Abstract:
The stirrer comprises a driving magnetic portion ( 13 ) and a driven magnetic portion ( 14 ). The driven portion is for placing on the bottom of a receptacle ( 5   1 ) containing a liquid to be stirred. The driving portion ( 13 ) is controlled so as to drive the driven portion ( 14 ) in rotation about a predetermined axis of rotation ( 8   1 ) by means of magnetic coupling with the driving portion. The stirrer is characterized in that the driving and driven portions are configured in such a manner as to encourage the field lines ( 15 ) that result from the magnetic coupling to extend substantially perpendicularly to the axis of rotation in the vicinity of the driven portion. Axial attraction between the driving and driven portions is thus considerably reduced for given torque transmission to the driven portion, such that the driven portion ( 14 ) and the bottom of the receptacle ( 5   1 ) are subject to less wear during rotation of the driven portion.

Description:
This application is a 35 U.S.C. sec. 371 of PCT Application Ser. No. PCT/FR99/02385, filed on Oct. 6, 1999. 
     The present invention relates in general to liquid stirrers, and more particularly to stirrers with magnetic coupling. 
     DISCUSSION OF THE RELATED ART 
     Stirrers with magnetic coupling generally transmit rotary motion without contact from a driving magnetic portion to a driven magnetic portion. The driving and driven portions are disposed respectively outside and at the bottom of a receptacle of liquid to be stirred. This transmission of motion can be described as “coupling of the axial type without guidance for the driven portion”. 
     Compared with less-recent stirrers having mechanical coupling, transmitting rotary motion without contact makes it possible to avoid having passages through the walls via mechanical rotary gaskets which present a risk of leakage. 
     FIGS. 1A and 1B show two known stirrers with magnetic coupling. 
     The stirrer of FIG. 1A comprises a driving magnetic portion  1  which can be rotated about an axis by a motor  2 , in particular via transmission means  3 , and a driven magnetic portion  4 . The driven portion  4  is placed on the bottom of a receptacle  5  which is supported by means of a support  6  over the driving portion  1 . The receptacle  5  and the support means  6  are made of non-magnetic material. The driving and driven portions  1  and  4 , the transmission means  3 , and the motor  2  are centered on a vertical axis of symmetry  8 . The driven portion  4  is typically a permanent magnet in the form of a bar having a pair of north and south magnetic poles. The driving portion  1  is constituted by a U-shaped permanent magnet whose magnetic poles face the magnetic poles of the bar  4  when the stirrer is at rest. The magnets  1  and  4  are generally made of materials such as ferrites or alnicos (aluminum, nickel, cobalt). 
     When the motor  2  is in operation, the driven portion  4  is rotated about the axis of symmetry  8  by magnetic coupling with the driving portion  1  via an air gap  7 . More precisely, when the driving portion  1  rotates about the axis  8  under drive from the motor  2 , torque is transmitted to the driven portion  4 , thereby causing it to turn about the same axis. 
     The receptacle  5  contains chemical reagents in liquid form. Rotation of the bar magnet  4  about the axis enables the liquids contained in the receptacle  5  to be stirred and, for example, favors the production of precipitates which are evacuated via an evacuation outlet  50  on a side face of the receptacle  5 . With such a precipitation reaction, the stirrer shown in FIG. 1A can be referred to as a“precipitator”. 
     FIG. 1B shows another known stirrer. In FIG. 1B, those elements which are identical to elements of FIG. 1A are designated using the same references. The stirrer of FIG. 1B differs from the stirrer shown in FIG. 1A in that the driving portion  1 , the motor  2 , and the transmission means  3  are replaced by a static drive  9  powered by an alternating (square wave) voltage source  10 . The static drive  9  has vertically disposed electromagnets (one of which is shown diagrammatically in FIG. 2B) which are powered by a voltage delivered by the source  10  and are switched in alternation. The static drive  9  produces the same effect as the elements  1 ,  2 , and  3  in FIG. 1A, i.e. it produces a rotating magnetic field which causes the bar magnet  4  to rotate by magnetic coupling about a vertical axis of symmetry  8 ′. 
