Improvements relating to thermal switches

Abstract

 A snap-acting bimetal blade for use as a thermally-responsive switch actuator has a generally rectangular outer shape and has a generally rhomboidal cut-out defining two spaced-apart and tapering legs connected together at their opposite ends. Such a blade shape has advantageous characteristics as regards the forces and movement that can be developed in switching operations. A thermally-responsive switch incorporating such a bimetal blade has first and second spaced-apart metal parts, one of which is formed of rigid, electrically-resistive material and the other of which is a spring member, first and second contacts provided on such parts, a bimetal blade as aforesaid welded to one of the parts, and an insulating push rod arranged for transferring switch-­operating movement of the bimetal to the spring member for operating the switch. When used as a motor protection switch, the bimetal operation is dependent upon heat generated by current flow in the electrically-resistive part of the switch and the switch can be made relatively insensitive to large currents flowing for short durations, but sensitive to longer term lower currents, and can have a high ratio of "off" time to "on" time.

Description

[0001] This invention generally concerns improvements relating to thermal switches and more particularly concerns switches employing bimetallic elements as thermal actuators.

[0002] 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 arranged to open a pair of switching contacts via a push-rod arrangement with the bimetal simply exposed to heat developed in an appliance or other apparatus controlled by the switch so as to effect an overtemperature control function. Development of the snap-acting bimetallic actuator provided a major advance in the art and there are various known forms of snap-acting bimetallic actuators, such as those disclosed in British Patent Specifications Nos. 600055, 657434, 1064643 and 1542252 for example, and various known forms of electric switches employing such snap-acting actuators. Other forms of bimetallic actuator, such as the pear-shaped blade of our British Patent Specification No. 2124429 for example, have been developed for the provision of operating characteristics to suit particular applications and in this regard the amount of blade movement and the force that the blade can develop have been of particular interest as indeed has the current carrying capability of the blade, the latter being an important parameter in those applications where the blade is required to be current carrying in use and where the heat developed in the blade by current flow therein at least contributes to switching of the blade.

[0003] The bimetallic blade of our British Patent Specification No. 2124429 is particularly well suited to utilization in the electric motor protection switch that is described in our British Patent Specification No. 2133931. As is described in Specification No. 2133931, the subject motor protection switch has the principal characteristic of being relatively insensitive to high currents of short duration but being sensitive to lower values of current flowing for relatively long time periods, this being the characteristic required for example of a motor protection switch adapted for use with an automotive window lift motor where the initial operating current of the motor is high and then reduces as the heating of the motor windings causes an increase in the resistance of the winding. In the motor protection switch described in Specification No. 2133931 this characteristic is achieved by judicious combination of selected bimetal characteristics with selected characteristics of terminal, contact, and/or bimetal mounting parts of the switch, so that at relatively high overload current levels the bimetallic heating predominates in determining the operation of the switch and resistance heating in other switch parts has a substantially negligible effect, whereas at current levels barely of an overload level the resistance heating of such other switch parts contributes significantly to the switch operation.

[0004] Whilst the motor protection switch of Specification No. 2133931 is effective in operation and has been commercially successful, for some applications the triple-legged blade gives rise to undesirable dimensional constraints. We have experimented with other blade shapes, such as the lozenge-shaped or diamond-shaped blades utilized by other manufacturers for example, but have been unable to obtain the requisite operating characteristics, particularly the necessary balance between force and movement. We have found, however, that by modifying the blade shape disclosed in Specification No. 2124429 by removal of the centre leg but with retention of the tapered outer legs a blade configuration having advantageous characteristics can be obtained. More particularly, we have found that a blade having a generally rectangular outer form and with a tapering (rhomboidal) aperture cut in the centre, when mounted by a weld on the short side, at the narrow end of the aperture, gives forces and movement comparable to the three-legged blade of Specification No. 2124429 and has similar current carrying capability. The tapered blade aperture has been found to optimize the stiffness of the blade and enhances the useful movement of a rectangular blade.

