A magneto-thermic switch having thermal protection which can be calibrated mechanically and associated method of calibration

Abstract: A magneto-thermic switch having thermal protection which can be calibrated mechanically, in which the casing (50) is provided with an opening (25) for detecting, with a sensor (33) introduced into the opening (25), the rest position (with respect to the casing) of the free end of a bimetallic element (10) which actuates the thermal protection, fitted on an adjustable cantilever support (9,67) thus allowing the cantilever support (9,67) to be adjusted to make the free end of the bimetallic element (10) assume a predetermined rest position with respect to the casing (50).

Description: The present invention relates to a magneto-thermic switch according to the preamble of claim 1, as for example known from DE-A-30 03 288 and to methods of calibration of such a switch. It is known that magneto-thermic switches are automatic switching devices in which two contacts are closed by a manual arming device (which can also be controlled manually to open them) and which must open automatically when the switch is overloaded whereby to ensure protection of electrical installations against overloads and short circuits. To this end two types of protective action are in general envisaged: protection against overloads, protection against short circuits. Protection against short circuits is ensured by an electromagnetic actuator having a movable core excited by the load current which, in the event of a short circuit, acts on a release device causing immediate and rapid opening of the contacts. Response speed is an essential requirement for short circuit protection. Protection against overloads is ensured by a bimetallic element which, when heated by the load current which flows through it, bends until it acts on a release device and thus causes opening of the contacts. As opposed to the magnetic protection, thermal protection requires precision and repeatability of operation and must ensure that for a predetermined overload condition, corresponding for example to a current of 20A, lasting for a predetermined time, for example 20 seconds, opening of the contacts will occur. Thermal protection of automatic switches therefore requires a calibration operation which must be performed on each individual switch and must ensure repeatability of action and prevent possible miscalibrations. The bimetallic element, which constitutes the overload sensor, is generally in the form of a rigid beam mounted in a cantilever fashion on a relatively flexible support and the free end of which acts on the release device. A calibration screw accessible from outside the switch acts on the flexible support close to the bimetallic beam mount and allows the angle of the mount to be modified so as to rotate the beam at the mount and displace the end of the beam. Calibration involves mechanical and electrical operations. First of all it is necessary to connect the switch to a source of a predetermined current, to apply the current to the switch for a predetermined time interval and to test if, at the end of the time interval, the protective intervention has occurred. If the protective intervention takes place prematurely, or does not take place, it is necessary to modify the orientation of the bimetallic beam by acting on the calibration screw and repeat the operation of applying current. Between one application of current and another it is necessary to wait a not-negligible time, of the order of several minutes, to ensure that the bimetallic element, heated by the current, cools to a value very close to the ambient temperature (from 20 to 30°C). Once calibration is completed it is necessary to fix the calibration screw with adhesive to prevent vibrations or repeated thermal excursions from modifying the calibration state. The adhesive is introduced into the switch through the same aperture used for acting on the calibration screw. It is therefore evident that the calibration operations are relatively complex, time consuming and laborious and significantly affect the cost of the switch. Moreover, due to the complexity and variety of the operations to be performed it lends itself only with difficulty to automation, and because it requires a variable time for performance of the calibration, with a number of tests, it does not lend itself to integration of the calibration process into a mass production line. A further disadvantage lies in the fact that calibration thus performed is capable of corruption: the glue on the calibration screw does not in fact prevent it from being screwed or unscrewed forcibly by the installer or by a careless user. All these limitations are overcome by the magneto-thermic switch which is the subject of the present invention, which allows an exclusively mechanical repeatable calibration to be performed in a short time and, therefore, as well as being simple, fast and low cost, is capable of automation and integration into the production line. These results are achieved by providing a switch in which the casing is formed with an aperture for passage of a feeler probe for testing the position of the end of the bimetallic beam. The probe is coupled to a transducer the signal from which is used to control the automated screwing or unscrewing of the calibration screw. In a further aspect of the present invention a second aperture is also provided in the switch casing disposed in correspondence with the bimetallic beam mount to allow the introduction into the casing