GB2388479A

GB2388479A - Permanent magnet rotor assembly suitable for mounting on a turbine shaft

Info

Publication number: GB2388479A
Application number: GB0209655A
Authority: GB
Inventors: David Ainsworth; Martyn Harris; Alexander Kenworthy Molyneaux; Ronald William Woolley
Original assignee: Bowman Power Systems Ltd
Current assignee: Bowman Power Systems Ltd
Priority date: 2002-04-26
Filing date: 2002-04-26
Publication date: 2003-11-12
Anticipated expiration: 2022-04-26
Also published as: GB0209655D0; WO2003092140A3; GB2388479B; AU2003227883A1; WO2003092140A2

Abstract

A rotor construction is disclosed for an electromagnetic machine. The machine may be a turbo-alternator in which the rotors of a compressor, a turbine and an alternator are mounted for rotation in unison on a common shaft (fig. 12). The rotor 102 has a central shaft 100, magnet poles 114 and in-fills 108 located between the magnet poles 114. Each magnet pole 114 is made up of a series of samarium-cobalt magnet segments extending axially along the rotor. The in-fills 108 are of solid or rigid material, e.g. a ceramic such as alumina silicate. The magnet poles 114 and in-fills 108 are bonded to the central shaft 100 and are retained in position with a carbon fibre retention sleeve 130. The in-fill material has good matching of Young's modulus and coefficient of thermal expansion to the magnet segments, and thus the sleeve is not adversely stressed at high speed. Apparatus for positioning the sleeve on the rotor body is also disclosed. The sleeve is expanded by hydraulic pressure and located on the rotor body. The hydraulic pressure is then removed.

Description

( ROTORS FOR ELECTROMAGNETIC MACHINES

The present invention relates to rotors for electromagnetic machines, such as to rotors for alternators or motors. The invention also relates to 5 electromagnetic machines, such as generators and motors, incorporating such rotors. The invention also relates to turbo alternators incorporating such rotors, such as to turbo alternators comprising a gas turbine in which a rotor of an alternator is connected to and spins at the same speed as a turbine stage of the gas turbine.

10 The present invention also relates to a method of assembling a rotor for an electromagnetic machine, such as a motor, or a generator e.g. for a turbo-alternator, such as a turbo-alternator in which rotors of an alternator, compressor and turbine are mounted for rotation in unison on a common shaft. The invention also relates to a rotor assembly for an electromagnetic 15 machine such as a motor or generator.

It is known to provide a rotor for an electromagnetic machine having a central rectangular shaft, with four or six loaf shaped magnet poles, one magnet pole being located on each side surface of the rectangular central shaft. The magnets of the magnet poles are held in place by a cylindrical 20 sleeve. The circumferential gaps between the adjacent magnets are filled with epoxy resin. Unfortunately, the known prior art rotors are subject to

mechanical stress problems in the sleeve. These problems are particularly evident in rotors with loaf pole structures and, in particular, with four pole and six pole structures with high side walls, since there is a substantial angle 25 subtended by the space between the magnets at the inner diameter of the sleeve. When that angle exceeds about 10 , as a guideline, the sleeve is subject to an undesirable non-uniform stress distribution, particularly

( sigruficant sheer stress between layers of the sleeve. This predisposes the sleeve to failure during service. The non-uniform stress may be introduced during application of the sleeve to the rotor body by any of the known methods of application and also by temperature change during operation.

5 The subtended angle is typically also large with six pole and tends to be a reducing problem with higher pole numbers, but there may still be similar problems. Furthermore, it is known to apply a retention sleeve to a rotor body including radial field magnets by winding the component under tension on to

10 the magneted rotor body, so that, when the rotor is spun in use at high speed, the magnets are retained in place. The wound material of the sleeve has to be applied at high tension and the process is complicated and expensive. The number of specialist winding companies is severely limited and the capital cost of the equipment for the process is high.

15 GB2222301 and GB2278504 relate to axial field rotors in which

numerous short axial length rotor parts are assembled on a single shaft.

Each rotor part is made by fitting a short axial length hoop on to a magnet/shaft assembly using a hydraulic expansion method. Due to the structure of the axial field rotor' the hydraulic expansion method must be

20 carried out numerous times for a completed rotor assembly incorporating several rotor parts each having a retention hoop. One would assume this technology would not assist in the construction of radial field magnets which

can consist of one long cylindrical body, since it is unclear how the hoop application device described in GB227g504 could be used to apply several 25 short axial length hoops on the long body of a radial field rotor.

Furthermore, these two publications appear to disclose the requirement for an aluminium hoop other spacer between the magnet segments and the outer

retention hoop, said aluminium hoop or other spacer adding to the mass of the rotor and increasing dynamic forces undesirably.

The present invention claims to alleviate at least some problems of the prior art.

5 Various aspects of the invention are set out in independent claims 1 to 3. Various preferred features are set out in the dependent claims.

A substantial advantage of the infill is that by matching the Youngs modulus and/or thermal coefficient of expansion of instill material and magnet material the sleeve remains more nearly circular as it is being wound 10 on or otherwise fitted to the rotor and also during thermal cycling of the rotor as part of normal operation. Minimum discontinuity of the sleeve profile over the infill region is maintained so that the occurrence of shear stress in the sleeve material is minimised. Thereby, the sleeve can be applied over infills of greater subtended angle without risk of sleeve failure 15 in service.

