EP2583292B1

EP2583292B1 - Thermally responsive electric switches

Info

Publication number: EP2583292B1
Application number: EP11741680.0A
Authority: EP
Inventors: Mark Heywood; Andrew Rixham; Mark Sherratt
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2010-06-17
Filing date: 2011-06-14
Publication date: 2017-09-27
Anticipated expiration: 2031-06-14
Also published as: GB2481263B; CN202142471U; GB2481240A; WO2011158023A2; GB2481240B; GB201016712D0; GB2481263A; CN102947907A; WO2011158023A3; CN102947907B; GB201010182D0; EP2583292A2

Description

Field of the Invention

This invention concerns improvements relating to thermally responsive electrical switches employing thermal actuators such as bimetallic elements, and more particularly, but not exclusively, concerns thermally responsive switches employed in the overheat protection of electrical motors.

Background of the Invention

Many kinds of electrical switches employing bimetallic actuators are known and likewise many different forms of bimetallic switch actuators are known. Early bimetallic switches simply employed a plain bimetal blade which moved relatively slowly in response to temperature changes and gave rise to arcing problems in the switch. The development of the snap-acting bimetallic actuator, constructed as a dished bimetallic element capable of moving between oppositely curved configurations with a snap action, provided a major advance in the art. Various forms of snap-acting bimetallic actuators are known, such as those disclosed in GB-A-600055 , GB-A-657434 , GB-A-1064643 , GB-A-1542252 and GB-A-2124429 , for example.
Likewise, various forms of electric switches employing such bimetallic actuators are known; GB-A-2124429 abovementioned, for example, discloses the utilisation of a pear-shaped snap-acting bimetallic actuator in a current-sensitive switch, where the heating of the bimetal by flow of electric current therethrough is designed to trip the switch in a current overload situation.
In WO-A-92/20086 there is described a miniature electrical switch employing a snap-acting bimetallic actuator, the switch being well suited to automatic manufacture and installation and comprising a small number of parts. The switch comprises a moulded plastics body portion, which captures therein first and second terminal conductors; a snap-acting bimetallic actuator is secured to one of the two conductors and carries a contact which constitutes the moving contact of the switch and is arranged for cooperation in switching operations with the other of the two conductors. The possibility is further described of providing a silver or silver alloy coating, for example a silver antimony coating as described in WO-A-92/14282 , on the terminal conductor which co-operates with the moving switch contact carried by the bimetal so as to enable an otherwise plain conductor to be utilised without need for attachment of a discrete contact to the conductor. To enhance the current sensitivity of the switch, the possibility is further disclosed to provide a series-connected heating element in the switch for injecting heat into the bimetallic actuator when the switch is in closed condition, and in a particularly convenient arrangement, this is achieved by forming the heating element as a portion of one or other, or both, of the two terminal conductors.
In GB-A-2275823 a switch with similar construction to WO-A-92/20086 is described, in which the moulded plastic body is formed of a polymeric PTC material, such that the break and remake characteristics of the thermally responsive switches can be tuned for specific applications.
In GB-A-2280785 there is disclosed a further variation of a thermally responsive switch in which the components are designed specifically for automatic assembly (commonly known as lead frame) whereby the major components are stamped out of continuous metal strip. Welding, riveting and plastic insert or over moulding can all be carried out in the strip form and brought together in strip form to the final assembly machine. Variations in, for example, terminal configurations can be included in the strip form with redundant parts cropped ahead of or during the final assembly. GB-A-2280785 also deals with the issues of reducing the working stress in bimetals to increase the life of the thermally responsive switches.
The above patents described methods of continuously improving assembly and material characteristics aiming to provide solutions for a wide range of motor application requirements, whilst effectively minimising the tooling and parts count. Generally the above patents disclose designs that employ the use of plastics and polymers for manufacturing, assembly, electrical insulation and the isolation and protection of the finished switch and especially the insert or over moulding of plastics and polymers to facilitate electrical insulation between metal parts.
Other patents, for example US-A-4476452 , rely upon mainly metal materials in the design and rely upon separate gaskets for electrically insulating electrical parts. This in itself provides problems in ensuring the rigidity of the separate parts and also the ingress of dirt and dust into the working parts of the thermally responsive switch. These problems are increased when the thermally responsive switch also includes a series-connected heating element within the design.
GB-A-1526275 has first and second metal plates forming part of the switch housing, with an insulating spacer between. A bimetallic sensor is welded to one of the plates, which also has a bulge or protuberance acting on the bimetallic sensor.
US-A-5268664 teaches the use flat and thin insulating material to electrically isolate a base plate from a cover plate. However, the flat and thin insulating material does not provide support for the cover plate. Instead, folds on the base plate provide support for clench tabs on the cover plate. The problem with such arrangement is that during assembly of the component, the forces exerted by the clench tabs are transferred to the folds of the base, which may in turn cause the base plate to distort. Distortion on the base may have an adverse effect on the reliable operation of the thermostat, and may cause the thermostat blade to operate out of its designed operating parameter.
In recent years the size, performance and assembly methods of electrical motors have changed. Thermally responsive switches employed in the over heat protection of these electrical motors can be subjected to higher temperatures during the assembly of the motor; for example, the use of lead free solders has resulted in increased processing temperatures.
Furthermore, electrical motors have become smaller and lighter and therefore more prone to overheating. Yet at the same time the thermally responsive switches are expected to function alongside electronic control circuits which may necessitate longer energisation times in the fault condition enabling the electronic controls to run through a predetermined logic sequence ahead of the bimetal switching.
There may be benefits, in higher ambient temperatures, for thermally responsive switches made from mainly metal materials; however in these circumstances there may be compromises in the production processes, electrical insulation and long term reliability compared to thermally responsive switches that rely in some part on components made from plastics and polymers.
Care needs to be taken when soldering wires or conductors onto terminals to ensure that flux does not transfer along the terminal and contaminate internal parts of the switch. Also, elevated temperatures during normal or abnormal use may cause excess flux on the terminal to melt and flow to subsequently contaminate the internal parts of the switch.
Patent publication US-A-4 755 787 discloses a switch according to the pre-characterising portions of the appended claim 1.

