Flexible coupling for rotor elements of a superconducting generator

A flexible coupling for connecting pairs of concentric rotor elements in the rotor of a superconducting generator. Various combinations of flexible couplings, each of which is disposed normally to the rotor axis, are used at one or both ends of the superconducting rotor to connect the inner and outer rotors and to connect the inner rotor to a concentric insulating shield. The flexible coupling utilizes a relatively thin ring. When used to connect the inner rotor to the insulating shield the ring may be constructed of a material that retains flexibility at temperatures down to at least about 100.degree. K. The ring may be laminated to increase flexibility and may be made up of joined segmental portions. Securing devices are annularly distributed about the ring. The securing devices are spaced apart a sufficient distance to permit deflections of the ring in an axial direction. The rings are dimensioned to provide radially and torsionally rigid connections between the rotor elements, maintaining their concentricity, while permitting relative axial movements between the rotor elements to accommodate relative thermal contractions and expansions of the rotor elements during cool-down and warm-up of the superconducting rotor.

ACKNOWLEDGEMENT 
This invention was made under contract with or supported by the ELECTRIC 
POWER RESEARCH INSTITUTE, INC. of Palo Alto, California. 
BACKGROUND OF THE INVENTION 
It is a well-known phenomenon that many metals, alloys and chemical 
compounds substantially lose all of their electrical resistance and become 
superconductive at temperatures near absolute zero. This phenomenon is 
advantageously employed in electrical alternators or generators by cooling 
the rotor of the generator to a sufficiently low temperature. By operating 
a generator in its superconducting state, losses in the windings of the 
rotor are substantially eliminated and the generator efficiency is 
correspondingly increased. Additionally, the generator rotors and stators 
can be built to a much smaller dimension. This results in a reduction in 
weight of the generator. Moreover, when the rotor is smaller, there is a 
resultant reduction in operating problems such as vibrations, high 
material stresses and the like frequently encountered in high-speed 
rotors. 
In general, superconducting rotors consist of a number of cylindrical 
concentric elements. On the outside there is a damper shield and a damper 
shield support which are supported by an outer rotor. Inside the outer 
rotor is an inner rotor including superconducting field windings or coils 
immersed in a helium refrigerated annulus. This helium refrigerated 
annulus typically maintains the temperature of the superconducting coils 
at 4.3.degree. K. or below so that superconductivity takes place. 
Intermediate the two rotors and concentric therewith, is a thermal 
radiation or insulating shield, designed to minimize radiant heating of 
the cold inner rotor. The term "rotor" will be used to refer to the inner 
rotor, the outer rotor, and the insulating shield. 
An inherent problem in the design of superconducting generators is the 
accommodation of the relatively large thermal deflections between the cold 
inner rotor and the relatively warm outer rotor and between the cold inner 
rotor and the insulating shield. The present invention accommodates these 
differential axial thermal expansions and contractions. 
The outer damper shield and damper shield support serve two functions. 
First, they comprise the strongback outer thermal jacket of the 
super-cooled rotor. Second, they prevent back electromotive forces from 
the stator from penetrating to the superconducting coils. If penetration 
to the superconducting coils of the back electromotive forces occurs, the 
coils of the windings become heated. When they become heated above a 
critical temperature, they lose their superconductivity and the designed 
field is lost. 
During normal operation, the inner rotor is first subjected to "cool-down". 
In cool-down, liquified helium is introduced into the vicinity of the 
superconducting coils. The inner rotor undergoes substantial thermal 
contraction in an axial direction. Taking the case of a superconducting 
rotor 132 inches long, a thermal contraction of 3/10 of an inch or more 
can occur. In a longer superconducting rotor on the order of 275 inches 
long, thermal contractions of as much as 7/10 of an inch or more can 
occur. Simultaneously, the insulating shield, which will be cooled to an 
intermediate temperature of about 100.degree. K., contracts axially, but 
normally in an amount less than that of the inner rotor. 
At the same time this axial shrinkage is accommodated, any tendency of the 
inner rotor to move rotationally with respect to the outer rotor must be 
prevented. Otherwise, this relative movement between inner and outer 
rotors will generate undesired back electromotive heating of the 
superconducting coils and can result in the loss of their 
superconductivity. 
