Abstract:
An electrical machine configured to operate in a power range of several MVA includes a rotor configured to rotate about an axis. The rotor includes a rotor winding disposed in a rotor lamination stack and having an exposed winding head outside of the rotor lamination stack. The winding head includes a winding head retention system having a plurality of radially oriented retention elements each including a locking device configured to secure a respective one of the retention elements against at least one of an unintentional loosening and a flying away in an event of a breakage.

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
     This is a continuation of International Patent Application No. PCT/EP2010/055111, filed on Apr. 19, 2010, which claims priority to German Application Nos. DE 10 2009 018 552.6, filed on Apr. 24, 2009, and DE 10 2009 037 989.4, filed on Aug. 20, 2009. The entire disclosure of each of the applications is hereby incorporated by reference herein. 
     FIELD 
     The present invention relates to the field of electrical machines. 
     BACKGROUND 
     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, there are three types of winding head retention systems: 
     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. 
     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 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 ′. The rotor winding head  16 , which is shown once more enlarged in vertical orientation in  FIG. 2(   a ), is equipped with a winding head retention system  29  which, according to  FIG. 2(   a ), is fitted with retention elements which run radially through the rotor winding head  16  in the form of threaded rods  20  or threaded bolts or similar, and which are fitted with nuts  21 . 
     The retention elements  20 ,  21  absorb the centrifugal forces acting on the rotor winding head  16  during the operation of the machine and pass them into the solid part of the rotor arranged below the winding head. It can now come about that the heavily mechanically loaded retention elements  20 ,  21  break or lose their mechanical integrity in some other manner during operation over a longer period. However, it may also be that the nuts  21  or screw fittings come loose and in this way the function of the retention elements  20 ,  21  is lost. If, as part of this failure, some of the retention elements  20 ,  21  come loose and are flung outwards, the machine can incur consequential damage which leads to extended downtime of the machine and therefore to high consequential costs. 
     It is desirable to either prevent such a failure of the retention elements  20 ,  21  altogether or at least keep the consequential damage within limits. Furthermore, it would be of advantage if abnormal changes in the winding head could be detected in good time and the machine could be switched off at an early stage in order to limit damage or to prevent it occurring in the first place. 
     SUMMARY OF THE INVENTION 
     In an embodiment, the present invention provides an electrical machine configured to operate in a power range of several MVA including a rotor configured to rotate about an axis. The rotor includes a rotor winding disposed in a rotor lamination stack and having an exposed winding head outside of the rotor lamination stack. The winding head includes a winding head retention system having a plurality of radially oriented retention elements each including a locking device configured to secure a respective one of the retention elements against at least one of an unintentional loosening and a flying away in an event of a breakage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  shows in a simplified representation a section of an asynchronous machine with stator and rotor winding and a winding head retention system for the rotor winding head according to the prior art; 
         FIG. 2  shows in two part figures the enlarged section of the rotor winding head from  FIG. 1  in vertical orientation ( FIG. 2   a ) and two types of safety devices for the retention elements according to exemplary embodiments of the invention ( FIG. 2   b ); 
         FIG. 3  shows in two part figures the section through ( FIG. 3   a ) and the plan view on ( FIG. 3   b ) a wire locking device of retention elements comprising threaded rods and nuts according to another exemplary embodiment of the invention; 
         FIG. 4  shows by way of example the incorporation of monitoring means into the retention system according to another exemplary embodiment of the invention; and 
         FIG. 5  shows the schematic diagram of a monitoring device which works with the monitoring means of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     In an embodiment, the present invention provides an electrical machine of the kind mentioned in the introduction so that damage to the rotor winding head due to the centrifugal forces acting can be prevented or at least limited. 
     An embodiment of the invention includes retention elements provided with locking means which secure the retention elements against unintentional loosening and/or flying away in the event of a breakage. 
     An embodiment of the invention provides retention elements designed as threaded rods or threaded bolts which extend through the winding head in a radial direction and the outer end of which is provided with a nut or a screw head for supporting the winding head. 
     In an embodiment, the locking means can comprise a textile locking device which is stretched, particularly in the form of a textile tape, over the outsides of the nuts or screw heads, preferably in close contact therewith and in a circumferential direction. 
