Note: Descriptions are shown in the official language in which they were submitted.
CA 0221180~ 1997-07-29
WO96123146 PCT/GB96/00131
ROTORS
A This invention relates to rotors, and in particular to
rotors constructed from fibre reinforced composite material.
5 More particularly, the invention is applicable to a rotor
for use in an energy storage and conversion device.
Flywheels for energy storage and conversion devices may
be constructed from a variety of materials. Traditionally,
however, flywheels have consisted of heavy wheels rotating
l0 at relatively slow speeds. The heavier the flywheel, the
more energy can be stored. This is because the energy
stored in the flywheel is given by the equation
Energy = ~ I ~ (l)
where I is the moment of inertia of the flywheel and
w is the angular velocity of the flywheel.
Hence, for a given angular velocity, the energy stored is
proportional to the moment of inertia, and thus the mass,
of the flywheel.
Flywheels constructed of traditional materials have two
main disadvantages, namely their weight and their large
volume. If, however, the angular velocity of the flywheel
can be increased rather than its weight, a much greater
energy storage capacity is achieved because, for a given
mass, the energy storage capacity is proportional to the
square of the angular velocity (cf. equation l above).
Unfortunately, the maximum angular velocity of a flywheel is
limited by the strength of the material from which it is
made.
In the light of the foregoing, the best materials for
maximising specific energy and energy density are those with
the highest strength to weight ratio. Hence, glass or
carbon fibres can be used to produce excellent flywheels for
energy storage and conversion devices. An energy storage
and conversion device employing such a flywheel, in the
shape of a cylindrical/tubular shaped rotor, is described in
the applicant's earlier patent applications, numbers
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W O96/23146 PCTIGB96/00131
W0 95/02269, WO 95/02271 and W0 95/02270.
In such applications, the glass or carbon fibres are
wound in a resin binding material helically or in hoops to
give a composite construction having appropriate mechanical
properties. Although the rotor of the applicant's energy
storage and conversion device is substantially
cylindrical/tubular in shape, it should be understood that
the invention of this patent application can be applied to
any shape of flywheel/rotor.
As will be appreciated, in any flywheel/rotor there is
a difference in surface velocity between the inner and outer
parts or surfaces of the rotor. Thus, as the forces due to
rotation are proportional to the surface speed squared and
inversely proportional to the distance from the center of
rotation of the rotor, the hoop strain induced in the layers
of the flywheel vary significantly across the section of the
flywheel. This variation induces a radial strain into the
composite material of the flywheel which tries to pull apart
the layers of the composite, thereby resulting in a
delaminating force. The delaminating strain is, however,
significantly reduced if the rotor is formed as a thin
walled cylinder with a hollow tubular section.
As mentioned above, higher energy storage capacity is
achieved by adding greater mass to a rotor, which means
increasing the wall thickness. This in turn increases the
strain differential across the wall with the consequences
outlined above. If no measures are taken to reduce the
radial strain differential (or r,ismatch) across the wall
thickness, the overall radial strain must be reduced by
running the rotor at a lower speed, thus reducing the energy
storage capacity.
Various methods of reducing radial strain mismatch have
been described in the prior art. For example, the rotor can
be constructed from concentric cylinders with interference
fits, as disclosed in paper by D.M. Ries (FARE Inc) and J.A.
Kirk (University of Maryland) from the proceedings of the
27th Intersociety Energy Conversion Engineering Conference
P.4.43-4.48, Vol 4, published 1992, or by winding concentric
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cylinders directly one onto another as disclosed in GB-
1534393. Further, a paper entitled "Feasibility Assessment
of Electromechanical Batteries for Electric Vehicles",
reference number UCRL-ID-1094Z2 dated May 1992 by Lawrence
Livermore National Laboratory, U.S.A., discloses the idea of
having a series of concentric cylinders separated by pliable
layers to reduce radial strain. Solutions such as these
provide only an averaging of the radial strain, not a
complete elimination of strain mismatch.
WO 86/03268 discloses the possibility of progressively
varying the winding tension of fibres during manufacture of
a rotor to reduce/manage the hoop strain variation induced
by rotation of the rotor. Further, NL 9002415 teaches the
use of adding high density powder to the fibre reinforced
composite matrix progressively during winding of the rotor
to achieve a similar result.
Although the prior art discloses various ways of
avoiding/reducing strain mismatch across a rotor of an
energy storage and conversion device, none of the prior art
arrangements are ideal. Hence, the present invention has
been made to improve upon the known prior art.
AME~D~O ~
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According to the present invention, there is provided
a method of producing a rotor from a fibre composite
material comprising the steps of
(a) providing a tow of fibres;
(b) applying a resin to the tow; and
(c) winding the tow onto a mandrel to produce a rotor;
wherein at least some of the fibres of the tow are broken
during manufacture to vary the modulus of the fibre
composite material.
Preferably the fibres are broken after step (b) and
before step (c). The fibres may be broken by cutting or
simply by giving the fibres a sharp tap or strike.
The modulus of the fibre composite material preferably
decreases progressively from the outside of the rotor to the
inside of the rotor. Further, the fibres on the outside of
the rotor are preferably unbroken.
The fibres usea in the fibre composite material may be
carbon fibres or glass fibres. Other suitable fibre
materials may, alternatively, be used.
A spreading device may act on the tow during winding to
cause the broken fibres to spread in different directions.
