GB1559228A - Electric automatic cut out for overload and short-circuit current tripping - Google Patents

Description

(54) ELECTRIC AUTOMATIC CUT-OUT FOR OVERLOAD AND SHORT-CIRCUIT CURRENT TRIPPING (71) We, BROWN, BOVERI & CIE AG, of D 6800 Mannheim-Kafertal, Kallstadter Strasse 1, Germany, a German body corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to an electric automatic cut-out for overload and short-circuit current tripping with an overload trip means having a thermal bimetal strip and a shortcircuit trip device having a magnet core, surrounded by a coil, and magnet arm2- ture which acts on a lever for opening contacts.

Electric automatic cut-outs of conventional kinds have an electromagnetic trip device and a thermal trip device of which the electromagnetic trip device is used for tripping in the event of short-circuit currents and the thermal trip device responds to the appearance of so-called "overloads" and thus acts on the latch of the automatic cutout for opening the contacts. In general, the electromagnetic trip device comprises a coil, a fixed magnet core surrounded by the coil and a movable magnet armature which is normally constructed as a plunger armature and is drawn into the coil in the event of a short-circuit. In the course of this action the plunger armature releases the latch; when functioning as a so-called "impact armature", it can also act directly on - the movable contact member. Generally, the thermal overload trip device is provided with a bimetal strip which is heated in the event of an overload and thus bends: when the himetal strin has bent through a snecific di- tance, it acts on a latching place of the latch and unlatches the latter so that the movable contact is switched off or operated.

The two trip devices, the magnetic trip device and the thermal trip device, act independently of each other in normal commercial automatic cut-outs.

The prior art discloses an overload trip device (Offenlegungsschrift 1513167) in which the thermal trip device and the electromagnetic trip device together practically form one unit. To this end, the bimetal is disposed between a magnet core and a magnet armature so that owing to its thermal inertia the bimetal does not come into operation when a short-circuit current appears, but the movable magnet armature is attracted by the magnet core and thus unlatches a latch for the purpose of opening the contacts. If an overload occurs, the magnet core moves away from the magnet armature due to the motion of the bimetal.

The German Offenlegungsschrift 1 513 133 also discloses an overload switch in which the bimetal strip and the electromagnet trip device also form one unit in the sense that the plunger armature extends through the free end of the bimetal strip.

In known automatic cut-outs with a thermal bimetal overload tripping arrangement, the tripping action is therefore performed solely by the mechanical energy of the thermal bimetal strip. Dispersions and difficulties frequently arise, more particularly if the force required for unlatching alters due to some influences within the latch, since the aforementioned energy depends on the size of the bimetal strip employed and the thermal bimetal strips must be as small as possible for reasons of cost and space.

It is the object of the invention to combine the thermal and the electromagnetic trip devices with each other so that a greater energy is made available for thermal overload tripping, either to achieve higher reserves of force or to permit a further reduction of the size of the bimetal strip.

According to the present invention, there is provided an electric automatic cut-out for overload and short-circuit current tripping with an overload trip means having a thermal bimetal strip and a short-circuit tripping device having a magnet core, surrounded by a coil, and a magnet armature which acts on a lever for opening contacts, wherein the magnet core is slidably sup ported within the coil and is connected to the thermal bimetal strip so that the magnet core is displaceable by the thermal bimetal strip, when this is heated and bends, towards the magnet armature to reduce the air gap between the magnet core and the magnet armature.

The invention makes use of the fact that the response limit, i.e. the response current, of the magnet trip device is continuously reduced with a diminishing air gap. This is simply achieved by the thermal bimetal strip displacing the fixed magnet core towards the movable magnet armature; this reduces the air gap and thus lowers the response limit.

The method of operation of the tripping device is as follows: due to the dimensions of the trip device, no tripping takes place when the current flow is less than the permissible continuous current of the apparatus even if the maximum continuous current flows and if the thermal bimetal strip is bent to its corresponding limit. However, if the current rises above the intended limit value, the air gap, i.e. the distance between the movable magnet armature and the fixed magnet core, is reduced to such an extent that the current causes magnetic attraction of the magnet armature which therefore unlatches the latch on which the magnet armature acts. This ensures that for the same size and the same temperature the thermal bimetal strip will have higher reserves of force obtained bv a kind of "magnetic amplification" while conversely, the size of the bimetal strip in its entirety can be reduced if an increased force is necessary.

