Rotating machine having superconducting windings

A superconducting rotating machine includes a direct current field excitation source and an alternating current armature winding mounted on a stationary support member, at least one of which includes a superconducting material, a core member formed of a magnetic permeable material and rotatable around the static support member, and a refrigerator unit which cryogenically cools at least one of the field excitation source and the armature winding. The superconducting rotating machine may have a construction for providing polyphase (e.g., three-phase) power.

BACKGROUND OF THE INVENTION 
The invention relates to superconducting rotating machines (e.g., a 
superconducting electric generator or motor) and their constructions. 
The worldwide demand for additional electrical generation is ever 
increasing. To meet these demands, larger and more efficient electrical 
generators are being developed. Electric generators convert rotational 
mechanical input energy (e.g., that from a steam or gas turbine) into 
electricity by rotating a rotor field within stationary armature 
conductors. In conventional generators, the generator field is produced 
with copper windings or permanent magnets. 
The overall efficiency of an electrical generator is affected by the losses 
in the rotor windings and in the armature windings. By using 
superconducting wire for the field windings, these losses become almost 
negligible. Moreover, the overall volume of an electrical generator using 
high temperature superconductor (HTS) generator can be as much as 1/3 the 
volume of its conventional equivalent. 
Such superconducting generators are also finding application in power 
plants where expansion is difficult (e.g., shipboard or locomotive power). 
Smaller, lighter HTS generators use an "air core" design, eliminating much 
of the structural and magnetic steel of a conventional equivalent. 
Construction, shipping, and installation are all simplified and less 
costly. 
SUMMARY OF THE INVENTION 
The invention features a superconducting rotating machine which produces 
increased electric power with significantly lower losses while being 
smaller and lighter than conventional equivalent electric rotating 
machines. 
In a general aspect of the invention, the superconducting rotating machine 
includes a direct current field excitation source and an alternating 
current armature winding mounted on a static support member, at least one 
of the excitation winding and armature including a superconducting 
material, a core member formed of a magnetic permeable material and 
rotatable around the static support member, and a refrigerator unit which 
cryogenically cools at least one of the excitation winding and armature. 
In the above arrangement, the field excitation source and armature winding 
are mounted statically to a support member and the core is rotated about 
the field excitation source and armature winding. This arrangement has 
numerous advantages. Specifically, because the field excitation source and 
armature winding are not mounted on a rotating, or otherwise moving 
member, difficulties associated with cooling moving parts are eliminated. 
Thus, either or both of the field excitation source and armature winding 
can be more easily cooled, for example, with a cryocooler. Because cooling 
is easier, either or both of the field excitation source and armature 
winding, can be formed of superconducting material. This advantage is 
important because a significant amount of the total electrical losses in 
an electric rotating machine are associated with the field excitation 
source and armature winding. Further, because the electrical losses, 
weight, and volume of the rotating machine are significantly reduced, the 
overall efficiency and reliability of the machine is increased. Moreover, 
installation, as well as retrofitting, of this construction is simplified 
and less costly. 
Embodiments of this aspect of the invention may include one or more of the 
following features. 
For example, in one embodiment, the field excitation source is a 
non-superconducting permanent magnet, with the armature winding including 
the superconducting material. Alternatively, the field excitation source 
is in the form of a coil (superconducting or non-superconducting). 
The superconducting material is a high temperature superconductor (HTS) and 
may be in the form of a tape having a thickness and a width greater than 
the thickness. In embodiments utilizing HTS tape, the field excitation 
source is a pancake coil, and preferably a double pancake coil. The double 
pancake coil is preferably a saddle-shaped racetrack coil. Because HTS 
materials are typically ceramic-based (e.g., BSCCO), such materials are 
intrinsically less flexible. The saddle-shaped racetrack configuration is 
well-suited for providing pancake coils with a shape which conforms to 
rounded support structures. 
The superconducting rotating machine may include a plurality of field 
excitation sources, circumferentially spaced from each other and mounted 
on the static support member. In preferred embodiments, adjacent ones of 
the excitation sources have polarities of opposite sense. Thus, when the 
core member rotates past the excitation sources, the alternating polarity 
of the magnetic flux causes an AC voltage to be generated. 
The core member includes salient members extending in a direction 
substantially parallel to the longitudinal axis. The salient members, in 
essence, are extended portions of the core member closely spaced from the 
excitation winding and armature. First and second groups of salient 
members are spaced from the longitudinal axis of the core member by first 
and second radial distances, respectively, with the second radial distance 
being greater than the first radial distance. This arrangement provides a 
pair of salient poles between which armature and excitation winding pass, 
thereby ensuring a good magnetic flux path. The core member is in the form 
of a radially-stacked lamination of the magnetic permeable material to 
reduce lossy eddy currents. 
In another aspect of the invention, a polyphase rotating machine (e.g., 
three-phase machine) includes a plurality of direct current excitation 
source groups and a plurality of alternating current armature windings. 
