Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
  • H01H37/00—Thermally-actuated switches
  • H01H37/02—Details
  • H01H37/32—Thermally-sensitive members
  • H01H37/52—Thermally-sensitive members actuated due to deflection of bimetallic element
  • H01H37/54—Thermally-sensitive members actuated due to deflection of bimetallic element wherein the bimetallic element is inherently snap acting
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
  • H01H37/00—Thermally-actuated switches
  • H01H37/02—Details
  • H01H37/32—Thermally-sensitive members
  • H01H37/52—Thermally-sensitive members actuated due to deflection of bimetallic element
  • H01H37/54—Thermally-sensitive members actuated due to deflection of bimetallic element wherein the bimetallic element is inherently snap acting
  • H01H2037/5454—Thermally-sensitive members actuated due to deflection of bimetallic element wherein the bimetallic element is inherently snap acting with separate spring biasing the bimetal snap element against the heat transfer surface

Definitions

• This invention concerns improvements relating to thermally-responsive controls and, more particularly, concerns bimetallic controls as utilised to control the operation of an electric heating element wherein a bimetallic actuator is held in thermal contact with the heating element to be responsive to the temperature thereof.
• bimetallic controls are well known and find application inter alia in water heating appliances such as electric kettles and hot water jugs.
• Heating element overtemperature protection controls are well known in the field of domestic kettles and hot water jugs where they are commonly used . to protect the heating element against overheating in the event of the kettle or hot water jug being switched on without first being filled with water.
• Such controls commonly employ snap-acting bimetallic actuators comprising a dished bimetallic element which is moveable with a snap-action between two oppositely dished conditions.
• the carrier was made of a material selected to soften or melt at a predetermined temperature so that in the event of the bimetal failing properly to operate its associated switch in a heating element overtemperature situation, so that the heating element temperature continued to rise, the carrier would be collapsed by its biasing springs towards the heating element.
• the collapse of the carrier was arranged to effect a secondary control function.
• the XI control and its successors, the X2 and X3 controls, are well known.
• the X2 control is substantially as described in GB 2283156 and the X3 control is substantially as described in GB 2248724.
• the bimetal was simply placed with its convex face towards the heating element and movement was achieved when the bimetal snapped to its opposite curvature, with the centre moving towards the control and the edges snapping down onto the heating element surface to provide an abutment against which the output force could be generated. Because of the way that the XI gave compensation for distortion of the heating element head, adequate heat transfer and movement were available to make the control function satisfactorily, although it was always known that the heat transfer would be improved if the bimetal could be pressed more positively against the heater. In the design of the X3 it was elected to place small hooks at the corners of the bimetal to retain it in the carrier, since for use with flat heating elements the snap clips provided in the
• XI control at the tip of the push rod were preferably not provided so as to avoid interference with the heating element surface.
• the hooks tended to hold the bimetal away from the heating element, giving the X3 initially a slower response than the XI.
• shoulders were placed behind the hooks so that the bimetal was trapped between the shoulders and the hooks.
• the X-series of controls were originally designed for use with conventional sheathed heating elements used in an immersion or underfioor heating mode.
• the set temperature of the bimetal needed to avoid nuisance operation during the life of the product, especially after the heating element has suffered scaling has increased to the level where it is difficult to manufacture.
• the speed of response of the bimetal depends on its set temperature and the high settings tend to give a slow response.
• the X4 does not have the benefit of a floating carrier to absorb element distortion, so the movement of the bimetal has to be optimised to guarantee opening the contacts. This is especially true of the cycling versions, as opposed to the latched version.
• the comers of the bimetal are retained by a metal clip which is quite thin (0.2mm), and the clip does not add much to the effective bimetal movement, as is the case with the X3.