     The stirrer with static drive shown in FIG. 1B presents several advantages over the stirrer with rotary motor of FIG.  1 A. In particular, it does not require mechanical moving parts to be used and it is more compact. In addition, with the stirrer with static drive it is possible to vary the torque transmitted to the driven portion merely by modifying the amplitude of the current delivered to the coils of the electromagnets. In the case of the stirrer with rotary motor, the transmitted torque can be adjusted only by physically varying the size of the air gap by means of a mechanical device. 
     A major drawback of prior art stirrers as shown in FIGS. 1A and 1B lies in the fact that the amount of torque that can be transmitted to the driven portion is limited. Increasing this torque increases the attraction force pulling the bar  4  against the bottom of the receptacle and increases wear by friction both of the bar and of the bottom of the receptacle. 
     FIGS. 2A and 2B are diagrammatic front views of the relative positions of the driving and driven portions when the driving portion is constituted by a permanent magnet  1  (FIG. 2A) and when it is constituted by electromagnets  9  (FIG.  2 B). FIG. 2C is a diagrammatic plan view showing the arrangement of FIG.  2 A. As shown in FIG. 2C, when the stirrer is in operation, the driven portion  4  continuously lags behind the rotary field produced by the driving portion by an angle α. The field lines, represented by arrows  11  (FIGS. 2A and 2C) and  12  (FIG. 2B) between the poles of the driving and driven portions have a horizontal component (FIG. 2C) which contributes to the torque transmitted to the driven portion, and a vertical axial component (FIGS. 2A and 2B) parallel to the axis of rotation  8 ,  8 ′. The axial force due to the axial component constitutes a very large fraction of the interaction energy between the driving and driven portions. Any increase of the torque transmitted to the driven magnet  4  automatically increases the axial attraction between the driving and driven portions and thus increases wear of the driven magnet  4  and of the bottom of the receptacle  5  because of the presence of a significant axial component in the field lines. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a liquid stirrer which, compared with prior stirrers, makes it possible to reduce the wear on the driven portion and the bottom of the receptacle for given torque transmitted to the driven portion. 
     To this end, the invention provides a liquid stirrer with magnetic coupling comprising a driving portion and a driven portion, the driven portion being for placing on the bottom of a receptacle containing a liquid to be stirred, and control means for controlling the driving portion so as to drive the driven portion in rotation about a predetermined axis of rotation by means of magnetic coupling with the driving portion, the stirrer being characterized in that the driving and driven portions are configured so as to encourage the field lines that result from the magnetic coupling to extend substantially perpendicularly to said axis of rotation in the vicinity of the driven portion. 
     In practice, the driving and driven portions are preferably configured so as to encourage the field lines resulting from the magnetic coupling to extend substantially parallel to a longitudinal axis of the driven portion in the vicinity of the driven portion. 
     The predetermined axis of rotation is typically, but not necessarily, a vertical (virtual) axis of symmetry of the driven portion and/or of the driving portion. When the stirrer is in operation, the driven portion is merely placed on the bottom of the receptacle and is therefore subject only to its own weight, to friction forces with the bottom of the receptacle, and to the electromagnetic forces generated by the driving portion through the receptacle. The receptacle, or at least a portion thereof close to the driving portion, is made of a non-magnetic material, so as to allow the field lines to pass through. 
     Thus, contrary to prior stirrers, in the stirrer of the invention, a large part of the field lines in the vicinity of the driven portion have an axial component (parallel to the axis of rotation) which is small compared to their horizontal component. The axial attraction force which is an undesirable force because of the wear phenomena to which it gives rise both on the driven portion and on the bottom of the receptacle, is therefore smaller for identical torque. Consequently, a larger torque can be transmitted to the driven portion without increasing its wear, and without increasing wear on the bottom of the receptacle. Experimentally, the present inventors have observed that under certain conditions it is possible to increase torque by about 30%. 
     The increase in torque made possible by the present invention makes it possible to obtain greater stirring capacity, and, for example, to deal with the bottom of the receptacle becoming caked and with possible variations in the viscosity of the liquids contained in the receptacle. It also makes it possible to increase the air gap between the driving and driven portions, e.g. to allow the use of receptacles that are of greater thickness. 
