U.S. patent application number 13/260678 was filed with the patent office on 2012-05-10 for flywheel.
This patent application is currently assigned to Ricardo UK Limited. Invention is credited to Andrew Atkins, Joshua Dalby.
Application Number | 20120111689 13/260678 |
Document ID | / |
Family ID | 40671863 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120111689 |
Kind Code |
A1 |
Atkins; Andrew ; et
al. |
May 10, 2012 |
FLYWHEEL
Abstract
The invention relates to an apparatus and method for
constructing a magnetic gear coupling for transmitting torque from
a flywheel to a driveshaft, the coupling employing permanent magnet
arrays and magnetic flux coupling elements therebetween, and at
least one of the arrays and coupling elements being
circumferentially staggered or spread, so as to avoid complete
alignment of the arrays and elements, and thereby mitigate torque
coupling capability variations.
Inventors: |
Atkins; Andrew; (West
Sussex, GB) ; Dalby; Joshua; (Chesterfield,
GB) |
Assignee: |
Ricardo UK Limited
West Sussex
GB
|
Family ID: |
40671863 |
Appl. No.: |
13/260678 |
Filed: |
March 26, 2010 |
PCT Filed: |
March 26, 2010 |
PCT NO: |
PCT/GB2010/000592 |
371 Date: |
January 19, 2012 |
Current U.S.
Class: |
192/84.3 ;
29/428 |
Current CPC
Class: |
Y02E 60/16 20130101;
H02K 49/102 20130101; H01F 7/02 20130101; H01F 41/0253 20130101;
H02K 49/106 20130101; H02K 2201/06 20130101; Y10T 29/49826
20150115; H02K 7/025 20130101; F16F 15/30 20130101 |
Class at
Publication: |
192/84.3 ;
29/428 |
International
Class: |
H02K 49/10 20060101
H02K049/10; B23P 11/00 20060101 B23P011/00; F16F 15/30 20060101
F16F015/30 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2009 |
GB |
0905343.0 |
Claims
1. Apparatus for coupling force between first and second movable
members, each of the first and second members having an array of
alternating magnetic poles, the alternating poles having a spacing
therebetween in a direction of relative motion between the members,
a magnetic flux coupling element being arranged between the first
and second members, thereby providing a flux density region of
relatively high magnetic flux density between a pole of the first
member and a pole of the second member, wherein: the first and
second members and the coupling element are arranged relative to
one another to prevent complete alignment.
2. The apparatus of claim 1 in which one of first and second pole
arrays are split along their axis into split pole parts and one of
the split pole parts is differently aligned in the direction of
motion than another split pole part.
3. The apparatus of claim 2 in which the split pole part alignments
form a symmetrical pattern.
4. The apparatus of claim 3 in which the split pole part alignments
form a chevron or a sinusoidal pattern.
5. The apparatus of claim 1 in which the coupling element is split
along its axis and one of the split coupling parts is arranged with
a different alignment in the direction of motion relative to one of
the poles of the first and second members than another of the split
coupling parts.
6. The apparatus of claim 5 in which the split pole part alignments
form a symmetrical pattern.
7. The apparatus of claim 6 in which the split pole part alignments
form a chevron or a sinusoidal pattern.
8. The apparatus of claim 1 in which the first and second members
are rotatable, and the first and second spacings are angular
spacings.
9. The apparatus of claim 1 in which the first array is arranged
inside the second array.
10. The apparatus of claim 1 in which the first array is arranged
opposite the second array.
11. The apparatus of claim 1 in which the first array is arranged
alongside the second array.
12. The apparatus of claim 1 in which the first and second members
are linearly movable, and the first and second spacings are linear
spacings.
13. The apparatus of claim 1 in which one of the first and second
members is rotatable, the corresponding one of first and second
spacings is an angular spacing, the other one of the first and
second members is linearly movable, and the corresponding other one
of first and second spacings is a linear spacing.
14. The apparatus of claim 1 in which the coupling element
comprises one or more coupling elements of material having a high
relative magnetic permeability.
15. The apparatus of claim 14 in which the relative permeability is
greater than about 400.
16. The apparatus of claim 1 in which the coupling element has a
low electrical conductivity.
17. The apparatus of claim 14 in which each coupling element is
incorporated in a membrane separating the first and second
members.
18. The apparatus of claim 17 in which each coupling element is
substantially the same thickness as the membrane or thicker than
the membrane.
19. The apparatus of claim 14 in which a dimension of each coupling
element increases with radius from an axis of rotation.
20. The apparatus of claim 14 in which between each coupling
element is a material having a low magnetic permeability and low
conductivity.
21. The apparatus of claim 1 in which the poles of each array are
substantially equally spaced with respect to other poles in the
same array.
22. The apparatus of claim 1 in which the poles of the first array
have substantially the same spacing as those of the second
array.
23. The apparatus of claim 1 in which poles of first array have a
greater or lesser spacing than the spacing of poles in the second
array.
24. A method of making a coupler for coupling force between first
and second movable members, the method comprising: arranging an
array of alternating magnetic poles on each of first and second
members, the poles having a spacing therebetween in a direction of
relative movement between the members; arranging magnetic flux
coupling means between the first and second members, thereby
providing a flux density region of relatively high magnetic flux
density between a pole of the first member and a pole of the second
member; and arranging at least one of the first member, the second
member and the coupling means so as to prevent complete
alignment.
25. (canceled)
Description
[0001] The invention relates to a flywheel and a method for
constructing a flywheel for energy storage.
BACKGROUND OF THE INVENTION
[0002] Flywheels are known for the storage of energy in the form of
kinetic energy, for example for use in vehicles. In such instances
it is known to use a flywheel to store the energy which would
otherwise be converted to heat in the vehicle's braking system when
the vehicle decelerates, this stored energy then being available
for use to accelerate the vehicle when desired.
[0003] An existing type of flywheel according to FIG. 1 has a
central metallic support section (1) which can be mounted on a
central support such as a shaft. At least one composite ring (2) is
mounted on the central support section. The composite ring in this
type of flywheel is filament wound from carbon fibre. When the
flywheel is in rotation, the ring will tend to expand in diameter
due to the centrifugal forces acting on it. The ring has high
strength in hoop for re-acting the centrifugal forces when the
flywheel is in rotation. However, the outer ring can become a loose
fit on the central support section and potentially (dangerously)
become dismounted from the central support section. In addition the
radial stress can result in failure of the composite ring.
[0004] In order to counteract the tendency of the ring to grow, the
ring is typically machined with a smaller inner diameter than the
outer diameter of the central support section and is then mounted
onto the central support section with an interference fit. The
mismatch in diameters results in a pre-load such that that ring
exerts an inward force onto the central support section. This
inward preload is greatest when the flywheel is not rotating and
results in a requirement for the central support section to be
sufficiently structurally strong that it can withstand the preload
force when the flywheel is stationary. It is known for more than
one composite ring to be pressed together and further mounted onto
the central support. The pre-load increases towards the centre of
the flywheel and with the number of rings pressed together.
Consequently a large amount of material may be required in the
central support section of the flywheel in order to counteract this
pre-load force, and this material, being near the centre of the
flywheel, adds only very inefficiently to the rotational inertia of
the flywheel. Further, if the hub is stiffer than the composite
ring, as the speed of the flywheel increases and the pre-load
reduces then the increased mass will lead to stress management
problems in the hub.
[0005] Yet further, in the existing system, exceeding the maximum
stress rating of the composite ring will result in failure. In the
flywheel type above, the central support section exerts an outward
force on the composite ring due to the pre-load. This force is in
the same direction as the centrifugal forces acting on the ring
when the flywheel is in rotation. Then, if the stiffness of the hub
is lower than the composite ring, the ring must be strong enough to
counteract the sum of the preload force and the centrifugal forces
when the flywheel is rotating at maximum speed. A further problem
with this type of flywheel is therefore that the preload reduces
the maximum rotation speed of the flywheel.
[0006] A further problem with existing systems is that if a
flywheel is to be coupled to, for example, a vehicle transmission,
a splined coupling is normally required in order that high
transient torque levels (for example when the vehicle gearbox ratio
is changed quickly, thus requiring the flywheel to accelerate or
decelerate rapidly) may be transmitted to the flywheel without
slippage.
[0007] A flywheel of the type described in UK patent application
0723996.5, filing date 7 Dec. 2007, overcomes the aforementioned
limitations by providing a flywheel having a drive transfer element
and a rim comprising a mass element, where the rim and the drive
transfer element are coupled by a winding. However, it is desirable
with this type of flywheel to have an indication of stress in the
flywheel components as the flywheel is rotated at increasing
speed.
[0008] UK patent application 0902840.8 provides such an
aforementioned indication of stress in the flywheel components by
incorporating a warning, or indicator, ring into the flywheel. The
indicator ring can be mounted to the flywheel with an interference
fit, such that residual stresses are set up between the ring and
the flywheel. The level of interference fit, or preload, and the
relative stiffnesses of the ring and the part of the flywheel onto
which the ring is mounted, are chosen such that when the flywheel
is rotated at or in excess of a pre-determined trigger speed, the
preload is substantially overcome by centrefugal forces, causing
the ring and support member to at least partially separate. The
ring is then able to move on the flywheel, causing an "out of
balance" condition, resulting in a vibration which is detectable as
an indication of stress in the flywheel components.
