Abstract:
The invention relates to an electric machine ( 2 ) comprising a rotor ( 6 ) that contains a shaft ( 18, 22 ) that is located, when in operation, in the ambient temperature range, and a superconducting rotor winding ( 30 ) that is cooled to a cryogenic temperature when in operation and which is arranged on the shaft ( 18, 22 ). Said machine also comprises a cooling system ( 8 ) for cooling the rotor winding ( 30 ), said system comprises a compensation pressure reservoir ( 50, 64 ) for a cooling medium ( 42 ). In said electric machine, the compensation pressure reservoir ( 50, 64 ) is arranged on the shaft ( 18, 22 ).

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2008/058020, filed Jun. 24, 2008, which designated the United States and has been published as International Publication No. WO 2009/003877 and which claims the priority of German Patent Application, Serial No. 10 2007 030 474.0, filed Jun. 29, 2007, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to an electrical machine having a rotor which contains a superconducting rotor winding which is cooled to cryogenic temperature during operation. 
     Electrical machines, to be precise generators or electric motors, for example synchronous machines, can nowadays be equipped with superconducting components, for example a rotor winding. A cooling system is required in order to cool the rotor winding down to the operating temperature. A normal embodiment of such cooling systems uses, for example, a so-called thermosiphon and operates with a two-phase cooling medium, for example helium, hydrogen, neon, nitrogen or argon. In general, a corresponding rotor has a cooling chamber which is arranged in the area of the rotor winding and into which liquid cooling medium is introduced, where it is vaporized for cooling and is then fed back to a cold head in order to once again liquefy the cooling medium. 
     When the machine is being started up, all the components of the machine and of the cooling system are initially at room temperature. After the cooling system has been switched on, the so-called cold heads are cooled down first of all, and the liquefaction of the working gas starts when the temperature is sufficiently low. 
     Since the density of the working gas increases as the temperature falls, and in particular when it is liquefied, it is known for an appropriately large gas volume to be kept available at room temperature in an external supply container which is fitted to the cooling system in order to ensure that a sufficient amount of liquefied working gas, that is to say liquid coolant, is available during operation of the machine. In the equalization pressure container, the cooling medium is generally at ambient temperature in the gaseous state both during operation and when the machine is at rest. A buffer system such as this results at least in a system which operates “at the push of a button”, that is to say the user does not need to worry about replenishment of cooling medium, or allowing cooling medium to escape. In the end, he just has to switch on the machine, including the cooling system, after initial filling. 
     In order to keep the dead volume, that is to say the cooling circuit volume which is actually not required during operation in an arrangement such as this, small, it is desirable for the initial pressure at room temperature to be as high as possible. The equalization vessel is therefore generally in the form of a pressure container. The equalization pressure container is arranged outside the electrical machine and is connected via an appropriate connecting line to the cooling system, which leads into the rotating rotor of the machine. 
     Alternatively, it is known for the working gas to be replenished from a reservoir when the machine is being started up. This means that the installation operator has to ensure that the gas is replenished. 
     When the machine is shut down, the vaporizing working gas must then once again be stored in the supply container, for example a gas cylinder or a pressure vessel, or alternatively must be blown out. Particularly during replenishment and when working gas is escaping, the user must ensure the supply of working gas. In addition, the working gas is generally expensive, which means that blowing out should be avoided. It is therefore also known for the blown-out working gas to be fed back again into a supply cylinder, but this requires the use of an external compressor. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to specify an improved electrical machine having a superconducting rotor winding. 
     The object is achieved by an electrical machine having a rotor, with the rotor comprising a shaft which is at ambient temperature during operation. Furthermore, the machine contains a superconducting rotor winding which is arranged fixed on the shaft and is cooled to cryogenic temperature during operation. The electrical machine furthermore comprises a cooling system for cooling the rotor winding, wherein the cooling system comprises an equalization pressure container for cooling medium. According to the invention, the equalization pressure container is arranged on the shaft of the rotor of the electrical machine. 
     The shaft is the part of the machine which has to supply cooling medium during operation. The arrangement of the equalization pressure container directly on the shaft avoids the need for an external connecting line leading to the equalization pressure container. The pressure container that is arranged on the shaft therefore holds the working gas as close as possible to the location where it is also required when the machine is switched on and operated. 
