Abstract:
A method of determining the magnetization level of permanent magnets of an electric machine, whereby a probe winding is placed in an electric machine having a stator with a plurality of stator winding, and a rotor with a plurality of permanent magnets; the probe winding is fixed with respect to the stator and links a magnetic flux produced by the permanent magnets; the rotor is rotated at an angular speed; an induced electric quantity is determined at terminals of the probe winding in response to passage of the permanent magnets; and the magnetization level of the permanent magnets is determined on the basis of the induced electric quantity detected.

Description:
PRIORITY CLAIM 
       [0001]    This application is a national stage application of PCT/IB2011/055469, filed on Dec. 5, 2011, which claims the benefit of and priority to Italian Patent Application No. MI2010A 002246, filed on Dec. 3, 2010, the entire contents of which are each incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    As is known, the efficiency of a rotary, permanent-magnet electric machine, such as a wind turbine alternator, is strongly affected by its magnetization level, which may vary over time. During operation of the machine, the original magnetization condition may be altered, for example, by breakages, exposure to high temperature or intense electromagnetic fields, or other factors. 
         [0003]    It is also important to bear in mind that permanent magnets are normally magnetized before the machine is installed, and often even before the machine is assembled; and assembly and installation may alter magnetization of the permanent magnets, thus greatly affecting performance of the machine by the time the machine is ready to go into operation. 
         [0004]    In fact, it is not unusual for the efficiency of the electric machine to be insufficient or less than predicted. 
         [0005]    On the other hand, certain currently used methods of determining the magnetization of permanent magnets are relatively complicated and expensive, such as normally involving dismantling the machine and often also the magnets. 
         [0006]    As a result, magnetization of the permanent magnets cannot be checked as often as it should. 
         [0007]    Maintenance is therefore not organized properly, the machine is not run to its full potential, and reassembling the magnets involves the same risks as prior to installation of the machine. That is, the risk of altering the magnetization of even perfectly functioning magnets. 
         [0008]    PCT Published Patent Application No. WO 2008/116463 discloses an electric machine having a magnetization sensor fixed to the stator and arranged to link a magnetic flux produced by permanent magnets of the rotor. A measuring means detects currents induced in the magnetization sensor in response to passage of the permanent magnets during rotation of the rotor. A processing unit determines the magnetization level of the permanent magnets on the basis of the induced currents detected when the rotor is rotating. 
       SUMMARY 
       [0009]    The present disclosure relates to an electric machine. 
         [0010]    It is an advantage of the present disclosure to provide an electric machine, configured to eliminate certain of the drawbacks of known electric machines. 
         [0011]    According to one aspect of the present disclosure, there is provided an electric machine including a stator having a plurality of stator windings, a rotor having a plurality of permanent magnets, and a probe winding fixed with respect to the stator and located close to the rotor to link a magnetic flux produced by at least one of the permanent magnets. The electric machine of this embodiment includes a drive member configured to rotate the rotor at an angular speed, and a detector configured to detect an induced electric quantity at at least one terminal of the probe winding, said induced electric quantity detected in response to passage of the permanent magnets close to the probe winding. The electric machine of this embodiment includes a processing unit configured to: (i) determine a magnetization level of the permanent magnets based on the induced electric quantity detected when the rotor is rotating, (ii) set the stator windings to an open-circuit condition, and (iii) determine the magnetization level of the permanent magnets based on the induced electric quantity detected when the rotor is rotating with the stator windings in the open-circuit condition. The electric machine of this embodiment includes a switch connected to the processing unit and controllable to alternatively: (i) connect the stator windings to at least one external electric equipment, and (ii) set the stator windings to the open-circuit condition. 
