Abstract:
An electrical machine configured to operate at a power range of several MVA includes a rotor is configured to rotate about a rotor axis. The rotor includes a rotor lamination stack. A stator includes a stator lamination stack concentrically surrounding the rotor lamination stack, wherein a ring-like air gap separates the rotor lamination stack from the stator. An air gap monitoring device extends through the air gap in an axial direction and is configured to detect at least one of a change in a rotor geometry and a presence of debris in the air gap.

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
       [0001]    This application is a continuation of International Patent Application No. PCT/EP2010/055132, filed on Apr. 19, 2010, which claims priority to German Patent Application Nos. DE 10 2009 018 553.4, filed on Apr. 24, 2009, and DE 10 2009 037 990.8, filed on Aug. 20, 2009. The entire disclosure of each application is hereby incorporated by reference herein. 
     
    
     FIELD 
       [0002]    The present invention relates to the field of electrical machines. 
       BACKGROUND 
       [0003]    Double-fed asynchronous machines in the power range from 20 MVA to 500 MVA can be used for variable-speed energy production. These machines are distinguished by a distributed three-phase winding on the rotor. The rotor winding consists of individual bars which are embedded in slots in the rotor laminations. The individual bars are connected in the winding head to form a winding. The arrangement of the bar connections is uniformly distributed around the circumference. As a result of the rotation of the rotor, the winding heads are subjected to centrifugal forces, against which they have to be mechanically secured by means of winding head retention systems. In principle, three types of winding head retention systems can be used:
   1. Fixing by means of a steel cap, as is the case with turbogenerators.   2. Fixing wherein a steel cable, wire or plastic film is wrapped around the whole winding head.   3. Fixing by means of bolts, screws or U-shaped brackets.   
 