     In the invention, the driven portion comprises a permanent magnet which is preferably made of neodium-iron-boron or of samarium-cobalt. These materials withstand demagnetization very well, unlike the materials conventionally used in prior stirrers which have a tendency to become demagnetized quite easily under the effect of an opposing magnetic field, which means that the magnets need to be replaced regularly, thereby increasing maintenance costs of such stirrers. 
     In a first embodiment of the present invention, the driving portion comprises at least one permanent magnet having at least one pair of magnetic poles whose active faces are substantially parallel to the axis of rotation. The magnetic poles of a given pair are typically of opposite polarities. 
     Advantageously, at least the poles of the permanent magnet of the driving portion are made of an anisotropic material. The permanent magnet is placed in such a manner that the magnetization direction of the anisotropic material is substantially perpendicular to the axis of rotation of the driven portion. The anisotropic material comprises, for example, strontium ferrite. 
     The magnetic poles of the permanent magnet of the driving portion are optionally united by a central part made of a ferromagnetic material such as soft iron. This central part makes it possible to avoid magnetic leakage in a direction parallel to the axis of rotation between the poles of the permanent magnet. 
     The control means comprises drive means for rotating the driving portion, which drive means includes a motor and transmission means for coupling the motor to the driving portion. 
     In a second embodiment of the invention, the driving portion comprises at least one electromagnet having at least two pairs of magnetic poles whose active faces are substantially parallel to the axis of rotation. The control means then comprises power supply means for feeding said at least one electromagnet with AC. 
     Said at least one electromagnet is typically constituted by an integer number p of electromagnets where p is greater than or equal to 2, and the power supply means feeds said p electromagnets with p-phase AC. The p electromagnets are arranged in a cross, each electromagnet constituting one of the branches of the cross. 
     In a third embodiment of the present invention, the driving portion comprises at least one permanent magnet having at least one pair of magnetic poles, and the distance between the magnetic poles of a given pair is substantially equal to or greater than the size of the driven portion in any direction perpendicular to the axis of rotation. The term “substantially equal” is used to mean a distance which is equal to, slightly greater than, or slightly less than the size of the driven portion in any direction perpendicular to the axis of rotation. The distance between the magnetic poles of a given pair is measured between the facing internal faces of the pair of poles. 
     The active faces of said at least one pair of magnetic poles are preferably substantially perpendicular to the axis of rotation. 
     The driving portion is rotated by drive means constituted by a motor and transmission means for coupling the motor to the driving portion. 
     In a fourth embodiment of the invention, the driving portion comprises at least one electromagnet having at least two pairs of magnetic poles, and the distance between the magnetic poles of a given pair is substantially equal to or greater than the size of the driven portion in any direction perpendicular to the axis of rotation. The control means comprises power supply means for feeding said at least one electromagnet with AC. Said at least one electromagnet is constituted, for example, by an integer number p of electromagnets where p is greater than or equal to 2, and the power supply means feeds said p electromagnets with p-phase AC. The driving portion comprises a yoke having p pairs of teeth extending substantially parallel to the axis of rotation, each pair of teeth constituting cores having coils of an electromagnet wound thereabout. The yoke is made up of laminations that are generally cylindrical in shape, concentric, and stacked radially. 
     In the above embodiments, the driven portion may be in the form of a bar having at least a central portion that is cylindrical. In addition, a magnetic element capable of producing asymmetry in the field lines can be provided so that the driven portion is also rotated about a longitudinal axis thereof, perpendicular to said axis of rotation, by magnetic coupling. This magnetic element can be situated on the driven portion or on the driving portion, and more precisely it can be situated on or close to one of the magnetic poles of the driving portion or of the driven portion. The magnetic element is a part made of a ferromagnetic material such as soft iron. 
