Abstract:
The invention relates to the electrical power supply of aircraft and notably of large commercial aircraft. 
     According to the invention, the aircraft is equipped with an AC-DC converter that distributes power over a DC network starting from a three-phase alternating voltage of 230 volts applied to its main inputs (E 1 , E 2 , E 3 ). The converter comprises an autotransformer which preferably has nine outputs (A 1 , A 2 , A 3 , B 1 , B 2 , etc.) for a nine-phase rectification. These outputs are applied to a rectifier bridge with 18 diodes. When the aircraft is on the ground, the AC power is delivered at 115 volts from a ground generator; it is applied via a three-phase connector (CAUX) to auxiliary inputs (M 1 , M 2 , M 3 ) connected to intermediate taps of the three-phase windings forming the AC-DC converter.

Description:
RELATED APPLICATIONS 
     The present Application is based on International Application No. PCT/EP2005/054362, filed on Sep. 4, 2005, which in turn corresponds to French Application No. 04 10150 filed on Sep. 24, 2004, and priority is hereby claimed under 35 USC §119 based on these applications. Each of these applications are hereby incorporated by reference in their entirety into the present application. 
     FIELD OF THE INVENTION 
     The invention relates to the electrical power supply of aircraft and notably of large commercial aircraft. 
     BACKGROUND OF THE INVENTION 
     In large-sized aircraft such as airplanes for transporting tens or hundreds of passengers, the supply of electrical power becomes a very significant element in the general design of the aircraft. The reason for this is that the amount of electrical equipment installed on board and used either for the operation of the aircraft or for the onboard services is growing and is consuming more and more power. 
     This energy is produced by alternators coupled to the engines of the aircraft and the alternators usually supply a three-phase voltage of 115 volts r.m.s. between neutral and phase, at a frequency of 400 Hz. This voltage is carried inside the aircraft by electrical cables whose cross-section is proportional to the square of the value of the current that these cables must be able to carry. Typically, several hundred meters of cable capable of carrying several kilowatts are needed. This results in a very large weight of copper or aluminum to be installed into the aircraft. 
     In consequence, it was seen that it could be preferable to now design aircraft in which the power is carried at a minimum of 230 volts, in order to divide substantially by 4 the cross-section of the cables carrying the power. The alternators of such aircraft will therefore be designed to directly supply a three-phase voltage at 400 Hz and 230 volts r.m.s. between phase and neutral. In addition, these modern aircraft will now be equipped with a distribution network of DC electrical power, typically at 540 volts (plus or minus 270 volts with respect to the metallic structure of the aircraft). The advantage of the DC power distribution is to allow, by means of variable-frequency inverters, an individual speed control to be effected for certain synchronous or asynchronous motors present in the aircraft (compressors, air-conditioning units, fuel pumps, etc.). 
     Furthermore, aircraft have to consume electrical power when they are stationary on the ground at an airport with engines stopped. This power is required in order to provide lighting, air-conditioning, maintenance, starting, etc. functions. 
     They are therefore connected, via a three-phase connector accessible on the exterior of the aircraft, to electrical power generator units situated on the ground, administered by airports. The generator units all supply three-phase power at 115 volts r.m.s. since the majority of airplanes are equipped for operation with 115 volts r.m.s. It may be envisioned that, in the future, airports equip themselves with generators supplying both 115 volts and 230 volts, or that special units supplying 230 volts be provided for the case where an airplane equipped with 230 volt systems might land. However, that implies a cost that the airports do not wish to carry and this solution is only to be envisioned in the very long term when the number of aircraft equipped with 230 volt systems will be very significant. 
     In the immediate term, the solution is to provide a three-phase transformer on the aircraft placed between an external power supply connector (designed to be hooked to the ground generator) and the 230 volt power supply network on the airplane. This transformer adds more weight and takes up additional space solely for this reason of airport logistics. 
     The present invention proposes a solution for limiting the drawbacks resulting from this situation. This solution is applicable when the aircraft possesses a DC electrical power distribution network receiving the power from a three-phase alternator and an AC-DC converter (in practice an autotransformer followed by a rectifier bridge) in order to transform the AC power into DC power. 
     SUMMARY OF THE INVENTION 
     According to the invention, the AC-DC converter, designed to be connected by three main inputs to a supply of three-phase voltage of given amplitude Va, comprises an autotransformer with three main inputs and more than three outputs and a rectifier bridge with more than three phases connected to the outputs of the autotransformer, the autotransformer comprising a magnetic core with three branches and on each branch a main winding and auxiliary windings, the auxiliary windings of one branch being electrically connected at one end to terminals of the main windings of this branch or of another branch and at the other to a respective output of the autotransformer, all of the connections exhibiting a circular permutational symmetry allowing a balanced three-phase operation. The converter is characterized in that it comprises three auxiliary power supply terminals, each one on a respective winding according to a three-phase permutational symmetry, at a location where the amplitude of the voltage is Va/g when the power supply voltage present on the inputs is Va, these three auxiliary power supply terminals being connected to a three-phase connector in order to form an auxiliary three-phase input allowing AC electrical power to be occasionally received at a voltage Va/g of different amplitude from the amplitude Va to be applied to the main inputs. 