It is believed that the prior art aluminium hoop disclosed in

GB222203 1 may have the effect of reducing stress in and/or other deleterious effects of localised deformation of the retaining hoop thereof In the present invention, the nature of the infills has substantial stress reduction 20 advantages and may provide these advantages without the need for an aluminium layer adding weight to the rotor. However, such a layer may be employed if especially desired in a particular application. However, in some applications an electrically conducting screen (e.g. below the sleeve), preferably of the aluminium may be included for the purpose of screening 25 the magnet segments and hub/shaft steel from high frequency harmonic magnetic fields and thereby minimising eddy current power loss in the rotor

parts.

( According to another aspect of the present invention there is provided a rotor for an electromagnetic machine comprising a central shaft, at least two magnet poles adjacent the shaft, and an infill located between the magnet poles, wherein the infill is of a solid material.

5 Preferably, the instill is of rigid material. Preferably, the infill is of non-viscous material.

According to another aspect of the present invention there is provided a rotor for an electromagnetic machine comprising a central shaft, and at least two magnet poles adjacent the shaft, and an infill located between the 10 magnet poles, wherein the infill is of a rigid material. Preferably, the instill is of a solid material.

A number of further preferred, optional features will now be described. Preferably, a said infill fills a gap located between two adjacent said IS magnet poles in a circumferential direction.

Preferably, each said infill is of ceramic material. The use of ceramic infill material instead of the known practice of using epoxy is considered highly advantageous and inventive. An advantage of using ceramics is that they can have low density, thus resulting in lower centrifugal forces at high 20 revolution speeds and being substantially non-magnetic and having negligible electrical conductivity. Furthermore, the Youngs modulus and linear coefficient of thermal expansion can be relatively closely matched to that of magnet material in the rotor. Therefore, stresses in any surrounding retention sleeve applied to the rotor may be maintained relatively uniform at 25 all times.

In a number of preferred embodiments, each infill is of alumina silicate. Each infill may be of rescor 902, available from Cotronics

s Corporation, but a more tailored ceramic may be used.

Preferably, each infill has a Youngs modulus of more than lOOGpa more preferably more than 120 GPa. Each infill may be of material having a Youngs modulus of about 200Gpa. Preferably, the Youngs modulus is 5 within +15%,+20%,+75%or +80%,e.g. within 1O, 5 or 2% of that of the magnet material or the magnet poles. Preferably, the Youngs modulus of each infill is more than 60% of the Youngs modulus of the magnetic material, preferably more than 75 % thereof. Each infill preferably has substantially the same Youngs modulus as that of the magnet material.

10 Preferably, each infill is of a material having a linear coefficient of expansion of about 10-5 per Kelvin. Preferably the infill linear coefficient of expansion is within 20%, more preferably 15%, e.g. within 10, 5 or 2% of that of the material of the magnet poles. Preferably, the linear coefficient of expansion of each infill is less than 400% of the value of this coefficient for 15 the magnet material. Preferably each infill has this coefficient at more than 8.5% of the value of this coefficient for the magnet material, preferably being within +50%thereof, more preferably substantially the same thereas.

Preferably, each infill is of a material having an electrical resistivity greater than 104 Ohm metros.

20 Preferably, the density of each infill is less than 4.5 g/cc, for example about 2.5 g/cc. The low density means that the stress components in the sleeve required to overcome centrifugal force contributed by the infill is relatively low.

Preferably, each infill is of a material adapted to maintain 25 substantially constant physical characteristics set up to at least 160 Celsius, preferably up to at least about 180 Celsius. Thus, while the rotor is operating at about 160 o r 180 Celsius, the infill does not deform.

( Preferably, each infill is of material having a compressive strength greater than 200MPa.

Preferably, each infill is of grindable material.

Preferably, the central shaft has a middle portion thereof adjacent the 5 magnet poles, the middle portion having a regular polygonal section, and a said magnet pole is provided with a flat surface thereof adjacent a flat surface of said polygonal section. In this case, for example, said middle portion of the central shaft may have a square section with four said magnet poles provided, one on each of four flat surfaces of the middle portion.

10 However, other numbers of magnet poles are envisaged, such as six or eight or more.

The principle of the invention is applicable with advantage to loaf shaped magnets with all possible pole numbers and is particularly advantageous with pole numbers four and six where the angles subtended by IS the infills tend to be greatest. It is also applicable to arc shaped magnets of all possible pole numbers.

Preferably, each said magnet pole comprising a series of magnet segments arranged extending in a mutually axially ordered configuration along the rotor body. The material of the magnet poles may be samarium 20 cobalt. Preferably, each said magnet pole and each said magnet segment is arranged to provide a radial field.

Preferably, each said magnet pole has a loaf shaped cross section (in the case of the central shaft having a square (or e.g. hexagonal), polygonal or other section having flat magnet mounting surfaces), said cross section 25 consisting of a flat base surface located adjacent the central shaft, respective flat side surfaces being substantially perpendicular to the base surface, and a part-circularly cylindrical outer surface, the respective part circularly

( cylindrical surfaces of said magnet poles lying on a common imaginary cylindrical surface.

However, the central shaft may have a circular cross section and each magnet pole may have a part-ring-like or arc-like cross section with said 5 infills being located between adjacent ones of the magnet pole spaced about the central shaft each said infill having an arc-like cross section.