[0005] The invention, in one of its aspects, thus resides in a snap-acting bimetal blade as aforesaid having a generally rectangular outer shape and having a generally rhomboidal cut-out defining two spaced-­apart and tapering legs connected together at their opposite ends.

[0006] In another of its aspects the present invention provides a thermally responsive switch incorporating a bimetal blade as above defined. In one embodiment of such a thermally responsive switch, the switch comprises first and second spaced-apart metal parts one of which is formed of rigid, electrically resistive material and the other of which is a spring member, first and second contacts provided on said first and second parts, a bimetal blade as above defined welded or otherwise affixed to one of the first and second parts, and an insulating push rod arranged for transferring switch-operating movement of the bimetal to the spring member for operating the switch. In accordance with an alternative embodiment of generally similar construction the moving contact of the switching contacts set is carried by the bimetal itself and both of the first and second metal parts of the switch are of rigid construction. These two embodiments are exemplary only and various modifications and variations are available without departure from the scope of the invention.

[0007] The invention, together with features and advantages thereof, will best be understood from consideration of the following detailed description which is given with reference to the accompanying drawings wherein:

Figs 1A and 1B are schematic views of a first exemplary kind of electrical switch in accordance with the present invention;

Figs 2A and 2B are similar schematic views of a second exemplary kind of electrical switch in accordance with the present invention;

Figs 3A to 3F are views of a first exemplary embodiment of motor protection switch in accordance with the present invention; and

Figs 4A to 4C are views of a second exemplary embodiment of motor protection switch in accordance with the present invention.

[0008] Referring first to Figs 1A and 1B of the drawings, the kind of switch shown therein is a current-sensitive switch wherein the operation of the switch is dependent upon the current flowing through the switch and, more particularly, through the bimetallic actuator incorporated into the switch. As shown, the switch 1 comprises a moulded plastics body portion 2 supporting first and second rigid metal parts 3 and 4, and a snap-acting bimetal 5 as outlined in the foregoing and as will be more particularly described hereinafter welded or otherwise conductively affixed to the upper metal part 3, the bimetal carrying at its free end a contact 6 constituting the moving contact of the switch and cooperating with a fixed switch contact 7 provided on the lower metal part 4. In the hot condition of the switch, as shown in Fig. 1A, the bimetal 5 holds the moving contact 6 spaced-apart from the fixed contact 7, whereas in the cold condition of the switch, as shown in Fig. 1B, the bimetal adopts its oppositely dished configuration and maintains the contacts 6, 7 in closed condition. As will be appreciated, the switch of Figs. 1A and 1B is adapted for use with the load current of the switch flowing through the bimetal 5 and can be constructed in accordance with the teachings of our British Patent No. 2133931 aforementioned such that the nature of the non-bimetal switch parts has an influence upon the operation of the switch.

[0009] An alternative form of switch in accordance with the present invention is shown in Figs. 2A and 2B and, as with the switch of Figs. 1A and 1B, this alternative switch 10 comprises first and second metal parts 11 and 12 supported in a moulded plastics body portion 13, a snap-acting bimetal 14, and fixed and moving contacts 15 and 16 respectively. However, in the switch 10 of Figs. 2A and 2B the upper switch part 11 is of rigid metal and carries the fixed contact 15 of the switch at its cantilevered end, the lower part 12 is formed of spring metal and carries the moving contact 16 of the switch, and the bimetal 14 is welded or otherwise affixed to the upper metal part 11 and operates through the intermediacy of an insulating push-rod 17 affixed to the spring metal switch part 12 for effecting switching operations. In this switch configuration, the bimetal 14 is separate from the current path through the switch and is heated indirectly, for example by provision of a separate heater (not shown) or by heat transfer from the main current carrying parts of the switch. The part 11 of the switch of Figs. 2A and 2B may thus be formed of a material which is designed to self-heat in current-­carrying operation of the switch so that heat conduction from the part 11 into the bimetal 14 influences the operation of the switch. In Figs. 2A and 2B, the hot or open condition of the switch is shown in Fig. 2A wherein the bimetal 14 is shown depressing spring metal part 12 via push-rod 17, and the cold or closed condition of the switch is shown in Fig. 2B.

