GB2181598A - Thermally-sensitive controls - Google Patents

Abstract

A thermally sensitive control 1 for an electric heater 2 comprises back-up protection means in the form of a pre-loaded spring mechanism comprising an actuating spring 25 which is held in normal use of the control in a compressed condition by retaining means in the form of push rod 27. The push rod 27 is adapted to soften and deform in the event of serious overheating of the heater consequent upon failure of a primary protector switch means of the control, whereby the spring 25 is released and moves an operating member 30 into engagement with a set switch contacts 15 to open the contacts and disable the heater. <IMAGE>

Description

SPECIFICATION Thermally-sensitive controls This invention relates to thermally-sensitive controls for electric heaters of containers such as electric kettles, jugs, urns, pans, coffee percolators, laboratoy equipment and the like (hereinafter referred to as "containers ofthe kind described"). The electric heaters of such containers are typically either immer- sion heaters, or similarly constructed heaters mounted externally to the container in good thermal contact with a wall thereof, for example by being brazed to the underside ofthe container base. Traditionally the heating elements of such heaters terminate in socalled "cold leads" by means of which electrical connections are made to the heater.

Thermally-sensitive controls for heaters of this type are well known which include electrical switch means operable to interrupt the power supply and thus deenergise the heater upon its element overheating, after four example having been switched on with insufficient liquid in the containerto coverthe element or having boiled dry. Such arrangements conventionally include a thermally responsive actuating means, typically a snap-acting bimetallicactuator, coupled mechanically to a set of breakable switch contacts wired in series with the element and operable to open the contacts upon the element overheating. Examples of such controls are described in British patent specifications Nos. 1,470,366 and 2,042,269.

In such arrangements failure ofthethermally responsive actuating means can represent a serious problem since if, for example, the container boils dry and the element overheats, the power supply will not be automatically interrupted, and in the absence of manual intervention the element will become hotter and hotter until it eventially burns out. This not only leadsto the expense of a replacement element but may also constitute a serious safety hazard in the case of plastic containers or containers having plastic parts, since the over-heating can cause melting and in the extreme burning of all or part ofthe container. The problem is of particular significance where the control is used with an appliance which is likely to be unattended, for example an automatic electric kettle organ early morning tea maker.

In an attempt to overcome this problem it has been proposed to provide back-up protection including means operable to interupt the power supply upon failure ofthe primary actuating means when the element overheats. One such arrangement is described in British patent specification No.2,149,217, wherein back-up protection is provided by a further thermally responsive actuating meansoperablein- deperidentlyoftheprimaryactuating meanstoopen a set of switch contacts and thus deenergise the heater in the event of serious over-heating consequent upon failure of the primary actuating means.In the arrangement described in British patent specification No.2,149,217 the primary actuating means comprises a snap-acting bimetallic actuatorwhilstthe back-up protection is provided by a slow acting creep bimetal,although it has also been proposed to in clue two snap-acting bimetallic actuators coupled to respective sets of switch contacts and calibrated to provide primary and back-up protection.

Whilst in many applications the use of a thermally responsive actuating means such as a bimetallic ac tuatorcoupledtoasuitableswitch means provides adequate back-up protection, in certain circumstances, particularly in the case of high wattage density heaters, experience has shown that reliable operation may not be achieved. For example, problems can be caused by the effects of the "over-shoot" heating which results from the residual heat in the element after the heater is deenergised.Such heating can cause melting and deformation of plastic parts ofthe control and this in turn can renderthe known type of back-up protection means ineffective, and in certain circumstances a secondary bimetallic actuator may operate initially to open the switch contacts but shortly thereafter distortion of the control consequent upon the over-shoot heating is such that the contacts reclose. Under such circumstances current is thus allowed to flow once more to the element and this clearly represents a serious hazard.

An alternative approach to providing back-up protection is the provision of an electrical fuse wired in series with the element. Whilst a suitably callibrated fuse can provide adequate protection and avoid problems discussed above concerning possible reclosing of switch contacts, the provision of a fusable link can undesirably increase manufacturing costs.

Viewed from one aspect the invention provides a thermally-sensitive control for an electric heater of a container ofthe kind described, said control including a thermally-sensitive actuating means arranged in use in good thermal contact with a part of the el ementandcoupledto a set of switch contacts of electrical switch means ofthe control so that the actuating means is operable to open the contacts and deenergise the heater upon over heating of the element, wherein back-up protection for the control is provided buy a pre-loaded spring mechanism associated with the same or a further set of switch contacts ofthe control and comprising an actuating spring which in normal use of the control is held in a pretensioned condition by a retaining means and an operating member coupled to the spring, said retaining means being adapted and arranged to release the spring in the event of serious over-heating of the control consequent upon failure of said thermallysensitive actuating means, whereupon the operating member is moved underthe force of the spring and co-operates with the switch contacts so as to open the contacts and deenergise the heater.

