Patent Publication Number: US-2011051480-A1

Title: 20º  PHASE-SHIFTING AUTOTRANSFORMER

Description:
The invention relates to autotransformers used notably for converting alternating (AC) electric power into (DC) direct power. Autotransformers may be used to reduce weight and space requirement if there is no requirement for insulation between the potentials on the side of the power supply network and the potentials on the side of use. 
     AC/DC conversion from a three-phase power supply network voltage uses rectifier bridges; in theory, a single bridge of twice three diodes would be sufficient to rectify three-phase voltage into direct voltage; but in practice the use of a single bridge supplied by the three-phase network produces a direct current affected by too great a residual oscillation, which is unacceptable for many applications. In addition, the rectification causes a reinjection of currents into the network, these currents having harmonic frequencies of the frequency of the power supply alternating current. These reinjections of harmonics are unacceptable if they are too great. 
     To reduce the direct voltage residual undulations and the harmonics of current reinjected into the network, it has already been proposed to increase the number of phases of the power supply current and the number of rectifier bridges. Therefore, typically, it is possible to transform the three-phase system, whose three phases are spaced at 120°, into a system with nine phases spaced at 40° which may be considered to be a system of three three-phase networks shifted by 40° relative to one another. Three six-diode bridges are used, each bridge being supplied by one of these networks. These eighteen-diode AC/DC converters are also called eighteen-pulse converters. The residual undulations become weak, the reinjections of harmonics also. The nine phases are produced from one autotransformer. Such an achievement is for example described in U.S. Pat. No. 5,124,904. This autotransformer comprises a magnetic core with three branches and a main coil on each magnetic branch. The three main coils are connected in a triangle and it has been noted that a considerable share of the supply power passes through the magnetic circuit of the autotransformer. 
     In addition, the direct voltage obtained from this nine-phase system is higher than that which would be obtained from three phases, for various reasons, including the fact that the residual undulation is weaker and the direct voltage depends on the average value of the residual undulation. For reasons of equipment compatibility for example (three-phase voltage imposed, direct voltage of use imposed) the user may wish that there is not this change of direct voltage level when six-diode rectification is replaced by eighteen-diode rectification. To prevent finishing up with a direct voltage that is higher than that which would give a simply three-phase rectification (for the same three-phase power supply voltage value) it is then necessary to provide additional voltage-reduction means in the autotransformer. In U.S. Pat. No. 5,124,904, one embodiment provides that these means are constituted by additional windings which increase complexity and weight and the rates of leakage reactants. 
     U.S. Pat. No. 5,619,407 proposes a different solution to reduce the direct voltage supplied at the output of the rectifier bridges. This solution does not use additional windings, but it is not very satisfactory because it results in a nonsymmetric autotransformer structure; this absence of symmetry leads to a harmonic distortion and therefore a greater reinjection of harmonics to the power supply network; the greater the percentage of voltage reduction (percentage relative to the direct voltage that would supply the simple three-phase rectification), the more significant is this distortion. 
     In addition, the systems described above do not provide a solution for increasing the direct voltage relative to that which would give simply a six-diode three-phase rectification. There are cases where a user may wish to increase the direct voltage rather than reduce it. 
     The object of the invention is to alleviate the defects of the systems described above by proposing a nine-phase autotransformer making it possible to choose a desired level of direct voltage (higher or lower than that which a simple three-phase rectification would give), while limiting the weight and space requirement of the autotransformer. 
     Accordingly, the subject of the invention is an autotransformer designed to be connected to a three-phase voltage supply of given amplitude supplying three first output voltages of identical amplitudes, and six other output voltages of the same amplitude as the first three output voltages and divided into pairs symmetrically phase-shifted by 20° relative to the first three output voltages, characterized in that the output voltages have greater or lesser amplitudes than the amplitude of the three-phase supply. 
     By phase-shifting six output voltages by only 20° instead of the 40° proposed in the prior art described above, it is possible to reduce the power passing through the magnetic circuit of the autotransformer. At constant power of the autotransformer, it is possible to reduce the weight of the magnetic circuit. 