Further, any tendency of the rotors to move out of concentric alignment 
must be avoided. Even a minute eccentricity of the rotors may result in 
substantial resonances and unbalanced forces during high-speed rotation. 
Therefore, the connection must have sufficient lateral (radial) stiffness 
and strength to maintain the rotors in concentric alignment. 
In addition, the regions between the rotor elements are in a vacuum which 
adversely affects the operation of a sliding coupling. At low vacuum 
temperatures and at high rates of rotation, a rapid "fretting corrosion" 
of the sliding parts normally occurs. Also, in a vacuum, rubbing surfaces 
frequently gall and seize or weld. 
In the past, it has been proposed to connect the rotor elements rigidly 
both axially and torsionally. However, this design leads to excessive 
axial stresses in large generators. 
Prior art patents do not address themselves to the particular needs of a 
coupling between the rotors of a superconducting generator. There are, 
however, a number of prior art patents which disclose a variety of 
couplings designed to connect misaligned shafts in end-to-end relation. 
Typical of these are U.S. Pat. Nos. 3,798,924, 3,874,195, 3,759,064, 
3,703,817, 1,947,052, and 3,405,760. None of the couplings disclosed in 
these patents is adapted to solve the problems encountered when connecting 
concentric structures of a superconducting rotor. 
In general, the couplings disclosed in these patents provide a driving 
connection only between the shafts since the shafts are supported by 
independent bearings on each side of the coupling. These couplings are 
well adapted to accommodate misalignments between rotating shafts. 
However, they are generally incapable of accommodating appreciable axial 
movement between the shafts, particularly at the low temperatures 
encountered in super-cooled generators and they are even less capable of 
radially supporting a pair of shafts, especially when the weight of the 
rotary element is as large as that of the rotor of a generator. 
SUMMARY OF THE INVENTION 
The present invention provides a flexible coupling for connecting pairs of 
concentric elements in a superconducting rotor. The coupling is preferably 
constructed of a relatively thin ring and its provides the sole support 
between the rotors at one or both ends of the superconducting rotor. When 
used to connect the inner rotor to the insulating shield, the ring is 
constructed of a material that retains flexibility at temperatures down to 
at least about 100.degree. K. The ring may be laminated to increase its 
flexibility and may comprise a number of joined ring segments. Spaced 
apart securing devices alternately affix the ring to the rotors which it 
connects. The spacing between the securing devices is sufficient to permit 
deflections of the ring in an axial direction. 
Thus, the flexible coupling of the present invention provides a radially 
and torsionally rigid connection between pairs of concentric elements in a 
superconducting rotor. The coupling prevents vibrations, can withstand 
short-circuit torques, and permits relative axial movements between the 
rotors caused by thermal contractions and expansions thereof during 
cool-down and warm-up. The coupling of the present invention maintains the 
rotors in concentric alignment. 
An improved superconducting rotor design is also provided which is capable 
of accommodating large relative axial deflections between the cold inner 
rotor and (1) the relatively warm outer rotor, and/or (2) the insulating 
shield.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1a, a rotor 2 is disposed within a stator (not shown), 
and is driven by a stub shaft 6 and has at its end opposite the stub shaft 
a cryogenic transfer system 8. Rotor 2 revolves at high speed about its 
axis 4 to generate electricity in windings disposed in the stator. 
Cryogenic transfer system 8 transfers liquid helium to and from the 
interior core space 10 of an inner rotor 12. This system is fully 
described in an article entitled, "A Relatively Rotatable Cryogenic 
Transfer System", dated July 13, 1972, in a technical publication 
available at the Massachusetts Institute of Technology, marked MT-125J. In 
addition to and adjacent transfer system 8, rotor 2 includes conventional 
collector rings 14 which transfer current to super-cooled field windings 
16 disposed in inner rotor 12. 
Inner rotor 12 includes torque tubes 18 and 20 which are disposed at 
respective ends of the inner rotor 12. Torque tube 18 has a flange 22 and 
torque tube 20 has a flange 24. 
Surrounding inner rotor 12 and concentric therewith is a cylindrical 
insulating shield 26. The insulating shield minimizes the radiant heating 
of cold inner rotor 12 by the warmer outer rotor 32. An annulus 28 which 
surrounds inner rotor 12 and an annulus 30 which surrounds insulating 
shield 26 are maintained in a vacuum state during operation of the 
generator. 
