Patent Publication Number: US-2023137395-A1

Title: Method for controlling a rectifier connected to a permanent-magnet synchronous electric generator to provide a direct voltage, corresponding device and computer program

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for controlling a rectifier connected to a synchronous electric generator with permanent magnets to provide a direct current voltage, and to a corresponding device and computer program. 
     BACKGROUND OF THE INVENTION 
     The prior art comprises in particular the documents WO-A2-2014/173954, US-A1-2013/106368 and US-A1-2013/335041. 
     To control an inverter connected between an electric motor and a direct current voltage source, it is known to use an inverter control method, comprising:
         determining a first setpoint of a first vector component of the phase currents along a first axis of a rotating reference frame linked to a rotor of the electric machine, and a second setpoint of a second vector component of the phase currents along a second axis of the rotating reference frame, this second vector component of the phase currents being intended to drive a defluxing of the rotor; and   controlling the inverter from the first and second setpoints of the vector components of the phase currents.       

     This known method uses a direct current voltage measurement, a torque setpoint of the electric motor and a measurement of the instantaneous rotational speed of the rotor to determine phase current setpoints in a reference frame d-q attached to the rotor. More specifically, tables are provided which give the direct current setpoint and the quadrature current setpoint respectively from the measurement of the direct current voltage, the torque setpoint and the measurement of the instantaneous rotational speed. 
     Furthermore, for the electric machines operating as a generator, it is known to use a method for controlling a rectifier connected to phases of the synchronous electric generator with permanent magnets to provide a direct current voltage, the phases being designed to have phase currents flowing through them when the electric generator is driven, the method comprising:
         determining a first setpoint of a first vector component of the phase currents along a first axis of a rotating reference frame linked to a rotor of the electric generator, and a second setpoint of a second vector component of the phase currents along a second axis of the rotating reference frame, this second vector component of the phase currents being intended to drive a defluxing of the rotor; and   controlling the rectifier from the first and second setpoints of the vector components of the phase currents.       

     It may thus be desired to provide a control method suitable for an electric machine operating as a generator and in particular a multi-star and multi-phase machine, i.e., a machine which comprises several stars and for which each star is at least three-phase. 
     SUMMARY OF THE INVENTION 
     A method for controlling a rectifier connected to phases of a synchronous electric generator with permanent magnets to provide a direct current voltage is therefore proposed, the phases being designed to have phase currents flowing through them when the electric generator is driven, the method comprising:
         determining a first setpoint of a first vector component of the phase currents along a first axis of a rotating reference frame linked to a rotor of the electric generator, and a second setpoint of a second vector component of the phase currents along a second axis of the rotating reference frame, this second vector component of the phase currents being intended to drive a defluxing of the rotor; and   controlling the rectifier from the first and second setpoints of the vector components of the phase currents;       

     characterised in that the first setpoint of the first vector component of the phase currents is determined from an external feedback loop designed to feedback-control a voltage of a direct current bus or to regulate a current from a battery connected to the direct current bus, 
     and in that the method further comprises:
         determining a first theoretical setpoint of the first vector component of the phase currents;   determining a second theoretical setpoint of the second vector component of the phase currents; and   if a magnitude of a vector sum of the setpoints of the vector components of the phase currents is less than or equal to a predefined threshold, providing the theoretical setpoints as setpoints of the vector components of the phase current; and   otherwise, limiting the first theoretical setpoint and/or the second theoretical setpoint so that a magnitude of a vector sum of the theoretical setpoints is less than or equal to the predefined threshold, and providing the theoretical setpoints after limiting the vector components of the phase currents as setpoints.       

