Patent Publication Number: US-2004051412-A1

Title: Alternator

Description:
[0001] The present invention relates to rotary electric machines and more particularly, but not exclusively to alternators for electricity generator units of power lying in the range, for example, a few kilowatts (kW) to a few tens of kilowatts, for example less than 30 kW.  
       BACKGROUND OF THE INVENTION  
       [0002] Patent application EP-A2-1 081 828 describes an alternator having a rotor that drives a fan. The fan rotates in a housing defining two passages each in the form of a volute for delivering the air driven by the fan. Each passage offers a path of increasing section for the air heading towards the outlet. The above-mentioned housing is defined by a separate piece fitted to the cylindrical body of the casing. In general, the electrical performance of an alternator depends on the efficiency with which it is cooled and it is desirable for cooling efficiency to be as high as possible.  
       [0003] Alternators for electricity generator units are also known that comprise:  
       [0004] a casing of elongate shape;  
       [0005] a rotor having a shaft capable of turning inside the casing about an axis of rotation; and  
       [0006] a fan rotated by the rotor.  
       [0007] In those prior art alternators, the grids protecting the air inlet and outlet openings are constituted by independent pieces, which implies assembly operations need to be performed on the casing during manufacture of the alternator, thereby increasing its cost price.  
       [0008] In addition, that can make fixing certain alternators on the driving engine relatively awkward when access to fasteners takes place through the openings via which the cooling air leaves. The protective grids need to be removed in order to give access to the fasteners, and subsequently they need to be put back into place.  
       [0009] Alternators for electricity generator units are also known that comprise:  
       [0010] a casing of elongate shape;  
       [0011] a rotor having a shaft capable of turning inside the casing about an axis of rotation; and  
       [0012] at least one alternator fixing flange at one end.  
       [0013] It is relatively awkward to fix certain alternators of that type to the engine since access to the fasteners takes place through the openings via which the cooling air leaves.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0014] There therefore exists a need for an alternator of low manufacturing cost that nevertheless presents satisfactory electrical performance and reliability.  
       [0015] There also exists a need to facilitate the flow of air for cooling the machine.  
       [0016] There also exists a need to have an electric machine, in particular an alternator, in which installing or removing the brush carrier is facilitated.  
       [0017] The present invention seeks in particular to satisfy all or part of these requirements.  
       [0018] In a first of its aspects, the invention provides a rotary electric machine comprising:  
       [0019] a casing of elongate shape;  
       [0020] a rotor having a shaft capable of turning inside the casing about an axis of rotation; and  
       [0021] a fan rotated by the rotor;  
       [0022] wherein the casing includes at least one air inlet grid and at least one air outlet grid, both of which are made integrally with the casing.  
       [0023] This avoids the need to fit the protective grids of prior art alternators on the casing, thus reducing the amount of labor needed for manufacturing the alternator.  
       [0024] In an exemplary embodiment of the invention, the casing is made as a casting, in particular out of injected aluminum.  
       [0025] The casing may include a flange at a first longitudinal end, and an end wall at a second longitudinal end opposite from the first, which end wall is made integrally with the casing and is perforated to form a grid to allow cooling air to enter into the casing.  
       [0026] On its inside face, the end wall may include a portion in relief enabling a brush carrier to be fixed. Said portion in relief may comprise a slideway and the brush carrier may be configured to slide in said slideway while it is being installed in the casing. Such a configuration makes it easier to install the brush carrier and makes it possible to further reduce the time taken to manufacture the alternator. Removing the brush carrier during maintenance is also made easier.  
       [0027] In a particular embodiment, the casing may have two side grids made integrally with the casing and situated respectively on the left and right sides of the casing when the alternator is observed along the axis of rotation of the rotor.  
       [0028] The casing may have at least one volute-shaped passage opening out to a side grid, and in particular it may have two volute-shaped passages associated respectively with the two above-mentioned side grids.  
       [0029] These volute-shaped passages can be made during molding of the casing, thus avoiding subsequent assembly operations.  
       [0030] At least one side grid may comprise bars, each bar having a longitudinal axis that is preferably curvilinear with a concave portion facing towards the fan. Each bar may be oriented substantially parallel to the plane perpendicular to the axis of rotation of the rotor. The bars may thus be substantially parallel to a vertical plane when the axis of rotation of the rotor is horizontal.  
