IMPROVEMENTS IN AND RELATING TO MAGNETIC BEARINGS

Abstract: The invention provides improved magnetic bearings intended to function successfully at elevated temperatures, particularly temperatures in excess of 600 DEG C. The invention provides the magnetic bearing in the form of a number of decoupled magnetic circuits with an increased slot area, and winding level, for the upper magnetic circuit so as to provide the upward force and regulating force. The extra windings available allow the level of losses sustained to be reduced as the biasing windings can be successfully operated at a current density approximately half that of the regulation windings.

Description: IMPROVEMENTS IN AND RELATING TO MAGNETIC BEARINGS This invention concerns improvements in and relating to magnetic bearings, particularly, but not exclusively, for high temperature applications. Magnetic bearings have a number of advantages over other bearing types. In particular they possess long service life, zero starting friction, low losses and most importantly do not require lubricants. These advantages are particularly apparent in high speed and / or high temperature environments. Magnetic bearings can also serve other functions, such as a variable damper unit. A magnetic bearing operated in this mode is provided to enable the effective stiffness of the shaft to be varied rather than to support its weight. The shaft stiffness may be varied, or even switched essentially instantaneously, thus skipping through critical speeds in the system. To date magnetic bearing systems capable of operating upto 450 °C only have been produced. Magnetic bearings capable of operating at higher temperatures are desirable for a number of applications, including; gas turbine bearings for power generation; and aircraft engines to allow for direct power generation from the jet so as to avoid mechanical links such as belts/chains which have a lower service life. Such a magnetic bearing will, however, need to operate successfully at a temperature in excess of 600 °C and preferably at greater than 800 °C. No such a system has yet been provided. High temperatures have a number of significant effects on the behaviour and performance of magnetic bearings and their associated components. The majority of magnetic materials, such as iron based ones, cross their Curie temperature (the point at which the material becomes non-magnetic) well below the desired performance envelope. Most iron based alloys have Curie temperatures in the range 650-700°C and so can only operate at 550-600°C as their limit. Additionally, even below the Curie temperature the saturation flux for the magnetic material tends to tail off; the value at room temperature may be 150% the value at 450°C. The resistivity of the materials used for the windings also increases considerably with temperature. As a consequence lower current densities must be applied. Otherwise the resistance heating, coupled with the ambient environment, causes the windings to approach their melting point, 1050 °C for Cu, giving rise to structural failure of the windings. Similar factors apply to the heating of the magnetic material itself as this too is close to the limit, its Curie temperature. The lower current density has significant implications in the level of force which can be obtained and as a consequence on the physical size of the unit. Two principal forces are involved in the system. Firstly there is the control/position force which is necessary to regulate the position of the shaft during use. Additionally, if the bearing is to support the weight, a relatively constant biasing force must be applied to the shaft to counteract its mass. The first force may be commonly double the latter, although the ratio may vary significantly between different applications. A far higher regulation force is employed in high speed applications or uses where the bearing is mobile, for instance aerospace applications. Conventional bearings comprise a plurality of teeth around the perimeter of the shaft, provided in as simple a structure as possible. The uppermost teeth provide the biasing force and also contribute to the regulation. These upper teeth are operated to their maximum performance within the environmental constraints in prior art systems. The other teeth are provided in a symmetrical arrangement and as a consequence are substantially under used as they are only involved in regulation. According to a first aspect of the invention we provide a magnetic bearing for a shaft comprising a plurality of poles of soft magnetic material, together with electrically conductive windings, at least one of the pole pairs being provided decoupled from the other pole pairs. According to a second aspect of the invention we provide a magnetic bearing for a shaft comprising a plurality of poles of soft magnetic material, together with an electrically conductive winding, at least one of the pole pairs being asymmetrical relative to the others. Other potential features for the invention include :- Preferably all the pole pairs are decoupled from one another. Preferably four pole pairs are provided. Preferably one or more of the pole pairs comprises a central pole together with two side poles. Preferably the side and central poles are connected by a bridging element. The central pole maybe