EP0170562A1 - Direct-current electromagnet, in particular for an electric commutator device - Google Patents

Abstract

A DC current electromagnet is provided developing a high pulling force with reduced current consumption. This electromagnet has the form of a cylindrical pot and comprises a mobile armature which slides along a yoke piece through a closure air gap -e, S-, whose reluctance is low with respect to the reluctance of the working air gap -E, s- when this latter is open and substantially constant during movement.

Description

• - un noyau central,
• - une bobine disposée dans ce pot concentriquement au noyau, et
• - une armature mobile soumise à l'action d'un ressort de rappel et dont des surfaces polaires périphériques, ayant une forme qui augmente leurs valeurs, coopèrent avec des surfaces de même forme portées par une jupe annulaire du pot, à travers un entrefer de travail qui se trouve placé magnétiquement en série avec un entrefer de fermeture du flux.

The invention relates to a direct current electromagnet, in particular for an electrical switching device, comprising a magnetizable pot-shaped yoke having:

• - a central core,
• - a coil arranged in this pot concentrically with the core, and
• - a movable frame subjected to the action of a return spring and whose peripheral polar surfaces, having a shape which increases their values, cooperate with surfaces of the same shape carried by an annular skirt of the pot, through an air gap of work which is placed magnetically in series with a gap closing the flow.

Such electromagnets which are, for example, known from French Patent No. 1,051,651, have advantages and disadvantages which the user must be satisfied with when considering their application in particular fields; among the advantages, it should be mentioned that the surfaces of the two air gaps participate at the same time in the generation of the initial calling force; however, due, on the one hand, to the ratio of the surfaces of these two air gaps and, on the other hand, due to the presence of two air gaps in series in the magnetic circuit, it can be seen that the initial flux is not very high and that, if the induction is relatively high in the nucleus, its value on the peripheral air gap remains low, so that for given ampere-turns, the initial calling force cannot reach interesting values, or that in order to reach these values, it becomes necessary to increase the volume of the coil .

The invention therefore intends to provide an electromagnet having the general constitution mentioned above, in which measures will be taken so that ampere-turns of low value (or, in other words, a power low coil excitation) are likely to impart to the armature an initial call force greater than that obtained with the prior techniques, when the stroke of the armature and the volume of the electromagnet are practically fixed in advance.

According to the invention, the aim is achieved thanks to the fact that this closing air gap (e, S) has a large surface area and has a low first reluctance opposite the second reluctance presented by the working air gap when the latter is open, and has cylindrical surfaces, parallel to the direction of movement of the armature so that this first reluctance is not substantially affected by this movement.

Already known, for example, from the published German patent application DE.AS-NR 1,097,563, a direct current electromagnet in which a central flux-closing air gap having surfaces parallel to the direction of movement of the the armature is placed magnetically in series with two lateral working gaps, the surfaces of which are inclined relative to said direction; as the magnetic circuit used here takes the classic form of two _E- shaped parts, we see that not only the benefits that the present invention provides cannot be obtained here, but that, moreover, an attempt to obtain these benefits by giving. parts of the magnetic circuit a form of revolution would be doomed to failure due to the respective proportions that these two air gaps initially present; such an attempt would, moreover, constructively oppose the installation of the guide and transmission means described therein.

Indeed, in one of the embodiments presented where the closing air gap is placed outside the coil, the reluctance of the closing air gap, which should favor the importance of the flux, cannot here bring force. of initial attraction, a significant improvement insofar as the polar surfaces of the working air gap cannot exceed a certain value without causing a parallel increase in the volume of the coil, and where the covering surfaces of the closing air gap are weak.

In a second proposed embodiment, where the working air gaps are placed outside the coil and where the closing air gap is placed inside the latter, the value of the initial reluctance is very high due to the fact that the movable surfaces facing each other in the closing air gap are extremely reduced; moreover, any increase in their overlap would cause a substantial reduction in the stroke, which would oppose the goals which the mechanism associated with this electromagnet proposes to achieve.