       [0031] In another aspect of the invention, each side grid has no bars extending substantially parallel to the axis of rotation of the rotor, thereby reducing head losses in the air passing through the grid and thus improving the efficiency of the fan and the effectiveness of the cooling.  
       [0032] In order to make the bars of the grid easier to make, they may be molded together with a web of material which connects them together on their radially inner side. This web, which may be relatively thin, serves to reinforce the mechanical strength of the bars while the material is cooling down after molding. On leaving the foundry, the web is subsequently removed during an operation of machining the inside surface of the casing. Advantageously, after the casing has been molded, the method of manufacturing the casing includes the step which consists in progressively eliminating the web extending between the bars by machining in a single pass. This avoids a second pass, which might damage the bars.  
       [0033] The inside of the casing may be machined with the stator already in place therein so as to make it easier to ensure that the machined surfaces are concentric.  
       [0034] The casing may include extensions for supporting a protective cover for an electric circuit, in particular an electric circuit for regulating the alternator and for making connections thereto, said extensions including air inlet openings. The casing may have at least one opening that opens out beneath the cover so as to allow air to be sucked in from beneath it while the alternator is in operation. The increase in the efficiency of the fan due in particular to the side grids having no bars parallel to the axis of rotation of the rotor and to the presence of the volute-shaped passages serves to obtain a relatively large flow of cooling air beneath the cover.  
       [0035] The casing may have axial splines that are not machined on which the stator rests. The fact that these splines are not machined makes it possible to further reduce the cost of manufacture by avoiding a specific machining operation.  
       [0036] The casing may comprise a cylindrical body and the flange may have passages for fasteners on axes that are situated radially outside the envelope of the cylindrical body. It is thus possible to have easy access to the fasteners that are used for fixing the alternator to an engine, for example, without it being necessary to remove protective grids.  
       [0037] In another of its aspects, the invention provides independently or in combination with the above, a rotary electric machine comprising:  
       [0038] a casing;  
       [0039] a rotor capable of rotating inside the casing about an axis of rotation;  
       [0040] a fan fixed on the rotor; and  
       [0041] at least one outlet grid for air driven by the fan;  
       [0042] the grid having no bars that extend substantially parallel to the axis of rotation of the rotor or having a number of such bars that is less than or equal to the number of bars extending substantially parallel to a plane perpendicular to the axis of rotation.  
       [0043] Unlike prior art machines having grids that are fitted to the casing, these grids having a large number of mutually perpendicular bars, in the invention at least one of the grids has no bars that are substantially parallel to the axis of rotation or has a relatively small number thereof.  
       [0044] As a result, the head loss to which air passing through the grid is subjected is smaller than it would be if the grid had a large number of bars extending substantially parallel to the axis of rotation of the rotor. The bars can thus be made of a cross-section that is larger for equivalent head loss, thus making them easier to cast integrally with the casing.  
       [0045] The grid preferably has no bars extending substantially parallel to the axis of rotation of the rotor.  
       [0046] The casing may have air outlet passages which offer a section to the air flow that increases on approaching the outlet. These passages may be volute-shaped, for example.  
       [0047] The air may leave radially.  
       [0048] The bars of the grid may be made integrally with the casing, as mentioned above.  
       [0049] The bars may present respective central portions having a curvilinear longitudinal axis that is concave towards the fan. These central portions may be connected to the casing via connection portions which co-operate with the central portion-to form a concave side facing outwards. The connection portions may be adjacent to an enlarged portion of the air outlet passage.  
       [0050] The bars of the grid may present a radially inner side that is machined.  
       [0051] The grid may comprise two bars.  
       [0052] At one end, the casing may have a transverse wall that is perforated to enable air to be sucked in, and the casing may comprise a cylindrical body provided with an opening that opens out beneath a protective cover, extensions on which the cover bears being also made on the casing, these extensions being provided with openings, air being sucked in during operation of the fan through the perforated end wall, via the openings in the extensions, and through the opening made in the cylindrical body of the casing, and delivered through two air outlet grids as defined above.  
       [0053] In another of its aspects, the invention also provides independently or in combination with the above, an assembly comprising an alternator and a connection member enabling the alternator to be fixed on an engine, the alternator having a casing with a cylindrical body and a flange, wherein the flange has passages for fasteners for fastening the alternator to the connection member, these passages having axes situated outside the envelope of the cylindrical body of the casing, the connection member having a first set of holes for fastening the connection member to the engine, and a second set of holes disposed in such a manner as to be superposable on the passages through the flange for fastening the alternator to the connection member.  