     In another aspect of the invention, the stirrer includes a confinement wall made of a non-magnetic material in particular for confining the receptacle and protecting the driving portion from the liquid contained in the receptacle. The stirrer can thus be used to stir reagents that are dangerous, e.g. including nuclear materials. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the present invention will appear on reading the following detailed description of various embodiments made with reference to the accompanying drawings, in which: 
     FIG. 1A is a diagram of a first known liquid stirrer with magnetic coupling; 
     FIG. 1B is a diagram of a second known liquid stirrer with magnetic coupling; 
     FIG. 2A is a diagram of an arrangement for the driving and driven portions as used in the stirrer shown in FIG. 1A; 
     FIG. 2B is a diagram of an arrangement for the driving and driven portions used in the stirrer shown in FIG. 1B; 
     FIG. 2C is a diagrammatic plan view of the arrangement shown in FIG. 2A; 
     FIG. 3A is a diagram showing a liquid stirrer constituting a first embodiment of the present invention, and more particularly showing an arrangement for the driving and driven portions of the stirrer; 
     FIG. 3B is a diagrammatic plan view of the arrangement of driving and driven portions shown in FIG. 3A; 
     FIG. 4A is a diagram showing a liquid stirrer constituting a second embodiment of the present invention, and more particularly showing an arrangement for the driving and driven portions of the stirrer; 
     FIG. 4B is a diagrammatic plan view of the arrangement of driving and driven portions shown in FIG. 4A; 
     FIG. 5 is a diagram showing a liquid stirrer constituting a third embodiment of the present invention, and more particularly showing an arrangement for the driving and driven portions of the stirrer; 
     FIG. 6A is a diagram showing a liquid stirrer constituting a fourth embodiment of the present invention, and more particularly showing an arrangement for the driving and driven portions of the stirrer; 
     FIG. 6B is a diagram showing a magnetic circuit used as the driving portion in the stirrer shown in FIG. 6A; 
     FIG. 7A is a diagrammatic perspective view of a driving permanent magnet used in the stirrer shown in FIG. 3A; 
     FIG. 7B is a diagrammatic perspective view of a driving permanent magnet used in the stirrer shown in FIG. 5; 
     FIG. 8A is a diagram showing a driven magnet suitable for use in the stirrers shown in FIGS. 3A,  4 A,  5 , and  6 A; 
     FIG. 8B is an end view of the driven magnet shown in FIG. 8A; and 
     FIG. 9 is a diagram showing the effect produced on a driven magnet by a magnetic element for providing asymmetry included in the stirrers shown in FIGS. 3A,  4 A,  5 , and  6 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 3A and 3B are diagrams respectively showing a front view and a plan view of a liquid stirrer constituting a first embodiment of the present invention. FIGS. 3A and 3B show mainly the driving portion, the driven portion, and the receptacle, respectively referenced  13 ,  14 , and  5   1 . The stirrer of the invention also comprises, in particular, elements of the same type as the elements  2 ,  3 , and  6  shown in FIG.  1 A and not shown again in FIGS. 3A and 3B for reasons of clarity. More particularly, the driving portion  13  placed beneath the receptacle  5   1  which has the driven portion  14  placed on the bottom thereof is itself driven in rotation by a motor about a vertical axis of symmetry  8   1 , thereby rotating the driven portion  4  about the same axis by contactless magnetic coupling. The receptacle  5   1  or merely a bottom portion  50   1  close to the driving portion  13  is made of a non-magnetic material. 
     The driven portion  14  is a magnetic bar having a south pole  140  and a north pole  141 , both of which can be tapering in shape, for example, together with a cylindrical central portion  142 . The magnetic bar  14  is made of neodium-iron-boron, for example. 
     The driving portion  13  comprises a permanent magnet whose active faces  130  and  131 , i.e. the faces through which the major fraction of the magnetic field produced by the magnet passes, are parallel to the axis of rotation  8   1 . The permanent magnet is constituted by a north pole piece  132  and a south pole piece  133  united by a core  134  of ferromagnetic material such as soft iron, and a non-magnetic support  135  supporting the elements  132  to  134 . The lengths of the driving and driven portions  13  and  14  are preferably substantially equal so that the north and south pole pieces  132  and  133  of the driving portion  13  are situated respectively beneath the south and north poles  140  and  141  of the magnetic bar  14 . As soon as it has been placed on the bottom of the receptacle, the magnetic bar  14  takes up a position such that its vertical axis of symmetry and of rotation  8   1  coincides with the vertical axis of symmetry of the driving portion  13 . 
     In the invention, the pole pieces  132  and  133  of the driving portion  13  are made of an anisotropic material, e.g. strontium ferrite. The anisotropic material is cut in such a manner that its magnetization direction  136 , and thus its remanent field, are oriented parallel to the magnetic bar  14 , i.e. perpendicularly to the axis  8   1 . In practice, the axis  8   1  is vertical and the direction of magnetization  136  is horizontal. 
