Abstract:
The machine device comprises a motor ( 2 ) with a rotor ( 5 ) rotating about an axis (A), the superconducting winding of which is coupled with thermal conduction to a central refrigerant cavity ( 12 ). The cavity ( 12 ) forms a single-tube system, together with the line sections ( 22 ) laterally connected thereto and a condenser chamber ( 18 ) of a refrigeration unit, located outside the motor ( 2 ), in which a refrigerant (k, k′) circulates as result of a thermal siphon effect. According to the invention, the refrigerant supply to the central cavity ( 12 ) is maintained, even with inclined positions (d) for the rotor ( 5 ), whereby pressurization means are provided, which generate pressure pulses of gaseous refrigerant, acting on the liquid refrigerant (k) in the condenser chamber ( 18 ) or the connected line sections ( 22 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a superconducting machine device with
         a rotor supported for rotation about a rotation axis, the rotor having at least one superconducting winding with a conductor that is coupled for heat transmission to a central cylindrical cavity extending in the axial direction,   a stationary cooling unit located outside the rotor having a condenser space,
 
and
   tubular line sections extending between the center cavity of the rotor and the condenser space of the cooling unit.       

     The cavity, the tubular line sections and the condenser space form a closed single-tube line system with a coolant disposed therein, wherein by way of a thermosiphon effect, coolant condensing in the condenser space reaches the central cavity through the tubular line sections and coolant evaporating in the cavity is returned to the condenser space via the line sections. A machine of this type is disclosed in DE 100 57 664 A1. 
     Metal oxide superconducting materials with superconducting transition temperatures T c  above 77 K are known since 1987. These materials are also referred to as High-T c -Superconductors or HTS materials and can in principle be cooled with liquid nitrogen (LN 2 ). 
     Conductors using HTS materials have been employed on an experimental basis for fabricating superconducting windings for machines. Disadvantageously, however, conventional conductors of this type have a relatively small current carrying capacity when the magnetic fields reach several Tesla. In many cases, the temperature of the conductors of such windings must be maintained at temperatures below 77 K, for example between 10 and 50 K, in spite of the high superconducting transition temperatures T c  of the employed materials, because these windings would otherwise not be able to carry significant currents in large magnetic fields. However, this temperature level is significantly higher than 4.2 K which corresponds to the boiling point of liquid helium (LHe), which is used to cool conventional metallic superconducting materials with relatively low superconducting transition temperatures T c , so-called Low-T c -Superconductors or LTS materials. 
     Preferably, cooling systems in the form of so-called cryo-coolers with a closed He compressed gas loop can be used for cooling windings with HTS conductors in the aforementioned temperature range below 77 K. In particular, such cryo-coolers operate according to the Gifford/McMahon or Sterling principle, or are formed as so-called pulsed tube coolers. Advantageously, they can immediately produce a cooling effect by pushing a button and avoid handling of cryogenic liquids. These cooling devices cool the superconducting winding only indirectly via thermal conduction to a cold head of a refrigerator (see, for example, “Proc. 16 th  Int. Cryog. Engng. Conf. (ICEC 16”, Kitakyushu, J P, 20-24 May 1996, Published by Elsevier Science, 1997, pages 1109 through 1129). 
     A similar cooling technique is also used for the rotor of an electric machine, as disclosed, for example, in the aforementioned DE 100 57 664 A1. The rotor includes a rotating winding made of HTS conductors which are located in a thermally conductive winding support. The winding support includes a cylindrical central cavity extending in the axial direction, to which tubular line sections extending from the side of the winding support are connected. These line sections are routed to a raised condenser space of a cooling unit and form in conjunction with the condenser space and the central rotor cavity a closed single-tube line system. A coolant, which circulates by way of a so-called thermosiphon effect, is disposed in the line system. Coolant condensing in the condenser space flows via the tubular line sections to the central cavity, where the condensed coolant absorbs heat due through thermal coupling to the winding support and hence also to the HTS winding. The coolant then evaporates. The evaporated coolant returns via the same line sections to the condenser space where it condenses again. The required cooling power is generated by a refrigerator engine having a cold head which is thermally coupled to the condenser space. 
     The return flow of the coolant is driven towards the sections of the refrigerator engine operating as the condenser by a small overpressure in the central cavity, which acts as an evaporator section. The overpressure produced by the generation of gas in the evaporator section and the condensation in the condenser space thereby causes the desired return flow of the coolant. Similar coolant flow patterns are generally known in association with so-called “heat pipes.” 