       [0054] By using such a connection member, fastening the alternator to the engine is facilitated and, for example, it is possible to avoid having to access fasteners through the outlet openings for cooling air leaving the alternator. There is no longer any need to provide removable protective grids, and the casing may have bars extending across the air outlet openings that are made integrally with the casing, e.g. as a casting.  
       [0055] The connection member may include nuts fixed on one face of a plate, said face being situated facing the engine, the nuts being suitable for receiving screws inserted through holes of the second set of holes.  
       [0056] The connection member may have a first plane annular portion in which the holes of the first set of holes are made and a second plane annular portion in which the holes of the second set of holes are made, the second annular portion being axially offset relative to the first annular portion.  
       [0057] The offset between the first and second annular portions may be greater than the thickness of the nuts.  
       [0058] The connection member may include reinforcing ribs.  
       [0059] The connection member may have projecting portions in the form of circular arcs for co-operating with the alternator casing in order to contribute to centering the connection member relative to the alternator.  
       [0060] The alternator flange may be made integrally with the casing.  
       [0061] In another of its aspects, the invention also provides, independently or in combination with the above, an electricity generator unit comprising an alternator and an engine, wherein the alternator is fixed to the engine via a connection member.  
       [0062] The connection member may be configured to be suitable initially for fixing on the engine, and subsequently for fixing the alternator on the connection member.  
       [0063] The connection member may have holes enabling it to be fixed on the engine and nuts for receiving screws for fastening the alternator to the connection member.  
       [0064] In another of its aspects, the invention also provides independently or in combination with the above, a connection member for fixing an alternator on an engine, the connection member comprising:  
       [0065] a plate provided with an opening enabling a rotor alternator shaft of the alternator to be coupled with the engine;  
       [0066] a first set of holes for fixing the plate on the engine; and  
       [0067] a second set of holes situated radially outside the first set for fixing the alternator, on the plate.  
       [0068] In another of its aspects, the invention also provides independently or in combination with the above, a rotary electric machine comprising a stator and a rotor suitable for rotating about an axis of rotation inside the stator, the rotor comprising a yoke having windings disposed thereon, together with winding-retaining spacers, at least one spacer when seen in section in a plane perpendicular to the axis of rotation defining a concave side that is open towards the stator.  
       [0069] The presence of such a concave side facilitates the passage of cooling air so that cooling of the machine is thereby improved.  
       [0070] In another of its aspects, the invention also provides, independently or in combination with the above, a rotary electric machine comprising:  
       [0071] a rotor suitable for rotating about an axis of rotation, the rotor comprising:  
       [0072] a magnetic yoke having slots for receiving windings; and  
       [0073] damper windings comprising electrical conductors passing along corresponding passages in the magnetic yoke;  
       [0074] wherein said passages, when the rotor is observed in a plane perpendicular to the axis of rotating, are disposed in a manner that is not symmetrical about any plane containing the axis of rotation.  
       [0075] By disposing the passages in this way, it is possible to improve harmonic attenuation and/or the flow of magnetic flux.  
       [0076] On going circumferentially around the axis of rotation of the rotor on one side of a midplane, the spacing between consecutive passages need not be constant. In particular, this spacing may vary, e.g. monotonically, increasing on going circumferentially in the direction of rotation of the rotor. This enables the operation of the machine to be improved, particularly when loaded.  
       [0077] In another aspect of the invention, at least one passage may present a cross-section that is not circular. In particular, at least one passage may present a cross-section that is oblong, having a major axis that is oriented substantially radially. This disposition can facilitate the passage of flux.  
       [0078] The invention also provides, independently or in combination with the above, a rotary electric machine comprising:  
       [0079] a casing;  
       [0080] a rotor capable of rotating inside the casing; and  
       [0081] a brush carrier having brushes configured to come into electrical contact with the rotor;  
       [0082] wherein the brush carrier is fixed on an inside face of a transverse wall of the casing that is made integrally therewith and that includes a slideway enabling the brush carrier to be fixed.  
       [0083] Such a configuration facilitates installation of the brush carrier and can reduce the time required to manufacture the machine.  
       [0084] The brush carrier is configured to be capable of sliding in the slideway while being installed in the casing. Removing the brush carrier during a maintenance operation is thus also made easier.  
       [0085] Where appropriate, the slideway may also make it possible to position the brush carrier radially relative to the shaft of the rotor.  
