Abstract:
A monodirectional impeller for centrifugal electric pumps having a permanent-magnet synchronous motor, having vanes which are deformable at least along part of their extension so as to change their curvature, when loaded, in one direction of rotation, so that the power required for rotation in that direction is greater than the maximum power that can be delivered by the motor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a monodirectional impeller for centrifugal electric pumps having a permanent-magnet synchronous motor.  
           [0002]    It is known that permanent-magnet synchronous electric motors have a general structure which comprises a stator, provided with an electromagnet constituted by a lamination pack and by corresponding windings, and a rotor, which is arranged between two pole shoes formed by the stator and is crossed axially by a shaft which is rotatably connected to a supporting structure.  
           [0003]    These motors are bidirectional, i.e., at startup the rotor can be induced equally to turn clockwise or counterclockwise.  
           [0004]    This characteristic depends on a plurality of factors, including the arrangement of the polarities of the rotor with respect to the magnetic field generated between the pole shoes of the stator pack when the induction windings are supplied with AC current.  
           [0005]    For this reason, permanent-magnet synchronous motors are currently widely used where the direction of rotation is not important; accordingly, for example they are coupled, in centrifugal pumps, to radial-vane impellers which ensure the same performance in both directions of rotation.  
           [0006]    In order to increase the efficiency of synchronous-motor electric pumps without resorting to the use of particular electronic starting devices, it is convenient to use vanes which are orientated with a certain curvature profile, which clearly presumes a single direction of rotation of the motor.  
           [0007]    Accordingly, electronic starter devices have been devised which guide the motor so that it starts in a single direction of rotation; as an alternative thereto, mechanical devices have been devised which block the rotor when it tends to start in the wrong direction of rotation (reference should be made for example to patent application PD98A000003 of Jan. 8, 1998 in the name of this same Applicant).  
           [0008]    In this manner, monodirectional behavior is ensured in any operating condition assumed by the electric pump.  
           [0009]    However, the system may generate noise during starting and is a limitation as regards reliability (for high-power pumps), since there is a mechanical device which is subjected to repeated stresses, especially during starting.  
           [0010]    A particularly important alternative for a monodirectional synchronous electric pump without mechanical devices for stopping the rotor and without electronic devices (which are reliable but expensive) is constituted by what is disclosed in patent application PD 
           [0011]    [0011] 98 A000058 of Mar. 19, 1998 in the name of this same Applicant.  
           [0012]    This patent application discloses a device which is able to start, with limited power levels, loads which have high moments of inertia, such as impellers with orientated vanes of a centrifugal pump.  
           [0013]    In particular, this is a driving device with a larger angle of free rotation between the rotor and the impeller, so as to obtain, with respect to conventional mechanical couplings, several advantages:  
           [0014]    reduction of the starting torque for starting the motor;  
           [0015]    a consequent reduction of the level of vibrations generated during synchronous operation;  
           [0016]    the motor is rendered monodirectional by means of the correct design of the vanes of the impeller, so that the power absorbed by the load in one direction of rotation is greater than the available power of the motor and is smaller in the opposite direction of rotation.  
           [0017]    Therefore, by designing the motor and the vanes of the impeller so that the power absorbed by the load in one direction of rotation is greater than the available power of the motor and smaller in the opposite direction of rotation, in the first case the impeller goes out of step with respect to the motor, is halted and automatically reverses its motion, whereas in the second case it is driven normally.  
           [0018]    It is thus possible to render the pump monodirectional by utilizing the difference in power between what the motor is able to deliver and the power absorbed by the load in the two directions of rotation (the rotor stops because the power required by the impeller in the wrong direction of rotation is greater than the power that the motor can deliver).  
           [0019]    Although this system provides a fundamental advantage with respect to the prior art, it still has limitations, because monodirectionality is ensured only within a flow-rate/head range; accordingly, it is used in applications where the hydraulic working point does not vary beyond certain limits or, in other words, where the characteristic curve of the duct does not undergo significant variations (this is the case, for example, of washing pumps for dishwashers).  
