FR2780579A1 - Magnetic structure for motor vehicle alternator - Google Patents

Abstract

The alternator has structure including magnets and non-magnetic regions to reduce losses. The electric machine comprises a stator (1) and a rotor (2), the stator including a coil (14) housed in slots (13). The rotor includes successive excitation magnets which generate in the rotor two magnetic fluxes having components in tangentially opposite directions. The slots between projecting poles are occupied by only one, or a few strands of coils around a single projecting pole. The rotor shaft is made of magnetic material, and in the proximity of the shaft and magnets (27) the machine comprises at least one non-magnetic zone (28,28a,30). This zone has the effect or reducing to a minimum the leakage of flux towards the rotor shaft. Between two successive magnets there may be oblong shaped holes formed.

Description

The present invention relates generally to machines

  rotating such as alternators for motor vehicles.

  The single or multi-phase generator that constitutes a conventional motor vehicle alternator generally comprises a stator inside which rotates a rotor provided with an excitation winding. This winding is powered by brushes in contact with two rings

  manifolds provided on a projecting part of the rotor shaft.

  There are already known in particular from EP-A-0 707 374 rotating machines in which, with a view in particular to increasing their efficiency, the excitation field of the rotor is produced both by permanent magnets and by coils (we speak in general of "mixed" excitation), and in which the current delivered by the armature is controlled by means of switching means at the excitation windings, these switching means making it possible to selectively reverse the direction of the excitement

  to reduce, or even substantially cancel, the flux of the magnets.

  This need to reverse the direction of the excitation current means that a so-called "H" semiconductor switching bridge must be used, the

  cost is high and therefore strikes the cost price of the machine.

  It has recently been proposed by the applicant in its application FR 98 02886 a structure making it possible to overcome this drawback. An alternator is described in this application, the armature current of which is regulated by acting on the excitation of rotor coils supplied unidirectionally by an excitation current, the intensity of which varies between an essentially zero value and a maximum value. to deliver respectively a limited, or even essentially zero, energy and a

maximum energy.

  The structure described in this patent application is of the type which is illustrated in Figures la and 1b. It includes a stator 1 and a

rotor 2.

  The stator 1 has a carcass 12 defining an annular structure at the inner periphery of which is formed a plurality of notches 13 which are defined between poles 15 of the rotor and which receive

armature winding strands 14.

  The rotor 2 has a carcass 22 on which are also formed a plurality of notches 23 which are separated by poles 25 and which receive permanent magnets 27 whose flux lines are oriented tangentially along the rotor structure, as well as strands of an excitation winding 26. Thus, in succession in the adjacent notches 23 of the rotor: - a magnet 27 having a first polarity, - two strands of a winding 26, received in two adjacent notches, - a second magnet 27 having a second polarity opposite the first, - again two strands of a coil 26, capable of being crossed by currents opposite to those flowing in the strands of the coil 26

previous, etc ...

  The number of poles on the rotor and on the stator is for example identical and

equal to 12.

  With such a structure, the rotor adopts, in the absence of currents in the strands 26, a polarity diagram ... S-N-N-N-S-S-S-N-N-N, etc.

(figure la).

  Thus, in the absence of current in the windings 26, the N-S polarity diagram has a pitch which is equal to three times the pitch of the poles of the stator and

  the energy transferred between the rotor and the stator is minimal.

  On the other hand, when a current flows in the windings of the rotor, the polarities of this one then become ... N-S-N-S-N-S, etc. with a pitch which corresponds to that of the different phase windings on the stator; a higher magnetic energy, which increases with the intensity of the current

  excitation, is transferred between the stator and the rotor.

  To limit the leakage of the magnetic flux and prevent the flux of the magnets from closing on the machine shaft, it is proposed in FR 98 02 886 to produce the rotor shaft in a non-magnetic material and to conform this shaft and the rotor sheets so as to reduce as much as

  possible the thickness of the carcass below the magnets.

  However, the use of non-magnetic materials (titanium by

  example) can lead to significant additional manufacturing costs.

  In addition, mechanical strength constraints prevent the thickness of the carcass between the magnets and the rotor shaft from being reduced as much as would be desirable.

  The invention overcomes these drawbacks.

