GB2239987A

GB2239987A - Motor protection switches

Info

Publication number: GB2239987A
Application number: GB9027733A
Authority: GB
Inventors: David Walter Humphreys; Nigel Anthony Harby; Paul Douglas Gordon Harrison; Stephen William Harrison
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1989-12-20
Filing date: 1990-12-20
Publication date: 1991-07-17
Anticipated expiration: 2010-12-20
Also published as: GB2239987B; GB8928718D0; GB9027733D0

Abstract

A motor protection switch for use with automobile window lift motors for example is constructed to provide advantageous fast break, slow remake characteristics. The switch comprises a plastics body portion (1) into which are moulded first and second rigid metal parts (2, 3) which define, on one side of the body portion, the electrical supply terminals of the switch (2', 3') and on the other side a mounting (3'') for a fixed contact (6) and an elongate heater bridge (2'') from the distal end of which an elongate snap-acting bimetal (4) is cantilevered with the bimetal extending back towards the body portion and carrying at its end a moving switch contact (5) which co-operates with the fixed switch contact (6). The heater bridge (2'') is formed of a resistive material, eg nickel silver, such as to heat up during current-carrying operation of the switch and serves to pump heat into the bimetal (4) during switch operation so as to ensure a fast break and a slow remake. <IMAGE>

Description

IXPROVEMENTS RELATING TO THERMAL SWITCHES This invention concerns improvements relating to thermal switches and more particularly concerns thermal switches employing bimetallic elements as thermal actuators.

Our British Patent Application No. 8830299 filed 28 December 1988 and published as GB 2227884A relates to an improved form of snap-acting bimetallic switch actuator and to an improved switch employing such actuator, the switch being especially suited to utilization as a motor protection switch. As is described in our said Application, motor protection switches are customarily connected in series with the windings of small electric motors such as are commonly used as automotive window lift motors for example and where the initial operating current of the motor is high and then reduces as the heating of the motor windings causes an increase in their resistance.The requirement of such a motor protection switch for an automotive window lift motor is that the switch should be insensitive to high currents of short duration but sensitive to lower values of current flowing for relatively long time periods. Our said Application contains a discussion of the required characteristics of motor protection switches and inter alia makes reference to our British Patent Specifications Nos.

2133931 and 2124429.

It has been customary in the art of motor protection switches to have different switch types and different switch configurations for use with different kinds of electric motors, and it has been a constant aim of researchers in this field to develop a universal motor protection switch capable of providing high level protection for all kinds of electric motors. The present invention aims to satisfy this requirement.

According to the present invention there is provided a motor protection switch comprising a body portion formed of electrically insulating material, first and second rigid metal parts secured to said body portion and defining on one side thereof terminals for the switch and on the other side thereof a carrier for a bimetallic switch actuator and a carrier for a fixed contact of the switch respectively, the first metal part being elongate on said other side of the switch as compared to the second metal part and being formed of an electrically resistive material such as to self heat in use of the switch, an elongate snap-acting bimetallic blade cantilevered from the end of the first metal part furthest from the body portion of the switch on said other side thereof and extending towards the second metal part, a moving switch contact secured to the end of the elongate bimetallic blade remote from the cantilevered end thereof, and a fixed switch contact affixed to said second metal part on said other side of the body portion, said moving and fixed switch contacts being arranged for co-operation with each other in switching operations of the switch, and the electrically resistive material of said first switch part and/or the configuration thereof being such as significantly to influence the switching characteristics of the switch under current load.

As will become apparent hereinafter, the elongate first metal part to one end of which the bimetallic blade is cantilevered acts as a thermal pump to drive heat into the bimetal when the switch is closed, and as a thermal store to retain heat within the bimetal when the switch has opened. This results in the switch being relatively insensitive to short lived high currents arising when a window lift motor for example is first switched on, in that insufficient heat is pumped into the bimetal in the relevant period to operate the switch, but being sensitive to longer term lower currents, which can result in sufficient heat being pumped into the bimetal to cause switch operation. Also the heat retained in the elongate first metal part ensures that the bimetal cools only slowly once the switch has opened so that a relatively long reset time period results.

The bimetal itself is preferably a dished bimetallic blade having no cut-outs, though one or more cut-outs may be provided for increased current sensitivity. A generally rectangular blade with rounded corners for avoiding stress induced cracking is preferred and might include tabs at its opposite ends for attachment of the blade to its cantilever support and for attachment of the moving contact to the blade.

The first and second metal parts are preferably formed as integral metal stampings, though the first metal part in particular could be formed as a composite of a low resistance part in contact with the switch body and a relatively high resistance part spaced apart from the switch body. By virtue of such an arrangement, heat developed in use of the switch in the high resistance part can be kept away from the switch body which may conveniently be formed of a synthetic plastics material.

The above and further features of the present invention will become apparent from consideration of the following description of exemplary embodiments of the present invention wherein: Figs. 1A to 1D are, respectively, a bottom plan view, a side elevation view, a top plan view and an end elevation view of a first exemplary switch embodying the present invention; Fig. 2 is a perspective view of the switch of Fig.1; Fig. 3 is a plan view of the bimetal blade used in the first embodiment; Figs. 4A, 4B and 4C are, respectively, a bottom plan view, a side elevation view and a top plan view of a second exemplary embodiment of the present invention; Fig. 5 is a perspective view of the switch of Figs. 4A, 4B and 4C; and Figs 6A and 6B are plan views of alternative bimetal blades usable in the second embodiment.