           [0020]    In the accompanying drawings FIG. 1 plots, for both directions of rotation of the motor, the power absorbed by the motor as a function of the required flow-rate.  
           [0021]    The line A plots the correct direction of rotation, the line B plots the wrong direction of rotation, and the straight line C represents the maximum power that can be delivered by the motor.  
           [0022]    The chart shows three flow-rates Q1, Q2 and Q3, which correspond to three working points, and it is clear that only Q1 and Q2 are the flow-rates for which a single direction of rotation is ensured, since the maximum power that the motor is able to deliver (straight line C) is greater than the power required by the impeller when it turns in the correct direction of rotation (line A) and is smaller than the power required by the impeller when it turns in the opposite direction (line B).  
           [0023]    For the flow-rate Q3, instead, there is a condition in which both power levels, in both directions of rotation, are lower than the maximum deliverable power and therefore monodirectional behavior is not possible.  
         SUMMARY OF THE INVENTION  
         [0024]    The aim of the present invention is therefore to eliminate the above-noted drawbacks of the above-cited device related to patent application  
           [0025]    Within this aim, a consequent primary object is to provide a pump which is monodirectional over the entire available flow-rate range.  
           [0026]    Another object is to provide all of the above in a constructively simple manner.  
           [0027]    Another object is to have no effect on noise levels.  
           [0028]    Another object is to provide an impeller, if necessary, with deformable vanes enclosed between a double fluid conveyance wall (closed impeller).  
           [0029]    This aim and these and other objects which will become better apparent hereinafter are achieved by an impeller for centrifugal electric pumps having a permanent-magnet synchronous motor, characterized in that its vanes are deformable at least along part of their extension and can change their curvature, when loaded, in one direction of rotation, so that the power required for rotation in that direction is greater than the maximum power that can be delivered by the motor.  
           [0030]    Conveniently, in one embodiment, this aim and these objects are achieved by an impeller for centrifugal electric pumps having a permanent-magnet synchronous motor, characterized in that it comprises:  
           [0031]    a first disk-like element provided with curved nondeformable vanes which are monolithic therewith,  
           [0032]    an annular element, whose dimensions are contained within the inlet dimensions of said nondeformable vanes and which is provided with means for coupling to said first disk-like element, said annular element being provided with flexibly deformable vanes which cantilever outward, are interposed between the nondeformable ones, and are adapted to modify, when loaded, their curvature in one of the directions of rotation so that the power required for rotation in that direction is greater than the maximum power that can be delivered by the motor,  
           [0033]    a second disk-like element, which encloses, together with said first disk-like element, the set of vanes and is rigidly coupled to said nondeformable vanes, leaving the deformable ones free. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    Further characteristics and advantages of the invention will become better apparent from the detailed description of embodiments thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein:  
         [0035]    [0035]FIG. 1 is a chart which plots, for conventional centrifugal pumps, the flow-rate as a function of the power required in the two directions of rotation;  
         [0036]    [0036]FIG. 2 is a sectional view of an impeller according to the invention in a first embodiment, arranged inside a volute of a centrifugal pump;  
         [0037]    [0037]FIG. 3 is an exploded view of the components of FIG. 2;  
         [0038]    [0038]FIG. 4 is a plan view of an impeller according to the invention in a second embodiment;  
         [0039]    [0039]FIG. 5 is a side view of the impeller of FIG. 4;  
         [0040]    [0040]FIG. 6 is a sectional view of an impeller according to the invention in a third embodiment, arranged inside a volute of a centrifugal pump;  
         [0041]    [0041]FIG. 7 is a chart which plots, for centrifugal pumps with impellers according to the invention, the flow-rate as a function of the power required in the two directions of rotation;  
         [0042]    [0042]FIG. 8 is a side view of another impeller according to the invention;  
         [0043]    [0043]FIG. 9 is a front view of the impeller of FIG. 8;  
         [0044]    [0044]FIG. 10 is an exploded perspective view of the impeller of FIG. 8. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    With reference to FIGS. 2 and 3, in a first embodiment the impeller according to the invention comprises a disk  10  with a central hollow cup-shaped body  11  which is a component of a driving device  12  described in greater detail hereinafter.  
