GB2201319A

GB2201319A - Electrical heating device controlled by a Curie point sensor

Info

Publication number: GB2201319A
Application number: GB08800331A
Authority: GB
Inventors: Gerhard Schulze
Original assignee: Bosch Siemens Hausgerate GmbH
Current assignee: BSH Hausgeraete GmbH
Priority date: 1987-02-17
Filing date: 1988-01-07
Publication date: 1988-08-24
Also published as: DE3705009A1; GB8800331D0

Abstract

A heating device comprises electrically heated radiant heating elements (1, 2) and a temperature-checking equipment which comprises a temperature-measuring sensor (8) and electrical evaluating equipment (9) connected to output means of the sensor and which interrupts or permits the supply of electrical energy on a given critical temperature being exceeded or fallen below, respectively. The sensor (8) comprises a magnetic material body which changes its reluctance on the attainment of the Curie temperature which is critical to that material, as well as an electronic circuit element influenced by the reluctance. As shown ceramic hot plate 3 is provided with a reflector 6. The magnetic material may be cobalt or a ferrite and may be the core of a sensing coil (Fig. 2) or of a transformer (Fig. 3). <IMAGE>

Description

HEATING DEVICE The present invention relates to a heating device.

Heating equipment with electrically heated radiant heating elements can be so constructed, for example, that radiant heating bodies with radiant heating elements loadable by electrical energy are arranged below a plate, for example a glass-ceramic plate, permeable by radiant heat. The heating elements can be designed for different operating temperatures.

Appropriate protective devices should be provided for protection against overheating. Thermal protection switches, which are of a relatively large size and extend over the entire heating surface, are known from German published specifications DE-OS 28 06 369 and 30 04 187.

Apart from their size, the overheating protection devices used in known heating equipment are designed for relatively low temperature regions.

There is thus a need for a heating device with a temperature sensor which can be constructed to be relatively small and to function with relatively high operating temperatures.

According to the present invention there is provided a heating device comprising electrically operable radiant heating means, a temperature sensor to sense temperature in the region of the heating means and comprising a body of magnetic material which substantially changes its reluctance on attainment of the Curie temperature of that material, and means responsive to such change in reluctance to influence the supply of operating current to the heating means.

In a preferred embodiment, the heating device comprises electrically heated radiant heating elements and temperature-checking equipment, which comprises a temperature-measuring sensor and electronic evaluating equipment connected to output means thereof and which interrupts or permits the supply of electrical energy on a given critical temperature being respectively exceeded or fallen below. The temperature-measuring sensor contains å magnetic material body which significantly changes its reluctance on the attainment of the Curie temperature given as critical and contains an electronic circuit element influenceable by this reluctance.

A heating device embodying the present invention måy have several advantages. The electronic evaluating equipment connected to the temperature-measuring sensor can be constructed in simple manner since the change in reluctance on attainment of the Curie temperature leads to clear and thus easily evaluated electrical signals. The circuit components of the evaluating equipment can thus be exposed to relatively high ambient temperatures. A further advantage is that an adjustment of the sensor and evaluating equipment is not required.

In one preferred embodiment, the sensor is constructed as an electrical coil with the magnetic material body arranged as a core.

In another preferred embodiment, the sensor is constructed as an electrical transformer with the magnetic material body arranged as a core in the transformer, the transformer comprising a primary winding connected to signal generating means and a secondary winding connected to current supply control means.

Preferably, the coil or the primary and secondary windings, as the case may be, are arranged at a spacing from the place of temperature measurement and the core extends from the place of temperature measurement to the location of the coil or the windings, an air gap being present between the core and the coil or the windings.

These embodiments may have the advantage that the coil or the primary and secondary windings as well as the feed lines are not exposed to the temperature prevailing at the place of temperature measurement.

In yet another embodiment, the temperature sensor may be constructed as a reed switch. This form of sensor may function with particularly simple evaluating equipment.

For preference, the magnetic material body consists of cobalt.

This is suitable for very high temperatures and thereby usable for inter alia monitoring heat levels produced by halogen radiators.

Embodiments of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic sectional side elevation of a heating device embodying the invention; Fig. 2 is a diagram of a heating device embodying the invention, in which a temperature-measuring sensor is constructed as an electrical coil; and Fig. 3 is a diagram of a heating device embodying the invention, in which a temperature-measuring sensor is constructed as an electrical transformer.

Referring now to the drawings, there is shown in Fig. 1 a heating device comprising radiant heating elements 1 and 2'. The elements 1 and 2 radiate heat energy in the direction of a heating plate 3, which is, for example, a glass-ceramic plate with good transmission properties for radiant heat. The heating elements 1 and 2 are arranged in a housing shell 4.

The heating element 1 can be constructed as, for example, a normal temperature element and the heating element 2 as a high temperature element. Preferably, the element 1 is disposed in a temperatureresistant, thermally insulating and electrically non-conductive material 5. The heating element 2 is preferably mounted punctiformly and selfsupportingly over a wide area. This mounting permits the arrangement of a radiation reflector 6 below the element 2 to upwardly reflect downwardly radiated thermal energy.

A temperature sensor 8 is arranged in the region between the elements 1 and 2 and the plate 3 or directly at the heating plate 3.

