Abstract:
A three-phase/two-phase rotary transformer includes a first single-phase rotary transformer and a second single-phase rotary transformer the first transformer including a first body defining a first slot, a first coil in the first slot, a second body defining a second slot, and a second coil in the second slot the second transformer including a third body defining a third slot, a third coil in the third slot, a fourth body defining a fourth slot, and a fourth coil in the fourth slot, wherein one terminal of the first coil is connected to the midpoint of the third coil, said the first body the first coil the third body, and said the third coil forming a three-phase portion of the transformer the second body the second coil the fourth body, and said fourth coil forming a two-phase portion of the transformer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the general field of transformers. In particular, the invention relates to a three-phase/two-phase transformer. 
         [0002]    In certain situations, it may be necessary to transfer energy or signals in balanced manner from a three-phase source to a two-phase source. There exist three-phase/two-phase transformers that are stationary, and in particular one known as a “Scott connection” and another known as a “Leblanc connection”. 
         [0003]      FIG. 1  is a diagram of the Scott connection. Two single-phase transformers  1  and  2  are used. The transformer  1  has a primary  3  with n 1  turns and a secondary  6  with n 2  turns. The transformer  2  has a primary  4  with n′ 1  turns and a secondary  7  with n 2  turns. 
         [0004]    In  FIG. 1 , there can be seen:
       A, B, and C, which are the points for connection to the three-phase network;   I a , I b , and I c , which are the three-phase currents entering via the points A, B, and C; and   V 1 , I 1 , V 2 , I 2 , which are the two-phase voltages and currents.       
 
         [0008]    The transformer  1  has its n 1 -turn primary  3  connected between the terminals A and B of the three-phase network. The transformer  2  has its n′ 1 -turn primary  4  connected between the terminal C of the three-phase network and the midpoint  5  of the primary  3  of the transformer  1 . 
         [0009]    The primary voltages are in quadrature, as are the secondary voltages V 1  and V 2 . 
         [0010]    For a ratio n′ 1 =(√3/2)n 1 , the secondary voltages V 1  and V 2  have the same value and they are in quadrature. The ratio of the currents is given by: 
         [0000]    
       
         
           
             
               