Preferably, a said infill is provided between each two adjacent said magnet poles. Each said infill may, for example in the case of a four pole loaf structure, have a generally quadrant shaped section. Each said infill 10 may have a section comprising two mutually perpendicular surfaces, each being located adjacent a said magnet pole. The infills may be sextant shaped when six magnet poles are provided on a hexagonal shaft.

Optionally, the rotor has a central axis and each infill (before or after an optional grinding step once mounted with the magnet poles) has edges in 15 the region of the circumference of the rotor, and (viewed along the axis)an imaginary angle formed between two imaginary lines between the axis and each said edge subtends at an angle of more than 10 . Said subtended angle, e.g. in a four pole design, may be about 20 or more than 20O, for example more than 30 or about 35 to 40 , 18.7 and 27.7 being two 20 examples. In a six pole design, the subtended angle may, for example, be about 15 to 20 .

Each said infill may, after preferred grinding/finishing steps, have a curved surface, the curved surface having a radius substantially equal to that of a curved surface of a magnet pole adjacent said infill.

25 Preferably, a plurality of said magnet poles and infills are provided around said central shaft, radially outwardly facing surfaces of the magnet poles and infills forming a substantially cylindrical outer surface, preferably

before or after preferred grinding/finishing steps.

Preferably, the rotor includes a tubelike retention sleeve located around said magnet poles and infills. An inner surface of said sleeve may abut against said magnet poles and/or each said infill. The nature of said 5 infills may thus mean that there is in preferred embodiments no need for an aluminium hoop or other spacer, adding weight and increasing dynamic forces. Said retention sleeve may be formed of carbon fibre material.

Preferably, said substantially cylindrical outer surface is, due to the mechanical properties of the instills, in particular the close matching of the 10 Youngs modulus and/or coefficient of thermal expansion thereof to that of the magnet poles, is closely retained when said sleeve is applied.

Each said magnet pole may comprise a plurality of magnet segments, for example two or three magnet segments, extending axially along the rotor.

According to a further aspect of the present invention there is 15 provided a rotor for an electromagnetic machine comprising a central shaft having a square or hexagonal cross section, four or six magnet poles, each being located on one surface of said shaft, and an infill located between each adjacent two magnet poles, each infill having edges in the region of a circumference of the rotor, an angle (viewed along an axis of the rotor) 20 formed between two imaginary lines between the axis and each said edge subtending at an angle of equal to or greater than 10 .

Thus, in preferred embodiments in accordance with the invention, whereas the present inventors have worked out that sleeve failure problems were at least partly due to large differences in the mechanical properties of 25 Youngs modulus and thermal expansion coefficient between the infills (which were epoxy) and the magnet material, these properties may be more closely matched, for example by providing a ceramic material for the infills, so that

such significant stresses are not present in any retention sleeve either from the time of application, due to compression of the infill material during sleeve application, or during operation. The invention is particularly beneficial for four pole and six pole loaf structures, these being the usual S pole numbers and magnet configuration of choice for a large proportion of high speed machine designs up to perhaps 200Kw and maybe more. When a ceramic material is used for the infill, it may be described as being similar to those in the Cotronics range, for example, but an important point is that it is possible to tailor the mix and thus mechanical properties of the ceramic, 10 without unacceptable price penalty, so as to closely match those of the magnet material. A typical samarium-cobalt magnet material as a Youngs modulus of 200Gpa and has a linear coefficient of thermal expansion of 10 5/oC. These figures may be matched in the ceramic, in the preferred embodiments, to within, for example, + 15%, which is amply good to 15 provide little distortion of the sleeve during assembly of the sleeve on to the rotor or during the subsequent thermal cycling that occurs during operation.

By contrast, the present inventors have noted that epoxy is orders of magnitude out on both counts. At the same time, the strength of the ceramic material, when chosen, may be amply good enough to withstand the pressure 20 applied by the retention sleeve, which may be pre-stressed, and the density of the ceramic material may typically not be more than about 2.5gms/cubic centimetre5 which compares with about 8.3 for a typical samarium-cobalt magnet material. Preferably, the retention sleeve is pre-stressed to minimise the possibility of the magnet segments of the magnet poles "lifting off" and 25 moving radially outwards during rotation at high speed.

According to a further aspect of the invention there is provided an electromagnetic machine including a rotor as specified above. A further

( aspect of the invention provides a generator or motor including a rotor as specified above.

According to another aspect of the present invention, there is provided a method of assembling a rotor for an electromagnetic machine as set out in 5 claim 37. Further aspects of the invention are set out in claims 38, 39 and 40. The invention's features are highly advantageous since, in the preferred embodiment, the sleeve may be pre-formed by winding materials at low tension and the cost of the process may be substantially reduced. The magnets are not damaged during assembly and one sleeve may suffice for the 10 whole rotor. A single or unitary sleeve may be used for the whole rotor. A radial field rotor body, which may have a relatively long body may be

surrounded by a pre-forrned retention sleeve. The long body of the sleeve, which may bow if subjected to hydraulic pressure may be controlled during sleeve application if a monitoring step is employed.