     The soft iron core  134  makes it possible to ensure that a fraction of the magnetic field between the poles  132  and  133  does not escape towards the bar  14  along the axis  8   1 , which would increase the axial force between the driving and driven portions. The magnetic field inside the core  134  is oriented horizontally. 
     In the invention, the driving magnets  132 - 133 - 134  and the driven magnet  14  produce a coupling magnetic field whose field lines  15  generally leave the active side face  130  of the north pole piece  132  substantially horizontally to reach the north pole  140  of the driven magnetic bar  14  also substantially horizontally, and leave the north pole  141  of the driven bar  14  substantially horizontally to reach the active side face  131  of the south pole piece  133  substantially horizontally so as to form a loop, as shown diagrammatically in FIG.  3 A. The field lines  15  extend more particularly in the vicinity of the poles of the driven magnet  14  substantially parallel to the longitudinal axis of the driven magnet, referenced  143 . Given that the number of field lines extending in a direction having a significant axial component in the vicinity of the magnetic bar  14  is greatly reduced compared with the arrangement shown in FIG. 2A, the axial force between the driving and driven portions is minimized. 
     In the present invention, a leakproof protective wall  145  is provided between the driving portion  13  and the receptacle  5   1 . The protective wall  145  is made of non-magnetic material and serves to protect the driving portion  13  and the associated elements (motor, transmission means) from the chemical reagents contained in the receptacle  5   1 . This characteristic of the present invention is of particularly advantageous application in the nuclear field. When the reagents contained in the receptacle are nuclear materials, the protective wall  145  can be used to confine the dangerous portion of the stirrer, comprising in particular the driven portion and the receptacle. 
     FIGS. 4A and 4B are respectively a simplified diagrammatic front view and a plan view of a second embodiment of the liquid stirrer of the invention. In this second embodiment, the stirrer comprises in particular a driven portion  16  identical to the driven portion  14  shown in FIGS. 3A and 3B and placed in a receptacle  5   2 , a protective wall  160 , and a static driving portion  17  beneath the receptacle  5   2  and the protective wall  160 . The width of the drive portion  17  is preferably substantially equal to the length of the driven portion  16 . The drive portion  17  comprises three stationary electromagnets  170 ,  171 , and  172  extending horizontally and powered from a multiphase sinusoidal alternating current (AC) power supply (not shown). The active faces  170   a - 170   b ,  171   a - 171   b , and  172   a - 172   b  of the respective electromagnets  170 ,  171 , and  172  are parallel to the axis of rotation  8   2  of the driven portion. 
     The electromagnets are disposed in a three-branch cross configuration, with each electromagnet forming one of the branches of the cross. Each electromagnet  170 ,  171 , and  172  is constituted by a horizontal core having a pair of coils  170   c - 170   d ,  171   c - 171   d , and  172   c - 172   d  wound on respective ends thereof. The coils of each electromagnet are interconnected as represented by wire  17 ′ in FIG. 4A in respect of electromagnet  170 . 
     The coils of the electromagnets  170 ,  171 , and  172  are powered using three-phase AC. More precisely, the pair of coils  170   c - 170   d , the pair of coils  171   c - 171   d , and the pair of coils  172   c - 172   d  are powered respectively with alternating currents that are phase-shifted relative to one another by 120°. Each active face of an electromagnet constitutes a magnetic pole whose polarity varies as a function of the phase of the AC applied to the corresponding coils. This produces a rotating electromagnetic field which is equivalent to the field produced by the rotating magnet  13  in FIGS. 3A and 3B. Because the electromagnets are disposed horizontally, the field lines  18  between the active faces  170   a ,  170   b ,  171   a ,  171   b ,  172   a , and  172   b  and the magnetic poles of the driven magnets  16  are comparable to the field lines  15  of FIGS. 3A and 3B. In particular, the field lines  18  generally extend substantially parallel to the longitudinal axis of the driven magnet  16  in the vicinity thereof. 