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         [0011]    When it is desired to transfer energy or signals in balanced manner from a three-phase source to a two-phase source in reference frames that are rotating relative to each other, one solution consists in using a stationary three-phase/two-phase transformer and two single-phase rotary transformers. Another solution consists in using three single-phase rotary transformers in a Leblanc connection. 
         [0012]    Nevertheless, both of those solutions require considerable weight and volume. Furthermore, the first solution encounters current inrush problems when switching on and also problems of residual magnetization. 
         [0013]    There thus exists a need for an improved solution that enables energy to be transferred in balanced manner from a three-phase source to a two-phase source in reference frames that are rotating relative to each other. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0014]    The invention provides a three-phase/two-phase rotary transformer, characterized in that it comprises a first single-phase rotary transformer and a second single-phase rotary transformer, 
         [0015]    the first transformer comprising a first body made of ferromagnetic material defining a first annular slot of axis A, an n′ 1 -turn first toroidal coil of axis A in the first slot, a second body made of ferromagnetic material defining a second annular slot of axis A that is open towards the first slot, and an n 2 -turn second toroidal coil of axis A in the second slot; 
         [0016]    the second transformer comprising a third body made of ferromagnetic material defining a third annular slot of axis A, an n 1 -turn third toroidal coil of axis A in the third slot, a fourth body made of ferromagnetic material defining a fourth annular slot of axis A that is open towards the third slot, and an n 2 -turn fourth toroidal coil of axis A in the fourth slot, 
         [0017]    wherein one terminal of the first coil is connected to the midpoint of the third coil, 
         [0018]    the first body, said first coil, the third body, and the third coil being stationary relative to one another and forming a three-phase portion of the transformer, 
         [0019]    the second body, said second coil, said fourth body, and the fourth coil being stationary relative to one another and forming a two-phase portion of the transformer, and 
         [0020]    the three-phase portion and the two-phase portion being movable in rotation about the axis A relative to each other. 
         [0021]    Since the same transformer made up of two single-phase rotary transformers serves firstly to perform three-phase/two-phase transformation and secondly to provide transmission between two reference frames that are rotating relative to each other, these two functions are performed with limited volume and weight. Furthermore, it has been found that this connection makes it possible to obtain transfer that is balanced. 
         [0022]    In an embodiment, n′ 1 =(√3/2)n 1 . 
         [0023]    The ratio between the section of the electrically conductive material of the first coil and the section of the electrically conductive material of the third coil may be equal to √3. It is thus possible to compensate for the different numbers of turns between the two coils. This enables resistances to be balanced. In the event of the coils being at different distances from the axis of rotation, this ratio should be reevaluated accordingly. 
         [0024]    In an embodiment, the second coil comprises a first half-coil and a second half-coil that are joined together by the midpoint, the winding directions of the half-coils corresponding to magnetic potentials of opposite directions for currents entering via the terminals of the second coil. 
         [0025]    The two-phase portion further includes at least one set of three-phase coils. This makes it possible to provide a transformer having a plurality of secondaries that can power an arbitrary number of loads greater than one in balanced manner. 
         [0026]    The three-phase portion may surround the two-phase portion relative to the axis A, or vice versa. This corresponds to a “U-shaped” embodiment. 
         [0027]    The three-phase portion and the two-phase portion may be situated one beside the other in the direction of the axis A. This corresponds to a “E-shaped” or “pot-shaped” embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Other characteristics and advantages of the present invention appear from the following description made with reference to the accompanying drawings, which show embodiments having no limiting character. In the figures: 
           [0029]      FIG. 1  is an electric circuit diagram of a prior Scott connection three-phase/two-phase stationary transformer; 
           [0030]      FIG. 2  is a section view of a three-phase/two-phase rotary transformer in a first embodiment of the invention; 
           [0031]      FIGS. 3A and 3B  are electric circuit diagrams showing a plurality of variant connections for the coils of the  FIG. 2  transformer; 
           [0032]      FIG. 4  is a section view of a three-phase/two-phase rotary transformer in a second embodiment of the invention; 
           [0033]      FIG. 5  is a section view showing a variant of the  FIG. 2  transformer having a plurality of secondaries; and 
           [0034]      FIG. 6  is a section view of a variant of the  FIG. 4  transformer, having a plurality of secondaries. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0035]      FIG. 2  is a section view of a transformer  10  in a first embodiment of the invention. The transformer  10  is a three-phase/two-phase rotary transformer. 
         [0036]    The transformer  10  comprises two single-phase rotary terminals, namely a transformer  11  and a transformer  21 . 
         [0037]    The transformer  11  comprises:
       a body  12  made of ferromagnetic material in the form of a ring of axis A and having a slot  14  formed therein that is open towards the axis A;   an n′ 1 -turn toroidal coil  16  of axis A in the slot  14 ;   a body  13  made of ferromagnetic material, in the form of a ring of axis A surrounded by the body  12  about the axis A and having formed therein a slot  15  that is open towards the slot  14 ; and   an n 2 -turn toroidal coil  17  of axis A in the slot  15 .       
 
         [0042]    The bodies  12  and  13  are movable in rotation relative to each other about the axis A. 
         [0043]    In corresponding manner, the transformer  21  comprises:
       a body  22  made of ferromagnetic material, in the form of a ring of axis A and having formed therein a slot  24  that is open towards the axis A;   an n′ 1 -turn toroidal coil  26  of axis A in the slot  24 ;   a body  23  made of ferromagnetic material, in the form of a ring of axis A, surrounded by the body  22  about the axis A and having formed therein a slot  25  that is open towards the slot  24 ; and   an n 2 -turn toroidal coil  27  of axis A in the slot  25 .       
 