of cold polymerisable resin which sets the beam mount in the calibration position and thus prevents subsequent manipulation of the calibration screw from modifying the position) of the bimetallic element. The characteristics and advantages of the invention will become clearer from the following description of a preferred embodiment and from the attached drawings in which: Figure 1 is a sectional view of the structure of a magneto-thermic switch thermal protection which can be calibrated mechanically formed in accordance with the present invention; Figure 2 is a sectional view on an enlarged scale of a portion of the switch of Figure 1; Figure 3 is a schematic perspective view of a calibration bed for mechanically calibrating the thermal protection of the switch of Figure 1 in accordance with the method of the present invention; and Figure 4 is a variant of the calibration bed control system of Figure 3 for putting into practice a variant of the mechanical calibration method of the present invention. With reference to Figure 1, an automatic switch according to the invention comprises a casing 50 of generally rectangular form constituted by two coupled half-shells one of which is shown in section along the plane of coupling in order better to show the internally ribbed structure of the half-shells and to make more clearly visible the different apertures formed by these. The internal frames and ribs of the two half-shells suitably interpenetrate and hold the two half-shells and the different components housed within the casing together in a precise manner. The two half-shells are fixed together by means of screws or rivets passing through apertures 51, 52, 53, 54, 55 perpendicular to the plane of the drawing. The switch shown is of modular type for installation on rails next to adjacent modules positioned with the casing faces parallel to the coupling plane of the half-shells. The switch has on its rear wall a recess 56 intended to receive a rail of DIN regulation type to which the switch is engaged by means of toothed connector guides not shown. Within the casing a plurality of mechanical and electrical components are housed and positioned with precision, and in particular these comprise: first and second terminals 57, 58 respectively for electrical connection with external conductor terminations, a bimetallic strip 10, a pull-in electromagnet 59, an arc quenching cell 60, a manual arming lever 61 pivoted to a pin 62 in a position fixed in the casing and coupled to an arming link 2, a fixed contact 3 supported by a rigid metal projection 69 of the electromagnet 59 and electrically connected to a terminal of the winding of the electromagnet 59, a movable contact 4 at the end of a contact arm 5 of conductive material electrically connected to the bimetallic strip by a flexible copper braid 6, a release device formed by a support lever for the contact arm 5, a spring 11 acting on the contact arm 5 to maintain it in open position, a latch pawl 6 and a trip lever or member 7 (more details on the release device will be provided hereinafter), a guide 8 for uni-directional mechanical coupling between the bimetallic strip 10 and the release device. The terminal 57 is clamped by means of a clamping screw the head of which is accessible through an opening 63 in the casing co-axial with the screw (and open to the front of the casing). The external electrical terminations to be clamped by the terminal can be interposed between the jaws of the terminal through an aperture 64 of the casing (open on to the face of the casing which, in relation to normal installation conditions of the switch, corresponds to the lower face). Similar apertures 65, 66 are respectively provided for clamping and introduction of electrical terminations into the terminal 58. The terminal 57 is provided with a metal projection 9 the end of which is electrically connected to one end of the bimetallic strip 10 and to one end of a arc conveyor guide 67 for conveying the arc towards the arc quenching labyrinth. The walls of the casing 50 have suitable internal ribs ensure the positioning of the terminal 57 and the guide 67 within the casing, with limited play. The ends of the projection 9 and the guide 67 provide a cantilever support for the bimetallic element 10. On the cantilever support acts the end of a calibration screw 1 housed, together with a nut forming a threaded screwing seat, in a housing conveniently formed in the casing 50. The actuation of the screw 1 is effected through an opening 68 in the lower wall of the casing and causes rotation of the cantilever support of the bimetallic element 10 the free end of which turns proportionally. The projection 9, the bimetallic strip 10 and the flexible copper braid 6 ensure electrical continuity between the terminal 57 and the movable contact arm 5. The projection 69 and the winding of the electromagnet 59 one end 70 of which is connected to the terminal 58 ensure electrical continuity between fixed contact 3 and terminal 58. Therefore, when the contacts 3 and 4 are closed the current which flows in the electromagnet and in the bimetallic strip can pass through the switch. The current is interrupted by opening of the contacts 3 and 4. In Figure 1 the contacts are shown in the open position to which they are biased by the thrust exerted by the spring 11 on the movable contact arm 5 and through this on the entire trip device. The contacts 3 and 4 are closed upon arming of the release device, effected manually by rotation of the lever 61 in the direction indicated