Abstract:
The present disclosure relates to a stator assembly for an electrical machine, and more in particular relates to a support element utilized within the stator assembly in order to improve its performance. The support element allows the thermal expansion of round connection arms while detuning the Eigen-frequency and reducing the amplitude of the vibrations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT/EP2014/073792 filed Nov. 5, 2014, which claims priority to European Patent Application No. 14152324.1 filed on Jan. 23, 2014, both of which are hereby incorporated in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a stator assembly for an electrical machine, and more in particular relates to a support element utilized within the stator assembly in order to improve its performance. 
       BACKGROUND 
       [0003]    State-of-the-art electric energy conversion relies on a three-phase power is network with alternating currents (AC) at 50 Hz or 60 Hz frequency and voltage levels ranging from several hundreds of Volts to hundreds of thousands of Volts. The conversion of rotating mechanical energy into electric energy and vice versa is done by generators and by motors, respectively. Those rotating machines can be divided into asynchronous and synchronous apparatuses. 
         [0004]    Motors and generators comprise a stator and a rotor. The rotor of the machine rotates inside the stator bore of the stator. Rotating machines generate the magnetic field typically through rotor pole windings. The number of rotor poles and the frequency of the stator magnetic field define the number of revolutions per minutes (rpm) of the rotating machine. 
         [0005]    As known, the stator-bars of generators are generally made of a top and a bottom layer. The bars of top and bottoms layer are connected to each other by copper pieces called lugs, such to form electrical coils. An electrical connection is then needed between the electrical coils and the terminals of the generator, which lead the power to the outside through the generator casing. For this purpose, this connection is established by copper bars (of rectangular or round cross-sections) bent around the support structure on its outermost diameter. 
         [0006]    Generally, the first and the last bar of a coil of the winding are connected to the terminals. These bars (usually six per layer, equally distributed over the circumference) are called phase bars and are specially designed to lead the generated electrical power of the generator into an external grid. The connection between the phase bars and the terminals, which lead the power through the generator&#39;s casing and are called round connections or phase rings. 
         [0007]    They are electrically insulated against each other, and quite sophisticated shapes are required for reaching the phase bars and connecting them to the generator terminals whilst maintaining the required insulation distances. 
         [0008]    The round connections are generally fixed to the support structure to prevent high vibrations. 
         [0009]    The end winding support structure of the generator is located at the front of the press plates and the outer side of the end winding of the stator bars. This support structure is used for the fixation of the stator bars ends, to apply mechanical strength to the end winding and withstand the forces occurring during normal operation and accidental incidents. The support usually consists of brackets mounted in axial direction and often of supporting rings to stiffen the structure. 
         [0010]    The ends of the round connections, which are foreseen to be connected to the phase bars, sometimes cannot be fixed to the supporting structure over a long distance, since the position of the phase bars might be located between the supporting brackets. 
         [0011]    As a result of this, round connection ends have a high risk of vibrations. 
         [0012]    As a consequence, an important problem is the vibration of round connection arms to the phase lugs. The Eigen frequency of these arms often need to be detuned (e.g. no resonance between 100 and 140 Hz for 60 Hz applications) or at least the vibration levels have to be reduced to a minimum. 
         [0013]    Cracks in the strands close to the phase lugs and in the lugs can be found in several generators. These cracks are mostly caused by high vibration levels of the round connections arms. Some round connections arms have Eigen frequencies close to 120 Hz (for 60 Hz applications) or 100 Hz (for 50 Hz applications). 
         [0014]    Up to date, some existing solutions suggest applying a very stiff support between the round connection arm and the support ring with cords or tape together with resin. This solution detunes the Eigen-frequency out of the range of 100 to 140 Hz very well and reduces the amplitude of the vibrations. 
         [0015]    On the other hand thermal expansion, which does occur during operation of the machine, should not be blocked. Such thermal expansion mainly occurs along the axial direction (usually causing a displacement of the connection arm of about 2-3 mm), but also in radial and tangential directions. Therefore a “stiff” solution is not possible. 
         [0016]    The stiff solution hinders the thermal expansion, which increases the stress at the strands close to the lug and considerably increases the danger of cracks in the strand. On the other hand, the high stress to the cords/tapes of the stiff support may lead to a loosening of the connection over time, which renders this support useless for reducing the vibrations. 
         [0017]    Generally, the stiff support according to the teachings of the prior art entails a high risk of breakage caused by the blocking of the thermal expansion. The present disclosure is oriented towards providing the aforementioned needs and towards overcoming the aforementioned difficulties. 
