Note: Descriptions are shown in the official language in which they were submitted.
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A ROTOR OR A STATOR FOR A SUPERCONDUCTING
ELECTRICAL MACHINE
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. Patent Application
Serial No.
12/288,669, filed October 22, 2008, now pending.
FIELD OF THE INVENTION
[0002] The present invention relates to superconducting rotating electrical
machines. In
particular, the present invention provides a rotor or a stator that is
suitable for high
temperature superconducting electrical machines. The rotor or stator has high
strength
and low thermal conduction between the superconducting field coils and the
mounting for
the field coil support structure.
BACKGROUND OF THE INVENTION
[0003] Electrical machines typically comprise a cylindrical rotor mounted to
rotate
around or within a cooperating stator. That is, an electrical machine may have
an internal
substantially cylindrical rotor mounted within an external stator or an outer
cylindrical
rotor mounted around an internal stator. In superconducting electrical
machines the stator
and/or the rotor may include a plurality of superconducting field coils
mounted on a field
coil support structure, thereby forming a surface that is substantially
adjacent to, but
spaced apart from, a surface of the other of the stator or rotor.
[0004] There are several problems that must be overcome in the construction of
such
rotors and stators. First of all, the superconducting field coils and the
field coil support
structure must be kept at cryogenic temperatures when operating. In contrast,
it is
preferable that the mounting upon which the field coil support structure is
mounted is
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kept as close to ambient temperature as possible in order to allow the rotor
or stator to
couple to other equipment or components which operate at ambient temperature.
Therefore, there is the need for an improved rotor and stator construction
that provides a
very good thermal barrier between the field coil support structure and the
mounting for
the field coil support structure.
[0005] It is also necessary that any such rotor and stator construction
efficiently transfers
torque and other stresses from the field coils to the mounting for the field
coil support
structure. It will be readily appreciated that the rotor or stator must be
capable of
withstanding all the forces that it may be subject to during operation of the
electrical
machine. This includes short circuit torque, which is oscillatory and is
typically many
times greater than normal rated torque.
[0006] The rotor or stator must also be capable of being built at ambient
temperature and
subsequently having the field coils cooled down to the cryogenic temperatures
at which
they will operate. This cooling produces large contractive forces which the
rotor or stator
must withstand.
[0007] There is therefore a need for a rotor or stator for superconducting
electrical
machines that provides an adequate thermal barrier between the superconducting
field
coils and the mounting for the field coil support structure and that is also
capable of
withstanding the mechanical and thermal stresses that it will be subject to
during
construction and operation.
SUMMARY OF THE INVENTION
[0008] The present invention provides a rotor or stator for a superconducting
electrical
machine comprising a mounting maintained at substantially ambient temperature
during
operation of the electrical machine, a field coil support structure, a
plurality of
superconducting field coils maintained at cryogenic temperatures during
operation of the
electrical machine and supported by the field coil support structure, and at
least one
coupling element fixing the support structure to the mounting.
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[0009] Preferably, the field coils are high temperature superconducting (HIS)
coils that
are formed in a manner known to a person skilled in the art. However, it is
also possible
that the field coils are low temperature superconducting (LTS) coils.
[0010] The field coil support structure may be substantially a cylindrical
sleeve that is
formed around or within the mounting and has a first axial end and a second
axial end.
[0011] If the rotor or stator of the present invention is substantially
internal to and
mounted within the other of the rotor or the stator of the same electrical
machine then the
field coils may be formed around, and are supported by, a radially outer
cylindrical
surface of the field coil support structure. Alternatively, if the rotor or
stator of the
present invention is external to and mounted around the other of the rotor or
the stator of
the same electrical machine then the field coils may be formed around, and are
supported
by, a radially inner cylindrical surface of the field coil support structure.
[0012] It will be readily appreciated that if the rotor or stator of the
present invention is
the radially inner member of an electrical machine then the mounting will be
central
within the electrical machine and may be a substantially cylindrical rod,
shaft or tube
sleeved inside the field coil support structure and fixed thereto. Such a
central mounting
may be solid but will preferably be substantially tubular in order to minimize
its weight.