In the two embodiments, it is finally noted that the freedom which must be given to the two moving parts of the frame, as well as the space necessary for the passage of a drive shaft, so considerably reduce the amp-turns applied to the circuit that the filling factor of the window cannot exceed 30% here.

We currently encounter the problem of accommodating in a small volume, usually occupied by a small contactor alternating electromagnet, an electromagnet supplied with direct current whose consumption is very low (or in any case, close to that of 'a signaling relay), so that such electromagnets can be supplied in large numbers by the output stages of modern electronic information processing devices which are, for example, programmable controllers.

The difficulties of the solution to this problem can be evaluated when it is known that such small contactors must be able to close simultaneously, on the one hand three power switches where sometimes the contact pressure forces must allow the '' supply of motors whose power is around 1.5 KW at 220 V, and whose strokes must be sufficient to ensure, if necessary, good insulation for supply voltages of around 600 V and , on the other hand, up to three auxiliary signaling switches; we know, moreover, that it is difficult to manufacture, with equal performance, two electromagnets of the same volume, one of which is supplied with alternating current, and the other with direct current, taking into account the significant physical differences which distinguish their respective operations.

These data can be specified, for example, by the observation that, when the imposed volume is less than 20 cm ³ , that insulation as specified above requires opening strokes of 3 to 4 mm, that obtaining a pressure of the order of 250 g on closed contacts is required by the presence of the switches mentioned, and that a power of 0.5 W to power the coil is currently an objective targeted by the manufacturers, we cannot have only about 130 ampere-turns so that the electromagnet develops on call a force of the order of 100 cN (even when there is a drop in voltage or a significant rise in temperature) so that the obligation to keep all of the moving masses in their rest position is fulfilled.

Furthermore, modern trends towards reducing technical mounting costs and increasing reliability mean that we want to transmit directly to the contacts, that is to say without means of multiplication of movement, the displacements that '' performs the armature of the electromagnet.

In contactors of this type, where the number of operations can reach very high figures, it is necessary to reduce damage and wear that could be caused by repeated percussion of the armature on the fixed cylinder head.

In an advantageous embodiment of the invention, where the reduction in the intensity of these shocks has been taken into account, and which is accompanied by a further reduction in the volume of the coil going in the same direction as the objectives defined above, the reduction in the intensity of the terminal attraction force is determined by a choice of the diameter of the nucleus which locally causes there to appear saturation phenomena which take on a preponderant importance only when the reluctance of the air gap of work decreases.

According to a particular execution of these last measurements which aims to give the attraction curve a substantially linear increasing pace, these saturation phenomena substantially change the induction in the nucleus in a ratio of 1 to 2.

The invention will be better understood on reading the description below.

• Les figures 1, 2 et 3 représentent un premier mode de réalisation de l'invention, dans lequel l'entrefer de fermeture est disposé à l'intérieur de la bobine, tandis que l'entrefer de travail est situé à la périphérie du pot ;
• La figure 4 illustre un second mode de réalisation de l'invention, dans lequel les entrefers de travail et de fermeture sont placés à la périphérie du pot ;
• La figure 5 montre les dimensions que peut prendre un électro-aimant selon l'invention pour répondre à des données numériques définies par ailleurs ;
• La figure 7 illustre une répartition du flux développé dans le circuit pour une certaine position de l'armature ;
• La figure 6 représente un diagramme de l'évolution des forces d'attraction et des forces résistantes rencontrées par un électro-aimant selon l'invention, lorsqu'il est appliqué à un petit contacteur ;
• La figure 8 représente, à des fins de comparaison, un électro-aimant en forme de pot connu dans l'art antérieur ;
• La figure 9 montre un électro-aimant conforme à l'invention, dans lequel l'augmentation de la surface de l'entrefer de travail a été obtenue par des voies différentes des précédentes ;
• La figure 10 montre un exemple de mesures pouvant être prises pour diminuer la réluctance de l'entrefer de fermeture ;
• La figure 11 représente, dans une vue extérieure en élévation, un électro-aimant dans lequel la réluctance de l'entrefer de fermeture évolue pendant la course de l'armature ;
• Les figures 12 et 13 illustrent, dans une vue en élévation extérieure et, respectivement, dans une vue de dessus coupée par le plan TT', un électro-aimant ayant les mêmes propriétés que le précédent, mais où l'évolution de la réluctance peut être choisie parmi l'une de deux évolutions possibles.