The sensor is disposed at a point at which, in normal operating conditions, the maximum prevailing temperature will be the Curie temperature critical for a magnetic material body present in the sensor. This material, below the Curie temperature, possesses its normal magnetic property,iwhich it loses above the Curie temperature.

The sensor 8 is connected with evaluating equipment 9 (not shown in Fig. 1). The heating means can include more than one temperature sensor 8, wherein each sensor 8 in co-operation with one or more evaluating devices regulates the supply of electrical energy.

The sensor 8 comprises a magnetic material body which significantly changes its reluctance on the attainment of the Curie temperature given as critical as well as an electronic circuit element influenceable by this reluctance. The sensor can be constructed as an electrical coil, with the magnetic material body arranged as a core, or it can be constructed as an electrical transformer with the magnetic material body again arranged as a core. As explained by reference to Fig. 3, the primary winding of the transformer can be connected with a signal generator and the secondary winding of the transformer with the evaluating equipment.

The temperature-measuring sensor 8 can also be constructed as a reed switch. Reed switches are on/off switches. The switching-on and the switching-off operations take place on attainment of the Curie temperature in the course of a temperature increase and a temperature decrease, respectively. These switching operations can be recognised and electronically evaluated in particularly simple manner.

The magnetic material body of the sensor 8 preferably consists of cobalt, which has a Curie temperature of 11210C. The sensor is thus usable for the relatively high temperatures which arise in the environment of halogen radiators.

Fig. 2 shows a sensor 8 constructed as an electrical coil and Fig. 3 shows a sensor 8 constructed as an electrical transformer.

The coil or the transformer contains, as already explained, a core of a magnetic material which changes its reluctance significantly on the attainment of the Curie temperature. These materials include certain ferrites, in the structure of which iron atoms are replaced by manganese zinc or nickel zinc. They lose their magnetic property above the Curie temperature.

A particularly good heat transfer between the measurement object, in particular the heating plate 3, and the sensor 8 is achieved if the magnetic material body is mounted directly on the measurement object. The sensor 8 can, however, be arranged in close proximity to the measurement object without being directly connected thereto.

The sensor 8 can be distinguished by small geometric dimensions and high reversability. Moreover, long-term stability is provided by ageing of the magnetic material.

In the case of temperatures which could damage the coil or the primary and the secondary winding of the transformer or the feed lines to the coil and the transformer windings, the coil or the windings can be arranged remote from the placeof temperature measurement and at, in particular, a location where lower temperatures will prevail.

The coil or windings are then not wound directly onto the magnetic material body; rather, an air gap is present between the magnetic material body and the coil or windings. The magnetic material body can then achieve the critical Curie temperature while the coil or windings have a lower temperature. The magnetic material body will be so shaped and arranged that it extends from the place of temperature measurement to the coil or windings. For example, the coil or windings can be arranged externally of the housing shell 4 (Fig. 1), whilst the magnetic material body projects into the shell.

The magnetic material body can in that case have a closed, especially circularly round or oval, shape. When the body loses its magnetic properties on the attainment of the Curie temperature, it behaves as if it has an air gap at the location of temperature measurement. The inductance of the coil or the electrical properties of the transformer change in dependence on the reluctance of the magnetic material body.

The evaluating equipment 9 preferably comprises an alternating current bridge which serves for the recognition of the temperaturedependent impedance of the coil or of the primary winding of the transformer. The evaluating equipment is connected with an on/off switch - indicated generally by 10 - arranged in an electrical circuit with a heating voltage source and the heating elements 1 and 2. Each element 1 and 2 can be operated separately. The evaluating equipment controls the on/off switch 10 in such a way that the circuit is closed below the Curie temperature, the heating element concerned thus being coupled to the respective voltage source, and opened above the Curie temperature, the heating element concerned thus being separated from the voltage source.

The temperature-measuring sensor 8 constructed as an electrical transformer is, as illustrated in Fig. 3, connected at its primary winding with a signal generator 11 and at its secondary winding with the evaluating equipment 9. It acts on the on/off switch 10 in the afore-described manner.

Claims

1. A heating device comprising electrically operable radiant heating means, a temperature sensor to sense temperature in the region of the heating means and comprising a body of magnetic material which substantially changes its reluctance on attainment of the Curie temperature of that material, and means responsive to such change in reluctance to influence the supply of operating current to the heating means.

2. A device as claimed in claim 1, wherein the sensor comprises an electrical coil and the magnetic material body is arranged as a core within the coil.

3. A device as claimed in claim 1, wherein the sensor is constructed as an electrical transformer with the magnetic material body arranged as a core in the transformer, the transformer comprising a primary winding connected to signal generating means and a secondary winding connected to current supply control means.

4. A device as claimed in either claim 2 or claim 3, wherein the coil or the windings, as the case may be, is or are disposed in a location spaced from the temperature sensing region and the magnetic material body extends from said region to said location, an air gap being present between the body and the coil or windings.

5. A device as claimed in claim 1, wherein the sensor is constructed as a reed switch.

6. A device as claimed in any one of the preceding claims, wherein the magnetic material is cobalt.

7. A heating device substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

8. A heating device substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

9. A heating device substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.