The central mounting will also have a first axial end and a second axial end
that are
formed in the same orientation as the field coil support structure.
[0013] Alternatively, if a rotor or stator according to the present invention
is the radially
outer member of an electrical machine then the mounting will be external to
the field coil
support structure and may be a substantially cylindrical tube sleeved around
the support
structure and fixed thereto. In the case where the present invention is a
stator then it will
remain stationary in use and the mounting may have a substantially cylindrical
inner
surface but may be otherwise formed to any suitable shape.
[0014] In order to operate properly, the plurality of superconducting field
coils must be
cooled to suitable cryogenic temperatures. The field coil support structure
will
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necessarily also be maintained at cryogenic temperatures during operation of
the
machine. However, the field coil support structure will be formed at
substantially ambient
temperatures and when the electrical machine is not operating both the field
coils and the
field coil support structure may be allowed to warm to substantially ambient
temperatures. The mounting of the rotor or stator of the present invention
will typically
be maintained at a substantially ambient operating temperature in order to
preserve its
necessary physical properties such as strength and toughness and in order to
enable it to
be connected to other equipment that is at ambient temperature.
[0015] When cooled to cryogenic temperatures, the field coil support structure
will
contract in both the axial and radial directions. Since the mounting is not
cooled and is
maintained at a higher operating temperature, a contraction or expansion
stress will be
imposed on the, or each, coupling element depending upon whether the field
coil support
structure is internal or external to the mounting. It is necessary that the,
or each, coupling
element can withstand this stress and continue to function properly over an
extended
period. The contraction or expansion stress may be particularly acute in
rotors or stators
with a large diameter. Preferably, the stress is minimized through the design
of the rotor
or stator in general and the coupling elements in particular.
[0016] Preferably, the, or each, coupling element is also designed to minimize
the heat
flow from the mounting to the field coil support structure during operation of
the
electrical machine. This is in order to minimize the amount of cooling
required to
maintain the field coil support structure and the field coils at cryogenic
temperatures.
[0017] Minimizing the cooling required to maintain the field coil support
structure and
the field coils at cryogenic temperature is a crucial design factor for
superconducting
electrical machines. For example, using current technology, 100 Watts is
required to
remove 1 Watt from a component maintained at 30K. Improvements to cooling
systems
are reducing this power requirement towards a theoretical minimum of 9 Watts.
However, there will always be a real need to reduce any unnecessary heating of
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cryogenically cooled components. This heating may be internally generated,
externally
conducted or radiated into the cryogenic components.
[0018] Therefore, in order to minimize heat flow by conduction between the
field coil
support structure and the mounting the two components may be substantially
separated
over their respective axial lengths by a vacuum gap.
[0019] Furthermore, it is also preferable that any coupling elements used to
fix the
mounting to the field coil support structure minimize heat flow between the
mounting and
the support structure. Thus, the, or each, coupling element may be in contact
with only a
small portion of the surface of both the mounting and the field coil support
structure. For
example, it may be preferable that there are a plurality of coupling elements
and that the
mounting and field coil support structure are only fixed together at their
respective first
and second axial ends. The mounting and the support structure may then be
separated
from one another over the remainder of their axial length by a vacuum gap.
[0020] In order to further reduce the heat flow between the field coil support
structure
and the mounting, the, or each, coupling element of the present invention may
be made of
a suitable material with a relatively low thermal conductivity. The, or each,
coupling
element may be made of carbon fiber or glass fiber, for example.
[0021] Preferably, the mounting and the field coil support structure will only
be joined
together at substantially their respective first and second axial ends. In
order to enable
this, the mounting and the field coil support structure may have at least one
radially
extending tab or flange formed at each of said ends where they are joined. The
coupling
elements may be fixed to any radially extending tab or flange formed on either
the
mounting or field coil support structure.
