Active element for an electromagnetic machines, a method of fabricating such an active element, and an electromagnetic machine including such an active element

The invention provides an active element for an electromagnetic machine, the element comprising an alternating succession in a main direction of portions presenting a first magnetic property and of portions presenting a second magnetic property, wherein the element includes a non-magnetic covering that is thin relative to a thickness of the portions and that extends to cover a substantial fraction of an outside surface of the active element, the covering being secured to at least some of the portions and presenting sufficient strength to form a member for mechanically transmitting the magnetic forces to which the portions are subjected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Application No. PCT/FR2007/001088 filed on Jun. 28, 2007 and French Patent Application No. 06 06335 filed on Jul. 12, 2006.

FIELD OF THE INVENTION

The invention relates to an active element for an electromagnetic machine, to a method of fabricating such an active element, and to a machine including such an active element.

BACKGROUND OF THE INVENTION

Linear electromagnetic machines are known that have two parts that move relative to each other in a sliding direction. At least one of the parts extending in the sliding direction includes active elements, each made up of a succession of portions having different magnetic properties.

In variable-reluctance machines, the portions of each active element alternate between ferromagnetic portions and non-magnetic portions. In permanent-magnet machines, the active elements of one of the parts comprise portions that alternate between ferromagnetic portion and non-magnetic portions, while the active elements of the other part comprise magnetized portions that alternate between portions magnetized in a first direction and portions magnetized in a second direction opposite to the first.

The two parts are placed to interact electromagnetically, one of the parts being associated with means for generating a magnetic field. Magnetic-field generation causes magnetic forces to appear that tend to move the parts relative to each other in the sliding direction. Alternatively, the machine may be operated as a generator by imparting a relative movement between the two parts.

In a first type of electromagnetic machine, the portions making up the active elements are in the form of plates or blades. The active elements of the two parts are parallel and form respective interleaved combs such that an active element of one of the parts extends between two active elements of the other part (naturally with the exception of the outermost active elements).

In an embodiment shown in FIG. 5 of document FR 2 588 133, the portions forming the active elements of the movable part are subdivided into two sub-portions extending on either side of a central support that holds the sub-portions at one end thereof and that takes up the magnetic forces to which the portions are subjected. The portions forming the active elements of the stationary part are likewise subdivided into two sub-portions, each being held via one of its ends by an external support.

Those supports are complex to fabricate and they receive each of the sub-portions via one end only, which means that the sub-portions are cantilevered out and that they are therefore subjected to stresses that tend to separate them from the supports that receives them. In addition, the end of each sub-portion that is engaged in a support is not in magnetic interaction with the sub-portions of the facing active elements, and those ends therefore do not participate in the operation of the machine. In addition, the presence of central supports and of external supports increases the overall size of the machine. The supports do not participate in the magnetic interaction between the active elements, so they take up precious space and tend to limit the volume power density of the machine.

In a second type of electromagnetic machine, as illustrated in the article “Actionneur linéaire synchrone machine à aimants permanents multi-tiges” [Synchronous linear actuators with multiple-rod permanent magnets], presented to the colloquium “Electrotechnique du futur, SUPELEC, Dec. 9-10, 2003”, the active elements of the movable part are in the form of rods and comprise cylindrical portions threaded onto a central support. The central support holds the portions and mechanically takes up the magnetic forces to which the portions are subjected, and it stiffens the rod.

As with plates, the central support does not contribute to the magnetic interactions and it occupies precious space, tending to limit the volume power density of the machine.

As suggested in document FR 2 588 133, it is possible to omit the support and to bond the portions of a given active element to one another by adhesive or by brazing. Nevertheless, that method assumes that the portions are mutually adjacent and that they are suitable for being bonded together by brazing or adhesive. The magnetic forces are then transmitted mechanically in traction through the bonds made in this way, which is not ideal from a mechanical point of view.

OBJECT OF THE INVENTION

An object of the invention is to provide a novel active element that provides greater efficiency.

BRIEF SUMMARY OF THE INVENTION

In order to achieve this object, the invention provides an active element for an electromagnetic machine, the element comprising an alternating succession in a main direction of portions presenting a first magnetic property and of portions presenting a second magnetic property. According to the invention, the active element includes a non-magnetic covering that is thin relative to a thickness of the portions and that extends to cover a substantial fraction of an outside surface of the active element, the covering being secured to at least some of the portions and presenting sufficient strength to form a member for mechanically transmitting the magnetic forces to which the portions are subjected.

Thus, the covering is placed on the outside of the active element, i.e. in a zone of greatest second moment of area, such that the covering contributes greatly to stiffening the active element in spite of being thin. The electromagnetic interactions then take place through the covering.

The magnetic forces acting on the portions are transmitted mechanically in shear by the covering, and that is highly favorable from a mechanical point of view. The covering thus forms an effective force transmitter that is very compact.

Thus, in the context of an active element in the form of a plate or a blade, it is advantageous to provide a covering that has two thin walls substantially covering the two large faces of the active element through which the electromagnetic interactions take place. In the context of an active element in the form of a rod, it is advantageous to provide a covering that comprises a tubular thin wall that covers the cylindrical portions.