Each armature winding associated with and magnetically coupled to a 
corresponding one of the plurality of excitation source groups. Each 
excitation source group is mounted on the static support member and has at 
least one excitation source including a superconducting material. Each 
excitation source from a first one of the excitation source groups is 
radially spaced from an excitation source of a second one of the 
excitation source groups. The polyphase rotating machine also includes a 
core member formed of a magnetic permeable material and rotatable about a 
longitudinal axis and around the static support member, as well as a 
refrigerator unit which cryogenically cools the excitation windings. The 
core member is disposed adjacent to the excitation windings of the phase 
winding groups and armature windings. 
Embodiments of the polyphase rotating machine may include one or more of 
the features described above as well as the following additional feature. 
The core member includes salient member groups, each group extending in a 
direction substantially parallel to the longitudinal axis and radially 
spaced from another of the groups of salient members, each of the phase 
winding groups positioned between the groups of salient members. This 
arrangement provides salient members on either side of each excitation 
source and armature winding. 
Other advantages and features of the invention will become apparent from 
the following description and claims.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, a superconducting rotating machine 10 includes 
a stator assembly 12 and a rotor assembly 14 positioned along a rotational 
axis 16 of the machine. As will be discussed in greater detail below, 
stator assembly 12 includes superconducting field excitation windings 18 
and a superconducting armature winding 20, both of which are enclosed 
within a cryocooled refrigeration unit, such as a cryostat 22. Cryostat 22 
is thermally coupled to a cryocooler 134 (FIG. 5) to maintain the field 
excitation windings 18 and armature winding 20 at cryogenic temperatures 
(e.g., less than 120.degree. K.). In operation, rotor assembly 14 revolves 
around statically-mounted windings 18, 20 of stator assembly 12. 
Stator assembly 12 includes a cylindrical support tube 24, upon which 
superconducting excitation field windings 20 are mounted. The excitation 
field windings are equally spaced around the periphery of tube 24. 
Excitation field windings 20 are connected with superconducting wire 25 in 
a manner to produce alternating north and south poles. In this embodiment, 
eight saddle-shaped racetrack windings are excited with a direct current 
(DC). Superconducting armature 20 is in the form of a circular coil, wound 
around the periphery of tube 24 and is magnetically coupled to excitation 
windings 18 through iron core 26. Armature 20 carries a single-phase AC 
signal. 
Rotor assembly 14 includes a core 26 formed of a high permeability 
material, such as iron. Because iron is a high permeability, high 
saturation flux density material, it acts, in essence, as a magnetic short 
circuit for flux generated by excitation windings 18 and armature winding 
20. The individual laminations are stacked in the radial direction and are 
insulated from each other and bonded together, for example, by mill scale, 
lacquer, or japanning, to minimize the flow of eddy currents in the core. 
In alternative embodiments, amorphous metal cores, which do not have a 
preferential direction for magnetic flux flow may be used, at the expense 
of a generally slightly lower permeability characteristic. 
Core 26 includes an inner group of salient arms 28 and an outer group of 
salient arms 30, both of which extend coaxially along the length of 
assembly 14 and are connected to a common yoke 31 (FIG. 2). As shown in 
FIG. 2, each of the outer and inner groups includes four salient arms, 
radially spaced from each other by a distance sufficient for allowing 
support tube 24 with excitation windings 18, armature winding 20 and 
cryostat 22 to pass between associated pairs of the salient arms of the 
inner and outer groups. The salient arms, in essence, act as high 
permeability extensions of yoke 31, spaced by a small air gap from 
extension windings 18 and armature 20. The salient arms, therefore, 
provide a low reluctance path for magnetic flux generated by windings 18 
and armature 20. A shaft 32, attached to core 26 and extending along axis 
16, is used to provide a mechanical coupling to, for example, a drive 
assembly (not shown). 
Referring to FIG. 3, each excitation coil 18 includes racetrack double 
"pancake" coils 34 (here, five in number) wound positioned within a coil 
support shell 36. Each double pancake coil has co-wound conductors, in the 
form of superconducting tape, wound in parallel which are then stacked 
coaxially on top of each other. As shown here, one of more of the double 
pancake coils 34 may include a pancake coil having a diameter smaller than 
its associated pancake coil of the double pancake, the two coils of a pair 
being wound from the same continuous length of superconducting tape. A 
technique for providing double pancake coils in this manner is described 
U.S. Pat. No. 5,581,220, which is incorporated herein by reference. In 
certain applications, the double pancake coils may be wound with a 
variable profile with the approach described in U.S. Pat. No. 5,581,220, 
which is incorporated herein by reference. 
Both excitation windings 18 and armature windings are wound with 
superconducting tape, formed of a high temperature superconductor (HTS), 
such as those made from ceramic or metallic oxides. HTS tape is typically 
anisotropic, meaning that they generally conduct better, relative to the 
crystalline structure, in one direction than another. Anisotropic high 
temperature superconductors include, but are not limited to, the family of 
Cu--O--based ceramic superconductors, such as members of the 
rare-earth-copper-oxide family (YBCO), the 
thallium-barium-calcium-copper-oxide family (TBCCO), the 
mercury-barium-calcium-copper-oxide family (HgBCCO), and the bismuth 
strontium calcium copper oxide family (BSCCO). These compounds may be 
doped with stoichiometric amounts of lead or other materials to improve 
properties (e.g., (Bi,Pb) .sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10). 