     A phase shift of 20° makes it possible to limit the output harmonic distortion of an AC/DC converter using an autotransformer according to the invention. Phase-shifting by 20° means a real phase shift that is able to depart slightly from a nominal value of 20°. It has been shown that the phase shift could lie in a range of 20°+ or −10% while retaining an acceptable distortion value. 
     Furthermore, the fact that all the output voltages have identical, or of course substantially identical, amplitudes makes it possible to use an autotransformer according to the invention in an AC/DC converter comprising a diode rectifier that is much more simple to use than a controlled rectifier, for example a thyristor-controlled rectifier. Specifically, when the output voltages of the autotransformer are not equal, the use of an uncontrolled rectifier induces a strong harmonic distortion that can be reduced with the aid of a controlled rectifier. 
    
    
     
       The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given as an example, the description illustrated by the attached drawing in which: 
         FIG. 1  represents a simplified diagrammatic view of a transformer with three magnetic branches designed for three-phase use; 
         FIG. 2  represents a vector composition making it possible to define the features of a voltage step-up autotransformer in a first star embodiment according to the invention; 
         FIG. 3  represents another vector composition making it possible to define the features of a voltage step-down autotransformer, in a second star embodiment according to the invention; 
         FIG. 4  represents the coils provided on a magnetic branch of the autotransformer of  FIG. 2  and  FIG. 3 ; 
         FIG. 5  represents the installation of the autotransformer making it possible to produce the two vector compositions of  FIG. 2  and  FIG. 3 ; 
         FIG. 6  represents another vector composition making it possible to define the features of a voltage step-up autotransformer, in a first triangle embodiment according to the invention; 
         FIG. 7  represents the installation of the autotransformer making it possible to produce the vector composition of  FIG. 6 ; 
         FIG. 8  represents another vector composition making it possible to define the features of a voltage step-down autotransformer, in a second triangle embodiment according to the invention; 
         FIG. 9  represents the installation of the autotransformer making it possible to produce the vector composition of  FIG. 8 ; 
         FIG. 10  represents an AC/DC converter using an autotransformer according to the invention. 
     
    
    
     For the purposes of clarity, the same elements will bear the same reference numbers in the various figures. 
     A few general principles are given first. 
       FIG. 1  shows the conventional principle of a three-phase transformer formed by coils placed around branches of a closed triple magnetic circuit. The closed triple magnetic circuit comprises a ferromagnetic core with a central branch M 1  to receive the coils corresponding to a first phase, and two lateral branches M 2  and M 3  connected to the central branch on either side of the latter to receive the coils of a second and a third phase respectively. The central branch M 1  and one of the lateral branches form a first closed magnetic circuit; the central branch and the other lateral branch form a second closed magnetic circuit; the two lateral branches M 2  and M 3  form a third closed magnetic circuit. 
     Several coils are wound onto each branch, certain of them forming transformer primaries and others forming secondaries. The installation is identical for the three branches, that is to say that the coils playing the same role on the various branches comprise the same number of turns and the same direction of winding. 
     As a simplified diagram,  FIG. 1  shows a respective main coil B 10 , B 20 , B 30  and a respective auxiliary coil S 1 , S 2 , S 3  on each branch of the magnetic core. The coils of one and the same magnetic branch are travelled over by the same magnetic flux. For greater convenience of representation, the auxiliary coils are represented beside the main coils although in reality the two coils are placed in the same location (one around the other, or even the layers of one are inserted between the layers of the other) in order to be traversed exactly by the same magnetic flux. 
     In the simplest connection diagram that can be imagined, transforming one three-phase voltage into another three-phase voltage, the main coils could be the primary windings of a transformer and the auxiliary coils would be secondary coils. The primary coils could be connected in a triangle or in a star, in order to receive the three-phase voltage to be converted. The secondary coils would also be connected either in a triangle or in a star in order to produce a three-phase voltage. The magnetic fluxes that travel in the three branches are identical but phase-shifted by 120° from one another. In the production of a transformer converting a three-phase voltage into a nine-phase voltage, the installation is more complex and uses a larger number of coils as will be seen, but the principle of a magnetic circuit with three symmetrical branches is retained in which the magnetic fluxes of the various branches are phase-shifted by 120° from one another and in which the coils of one and the same branch are all travelled over by the same magnetic flux. 
