Abstract:
Disclosed herein are pressure attenuator devices for rotating electrical machines, The pressure attenuation device contains a screen with a plurality of openings, wherein the screen is positioned within the internal cavity and configured to segregate a flame front propagating from an ignition source formed by a combustion process originated in the cavity. Thus, with the use of the pressure attenuation device, an expressive reduction of pressure inside the cavity of the rotating electrical machine is obtained.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of and the priority to U.S. Provisional Application No. 62/055,755, filed Sep. 26, 2014. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates to an internal pressure attenuator device for rotating electrical machines able to operate in explosive atmospheres and, more specifically, to an apparatus that allows for reducing the internal pressure in the machine casing, enabling significant reductions of the pressure level inside the machine and preventing an eventual combustion process. 
       BACKGROUND 
       [0003]    Rotating electrical machines are widely known in the art, and basically include a static part (stator) and a rotating part (rotor). In a simplified version, the stator is made up of a slotted magnetic core where coils or windings are placed, and the rotor consists of a slotted magnetic core that can be filled with coils or windings, or filled with liquid metal and then so sealed. This assembly is mounted on an axle, thus producing the rotor. 
         [0004]    When used in areas classified as “explosive atmosphere”, electrical machines may present the risk of acting as an ignition source. 
         [0005]    Explosive atmospheres are those that contain, in addition to oxygen, a proportion of gas, steam, dust or fibers, where a spark from an electrical circuit or the heating of a device can be an ignition source or cause an explosion. This kind of atmosphere is found, for example, in petrochemical, food, pharmaceutical, paper, and textile industries. 
         [0006]    A “combustion process” depends upon the rate of flame propagation and to the generated pressures. Usually, combustion processes with flame propagation speeds of up to 2 m/s and pressures below 4 bar are referred to as “burning”. For speeds between 0.5 and 4.0 m/s at pressures ranging between 4.0 and 10.0 bar the term “explosion” is used. The term “blasting” is adopted when the pressure exceeds 10.0 bar and speeds exceed 30 m/s and even exceed the supersonic speed. As used herein, the term “explosion” encompasses any type of combustion process. 
         [0007]    The electrical machines designed to operate in this kind of atmosphere are built to avoid or reduce the risk of explosion. One known way to achieve this is to confine the parts that can cause the ignition of an explosive atmosphere within a casing capable of withstanding the pressure generated by an internal explosion without allowing it to spread to the external environment. The machine casing is capable of withstanding the pressure wave generated by the explosion inside. Usually, the thickness of the casing is increased to make it able to withstand the mechanical stresses from this explosion process. Thus, the intention is to limit the effect—in this case, the pressure generated by the combustion process, and not the cause—the increase in pressure. 
         [0008]    Increasing the thickness of the casing causes an increase in weight and volume of the rotating electrical machine, and consequently, operational restrictions and costs increase. In some cases, performing prior tests is required to ensure that the components of the casing are able to withstand the occurrence of explosion. These tests, which can be performed on the electrical machine components and/or on the electrical machine after assembled, result in a higher manufacturing costs and higher manufacturing times, thus directly impacting the competitiveness of the product. 
         [0009]    European document EP 0113628 discloses an electrical rotary machine with an explosion-proof casing formed with shields. The shields include a surface substantially perpendicular to the axis of the rotor, and an outer cylindrical surface, and an inner cylindrical surface, each of these cylindrical surfaces being parallel to the axis of the rotor. The explosion-proof casing is constituted by the casing and fixed shields, the cylindrical surfaces being at the same side of their plane surface and the outer cylindrical surface being situated in the immediate vicinity of the casing. 
         [0010]    These shields may perhaps be a more economical solution than the one involving a casing with a greater thickness, but use of the reinforcement shields still causes a significant increase of the whole weight of the machine. 
