GB2092869A - An electric cooker - Google Patents

Abstract

An electric cooker comprising a glass ceramic cooking surface having one or more electrical heaters arranged on the underside of the cooking surface has means for warning a user when the cooking surface is above a safe touching temperature. The warning means comprises a temperature monitoring device thermally coupled with at least one of said one or more electrical heaters and comprising a switch (42) including a thermally sensitive mechanical switching element such as a bi-metallic snap disc (52), an auxiliary electrical heater (38), for example of PTC material, for supplying additional heat to the switching element, and a thermal buffer (44), such as a disc of mild steel, between the auxiliary heater and the switching element. A neon lamp or a filament lamp is provided in series with the switching element for indicating when the switching element is at or above a predetermined temperature. <IMAGE>

Description

SPECIFICATION Electric cooker having temperature warning means The present invention relates to an electric cooker having means for warning the user when one or more of the cooking surfaces is above a safe touching temperature, and more particularly, but not exclusively, the present invention relates to glass ceramic top cookers having such warning means.

Problems can arise with electric cookers if there is no visible or other indication when the temperature of the cooking surface becomes too hot to touch without causing burns. Most cookers have a pilot light system to indicate when any of the heaters is electrically energised, but this does not give an adequate indication of a hazardous surface temperature, particularly in the case of glass ceramic top cookers.

The pilot light is illuminated immediately the cooker is switched on, but the cooking surface will take some 15 to 50 seconds to reach a hazardous temperature, for example 50 to 60" C. More importantly, as soon as the heater is switched off, the pilot light is turned off, but the cooking surface remains hot for some time and, depending on the construction of the cooker and the time for which the heater has been operating, it can take from 20 to 80 minutes for the cooking surface to cool down sufficiently for it to be touched with safety.

Various devices have been used in commercial cookers to indicate a hazardous temperature of the actual cooking surface. It has been proposed to provide an electronic timer which energises a warning light as soon as the heater is switched on and which keeps the warning light illuminated for a predetermined time after the heater has been switched off.

This timer, however, has the disadvantage that it indicates a hazardous temperature even if the heater has been energised for a very short time; for example, the heater may have been switched on in error without the cooking surface reaching a hazardous temperature. This results in the warning light losing credibility and in it being ignored by the user because he knows from experience that the cooking surface has not become hot. Nevertheless, if the cooking surface has been in use for a longer period, the warning light is necessary. These differences, however, are not always readily discernible to the user, and lead to confusion and to consequent danger to the user.

It has also been proposed to simulate temperature variations in the cooking surface and to operate a warning light switch in response to these simulated temperature changes. However, simulation devices are expensive and bulky, and often require more space than is available inside the cooker housing.

According to a further proposal, a warning light is actuated in direct dependence on the actual temperature of the heater or of the support of the heater.

This design necessitates a slow response time because the heater as a whole has first to reach a predetermined temperature before the temperature responsive element operates and switches on the warning light. Auxiliary heaters have therefore been provided to reduce the response time of the temperature responsive element, but these still have not enabled a sufficiently accurate response to the temperature of the cooking surface to be achieved. A particular problem exists in the period following the initial response time and before the heater as a whole reaches a sufficiently high temperature to maintain the warning light illuminated, that is, in the period from about 20 seconds after twitching on to about 5 minutes after switching on.During this period it is necessary to maintain the illumination of the warning light for up to about 20 minutes until the temperature of the glass ceramic top fall to a safe touching temperature. However, if the power output of the auxiliary heater is increased in order to reduce the initial response time, the duration of the period of illumination of the warning light is also extended, which results in the light being illuminated when the glass ceramic top is cool and consequently there is a risk of the light being ignored by the user.

It is therefore an object of the present invention to provide a warning means which incorporates an auxiliary heater and which enables a warning device to be actuated promptly, but which cancels the warning device as soon as the temperature of the cooking surface falls to a safe level.