       [0086] The transverse wall may be situated at one longitudinal end of the casing, and it may be pierced by a plurality of air inlet openings, for example.  
       [0087] The casing may comprise a cylindrical body pierced by an opening, for example adjacent to the above-mentioned slideway, through which the brush carrier can be installed or removed.  
       [0088] This opening may also serve as an air inlet into the casing for cooling purposes.  
       [0089] The brush carrier may be held in place in the slideway by a fastener such as a screw, for example, inserted through an extension of the brush carrier and screwed into the casing. The above-mentioned extension may be configured so as to enable the radial position of the brush carrier to be adjusted to a small extent.  
       [0090] The brush carrier may include stiffening webs extending parallel to a plane containing the axis of rotation of the rotor.  
       [0091] The machine may constitute an alternator, in particular for an electricity generator unit.  
       [0092] The slideway may be oriented radially.  
       [0093] The machine need not necessarily include brushes and a brush carrier. If it has no brushes, the field windings may be connected to capacitors.  
       [0094] The various aspects of the invention defined above may advantageously be implemented within the same machine, in particular an alternator, however that is not necessarily so, and any one of the aspects may be implemented independently. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0095] The invention can be better understood on reading the following description of a non-liming embodiment and on examining the accompanying drawings, in which:  
     [0096]FIG. 1 is a diagrammatic perspective view showing an example of an alternator casing made in accordance with the invention;  
     [0097]FIG. 2 is a fragmentary and diagrammatic axial section view of an alternator including the casing of FIG. 1;  
     [0098]FIG. 3 shows the casing of FIG. 1 in isolation, and in side view;  
     [0099]FIG. 4 is a diagrammatic and fragmentary plan view seen looking along arrow IV of FIG. 3;  
     [0100]FIG. 5 is a cutaway diagrammatic and fragmentary face view seen looking along arrow V in FIG. 4;  
     [0101]FIG. 6 is a diagrammatic and fragmentary face view seen looking along arrow VI of FIG. 4;  
     [0102]FIG. 7 shows a brush carrier in isolation, in section on a midplane;  
     [0103]FIG. 8 is a diagrammatic and fragmentary plan view seen looking along arrow VIII in FIG. 7;  
     [0104] FIGS.  9  to  11  are face views showing various examples of prior art rotor laminations;  
     [0105]FIG. 12 is a face view of a rotor lamination with spacers;  
     [0106]FIG. 13 is a perspective view showing a spacer in isolation;  
     [0107]FIGS. 14 and 15 are fragmentary and diagrammatic face views showing respective ends of the rotor with the short-circuit “rings”;  
     [0108]FIG. 16 is a face view of the connection piece shown in isolation; and  
     [0109]FIG. 17 is a section view on XVII-XVII of FIG. 16.  
    
    
     MORE DETAILED DESCRIPTION  
     [0110]FIGS. 1 and 2 show an alternator  1  comprising a metal casing  2  and a cover  3  fitted on the casing  2 . In the example described, the alternator  1  is designed to be fixed to an engine (not shown) in order to form an electricity generator unit. The cover  3  can be made of metal or of plastics material and serves, for example, to house a conventional electric circuit (not shown in the figures) for regulating and connecting the alternator.  
     [0111] As can be seen in FIG. 2, inside the cover  3 , the alternator  1  comprises a stator  4  and a rotor  5  capable of rotating inside the stator  4  about an axis of rotation X.  
     [0112] The rotor  5  comprises a shaft  6  on which a magnetic yoke  7  is fixed that receives field windings  8 .  
     [0113] The rotor  5  rotates a fan  9  for generating a flow of cooling air.  
     [0114] Casing  
     [0115] The casing  2  comprises a circularly cylindrical body  12  which is connected at a first longitudinal end via an enlarged portion  20  to a flange  13  for fixing the alternator on the engine via a connection member, as described in greater detail below.  
     [0116] The flange  13  has holes  40  of respective axes Y situated radially outside the envelope of the cylindrical body  12 .  
     [0117] At a second longitudinal end opposite from the first, the casing  2  also comprises a perforated transverse wall  14  and a bearing  15  for supporting the rotor shaft.  
     [0118] Reinforcing ribs  16  extend radially between the bearing  15  and the cylindrical body  12 .  
     [0119] The flange  13  and the bearing  15  are made integrally with the remainder of the casing, as an aluminum casting in the example described.  
