Abstract:
A superconductor motor operates as a squirrel cage induction motor. The rotor is covered with a thin film of superconducting material and the magnetic field created by the stator is strong enough to quench the superconducting material to its normal state at periodic spots on the rotor. This periodic quenching both creates a squirrel cage configuration of superconducting material on the rotor and allows the stator field to penetrate the rotor to induce a current. Once the squirrel cage is “created” by the stator field and a current induced, the motor operates as a conventional squirrel cage induction motor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The field of the invention is superconducting motors and specifically induction motors of a type commonly known as squirrel cage motors.  
           [0002]    Electric induction motors operate by using a magnetic field produced by a stator to induce a current in a rotor. The stator consists of coil windings distributed around the circumference of the rotor. As these coils are excited with an AC current, a magnetic field that varies with time in a sinusoidal fashion is produced. The peak(s) of this magnetic field travels around the circumference of the stator at a rate determined by the frequency of the AC current and the number of poles in the stator.  
           [0003]    If there is no load on the motor, the rotor will turn at the same rate that the magnetic field is rotating around the stator. As the load on the rotor increases, the rotor speed will decrease relative to the speed of rotation of the stator field. The difference in the speed of rotation between the rotor and the stator field induces a current in the rotor, since the stator field is no longer fixed with respect to the rotor. This induced current in the rotor creates its own magnetic field that interacts with the magnetic field produced by the stator to produce mechanical forces. The larger the difference in rotational speed between the rotor and the stator field (known as slip), the more current that is induced in the rotor and consequently the greater the torque produced.  
           [0004]    One of the most common types of induction motors is known as a squirrel cage motor. This type of motor gets its name because of the configuration of the windings of the rotor. The windings in which a current is induced consist of bars of electrically conducting material running parallel to the axis of the rotor. These bars are short-circuited at both ends of the rotor by conducting rings. The combination of pairs of bars and end rings are the equivalent of one-turn coils. Their configuration resembles the rotating cage in which squirrels and other animals might exercise, hence the name. Squirrel cage motors are sturdy, simple to construct, and of relatively low cost to manufacture.  
           [0005]    The efficiency and torque characteristics of squirrel cage motors can be improved by using superconducting material in the rotor. An immediate consequence of the use of this material is the elimination of the Ohmic resistance of the conductors of the rotor. This would cause the induced current in the squirrel cage to increase. As a result, more torque would be produced for a given power input, i.e. the motor can be made more compact. There is a definite commercial advantage in using high temperature superconductors (HTS) since these materials exhibit superconductivity at temperatures below 90 degrees Kelvin. The cooling system required to operate at this temperature is considerably less expensive than that needed for the low temperature superconductors that have to be cooled down to temperatures below 10 degrees Kelvin. The materials that act as HTS, however, are very brittle.  
           [0006]    Unfortunately, two problems are encountered when using a squirrel cage rotor constructed with bars of high temperature superconducting material. The first problem is the difficulty of securing electrical contacts at the junction of the bars and end rings of the squirrel cage that preserve the superconducting properties. Furthermore, assuming that a perfect superconducting rotor cage can indeed be constructed, the “coils” of the squirrel cage rotor would behave as diamagnetic bodies. Hence, they could not be penetrated by the stator magnetic field and no induced current would be able to circulate in the rotor.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention overcomes the two problems mentioned above by finding a simple method of constructing a squirrel cage structure of high temperature superconducting material free of imperfect electrical contacts and by obtaining a means of magnetically linking the superconducting rotor coils to the stator field.  
           [0008]    According to the theory of superconductivity, all superconducting materials can be quenched to a normal, non-superconducting state if exposed to a sufficiently strong magnetic field. The value of this “quenching” field varies with the composition of the superconducting material and with the temperature.  
           [0009]    Superconducting material that is exposed to a magnetic field stronger then that material&#39;s quenching field value will become non-superconducting and will therefore allow that magnetic field to penetrate it. If a portion of otherwise superconducting material is quenched to the normal state by a strong time-varying magnetic field, a current will be induced in that portion. In the present invention a rotating magnetic field strong enough to quench the superconducting material on the rotor of the present invention is used to induce a current in the superconducting material.  
