Rotor of a reluctance synchronous motor, with axial lamination magnetic poles

Abstract

A rotor (3) of a synchronous reluctance motor (1), with axially laminated magnetic poles (4), is provided with a support core (10) having a plurality of recesses (17) along its outer periphery, so as to house respective sheet stacks (19) defining the magnetic poles (4); the rotor is also provided with a respective retention bar (30) for each recess (17); these retention bars (30) are parallel to the axis (6) of the rotor (3) and are fixed to the support core (10) by threaded stems (34), which radially clamp the sheet stacks (19) between the retention bars (30) and the support core (10); the rotor (3) has two head rings (40), which are arranged at opposite ends of the rotor (3), coaxially to the support core (10), and are coupled directly to axial ends of the retention bars (30) in such a way as to constrain the latter with respect to the head rings (40) in a fixed position.

Description

[0001] "ROTOR OF A RELUCTANCE SYNCHRONOUS MOTOR, WITH AXIAL LAMINATION MAGNETIC POLES"

[0002] Cross-Reference To Related Applications

[0003] This patent application claims priority from Italian patent application no . 102024000007135 filed on March 29 , 2024 , the entire disclosure of which is incorporated herein by reference .

[0004] Technical field

[0005] The present invention relates to a rotor of a synchronous reluctance motor, with axial lamination magnetic poles .

[0006] Background

[0007] As is known, reluctance electric motors are synchronous motors that are sometimes used as an alternative to permanent magnet motors and asynchronous motors , as they have some advantages , essentially due to the absence of windings and, actually, permanent magnets in the rotor . In particular, reluctance motors are relatively simple and therefore cheap and have a relatively high ef ficiency, i . e . low losses , especially in the rotor, so they are subj ect to relatively low heating and, therefore , temperatures ( and therefore do not need special cooling systems ) .

[0008] In recent years , some solutions with relatively high power have been presented on the market , and solutions suitable for use as electric traction motors in the railway or tramway field, and not only for industrial applications .

[0009] In general , there are two categories of rotors for reluctance motors , i . e . rotors with transversely laminated poles ( i . e . with magnetic poles formed by sheet metal stacks of magnetic material , laminated according to planes orthogonal to the axis of the motor ) , and rotors with axially laminated poles ( i . e . with magnetic poles formed by sheet metal stacks of magnetic material , laminated according to surfaces parallel to the axis of the motor ) .

[0010] In order to achieve relatively high powers and drive torques , it is preferable to adopt this second category of rotors , axially laminated, even i f it has greater construction di f ficulties . In fact , the torque density and the goodness factor of a reluctance motor are dependent on the saliency ratio ( defined by the ratio Ldm / Lqm, between the direct axis magneti zing inductance Ldm and the quadrature axis magneti zing inductance Lqm) : for transversely laminated rotors , this ratio typically reaches values between 3 and 10 , with the highest values reached by multiple flux barrier (MFB ) synchronous reluctance motors , while it reaches decidedly higher values ( even around 30 ) for reluctance synchronous motors provided with axially laminated rotors .

[0011] In addition, the power factor of the synchronous reluctance motor, with axially laminated anisotropic rotor, is considerably better than the one of the synchronous reluctance motor with multiple flux barrier rotor . In fact , the variable reluctance synchronous motors with axially laminated rotor can achieve a higher power factor (with a di f ference of about or even more than 80% , for peak continuous torque in railway traction applications ) . A higher power factor allows the motor to operate at a lower current for a given torque and given speed, thereby reducing losses in the motor copper and conduction and switching losses in the inverter, and a lower apparent rated power, which results in a lower cost of the inverter .

[0012] It is therefore felt the need to adopt constructive measures that allow to obtain a relatively simple axially laminated rotor . In this regard, the known solutions have a non-magnetic core , usually indicated with the term " spider" , which is shaped like a spider or a star so as to define a series of recesses along its outer periphery for housing respective magnetic poles . The latter, as mentioned above , consist of sheet metal stacks or sheets of magnetic material , alternated with non-magnetic material , and held in a fixed position against the surface of the aforementioned recesses by radial screws , which are also made of non-magnetic material and pass through the same sheet metals .

[0013] A solution of this type , for example , is shown in US4459502A, which corresponds to the preamble of claim 1 . In this document , the head of the screws is carried by longitudinal inserts , which are parallel to the axi s of the rotor, are arranged along the outer periphery of the aforementioned recesses , and have a wedge-shaped cross section . In practice , each sheet metal stack remains radially retained between the corresponding longitudinal insert and the spider core , thanks to the clamping of the screws .