As a result of this, a form of "matting" effect may be
produced around the broken fibres which results in a
lowering of the modulus of the fibre material.
An explanation as to the reasoning behind the
development of the present invention and a specific method
of manufacturing a rotor as herein claimed will now be
described in detail.
The parameters that determine the strain behaviour of
a rotor are the composite specific modulus (i.e. the ratio
of modulus E (E = stress (a)/strain (~)) to density (p), and
~Jl~L~ r~
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WO96/23146 PCT/GB96/00131
the usable strain range of the material. The specific
strength of the material, i.e. the ratio of strength to
density, gives an indication of how a composite fibre
material will resist the centrifugal forces due to the
weight of the composite material as the rotor rotates.
Reducing the specific strength by reducing the strain range
does not benefit the radial strain problem. However,
reducing the specific strength whilst maintaining the
overall strain range (i.e. reducing the modulus of the
material), does benefit the radial strain distribution.
Thus, in order to reduce the radial strain induced by
the differences in hoop strain, a method of reducing the
effective hoop modulus of the layers in a controlled ~.anner
is presented.
The modulus of a fibre composite material in a multi-
ply-lay-up is determined by the angle of the fibres (or
filaments) relative to the direction of the applied force
and the number of fibres. By introducing a procedure
immediately prior to lay-up (i.e. winding) which cuts a
proportion of the fibres and spreads them so that their
effective axes of lay-up are out of alignment to the bulk of
the fibres, the effective modulus of the material produced
is reduced. Hence, by varying the number of fibres treated
in this manner from layer to layer, the effect is to
generate, from one source of material, a rotor with a
modulus which varies across its wall thickness (or section).
Further, the modulus of the material can be arranged
virtually to eliminate this source of radial strain
mismatch.
With the foregoing in mind, an apparatus and method for
putting the present invention into practice are as follows.
~ Firstly, a guide is provided to position a tow of carbon or
glass fibres accurately in the apparatus. Means are
provided ror applying a resin, such as an epoxy resin, to
the fibre tow. A blade or chopping element is then provided
for chopping the tow at regular intervals defined by a
metering or regulating device which regulates the frequency
of the chopping operations duriny winding. As a result of
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W O96123146 P~ 96100131
the chopping operation, a pre-determined percentage of the
fibres in the tow are broken; the complete tow is not cut
through, since this would cause the winding operation to
fail. Further, the cut length defined by the metering
element is never less than the "pull out" length of the
fibres for the particular fibre and resin system involved.
A "pull out" length is defined as the length of fibre in
which the sheer strength of the bond between the fibre and
the resin is equal to the strength of the fibre.
once the chopping step has been completed, the tow is
applied to a mandrel or other support which is steadily
rotated to receive the fibres in a chosen fashion of helical
and hoop windings. As the tow is applied to the rotor, a
spreading device bears up against the tow. Although the
uncut fibres of the tow lie in the winding direction, the
cut fibres of the tow will be re-aligned by the spreading
device to lie in a random manner, some ends of the cut
fibres being perpendicular to the uncut fibres.
As will be appreciated, the modulus of the fibre
composite material will be dependent upon the number and
frequency of the chopped fibres wound onto the rotor.
Hence, the metering or regulating element needs only to be
controlled to provide a winding, and hence a rotor, having
exactly the desired modulus. An improved rotor can,
therefore, be produced.
During initial winding of the rotor, the inner layer
provided on the mandrel will include tows that are chopped
at frequent intervals to produce a "matted" lay of fibres
with many fibres oriented randomly in the resin matrix.
This will produce a fibre composite material having a very
low modulus. As winding progresses, the chopping intervals
are gradually reduced until the outer layers of the rotor
are reached, where no chopping of tows is undertaken and the
tows are laid undamaged onto the rotor. These outer layers
will provide the rotor with a significant degree of
strength.
By using the method of the present invention, the
resulting rotor is arranged to have a substantially constant
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WO 96/23146 PCr/~D5Gt00131
radial strain loading across the thickness of the rotor wall
during running of the rotor at high speed. As a result, no
layer separation will occur. The rotor radial strength is
also greatly improved by the random layering of fibres in
the inner region.
Finally, as is well known, carbon and glass fibres are
extremely strong in tension along their length yet very weak
when loaded from the side. Indeed, a slight shock load
against the side of a fibre tow can cause individual fibres
to break. Hence, the chopping device used to break the
fibres of a tow as described above could be replaced by a
simple device which strikes the side of the tow to break the
required number of fibres. Physical cutting of the fibres
is not, therefore, required.
As will be appreciated, a rotor according to the present
invention is extremely strong and robust in comparison with
the prior art, and can be operated up to very significant
angular velocities. The actual angular velocities
achievable are limited only by the strength of the material
from which the rotor is made, and not by limitations caused
by internal strain mismatch in the rotor.
Although, as mentioned above, a rotor according to the
present invention is suitable for use in many different
applications, it is particularly suitable for use in an
energy storage and conversion device of the type being
developed by the present applicant. More particularly, the
energy storage and conversion device comprises a stator
mounted within~ a cylindrical rotor, the stator being
energised to drive the rotor about the stator to store
energy as kinetic energy of the rotor, and the stator and
rotor in combination being able to act as a generator to
release energy from the rotor via the stator as electrical
energy.
It will of course be understood that the present
invention has been described above purely by way of example,
and that modifications of detail can be made within the
scope of the invention.