           [0010]    Specifically, the present invention consists of a conventional stator with multiphase coil windings to produce a rotating magnetic field in a manner well known in the art. It is thus one object of the invention to produce a magnetic field that rotates around the rotor at a fixed speed. The present invention also includes a cylindrical rotor comprised of a lightweight central portion surrounded by a ceramic shell. On the outer surface of the shell is deposited a thin film of superconducting material. It is thus another object of the invention to provide a lightweight rotor of superconducting material.  
           [0011]    The present invention also includes a cryogenic cooling system to cool the rotor to a temperature below which the outer material becomes superconducting. It is thus another object of the invention to maintain the temperature of the rotor below the critical temperature of the superconducting material.  
           [0012]    The superconducting material on the rotor of the present invention is chosen to have a reasonably low quenching magnetic field. One HTS compound that exhibits the desired properties is YBa 2 Cu 3 O 7 . This compound is quenched to the normal state by a relatively weak magnetic field of the order of 1 Tesla. It is thus another object of the invention to allow the superconducting material to quenched to a normal state by a relatively weak magnetic field.  
           [0013]    The operation of the present invention was designed to maintain the simplicity and reliability typical of conventional squirrel cage induction motors while providing higher efficiency by reducing resistive losses in the rotor. First, the motor is cooled to a temperature below the critical temperature of the superconducting material. The rotor is thus superconducting at this point. Second, the stator coils are excited with an AC current to produce a rotating magnetic field. The strength of this field is calibrated so that it is strong enough to cause the superconducting material on the rotor to be quenched to a normal state at periodic places. The regions of this normal state are half a pole pitch apart. Since the magnetic field generated by the stator is rotating around the rotor at the synchronous speed, the quenched regions on the rotor also rotate at the same speed if the rotor is prevented from rotating within the stator. The pattern of the quenched spots on the rotor resembles a conventional squirrel cage where the superconducting strips between the quenched regions and the superconducting regions circling the ends of the rotors are the bars and end rings of the squirrel cage. To ensure the formation of the desired pattern, the axial length of the superconducting material must be greater than that of the stator coils.  
           [0014]    Once portions of the superconducting material are quenched to a normal state, the stator field can penetrate these regions. This allows a current to be induced in the non-superconducting regions that can then migrate to the superconducting regions. Thereafter the present invention acts as an induction motor. As the motor begins to rotate, the induced current on the rotor “bars” will decline in frequency. Eventually, equilibrium is reached and the current is stabilized at a slip frequency fixed by the load torque.  
           [0015]    The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is perspective side view of an electric motor showing a housing containing a stator and a rotor as well as a cooler and power source;  
         [0017]    [0017]FIG. 2 is a longitudinal cross-sectional view of the present invention showing a cylindrical rotor inside and coaxial with a cylindrical stator, both of which are contained within a housing with openings at both the left and right sides to allow the shaft of the rotor to penetrate the housing and connect to an external device to be supplied with mechanical torque by the rotor;  
         [0018]    [0018]FIG. 3 is a transverse cross-sectional view of the present invention, taken along the line  2 - 2 , FIG. 2, showing the orientation of rotor within the stator and the rotating magnetic field produced by the stator coil;  
         [0019]    [0019]FIG. 4 is a detail view of a portion of the rotor of FIG. 3, showing the various layers of the materials that comprise the rotor.  
         [0020]    [0020]FIG. 5 is a perspective side view of the rotor of FIG. 2, showing the shaft extending beyond the ends of the rotor;  
         [0021]    [0021]FIG. 6 is the same view as FIG. 5, showing the location of the portions of the superconducting material that are quenched to a normal state by the stator field;  
         [0022]    [0022]FIG. 7 is a graph showing the magnitude of the stator field at a fixed point in time and the locations where this field exceeds the critical field value of the superconducting material.  
         [0023]    [0023]FIG. 8 is a graph showing the longitudinal position of the stator field that exceeds the critical field value of the superconducting material.  
         [0024]    [0024]FIG. 9 is a detailed view of a portion of the outer circumference of the rotor showing the orientation of the quenched regions as the stator field instantaneously quenches the superconducting film.  