         [0007]    Such an asynchronous machine  10  is reproduced in section in highly simplified form in  FIG. 1 . It comprises a rotor  19  which can be rotated about an axis  18  and is encompassed concentrically by a stator with a stator lamination stack  14  with corresponding stator winding and a stator winding head  17 . The rotor  19  comprises a central body  11  which merges with a shaft  11 ′ at each end. The central body  11  is surrounded by a rotor lamination stack  12  in which the rotor winding  13  runs. Slip rings  15 , which are used to supply the rotor winding  13  with current, are arranged on one of the shafts  11 ′. A cylindrical air gap  21 , which extends through the machine in the axial direction, is provided between the rotor lamination stack  12  and the stator lamination stack  14 . 
         [0008]    The air gap can have a different geometry in different operating phases depending on temperature, speed and other variable parameters. These changes do not substantially affect the function of the machine. However, it is also possible for the geometry of the rotor to permanently change in an intolerable manner or for debris to get into the air gap. In order to prevent consequential functional faults and to subject the machine to an inspection as soon as possible, it is necessary to detect changes of this kind in the air gap in good time. 
       SUMMARY OF THE INVENTION 
       [0009]    In an embodiment, the present invention provides an electrical machine configured to operate at a power range of several MVA. A rotor is configured to rotate about a rotor axis and includes a rotor lamination stack. A stator includes a stator lamination stack concentrically surrounding the rotor lamination stack, wherein a ring-like air gap separates the rotor lamination stack from the stator. An air gap monitoring device extends through the air gap in an axial direction and is configured to detect at least one of a change in a rotor geometry and a presence of debris in the air gap. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. Other features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following: 
           [0011]      FIG. 1  shows in a simplified representation a section of an asynchronous machine with stator and rotor winding and an air gap between rotor and stator according to the prior art; 
           [0012]      FIG. 2  shows an asynchronous machine according to  FIG. 1  with a monitoring track fixed on the rotor side according to a first exemplary embodiment of the invention; 
           [0013]      FIG. 3  shows in a comparable arrangement to  FIG. 2  a synchronous machine with a monitoring track fixed on the rotor side according to a second exemplary embodiment of the invention; 
           [0014]      FIG. 4  shows an asynchronous machine according to  FIG. 1  with a monitoring track fixed on the stator side according to a third exemplary embodiment of the invention; 
           [0015]      FIG. 5  shows in a comparable arrangement to  FIG. 4  a synchronous machine with a monitoring track fixed on the stator side according to a fourth exemplary embodiment of the invention; and 
           [0016]      FIG. 6  shows in a comparable arrangement to  FIG. 4  an asynchronous machine with an optical monitoring track with higher spatial resolution fixed on the stator side and formed by a plurality of light beams. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    An embodiment of the invention provides an electrical machine such that critical changes in the geometry of the air gap are detected in good time. 
         [0018]    In an embodiment according to the invention, means are provided for monitoring the air gap which extend through the air gap in the axial direction and with which a change in the rotor geometry and/or the presence of debris in the air gap can be detected. 
         [0019]    An embodiment of the invention is characterized in that the monitoring means comprise a monitoring track which extends through the air gap in the axial direction. 
         [0020]    Preferably, the monitoring track runs parallel to the axis. 
         [0021]    According to an embodiment, the monitoring track is formed by at least one taut wire. 
         [0022]    In particular, means are provided for monitoring the mechanical strain in the wire. 
         [0023]    According to another embodiment, the monitoring track is formed by at least one light beam. In particular, means are provided for monitoring the integrity of the light beam. 
         [0024]    In order to achieve a better resolution, it can be of advantage here when a plurality of parallel light beams distributed over the thickness of the air gap form the monitoring track. 
         [0025]    In an embodiment, the at least one wire can be fixed to the rotor by means of suspension means. 
         [0026]    In an embodiment, the at least one wire can also be fixed to the stator by means of suspension means. 
         [0027]    In a corresponding manner, the at least one light beam can be connected to the rotor and fixed to rotate therewith. 
         [0028]    In an embodiment, the at least one light beam can also be designed to be stationary. 
         [0029]    In an embodiment of the present invention, at an early stage, the presence of debris in the air gap or a change in the rotor geometry can be detected with the help of a monitoring track in the form of a taut wire or in the form of optical means (laser). The signal generated by the associated monitoring device can be used for protection purposes or only for monitoring purposes. At the same time, differentiation can be made between a system which is mounted on the rotor and fixed to rotate therewith and is used to monitor the stator, and a system fixed to the stator, which is used to monitor the rotor. However, not only can such a system be used with advantage in asynchronous machines, but also in conventional synchronous machines. 
         [0030]      FIGS. 2 and 3  show a first exemplary embodiment of a monitoring system according to the invention for an asynchronous machine according to  FIG. 1  ( FIG. 2 ) and for a synchronous machine ( FIG. 3 ), wherein here, in contrast to  FIG. 1 , the machine axis is shown in a vertical orientation. In both cases, a monitoring track  23  is fed parallel to the axis through the air gap  21  between rotor lamination stack  12  and stator lamination stack  14 . As already mentioned, the monitoring track  23  can be formed by a wire under mechanical strain or by a light beam which runs between an appropriate optical transmitter and receiver and which is weakened or completely interrupted when debris or geometrical changes in the air gap affect the light beam. 
         [0031]    The exemplary embodiment of  FIG. 2  starts from a monitoring track  23  in the form of a wire. A wire made from a suitable material (Perlon, Kevlar, aluminum etc.) is mounted on the rotor  12  with the help of suspension means  22  so that it runs in the air gap  21  parallel to the machine axis  18 . In doing so, the mountings can be fixed to the rotor rim or attached directly to the pole end plates. The mechanical stress in the wire is monitored. For this purpose, a strain sensor  26  which measures the mechanical strain in the wire and outputs appropriate signals to a processing unit  27  is arranged on the suspension means  22  or on the wire itself (shown dotted). 
         [0032]      FIG. 3  shows an analogous arrangement for a synchronous machine  20  with a stator lamination stack  12 ′ and suspension means  22 ′. The monitoring means  26 ,  27  are omitted here for the sake of simplicity. 
         [0033]    Here too, an optical system, such as a laser and an appropriate receptor, can be used instead of the wire. The optical system behaves like a light barrier. 
         [0034]    If parts of the stator come loose (for example ventilation plate spacers) or if the geometry of the stator changes significantly, contact is made with the wire rotating with the rotor, or the light barrier rotating with the rotor is interrupted. 
         [0035]    Two other exemplary embodiments, which correspond to those in  FIGS. 2 and 3 , are shown in  FIGS. 4 and 5 . In this case, the wire or light barrier (monitoring track  23 ) is fixed to the stator. In doing so, the suspension means  24  and  24 ′ respectively required for this can be mounted directly on the spring fingers, on the air cowlings or on the stator housing. The principle of operation corresponds to that of the monitoring fixed to the rotor in  FIGS. 2 and 3 . If parts on the rotor come loose or if the geometry of the rotor changes significantly, contact is made with the wire, or the light barrier is interrupted. 
         [0036]    With synchronous machines  20  ( FIG. 5 ), the monitoring system described can detect a change in rotor geometry such as a change in the radial position of a pole, an unequal expansion of the rotor rim and also deformations of the pole connections of the excitation and damping winding. 
         [0037]    With the double-fed asynchronous machine ( FIG. 4 ), as well as the roundness of the rotor lamination stack, particularly the retaining device for the winding head can be monitored. 
         [0038]    In all the cases described, according to  FIG. 6  it is possible to replace the beam-shaped light barrier by a plurality of parallel-running light beams  23   a,    23   b  and  23   c  with an appropriate optical transmitter/receiver  25 . In this case, the change in air gap geometry and/or the unwanted presence of bodies can be detected in steps. This can be used in a first step, for example, to only trigger an alarm, and—if the situation deteriorates—to stop the machine in an emergency. If the signals of the system fixed to the stator according to  FIG. 6  are synchronized with the signal of a keyphasor, the evaluation of the data allows the radial position of each individual pole to be measured. The expansion of the rotor under different operating conditions can also be monitored (no-load, full-load . . . ). 
         [0039]    While the invention has been described with reference to particular embodiments thereof, it will be understood by those having ordinary skill the art that various changes may be made therein without departing from the scope and spirit of the invention. Further, the present invention is not limited to the embodiments described herein; reference should be had to the appended claims. 
       LIST OF REFERENCE NUMERALS 
       [0040]      10  electrical machine (asynchronous machine) 
         [0041]      11  central body 
         [0042]      11 ′ shaft 
         [0043]      12 , 12 ′ rotor lamination stack 
         [0044]      13  rotor winding 
         [0045]      14  stator lamination stack 
         [0046]      15  slip ring 
         [0047]      16  rotor winding head 
         [0048]      17  stator winding head 
         [0049]      18  axis 
         [0050]      19  rotor 
         [0051]      20  electrical machine (synchronous machine) 
         [0052]      21  air gap 
         [0053]      22 , 22 ′ suspension means (rotor side) 
         [0054]      23  monitoring track (wire, light beam) 
         [0055]      23   a,b,c  light beam 
         [0056]      24 , 24 ′ suspension means (stator side) 
         [0057]      25  optical transmitter/receiver 
         [0058]      26  strain sensor 
         [0059]      27  processing unit