  To this end, the invention provides an electric machine comprising a stator and a rotor, the stator comprising at least one armature winding housed in at least one pair of notches, the rotor comprising means capable of selectively establishing magnetic circuits. closed passing around the strands of the armature winding (s), characterized in that said means comprise at least two permanent excitation magnets capable of establishing two magnetic fluxes having, in the direction of movement of the rotor, components of opposite directions, and, between each pair of successive magnets, at least one excitation winding having two strands and able to generate in an adjustable manner two flux components capable of opposing the fluxes generated in the magnets of said pair, the winding strands being housed in notches each extending between two successive rotor poles and in that the shaft is made of a magnetic material, said machine comprising in the vicinity at least one nonmagnetic zone is swimming between said shaft and the magnets which minimizes the leakage fluxes towards the rotor shaft. In particular, according to a preferred embodiment, the rotor carcass has a plurality of holes which extend in the vicinity of the rotor shaft and which are distributed between the magnets, these holes constituting non-magnetic zones which define on the one hand and on the other, magnets,

  choke surfaces limiting flow leakage to the shaft.

  In yet another embodiment, the tree has a non-magnetic coating which constitutes a spacer separating it from

magnets.

  Advantageously then, the sheets which constitute the rotor carcass are made up of n independent sectors juxtaposed angularly, where n is the number of permanent magnets, means

  allowing to maintain said sectors relative to each other.

  Other characteristics and advantages of the invention will emerge

  still from the description which follows. This description is purely illustrative

  and not limiting. It must be read with reference to the appended drawings in which: - FIGS. 1 a and 1 b, already analyzed, are schematic diagrammatic representations of the rotor and the single-phase stator of a machine of the type described in the patent application FR 98 02886, on which the flux lines have been shown in the absence or absence of excitation on the windings of the rotor; - Figure 2 is a schematic representation of the developed rotor of a machine according to a possible embodiment of the invention; - Figure 3 is a representation in cross section of the rotor of the machine of Figure 2; - Figure 4 is a schematic representation of the developed rotor of a machine according to another possible embodiment of the invention - Figure 5 is a schematic representation in cross-section of the rotor of a machine according to a other mode of

  possible realization of the invention.

  The machine, the rotor of which is illustrated in FIGS. 2 and 3, comprises a single-phase stator which is analogous to that of the machine illustrated in FIG. 1 (and which has not been shown in order not to weigh down the figures) or a polyphase stator . It further comprises a rotor 2 which, in the same way as that of the machine illustrated in FIG. 1, comprises a carcass of generally cylindrical shape in which are formed a plurality of notches 23 which are separated by salient poles 25 and which receive permanent magnets 27 alternating with pairs of strands of a

excitation winding 26.

  More particularly, the permanent magnets 27 are distributed with alternating polarity around the periphery of the rotor 2, the fluxes generated by two successive magnets 27 having, according to the direction of movement of the rotor 2,

opposite components.

  In the excited state, the rotor winding is supplied unidirectionally so that the flux generated by the current flowing in the two strands between two successive permanent magnets 27 opposes the fluxes of said magnets 27 respectively. polarity

  N-N-N-S-S-S-etc. in the absence of excitation at an N-S-N-S-etc polarity.

  when the winding 26 of the rotor is excited. The direction of the excitation currents and the flows generated by the currents flowing in the strands of the winding 26 have been shown in FIG. 2. In the embodiment according to the invention illustrated in these FIGS. 2 and 3, the rotor shaft, referenced by A, is a cylindrical shaft made of a material

magnetic, for example steel.

  The carcass has four oblong holes 28 which extend at the

  in the vicinity of the rotor shaft A and which are distributed between the magnets 27.

  These holes 28 constitute non-magnetic zones which define, on either side of the magnets 27, choke surfaces 29 limiting the flow leaks towards the shaft A. The thickness of the sheet under the magnet can be

  so great without harming the electrical performance of the machine.

  In addition, the mechanical strength of such a structure is reinforced compared to the mechanical strength of a structure of the type illustrated in FIGS. 1a and 1b. In particular, the thicker zone under a magnet and the throttling zone 29 between a hole 28 and a magnet 27 of a structure of the type illustrated in FIG. 2 contribute more effectively to the joining of a "sector" of sheets in relation to the immediately neighboring sector. This is not the case for the constriction zone of small thickness under a magnet of a structure of the type of that of FIGS. 1a and 1b, zone capable of breaking at high speeds of rotation of the rotor. The ends of the holes 28 are preferably rounded in shape. Rectangular shapes can nevertheless be considered

for said holes 28.