• the invention aims to do this by improving the heat transfer between a bimetallic actuator and also optimising the movement available from the actuator.
• the present invention proposes a bimetallic control wherein a bimetallic actuator of the control is mounted so as to be subjected by its mounting in use of the control to forces tending to increase its curvature, the mounting furthermore being such as in use of the control with a heating element to enable the actuator to be biased against the heating element with a force sufficient to reduce its curvature.
• Figure 1 shows a perspective view of an X4 control as described in our British Patent Application No. 9909171.2;
• FIGS. 2A, 2B and 2C are different perspective and side elevation views of a spring metal bimetal mounting carrier for an X4 control modified in accordance with the present invention.
• Figure 3 illustrates schematically the forces arising in use of the bimetal mounting carrier of Figures 2A, 2B and 2C in an X4 control as shown in Figure 1.
• FIG. 1 shown therein is an X4 control as described in our British Patent Application No. 9909171.2 to which reference may be made for a more detailed description than will be given herein.
• the control is an integrated 360° inlet connector for a cordless appliance, based upon our
• the 360° inlet connector is designated in Figure 1 and is integrated with first and second bimetallic overtemperature protection controls designated 2 and 3.
• the controls 2 and 3 are each adapted to disconnect the electricity supply through the inlet connector 1 to an associated heating element (not shown) in the event of the heating element temperature rising above a certain level, for example because a water boiling vessel fitted with the heating element has been switched on without first being filled with water or, alternatively, has been allowed to boil dry on account of having been boiled with the lid off or open thereby bypassing a steam sensor provided in the vessel.
• the two bimetals may be set to operate at the same temperature or, more preferably, there may be a temperature difference between the settings of the two bimetals so that one operates normally as a dry boil protector and may be automatically resetting, whereas the other provides a secondary protection facility operable only at a higher temperature in the event of failure of the dry boil protector and may be arranged to latch in its operated condition.
• either or both of the controls 2 and 3 may have associated therewith a latching arrangement such as to be reset by the operation of removing the associated cordless appliance from its base and then replacing it; this concept of latching out a bimetallic heating element overtemperature protection control when it operates in response to a sensed overtemperature and enabling it to be reset by disconnection and reconnection of the plug and socket connector set of the appliance was first disclosed in GB 2176055.
• Each of the controls 2 and 3 has a generally rectangular bimetal 4 formed as shown with a generally X-shaped cut-out 5 and the respective bimetals are each supported in a spring metal carrier 6 which at one (lower) level is secured to a plastics moulding of the X4 control and at another (upper) level has formations at its opposite ends which locate and support opposite ends of the bimetal.
• This method of supporting the bimetals enables them not only to accommodate dimensional variations between one heating element and another caused by unavoidable manufacturing tolerances, but also enables them to accommodate heating element distortions caused by thermal expansion/contraction effects during operation.
• Each bimetal has an associated push-rod (not shown) which, when the bimetal snaps from its cold condition (when its curvature is convex towards the surface of an associated heating element) to its oppositely curved hot condition, causes a respective set of switch contacts (not shown) within the control to open thereby breaking the current supply path through the control.
• one at least of the bimetal mounting springs is formed integrally at its lower level with an overcentre arrangement interfacing with the push-rod so as to latch the push-rod in its operated condition once the bimetal has operated.
• an arrangement for resetting such a latched out arrangement by lifting and replacing a cordless vessel on its base may be provided.
• FIG. 1 shows the carrier 10 in perspective view as it would appear on the control
• Figure 2B is an inverted perspective view
• Figure 2C shows the carrier 10 in side elevation view with a bimetal 4 supported in the carrier.
• Figure 2A shows the spring metal carrier 10 as comprising a generally rectangular base position 12 having long sides 13, which are apertured at 14 for mounting to the plastics material body of the X4 control, and short sides 15, which terminate in upstanding portions 16 from the upper edges whereof there are generally inwardly extending formations 17, 18 and 19.
