Abstract:
A method for obtaining, in a quick way, a strong, compact and waterproof permanent-magnet rotor ( 3 ), intended for a synchronous motor ( 1 ), particularly for pumps of washing machines for industrial and domestic use and the like, of the external stator ( 2 ) type, comprising a cylindrical hollow core ( 6 ) surrounded by a plurality of permanent magnets ( 10 ) comprising the steps of arranging a cup-like body ( 7 ) with a base end ( 8   a ), a free end ( 8   b ) and a side wall ( 9 ) exhibiting a plurality of passing longitudinal recesses ( 12 ) which define, between the same, a plurality of positioning seats (13) for the magnets ( 10 ), providing to insert the core ( 6 ) into the cup-like body ( 7 ) and arrange the magnets ( 10 ) in said seats ( 13 ) and injecting a plastic material with obtainment of a cage-like structure ( 50 ) having opposite bottoms ( 27, 28 ) abutting at the ends ( 8   a,    8   b ) of the cup-like body ( 7 ) as well as columns ( 29 ) extended between the opposite bottoms ( 27, 28 ) housed in the recesses ( 12 ).  
     The invention also relates to a rotor ( 3 ) obtained by means of the present method.

Description:
FIELD OF APPLICATION  
       [0001]     The present invention relates to a method for obtaining a permanent-magnet rotor for a synchronous motor, particularly for a pump of a washing machine for industrial, domestic and similar use.  
         [0002]     More in particular, the present invention relates to a rotor comprising a cylindrical central core, with axis X-X and an axial passage, surrounded by a plurality of permanent magnets, having an extrados, an intrados and side edges.  
         [0003]     The invention also relates to a rotor obtained according to the present method.  
       PRIOR ART  
       [0004]     In the specific technical field of permanent-magnet synchronous motors with rotor inside the stator, using rotors wherein magnets are associated with a central core by means of, for example, an adhesive is known.  
         [0005]     These rotors, during the operation of the motor, are subjected to a high centrifugal force with the consequence that the magnets tend to detach from the core.  
         [0006]     A possible known solution is that of inserting the core with the magnets being glued, into a sleeve which allows to contain the magnets during the motor operation.  
         [0007]     Usually the sleeve is realised in metallic material, generally stainless steel sheet, of minimal thickness so as to have the greatest structural resistance and to maintain a magnetic gap, i.e. a distance between stator and rotor, of the lowest value possible.  
         [0008]     The drawback this solution however exhibits is that the sleeve in metallic material in the presence of the magnetic field induced between rotor and stator, causes induced currents with a subsequent dissipation of energy. Moreover, as a consequence of the heating of the permanent magnets, due to the parasite currents, there is a reduction of the magnetic induction of the rotor field.  
         [0009]     In the case of rotors immersed in a fluid, realising a system of protection of the rotor is also known, so as to avoid that the core and the magnets can be deteriorated through corrosion. In fact, it had been suggested incorporating the components of the rotor for example with co-moulding techniques.  
         [0010]     Although being advantageous, the co-moulding is not exempt from drawbacks, in fact, the presence of rotor components, realised in metallic and massive ferromagnetic material, requires a previous heating step before the insertion of the rotor components into the mould to prevent, in the successive solidification and/or crystallisation process of the plastic material used, thermal discontinuity areas from being formed, such areas altering and jeopardising the structural resistance of the rotor especially with the high rotation speeds being usual during the pump operation.  
         [0011]     In fact, during the operation of the pump, the high centrifugal forces applied to the rotor magnets with possible faults induced into the material by a non conform co-moulding process, determine an unbalance of the rotor with greater wears of the supports and possible breakages of the rotor itself, as well as a noise, both mechanical and magnetic, with a subsequent annoying acoustic emission.  
         [0012]     The pre-heating step of the rotor components implies further drawbacks during a productive process in automation. These drawbacks are linked to the pre-heating time of the components themselves and they are not technically easy to be solved and they imply an increase of the rotor final costs due to the negative incidence of this latter step of the productive process.  
         [0013]     A further drawback of the rotors, realised according to the known methods, is linked to the fact that the magnets, associated with the core, can, due to the flow of the plastic material under high pressure and temperature during the co-moulding, move, causing a loss of the electromechanical performances of the rotor and a mechanical unbalance of the same.  
         [0014]     It is good also to note that the co-moulding defines material thicknesses which imply an increase of the rotor volume and a subsequent reduction of the magnetic gap, with a decrease of the torque generated and a subsequent loss of performances of the motor.  