15 The electromagnetic machine may comprise an alternator which may act as a generator (or motor). The alternator may comprise part of a turbo alternator in which rotors of the alternator and a compressor and turbine of the turbo-alternator are mounted for rotation together in unison on a common shaft. 20 The method may include pre-forming the retention sleeve as a carbon fibre sleeve. Other materials, such as kevlar or similar materials may be used in addition to or as alternatives to carbon fibre. The sleeve may be formed by winding fibre filaments or ribbon.

The method may include pre-forming the retention sleeve as a 25 circularly cylindrical hollow body.

The method may include providing the rotor body with a generally regular polygon-sectioned central shaft. The method may include providing

the central shaft with an internal bore.

The method may include locating at least one magnet pole (preferably comprising one or more magnet segments which may be axially aligned) on the central shaft, preferably on a generally flat surface thereof. The method 5 may include providing each said magnet segment with a generally flat surface adjacent the central shaft and a radially outwardly convex surface.

The method may include providing each said magnet pole with a generally loaf-shaped cross section. The magnet poles may have other sections such as loll chordal in which the side walls of the loaf are reduced to zero.

10 The method may include providing the central shaft with a square section and providing four magnet poles or a hexagonal section and six poles, each magnet pole being located on a respective flat side of the central shaft and comprising one or more said magnet segments extending axially along the rotor body.

1S The method may alternatively comprise providing the central shaft with a circular section and providing one or more arc-shaped magnet poles on the shaft.

The method may include assembling a plurality of magnet poles around a central shaft of the rotor body, each magnet pole having a part 20 circularly cylindrical surface, the part circularly cylindrical surfaces of the magnet poles lying on a common imaginary circularly cylindrical surface.

Preferably, each magnet pole is a radial field magnet pole.

The method may include providing an infill between each two adjacent magnet poles.

25 One method of applying the sleeve with a pre-stress is to wind on under tension a ribbon or tow of carbon fibre other material in filament form, the filaments being preferably pre-impregnated with a suitable matrix

( material which after curing binds the total matrix/filament structure.

In a preferred embodiment, the method includes expanding the pre formed sleeve during or before location of the sleeve onto the rotor body.

Preferably, the sleeve is expanded so as to place material thereof in tension 5 when expanded to an expanded state thereof.

The method may include providing the rotor body as a circularly cylindrical body with a first diameter and pre-forming the sleeve with an inner diameter of a second diameter smaller than the first diameter, and expanding the sleeve's inner diameter during or before the location of the 10 sleeve on the rotor body to a diameter equal to or greater than the first diameter. The method may include inserting the rotor body into the sleeve while the sleeve is in an expanded state.

Most preferably, the method includes expanding the sleeve under the 15 pressure of a fluid. The sleeve may be expanded under the pressure of a gaseous fluid. However, the sleeve is preferably expanded hydraulically such as with a hydraulic fluid, e.g. water, mineral oil or silicone fluid. The method preferably includes removing the pressure of the fluid once the sleeve is located on the rotor body.

20 It is also envisaged that relative expansion of the sleeve compared to the rotor body may be effected by heating and/or cooling effects applied appropriately, bearing in mind the expansion coefficients of the materials concerned to achieve in whole or in part the relative expansion.

Preferably, the location of the sleeve on the rotor body leaves material 25 of the sleeve under tension in order to provide a compression force on the rotor body. The tension in the sleeve may be in a circumferential direction around the sleeve and the compression force on the rotor body may be

applied to the rotor body by the sleeve in a radially inward direction. Thus, the sleeve may provide radially inward compression forces to hold the magnet poles in position when rotating at high speed.

Preferably, the sleeve is formed by winding a structural component 5 thereof on a cylindrical body having a diameter less than an outer diameter of the rotor body. This is highly advantageous since the structural component of the sleeve may be wound onto the cylindrical body at low tension with inexpensive equipment. The tension applied to the structural component during winding may be varied during the winding process. The 10 structural component, such as a filament or ribbon, may be wound in a pattern which is at least part helical. The sleeve may include some structural components extending in a generally longitudinal direction of the sleeve, in addition to other components thereof. The structural component may comprise a carbon fibre filament or carbon fibre ribbon.

15 The method may include machining, polishing or otherwise working the sleeve prior to location of the sleeve on the rotor body. The method may include machining, polishing or otherwise working a cylindrical inner surface of the sleeve before locating the sleeve on the rotor body. The method may include machining, polishing or otherwise working end surfaces 20 of the sleeve to length before locating the sleeve on the rotor body.

Preferably, a taper or chamfer may be provided on an interior entrance to the sleeve to assist in insertion of the rotor body into the sleeve.

Such are the advantages of some aspects of the invention that in a preferred embodiment a pre-formed carbon fibre sleeve may be expanded 25 under high pressure hydraulic force, for example, in a sealed container.

Once the sleeve has been expanded, an alternator rotor shaft or rotor body including attached magnet segments may be inserted into the sleeve. Once

( the rotor body including the rotor shaft is in a correct axial position, the hydraulic force may be relaxed, and the sleeve tension, e.g. carbon fibre sleeve tension, at the installed condition may be selected to be sufficient for holding the magnets on the rotor shaft without radial movement while the 5 shaft is rotating at a selected speed in use.