[0010] Referring now to Figs. 3A to 3F, an exemplary switch of the kind described above with reference to Figs. 2A and 2B is shown therein with corresponding parts bearing the same reference numerals as were used in the description of Figs. 2A and 2B. The form of the bimetal 14 can be seen clearly from Figs. 3A, 3B and 3F and exemplary dimensions are given in Fig. 3F. The bimetal, as shown, comprises a generally rectangular blade 20 provided with curved, stress-­relieving corners and with a tapering, rhomboidal aperture 21 cut in the centre of the blade so as to define two tapering outer legs 22, 23 bridged at their opposite ends by thin-end bridging portion 24 and thich-end bridging portion 25. The blade 20 is dished so as to be movable with snap-action between two oppositely dished configurations as is well known in the art, and the dishing of the blade may as is also well known be achieved by press-forming the blade during its manufacture for example or alternatively may be achieved by impressing a crimp deformation in the thin-end bridging portion 24. The tags 26 shown projecting outwardly from the bridging portions 24 and 25 merely result from the formation of the blade 20 as one of a plurality of such blades by a continuous manufacturing process wherein the blades are stamped from a continuous strip of suitable bimetal material. The bimetal blade 20 is mounted on to the upper metal part 11 of the switch by means of a weld at its thick-end bridging portion 25 and as thus mounted has been found to provide forces and movement comparable to those obtainable with a three-legged blade of the kind described in our British Patent No. 2133931 and with similar current-carrying capability, but in a substantially smaller blade. The provision of a tapered aperture 21 in a generally rectangular bimetal blade has been found to optimise the stiffness of the blade and to enhance the useful movement of a rectangular blade of such size.

[0011] The operation of the switch of Figs. 3A to 3F is substantially as above described with reference to the switch of Figs. 2A and 2B. The bimetal 14 is separate from the current path through the switch and is heated by the heat developed in the main current-carrying members of the switch, particularly the upper metal part 11 which may be formed of a resistive material so as to be self-heating in use. By welding the bimetal to its source of heat consistent and controllable heat transfer is assured and furthermore it will be seen that contacts-opening movement of the bimetal 14 brings it into closer proximity with its heater thereby holding the bimetal in its operated state for a longer period and giving a desirable increase in the off time of the switch. The illustrated switch configuration is characterized by being insensitive to high values of initial current, but being sensitive to lower values of current applied for relatively long periods of time, and having a high ratio of "off" time to "on" time. Both of these features are desirable in the protection of short term rated DC motors used in automobiles (window lift motors for example).

[0012] Figs. 4A, 4B and 4C show a further exemplary embodiment of the kind of switch that is illustrated in Figs. 2A and 2B, Figs 4A and 4B being exploded perspective views of the switch taken from different positions, and Fig 4C being a series of plan and elevation views showing stages in the assembly of the switch. The switch of Figs. 4A to 4C is similar to that of Figs. 3A to 3F and differs therefrom only in regard to the different shapes of its constituent parts and the provision of a moulded plastics cover for the switch, the bimetal being identical to that previously described with reference particularly to Fig. 3F. Accordingly, the same reference numerals are used in Figs. 4A, 4B and 4C as were used to describe like parts of the switch of Figs. 3A to 3F.

[0013] As shown, the switch comprises moulded plastics cover parts 30 and 31 with the part 30 incorporating an integral boss 32 shaped to receive thereupon the switch component parts 12, 13 and 11 in the order shown in Figs. 4C(ii), 4C(iii) and 4C(iv) and then to be received in a complementarily shaped aperture 33 formed in cover part 31, the tip of the boss 32 being arranged to project through and beyond the aperture 33 so as to be thermally deformable to secure the two cover parts 30 and 31 together.