The provision in accordance with the invention of a pre-loaded spring mechanism adapted and arranged to be released in the event of serious over-heating consequent upon failure ofthe actuating means providing primary protection is advantageous to the ex- tentthat reliable operation may be achieved despite deformation of plastic parts ofthe control dueto over-shoot heating. Thus, a spring loaded mechanism can provide a stronger, more positive action for opening the contacts than is possible with bimetallic actuators used in the priorart proposals, and the possibility in extreme conditions ofthe contacts reclosing may be avoided.

In the design ofthermally-sensitive controls ofthis type there are generally size constraints forthe various components having regard to the desired overall compactness ofthe control, and a preloaded spring mechanism is also advantageous in this respect since such a mechanism can be designed relatively compactly. Moreover, such a spring mechanism can be incorporated in a control without substantially increasing the manufacturing costs.

The preloaded spring mechanism make take any convenientform, and it could for example comprise atension spring normally held in an extended condition bythe retaining means and adapted to be released in the event of serious over-heating so that the switch contacts are opened. However, since it is des arable to povide a relatively strong biasing force gen rally within relatively narrow size constraints the preferred preloaded spring mechanism comprises a compression spring which in normal use is held in a compressed state by the retaining means.

The form ofthe retaining means may vary depending on the general configuration of the control and on the nature ofthe actuating spring provided. Forexample, however, it is envisaged that such means may includea retaining elementfixedlysecured to or moulded integrallywith a body portion ofthecontrol which is adapted to mount the spring, the element being formed of an appropriate material and being so arranged as to deform, melt or break so as to release the spring when heating of the control has reached a predetermined level consequent upon failure of the primary protector.In the case of a compression spring, such may be mounted during assembly of the control within a suitable recess in the body portion and held undertension therein by means ofthe retaining element located at one end of the recess from which end the spring thus projects when released upon deformation ofthe retaining element.

In a preferred embodiment, however, the retaining menas comprises a displaceable means coupled to orcarried bythespring and arranged to co-operate with part of an external element defining the mounting location for the control so as to be displaced in such a way as to pre-load the actuating spring when the control is secured in place. Thus, the retaining element is resiliently urged by the actuating spring againstpartofthe mounting locationforthecontrol, and such part is arranged in suitable thermal contact with the element so that upon serious over-heating consequent upon failure ofthe primary protectorthe end of the retaining element remote from the spring melts or deforms sufficiently to release the spring mechanism and open the contacts.In a particularly preferred such embodiment where the spring mechanism includes a compression spring, the retaining element may conveniently comprise a slidable push rodwhich atone end is coupledto the spring and which at the other end projects externally of the control for engagement with part of the mounting location therefor.

The mounting location forthe control will varydepending on the nature of the heater and the type of container. For example, the control may be adapted for use with an immersion heater having a heating elementcarried buy a head which issecurableina watertight manner in or adjacent an opening in the container wall. In such an embodiment, the heater head maydefinethe mounting locationforthecon- trol, and in the arrangement discussed above wherein the retaining element comprises a slidable push rod, the push rod may conveniently be arran gedforengagementwith part of the head when the control is mounted thereto.Thus, the push rod and spring mechanism are advantageously arranged so thatupon mounting ofthe control to the heaterhead the push rod engages the head and is displaced axially so as to compress and preloadthespring. In the event of fail ure of the primary protector, when the head reaches a certain temperaturetheend ofthe push rod softens sufficiently to be distorted under the force ofthe spring which is thus released to open the contacts.

The operating memberwhich is adapted to be moved undertheforce ofthespring into engagement with the contacts may take any convenient form, and ina preferred embodimentwherethe spr- ing mechanism comprises a compression spring such member make conveniently comprise an arm which extends laterally from one end of the spring. In the case of a coil spring, such arm may conveniently beformed integrallytherewith.Themannerinwhich the operating member co-operates with the contacts of such a control may vary depending on the configuration ofthe switch means. Typically however, the switch contacts include interengaging contact members, at least one of which is carried by a re silientconducting strip or leaf spring so as to be displaceable away from the other contact member.In such an arrangement, the operating member may conveniently be arranged to engage the leaf spring carrying the movable contact member.

The general configuration of the switch means of the control may take any convenientform, and the pre-loaded spring mechanism may be associated either with the switch contacts which are adapted to be opened in response to operation ofthethermallyresponsive actuating means, or alternatively the control may comprise a furtherset of contacts arranged to be acted on bythespring mechanism. In oneemb- odiment, the control additionally comprises means wherebythe heater is disabled upon boiling of liquid within an associated container, such means including a furtherthermally-responsive actuating means exposed to steam orvapour emitting from the container and coupled to a set of switch contacts via an over centre spring mechanism. In such an embodiment, the pre-loaded spring mechanism can alternatively be associated with the contacts forming part of the steam sensitive switch mechanism.

