TECHNICAL FIELD

The present invention relates to an electric switching devi¬ ce of the kind described in the preamble to claim 1. The invention is primarily intended for use in different types of electric switching devices such as contactors, load switches, etc., for low voltage (up to a rated operating voltage of 1000 V) .

BACKGROUND ART

When an electric switching device, for example a contactor, with antiparallel current paths is traversed by currents above the normal operating range, the contact system is in¬ fluenced by repulsion forces which may exceed the static force of the contact pressure springs. This leads to the occurrence of contact separation, and the arc which is then created may cause fusion of contact material with an ensuing breakdown as, for example, contact welding.

For contactors and other electric switching devices which are intended to be connected into a motor circuit, inter¬ national standards (IEC 158-1) require weldless contacts in the direct on-line current region of the motor (up to about 10 times the rated current). In addition, it is de¬ sirable that contact welding be avoided also in connection with a short-circuit.

From US Patent No. 2,679,561 a contactor is previously known in which the movable contact is supported by a rotatably journalled contact carrier which is connected to an opera¬ ting magnet, the movable contact being movable in relation to the contact carrier against the action of a contact pres¬ sure spring. For increasing the contact pressure at high currents, the contactor is provided with a magnetic circuit,

excited by the current through the contacts, consisting of a core which is rigidly connected to the contact carrier, and an armature which is rigidly connected to the movable contact. By such a contact pressure-increasing magnetic circuit, the electrodynamic repulsion- forces can be balanced so as to avoid contact separation within a relatively large current range. Further, the spring force of the contact pressure springs can be reduced, and therefore a smaller size of the operating magnet can be chosen, which entails savings of costs. A drawback with the known design, however, is that the magnetic core which is rigidly connected to the contact carrier gives a considerable increase of the mass of the movable contact system which, upon closing, impinges against the core of the operating magnet. This causes major wear of the pole surfaces, increased propensity to contact bouncing, etc.

From US Patent No. 4,513,270 a contactor is previously known comprising a transversally movable contact bridge, arranged between two fixed contacts, the contact bridge being influ- enced by a contact pressure-increasing magnetic circuit. Also in this design, the core of the magnetic circuit is rigidly connected to the contact carrier by means of mecha¬ nically rigid joints. This means that the device exhibits the same drawbacks as the device described above.

From US Patent No. 3,887,888 it is known to arrange, in imme¬ diate association with each contact unit in a contactor, a magnetic circuit for increasing the contact pressure at over- currents. In this case the magnetic circuit consists of a U- shaped magnetic core fixed to the fixed contact and an arma- ture fixed to the movable contact. With such a contactor, the magnets will compress the contacts at high currents so as- to prevent contact welding. At the same time, however, this design entails the drawback that the contactor, because of the contact attraction, cannot be used for breaking of currents exceeding a relatively moderate overcurrent value. Furthermore, by placing the magnets in immediate proximity

to the contact unit, the possibility of blowing out the arc in the extinguishing chamber of the contactor with the aid of the magnetic field generated by the current is eliminated. One reason for this is that the contact pressure-increasing magnets shield the field around the contact unit, and another reason is that with this location of the magnets it is neces¬ sary to design the contacts in such a way that the current passes right through the magnetic circuit and not in a loop- shaped path, which would have given a blowing of the arc tow- ards the set of quenching laminations.

It is further known, in contactor-like devices, to arrange U- shaped attraction magnets to increase the contact pressure at moderate overcurrents (see e.g. US patent No. 4,454,490). These attraction magnets, in cooperation with repulsion mag- nets, are intended to render these devices suitable for break¬ ing short-circuit currents as well. The attraction magnets are dimensioned so as to become saturated when the overcurrent exceeds a value corresponding to normal motor starting cur¬ rents, for example 10 times the rated current. These devices ^a ^e specially designed and, therefore, relatively expensive compared with ordinary contactors.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an electric switching device with a current-depending contact pressure increasing magnetic circuit, which prevents contact separa¬ tion in the critical overcurrent range up to about 30 times the rated current, but which at the same time enables the switching device to be used for breaking of currents up to the limit which is determined by the breaking capacity of the device. In addition, the switching device is to be designed so as to avoid the above-mentioned drawbacks of comparable prior art designs. This is achieved according to the present invention by an electric switching device with the characte¬ ristic features described in the characterizing part of claim 1.

By arranging the core of the contact pressure-increasing mag¬ netic circuit to be retained by the contact pressure spring against a stop provided on the contact carrier, several ad¬ vantages are gained. For one thing, the assembly is simpli- fied and no extra assembly parts are required, and for another, the impact upon closing is reduced, since the mass of the magnetic core is slowed down by the spring and thus does not take part in the impact process. This in turn re¬ duces the tendency to contact bouncing and increases the life of the device.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail by way of examples with reference to the accompanying drawing, wherein

Figure 1 shows a contactor, designed according to the invention, in longitudinal section,

Figure 2 shows half of the same contactor in cross section along the line II-II in Figure 1 ,

Figure 3 is a perspective view of the contact assem¬ bly of the contactor,

Figures 4 and 5 show on an enlarged scale the central part of the movable contact assembly in cross section and longitudinal section, respecti¬ vely,