         [0025]    [0025]FIG. 10 is the same view as FIG. 9, showing the migration of the quenched regions as the stator field rotates in relation to the rotor.  
         [0026]    [0026]FIG. 11 is a schematic cross-sectional view of a portion of the stator and rotor showing the orientation of the magnetic flux lines at motor startup.  
         [0027]    [0027]FIG. 12 is the same view as in FIG. 11 showing the orientation of the magnetic flux lines as they begin to migrate into the superconducting regions.  
         [0028]    [0028]FIG. 13 is the same view as in FIG. 11 showing the increased displacement angle between the rotor and the stator as the rotor reaches its operating speed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring now to FIG. 1, the present invention is an electric motor  10  connected via a shaft  14  to a machine  16  to which the motor  10  provides mechanical power. The shaft  14  penetrates, at one or both ends, a rectangular housing  12  that forms the outer portion of the motor  10 . External to the housing  12  are a power source  24  to supply an AC current through a set of wires  22  that penetrate the housing  12  to connect to a stator (shown in FIG. 2), and a cooler  18  to supply a coolant (not shown) through a tube  20  that penetrates the housing  12 .  
         [0030]    Referring now to FIGS. 2, 3, and  4 , the housing  12  surrounds a stator  26  and a rotor  28 . The rotor  28  is of a cylindrical shape surrounding and coaxial with the shaft  14  that penetrates the housing  12 . A bearing  30  is located at each point where the shaft  14  penetrates the housing  12  to support the rotor  28  and to allow it to rotate relative to the housing  12  while preventing the coolant from escaping. The rotor  28  consists of a cylindrical torque tube  38  surrounding and bonded to the circumference of the shaft  14 . The torque tube  38  consists of a stack of laminations of magnetic steel. On the outer circumference of the torque tube  38  is a ceramic shell  36  on which a superconducting film  34  is deposited. The superconducting film  34  is not deposited directly on the shell  36 ; rather, a substrate  35  is sandwiched between the superconducting film  34  and the shell  36  to provide a mounting surface that is compatible with the structure of the superconducting film  34 .  
         [0031]    Also inside the housing  12  is a stator  26 , coaxial with the rotor  28 , comprised of a hollow cylinder of coil windings (not shown). The rotor  28  is located in the interior of the stator  28  and is longer than the stator  26  so that both ends of the rotor  28  extend beyond the ends of the stator  26 . The stator  26  is connected to the interior of the housing  12  by supports  25  that fix the position of the stator  26  relative to the housing  12 . The rotor  28  is thus able to rotate freely relative to the stator  26  and is not connected to the stator  26  but is separated by an air gap  27 .  
         [0032]    Referring now to FIGS. 2 and 3, the stator  26  is connected to an AC power source  24  located outside the housing  12  by a set of wires  22  to provide an AC current to the stator  26 . The stator  26  is of a type well known in the art. When the stator  26  is excited with an AC current, it generates a magnetic field that varies with time in a sinusoidal fashion. The effect of the sinusoidal variation is the creation of a rotating magnetic field that rotates around the stator  26  at a fixed speed known as the synchronous speed that is determined by the construction of the stator  26  and the frequency of the AC current.  
         [0033]    Referring now to FIGS. 5, 6,  7 , and  8 , the rotor  28  is covered on its outer circumference by a superconducting film  34 . If the magnitude of the rotating magnetic field described above exceeds the critical value for the superconducting film  34 , then the superconducting film  34  is quenched to a normal state. The size and shape of the quenched regions  40  can be controlled by adjusting the strength of the stator current.  
         [0034]    Since the magnetic field produced by the stator  26  is a traveling wave that varies with time in a sinusoidal manner, it can be represented at a fixed point in time by a curve  42  that resembles a sine wave. This is illustrated in FIG. 7, where the horizontal axis represents the radial position on the rotor  28 , and the vertical axis represents the strength of the magnetic field. The stator current is adjusted so that the amplitude a of the curve exceeds the critical quenching value β of the superconducting film  34 , represented by the broken line  44 , only near the peaks and valleys of the sinusoidal curve  42 . The width of the quenched regions  40  is thus determined by the area under the curve  42  that is greater than the broken line  44 .  