     In practice, the auxiliary terminals are situated at a location where the amplitude is Va/2. Thus, in normal operation of the aircraft, with the engines running, the three main inputs receive the AC power at 230 volts 400 Hz supplied by the engine alternators. In operation on the tarmac, with engines stopped power can be transmitted by the ground generators toward the auxiliary inputs and, if this power is at 115 volts, the autotransformer will continue to operate exactly as if it were receiving power at 230 volts on its main inputs. This results from the choice of the position of the intermediate taps and from the principle of reversibility of autotransformers. 
     The amplitude Va is, for example, the amplitude of the single-ended voltage present between one input of the three-phase system and a physical or virtual neutral of the circuit, and in this case of course the amplitude Va/g is also a single-ended voltage. However, the reasoning is the same if the amplitudes between phases of the main inputs are considered: the auxiliary terminals are chosen at the locations where the amplitude between phases of the three-phase system is divided by g. The number g will, in general, be greater than 1, the main advantage of the system being when g is equal to 2 or thereabouts. 
     The principle of the invention may be adapted to any type of autotransformer construction, to 12 phases or 18 phases or more. For some constructions of autotransformer, g cannot have too high a value. The very simple principle for determining the positions of the auxiliary terminals, and how that depends on the construction of the autotransformer, will be explained hereinbelow. 
     In one preferred exemplary embodiment, the main windings are electrically connected in a ‘delta’ configuration between the three main inputs, the auxiliary windings of one magnetic branch are each connected to a tap of a main winding of another branch, and the auxiliary power supply terminals are each placed on a main winding. In order to allow an auxiliary supply of power at a voltage Va/2 to occasionally be used, the auxiliary terminals are then center-tap terminals placed in the middle of the main windings. 
     In other embodiments, the main windings are not necessarily directly connected to the main inputs; an auxiliary winding may for example be connected between a main input and one end or an intermediate tap of a main winding. In this case, the terminal that allows, for example, a supply of voltage at Va/2 to be used is not situated in the middle of the main winding. It will be explained hereinbelow how the position of the auxiliary terminals is very simply defined by using a vector construction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the invention will become apparent upon reading the detailed description that follows which is presented with reference to the appended drawings in which: 
         FIG. 1  shows the schematic construction of a three-phase transformer; 
         FIG. 2  shows a vector composition allowing the characteristics of a step-down autotransformer to be defined, in one preferred embodiment of the invention; 
         FIG. 3  shows the windings provided on one magnetic branch of the autotransformer; 
         FIG. 4  shows the configuration of the autotransformer corresponding to the vector composition in  FIG. 2 ; 
         FIG. 5  shows the AC-DC converter using the autotransformer in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A few general principles on AC-DC converters, formed by means of a three-phase autotransformer and a rectifier bridge, will firstly be recalled. 
     The autotransformer has three inputs for receiving a three-phase voltage and a higher number of outputs that are phase-shifted in order to supply an AC voltage with a larger number of phases allowing rectification with a lower residual ripple and with a lower re-injection of harmonic currents. Thus, typically, the three-phase system, whose three phases are separated by 120°, can be transformed into a system with nine phases separated by 40° which may be considered as a system of three three-phase networks offset by 40° with respect to one another. At the output of the autotransformer, three bridges of six diodes are used, each bridge being supplied by one of these networks. These AC/DC converters with eighteen diodes are also referred to as 18-pulse converters. 
     In the autotransformer, there are essentially three main windings wound onto three different magnetic branches of a three-branch magnetic core, and there are several auxiliary windings wound onto these three branches and connected to intermediate taps provided on the windings of one or the other of the other branches (or sometimes even on the same branch). 
       FIG. 1  serves as a reminder of this principle of a three-phase transformer with three magnetic branches M 12 , M 23  and M 31 , three corresponding main windings B 12 , B 23  and B 31  and an example of auxiliary winding S 12 , S 23 , S 31  on each branch. The windings are shown without mutual connections, the object of  FIG. 1  being only to recall the principle of a three-phase magnetic circuit. The following figures will show the connections between the various main and auxiliary windings of the autotransformer. 
     For display convenience, the auxiliary windings are shown next to the main windings, although, in reality, the various windings on the same magnetic branch are disposed at the same location (one around the other, or even the layers of one interspersed between the layers of the other) in order to have exactly the same magnetic flux passing through them. 
     A voltage is created across the terminals of an auxiliary winding of a magnetic branch that is in phase with the voltage across the terminals of the main winding of the same branch. The voltage generated within the auxiliary winding depends
         on the voltage value across the terminals of the associated main winding,   on the ratio between the number of turns in the main winding and in the auxiliary winding, and   on the direction of rotation of the current in the auxiliary winding with respect to the direction of the current in the main winding.       