     The equalization pressure container may, for example, be flange-connected to the shaft, for example at the non-drive end of the electrical machine which is generally unused. 
     At least in one subsection, the shaft may be in the form of a hollow shaft and may have a cutout there in its hollow area for the equalization pressure container. This refinement of the invention is based on the idea of widening the diameter of an electrical machine shaft which is present in any case, of designing it to be in form of a hollow shaft, or of enlarging the cavity in a hollow shaft which exists in any case, or of using it, in order to arrange the equalization pressure container there. Inter alia, this avoids the need for the additional volume for an external pressure container. The wall thickness of the shaft therefore just needs to be designed to be adequate for its supporting function. 
     A further material saving is achieved if the cutout mentioned above forms the equalization pressure container. No separate equalization pressure container is therefore used in the hollow shaft and, instead, the hollow shaft is itself used as the equalization pressure container. 
     The cutout may be cylindrical and concentric with respect to the center longitudinal axis of the rotor, thus allowing the hollow shaft to be configured in a particularly simple form, which is advantageous for the electrical machine. 
     Since the equalization pressure container must connected to the cooling system by means of a line, in order to allow working gas to be interchanged, the equalization pressure container can also be connected to the cooling system by means of a channel, wherein the channel is a cutout in the rotor. This also saves on separate components for the channel or the connecting line. 
     The channel may be concentric with respect to the center longitudinal axis of the rotor. In this case as well, the concentric arrangement of the channel results in particularly simple production and advantageous mechanical characteristics for the rotating shaft. 
     A thermosiphon system generally has a cooling tube, which runs centrally along the center longitudinal axis in the rotor, for transportation of cooling medium into the area of the rotor winding. The equalization pressure container can therefore be designed such that a cooling tube such as this passes through it. The transportation of cooling medium is then not impeded by the equalization container. 
     In the case of a cooling tube, the connecting channel between the equalization pressure container and the cooling system may concentrically surround the cooling tube. 
     Since the non-drive end of the rotor is generally unused or is used only for connection of the cooling system, the equalization pressure container can be arranged at the non-drive end of the rotor. 
     In electrical machines, it is known for the diameter of the non-drive end of the rotor to be reduced in comparison to that of the drive end, since this shaft has only a supporting function and does not transmit torque. As a result of the integration of the equalization pressure container, in the machine according to the invention, the non-drive end of the rotor can have the same diameter as the drive end, which on the one hand offers space for the equalization pressure container and a corresponding cavity, and on the other hand results in the advantage that the bearing shells for the drive-end of the shaft and the non-drive end of the shaft are of the same size, thus simplifying their storekeeping. 
     When a cooling tube is passed through, the internal volume of the hollow shaft, that is to say the equalization pressure container, can be sealed from the external area, that is to say the atmosphere side, by means of a rotating bushing which is required in any case for this purpose. The equalization pressure container is therefore sealed from the atmosphere at the shaft end. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In order to describe the invention further reference is made to the exemplary embodiments in the drawing in which, in each case in the form of a schematic outline sketch: 
         FIG. 1  shows an electrical machine with an equalization container integrated in the rotor. 
         FIG. 2  shows an alternative embodiment for the non-drive-end shaft of the machine shown in  FIGS. 1 , and 
         FIG. 3  shows an alternative embodiment with a separate, but integrated, equalization container. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As an example of an electrical machine,  FIG. 1  shows an electric motor  2  with a stator  4 , a rotor  6  and a cooling system  8  which acts on the rotor  6 . 
     The stator  4  comprises a stationary stator housing  10  in which a stator winding  12  is firmly anchored. A bearing  14   a ,  14   b  is respectively attached both to the drive end A and to the non-drive end B of the electric motor  2  in the stator housing  10 , in which bearing  14   a, b  the rotor  6  is borne such that it can rotate about its center longitudinal axis  16 . 
     At the drive end, the rotor  6  has a drive-end shaft  18 , which is fixed in the bearing  14   a  and is used to emit the torque that is produced by the electric motor  2 . The rotor  6  essentially has three subelements which are located one behind the other axially. In the interior of the electric motor  2 , the drive-end shaft  18  is connected to a core piece  20  which is firmly connected both to the drive-end shaft  18  and to the non-drive-end shaft  22 , which is in turn connected thereto. The non-drive-end shaft  22  is in turn held in the bearing  14   b  in the stator  4 . 