         [0048]    The term “toroidal” is not used restrictively in the sense of a solid generated by rotating a circle about an axis. On the contrary, as in the example shown, the section of a toroidal coil may, in particular, be rectangular. 
         [0049]    The coil  26  is made up of two half-coils  26   a  and  26   b  each having n 1 /2 turns. The bodies  22  and  23  are movable in rotation relative to each other about the axis A. 
         [0050]    In the transformer  10 , the bodies  12  and  22  and the coils  16  and  26  are stationary relative to one another. The coils  16  and  26  may be connected to a three-phase source. The bodies  12  and  22  and the coils  16  and  26  thus form parts of a three-phase portion  31  of the transformer  10 . Likewise, the bodies  13  and  23  and the coils  17  and  27  are stationary relative to one another. The coils  17  and  27  may be connected to a two-phase source. The bodies  13  and  23  and the coils  17  and  27  thus form parts of a two-phase portion  32  of the transformer  10 . 
         [0051]    The three-phase portion  31  and the two-phase portion  32  are movable in rotation about the axis A relative to each other. For example, the three-phase portion  31  may be a stator and the two-phase portion  32  a rotor, or vice versa. In a variant, both the three-phase portion  31  and the two-phase portion  32  are movable in rotation relative to a stationary reference frame (not shown). 
         [0052]    Furthermore, the magnetic circuit of the transformer  11  as formed by the bodies  12  and  13  is separated from the magnetic circuit of the transformer  21  as formed by the bodies  22  and  23  by a space  33 . In other words, said transformers  11  and  12  are magnetically segregated. 
         [0053]      FIG. 2  also shows the magnetic core  18  of the transformer  11  and the magnetic core  28  of the transformer  21 . The term “magnetic core” is used to mean a portion of the magnetic circuit in which the same-direction flux created by a coil is the greatest. 
         [0054]      FIG. 3A  is an electric circuit diagram showing the way the coils  16  and  26  are connected. 
         [0055]    In  FIG. 3 , there can be seen:
       Ap, Bp, and Cp, which are the terminals of the coils  16 ,  26   b,  and  26   a,  respectively, that are connected to the three-phase network;   Oap, Obp, Ocp, which are the terminals of the coils  16 ,  26   b,  and  26   a,  respectively, that are opposite from the terminals Ap, Bp, and Cp;   Iap, Ibp, and Icp, which are the three-phase currents entering the terminals Ap, Bp, and Cp, respectively;   Pa, which is the magnetic potential in the magnetic core  18  corresponding to the current Iap;   Pb which is the magnetic potential in the magnetic core  28  corresponding to the current Ibp; and   Pc which is the magnetic potential in the magnetic core  28  corresponding to the current Icp.       
 
         [0062]    As shown in  FIG. 3A , the terminal Oap of the coil  16  is connected to the terminals Obp and Ocp of the coils  26   b  and  26   c,  which thus constitutes the midpoint of the coil  26 . 
         [0063]    Furthermore,  FIG. 3A  shows the winding directions of the coils  16 ,  26   a,  and  26   b  by means of black dots, using the following convention:
       if the black dot is on the left and the current enters on the same side as the black dot, then the corresponding magnetic potential goes to the right;   if the black dot is on the left and the current enters from the side opposite from the black dot, then the corresponding magnetic potential goes to the left;   if the black dot is on the right and the current enters on the same side as the black dot, then the corresponding magnetic potential goes to the right; and   if the black dot is on the right and the current enters from the side opposite from the black dot, then the corresponding magnetic potential goes to the left.       
 