     Thus, with the invention, the first setpoint is determined from the external feedback loop, so that it is not necessary to know the operating characteristics of the electric machine. In particular, there is no need to use a torque setpoint of the electric machine or a measurement of the instantaneous rotational speed of the rotor, as is necessary in the known method adapted to the electric motor. 
     Optionally, the control of the rectifier from the first and second setpoints comprises:
         determining setpoints of phase voltages of the electric generator; and   controlling the rectifier from the setpoints of the phase voltages;       

     and the second setpoint of the second vector component of the phase currents is determined from the setpoints of the phase voltages. 
     Optionally, the second setpoint of the second vector component of the phase currents is set equal to a predefined constant. 
     Optionally, the second setpoint of the second vector component of the phase currents is determined from the first setpoint of the first vector component of the phase currents. 
     Optionally also, the second setpoint of the second vector component of the phase currents is determined by means of a table associating values of the second setpoint of the second vector component of the phase currents to values of the first setpoint of the first vector component of the phase currents. 
     Optionally also, the second setpoint of the second vector component of the phase currents is determined by means of a function associating values of the second setpoint of the second vector component of the phase currents to values of the first setpoint of the first vector component of the phase currents. 
     Also proposed is a computer program downloadable from a communication network and/or stored on a computer-readable medium, characterised in that it comprises instructions for the execution of the steps of a control method according to the invention, when said program is executed on a computer. 
     Also proposed is a control device for a rectifier connected to phases of a synchronous electric generator with permanent magnets in order to provide a direct current voltage, the phases being designed to have phase currents flowing through them when the electric generator is driven, the control device comprising:
         a module designed to determine a first setpoint of a first vector component of the phase currents along a first axis of a rotating reference frame linked to a rotor of the electric generator, and a second setpoint of a second vector component of the phase currents along a second axis of the rotating reference frame, this second vector component of the phase currents being intended to drive a defluxing of the rotor; and   a module for controlling the rectifier on the basis of the first and second setpoints of the vector components of the phase currents;       

     characterised in that the first setpoint of the first vector component of the phase currents is determined from an external feedback loop designed to feedback-control a voltage of a direct current bus or to regulate a current from a battery connected to the direct current bus, 
     and in that the module is further designed to:
         determining a first theoretical setpoint of the first vector component of the phase currents;   determining a second theoretical setpoint of the second vector component of the phase currents; and   if a magnitude of a vector sum of the setpoints of the vector components of the phase currents is less than or equal to a predefined threshold, providing theoretical setpoints as the setpoints of the vector components of the phase currents; and   otherwise, limiting the first theoretical setpoint and/or the second theoretical setpoint so that a magnitude of a vector sum of the theoretical setpoints is less than or equal to the predefined threshold, and providing theoretical setpoints after limiting the vector components of the phase currents as setpoints.       