     The embodiment shown in FIGS. 4A and 4B uses three electromagnets. Nevertheless, a different number of electromagnets could be used, where said number is greater than or equal to two. In general, for an integer number p of electromagnets, where p is greater than or equal to 2, the electromagnets are arranged in a p-branch cross and p-phase AC is fed to the electromagnets. Each electromagnet is then powered with AC at a phase that is offset by +(360°/p) and −(360°/p) relative to the two closest electromagnets, respectively. 
     FIG. 5 shows a liquid stirrer constituting a third embodiment of the present invention. The stirrer in this third embodiment comprises in particular a driven magnet  19  identical to the magnets  14  and  16  and placed in a receptacle  5   3 , a U-shaped driving magnet  20 , a receptacle  5   3 , elements (not shown) identical to the elements  2  and  3  in FIG. 1A for rotating the driving magnet  20  about a vertical axis of symmetry  8   3  of the driving and driven magnets, and a protective wall  190 . The driving magnet has a north magnetic pole  200  and a south magnetic pole  201  disposed symmetrically about the axis  8   3  and having active faces  202  and  203  that are perpendicular to the axis  8   3 , i.e. parallel to the driven magnet  19 . 
     In the present invention, the distance D between the poles  200  and  201  of the driving magnet  20  is at least substantially equal to the length L of the driven magnet  19 , i.e. it is at least substantially equal to or greater than L. In this manner, the field lines  21  close to the driven magnet  19  extend generally substantially perpendicularly to the axis of rotation  8   3  of the driven magnet, and more precisely in a manner that is substantially parallel to the longitudinal axis of the driven magnet. 
     FIG. 6A is a simplified front view of a liquid stirrer constituting a fourth embodiment of the present invention. The stirrer in this fourth embodiment comprises in particular a driven magnet  22  identical to the magnets  14 ,  16 , and  19  and placed in a receptacle  5   4 , together with a static driving portion  23  and a protective wall  220 . 
     The driving portion  23  comprises a plurality of vertically extending electromagnets (only one electromagnet is shown in FIG.  6 A). Each electromagnet has a pair of coils  23   a - 23   b  wound around two vertical cores disposed symmetrically about a vertical axis of symmetry  8   4  of the driven magnet  22  and of the driving portion  23 . The coils of each electromagnet are interconnected, as represented by wire  23 ′. 
     Each electromagnet is fed with AC from a power supply (not shown) and has two magnetic poles of varying polarity whose active faces  23   c ,  23   d  are perpendicular to the axis  8   4 . The polarity of each magnetic pole varies periodically as a function of the phase of the AC powering the electromagnet, such that a rotating electromagnetic field is produced in the air gap between the driving and driven portions. 
     FIG. 6B is a perspective view showing an example of a magnetic circuit used for providing static drive  23 . In FIG. 6B, the coils are omitted for reasons of clarity. The magnetic circuit comprises a yoke made up of three pairs of vertical teeth  230 — 231 ,  232 - 233 , and  234 - 235  disposed in a circle on a cylindrical base  236 . Each tooth  230  to  235  constitutes a core around which a coil is wound. Thus, each pair of teeth  230 - 231 ,  232 - 233 , and  234 - 235  is to receive a pair of coils symmetrically about the axis  8   4  so as to form an electromagnet. The yoke is made up of laminations in the form of concentric cylinders  237  that are stacked radially. 
     In the present invention, the inside diameter DI of the yoke is substantially equal to the length LO of the driven magnet  22  or is greater than said length, so as to ensure that the field lines  22 ′ extend substantially perpendicularly to the axis  8   4  in the vicinity of the poles of the driven magnet  22 . 
     In the example shown in FIG. 6B, three pairs of coils are used. These three pairs of coils are powered by three-phase AC, i.e. each pair of coils receives AC that is offset by 120° and −120° relative to the other two pairs of coils, respectively. In general, as described above with reference to FIGS. 4A and 4B, the magnetic circuit constituting the driving portion  23  can have an integer number p of pairs of coils where p is greater than or equal to 2 (i.e. it can have an integer number p of electromagnets where p is greater than or equal to 2) and the pairs of coils are fed with p-phase AC. 