Superconductor tape has a relatively high aspect ratio (i.e., width 
greater than the thickness) and is fabricated as a multi-filament 
composite superconductor including individual superconducting filaments 
which extend substantially the length of the multi-filament composite 
conductor and are surrounded by a matrix-forming material (e.g., silver). 
The ratio of superconducting material to the total amount of material 
(i.e., the matrix-forming material and superconducting material) is known 
as the "fill factor" and is generally less than 50%. Although the matrix 
forming material conducts electricity, it is not superconducting. 
Together, the superconducting filaments and the matrix-forming material 
form a composite multi-filament high temperature superconducting 
conductor. 
Referring to FIG. 4, excitation coils 18 are arranged on support tube 24 so 
that adjacent windings have opposite polar sense. In other words, windings 
18 are wound and positioned such that, in operation, they produce 
alternate north and south magnetic poles. When an excitation winding 18 
having a magnetic "north" polarity passes between a corresponding pair of 
inner and outer salient poles 28, 30, the peak flux is magnetically 
coupled to core 26. On the other hand, when an adjacent excitation winding 
18 having a magnetic "south" polarity passes between the same pair of 
inner and outer salient poles 28, 30, peak magnetic flux of the opposite 
sense is coupled to core 26. AC flux is produced in the rotating core 26 
as excitation windings 18 sweep by salient poles 28, 30, and current is 
generated in armature winding 20 and is delivered to a load connected to 
terminals 40, 42. 
Because the majority of electric power produced in the United States is by 
three-phase generators, the concept of the invention is particularly 
advantageous when applied to polyphase systems. 
Referring to FIGS. 5 and 6, to provide a three-phase superconducting 
rotating machine 100, for example, three groups of DC field excitation 
windings 102a, 102b, 102c are mounted together on the same cylindrical 
support tube, but spaced from each other in both the radial and 
circumferential directions. As shown in FIG. 6, in particular, each 
excitation winding of one group is spaced from a corresponding excitation 
winding of an adjacent group of an electrical angle of by 120.degree.. 
Positioned on support tube 104, as well, are three AC armature windings 
106a, 106b, 106c, each of which corresponds to one of the three groups of 
DC excitation windings. DC filed excitation windings 102a, 102b, 102c, and 
AC armature windings 106a, 106b, 106c are positioned within an internal 
volume 130 of a cryostat 132. A cryocooler 134 is connected to cryostat 
132 and includes a cold finger element 136 thermally coupled to the field 
excitation windings and the armature windings, via conductors 138. 
For example, a field excitation winding 102a' of group 102a has a north 
polarity and is electrically spaced from a corresponding field excitation 
winding 102b' of group 102b which also has a north polarity. 
Three-phase superconducting rotating machine 100 includes a rotating core 
assembly 110 having an inner group of salient arms 112, an outer group of 
salient arms 114, and a pair of intermediate groups of salient arms 116, 
118, all extending from an iron yoke 120 having a shaft 122. 
Referring to FIG. 7, the operation of the 3-phase machine will now be 
explained. With reference to FIG. 5, a phase A voltage 140 is generated 
when, for example, winding 102a passes by salient poles 112, 118 of core 
110. Because excitation winding 102b is physically displaced by a 
120.degree. electrical angle (60.degree. physical angle for a 4-pole 
machine) and because salient arms 116 and 118 are similarly displaced by 
120.degree. with respect to arms 114 and 116, a phase B voltage is 
generated that lags phase A voltage by 120.degree.. Similarly, excitation 
winding 102c is displaced by 120.degree. with respect to 102b and salient 
arms 114, 116 are similarly displaced with respect to salient arms 118 and 
112. This produces phase C voltage 142 that lags phase B voltage by 
120.degree.. Thus, a 3-phase power is generated in AC armature coils 106a, 
106b and 106c. 
Other embodiments are within the scope of the claims. For example, field 
excitation windings 18 in the embodiment described above in conjunction 
with FIG. 1 were double pancake coils. Referring to FIG. 8, in an 
alternative embodiment, the excitation windings are replaced with 
permanent magnets 50. As was the case with the embodiment of FIG. 1, 
adjacent magnets 50 have a polarity of opposite view. 
Also excitation windings 18 and armature windings 20 in the embodiment 
described above in conjunction with FIGS. 1 and 2, were both formed of 
high temperature superconducting material. Thus, cryocooling both types of 
windings was required. In other applications, however, only the excitation 
windings 18 may be formed of superconducting material with the armature 
winding formed of conventional non-superconducting material (e.g., 
copper). In this case, only excitation windings would require cryocooling. 
Similarly, excitation windings 18 can be formed of copper with armature 
winding formed of HTS material and cryocooled. 
The foregoing descriptions of embodiments of the invention have been 
presented for purposes of illustration and description. They are not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed. The particular chosen embodiments are described in order to 
best explain the principles of the invention.