     [0120] In FIGS. 1 and 3 it can be seen in particular that the casing  2  has two side extensions  18  in its upper portion extending over the major part of the cylindrical body  12 , with the cover  3  resting thereon, each of said extensions  18  being provided with a plurality of openings  19  allowing air to enter beneath the cover, as explained in greater detail below.  
     [0121] The enlarged portion  20  which is made integrally with the remainder of the casing defines a housing for the fan  9 .  
     [0122] Two side openings  21  are made in the enlarged portion  20  to allow cooling air to escape. Only one of these openings can be seen in FIGS. 1 and 3. Protective bars  22  are present in each of the openings  21  to protect people against making contact with the fan in operation. The bars  22  are made integrally with the casing  2 , by casting. Prior to machining, the bars  22  are connected together on the inside by a web of cast material.  
     [0123] After being cast and fixed on the stator, the casing  2  is subjected to machining, in particular for the purpose of making a circularly cylindrical surface about the axis X inside the bearing  15  so as to receive a cage  11  containing ball bearings for supporting the rotor shaft.  
     [0124] The radially inner sides of the bars  22  are machined to remove the above-mentioned web. This machining is performed in a single pass so as to avoid a second pass which might damage the bars  22 .  
     [0125] In FIG. 5, there can be seen splines  41  projecting from the radially inner surface of the cylindrical body  12  for fixing the stator. The splines  41  are made by casting and they are made with sufficient precision to avoid any need to machine them.  
     [0126] The transverse wall  14  has perforations  43  to allow cooling air to enter into the casing.  
     [0127] Air Outlet Grids  
     [0128] Each bar  22  presents a longitudinal axis Z which extends, as can be seen in particular in FIG. 3, parallel to a plane perpendicular to the axis of rotation X of the rotor, i.e. substantially vertically in the example described. The axis Z is not rectilinear, each bar  22  having a curvilinear central portion  22   a  with an inside that is concave towards the axis X.  
     [0129] On examining FIG. 3, it can also be seen that the bars  22  are connected at their longitudinal ends to the remainder of the casing  2  and that between their longitudinal ends they do not have any connection between one another all along their length. The grids formed by the bars  22  in the example described thus have no horizontal bars. This disposition serves to reduce head losses in the air flow passing through the openings  21  and thus to improve the effectiveness of ventilation.  
     [0130] In FIG. 5, it can be seen that the enlarged portion  20  defines two passages  91  forming a volute. In the example described, these two passages  91  are substantially images of each other in axial symmetry about the axis X.  
     [0131] Each passage  91  presents a flow section for passing air which increases on coming closer to the outlet.  
     [0132] In FIG. 5, the fan turns clockwise.  
     [0133] Each passage  91  has an enlarged portion  91   a  adjacent to a portion  22   b  of the bars  22 . This portion  22   b  is connected at one end to the casing  2  and at the other end to the corresponding central portion  22   a  and co-operates therewith to form a concave shape facing outwards. This makes it possible to have the connection portion  22   b  directed substantially perpendicularly to the air flow direction.  
     [0134] There are two bars  22  per side opening  21  in the example described, however this number could naturally be different.  
     [0135] In its upper portion, the casing  2  has a first opening  24  as can be seen in FIG. 4, said opening serving to enable a brush carrier  30  (as shown diagrammatically in FIGS. 7 and 8) to be fixed inside the casing.  
     [0136] The casing  2  also includes, in its upper portion, a second opening  25  for co-operating with the opening  24  to allow cooling air to flow and to pass the electrical conductors of the stator.  
     [0137] Brush Carrier  
     [0138] In FIGS. 7 and 8, it can be seen that the brush carrier  30  comprises firstly a support portion  31  having conventional brushes  32  fixed thereon, for coming into contact via one end  32   a  with rotor slip rings  110 , and secondly a fixing portion  33  for engaging in a slideway  35  formed in the inside face of the transverse wall  14  of the casing  2 , the slideways  35  being vertical in the example shown. The brushes  32  are connected to connection tabs  32   b  for electrical conductors (not shown) for powering the rotor.  
     [0139] A groove  39  is formed between the support portion  31  and the fixing portion  33  to co-operate with rims  36  on the slideway  35 .  
     [0140] The fixing portion  33  has an extension  37  provided with a hole  38  for passing a lock screw that is engaged in the transverse wall  14 .  