provided as a linear element. Preferably the gap between the side and central poles reduces towards the shaft. Preferably the gap has a substantially constant width portion and a tapering portion. Preferably the tapering portion is closer to the shaft that the constant width portion. The reduced gap may be defined by a portion of the side pole angled towards the central pole. The constant gap portion may be defined by a portion of the side pole substantially parallel to the central pole. Preferably the gap between the shaft and opposing surface of the poles is less than 10mm, for instance between 0.1 and 1.0mm. Preferably the gap between the poles of a pair is less than 5mm, for instance between 0.5 and 10mm at its minimum. The gap may be at least 5 times the gap between shaft and opposing surface of the poles. Preferably the gap between poles of adjoining pairs is equivalent to that between poles of a pair, for instance 0.5 to 5mm at its minimum. Preferably the bridging portion is substantially perpendicular to the portion of the central pole and/or the portion of the side poles connected to it. The side and central poles may be moveable relative to one another in a manner to increase the minimum gap between them. This may be effected by detaching the side poles from the central pole and/or bridging portion. Alternatively other movement maybe facilitated, such as a sliding increase in separation or rotation of one component relative to another or other mode. Preferably the windings are provided on the central pole in prewound form. The windings may support themselves or be provided on a carrier/support. Alternatively the windings maybe wound directly onto the pole or a support provided thereon. Preferably the biasing pole pair, most preferably the upper pole pair, is provided with a larger slot area than one or more of the other pairs. Preferably the slot area is greater than the slot area of any of the other pole pairs. The biasing slot area maybe up to 500% that of the other slot areas. Ratios of 4:1 or less, more preferably 3:1 or less and even 2:1 or less bias regulation coil area are envisaged. The increased slot area may arise as a result of a greater gap between the central and side poles and/or because of a greater radial extent of the poles. The uppermost pole may extend further axially than one or more, and most preferably all, of the other pole pairs. The axial increase in extent may be at least 30%, at least 40%, at least 50% or even up to 300% that of the other pole pairs. Preferably the magnetic bearing functions at temperatures in excess of 650°C, more preferably in excess of 700°C and ideally in excess of 800°C or even 900°C. Even at ambient or moderately elevated temperatures the invention provides significant advantages, for instance in terms of reduced overall weight. The soft magnetic material may comprise or be cobalt. Preferably a cobalt alloy is employed. 20% or greater, 45% or greater and even 95% cobalt alloys maybe employed. A cobalt content of between 20 and 35% is preferred. The cobalt may be alloyed with one or more of iron, chromium, vanadium or tantalum Ta. The soft magnetic material may comprise iron, iron alloys, iron silica alloys, iron nickel alloys and other such materials, particular where ambient or moderately elevated temperatures are concerned. The windings may be of copper, silver or alloys thereof, such as beryllium copper. The principal conducting material of the winding maybe coated with a further layer for support, such as by stainless steel or ceramic. An insulating layer of ceramic or braid may be provided, which layer may also provide support. The windings may be provided with varnish, plastics, polyester or other insulating materials, particularly where ambient or moderately elevated temperatures only are encountered. The level of windings provided may equate to a packing factor of at least 20% and preferably of at least 50% or even 70% or greater. According to a third aspect of the invention we provide a method of bearing or damping a shaft comprising a system according to the first or second aspect of the invention. According to a fourth aspect of the invention we provide a method of bearing or damping a shaft comprising a plurality of poles of soft magnetic material, together with electrically conductive windings, wherein one of the pole pairs is provided with a lower current density than the others. Preferably the lower current density is applied to the biasing and / or upper pole pair. According to a fifth aspect of the invention we provide electrical power provided by or arising from a magnetic bearing according to the first and / or second aspect of the invention and/or using a method according to the third and / or fourth aspect of the invention. Various embodiments o the invention will now be described, by way of example only, and with reference to the accompanying drawings in which : - Figure 1 shows a prior art bearing assembly; Figure 2 shows a bearing assembly according to a first embodiment of the present invention; Figure 3a shows a side view of a pole pair in a second embodiment of the present invention; Figure 3b to 3d show partial views of further embodiments of poles; and Figure 4 shows a comparison of a pole pair between a prior art bearing system and a bearing system according to