         [0015]     The technical problem underlying the present invention is that of devising a simple and quick method to manufacture a particularly compact permanent-magnet rotor, which allows to overcome the drawbacks cited with reference to the prior art.  
       SUMMARY OF THE INVENTION  
       [0016]     The solution idea underlying the present invention is that of positioning the magnets and the core and of maintaining them positioned, according to the final desired position, with a co-moulding which minimises the volume of the rotor maximising the structural resistance and the reliability of the rotor during the high speed operation.  
         [0017]     On the basis of this solution idea, the technical problem is solved by a method to obtain a permanent-magnet rotor ( 3 ), for a synchronous motor ( 1 ), particularly for a pump of a washing machine for industrial and domestic use and the like, with external stator ( 2 ), comprising a cylindrical central core ( 6 ), with axis X-X and an axial passage ( 11 ), surrounded by a plurality of permanent magnets ( 10 ) having an extrados ( 16 ) and an intrados ( 17 ) and side edges ( 18 , 19 ), characterised in that it comprises the following steps: 
    arranging a cup-like body ( 7 ), in a plastic material, comprising a base end ( 8   a ), an open end ( 8   b ), a side wall ( 9 ) and a plurality of passing longitudinal recesses ( 12 ) formed in the side wall ( 9 ) and defining positioning seats ( 13 ) for a positioning of the magnets ( 10 );     inserting the central core ( 6 ) in the cup-like body ( 7 ) and arranging, in correspondence with the positioning seats ( 13 ), the magnets ( 10 );     obtaining, by injection of a plastic material, a cage-like structure ( 50 ) having opposite bottoms ( 27 , 28 ) and columns ( 29 ) extended between said opposite bottoms ( 27 ,  28 ), wherein said opposite bottoms ( 27 , 28 ) abut at the ends of the cup-like body ( 7 ) and the columns ( 29 ) are housed in the recesses ( 12 ).    
 
         [0021]     The invention also relates to a permanent-magnet rotor for a synchronous motor as defined by claim  6 .  
         [0022]     Further characteristics and the advantages of the method and of the rotor obtained according to the invention will be apparent from the following description of a preferred embodiment thereof given by way of indicative and non limiting example with reference to the annexed drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     In the drawings:  
         [0024]      FIG. 1  schematically shows a section view of an electrical pump for washing machines and similar household appliances, incorporating a rotor realised according to the present invention;  
         [0025]      FIG. 2  shows a partially sectioned view from above of a rotor realised according to the present invention;  
         [0026]      FIG. 3  shows a view of the rotor of  FIG. 2  realised along the section lines I-I;  
         [0027]      FIG. 4  schematically shows a view ideally made with detached parts, of the rotor of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0028]     With reference to the annexed figures, a fluid circulation electrical pump is globally and schematically shown with  100  comprising a synchronous electric motor  1 , wherein an external stator  2  is centrally crossed by a rotor  3 .  
         [0029]     As shown in  FIG. 1 , the permanent-magnet rotor  3  is keyed on a driving shaft  4  which brings in rotation at one end a plurality of kinematically connected impellers  5 . In the embodiment shown, the impellers  5  are three.  
         [0030]     The rotor  3  is induced in rotation by the electromagnetic field generated by the stator  2 , equipped with pole pieces with relative windings, and it comprises a central core  6 , realised like a lamination package, externally surrounded by a plurality of permanent magnets  10 .  
         [0031]     As shown in  FIGS. 2 and 3 , the central core  6  exhibits a cylindrical shape, of axis X-X, with an axial passage  11  suitable to allow the crossing of the driving shaft  4 .  
         [0032]     In the present embodiment, the rotor  3  exhibits four permanent magnets  10 .  
         [0033]     Each magnet  10 , substantially “tile-like” shaped as shown in  FIG. 4 , exhibits an extrados  16 , an intrados  17  and side edges  18 ,  19 .  
         [0034]     Advantageously, according to the present invention, the rotor  3  exhibits a cup-like body  7 , realised in a suitable plastic material, comprising a base end  8   a,  substantially circular, an opposite open end  8   b  and a side wall  9  exhibiting a plurality of passing longitudinal recesses  12 .  
         [0035]     Suitably, the symmetrically arranged recesses  12  define, between each other and inside the cup-like body  7 , a plurality of positioning seats  13  suitable to house the magnets  10 .  
         [0036]     Conveniently, the recesses  12  exhibit, along a section transversal to the axis X-X, as shown in  FIG. 2 , a substantially isosceles trapezium-like shape, comprising a base  40  connected to the side wall  9  by a pair of sides  21 .  