[0009] A further problem with existing flywheels is the need to
finely balance the rotating mass of the flywheel. Since the kinetic
energy stored in a rotating flywheel is proportional to {acute over
(.omega.)}.sup.2 (where {acute over (.omega.)} is the angular
velocity of the flywheel), increasing the maximum rotational speed
of a flywheel allows more energy to be stored in a flywheel of a
given mass, and thereby increases the energy storage density of
such a flywheel. However, as the rotational speed increases, the
balance of the assembly becomes more critical, as does proving the
structural integrity of the flywheel. Furthermore, the cost of
balancing a flywheel generally increases with the level of accuracy
of balance required.
[0010] A further problem when balancing composite flywheels, such
as the type described in UK patent applications 0723996.5 and
0902840.8, is that only a limited amount of machining/processing
can be performed on the composite component (i.e. the mass bearing
rim) without severely affecting the structural integrity of the
composite. This thereby affects the simplicity of the balancing
process, since material has to be removed from the flywheel at a
location away from the composite rim.
[0011] A further problem is that existing methods for balancing
flywheels generally incorporate machining and/or grinding and/or
drilling of material from the flywheel. Not only can (as previously
mentioned) such machining and flash or grinding and/or drilling of
material from a composite flywheel compromise the structural
integrity of the composite part, but furthermore, such machining
limits the accuracy of balancing obtainable in at least the
following two ways. Firstly, the accuracy of the balancing
operation is limited by the trueness of the lathe shaft onto which
the flywheel is mounted during the machining operation, and by the
accuracy of the mounting of the flywheel mass to the lathe shaft.
Secondly, the balancing accuracy is limited by the minimum
thickness of material which can be removed in the
machining/grinding/drilling process, which in turn may be affected
by the skill of the operator and/or (if the machine tool is
computer numerically controlled) by the precision of the CNC
machine. This is made more acute, since the material removed from
the flywheel is necessarily dense (in order to maximise the energy
storage density of the flywheel).
[0012] It is desirable therefore for a method to be found for
simply and quickly balancing such a flywheel to a high degree of
accuracy. It is also desirable that the method should
simultaneously prove the structural integrity of the flywheel. Such
a method would save time, production cost, capital cost, and would
also increase the performance and reliability of the flywheel.
[0013] Existing flywheels are sometimes constructed such that the
rotating mass of the flywheel rotates inside a chamber containing a
vacuum. Operating the rotating mass inside a vacuum is advantageous
since it reduces energy losses due to air resistance (also known as
windage). However, in order to transfer energy into and out of the
rotating flywheel mass, a coupling means is required. Some existing
flywheels use a rotating shaft passing through a rotating seal in
the vacuum chamber to couple torque from an energy source to the
flywheel energy storage means. Rotating seals are never perfect,
however, since they inevitably leak and therefore require an
environmental management system to be coupled to the vacuum chamber
in order to maintain the vacuum despite leakage. Furthermore, the
seals become more "leaky" with age and as rotational speed
increases, and also wear more quickly at higher speeds. The use of
rotating seals is therefore undesirable. The mass, volume and cost
of such an environmental management system is undesirable.
[0014] Magnetic couplings can be used with flywheels to transfer
torque through a vacuum chamber wall, thereby obviating the need
for rotating seals. However, the torque transmission capability of
such magnetic couplings using permanent magnets has previously been
found to be lacking in torque transmission capability.
[0015] This has been found to be at least partly because the
magnetic flux which passes between the poles of the two rotating
members, for a given magnetic pole strength, is limited by the "air
gap" between the two members. The air gap in fact, comprises the
air gap between the outer rotating member and the vacuum wall, the
vacuum wall itself, and a vacuum gap between the vacuum wall and
the inner rotating member. Since the vacuum chamber wall must be
structurally strong enough to support atmospheric pressure, its
thickness is necessarily significant, resulting in a large "air
gap" between the inner and outer rotating members.
[0016] Existing arrangements have sought to overcome this limited
torque coupling capability by employing electromagnetic poles in
order to increase the magnetic strength and thereby increase torque
coupling capability. However, the use of electromagnetic poles
requires an energy conversion, thereby reducing the efficiency of
the energy storage flywheel (since the electromagnets require
electrical power to operate them, which must be sourced from the
energy stored in the flywheel). Furthermore, the additional control
and power electronics associated with electromagnetic couplings
significantly increases the size, and weight of a flywheel energy
storage system incorporating such an electromagnetic coupling,
thereby further reducing the energy storage density of such a
flywheel energy storage system, both in terms of mass and volume. A
method of coupling energy into and out of an energy storage
flywheel operating in a vacuum chamber, which is efficient in terms
of mass, volume and energy is therefore required.
[0017] A further problem with existing flywheels is that while the
flywheel itself should be able to rotate at a high angular
velocity, the drive shaft which invariably couples the flywheel to
an energy source or sink (such as an engine or transmission) and
associated components which are outside of the vacuum chamber
suffer losses associated with air resistance (or "windage").
[0018] Magnetic gears use arrays of magnets (for example, permanent
magnets) and stationary pole pieces to transfer torque between
rotatable members, for example driveshafts. They exhibit reduced
wear when compared to conventional mechanical gears. However, their
torque transmission capability is dependent on the rotational
position of the magnets with respect to each other, and therefore
varies as the shafts rotate. For example, when torque transfer
capability is plotted on a graph against angular position, severe
peaks and troughs in the torque curve can be exhibited. This is
known as "cogging" and leads to a set of undesirable
characteristics.
[0019] Firstly, peaks and troughs in the torque curve lead to the
magnetic gear having a variable "pull-out" torque with meshing
position. That is, the torque required before the gears will slip
out of mesh varies depending on the rotational meshing position.
Therefore, such a gear set for transmitting a given level of torque
must be designed such that its minimum torque coupling capability,
as represented by one of the troughs (shown at around 20 Nm in FIG.
26) in the torque curve, is greater than the design torque handling
figure. For this, the magnet arrays must be sized appropriately
larger, and this also normally results in excess torque coupling
capability at certain meshing positions, representing an
inefficiency. Thus, the magnet arrays are normally sized larger
than that which would be necessary if the torque curve more closely
followed the mean torque handling capacity, thereby increasing
their cost and size, and reducing the energy storage density of a
flywheel incorporating such a magnetic gear.
[0020] Furthermore, since the angular offset between the input and
output shafts of a magnetic gear varies according to the torque
applied and to the torque coupling capacity at a given meshing
position, if the torque coupling capacity varies with meshing
position then this will result in a torsional vibration in the
shafts. Such a torsional vibration can reduce the life of the
associated mechanical components, and/or can result in failure
and/or disengagement. This is an especially serious problem if the
rotational speed is such that the frequency of the torsional
vibration coincides with a resonance of the mechanical system. It
would therefore be advantageous if the variation between the peaks
and troughs in the torque curve could be reduced or eliminated.
This would allow smaller, cheaper, magnet arrays to be used, since
the minimum torque coupling capability would then be much closer to
the mean torque coupling capability. Torsional vibration of the
shafts would also be reduced, allowing cheaper, lighter and smaller
components to be used. A flywheel energy storage system employing
such smaller, cheaper and lighter components would have a higher
energy storage density.
BRIEF DESCRIPTION OF THE INVENTION
[0021] The invention is set out in the claims.
[0022] In a first aspect of the invention there is provided an
apparatus for coupling force as defined in claim 1 of the appended
claims.
[0023] In a second aspect of the invention there is provided a
method as defined in claim 24 of the appended claims.
FIGURES
[0024] Embodiments of the invention will now be described with
reference to the drawings, of which:
[0025] FIG. 1 is a representation of a known flywheel;
[0026] FIG. 2 is an isometric view of an embodiment of the present
invention;
[0027] FIG. 3 is a cross-sectional view of the embodiment of FIG.
2;
[0028] FIG. 4 is a view of a shaft construction;
[0029] FIG. 5 is a detailed view of a winding pattern;
[0030] FIG. 6 is a view of the winding at a shaft;
[0031] FIG. 7 is a view showing alternative winding methods at a
rim;
[0032] FIG. 8 is a cross-sectional view of a flywheel incorporating
a warning ring;
[0033] FIG. 9 is a side view of the embodiment of FIG. 8;
[0034] FIG. 10 is a view of another embodiment incorporating a
warning ring;
[0035] FIG. 11 is a side view of the embodiment of FIG. 10;
[0036] FIG. 12 is a view of a further embodiment having a warning
ring;
[0037] FIG. 13 is a side view of the embodiment of FIG. 12;
[0038] FIG. 14 is a view of a yet further embodiment incorporating
a warning ring;
[0039] FIG. 15 is a view of a flywheel constructed inside a vacuum
chamber;
[0040] FIG. 16 is a cross sectional view of the flywheel of FIG.