the present invention. Typical radial magnetic bearings according to the prior art, designed to operate well below 500°C, follow the simple configuration illustrated in Figure 1. The bearing provides four pairs of poles, 2,4,6,8, respectively in an integral structure. The pairs each consist of two teeth 10 wound with series connected coils, the coils being provided in opposite senses. The four pairs provide control in both the vertical and horizontal axes for the shaft 12. As the upper most pair of teeth 2 must provide both the upward force and a regulating force it is operated to its full capacity. However, the other three pairs 4, 6, 8 need only provide the regulating force. As a result they are substantially under used. The system contains excess weight and volume as a result. The under utilisation of the other three pairs 4, 6, 8 is even more extreme as these coils are only contributing for a small proportion of the time, as and when regulatory force is required. The upward force on the other hand is required continuously. These wasteful characteristics become even more significant at high temperatures. The embodiment of the invention illustrated in Figure 2 provides four decoupled magnetic circuits 20,22,24,26 enclosing a shaft 12. The circuits each consist of a central tooth 28 and two side teeth 30. In the case of the uppermost circuit 20 the teeth are 50% longer in the radial direction than in the other circuits. The teeth may also extend further in an axial direction than the other poles. This additional slot area is used to accommodate the additional windings needed to provide the biasing force. The extra windings available allow the level of losses sustained to be reduced as the biasing windings can successfully be operated at a current density approximately halve that of the regulation windings. The soft magnetic materials suitable are restricted by the Curie temperature which needs to ideally be significantly higher than the operating temperature. Cobalt, with the highest known Curie temperature 1121°C, and its alloys are particularly suited. Suitable materials include: 24% Co, 75.25% Fe, 0.75% Cr; 49%Co, , 49% Fe, 2%V; 49% Co, 50.65% Fe, 0.35% Ta; 95% Co, 5% Fe. Telcon Limited offer one such suitable material under the trade mark Totelloy 5. For lower temperature applications iron alloys, such as silicon iron laminates may be employed. The circuits in this embodiment are wound with coated wires threaded through the gaps. A variety of potential winding materials and constructions are possible. Pure copper wires can be coated in an alumina ceramic dried and cured. The wires may be wound dry and heat cured in their wound state. The ceramic itself can provide the necessary insulation or a woven braid may be introduced around the wire. Alternatively the copper wires can be coated in stainless steel and / or nickel with an outer insulating layer also provided. More conventional insulation can be provided in ambient or more moderate temperature applications. Whilst this embodiment is described in relation to the magnetic bearing supporting the weight of the shaft 12, the term magnetic bearing also encompasses situations where the magnetic force merely provides a control function to the shaft, for instance damping in a regulatory way. Support for the weight in such situations may be provided by conventional mechanical bearing systems and / or prior art magnetic bearings. The second embodiment of the invention illustrated in Figure 3 provides the central tooth 32 and side teeth 34 of each circuit as separate components. This enables the windings for the teeth to be prewound onto a carrier or bobbin and then inserted over the central tooth 32. The side teeth 34 can then be fastened to the respective sides of the central tooth 32 to complete the circuit. Figure 3b, 3c and 3d illustrate further alternatives for mounting the central tooth or a support therefore, directly to the side teeth or to a support therefore. Figure 3b provides a side tooth 34 with two laterally extending arm portions 50 between which a single arm portion 52 on a support element 54 to which the central tooth (not shown) can be attached, is inserted. Figure 3c illustrates a central tooth 32 provided with a protrusion 58 at its base. This protrusion 58 is designed to be accommodated within a similarly dimensioned gap on the support or on an element extending from the side tooth (not shown) with pins being inserted into aperture 60 to fix the tooth in location. A further possibility is illustrated in Figure 3d where two side protrusions 62 are provided on the central tooth 32, with a protrusion on the support (not shown) extending into the gap therebetween. Once again pins can be introduced into apertures 64 to fasten the system in its assembled state. This option enables the advantages, such as higher packing factors, of pre-winding to be obtained. Pre-winding is not feasible with some systems as the gaps between the teeth preclude the insertion of sufficient windings. The side teeth and central tooth are connected together by suitable key surfaces and fasteners. A comparison of one pole of equivalent coupled 36 and de-coupled 38 bearings with the same pole face area 40 the same reluctance and slot area is provided in Figure 4. The slot area being defined in a radial plane