       SUMMARY 
       [0018]    According to preferred embodiments, the object of the present invention is to provide a support element as substantially defined in independent claim  10 , placed between the support structure and the connection arm. 
         [0019]    The support element, as it will be described in details with reference to preferred but not limiting embodiments, allows the thermal expansion while detuning the Eigen-frequency in the arms and reducing the amplitude of the vibrations. 
         [0020]    Said differently, the support element is such to allow axial, radial and tangential displacement of the round connection arm due to thermal expansion whilst hindering vibrations. 
         [0021]    Further object of the present invention is to provide a stator assembly, as substantially defined in claim  1 , which comprises the support element. As it will be appreciated, the stator assembly might comprise a number of support elements disposed along its circumferential development as required by the particular configuration of the machine, in order to ensure the correct functioning of the connection arms and allow the thermal expansion thereof. 
         [0022]    Preferably, the support element shall be made from glass fibre reinforced insulation material with a lamination direction along the contour. 
         [0023]    According to an aspect of the invention, the upper vertical part comprises screw-type connection means to the support ring. Advantageously, the upper vertical part is thicker than the lower vertical part in order to allow movement caused by vibration between the lower part of the support element and the annular ring. 
         [0024]    Differently, the horizontal part of the support element is glued with resin to the lower side of the round connection arm. Additionally, it also may be fixed by cords or tapes around the connection arm and the support element to ensure a very stiff contact therebetween. 
         [0025]    The proposed solution was developed after extensive studies of the winding head and round connection vibration behaviour. The invention reduces the vibration amplitudes and therefore the stresses due to vibrations in the affected parts considerably. It is easy to install and comparatively cost efficient. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]    The foregoing objects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
           [0027]      FIG. 1  is a prospective view of a stator assembly according to the present invention; 
           [0028]      FIGS. 2A-2C  show different views of a support element according to the present invention; 
           [0029]      FIGS. 3, 4 and 5  show different views of the support element of  FIGS. 2A-2C  when mounted on a stator assembly; and 
           [0030]      FIG. 6  shows a front view of an example of how the support element can deform in order to allow thermal expansion of the round connection arm in exaggerated scale in order to show the effect. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    With reference to  FIG. 1 , a prospective view of a stator assembly is shown, generally indicated with the reference number  1 . The stator assembly  1 , part of an electrical machine (not shown) comprises internally a plurality of stator winding bars  4  in which voltage is induced. These bars cooperate together with the rotor (not shown) in the process of energy conversion. In particular, the generated power is made available on external terminals (not shown), which are then connected to a grid to feed. The connection established between the stator winding bars  4  and the external terminals is operated by specially designed connection bars, generally referred to as “round connection”, of which an arm  3  is indicated in the figure. 
         [0032]    The stator assembly  1  may comprise a plurality of connection arms  3 , disposed along its circumferential development, in a number generally depending on the particular type of the generator used. In the example here described, the stator assembly  1  comprises six connection arms for each top and bottom layer of the stator bars equally spaced from each other, so twelve arms in total. Six arms each for top and bottom layer are a typical arrangement for three-phase generators with two parallel circuits. 
         [0033]    The stator assembly further comprises a support structure, which in turn comprises an annular ring  2 . The arrangement of the stator assembly  1  is well known to those skilled in the art, and therefore it won&#39;t be disclosed with further details. 
         [0034]    As mentioned above, connection arms  3  are often subjected to high vibrations. In order to hinder the high vibrations, the stator assembly according to the invention comprises a support element  6 , positioned between the support structure  2  and the connection arm  3 . 
         [0035]    As explained, the figure shows several connection arms  3  each one linked to the support structure  2  by a respective support element  6 . It will be appreciated that what will be hereafter described about a single connection arm  3  and the respective supporting element  6  will stand for every connection arm and support element of the stator assembly. 
         [0036]    The support structure is generally made of an electrical insulating material to avoid unwanted damaging current flows. For the same reasons, the support element  6  is made of an electrically insulating material as well. Preferably, the support element  6  is made of a reinforced glass fibre. 
         [0037]    Advantageously, the support element  6  comprises a resilient module configured to allow displacements of the connection arm due to its thermal expansion because of high temperatures, which occur during operation, whilst hindering high vibrations. 
         [0038]    According to a preferred embodiment, the support element  6  comprises a radial member  61  disposed along axial direction A of the electrical machine and an axial member  62  in turn disposed along a radial direction R. The axial member  62  of the support element  6  is connected to the connection arm  3 , while the radial member  61  is connected to the annular ring  2 . 