Referring first to Figs. 1 to 3 of the drawings, wherein the shown dimensions are in millimetres and are in all respects exemplary, the switch comprises a synthetic moulded plastics body portion 1 which captures a first rigid metal part 2 and a second rigid metal part 3, the first part 2 being formed to define a terminal part 2' on one side of the body portion 1 and an elongate heater bridge 2'' on the other side, and the second part 3 being formed to define a terminal part 3' and a contact carrying part 3tut. The first part 2 is formed of nickel silver (NS 107) for example and the second part 3 may, for convenience in manufacture of the switch, be formed of the same material.A snap-acting dished bimetallic blade 4 is schematically shown in Fig.lB attached at one end thereof to the end of the heater bridge 2'' furthest from the body 1 of the switch, and cantilevers towards the contact carrying portion 3'' of the second metal part, and moving and fixed switch contacts 5 and 6 respectively are attached to the bimetallic blade 4 and to the metal part 3''. A portion 2''' of the part 2'' defines a back stop for the bimetal blade. For clarity of illustration, the bimetal blade 4 is shown only in Fig.lB of the Fig.l series, though a plan view of the blade is shown in Fig.3 from which it can be seen that the blade is generally rectangular with. rounded corners and has attachment tabs at its opposed ends.

The body portion l is formed to give sufficient support to the first and second metal parts 2 and 3 to ensure that the switch characteristics will not vary with age as a result of relative movement between the parts 2 and 3, note in particular the additional support provided for the elongate part 2 as compared to the shorter part 3, and the metal part 2 is itself formed as shown so as to be resistant to deformation.

A moulded plastics cover (not shown) is adapted to be fitted over the active parts of the switch and may if desired make a snap-fit with the body portion 1.

In use of the switch to protect the windings of an automobile window lift motor for example, the switch is connected in series with the motor windings and is located in close proximity to the windings.

When the motor is operated, the initial high start-up current that flows in the motor windings will not cause the switch to operate and neither will the subsequent lower normal operating current of the motor. However, if the motor is stalled, for example on account of the window being jammed, an excessive current will flow in the motor windings and, on account of self-heating of the bimetal blade 4, will cause the switch quickly to switch off. As the bimetal moves into its open-contacts position, so it moves into closer heat transfer relation with the heater bridge 2'' (which will itself have been heated by the excess current in the motor windings) and as a result the bimetal cools more slowly than would otherwise be the case. An advantageous fast break, slow re-make switch operation thus results.

Figs. 4A, 4B and 4C illustrate a second embodiment of the invention which is very similar to the first embodiment described hereinabove. The same reference numerals are used in Figs. 4A, 4B and 4C as were used to designate like parts in the first embodiment. As compared to the first embodiment, the second embodiment has a thinner heater bridge portion 2'' which extends closely adjacent to the bimetal blade 4 for enhanced heat exchange therewith, which is beneficial for motor protection applications since it results in longer first break times with a larger on:of ratio in a stabilized stall condition. As shown in Fig. 6A the bimetal blade 4 can be formed with a central cut-out 10 for increased current sensitivity.

The switch of Figs. 4A, 4B and 4C otherwise functions in the same manner as the first embodiment.

The switch according to the present invention, particularly as hereinbefore described, is well suited to automatic assembly and, by changing the metal of the heater bridge portion and/or changing the bimetal resistivity, switches having a range of characteristics suitable for different applications can readily be obtained.

Claims

CLAIMS:

1. A motor protection switch comprising a body portion formed of electrically insulating material, first and second rigid metal parts secured to said body portion and defining on one side thereof terminals for the switch and on the other side thereof a carrier for a bimetallic switch actuator and a carrier for a fixed contact of the switch respectively, the first metal part being elongate on said other side of the switch as compared to the second metal part and being formed of an electrically resistive material such as to self heat in use of the switch, an elongate snap-acting bimetallic blade cantilevered from the end of the first metal part furthest from the body portion of the switch on said other side thereof and extending towards the second metal part, a moving switch contact secured to the end of the elongate bimetallic blade remote from the cantilevered end thereof, and a fixed switch contact affixed to said second metal part on. said other side of the body portion, said moving and fixed switch contacts being arranged for co-operation with each other in switching operations of the switch, and the electrically resistive material of said first switch part and/or the configuration thereof being such as significantly to influence the switching characteristics of the switch under current load.

2. A motor protection switch as claimed in claim 1 wherein the bimetal comprises a dished bimetallic blade.

3. A motor protection switch as claimed in claim 2 wherein the bimetallic blade is formed with a cut-out thereby enhancing the current sensitivity of the switch.

4. A motor protection switch as claimed in claim 2 or 3 wherein the bimetallic blade is generally rectangular but with rounded corners.

5. A motor protection switch as claimed in claim 4 wherein the bimetallic blade is formed with tabs at its opposed ends, one of said tabs serving for attachment of the bimetallic -blade to said first metal part and the other serving for attachment of the moving switch contact to the bimetallic blade.

6. A motor protection switch as claimed in any of the preceding claims wherein said first metal part has a portion which serves as a back-stop for the contact carrying end of the bimetallic blade.

7. A motor protection switch as claimed in any of the preceding claims wherein said first metal part extends closely adjacent to the bimetal for efficient heat transfer thereto.

8. A motor protection switch substantially as herein described with reference to Figs. 1A to 1D, 2 and 3 or Figs. 4A to 4C, 5 and 6 of the accompanying drawings.