     In an embodiment, the locking means can also comprise a wire locking device which is stretched in the form of a wire over the outsides of the nuts and/or threaded rods or screw heads, preferably in close contact therewith and in a circumferential direction. 
     In an embodiment, slots, in which the wire locking device, which simultaneously safeguards the retention elements against twisting, is inserted, are preferably provided on the retention elements. 
     Another embodiment of the invention is distinguished in that the winding head retention system is equipped with monitoring means, with which the proper functioning of the winding head can be monitored. 
     In an embodiment, the monitoring means preferably comprise strain measuring devices, which measure the mechanical strain of the locking means in the winding head retention system and transmit it to a processing unit for evaluation. 
     According to an embodiment of the invention, the bolts or screws of the winding head retention system  29  must be secured in the rotor winding head  16 . If the retention elements according to  FIG. 2   a  and  FIG. 3  are threaded rods  20  with nuts  21 , the nuts  21  must be secured against unintended loosening. However, if a screw or bolt should accidentally break, the parts which come free must be retained in order to prevent damage to the machine. 
     According to an embodiment of the invention, safeguarding against flying away can be achieved by a wire (wire locking device  23  in  FIG. 2   b ) or a textile (textile locking device  22  in  FIG. 2   b ) which is stretched over the screw heads or nuts  21  in a circumferential direction. In normal operation, the screws (or threaded rods  20 ) cannot move in the radial direction. The forces, which are absorbed by the screws (or threaded rods  20 ), are large, as these must be able to carry the centrifugal forces which result from the weight of the winding head  16 . If a screw (or threaded rod  20 ) should break, only the centrifugal force of the mass of the piece of the screw which comes free acts on the safety device. This force is relatively small. For this reason, it is possible to retain a broken screw or threaded rod  20  by means of a wire or textile. A highly tear-resistant textile which is stable even at higher temperatures can be used as a suitable material for the textile or textile tape. 
     While the textile tape of the textile locking device  22  according to  FIG. 2   b  can be fed over the width of the nuts  21  or screw heads, when using the wire locking device  23 , it can be of advantage to provide slots  24  in which the wire locking device  23  is laid and which run in a circumferential direction in the nuts  21  or threaded rods  20  or in the screw heads according to  FIG. 3 . As a result of the wire running in the slots  24 , the nuts  21  or threaded bolts are unable to turn. At the same time, the wire itself is safeguarded against slipping. 
     Advantageously, monitoring can simultaneously be combined with the safeguarding of the retention system. The monitoring is used to protect the machine. It is there to detect and signal accidental breakage of the screw or bolt or an inadmissible expansion of the rotor winding head  16  so that the machine can be stopped in good time and repairs initiated. For this purpose, according to  FIGS. 4 and 5 , the strain (tension) in the wire or textile tape is monitored by means of a suitable electronic sensor (strain measuring device  25 ) (for example, by means of a load cell). If part of the screw comes loose due to a breakage, the mechanical strain in the wire (wire locking device  23 ) increases and can be detected. At the same time, the sensor or strain measuring device  25  can be integrated running around the circumference directly in the wire (top part in  FIG. 4 ), or can measure the strain of two safety wires in that the sensor is arranged perpendicular to the wires (bottom part of  FIG. 4 ). Furthermore, the wires can be fed with appropriate deflections through the crossing points of the winding bars so that the strain can be measured inside the rotor body. 
     The signals measured by the different sensors or strain measuring devices  25  in the rotor winding head  16  are fed to an appropriate monitoring device ( 28  in  FIG. 5 ) of a central processing unit  26  where they are evaluated. If abnormal changes in the strain pattern of the safety devices of the winding head retention system  29  are detected during operation, a signal is output to and displayed on a downstream display unit  27 . The signal can simultaneously be fed to a central controller for the machine where it can initiate an automatic shutdown so that the machine can be inspected in good time. 
     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 
     
         
           10  electrical machine (asynchronous machine) 
           11  central body 
           11 ′ shaft 
           12  rotor lamination stack 
           13  rotor winding 
           14  stator lamination stack 
           15  slip ring 
           16  rotor winding head 
           17  stator winding head 
           18  axis 
           19  rotor 
           20  threaded bolt 
           21  nut 
           22  textile locking device 
           23  wire locking device 
           24  slot 
           25  strain measuring device 
           26  processing unit 
           27  display unit 
           28  monitoring device 
           29  winding head retention system