Retention sleeves, such as carbon fibre sleeves, can be wound under low tension to a nominal size, machined to the correct size and assembled hydraulically. Accordingly, sleeves can be procured from multiple sources as they do not require high technology equipment to obtain the required pre 10 stress when directly wound on the rotor body/shaft. The invention extends to the application of carbon fibre and non-carbon fibre magnet retention sleeves. Where the sleeve is expanded under force of a fluid, a lubricant may be employed on at least one surface of the tube or sleeve and the rotor body 15 for assisting in assembly, particularly when the method includes driving the sleeve over a mandrel, so that damage to the magnets during application is avoided. In some embodiments, hydraulic fluid, e.g. an oil, may also provide a lubricating function.

In preferred embodiments, the present invention allows pre-forrned 20 tubes or retention sleeves, such as carbon fibre tubes or retention sleeves, to be wound under a first or low tension and then applied to a rotor body (e.g. an assembled alternator shaft including at least one magnet) such that the tube when installed is under tension, which is preferably higher than the initially wound tension thereof, and thereby allows each said magnet to be 25 retained when rotating at high speed. In use, depending upon machine size, the rotor shaft may rotate at for example 15,000 to 500,000 RPM.

It is envisaged that when fluid pressure is used to expand the sleeve,

( pressure of about 5000 to 50,000 psig (psi gauge pressure) may be employed. Preferably more than 10,000 psig maybe employed, some examples being about 12,000 to 13,000, 20,000 and 25,000 psig being some examples.

5 According to further aspects of the invention there is provided a rotor assembly as set out in one or more of claims 74, 75 and 76. A further aspect of the invention provides an apparatus as set out in claim 94. Various optional features are set out in the dependent claims. The invention also envisages and extends to any combination of the optional features and aspects 10 thereof which is not specifically set out herein. In particular the concepts of claims 37 to 73 (assembly method) and/or claim 94 (apparatus) and their dependent claims may be applied when assembling the rotors and rotor assemblies of claim 1 to 35 and 74 to 93.

The present invention may be carried out in various ways and various 15 preferred embodiments of rotors in accordance with the invention and electromagnetic machines incorporating such rotors will now be described by way of example with reference to the accompanying drawings, in which; Figure 1 is an isometric view of a central shaft or hub of a rotor; Figure 2 is a side elevation of a preferred ceramic infill for the rotor, 20 being a view in the direction of arrow 2 in Figure 3, the length of the infill shown foreshortened for clarity; Figure 3 is an end view of the ceramic infill shown in Figure 2, being a view in direction 3 in Figure 2; Figure 4 is an isometric view of the ceramic infill shown in Figures 2 25 and 3, the length of the infill shown foreshortened for clarity; Figure 5 is an elevation of the infill of Figures 2 to 4, located with three others on the central shaft of Figure 1;

( Figure 6 is a section on the line 6-6 in Figure 5; Figure 7 is a section on the line 7-7 in Figure 5; Figure 8 is an elevation showing the components of Figures S to 7 once a retention sleeve has been applied; 5 Figure 9 is a section on the line 9-9 in Figure 8; Figure 10 is a section on the line 10-10 in Figure 8; Figure 11 is a section through a second preferred embodiment of a rotor in accordance with the present invention, having a somewhat different configuration to that shown in Figures 1 to10; 10 Figure 12 is a schematic view of a micro-turbine incorporating the rotor shown in Figures 1 to 10; Figures 13 and 14 show two further preferred embodiments of rotor central shaft, magnet pole and instill cross sections.

Figure 15 is a schematic part sectional end view of a preferred IS embodiment of a rotor assembly for an electromagnetic machine in accordance with the present invention, although this is somewhat schematic since the cross-dimension of the square hub will usually be larger compared to the outer radius of the magnets, such that the infills will be proportionately larger; 20 Figure 16 shows a section on the line III-III' in Figure 15; Figure 17 is a schematic view of a turbo- alternator including the assembled rotor; Figure 18 shows a second preferred apparatus and method for assembling rotor components the same as those shown in Figures l to 11, 25 13, 14 and 15; and Figure 19 shows a preferred sleeve of the rotor assembly shown in Figure 15 being formed on a cylindrical body.

( Figure 1 shows a square-sectioned central shaft 100 of a preferred embodiment of a rotor assembly 102 (see Figure 10) in accordance with the present invention. The central shaft 100 incorporates generally circular end flanges 104 and has a central through bore 106 for mounting the central shaft S 100 on an inner drive shaft 166.

Figures 2 to 4 show a ceramic infill 108, four of which are employed in the rotor. The infill is shown foreshortened in Figures 2 and 4 compared to typical embodiments. In one example the length of the infill is approximately 80mm and the height of the side wall approximately Smm.

10 As shown in Figure 3, the infill 108 has a generally quadrant shaped cross section, an outer surface 110 of the infill 108 having substantially the same radius as the radius of an outer surface 112 of a magnet pole 114 (Figure 6) of the rotor assembly 102. The ends of the infill are typically chamfered to remove sharp corners which might otherwise interfere with rounded internal 15 corners of the rotor hub or shaft. The infills may be made with a size larger than the magnet poles and once magnet poles and infills are attached to the shaft this assembly may be turned on a lathe. Each magnet pole 114 is made up of a series of samarium-cobalt magnet segments extending in line axially along the rotor, each magnet segment providing a radial field and having a

20 Youngs modulus of 200Gpa and a coefficient of linear thermal expansion of 10-5/ C and a density of about 8.3g/cc. As shown in Figure 3, the instill 108 also has two generally mutually perpendicular surfaces 116, each extending inwardly from the outer surface 110, towards a generally flat inner surface or chamfer 118. The chamfer 118 may provide clearance over any 25 residue of adhesive left by the bonding of the magnet segments to the hub or may be more substantial for clearing large upstanding hub edges, as shown in Figure 13. Each end 120 of the instill 108 is defined by two inwardly

( tapering surfaces 122 and a flat end surface 124. For the purposes of clarity, it will be appreciated that the infill shown in Figures 2 to 4 is somewhat shortened in length. Furthermore, before application to the central shaft 100, the ends 120 of the infill 108 are machined off.