       BRIEF SUMMARY 
       [0011]    The systems and methods disclosed herein may address the above-mentioned shortcomings in the art, and provide a variety of other benefits. The invention disclosed herein provides for a new rotating electrical machine that allows the attenuation of the internal pressure in the machine casing. This new machine is thus able to operate in explosive atmospheres and provides great economies in the weight and costs of the electrical machines. 
         [0012]    Embodiments of the invention provide an internal pressure attenuation solution that does not require increasing the thickness and mass of the rotating electrical machine casing. Embodiments of the invention provide an internal pressure attenuation solution that can be used in several different types of rotating electrical machines. Embodiments of the invention provide an internal pressure attenuation solution that is able to avoid flame propagation to the exterior of the casing. 
         [0013]    Embodiments of the invention include a pressure attenuator device for a rotating electrical machine of the type having a casing that defines a cavity along with the rotor stator assembly. The pressure attenuator device is positioned within the cavity and comprises a screen with a plurality of openings, which segregate a flame front propagating from an ignition source formed by a combustion process originating in the cavity. The segregation of the frame front prevents compression of the gases ahead of the flame front, which have not yet burnt. This displacement of the gases before they are burnt affects the pressure within the cavity. An expressive reduction of pressure inside the cavity of the rotating electrical machine is thus obtained. 
         [0014]    The term “openings” refers to any material discontinuity, such as slots, holes, orifices, or hollow spaces in the device, having any geometry or dimensions, provided that they allow the segregation of the flame front. 
         [0015]    In certain embodiments, the pressure attenuator device is a screen. In certain embodiments, the screen includes a circular ring defined by a cylindrical wall, with the plurality of openings being disposed on the ring wall. In certain embodiments, the openings have a circular geometry. 
         [0016]    In certain embodiments, the screen is a polyhedron-shaped structure defined by a plurality of wall sides, with the plurality of openings being disposed on the wall sides. 
         [0017]    In certain embodiments, the screen is a disk defined by a flat wall and an inclined wall which projects from the flat wall, with the plurality of openings being disposed on the inclined wall. In certain embodiments, the openings have a circular geometry. 
         [0018]    In certain embodiments, the screen is positioned concentrically and co-radially to the center of the stator coils of the rotor-stator assembly. However, in an alternative embodiment, the screen is positioned under or over the stator coil head of the rotor-stator assembly in the radial direction. 
         [0019]    Certain embodiments include a rotating electrical machine of the type that has a casing with a cover defining a cavity and a rotor-stator assembly inside the casing. The electrical machine includes a pressure attenuator device inside the cavity, such as a screen with a plurality of openings. These openings segregate a flame front from an ignition source formed by the combustion process originated in the cavity. 
         [0020]    In certain embodiments, the screen may be a circular ring defined by a cylindrical wall, with the plurality of openings being disposed on the ring wall. In certain embodiments, the pressure attenuating device can be a disk defined by a flat wall and a inclined wall which projects from the flat wall, with the plurality of openings being disposed on the inclined wall. In certain embodiments, the pressure attenuating device can be a screen defined by a polyhedron-shaped structure with a plurality of wall sides, with the plurality of openings being disposed on the wall sides. 
         [0021]    In an embodiment, the pressure attenuator device is attached to one of the covers of the rotating electrical machine. In certain embodiments, the pressure attenuating device can be attached to any location inside the cavity, for example, the coil or the axle of the electrical rotating machine. 
         [0022]    In certain embodiments, the pressure attenuating device is positioned concentrically and co-radially to the center of the stator coils of the rotor-stator assembly. In certain embodiments, the pressure attenuating device is positioned under or over the stator coil head of the rotor-stator assembly in the radial direction. 
         [0023]    Additional features and advantages of an embodiment will be set forth in the description which follows, and in part will be apparent from the description. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the exemplary embodiments in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The accompanying drawings constitute a part of this specification and illustrate an embodiment of the invention and together with the specification, explain the invention. 
