GB2209100A - Switching system for electric heating assembly - Google Patents

Description

1 i 1 ELECTRIC HEATER ASSEMBLIES z0 g- ^ r 4 UC1J1100 The present

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

Electric heater assemblies are known which comprise a metal dish containing a base layer of thermal and electrical insulating material, a peripheral wall of insulating material and a plurality of heating elenents The heating elements may re of bare form of infra-rec lamps resistance wire or mnay be in the A thernal cut-out device is provided tc disconnect all power to the theating elements in the event of overheating so as to protect the glass ceramic surface from discoloration or even breakaae. Where, as hitherto has Deen the case, the rating of the heating elements does not exceed about 2500 watts a thermal cut-out device having a standard current rating of ten amps is employed. However, where the currenzi exceeds ten amps it is necessary to provide some means for accommodating the higher current in order to avoid premature failure of the thermal cut-out device. one way in which the problem could be overcome would be to use an alternative thermal cut-out device havin a higher rating or additional contacts of the same rating. However, the expense involved in developing and life testing such a device is unlikely to be n A - commercially economic.

1 It is an object of the present invention to provide an electric heater assembly which is capable of accommodating relatively high current.

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

a Dlurality of electric heatinQ elements; a thermal cutout device incorporating a first set of contacts for controlling the supply ot electric current to at least one of the heating elements; and m eans associated with the thermal cut-ou"--- device and incorporating a second set of contacts for controlling the supply of electric current to at least another of the heating elements, wherein in the event of the thermal cut-out device detecting a first predetermined temperature the first set of contacts, moves from a first position to a second position thereby causing the second set of contacts to move vl> 11 11 f rom a f irst position to a second positi...)n and in co-operation with the second set of contacts c)nnects the heating elements in a lower power configuration or cuts off the supply of electric power to the heating elements and wherein when the thermal cut-out device subsequently detects a second predetermined temperature. lower than the first predetermined temperature the first set of contacts moves from its second position to its first position thereby causing the second set of contacts tc move f rom its second position to its f irst position anc, connecting the heating elements in tneiE or,-cinal configLrai.oi-,.

Th e heating elements r-., a y r_. e 1 n the f, _), - _ c _ infra-red lamps. Four electric heatinc elements mav be orc.vided.

The means associated with the t.nenal cut-out device may comprise a relay. The relav nav incorporate a coil which is connected in parallel wit"-, tne first- set, cf contacts.

Alternatively, the relay nay incorporate a coil which is connected in parallel with one of the heating elements. In this case, a voltage dependent resistor may be connected in series with the relay coil.

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 4 - accompanying drawings -in whi-h:

Figure 1 is a perspective schematic view, partly cut away, of an electric heater assembly arranged beneath a glass ceramic cooking surface; Figure 2 is a circuit diagram illustrating one embodiment of an electric heater assembly according to the present invention; Figure 3 is a circuit diagram i'Llustrating another embodiment of an electric heater assembly according to the present invention; and Figure 4 is a circuit diagram illustrating means for ensuring operation of a relay.

Figure 1 shows a known radi-ant electric heater 2 arranged beneath a glass cerar-nic cooking surface '.

L The heater comprises a dish 3, for example made of metal, the dish containing a base layer 4 of thermal and electrical insulation material such as a microporous material sold under the Registered Trade Mark MICROTHERM. 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-red lamps extend above the li base la.,.-r 4 and a thernal cutout device 7 in ti",e turm of a probe-r- ype differential expansion switch extends ber-ween the heating elements 6 and operates to disconnect the heating elements from a source of electrical energy (not shown) should the probe of the thermal cut-out device exceed a first predetermined temperature so asto protect ceramic Af ter disconnect the glass temperatures operated to temperature will, decrease and at a second oredetermined temperat,:-e, lower than tile first Preaetermined temperature, the theri-aal cut-out device Wi- 1 11 operace to connect the source of electrical energy once again.