• the portions 17 and 18 are at the comers of the spring carrier 10 and terminate, at their free ends, in a generally flat portion 20 from the outer edge of which there extends a downwardly-depending finger 21.
• the portions 20 overlie the comers of the bimetal when it is mounted in the carrier, and fingers 21 serve to locate the longitudinal edges of the bimetal and the portions 19 underlie the bimetal and retain it in position. This can be seen in Figure 2C where the bimetal is designated 30.
• FIGS. 2A, 2B and 2C Also shown in Figures 2A, 2B and 2C is the provision on the portions 20 of the carrier spring 10 of projections or pips 22 which extend from the portions 20 for contacting the underlying bimetal.
• These pips 22 serve a dual function in keeping with the teachings of the present invention, namely they increase the available movement from the bimetal and, as described hereinafter, they assist in the flattening of the bimetal when, in use of the control, it is pressed against a heating element with a force sufficient to cause the bimetal to flatten as compared to its normal, rest condition curvature.
• the contact point of the spring portion it should be noted that the contact point of the spring portion
• FIG. 3 this schematically shows the situation when the bimetal 30 is pressed against the heating element surface 50 with a force sufficient to flatten the bimetal curvature.
• the pips 22 are schematically shown and also schematically shown are the locations 100 where the spring carrier portions 19 apply forces to the bimetal, it being noted that these locations are inwardly of the points of contact of the pips 22 with the bimetal so that schematically-illustrated force couples 200 are applied to the bimetal.
• the flattening of the bimetal curvature against the heating element surface is achieved by mounting the control to the heating element so that the bimetal mounting carriers 10 are subjected to substantial forces, in the region of 800 grams, which bends the portions 17 and 18 of the bracket downwardly against the bimetal and at the same time develops the reaction forces 100 from the portions 19 and flattens the bimetal.
• the effect of the forces applied to the bimetal and its flattening against the heating element cause its effective set operating temperature to be raised, thereby enabling a more practical, lower manufacturing setting to be used whilst still avoiding nuisance operation, and increases the thermal contact of the bimetal with the heating element thereby giving a more rapid response which more than compensates for the rise in set temperature with the result that a higher resistance to nuisance operation is in fact achieved.
• Nuisance operation for avoidance of doubt, is when the set operating temperature of the control is so close to the temperatures encountered by the control during normal boiling operation of an associated vessel that the control operates under normal boiling conditions rather than operating only in response to a heating element overtemperature condition.
• the problem of nuisance tripping of heating element overtemperature protection controls is exacerbated by the build up of lime scale on the heating element, since the lime scale tends to thermally insulate the heating element from the water to be heated so that the heating element has to operate at a higher temperature in order to boil the water, which reduces the differential between the normal boiling temperature of the heating element and the set temperature of the element protection control.

Landscapes

• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Thermally Actuated Switches (AREA)
• Control Of Combustion (AREA)
• Air Conditioning Control Device (AREA)
• Inorganic Insulating Materials (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Control Of Resistance Heating (AREA)
• Temperature-Responsive Valves (AREA)

Applications Claiming Priority (3)

Publications (2)

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ID=10852272

Family Applications (1)

Country Status (7)

Families Citing this family (5)

Family Cites Families (7)

• 1999
  • 1999-04-26 GB GB9909563A patent/GB2349508B/en not_active Expired - Fee Related
• 2000
  • 2000-04-25 AT AT00920934T patent/ATE282245T1/de not_active IP Right Cessation
  • 2000-04-25 AU AU41344/00A patent/AU4134400A/en not_active Abandoned
  • 2000-04-25 WO PCT/GB2000/001581 patent/WO2000065625A1/fr active IP Right Grant
  • 2000-04-25 EP EP00920934A patent/EP1173863B1/fr not_active Expired - Lifetime
  • 2000-04-25 DE DE60015746T patent/DE60015746T2/de not_active Expired - Fee Related
  • 2000-04-25 CN CN00809498.5A patent/CN1258792C/zh not_active Expired - Fee Related

Non-Patent Citations (1)

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