In the attached drawing:

• Figures 1, 2 and 3 show a first embodiment of the invention, in which the closing gap is arranged inside the coil, while the working gap is located at the periphery of the pot;
• FIG. 4 illustrates a second embodiment of the invention, in which the working and closing air gaps are placed at the periphery of the pot;
• FIG. 5 shows the dimensions that an electromagnet according to the invention can take to respond to digital data defined elsewhere;
• FIG. 7 illustrates a distribution of the flow developed in the circuit for a certain position of the armature;
• FIG. 6 represents a diagram of the evolution of the forces of attraction and of the resistant forces encountered by an electromagnet according to the invention, when it is applied to a small contactor;
• FIG. 8 represents, for comparison purposes, a pot-shaped electromagnet known in the prior art;
• FIG. 9 shows an electromagnet according to the invention, in which the increase in the surface of the working air gap has been obtained by means different from the previous ones;
• Figure 10 shows an example of measures that can be taken to decrease the reluctance of the closing air gap;
• FIG. 11 represents, in an external elevation view, an electromagnet in which the reluctance of the closing air gap evolves during the stroke of the armature;
• Figures 12 and 13 illustrate, in an external elevation view and, respectively, in a top view cut by the plane TT ', an electromagnet having the same properties as the previous one, but where the evolution of the reluctance can be chosen from one of two possible developments.

An electromagnet l 'in accordance with the invention and visible in FIG. 1 has two magnetizable parts of revolution 2, 3, movable relative to each other along a common axis of revolution XX', and engaged in each other. These parts take on, once engaged, the appearance of a pot whose internal volume 18 is occupied by a coil 4 having current leads 5, 6, which pass through, for example, part 3 considered here as fixed, while the reverse would also be possible.

The part 3 has a substantially flat bottom 7 in the center of which projects a cylindrical core 8 provided on its outer surface with a thin layer 9 of a non-magnetic material having good friction properties. An annular skirt 10 extends externally the bottom parallel to the core and has at its free end a conical surface 11 of revolution making an angle a with the axis.

The second part 2, which is therefore assumed here to be mobile, has a shape comparable to that of the part 3, but has at the center of its bottom 16 a tubular extension 12, the bore 13 of which passes through the core 8 and slides over it with gentle friction, while its short skirt 14 has a conical surface 15 of angle a which is parallel to the previous one, opposite which it is located.

Due to the properties and the very function of the electromagnet, a return spring 17, which, for example has been placed inside the volume 18 comprised between parts 2 and 3, but which could be placed outside, communicates to the piece 3 a restoring force towards a rest position where it meets a stop 19 and where a distance "c" equal to the relative stroke of the pieces 2 and 3 separates the conical surfaces parallel to axis XX '.

The moving part 2, which will henceforth be called the armature, can be mechanically coupled to any part or device to communicate to it a displacement of amplitude equal to "c" which, to occur, requires that the amp-turns circulating in the coil are sufficient to overcome, initially an initial resistant force, thanks to an initial calling force developed by the attraction of the reinforcement, and are then used effectively to overcome other resistant forces which appear during displacement , for example to operate the compression of pressure springs, if the armature is associated with movable contacts of switches (not shown).

We see that the amp-turns developed by a coil are essentially used to magnetize, on the one hand, the working air gap E which separates the conical pole surfaces and, on the other hand, the inevitable closing air gap -e- that the magnetic flux has to cross.