The primarythermally-responsive actuating means and, if provided, the actuating means forthe steam sensitive switch mechanism, may both comprise snap acting bimetallic actuators of any con venient fo rm, for example a stressed sheet of bimetal having a generally domed configuration and a cen traltoungesuch as that described in British patent 1,542,257.The control may be fabricated from the suitablethermoplastics material, and in a preferred embodiment wherein the retaining element comprises a push rod which is arranged to be engaged and axially displaced by the heater head,the body part ofthe control which confronts the head and mounts the push rod may if necessary be formed a material having a higher heat resistance than other plastic parts ofthe control and than the push rod itself. This avoids the possibility ofthis body portion being distorted in the event of serious overheating before release of the spring mechanism in such a way as to impairthe movementofthe push rod.

An embodiment ofthe present invention will now be described, byway of example only, with reference to the accompanying drawings, wherein: Figure lisa partly schematic vertical crosssectional illustrating a thermally sensitive control secured to the head ofan immersion heater; and Figures2a and 2b are respectively diagrammatic cross-sectional elevational views illustrating in more detail a pre-loaded spring mechanism.

Referring firstly to Figure 1, a thermally-sensitive control 1 is mounted to the head 2 of an electric immersion heater, the control and heater head being interconnected by means of bolts (not shown) and thus being clamped within an aperture formed in the wall 3 of an associated container such as a kettle or hotwaterjug. The general configuration ofthe heater will be well known to those skilled in the art, and as is conventional the heater includes a heating element terminating in a pair of cold leads 4 which extend through the plate-like head 2 and having a so-called hot return part which is brazed to the head in good thermal contact therewith. A resilient annular sealing ring 6 is tightly engaged between the head and the control so asto provide a water-tight seal with the container wall 3.

The control 1 includes first and second body portions 7, 8which interengage axially and are so shaped to define cavities therebetween within which electrical switch means ofthe control are mounted. The second body portion 8 includes three terminal pins 9 arranged in a triangular configuration and adapted to be engaged with a separate female socket connector forthe supply of electrical powerto the heater, as is conventional with this type of control. The live and neutral ones ofthe terminal pins 9 are connected via electrical switch means to the respective cold leads 4 ofthe element, the free end portions of which extend through apertures 10formed in thefirst body portion 7 ofthe control.

Those skilled in this artwill befamiliarwiththe basic components of the illustrated control and will recognise thatthe general layoutcould be varied.

Similartypes of control are known,for examplefrom British Patent Specifications Nos. 1470366,2149217 and from ourco-pending British PatentApplication No. 8502170.The improvement of the presentinven- tion is applicable to all of these and other known controls.

The control illustrated in Figure 1 furthercom- prises a primary overheat protector including a snapacting bimetallic actuator 11 mounted in good thermal contact with a dimple 1 2formed in the head adjacent the hot return 5 ofthe element. The actuator 11 is of the general type described in British Patent Specification No. 1,542,257 and thus includes a central tongue 13 which is secured to the first body portion 7 ofthe control by means such as a hammer drive screw, and an upper peripheral portion 14. The upper portion 14 is arranged for engagement with an axially slidable push rod (not shown) extending through an aperture in the first body portion 7 and co-operating with a set of switch contacts (not shown) wired in series between eitherthe live or neutral terminal pin 9 and a respective cold lead 4.The actuator 11 is calibrated with a nominal operating temperature of 1 400Cso as to undergo snap action when the element overheats, for example having been switched on with insufficient water in the containerto coverthe element or in the event that the container boils dry. Upon operation of the actuator 11 it reverses its curvature from the configuration shown in Figure 1 and the upper peripheral portion 14 thereof is moved rightwardly ie, away from the heater head, so as to engage the push-rod which in turn opens the switch contacts so as to interruptthe power supply to the element.This general arrangement of primary protector is well known, and the par ticularconfiguration illustrated is similartothat described in more detail in our co-pending application no.8502170.

The control illustrated in Figure 1 furthercomprises a second set of switch contacts 15 arranged in series between the other cold lead 4 ofthe element and a respective terminal pin 9. The contacts 15 include contact members 16 carried byconductive leaf spring elements 17,18, one of which 17 is connected to the base portion ofthe respective terminal pin 9 and the other of which 18 is connected via a further conductive strip elementtothe respective cold lead 4. Such a contact arrangement is of the same general type as described in ourco-pending application no.

8502170, but many other similar configurations are possible.