         [0012]    Additional features and advantages are described in, and will be apparent from the following Detailed Description and the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A number of non-limiting embodiments of the present disclosure will be described by way of example with reference to the accompanying drawings, in which: 
           [0014]      FIG. 1  shows a partly sectioned side view, with parts removed for clarity, of a wind turbine comprising an electric machine in accordance with one embodiment of the present disclosure; 
           [0015]      FIG. 2  shows a larger-scale, partly sectioned side view, with parts removed for clarity, of a detail in  FIG. 1 ; 
           [0016]      FIG. 3  shows a simplified front view of a portion of the  FIG. 1  electric machine sectioned along line III-III in  FIG. 2 ; 
           [0017]      FIG. 4  shows a simplified block diagram of the  FIG. 1  electric machine; 
           [0018]      FIG. 5  shows a simplified block diagram of a component part of the  FIG. 1  electric machine; 
           [0019]      FIG. 6  shows a flow chart of steps in a method of determining the magnetization level of permanent magnets of an electric machine in accordance with one embodiment of the present disclosure; 
           [0020]      FIG. 7  shows a simplified block diagram of an electric machine in accordance with a different embodiment of the present disclosure; 
           [0021]      FIG. 8  shows a simplified block diagram of an electric machine in accordance with another embodiment of the present disclosure; 
           [0022]      FIG. 9  shows a simplified front cross section of a portion of an electric machine in accordance with another embodiment of the present disclosure; and 
           [0023]      FIG. 10  shows an enlarged, three-quarter view in perspective of a detail of the  FIG. 9  electric machine. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the example embodiments of the disclosure described below, reference is made to permanent-magnet electric generators used in wind turbines, in which the disclosure may be used to particular advantage. This, however, shall in no way be construed as a limitation of the disclosure, which applies to any rotary, permanent-magnet electric machine, particularly synchronous generators, coupled to any type of motor (especially gas, steam, and hydraulic turbines) and electric motors. 
         [0025]    Referring now to the example embodiments of the present disclosure illustrated in  FIGS. 1 to 10 , number  1  in  FIG. 1  indicates as a whole a wind turbine comprising a pylon  2 , a nacelle  3 , a hub  4 , a plurality of (in the example shown, three) blades  5 , and an electric machine  6 . 
         [0026]    Blades  5  are fitted to hub  4 , which in turn is fitted to nacelle  3 . Nacelle  3  is fitted to pylon  2  to rotate about an axis A 1  and position blades  5  facing the wind; and hub  4  is mounted to rotate about an axis A 2  with respect to nacelle  3 . 
         [0027]    With reference to  FIGS. 2 and 3 , hub  4  comprises a hollow shaft  9  and a body  10  connected rigidly to each other. Hollow shaft  9  is fitted to nacelle  3  and, in the embodiment described, is connected directly to electric machine  6 . 
         [0028]    In the embodiment described, electric machine  6  is a synchronous generator, and comprises a stator  12  and a rotor  13  separated by an annular gap  14 . Stator  12  forms a portion of nacelle  3 ; rotor  13  is fixed directly to hollow shaft  9 ; and rotor  13 , hub  4 , and blades  5  define a rotary assembly  15 , which rotates with respect to nacelle  3  about axis A 2 , and is rotated by the wind about axis A 2  at an angular speed Ω. 
         [0029]    As shown in  FIGS. 3 and 4 , stator  12  has a plurality of stator windings  16 , each connectable selectively to an electric load  17  by a respective switch  18 . When switches  18  are open, the corresponding stator windings  16  are set to an open-circuit condition, isolated from load  17 , which may, for example, be an electric power distribution network, to which user devices (not described in detail), are connected. 
         [0030]    Rotor  13  has a plurality of permanent magnets  20 , which face stator  12  across gap  14 , are configured and arranged to produce a substantially sinusoidal magnetic field along a circle concentric with axis A 2  of rotor  13 , and may be magnetized radially or tangentially. 
         [0031]    A probe winding  21  is housed between stator  12  and rotor  13 , is fixed with respect to stator  12 , and, in one embodiment, is fitted to a pole piece  22  projecting towards rotor  13  from a casing  23  of stator  12 , and located between two adjacent stator windings  16 . 
         [0032]    Probe winding  21  is oriented to link the magnetic flux generated by permanent magnets  20  passing close to probe winding  21 . 
         [0033]    Electric machine  6  also comprises a voltage detector  24 , a peak detector  25 , an angular position transducer  26 , a temperature sensor  27 , and a memory  28 . In one embodiment, electric machine includes a non-volatile processing unit  30 . 