     The coolant is transported in the conventional machine with thermosiphon cooling from a corresponding cooling unit only by gravity, thereby obviating the need for additional pumping systems. A Machine device employed on ships or offshore installations may frequently experience a static tilt, also referred to as “trim”, of up to ±5° and/or dynamic tilt of up to ±7.5° in the longitudinal direction. Before a series of these machines can be certified for installation on a ship, a reliable cooling performance of the cooling system of such a machine device under these conditions on board of a marine vessel must be ensured. If the machine were tilted in the aforementioned manner, a region of the tubular line sections between the central rotor cavity and the cooling units may be located at a lower level than the central rotor cavity. The coolant can then no longer reach the rotor cavity and cool the rotor cavity by gravity alone, so that there would be no guarantee that the machine could be adequately cooled and would operate reliably. 
     Several proposals have been made to eliminate this risk: 
     The simplest solution is to install the machine with a tilt relative to the horizontal so that there would still be a downward slope in the thermosiphon line system in the direction towards the rotor cavity even at the largest assumed trim position or oscillation amplitude. However, such tilted arrangement is undesirable for longer machines, in particular for shipboard installation because of the increased space requirements. 
     Instead of a single-tube line system, where the liquids and the gaseous coolant flow through the same tube sections, dual-tube line systems can be employed when a coolant is circulated by a thermosiphon effect (see, for example, WO 00/13296 A). However, in this case, an additional tube for the gaseous coolant must be provided in the region of the hollow rotor shaft. 
     In principle, the coolant could also be forced-circulated by a pumping device. However, this approach requires additional equipment, in particular when the coolant must be maintained at a temperature of, for example, 25 to 30 K. Such pumping systems also experience significant thermal losses and may be unable to satisfy the service life requirements for shipboard installations with their long maintenance intervals. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a machine device which includes a machine and an associated cooling unit with the aforementioned features, so that the coolant can still reach the central rotor cavity even if the rotor is tilted, such as in an shipboard or offshore installation. 
     According to the invention, the machine unit should include means for increasing the pressure which produce a short-term pumping effect on the liquid coolant in the direction towards the central rotor cavity by way of pressure pulses on a gaseous coolant. 
     The invention is based on the idea that the coolant need not necessarily be supplied continuously to the central cavity. Based on the thermal inertia of the system, it is sufficient if the coolant is reintroduced into the rotor cavity during a short time period (=“briefly”) at certain time intervals. With the configuration of the machine device according to the invention it is advantageously possible adequately fill the central cavity with liquid coolant, so that the superconducting winding can be safely cooled by repeatedly applying any number of times brief pressure pulses to the liquid coolant, even if the rotor of the machine is tilted. Briefly applying the pressure (typically only for one or more seconds) prevents evaporation of large quantities of liquid coolant which would otherwise noticeably reduce the cooling efficiency on the winding. 
     Advantageous modifications of the machine device of claim  1  are recited in the dependent claims. 
     For example, the means for increasing the pressure can preferably affect the region of the condenser space or of the tubular line sections. The liquid coolant must be reliably supplied to the central rotor cavity even during a realistic tilt. 
     According to a preferred embodiment for achieving the short-term pumping effect, a buffer volume, which is filled with coolant under overpressure, can be connected to the condenser space or the connected line sections via a pump supplying the gaseous coolant. 
     Instead, a heating apparatus which operates on the liquid coolant and is activated for the short-term pumping effect during a corresponding time period may be installed on the tubular line sections. The heating apparatus can advantageously be arranged in a buffer volume which is at least partially filled with the liquid coolant. Corresponding means for increasing the pressure can be relatively easily realized. 
     Advantageously, a permanently pulsating pressure boost can be provided for supplying the liquid coolant to the central rotor cavity. Only relatively insignificant changes to the construction are required to implement a respective conveyance. 
     Instead or in addition, a pressure increase can be initiated by a sensor. For this purpose, a position sensor can be used which detects a tilt of the rotation axis relative to the horizontal, or a fill level sensor, which initiates the pressure increase, can be disposed on the tubular line sections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will be described hereinafter in more detail with reference to the drawings which two preferred embodiments of a machine device. 
         FIG. 1  shows schematically a longitudinal section of a first embodiment of a machine of this device with an associated cooling unit, and 
         FIG. 2  shows schematically in longitudinal cross-section another embodiment of the cooling unit for the machine. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Like elements in the figures have the same reference symbols. 
