Rotary electrical machines having a superconducting field winding
BACKGROUND OF THE INVENTION 
     The invention relates to rotary electrical machines having a superconducting field winding. 
     Electrical machines adapted to operate as A.C. motors or generators have been described which comprise a superconducting field winding constructed to be supplied with direct current for producing a magnetic field having n pairs of poles (n being an integer), an armature having polyphase windings of normal electrical conductivity which deliver a field having n pairs of poles which rotate with respect to the armature when they are circulated by polyphase currents, and auxiliary windings of a material having normal electrical conductivity which deliver a field having n pairs of poles when D.C. currents circulate therein. Such machines may be of the synchronous type, as described in U.S. patent application Ser. No. 443,014, now abandoned. They can also be asynchronous, as described in U.S. Pat. No. 3,742,265 (Smith). The field winding, auxiliary windings and armature windings are located concentrically or coaxially in side by side relation. 
     In the above constructions, the field winding rotates in operation. As a consequence, sliding seals should be provided on the circuit for feeding the cooling fluid (generally liquid helium) to the cryostat of the field winding and returning it. The design of sliding seals adapted to operate satisfactorily at the cryogenic temperatures is a problem. 
     On the other hand, British patent specification No. 1,289,092 describes an A.C. generator having a stationary superconducting winding, an armature winding and supplementary coils. In operation, substantial axial forces are exerted on the superconducting field winding and the structure supporting the field winding should be so massive that it constitutes a heat flow path between parts at cryogenic temperatures and parts at ambient temperature which results in losses of such magnitude that a large cryogenic supply is necessary. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a rotary electric machine having a superconducting field winding which is stationary and is not subjected to substantial torque or axially directed forces in operation. 
     For that purpose, there is provided a synchronous rotary electric machine having, in combination, a stationary frame; a superconducting field winding located in a cryostat enclosure and having an axis and carried by said frame against relative axial movement thereof with respect to said frame, said field winding being constructed to deliver a magnetic field having n pairs of poles, n being an integer, when D.C. currents circulate therein; a mechanical torque transmission shaft rotatable with respect to said frame about said axis; normally conducting polyphase windings for providing a rotary field having n pairs of poles when A.C. currents circulate therein, carried by said shaft and connected to sliding external contacts for exchanging electrical power with an external A.C. circuit; auxiliary windings securely connected to said frame, constructed to deliver a magnetic field having n pairs of poles when D.C. currents circulate therein; and means for adjusting the currents in said auxiliary windings at a value for which the torques and axial forces tending to move said field winding with respect to said frame are lower than a predetermined value. 
     The polyphase currents which circulate in the armature should be transmitted from an external source (when the machine operates as a motor) or to a load (when the machine operates as a generator) through sliding contacts. 
     The use of sliding contacts puts a limit on the value of the currents which can be accepted in the armature windings. That limit is of about 10 KA. Technological limitations with respect of the voltage are also to be considered. As a result, the invention is of particular interest in the design of rotary machines whose power does not exceed 25 MW. 
     Since the field winding, the auxiliary windings and the armature winding are located in side by side relation in the axial direction, the number of pairs of poles may be high. Consequently, the speed may be low and the machine may be used as a motor for ships. 
    
    
     SHORT DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a motor according to an embodiment of the invention, in vertical cross-section along the axis for making apparent the relative arrangement of the components; 
     FIG. 2 illustrates the relative arrangement of the windings of the machine of FIG. 1, as seen in the tangential direction. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a motor whose main components are located in side by side relation, that is, according to a &#34;flat&#34; geometry. The motor comprises a stationary frame 10 which carries the cryostat 11 which contains the superconducting field winding 12. Auxiliary windings 13 are securely connected to the frame on one side of the field winding. A bearing means 14 is carried by the frame on the other side of the field winding and supports the output shaft 15 of the motor. The shaft has a terminal disk-shaped enlargement 16 formed with slots which receive the electrical conductors which constitute a polyphase winding 17 (three phase winding for instance) adapted to deliver a magnetic field having n pairs of poles. n may easily be 10 or more. The winding 17 will be designated as the armature winding for more simplicity in the following. The stationary frame 10 and the terminal enlargement 16 of the shaft may constitute a magnetic circuit. Then the terminal portion should be of laminated construction. Since however the magnetic flux may be very high due to the use of a superconducting field winding, the magnetic materials are saturated and it may be sufficient for the frame 10 to play the part of a mechanical structure of retaining the field winding 12 and the auxiliary windings 13. 
     The shaft 15 carries the rotary components of a device for transmission of polyphase currents from an external source. That device, schematically illustrated as sliding rings 18 on FIG. 1, has a structure which is selected to fit the current values to be transmitted. If the current values are very high, liquid contacts (for instance mercury-indium contacts) may be necessary. 