     The operation of an autotransformer is conventionally represented by a vector composition of the voltages across the terminals of the various windings. The phase and the amplitude of the voltage (single-ended voltage present at one 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 AC voltage (single-ended or differential) and whose orientation represents the phase from 0° to 360° of this AC voltage. 
     In the following, the invention will be described using one exemplary embodiment where the autotransformer converts a three-phase voltage of given amplitude into a voltage with nine phases separated by 40°, of slightly lower amplitude than that of the three-phase voltage. 
     For the formation of such an autotransformer, the vector compositions are determined which, starting from the initial three phases, allow the nine phases sought to be constructed. 
     In this example, the main windings B 12 , B 23 , B 31  are connected in a delta configuration directly between the main inputs E 1 , E 2 , E 3  receiving the three-phase AC voltage of 230 volts r.m.s. to be converted.  FIG. 2  shows a vector composition corresponding to such an autotransformer. 
     The vectors used in the composition are obtained, on the one hand, starting from points representing the terminals of main or auxiliary windings and, on the other, using points representing intermediate taps of these windings. The voltage obtained between two intermediate taps of a main winding is in phase with the voltage on the main winding (the vectors are therefore collinear); its amplitude is a certain fraction of the voltage across the terminals of the main winding, this fraction being a function of the ratio between the number of winding turns situated between the intermediate taps and the total number of turns of the main winding; the relative length of the vector representing the voltage between two intermediate taps of a winding is determined by this ratio of number of turns. 
     According to the same principal, the voltage obtained across the terminals of an auxiliary winding associated with the main winding (in other words with the same magnetic flux passing through it hence wound at the same location on the same magnetic branch) is in phase with the voltage across the terminals of the main winding (the vectors are therefore parallel) and its amplitude is also determined by the ratio between the number of turns in the auxiliary winding and the number of turns in the main winding; the length of the vector representing the voltage on the auxiliary winding is therefore, relative to the length of the vector representing the voltage on the main winding, in the ratio of the number of turns. 
     For convenience, in the following, the same letters (for example E 1  and E 2 ) will denote both the terminals of a winding (in the figures showing windings) and the ends of the vector representing the voltage across the terminals of this winding (in the figures showing the vector compositions). 
     For the vector composition, a neutral point of origin O is arbitrarily defined and the single-ended input and output voltages of the autotransformer will be referenced with respect to this point. Thus, the vector OE 1  represents the amplitude and the phase of the single-ended voltage present on the terminal E 1  of the three-phase power supply. The neutral point O is a virtual point (input and output via delta configuration) of the circuit; if it is assumed that the three-phase power supply applied to E 1 , E 2 , E 3  is well balanced, the neutral point represents the reference point where the vector sum of the voltages OE 1 , OE 2 , OE 3  is zero. In the vector representation, the point O is the center of an equilateral triangle whose corners are the points E 1 , E 2 , E 3 . The vectors OE 2  and OE 3 , of same amplitude as the vector OE 1 , are respectively oriented at +120° and −120° to the reference vector OE 1 . If the power supply applied to the terminals E 1 , E 2 , E 3  is a three-phase power supply in delta configuration (preferred case), the vectors E 1 E 2 , E 2 E 3 , E 3 E 1  represent the amplitudes and phases of the voltages between power supply lines, applied to the terminals of the main windings. They are at 120° with respect to one another. In order to simplify the vector notation, in all that follows, the first letter of a vector is considered as the origin of the vector and the second letter is the arrival point of the vector; thus, OE 1  represents the vector starting from O and going as far as E 1 , and not the reverse. 
     In  FIG. 2 , the phase of the single-ended voltage OE 1  (vertical direction) is chosen as phase reference. The direction of the vector E 1 E 2  is at +150°; that of the vector E 2 E 3  is at +270°, and that of the vector E 3 E 1  is at +30°. 
     The vector composition in  FIG. 2  allows nine voltages of phases at 40° to one another and of same amplitude, lower than that of the three-phase power supply voltage, to be constructed. 
     Three of the nine phases are aligned with the phases OE 1 , OE 2 , OE 3  of the three-phase power supply of the autotransformer. 
     Starting from an initial assumption of coefficient k representing the ratio between the value Va′ of the voltage of the nine phases and the value Va of the input voltage (single-ended OE 1 , OE 2 , OE 3 ), the procedure is as follows: starting from the neutral point O, three systems are traced of three vectors of same amplitude Va′ equal to the amplitude of OE 1  multiplied by the reduction ratio k:
 