15;
[0041] FIG. 17a is a view of a type of magnetic coupling;
[0042] FIG. 17b is a view of a magnetic coupling having a coupling
element;
[0043] FIG. 18a is a view of an epicyclic magnetic gear
coupling;
[0044] FIG. 18b is a close up view of part of the coupling of FIG.
18a;
[0045] FIGS. 19a to 19c are sequential views of part of the
coupling of FIG. 18a as it rotates through a sequence of three
positions;
[0046] FIG. 20 is a cross sectional view of a magnetic coupling
such as that shown in FIG. 18a when incorporated in a vacuum
chamber wall;
[0047] FIG. 21 is a view of a magnetic gear coupling incorporating
staggered poles;
[0048] FIG. 22 is a view of an epicyclic magnetic gear coupling
incorporating staggered poles;
[0049] FIG. 23 is a view of an epicyclic magnetic gear coupling
incorporating a spread coupling element;
[0050] FIG. 24 is a view of a contra-rotating magnetic gear
coupling incorporating staggered coupling elements and a spread
coupling element;
[0051] FIG. 25 is a cross sectional view of a flywheel
incorporating an epicyclic magnetic gear coupling;
[0052] FIG. 26 is a graph showing two curves of torque coupling
capability versus gear meshing position.
DETAILED DESCRIPTION
[0053] In overview, the apparatus and method described herein
relates to a flywheel energy storage device where material used in
its construction is deployed in an inertially efficient manner, and
where the support structure is under tension, a rim comprising a
mass element is held in place on its outer surface by a winding
which also passes around a drive transfer element, rather than for
example by a compressive interference fit to its inner surface.
[0054] In other embodiments a support element can surround the rim
to counteract centrifugal forces and a torsionally compliant or
resilient drive transfer element such as shaft can be provided.
[0055] The winding may be configured in a number of ways as
described below and may also be pre-tensioned. The drive transfer
element may be a shaft, which may be hollow and may be constructed
from wound carbon fibre. The rim may comprise a circumferential
support member (also referred to as a support element) and a mass
element mounted radially inwards of the support member.
[0056] In embodiments the rim may be constructed of a composite
material, for example a wound carbon fibre and resin. The mass
element may be a ring, pressed or moulded into the reinforcing
element. Alternatively, the mass element may comprise one or more
dense elements, which may be linked as in a chain, incorporated
into the rim by moulding, drilling, pressing or adhesive attachment
to the inside of the reinforcement element.
[0057] The drive transfer element may be a hollow shaft for
example, and this may be formed from a wound carbon fibre
composite. The composite may be wound with fibres oriented in
directions arranged such that both bending of the shaft and
twisting of the shaft result in a change in the length of the
fibres, these deformations therefore being resisted by the fibre's
natural tendency to resist changes in length. The shaft may thereby
be formed so that it is compliant to a twisting motion.
[0058] A warning or indicator ring can be mounted to the flywheel
rim, and the flywheel can be arranged such that at least one of the
warning ring and other components move, expand, contract, deform or
distort relative to the other under centrifugal force of sufficient
magnitude. This can affect rotation of the assembly, for example by
unbalancing it, which can be monitored or detected to provide an
indication of overload.
[0059] Because a warning or indicator ring is incorporated with the
flywheel to behave differently under rotation, when the flywheel
reaches undesirable rotational speeds a detector can detect
consequences of the different behaviour, for example, imbalance in
the flywheel.
[0060] The indicator ring may be mounted to the flywheel with an
interference fit, and is supported either by the support member or
by the drive transfer element (for example, a shaft). The ring can
be constructed from circumferentially wound fibre (for example,
carbon fibre), or can be another material with sufficient strength
in hoop to enable it to be rotated at the maximum designed flywheel
speed without failing, and with a suitable stiffness as further
described below. When mounted to the support member, the ring can
be radially disposed inside or outside of the support member. When
the ring is radially disposed inside of the support member, the
ring has a stiffness which is greater than or substantially the
same as that of the support member. When the ring is radially
disposed outside of the support member, the ring has a stiffness
which is lower than or substantially the same as that of the
support member. The support member comprises circumferentially
wound fibre, for example carbon fibre. When mounted to the drive
transfer element (for example, a shaft), the ring has a stiffness
which is lower than or substantially the same as that of the drive
transfer element.
[0061] The interference fit results in a pre-load between the ring
and its mounting (for example, the support member) when the
flywheel is at rest. The level of preload and the relative
stiffnesses of the ring and the ring mounting are chosen such that
when the flywheel is rotated at or in excess of a predetermined
trigger speed, the preload is substantially overcome by centrifugal
forces, causing the ring and support member to separate. Generally,
the less stiff component will tend to stretch and "grow" more than
the stiffer component. Notably, however, in the case where the ring
and its mounting have substantially the same stiffness, the two
components will nevertheless tend to separate under rotation
because greater forces act upon the component which is at the
greater radius from the axis of rotation. The combination of radial
position and material stiffness can be adjusted accordingly to
achieve separation at the desired predetermined speed. The
predetermined speed is chosen to be lower than the speed at which
flywheel failure is to be expected. The ring is fitted to its
mounting by a press fit which results in a non-uniform stress
distribution at the interference boundary.
[0062] Referring to FIGS. 2 and 7, in order to effectively provide
a high inertia, a rim (50) including a mass element (10)
comprising, for example, a ring of relatively massive material is
disposed at a relatively large radius, compared with the size of
the flywheel (30), from a drive transfer element such as a shaft
(60) providing a central rotating axis (20). The mass element (10)
has a high density in order to effectively provide inertia.
Suitable materials may be lead or steel for example, although other
materials could be used. The mass element (10) is subject to stress
when the flywheel (30) is in rotation, this being induced by
centrifugal forces.
[0063] An outer circumferential support member (40) is located
radially outside the mass element. The support member (40) has a
high hoop strength and is able to counteract the centrifugal forces
acting on the mass element (10) when the flywheel (30) is in
rotation. The support member (40) is preferably a carbon fibre
composite, wound in a circumferential direction so as to impart a
high strength in hoop. In the embodiment shown the support member
(40) is pressed onto the mass element (10) with a small
interference preload such that the two are effectively joined,
forming a rim (50). The preload only needs to be small since it
merely functions to hold the two elements together in an
interference fit when the flywheel is stationary. Alternatively,
the two may be joined by an adhesive bond or similar. The more
efficient placement of mass, concentrating mass near the rim of the
flywheel results in a lighter flywheel for a given energy storage
capacity. Although the mass element is shown in FIG. 2 as a
continuous ring, alternatively it may be separate ring segments, or
may be discrete elements of mass. For example, an alternative
arrangement is shown in FIG. 7 wherein the mass may be inserted or
moulded into the support member (40), either as a ring, or as
discrete elements into receiving holes in the support member
(40).
[0064] Referring to FIGS. 2, 5 and 6, a winding couples the rim
(50) to the shaft (60). The winding is configured such that it
consists of substantially or partially radial portions (80)
extending from the shaft (60) to the rim (50) and substantially
axial portions (90) extending around the rim (50). In the
embodiment shown the winding is filament wound in a winding
operation proceeding as follows: radial portions from shaft (60) to
rim (50), axial portions over the rim (50) so as to form a `sling`,
and then radial portions back from rim (50) to shaft (60), in a
repeating fashion. The winding (87) may pass at least partially
around the circumference of the shaft (60) between some but not
necessarily all iterations of the winding operation. The winding
(80, 90) will stretch slightly as the rim (50) grows under
centrifugal forces and will exert a counteracting force on the rim
(50). Thereby, the winding (80, 90) assists the support member (40)
of the rim (50) in resisting the centrifugal forces acting on the
mass element (10) and in resisting the radial growth of the rim
(50). The winding (80, 90) could be made of a fibre, including
carbon, glass fibre, Kevlar, Zylon or nylon, or could be made of a
metal wire in low stress applications. As a result more massive
mounting arrangements such as a central support section or spokes
are not required.
[0065] In embodiments where the mass element comprises a ductile or
malleable material, the support member (10) and the winding (80,
90) can be pre-tensioned during manufacture by the following
method: The flywheel is assembled in the way herein previously
described, with drive transfer element (60) and rim (50) coupled by
a winding, the rim (50) comprising a mass element (10) and an outer
support member (40). No or negligible pre-load inwardly need be
applied at this stage. The flywheel is then spun at an angular
velocity sufficiently high that the centrifugal forces on the mass
element (10) are sufficient to cause it to yield and smaller than
its ultimate tensile strength. As a result, the mass element (10)
yields outwardly and its circumference increases. The increase in
circumference of the mass element (10) results in a secure
interference fit between mass element (10) and support member (40),
thereby stretching and pre-tensioning the support member (40) and
also stretching and pre-tensioning the winding (80, 90). The mass
element (10) has a low to moderate Young's modulus, which is less
than that of the support member (40), such that the mass element's
(10) tendency to deform under centrifugal forces is greater than
that of the support member (40). This operation results in a
pre-tensioning of both the support member (40) and the winding (80,
90). In this way, both the support member (40) and the winding (80,
90) are pre-tensioned, compared to the result of fitting the mass
element (10) to the support member (40) with an interference fit
before adding the winding, which would result in a pre-loading of
the support member (40) only. In other embodiments the above method
can be used to pre-tension the support member alone.