     In the embodiment of FIG. 1, the field winding 12 and the cryostat 11 are secured against rotation as well as against axial movement and the auxiliary windings are designed for lowering the resultant axial force and torque exerted on the field winding under a predetermined value, which is selected as a function of the mechanical resistance of the structure supporting the field winding 12. With such a design, the cryostat is stationary and the circuit for circulating a cryogenic fluid (generally liquid helium) necessary for maintaining the field winding 12 in superconducting condition may be of relatively simple design. In the embodiment of FIG. 1, a central pipe 19 delivers liquid helium directly into cryostat 11. Pipe 19 is located in a larger pipe 20 for return of helium in vapor phase. Last, pipe 20 is thermally insulated by a space 21 limited by a pipe 22 and in which is a pump 23 maintains a primary vacuum. The evacuated annular space 21 may be connected to the space limited by the internal wall 24 of the cryostat (which may be of conventional construction) and the external wall 25. The surface of the external wall 25 which confronts the armature 17 is preferably provided with a shield 26 of a material having normal electric conductivity for protecting the field winding against the field changes due to the transient phenomenae and to the harmonics of the armature field. 
     For lowering the heat flow between the field winding 12 at cryogenic temperature and the frame 10 at ambient temperature to a minimum, the field winding 12 is mechanically connected to the frame by a structure having a low mechanical resistance and a low thermal conductivity. As a consequence, the various forces which are exerted on the field winding and tend to move it with respect to the frame should be maintained at a value lower than a predetermined threshold, depending on the mechanical resistance. 
     In the embodiment of FIG. 1, two types of electro-magnetic efforts are applied to the field winding due to the interaction between its field and the armature field, namely a torque tending to rotate the winding about its axis and an axially directed force. For neutralizing the resultant force and torque to a level which is low enough, the auxiliary windings comprise, as indicated on FIG. 2: 
     first D.C. coils 27 for generating a field having the same number of pairs of poles as that delivered by the field winding; the coils 27 are located for their field to have poles offset by (π/2n ) with respect to the poles of the field winding; the first coils are associated with current supply means which delivers a D.C. current having a value adjusted for the torque on the field winding not to exceed a predetermined value; 
      second D.C. coils 28 for generating a field having the same number of pairs of poles as the field winding, and having poles facing the poles of the field winding; the second coils are associated with current supply means which delivers a D.C. current having a value adjusted for the axial force on the field winding not to exceed a predetermined value. 
     The currents in coils 27 and 28 are automatically adjusted by systems which may be of the open-loop or closed-loop type. 
     Closed loop regulating circuits are used in the embodiment of FIGS. 1 and 2. Appropriate transducers (for instance strain gauges) are carried by the connecting structure, which is illustrated in simplified form as consisting of small diameter rods 29. First transducer means are located on structure 29 and put in circuit for delivering an electric signal which is a direct function of the torque taken by structure 29. That signal is applied to a first input of a comparator 30. The comparator 30 has a second input which receives an adjustable electric signal representing the maximum acceptable value of the torque. That signal is delivered by a manually adjustable set generator 31. The comparator 30 may be of conventional design which delivers a signal of appropriate polarity to a control input of a D.C. generator 32 as long as the signal from the transducer means exceeds the threshold for modifying the current delivered by the D.C. generator until the torque representative signal has been lowered under the threshold set by generator 31. The D.C. generator 32 may be of any conventional design. It may for instance consists of an adjustable self transformer fed by an external A.C. source, a control motor for adjusting the step down ratio of the transformer, and rectifiers. The output current from generator 32 is circulated in the torque neutralizing coils 27 (FIG. 2). 
     Similarly, transducer means sensing the axial forces is associated with a servo-circuit consisting of a comparator 33, threshold generator 34 and D.C. generator 35. 
     Alternately, the current adjusting circuits may consist of D.C. generators associated with means for measuring the values of the active current and reactive current which circulate in the armature winding. The D.C. generators adjust the currents in coils 27 and 28 at values which are proportional to the active power and reactive power of the motor, respectively. Such a system is of the open-loop type and relies on prior determination of the proportionality factors for reducing the torque and force on the field winding to a value lower than the threshold. 
     In a modified embodiment (not shown), the field winding is not secured to the frame directly. It is carried by an annular suport whose angular position about the axis may be adjusted by a mechanical linkage. The auxiliary windings comprise a single set of D.C. coils for torque and axial force neutralization. For compensation to be achieved with a single set of coils, the field winding should be located at an appropriate angle with respect to the auxiliary winding. The electrical angle α between the poles of the coils of the auxiliary windings and the poles of the field winding should be equal to φ (cos φ being the power factor of the machine). In other words, the geometrical angle between the poles should be (φ/n). Then, the same current which compensates for the axial forces also compensates for the torque. It is then sufficient to determine the force which tends to move the field winding axially and to adjust the currents in the set of coils for maintaining that force under a predetermined value for correlatively maintaining the torque under another threshold. 
     Still another possibility consists in leaving the field winding free to move angularly within a limited angular extent while adjusting the current to a value which is appropriate for neutralization of the axial forces. Then, the field winding will automatically move to a position where balance of the forces due to interaction between the various field is achieved. It should be noted that a range of variation of some degrees may be sufficient for a motor having ten pairs of poles or more. 
     Still another possibility consists of using a symetric coil arrangement. Compensation windings are located on both sides of the field winding and are carried by U-shaped frame. The armature winding also comprises two sets of windings which straddle the auxiliary windings and the field winding. Then, the axial forces are balanced and the coils 28 may be omitted. 
     The motor should be associated with ancillary components for circulating the field generating current in the field winding, and then short-circuiting the field winding for steady state operation. Such components are conventional and a description thereof may be found in prior art documents, for instance U.S. Pat. application Ser. No. 443,014, now abandoned referred to above.