5 The magnet segments are bonded to the central shaft using Ciba Geigy AV119. The four infills are then bonded to the shaft 100 and magnet poles 114 (which have a loaf cross section - see Figure 6) using Eccobond 104.

All voids and interstices between the magnet poles 114 and infills 108 are filled with Eccobond 104. Although the mechanical properties of Eccobond 10 104 do not closely match those of the magnet segments, this is not critical and, overall, mechanical properties are dominated by the infills. Importantly though, Eccobond 104 remains rigid up to the working temperature. The resultant outer surface defined by the magnet poles 114 and infills 108 is then ground to required dimensions.

15 It will be appreciated that each magnet pole 114 may consist of several magnet segments 115 extending axially along the rotor body, 3 being the number in each magnet pole in this embodiment. The assembled shaft 100, magnet poles 114 and infills 108 may then be ground to a circular shape. In practice, the infills 108 (which may preferably be of ceramic 20 material) may normally be made slightly larger than the finished diameter of the magnet poles and thus then may be finshed/ground in-situ, so that the final surface of the is determined by the grinding. Thus, the instill shape before grinding may to a certain extend be of secondary interest, only as far as it reduces grinding effort. The idylls 108 do not have to be very closely 25 mated to the surface of the magnets and this is an important application point. As long as a high temperature epoxy like Eccobond 104 is used that does not soften at the full working temperature of the magnets, then this

epoxy fills any gaps due to a poor fit and since the epoxy only occupies a small fraction of the whole volume to be filled in between the magnets, its lack of good matching mechanical properties is of little importance.

As shown in Figures 8 to 10, a retention sleeve 130 is then fitted over S the assembled shaft 100 magnet poles 114 and infills 108. The assembled rotor as shown in Figures 8 to 10 may then be balanced by removing material from the end flanges 104, as required.

The infills 108 are made of a tailored material, having electrical resistivity greater than 104 Ohm metres, a linear coefficient of thermal 10 expansion of 10 x 10-6 per Kelvin, a compressive strength of 38,000 psi (262 MPa) and a Youngs modulus of 200 GPa. Thus, the infill material, which is an alumina silicate is a relatively good insulator which is nonmetallic. The operating temperature is only up to about 180 C and the infills do not noticeably soften during use because they have a substantially higher service 1S temperature. Cotronics Rescor 902 may be used in some embodiments.

The infill material also has relatively low density, being less than 4g/cc namely about 2.5g/cc. Furthermore, the infill material is substantially non-magnetic. Thus, the infill material has a relatively good matching of Youngs modulus and coefficient of thermal expansion to the magnet 20 segments and then, the sleeve will not be adversely stressed by shape distortion with temperature and the low density allows high operation speed.

As shown in Figure 9 in this preferred embodiment, each infill 108 (it will be appreciated that there are four similar instills 108 and four similar magnets 114) subtends an angle of 20 when two imaginary lines are drawn 25 between circumferential edges 140 of the infill 108 and a central axis 142 of the rotor assembly 102.

Figure 11 shows another embodiment in which the subtended angle is

( 30 . In other embodiment s the subtended angle is 18.7 o r 27.7 .

In the schematic view of Figure 12, the rotor assembly 102 of Figures g to 10 is incorporated in a turbo alternator 150 of a micro-turbine 152. The micro-turbine has a gas fuel inlet 154 and an air inlet 156. The air inlet 156 5 feeds a compressor 158. The gas inlet feeds a combustor 160, optionally via a gas boost compressor 162. The compressor 158 has a compressor stage rotor 164 on a common shaft 166 with a turbine 168 and the rotor 102. The rotor 102 as mentioned before is part of a turbo alternator 150 and thus drives an electrical load 170 via a power module 172 which preferably 10 includes a rectifier 174, inverter 176 and filter 178. The power module 172, rotor 102, turbine 168, compressor 158, combustor 160 are located inside a rectangular transportable cabinet 180 which has feet 182 and is approximately 3m long by 2m wide by 2m high. The micro-turbine 152 may optionally heat water in a boiler 190 using the exhaust gases passing from the 15 turbine 168 to an exhaust stack 192, the micro-turbine 152 thus being a co generation micro-turbine, for the co-generation of electricity and heat.

Under maximum rated operation conditions, the rotor assembly 102 is adapted to drive the power module 172 at up to about 100 kWe.

Figure 13 shows how the central shaft 100 of the rotor assembly 102 20 may be formed with locating recesses 103/upstanding edges 105 which may extend part or all of the way along the central shaft 100, for locating the magnet segments 115 of the magnet poles 114 in position on the shaft 100.