[0066] In other embodiments, a material with an extremely low
Youngs's modulus comprises the mass element (10), such as Lead. The
use of a dense liquid such as Mercury results in a flywheel in
which the mass element (10) is self-balancing. The support member
(40) constrains the mass element (10) radially inside the support
member (40).
[0067] Suitably ductile or malleable materials for use in
comprising the mass element (10) have a large ultimate tensile
strength compared with their first point of yield strength,
defining a sufficiently large ductile region that the yield point
of the material can be exceeded during the manufacturing operation
detailed above without a risk of exceeding the ultimate tensile
strength of the material. A suitable ratio of yield strength to
ultimate tensile strength would be close to 1:2. The material used
for the mass element (10) also has a first point of yield which is
sufficiently low that it may be exceeded at moderate flywheel
speeds such that failure of other parts of the flywheel is avoided,
such parts being for instance the outer support member (40) and
winding (80, 90). The material also has properties such that the
centrifugal forces resulting in the pre-loading process cause a
sufficiently large circumferential deformation of the mass element
(10) that the resulting deformation of the support member (40) and
winding (80, 90) results in a pre-load which significantly
counteracts centrifugal forces acting on the mass element (10) when
it is rotating at the typical rotational speeds encountered during
normal operation.
[0068] In embodiments where the mass element (10) is not ductile
and is not pre-loaded using the above method, the ultimate tensile
strength of the mass element is optimally close to that of the
support member (40) and the yield strength of the mass element (10)
is as close as possible to the ultimate tensile strength of the
support member (40).
[0069] Referring to FIG. 5, the angle of the winding portions (80)
from shaft (60) to rim (50) may be selected to determine the
characteristics of torque transfer between shaft (60) and rim (50).
The angle used may be selected between i) a tangent to the shaft
circumference and ii) perpendicular to the shaft circumference. A
selected angle which is close to a perpendicular angle to the shaft
(60) will enhance the contribution made by the winding (80) to the
counteraction of centrifugal forces acting on the mass element
(40). A selected angle which is close to a tangent to the shaft
circumference will enhance the ability of the winding (80) to
transfer torque between the rim (50) and the shaft (60). A
compromise angle within the range of angles above can be selected
in order to optimise the contribution made by the winding. Since
the winding (80, 90) is only able to transfer torque when in
tension, the radial winding portions (80) can be arranged in both
clockwise (85) and anticlockwise (86) directions such that either
clockwise (85) or anticlockwise (86) winding portions are in
tension, depending on whether the flywheel is accelerating or
decelerating. Yet further the axial position along the shaft about
which the winding is arranged can be varied to vary the strength of
tensile support.
[0070] Referring to FIG. 2, the number of turns of the winding (80,
90), and as a result its strength, may be varied. Likewise, the
number of turns of fibre in the carbon support member (40) may be
varied so as to alter its strength. Since the reaction against the
centrifugal forces acting on the mass element (10) is a combined
reaction from the support member (40) and the winding (80, 90), the
relative contribution from each can be varied by altering the
number of turns in the winding (80, 90) and the number of turns in
the support member (40). In one aspect, the support member (40) can
be removed altogether, the centrifugal forces acting on the mass
element being counteracted solely by the winding (80, 90). Yet
further the winding can extend continuously around the whole
circumference or can be interrupted with gaps in the
circumferential direction between individual or groups of fibres,
providing a "spoke-like" arrangement. For example, in the case
where the mass element is a number of discrete elements the winding
at the rim (90) can be aligned with the discrete mass elements.
[0071] Referring to FIG. 3, the rim (50) is at least temporarily
supported by a carrier portion (70) on the drive transfer element,
which may be a shaft (60). The carrier portion is preferably made
of a lightweight material so as to reduce overall flywheel mass and
concentrate mass at the periphery. The carrier portion may for
example be made of wood, wax, resin or other lightweight material.
The carrier portion allows mounting of the rim on the drive
transfer element while the winding is being applied during
manufacture. The carrier portion may be removed or removable after
the winding has been applied to the rim and drive transfer
elements, by way of erosion, dissolution, melting or
sublimation.
[0072] The winding and the carrier portion are relatively light
compared with the rim, thereby the flywheel may thus be configured
with a rim comprising a mass element such that the majority of the
mass of the flywheel is near the rim where it is most inertially
efficient. The carrier portion (70) may be glued to the shaft (60)
and/or the rim (50).
[0073] Referring to FIG. 4, the shaft (60) may be solid, but is
preferably hollow so as to reduce its mass. The shaft (60) is
preferably a carbon fibre composite, woven such that it is
torsionally compliant and axially stiff The shaft could however be
made of other materials such as glass fibre, steel, titanium, other
metals or composites. In the case of a fibre composite shaft, the
weave pattern of the fibres may, be altered so as to influence the
degree of resistance to bending and twisting, and to fine tune the
torsional compliance of the shaft. The shaft may have one or more
bearing surfaces (65) pressed or glued onto it. One or more bearing
surfaces (65) may also incorporate a drive coupling (66), or a
separate drive coupling may be glued or pressed onto the shaft. The
torsional compliance of the shaft has the effect of limiting peak
torque levels at the drive coupling and therefore allows the use of
drive couplings with lower peak torque handling capability than
that of splined drive couplings, for example frictional or magnetic
couplings.
[0074] Manufacture of the flywheel can be further understood by
referring to FIGS. 3 and 6. The winding (80, 90) can be formed by a
`wet winding` process whereby a binder is provided for example
using a resin or adhesive. The fibre which forms the winding (80,
90) can be impregnated with a resin or adhesive and can be wound
while the resin or adhesive was still `wet`, that is to say that
the resin or adhesive is in the uncured state. Alternatively, the
support member (40) can be coated with a resin or adhesive before
or during the process of forming the winding (80, 90) such that the
winding (80, 90) adheres to the support member (40). Likewise, the
shaft (60) can be coated with a resin or adhesive prior to or
during the winding process such that the winding (80, 90) adheres
to the shaft (60). These techniques enhance the transfer of torque
between shaft (60) and rim (50). Alternatively, an interference fit
between winding (80, 90), shaft (60) and rim (50) could be
used.
[0075] Referring to FIG. 7, the winding (80, 90) and the support
element (40) have henceforth been described as separate wound
elements. However, it would be possible to combine both elements
by, for instance, interleaving turns of the winding (80, 90) and
turns of the support element (40). It would also be possible to
first form the support element (40), form holes (45) through it,
and then form the winding (80, 90) with the winding portions (80,
90) passing through the holes (45) in the support element (40). The
shape of the support element (40) may be hemispherical or parabolic
in order to spread stress in the portion of the winding (90) which
contacts the support element (40). Any smooth sectional outline
shape is envisaged as being suitable.
[0076] Referring to FIGS. 2 and 5, spaces may be left between the
winding portions at the rim (80, 90) such that access to the
carrier portion (70) remains. The carrier portion (70) may be left
in place or may be removed by blasting, erosion, dissolution,
melting or sublimation, after the winding (80, 90) has been formed.
The carrier portion could for example be made of ceramic, resin,
wax or other suitable material to enable this operation. Removing
the carrier portion (70) would result in an even lighter flywheel
having an even lower proportion of inertially inefficient mass.
With the carrier portion removed, the winding provides the only
substantial means of support for the rim on the drive transfer
element.
[0077] In alternative approaches the flywheel can be constructed
with the circumferential support member providing hoop strength but
the ring being mounted using a conventional central support section
rather than a winding.
[0078] In use the flywheel may be mounted in a vehicle or any other
appropriate setting for storage of energy or other purpose such as
stabilisation and coupled or decoupled from a drive-providing or
receiving component such as a motor, engine or dynamo as
appropriate via the drive transfer element.
[0079] Referring to FIGS. 8 and 9, which show a first embodiment of
a flywheel (30) having a warning or indicator ring (800), it can be
seen that the warning ring (800) is mounted on the outer periphery
of the support element (40). The warning ring (800) is mounted
radially outside the support element (40), using an interference
fit, and is typically pressed into place. The interference fit
between the warning ring (800) and the support element (40) results
in a pre-load force between these two components when the flywheel
(30) is at rest. The assembly of warning ring (800) to support
element (40) results in a residual non-uniform stress between the
two. The winding (80) passes around the warning ring (800), support
element (40) and mass element (10). The flywheel is finely balanced
to avoid vibration when rotating. During manufacture, the balancing
operation is performed after the warning ring is assembled such
that it is balanced with the warning ring in place.
[0080] As shown in FIGS. 8 and 9, the winding (80) passes around
the warning ring (800) and support element (40) Thus, the winding
tends to hold the warning ring (800) in contact with the support
element (40), counteracting the warning ring's tendency to grow
away from the support element (40). However, by selecting the
stiffnesses of the warning ring (800), winding (80) and support
element (40) appropriately it is possible to ensure that the
warning ring (800) is able to move radially (i.e. grow) away from
the support element (40) under centrifugal forces. In other
embodiments (such as shown in FIGS. 10 and 11) the warning ring
(800) is pressed onto the outside of the support element (40) and
radially outside the winding (80).