     At the terminals of a secondary coil of a magnetic branch there appears a voltage that is in phase with the voltage at the terminals of the primary coil of the same branch. The voltage generated in the secondary coil depends
         on the voltage value at the terminals of the associated primary,   on the ratio between the numbers of turns of the primary and of the secondary,   and on the direction of rotation of the current in the winding of the secondary coil relative to the direction of the current in the primary coil (the voltage phase is inverted if the directions are inverted).       

     For a transformer with insulation between potentials of the primary and potentials of the secondary, the terminals of the secondary coils are not connected to the terminals of the primary coils or to other circuit elements on the primary side. For an autotransformer (a transformer with no insulation), the terminals of the secondary coils may be connected to the terminals of the primary coils or to intermediate power outlets formed in the primary coils. The invention relates to autotransformers. 
     The principle of vector representation will now be described making it possible to describe the operation of a more complex transformer and notably an autotransformer capable of supplying nine secondary phases from the three phases of the primary power supply. 
     The phase and amplitude of the voltage (simple voltage present at a point of the circuit or differential voltage present between two points of the circuit) may be represented by a vector whose length represents the amplitude of the alternating (simple or differential) voltage and whose orientation represents the phase from 0° to 360° of this alternating voltage. 
     For the constitution of an autotransformer capable of producing nine phases from three phases spaced at 120°, the user looks for vector compositions which, from the initial three phases, make it possible to produce the nine phases sought. 
     The vectors used in this composition are obtained on the one hand from points representing the terminals of main or auxiliary coils and, on the other hand, from points representing intermediate power outlets of these coils. The voltage obtained between two intermediate power outlets of a main coil is in phase with the voltage of the main coil (the vectors are therefore collinear); its amplitude is a fraction of the voltage at the terminals of the main coil, this fraction being a function of the ratio between the number of winding turns situated between the intermediate power outlets and the total number of turns of the main coil; the relative length of the vector representing the voltage between two intermediate power outlets of a coil is determined by this ratio of number of turns. 
     According to the same principle, the voltage obtained at the terminals of an auxiliary coil associated with the main coil (that is to say travelled over by the same magnetic flux, so wound at the same location on one and the same magnetic branch) is in phase with the voltage at the terminals of the main coil (the vectors are therefore parallel) and its amplitude is also determined by the ratio between the number of turns of the auxiliary coil and the number of turns of the main coil; the length of the vector representing the voltage in the auxiliary coil is therefore, relative to the length of the vector representing the voltage in the main coil, in the ratio of the numbers of turns. 
     In this patent application, the term “main coil” will be used to designate a coil having two ends and intermediate power outlets, this term nevertheless not meaning that the main coil is necessarily a primary coil of the autotransformer. Specifically, in certain embodiments (voltage step-down transformer), the main coil will effectively be a primary coil in the sense that it is supplied directly by a voltage to be converted; but in other embodiments (step-up transformer), the main coil will not be a primary coil because the three-phase supply to be converted will not be applied between the two ends of this coil. 
       FIG. 2  represents a vector composition which makes it possible to lead to the present invention, in the case of a voltage step-up autotransformer. The autotransformer comprises three main coils B 10 , B 20 , B 30  connected in a star installation. The three main coils B 10 , B 20 , B 30  have a common terminal N forming the neutral of the autotransformer. The three-phase supply of the autotransformer is applied to three input points K″ 1 , K″ 2 , K″ 3  each belonging to one of the three main coils, respectively B 10 , B 20 , B 30 . 
     For convenience, in the following, the same letters (for example K″ 1 , K″ 2 , K″ 3 ) will designate both the terminals of a coil (in the figures representing coils), the ends of the vector representing the voltage at the terminals of this coil (in the figures representing the vector compositions) or else the voltage present between this terminal and a point situated at the origin of the corresponding vector diagram. 