     [0141] The brush carrier includes stiffening webs  31   a  and  33   a  associated respectively with the support portion  31  and with the fixing portion  33  and which extends substantially parallel to a plane containing the axis of rotation X and coinciding with the section plane of FIG. 2.  
     [0142] The support and fixing portions  31  and  33  in the example described are made as a one-piece molding of insulating plastics material.  
     [0143] The brush carrier  30  is fixed to the casing  2  by being initially inserted through the first opening  24  into the casing  11 , and then causing the fixing portion  33  to slide in the slideway  35 , after which the brush carrier  30  is prevented from moving axially along the slideway  35  by inserting a screw into the hole  38  and causing it to co-operate with the transverse wall  14 .  
     [0144] Rotor  
     [0145] The yoke  7  of the rotor  5  is laminated in the example described and comprises a stack of magnetic laminations  60  each having slots  61  in which the wires of windings  8  are engaged. The yoke  7  need not be laminated.  
     [0146] In the example described, the rotor  5  is of the projecting pole type.  
     [0147] FIGS.  9  to  11  show various shapes of rotor lamination that are to be found in prior art alternators, for a rotor having two pole portions. In these examples, each lamination has slots  50  for receiving the field windings and passages  51  for damper windings for attenuating the generation of harmonics.  
     [0148] It can be seen that the slots  50  are disposed symmetrically on either side of a midplane M containing the axis of rotation, and that each lamination can have, for example, eight slots as shown in FIG. 9, or four slots as shown in FIGS. 10 and 11.  
     [0149] It can also be seen that the passages  51  for the damper windings are circular in prior art alternators and disposed close to the radially outer edges of the pole portions in a manner that is symmetrical about the midplane M.  
     [0150] In the embodiment described, each magnetic lamination  60  is of the shape shown in FIG. 12 and presents axial symmetry about the axis X.  
     [0151] The slots  61  are disposed symmetrically on either side of a midplane M containing the axis X for receiving the field windings  8  (not shown in FIG. 12), which windings may be electrically connected to the slip ring  110  as in the example shown, with the above-described brushes  32  coming to bear against the slip rings.  
     [0152] Each lamination  60  defines two opposite pole portions  75 , each of which is provided with two pole horns  68 .  
     [0153] Spacers for Holding the Field Windings  
     [0154] The windings are held in the slots  61  by spacers  63 , one of which is shown in isolation in a perspective view in FIG. 13.  
     [0155] In the example described, each spacer  63  is made of insulating plastics material and presents, in a plane perpendicular to the axis X, a V-shape with two wings  64 . The free end  65  of each wing  64  bears against the inside edge  67   a  of a pole shoe  68  close to its free end.  
     [0156] It can be seen in the figures that each spacer  63  has two beads of material  100  projecting from the outside faces  69   c  of the wings  64  close to their free ends.  
     [0157] Each wing  64  presents a generally concave inside face  69   a  facing towards the corresponding pole portion  75 .  
     [0158] The base of each spacer  63  bears or nearly bears against the stack of rotor laminations  60 , possibly via an insulating sheet put into place in the slot  61  before the windings. A spacer  63  thus acts in a slot  61  to define two regions, and these regions receive windings associated respectively with the two pole portions  75 . The base of the spacer  63  presents a generally flat face  69   b  which takes up a position facing an edge  67   b  of the slot  61 , said edge  67   b  being generally outwardly convex. In FIG. 12, it can be seen that given the shape of the edge  67   b , the section offered for passing magnetic flux does not pass through a minimum in the plane P containing the axis X and perpendicular to the midplane M.  
     [0159] Given their shape, the two wings  64  of each spacer  63  leave between them and the stator a space  70  of non-negligible section which allows increased air flow between the rotor and the stator. This makes it easier to cool the rotor.  
     [0160] In the example described, the section S e  defined by the circularly cylindrical envelope E of the rotor touching the pole shoes  68  and the spacer  63  is about one-fourth greater than the section S i  of the slot on the inside of the spacer  63 . A plurality of spacers  63  following one another along the axis X may be associated with each slot  61 .  
     [0161] Disposition of the Damper Windings  
     [0162] Each lamination  60  has passages  71  for the damper windings.  
     [0163] In the example shown, these passages  71  are non-circular in shape.  