by the arrow 71. The link 2 then presses the trip device into the armed position overcoming the thrust of the spring 11. Figure 2 is an enlarged and simplified partially sectioned view of the trip device and the elements interacting with it for a better understanding of the operation of the trip device and the thermal protection. As already mentioned, the trip device is formed by a support lever 12 for the contact arm 5, articulated at one end on a pin 13 engaged in a predetermined fixed position in the casing. The contact arm 5 is pivoted at its end opposite from the contact 4 on a pin 14 rigidly connected to the free end of the support lever 12. The contact arm 5 is capable of relative rotation with respect to the support lever 12 on the pin 14, limited by a slot 15 in which the pin 13 is inserted. The lever 12 is provided with an arming tooth 16 on which the end of the link 2 presses. A latch pawl 6, which for simplicity is represented as pivoted on the pin 14 (but which could be also pivoted in a different position) prevents the end of the link 6 from sliding on the tooth 16. Rotation of the arming lever 61 into the position shown and defined by an abutment 72 of the casing, acts through the link 2 on the support lever 12 and, by overcoming the action of the spring 11, causes a predetermined rotation of the pin 13 thereby making it assume a stable and defined armed position in relation to the casing. Rotation of the support lever 12 is accompanied by a rotation of the contact arm 5 which closes the contacts 3 and 4. A release lever or member 7 pivoted on the pin 13 is provided with a release tooth 17 which interacts with a tooth of the latch pawl 6 to cause its rotation and consequent release of the end of the link 2 from the tooth 16. The release member 7 is provided with an arm 18 (on which acts a striker 19 operated by the electromagnet) ending in a tooth 20 which interacts with the guide 8. The guide 8 is provided at its end with two teeth 22, 23 which interfere respectively with the end of the bimetallic strip 10 and with the tooth 20. When the bimetallic strip 10 is bent by the effect of the heating caused by a current which flows in the strip, in the direction indicated by the arrow 24, the end of the strip acts on the tooth 22 and causes translation of the guide 8 in the same direction. Consequently the tooth 23 of the guide acts on the tooth 20 of the trip member 7 and causes a rotation thereof which in turn provokes release of the latch pawl 6. It is to be noted that the resistance of the trip device to movement of the guide 8 is entirely negligible. The stroke which the guide 8 must perform and the position which it must assume to ensure release of the latch pawl 6 are uniquely defined in relation to the casing by the geometry of the release mechanism and known from the design, with the single uncertainty due to the working tolerances, which must be kept within predetermined limits. The position which the end of the bimetallic strip 10 must assume in order to ensure tripping of the release device is therefore unequivocally defined. In the same way the bending caused in the bimetallic strip 10 by a predetermined current flowing for a predetermined time can be calculated theoretically and/or measured experimentally. Consequently, the position which the end of the bimetallic strip 10 must occupy in rest conditions at ambient temperature, when the bimetallic strip has no current flowing through it, is also defined in relation to the casing. The problem of calibration of the thermal protection device can be reduced therefore to positional adjustment of the free end of the bimetallic strip 10 in rest conditions. According to one aspect of the present invention the adjustment is easily achieved by providing in the casing 1 a rectilinear opening 25, preferably of circular cross-section which extends from one wall (the lower wall) of the casing up to the end of the bimetallic strip 10 in a direction substantially perpendicular to the length of the bimetallic strip. Through this opening it is possible to introduce into the casing a measurement rod 33 coupled to a transducer, for example a differential inductance transducer, which, when thrust with a minimum force into contact with the bimetallic element 10 allows measurement with high precision of the distance D of the end of the bimetallic strip from a reference plane of the casing perpendicular to the direction of heat-induced bending of the metal strip. The reference plane can be the lower face of the casing into which the hole 25 opens, or more advantageously a parallel plane closer to the pin 13 of the trip mechanism in order to reduce to the minimum imprecisions due to the shrinkage of the casing 1 which is generally made of moulded plastics material, which shrinkage is subject to a certain variability. For example the head of the switch in which the trip device is partially housed generally has a reference plane 26 particularly suitable for the performance of this measurement. By acting on the calibration screw 1 it is possible, with a simple and rapid operation, to modify the position of the mount of the bimetallic strip in such a way as to make the free end of the bimetallic strip assume the desired position. Figure 3 schematically illustrates apparatus which allows the calibration operation to be performed automatically by putting into operation the method described. A switch 27 is disposed, in this case with automatic gripping devices, on a test bed 28 provided with a reference tooth 29 against which