[0014] In this kind of solutions , the need is felt to retain the sheet metal stacks in a fixed position against centri fugal actions , even when the motor operates at relatively high rotational speeds ( for example between 4000 and 6000 rpm for larger diameter motors ) . In this regard, the radial screws alone are not enough .

[0015] To improve radial retention, one of the solutions of US4459502A shows rings that are mounted around the periphery of the rotor, so as to be housed in respective circular grooves formed along said periphery . However, a solution of this type is unsatis factory, as it increases the number of components and complicates the construction of the rotor .

[0016] Aim of the present invention is , therefore , to improve the known solutions described above , in such a way as to resist centri fugal actions and, at the same time , to obtain a rotor that is relatively light and / or has a relatively low number of components ; preferably, a further aim of the present invention is to obtain a robust rotor that can be manufactured by a simple and economical process .

[0017] Summary of the invention

[0018] According to the present invention, a rotor of a synchronous reluctance motor, with axially laminated magnetic poles , as defined in claim 1 is provided . The present invention, moreover, relates to a reluctance synchronous motor according to claim 11 , and to a process for making the aforementioned rotor, as defined in claim 12 . The dependent claims relate to pre ferred embodiments of the present invention .

[0019] Brief description of the drawings

[0020] To better understand the present invention preferred embodiments thereof will be now described, for merely exemplary and non-limiting purposes , with reference to the appended drawings , wherein :

[0021] Figure 1 is a perspective view of a preferred embodiment of the rotor of a synchronous reluctance motor, with axially laminated magnetic poles , according to the present invention;

[0022] Figure 2 is an exploded view showing some parts of the rotor of Figure 1 ;

[0023] Figure 3 is a cross-sectional view, on enlarged scale , of the rotor of Figures 1 and 2 ;

[0024] Figure 4 is a split perspective view showing a variant of the rotor of Figures 1-3 ;

[0025] Figure 5 is analogous to Figure 1 and shows , with parts removed for the sake of clarity, a further embodiment of the rotor of a synchronous reluctance motor according to the present invention; and

[0026] Figures 6a and 6b schematically show two steps of the process for making the rotor of the present invention .

[0027] Detailed description of preferred embodiments

[0028] In Figure 1 , reference numeral 1 denotes a synchronous reluctance motor, shown in a schematic and simpli fied way . The motor 1 comprises a stator 2 , shown schematically in dotted line , and a rotor 3 , which is housed in the stator 2 and is provided with a plurality of magnetic poles 4 ( for example , four or six poles ) , made of axially laminated magnetic material .

[0029] The motor 1 further comprises a shaft 5 , which extends along an axis 6 coaxially to the stator 2 and the rotor 3 , supports the rotor 3 and, together with the latter, is rotatable about the axis 6 with respect to the stator 2 .

[0030] With reference to Figure 3 , the rotor 3 comprises a support core 10 , made of non-magnetic material , for example brass , keyed in a fixed position on a portion 11 of the shaft 5 . In particular, the portion 11 is defined by an intermediate segment of the shaft 5 , having externally a cylindrical surface 12 . As shown in Figure 1 , the shaft 5 further comprises two portions 13 and 14 , which protrude along the axis 6 with respect to the core 10 in opposite directions to each other . Preferably, the shaft 5 consists of a single piece , whereby the portions 13 and 14 extend seamlessly with respect to the portion 11 . According to a variant that is not shown, the shaft 5 has a single protruding portion ( 13 or 14 ) , at one of the two axial ends of the rotor 3 .

[0031] Still with reference to Figure 3 , the core 10 has an axial hole , defined by an inner cylindrical surface 16 which, in particular, is fixed by interference fitting on the cylindrical surface 12 of the portion 11 . This interference fitting is achieved during the manufacturing process of the motor 1 , by heating the core 10 , by a subsequent insertion of the portion 11 into the axial hole of the core 10 , and finally by cooling so as to achieve a radial forcing on the cylindrical surface 12 .

[0032] Outwardly, the core 10 is shaped like a spider or a star, in such a way to have a plurality of recesses 17 along its periphery, equally-spaced about the axis 6 and corresponding, each, to a respective pole 4 . The recesses 17 are defined by respective surfaces 18 of the core 10 , which are parallel to the axis 6 and have , for example , a U-shape or V-shape in cross-section .