         [0039]    Making now reference to  FIG. 2 , it is shown the support element  6 , object of the present invention, in three different views. In particular,  FIG. 2A, 2B and 2C  show the support element  6  respectively in a lateral, front and perspective view. 
         [0040]    As clearly shown in the figure, the support element  6  is substantially L-shaped and comprises the radial member  61  and the axial member  62 , disposed along respectively radial and axial directions of the electrical machine when mounted on the stator assembly (as shown in preceding  FIG. 1 ). 
         [0041]    More in particular, the radial member  61  comprises a first portion  611  and a second portion  612 , the latter being interposed between the axial member  62  and the first portion  611 . When mounted on stator assembly, the first portion  611  is connected to the annular ring (not shown) whilst the second portion  612  is released from it such that the support element  6  has a resilient reaction in response to a rotation of second member  612  about a linear pivot region  8 . The linear region  8  delimits the first portion  611  (connected to the annular ring by means of a screw-type connection  11  indicated in  FIG. 2B ) from the second region  612  which is released from the same. 
         [0042]    Advantageously, the second portion  612  has a thickness which is smaller than that of the first portion  611  such to define a step in correspondence of the linear region  8 , so that second portion  612  has its inner surface that, when mounted on the stator assembly and facing the annular ring, is not in direct contact with it. This way, friction between the two parts caused by vibration is avoided. Preferably, the step has a rounded edge so that three-axial tension states, which could arise during resilient displacements of second portion  612 , are avoided. 
         [0043]    Moreover, the support member  6  comprises a U-shaped junction  9 , interposed between the axial member  62  and the radial member  61 . The U-shaped junction  9  offers an additional pivot region which allows the support element to resiliently react to a displacement of the axial member  62  around a region  12 , which is substantially located where a curvature of the junction merges with the axial member  62 . 
         [0044]    With now reference to following  FIGS. 3-5 , it is shown two different views of the support element  6  mounted on the stator assembly. In particular, it is readily understood that the second member  612 , acting as a resilient module as the first member  611  is attached to the annular ring  2  while the second member is released from it, provides for a resilient reaction to an expansion of the connection arm  3  along the axial direction A, as the connection arm is connected to the axial member  62  and thus forces the second portion  612  of the radial member  61  to rotate about a linear region  8 , as previously explained. Similarly, the U-shaped junction  9  acts as a further resilient module as it provides a resilient reaction of the support element  6  caused by the expansion of the connection arm  3  along a radial direction R. In particular, the axial member  62  can resiliently rotate around a linear region  12  this way allowing a thermal expansion of the connection arm  3  in turn connected to the axial member  62 . Preferably, the connection arm  3  is glued to the axial member  62  with a resin or fixed with tapes or cords. 
         [0045]    With particular reference to  FIG. 5 , it is well visible that second portion  612  of radial member  61  has a thickness which is smaller than the first portion  611 . So there is no contact between second portion  612  and the support structure, thus avoiding friction which would be caused by high vibrations. 
         [0046]    With reference to last  FIG. 6 , it is shown the support element  6  in operation in a deformed state. In particular, it is shown a possible displacement of the second portion  612  indicated by arrow D 1  caused by the expansion of the connection arm (not shown) which forces the axial member  62  to move along the axial direction A and consequently forces the second portion  612  to rotate about a linear region  8  of radial member  61 . The resilient reaction of second member  612 , which is released from the support structure, allows such thermal expansion without leading to breakage of the support element. 
         [0047]    In a similar way, it is shown a possible displacement of the axial member  62  indicated by arrow D 2  caused by the expansion of the connection arm, end-winding and stator bars (not shown) which forces the axial member  62  to move along the radial direction R and consequently the axial member  62  to rotate about a linear region  12  which is located where the U-shaped junction  9  merges with the axial member  62 . The resilient reaction of the U-shaped junction allows the thermal expansion of the connection arm along the radial direction without leading to breakage of the support element. 
         [0048]    It will be appreciated that thermal expansions can generally derive from the heating up of stator components. 
         [0049]    While the stator bars in the active part can expand mainly in axial direction, for the end-winding part expansions along radial and tangential directions can occur as well. 
         [0050]    Therefore, the thermal expansion experienced by the connection art can also be caused typically by the bars of the stator assembly. 
         [0051]    Also given by the arrangement of the bars in the end-winding, there may be a tangential force due to thermal expansion (not shown). The resilient reaction of the U-shaped junction allows the thermal expansion of the connection arm along the tangential direction without leading to breakage of the support element. 
         [0052]    Although the present invention has been fully described in connection with a preferred embodiment, it is evident that modifications may be introduced within the scope thereof, not considering the application to be limited by these embodiments, but by the content of the following claims.