    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be better understood with the aid of the following description, given only by way of example and made with reference to the attached drawings in which: 
         FIG.  1    is a schematic representation of an electrical installation according to a non-limiting example of implementation of the invention, comprising a rectifier connected to an electric generator, as well as a control device for the rectifier, 
         FIG.  2    is a schematic representation of modules of the control device of  FIG.  1   , according to a first example of embodiment, 
         FIG.  3    is a block diagram illustrating steps in a method for controlling the rectifier of the electrical installation of  FIG.  1    with the control device of  FIG.  2   , 
         FIG.  4    is a schematic representation of modules of the control device of  FIG.  1   , according to a second example of embodiment, 
         FIG.  5    is a block diagram illustrating steps in a method for controlling the rectifier of the electrical installation of  FIG.  1    with the control device of  FIG.  4   , 
         FIG.  6    is a schematic representation of modules of the control device of  FIG.  1   , according to a third example of embodiment, 
         FIG.  7    is a block diagram illustrating steps in a method for controlling the rectifier of the electrical installation of  FIG.  1    with the control device of  FIG.  6   , 
         FIG.  8    is a schematic representation of modules of the control device of  FIG.  1   , according to a fourth example of embodiment, and 
         FIG.  9    is a block diagram illustrating steps in a method for controlling the rectifier of the electrical installation of  FIG.  1    with the control device of  FIG.  8   . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG.  1   , a non-limiting example of electrical installation  100  implementing the invention will now be described. 
     The electrical installation  100  is for example designed to be part of an aircraft, in particular a VTOL (vertical take-off and landing) aircraft. 
     The electrical installation  100  comprises a power supply system  102  and a load  104  powered by the power supply system  102 . For example, the load  104  comprises various aircraft items of equipment. 
     The power supply system  102  comprises an electrical generation chain  106  comprising firstly a synchronous electric machine  108  with permanent magnets, designed to operate as a generator. The electric generator  108  comprises a stator  110  and a rotor  112  designed to be driven in rotation relative to the stator  110 , about an axis of rotation Δ. The rotor  112  comprises one or more permanent magnets (not shown) designed to generate a rotor magnetic field along an axis d, referred to as direct axis, attached to the rotor  112 . The electric generator  108  further has an input shaft  114  secured to the rotor  112 . In order to drive the rotor  112  in rotation, the electrical generation chain  106  further comprises, in the example described, a gas turbine  116  connected to the input shaft  114 . 
     In the example described, the electric generator  108  is multi-star (in this case two stars) and each star is three-phase star with a single neutral. For the sake of clarity, only one star is shown in  FIG.  1   . The person skilled in the art will appreciate that the invention applies, in general, to a synchronous electric machine with multi-star (i.e., the number of stars is greater than or equal to 1) and multi-phase (i.e., the number of phases per star is greater than or equal to 3) permanent magnets. The stator  110  comprises three phases A, B, C whose magnetic axes are oriented respectively in three directions a, b, c transverse to the axis of rotation Δ of the rotor  112  and intersecting on this axis of rotation Δ. The electric generator  108  is preferably balanced so that the three directions a, b, c are separated from each other by an angle of 120°. The three phases A, B, C have respective first ends connected together to a single neutral point N. 
     During the rotation of the rotor  112 , the phases A, B, C are designed to have phase currents Ia, Ib, Ic flowing through them, generating magnetic fields in the three directions a, b, c respectively. 
     Furthermore, during operation of the electric generator  108 , the phases A, B, C have respective phase voltages with respect to the neutral point N, noted Va, Vb, Vc. 
     As will be described in more detail later, the electric generator  108  is designed to be vector-controlled. Thus, each phase current Ia, Ib, Ic is associated with a phase current vector having a magnitude equal to the phase current Ia, Ib, Ic and extending in the direction a, b, c of the phase A, B, C through which this phase current Ia, Ib, Ic flows. The phase currents Ia, Ib, Ic are thus expressed in a reference frame formed by the axes a, b, c, referred to as the reference frame abc. The phase currents Ia, Ib, Ic are therefore globally represented by a global vector of phase current equal to the vector sum of the phase current vectors. 
     To simplify control by using substantially continuous quantities rather than directly the alternating phase currents Ia, Ib, Ic, the global vector of phase current is expressed in a rotating reference frame R attached to the rotor  112  and comprising the direct axis d and a quadrature axis q, perpendicular to the direct axis d in the case where the rotor  112  has a single north-south pole pair. In general, the invention applies to a rotor with a number of pole pairs greater than or equal to one. Thus, the global vector of phase current is expressed by two components: a direct component Id along the direct axis d and a quadrature component Iq along the quadrature axis q. In this way, the phase currents Ia, Ib, Ic are represented by the components Id, Iq. To change from the phase currents Ia, Ib, Ic to the components Id, Iq, the Park transform or the dqo transform can be used. The transition from the components Id, Iq to the phase currents Ia, Ib, Ic is done for example by means of inverse transforms. 
     Similarly, the phase voltages Va, Vb, Vc in the reference frame abc are represented by direct Vd and quadrature Vq components in the rotating reference frame R. 
