GB2209099A

GB2209099A - Switching system for radiant electric heater assembly

Info

Publication number: GB2209099A
Application number: GB8819346A
Authority: GB
Inventors: George Anthony Higgins
Original assignee: Micropore International Ltd
Current assignee: Micropore International Ltd
Priority date: 1987-08-25
Filing date: 1988-08-15
Publication date: 1989-04-26
Anticipated expiration: 2008-08-15
Also published as: DE8810596U1; GB8819346D0; GB8720054D0; GB2209099B; US4908496A

Description

- J RADIANT ELECTRIC HEATER ASSEMIBLIES 2209099 The present invention

relates to radiant electric neater assemblies such as may be used in electric cookers having a glass ceramic cooking surface.

Radiant electric heater assemblies are known which comprise a metal dish containing a base layer of thermal and electrical insulatinc material, a peripheral wa.-1 of insulating material anc. a plurality of heating elenents. The heating elenents i,:a, De of tare resistance wire ir may be i n the f orm of n:- r a - r eli -. i ar,-.ps A. thernal cut-out device is provioe(-i te, cisconnect- all power to the h-ating elements in the event ct overheating so as to protect the glass ceramic su.rfac--- f,,o,--, discclora-.lon or even oreakage and avoid heater failure. However, in certain moces of operation the therr-.al cut-out device can operate so frequently that the service life of the heater assembiy may be reduced. Ir. additicr. the operation of the cutout and the complete de-energisation of the heater may create the impression that the heater has malfunctioned.

It is an object of the present invention to provide a radiant electric heater assembly which alleviates these problems.

According to the present invention there is provided a radiant electric heater assembly for a glass cerairic top cooker comprising:

four radiant electric heating elements; a thermal cut-out device; and means associated with the thermal cut-out device adapted to connect the four heating elements in parallel with each other to Qive a first power lev(-! setting, which in the event of the thermal cut-out device detectinc a first tne heatinQ which the in series a pair of and which detects a tne first the predetermined temperature is adapted to connect elements in a lower power configuration in elements are connected as two elements connected with each other and connected in series with elements connected in parallel with each other, when the thermal cut-out device subsequently second predeternined temperature lower than predetermined temperature is adapted to reconnect heating elements in the configuration of the first powe level setting.

4 In addition. to extending the service life, the radiant electric heater assembly according to the present invention can also enable at least one of the heating elements to be -3 energised at &II heater is on.

times giving a visual indication tnat the The means associated with the thermal cut-out devicemay include switch means for selecting between the configuration of the first power level setting and a second, lower, power level setting in which the heating elements are connected as two elements connected in parallel with each other and connected in parallel with two elements connected in series with each other and which in theevent of t-ie -Iherr-.,al cut-out device cetectinc rhe first n,recieterr-iine----ei-,,1-.t-rature is adapted to connect heating elements in a lower power configuration in which +Che four elements are ec)nnectec,. ir, series with eac other, and which when the r-ner,,al cut-out device subsequently deects the second p.,edeter7ined ter,-,r)erature is adapted to reconnect the heatinc elements in the conficuration of the second power level setinc.

The two heating elements which are connected in series with each other in the lower power configuration of the first power level setting preferably correspond to the two heating elements which are connected in parallel with each other in the higher power level configuration of the second power 1Evel setting.

The electric heatino elements are prefer;-zt.,,lv in t..e forr. of infrared lamps.

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 perspective view, partly cut away, of a radiant electric heater assembly arranged oeneatr.. a glass ceramic COOKing surface; Figure 2 is a circuJLt diagrar. illust-rat-inq one ei-iiDouiment of a radiant electric heater assembly accorcinc r)resent invention; 3 t- ( the E'i g u r e illustratinc another is a circuit diagram embodiment of a radiant electric heater assej-,---'lv n-ccording to the present invention; Figure 4 is a circuit diagram showing in more detail the circuit diagram of Figure 3; and Figure 5 shows the switch positions for each power level setting of the arrangement shown in Figure 4.

Figure 1 beneath a comprises containing shows a known radiant electric hcater 2 arranged glass cerar-,,ic cookinc surface- The heater a dish 3, for example made of metal, the dish a base layer 4 of thermal and electrical insulation material such as a microporous material sold under the Registered Trade Mark MICROTHERV.. A peripheral wall 5, for example of ceramic fibre material, extends around the internal periphery of the dish 3. Four heating elements 6 in the form of infra-rec; lamps -17-xtend above the base layer 4 and a thermal cur-o,.,t-- device - in the form of a probe-type differential expansion extends Detween the heatina elements C and operates c 51sconnect the heating elements f rom a source c.' electr., cal energy (not shown) should the probe of th- t-.erma-', cut-out device exceed a f irst predeterr-.lned tLc-r7i,.era,.ure s', as to protect the qlass ceramic cookinc Sjrface trol. excessive temperatures. Af ter the tl-,e,---;a.' cur -0J11 device has operated to disconnect the source of electrical energy the temperature will decrease and au a second, predetermined temperature, lower than the f i r s t predetermined temperature, the thermal cut-out device will operate to connect the source of electrical energy cnee again.