         [0068]    Given the winding directions of the coils  26   a  and  26   b,  it can thus be seen that the magnetic potentials Pb and Pc in the magnetic core  28  are in opposite directions.  FIG. 3B  shows a variant for the winding directions, that likewise makes it possible to obtain magnetic potentials Pb and Pc in opposite directions. 
         [0069]    Below, V 1 , I 1 , V 2 , and I 2  designate the two-phase voltages and currents in the coils  17  and  27 . 
         [0070]    It can be seen that the transformer  10  is a Scott connection three-phase/two-phase rotary transformer. In similar manner to the Scott connection three-phase/two-phase stationary transformer  1  of  FIG. 1 , the primary voltages are in quadrature, and the same applies to the secondary voltages V 1  and V 2 . 
         [0071]    For a ratio n′ 1 =(√3/2)n 1 , the secondary voltages V 1  and V 2  have the same value and are in quadrature. The ratio of the currents is given by: 
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         [0072]    Resistances are balanced by appropriately selecting the sections for the conductive materials of the coils  16 ,  26   a,  and  26   b : the sections of the coils  26   a  and  26   b  are equal if their mean distances from the axis of rotation are equal. The section of the coil  16  is √3 times the section of the coils  26   a  and  26   b  for the same mean distance from the axis of rotation. If it is desired to conserve balanced resistances in the phases, the longest phase must also have a larger section in order to compensate for its greater length. The magnetic coupling performed by the magnetic circuit of the single-phase rotary transformer  21  possesses two phases, thereby making it possible to obtain a coupling coefficient of √3 for the fluxes created compared with a single-phase transformer per phase. This coefficient makes it possible either to reduce the number of coil turns per phase, or else to reduce the magnetizing current that is absorbed. 
         [0073]    The transformer  10  presents several advantages. It makes it possible to transfer energy or signals between a three-phase source and a two-phase source in reference frames that are rotating relative to each other, and to do so without contact and in balanced manner. Furthermore, the volume and the weight of the transformer  10 , corresponding to the volumes and to the weights of the two single-phase rotary transformers  11  and  21 , can be reduced compared with the three-transformer solution mentioned in the introduction, in which the three-phase/two-phase transformation is performed by a first transformer that is stationary, and then the change of reference phase is performed by two single-phase rotary transformers. Finally, it requires only toroidal coils of axis A, which are particularly simple in structure. 
         [0074]    In  FIG. 2 , the coils  26   a  and  26   b  are shown as being one beside the other, however other positions may be suitable. For example, in the slot  24 , the coils  26   a  and  26   b  may be one beside the other in the axial direction, one around the other relative to the axis A, or they may be mixed together. 
         [0075]    The transformer  10  may be considered as a U-shaped variant in which the three-phase portion surrounds the two-phase portion relative to the axis A. In a variant, the two-phase portion may surround the three-phase portion relative to the axis A. 
         [0076]      FIG. 4  is a section view of a transformer  110  in a second embodiment of the invention. The transformer  110  is a three-phase/two-phase rotary transformer and it may be considered as being an “E-shaped” or a “pot-shaped” variant of the “U-shaped” transformer  10 . In this variant, the three-phase portion and the two-phase portion are situated one beside the other in the direction of the axis A, and the slots  14  and  15  are open towards each other in the direction of the axis A. In  FIG. 4 , the same references as in  FIG. 2  are used again without risk of confusion for designating elements that correspond, and a detailed description is therefore not necessary. 
         [0077]    In known manner in the field of transformers, a transformer may have a plurality of secondaries. Thus, a transformer in accordance with the invention may comprise for its primary, a three-phase portion of the same type as the three-phase portion  31  of the transformer  10  or  110 , and for its secondary, a two-phase secondary portion of the same type as the two-phase portion  32  of the transformer  10  together with at least one set of additional three-phase or two-phase coils. 
         [0078]    This makes it possible to power an arbitrary number of loads in balanced manner from a three-phase source. For example, in order to power 11 loads, it is possible to use three loads on the three-phase secondary and two loads on the two-phase secondary (11=3*3+2). 
         [0079]      FIG. 5  shows an example of a transformer  210  having a plurality of secondaries. The transformer  210  may be considered as a variant of the transformer  10  and it further comprises a set of three-phase coils for its secondary. Elements corresponding to embodiments of the transformer  10  are designated by the same references, without risk of confusion. The transformer  210  also has an n√3-turn toroidal coil  40  of axis A in the slot  15  and an n 3 -turn toroidal coil  41  of axis A in the slot  25 . The coil  41  is made up of two half-coils  41   a  and  41   b,  each having n 3 /2 coils. The coils  40 ,  41   a,  and  41   b  are connected to one another and to the secondary three-phase source in a manner that corresponds to the connection of the coils  16 ,  26   a,  and  26   b.    
         [0080]    In corresponding manner,  FIG. 6  shows another example of a transformer  310  having a plurality of secondaries. The transformer  310  may be considered as being a variant of the transformer  110 , and it further comprises a set of three-phase coils for its secondary. Elements that correspond to elements of the transformer  110  are designated by the same references, without risk of confusion. The transformer  310  also has an n√3-turn toroidal coil  50  of axis A in the slot  15 , and an n 3 -turn toroidal coil  51  of axis A in the slot  25 . The coil  51  is made up of two half-coils  51   a  and  51   b,  each having n 3 /2 turns. The coils  50 ,  51   a,  and  51   b  are connected to one another and to the secondary three-phase source in a manner that corresponds to the connection of the coils  16 ,  26   a,  and  26   b.