According to the present invention there is provided an electric cooker comprising: a cooking surface; one or more electrical heaters arranged on the underside of the cooking surface; a temperature monitoring device thermally coupled with at least one of said one or more heaters and comprising a switch including a thermally sensitive mechanical switching element, an auxiliary electrical heater for supplying additional heat to the switching element, and a thermal buffer located between the auxiliary heater the switching element; and means for indicating when the switching element is at or above a predetermined temperature.

The cooking surface is preferably made of glass ceramic and the or each electrical heater is preferably a radiant heater unit.

The or each heater preferably has a thermally conductive outer cover in the form of a metal dish, the temperature monitoring device being mechanically attached to the metal dish in heattransmissive relation thereto.

The temperature monitoring device may further comprise a hollow cylindrical housing which is closed at one end, the auxiliary heater, the thermal buffer and the switch being mounted in the housing.

Preferably, the housing is made of a ceramic material. The housing may be formed with two opposing axially-extending slits for the passage of connectors for the auxiliary heater and for the switch. The auxiliary heater, the buffer and the switch may be confined within the housing by means of a strip of spring steel which extends over the housing and engages with a pair of slots formed in a metal plate The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

which is located across tne open end of th -S housing.

The temperature monitoring device is preferably mounted on the heater by means of the plate. A!4 ai-- natively, the housing may be made of aluminium or aluminium alloy, the auxiliary heater, the buffer arid the switch being mounted within the housing by means of a curable composition. The curable corn- position may be a silicone rubber.

The auxillary heater may be made from a positive temp'--rau a.urs coefficient material. A suitable material may have a resistance of from 500 to 2500 ohms at 2.? C and a switching temperature (Tr) of at least about 1850 C. A particularly suitable material has a resistance of 1200 ohms at 25 C and a switching temperature (Tr) of about 1350 C.

The switching element preferably comprises a domed bimetallic disc which moves with a snap action from one configuration to another at a pre determines temperature thereby opening or closing set of CGnfacts for controlling the indicating means.

The thermal buffer is preferably made from a sheet of mild steel having a thickness of substan 'dally 0.8 morn.

Preferably, the auxiliary heater is electrically connected in parallel with a heating element of the heater with which it is associated.

The indicating means may comprise a discharge lamp or a filament lamp.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a sectional view through one embodiment of a part of an electric cooker according to the present invention; Figure 2 is a plan view of the radiant heater incorporated in the cooker of Figure 1; Figure 3 is a sectional view, on a larger scale, taken along the line Ill-Ill in Figure 2 and showing the temperature monitoring device in detail; and Figure 4 is an exploded view showing the components of the temperature monitoring device of Figure 3.

The electric cooker shown in the drawings comprises a glass ceramic top 2 having a radiant heater 4 arranged immediately beneath the top. The heater4 comprises a metal dish 6 containing a base layer 8 of electrical and thermal insulation material. Against the side 10 of the metal dish there is located a peripheral wall 12 of thermal insulation. The base layer 8 preferably comprises a microporous insulating material such as a mixture of silica aerogel, an opacifier and, if necessary, reinforcing fibres of alumina or aluminium silicate, and the peripheral wall 12 preferably comprises ceramic fibres. Set in grooves formed in the base layer 8 are two electric heating elements 14 and 16 which are separated from each other by a dividing wall 18, for example of ceramic fibres.Each heating element 14,16 is in the form of a helically wound coil of bare wire. Extend ing over the inner element 14 is a thermal cut-out device 20 which is operable to switch off both ele ments in the event of overheating.

Each element 14 and 16 is energisable separately by way of terminal connectors 22 and 24, respectively, to enable a relatively small circular pan or other utensil to be heated solely by element 14 and to enable a larger casserole or other utensil to be heated by both elements 14 and 16. Typically the element 14 is rated at 140G watts and the element 16 is rated at 800 watts. Each coil is secured in the base layer 8 by means of staples (not shown) and is preferably made from an iron-chromium-aluminium resistance heating wire.