Surrounding insulating shield 26 is an outer rotor 32 which is co-axial 
with axis 4. A damper shield 34 is juxtaposed between and it is supported 
by outer rotor 32 on its inner side and by a damper support 36 on its 
outer side. 
During operation of the generator, space 10 of rotor 12 and field windings 
16 are cooled down to a temperature of approximately 4.3.degree. K. Field 
windings 16 thus become superconducting. Insulating shield 26 is cooled to 
a temperature of about 100.degree. K. Torque tubes 18 and 20 exhibit a 
temperature gradient along their axial lengths. For example, during 
operation, points 38 and 40 of the torque tubes are at approximately the 
same temperature as field windings 16. Points 42 and 44 are at 
approximately room temperature. Between points 38 and 42 and between 40 
and 44 on the respective torque tubes, the temperature rises from about 
4.degree. K. to room temperature moving axially outward from points on the 
torque tubes closest to intermediate point 46. To effect the described 
temperature gradient the interiors 18' and 20' of torque tubes 18 and 20 
are thermally insulated from space 10 by providing thermally insulating 
barriers at each end of space 10 (not shown in the drawings). The exact 
proportions of the superconducting rotor elements and the couplings and 
the design of the cooling apparatus within inner rotor 12 are not shown. 
Outer rotor 32 is at room temperature or above (300.degree. K. plus). 
Thus, in general, it can be said that during "cool down" inner rotor 12 and 
insulating shield 26 undergo axial thermal contraction and since the 
former contracts more than the latter, there is also a relative thermal 
contraction between the two. Outer rotor 32 does not undergo thermal 
contraction although it may expand relative to the axis if it becomes 
heated above room temperature during operation. 
Referring to FIGS. 1a-2, the present invention provides a flexible coupling 
48 for connecting flange 22 of inner rotor 12 to an end wall 33 of outer 
rotor 32. Coupling 48 preferably includes a ring 48a , best shown in FIG. 
2, having spaced apart apertures 52-57 located intermediate an outer 
periphery 64' and an inner diameter 66' of the ring. 
Referring to FIG. 1d, coupling 48 includes means, such as bolt sets, each 
of which consists of a bolt 68, a spacer 69, and a nut 68'. Three equally 
spaced bolt sets secure ring 48a to flange 22 of inner rotor 12, and three 
equally spaced bolt sets secure rings 48a to end wall 33 of outer rotor 
32. Spacers 69 and 71, space ring 48a from flange 22 and end wall 33, 
respectively. Bolts 68 and 70 are secured by nuts 68' and 70'. In this 
manner, end wall 33 of outer rotor 32 is secured to ring 48a at three 
spaced points around the latter and inner rotor 12 is secured at 
alternative points about ring 48a midway between the outer rotor 
attachments. 
Axial deflection between the inner and outer rotors is accommodated by 
out-of-plane bending of ring 48a which causes a wave-like deformation of 
the ring while the concentricity of the inner and outer rotors is 
maintained. 
Instead of employing nuts to secure the bolts to ring 48a, the ring can be 
provided with bosses 62-67 that surround the ring apertures and the 
apertures can be threaded for direct engagement by the bolts to eliminate 
the need for the spacers and separate nuts. The bosses additionally 
strengthen the ring at the points where the greatest stresses occur. 
The ring can be constructed of multiple laminations to enhance its axial 
deformability. Alternatively, a series of overlying, separate rings can be 
employed. 
Coupling 48 must maintain inner rotor 12 and outer rotor 32 torsionally 
rigid during operation, that is, it must prevent relative angular 
movements between the inner and the outer rotors. Otherwise, undesired 
back electromotive heating can cause a temperature rise in the 
superconducting coils which in turn can lead to a loss of 
superconductivity. Additionally, coupling 48 must be radially rigid, that 
is, it must maintain the concentricity of the inner and outer rotors to 
avoid vibrations. 
For coupling 48 to have these characteristics, ring 48a must have an axial 
thickness and a radial width which assure the necessary torsional and 
radial rigidity, while at the same time ring 48a must retain the ability 
to deform out of the ring plane to accommodate relative axial contractions 
between the rotors. An axial thickness of about 0.3 to 0.4 inch and radial 
width of about 4 inches for a ring having an outside diameter of about 25 
inches and constructed of titanium accommodates axial deflections of 
between 0.4 inch to 0.7 inch. The ring can also be made out of aluminum, 
aluminum alloy, ferro alloy, titanium alloy, or reinforced composite. 