     In order to convert the phase voltages Va, Vb, Vc into a voltage that can be applied to the load  104 , the electrical generation chain  106  further comprises a rectifier  118  designed to convert the phase alternating voltages Va, Vb, Vc into a direct current voltage Vdc. The rectifier  118  comprises, for each phase voltage Va, Vb, Vc, a respective switching arm comprising a high-side switch and a low-side switch connected to each other at a middle point to which the phase voltage Va, Vb, Vc under consideration is applied and receiving the phase current Ia, Ib, Ic of this phase A, B, C. 
     The electrical generation chain  106  also comprises a bus  120  comprising two power lines (positive and negative respectively) between which the switching arms of the rectifier  118  are connected and distributing the direct current voltage Vdc to the various items of equipment forming the load  104 . The bus  120  is generally referred to as HVDC bus (High Voltage Direct Current) and the direct current voltage Vdc that it carries is generally a high voltage, i.e., equal to 540 V (standard voltage in aeronautics), or even higher to 540 V. To smooth the direct current voltage Vdc, the rectifier  118  preferably comprises a capacitor Cp between the power lines of the bus  120 . 
     In the example described, the electrical generation chain  106  further comprises a battery  122  connected to the bus  120  to participate in the generation of the direct current voltage Vdc. The battery  122  has a state of load that varies over time and therefore influences the direct current voltage Vdc of the bus  120 . 
     The rectifier  118  is sometimes referred to as active rectifier in that it is designed to be controlled so that the direct current voltage Vdc on the bus  120  can be maintained close to a setpoint Vdc_ref despite the variations in the load of the battery  122  and the phase voltages Va, Vb, Vc. This setpoint Vcd_ref can be varied over time, according to the needs of the items of equipment forming the load  104 . 
     In addition, the rectifier  118  may also be used to maintain a current Ibat in the battery  122  substantially equal to a current setpoint Ibat_ref regardless of the variations in the voltage Vdc and the phase voltages Va, Vb, Vc. 
     Thus, in order to drive the rectifier  118 , the electrical generation chain  106  further comprises a measuring device  121  designed to provide a measurement Vdc_mes of the direct current voltage Vdc, a measuring device  123  designed to provide respective measurements Ia_mes, Ib_mes, Ic_mes of the phase currents Ia, Ib, Ic and a measuring device  127  designed to provide a measurement ST of a state of the electric generator  108 . 
     In particular, the measuring device  127  may be, for example, a position sensor, such as a resolver, which directly measures the angular position information of the rotor  112  or a synchronous generator mounted on the shaft of the electric generator  106  and rotating at no load. In this case, the angular position of the rotor  112  is deduced from measurements of its no-load electromotive forces. 
     In addition, the power supply system  102  comprises a control device  124  for the rectifier  118  more specifically designed to provide respective switching commands CM D to the switches of the rectifier  118 , in order to connect each phase A, B, C alternately to the positive line and the negative line of the bus  120 . 
     The control device  124  firstly comprises a module  126  designed to determine a setpoint Id_ref of the direct component Id of the phase currents Ia, Ib, Ic and a setpoint Iq_ref of the quadrature component Iq of the phase currents Ia, Ib, Ic. 
     Different possible embodiments of the module  126  will be described later with reference to  FIGS.  2 ,  4 ,  6  and  8   . 
     The control device  124  further comprises a module  128  for controlling the rectifier  118  on the basis of the setpoints Iq_ref, Id_ref. 
     The module  128  comprises a module  130  designed to determine an angular position θ of the rotor  112  from the measurement ST of the state of the electric generator  108 . 
     The module  128  further comprises a module  132  designed to convert, from the angular position θ of the rotor  112 , the measurements Ia_mes, Ib_mes, Ic_mes into measurements Id_mes, Iq_mes in the rotating reference frame R. The module  132  uses, for example, the Park transform, the dqo transform or the Fortescue transform generalized to a given number of phases. 
     The module  128  further comprises a module  134  designed to determine setpoints Vd_ref, Vq_ref of the direct and quadrature components Vd, Vq of the phase voltages Va, Vb, Vc, from the setpoints Id_ref, Iq_ref and the measurements Id_mes, Iq_mes. The setpoints Vd_ref, Vq_ref thus form a vector setpoint of the phase voltages Va, Vb, Vc. The module  134  is, for example, designed to determine an error from a difference between the setpoints Id_ref, Iq_ref and the measurements Id_mes, Iq_mes and to use correctors, for example proportional integral correctors, to determine the setpoints Vd_ref, Vq_ref from the error. 
     The module  128  further comprises a module  136  designed to convert, from the angular position θ, the setpoints Vd_ref, Vq_ref into setpoints Va_ref, Vb_ref, Vc_ref in the reference frame abc. The module  136  uses, for example, the inverse Park transform, the inverse dqo transform or the Fortescue transform generalized to a given number of phases. 
     The module  128  further comprises a module  138  designed to determine the switching commands CMD of the switches of the rectifier  118  from the setpoints Va_ref, Vb_ref, Vc_ref. 
     In the example described, the control device  124  comprises a computer system comprising a processing unit  140  (such as a microprocessor) and a memory  142  (such as a main memory) in which is stored a computer program containing computer program instructions designed to be executed by the processing unit  140 . Thus, the modules described above are implemented in the example described in the computer program as software modules. 
     Alternatively, some or all of the modules could be implemented as hardware modules, i.e., in the form of an electronic circuit, e.g., micro-wired, not involving a computer program. 
     With reference to  FIG.  2   , a first example of embodiment of the module  126  will now be described in more detail. 
     The module  126  firstly comprises a module  201  designed to determine a theoretical setpoint Iq_ref* of the quadrature component Iq of the phase currents Ia, Ib, Ic, from the measurement Vdc_mes and the setpoint Vdc_ref. For example, the theoretical setpoint Iq_ref* is determined by the equation: 
     