The initial induction B _i in the air gap E, as well as the dimensions of the pole surfaces 15 and 11, must themselves be sufficient for the initial calling force f to be greater than the initial resistive forces R _i .

The dimensions of an electromagnet capable of developing such a calling force, when a stroke "c" must be carried out by exceeding initial resistance forces whose subsequent variation is known in advance, can be obtained without difficulty by known means and calculation, when the power and volume available to the electromagnet are not subject to any particular constraint.

It is not the same when, for example, it is proposed either to place, in a contactor box using an electromagnet with alternating current, an electromagnet powered by a direct current, because the volume of this the latter is generally about 50% higher, that is to limit the power of an electromagnet, for example to a power of half a Watt, while asking it to develop at the call of the initial forces of the order of 100 cN, then making a stroke of the order of 3 to 4 mm.

We also know that, unlike the behavior of electromagnets supplied with alternating current, the armature of an electromagnet supplied with direct current is only slightly attracted by the call, the latter then undergoes a very rapid increasing evolution, the excess of which relative to the resistant forces becomes not only useless, but also harmful, due to the final speed which the frame then reaches when no auxiliary means or phenomenon comes thwart its evolution.

Another electromagnet has it, represented in FIG. 2, where the same references relate to bodies having the same functions, derives directly from the previous one by reversing the position of the core 8 ′ and of the tubular extension 12 ′ which are now connected to the frame 2 'and to the cylinder head pot 3'. In the two embodiments of FIGS. 1 and 2, the axial length along which the air gap extends -e-, respectively -e'- will be as large as possible for reasons explained later; in all cases, this length will always be close to the height -H- of the coil 4.

The electromagnet 1b shown in FIG. 3 shows how the position of the working air gap E _b can be modified in one or other of the embodiments illustrated in FIGS. 1 or 2, by bringing this air gap closer to the bottom 7b of the fixed cylinder head 3b to reduce the leakage fluxes which could not contribute to the attraction of the frame; the skirt 14 'of the movable frame 2b is here longer than in the previous case and the skirt 10' is, on the other hand, shorter.

Another 1 "electromagnet according to the invention, and visible in FIG. 4, comprises a fixed yoke 20 which is formed by the assembly of two magnetizable revolution parts 21 and 22, a movable magnetizable armature 23, a coil 24 and a spring 17 '.

Once assembled, the cylinder head 20 retains, in an internal volume 80, the coil which was previously introduced around a solid core 25 of the concentric piece 21 and secured to a bottom 26 carrying at its periphery a conical surface 81 forming an angle a with the axis YY '.

The free end 27 of the core is here anchored in the flat bottom 28 of the part 22, which is extended by an annular skirt 29 whose surface is preferably covered with a thin layer of non-friction anti-friction material 30 and which extends parallel to the core 25.

The movable frame 23 takes the form of a ring, the internal cylindrical surface 31 of which slides with an appropriate clearance on the layer 30 and the end of which faces the surface 81 has a parallel conical surface 32; the length "m" of this frame is preferably greater than the height "h" of the skirt 29, to allow it to slide without the contact surface being reduced and therefore the reluctance increased; it will be seen later that certain adjustments can however be made to the value thereof to meet particular objectives. liers. It can be seen that, thanks to this arrangement, the entire internal volume 80 of the cylinder head 20 is occupied by the coil 24.

In the two embodiments of the invention, the ampere-turns -ni- developed by the coil when the latter is traversed by the current, will cause a flux développer which will circulate in the magnetizable parts, crossing, d 'on the one hand, a working air gap, which is placed between the conical surfaces 81, 32, respectively 11, 15 and whose dimension is - E - and, on the other hand, a closing air gap, which is materialized by the thickness - e - of the layer of anti-friction material.

The air gap E whose surface is - s - and the air gap - e - whose surface is - S -, define reluctances R ₁ and R ₂ which give the flux a value φ revealing inductions B ₁ and ^B ₂ .