         [0046]    A plurality of vanes  13  protrudes from a ring  16  which is located on the outside of the cup-shaped body  11  in a corresponding seat  10   a  of the disk  10 .  
         [0047]    The vanes  13  are monolithic with respect to the ring  16 , which affects only their part that lies closest to the center.  
         [0048]    The peripheral part can therefore perform flexing movements arising from the elastic characteristics of the material of which they are made.  
         [0049]    The vanes  13  can also be rigidly coupled to the ring  16  (axial and torsional retention) in various manners: by interlocking and/or interference, ultrasonic welding, adhesive bonding.  
         [0050]    The peripheral regions  14  of the vanes  13  are therefore flexibly deformable, as mentioned, and said deformation is greater for the wrong direction of rotation and is optionally limited by the stroke limiting teeth  15  which protrude from the disk  10  alternately with the vanes  13 .  
         [0051]    In order to center the vanes  13  with respect to the teeth  15 , the ring  16  has axial teeth  17  to be inserted in appropriately provided holes  18  of the disk  10 .  
         [0052]    As regards the driving device  12 , it comprises said hollow body  11  and a cover  19  which can also be rigidly coupled to the ring  16  with the vanes  13 .  
         [0053]    The hollow body  11  is provided with an axial hole  20  for the shaft  21  of the rotor, not shown in the figures, of the motor.  
         [0054]    An O-ring gasket  23  acts on the shaft  21  and is accommodated in a corresponding seat of the hollow body  11 .  
         [0055]    The hermetic seal of the device  12  is ensured not only by the gasket  23  but also by the closure of the lid  19 , which is provided by ultrasonic welding, adhesive bonding or other known methods on the hollow body  11 .  
         [0056]    It is possible to provide alternative embodiments which are not hermetic or in which the lid  19  is monolithic with the ring  16 .  
         [0057]    In said ring, a tooth  24  protrudes from the inner wall and is therefore rigidly coupled to the impeller assembly; said tooth  24  interacts with a tooth  25  which protrudes from a ring  26  which can rotate about a shank  27  which is mounted with interference on the shaft  21  and is rigidly coupled thereto.  
         [0058]    A tooth  28  protrudes radially from the shank  27  and interacts, in its rotation, with the tooth  25  of the ring  26 , whose axial extension is such as to affect the path of the rotation of both teeth  24  and  25 .  
         [0059]    Said teeth are arranged axially so that they do not interfere with each other.  
         [0060]    Accordingly, the rotation of the shaft  21  starts the rotation of the tooth  28 , makes said tooth interact with the tooth  25 , turning it until it interferes with the tooth  24 , and finally makes the rotor turn the impeller.  
         [0061]    Grease, with a shock-absorbing function, can be conveniently placed inside the hollow body  11 .  
         [0062]    [0062]FIGS. 2 and 3 also illustrate the volute  29  in which the impeller is arranged.  
         [0063]    With reference now to FIGS. 4 and 5, an impeller according to the invention, in a second embodiment which is simplified with respect to the preceding one, comprises a disk  110 , from which a coaxial shank  111  with a hole  112  for the shaft of the rotor (not shown for the sake of simplicity) protrudes centrally on one side, and from which a plurality of vanes  113  with a curved profile protrudes on the other side.  
         [0064]    The impeller as a whole is formed monolithically.  
         [0065]    According to the invention, the vanes  113  are flexibly deformable along at least part of their extension, so as to modify their curvature, when loaded, in one of the two directions of rotation so that the power required for rotation in that direction is greater than the maximum power that can be delivered by the motor.  
         [0066]    The deformability of the vanes arises from the flexibility of their peripheral regions  114 , which are provided separately from the disk  110  by the molding step by way of an appropriate shaping of the mold.  