Statement of the Invention

According to one aspect of the invention, there is provided a thermally responsive switch according to claim 1. According to another aspect of the present invention, there is provided a method of assembling a thermally responsive switch, according to claim 2.
A proportion of the component parts may be manufactured in advanced plastic polymers suitable for ambient temperatures in excess of 320°C.
A proportion of component parts may be manufactured in advanced plastic polymers suitable for ambient temperatures in excess of 320°C and the polymer assists in the isolation of the internal electrical parts.
A proportion of metal parts and the advanced plastic polymers may be assembled together by an insert or over moulding process.
The switch may comprise two sub-assemblies that are assembled together by a mechanical clamping process, the first assembly being reliant upon insert or overmoulding of high temperature polymers to provide a moulded unit to enclose and or attach first electrical connection means, base portions, an in-series electrical heater and a fixed contact, the second sub-assembly being made up of a second electrical connection means, a bimetal blade part including a second contact and an integral metal housing, such that the two sub-assemblies are electrically connected when the bimetal blade is in the cold state and electrically separate when the bimetal blade part is in the hot state.
One side of the assembly may be sealed against ingress of dirt and debris by an insert moulded bridge.
A bimetal blade part may be assembled to the blade mount part in such a way as to reduce the working stress in the bimetal.
The first break time of the switch may be greater than four seconds in a motor with a 30 amp stall current and the subsequent cycling of the control limits the winding temperature of the motor to less than 300°C for a short term duration and 250°C for a long term duration.
A portion of the terminal surface extending inside or outside a housing of the switch may be textured and/or formed to prevent or restrict the flow of flux or solder from the external portion of the terminal into the internal areas of the switch.
Embodiments of the present invention provide the ability to develop a suite of thermally responsive controls designed to protect a range of electrical motors so that only a minimum of parts are required to be modified to cover the range.
Embodiments of the invention may provide thermally responsive switches to meet the new requirements in the art whilst enabling the switch to be manufactured in an automated process with the minimum of components.

Brief Description of the Drawings

There now follows, by way of example only, a detailed description of preferred embodiments of the present invention, with reference to the figures identified below.