[0033] Each pole 4 comprises at least one stack 19 of sheets , which are laminated parallel to the axis 6 as mentioned above . Each sheet stack 19 consists of sheets of magnetic material 20 ( for example defined by magnetic steel sheet metals , grain-oriented) and sheets of non-magnetic material 21 ( for example defined by polyester or Teflon films ) ; the sheets 20 and 21 are alternated with each other, are parallel to the surface 18 of the corresponding recess 17 and rest on each other . Preferably, they are glued together, according to techniques known in the field of stator and rotor stack production .

[0034] As can be seen in Figure 2 , in the preferred embodiment shown the rotor 3 comprises a plurality of modules , for example three , indicated by 3a, 3b and 3c, distinguished and arranged in positions aligned with each other along the axis 6 . The core 10 comprises a plurality of sectors 10a, 10b and 10c, one for each of the modules 3a, 3b and 3c . Each sector 10a, 10b and 10c supports , for each pole 4 , a respective sheet stack 19a, 19b, 19c .

[0035] In particular, the modules 3a, 3b and 3c can be axially separated by intermediate plates 22 , made of non-magnetic material , which are mechanically coupled to the adj acent sectors 10a, 10b, 10c ( for example by pins and / or welding) .

[0036] The recesses 17 of the sectors 10a, 10b and 10c are axially aligned with each other . According to a variant that is not shown, the solution of Figure 2 is devoid of the intermediate plates 22 .

[0037] The intermediate plates 22 are also provided in the embodiment of Figure 5 : here , as will be better described below, the sectors 10a, 10b and 10c are of fset with respect to each other about the axis 6 with a predefined angular pitch, so that the recesses 17 of each said sector 10a, 10b, 10c are not aligned with the recesses 17 of the adj acent sectors . This solution of Figure 5 allows to achieve better performance in terms of torque ripple , at the price of reducing the average value of the electromagnetic torque by 3-4 % .

[0038] In a variant that is not shown, the reduction of the torque ripple and losses in the rotor is achieved without of fsetting the sectors 10a, 10b , 10c, but thanks to a stator provided with fractional slot winding, designed so as to reduce or cancel the subharmonics of the air gap induction produced by the same stator . In this way, the torque oscillation and the losses in the rotor due to the subharmonics are simultaneously reduced, without a signi ficant reduction in the average value of the electromagnetic torque .

[0039] The following discussion will refer to a single sheet stack 19 and a single recess 17 , for the sake of simplicity, taking into account that the configuration is the same for all .

[0040] As shown in Figure 3 , the sheet stack 19 has at least one radial through hole 23 , which is radially aligned with a threaded hole 24 , made in the core 10 starting from the surface 18 ; in particular, the sheet stack 19 has two radial through holes 23 , which are parallel , are axially spaced from each other, and are aligned with respective threaded holes 24 of the core 10 . Preferably, the threaded holes 24 are through holes , so that they end at the inner cylindrical surface 16 .

[0041] The recess 17 also houses a retention bar 30 , in nonmagnetic material ( for example in non-magnetic steel ) , which is parallel to the axis 6 and is arranged along the periphery of the rotor 3 and, in a circumferential direction, in the centre of the recess 17 , in such a way that the sheet stack 19 remains interposed ( in a radial direction and in a tangential direction) between the retention bar 30 itsel f and the surface 18 .

[0042] The retention bar 30 has a teardrop-shaped or wedge- shaped cross-section, for example . Speci fically, the retention bar 30 is radially delimited by : an outer surface 31 , defining part of an outer cylindrical surface 32 of the rotor 3 (preferably, without any other element provided around the outer surface 31 ) ; and an inner surface 33 , substantially parallel to the surface 18 and resting against the sheet stack 19 .

[0043] The retention bar 30 retains the sheet stack 19 against the surface 18 under the action of at least one threaded stem 34 , for example two threaded stems 34 . The latter are made of non-magnetic material ( e . g . non-magnetic steel ) , are supported by the retention bar 30 , engage the radial through holes 23 , and are screwed into the threaded holes 24 so as to radially clamp in a sandwich-like manner the sheet stack 19 .

[0044] The threaded stems 34 , preferably, define part of respective screws or bolts 36 , each having a head 37 which is preferably completely housed in a seat 38 made along the outer surface 31 of the retention bar 30 , in such a way as not to protrude beyond the outer cylindrical surface 32 of the rotor 3 .