Figure 6 shows schematically a magnetic circuit arranged around the movable contact of the contactor, and the associated magnetic field configuration,

Figure 7 shows curves of the forces acting on the movable contact assembly as a function of the current,

Figure 8 shows a tripping curve for a fuse which is connected in series with the contact, and

Figures 9 and 10 show the central part of an alternative embodiment of the movable contact assembly, in cross section and longitudinal section, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The contactor shown in Figures 1 and 2 is of three-pole de¬ sign and has a rated voltage of, for example, 660 V. The rated current of the contact may, for example, lie in the range 100 - 1000 A. The contactor is built up on a stand 1 of pressure-cast light metal. The stand 1 supports a holder 2 of plastic with connecting bars 3, 4 for connection of the contactor into an external main circuit.

The contact means of the contactor, which in the drawing is shown in the open position, is surrounded by an arc chute 5 of plastic provided with extinction plates 6. The contact means comprises, in each pole, two electrically series- connected breaking units, which are each arranged in a res- pective extinguishing chamber 7, 8 formed in the arc chute 5. Each breaking unit has a fixed contact 9 and 10, res¬ pectively, which is fixed to the connecting bar 3 and 4, res¬ pectively. In closed contact position, the fixed contacts are connected to each other through a contact bridge consis- ting of a movable contact 11 and an iron mounting 12, posi¬ tioned on top of the movable contact, for mechanical stiffe¬ ning of the contact.

The contact bridge is mounted in an opening 13 in a contact carrier 14 which is connected to the armature 15 of the ope- rating magnet of the contactor, the magnetic core and coil of the operating magnet being designated 16 and 17, respec¬ tively. The armature 15 is influenced by opening springs 18, only one of which is shown. When the coil 17 is de-energized,

these springs 18 bias the armature 15 to the position shown in Figures 1 and 2. The contact bridge 11, 12 passes between the legs 19 of a U-shaped yoke 20 (Figure 3) arranged in the contact carrier 14. The free ends of the legs 19 of the yoke 20 are interconnected through a pin 21. Between this pin 21 and the movable contact bridge 11, 12, a leaf spring 22 is mounted. A contact pressure spring 23 is arranged in the yoke 20.

The- movable contact 11 is surrounded by a magnetic circuit consisting of a magnetic core 25, which is fixed to the con¬ tact carrier 14, and a magnet armature 26 which is fixedly mounted to the movable contact 11. As is best illustrated in Figures 4 and 5, the fixation of the magnetic core 25 is brought about by the contact pressure spring 23 keeping the magnetic core 25 pressed against a collar 24 in the contact carrier 14. The yoke 20 is axially movable in the contact carrier 14 against the action of the springs 22 and 23.

When, in the closed position of the contactor, current passes through the movable contact 11, the magnetic circuit 25, 26 (Figure 6) is magnetized, whereby an attraction force F arises between the magnetic core 25 and the contact bridge

11, 12. This attraction force is composed of a relatively great force Fa.i between the core 25 and the armature 26 and a smaller force Fa2„ between the core 25 and the contact 11. The attraction force F 3. cooperates with the static spring force F exerted by the springs 22 and 23. At the same time, the contact bridge is influenced by an electrodynamic repul¬ sion force Fr.

To achieve a higher saturation value for the magnetic circuit 25, 26 without having to increase the dimensions of the elec¬ tric switching device, as will be clear from Figure 4 the mag¬ netic core 25 has been designed with a larger cross-sectional area in the bottom part 25a than in the legs 25b. In this way, F _ will increase with the current.

Figure 7 shows how the above forces F3., FS and F•£ , acting on the contact bridge, vary as a function of the instantaneous value i of the current. The diagram shows the "lifting current" im of the contact assembly,' i.e. the current value at which contact separation occurs. As will be seen, this value is considerably higher than the lifting current i of a corresponding contact assembly without the magnetic cir¬ cuit 25, 26.

Figure 8 shows the tripping curve of a fuse intended for series connection with the contactor, i.e. the tripping time t as a function of the R.M.S. value I of the current. The current scale in Figure 8 is then directly comparable with the current scale in Figure 7. With a contact assembly without the described magnetic circuit, contact separation occurs at the current I _? . At this current, the fuse has a tripping time t during which burns on contacts and possibly contact welding may occur before the current is broken. With a contact assembly which is provided with a magnetic circuit 25, 26 according to the present invention, the lifting current I is considerably higher, and at this current the tripping time tm of the fuse is so short (< 50 ms) that', if contact separation at all occurs before the current is broken, no fusion of the material in the contact surfaces - or only greatly reduced fusion - will occur.

Figures 9 and 10 show an alternative embodiment of the central part of the movable contact assembly. In this design the con¬ tact pressure spring 23 is arranged on the upper side of the movable spring 11, and the magnetic core 25 is retained against the stop 24 of the contact carrier with the aid of a separate spring 27, which supports against the armature 15 of the operating magnet. Otherwise, the design according to Figures 9 and 10 do not differ significantly from that shown in Figures 4 and 5 and operates, in principle, in the same way as this.

The invention is not limited to the embodiments described

above, but several modifications are feasible within the scope of the appended claims.