       
         
           
             
               
                 
                   
                     
                       Iq 
                       ⁢ 
                       _ 
                     
                     ref 
                     * 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           V 
                           dc_ref 
                           2 
                         
                         - 
                         
                           V 
                           dc_mes 
                           2 
                         
                       
                       ) 
                     
                     ⨯ 
                     
                       ( 
                       
                         Kp 
                         + 
                         
                           Ki 
                           s 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     where s is the Laplace operator, Kp is a proportional action gain and Ki is an integral action gain. 
     The module  126  further comprises a module  202  designed to determine a magnitude V_ref of the vector setpoint of the phase voltages. In the example described where the vector setpoint of the phase voltages expressed in the rotating reference frame R by the components Vd_ref, Vq_ref, the module  202  is for example designed to determine the magnitude V_ref according to the following equation: 
         V _ref=√{square root over ( Vd _ref 2   +Vq _ref 2 )}  [Math. 2]
 
     The module  126  further comprises a module  204  designed to determine a difference D between the magnitude V_ref and a threshold V_max corresponding to a maximum permissible voltage for the electric generator  108  defined for a given measured direct current voltage Vdc_mes and a given modulation strategy used by the module  138 . For example, the module  204  is designed to determine the difference D according to the following equation: 
         D=V _max− V _ref  [Math. 3]
 
     The module  126  further comprises a module  206  designed to receive the difference D and to determine, from this difference D, a theoretical setpoint Id_ref* of the direct component Id of the phase currents Ia, Ib, Ic. 
     More precisely, if this difference D is negative, i.e. if the magnitude V_ref is greater than the threshold V_max, then the electric generator  108  must be controlled in defluxing mode and the module  206  is then designed to provide a non-zero theoretical setpoint Id_ref* intended to drive a defluxing of the rotor  112 , i.e. to drive the generation of a defluxing magnetic field along the direct axis d which opposes the magnetic field produced by the rotor  112  along the direct axis d. In the example described, the electric generator  108  is modelled in motor convention, so that the theoretical setpoint Id_ref* is negative to obtain the defluxing magnetic field. For example, the module  206  uses a proportional integrator to determine the theoretical setpoint Id_ref*. In particular, this integrating proportional corrector can be applied to D according to the equation: 
     
       
         
           
             
               
                 