Figure 1 shows an exploded view of the complete assembly of the first embodiment.
Figure 2a shows an exploded view from above of the two main sub-assemblies, which are the base sub-assembly and the cover sub-assembly.
Figure 2b shows an exploded view from the underside of the sub-assemblies in 2a.
Figures 3a to 3c show an assembly sequence of the cover sub-assembly.
Figures 3d to 3f show an alternative assembly sequence of the cover sub-assembly
Figure 3g shows a perspective cross section of a variant with a discrete rivet.
Figure 4a shows a plan view of the base sub-assembly of the first embodiment including a heater.
Figure 4b shows a plan view of the base sub-assembly of a second embodiment that does not include a heater.
Figure 5a shows a cross section of the first embodiment.
Figure 5b shows a cross section a second embodiment.
Figure 6a shows a top view of the complete assembly whereby the left hand terminal of the cover sub-assembly is redundant so the terminals are at the same end of the assembly.
Figure 6b shows a top view of the complete assembly whereby the right hand terminal of the cover sub-assembly is redundant so the assembly terminals are in line.
Figure 6c shows a variant of the embodiment of Figure 6a with an aperture and indentation in the area of the terminal.
Figure 6d shows a variant of the embodiment of Figure 6a with an aperture and indentation in the area of the terminal.
Figure 7a shows an underside view of the mould retention features in the base sub-assembly of the first embodiment.
Figure 7b shows an underside view of alternative mould retention features in the base sub-assembly of the first embodiment.
Figure 7c shows an underside view of alternative mould retention features in the base sub-assembly in an alternative configuration of the first embodiment.
Figure 8a shows an underside view of the cover and in line terminal showing a textured surface detail.
Figure 8b shows an underside view of the cover and in line terminal showing an alternative textured surface detail.
Figure 8c shows an underside view of the cover and in line terminal showing another alternative textured surface detail.
Figure 9a shows an underside view of the cover and in line terminal showing a variant of the textured surface detail of the embodiment of Figure 8a.
Figure 9b shows an underside view of the cover and in line terminal showing a variant of the textured surface detail of the embodiment of Figure 8b.
Figure 9c shows an underside view of the cover and in line terminal showing a variant of the textured surface detail of the embodiment of Figure 8c.
Figure 9d shows an underside view a complete assembly showing a textured surface detail of the embodiment of Figure 9b.

Detailed Description of the Embodiments

In the following description, functionally similar parts carry the same reference numerals between different embodiments. Reference is made to the proprietor's above-mentioned patent publications GB-A-2124429 , WO-A-92/20086 , GB-A-2275823 and GB-A-2280785 which describe the functions of the various control types; the following description will focus on improvements to these control types.