the switch 27 is pressed by a movable jaw 30 moved by an actuator 31. On the bed 28 is installed a position sensor 32 provided with a measuring rod 33 which is inserted into the aperture 25 of the switch until it comes into contact with the bimetallic strip. On the bed 28 there is also installed a motorised screw driver 34 the screw driver blade 35 of which is inserted into the aperture 68 of the switch to engage with the calibration screw. A sensor 32 provides a comparator 36 (which can be either of the analogue or digital type), with a signal indicating of the detected position of the bimetallic strip. This signal is compared with a reference value REF corresponding to the desired position of the bimetallic strip. The error signal ERR generated by the comparator is applied to the input of an amplifier 37 which supplies the motorised screw driver 34 with an electrical signal of suitable magnitude and sense to cause screwing or unscrewing of the calibration screw until the error signal becomes zero. With an open loop adjustment the same apparatus can serve, by utilising the sensor 32, to measure the thermal deflection caused by a predetermined current in the bimetallic strip for a predetermined time, and to experimentally determine, on a certain number of samples, the position which the bimetallic strip assumes in order to cause the trip device to act, and the corresponding transducer signal. This measurement method although sufficiently precise for the practical requirements of calibration does not allow compensation of uncertainties and imprecision due to dimensional tolerances of different components. For this reason it is suitable for the calibration operations to be followed by a test operation which comprises applying to the switch a current of predetermined value and testing if the thermal protection intervenes with a delay lying between two predetermined values. The method is however capable of variations which allow integral compensation of the production tolerances on each individual calibrated switch. In fact, the magnitude of the thermal deflection of the bimetallic element is substantially independent of the production tolerances which to some extent influence only the position of the bimetallic strip relative to the reference plane of the casing, for which position intervention of the trip device occurs. As schematically shown in Figure 4 it is therefore possible to perform the calibration operations on each individual switch by effecting two measurements. Preferably in this case the test bed includes a microprocessor 38 which controls the necessary operations in sequence. The switch to be calibrated is located on the test bed and armed. In a first measurement phase the calibration screw is actuated to cause rotation of the cantilever support of the bimetallic strip to simulate a thermal deflection thereof. Simultaneously the position assumed by the end of the bimetallic strip is measured and the indication memorised (after conversion from analogue to digital form in a convertor 39) corresponding to the position in which triggering of the overload protector occurs. In a second calibration phase the calibration screw is turned in an opposite sense to return the bimetallic strip to a rest condition in which the end of the strip is located at a predetermined distance from the now-identified trip position, equal to a predetermined thermal deflection. This calibration operation is extremely precise and not affected by manufacturing tolerances. Since it is performed without effective thermal stress, but only with simple and quick mechanical operations it can easily be integrated into a continuous production and inspection cycle. Figure 3 shows a further advantageous characteristic of the switch formed in accordance with the present invention. Conveniently an aperture 40 is provided in a side face of the casing in correspondence with the cantilever support of the bimetallic strip 10. The same aperture is indicated in broken outline in the section of Figure 2. Through this aperture is introduced or cast into the casing, once calibration of the switch has been performed, a metered quantity of polymerisable resin of suitable viscosity. Preferably, but not necessarily, injection of resin takes place with the switch disposed in a plane with the rear face orientated downwardly and this position is maintained for the time necessary for hardening of the resin. In this way a block of hardened resin which encapsulates and rigidly retains the fulcrum of the bimetallic element 10 in the calibrated position is formed between the ribs of the casing which operate as a containment vessel as shown in Figure 2, and this prevents tampering by subsequent actions on the calibration screw. High repeatability of the intervention of the thermal protection over time is thus ensured. The preceding description relates only to a preferred embodiment of the switch and the calibration method of the invention and many variations can be introduced. For example the trip device, rather than having a latch pawl and an interacting trip member can have a simple hook pawl pivoted on the pin 13 or eccentrically thereof, acting directly on the end of the link and actuated by the guide 8. The aperture 25 orientated in the direction of the thermal deflection can be replaced by a pair of facing openings formed in both the half-shells of the casing at the free end of the bimetallic strip to detect its position using optical apparatus or mechanical sensors of different type from the feeler rod described.