         [0037]     Advantageously, the core  6  is realised with a plurality of longitudinal recesses  15  suitable to house the intrados  17  of each magnet  10 .  
         [0038]     Suitably, each recess  15  exhibits, in a section transversal to the axis X-X, a substantially trapezium-like shape, with lateral containment edges  23 .  
         [0039]     According to the present invention, each of the side edges  18 , 19  of each magnet  10  comprises a first bevel  20 , next to the extrados  16 , a second bevel  22 , next to the intrados  17 , connected by a bulkhead  24 .  
         [0040]     In particular, each side edge  18 ,  19  exhibits the first bevel  20  having dimensions substantially greater than the second bevel  22 .  
         [0041]     In practice, each side  21 , of each recess  12 , is conjugated to a first bevel  20  of the magnets  10  and each lateral containment edge  23 , of each recess  15  is conjugated to a second bevel  22 .  
         [0042]     Conveniently, the cup-like body  7  and in particular the positioning seats  13 , according to the present invention, allow a stable housing and according to the final desired position of the permanent magnets  10 , while the recesses  15 , of the central core  6 , allow to house the intradosses  17  improving the core  6  stability with respect to the magnets  10 .  
         [0043]     Advantageously, the cup-like body  7  exhibits at least one longitudinal rib  25  projecting from each recess  12 .  
         [0044]     In particular, according to the present embodiment, the cup-like body  7  exhibits a pair of ribs  25  projecting from the base  40 , of each recess  12 , outside the cup-like body  7  and a second rib  26  projecting inside the body  7  itself.  
         [0045]     In practice, during the mounting step the central core  6  is inserted, with limited clearance, into the cup-like body  7 , subsequently the magnets  10  are inserted, with a limited clearance, into the positioning seats  13 , between pairs of recesses  12  with the extrados  16  facing the side wall  9 , the intrados  17  housed in each recess  15  of the core  6 , with the first bevels  20  conjugated to the sides  21  and with the second bevels  22  conjugated to the lateral containment edges  23 .  
         [0046]     Advantageously, according tot he present invention, the rotor  3  exhibits a substantially cage-like shape structure  50  comprising a plurality of columns  29  extended between opposite bottoms  27 , 28 , wherein the core  6 , the magnets  10  and the cup-like body  7  are incorporated.  
         [0047]     Suitably, the opposite bottoms  27  and  28  respectively abut at the base end  8   a  and at the free end  8   b  of the cup-like body  7  while the plurality of columns  29  are housed in the recesses  12  and they are counter-shaped to them.  
         [0048]     Suitably, the cage-like structure  50  is obtained through injection of a suitable plastic material, in the cup-like body  7 , and outside it in correspondence with said recesses  12 .  
         [0049]     Advantageously, according to the present invention, the cage-like structure  50  further comprises a plurality of second columns  31  inside the cup-like body  7  which extend between said opposite bottoms  27 , 28  and are interposed between adjacent magnets  10  and allow to increase the structural stiffness of the rotor  3 .  
         [0050]     Preferably, the plastic material used to obtain the structure  50  is the same plastic material with which the cup-like body  7  is realised, or, it is a chemically compatible plastic material and it allows to improve the adhesion between the same to obtain a rotor  3  with particularly massive structure.  
         [0051]     The pair of ribs  25  and the second rib  26  allow, besides stiffening the cup-like body  7 , also to improve the hold of the plastic material at the recesses  12 , making the rotor  3  structurally more massive and strong.  
         [0052]     Conveniently, the base end  8 a of the cup-like body  7  exhibits a plurality of spacers  32  which, during the injection of the plastic material, are covered by the material itself to define the bottom  27  of the cage-like structure  50 .  
         [0053]     Advantageously, the cage-like structure  50  exhibits an inner sleeve  42 , arranged along the axis X-X, which radially covers the axial passage  11  of the core  6  and extends between the opposite bottoms  27  and  28 .  
         [0054]     The cup-like body  7 , the sleeve  42  and the opposite bottom  27  and  28 , realised according to the present invention, define a single body and make the rotor  3  obtained waterproof.  
         [0055]     In particular, the sleeve  42  exhibits an inner diameter suitable to define a hole  33  which allows the forced housing of the driving shaft  4 . In the present embodiment, the shaft  4  comprises a longitudinal groove  60  suitable to allow a passage of water or of cooling liquid for a bush placed next to the bottom  28  of the rotor  3 .  
         [0056]     Further, the cage-like structure  50  exhibits, next to the perimeter of said opposite bottoms  27 ,  28  respectively a first  34  and a second  35  annular flange, suitable to contain a plurality of first  36  and second  37  housing seats for counter-shaped balancing masses.  