First Embodiment - Thermally Responsive Switch with Integral Heater

Figure 1 is an exploded view of the first embodiment. The individual components are as follows. A base terminal plate 11 is formed from an electrically conductive strip material and includes the base terminal 12. The base terminal plate 11 includes mould retention features 16 and a heater pad 14 for a first connection of the in-series electrical heater 8. A base contact plate 13, which is formed from the same electrically conductive strip material as the base terminal plate 11. The base contact plate 13 includes a heater pad 14 for a second connection of the in-series electrical heater 8 and a platform 15 to which the fixed contact 10 will be assembled.
A fixed electrical contact 10 is assembled to the base contact plate 13 by a welding process, by riveting or any other suitable method. The contact can be made of a silver alloy, silver-plated copper, silver wire, layered contact tape or any other low resistance material suitable for meeting the electrical switching requirements.
A moulded unit 9 is formed during an overmoulding process which takes place whilst the base plates 11 & 13 are in the strip form. The moulded unit 9 serves to support the base plates 11 & 13 by the formation of moulded plastic rivets 27 formed by molten plastic during the moulding process and/or the molten plastic flowing around preformed chamfers 23 or stamped profiles 28 around the edges of the base plates 11 & 13. The moulded unit 9 has four vertical sides and the moulding process fills and seals the gap between the two base plates 11 & 13. In use, the moulded unit 9 will be subjected to radiated and conducted heat from the in-series electrical heater 8 along with ambient heat from the electrical motor and the motor assembly processes, therefore the plastic should preferably have a heat deflection temperature above 320°C and melt temperature of greater than 375°C.
The in-series electrical heater 8 is connected to the heater pads 14. This heater 8, in collaboration with specially selected bimetal specifications, can be used to fine tune the break and remake characteristics of the assembled thermally responsive switch.
A cover 4 is formed from an electrically conductive strip material. This may be the same conductive strip material from which the base plates 11 & 13 are formed or it may be a second conductive strip material. The cover 4 includes the second electrical terminal 24 or 25. The cover may include a sidewall 35 extending from at least one edge and may further include clench features 38. The cover 4 may also include a blade mount platform 3 from which a rivet 18 is formed. In an alternative embodiment a discrete rivet 18 may be utilised. The rivet 18 may be formed by cropping a continuous feed of wire material, for example an alloy wire with both ends being flattened or 'coined' over during the assembly process.
A bimetal blade 6 may be stamped out of a coil of suitable bimetal material. The blade 6 includes a reciprocating rivet hole 5. The blade 6 is subjected to a process, known as tooling, that will set the make and break characteristics of the bimetal. The tooling may take place whilst the blade 6 is in the strip form.
A moving contact 7 is assembled to the bimetal blade 6 by a welding process, or alternatively by a rivet or other suitable attachment. As with the fixed contact 10, this can be manufactured from a suitable low resistance material.
The base terminal plate 11, base contact plate 13, moulded unit 9, heater 8 and fixed contact 10 form the first sub-assembly 1a. The cover 4, rivet 18, blade 6 and moving contact 7 form the second sub-assembly 1b.
Figure 2a shows the view from above of the two sub-assemblies 1a, 1b before the final assembly. In this view, the internal parts of the first sub-assembly can be clearly seen in place. Any or all of the heater pads 14, contact platform 15, fixed contacts 10 and in-series heater 8 may be added to the base plates 11 & 13 whilst still in the strip form either before or after the moulding process.
Figure 2b shows the view from the underside of the two sub-assemblies 1a, 1b before the final assembly. In this view, the moulded rivet 27 features of the first sub-assembly are clearly visible, together with the bridge 26 between the two base plates 11, 13. This bridge 26 prevents access to live parts and also prevents ingress of debris into the contact and bimetal area of the switch.