Claims: A magneto-thermic switch with thermal overload protection activated by a beam-type bimetallic element (10) fitted at one end to a cantilever support (9,67) adjustable by means of a calibration screw (1), the other, free end of the beam (10) acting on a contact-opening release device (6,7,11,12,8) by the effect of a thermally-induced deflection of the said beam (10) which displaces the said free end from a rest position, the said bimetallic element (10), the said cantilever support (9,67) and the said calibration screw (1) being housed in a closed casing (50) having a first access opening (68) for the said calibration screw (1), characterised in that the said casing (50) includes a second opening (25) for detecting the rest position of the said free end of the beam (10) with respect to the said casing (50) using detection devices through the said second opening (25). A switch as in Claim 1, in which said second opening (25) in the said casing (50) extends in the direction of the said thermal deflection and is aligned with the said free end to detect the said rest position with a measurement rod (33) inserted in said second opening (25). A switch as in Claim 1 or Claim 2, in which the said casing (50) includes ribs forming a housing tray for the said cantilever supported mount end of the said beam, a third opening (40) in the said casing (50), open into the said tray and a polymerisable resin block injected into the said casing through the said third aperture (40) and hardened in the said tray, the said block encapsulating and making the said cantilever supported mount end of the bimetallic strip (10) rigid in a fixed position in relation to the said casing (50). A method of calibrating the thermal protection of the switch according to claims 1 to 3, activated by a bimetallic beam element (10) fitted at one end on a cantilever support (9,67) adjustable by means of a calibration screw (1), the other, free, end of the beam (10) acting on a contact-opening release device (6,7,11,12,8) by the effect of a thermally-induced deflection of the said beam which displaces the said free end from a rest position, the said bimetallic element (10), the said cantilever support (9,67) and the said calibration screw (1) being housed in a casing (50), consisting in detecting the rest position of the said free end in relation to the said casing (50) by a detecting device (33), inserted through a second opening (25) in the casing (50) and in turning the said calibration screw (1) to cause the said detected rest position to coincide with a predetermined position in relation to the said casing (50). A method of calibrating the thermal protection of the switch according to claims 1 to 3 activated by a beam-type bimetallic element (10) mounted at one end on a cantilever support (9,67) adjustable by means of a calibration screw (1), the free other end of the beam (10) acting on a contact-opening release (6,7,11,12,8) device by the effect of a thermally-induced deflection of the said beam (10) which displaces the free end from a rest position, the said bimetallic element (10), the said cantilever support (9,67) and the said calibration screw (1) being housed in a casing (50), consisting in arming the said contact-opening release device (6,7,11,12,8), turning the calibration screw (1) until the said free end assumes a first position tripping said contact-opening release device (6,7,11,12,8), detecting the said first position assumed by the said free end in relation to the said casing by a detecting device (33), inserted through a second opening (25) in the casing (50) and turning the calibration screw until the said free end assumes a second rest position a predetermined distance from the said first position in relation to the said casing. A method as in Claim 4 or Claim 5, in which the position of the said free end is detected by means of a feeler rod (86) inserted in the said casing (50) and brought into contact with the said free end, the said feeler rod (33) being coupled to a measurement transducer (32) which generates a signal correlated to the said position of the free end with respect to the said casing (50). A method as in Claim 4, 5 or 6, including a further step of applying a predetermined current to the switch and testing whether the intervention of the thermal protection occurs with a delay lying between two predetermined values. A method as in Claim 4, 5, 6 or 7, including the further step of introducing into the said casing (50) a hardening resin which rigidly encapsulate the said end of the bimetallic element (10) fitted on the cantilever support (9,67).

Inventors: PIANEZZOLA SERGIO, ROSSETTI GIANPAOLO, PIANEZZOLA, SERGIO, ROSSETTI, GIANPAOLO
Applicants: BTICINO SPA, BTICINO S.P.A.
Application year: 1995
Main classification: H01H73
IPC codes: H01H71, H01H69