[0022] However, it is to be appreciated that coupling elements of rotors or
stators
according to the present invention may also be directly attached to the
adjacent
cylindrical surfaces of the mounting and field coil support structure. For
example,
coupling elements may be attached to an outer cylindrical surface of the
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innermost of the mounting and the field coil support structure at a first end
and to an
inner cylindrical surface of the radially outermost of the mounting and the
field coil
support structure at a second end. Furthermore, it is also possible that each
coupling
element is directly attached to a cylindrical surface of the mounting or field
coil support
structure at a first end but is attached to a radially extending tab or flange
of the other of
mounting or field coil support structure at a second end. Coupling elements
fixed to the
mounting and the field coil support structure in either of these manners may
have
substantial length (i.e., be substantially elongate) and may extend
substantially
circumferentially around the mounting and field coil support structure.
Alternatively, the
coupling elements may extend substantially radially from the mounting to the
field coil
support structure.
[0023] Any tab or flange formed on the radially innermost of the mounting and
the field
coil support structure may extend radially outwardly whilst any tab or flange
formed on
the radially outermost of the mounting and the field support structure may
extend radially
inwardly. The, or each, coupling element may then be fixed at a first end to a
tab or
flange of the mounting and at a second end to a tab or flange of the field
coil support
structure.
[0024] Preferably, both the mounting and the field coil support structure will
have
continuous radially extending flanges formed around their respective first and
second
ends. Each flange of the radially innermost of the mounting and the field coil
support
structure may extend radially outwardly a distance less than the separation
between the
mounting and the field coil support structure. Likewise each flange of the
radially
outermost of the field coil support structure and the mounting may extend
radially
inwardly a distance less than the separation between the mounting and the
field coil
support structure. As an alternative, there may be a plurality of tabs formed
on each of the
mounting and the field coil support structure; the number of tabs on each
being equal to
the number of coupling elements that are used to fix the two components parts
together.
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In this manner each coupling element may be joined to a separate tab around
each of the
mounting and the field coil support structure at each of their ends.
[0025] A plurality of coupling elements are preferably used to fix the
mounting to the
field coil support structure at both of their respective first and second
axial ends. Each of
the coupling elements may be a strut with a substantial length and may be
fixed at a first
end to a flange or tab of the mounting and at a second end to a flange or tab
of the field
coil support structure such that they extend in a substantially
circumferential direction
around the rotor or stator. The struts may be curved or substantially straight
and are
preferably rigid.
[0026] Alternatively, the coupling elements may extend substantially radially
from the
mounting to the field coil support structure. As a further alternative, any
flanges or tabs of
the field coil support structure and the mounting may be formed at relative
axial positions
such that each coupling element may be fixed at a first end to a tab or flange
of the
mounting and extend substantially parallel to an axis of the mounting and
field coil
support structure to a second end where they are fixed to a tab or flange of
the field coil
support structure.
[0027] Advantageously, the coupling elements may be slidably or pivotably
fixed to the
mounting and/or the field coil support structure to enable partial rotation
and/or sliding
movement to take place during relative thermal expansion or contraction of the
mounting
and the field coil support structure. This may enable the coupling elements to
accommodate the radial and axial thermal expansion and contraction of the
field coil
support structure without becoming overly stressed.
[0028] If the coupling elements have a substantial length and extend
substantially
circumferentially around the rotor or stator they will be better able to
withstand any
thermal contraction stress they are subjected to during operation of the
electrical machine.
Generally, the longer the coupling elements are the greater their resistance
to contraction
stress will be. This is because the degree of bending of the circumferential
coupling
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elements as a result of thermal contraction of the field coil support
structure is reduced as
the length of the coupling elements is increased.