Figure 2b also shows the internal details of the second sub-assembly in its assembled form. The bimetal blade 6 is attached to the cover 4 by two separate methods each with its own purpose. The rivet 18 is formed from the cover 4 so as to remove the need for an additional component. The rivet 18 provides excellent mechanical strength, essentially forming a pivot or fulcrum about which the bimetal blade 6 moves. However it is known that the constant movement of a rivet over the life of an appliance can compromise the electrical connection through the rivet. The weld 17 provides excellent electrical contact but it is known that a weld 17 causes stress in a bimetal which can compromise the mechanical strength of the bimetal 6 over the life of the appliance. By placing the rivet 18 between the active, moveable part of the bimetal 6 and the weld 17, the number of cycles achieved before failure can be increased, for example by 50%.
The rivet hole 5 may be positioned on the centre line of the bimetal blade 6 with the weld 17 located within an arc-shaped zone beyond the outer perimeter of the rivet 18, the arc being a maximum of 60° either side of the centre line with the apex of the arc at the centre point of the rivet 18. The weld 17 should preferably be as close as possible to the outside rim of the rivet 18 without actually being in contact with the rivet 18. This may enable the overall length of the inactive part of the blade 6 to be minimised, which in turn may reduce the overall package size of the finished switch and the cost of the materials.
In another alternative, there may be an interference fit or engagement between a part of the blade mount platform 3 and the blade 6, without the need for a rivet. For example a strip of material 19 may be formed at the rear of blade mount platform 3 which may be folded over onto the end of the bimetal blade 6 before the weld process. The bimetal blade 6 is then welded through this strip 19 onto the blade mount platform 3. The additional strip 19 may become the equivalent of a 'weld slug' which would normally need to be supplied as a separate part. The strip 19, after welding, may provide the fulcrum for the bimetal blade 6 without the need for an additional rivet. This strip of material 19 would be formed from what would otherwise be waste material in the strip that forms the cover 4.
Figures 3a to 3c show the assembly sequence of the integral rivet 18 and weld 17. In embodiments that include either an integral or discrete rivet 18, the preferred sequence is to make the rivet 18 first followed by the weld 17; however, this sequence may be reversed if required. The moving contact 7 may be fixed to the bimetal blade 6 whilst the bimetal is in the strip form.
Figure 3g is a cross sectional view of the previously described variant with a discrete rivet.
In an alternative embodiment, the rivet 18 may be replaced by another suitable fixing means, such as an interference fit or engagement between a part of the blade mount platform 3 and the blade 6. In an alternative embodiment, the weld 17 may be replaced by a braze or other suitable join.
The bimetal blade 6 is the active element of the thermally responsive control and its function is well known. Briefly, in a closed position the moving electrical contact 7 contacts the fixed contact 10 on the base contact plate 13, which closes the circuit through the switch. The bimetal 6 is both temperature and current sensitive and is caused to dish or snap in the opposite direction when the ambient temperature, the self-heating properties of the bimetal due to the current, or a combination of the two reach a set limit. The movement of the blade 6 moves the moving electrical contact 7 out of electrical contact with the fixed contact 10 so that the electrical contact breaks and the switch opens.
The bimetal blade 6 reverts to its original position when it has cooled below the specified reset temperature. Different bimetal materials with different resistivity can be used. In some circumstances the bimetal materials may have more than two layers.
Figure 4a shows a plan view of the heater 8 attached to the base plates 11 & 13. The heater 8 can be produced from any of a range of materials, or in any of a range of thicknesses, lengths and/or shapes, to give the desired resistance value. This enables the performance of the thermally sensitive switch to be tuned to the requirements of the particular application. The heater 8 has two ends which are welded to the individual heater pads 14, bridging the electrical circuit between the two base plates 11 & 13. Alternative heaters may be as described in WO-A-92/20086 or GB-A-2275823 , for example.
The materials from which the individual components are manufactured also influence the way in which the switch performs; for example the cover 4 may either assist or inhibit the heat loss during the cycle. Alternatively the material of the cover 4 and/or base plates 11 & 13 may influence the electrical resistance and subsequently the heat generation of these parts.
The following criteria can be modified to fine tune the performance of the switch:

Choice of bimetal material.
Tooling of the bimetal.
Heater resistance and output
Material of the cover 4
Material of Base Plate
Material and mass of Contacts

The performance characteristics of the switch are expected to be within the following broad ranges, with the aim to provide an average off/on ratio of four to one.

Carry current:	3 to 20 Amps
Stall current:	7 to 50 amps
First break time under fault conditions:	up to 10 seconds
First remake:	less than 10 seconds
Second remake:	greater than 10 seconds
Winding temperature - short term:	maximum 300°C
Winding temperature - long term	maximum 250°C

It can be seen that the heater 8, moulded unit 9 and the fixed contact 10 are provided in the first sub-assembly and the bimetal blade 6, moving contact 7 and metal cover 4 are provided in the second sub-assembly. The first and second sub-assemblies are brought together as part of the final assembly when the cover 4 is clenched onto the moulded unit 9 to form the complete switch as shown in Figure 5. The cover 4 may be clenched onto the moulded unit 9 by the clench features 38 which may be pre-formed from the sidewall 35. Alternatively or additionally other clench features (no shown) may be punched or formed in the sidewall 35 as part of the clenching process. The plastic material specified for the moulded unit 9 is preferably capable of withstanding the process of clenching and also is capable of withstanding the pull off forces that can be exercised on the moulded unit 9 by both the cover 4 and the base plates 11 & 13. Suitable plastic material for the moulded unit 9 preferably has a low brittleness and a high flexural strength, preferably greater than 110 MPa or more preferably greater than 120 MPa.
The strip material provides the option to include additional features on the metal parts in which the redundant parts can be cropped as necessary, dependant upon which configuration is required. Figure 6a and 6b shows an example of this which provides the option of the second terminal 24 on the same side as the first terminal 12 or the second terminal 25 in line with the first terminal 12.
Figures 6c and 6d show stiffening and or strengthening indents 37 which may be provided in the portion overlapping the terminal 25 and the cover 4 so that the narrow portion of the terminal 25 may be able to withstand forces applied, for example, during the assembly of a male tab terminal (not shown) onto the terminal 25. It is preferred that the indents 37 are positioned on an outside facing part of the cover 4 so that the indents 37 do not provide a track, for debris or other contaminants to enter the internal part of the switch assembly 1. It is also preferred that the indents 37 are formed within the depth of the material of the cover 4 so that there is no witness of the indents 37 on the inward facing part of the cover 4. Instead of indents 37, raised or corrugated portions may be provided.
Figure 6c and 6d also illustrate apertures 36 which may be provided in the terminal 25 to assist, for example, with the retention of wires (not shown) prior to or during, for example, the soldering process. The apertures may also be used to engage with dimples on, for example, cooperating female tab terminals.
The indents 37 and apertures 36 may be provided on or in the area of other terminal portions, for example, terminals 12 or 24.
Figures 7a, 7b and 7c show isometric views of the cover 4 clenched to the moulded unit 9 by clench features 38.
Figure 7a shows the countersunk hole detail 28 of the moulded rivets 27 that are formed during the moulding process.
Figure 7b shows an alternative embodiment to Figure 7a in which the retention of the moulded units 9 to the base plates 11 & 13 is achieved by the moulded unit 9 being formed around the chamfered edges 23 of the base plates 11 & 13.
Figure 7c shows an alternative configuration to Figure 7b in which the edges of the base plates 11 & 13 include a stamped profile 29 instead of the chamfered edge. The profile 29 is designed to provide enhanced retention between the base plates 11 & 13 and moulded unit 9. Other profiles may be utilised and further embodiments may include a combination of rivet, chamfer and stamped profile.
In Figures 7a, 7b and 7c the molten plastic forms a bridge 26 between the two base plates 11 & 13 to enhance the rigidity of the assembly and/or to close the gap against ingress of foreign particles from the motor assembly, such as carbon dust.
The motor assembly industry is moving towards lead free soldering which melts at a higher temperature than the leaded varieties; this requires the thermally responsive switch to withstand elevated temperatures during installation. In addition, automated motor assembly lines may employ belt ovens to process the lead free solder, which in turn requires the switch to withstand elevated temperatures for longer periods.
As previously detailed, the moulded unit 9 also will be subjected to radiated and conducted heat from the in-series electrical heater 8 along with the ambient heat from the electrical motor, therefore the polymer material of the moulded unit 9 should preferably have a heat deflection temperature of 320°C or above and a final melt temperature of over 375°C. The moulded unit 9 will also need to be able withstand the pull off forces that can be exercised on the moulded unit 9 by both the cover 4 and the base units 11 & 13. It has been found that a high temperature liquid crystal polymer (LCP) is suitable for this purpose.

