circuits on a voltage source is connected and either clocked or the power can be set by the type of connection.

On the one hand, a contact hotplate can be a so-called cast hotplate, as is common. The heater or heaters can run between ribs which are molded onto the underside of the cast-iron hotplate. As described above, the heaters can run in an insulation and, under certain circumstances, also in the form of a tubular heater.

As an alternative to a cast hotplate, it is possible to use and attach a previously mentioned heater, advantageously together with other conventional heaters, to a contact hotplate made of ceramic material. It is advantageous here if the heater has a flat wire shape and is attached to the underside of the ceramic with good heat conduction, preferably flat. A flat wire shape can extend up to a wide foil shape in order to create a balanced relationship between heater area, heater cross-section and heater output.

These and further features of preferred developments of the invention are evident not only from the claims but also from the description and the drawings, the individual features being realized individually or in groups in the form of sub-combinations in one embodiment of the invention and in other fields be able to present advantageous and protectable versions for which protection is claimed here

Brief description of the drawings

Exemplary embodiments of the invention are shown schematically in the drawings and are explained in more detail below. The drawings show: 1 is a side view of a ceramic plate with flat heaters,

2 is a plan view of the underside of the ceramic plate from FIG. 1,

3 shows a section through a cast-iron hotplate with embedded heaters on the underside,

4 shows a further section through a further hotplate with heaters which run in a tube which is pressed onto the underside of a hotplate, FIGS. 5 and 6 wiring examples for several heating conductors.

Detailed description of the exemplary embodiments

In Figure 1, a heater 11 is shown from the side. This has a contact hotplate 12, which can consist, for example, of a ceramic material. Such a ceramic hotplate is known for example from EP 069 298 A1.

Heating conductors 14 are arranged on the underside of the hotplate 12. The course of the heating conductor 14 is particularly clear from Figure 2. It can be seen here how, starting from connections 15, a heating conductor 14 is applied as a continuous band in double spiral shape to the underside of the hotplate 12.

The heating conductor 14 is divided into several different areas, which can basically be divided into two types. The heating conductor regions 14a are formed from so-called normal or conventional heating conductor material, for example a FeCrAI alloy. The

Heater regions 14b, which together as the heater regions 14a form the entire, continuous heat conductor 14 as short pieces, are made of the metallic material according to the invention with PTC

Properties formed, for example an iron-based alloy, such as it is described above. The heating conductor regions 14a and 14b are connected to one another in such a way that there are no disturbing contact resistances and also no other boundary layer problems.

There are various options for applying the heating conductor 14. On the one hand, it is possible to produce a continuous band, for example in the form of flat wire, from the heating conductor regions 14a and 14b by joining them together. This can be attached to the underside of the hotplate 12 in almost any manner, for example by gluing or embedding using ceramic adhesive. Alternatively, it is conceivable to produce so-called thick-film pastes from the respective materials and to melt them onto the hotplate 12.

In FIG. 3, a heater 111 is shown in section with a cast hotplate 112 on which a cooking vessel stands. The cast hotplate 112 has, according to known designs, concentric projecting ribs 118 on the underside. In each of the spaces formed by these ribs 118, a heating conductor 114 is embedded in a spiral shape by means of the insulating embedding 117.

A further heater 211 is shown in FIG. A heating conductor 214 is also provided there in a coiled form. However, here the heating conductor 214 is embedded in a tube 216 together with an insulating embedding 217. Tube 216 and heating conductor 214 form a so-called tubular heating element and are approximately triangular in cross section here. This makes it possible to ensure a larger direct contact surface for heat transfer to a hotplate 212 located above. Methods for deforming a tube 216 into such a triangular shape are known to the person skilled in the art. The tube 216 is advantageously surrounded on the underside of the hotplate 212 with a thermal insulation or the like, which particularly preferably has a certain reflective effect upwards. As a result, a large part of the heat generated by the tube 216 or the hot conductor 214 can be directed upwards to the hotplate 212.