         [0057]     Suitably, according to the present embodiment, the first seats  36  and the second seats  37  exhibit a cylindrical shape.  
         [0058]     The bottom  27  also comprises a pair of projections  38 , symmetrically arranged next to the hole  33 , which exhibit a substantially “C-like” shape, suitable to allow the insertion of a possible axial thrust bearing bush.  
         [0059]     Significant versions can be realised, with respect to the described solution, in particular it is possible to realise the containment element, the core and the magnets so that the core and the magnets are inserted in the cup-like body with a thrust coupling.  
         [0060]     The present invention also relates to a method for obtaining the permanent-magnet rotor  3 , as previously described.  
         [0061]     The method according to the present invention comprises the steps of:  
         [0062]     arranging the cup-like body  7 , realised in a suitable plastic material; inserting the central core ( 6 ) into the cup-like body  7  and arranging, with limited mutual clearance, the permanent magnets  10  in the positioning seats  13 .  
         [0063]     Subsequently, after the insertion of the cup-like body  7  in a measure-to-measure mould, advantageously the method according to the present invention provides to obtain a cage-like structure  50  by injecting a suitable plastic material into the cup-like body  7  and into the recesses  12 . The cage-like structure  50  exhibits the opposite bottoms  27  and  28  abutting at the ends  8   a  and  8   b  of the cup-like body  7 , and the columns  29 , extended between the opposite bottoms  27  and  28 , housed in the recesses  12 .  
         [0064]     Suitably, in the cage-like structure  50  the core  6 , the magnets  10  and the cup-like body  7  are incorporated.  
         [0065]     Advantageously, during the injection step, the plastic material is injected so as to occupy all the accessible spaces, i.e. the possible gaps between core  6 , magnets  10  and containment element  7  to realise a plurality of fillings that makes the rotor  3  obtained particularly compact and strong.  
         [0066]     In particular, thanks to the shape of the side edges  18 ,  19  of the magnets  10  and of the recesses  12 , according to the present invention, between adjacent magnets  10  a channel is realised, arranged along the axis X-X, which allows to realise a plurality of second inner columns  31  extended between the opposite bottoms  27  and  28 .  
         [0067]     Suitably moreover, during the injection step, the cage-like structure  50  is realised with the longitudinal sleeve  42  which radially covers the entire axial passage  11  of the core  6  with an inner diameter suitable for the forced housing of the shaft  4 .  
         [0068]     Moreover, advantageously, the injection step of the polymeric material occurs with injection points arranged symmetrically with respect to the axis X-X and by suitably regulating the injection pressure, this allows to fill, in an excellent way, the interspaces between core  6 , cup-like body  7 , magnets  10  and mould.  
         [0069]     Main advantage of the method according to the present invention is that it allows to obtain, in a quick way, a structurally massive rotor with the magnets steadily housed, according to a final position, in the positioning seats without the help of further components or of adhesives, as well as to maintain them in position, in a stable way, during the injection step.  
         [0070]     A further advantage of the method according to the present invention is that of obtaining, in a simple way, a particularly massive and strong rotor with a cup-like body realised in a plastic material and a cage-like structure, obtained by injection, which allows to strengthen the rotor, thanks to the presence of the columns housed in said recesses and extended between the opposite bottoms, which allow to maintain the minimal space outside the rotor, i.e, identical to the one of the cup-like body, maximising in the meantime the structural resistance of the rotor and the reliability also during high speed operation.  
         [0071]     An advantage of the rotor realised according to the present invention is linked to the particular shape of the cup-like body which, thanks to the positioning seats, exhibits stable housings according to the final position for the magnets, also during the injection step.  
         [0072]     Another advantage of the rotor realised according to the present invention is obtained thanks to the recesses in the core which allow to house, with limited clearance, the intradosses of the magnets thus allowing the core and the magnets themselves to maintain, during the injection of the plastic material, a mutual stable position according to the final position.  
         [0073]     Another remarkable advantage of the present invention is that of obtaining a waterproof rotor, in fact, the shape of the cage-like structure, with the inner sleeve extended between the opposite bottoms, allows to incorporate the cup-like body insulating the inner magnets and the core from the water or from the cooling fluid wherein the rotor is immersed.  
         [0074]     Another advantage of the rotor obtained according to the present invention is the easiness with which the balancing of the same can be carried out, in fact the plurality of first and second seats realised in opposite bottoms of the cage-like structure allow to use counter-shaped masses correcting, in a quick way, possible balancing faults, both static and dynamic, of the rotor obtained.