In the embodiment of the radiant electric heater assembly according to the present invention illustrated by the circuit diagram of Figure 2 there are four heating elements in the form of infra-red lamps which may be rated at, for example, 1000 watts each and which are arranqed in a heater dish having a diameter of for example 30Omm. Different power settings for the heater can be obtained in a number of ways. For example in one embodiment the different power levels can be obtained by a switch which connects the lamps in various parallel and series configurations, the configuration shown in Figure 2 corresponding to full power. Alternatively, the different power levels of the heater can be obtained by connecting 11 and 12 of the heater shown in Figure 2 to a 1 device such as an electronic phase control or mark-to-space ratio regulator. The thermal cut-out device incorporates four sets of contacts 13,14,15 and 16.

However, in practice, only contacts 13 and/or 14 need to be incorporated into the thermal cut-out device and the remaining contacts 15,16 and possibly 14 or 13 can be incorporated into one or nore separate relays which can be triggered b.,., the opening or closing of the contacts 13 and/or 14. When the heater is first energised the thermal cut-out device is in its normal position and allows current to pass to the lamps 10 which in the full power configuration illustrated are connected in parallel with each other. - Contacts 13,14 and 15 are closed in this configuration.

connections suitable contro Z R - 1 I f' tne temperature of the probe of the thermal cutout device exceeds a first predetermined value the cut-out device operates, not to disconnect all of the heating elements from the source of electric power, but to open the contacts 13,14 and 15 and to close contacts 16. This connects the lamps in a configuration in which, two lamps are connected in series and are connected in parallel to the remaining two lamps which are also connected in series. This reduces the power to substantially 34 per cent of full power and allows the temperature of the glass cerar-,,Lc to fall anc., the temperature of tne proiDe of the thern,al cutout device te fall to a second predetermined temperature at,jhict-. the contacts 16 art: opened and the contacts 13,14, and lS are closed to restore full power to is the heating elements.

It is important in this embodinent tha the contacts 15 should open prior to the closing of the contacts 16 and that the contacts 16 should not close until after any arc caused by the opening of the contacts 15 is extinguished.

Similarly the contacts 16 should open and any arc should be extinguished before the contacts 15 close. The tin, ing of the opening and closing of the contacts 13 and 14 is not critical.

We have found that, in circumstances where heat is not discipated frr)r the cooking surface 1 quickly enough to - 8 avoid overheating, the arrangement whereby the action of the thernial cut- out device reduces the power of ihe- lamps -rather than disconnecting all power to the lamps reduces the number of times the thermal cut-out device switches in a given period and thus increases the useful life of its contacts. Moreover, for any given period during which the heater is energised the lamps 10 will run at full power for a lower proportion of the time. This leads to an increased service life for the lamps. Nevertheless, for the embodiment of the invention described with reference to Figure 2 it is vitall that the contacts 15 should open before the contacts 1E close and that the contacts 16 should open before the contacts 15 close.

In the enbodiment of rhe radiant electric heater assembly according tc the p-esentl- invention illustrated by the circuit diagram of Figure 3 there are four heating elements as with the embodiment of Figure 2. The conf iauration shown in Figure corresponds to full power.iile the contacts 20,21 of the thermal cut-out device are closed.

If the temperature of the probe of the thermal cut-out device exceeds a fitst predetermined value the thermal cut-out device operates to open the contacts 20,21 and thus to connect two of the lamps (the lower pair in the Figure) in parallel with each other and the remaining lamps (the z upper pair the power allows the temperature fall to a contacts 20 elements.

in series. with the parallel pair. This r.i-duces to substantially, 18 per cent of full pow-r and temperature of the glass ceramic to fall a.,-, d the of the probe of the thermal cut-out device to second predetermined temperature at which the 01 21 close to restore full power to the heating With this embodiment, in addition to increasing the service life of the lamps and the contacts, the timing- of the opening and closing of the contacts is not crucial because no short circuit configuration can exist. The contacts 21 carry the current of three lamps whereas the contacts 20 carry the current of only, two lamps.

in practice it is nct necessary. for both contacts 20,21 to be incorporated intc tie thermal cut-out device and either set or both sets of contacts could be incorporatec into a separate relay.