     Machine devices according to the invention include a machine or a motor with an associated cooling unit. The embodiments of the machine described hereinafter with reference to the figures can include, in particular, a synchronous motor or a generator. The machine includes a rotating, superconducting winding which typically employs a metallic LTS material (Low-T c -Superconducting material) or an oxide HTS material (High-T c -Superconducting material). The following embodiments employ the latter material. The winding can be made of a coil or an assembly of coils with a two-pole, four-pole, or another multi-pole arrangement. The basic configuration of a corresponding synchronous motor is shown in  FIG. 1 , which is based on an embodiment of such a machine described in the aforementioned DE 100 57 664 A1. 
     The machine designated with reference symbol  2  includes a stationary outer housing  3  at room temperature with a stator winding  4 . A rotor which is located inside the outer housing and enclosed by the stator winding  4  is supported in bearings  6  for rotation about a rotation axis A. These bearings can be conventional mechanical bearings or magnetic bearings. The rotor also includes a vacuum vessel  7 , in which a winding support  9  with an HTS winding  10  is supported, for example, on hollow cylindrical, torque-transmitting suspension members  8 . A central cavity  12  which extends concentrically with a rotation axis A in the axial direction is disposed in the winding support. The cavity  12  can have a cylindrical shape. The winding support is vacuum-tight with respect to the cavity and closes the cavity on the side of the rotor. The rotor is on this side supported by a solid axial rotor shaft section  5   a . On the opposite side, the central cavity  12  is connected to a lateral cavity  13  having a comparatively smaller diameter. The lateral cavity extends from the region of the winding support to the exterior of the outer housing  3 . The reference symbol  5   b  designates a tubular rotor shaft section which encloses the lateral cavity  13  and is supported in one of the bearings. 
     A cooling unit is provided to indirectly cool the HTS winding  10  via thermally conductive elements. Only the cold head  16  of the cooling unit is shown. The cooling unit can be a cryo-cooler of the Gifford-McMahon type, or more particularly a regenerative cryo-cooler, such as a pulse tube cooler or a Split-Sterling cooler. The cold head  16  and hence all essential additional elements of the cooling unit are located outside the rotor  5  or its outer housing  3 . 
     The cold section of the cold head  16 , which may be arranged, for example, several meters on the side of the rotor  5 , is located in a vacuum vessel  23  and makes excellent thermal contact via a heat transmitting element  17  with a condensing unit for the coolant, which has a condenser space  18 . A stationary vacuum-insulated heat pipe  20  is connected to the condenser space, with the heat pipe extending laterally in an axial region into the lateral, co-rotating cavity  13  or the central cavity  12 . The heat pipe  20  is sealed against the lateral cavity  13  by a sealing device  21  (not shown in the Figure) having at least one sealing element which may be implemented as a ferrofluidic seal and/or as a labyrinth seal and/or as a diaphragm gland. The central cavity  12  and the heat exchanger region of the condenser space  18  are sealed gas-tight to the outside by the heat pipe  20  and the lateral cavity  13 . The tubular sections extending between the central cavity  12  and the condenser space  18 , which are adapted to receive a coolant, are generally referred to as line sections  22 . These line sections together with the condenser space  18  and the central cavity  12  are referred to as a line system. 
     These cavities of the line system are filled with a coolant which is selected based on the desired operating temperature of the HTS winding  10 . For example, hydrogen (condensation temperature 20.4 K at normal pressure), neon (condensation temperature 27.1 K at normal pressure), nitrogen (condensation temperature 77.4 K at normal pressure), or argon (condensation temperature 87.3 K at normal pressure) can be employed. Mixtures of these gases can also be used. The coolant is hereby circulated through the so-called thermosiphon effect. The coolant condenses on the cold surface of the cold head  16  in the region of the condenser space  18 . The condensed coolant, indicated by k, flows through the line sections  22  to the central cavity  12 . The condensed coolant is transported by gravity. The heat pipe  20  can advantageous be tilted slightly (by several degrees) relative to the rotation axis A so as to increase the outflow of the liquid coolant k from the open end  20   a  of tube  20 . The liquid coolant then evaporates inside the rotor. The vaporized coolant is indicated by k′. The coolant, which evaporates by absorbing heat, then flows back through the inside of the line sections  22  to the condenser space  18 . The return flow is supported by a slight overpressure in the cavity  12 , which operates as an evaporator, in the direction to the condenser space  18 , caused by the generation of gas in the evaporator and condensation in the condenser space. Because the liquefied coolant circulates from the condenser space  18  to the central cavity  12  and the evaporated coolant k′ returns from the same cavity to the condenser space through the line system formed by the condenser space  18 , the line sections  22  and the cavity  12 , the system can be viewed as a single-tube system where the coolant k, k′ circulates due to a thermosiphon effect. 