The inner surface or chamfer 118 of each infill 108 therefore serves to clear the corresponding upstanding edge 105.

25 Figure 14 shows an embodiment in which the central shaft 100 of the rotor assembly 102 has a circularly cylindrical exterior surface 300. The shaft 100 is surrounded by four arc-sectioned magnet poles 302 consisting of

a number of axially aligned magnet segments 304, the four arc-shaped magnet poles 302 being spaced apart by four arc-shaped solid or ceramic infills 306.

The pre-assemblies shown in Figures 13 and 14 depict the rotor body 5 of each rotor, once ground to have a circularly cylindrical shape and before application of a tubelike carbon fibre retention sleeve like the sleeve 130 shown in Figures 9 and 10.

The sleeve, in all of the aforementioned embodiments may be applied by any known method, but is preferably applied as now described with 10 reference to Figures 15 to 19.

With reference to Figures 15 and 16, an assembled rotor assembly 10 in accordance with a preferred embodiment of the present invention includes a rotor body 12 comprising a steel central shaft 14 having two end flanges 16, each having a circularly cylindrical outer edge 18, and IS an elongate middle portion 20' having a square outer section 24 and a circularly cylindrical central bore 26. The square outer section 24 is defined by four generally flat surfaces 28'. Four loaf-shaped sectioned magnets 30 are located on the flat surfaces 28, regularly spaced around an imaginary central axis 32' of the rotor assembly. Each magnet 30 20 has a flat contact surface 34' located adjacent one of the flat surfaces 28, two side surfaces 36 and an outwardly convex domed surface 38. The convex domed surfaces 38' of the various magnets lie on a corrunon imaginary circularly cylindrical surface 40. Infills 42 of epoxy or other material are located between each two adjacent magnets 25 30. A carbon Able retention sleeve or tube 44 is located around the rotor body 12'. The retention sleeve 44' is located on the rotor body 12' under tension, such that the sleeve 44 provides radially inward

( compressive forces on the rotor body 12' such that the magnets 30' are retained in place while the rotor assembly 10' rotates in use at speeds of up to at least about 80,000 to 150,000 rpm.

The retention sleeve 44' includes an inner circularly cylindrical 5 surface 46' and an outer circularly cylindrical surface 48', as well as annular end surfaces 50' which are flat.

To assemble or manufacture the rotor assembly 10' shown in Figures 15 and 17, the magnets 30' are located on the central shaft 14' and may be attached thereto by adhesive or material with an adhesive 10 property. Then, the infills 42' are added to complete the rotor body.

Separately, the retention sleeve 44 is fanned by winding carbon fibre filament or ribbon elements on a circularly cylindrical body 52' (Figure 19), the diameter D1 of the cylindrical body 52 being less than the diameter D2 of the circularly cylindrical rotor body 12' formed by the 15 central shaft 14', magnets 30' and infills 42'. The sleeve 44' is wound on the circularly cylindrical body 52 at a first relatively low tension and may be removed from the body 52 once resin associated with the carbon fibre filament/ribbon components has suitably set.

The sleeve 44' may, once removed from the body 52', be 20 worked, for example, to effectively cut it to length, and/or to machine or polish or otherwise work the circularly cylindrical interior 56' and/or exterior 58 surfaces thereof, as well as the end surfaces 50 thereof.

End surfaces 50 may be formed with a chamfer 237 to assist with rotor body insertion. A long length of retention sleeve tube 44' may be formed 25 on the body 52 and then cut into shorter lengths in order to form a plurality of retention sleeves with only one winding step on the circularly cylindrical body 52'

In Figure 18, a pressure vessel 200 has a cylindrical internal bore 202 a measurement zone 204 of slightly larger diameter than the internal bore 202 and a lid 206. The lid 206 is sealed to a main body 208 of the vessel by a seal 210. The lid 206 is releasably attached to the main body 208 by bolts 5 (not shown), and includes an internal annular sealing ledge 212 having a seal 214. Spaced along the pressure vessel 200 from the annular ledge is a further seal 216 formed at the internal bore 202. The internal bore 202 has the same internal diameter as the annular ledge 212. The lid 206 has an inlet port 218. The pressure vessel 200 at an end thereof opposite the lid 206 10 incorporates a rotor insertion or positioning element 222 which may be axially driven by a screw-thread system or hydraulic ram or other suitable means axially along the pressure vessel 200. In use, the lid 206 is removed from the main body 208 and the rotor body 10 is inserted into the pressure vessel 200 mounted on a shaft 223 therefor. The positioning 15 element 222 is retracted to the position in which it is shown in Figure 18 and in which the rotor body 10 is shown in dotted lines. Then, the sleeve 44 is inserted into the pressure vessel 200 to the position shown in which end portions thereof seal against the seals 214,216, once the lid 206 has been secured in position. The pressure vessel 200 is then filled with fluid such as 20 water or mineral oil through the inlet 218. The fluid is then pressurised until the sleeve 44 takes up a somewhat expanded configuration inside the vessel 200. The positioning element 22 is then pushed forwards to insert the rotor body 10 into the sleeve 44. It will be noted that the bend regions of the sleeve 44 have internal chamfers 237 formed therein for easy 25 insertion of the rotor body 10'. While the pressure vessel is under pressure, the state of the sleeve 44' may be monitored using one or more displacement sensors 230 which may be inserted through various apertures