[0014] The operation of the switch of Figs. 4A, 4B and 4C is the same as that previously designed herein for the switch of Figs. 3A to 3F and therefore need not be described again.

[0015] There has thus been described an improved bimetal configuration which is a departure from the conventional three-legged Otter Controls bimetal as described for example in our British Patent No. 2133931 aforementioned, but is a development of it designed to provide forces and movement comparable with the three-legged blade and with similar current carrying capability all in a unit of lesser size than the conventional three-legged blade. Also described herein are exemplary switch configurations incorporating the newly developed blade and, as described, the featured switches are advantageously well adapted for use in the protection of short term rated DC motors. Other advantages accrue from the described configurations. For example, it is contemplated that all characteristics and sensitivities within the performance of the subject switches are obtainable from a single bimetal type (that is to say a single type of bimetal material) and within a wide range it will not matter which type is used. Performance variations may be achieved by altering the bimetal and/or its operating temperature, and/or by altering the resistance of the unit metal parts either by material changes or by laser trimming of the form of the unit metal parts, the latter being an especially attractive option where laser welding of the bimetal blade to its carrier is utilized. The invention thus contemplates the provision of a range of small size and low cost cut-out products having wide application and assemblable all on the same production machinery.

[0016] Various modifications and variations may occur to those possessed of appropriate knowledge and skills from a reading of the foregoing and it is to be appreciated that the described embodiments are in all respects exemplary of the invention and are not to be read in a limiting sense. For example, whereas the described embodiments have used an insulator as the push-rod 17, it is within the scope of the invention to employ a material for the push-rod 17 which permits a small current to flow even after the contacts have broken such current causing self-heating of the bimetal and ensuring that the switch remains in its contacts-open condition until manually reset. Such a material might advantageously comprise a PTC thermistor material.

Claims

1. A snap-acting bimetal blade having a generally rectangular outer shape and having a generally rhomboidal cut-out defining two spaced-apart and tapering legs connected together at their opposite ends.

2. A snap-acting bimetal blade as claimed in claim 1 wherein the blade is dished so as to be movable with a snap-action between two oppositely curved configurations.

3. A snap-acting bimetal blade as claimed in claim 2 wherein the dishing of the blade is a result of press-­forming the blade.

4. A snap-acting bimetal blade as claimed in claim 2 wherein the dishing of the blade is a result of the provision of a crimp deformation in a portion of the blade bridging the ends of the two legs.

5. A thermally responsive switch incorporating a bimetal blade as claimed in any of the preceding claims.

6. A thermally responsive switch as claimed in claim 5 wherein the bimetal blade is mounted in the switch by means of the bridging portion interconnecting the ends of the legs at the narrow end of the aperture defined by the cut-out.

7. A thermally responsive switch as claimed in claim 5 or 6 which comprises first and second spaced-apart metal parts one of which is formed of a rigid, electrically resistive material and the other of which is a spring member, first and second contacts provided on said first and second parts, the bimetal blade being affixed to one of the first and second parts, and a push rod arranged for transferring switch-­operating movement of the bimetal to the spring member for operating the switch.

8. A thermally responsive switch as claimed in claim 7 wherein the push rod is formed of a material such as to permit a small current to flow even after the switch contacts have broken, such current causing self-heating of the bimetal and holding the switch in its contacts-open condition.

9. A thermally responsive switch as claimed in claim 8 wherein said material comprises a PTC thermistor material.

10. A thermally responsive switch as claimed in claim 5 or 6 which comprises first and second spaced-apart metal parts of rigid construction, the bimetal blade being affixed to one of said metal parts, a moving switch contact carried by the bimetal, and a fixed switch contact carried by the other metal part.

11. A snap-acting bimetal blade substantially as herein described with reference to the accompanying drawings.

12. A thermally responsive switch substantially as herein described with reference to any of the accompanying drawings.

Drawing