Other configurations are also possible Although for simplicity we have described above the power being reduced only from full power to a lower power configuration, and indeed for heater control by phase control or mark-to-space ratio regulators no further 10- configuration may be needed. However, for heater control by multi- position switch it is also possible to connect the heating elements so that in the event of the thermal cut-out device having detected a temperature which exceeds the first predetermined value and with the multi-position switch set to any intermediate power setting the heating elements are connected in a lower power configuration or the same power configuration where the heater power setting is low. This is illustrated in Figure 4 in which the same reference numerals as those used in Figure 3 are employed to denote the sane components. Figure 4 shows a more complete circuit diagram in which the contacts 20, 21 are used to connect the lamps 10 in a lower power configuration at a number of power level settings if the temperature, of the probe of the thermal cut-out device exceeds a first value. Figure 4 also shows contacts 22, 23, 24, 25, 2r6S, 27 and 28 which are incorporated into a multi-position switch and which in co-operation with diodes 29 and 30 are used to connect the lamps in six different giving six different power levels plus an off setting. The power levels are level 1 with all four lamps connected in series with each other and with both diodes (7 per cent of full power), level 2 with all four lamps connec.ted in series (12 per cent of full power), level 3 with two pairs of lamps connected in series with each other and with a diode, the two pairs being connected predetermined configurations - 1 1 - in parallel such -that the diodes conduct in alternate half cycles of the power supply (20 per cent of full power), level 4 with two pairs of lamps connected in series with each other, the two pairs being connected in parallel and supplied via parallel opposite-poled diodes (34 per cent of full power), level 5 with two lamps (the lower pair in Figure 4) connected in series, the two lamps being connected in parallel with the remaining two lamps (67 percent of full power) and level 6 in which all four lamps are connected in parallel (100 per cent of full power). Figure 5 shows which of tne contacts 22 - 28 should be open and which should be c-',ose.j to give any particular power level. At levels 1 and 2 itis considered unnecessary to switch to a lower power configuraticn should the is temperature of the prooe of the thermal cut-out device exceed the f irst or second predeterr-,,irie,-; values Decause this situation shoulc only arise it the!eve-', ischanged fron a high level to a lower level while the temperarure is in excess of one of the said values anc will quickly fall to below the second predetermined value. At level 3 the contacts 20, 21 co-operate to connect the lamps so that two lamps (the upper pair in Figure 4) are connected in series with each other and with the diode 30 in the event of an excessive temperature, the remaining two lamps being de-energised. Similarly at level 4 the contacts 20, 21 co-opera-te to connect two lamps (again the upper pair in Figure 4) in series with each other in the event, of an excessive temperature with the remaining two Lar-iL:L-- r)eing de-energised. At level 5, the contacts 20, 21 co-operate to connect the four lamps in series with each other in the event of an excessive temperature, while level 6 has already been described in connection with Figure 3. It can be seen that the lamps which carry a higher current in normal operation at level setting 5 (the upper pair in Figure 4) are the sane as the lamps which carry all the current at level setting 6 with the linitel. contacts open (though thev are then connected ir series) i s facilitates arranc(ement of the lamps aestheticalIv pleasing appearance at settings and states of limiter operation.

d c) provide an different power k _1 -13

Claims

CLAIMS:

1 1. A radiant electric heater assembly for a glass ceramic top cooker comprising:

four radiant electric heating elements; a thermal cut-out device; and means associated wit, the thernal cut-out device adapted to connect tne four heating elements in parallel with each other to give a fl-'sr power level setting, which in the event of the ther--l cut-out device detectinq a f irst predeternineo t-e7iper,ture is adapted to connect elements in a -1ow- power configuration in elements are connected as two elei-,,ents connected with each otner anc. connected in series with the Ineating which the in series a pair of elements connected in parallel with each other, and which when the thermal cut-out device subsequently detects a second predetermined temperature lower than the first predetermined temperature is adapted to reconnect the heating elements in the configuration of the first power level setting.

2. A radiant electric heater assembly as claimed in claim 1, wherein the means associated with the thermal cut-out device includes switch means for selectinq between the configuration of It---he first power level setting and a second, lower, power level setting in which the heating elements are connected as two elements connected in parallel with each other and connected in parallel with two elements connected in series with each other and which in the event of the thermal cut-out device detecting the first predetermined temperature is adapted to connect the heating elements in a lower power configuration in which the four elements are connected in series with each other, and which when the thermal cut-out device subsequently detects the second predetermined temperature is adapted to reconnect the heating elements in the configuration of the second power level setting.

3. A radiant electric heater assemoly as claimed claim 2, wherein the two heating elements which are connected in series with each other in the lower power configuration of the first power level setting correspond to the two heating elements which are connected in parallel with each other in the higher power level configuration of the second power level setting.

4. A adiant electric heater assembly as claimed in claim 1,2 or 3,wherein the electric heating elements are in the form of infra-red lamps.

v 4 A radiant electric heater assembly as clair,e-c in and substantially as hereinbefore describec:- with reference to the accompanying drawings.

5.

c 1 a i m., 1 Published 1985 a! The Patent Office. State Housc 66 7' Hi'. Hc)'b:rn- London WC1R 4TP F-arther C::-le may be obtaine froin The Patent 051ce. Sales Branch, St Man- Cray. Orpingtcr.. Kent BR5 3RD Printed by Multiplex techniques ltd. St Mary Cray, Kent. Con- 1,87.