When a short-circuit occurs, the bimetal strip has a specific thermal inertia so that the magnet armature is actually retained by the bimetal strip and the magnet trip device functions as a pure short-circuit trip device.

It is possible to construct the magnet core so that it can slide only along a limited pathwav. To this end, a stop is provided on the guide of the magnet core to prevent further reduction of the air gap if a specific maximum deflection of the strip is exceeded.

However, free motion can be provided between the thermal bimetal strip and the magnet core so that the thermal bimetal strip itself is not exposed to an unnecessarily high force; that is achieved bv providing the magnet core with a proiection which extends through an opening of the bimetal strip and the length of the projection corresponds to at least twice the thickness of the bimetal strip, a thickened portion of the magnet core is resiliently urged in the deflection direction of the bimetal strip against the latter, When the thermal bimetal strip is deflected, the magnet core will be entrained because of the resilient thrust until it bears against the stop. Since the projection is longer than the thickness of the bimetal strip, the latter can continue to bend before it bears on the connecting portion between the projection and the magnet core and thus again acts directly on the magnet core. The projection can be made sufficiently long so that the thermal bimetal strip can bend freely until it achieves its maximum deflection after the magnet core bears against the stop.

Two embodiments of the invention will be explained hereinbelow by reference to the accompainying drawing, in which: Fig. 1 shows a first embodiment of the combined thermal and magnetic tripping device, and Fig. 2 shows another embodiment of a tripning device according to Fig. 1.

Fig. 1 shows a tripping mechanism which can be inserted into an electric automatic cut-out of conventional construction which has to be imagined. This con contain a latch and movable contacts operated thereby.

The mechanism, referenced in its entirety with the numeral 11 in Fig. 1, has a support 12 and a thermal bimetal strip 13 mounted thereon, the free end of which has an opening 14. The tripping mechanism also has a magnet system in which the magnet core 15 and a movable magnet armature 16, the latter in the form of a plunger armature, are surrounded by a former 17 with a winding 18. The former 17 and the winding 18 itself are surrounded by a magnet yoke 19 each of whose free members 20 and 21 have an opening 22 and 23 for guiding the magnet core 15 and the magnet armature 16. The portion of the magnet core 15 which projects beyond the member 20 of the magnet yoke 19 has a projection 24 the diameter of which is smaller than that of the magnet core and whose free end is provided with a thickened portion 25. The projection 24 extends through the opening 14 on the bimetal strip 13. The magnet armature 16 also has a magnet armature projection 26 which terminates in a plate-shaped portion 27 and the projection 26 extends through an opening 28 on a latch member 29; the external diameter of the plate-shaped portion 27 is greater than the diameter of the opening 28 associated with the latch member 29. Under the influence of a tension spring 30, the latch member 29 bears on a latch lever 31. If the bimetal strip 13 together with the magnet armature 15 is displaced in the direction of the arrow A in the event of an overload, the air gap L between the magnet core 15 and the magnet armature 16 will be reduced, so that the operating current of the magnetic strip is continuously reduced and the sensitivity is continuously increased. The movable magnet armature is attracted as soon as the current exceeds a specific limit value and said armature releases the latching between the latch member 29 and the latch lever 31 via the portion 27.

Fig. 2 shows a magnet core 15 with a projection 35 which has approximately twice the size of the projection 24 in Fig. 1. The magnet core 15 is biased by a spring 36 so that the thickened portion 25 is always urged against the bimetal strip. Bending of the bimetal strip causes the magnet core 15 to move therewith under the influence of the spring 36 until it bears against a stop, not shown, so that the air gap then remains constant even if the bimetal strip continues to be deflected; the bimetal strip can then continue to bend in the direction of the arrow A until it bears against the surface 15a. Generally, however, the bimetal strip will be designed so that the last-mentioned event does not take place. A trip device of this kind can of course also be constructed as a pivoting armature tripping device. In this case, the magnet core 15 is extended so that it projects practically through the entire magnet coil.