  Alternatively, to further increase the mechanical strength of the structure, an oblong hole 28 can be replaced between two successive magnets 27 by a plurality of holes 28a (for example, two, three or

  four). This is what has been illustrated in FIG. 4.

  The width which separates a hole 28a from the neighboring hole 28a or from the magnet 27 to which it is juxtaposed is such that the throttling surfaces thus

  defined is sufficient to limit flow leaks to the rotor shaft A.

  In the embodiment illustrated in FIG. 5, a non-magnetic material is interposed between on the one hand the magnetic shaft A and on the other hand the magnets 27 and the sheets 31 of the carcass. For example, the magnetic shaft A can carry around its entire periphery a coating 30 of a non-magnetic material (resin) which defines a spacer which separates the said shaft A from the magnets 27. As a variant, it can be fitted into a tube, for example in aluminium. The spacer may externally bear effusions defining dovetails on which the sheets 31 are fixed, and may include dovetail shoulders which

help hold the sheets.

  In either of these two cases, the sheets 31 which constitute the carcass of the rotor 2 then consist of n sectors 31a, 31b, etc. independent juxtaposed angularly, where n is the number of permanent magnets, the notches 23 receiving the magnets 27 being defined by

  shapes opposite two sectors 31 a, 31 b, etc. successive.

  In the absence of the dovetail elements, these sectors 31a, 31b, etc. are for example held by a molding 32 in a non-magnetic resin, or by end flanges in a non-magnetic material, these flanges cooperating with magnetic tie rods 33

crossing the sheets.

  These two means can, as shown in the

  Figure 5, be used simultaneously.

  As will be understood, a structure of the type illustrated in FIG. 5 makes it possible to completely resolve the problems of flow leakage towards the shaft A and to avoid the problems of mechanical strength which may arise.

  install with structures of the type illustrated in Figures 1a and 1b.

  Of course, the present invention also applies to alternators performing other functions such as the start function of the

  heat engine of a motor vehicle called an "alternator-starter".

Claims (10)

  1. Electric machine comprising a stator (1) and a rotor (2), the stator comprising at least one armature winding (14) housed in at least one pair of notches (13), the rotor comprising means capable of selectively establish closed magnetic circuits passing around the strands of the armature winding (s), characterized in that said means comprise at least two permanent excitation magnets (27) capable of establishing two magnetic fluxes having in the direction of movement of the rotor components in opposite directions, and, between each pair of successive magnets, at least one excitation winding having two strands (26, 26) and able to generate in an adjustable manner two flux components capable of opposing the generated fluxes in the magnets of said pair, the winding strands being housed in notches (23) each extending between two successive rotor poles (25) and in that the shaft (A) of the rotor is made of a magnetic material, said thing e comprising in the vicinity of said shaft (A) and magnets (27) at least one non-magnetic zone (28, 28a; 30) which minimizes or removes   leakage flow to the rotor shaft.   2. Machine according to claim 1, characterized in that the rotor carcass has a plurality of holes (28, 28a) which extend in the vicinity of the rotor shaft and which are distributed between the magnets (27), these holes (28, 28a) constituting non-magnetic zones which define, on either side of the magnets, choke surfaces limiting leakage of flow to the tree.   3. Machine according to claim 2, characterized in that it comprises between two successive magnets (27) a single hole (28) which is oblong shape.   4. Machine according to claim 2, characterized in that it   comprises several successive holes (28a) between two magnets (27).   5. Machine according to claim 1, characterized in that a non-magnetic tubular spacer (30) is interposed between the shaft (A) and the magnets. 6. Machine according to claim 5, characterized in that the sheets which constitute the carcass of the rotor consist of n independent sectors (31a, 31 b) juxtaposed angularly, one is the number of permanent magnets, means (32, 33 ) allowing to maintain said sectors one to another.   7. Machine according to claim 6, characterized in that the non-magnetic spacer is constituted by a coating carried by the shaft (A). 8. Machine according to claim 6, characterized in that   the spacer has dovetailed shoulders.   9. Machine according to one of claims 6 or 7, characterized in that   that said means comprise end flanges (33) of a non-magnetic material, these flanges cooperating with tie rods magnetic crossing the sheets.   10. Alternator or alternator-starter of a motor vehicle, characterized in that it is constituted by a machine according to one of claims 1 to 8.