[0029] The length of the coupling elements may also help to reduce the heat
flow
between the mounting and the field coil support structure. The greater the
length of the
coupling elements, the lower the heat flow between the mounting and the field
coil
support structure will be. Similarly, if the aggregate cross-sectional area
perpendicular to
the direction of the heat flow of the coupling elements is reduced then the
heat flow will
also be reduced. It is therefore preferable that the length of coupling
elements in rotors or
stators according to the present invention is maximized and the aggregate
cross-sectional
area of the coupling elements is minimized. However, it is important that the
coupling
elements are strong enough to effectively transfer torque between the mounting
and the
field coil support structure during operation of the electrical machine. The
coupling
elements must also be able to withstand all the other stresses to which they
will be
subjected, for example, short circuit torque which may be high and
oscillatory.
[0030] The coupling elements are preferably adapted to carry substantially all
of the
torque that is experienced during operation of the electrical machine. The
design and
mounting of the coupling elements (e.g., the linear struts) and their
substantially
circumferential orientation around the rotor or stator are such that they can
carry the
torque in tension or compression. The coupling elements are preferably made
with
sufficient length and small enough cross-section to be the only means of
transmitting
substantially all the torque between the support structure and the mounting,
whilst
minimizing thermal transfer along the coupling elements. The torque that is
carried by the
coupling elements can be the full rated torque of the electrical machine, as
well as any
overloads and transient and oscillatory torques which might be produced by the
electrical
machine or any external unit or device to which the machine is mechanically
connected.
As mentioned elsewhere, the coupling elements can be pre-tensioned if required
to help
carry compressive torque without incurring additional deflection.
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[0031] In an arrangement where the back iron does not project through the
field coils into
the air gap region (e.g., where the back iron is below the field coils and is
typically
maintained at ambient temperature), then the field coils and the support
structure will
carry substantially all of the torque. In an alternative arrangement where the
back iron
projects into the air gap region and is maintained at cryogenic temperatures,
then the flux
will be shared with the field coils and the torque will be shared according to
how much
useful flux is distributed between the field coils and the back iron
components. In both
arrangements, substantially all of the torque that is produced on the field
coils, on the
field coil support structure, and where applicable on the poles and/or the
back iron, would
be carried by the coupling elements.
[0032] As will be appreciated by the person skilled in the art, the number of
coupling
elements present in any rotor or stator according to the present invention
will depend on
the size, design and intended use of the specific rotor or stator. The design
and number of
coupling elements will be dependent upon the stresses the coupling elements
will be
subject to and the heat flow between the mounting and the field coil support
structure.
[0033] Preferably, the coupling elements will be substantially linear struts
formed of
carbon fiber or glass fiber. Carbon fiber or glass fiber is particularly
suitable as it has a
relatively low thermal conductivity, is lightweight and may be formed to be
particularly
strong in one or possibly two directions. If linear struts are formed of
carbon fiber it may
be preferable that they are formed such that they are strongest along the
axial direction of
the struts (i.e., that the primary direction of fiber lay is substantially
along the axis of the
struts). Alternatively, it may be preferable that the linear struts are formed
such that the
primary direction of fiber lay minimizes their thermal expansion and
contraction during
use of the electrical machine.
[0034] Forming the coupling elements as linear struts is particularly
preferred as struts
may transmit both positive and negative torque from the field coil support
structure to the
mounting. They may also provide a long, low thermal conductivity path between
the
mounting and the field coil support structure. They may also minimize the
radial and
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axial stress due to the cooling of the field coil support structure if they
are mounted
substantially circumferentially around the rotor or stator and have sufficient
length to
minimize deflection stresses.
[0035] When under tensile stress, coupling elements formed as linear struts
will act in
substantially the same manner as tie rods. When under compressive stress, the
coupling
elements will act as bracing struts. To improve the capability of the coupling
elements to
act as bracing struts and withstand compressive forces they may be pre-
tensioned. As will
be understood by those skilled in the art, pre-tensioning may be applied to
the coupling
elements either during assembly of the rotor at ambient temperatures, or
during cooling
of the field coil support structure to cryogenic temperatures.
[0036] In a preferred arrangement, each rotor or stator may have eight
substantially linear
struts to fix the mounting and the field coil support structure together at
each of their
respective first and second axial ends. The mounting and the field coil
support structure
may then be separated by a vacuum gap.