The thermal cutout device is of ths differential expansion type and comprises a quartz tube contain- ing a metal rod. Differential expansion between the tube and the rod as a consequence of overheating operates a mechanical switch 25 to disconnect both elements 14 and 16 from the popper source.

Although a thermal sut-out device could be located adjacent to each element 14, 16, it has been found that one cut-out device over the element 14 is quite satisfactory.

The radiant heater4illustrated in the drawings has a step junction 28 between the underneath and the side of the metal dish gto facilitate mounting of the heater in the cooker, for supporting the thermal cutout device, and for mounting a temperature monitoring device as will be described hereinafter. The step junction may be provided with screw holes for use in securing the heater.

The peripheral wall 12 and the dividing wall 18 are located against the underside of the glass ceramic top 2. Thus, the heater defines on the surface of the glass ceramic two concentric heating zones heated by the element 14 and the element 16. The influence of the element 14 is thus confined to a central region of the heater by the base and the dividing wall and direct thermal communication with a temperature monitoring device is effectively precluded unless the device is mounted at the base of the heater where there is generally insufficient space available.

The temperature of the glass ceramic top 2 is monitored by means of a temperature monitoring device 30 which is shown in more detail in Figures 3 and 4.

The temperature monitoring device 30 comprises a housing 32, for example of ceramic material, of generally hollow cylindrical construction, but closed at one end. Openings 34 are formed in the housing 32 for the passage of connections 36 for an auxiliary heater 38 and for the passage of connections 40 for a temperature responsive switch 42. In the preferred embodiment illustrated, the auxiliary heater 38 comprises a material which has an electrical resis- tance which rises with temperature or which rises substantially at a given temperature. Such materials are referred to herein as positive temperature coefficient (FTC) materials and include n-type semiconductor materials such as doped barium titanate, which is barium titanate (which is normally insulative) doped with strontium. This material has a resistivity which falls as its temperature rises to about 200 C, but which increases rapidly as the temperature reaches this level. This characteristic of the mat erial enables the auxiliary heater 38 to act also as a thermostat, maintaining the temperature of the switch 42 at about 200 C. In this way, overheating of the temperature monitoring device can be avoided in a simple manner, as can energy wastage due to the continuous passage of electricity through the auxiliary heater circuit.A particularly suitable PTC element has a resistance of 1200 ohms at 25"C and a switching temperature (Tr) of about 185 C.

The connections 36 comprise sheet metal plates between which the heater 38 is sandwiched, the plates having radial projections which extend through the respective openings 34 for securing connecting leads (not shown).

Thermal contact between the auxiliary heater 38 and the temperature responsive switch 42 is via a buffer 44. The buffer 44 determines the time lapse between enes-gisation of the auxiliary heater and switch-over of the switch 42 and, together with the heater 38, determines the time lapse between deenergisation of the auxiliary heater and the return of the switch 42 to its initial position in the situation in which the body of the heater 4 has not become sufficiently hot to control the switch 42. The initial response time before the switch 42 reaches its switch-over temperature depends on the rate at which the buffer 44 is heated by the auxiliary heater 38.Thus, for any particular auxiliary heater 38,the properties of the buffer 44 can be selected such that the initial response time corresponds to that required by the glass ceramic top 2 to reach a hazardous temperature of about 50 to 60 C. The delay before the switch 42 returns to its initial position depends on the thermal capacity of the components of the temperature monitoring device, particularly the buffer 44.