     The three-phase supply comes from an alternating current power distribution network at a frequency that depends on the applications. In aviation, where the invention is of particular value because the requirements of weight, space requirement and suppression of harmonics are strong, the frequency is often 400 Hz and it may also be 800 Hz. 
     For the vector composition, the point N is chosen as the origin. The simple input and output voltages of the autotransformer will be referenced relative to this point. So, the vector NK″ 1  represents the amplitude and the phase of the simple voltage present on the terminal K″ 1  of the three-phase supply. If it is supposed that the three-phase supply applied at K″ 1 , K″ 2  and K″ 3  is well balanced, the neutral point N represents the point of reference at which the vector sum of the voltages NK″ 1 , NK″ 2 , NK″ 3  is zero. The vectors NK″ 2  and NK″ 3 , of the same amplitude as the vector NK″ 1 , are respectively oriented at +120° and −120° from the reference vector NK″ 1 . To simplify the vector notation, in all that follows the first letter of a vector is considered to be the origin of the vector and the second letter is the end of the vector; therefore, NK″ 1  represents the vector leaving N and going to K″ 1  and not the reverse. 
     In  FIG. 2 , the chosen phase reference is the phase of the simple voltage NK″ 1  (horizontal direction). The angles are measured in the clockwise direction. The direction of the vector NK″ 2  is at +120° and that of the vector NK″ 3  is at +240°. The other vector compositions use the same conventions of representation. 
       FIG. 4  represents the coils provided on the magnetic branch M 1  of the autotransformer. The coils of the other two branches M 2  and M 3  are similar and are deduced by replacing the reference numbers  1  by  2  or  3  depending on the branch. 
       FIG. 5  represents the installation of the autotransformer that makes it possible to produce the two vector compositions of  FIG. 2  and  FIG. 3 . 
     Each of the main coils B 10 , B 20  and B 30  comprises a first and a second terminal. The first terminals are connected at N. The second terminals are called respectively K′″ 1 , K′″ 2  and K′″ 3 . Each main coil B 10 , B 20  and B 30  comprises three intermediate power outlets K 1 , K′ 1  and K″ 1  for the coil B 10 , K 2 , K′ 2  and K″ 2  for the coil B 20  and K 3 , K′ 3  and K′ 3  for the coil B 30 . In the embodiment represented in  FIG. 2  (voltage step-up), the three three-phase input voltages are applied to the power outlets K″ 1 , K″ 2  and K″ 3 . The first three output voltages are in phase with the three-phase input voltages and are available at the second terminals K′″ 1 , K′″ 2  and K′″ 3  of the main coils B 10 , B 20  and B 30 . A coefficient k represents the ratio between the amplitude Va′ of the voltage of the nine output phases and the amplitude Va of the three three-phase input voltages 
     
       
      
       Va′=Va×k  
      
     
     The intermediate power outlets K 1 , K 2  and K 3  may be used to apply three-phase input voltages that differ from those provided on the power outlets K″ 1 , K″ 2  and K″ 3 . This arrangement is of value for example in the aviation sector. 
     In large-sized aircraft such as aircraft for carrying tens or hundreds of passengers, the electric power supply becomes a very important element in the general design of the craft. Specifically, the electric apparatus placed onboard and used either for the operation of the craft or for the onboard services are increasingly numerous and consuming more and more energy. 
     This energy is generated by alternators coupled to the engines of the aircraft and the alternators usually supply a three-phase voltage of 115 effective volts between neutral and phase, at a frequency of 400 Hz. This voltage is transported inside the aircraft by electric cables whose section is proportional to the square of the value of the current that must be able to be transported by these cables. Typically, it may be necessary to have several hundred meters of cables capable of transporting several kilowatts. The result of this is a considerable weight of copper or aluminum to be installed in the aircraft. 