[0037] Further features and advantages of the present invention will become
apparent
from the specific embodiment of the present invention that is discussed with
reference to
the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] Exemplary embodiments of the invention will now be described, with
reference to
the accompanying drawings, in which:
[0039] Figure 1 shows a cutaway view of a rotor according to the present
invention;
[0040] Figure 2 shows a partial cross-section of the rotor of Figure 1;
[0041] Figure 3 shows a plan view of a coupling element connecting a flange of
a
mounting and a flange of a field coil of the rotor of Figures 1 and 2;
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[0042] Figure 4 shows a single coupling element as used in the rotor of
Figures 1 to 3;
and
[0043] Figure 5 shows two alternative coupling elements.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] A cutaway view of a rotor 1 according to the present invention is shown
in Figure
1. The rotor 1 substantially consists of a mounting 2, a field coil support
structure 3, a
plurality of field coils 4 and a plurality of coupling elements 5. The rotor 1
may form part
of an electrical machine (not shown) in a manner that will apparent to a
person skilled in
the art.
[0045] The mounting 2 is substantially hollow and tubular and has a
cylindrical outer
surface 6. The mounting 2 forms the central shaft of the electrical machine 1.
During
operation of the electrical machine, the mounting 2 will be maintained at a
substantially
ambient operating temperature in order to maintain its 5 necessary physical
properties
such as strength and torque resistance.
[0046] A pair of circumferentially continuous radially outward extending
flanges 7 is
formed on the outer cylindrical surface 6 of the mounting 2. Each flange 7
extends
radially outward from the mounting 2 a distance that is less than the radial
separation
between the outer surface 6 of the mounting 2 and a radially inner surface of
the field coil
support structure 3. This can be seen most clearly in Figure 2.
[0047] The field coil support structure 3 is substantially cylindrical and is
sleeved around
the mounting 2 so as to be at a constant radial separation from the outer
surface 6 of the
mounting 2 about its circumference. The plurality of field coils 4 are formed
around, and
supported by, the radially outer surface of the field coil support structure
3. The field
coils 4 are high temperature superconducting field coils and may be formed
from any
suitable high temperature superconducting material and in any manner apparent
to a
person skilled in the art.
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[0048] The field coil support structure 3 and field coils 4 are constructed
and fixed to the
mounting 2 whilst the rotor is at substantially ambient temperature. However,
during
operation of the electrical machine, the field coils 4 and the field coil
support structure 3
are maintained at a suitable cryogenic temperature by a cooling system (not
shown) in a
manner that will be immediately apparent to a person skilled in the art. When
the rotor 1
is idle, the field coils 4 and the field coil support structure 3 may be
allowed to warm up.
During warming from, and cooling to, cryogenic temperatures the field coils 4
and the
field coil support structure 3 will undergo significant thermal expansion and
contraction.
This places a radial and axial stress upon the coupling elements 5.
[0049] The field coil support structure 3 has a pair of circumferentially
continuous
radially inward extending flanges 8 formed on its radially inner surface. Each
flange 8
extends radially inwardly from the field coil support structure 3 a distance
that is less than
the radial separation between the outer surface 6 of the mounting 2 and the
radially inner
surface of the field coil support structure 3. Once again, this can be seen
most clearly in
Figure 2.
[0050] Each of the plurality of coupling elements 5 is substantially
identical. An
individual coupling element is shown in Figure 4. Each coupling element 5 is 5
formed as
a substantially linear strut and has a first end 9 and a second end 10. At
both the first end
9 and the second end 10 there is a hole or opening 11 to enable the coupling
elements 5 to
be attached to the flange 8 of the field coil support structure 3 or the
flange 7 of the
mounting 2. The coupling elements 5 are formed of carbon fiber such that the
primary
direction of fiber lay extends along the axis of the coupling 10 elements from
the first end
9 to the second end 10. In this manner the coupling elements 5 are each
strongest along
their axis and may withstand the forces that they are subject to during
operation of the
electrical machine. The coupling elements 5 carry all of the torque that is
experienced by
the electrical machine during its operation.