Thus, the configuration and composition of the buffer 44 are important to the correct operation of the temperature monitoring device. The buffer 44 could be made of a number of high temperature resistant materials, such as ceramics and high temperature plastics, but it has been found that mild steel is particularly effective. Furthermore, it has been found that a disc corresponding in shape to the shape of the auxiliary heater 38 and the switch 42 and having a thickness of about 0.88 mm is most suitable. A disc 0.8 mm thick gives an initial response time of about 20 seconds, whilst at the same time allowing an adequate period before the switch returns to its initial position.For example, if the heater 4 is switched on for 30 seconds, the switch will remain actuated for about 5 to 9 minutes, whereas if the heater is switched on for 5 minutes, the switch will remain actuated for about 24 to 36 minutes. As shown in Figures 3 and 4, the buffer is positioned between the auxiliary heater 38 and one of the connections 36. Clearly this is acceptable when the buffer is electrically conducting, but if the buffer is not electrically conducting it must be positioned on that side of the connection 36 remote from the auxiliary heater 38.

The components of the temperature monitoring device are confined within the housing 32 by a strip 46 of spring steel which slots into and engages with a pair of slots formed in a plate 48 which extends across the open end of the housing 32. The engagement of the strip 46 in the plate 48 urges the switch 42, buffer 44, auxiliary heater 38 and connections 36, 40 towards the closed end of the housing 32 to hold them in position. Alternatively, a spacer member (not shown), for example of steel, may be used to urge the components towards the closed end of the housing 32. The strip 46 lies in a groove formed in the outer surface of the housing 32 and the ends of the strip are bent over to maintain the strip in engagement with the plate 48.The plate 48 extends from the housing 32 to provide the means by which the temperature monitoring device is mounted on the heater (see Figure 1) and is provided with a ridge to ensure accurate positioning of the device relative to the heater. The device may be mounted on the heater by means of a screw 50 which engages in one of the holes formed in the step junction 28. The plate 48 in the illustrated embodiment is also armed with a series of indentations in order to facilitate secure mounting of the switch 42 on the plate, although other means of mounting the switch may be employed depending on the type of switch used.

The switch 42 in the illustrated embodiment is model 63TM02 available from Therm-O-Disc, Incorporated of Mansfield, Ohio, U.S.A. and comprises a bimetallic disc 52 which moves from one position to another with a snap action at a predetermined temperature. The bitmetallic disc operates on contacts 54 by way of a rod 56 so as to close the contacts when the disc 52 reaches said predetermined temperature.

The housing 32 need not extend entirely over the switch 41, provided that the switch is held securely in place.

As shown diagrammatically in the figures, the auxiliary heater 38 is connected in parallel with the element 14 of the radiant heater 4 and the switch 42 is connected to a warning means such as a neon lamp 58, although other warning means, visible and/or audible, may be used.

In use, when the heating element 14 is energised, or when both heating elements 14 and 16 are energised, the auxiliary heater 38 is also energised and the temperature of the switch 42 rises until its switch-overtemperature is reached. The rate of temperature rise is, however, regulated by the buffer 44. When the switch-over temperature is reached, the contacts 54 are closed and the neon lamp 58 is illuminated. For so long as the heating element 14, and possibly the heating element 16, is energised and until thermal equilibrium is reached, the body of the radiant heater 4 increases in temperature, while the auxiliary heater 38 maintains the switch 42 at or above its switch-over temperature. However, when the heating element 14, and possibly also the element 16, is de-energised at the completion of cooking, the auxiliary heater 38 is also de-energised. The switch 42 will then cool, but the rate of cooling will be slowed by the heat retained in the temperature monitoring device 30 by the buffer 44, and by heat transmitted from the radiant heater 4 by radiation and convection from the metal dish 6 and by conduction along the plate 48. In this way, the temperature monitoring device 30 can be constructed such that the temperature of the switch 42 does not fall to its switch-over temperature until the temperature of the glass ceramic top 2 falls to a safe level.

In general, because of the relatively high heat output of the heating element 14 in the embodiment described, it has been found that a single tempera ture monitoring device can adequately monitor the temperature of the glass ceramic top both when only the heating element 14 is energised and when the heating elements 14 and 16 are energised simul- taneously. It should be noted, though, that the construction of the radiant heater 4 is not restricted to two concentric heating elements and that many alternative constructions may be used.