     Consequently, it appeared that it could be preferable to now design aircraft in which the transported energy travels at 230 volts at least, in order to substantially divide by 4 the section of the cables transporting the energy. The alternators of such aircraft will therefore be designed to directly supply a three-phase power supply from 400 Hz to 800 Hz and 230 effective volts between neutral and phase. In addition, these modern aircraft will now be fitted with a DC electric power distribution network, typically at 540 volts (plus or minus 270 volts relative to the metal structure of the aircraft). The value of DC energy distribution is to make it possible, by means of variable-frequency inverters, to achieve an individual control of speed of certain synchronous or asynchronous motors present in the craft (compressors, air conditioners, fuel pumps etc.). 
     Furthermore, the aircraft must consume electric power when they are immobilized on the ground at an airport, with the engines stopped. This power is necessary for performing functions of lighting, air conditioning, maintenance, startup, etc. 
     They are therefore connected by means of a three-phase connector that can be accessed from outside the aircraft to electric power generator sets placed on the ground, administered by the airports. The generator sets supply all the three-phase power at 115 effective volts since most of the aircraft are fitted out to operate with 115 effective volts. It is possible to imagine that, in the future, the airports are provided with generator sets supplying 115 volts and 230 volts or that special generator sets supplying 230 volts are provided for the case in which an aircraft fitted with 230 volts should land. But this involves a cost that the airports do not wish to bear and this solution can be envisaged only in the very long term when the number of aircraft fitted with 230 volts is very significant. 
     In the immediate future, the solution is to provide on the aircraft a three-phase transformer placed between an outside power supply connector (designed to be connected to the generator on the ground) and the aircraft&#39;s 230 volt power supply network. This transformer adds additional weight and space requirement only for this airport logistics reason. 
     To remedy this problem, an autotransformer according to the invention may be supplied either at 115 V by the power outlets K 1 , K 2  and K 3  or at 230 V by the power outlets K″ 1 , K″ 2  and K″ 3 . 
     The other six output voltages are divided into pairs symmetrically phase-shifted by 20° relative to the first three output voltages. In order to produce them, the autotransformer comprises on each magnetic branch M 1 , M 2  and M 3  two auxiliary coils X 1  and Y 1  for the branch M 1 , X 2  and Y 2  for the branch M 2  and X 3  and Y 3  for the branch M 3 . The first output voltage A 1  is phase-shifted by −20° relative to the voltage K′″ 1  and is obtained in the following manner: a first terminal of the auxiliary coil Y 2  is connected to the power outlet K′ 1  and the second terminal of the auxiliary coil Y 2  forms the point A 1 . Similarly, the second output voltage B 1  is phase-shifted by +20° relative to the voltage K′″ 1  and is obtained by connecting a first terminal of the auxiliary coil X 3  to the power outlet K′ 1 . The second terminal of the auxiliary coil Y 2  forms the point B 1 . 
     A similar arrangement is made to obtain the last output voltages. The voltages A 2  and B 2  are phase-shifted respectively by −20° and +20° relative to the voltage K′″ 2  and the voltages A 3  and B 3  are phase-shifted respectively by −20° and +20° relative to the voltage K′″ 3 . The voltage A 2  is obtained by connecting a first terminal of the auxiliary coil Y 3  to the power outlet K′ 2 . The second terminal of the auxiliary coil Y 3  forms the point A 2 . The voltage B 2  is obtained by connecting a first terminal of the auxiliary coil X 1  to the power outlet K′ 2 . The second terminal of the auxiliary coil X 1  forms the point B 2 . The voltage A 3  is obtained by connecting a first terminal of the auxiliary coil Y 1  to the power outlet K′ 3 . The second terminal of the auxiliary coil Y 3  forms the point A 3 . The voltage B 3  is obtained by connecting a first terminal of the auxiliary coil X 2  to the power outlet K′ 3 . The second terminal of the auxiliary coil X 2  forms the point B 3 . 