[0051] As can be seen most clearly in Figures 2 and 3, the flanges 7, 8 and
the coupling
elements 5 serve to fix the field coil support structure 3 to the mounting 2.
The first end
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of each coupling element 5 is pivotably connected to the flange 8 of the field
coil support
structure 3 by a bolt 12. Similarly, the second end of each coupling element 5
is pivotably
connected to the cooperating flange 7 of the mounting 2 by a bolt 12. The
coupling
elements 5 extend substantially circumferentially around the rotor 1 and may
rotate
slightly relative to the rest of the rotor 1 during thermal expansion and
contraction of the
field coil support structure 3. In this embodiment of the invention, eight
coupling
elements 5 are fixed around each pair of cooperating flanges 7, 8.
[0052] The coupling elements 5 may be axially spaced apart from the
cooperating flanges
7, 8 as shown in Figure 3.
[0053] A cylindrical iron core 13 is provided between the mounting 2 and the
field coil
support structure 3. During operation of the rotor 1, the iron core 13 is
maintained at
ambient temperature. The iron core 13 is attached to the radially outer
surface of the
mounting 2 and is separated from the field coil support structure 3 by a
vacuum gap.
[0054] The construction of the rotor 1 shown in Figures 1 to 4 provides a
number of
advantages over the prior art. First of all, the mounting 2 and the field coil
support
structure 3 are only fixed together by the coupling elements at their
cooperating flanges 7,
8. This minimizes the total cross-sectional area of the thermal pathway
between the two
components. Each individual coupling element 5 has a relatively small cross-
sectional
area between its first end 9 and second end 10 in order to further reduce this
thermal
pathway. Furthermore, the coupling elements 5 have a substantial length and
are made of
carbon fiber, which has a relatively low thermal conductivity. These factors
also further
serve to minimize heat flow between the field coil support structure 3 and the
mounting
2.
[0055] The coupling elements 5 provide adequate compressive capability
together with
axial and radial stiffness. This eliminates the need for any additional axial
or radial
supports.
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[0056] The coupling elements 5 and flanges 7, 8 also serve to effectively
transfer the
torque between the field coil support structure 3 and the mounting 2 and may
withstand
all of the forces that they may be subjected to during operation of the rotor
1. In
particular, they may withstand both tensile and compressive forces, such as
the strong
oscillatory forces produced by short circuit torque. When under tensile
stress, the
coupling elements 5 will act as tie rods, whilst under compressive stress the
20 coupling
elements will act as bracing struts. To improve their capability to act as
bracing struts and
withstand compressive forces, the coupling elements 5 are pretensioned. Pre-
tensioning is
applied during assembly of the rotor at ambient temperatures.
[0057] The length of the coupling elements 5 and their mounting in a
substantially
circumferential direction around the rotor 1 minimizes the axial and radial
stresses they
are subjected to as a result of the thermal expansion and contraction of the
field coil
support structure 3.
[0058] Two alternative coupling elements 5a, 5b are shown in Figure 5.
Coupling
element 5a is suitable for use as a radial strut to be bolted directly through
a cylindrical
surface of the mounting at a first end 9a and through a cylindrical surface of
the field coil
support structure at a second end 10a. Alternatively, the coupling element 5a
may be used
as a substantially axially extending strut. It may be fixed at a first end 9a
to a flange or
tab of the mounting and at a second end 10a to a flange or tab of the field
coil support
structure such that it extends in a direction substantially parallel to the
axis of the field
coil support structure and the mounting.
[0059] Coupling element 5b has a first end 9b that is the same as the first or
5 second end
9, 10 of the coupling element 5 of Figure 4 and can thus be fixed to either
the field coil
support structure or the mounting in the manner described above for coupling
element 5.
The coupling element 5b has a second end 10b that is the same as the first or
second end
9a, 10a of the coupling element 5a and can thus be fixed to either the field
coil support
structure or the mounting in the manner described above 10 for coupling
element 5a.
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