In the electromagnets according to the invention, it is arranged so that the reluctance R ₂ is as low as possible in front of the reluctance R1 when the armature is in the open position and the product of the square of the initial induction B ₁ by the surface - s - (or initial attraction force - F _i ) is as high as possible when the reinforcements are at rest, so as to easily overcome the initial resistance forces R _i .

It will be noted that this initial induction B ₁ must be chosen so that the current attractive force is able to evolve while remaining, for example, greater than the progressive or stepped resistance forces which are encountered successively or simultaneously when the armature actuates moving contacts of a contactor. Indeed, if the initial induction was already high, the appearance of saturation in the magnetic circuit would give a weak growth to the forces of attraction when the armature moves.

When the initial induction B ₁ is suitably chosen, saturation appears (if it appears) on surfaces - s - only when the armature is very close to the cylinder head, and the effects of saturation develop before either in the core 8 of section - f -, respectively 25, or in the regions 34, 35, respectively 36, 37, so that the growth of the forces of attraction is just greater than the growth of the resistant forces.

The advantage of showing saturation at these levels by giving them an appropriate section results from the fact that any reduction in the section of the core has a favorable effect on the volume of the coil, and therefore on the power which will be necessary for display the ampere-turns. Such a reduction, which also has repercussions on the total volume of the electromagnet, is also favorable for the evacuation of Joule losses, because, at constant current density, the external surface of the electromagnet decreases proportionally less than the internal volume.

The dimensions and parameters of an electromagnet 1 "'according to the invention, which are given in FIG. 5, and the evolution of the attraction force, which is given in the graph of FIG. 6 show that, on the one hand, the volume of such an electromagnet (supplied with direct current) can compete with that of an electromagnet supplied with alternating current and that, on the other hand, the power necessary for its excitation, which is of the order of 0.5 W, enables a large number of small contactors which use these electromagnets to be supplied directly by the output stages of an electronic control unit such as a programmable sequencer.

It will be noted, on examining FIG. 7, which shows the distribution of the open circuit flux, that the leakage fluxes which may appear between the regions L, M, N, Q are very small; this good result is due to the fact that the dry The magnetic masses other than the nucleus are chosen so as not to saturate.

It will also be noted that great freedom is given to the designer and / or to the user to choose the value of the surface - s - best suited to his needs, thanks to a more or less pronounced alpha inclination of the conical surfaces.

In practice, the air gap E = .... is determined by the value of the stroke - C - necessary to operate a good isolation and a good compression of the movable contacts associated with the frame and the inclination has as a result, while the value of the air gap - e - cannot drop below a reasonable threshold, which is determined by the means and materials used in economic mass production; one can for example choose 0.1 mm ^< e ^< 0.5 mm.

A quick and simplified calculation, the details of which are given below, shows that the performances of a pot-shaped electromagnet of conventional shape, which is illustrated in FIG. 8, are of equal dimensions significantly smaller than those of an electro- magnet according to the invention; the flexibility of the control of the induction B ₃ of the core which is acquired in the invention is done without detriment, neither for the force of attraction, nor for the volume of the coil, advantage which cannot be found in the electromagnet according to figure 8, due to the fact that the voluminous internal region of the nucleus which is used to develop a significant part of the force of attraction is also that where the phenomena of saturation will first appear.

Let - ni - be the available ampere-turns, and - R - the reluctance of the magnetic circuit considered as only represented by the air gaps, therefore taking no account of the saturation; constant coefficients are represented by the letter G.

d'où :

If we set E = K ₁ e and S = K ₂ s, R becomes

from where :

The initial attraction force Fi is given by:

In the invention, where the armature stroke is 3 mm and the angles a are 30 °, a closing air gap - e - was chosen equal to 0.3 mm and a working air gap - E - equal at 1.5 mm results from the geometric choices; consequently, K ₁ = 5.

