ROTORS

Abstract: A rotor manufactured from a fibre composite material wherein the modulus of the fibre composite material decreases progressively from the outside of the rotor to the inside of the rotor. More specifically, the modulus of the fibre composite material is decreased by increasing the number of fibres in a tow of the material which are broken. As a result, radial strain across a rotor formed from fibre composite material can be controlled during manufacture to improve the performance of the rotor, especially when used in an energy storage and conversion device.

Description: ROTORS This invention relates to rotors, and in particular to rotors constructed from fibre reinforced composite material. 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 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 ω 2 (1) where I is the moment of inertia of the flywheel and ω 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 1 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 WO 95/02269, WO 95/02271 and WO 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 delaπinating 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 mismatch) 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 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-109422 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. According to the present invention, there is provided a rotor manufactured from a fibre composite material wherein the modulus of the fibre composite material decreases progressively from the outside of the rotor to the inside of the rotor. As a result of the present invention, a rotor is provided wherein radial strain mismatch is significantly reduced, if not eliminated. The rotor can, therefore, be rotated at much higher speeds without delamination occurring. A significant increase in stored energy capacity can therefore occur. Preferably the modulus of the fibre composite material is decreased by increasing the number of fibres in a tow of the material which are broken. It should be appreciated, however, that the fibres on the outside of the rotor are preferably substantially unbroken, thereby providing the rotor with an extremely strong external surface. Ideally the fibre composite material forms a thin wall of the rotor. More preferably, the fibre composite material forms a hollow cylinder of the rotor, the rotor comprising solely the cylinder. In particular embodiments of the present invention, the fibre composite material may comprise carbon fibres, glass fibres or a combination of both. Other suitable fibres may of course alternatively be used. The present invention further provides 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 cf 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 used 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 (σ) /strain (e)) to density ( P ) , and 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 manner 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 for 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 during winding. As a result of 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 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.

Claims: CLAIMS 1. A rotor manufactured from a fibre composite material wherein the modulus of the fibre composite material decreases progressively from the outside of the rotor to the inside of the rotor. 2. A rotor as claimed in claim 1, wherein the modulus of the fibre composite material is decreased by increasing the number of fibres in a tow of the material which are broken. 3. A rotor as claimed in any preceding claim, wnerem the fibres on the outside of the rotor are substantially unbroken. 4. A rotor as claimed in any preceding claim, wherein the fibre composite material forms a thin wall. 5. A rotor as claimed in any preceding claim, wherein the fibre composite material forms a hollow cylinder. 6. A rotor as claimed in any preceding claim, wherein the fibre composite material comprises careen fibre, glass fibre or a combination of both. 7. A rotor as claimed in any preceding claim, wherein the fibre composite material comprises a resin for holding the fibres together. 8. A rotor according to claim 1 substantially as hereinbefore described. 9. An energy storage and conversion device comprising a rotor as claimed in any preceding claim and a stator mounted within the rotor for driving the rotor to store energy as kinetic energy of the rotor. 10. A method of producing a rotor from a 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. 11. A method as claimed in claim 10, wherein the fibres are broken after step (b) and before step (c) . 12. A method as claimed in claim 10 or claim 11, wherein the fibres are broken by cutting or by striking. 13. A method as claimed in any one of claims 10 to 12, wherein the modulus of the fibre composite material decreases progressively from the outside of the rotor to the inside of the rotor. 14. A method as claimed in any one of claim 10 to 13, wherein the fibres on the outside of the rotor are unbroken. 15. A method as claimed in any one of claims 10 to 14, wherein the fibres are carbon fibres, glass fibres or both. 16. A method as claimed in any one of claims 10 to 15, wherein a spreading device acts on the tow during winding to cause the broken fibres to spread in different directions. 17. A method of producing a rotor from a fibre composite material according to claim 10, substantially as hereinbefore described.

Inventors: TARRANT COLIN DAVID, TARRANT, COLIN, DAVID
Applicants: BRITISH NUCLEAR FUELS PLC, TARRANT COLIN DAVID, TARRANT, COLIN, DAVID
Application year: 1996
Main classification: F16F15
IPC codes: B29C70, B29D31, B29K105, F16C15, F16F15