 Va′=Va*k  
 
     It should be noted that k is less than 1 and may go as low as about 0.56. 
     The vectors of the first system define three points A 1 , A 2  and A 3  on the circle with center O and of radius Va′=k*Va. The vectors OA 1 , OA 2 , OA 3  are aligned with the vectors OE 1 , OE 2 , OE 3 , respectively, and hence are separated by 120° with respect to one another. The vectors of the second system define three points B 1 , B 2 , B 3  on the same circle with center O and of radius Va′. The vectors OB 1 , OB 2 , OB 3  can be deduced from the vectors OA 1 , OA 2 , OA 3  by rotation through +40°. Lastly, the vectors of the third system OC 1 , OC 2 , OC 3  can be deduced from the vectors OB 1 , OB 2 , OB 3  by a further rotation through +40° (it could also have been said that the vectors of the third system can be deduced from the vectors OA 1 , OA 2 , OA 3  by a rotation through −40°, which amounts strictly to the same thing by reversing the denotations C 1  and C 3 ). 
     The final result is therefore nine vectors separated by 40° and having as amplitude Va′=k*Va. 
     Three intermediate points K 1 , K′ 1 , K″ 1  are defined on the vector E 1 E 2  which will physically constitute intermediate taps of the main winding B 12 . 
     The point K 1  is the point of intersection between the vector E 1 E 2  and a straight line having its origin at the point A 1  and being parallel to the vector E 3 E 1 . 
     The point K′ 1  is the point of intersection of the vector E 1 E 2  with a straight line starting from the point B 1  and traced parallel to the vector E 2 E 3 . 
     Finally, the point K″ 1  is the point of intersection of the vector E 1 E 2  with a straight line starting from the point C 1  and traced parallel to the vector E 3 E 1 . 
     In the same manner, by circular permutation, the intermediate taps K 2 , K′ 2 , K″ 2 , K 3 , K′ 3 , K″ 3  are determined. 
     On this construction, or by making a trigonometric calculation whose reproduction would be tedious and which is trivial since all the angles and also the respective lengths of OA 1  and OE 1  are known, the lengths of the vectors E 1 K 1 , A 1 K 1 , E 1 K′ 1 , B 1 K′ 1 , K″ 1 C 1  and E 1 K″ 1  are measured. The lengths of the other vectors, obtained by circular permutation, are clearly identical. 
     These lengths, referenced to the length of the vector E 1 E 2 , will define numbers of winding turns referenced to the total number N of turns on the primary winding. 
     Thus, the intermediate tap K 1  in the main winding B 12  is at a position such that the ratio n 1 /N between the number n 1  of the turns falling between E 1  and K 1  and the total number N of turns on the primary winding B 12  is:
 