For an outside diameter of 30 mm and a height - h - equal to 19 mm, we have = 4.3 cm ² , S = 15 cm ² ; therefore, ^K ₂ = 3.5 hence

If we give to a known electromagnet according to FIG. 6 the same external dimensions, the same stroke, the same working air gap dimensions (E, s), and that a air gap of fermetur.e E placed on a nucleus of the same section - f - where the angle a = 30 °, we have

The application of the same formulas for identical -ni- and taking into account the fact that an induction B ' ₁ applies to - s - and an induction B' ₂ applies to S 'through two air gaps - E - of values equal to 1.5 mm, gives a total initial force:

Between Fi and F'i, there is therefore a multiplication ratio equal to 170 44 = 3.9.

The comparison between an electromagnet l 'according to figure l or 2, for example S = 5.6 cm ² when H = 1.9 cm and therefore K ₂ = 1.3 and a conventional electromagnet leads to a proportion still very favorable of 143 44 = 3.25.

The reserve of amplification of the initial attraction force provided by the electromagnet according to the invention makes it possible to envisage multiple applications where, not only this force, but also its evolution can be easily adapted to the needs, by modifying not only the ratios _K1 and _K2 but also, possibly, the ratio K ₃ which links the polar working surface - s - and the section - f - of the nucleus according to the equality s = K ₃ f.

This ratio K ₃ , the increase of which for a surface - s - and a given coefficient K ₂ , reveals the saturation of the nucleus sooner, for example makes it possible to define the terminal induction B _l t, therefore the force of terminal attraction ; in the embodiment of FIG. 5, K ₃ = 6.8.

An increase in K ₂ which reduces the reluctance R ₂ increases the initial attraction force, as well as the growth rate; its adjustment also makes it possible to fix the force of attraction mid-course if necessary, just before the effects of saturation can be felt significantly.

In the exemplary embodiment according to FIG. 5, where we have given ourselves the objective of making the electromagnet of a small contactor overcome the resistant forces -Q-, the calculation and the reading allow to see that (K ₂ being equal to 3.5) the force of attraction F is close to half of its maximum value after an initial stroke ranging from 30 to 40% of the total stroke and that the initial force is close to 100 cN.

The implementation of very inclined polar work surfaces, which could possibly create some difficulties. Cooperation tees at closing, is only necessary if it is proposed to reach significant initial forces, without the external dimensions and the mass of the armature becoming excessive.

The fact of replacing the conical surfaces of the electromagnet according to FIG. 5 by straight surfaces separated from the value of the stroke, would modify both the ratios: S + K ₂ s by giving K ₂ = 7 and E = K ₁ e by giving K ₁ = 10; these values no longer give the initial attraction force an interesting amplification compared to the prior art; we note the important role played by the inclination of the surfaces of the working air gap.

In the simplified calculations used above to make comparisons, the friction forces have not been taken into account; it is clear that if these forces are expressed by the effects of a lateral attraction force which friction against the surface of the skirt 29 of the pot can impart to the annular armature 23, an optimization calculation can be carried out for find the most appropriate values of K _l , K ₂ and ^K ₃ .

The expression of the useful net force Fu then includes a coefficient of the form (1- r K) which shows, if there were 2, that this force is greater when the coefficient of friction r is low and that K ₂ is high.

Simulations have made it possible to choose an evolution of the induction B ₃ in the nucleus which is advantageously between 0.7 Tesla and 1.6 Tesla (i.e. a ratio close to 2) when the armature closes, this evolution being indeed suitable for application to contactors.

The invention may know, in a variant 56 of the electromagnet, embodiments which always place it in the context of obtaining a high initial attraction force; instead of using surfaces inclined governed by cones, the pole surfaces of the working air gap could each have a series of teeth 50, respectively 51, with inclined sides, coming in cooperation by penetration of the projections of one in the intervals of the other; angular orientation means 52, 53, for example with axial groove and transverse pin, may then prove to be necessary to prevent rotation of the armature 54 relative to the yoke 55 when the coil is excited, see figure 9.

An increase in the area S that the closing air gap must have (and therefore of K ₂ ) can be obtained for example using an extension 59 of the frame 60 cooperating with an extension 57 of the skirt 61 belonging at cylinder head 58, see Figure 10.