[0045] According to a variant that is not shown, the threaded stems 34 constitute part of respective stud screws , which are carried by the core 10 and protrude radially into the recesses 17 to engage the axial through holes 23 , while the heads 37 are replaced by threaded nuts screwed onto the outer ends of such stud screws .

[0046] Preferably, the inner ends of the threaded stems 34 do not protrude radially beyond the inner cylindrical surface 16 . In other words , the cylindrical surface 12 of the shaft 5 is free of holes for housing and / or fixing said ends ; therefore , the threaded stems 34 are only fixed to the core 10 . A coupling system other than the threaded stems 34 ( e . g . , the interference coupling mentioned above ) will be provided to fix the core 10 to the portion 11 of the shaft 5 . This avoids weakening the shaft 5 from a structural point of view .

[0047] With reference to Figures 1 and 2 , the rotor 3 further comprises two head rings 40 , in non-magnetic material ( for example in non-magnetic steel ) , preferably defined by annular plates orthogonal to the axis 6 , and arranged at the opposite axial ends of the rotor 3 , coaxially to the core 10 .

[0048] According to the present invention, the head rings 40 are coupled directly to the axial ends of the retention bars 30 so as to be constrained to the latter in fixed radial positions and, therefore , form overall a cage 41 ( Figure 1 ) , which houses and retains the poles 4 formed by the sheet stacks 19 . In this way, the head rings 40 constrain the retention bars 30 to each other ( in addition to the constraint already defined by the threaded stems 34 screwed into the core 10 ) , thereby helping to keep the retention bars 30 ( and thus the sheet stacks 19 ) in a fixed position against centri fugal actions caused by rotations of the rotor 3 at relatively high speeds .

[0049] Preferably, the coupling between the axial ends of the retention bars 30 and the head rings 40 is defined by welds 42 ( schematically shown by a corresponding line in Figure 1 ) •

[0050] According to a variant that is not shown, the coupling between the axial ends of the retention bars 30 and the head rings 40 could be defined by screws parallel to the axis 6 , screwed into said ends ( this solution however has the drawback of having to provide a cross-section with a relevant area for the retention bars 30 , in order to be able to screw said screws ) . According to a further variant , not shown, the aforementioned coupling could be defined by j oints configured so as to radially retain the retention bars 30 in fixed positions with respect to the head rings 40 . According to a variant that is not shown, the head rings 40 can also be coupled to the core 10 ( for example through the use of pins ) , as well as to the axial ends of the retention bars 30 .

[0051] Preferably, the inner diameter of the head rings 40 is greater than or equal to the central hole of the core 10 , in such a way as not to interfere with the shaft 5 during the coupling of the rotor 3 to the same shaft 5 . This diameter, as well as the axial thicknesses of the head rings 40 and the intermediate plates 22 , are then established by design based on the di f ferent applications .

[0052] As can be seen in Figures 1 and 2 , preferably, the retention bars 30 have an axial length such as to engage all the modules 3a, 3b and 3c, i . e . they extend from one head ring 40 to the other .

[0053] According to an alternative , not shown, in each module 3a, 3b, 3c, each recess 17 has a respective retention bar 30 , axially separated from those of the adj acent modules . In this case , in each module 3a, 3b and 3c, each retention bar 30 has an axial length equal to that of the corresponding recess 17 , and the axial ends of the retention bars 30 are fixed (preferably welded) to the intermediate plates 22 , and not only to the head rings 40 .

[0054] This latter option (with separate retention bars 30 for each module 3a, 3b, 3c ) is necessary in the embodiment of Figure 5 , where for the sake of s implicity the same reference numbers used in the previous Figures have been used . As mentioned above , in this embodiment the sectors 10a, 10b and 10c of the core 10 are angularly of fset from each other ( for example to limit the ripples , of the driving torque provided by the motor 1 ) . Obviously, even i f not shown, the retention bars 30 of each module 3a, 3b, 3c are also of fset angularly with respect to the retention bars 30 of the adj acent modules .

[0055] In the variant of Figure 4 , each threaded stem 34 coaxially engages a respective sleeve 43 , which is clamped ( in a radial direction with respect to the axis 6 ) between the inner surface 33 of the retention bar 30 and the surface 18 of the recess 17 , in such a way as to define a spacer of predetermined length between the core 10 and the retention bar 30 itsel f . This predetermined length precisely defines the space radially dedicated to the sheet stack 19 during the process of construction and / or assembly of the rotor 3 , and therefore the clamping action acting on the same sheet stack 19 .