 n 1 /N=E 1 K 1 /E 1 E 2
 
     Similarly, the intermediate taps K′ 1  and K″ 1  are placed at positions such that the ratio between the number n′ 1  of turns situated between E 1  and K′ 1  and the total number N of turns is
 
 n′ 1 /N=E 1 K′ 1 /E 1 E 2
 
and the ratio between the number of turns n″ 1  situated between E 1  and K″ 1  and the total number of turns N is:
 
 n″ 1 /N=E 1 K″ 1 /E 1 E 2
 
The points A 1 , B 1  and C 1  are determined starting from the vectors K 1 A 1 , K′ 1 B 1  and K″ 1 C 1  whose orientations are not those of the vector E 1 E 2 . The voltages corresponding to these vectors will therefore be defined using the auxiliary windings; the auxiliary windings are placed on the two other magnetic branches M 23  and M 31  of the magnetic circuit. These windings will have a first end connected to an intermediate tap, K 1 , K′ 1  or K″ 1  respectively, of the main winding B 12  and a second end which will form an output A 1 , B 1  or C 1 , respectively, of the autotransformer.
 
     Thus, an auxiliary winding placed on the third magnetic branch M 31  of the magnetic circuit (that which carries the third primary winding B 31  connected between E 3  and E 1 ) will be used to establish a voltage represented by the vector K 1 A 1  since this vector is parallel to the vector E 3 E 1 . This winding will have one end connected to the tap K 1  and its other end will form an output terminal A 1  of the autotransformer. Similarly, an auxiliary winding placed on the second branch of the magnetic circuit (that which carries the second main winding B 23  connected between E 2  and E 3 ) will be used to establish a voltage represented by the vector K′ 1 B 1  since the vector K′ 1 B 1  is parallel to E 2 E 3 . This winding will have one end connected to the tap K′ 1  and its other end will form a second output B 1  of the autotransformer, phase-shifted by 40° with respect to the output A 1 . Similarly again, an auxiliary winding placed on the third magnetic branch M 31  (that which carries the main winding B 31  connected between E 3  and E 1 ) will be used to establish the voltage K″ 1 C 1 . This winding will have one end connected to the intermediate tap K″ 1  and another end defining a third output C 1  phase-shifted by 40° with respect to the second. 
     The other outputs A 2 , B 2 , C 2  then the outputs A 3 , B 3 , C 3  are formed according to the same principle, by circular permutation. 
     Finally, an auxiliary power supply terminal M 1  is provided on the winding B 12 , in the middle of the latter, and in the same way, auxiliary terminals M 2  and M 3  with the same positioning as the terminal M 1  but on the winding B 23  and the winding B 31 , respectively. The three terminals M 1 , M 2 , M 3  form, on the vector composition in  FIG. 2 , an equilateral triangle whose sides have half the length of the length of the vectors E 1 E 2 , E 2 E 3  and E 3 E 1 . 
     When the terminals E 1 , E 2  and E 3  are supplied by a three-phase voltage of amplitude Va, the voltages present on these terminals form a three-phase system of amplitude Va/2, phase-shifted by 60° with respect to the system of power supply voltages. 