DETAILED DESCRIPTION OF THE INVENTION

The invention is initially described with reference to a linear electromagnetic machine with multiple air gaps, such as the machine shown inFIG. 1. In known manner, such a machine comprises a cage1of ferromagnetic material, here receiving three coils2electrically powered so as to be offset successively by 120°. The magnetic field generated by the coils2is transmitted in an active zone of the cage1in which there extend in alternation:active elements in the form of stationary plates10that are mutually parallel and prevented from moving relative to the cage (only one stationary plate is referenced). Each of the stationary plates10comprises a succession of permanent magnets, as described in greater detail with reference toFIG. 2;active elements in the form of mutually parallel movable plates20, each extending between two stationary plates10so as to present a plurality of air gaps relative thereto (only one movable plate20is referenced). Each of the movable plates20comprises a succession of ferromagnetic portions and of non-magnetic portions, as described in greater detail below with reference toFIGS. 3 to 5. The movable plates20are secured to one another by means of pins30extending to pass through all of the movable plates20. The movable plates slide in a sliding direction X facing the stationary plates10.

As shown inFIG. 2, each stationary plate10comprises permanent magnets11oriented in a first direction disposed in alternation with permanent magnets12oriented in a second direction opposite to the first, as represented by the arrows drawn on the ends of the permanent magnets. The permanent magnets11and12are mutually touching bars and they form successive portions having alternating magnetic properties. According to an essential aspect of the invention, each stationary plate10has thin non-magnetic walls15that extend on either side of the stationary plate10over the large faces thereof and that are secured to the permanent magnets11and12, e.g. by adhesive. The large surfaces are made up of the adjacent side walls of the bars.

By way of example, the permanent magnets11and12present a thickness of about 1 millimeter (mm), while the thin walls15present a thickness of 0.1 mm. The thin wall15is made of bronze.

The thin walls15form support means for the permanent magnets11and12that are very compact. The full height of the permanent magnets11and12can thus interact electromagnetically with the movable plates20facing them through the thin walls15so that all of the magnets are used for producing mechanical forces on the movable plates20.

In addition, the thin walls15form the outer layers of a sandwich structure having its core constituted by the permanent magnets11and12, thereby giving the stationary plate10a high degree of stiffness in bending.

As shown inFIG. 3, each movable plate20comprises ferromagnetic portions21disposed in alternation with non-magnetic portions22. The portions21and22form touching bars. Each movable plate20also has non-magnetic ends23extending the portions21and22and serving to receive the pins30that secure the movable plates21to one another so as to form the movable element of the machine. According to an essential aspect of the invention, each movable plate20comprises thin walls25that extend on either side of the movable plate20over the large faces thereof, being secured to the portions21and22and also to the ends23, e.g. by adhesive.

In addition to their above-mentioned stiffening function, the thin walls also serve to transmit mechanically to ends23the magnetic forces to which the portions21and22are subjected. The mechanical transmission of these forces between the thin walls25and the portions21and22takes place in shear, which is an effective mode of transmission for adhesively-bonded assemblies. Here likewise, the thin walls25are made of bronze.

As shown inFIG. 4, and in a preferred embodiment, each movable plate20is obtained by using a sheet of non-magnetic material26having parallel windows27cut out therein for receiving the ferromagnetic portions21. To make a movable plate20, a thin wall25is adhesively bonded to one of the large faces of the cut-out sheet26, here on the bottom large face. Then the ferromagnetic portions21are placed in the windows27. One of the ferromagnetic portions21is shown while it is being put into place in one of the windows27. Thereafter, the other thin wall25is adhesively bonded to the top large face of the cut-out sheet26. The side margins of the cut-out sheet26are then cut away along the dashed lines so that the cut-out sheet26that remains between the ferromagnetic portions21constitutes the non-magnetic portions22. It then remains to drill the ends23in order to provide the orifices for receiving the pins30.

In the variant shown inFIG. 5, the cut-out sheet26is made of ferromagnetic material, while the windows are filled with liquid resin28in order to form the non-magnetic portions.

After the resin has set, the top thin wall25has been adhesively bonded, and the side margins have been cut away from the dashed lines, the cut-out sheet extending between the windows forms the ferromagnetic portions21, while the resin28forms the non-magnetic portions22.

The fabrication method shown inFIGS. 4 and 5can naturally be applied to a stationary plate, or indeed a plate without an end. Similarly, it is possible to leave the windows empty, with the air therein forming the non-magnetic portions.

In known machines, it is desired to keep empty spaces between the facing plates so as to ensure they do not touch, since the facing portions are not covered and therefore risk sliding one against another and catching one another because of the multiple edges at the boundaries between the portions. Keeping such spaces empty requires the movable plates to be externally guided relative to the stationary plates and gives rise to magnetic instabilities that tend to move the plates towards one another, and thus to stress them in bending, thereby leading to variable air gaps. These empty spaces enlarge the air gaps between the plates considerably, and thus reduce the efficiency of the machine.