Referring again to FIGS. 1a and 3-5, the present invention also provides a 
coupling 72 for connecting torque tube 18 of inner rotor 12 to insulating 
sheild 26. Normally shield 26 operates at approximately 100.degree. K.; 
however, abnormal conditions can cause larger thermal differences between 
the insulating shield and the interior of the rotor 10 which is at 
approximately 4.degree. K. during operation. Therefore, insulating shield 
26 must be attached in such a way that axial and radial expansions thereof 
resulting from temperature differentials of up to 350.degree. K. can be 
accommodated. Coupling 72 must also be capable of transmitting some torque 
and it must be radially rigid to avoid vibrations. 
FIGS. 1a, 1d, 3, 4, and 5 show the construction of coupling 72. A plurality 
of, e.g., two rings 73, each constructed of three 120.degree. ring 
segments 76 are secured to torque tube 18 with three right-angle brackets 
74 each of which is attached, e.g., bolted to one of the ring segments 76. 
The brackets are further secured, e.g., bolted to the outer surface of 
torque tube 18. A rigid, generally L-shaped, circular flange 78 is shrunk 
fit into the end of shield 26. The flange defines a flat end-face 84. 
Three pairs of lap plates 80 secure the ends of ring segments 76 to each 
other and secure rings 73 to the flat end face 84 of flange 78. It will be 
observed that a narrow gap 88 may be formed between each pair of adjoining 
ring segments 76, each set of aligned gaps may form a channel 86 between 
the inner and the outer lap plates, as best illustrated in FIG. 4. Bolts 
92 and nuts 94 tighten the lap plates 80 and the ring segments 76 against 
each other and against face 84 of flange 78. 
The laminated structure of the ring of coupling 72 yields greater 
flexibility and facilitates the accommodation of axial and radial 
expansions and contractions of insulating shield 26 with respect to inner 
rotor 12. Rings 73 of coupling 72 are separated from torque tube 18 by a 
space 98. 
The ring of coupling 72 must have an axial thickness and a radial width so 
that it has the requisite radial and torsional rigidity and the capability 
to deform out-of-plane between brackets 74 at operating temperatures 
between 90.degree.-130.degree. K. to accommodate axial movement of the 
inner rotor with respect to the insulating shield. For example, in a 
typical large rotor in which rings 73 have a mean diameter of 22 inches, 
the rings have a combined axial thickness of 0.6-0.8 inches and a radial 
width of 1.6 inches when made of steel alloy. 
As previously described, torque tube 18 has a temperature gradient in an 
axial direction so that the temperature increases moving in an axial 
direction indicated by arrow A. Insulating shield 26 has a length relative 
to inner rotor 12 so that coupling 72 is secured to torque tube 18 at a 
position where the normal operating temperature is between 
90.degree.-130.degree. K. 
For the actual construction of a superconducting rotor, the above described 
flexible couplings can be employed in several combinations depending on 
the size of the generator, the operating temperature, the materials of 
which the rotor is constructed and the like. In the embodiment shown in 
FIG. 1A, flexible couplings 48 and 72 are employed at one end of the rotor 
while the inner and outer rotors and the insulating shield are rigidly 
secured to each other at the other end. It should be noted, however, that 
couplings 48 and 72 are the only structures which hold the inner and outer 
rotors and the insulating shield in concentric alignment at that end of 
the rotor. 
FIGS. 1b and 1c show the use of couplings 48 and 72 and of coupling 72 
only, respectively at the right-hand end of the rotors. 
In practice, the inner rotor and the insulating shield may be assembled in 
a pre-stressed state so that couplings 48 and 72 deflect axially 
out-of-plane. In this manner, the maximum out-of-plane bending of the 
couplings is limited to almost one-half of what it would otherwise be. 
It is to be understood that variations of the present invention will occur 
to those having skill in the art. For example, it is possible that several 
flexible couplings can be used in series for larger axial deflections 
encountered in longer rotors, and that any combination of flexible 
couplings can be used at one or both ends of the superconducting rotor. 
Variations of the disclosed flexible coupling for attaching concentric 
elements in the rotor of a superconducting generator are within the spirit 
and scope of the present invention.