         [0067]    By providing the impeller as a single part made of plastics, with the peripheral regions  114  divided from the rest, said regions flex, when loaded, in the wrong direction of rotation and modify their curvature so that in practice they block the rotation.  
         [0068]    Conveniently, teeth  115  protrude from the disk  110  in the peripheral region, are alternated with the vanes  113 , and advantageously act as stop elements which avoid excessive curvatures of said vanes  113  in the wrong direction of rotation, thus avoiding excessive stresses thereto.  
         [0069]    The flexibility of the material would of course allow flexing in the correct direction of rotation as well, but the curvature of the vanes  113 , which matches the fluid threads that form during the rotation of the impeller, causes deformation in the correct direction of rotation to be very limited in practice.  
         [0070]    With reference to FIG. 6, in a third embodiment the impeller according to the invention comprises a disk  210  with a cup-shaped central hollow body  211  which is a component of a driving device  212  similar to the one of the first embodiment.  
         [0071]    A plurality of vanes  213  protrudes from a ring  216  which is arranged on the outside of the cup-shaped body  211  in a corresponding seat  210   a  of the disk  210 .  
         [0072]    The vanes  213  are monolithic with respect to the ring  216 , which affects only the part of said vanes that lies closest to the center.  
         [0073]    The peripheral part can therefore perform flexing movements arising from the characteristics of the material of which the vanes are made.  
         [0074]    The vanes  213  can also be rigidly coupled to the ring  216  (axial and torsional retention) in various manners: by interlocking and/or interference, ultrasonic welding, adhesive bonding.  
         [0075]    The peripheral regions  214  of the vanes  213  are therefore, as mentioned, flexibly deformable, and said deformation is greater for the wrong direction of rotation and is limited by teeth  215  which protrude from the disk  210  alternately with the vanes  213 .  
         [0076]    In order to center the vanes  213  with respect to the teeth  214 , the ring  216  has axial teeth  217  to be inserted in appropriately provided holes  218  of the disk  210 .  
         [0077]    Also in this case, the cover  219  is separate from the ring  216 , but it is also possible to provide alternative embodiments in which the cover  219  is monolithic with the ring  216 .  
         [0078]    In this embodiment, the lid  219  of the hollow body  211  has, at its end, a seat  230  for a first shim ring  231  made of ceramic material, sintered material or similar hard material.  
         [0079]    A second shim ring  232  made of ceramic material, sintered material or similar hard material is accommodated in a seat  233  provided at the end of a cylindrical support  234  which is supported by a bush  235  which is rigidly coupled, by means of radial spokes  236 , to a ring  237  which is inserted with interference in a corresponding seat  238  of the volute  229 .  
         [0080]    As an alternative, the support  234  can be monolithic with the bush  235 .  
         [0081]    The ring  232  acts as an axial thrust bearing in order to adjust, in cooperation with the ring  231 , the position that the impeller assumes in the volute  229  and maximize hydraulic efficiency.  
         [0082]    With reference now to FIG. 7, said figure is a chart which plots the flow-rate as a function of power and wherein:  
         [0083]    the line D is the curve related to an impeller with the flexible vanes according to the invention, with the wrong direction of rotation;  
         [0084]    the line C represents the maximum power that the motor can deliver;  
         [0085]    the line A plots the curve related to an impeller with flexible vanes, in the correct direction of rotation.  
         [0086]    The line D clearly shows that for any flow-rate in the wrong direction of rotation, the flexible vane requires more power than the motor can generate (straight line C).  
         [0087]    Accordingly, the motor cannot start in the wrong direction.  
         [0088]    FIGS.  8  to  10  illustrate another possible configuration of the impeller.  
         [0089]    In this case, the impeller according to the invention, which is entirely made of plastics, is generally designated by the reference numeral  310  and comprises a first disk-like element  311  (which is monolithic with respect to a bush  311   a ) which monolithically supports, in this case, three curved nondeformable vanes  312  which are angularly equidistant and, at the center, a rounded shank (which is separated from their inlet region).  