Second Embodiment - Thermally Responsive Switch without Integral Heater

In some applications it may not be necessary to include a heater 8 to achieve the performance requirements, therefore in the second embodiment there is provided a thermally responsive switch with a single base plate 22 as detailed in Figures 4b and 5b. Advantageously the design of the components may allow the moulding and assembly tooling for the single base plate 22 to be interchangeable with the two-piece base plates 11 & 13.
The resultant switch will to some extent be subjected to less heat than the first embodiment but will still be subject to the extremes temperatures of the production line and/or the application, and therefore may still benefit from the improvements to materials, retention features and production techniques detailed in the first embodiment.

Terminals with Textured Finish

In motor applications where wires or conductors are soldered onto the terminals 12, 24, 25 a textured finish may be added onto the terminals 12, 24, 25 to prevent and/or inhibit the flux flowing along the terminal 12, 24, 25 and into the internal part of the switch assembly 1. The texture may be formed by any of a number of methods, for example abrasion, shot blasting, etching or stamping, and the texture may be random (e.g. roughened) or formed by geometric shapes or may be formed by raised portions e.g. by chemical or plasma deposition on a surface. The distal area of the terminal 12, 24, and/or 25 onto which the wire or conductor is to be soldered is preferably kept free of the texture.
Figure 8a illustrates an embodiment where the inline cover terminal 25 includes a textured surface 32 between a distal area of the terminal 25 and the moulded unit 9 so as to prevent solder or flux entering the switch housing and passing into the area of the contacts 7, 10 during the assembly of, or during the life of the switch. As illustrated in Figures 8a to 8c, the textured surface 32 preferably substantially covers the width of the terminal 25 prior to entering the moulded unit 9 and may extend along the terminal 25 under the moulded unit 9 and may extend into the switch housing of assembly 1.The textured surface 32 could alternatively or additionally be positioned within the switch housing, for example between the moulded unit and the area of the contacts 7, 10. The textured surface 32 may form a barrier partially or completely around the contacts 7, 10.
As shown in Figure 8a, the textured surface may take the form of diagonal lines which are preferably stamped into the terminal 25 whilst the material is in strip form. It is preferred that the lines extend to the side edges of the terminal 25.
Figure 8b illustrates an alternative embodiment where the textured surface 32 takes the form of cross hatched diagonal lines stamped into the terminal 25.
In the embodiments of Figures 8a and 8b, the diagonal lines are at an angle of approximately 30° to the transverse direction of the terminal 25, but this angle may be between approximately 45° and 0°, so that any solder or flux that may flow during the assembly or during the life of the switch is directed away from the switch contacts 7, 10. Figure 8c shows an embodiment in which the lines extend transversely across the terminal.
The distance between adjacent lines may be between 0.1 and 0.9 mm and preferably approximately 0.3mm.
The lines may be between 0.05 and 0.15 mm wide and preferably approximately 0.1 mm wide.
The lines may have a depth or height of between 0.02 and 0.06mm and preferably approximately 0.04mm.
Figure 8b illustrates cross hatched lines which may form substantially parallelograms type portions with each portion having, for example, a first opposing angle of 60° and a second opposing angle of 120° so the diagonal sides of each portion are at an angle of approximately 30° to the transverse direction of the terminal 25. Other angles may be employed as described above.
The lines may be indented as grooves or channels, or raised as ridges or steps from the surface of the terminal 25.
In further embodiments the textured surface 32 may the take the form of non-linear indentations or raised portions or a series of non-linear indents or raised portions.
It is preferred that there is a small raised step or barrier 34 at the end of the textured surface 32 closest to contacts 7, 10, to act as a barrier if an excessive amount of solder or flux is used during the assembly. Alternatively or additionally, as shown in Fig. 8c, a raised step or barrier may be formed at the end of the textured surface towards the distal end of the terminal 25. The resultant line extends substantially perpendicularly to the expected direction of flow of the solder or flux, and is generally deeper or higher than the lines of the textured surface 32. The line may be curved, depending on the geometry of the terminal and switch. As an alternative to the raised step or barrier 34, there may be provided a groove or channel.
Figures 9a, 9b and 9c illustrate alternative positions for the textured surfaces 32 which overlap the portion between the end of the terminal 25 and the corresponding width of the portion of the cover 4.
As illustrated the textured portions 32 terminate substantially adjacent the contact area 7, 10. In further embodiments (not shown) the textured portion 32 may terminate between the end portion of the terminal 25 and contact area 7, 10.
In the case that, for example, the overlapping area of a terminal 12, 24 or 25 with a cover 4 or base plate 11 includes both a textured surface 32 and indent 37 it is preferred that the indent 37 is positioned on an outside facing part and the textured surface 32 on the inward facing part of the respective cover 4 or base plate 11 so that the indents 37 do not provide a track, for example, for the solder to enter the assembly 1.
Figure 9d illustrates a preferred embodiment of the motor protector assembly 1 including indents 37 and textured surface 32.

Alternative Embodiments

The present invention is not limited to the above embodiments. For example a PTC or NTC heater could be included in the assembly 1 to increase the remake time of the bimetal blade 6. Alternatively, the heat input from the PTC or NTC heater could be such that it is not possible to reactivate the switch until the power has been switched off from an external source and the assembly has been allowed to cool down.
In other embodiments, the contact 10 on the base plate 13 could be formed as part of the base plate 11 and, if required, that contact may be plated in a low resistance material.
In other embodiments, the bimetal blade 6 could be mounted on the base plate 22 or the two-part base plate 11 & 13, in which case the fixed contact 10 would be on the cover 4. The cover 4 could be made of insulating material 9, moulded around the terminal 24 on which the fixed contact 10 is mounted directly.
In other embodiments the moulded unit 9 may extend beneath one or both of the base plates 11 and 13, or the base plate 22, so that the moulding may form either an additional insulation layer and/or electrical isolation, for example if one side of the switch is close to an additional heat source or is to be housed close to live electrical parts.
The method of texturing the contact areas to prevent the flow of solder and flux on terminals is not limited to the present switch embodiments and can be employed on any soldered joint in any application.
The embodiments described above are illustrative of rather than limiting to the present invention. Alternative embodiments apparent on reading the above description may nevertheless fall within the scope of the invention, which is defined in the appended claims.