           [0025]      FIGS. 1A-1H  are graphical representations showing, over time, the flame front and the pressure inside the cavity of a rotating electrical machine, in accordance with the prior art. 
           [0026]      FIG. 2  is a perspective view of the pressure attenuator device according to an exemplary embodiment of a pressure attenuating device, as described herein. 
           [0027]      FIG. 3  is a sectioned view of an exemplary embodiment of a rotating electrical machine having a pressure attenuator device, as described herein. 
           [0028]      FIGS. 4A to 4L  are graphical representations showing, over time, the flame front and the pressure inside the cavity of an exemplary embodiment of a rotating electrical machine with a pressure attenuator device, as described herein. 
           [0029]      FIG. 5  is a perspective view of the pressure attenuator device, according to an exemplary embodiment, as described herein. 
           [0030]      FIG. 6  is a perspective view of the pressure attenuator device, according to an exemplary embodiment, as described herein. 
           [0031]      FIG. 7  is a sectioned view of a rotating electrical machine having a pressure attenuator device, according to an exemplary embodiment, as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Reference will now be made in detail to certain embodiments, examples of which are illustrated in the accompanying drawings. The embodiments described herein are intended to be exemplary. One skilled in the art recognizes that numerous alternative components and embodiments may be substituted for the particular examples described herein and still fall within the scope of the invention. 
         [0033]      FIGS. 1A-1H , are graphical representations showing, over time, the flame front and the pressure inside the cavity of a rotating electrical machine in accordance with the prior art. The point P corresponds to the ignition point.  FIGS. 1A, 1C, 1E, and 1G  show the flame front at a temperature of 2000 K.  FIGS. 1B, 1D, 1F, and 1H  show the pressure, in bars, within the cavity. 
         [0034]    The figures show the cavity inside the cover and part of the rotating electrical machine casing (e.g. an electric motor). 
         [0035]    As can be seen in  FIGS. 1A-1H , in the process of combustion in a cavity, the flame front tends to propagate spherically from the ignition point (point P in the figures) The propagation process of the flame front tends to compress the gas mixture, which has not been consumed, resulting in a peak pressure when this mass of compressed gas passes through the combustion process, as there is a concentration of the potential energy. 
         [0036]    To avoid accidents, in rotating electrical machines known from the prior art, the casing must be dimensioned to withstand the pressure peak. 
         [0037]    Embodiments of the present invention propose a new solution that allows the segmentation of the propagation of the flame front on several different fronts, avoiding the localized concentration of the gases that have not been consumed. This is achieved by a pressure attenuator device disposed within the cavity of the rotating electrical machine, the device comprising a screen with openings that enable the segregation of the flame front. 
         [0038]      FIG. 2  shows an exemplary embodiment of the pressure attenuator device. In this embodiment, the device  1  comprises a circular ring  2  defined by a cylindrical wall having a plurality of openings  3  disposed on the ring wall. 
         [0039]    The surface of the cylindrical wall may be smooth, as shown in the figures, or uneven, such, as for example, a corrugated or notched wall. 
         [0040]    The ring  2  is attached to the cover at the extremity of the rotating electrical machine. As known to those skilled in the art, the cover is the end portion of the machine casing, that is, the cover that closes the cylindrical housing of the machine. 
         [0041]    Thus, the device  1  may also include a substantially perpendicular edge  4  which helps to secure the attachment of the ring  2  (e.g. by means of screws  5 ). 
         [0042]    In certain embodiments, the device  1  is positioned concentrically and co-radially to the center of the stator coils, and in the axial direction between the stator head and the casing cover.  FIG. 3  is a sectioned view of an exemplary embodiment of a rotating electrical machine having a pressure attenuator device. 
         [0043]    However, it should be noted that this position may vary depending on the internal ventilation circuit of the rotating electrical machine. For example, the device  1  may be positioned to be under or over the coil head in the radial direction. 