     Reciprocally, if a three-phase voltage of amplitude Va/2 is applied to the terminals M 1 , M 2 , M 3 , a three-phase voltage of amplitude Va, phase-shifted by −60°, is again found on E 1 , E 2 , E 3 , and everything happens in the autotransformer as if the terminals E 1 , E 2  and E 3  were receiving a three-phase power supply voltage of amplitude Va, phase-shifted by +60° with respect to the voltages on the terminals M 1 , M 2 , M 3 . 
     Consequently, according to the invention, the auxiliary terminals M 1 , M 2 , M 3  are connected to a three-phase connector. This connector may be used in order to supply the AC-DC converter when the latter does not receive any supply of power on the terminals E 1 , E 2  and E 3 , and notably when the aircraft is at an airport with its engines stopped. 
     By using this type of autotransformer configuration and by supplying the terminals M 1 , M 2 , M 3  with the voltage of 115 volts available on the tarmac at airports, everything happens as if the converter were supplied with 230 volts at the inputs E 1 , E 2 , E 3  even though these inputs are not powered; neither dedicated power generators supplying 230 volts on the ground, nor a dedicated three-phase 115v/230v transformer in the aircraft are any longer required. 
     If the auxiliary terminals M 1 , M 2 , M 3  are not in the middle of the main windings but displaced to the right or to the left with respect to this center point, the voltages present on these terminals are in a ratio of g less than 2 with respect to the power supply voltage applied to E 1 , E 2 , E 3 . The system could therefore be used to supply the aircraft with an external voltage of amplitude Va/g with g less than 2. It should be noted that in this scenario no tap position can be found on the main winding that would allow a three-phase voltage below Va/2 to be obtained, in other words that g can, at most, be equal to 2. 
       FIG. 3  shows the windings situated on the first branch M 12  of the magnetic circuit: the main winding B 12  situated between the input terminals E 1  and E 2 , with its intermediate taps K 1 , K′ 1  and K″ 1  and the auxiliary power supply terminal M 1 ; and three auxiliary windings X 12 , Y 12  and Z 12 , which are situated on the same magnetic branch M 12  as the main winding B 12  and have the same magnetic flux passing through them, but which are not directly connected to the main winding B 12 . These auxiliary windings X 12 , Y 12 , Z 12  produce the voltages represented by the vectors K 2 A 2 , K′ 3 B 3  and K″ 2 C 2  which must all be in phase with the voltage on the main winding B 12 . These windings are therefore each connected between an intermediate tap K 2 , K′ 3  or K″ 2  on the main windings B 23  and B 31  and a respective output A 2 , B 3  or C 2  of the autotransformer. 
     The number of turns nx, ny and nz on these three windings X 12 , Y 12  and Z 12  are calculated relative to the number N of turns on the main winding as a function of the length of these three vectors:
 