     The lengths of the vectors represented in  FIG. 2  make it possible to define the number of turns of the various coils. First of all for the main coil B 10 , the ratio k between the amplitudes of the input voltage Va and output voltage Va′ makes it possible to define the ratio between the total number N of turns of the winding B 10  and the number of turns n″ 1  between the points N and k″ 1 : 
         N=n″ 1 ×k    
     The number of turns n 1  between the points N and K 1  is defined in the same manner. For example, if the autotransformer is supplied either at 230 V by the power outlets K″ 1 , K″ 2  and K″ 3  or at 115 V by the power outlets K 1 , K 2  and K 3 , this gives: 
         N=n 1×2 k  
 
     The numbers of turns n′ 1  between the terminal N and the power outlet K′ 1  and the number of turns of the auxiliary coils may be defined by geometric construction in  FIG. 2  or else by trigonometric computation. 
     In order to ensure the symmetry of the autotransformer, the numbers of turns of the other main coils B 20  and B 30  are defined in the same manner by changing the reference numbers  1  with  2  or  3  in the preceding determinations. For the same reason, the auxiliary coils all have the same number of turns. The symmetry of the autotransformer makes it possible to provide its reversibility and makes it possible to introduce no phase shift between the current and the voltage on the supply. 
     The direction of winding of the various coils on their respective magnetic core is given by the orientation of the vectors represented in  FIG. 2  or else by the points represented in  FIG. 5  in the vicinity of the first turn of each coil; as a reminder, for the main coils, the points indicating the first turns have been represented for each intermediate power outlet. This convention is also used for the other vector compositions and all the figures representing the installation of autotransformers. 
       FIG. 3  represents another vector composition making it possible to define the features of a voltage step-down autotransformer whose main coils B 10 , B 20  and B 30  are connected in a star. Unlike the embodiment represented in  FIG. 2 , the three-phase supply voltages are applied between the terminals K′″ 1 , K′″ 2  and K′″ 3  of the three main coils. The first three output voltages in phase with the input voltages are collected at the points K″ 1 , K″ 2  and K″ 3 . It is always possible to provide two possibilities for supplying the autotransformer, either by the terminals K′″ 1 , K′″ 2  and K′″ 3 , or by the intermediate power outlets K 1 , K 2  and K 3 . The rest of the vector construction of  FIG. 3  is achieved by retaining the same modulus for the various vectors representing the output voltages K″ 1 , K″ 2 , K″ 3 , A 1 , B 1 , A 2 , B 2 , A 3  and C 3 . The numbers of turns are computed and the direction of winding of the coils is determined by analogy with what has been presented in the embodiment shown in  FIG. 2 . 
       FIG. 6  represents another vector composition making it possible to define the features of a voltage step-up autotransformer. The connection of the coils necessary to produce this vector composition is represented in  FIG. 7 . The autotransformer comprises three main coils B 12 , B 23  and B 31  connected in a triangle and each wound on one magnetic branch, respectively M 1 , M 2  and M 3 . The terminals situated at the ends of the coil B 12  bear the reference numbers E 1  and E 2 . Similarly, the terminals situated at the ends of the coil B 23  bear the reference numbers E 2  and E 3  and finally the terminals situated at the ends of the coil B 31  bear the reference numbers E 3  and E 1 . The three-phase input power supply voltage may be applied either between the terminals E 1 , E 2  and E 3  or between the points I 1 , I 2  and I 3  forming the intermediate power outlets respectively of the coils B 12 , B 23  and B 31 . For one and the same output voltage amplitude of the autotransformer, the input voltage applied between the points E 1 , E 2  and E 3  will be double that applied between the points I 1 , I 2  and I 3 . 
     The autotransformer comprises on each magnetic branch M 1 , M 2  and M 3  five auxiliary coils P 12 , Q 12 , R 12 , S 12  and T 12  for the branch M 1 , P 23 , Q 23 , R 23 , S 23  and T 23  for the branch M 2  and P 31 , Q 31 , R 31 , S 31  and T 31  for the branch M 3 . 
     An output voltage C 1  is obtained in the following manner: a first terminal of the coil P 12  is connected to the terminal E 1  and an intermediate point of the coil P 12  is connected to a first terminal of the coil R 31 . The second terminal of the coil R 31  forms the point C 1 . 