relative to the shaft axis. The two systems have essentially the same force capability with the same copper losses as each other. The magnetic circuit in both cases is dimensioned to operate at a maximum flux density of 1.2T, which is approximately the limiting flux density imposed by considerations of saturation at 900 °C on the soft magnetic material. The coupled arrangement 36 has a mass some 1.75 times that of the de-coupled bearing 38. When account is taken of the fact that the other three poles of a de-coupled bearing can be considerably smaller since they only accommodate the position regulation windings, relative weight advantage is even more pronounced. In addition to the reduction in the weight the slot shape in the de-coupled topology is likely to make the winding easier than in the coupled topology. This enables a higher copper packing factor to be realised with a consequent reduction in the copper loss of the bearings. Losses are higher normally at elevated temperatures due to the increased resistivity of the windings. A proportion of the disadvantage in terms of weight of the coupled bearing can be attributed to the fact that the back iron between the four poles is essentially redundant in terms of its role within the overall magnetic circuit. The present invention enables the slot area to be further reduced by appreciating that the lower duty time from the regulation circuits, 4, 6, 8, enables them to be operated at a higher current density than the continuously operated upper circuit 2 without problems. Thus a two source reduction in mass is obtained firstly due to the asymmetrical pole configuration and secondly due to the differing operating current densities used between the biasing and regulating functions. A higher force level is also obtained in the present invention by increasing the pole area of the top bearing. The force is directly linked to this area as the saturation flux density for the soft magnetic material is the limiting factor. The effective pole area is not only reduced due to the lower gaps which can be obtained between adjacent teeth, but also by extending the upper circuit 2 axially, relative to the shaft. Axial extension is in some respects more effective than increasing the circumferential extent of the top bearing as the vertical component of the force decreases with the wire of the angle relative to the vertical. A 50% increase in axial extent is beneficial and gives a static to dynamic force capability of 1:2 without under utilisation.

Claims: CLAIMS : 1. A magnetic bearing for a shaft comprising a plurality of poles of soft magnetic material, together with electrically conductive windings, at least one of the pole pairs being provided decoupled from the other pole pairs . 2. A magnetic bearing for a shaft, preferably according to claim 1, comprising a plurality of poles of soft magnetic material, together with an electrically conductive winding, at least one of the pole pairs being asymmetrically relative to the others . 3. A magnetic bearing according to claim 1 or claim 2 in which the magnetic bearing functions at temperatures in excess of 700°C. . A magnetic bearing according to any preceding claim in which one or more of the pole pairs comprises a central pole together with two side poles. 5. A magnetic bearing according to any preceding claim in which the side and central poles are moveable relative to one another in a manner to increase the minimum gap between them. 6. A magnetic bearing according to any preceding claim in which the biasing pole pair, most preferably the upper pole pair, is provided with a larger slot area than one or more of the other pairs . 7. A magnetic bearing according to claim 4 or any claim depending thereon in which the gap between the side and central poles has a substantially constant width portion and a tapering portion, the tapering portion being closer to the shaft that the constant width portion. 8. A magnetic bearing according to any preceding claim in which the windings are provided on the central pole in prewound form. 9. A magnetic bearing according to any preceding claim in which the uppermost pole pair is provided with a larger slot area than the slot area of any of the other pole pairs. 10. A magnetic bearing according to any preceding claim in which the uppermost pole extends further axially than one or more of the other pole pairs . 11. A method of bearing a shaft or of damping a shaft comprising one or more magnetic bearings for the shaft, one or more of the magnetic bearings comprising a plurality of poles of soft magnetic material, together with electrically conducting windings, at least one of the pole pairs being provided decoupled from the other pole pairs. 12. A method of bearing a shaft comprising a plurality of poles of soft magnetic material, together with electrically conductive windings, wherein one of the pole pairs is provided with a lower current density than the others. 13. A method according to claim 10 in which the lower current density is applied to the biasing and / or upper pole pair.

Inventors: HOWE DAVID, JEWELL GERAINT WYN, HOWE, DAVID, JEWELL, GERAINT, WYN
Applicants: BRITISH NUCLEAR FUELS PLC, HOWE DAVID, JEWELL GERAINT WYN, HOWE, DAVID, JEWELL, GERAINT, WYN
Application year: 1997
Main classification: F16C39
IPC codes: F16C32, F16C39