[0056] With regard to this process , the rotor 3 is made by cutting sheets of magnetic material and sheets of nonmagnetic material , of flat shape , indicated by 20a and 21a in Figure 6a, in such a way as to have a first dimension LI always equal , equal to the axial length of the recesses 17 in the core 10 , and a second dimension L2 which is variable . The sheets 20a and 21a are then stacked, in alternating positions with each other and in such a way as to form a stack 19d in which the dimension L2 is progressively increasing, so that the stack 19d substantially has a truncated pyramid shape . As mentioned above , preferably the sheets 20a and 21a are glued together . Before or after forming the stack 19a, the sheets 20a and 21a are drilled to form the radial through holes 23 , described above .

[0057] The stack 19d is then subj ected to a bending operation, by means of an appropriate mould (not shown) , to deform the sheets 20a and obtain a stack 19e ( Figure 6b ) having a curvature compatible with the shape of the surface 18 of the recesses 17 .

[0058] After bending, the stacks 19e are arranged in the recesses 17 , between the corresponding surfaces 18 and the retention bars 30 , and then they are locked in this position by inserting and screwing the bolts 36 . After this clamping, the retention bars 30 retain the stacks 19e in a fixed position against the core 10 .

[0059] Regarding the variant of Figure 4 , the spacers defined by the sleeves 43 are arranged in the radial through holes 23 before coupling the retention bars 30 on the stacks 19e , and therefore before inserting and screwing the bolts 36 .

[0060] In the embodiments where several modules ( 3a, 3b and 3c ) are provided, before inserting the stacks 19e into the recesses 17 , the various sectors 10a, 10b and 10c of the core 10 and the intermediate plates 22 are assembled, by coupling these components in fixed relative positions ( for example by means of reference grooves , not shown in the attached drawings , made on the shaft 5 and on the sectors of the core 10 for insertion in the intended position) .

[0061] The stack 19e has two opposite ends , which are indicated by reference 44 in Figure 6b and, after clamping of the bolts 36 , overhang radially beyond the retention bars 30 outside the recess 17 ( in a way not shown) , as the dimension L2 of the sheets 20a and 21a is established by design in such a way as to be wider than necessary to occupy said recess 17 . A material removal machining is then carried out , for example by a lathe , to remove the ends 44 and accurately obtain the outer surface 32 of the rotor 3 .

[0062] Finally, the head rings 40 are mounted, fixing them to the axial ends of the retention bars 30 , so as to define the cage 41 that retains the poles 4 in the recesses 17 . Fixing of the head rings 40 to the axial ends of the retention bars 30 may be performed before or after removing the ends 40 , i . e . before or after machining the outer surface 32 of the rotor 3 .

[0063] In this regard, the threaded stems 34 are suf ficient to maintain the sheet metal stacks in a fixed relative position, both during mounting of the head rings 40 and the intermediate plates 22 , and during the turning operation .

[0064] At this point it is possible to mount the rotor 3 on the shaft 5 , for example by interference coupling already described above .

[0065] From the above , it is evident that the rotor 3 has a small number of components and requires a manuf acturing / assembly process that is relatively simple . At the same time , the cage 41 allows to ef fectively contain the centri fugal actions due , in use , to rotations with relatively high speeds .

[0066] In particular, fixing by means of the welds 42 is relatively simple , and can be performed by common equipment , available in production plants .

[0067] Furthermore , in the motor 1 the rotating parts have low centring tolerances , mainly thanks to the machining carried out to form the outer surface 32 of the rotor 3 , without adding other components around the retention bars 30 and the poles 4 ; thanks to the keying of the core 10 on the shaft 5 , which is in one piece ; and thanks to the precise positioning of the stacks 19 and the retention bars 30 , achieved by means of the possible sleeves 43 .

[0068] Other advantages are then evident to a person skilled in the art based on what is described above in detail . Finally, it is clear that modi fications and variants can be made to the rotor 3 and motor 1 described herein with reference to the attached figures , without thereby departing from the protective scope of the present invention, as defined in the claims below .

[0069] In particular, the rotor 3 may comprise a number of modules di f ferent from what is shown in the attached figures , for example a single module (with the core 10 consi sting of a single sector, therefore without intermediate plates 22 ) . Furthermore , the head rings 40 could have a di f ferent structure or shape from the plates shown by way o f example . Finally, the spacers between the core 10 and the retention bars 30 could be defined by elements other than the sleeves 43 ( and, for example , be housed in additional radial holes with respect to the radial through holes 23 ) .