         [0035]    [0035]FIG. 8 represents the length of the quenched regions  40 . In FIG. 8, the horizontal axis represents the strength of the magnetic field and the vertical axis represents the longitudinal position on the rotor  28 . The length l r  of the rotor  28  is longer than the length l s  of the stator  26  so that portion the length of the quenched regions  40 , defined by the area to the left of the curve  42  that is greater that the broken line  44 , does not extend the full length of the rotor  28 .  
         [0036]    Consequently, the magnetic field generated by the stator  26  creates quenched regions  40  on the surface rotor  28 . The quenched regions  40  are in the shape of elongated ellipses whose long axes are parallel to the axis of rotation of the rotor  28 . The quenched regions  40  are arrayed around the circumference of the rotor  28  at periodic locations determined by the number of poles on the stator  26 . Because the quenched regions  40  are non-superconducting, the area of superconducting film  34  that is superconducting takes the shape of a conventional squirrel cage with bars  46  of superconducting material interspersed between the quenched regions  40  and joined at both ends of the rotor  28  by end rings  48  of superconducting film  34  that cover the entire circumference of the rotor  28 .  
         [0037]    Because the magnetic field rotates around the stator  26  at the synchronous speed, if the rotor  28  is not rotating, the quenched regions  40  created by the magnetic field will rotate around that rotor  28  at the same speed. The formation of the quenched regions  40  allows the magnetic field to penetrate the superconducting film  34  in the quenched regions  40  and induce a current that circulates in the bars  46  and end rings  48  of the squirrel cage, described above, of superconducting film  34 .  
         [0038]    Referring now to FIGS. 9 and 10, as the stator is switched on to generate a magnetic field sufficiently strong enough to quench the superconducting film  34  to a normal state, quenched regions  40  appear on the outer circumference of the rotor  28 . As the stator field rotates in relation to the rotor  28 , which is stationary at this point in time, the location of the quenched regions  40  begins to rotate around the circumference of the rotor  28 . The quenching magnetic field does not immediately migrate out of the quenched regions  40 . Instead, there is a slight delay so that the even when the magnetic field generated by the stator (not shown) is not actively quenching the superconducting film  34 , the quenching magnetic field remains in the superconducting film  34 , creating residual quenching regions  41 . The combination of actively quenched regions  40  with residual quenched regions  41  creates aggregate quenched regions  51  that stretch from the leading edge of the traveling actively quenched regions  40  to the trailing edge of the residual quenched regions  41 .  
         [0039]    Referring now to FIGS. 11, 12, and  13 , the magnetic flux lines  52  created by the stator  26  create quenched regions  40  when the stator  26  is excited by an AC current. At this point, the magnetic flux lines  52  are aligned with the quenched regions  40  on the surface of the rotor  28 . As the magnetic flux lines  52  travel around the circumference of the stator  26  at the synchronous speed, the corresponding quenched regions  40  created by the magnetic flux lines  52  are unable to migrate around the rotor  28  at the same speed. Instead, a displacement is created between the location of the magnetic flux lines  52  on the stator  26  and the corresponding quenched region  40  on the rotor  28 . As the displacement increases, the magnetic flux lines  52  diffuse out of the quenched regions  40  into the non-quenched portions of the superconducting film  34 . As the magnetic flux lines  52  diffuse, the current induced in the quenched regions  40  on the surface of the rotor  28  migrates into the superconducting film  34 .  
         [0040]    The induced current in the superconducting film  34  on the rotor  28  creates its own magnetic field that interacts with the magnetic flux lines  52  generated by the stator  26 , causing the rotor  28  to rotate in the same direction as the rotation of the magnetic flux lines  52  generated by the stator  26 . As the rotor  28  rotates, the induced current in the superconducting film  34  on the rotor  28  will be at the slip frequency. As the motor  10  reaches equilibrium, a pull-in torque will cause the rotational speeds of the rotor  28  and the magnetic flux lines  52  generated by the stator  26  to be equal. Under this condition, the circulating current in the superconducting film  34  on the rotor  28  will remain fixed at a value determined by the load on the motor  10  and the strength of the AC current supplied to the stator  26 .