[0081] In the embodiments shown in FIGS. 8 to 11, the warning ring
(800) has a lower Young's modulus (is less stiff) than the support
element (40) such that in operation when the flywheel is rotated,
the warning ring (800) grows radially (under centrifugal forces) a
greater amount than the support element (40) grows, leading to
separation when the centrifugal force reaches a sufficient
magnitude. In the embodiments shown in FIGS. 8 and 9 where the
winding passes around the warning ring (800), the stiffness of
winding (80) and warning ring (800) are together low enough such
that the warning ring (800) and winding (80) grow more than the
support element (40) grows when the flywheel is rotated. The
warning ring (800) need only be a lightweight ring with relatively
low strength compared to the support element (40), since the
warning ring (800) does not substantially support the mass element
(10).
[0082] Expansion of the warning ring leads to a relaxing of the
pre-load between the warning ring (800) and the support element
(40). At a trigger rotational speed or centrifugal force magnitude
(predetermined by the amount of interference fit pre-load, and the
relative stiffnesses of the warning ring and the support element),
the pre-load is overcome and the warning ring (800) and support
element (40) at least partially separate. The separation is likely
to occur non-uniformly for example, because the interference fit
has a non-uniform stress distribution at the interference boundary,
leading to a movement off-centre and an imbalance in the rotating
mass. Furthermore, the residual non-uniform stresses between the
warning ring (800) and the support element (40) are at least
partially released by the movement of the warning ring (800) with
respect to the support element (40). This movement causes the
flywheel (which is finely balanced during manufacture) to go at
least slightly out of balance. The imbalance cause by relaxation of
the residual stresses is permanent (that is, the imbalance is
permanent unless the flywheel is subsequently at least partially
re-manufactured, for example by at, least performing the step of
re-balancing the flywheel and optionally, prior to rebalancing,
performing the steps of removing and re-mounting the warning ring
onto the support element such that the residual non-uniform stress
is restored, thereby restoring the capacity of the flywheel to go
out of balance if the pre-load is again overcome) and can be
considered to be evidence of a mechanical "fuse" having been
triggered.
[0083] The resulting imbalance causes a vibration when the flywheel
is rotating and the vibration can be detected by a vibration sensor
so as to give an indication of excessive flywheel speed, the
indication being separate from any indication derived from, for
example, a flywheel speed sensor. An example of a suitable
vibration sensor is a piezo-electric accelerometer. Thus, even if
the main flywheel speed sensor malfunctions, a separate and
independent indication of excessive flywheel speed is provided.
Furthermore, a permanent indication results showing that the
flywheel has at some point been operated above its design speed and
thus might fail at some point in the future.
[0084] In the second embodiment shown in FIGS. 10 and 11 the
warning ring passes outside the winding (80) and its relative
stiffness is selected accordingly to provide the same effects.
[0085] In a further embodiment, as shown in FIGS. 12 and 13, the
warning ring (800) is mounted with an interference fit, radially
inside the support element (40). The mass element (10) is
interposed between the support element (40) and the warning ring
(800) in this embodiment, but in other embodiments can be
incorporated in the support element (40) as previously described or
the warning ring can be interposed between the mass element (10)
and support element (40). In these further embodiments the warning
ring (800) has a higher Young's modulus (is stiffer) than the
support element (40).
[0086] In operation when the flywheel is rotated, the support
element (40) grows radially (under centrifugal forces) a greater
amount than the warning ring (800) grows. Similarly to the previous
embodiments, the pre-load between the warning ring (800) and
support element (40) is overcome by centrifugal forces, allowing
the warning ring (800) to move. When the support element (40) grows
radially such that the space within it is larger than the outside
diameter of the warning ring (800), the warning ring (800) is able
to move off-centre within the support element (40), leading to an
imbalance. Furthermore, under influence of the non-uniform residual
stresses (residual from the press-fitting assembly operation during
manufacture whereby the warning ring is pressed into the centre of
the support element), the warning ring (800) is caused to move
within the support element when the pre-load is overcome by
centrifugal forces, thereby causing the flywheel to go permanently
out of balance, causing vibration. As previously described,
vibration can be detected by a sensor and used as a warning
indication.
[0087] In a yet further embodiment, the warning ring (800) is
press-fitted to the drive transfer element (which is, for example,
a shaft) with an interference fit which results in a pre-load. As
before, the flywheel is finely balanced. The warning ring (800) is
less stiff than the shaft (60) and grows radially more than the
shaft grows when the flywheel rotates. At a predetermined speed,
the pre-load is overcome, allowing the warning ring (800) to move
on the shaft which causes an imbalance which can be detected prior
to mechanical failure.
[0088] The deliberate production of an imbalance when a flywheel
speed exceeds a trigger speed, and detection of a vibration caused
thereby, as described above, provides a warning that the flywheel
is being operated or has been operated at above its maximum safe
operating speed. This warning can be determined separately from a
primary flywheel speed monitoring system and thus provides a
fail-safe second indication of excessive flywheel speed in the
event that the primary speed monitoring system fails. It will be
noted that detection of overload can be triggered by setting at the
detector the level of imbalance signifying overload, or by
modifying the relative properties of the warning ring and/or other
rim components, or any combination thereof. The system can be
calibrated to indicate excessive speed when all or part of the
warning ring detaches, or when relative movement/dimension change
is sufficient to create a detectable or threshold-surpassing
imbalance.
[0089] The embodiments where the warning ring (800) is enclosed by
the winding (80) have the advantage that should the flywheel be
operated at a speed higher than the trigger speed, with the result
that the warning ring becomes loosened from the support element
(40), the warning ring (800) is contained within the winding (80)
and there is no danger of the warning ring (800) becoming
completely detached.
[0090] It will be seen that, as a result of the configuration
described above, a stronger, safer and more efficient flywheel can
be provided.
[0091] A method of balancing such a flywheel will now be described.
Referring to FIG. 15, a flywheel (30) can be placed inside a vacuum
chamber (1550). Operating a flywheel in a vacuum is advantageous
since it reduces frictional losses/overheating associated with our
resistance (or "windage"). The flywheel rim (50) inevitably has
surface irregularities (1630) which result from inadequacies in the
balancing operation which is performed on the flywheel during its
manufacture, and/or the methods used to construct the flywheel. It
has been found, as previously mentioned that these irregularities
result in an imperfect rotational balance of the flywheel (30).
[0092] In the embodiment shown, the flywheel is supported in the
vacuum chamber (1550) by bearings such that the flywheel is able to
rotate inside the vacuum chamber. The vacuum chamber is a sealed
chamber capable of withstanding forces exerted by a pressure
difference between atmospheric pressure and the pressure inside the
vacuum chamber. The thickness of the vacuum chamber wall is made
sufficient to give it enough strength to support atmospheric
pressure against the vacuum inside the chamber. The vacuum chamber
incorporates at least one of a gas inlet (1520) and a gas outlet
(1510). Optionally the gas inlet and gas outlet are combined as a
single port. Each of the gas inlet and gas outlets communicate with
the interior of the vacuum chamber.
[0093] A coupling (1566, 1567) is comprised of first and second
members and is arranged to couple torque between a rotatable
driveshaft (1570) and the flywheel shaft (60). A first member
(1566) is coupled to the flywheel shaft (60), and a second member
(1567) is coupled to the drive shaft (1570). The flywheel shaft
(60) is supported on bearings and is connected to the flywheel rim
(50) by means such as already described. The flywheel rim (50) is
comprised of a composite material which is preferably finely
balanced by machining, drilling, or grinding during manufacture.
The flywheel rim (50) in this embodiment is a composite constructed
using a circumferentially wound fibre and resin as previously
described. The rim is coupled to the shaft (60) by radial fibres
such that torque can be transmitted from the flywheel shaft (60) to
the flywheel rim (50).
[0094] The driveshaft (1570) is supported by bearings outside of
the vacuum chamber and is rotatable. The flywheel shaft (60) and
the driveshaft (1570) are supported such that the two coupling
members (1566, 1567) are arranged in close proximity with the wall
of the vacuum housing (1550) arranged therebetween. The members are
arranged so as to minimise the "air gap" between the coupling
members (1566, 1567) and the vacuum chamber wall (1550). The term
"air gap" is used to describe in general the total gap between the
two members of the coupling (1566, 1567). The vacuum chamber can be
constructed in any commonly known manner, e.g. casting, machining
etc.
[0095] Referring to FIG. 16, a valve (1610, 1620) is incorporated
in or attached to the inlet port (1520) and the outlet port (1510).
In operation, the valves can be opened or closed so as to
selectively seal the inside of the vacuum chamber (1550) from the
atmosphere, or allow communication between the interior of the
vacuum chamber (1550) and the atmosphere. The outlet port can be
connected in use to a vacuum pump (not shown).