It will 'c1e apparent, therefore, that the temperature monitoring device incorporating a buffer not only prevents the actuation of the neon lamp 58 or other warning device until the radiant heater has been energised for a predetermined time corresponding to the glass ceramic top exceeding a safe touching temperature, but also on de-energisation of the radiant heater prevents the switch 42 falling to its switch-over temperature until the temperature of the glass ceramic top has fallen to a safe level.

Although it is not shown in the drawings, the components of the temperature monitoring device can be contained in a housing made of aluminium or aluminium alloy, the components being mounted in the housing by means, for example, of a silicone rubber composition. In addition to providing mechanical support for the components, the silicone rubber also provides electrical and thermal insulation and can act as a heat store.

The temperature monitoring device need not be fixed directly to the side of a radiant heater, but, provided that it is thermally linked to one or more of the heaters, can be mounted at any convenient position within the cooker.

Claims (18)

1. An electric cooker comprising: a cooking surface; one or more electrical heaters arranged on the underside of the cooking surface; a temperature monitoring device thermally coupled with at least one of said one or more heaters and comprising a switch including a thermally sensitive mechanical switching element, an auxiliary electrical heater for supplying additional heat to the switching element, and a thermal buffer located between the auxiliary heater and the switching element; and means for indicating when the switching element is at or above a predetermined temperature.

2. An electric cooker as claimed in claim 1, wherein the cooking surface is made of glass ceramic and the or each electrical heater is a radiant heater unit.

3. An electric cooker as claimed in claim 1 or 2, wherein the or each heater has a thermally conductive outer cover in the form of a metal dish, the temperature monitoring device being mechanically attached to the metal dish in heat transmissive relation thereto.

4. An electric cooker as claimed in claim 1, 2 or 3, wherein the temperature monitoring device comprises a hollow cylindrical housing which is closed at one end, the auxiliary heater, thermal buffer and the switch being mounted in the housing.

5. An electric cooker as claimed in claim 4, wherein the housing is made of a ceramic material.

6. An electric cooker as claimed in claim 4 or 5, wherein the housing is formed with two opposing axially-extending slits for the passage of connectors for the auxiliary heater and for the switch.

7. An electric cooker as claimed in claim 4,5 or 6, wherein the auxiliary heater, the buffer and the switch are confined within the housing by means of a strip of spring steel which extends over the housing and engages with a pair of slots formed in a metal plate which is located across the open end of the housing.

8. An electric cooker as claimed in claim 7, wherein the temperature monitoring device is mounted on the heater by means of the plate.

9. An electric cooker as claimed in claim 4, wherein the housing is made of aluminium or aluminium alloy, the auxiliary heater, the buffer and the switch being mounted within the housing by means of a curable composition.

10. An electric cooker as claimed in claim 9, wherein the curable composition is a silicone rubber.

11. An electric cooker as claimed in any preceding claim, wherein the auxiliary heater is made of a positive temperature coefficient material.

12. An electric cooker as claimed in claim 11, wherein the positive temperature coefficient material has a resistance of from 500 to 2500 ohms at 25 C and a switching temperature (Tr) of at least about 185"C.

13. An electric cooker as claimed in claim 12, wherein the positive temperature coefficient material has a resistance of 1200 ohms at 25" C and a switching temperature (Tr) of about 185" C.

14. An electric cooker as claimed in any preceding claim, wherein the switching element comprises a domed bimetallic disc which moves with a snap action from one configuration to another at a predetermined temperature thereby opening or closing a set of contacts for controlling the indicating means.

15. An electric cooker as claimed in any preceding claim, wherein the thermal buffer is made from a sheet of mild steel having a thickness of substantially 0.8 mm.

16. An electric cooker as claimed in any preceding claim, wherein the auxiliary heater is electrically connected in parallel with a heating element of the heater with which it is associated.

17. An electric cooker as claimed in any preceding claim, wherein the indicating means comprises a discharge lamp or a filament lamp.

18. An electric cooker substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.