     As also seen in  FIG. 1 , a tilt can occur when the machine  2  is installed on ships or offshore installations, where the rotation axis A is tilted by an angle δ of several degrees with respect to the horizontal H. The coolant then still condenses in the condenser space  18 ; however, the coolant can then no longer reach the central cavity  12 , so that the line sections  22 , in particular in the region near the axis, increasingly fill up with liquid coolant k. The rotor cavity or the cavity  12  can become dry and would no longer be cooled, if the line system is filled only with a relatively small quantity of the coolant. When the line system is filled with a larger quantity, the accumulated liquid coolant blocks the return flow of the gaseous coolant k′ in the line sections  22  to the condenser space  18  after a certain time. This may prevent the rotor or its superconducting winding from being reliably cooled. According to the invention, the gas pressure on the condenser side is briefly increased under these conditions to a level, where the liquid coolant is pushed from the line sections  22  into the central cavity  12  against gravity in the presence of a tilt angle δ). 
     Such increase in pressure can be implemented according to the embodiment of  FIG. 1  by using a warm buffer volume PV w  and a pump  28 . In this way, the gas pressure in the condenser space  18  can be temporarily increased, pushing the liquid coolant k accumulated in the condenser space  18  and in the line sections  22  into the central cavity  12 . A control valve  29  is then arranged in a connecting line  24  between the buffer volume PV w , which is under an overpressure, and the condenser space  18 . The control valve  29  opens the connection to the pump  28  which then feeds the gas k′ from the buffer volume to the condenser space. A valve  30  is installed to return excess gas from the line system  20 . 
     The pressure oscillation produced in this way can be continuous, i.e., repeating in short time intervals (each oscillation having a brief time period), or the pressure oscillation can be controlled by a control unit  27  via a position sensor  26  of conventional design. The position sensor detects the tilt, i.e., the tilt angle δ, of the machine  2  and initiates the introduction of a pressure volume (gas pulse) via the control unit  27 . 
     For sake of clarity of the drawing,  FIG. 1  does not show additional elements for supplying and venting the gas, with the exception of a fill valve for filling the system with gaseous coolant via connecting line  24 , because these elements are generally known. Only the pressure relief valve  31  is shown, which is triggered by excess pressure in the system. 
     Of course, the elements or vessels enclosing the coolant k and k′, respectively, must be insulated to prevent influx of heat. Advantageously, a vacuum provides thermal insulation, whereby additional insulation, such as super-insulation or insulating foam, can be applied in the corresponding vacuum spaces. The volume enclosed by the vacuum vessel  7  is indicated in  FIG. 1  with the reference symbol V. The vacuum also surrounds the tube which surrounds the lateral cavity  13  and extends to the seal  21 . The vacuum surrounding the heat pipe  20 , the condenser space  18  and the heat transmitting element  17  is indicated with reference symbol V′. Optionally, a vacuum can also be provided in the interior space  32  which surrounds the rotor  5  and is enclosed by the outer housing  3 . 
       FIG. 2  shows a detail of another cooling unit which includes means according to the invention for increasing the pressure in the machine  2  of  FIG. 1 .  FIG. 2  only shows those elements of the cooling unit that are located outside the machine. In this embodiment, the pressure can be increased by placing a heating apparatus  34  in the lower section of the vacuum vessel  23  in a region of the connected line sections  22  or heat pipe  20  that is always filled with liquid coolant k. The heating apparatus, which is located in a cold buffer volume PV k  that is generally at least partially filled with liquid coolant k, is briefly activated when the liquid level in the corresponding vessel or the line sections  22  drops below the certain height. The valve  30  connected to the warm buffer volume PV w  is then closed. When the heating apparatus is activated, a small quantity of the liquid coolant k is rapidly evaporated. The heating apparatus  34  is controlled by a control unit  35  which can be connected to a liquid level sensor  36  located in the cold buffer volume PV k . The increase in pressure triggered by the heat pulse then pushes the remaining coolant k into the central cavity  12  of rotor  5 . 
     If in a particular embodiment according to  FIG. 2 , the liquid level need not be measured with a sensor  36  in the region of the cold buffer volume PV k  or of a corresponding region of the line sections  22 , in which case the heat pulse can also be applied periodically. The desired brief increase in pressure can then be repeated in regular time intervals. 
     It is, of course, also possible to combine the embodiment with the means for increasing the pressure, as depicted in  FIG. 2 , with the embodiment of  FIG. 1 , which includes a pump  28 .