[0096] In use, the vacuum chamber is sealed from the atmosphere by
closing the inlet valve (1620) so as to isolate the inlet port
(1520) which is at atmospheric pressure from the interior of the
vacuum chamber (1550). The outlet valve is typically connected to a
vacuum pump capable of producing a high, or hard, vacuum. The
outlet valve (1610) is opened so as to allow the outlet port (1510)
which is connected to the vacuum pump to communicate with the
interior of the vacuum chamber (1550). The vacuum pump is then run
until the vacuum chamber (1550) contains a hard vacuum. Preferably,
this vacuum is better than 1 mbar, typically 10.sup.-2 mbar. The
flywheel (30) is then rotated by the application of torque from the
driveshaft (1570) via the coupling (1566, 1567) to the flywheel
shaft (60). This in turn rotates the flywheel rim (50). The
flywheel is rotated at a speed such that the rim (50) surface is
travelling at a speed which is in excess of the speed of sound
(Mach 1). The flywheel surface speed is also referred to as the
"tip speed". Prior to this operation, as previously stated, the
flywheel will have been balanced by mechanical operations such as
grinding, drilling, or machining, to as high a degree as practical
(within cost constraints) or at least as high as necessary to
enable the flywheel to be rotated at such a speed without danger of
mechanical failure.
[0097] Next, while the flywheel is rotating at a perimeter speed of
at least Mach 1, the inlet valve (1620) is opened so as to allow an
amount of gas to enter the vacuum chamber. This gas is preferably a
non-reactive gas such as nitrogen, and is preferably a dry gas,
that is it does not contain significant amounts of water vapour, so
as to avoid introducing humidity into the assembly. If the gas is
to be anything other than plain air, the inlet port would first
need to be connected to a suitable supply of said gas. The amount
of gas admitted is sufficient so as to substantially reduce the
vacuum to a pressure substantially higher than 10.sup.-1 mbar, for
example as high as 0.5 bar. I bar works well. The rate of gas entry
has been found to be non-critical.
[0098] Upon admission of the gas into the vacuum chamber (1550),
shock waves are set up in the gas between the surface
irregularities (1630) and the wall of the vacuum chamber (1550).
The shockwaves, and also friction between the surface
irregularities (1630) and the gas, act to vapourise, melt,
sublimate, erode or abrade the surface irregularities (1630) so as
to reduce their size and thereby improve the balance of the
flywheel (30) to a higher degree of balance then that obtainable by
mechanical machining, drilling, or grinding alone.
[0099] Furthermore, a flywheel is normally designed for a maximum
safe operating speed. During manufacture, such a flywheel must be
proved to withstand the maximum rotational speed it is designed
for. This is typically done by rotating the flywheel at a speed
equal to the square root of 2 multiplied by the design speed.
Especially in composite flywheel construction types, it can be
safely assured that if the flywheel survives rotation at this
higher speed then it will always survive operation at the design
speed for its lifetime duration. It is further advantageous to
combine this proving operation with the balancing operation
described above.
[0100] The rate of gas admission has not been found to be critical.
After gas admission, the flywheel is allowed to slow to rest, but
continues to rotate at a tip speed of at least Mach 1 for around 10
to 60 seconds, typically 15 seconds. This has been found to be long
enough to remove the surface irregularities while avoiding
overheating the flywheel rim. The time taken for the flywheel to
come to rest was in one embodiment approximately 3 minutes. The gas
density is not homogenised throughout the vacuum chamber. A
non-reactive gas is preferred so as to avoid reaction of the gas
with the flywheel components. When the flywheel is rotated at a tip
speed in excess of Mach 1, the supersonic shock wave produced
causes a far better balancing effect than if subsonic flywheel
speeds are used.
[0101] In embodiments it is desirable to use magnetic coupling for
example to avoid the need for rotating seals isolating a vacuum.
FIG. 17a, shows a prior art magnetic coupling for coupling two
rotatable shafts (60, 1570). Each shaft is coupled to a coupling
member (1766, 1767) comprising an array of alternating magnetic
poles. The two arrays are arranged close to each other, such that
magnetic flux can pass from one to the other via an air gap (which
is preferably as small as possible). Torque can thus be usefully
transferred from one shaft to the other.
[0102] This can be particularly useful in a flywheel application
since the two coupling elements (1766, 1767) are not required to
touch in order to transmit torque therebetween. The wall of a
vacuum chamber (1550) can be placed between the coupling elements
(1766, 1767), thereby allowing torque to be coupled between a
flywheel (30) inside a vacuum chamber (1550) and a driveshaft
(1570) outside of the vacuum chamber. This allows the vacuum
chamber to be sealed without the use of rotating seals, as
described above. Running a flywheel in a vacuum is useful since it
avoids air resistance ("windage") related losses. Thus becomes even
more important if the flywheel runs at supersonic speeds. The
vacuum avoids supersonic shockwaves and/or overheating due to
friction with air. However, since the vacuum chamber wall thickness
forms part of the air gap between the coupling elements (1766,
1767), the ease with which magnetic flux is able to pass from one
coupling element to the other is reduced, therefore the flux
density is reduced, and the torque coupling capability is
resultingly reduced. The following embodiments solve this
problem.
[0103] Referring to FIG. 17b, a coupling element (1730) is placed
between the coupling elements (1766, 1767). The magnetic coupling
element (1730) has a high relative magnetic permeability (in excess
of 400) and therefore in operation magnetic flux passes easily
through it, from the poles (1710, 1720) of the first member (1766)
to the poles (1740, 1750) of the second member (1767) and vice
versa. The coupling element is effectively "transparent" to the
magnetic field. The coupling element (1730) is of a material having
a high magnetic permeability, for example soft iron. The coupling
element (1730) should also have as high as possible electrical
resistance, so as to reduce induced eddy currents and the losses
due to resistive heating associated therewith. Although a single
coupling element (1730) is shown for clarity, several coupling
elements are arranged between the first and second members (1766,
1767). Sufficient coupling members are present, so as to span at
least two north-south poles pairs of the member (1766, 1767) having
the widest spaced apart poles (1710, 1720, 1740, 1750). The space
between coupling elements has a much lower magnetic permeability
than the coupling elements, an example material is plastic. When
arranged thus, in use, magnetic flux is coupled via each coupling
element (1730) from the poles of each member (1766, 1767) and
thereby torque is coupled between the first and second members
(1766, 1767). Notably, in use, contrary to FIG. 17a in which the
first and second members contra rotate, the members of FIG. 17b
rotate in the same direction. The surfaces of the first and second
members of FIG. 17b actually pass in opposite directions relative
to each other.
[0104] When the coupling of FIG. 17b is incorporated in a vacuum
enclosed flywheel application, the coupling elements (1730) are
incorporated in the vacuum chamber (1550) wall. This has the
advantage that the vacuum chamber wall thickness does not
contribute to the total "air gap" between the poles of the first
and second members (1766, 1767). The total "air gap" is made up of
the gap between the surface of the first member poles and the
surface of the vacuum chamber wall, plus the vacuum chamber wall
thickness, plus the gap between the vacuum chamber wall and the
second member poles, minus the thickness of the coupling element.
Thus, the coupling element significantly reduces the total air gap.
A smaller air gap presents less resistance to magnetic flux,
thereby allowing a greater flux density between the poles of the
first and second members in use, and therefore a greater torque
coupling capability. This is highly advantageous over conventional
arrangements using magnetic couplings through a vacuum chamber
wall.
[0105] The magnetic poles (1710, 1720, 1740, 1750) are rare earth
magnets, since these exhibit high field densities for a given
volume of magnetic material. The magnets are smaller lighter, more
compact, and able to transmit greater torque. Rare earth magnets
have also been found to be good at withstanding compressive forces
and are therefore suitable for placing on the inner circumference
of a flywheel which rotates at high speed.
[0106] Referring now to FIG. 18a, a concentric embodiment of the
magnetic coupling shown in FIG. 17b is illustrated. FIG. 18a is a
cross-sectional view showing the first member (1766) concentrically
outside the second member (1767), and the vacuum housing (1550)
concentrically therebetween. Incorporated in the vacuum housing
(1550) are the coupling elements (1730). In this concentric
embodiment, the first and second members contra-rotate. In common
with the embodiment in FIG. 17b, the surfaces of the members move
in opposite directions relative to each other.
[0107] The number of coupling elements required, for an evenly
spaced distribution around the circumference of the vacuum housing
between the first and second members, is equal to the number of
north/south pole pairs of the first member (1766) added to the
number of north/south pole pairs of the second member (1767). The
coupling elements can be confined to particular regions around the
circumference of the vacuum housing, or can be distributed evenly
around its circumference. In the case where the coupling elements
are confined to particular regions, the coupling elements (1730)
are spaced with respect to each other as if the full number of
coupling elements were equally spaced around the vacuum chamber
wall, except that some elements are omitted. The positioning is
ideally chosen such that coupling elements are positioned
symmetrically around the vacuum chamber wall circumference, so as
to avoid net forces resulting. A minimum number of coupling
elements required is that which will span two pairs of north/south
pole pairs of whichever of the first and second members have the
greater pole spacing. This minimum number guarantees that torque
can be transferred between the members and that the relative
directions of rotation of the first and second member is well
defined.
[0108] Backing iron (1890) is arranged on the side of the poles
facing away from the coupling elements so as to aid the
transmission of magnetic flux between the mutual pole pairs of each
one of the first and second members. Further, the backing iron aids
the longevity of the permanent magnets.