In contrast, and in accordance with a particularly advantageous aspect of the invention, the machine ofFIG. 1is designed so that in operation the stationary plates10and the movable plates20slide with very little clearance with contact between the plates being allowed. The accurate guidance provided in this way enables the air gaps between the magnetic portions of two facing plates to be kept substantially constant and equal at most to the thickness of the thin walls rubbing against one another plus the clearance for sliding. The plates come into contact via their thin walls15and25which are continuous and without edges, thereby facilitating sliding.

In addition, the accurate guidance provided in this way increases the stiffness, and thus the buckling strength, of the movable plates20, so they can be long. The movable plates20can thus be floatingly mounted on the pins30.

The material used for the thin walls15and25(bronze in this example) allows the plates to slide relative to one another with a coefficient of friction that is low. In a variant, the thin walls15and25could be made of some other non-magnetic material, and if the material does not possesses a favorable coefficient of friction, then the thin walls15and25can advantageously be coated in a surface layer having a low coefficient of friction, for example of polytetrafluoroethylene (PTFE), and compatible with the operating conditions of the machine, and in particular its internal temperature.

The invention is illustrated below with reference to a linear electromagnetic machine having rods as shown inFIG. 6.

This type of machine likewise comprises a cage51receiving three coils52that are phase-offset at 120° intervals. The magnetic field generated by the coils52is transmitted in an active zone of the cage51within which there extend:a stationary core60made up of permanent magnets in the form of disks with alternating magnetizations; andmutually parallel movable rods70that extend through orifices in the stationary core60so as to present annular air gaps relative thereto (only one movable rod70is referenced). Each of the movable rods70comprises a succession of ferromagnetic portions and of non-magnetic portions, as described in greater detail below with reference toFIG. 7. The movable rods70are secured to one another by means of an end plate80to which the ends of the movable rods70are fastened. The rods slide in a sliding direction X.

The stationary core60performs the same function in this embodiment as the stationary plates10, and the movable rods70perform the same function as the movable plates20.

As can be seen inFIG. 7, each movable rod70has ferromagnetic portions71disposed in alternation with non-magnetic portions72. Each movable rod70also has threaded non-magnetic end portions73that extend in line with the portions71and72and that are designed to be received in one of the end plates80that secure the movable plates70to one another so as to form the moving element of the machine. According to an essential aspect of the invention, each movable rod70has a thin tubular wall75(shown partially cut away to clarify the figure) that extends around the movable rod70and that is secured to the portions71and72and also to the ends73, e.g. by adhesive. The electromagnetic interaction between the portions of the movable rods and the stationary core takes place through the tubular thin wall75.

As with the plates, the tubular thin wall75holding the portions71and72greatly stiffens the movable rod70and serves to transmit the magnetic forces to which the portions71and72are subjected mechanically to the ends73(transmission taking place in shear as above).

Furthermore, the absence of a central support makes it possible for the portions71and72to be made solid, having no central orifice, thereby improving the efficiency of the machine.

According to a particularly advantageous aspect of the invention, the movable rods70are slidably received in the orifices of the stationary core60with small clearance, thus allowing the movable rods70to come into contact with the stationary core60. As a result, the movable rods70, which may be long, are well guided, thereby increasing their stiffness and their ability to withstand buckling. Furthermore, the air gap is well controlled and substantially equal at most to the thickness of the tubular thin wall75plus the clearance for sliding. The tubular thin wall75is made of bronze, a material that presents a low coefficient of friction. The tubular thin wall75rubs directly against the orifice in the stationary core. Nevertheless, the thin wall75is continuous and without edges, thereby encouraging sliding of the associated movable rod.

The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims.

In particular, although the description above relates to machines in which some of the active elements have magnetic portions, the invention can naturally be applied to other types of machine, for example to variable reluctance machines without permanent magnets, the portions making up the active elements then alternating between being ferromagnetic and non-magnetic.

Although the portions of an active element are shown herein as being adjacent, empty spaces could be left between these portions. Furthermore, one portion could be constituted entirely by an empty space.

Although the active elements shown herein have an outer covering in the form of one or more continuous thin walls covering all of an outside face of the active element, the covering could have other forms. In particular, the covering need not comprise continuous walls but could comprise strips extending in the sliding direction of the active element along the surface thereof. It is then preferable for the strips of one active element to be arranged to face strips of the facing active elements. Under all circumstances, it is important for the covering to be sufficiently strong to be capable of mechanically transmitting the magnetic forces to which the portions of the active element are subjected.

Although it is stated that the covering is bonded to the portions adhesively, other connection methods could be used. For example, the covering could be obtained by coating the portions with a hot material that forms the covering on cooling. The covering may also be sprayed onto the portions in a gas.

Although it is stated that all of the portions are secured to the covering, it is possible for some of them not to be secured thereto, for example for every other portion not to be secured. The magnetic forces are then transmitted to the thin wall(s) solely by those portions that are secured thereto, still in shear.

Finally, although the invention is described with reference to linear electromagnetic machines, the invention can be applied to rotary machines, e.g. having active elements in the form of disks. Under such circumstances, the portions extending along angular sectors alternate in a main direction around a circle. The covering then comprises two thin walls extending over the two large spaces of the disk formed by the adjacent side faces of the portions.