                   
                     I_d 
                     ref 
                     * 
                   
                   = 
                   
                     D 
                     ⨯ 
                     
                       ( 
                       
                         Kp 
                         + 
                         
                           Ki 
                           s 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     where s is the Laplace operator, Kp is the gain of the proportional action and Ki is the gain of the integral action. 
     If the magnitude V_ref is less than or equal to the predefined threshold V_max (positive or zero difference D), then the electric generator  108  does not need to be controlled in defluxing mode and the module  206  is then designed so that the theoretical setpoint Id_ref* is positive or zero. 
     The module  126  further comprises a module  207  designed to determine the setpoints Iq_ref, Id_ref by limiting the theoretical setpoint Iq_ref* and/or the theoretical setpoint Id_ref* if necessary. 
     More precisely, the module  207  is firstly designed to limit the theoretical setpoint Id_ref* so that it does not risk driving the generation of a magnetic field along the direct axis d in addition to the magnetic field produced by the rotor  112  along the direct axis d. With the conventions used in the example described, the module  207  is designed to prevent the setpoint Id_ref from being positive by limiting the theoretical setpoint Id_ref* to zero. 
     The module  207  is further designed to prevent the setpoint Iq_ref from being positive by limiting the theoretical setpoint Iq_ref* to zero. Indeed, according to the convention adopted, the operation in generator mode of the electric machine requires a negative current Iq. 
     The module  207  is further designed to limit the theoretical setpoint Iq_ref* and/or the theoretical setpoint Id_ref* so that the magnitude of their vector sum after limitation remains less than or equal to a threshold I_max. This threshold I_max corresponds, for example, to a maximum current supported by the power electronics, in particular the switches of the rectifier  118 . 
     The module  207  is then designed to provide the theoretical setpoints Iq_ref*, Id_ref* after limitation as setpoints Iq_ref, Id_ref. 
     In the example described, in order to produce these different limitations of the theoretical setpoints Iq_ref*, Id_ref*, the module  207  firstly comprises a module  208  designed to limit the negative magnitude of the theoretical setpoint Id_ref* to the threshold −I_max and its positive magnitude to zero. Thus, the setpoint Id_ref is equal to the theoretical setpoint Id_ref* except when, on the one hand, the magnitude of the latter falls below the threshold −I_max, in which case the setpoint Id_ref is equal to −I_max, and, on the other hand, the theoretical setpoint Id_ref* is greater than zero, in which case the setpoint Id_ref is equal to zero. In particular, when the defluxing is not required (difference D greater than or equal to zero), the setpoint Id_ref is zero. 
     The module  207  further comprises a module  210  designed to determine a threshold Iq_ref_max of the setpoint Iq_ref, from the setpoint Id_ref and the threshold I_max, for example according to the equation: 
         Iq _ref_max=√{square root over ( I _max 2   −Id _ref 2 )}  [Math. 5]
 