         [0090]    The impeller  310  further comprises an annular element  314 , whose dimensions are contained within the inlet dimensions of said nondeformable vanes  312 ; said annular element has means  315  (described in greater detail hereinafter) for coupling to said first disk-like element  311 .  
         [0091]    The annular element  314  supports, so that they cantilever outward in this case, three curved flexibly deformable vanes  316  which are angularly equidistant and are to be arranged alternately with the nondeformable vanes  312 .  
         [0092]    The annular element  14  is in fact accommodated in a complementarily shaped seat  317  of the first disk-like element  311 .  
         [0093]    The flexibly deformable vanes  316  end externally with respect to the dimensions of the nondeformable vanes  312 , with respect to which they have slightly smaller axial dimensions.  
         [0094]    The flexibly deformable vanes  316  are adapted to modify, when loaded, their curvature in one direction of rotation so that the power required for rotation in that direction is higher than the maximum power that the motor (not shown for the sake of simplicity) can deliver.  
         [0095]    The impeller  310  further comprises a second disk-like element  318 , which encloses, together with said first disk-like element  311 , the set of vanes  312  and  316  and is rigidly coupled, by ultrasonic welding, adhesive bonding or other known methods, to the nondeformable vanes  312 , leaving free the flexibly deformable vanes  316 , which have slightly smaller axial dimensions.  
         [0096]    The second disk-like element  318  has a central hole and its edge  319  protrudes axially so as to form the inlet region for the fluid to be pumped.  
         [0097]    As regards the coupling means  315 , they comprise a shaped portion  320  which is for example polygonal (dodecagonal in the figures), is provided on the internal surface of the annular element  314 , and mates with a complementarily shaped surface  321  of the seat  317 .  
         [0098]    The coupling means  315  comprise a specific number of tabs  322  which are substantially radial, are angularly equidistant, protrude from the annular element  314 , are inserted between the vanes  316  and end with respective axially elongated hooks  323 , which engage by snap action, after elastic deformation, the first disk-like element  311  by insertion in suitable through holes  324  thereof.  
         [0099]    The seat  317  of course has a shape which also accommodates the tabs  322 .  
         [0100]    The hooks  323  inserted in the through holes  324  prevent any axial movement of the assembly constituted by the disk  314  and the vanes  316 .  
         [0101]    The coupling means  315  determine the exact mutual positioning of the vanes  312  and  316 .  
         [0102]    The peripheral part of the vanes  316  can thus perform flexing movements which arise from the elastic characteristics of the plastic material of which they are made.  
         [0103]    The deformation is greater for the wrong direction of rotation, and the vanes  316  modify their curvature so that in practice they block the rotation.  
         [0104]    The flexibility of the material would of course also allow flexing in the correct direction of rotation, but the curvature of the vanes  316 , which matches the fluid threads that form during the rotation of the impeller  310 , causes the deformation in the correct direction of rotation to be very small in practice.  
         [0105]    In practice it has been observed that the intended aim and objects of the present invention have been achieved.  
         [0106]    With the flexible-vane impeller, monodirectionality is in fact ensured for all flow-rates/heads.  
         [0107]    This is achieved in a constructively simple manner and has no effect on noise levels.  
         [0108]    The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept.  
         [0109]    Thus, for example, the change in the curvature of the vanes can be provided by means of a hinge, even of the film type, which connects each peripheral part to the central one.  
         [0110]    In the embodiment of FIGS. 8, 9 and  10 , even if the flexible vanes yield due to wear, the nondeformable vanes continue to give their constant contribution to the pumping action.  
         [0111]    All the details may further be replaced with other technically equivalent elements.  
         [0112]    In practice, the materials employed, so long as they are compatible with the contingent use, as well as the dimensions, may be any according to requirements.  
         [0113]    The disclosures in Italian patent applications Nos. PD2000A000176 and PD2001A000110, from which this application claims priority, are incorporated herein as reference.