While remaining within the framework of a high value of the surface of the closing air gap S, it may be desired, for applications where a small change in the attraction curve must meet specific requirements, that its value decreases during the frame stroke; such a result can be obtained by giving the skirt 63 of the electromagnet visible in FIG. 11 and its annular reinforcement 64, cutouts 66, respectively 67, preferably symmetrical, which modify the values of the surfaces S placed opposite when this frame moves. A progressive angular distribution of the cutouts 75, 76 would make it possible, if necessary, to choose one of several possible attraction curves, by giving the armature 70 of the electromagnet 69 before mounting, a particular angular orientation by relative to the cylinder head pot 71, orientation which would be ensured by guide means such as pin 72 and grooves 73, 74 visible in FIGS. 12 and 13.

We can finally observe that the choice of the orientation of the taper of the surfaces of the working air gap, visible in Figures 1, 2 and 3, was made so that the effects of a slight radial clearance present on the closing air gap does not produce an imbalance of the values which two portions of diametrically opposed working air gaps can take; such an imbalance, which would otherwise occur, could cause jamming, see Figure 3.

The orientation chosen can be defined as that which passes, in the vicinity of a central point 0, placed on the axis of symmetry XX ', respectively YY', a normal half-line raised on a concave conical surface such that 15 , respectively 81, see Figure 4.

The notion of cylindrical pot, which has been used in the previous examples, should not be limited to that of a cylinder of revolution, which however constitutes the most advantageous embodiment thereof.

One can also consider the use of straight cylinders whose external surface would be defined by the path of a generating line based on a curve or guiding figure different from a circle.

By means of the use of particular manufacturing means and methods, it is possible to produce electromagnet pots of quasi-prismatic shape, or, in other words, pots of square or rectangular section with rounded edges, in order to increase the areas which limit the call air gap; the production of the closing air gap can then make use of an insulating layer in the form of a film, which is bonded to the skirt of this pot and to a sliding frame having a corresponding section.

Claims (5)

1. Direct-current electromagnet, in particular for an electrical switching device, comprising a magnetizable yoke in the form of a pot having: - a central core, - a coil arranged in this pot concentrically with the core, and - a movable frame subjected to the action of a return spring and whose peripheral polar surfaces, having a shape which increases their values, cooperate with surfaces of the same shape carried by an annular skirt of the pot, through an air gap of work which is placed magnetically in series with a flow closing air gap,
characterized in that this closing air gap (e, S) has a large surface (S) and has a low reluctance "R ₂ " opposite that "R _l " presented by the working air gap (E, s) when it is open, and has cylindrical surfaces (13, 9), respectively (30, 31), parallel to the direction of movement of the armature (2), respectively (23), so that this first reluctance " R ₂ "is not significantly affected by this movement. 2. Electromagnet according to claim 1, characterized in that the terminal attraction force F _t of the armature (2), respectively (23), is essentially determined by the dimension of the central core (8, 12), respectively (25), whose section - f - is chosen so that the phenomena of magnetic saturation take on a preponderant importance there when the working air gap (E, s) decreases in reluctance. 3. Electromagnet according to claim 2, characterized in that the phenomena of saturation in the core (25) cause the induction to change substantially in the ratio of 1 to 2 when the armature (23) moves between its rest position and its working position. 4. Electromagnet according to claim 1, characterized in that the closing air gap (e, S), which is disposed between a tubular core (12, 12 ') and a solid core (8, 8') placed all two in the center of the coil and linked respectively to the fixed (3, 3 ') and mobile (2, 2') parts of the electromagnet (l ', a), extends substantially over the height - H- of the coil (4). 5. Electromagnet according to claim 1, characterized in that the closing air gap (e, S) is arranged between a cylindrical outer skirt (29) of a part (22) of the magnetic pot (20) and a frame annular mobile (23) which carries at one end pole surfaces (32) defining the working air gap (E, s).

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