     [0164] More precisely, in the example shown, each passage  71  presents a cross-section that is oblong in shape, as can be seen in FIG. 12, having its long dimension oriented radially, with two long sides  72  that diverge radially outwards, these sides  72  being united at their axial ends by semicircular edges  73  and  74 . The semicircular edge  74  has a tip close to the radially outer edge of the associated pole portion  75 . When compared with a passage of circular section, the shape of the passages  71  and the radial orientation thereof facilitates the flow of magnetic flux from one pole portion  75  to the other.  
     [0165] In FIG. 12, it may be observed that on either side of the plane P, the space between the passages  71  is not regular. More particularly, in the example described, the spacing between the passages  71  increases on moving circumferentially in the clockwise direction about the axis X. In FIG. 12, the direction of rotation R of the rotor is clockwise, the rotor being seen from the end remote from its end fixed to the engine. This disposition of the passages  71  serves to attenuate harmonics more effectively. The spacing between the passages  71  is, in particular, different from the pitch of the teeth of the stator, and where the spacing is greater it makes it easier for flux to pass. The operation of the alternator under load is improved thereby.  
     [0166] The spacing between the passages  71  can be determined by a finite element calculation, for example so that the flux passing between the passages  71  does not exceed a predetermined value.  
     [0167] In the example described, the passages  71  are five in number on each side of the plane P. The passages  71  disposed on one side of the plane P in each lamination  60  constitute an image of the passages situated on the other side of the plane P, in axial symmetry about the axis X.  
     [0168] The damper windings may be made by injecting a conductive metal, e.g. aluminum, while under pressure and in the fluid state into the passages  71  of the stack of laminations  60  so as to form electrically-conductive bars  140 . The laminations  60  are superposed with a small amount of angular offset between one another such that each of the bars  140  follows a path in the form of a portion of a helix, in conventional manner.  
     [0169] The bars  140  associated with each pole portion  75  are electrically interconnected at each axial end of the rotor by electrical short circuiting connection strips  190  and  195 , also referred to as short-circuit “rings”.  
     [0170]FIG. 14 shows the connection strips  190  at the same end as the bearing  15  and FIG. 15 shows the connection strips  195  at the same end as the fan  9 .  
     [0171] In these figures, it can be seen that each strip  190  or  195  electrically interconnects two bars  140  situated on one side of the midplane P with three bars  140  situated on the opposite side of said plane P.  
     [0172] For each end face, the two strips  190  or  195  are thus disconnected from each other and are of a shape that enables them to go round the shaft of the rotor.  
     [0173] Fixing the Alternator to the Engine  
     [0174] In the example described, the alternator is fixed to the engine by means of a connection member  120  which is shown on its own in FIGS. 16 and 17.  
     [0175] This connection member  120  comprises a plate  130  pierced by a central opening  121  for passing the end of the rotor shaft that is to be coupled with the engine shaft.  
     [0176] The plate  130  has a first set of holes  122  designed to enable it to be fixed to the engine, and a second set of holes  123  for fixing to the flange  13  of the alternator. In the example shown, nuts  125  are welded to the plate  120  in register with respective holes  123  on the side that is to face the engine.  
     [0177]FIG. 17 shows that the plate  120  is not plane, and that it may include stiffening ribs  131 . The plate  120  presents a first plane surface  126  in the vicinity of its outer periphery, for coming to bear against the flange  13  of the casing  2 , and a second plane surface  127  around the opening  121  for bearing against the engine. The two surfaces  126  and  127  are axially offset by a distance which is greater than the thickness of the nuts  125 .  
     [0178] Projecting portions  128  are designed to co-operate with the casing, for example by being welded to the plate  130 , so as to contribute to centering the connection member on the alternator.  
     [0179] In order to fix the alternator to the engine, the plate  130  is initially fixed thereto by inserting fasteners such as screws in the holes  122 , and then the alternator can be fixed to the plate  130  by inserting fasteners such as screws into the passages  40  and the holes  123 .  
     [0180] It should be observed that by placing the passages  40  outside the envelope of the cylindrical body  12 , it is very easy to insert the fasteners for holding the alternator casing to the connection member  120 . The fasteners may be tightened, e.g. by means of a wrench, without it being necessary to insert the wrench through an outlet opening for cooling air, as is the case in prior art alternators.  
     [0181] Naturally, the invention is not limited to the embodiment described above.  
     [0182] In particular, without going beyond the ambit of the present invention, it is possible to make casing differently.  
     [0183] Throughout the description, including in the claims, the term “comprising a” should be understood as being synonymous with “comprising at least one” unless specified to the contrary.