cooking surface from excessive the thermal cut-out device has the source of electrical energy the Although the arrangement described with respect tc, ic,,ire 1 has 1Deen found, satisfactory for lower power heaters,it is not suitaole for he,-,ters operating at higher power ratings, for example oil tne order of 4k',". The totai, current drawn at these higher powers exceeds the 10 ampere rating of normal commercially available limiters. It would be possible, as noted earlier, to provide the limiter with higher-rating or additional contacts. However, the development and life-testing of such a device would be very costly and time consuming. Another solution would be to use the limiter to operate a relay which in turn controls the heater. However, to interrupt the heater current such a relay would itself need either high current rating contacts or at least two sets of lower ratinc- coiit-.cts. In either case the relay would be relatively expensive. More complex control of the energisation of the heater lamps would require additional relay contacts for individual switching of different parts of the heater circuit, also increasing the relay cost. These problems are avoided or at least alleviated with the present invention.

In the embodiment of the electric heater assemibly, according to the present invention illustrated by the circuit diagram of Figure 2 are four heating elements 10a,10rj,10c and 10d in the form of infra-red lar,-ps which nay be rated at, for example, 1000 watts each and which are arranged in a heater having a diai-,leter of for example 30Omm. Different- power levels 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 paraIlel and series configurations p the configuration shown in Figure 2 corresponding to full power. Alternatively, the different power levels of the heater can be obtained by connecting connections 11 and 12 of the heater shown in Figure 2 to a suitable control device such -as a known energy regulator or an electronic phase control or mark-to-space ratio regulator.

1 - 1 A 1, 41 The contacts of the. thermal cut-out device are incorporated into the electrical circuit in such a way.ljat only part- of the electric current passing through the heater actually passes through the contacts of the thermal cut-out device.

In the circuit diagram illustrated in Figure 2, contacts 13 or 14 are incorporated into the thermal cut-out device with the contacts 15,16 and 14 or 13 incorporated into a separate relay (not shown as such in Figure 2) which is triggered by the operation of the contacts of the thermal cut-out device.

When the heater is f irst enercisec tne thermal cut-out device is in its norL,al position and allows current to pass to the lamps 10a, 101., 10c, and 10d whi& in. tne full power configuration illustrated are connected i n para'Llel with, each oti,er. Contacts 13f14 a n r-, 15 ar closed i n this configuration. If the temperature of the probe of the thermal cut-out device exceeds a f irst precetermined value the cut-out eevice operates, noi to cisconnecr- a11 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 lam, ps in a conf iguration in which two lamps are connected in series and are connected in parallel to the remalning 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 ceramic to f all and the temperature of the probe of the thermal cut-out device to fall to a second predleterj.iined temperature at which the contacts 16 are opened and the contacts 13,14 and 15 are closed to restore full power to the heating elements.

It is important in this embodiment that the contacts 15 should open prior to the closing of the contacts 16 and that the contacts 16 should not close until any arc caused by the opening of the contacts 15 is extinguished. Similarly th., conracts 16 should open anc any arc should be extinguished before the contacts 15 close. The timing of the opening and closing of tile contacts 13 ar.( 14 is not critical.

It is not essential to maintain energisation of the heating is elements (although at reduced power)if the temperature of the probe of the theri-iial cutout device exceeds the first predetermined value, and the heating elements could be completely disconnected from the supply of electrical energy under this condition.

We have found that splitting the power supply to the lamps between the thermal cut-out device and the relay optimises the use of the thermal cutout device and permits use of a less expensive relay than would otherwise be necessary.

7 1 11 In the embodiment of the electric heater assenoly according to the present invention illustratec:Dy the circuit diagram of Figure 3 there are four heating e.-lenents 10a,10b,10c and 10d as with the embodiment of Figure 2. The configuration shown in Figure 3 corresponds to full power while the contacts 20,21 of the thermal cut-out device and an associated relay are closed.

If the temperature of the prone of the thermal cut-out device exceeds a first predetermined value the thermal cut-out device operates to open thzcontacts, 20,21 and thus to connect two of tne lai-,ips in parallel with each other and the renaininc lai.. :)s in sefieS wit the parallel pair. This reduces the power to sunstantially 18 per cent of full oower and allows t temperature of the glass is ceramic to fall and t.e.er-,,;Derature of trie probe of the thermal cutout device to fall to a second predetermined temperature at which t,. e contacts 20 a n d 21 close to restore full power to the heating ele,-,e- nts.