     The core piece  20  comprises a rotor outer wall  24  which surrounds an insulating vacuum  26 . A rotor cold mass  28  is held in the insulating vacuum  26 , without touching the rotor outer wall. The rotor cold mass  28  represents, inter alia, the actual superconducting rotor winding  30 , which is cooled to cryogenic temperature during operation by the cooling system  8 . In order to transmit a torque to the drive-end shaft  18 , with this torque being produced on the rotor mass  28  by the electric motor  2 , this shaft is connected to the rotor cold mass  28  via a thermally insulating torque transmission element  32 . At the non-drive end, the rotor cold mass  28  is connected to the non-drive-end shaft  22 , simply for supporting purposes, via a cold part support  34 . 
     The cooling system  8  has a cooling chamber  36  which is arranged centrally with respect to the center longitudinal axis  16  in the rotor cold mass  28 . Liquid neon  42  is introduced as a cooling medium at a temperature of about −250° C. into this cooling chamber  48  in the direction of the arrow  40  through a cooling tube  38  which touches the stator  4 , and cools the rotor winding  30  in the cooling chamber  36  to a cryogenic temperature, is vaporized there and flows back again through the cooling tube  38  in the direction of the arrow  44 . The arrow  40  is therefore the flow direction of the liquid neon, and the arrow  44  that of the gaseous neon. Outside the rotor  6 , the cooling tube  38  leads to a cold head, which is not illustrated, and in which the gaseous neon  42  which was made to flow in the direction of the arrow  44  is once again cooled down to liquid neon  42 . Since the rotor  6  rotates together with the non-drive-end shaft  22 , the stationary cooling tube  38  is borne in the non-drive-end shaft  22  at the non-drive end  46 , by means of a gas-tight rotating bushing  48 , such that it can rotate about the center longitudinal axis  16 . 
     The cooling system  8  also has a working gas buffer volume  50  which is arranged in the interior of the non-drive-end shaft  22 , which is in the form of a hollow shaft. The working gas buffer volume  50  is connected by means of a channel  52 , which passes through the rotor outer wall  24  in the direction of the center longitudinal axis  16  and concentrically surrounds the cooling tube  38 . In other words, the channel  52  represents a radial gap between the cooling tube  38  and the rotor outer wall  24 . Neon  42  can circulate through the channel  52  between the working gas buffer volume  50 , which is always at the ambient temperature of the electric motor  2 , and the cooling chamber  36 . 
     When the electric motor  2  is started up, all of its parts are initially at ambient temperature. The neon  42  which is located at an increased pressure in the working gas buffer volume  50  then successively diffuses through the channel  52  into the cooling chamber  36  and via the cooling tube  38  to the cold head, which is not illustrated, in order to be successively cooled down there until a sufficient amount of liquid neon  42  is created, and the cooling chamber  36  has been cooled down to a cryogenic temperature. 
     After the electric motor  2  is switched off, it is once again gradually heated to ambient temperature, as a result of which all the liquid neon  42  is vaporized and flows back via the channel  52  to the working gas buffer volume  50 , in order to be stored there. 
     In one example of the design of the electric motor  2  with regard to non-drive-end shaft  22 , this shaft has, for example a length L of 50 cm and a diameter D of 30-40 cm. 
     In an alternative embodiment, which is shown in  FIG. 2 , the non-drive-end shaft  22  is formed in two parts, specifically with a cylindrical shaft part  60 , to which a flange  62  is connected which supports the rotating bushing  48 . A non-drive-end shaft  22  designed in this way can be produced considerably more easily but must be well sealed on the flange  62  in order to prevent neon  42  from being able to escape out of the working gas buffer volume  50 . 
     In a further embodiment, which is shown in  FIG. 3 , the non-drive-end shaft  22  is not itself in the form of a working gas buffer volume  50 , and instead the cavity which corresponds to the working gas buffer volume  50  is designed to hold a pressure container  64 . The pressure container  64  holds the gaseous neon  42  and is connected to the cooling chamber  36  via a separate connecting line  66 . Alternatively, the pressure container  64  can also be connected to the channel  52  from  FIG. 1 , in a manner which is not illustrated.