     The module  207  further comprises a module  212  designed to limit the negative magnitude of the theoretical setpoint Iq_ref* to the threshold −Iq_ref_max and its positive magnitude to zero. 
     With reference to  FIG.  3   , a first example of method  300  for controlling the rectifier  118  will now be described. In the example described, the method  300  is implemented by the control device  124 , with the module  126  of  FIG.  2   . Furthermore, the person skilled in the art will appreciate that the steps of the method  300  described may be executed in a different order from that shown in  FIG.  3    and may be executed, for example, concurrently. 
     In a step  302 , the control device  124  receives the measurements Vdc_mes, Ia_mes, Ib_mes, Ic_mes, ST and Ibat_mes. 
     In a step  304 , the module  130  determines the angle of rotation θ of the rotor  112  from the measurement ST of the state of the electric generator  108 . 
     In a step  306 , the module  132  converts, from the angular position θ of the rotor  112 , the measurements Ia_mes, Ib_mes, Ic_mes into measurements Id_mes, Iq_ref in the rotating reference frame R. 
     In a step  308 , the module  126  determines the phase current setpoints Iq_ref, Id_ref. 
     To this end, in a step  308 _ 2 , the module  201  determines the theoretical setpoint Iq_ref* from an external feedback loop for feedback-controlling the bus voltage  120  or for regulating the current of the battery  122 , i.e., for example, from the measurement Vdc_mes and the setpoint Vdc_ref of the direct current voltage Vdc or from the measurement Ibat_mes of the battery current and the battery current setpoint Ibat_ref. 
     In a step  308 _ 4 , the module  202  determines the magnitude V_ref of the vector setpoint of the phase voltages from the setpoints Vq_ref, Vd_ref. 
     In a step  308 _ 6 , the module  204  determines the difference D between the magnitude V_ref and the threshold V_max. 
     In a step  308 _ 8 , the module  206  determines, from the difference D, the theoretical setpoint Id_ref*. 
     In a step  308 _ 10 , the module  207  limits if necessary the theoretical setpoint Id_ref* and/or the setpoint Iq_ref* to provide the setpoints Iq_ref, Id_ref. The current value Id required to reach a given operating point is therefore limited, resulting in lower current magnitudes Ia, Ib and Ic and, consequently, less thermal losses in the power electronics and the electric machine. 
     Advantageously, the setpoints Iq_ref and Id_ref are obtained in the step  308  independently of operating parameters of the electric generator  108 , in particular independently of a torque and a rotational speed of the rotor  112  and without using a torque setpoint of the rotor  112 . In addition, in general, the operating parameters of the electric generator  108  may refer to electrical parameters, such as an inductance matrix, and mechanical parameters, such as the rotational speed of the rotor  112 . 
     In addition, it will be appreciated that the setpoint Id_ref is obtained by feedback regulation. 
     In a step  310 , the module  134  determines the setpoints Vd_ref, Vq_ref from the setpoints Id_ref, Iq_ref and the measurements Id_mes, Iq_mes. 
     In a step  312 , the module  136  converts the setpoints Vd_ref, Vq_ref into setpoints Va_ref, Vb_ref, Vc_ref in the reference frame abc. 
     In a step  314 , the module  138  determines the switching commands CMD and provides them to the switches of the rectifier  118 , so that the direct current voltage Vdc approaches the reference voltage Vdc_ref. 
     With reference to  FIG.  4   , a second example of embodiment of the module  126  will now be described in more detail. 
     This example is similar to that of  FIG.  2   , except that the module  206  is replaced by a module  402  designed to provide, when the difference D is negative, i.e. when the magnitude V_ref is greater than the threshold V_max, a setpoint Id_ref equal to a predefined constant, noted Id_ref°. This constant Id_ref° is chosen to be lower in absolute value than the threshold I_max. When the difference D is positive or zero, the module  402  is designed to provide the setpoint Id_ref equal to zero. It will be appreciated that the difference D is only used to determine the need to activate or not the defluxing. Its precise value is not used in this embodiment, just its sign. Thus, module  204  could provide the module  402  with a binary instead of the difference D, simply indicating whether or not defluxing is activated. 
     Moreover, with the choice of the constant Id_ref° lower (in absolute value) than the threshold I_max, the module  207  is simplified and no longer comprises the module  208 . In addition, the module  207  limits only the theoretical setpoint Iq_ref* if necessary, and not the setpoint Id_ref which remains constant. Finally, the person skilled in the art will appreciate that, in the case where the current sensors Ia, Ib and Ic are used in receiver convention, the constant Id_ref° is negative. 
     With reference to  FIG.  5   , a second example of method  500  for controlling the rectifier  118  will now be described. In the example described, the method  500  is implemented by the control device  124 , with the module  126  of  FIG.  4   . 
     The method  500  is similar to the method  300 , except that step  308 _ 8  is replaced by a step  308 _ 8 ′ in which the module  206  provides the setpoint Id_ref equal to zero or to the constant Id_ref°, depending on the sign of the difference D. 
     With reference to  FIG.  6   , a third example of embodiment of the module  126  will now be described in more detail. 
     In this third example, the setpoint Id_ref is determined from the setpoint Iq_ref, using a feedforward regulation. 
     More precisely, the modules  206  and  207  are replaced by a module  602  designed to provide, when the difference D is negative, a setpoint Id_ref determined by means of a table associating setpoint values Id_ref with values of the setpoint Iq_ref, such as the following table: 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Active power P [kW] 
                 Iq_ref [A] 
                 Id_ref [A] 
               
               
                   
               
             
            
               