     The other six output voltages are divided by pairs symmetrically phase-shifted by 20° relative to the first three output voltages C 1 , C 2  and C 3 . The first output voltage A 1  is phase-shifted by −20° relative to the voltage C 1  and is obtained in the following manner: a first terminal of the auxiliary coil P 12  is connected to the terminal E 1  and the second terminal of the auxiliary coil P 12  is connected to a first terminal of the coil Q 23 . The second terminal of the coil Q 23  forms the point A 1 . 
     Similarly, the second output voltage B 1  is phase-shifted by +20° relative to the voltage C 1  and is obtained by connecting a first terminal of the auxiliary coil S 31  to the terminal E 1 . The second terminal of the auxiliary coil S 31  is connected to a first terminal of the coil T 23 . The second terminal of the coil T 23  forms the point B 1 . 
     So as not to overload  FIG. 6 , only the connections necessary to obtain the voltages A 1 , B 1  and C 1  have been represented. The connections necessary to obtain the other voltages may be deduced by circular permutation. 
       FIG. 8  represents another vector composition making it possible to define the features of a voltage step-down autotransformer. The connection of the coils necessary to produce this vector composition is represented in  FIG. 9 . The autotransformer comprises three main coils B 12 , B 23  and B 31  connected in a triangle and each wound on one magnetic branch, respectively M 1 , M 2  and M 3 . The terminals situated at the ends of the coil B 12  bear the reference numbers E 1  and E 2 . Similarly, the terminals situated at the ends of the coil B 23  bear the reference numbers E 2  and E 3  and finally the terminals situated at the ends of the coil B 31  bear the reference numbers E 3  and E 1 . The coil B 12  comprises intermediate power outlets J 1 , J′ 1 , J″ 1  and J′″ 1 . Similarly, the coil B 23  comprises intermediate power outlets J 2 , J′ 2 , J″ 2  and J′″ 2 . Finally the coil B 31  comprises intermediate power outlets J 3 , J′ 3 , J″ 3  and J′″ 3 . The three-phase input power supply voltage may be applied either between the terminals E 1 , E 2  and E 3  or between the points J′ 1 , J′ 2  and J′ 3 . For one and the same output voltage amplitude of the autotransformer, the input voltage applied between the points E 1 , E 2  and E 3  will be double that applied between the points J′ 1 , J′ 2  and J′ 3 . 
     The autotransformer comprises, on each magnetic branch M 1 , M 2  and M 3 , three auxiliary coils X 12 , Y 12  and Z 12  for the branch M 1 , X 23 , Y 23  and Z 23  for the branch M 2  and X 31 , Y 31  and Z 31  for the branch M 3 . 
     An output voltage C 1  is obtained in the following manner: a first terminal of the coil Z 12  is connected to the point J′″ 3 . The second terminal of the coil Z 12  forms the point C 1 . 
     The other six output voltages are divided by pairs symmetrically phase-shifted by 20° relative to the first three output voltages C 1 , C 2  and C 3 . The first output voltage A 1  is phase-shifted by −20° relative to the voltage C 1  and is obtained in the following manner: a first terminal of the auxiliary coil X 23  is connected to the point J″ 3 . The second terminal of the coil X 23  forms the point A 1 . 
     Similarly, the second output voltage B 1  is phase-shifted by +20° relative to the voltage C 1  and is obtained by connecting a first terminal of the auxiliary coil Y 23  to the point J 1 . The second terminal of the coil Y 23  forms the point B 1 . 
     Whether the autotransformer is a voltage step-up or voltage step-down autotransformer, it may be used directly to produce an AC/DC voltage converter. 
     For this, as represented in  FIG. 10 , the three-phase power supply is connected to the inputs of an autotransformer AT and the outputs are connected to a three times six-diode triple-bridge rectifier. For greater convenience, the inputs are marked E 1 , E 2  and E 3  and for the star installations, the outputs in phase with the input voltages: C 1 , C 2  and C 3 . 