With this embodiment the timing of the opening and closing of the contacts is not crucial because no short circuit configuration can exist. Because the contacts 20 carry the current of three lamps whereas the contacts 21 carry the current of only two lamps we have found that with lkW lamps it is preferable to incorporate the co: zacts 21 into the thermal cut-out device and to incorpora -- the contacts 20 into a separate relay. With lower power lamps it would be feasible to interchange the arrangement of the contacts 20 and 21, so that the thermal cut-out oevice switches the larger current. once again it is not essential to maintain energisation of the heating elements at reduced power if the temperature of the probe of the thermal cut-out device exceeds the f irst predetermined value, and the heating elements could be completely disconnected frorn. the supply of electrical energy under this condition.

The relay incorporating the contacts 20 can be connected in 1 parallel to the contacts 21 of the ther7-.al cut-out device.

As an alternative the relay can oe connecrei in parallel to is the lamp 10a in Figure 3. Fowever in tne full power configuration, althouah the voltage across the relay assembly is 100 per cent of supply voltage when the contacts 21 are closed, the voltage on-ly falls to 40 per cent of supply voltage when the contacts 21 are open. This is not normally sufficiently low to ensure that the contacts 20 open when the contacts 21 open. This can be overcome by adopting the configuration illustrated in the circuit diagram of Figure 4 according to which the supply voltage V is applied to the relay coil 30 by way of a bridge rectifier 31 and a resistor 32 and,a voltage 311 0 11P 1:1 dependent resistor 33 whici, are connected in series. A capacitor 34 is connected in parallel with the relay coil 30.

Claims (8)

1. ' An electric heater assembly for a glass ceramic ton cooker comprising:

a plurality of electric heating elements; a thermal cut-out device incorporating a f irst set of contacts for controlling the supply of electric current to at least one of the heatina elements; and T., --- a n s associated w i t-- 1 the t h e r r., a 1 cut-out device and incorporating a second set of contacts for controlling the supply of electric current to at least anot,her o f the heating elements, wnerein in the event of the thermal cut-out device detecting a first precetermined ter-.ir)era,-i:re tine first set of contacts -oves from a first Position to a second position thereby causing the second set of contacts to move from a first position to a second position and in co- operation with the second set of contacts connects the heating elements in a lower power configuration or cuts off the supply of electric power to the heating elements and wherein when the thermal cut-out device subsequently detects a second predetermined temperature lower than the D first predete1-,lned temperature the "irsl,- se-.- ceritacts moves f rom itb secono posi t ion r_ 0 1 L. S I ' L p- C'S i t 1 on thereby causing the second se, of contacts te i.-tc)vc- froi,-, its second position to its first position and connecting the heating elements in their original configuration.

2. An electric heater assembly as claimed in claim 1, wherein the electric heating elements are ir; the form of infra-red lamps.

3. An electric heater assent.],,., as ir. clai,,-, 1 or 2, wtierei,,i fouL eiectric near-ine elc-ner,L-z

4. An electric heater assen.l., -"-,, a cla-,je(- in claim 1,2 or 13, whercin tne f"tear:s associ-;--tc-,- cut-out device co,,-,-,)r-ises a 5elav.

5. An electric heater asseT-t-ll, a--: 1:: clain 4, wherein the 1,elay incorporates a coi., i. in parallel with the first set of contacts.

6. An electric heater assembly as clained in claim 4, wherein the relay incorporates a coil which is connected in parallel with one of the heating elements.

29

7. An electric heater assembly as claime(l in claim 6 and including a voltage dependent resistor connected in series with the relay coi-'.

n

8. An electric heater assembly as claimed in claim 1 and substantially as hereinbefore described withreference to the accompanying drawings.

cl i 1988 at The Patent Offce. State House- 66 71 Ho'b::r.i. London WC1R 4TP F- ,Irther ecre= me-.- be obtained from The Patent 0'-"1--e.

Sales Branch, St. Mary Cray. Orpingtcr.. Kent BR5 3RD. Printed by Multiplex tecl,=ques itd. St Mary Cray, Kent. Con. 1187.