         [0044]    The pressure attenuator device in certain embodiments of the present invention can be attached anywhere in the cavity of the rotating electrical machine, including for example being attached to the cover, casing, coil or even to the machine axle. 
         [0045]    In the first embodiment shown in the figures, the diameter of the ring  2  of the device  1  is associated with the size of the rotating electrical machine, and can range from about 50 mm to 1000 mm. 
         [0046]    The geometry of the openings  3  of the circular ring can also assume different shapes, while still maintaining the system efficiency. Similarly, the number and size of openings  3  can be varied depending on the dimensions of the circular ring  2 . 
         [0047]    Accordingly, it should be noted that the term “openings” encompasses any material discontinuity, such as slots, holes, orifices, or hollow spaces in the device, having any geometry or dimensions, provided that they allow the segregation of the flame front. 
         [0048]      FIGS. 4A to 4L  are graphical representations showing, over time, the flame front and the pressure inside the cavity of a rotating electrical machine having the pressure attenuator  1  according to an exemplary embodiment of the present invention. Similar to that illustrated in  FIGS. 1A to 1H , the point P corresponds to the ignition point.  FIGS. 4A, 4C, 4E, 4G, 4I and 4K  show the flame front at a temperature of 2000 K.  FIGS. 4B, 4D, 4F, 4H, 4J, and 4L  show the pressure, in bars, within the cavity. 
         [0049]    As can be seen in  FIGS. 4A to 4L , the device  4  segregates the flame front, thus avoiding it compresses and directs the gases that have not yet passed through the combustion process. Thus, with the use of the device  1  of the present invention, an expressive reduction of pressure inside the cavity of the rotating electrical machine is obtained. 
         [0050]      FIG. 5  shows an embodiment of the invention, where the screen  10  is formed by a polyhedron-shaped structure  20  defined by a finite plurality of wall sides  20   a.  The plurality of openings is disposed on the plurality of wall sides  20   a.    
         [0051]    Although  FIG. 5  illustrates an embodiment of a polyhedral structure with  6  wall sides, it should be understood that the number of polyhedral wall sides could be larger or smaller without departing from the inventive concept of the invention disclosed herein. In certain embodiments, the finite number of sides of the polyhedron shaped structure is between 3 and 30 sides. 
         [0052]    The screen  10  may further comprise edges  40  substantially perpendicular to the side walls  20   a.  These edges  40  can be used for attaching the screen  10  to the machine, for example, by means of screws. 
         [0053]      FIGS. 6 and 7  show another exemplary embodiment of the pressure attenuator device.  FIG. 6  is a perspective view of the pressure attenuator device, according to an exemplary embodiment as described herein. The screen includes a disk  100  defined by a flat wall  200  and an inclined wall  201 , the inclined wall  201  having a plurality of openings  300 . 
         [0054]    The device  100  may further comprise an edge  400  that aids its attachment to the machine. 
         [0055]    In certain embodiments, the disk  200  is fixed to the end cover of the rotating electric machine. As known to those skilled in the art, the cover is the end portion of the machine casing, i.e., the element responsible for closing the casing in the axial direction. 
         [0056]    The attachment of the disk to the cover is performed by means of bolts and spacer sleeves. 
         [0057]    In certain embodiments, the device  100  is positioned concentrically and coradial to the center of the stator coils, and in the axial direction between the head coil and the casing cover. 
         [0058]    In another embodiment, the disk  100  is mounted on the axle E of motor. The attachment of the disk to the axle E may be performed by interference or by means of a fastening element.  FIG. 7  illustrates this arrangement. In this embodiment, the pressure attenuator  100  is installed on the axle E next to the bearing, in order to prevent the spread of flame to the outside of the casing, through the interstitial generated between the axle and the casing. 
         [0059]    The pressure attenuator device can be made of any suitable material such as steel, cast iron or thermoset. 
         [0060]    The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein. 
         [0061]    While various aspects and embodiments have been disclosed, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.