                     0 ≤ P ≤ −20 
                     0 ≤ Iqref ≤ −40 
                 −30 
               
               
                 −20 &lt; P ≤ −40 
                 −40 &lt; Iqref ≤ −75 
                 −40 
               
               
                 −40 &lt; P ≤ −60 
                  −75 &lt; Iqref ≤ −110 
                 −50 
               
               
                 −60 &lt; P ≤ −80 
                 −110 &lt; Iqref ≤ −140 
                 −60 
               
               
                 −80 &lt; P ≤ −90 
                 −140 &lt; Iqref ≤ −160 
                 −75 
               
               
                 −90 &lt; P ≤ −95 
                 −160 &lt; Iqref ≤ −178 
                 −90 
               
               
                   
               
            
           
         
       
     
     Thus, in the example described, ranges of contiguous values of Iq_ref are respectively associated with values of Id_ref. In this table, the associated setpoints Iq_ref, Id_ref are all chosen so that the magnitude of their vector sum is less than I_max. 
     In addition, in the particular case of a synchronous machine with smooth poles, the current Iq is directly proportional to the active power P and the range of variation of the current setpoint Iq_ref is thus directly deduced from the range of variation of the active power. 
     When the difference D is positive or zero, the module  602  is designed to provide the setpoint Id_ref equal to zero. Again, only the sign of the difference D is used by the module  602 . 
     With reference to  FIG.  7   , a third example of method  700  for controlling the rectifier  118  will now be described. In the example described, the method  700  is implemented by the control device  124 , with the module  126  of  FIG.  6   . 
     The method  700  is similar to the method  500 , except that the step  308 _ 8 ′ is replaced by a step  308 _ 8 ″ during which the module  602  provides the setpoint Id_ref either equal to zero or from the setpoint Iq_ref and the association table, depending on the sign of the difference D. Furthermore, the step  308 _ 10  is omitted because the limitation of the setpoints Iq_ref, Id_ref is provided for when the association table is established. 
     With reference to  FIG.  8   , a fourth example of embodiment of the module  126  will now be described in more detail. 
     In this fourth example, the setpoint Id_ref is also determined from the setpoint Iq_ref, according to a feedforward control. 
     More precisely, the module  602  is replaced by a module  802  designed to provide, when the difference D is negative, a setpoint Id_ref determined by means of a function associating setpoint values Id_ref with values of the setpoint Iq_ref. For example, the function is given by the following equation: 
         Id _ref=√{square root over (( Iq _max−Δ Iq ) 2   −Iq _ref 2 )}  [Math. 6]
 
     where Iq_max and ΔIq are predefined parameters taking into account in particular I_max, so that the magnitude of the vector sum of the setpoints Iq_ref and Id_ref is smaller than I_max. 
     When the difference D is positive or zero, the module  802  is designed to provide the setpoint Id_ref equal to zero. Again, only the sign of the difference D is used by the module  802 . 
     With reference to  FIG.  9   , a fourth example of method  900  for controlling the rectifier  118  will now be described. In the example described, the method  900  is implemented by the control device  124 , with the module  126  of  FIG.  8   . 
     The method  900  is similar to the method  700 , except that the step  308 _ 8 ″ is replaced by a step  308 _ 8 ′″ in which the module  802  provides the setpoint Id_ref either equal to zero or from the setpoint Iq_ref and the association function, depending on the sign of the difference D. 
     It is clear that a control method for a rectifier such as those described above is well suited to an electric machine operating as a generator. 
     It should also be noted that the invention is not limited to the embodiments described above. It will be apparent to the person skilled in the art that various modifications can be made to the above-described embodiments in the light of the instruction just disclosed. 
     In particular, in general, the electric generator  108  may have more than three phases, divided into several stars with respective neutrals disconnected from each other. For example, in an alternative embodiment, the electric generator  108  comprises three stars of three windings (phases) each, with three respective neutrals disconnected from each other. When several stars are present, a rectifier associated with a bus is provided for each star. 
     In the foregoing detailed presentation of the invention, the terms used should not be interpreted as limiting the invention to the embodiments exposed in the present description, but should be interpreted to include all equivalents the anticipation of which is within the reach of the person skilled in the art by applying his general knowledge to the implementation of the teaching just disclosed.