The second set of contacts 15 is arranged to engaged by a slidable push-rod 20 which in turn cooperates with a lever member 21 forming part of a steam-sensitive over-centre spring mechanism (not shown. The over-centre spring mechanism may take any convenient form, and conventionally comprises a second thermally responsive actuating means, for exam plea further snap-acting bimetallic actuator, which is exposed to steam or vapour emitting from the container during boiling and which is adapted to trip the over centre spring mechanism so as to deenergise the heater. Thus, in the arrangement illustrated in Figure 1, upon boiling the lever 21 is tripped to a second position (not shown) wherein the pushrod 20 is urged leftwardly, i.e. towards the head, and engages a free-end part 22 ofthe leaf spring 18 whereby the contacts 15 are broken. As is conventional means (not shown ) are provided whereby the over centre mechanism may readily be manually reset.

The illustrated control further comprises a preloaded spring mechanism associated with the second set ofcontacts 15which isarrangedto provide back-up protection in the event that the bimetallic actuator 11 should fail. One form of spring mechanism is shown in more detail in Figures 2a and 2from which it will be seen thatthe mechanism comprises a coil spring 25 which is mounted within a recess 26 formed in the second body portion 8 and which in normal use is held under tension by means of a retaining element in the form of a push-rod 27.

Push-rod 27 is formed forexamplefrom nylon and extends th rough and is axially moveable within a bore 28 formed in the first body portion 7. Thus, when the control is clamped to the heater head, the push-rod 27 engages the head and is urged rightwardly relative to the control so that the spring 25 is placed undertension.

As shown in Figure 2b the spring 25 includes an operating memberintheform of an arm 30formed integrally therewith which extends laterally and which is arranged for engagement with the leaf spring 18 adjacent the free end portion 22 thereof. However, as shown in Figure 1 in normal use when the spring is pre-loaded by means ofthe push-rod 27, the operating member 30 of the spring is spaced axially from the leaf spring 18 so that normally the contacts 15 are free to open and close in response to operation ofthe levermember21 forming partofthesteamsensitive over-centre spring mechanism.

However, in event of failure of the actuator 11,the arrangement is such that the consequent overheating of the head causes the end ofthe push-rod 27 which engages the head to soften and deform under the force ofthe spring whereby the spring is released sufficiently to enable the operating arm 30to engage the leaf spring 18 and open the contacts. The heater is thus disabled, and since the contacts 15 are open underthe positive action ofthe coil spring 25 the possibility of subsequent reclosing ofthe contacts as a resultfor example of distortion of part of the control caused by over-shoot heating is avoided.

Modifications ofthe control described and illustrated herein will be apparent to persons skilled in this art and the disclosure hereof is intended to encompass all such modifications.

Claims (9)

1. Athermally-sensitive control for an electric heater of the kind described, said control including a thermally-sensitive actuating means arranged in use in good thermal contact with a partoftheelement and coupled to a set of switch contacts of electrical switch means ofthe control sothatthe actuating means is operableto open the contacts and deener gise the heater upon over heating ofthe element, wherein back-up protection for the control is provided buy a pre-loaded spring mechanism associated with the same or a further set of switch contacts of the control and comprising an actuating spring which in normal use ofthe control is held in a pret ensiohed condition by a retaining means and an operating membercoupled to the spring, said retaining means being adapted and arranged to release the spring i n the event of serious overheati ng of the control consequent upon failure of said thermallysensitive actuating means, whereupon the operating memberis moved undertheforce ofthe spring and co-operates with the switch contacts so as to open the contacts and deenergise the heater.

2. A thermally-sensitive control as claimed in claim 1 wherein said preloaded spring mechanism comprises a compression spring.

3. A thermal Iy-sensitive control as claimed in claim 1 or 2 wherein the retaining means comprises a displaceable means coupled to or carried bythe spring and arranged to co-operate with part of an external element defining the mounting locationforthe control so as to be displaced in such a way as to preload the actuating spring when the control is secured in place.

4. Athermally-sensitivecontrol as claimed in claim 2 and 3 wherein the retaining means comprises a slidable push rod which at one end is coupled to the spring and which at one end is coupledtothespring and which at the other end projects externally ofthe control for engagement with part of the mounting location therefor.

5. A thermal Iy-sensitive control as claimed in claim 4 adapted for mounting to the head of an associated electric heater, the push rod and spring mechanism being arranged so that upon mounting ofthe control to the heater head the push rod engages the head and is displaced axiallyso asto compress and pre-load the spring.

6. A thermal Iy-sensitive control as claimed in claim 5 wherein a plastics body part ofthe control which confrontsthe heater head and mountsthe push rod is formed of a material having a higher heat resistance than other plastic parts of the control and than the push rod itself.

7. A thermally-sensitive control as claimed in any preceding claim wherein the operating member comprises an arm which extends laterally from one end of the spring.

8. A thermally-sensitive control as claimed in claim 6 wherein said arm is arranged for engagement with a leaf spring carrying a movable contact of switch means of the control.

9. A thermal Iy-sensitive control substantially as herein described with referencetotheaccompany- ing drawings.