WHAT WE CLAIM IS : - 1. An electric automatic cut-out for overload and short-circuit current tripping with an overload trip means having a thermal bimetal strip and a sort-circuit tripping device having a magnet core, surrounded by a coil, and a magnet armature which acts on a lever for opening contacts, wherein the magnet core is slidably sup- ported within the coil and is connected to the thermal bimetal strip so that the magnet core is displaceable by the thermal bimetal strip, when this is heated and bends, towards the magnet armature to reduce the air gap between the magnet core and the mag net armature.

2. An automatic cut-out as clailmed in Claim 1, wherein the magnet core passes through a member of a yoke which surrounds the coil and said magnet core has a projection to which the thermal bimetal strip is coupled.

3. An automatic cut-out as claimed in Claim 2, wherein the projection has a diameter which is smaller than that of the remaining magnet core and its end has a thickened portion, the thermal bimetal strip has an opening for receiving the projection and is situated between the thickened portion and the magnet core, and the length of the projection is equal to or greater than the thickness of the thermal bimetal strip.

4. An automatic cut-out as claimed in claim 2 or claim 3 wherein the length of the projection corresponds to at least twice the thickness of the thermal bimetal strip, wherein the magnet core is resiliently urged in the direction of deflection of the thermal bimetal strip against the latter and wherein the magnet core can slide only along a limited pathway and on reaching a stop abutment the thermal bimetal strip can continue to bend freely.

5. An automatic cut-out subsantially as herein described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. the latch member 29 and the latch lever 31 via the portion 27. Fig. 2 shows a magnet core 15 with a projection 35 which has approximately twice the size of the projection 24 in Fig. 1. The magnet core 15 is biased by a spring 36 so that the thickened portion 25 is always urged against the bimetal strip. Bending of the bimetal strip causes the magnet core 15 to move therewith under the influence of the spring 36 until it bears against a stop, not shown, so that the air gap then remains constant even if the bimetal strip continues to be deflected; the bimetal strip can then continue to bend in the direction of the arrow A until it bears against the surface 15a. Generally, however, the bimetal strip will be designed so that the last-mentioned event does not take place. A trip device of this kind can of course also be constructed as a pivoting armature tripping device. In this case, the magnet core 15 is extended so that it projects practically through the entire magnet coil. WHAT WE CLAIM IS : -

1. An electric automatic cut-out for overload and short-circuit current tripping with an overload trip means having a thermal bimetal strip and a sort-circuit tripping device having a magnet core, surrounded by a coil, and a magnet armature which acts on a lever for opening contacts, wherein the magnet core is slidably sup- ported within the coil and is connected to the thermal bimetal strip so that the magnet core is displaceable by the thermal bimetal strip, when this is heated and bends, towards the magnet armature to reduce the air gap between the magnet core and the mag net armature.

2. An automatic cut-out as clailmed in Claim 1, wherein the magnet core passes through a member of a yoke which surrounds the coil and said magnet core has a projection to which the thermal bimetal strip is coupled.

3. An automatic cut-out as claimed in Claim 2, wherein the projection has a diameter which is smaller than that of the remaining magnet core and its end has a thickened portion, the thermal bimetal strip has an opening for receiving the projection and is situated between the thickened portion and the magnet core, and the length of the projection is equal to or greater than the thickness of the thermal bimetal strip.

4. An automatic cut-out as claimed in claim 2 or claim 3 wherein the length of the projection corresponds to at least twice the thickness of the thermal bimetal strip, wherein the magnet core is resiliently urged in the direction of deflection of the thermal bimetal strip against the latter and wherein the magnet core can slide only along a limited pathway and on reaching a stop abutment the thermal bimetal strip can continue to bend freely.

5. An automatic cut-out subsantially as herein described with reference to and as illustrated in the accompanying drawings.