The tube 216 can either be attached to the underside of the hotplate 212 by means of glue or the like, for example as described above, it advantageously also providing good heat transfer here. Alternatively, it is possible to press pipe 216 and hotplate 212 flat against one another. Further possibilities can, for example, be to solder or bond the tube 216 to the underside of the hotplate 212, since the heating conductor 214 is electrically insulated from it. Such metallic transitions would additionally improve the heat transfer if the hotplate 212 were made metallic.

FIGS. 5 and 6 show possible types of interconnection of heating conductors 314a and 414a made from conventional heating conductor material and heating conductors 314b and 414b made from an iron-based alloy according to the invention. As can be seen, these can be connected in parallel, in series or in a mixed manner with one another in various ways.

function

The function is to be described essentially with reference to FIGS. 5 and 6. For this it is important at the outset that the heat conductors made of an iron-based alloy according to the invention have a PTC characteristic. This means that their specific electrical resistance increases sharply from a temperature of approx. 300 ° C and can more than double, for example up to 600 ° C. As described at the beginning, the increase in the electrical see resistance of a heating conductor the current flow through it is lower. This reduces the heat output, which in turn causes the resistance to drop due to the decreasing temperature.

Such self-regulating heating conductors, so to speak, can be used in various ways. In contrast to other PTC sensors or PTC elements, the heating conductors described here should primarily be used as heating conductors and not only as overtemperature protection devices.

FIG. 5 shows a parallel connection of two, so to speak, conventional heating conductors 314a. A heating conductor 314b made of an iron-based alloy is connected upstream of these. The heating conductors 314a and 314b are, for example, attached to a contact hotplate for heating in the manner described above.

In addition to the heating conductors shown, which are jointly supplied with voltage, further heating conductors can be attached to the same contact hotplate. For example, a separately switchable two-circuit radiator could be created.

It can be seen that when a temperature of the hotplate which is to be regarded as critical is reached, the heating conductor 314b heats because of the relatively uniform temperature distribution in the hotplate. This increases its electrical resistance due to the properties of the iron-based alloy used, which in turn causes a reduction in the current flowing through it. This means that less current flows through the conventional heating conductor 314a. This reduces their heating output and overall the heating of the hotplate. The temperature drops until the current flow through the heating conductor 314b increases again and thus also the heating power. Thus, the heating conductor 314b does not regulate only itself, but also the conventional heating conductor 314a. Furthermore, he himself contributes to heating the hotplate.

With a suitable design of the heating conductors, in particular the hot conductor 314b, it is possible to achieve a so-called steady state. The temperature and electrical resistance and thus the heating output remain approximately the same.

FIG. 6 shows a parallel connection of a conventional heating conductor 414a and a heating conductor 414b according to the invention with an iron-based alloy. Both heating conductors 414 are connected to the supply voltage. In the case of overtemperature, it is easy to see that only the heating conductor 414b reduces its output. The heating conductor 414a remains unaffected by this and continues to heat normally.

With this connection of the heating conductors 414, the heating conductor 414b according to the invention with the iron-based alloy regulates itself only. Thus, in the case of a hotplate on which the two heating conductors 414a and 414b are arranged, the total heat generation is reduced, but only by reducing the heat generation by the iron-based alloy heating conductor 414b according to the invention.

If a plurality of heating conductors according to the invention made of an iron-based alloy are arranged on a hotplate, which are in particular subjected to power together with a voltage, it is advantageous to arrange them over a hotplate in a surface-distributed manner. This makes it easier to detect local overheating and take this overheating into account. Furthermore, in certain types of hotplates, it is possible to replace conventional heating conductors with heating conductors according to the invention made of an iron-based alloy in usually particularly hot areas. The different heaters or heating conductors can have approximately the same output.

An embodiment in numbers

The mode of operation of a hotplate described above with a series connection of a normal heating conductor wire and a PTC resistor is to be illustrated below using a numerical example.

The hotplate has a normal heating conductor, the resistance of which increases from approx. 15 ohms at room temperature to approx. 20 ohms at 800 ° C. In addition, the hotplate has a PTC heating conductor, the resistance of which increases from 5 ohms at room temperature to approx. 32 ohms at 800 ° C. The two heating conductors should be connected in series and operated with a voltage of 230 V.