 nx/N=K 2 A 2 /E 1 E 2
 
 ny/N=K′ 3 B 3 /E 1 E 2
 
 nz/N=K″ 2 C 2 /E 1 E 2
 
     The second and the third magnetic branches M 23  and M 31  of the autotransformer are formed in the same manner. 
       FIG. 4  shows the three magnetic branches with their respective assemblies of main and secondary windings, and this time with the connections that completely establish the desired voltage amplitudes and phases allowing the outputs A 1 , B 1 , C 1 , A 2 , B 2 , C 2 , A 3 , B 3 , C 3  to represent a nine-phase system having the desired amplitude Va′ and able to directly supply a system of three rectifier bridges each with six diodes. In  FIG. 4 , in order to take account of the question of the relative direction of winding of the turns, it has been considered that all the windings rotate in the same direction when going from the left toward the right and this is the reason that, for example, the intermediate tap K 1  is connected to the right-hand terminal of the winding X 31 , the output A 1  being the left-hand terminal, because the vector K 1 A 1  has to be oriented in the opposite direction to the vector E 3 E 1 . 
     The center-tap auxiliary power supply terminals M 1 , M 2 , M 3  on the main windings are connected to an auxiliary three-phase connector CAUX for the 115 volt power supply from a three-phase ground generator. 
     The circuit diagram in  FIG. 4  and the vector diagram in  FIG. 2  are given purely by way of example of an autotransformer producing nine phases starting from three phases. Other solutions are possible and allow step-down autotransformers (this is the case in  FIG. 2 ) or step-up autotransformers to be made. 
     The autotransformer thus constructed is associated with a rectifier bridge with 18 diodes in order to form an AC-DC converter. 
     The direct outputs (A 1 , A 2 , A 3 ) of the autotransformer are connected to a first bridge PA of six diodes Da 1 , Da 2 , Da 3 , Da′ 1 , Da′ 2 , Da′ 3 . The outputs phase-shifted by +40° are connected to a second bridge PB of six diodes Db 1 , Db 2 , Db 3 , Db′ 1 , Db′ 2 , Db′ 3 , and the outputs phase-shifted by −40° are connected to a third bridge PC of six diodes Dc 1 , Dc 2 , Dc 3 , Dc′ 1 , Dc′ 2 , Dc′ 3 . 
     The three rectifier bridges have common outputs S and S′ which form the outputs of the converter. 
     The diode Da 1  is forward connected between the output A 1  and a positive terminal S forming one of the two DC output terminals of the converter. The diode Da′ 1  is reverse connected between the output A 1  and a negative terminal S′ forming the other DC output terminal of the converter. 
     The connection is the same for all the other diodes: the diode Da 2  and the diode Da′ 2  are forward and reverse connected, respectively, between A 1 , on the one hand, and S and S′, respectively, on the other. The diode Db 1  and the diode Bb′ 1  are forward and reverse connected, respectively, between B 1 , on the one hand, and S and S′, on the other, and so on; one forward-biased diode is connected between one output terminal of the autotransformer and the terminal S and one reverse-biased diode is reverse connected between this output terminal and the terminal S′. 
     Thus, one realistic exemplary embodiment of the invention has been described. Numerous variants may be envisioned, mainly depending on the type of vector composition that will have been used in the construction of the autotransformer. Starting from this vector composition, the auxiliary power supply terminals that need to be provided on the autotransformer are determined very easily: if g is the coefficient between the normal voltage Va that the converter should receive at its main inputs (for example 230 volts) and the occasional voltage Va/g that should be received by the auxiliary inputs, then on the vector composition of center O (virtual or real node of the three-phase power supply at E 1 , E 2 , E 3 ), which composition depends on the structure of the autotransformer, a circle of center O and of radius Va/g is traced and the intersections with vectors representing windings are noted. Three intersections forming an equilateral triangle of center O are chosen from amongst these intersections and these three points define auxiliary terminals that will be placed on these windings when the autotransformer is constructed. The three auxiliary terminals will be connected to an autotransformer. If the circle does not cross any vector representing a winding, it is because the value of g is outside of the feasible range for this type of structure. 
     This determination of auxiliary terminals may be applied to simple modifications of the diagrams in  FIGS. 2 and 4  or to diagrams that are widely different. Amongst the simple modifications, it may be envisioned for example that the output A 1  be obtained starting from an auxiliary winding of the branch M 23  rather than M 31  (and the same thing of course for the other outputs A 2  and A 3  by circular permutation). Amongst more complex modifications, it may be envisioned that the ends of the three main windings in delta configuration be not directly connected to the main inputs E 1 , E 2  and E 3 : an auxiliary winding may be connected between one main input and one end or an intermediate tap of a main winding. This auxiliary winding can be situated on the same magnetic branch as the main winding to whose end it is connected; it may also be envisioned (case of a step-up autotransformer) that the three main windings be connected in delta configuration but that the main inputs be connected to auxiliary windings connected to an intermediate tap of a main winding that is not situated on the same magnetic branch as the auxiliary winding. The variety of configurations is very wide and, depending on the configuration chosen, a range of values of g is possible. It is even possible that the auxiliary terminals be placed on auxiliary windings rather than on the main windings if the circle of radius Va/g crosses the vectors representing the auxiliary windings of the autotransformer.