[0109] Such a concentric magnetic geared coupling can be
constructed using standard machining techniques and using the
materials as described for the embodiment shown in FIG. 17b.
[0110] The first and second members (1766, 1767) can have the same
number of north/south pole pairs, or can have a different number of
north/south pairs. In the shown embodiment, the second member has a
lower number of north/south pole pairs than the first member. In
operation, when the first member (1766), having a number of
north/south pole pairs m, is rotated in a anticlockwise direction,
the second member (1767), having a number of north/south pole pairs
n, rotates in a clockwise direction. The second member rotates at a
speed relative to the rotational speed of, the first member
multiplied by a factor: n divided by m. FIG. 18b shows the lines of
magnetic flux (1880) which pass between the poles of the first and
second members, via the coupling elements (1730) which are embedded
in the vacuum chamber wall (1550).
[0111] FIGS. 19a to 19c show a sequence of a rotation of the first
and second members through three positions. FIG. 19a shows the
lines of flux between the poles of the first and second members in
a first position. FIG. 19b shows the top member having rotated
slightly in a clockwise direction, and the bottom member having
rotated slightly in an anti clockwise direction. The lines of flux
have accordingly moved position, and in particular a line of flux
(1880) has stretched. FIG. 19c shows a further rotation of the top
member in a clockwise direction and of the lower member in an anti
clockwise direction. The line of flux (1880) has now stretched so
far that it has broken and flux has switched to pass instead via
the left most coupling element (1895) to form a new line of flux
(1890). The torque transferred from the first member to the second
member is equal to the rate of change of flux as the lines of flux
switch from one route to another route in this way.
[0112] A further advantage from the use of rare earth magnets
results from their high flux density per unit size, particularly
when used in this way, since it is possible to arrange a large
number of pole pairs around the circumference of the first and/or
second members and thereby increase the rate of change of flux and
thereby increase the torque coupling capability.
[0113] Also, due to the relatively small size of rare earth magnets
for a given strength, it possible to have a large ratio between the
number of pole pairs on the first member and the number of pole
pairs on the second member, since many magnets can be packed into a
small size thereby delivering a high gear ratio in a compact size.
This has the advantage particularly in flywheel applications
employing a vacuum chamber (1550) in that the driveshaft and
associated components which run in air are able to be run at a
lower speed, thereby reducing losses associated with windage and
air resistance, while the flywheel inside the vacuum chamber (1550)
is geared by the magnetic coupling to run at a higher speed, so as
to increase the energy storage density of the flywheel.
[0114] Existing systems employ a gear box to allow the flywheel
inside the vacuum chamber to rotate at a high speed while the drive
shaft to the energy source/sync is able to rotate in air at a slow
angular velocity. However, gear boxes suffer frictional losses and
increase the cost, complexity and size of the energy storage
system.
[0115] FIG. 20 shows an embodiment of a flywheel (30) having a rim
(containing the majority of the mass) (50), mounted on a shaft
(60), coupled to a first element (1766) and housed within a vacuum
chamber (1550), the vacuum chamber incorporating coupling elements
(1730). In this embodiment, the drive shaft (1570) is coupled to
the second element (1767). The driveshaft and the flywheel shaft
are supported on bearings (2010). Each of the first and second
members (1766, 1767) has poles (1710, 1720, 1740, 1750). Thus, the
flywheel can be driven in a vacuum at high speed by the driveshaft
which is coupled to the flywheel via the first and second members
and the magnetic poles. Due to the gearing effect brought about by
the unequal numbers of pole pairs on the first and second members,
the drive shaft, which runs in air, is able to run at a lower speed
thereby reducing "windage", or air resistance related losses.
[0116] Furthermore, the coupling elements (1730) reduce the air gap
between the magnetic poles and enable permanent magnets to be used
to couple a high level of torque between the first and second
elements, avoiding the need for an energy conversion, as would be
required for example if electromagnets were used. By using the
coupling elements (1730) electromagnets are not required since the
more efficient arrangement allows the more limited field strength
of permanent magnets to be sufficient.
[0117] According to the approach described, the use of rotating
seals is completely eliminated, thereby eliminating the need for
environmental management apparatus to maintain the vacuum inside
the vacuum chamber (1550). The vacuum inside the vacuum chamber can
remain there indefinitely since the chamber is completely sealed,
using no rotating seals which can leak. The removal of the
associated environmental management equipment (for example a vacuum
pump, lubrication pump, associated pipe work and systems, control
systems/electronics) further reduces the flywheel storage system
weight and size and increases the energy storage density.
Furthermore, reliability of this simpler system is accordingly
improved and cost is reduced. Thereby a highly efficient flywheel
energy storage device is provided.
[0118] The coupling also has the advantage that if an over-torque
condition occurs, the coupling harmlessly slips while the
over-torque condition exists, and then later resumes normal
function with no adverse effects. Furthermore, due to Enshaw's Law,
only torsional energy is transferred via the coupling, therefore
the coupling gives axial and radial isolation in respect of
vibration. In an alternative embodiment, the coupling elements
could be supported in a third member which is driven by a shaft or
could drive a shaft, so as to provide further gearing ratios.
[0119] The removal of rotating seals also allows the flywheel to
rotate at a faster speed than would otherwise be possible due to
degradation rates of the seals (which become worse as rotation
speed increases), further increasing the energy storage density.
Parasitic losses due to shear in the seal lubrication fluid (which
is a necessary feature of rotating seals) will also be reduced by
removal of the seals.
[0120] As previously discussed, and referring to FIG. 17b, magnetic
gears can exhibit a variable torque coupling capability with
rotational meshing position of the first and second members (1766,
1767). This has been found to be a result of magnetic flux (as
shown in FIG. 19c) switching from a first path (1880) to a second
path (1890) as the first and second members (1766, 1767) move past
each other. A further cause of variation in the torque coupling
capability of a magnetic gear coupling is due to the varying
magnetic flux path lengths (shown in the sequence of FIGS. 19a to
19c) as the first and second members (1766, 1767) move past each
other. A longer magnetic flux pass experiences greater magnetic
reluctance, thereby reducing magnetic flux density and, as the
torque is proportional to the rate of change of flux, reducing the
torque coupling capability of the magnetic gear at that angular
meshing position is likewise reduced.
[0121] Following now to FIG. 26, the variation of torque coupling
capability for a particular physical implementation with respect to
the angle of an input shaft can be seen as the curve which exhibits
large exclusions of torque coupling capability (between
approximately 20 Nm and 50 Nm).
[0122] It has been found that variation of torque coupling
capability with meshing angle (or "cogging") can be reduced by
splitting each magnetic pole of a member, into "split parts" (2110,
2111, 2112, 2113, 2120, 2121, 2122, 2123). The split parts are
arranged in the direction of motion so as to form split arrays. The
split arrays are arranged side by side along an axis orthogonal to
the direction of motion, as shown in FIG. 21. Each split array is
offset in the direction of motion with respect to another split
array, such that a spread of relative positions is covered. The
spread of positions should cover approximately at least the
distance of a north-south pole pair of the member having the widest
pole spacing. Since the relative positions of the split arrays are
spread (or "staggered") over a range of positions, it is not
possible for a pole of each of the split arrays to each
simultaneously align completely with a coupling element and with a
pole of the other member, thus "complete alignment" is prevented.
Thereby, by splitting and staggering poles of one or both members,
and/or by splitting the coupling element and staggering the
positions of each split coupling element part, complete alignment
of the members and/or the couple elements can be prevented.
[0123] In the embodiment shown in FIG. 21, there are four split
arrays on the first member (1766). The result of this arrangement,
which prevents complete alignment, is that, referring back to FIGS.
19a to 19c, the position (an angular position in this embodiment)
at which flux lines (1880, 1890) switch from one coupling element
to another coupling element, or from one split pole to another
split pole, varies between each split array. If, as in the
embodiment shown in FIG. 21, there are four split arrays, and those
split arrays are offset in the direction of motion so as to prevent
complete alignment of the poles and coupling elements (rotationally
offset in this embodiment), then for a small movement (that would
otherwise have caused a transition in the whole field if complete
alignment was allowed) there will now be only one fraction of the
flux shown switching (one quarter in this embodiment). However, in
this embodiment there will be four times as many such transitions
for a particular movement distance of the assembly (e.g. a full
rotation). The torque transfer for that movement is thus in total
the same, but is delivered more continuously leading to lower
"cogging". For clarity, only a single coupling member is shown in
FIG. 17b. As shown in FIG. 21, this coupling member can also be
split into coupling parts (2130) to (2133). Splitting the coupling
member in this way reduces the interaction between the split arrays
of the members, but is not necessary for a reduction in "cogging"
to be achieved.