The temperature curve should be illustrated using the following table:

A pot with good heat extraction is heated at 300 ° C with an output of almost 2kW. If the pan is empty and the temperature of the hotplate rises sharply, the output is significantly reduced and you only get about half of it at the highest temperatures of almost 800 ° C Heating output of just over 1kW. This enables cooling or less intense heating again.

Claims

claims

1. Electric heater (11, 111, 211) with a contact hotplate (12, 112, 212) and with at least one wire-shaped heater (14, 114, 214, 314, 414) on the contact hotplate, characterized in that the heater (14 , 114, 214, 314, 414) consists of a metallic material with PTC properties.

2. Heater according to claim 1, characterized in that the heater (14, 114, 214, 314, 414) consists of an iron-based alloy as a metallic material with PTC properties, which maintains a cubic inner-centered crystal structure during all operating states of the heater.

3. Heater according to claim 1, characterized in that the heater (14, 114, 214, 314, 414) consists of a cobalt-iron alloy as a metallic material with PTC properties.

4. Heater according to claim 1, characterized in that the heater (14, 114, 214, 314, 414) consists of a tungsten alloy or molybdenum alloy as a metallic material with PTC properties.

5. Heating according to claim 1 or 2, characterized in that the metallic material, in particular the iron-based alloy, has a resistance coefficient greater than 6, preferably greater than 8 to 10, wherein a PTC effect from a temperature of about 300 ° C strong increases.

6. Heating according to claim 2, characterized in that the iron-based alloy contains vanadium, molybdenum or titanium, the proportion preferably being between 1% and 4%.

7. Heater according to one of the preceding claims, characterized in that the heater (14, 114, 214, 314, 414) on the underside of the contact hotplate (12, 112, 212) is arranged and applied, preferably with flat contact, wherein he in particular is surrounded by insulation (117, 217) in a tube (216) and the tube is placed on the underside.

8. Heater according to one of the preceding claims, characterized in that the heater (14, 114, 214, 314, 414) is airtightly surrounded by insulation, in particular insulation based on glass or ceramic, preferably polymer ceramic, in particular the Insulation with the heater is arranged on the underside of the contact hotplate (12, 112, 212).

9. Heater according to one of the preceding claims, characterized in that it has a plurality of wire-shaped heaters (14, 114, 214, 314, 414) for a single contact hotplate (12, 112, 212), preferably 2 or 3, in particular exactly one heater consists of an iron-based alloy mentioned above.

10. Heater according to claim 9, characterized in that the heater (14b, 314b, 414b) made of the above-mentioned metallic material with PTC properties is connected in series with at least one heater (14a, 314a, 414a) made of conventional resistance material.

11. Heater according to claim 9 or 10, characterized in that the heater (14b, 314b, 414b) made of the above-mentioned metallic material with PTC properties is connected in parallel with at least one heater (14a, 314a, 414a) made of conventional resistance material.

12. Heater according to one of the preceding claims, characterized in that the contact hotplate (12, 112, 212) has heater (14, 114, 214, 314, 414) in several separate branches, at least one heater (14b. In at least one branch) , 314b, 414b) is connected from a metallic material mentioned above with PTC properties and the branches can be acted upon together with electrical energy.

13. Heater according to claim 12, characterized in that the heaters (14, 114, 214, 314, 414) of the individual branches run in defined surface areas, preferably with a round or rounded shape and in particular concentrically to one another.

14. Heating according to claim 12 or 13, characterized in that branches with heaters (14, 114, 214, 314, 414) can be interconnected with a switching device in different circuits, in series and / or in parallel, and can be acted upon with electrical energy , preferably depending on the wiring with different amounts of energy.

15. Heater according to one of the preceding claims, characterized in that the contact hotplate (12, 112, 212) is formed without excess temperature securing means which have separating electrical contacts.

16. Heater according to one of the preceding claims, characterized in that the contact hotplate is a cast hotplate (112, 212), the heater or heaters (114, 214, 314, 414) preferably running between molded-on ribs on the underside of the cast hotplate.

7. Heating according to one of claims 1 to 15, characterized in that the contact hotplate (12) consists of ceramic, preferably the heaters (14, 314, 414) in flat wire form, in particular flat, are attached directly to the underside of the ceramic.