         [0034]    Voltage detector  24  is connected to terminals of probe winding  21  to detect an induced voltage VI in response to passage of permanent magnets  20 . Given the usual shape and arrangement of permanent magnets  20 , induced voltage V 1  is sinusoidal when rotor  13  rotates at constant angular speed a Ω. 
         [0035]    Peak detector  25  is connected to voltage detector  24  to determine peak values VPK and corresponding peak instants tK of induced voltage VI at each half-wave. Depending on the configuration of electric machine  6 , peak values VPK of induced voltage VI are caused by the magnetic field generated by one or a pair of permanent magnets  20 . For the sake of simplicity, unless otherwise stated, reference is made in the following description to peak values VPK of induced voltage VI caused by the magnetic field generated by one permanent magnet  20 , it being understood, however, that the same also applies to peak values VPK of induced voltage VI caused by the magnetic field generated by a pair of permanent magnets  20 . 
         [0036]    Angular position transducer  26 , which, in the embodiment described, is an absolute encoder, determines the angular position a of rotor  13  with respect to a reference angular position αREF, and supplies a corresponding angular position signal Sα to processing unit  30 . 
         [0037]    Temperature sensor  27  is located close to rotor  13 , in an angular position substantially corresponding to probe winding  21 , and supplies processing unit  30  with a temperature signal ST indicating a temperature T of rotor  13 . Temperature sensor  27  may, for example, be a thermoresistive sensor or a thermocouple; and, in one embodiment (not shown), temperature sensors are also installed on the rotor, close to respective permanent magnets. 
         [0038]    Memory  28  stores maximum values VJKMax and minimum values VJKMin of induced voltage VI as a function of the temperature T and angular speed Ω of rotor  13  (e.g., organized in tables  31 ,  32 , as shown in  FIG. 5 ). Maximum values VIJMax and minimum values VIJMin represent maximum and minimum acceptance thresholds for peak values VPK of induced voltage VI in normal operating conditions. In other words, when the magnetization level of the permanent magnet  20  passing close to probe winding  21  is appropriate, the peak values VPK of induced voltage VI range between maximum value VIJMax and minimum value VIJMin corresponding to the current temperature and angular speed Ω of rotor  13 . Conversely, when a peak value VPK of induced voltage VI is below minimum value VIJMin or above maximum value VIJMax in the current temperature and angular speed Ω conditions, a magnetization defect, directly attributable to one or a pair of permanent magnets  20 , depending on the structure of rotor  13 , is detected. Depending on the structure of rotor  13 , it is therefore possible to immediately identify the defective permanent magnet  20  or at least a subset (pair) of permanent magnets  20  including the defective permanent magnet  20 . 
         [0039]    Processing unit  30  receives angular position signal SΩ and temperature signal ST, is connected to memory  28  to access tables  31  and  32 , and controls switches  18 , utilizing a control signal Sc, to connect stator windings  16  to electric load  17 , or set stator windings  16  to an open-circuit condition. 
         [0040]    To determine the magnetization level of permanent magnets  20 , processing unit  30  opens switches  18  to set stator windings  16  to the open-circuit condition and disconnect electric load  17 , as shown in  FIG. 6  (block  50 ); and rotor  13  is then rotated. In one embodiment, rotor  13  is rotated at constant angular speed Ω (block  51 ). 
         [0041]    With rotor  13  rotating, induced voltage VI is detected (block  52 ), and its absolute peak values VPK detected by peak detector  25  (block  53 ). 
         [0042]    Using angular position signal Sα and temperature signal ST, processing unit  30  determines the angular position α, angular speed Ω, and temperature T of rotor  13  (block  54 ), and then accesses memory  28  to extract from tables  31  and  32  a maximum value VIJMax and minimum value VIJMin corresponding to angular speed Ω and temperature T (block  55 ). In one embodiment, maximum value VIJMax and minimum value VIJMin are updated whenever a new peak value VPK of induced voltage VI is determined. In other embodiments (not shown), however, maximum value VIJMax and minimum value VIJMin may be read from memory  28  at a predetermined rate, or only following variations in angular speed Ω and/or temperature T of rotor  13 . 