[0124] Referring now to FIG. 22, an epicyclic embodiment of the
invention is shown, this time having a first member with a first
array of magnetic poles (in pairs) arranged on it in the direction
of motion. A coupling element (2130) is arranged concentrically
between the first (rotatable) member and a second (rotatable)
member. The coupling element is also split, into multiple coupling
parts (again, three coupling parts in this embodiment). A single
coupling element is shown for clarity but a plurality of coupling
elements is employed, forming a barrel concentrically around the
first member. The second member is arranged concentrically outside
the first member and the coupling elements. The second member has
on its inner circumference a second array of magnetic poles, in
north-south pairs, arranged in the direction of motion. The second
array of poles is split into multiple split arrays (three split
arrays in this embodiment), arranged side by side along the axis of
rotation (which is orthogonal to the direction of motion). In use,
the first and second members contra-rotate. If one member is
rotated, magnetic flux couples between the poles of the first array
and the poles of the second array, through the coupling element,
and the other member is caused to contra-rotate and vice-versa.
[0125] It will also be appreciated from FIGS. 21 and 22 that,
instead of, or in addition to the splitting of first and/or second
pole arrays (into split arrays) along their axial length (the axis
is orthogonal to the direction of relative motion), and the
offsetting in the direction of motion of each split array, each
coupling element (2130) can optionally, alternatively or also, be
split into coupling parts along its axial length (2130, 2131, 2132)
as shown, and these coupling parts can accordingly also be offset.
One, or a combination of these features can be incorporated so as
to diversify the positions at which magnetic flux switches from one
path to another path as shown in FIG. 19c. This strategy may be
referred to as staggering the poles, or staggering the coupling
elements. Staggering the poles and/or coupling elements results in
a reduction of the variation of torque coupling capability when
plotted against position. This is shown in FIG. 26 as the curve
which exhibits a relatively small variation of torque coupling
capability, (around 25 to 35 Nm). This represents an improvement in
performance over conventional magnetic gear couplings, for the
following reasons.
[0126] The minimum torque coupling capability of the improved
magnetic gear is greater and does not fall below 25 Nm, shown in
FIG. 26. (In contrast, the prior art magnetic gear torque coupling
capability falls at some angular meshing positions to a figure of
less than 20 Nm). Accordingly, for a given design torque
capability, the size of the magnets used in the improved magnetic
gear can accordingly be reduced in size while still delivering the
torque coupling capability. The reduction in variation of torque
coupling capability thereby allows such an improved magnetic gear
to be designed with smaller, lighter and cheaper magnets.
[0127] A further advantage of the improved magnetic gear described
herein is that since the torque coupling capability has less
variation, in use, when a torque is applied to the improved
magnetic gear coupling, the resultant angular offset or "slippage"
(being proportional to the torque applied and the torque coupling
capability), is more constant than that which would result in a
prior art magnetic gear coupling. Thereby, torsional vibrations
caused by this variation are reduced. The reduced torsional
vibrations are less likely to cause severe resonance which might
damage components, require component strength to be uprated with
associated cost implications, or cause the coupling to slip out of
mesh and lose alignment.
[0128] A further embodiment is shown in FIG. 23 whereby the
coupling element (2330) follows a sinusoidal path along an axis
orthogonal to the direction of movement of the first and second
members (in this embodiment, along the axis of rotation of first
and second members) such that its position in the direction of
motion of the first and second members varies along the axis. The
shape of the coupling element is symmetrical between its ends,
along the axis so as to balance the axial forces resulting and
thereby cancel them. Thereby, the position at which magnetic lines
of flux switch position, as shown in FIG. 19c, varies with axial
position. Again, only a single coupling element (2330) is shown in
figure for clarity. However, multiple coupling elements will
normally be employed as described earlier.
[0129] Furthermore, although FIGS. 18a to 23 generally show
rotating embodiments, with the first and second members either
alongside each other or concentric with each other, as shown in
FIG. 24, an end-on alignment of first and second members is also
possible. In such an end-on embodiment, the coupling element (2430)
can either be curved, or can be split into parts (2431) to (2436)
which are staggered, and the coupling element and/or the poles of
the first and second members can also be split, this time rather
than being split along the axis of rotation, they are split in a
radial direction.
[0130] Furthermore referring to FIGS. 22 and 23, one or both of the
first and second members (1766, 1767) could be unrolled so as to
form a planar surface. Such an embodiment would resemble a rack and
pinion, or a pair of tracks slidable over each other, with the
coupling element being disposed therebetween. In such embodiments,
the first and/or second members and/or the coupling elements would
be staggered in a direction which is orthogonal to the direction of
movement and parallel to a surface between the members.
[0131] FIG. 25 shows a cross-sectional view of a practical
embodiment wherein a drive-shaft (1570) is coupled to the second
member (1767) which has magnetic poles (1740) arranged in an array
around its circumference. The poles of the second member (1740) are
split in a direction parallel to the axis of rotation (orthogonal
to the direction of rotation) into multiple split parts (eight in
this embodiment). This results in multiple split arrays of split
poles arranged on the second member. In this embodiment there are
eight split arrays, each arranged circumferentially around the
second member, and each split array side by side along the axis of
rotation. Each split array is positionally offset with respect to
another. In this embodiment, each split array is rotated with
respect to the other slightly so as to span a spread of angles at
least equal to the distance between north-south pairs of poles of
the member. Backing iron pieces (2580) are arranged concentrically
between the second member (1767) and the magnetic split poles
(1740).
[0132] Concentrically outside the second member and its pole arrays
is a wall of a vacuum chamber (1550) which also extends around the
axial ends of the device, thereby forming a toroidal shape, and
incorporates in its inner circumferential wall coupling elements
(2130, 2131). This allows very efficient packaging of the flywheel,
ease of manufacture and sealing. The coupling elements are located
concentrically between the first and second members and are
arranged in an array forming a barrel concentrically around the
second member and inside the first member. Each of the coupling
elements are also split along their axial length to form multiple
coupling parts per coupling element (eight coupling parts per
coupling element in this embodiment). Thus, the barrel formed by
the coupling elements is split into rings and each ring is
preferably offset (rotationally offset in this embodiment) from
another of the rings. Alternatively, the coupling elements, instead
of being split, can be shaped so as to still lie within the barrel
of the inner vacuum chamber wall, but to vary their position in the
direction of motion as the length of the barrel is traversed, for
example in a chevron or sinewave pattern.
[0133] Concentrically further outside, the poles (2110, 2111) of
the first member (1766) are supported inside a composite flywheel
rim (50) with backing iron pieces (2590) interposed between the rim
and the poles. Again, each of the poles (2110, 2111) of the first
member are split along the axial length of the device into multiple
split poles per pole (eight in this embodiment). The split poles
are arranged around the internal surface of the rim (50) to form
split arrays (eight in this embodiment). The first and second
members (1766, 1767) are supported on bearings (2010) such that
they are able to rotate. The first member (1766) is thereby able to
rotate inside the vacuum chamber (1550), and a second member is
able to rotate concentrically inside the vacuum chamber but outside
of the vacuum (e.g. in air), and separated from the first member by
the vacuum chamber wall (1550).
[0134] In use the vacuum chamber preferably contains a hard vacuum.
Although not shown, the first and second members have different
numbers of north/south pole pairs arranged radially around them,
such that a gear ratio results between them. In use, this allows
the second member (1767) (which operates in air) to rotate at a
relatively lower speed than the first member (1766) which is
operated in the vacuum. Thereby, losses associated with air
resistance (or windage) when the second member rotates are reduced
Also, the use of supersonic speeds for the first member and
flywheel compontents is enabled by use of a vacuum to house the
flywheel, since supersonic shock and frictional overheating are
avoided.
[0135] The vacuum chamber (1550) has no rotating seals and is
therefore able to be completely sealed without leakage (which is
unavoidable when rotating seals are used, and is worse at higher
rotational speeds), thereby obviating the need for equipment
associated with maintaining the vacuum, such as a vacuum pump,
control electronics, pipe work etc. Removal of rotating seals also
allows higher rotational flywheel speeds, and lower losses due to
elimination of drag. Thereby the energy storage density of the
flywheel is increased and the associated cost of such a flywheel is
reduced. Reliability is also improved due to the increased
simplicity of this arrangement, and due to the elimination of
rotating seals which wear in use (and wear especially rapidly at
high rotational speeds).
[0136] Furthermore, the "anti-cogging" features incorporated in
this embodiment, as previously described, allow the use of smaller
permanent magnets (due to the minimum torque coupling capability
being closer to the mean torque coupling capability) with
associated advantages of lower cost and weight, thereby increasing
the energy storage density of the flywheel. Smaller magnets also
enable a higher gearing ratio to be produced since a greater number
of north/south pole pairs can be packed into a flywheel of a given
size. This higher gearing ratio further reduces losses associated
with air resistance or windage, on the air side of the device,
further increasing efficiency of the flywheel and its energy
storage density. A further advantage of the anti-cogging features
previously described is an improvement in noise vibration and
harshness, and extended service life of components due to the
reduction in torsional vibration brought about by these features.
This will also allow components to be re-specified so as to use
cheaper material, or less material, thereby bringing about cost
and/or weight advantages. Manufacturing efficiencies may also be
gained from the ability to use materials which would not have
withstood torsional vibrations, but which are easier to machine or
process during manufacturing.
[0137] It will be seen that as a result of the features described
above, a stronger safer, lighter, more efficient and more effective
flywheel can be provided for energy storage.
* * * * *