Claims

CLAIMS1.- Rotor (3) of a synchronous reluctance motor (1) , with axially laminated magnetic poles (4) , the rotor having an axis (6) and comprising:- a support core (10) having a plurality of recesses (17) along its outer periphery, said recesses (17) housing respective sheet stacks (19) defining said magnetic poles (4) ;- a respective retention bar (30) for each said recess (17) ; said retention bars (30) being parallel to said axis (6) and being fixed to said support core (10) by threaded stems (34) , which extend radially with respect to said axis (6) , in such a way as to clamp each said sheet stack (19) between said support core (10) and the corresponding retention bar (30) ;- two head rings (40) , arranged at opposite ends of the rotor (3) and coaxially to said support core (10) ; characterized in that said retention bars (30) have axial ends that are coupled directly to said head rings (40) in such a way that they are constrained with respect to said head rings (40) in a fixed radial position.2.- The rotor according to claim 1, wherein said axial ends are welded to said head rings (40) .3.- The rotor according to claim 1 or 2, wherein said support core (10) has an axial hole, and said head rings (40) have an inner diameter that is larger than the diameter of said axial hole.4.- The rotor according to any one of the preceding claims, wherein said support core (10) has an axial hole, and said threaded stems (34) have respective ends that are completely housed in said support core (10) in such a waythat they do not protrude radially into said axial hole.5.- The rotor according to any one of the preceding claims, wherein said retention bars (30) have an outer surface (31) defining part of an outer cylindrical surface (32) of the rotor (3) , without further components arranged around said retention bars (30) .6.- The rotor according to any one of the preceding claims, wherein spacers with predefined length (43) are arranged radially between said support core (10) and said retention bars (30) .7.- The rotor according to any one of the preceding claims, wherein said support core (10) consists of a plurality of sectors (10a, 10b, 10c) , which are distinct from one another, are arranged along said axis (6) and have, each, a respective plurality of said recesses (17) ; each said recess (17) housing a respective one of said sheet stacks (19) .8.- The rotor according to claim 7, wherein said sectors (10a, 10b, 10c) are axially separated from each other by an intermediate plate (22) .9.- The rotor according to claim 8, wherein said retention bars (30) are fixed to said intermediate plates (22) .10.- The rotor according to any one of claims 7 to 9, wherein each said sector ( 10a, 10b, 10c) is offset about said axis (6) by a predetermined pitch with respect to the adjacent sectors.11.- Synchronous reluctance motor (1) comprising a stator (2) , a shaft (5) , and a rotor (3) , which is keyed in a fixed position on a first portion (11) of said shaft (5) and is defined according to any one of the preceding claims; wherein said shaft (5) comprises a second portion (13,14) , whichprotrudes axially with respect to said rotor (3) and extends coaxially and seamlessly with respect to said first portion (11) •12.- Process for making a rotor (3) of a synchronous reluctance motor (1) , with axially laminated magnetic poles, the rotor being defined according to any one of claims 1 to 10, the process comprising the steps of:- making a plurality of sheet stacks (19e) , each defined by an alternation of magnetic sheets (20a) and non-magnetic sheets (21a) ;- arranging each one of said sheet stacks (19e) in a corresponding one of said recesses (17) ;- clamping each one of said sheet stacks (19e) in the corresponding recess (10) , between said support core (17) and the corresponding retention bar (6) , by said threaded stems ( 34 ) ;- coupling the axial ends of said retention bars (30) directly to said head rings (40) in such a way as to constrain said axial ends with respect to said head rings (40) in a fixed radial position, after clamping said sheet stacks ( 19e ) .13.- The process according to claim 12, wherein said sheet stacks (19e) , after said clamping, have excess material protruding outside the corresponding recesses (17) , and wherein the process comprises lathing an outer cylindrical surface (32) of said rotor (3) to remove said excess material .14.- The process according to claim 12 or 13, wherein spacers with predefined length (43) are placed radially between each said retention bar (30) and said support core (10) before clamping said sheet stacks (19e) .15.- The process according to any one of claims 12 to 14, wherein each said sheet stack (19e) is made by:- cutting said magnetic sheets (20a) and said non-magnetic sheets (21a) according to predefined dimensions and in a flat shape;- stacking said magnetic sheets (20a) and non-magnetic sheets (21a) in alternating positions, so as to form a stack (19d) having a progressively increasing dimension (L2) ; - bending said stack (19d) in such a way as to achieve a curvature according to the shape of said recesses (17) .

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