         [0043]    Processing unit  30  then compares the last peak value VPK with the maximum value VIJMax and minimum value VIJMin extracted from tables  31  and  32  (block  56 ). 
         [0044]    If the peak value VPK ranges between the selected maximum value VIJMax and minimum value VIJMin (YES output of block  56 ), processing unit  30  determines whether the magnetization test is completed (block  57 ), and, if the magnetization test is completed (YES output of block  57 ), processing unit terminates the procedure (block  58 ). The test may be considered completed, for example, after a given or designated time interval or after a given or designated plurality of turns of rotor  13 . If the test is not yet completed (NO output of block  57 ), acquisition of induced voltage VI continues (block  52 ), and the procedure is repeated as described above up to comparison of the last peak value VPK of induced voltage VI with the maximum value VIJMax and minimum value VIJMin selected from tables  31  and  32 . 
         [0045]    If the peak value VPK of induced voltage VI is above maximum value VIJMax or below minimum value VIJMin (NO output of block  57 ), processing unit  30  acquires the angular position α of rotor  13  at a peak instant tK corresponding to the peak value VPK that has failed the test (block  59 ), and identifies the defective permanent magnet  20  by comparing the current angular position α of rotor  13  and the angular position of probe winding  16  with respect to the axis of rotor  13  (block  60 ). Finally, processing unit  30  indicates the presence and location of a defective permanent magnet  20  (block  61 ). 
         [0046]    In a different embodiment of the disclosure, shown in  FIG. 7 , one of stator windings  16  of an electric machine  100  is used as a probe winding and it is indicated with reference numeral  121 . In this case, the probe winding  121  is connectable to load  17  or to voltage detector  24  by a selector  118  controlled by processing unit  30  utilizing a control signal Sc′. During normal operation of electric machine  6 , selector  118  connects probe winding  121  to load  17 , and probe winding  121  operates as a normal stator winding  16 . To test magnetization, processing unit  30  switches selector  118  to connect probe winding  16  to voltage detector  24 . 
         [0047]    In a further embodiment of the disclosure, shown in  FIG. 8 , the probe winding  21  of an electric machine  200  is connected to a current detector  224 , which detects an induced current II in probe winding  21  in response to passage of a permanent magnet  20 . A peak detector  225  receives induced current II and determines its peak values IPK at each half-wave. The peak values IPK and corresponding peak instants tK are supplied to processing unit  30 . In this embodiment, memory  28  contains maximum values IIJMax and minimum values IIJMin for peak values IPK of induced current II as a function of the angular speed Ω and temperature T of rotor  13 . 
         [0048]    In this embodiment, electric machine  200  also comprises an angular speed detector  201  (e.g., a gyroscope, accelerometer, or inclinometer) which supplies processing unit  30  with an angular speed signal SΩ indicating the angular speed Ω of rotor  13 . 
         [0049]    In the  FIG. 9 and 10  embodiment of the disclosure, an electric machine  300  comprises a probe winding  321  housed in a through seat  301  formed in a tooth  302  supporting a stator winding  16 . More specifically, through seat  301  is a seat configured to house a stator tie rod configured to grip a portion of stator  12  corresponding to tooth  302 . Probe winding  321  is advantageously integrated in a stator tie rod of tooth  302 . 
         [0050]    Probe winding  321  comprises a conductor  303 ; and a bar-shaped core  304  made of ferromagnetic material, with a rounded cross section and diametrically opposite longitudinal grooves  305 . Conductor  303  is wound longitudinally about core  304  and housed inside grooves  305 . 
         [0051]    When probe winding  321  is inserted inside tooth  302 , its turns are arranged so as to link the magnetic flux generated by rotor  13 . 
         [0052]    Clearly, changes may be made to the method and electric machine as described herein without, however, departing from the scope of the present disclosure as defined in the accompanying Claims. In particular, more than one probe winding may be used in the same electric machine, and different types of probe windings may be used simultaneously. It should thus be understood that various changes and modifications to the presently disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.