Claims

A thermally responsive switch (1) comprising electrical terminals (12, 24; 25), a thermally responsive switch actuator (6), and a housing comprising at least first and second metal plates (4; 11; 13; 22) electrically connected to respective ones of the electrical terminals (12; 24; 25), the thermally responsive switch actuator (6) having a moving contact (7) for contacting a fixed contact (10) on one of the first and second metal plates (4; 11; 13; 22); wherein the thermally responsive switch actuator (6) is fixed to the other one of the first and second metal plates (4; 11; 13; 22) and characterised in that the housing further comprises a peripheral wall of insulating material moulded onto the first metal plate (11, 13; 22), and the second metal plate (4) is mechanically clamped or clenched onto the insulating material (9).
A method of assembling a thermally responsive switch, comprising:
a. providing first and second metal plates (4, 11, 13; 22), electrically connected to respective first and second electrical terminals (12, 24; 25); and

b. moulding a peripheral wall of insulating material (9) onto the first metal plate (11, 13; 22);

c. fixing a thermally responsive actuator (6) to one of the metal plates (4; 11;13; 22), the thermally responsive actuator having a moving contact (7) for contacting a fixed contact (10) on the other one of the first and second metal plates (4; 11; 13; 22); and

d. mechanically clamping or clenching the second metal plate (4) onto the insulating material (9), such that the insulating material (9) and the first and second metal plates (4, 11, 13; 22) form a housing for the switch (1).
The switch of claim 1, or the method of claim 2, wherein the insulating material (9) comprises vertical sides that are moulded onto the first metal plate (11, 13; 22) to form said peripheral wall, to which the second metal plate (4) is mechanically clamped or clenched, such that the insulating material (9) is disposed between the first metal plate (11, 13; 22) and second metal plate (4), and the first and second metal plates (4; 11; 13; 22) form the base and cover respectively.
The switch of claim 1 or claim 3, or the method of claim 2 or 3, wherein the second metal plate (4) includes clench features (38) that are clenched to the insulating material (9).
The switch of claim 1 or any one of claims 3 to 4, or the method of any one of claims 2 to 4, wherein the thermally responsive switch actuator (6) is fixed to one of the first and second metal plates (4; 11; 13; 22) by a first mounting means (18), which acts as a fulcrum for the switch actuator (6), and a second mounting means (17) which acts as an electrical connection to the switch actuator (6).
The switch or method of claim 5, wherein the first or second metal plate (4;11;13;22) includes a platform (3) to which the thermally responsive switch actuator (6) is fixed via the first and second mounting means (18, 17), whereby the platform (3) protrudes from the first or second metal plate (4;11;13;22).
The switch or method of claim 6, wherein the first mounting means (18) and second mounting means (17) are provided at one end of the thermally responsive switch actuator (6).
The switch or method of claim 6 or claim 7, wherein the first mounting means comprises a rivet (18) and/or the second mounting means comprises a weld (17).
The switch of claim 1 or any one of claims 3 to 8, or the method of any one of claims 2 to 8, wherein the insulating material (9) substantially seals a contact area (7, 10) of the switch (1) from the exterior of the housing.
The switch of claim 1 or any one of claims 3 to 9, or the method of any one of claims 2 to 9, wherein the first metal plate comprises a first metal plate portion (11) and a second metal plate portion (13) electrically connected together by a heater (8) for heating the actuator (6).
The switch or method of claim 10, wherein the first and second metal plate portions (11, 13) are separated by an insulating portion (26) integral with said insulating material (9).
The switch of claim 1 or any one of claims 3 to 11, or the method of any one of claims 2 to 11, wherein the insulating material (9) has a heat deflection temperature of at least 320°C, a melt temperature of over 375°C and/or a flexural strength greater than 110 MPa, and preferably greater than 120 MPa.
The switch of claim 1 or any one of claims 3 to 12, or the method of any one of claims 2 to 12, wherein the insulating material (9) is fixed to the first metal plate (11, 13; 22) by moulding to one or more fixing features (23, 28) thereof.
The switch of claim 1 or any one of claims 3 to 13, or the method of any one of claims 2 to 13, wherein at least one said terminal (12, 24; 25) has a surface portion (32, 34) extending inside and/or outside the housing, the surface portion being arranged to prevent flux or solder from entering a contact area (7, 10) of the switch (1) during or after a welding process, and the surface portion preferably comprises a textured surface (32) and/or a barrier (34).
The switch of claim 1 or any one of claims 3 to 14, or the method of any one of claims 2 to 14, wherein the first metal plate (11; 13; 22) includes a platform (15) to which a fixed contact (10) is attached.