     The autotransformer AT delivers three three-phase systems S 1 , S 2  and S 3 . Each system comprises three phases phase-shifted by 120° from one another. The device by rights comprises on each system a rectifier bridge, respectively P 1 , P 2  and P 3 , and smoothing means, respectively L 1 , L 2  and L 3 . The rectifier bridge P 1 , P 2  and P 3  and the smoothing means L 1 , L 2  and L 3  form rectifier means R of the device. 
     For each system S 1 , S 2  or S 3 , the smoothing means L 1 , L 2  or L 3  comprise a positive output, respectively L 1 +, L 2 + and L 3 + and a negative output, respectively L 1 −, L 2 − and L 3 −. The positive outputs L 1 +, L 2 + and L 3 + of each of the smoothing means are connected to one another to form a positive output R+ of the rectifier means. The negative outputs L 1 −, L 2 − and L 3 − of each of the smoothing means are connected together to form a negative output R− of the rectifier means. Between the outputs R+ and R− two capacitors Co 1  and Co 2  are connected in series. The common point of the two capacitors Co 1  and Co 2  is connected to a ground of the device. The smoothing means L 1 , L 2  and L 3  associated with the capacitors Co 1  and Co 2  make it possible mainly to limit the common mode voltage, and equally the differential mode voltage between the two outputs R+ and R−. The device is designed to supply a load Ch connected between the outputs R+ and R−. 
     Advantageously, the smoothing means L 1 , L 2  and L 3  each comprise two coils coupled to a single magnetic circuit respectively M 1 , M 2  and M 3 . It is well understood that the magnetic circuits M 1 , M 2  and M 3  are independent of one another. The coils bear the reference numbers L 11  and L 12  for the smoothing means L 1 , L 21  and L 22  for the smoothing means L 2  and finally L 31  and L 32  for the smoothing means L 3 . The two coils L 11  and L 12  of the smoothing means L 1  are represented as an example in  FIG. 2 . The smoothing means L 1 , L 2  and L 3  are independent of one another. Therefore, only the current specific to each rectifier bridge P 1 , P 2  or P 3  passes through each of the smoothing means L 1 , L 2  or L 3 . The current flowing in each coil, for example L 11  and L 12  of one and the same smoothing means is equal and saturation is not reached. This arrangement makes it possible to reduce the weight of the magnetic circuits M 1 , M 2  and M 3 . On each magnetic circuit, for example M 1 , the direction of winding of each coil L 11  and L 12  is defined so as to cancel out the amps per turn of the two coils. In  FIG. 1 , the direction of winding is symbolized by dots represented in the vicinity of the first turn of each coil and by a Z shape of each magnetic circuit. In other words, the two coils of each smoothing means are connected in common mode. The smoothing means mainly filter only the common mode voltage. The choke value of the smoothing means is reduced and the filtering of the differential mode voltage is provided by the leakage choke of the smoothing means. The smoothing means is defined so as to obtain a sufficient leakage choke value. 
     Advantageously, for each system S 1 , S 2  and S 3 , the associated rectifier bridge, respectively P 1 , P 2  and P 3 , comprises a positive output respectively P 1 +, P 2 + and P 3 +, and a negative output respectively P 1 −, P 2 −, P 3 −. For each rectifier bridge, the positive output is connected to a positive input of the smoothing means. Similarly, for each rectifier bridge, the negative output is connected to a negative input of the smoothing means. 
     Advantageously, the positive input of the smoothing means L 1 , L 2  and L 3  is formed by a first terminal of the first coil respectively L 11 , L 21 , L 31 , and the negative input of the smoothing means is formed by a first terminal of the second coil respectively L 12 , L 22 , L 32 . A second terminal of the first coil forms the positive output respectively L 1 +, L 2 + and L 3 + of the smoothing means L 1 , L 2  and L 3  and a second terminal of the second coil forms the negative output respectively L 1 −, L 2 − and L 3 − of the smoothing means L 1 , L 2  and L 3 .