     The above description of four embodiments relates to a driven magnet that is generally in the form of a cylindrical bar. It will be clear to the person skilled in the art that other shapes could be provided for the driven magnet. For example, it could have a horizontal cross-configuration with two pairs of magnetic poles. In the third and fourth embodiments shown in FIGS. 5 and 6A,  6 B, the distance D between the magnetic poles of the driving magnet  20  (FIG. 5) and the diameter DI of the yoke of the magnetic circuit  23  (FIG. 6B) are generally chosen so that they are substantially equal to or greater than the size of the driven magnet in any direction perpendicular to the axis of rotation. 
     In all of the embodiments of the present invention as described above with reference to FIGS. 3A to  6 B, when the driven magnet  14 ,  16 ,  19 , or  22  is in the form of a substantially cylindrical bar, it is rotated about an axis of symmetry  8   1  to  8   4  which is orthogonal to its own longitudinal axis. The portion of the driven magnet which is in contact with the bottom of the receptacle  5   1  to  5   4  at any given instant is a generator line, referenced  144  in FIG. 3A, extending parallel to the longitudinal axis of the driven magnet. 
     In a variant of the invention, in order to prevent the generator line that is in contact with the bottom of the receptacle from remaining unchanged while the driven magnet is being rotated, i.e. in order to prevent the driven magnet from suffering wear due to friction with the bottom of the receptacle essentially along a single generator line and in the vicinity thereof, a magnetic element for providing asymmetry or unbalance is disposed on the driven magnet or the driving portion. This magnetic element is preferably a part made out of ferromagnetic material such as soft iron and it seeks to produce asymmetry in the field lines between the driving and driven portions in order to cause the driven magnet to rotate about its longitudinal axis. In the first and third embodiments of the invention using a rotating permanent magnet as the driving portion (FIGS.  3 A and  3 B), the element for providing asymmetry is preferably situated on one of the magnetic poles of the driving magnet, a part which is not subject to wear, but it can also be placed on one of the magnetic poles of the driven magnet. In the second and fourth embodiments using static drive for the driving portion (FIGS. 4A,  4 B,  6 A and  6 B), the element for providing asymmetry is situated on one of the magnetic poles of the driven magnet. 
     With reference to FIG. 7A which is a perspective view of the driving magnet  3  in FIGS. 3A and 3B, the element for providing asymmetry, referenced  137 , can be placed in a suitable notch formed in the pole piece  133 . In comparable manner, in the third embodiment of the invention (FIG.  5 ), the element for providing asymmetry can be placed in a suitable notch formed in the pole piece  201  of the driving magnet  20 , as shown by reference  202  in FIG.  7 B. 
     FIGS. 8A and 8B show a magnetic bar  24  of the same type as the magnets  14 ,  16 ,  19 , and  22 , but having a magnetic element  240  for providing asymmetry placed thereon. More particularly, the element  240  is placed on the surface of one of the poles  241  of the bar  24  in a suitable notch and its shape matches that of the pole  241 . As mentioned above, the magnetic bar  24  carrying the element  240  is more particularly used as a driven magnet  16  or  22  when it is desired to cause the driven magnet to rotate about its longitudinal axis in embodiments that use static drive. 
     FIG. 9 is a plan view showing the effect produced by the magnetic element for providing asymmetry on the driven magnet, given overall reference  25 . 
     The element for providing asymmetry significantly alters the field lines between the driving and driven portions so that the “magnetic center of gravity”  250  of the driven magnet is offset from the center of symmetry  251 . The driven magnet then rotates about a vertical axis passing through the point  250  and not about a vertical axis passing through the point  251 . The friction forces F A  and F B  exerted on the magnet on either side of the point  250  by the bottom of the receptacle has different respective resultant magnitudes. The unbalance between these friction forces drives the magnetic bar  25  in rotation about its longitudinal axis  252 , as represented by arrow  253 , while the magnetic bar  250  is rotating about the vertical axis passing through the magnetic center  250 , as represented by arrow  254 . 
     Because the magnet rotates about its own longitudinal axis, the friction forces exerted by the bottom of the receptacle on the magnet are distributed over the entire cylindrical surface of the magnet during rotation thereof about its axis of vertical symmetry. As a result, wear on the magnet is not concentrated in a particular zone but is distributed uniformly over its entire surface.