232 in the main body 208 of the pressure vessel 200 in the measurement zone 204 thereof, in order to assess the expansion of the sleeve. The sensors 230 may be used to provide signals for controlling a pressure regulator or other means for controlling the pressure inside the pressure vessel 200. It 5 will be noted that insertion of the positioning element 222 into the pressure vessel 200 will, as shown in Figure 18, reduce the volume of the pressure vessel and, as the positioning element 222 is inserted, a pressure regulator or relief valve (not shown) may allow some of the fluid to exit the vessel 200 through the inlet 218 in order to maintain suitable pressure. Alternatively, 10 or additionally, the inlet 218 in other embodiments may be placed off centre in the lid 206 or elsewhere on the pressure vessel 200 and the positioning element 222 may incorporate a rod extending all the way through the pressure vessel such that axial movement of the positioning element 222 does not change the internal volume of the pressure vessel 200.

15 Once the rotor body 10' is located in a suitable position. the pressure may be relaxed, the lid 206 removed and, then the rotor body 10' and sleeve 44' removed. The sleeve 44' may then be trimmed to length.

In other embodiments, instead of expanding the sleeve to a position controlled by actual measurement, i.e. by the sensor or sensors 230, the 20 pressure vessel may be configured such that the sleeve 44 may be expanded up to a mechanical travel limit. Restraining the sleeve in some way, e.g. by measurement/pressure control and/or a mechanical limit, prevents it from expanding and yielding.

Although water may be used as the expansion fluid since it is cheap 25 and usable, mineral oils would provide sliding and lubrication benefits.

As shown in Figure 17, once the sleeve has been applied the rotor assembly 10' is secured on a turbine shaft 102' (of a turbo-alternator

104' having a turbine 106' and compressor 108' with respective rotors 110',112' secured on the shaft 102') by tightening a nut 114' onto a threaded end 116' of the shaft 102' in order to tighten the rotor assembly 10' up against a flange 118' formed on the shaft 102 for rotationally 5 securing the rotor assembly 10' in place on the shaft 102. The turbo alternator includes a combustor 120' which combusts fuel delivered from a fuel source 122' in compressed air delivered by the compressor 108' from an air source 124'. The turbine exhaust flows to an exhaust stack 126'. The rotor assembly 10' forms part of an alternator 130' which 10 provides an electrical output to a power conditioning unit 134 which includes a rectifier 136', inverter 138' and filter 140' for providing a regulated three phase or other desired supply to a load 142' Although the embodiment described with reference to Figure 15 includes four magnet poles, it will be appreciated that the invention is 15 applicable to rotor assemblies with other numbers of poles. such as six or eight poles.

Although the fluid pressure and rotor body insertion may be controlled or actuated manually, it will be appreciated that the process (pressure control, axial rotor movement and four rotor/sleeve loading/unloading 20 into/from the vessel) could be automated.

It will be noted that in the embodiment of Figure 18, the sleeve is longer than the surfaces to be engaged on the rotor body and this is to allow for the adjacent part of sleeve to bow and expand while under fluid pressure since the ends of the sleeve 44' are sealed to the seals 214,216 and do not 25 appreciably expand.

Instead of expanding the sleeve 44' by positive hydraulic pressure, it is anticipated that expansion by vacuum pressure could be employed for

( some applications.

Various modifications may be made to the embodiments described, it will be appreciated, without departing from the scope of the invention as defined in the claims as interpreted under patent law.

Claims

1. A rotor for an electromagnetic machine comprising a central shaft, at least two magnet poles adjacent the shaft and an infill located between the 5 magnet poles, the Youngs modulus of the infill being more than 2.5% of the Youngs modulus of the material of the magnet poles.

2. A rotor for an electromagnetic machine comprising a central shaft, at least two magnet poles adjacent the shaft and an infill located between the 10 magnet poles, the Youngs modulus of the infill being more than 5 GPa.

3. A rotor for an electromagnetic machine comprising a central shaft, at least two magnet poles adjacent the shaft and an infill located between the magnet poles the linear coefficient of expansion of the infill being less than 15 400% of the linear coefficient of expansion of the material of the magnet poles. 4. A rotor for an electromagnetic machine comprising a central shaft, at least two magnet poles adjacent the shaft and an infill located between the 20 magnet poles, wherein the infill is of a solid material.

5. A rotor as claimed in claim 4 in which the infill is of rigid material.

6. A rotor for an electromagnetic machine comprising a central shaft, at 25 least two magnet poles adjacent the shaft and an infill located between the magnet poles, wherein the idyll is of a rigid material.

( 7. A rotor as claimed in claim 6 in which the infill is of solid material.

8. A rotor as claimed in any preceding claim in which a said infill fills a gap located between two adjacent said magnet poles in a circumferential S direction. 9. A rotor as claimed in any preceding claim in which each infill is of ceramic material.

10 10. A rotor as claimed in claim 6 in which each infill is of alumina silicate. 11. A rotor as claimed in any preceding claim in which each infill has a Youngs modulus of more than 120 GPa.

12. A rotor as claimed in claim 11 in which each infill has a Youngs modulus of about 200 Gpa.

13. A rotor as claimed in any preceding claim in which the Youngs 20 modulus of each infill is more than 85% and less than 115% of the Youngs modulus of the material of each magnet pole.

14. A rotor as claimed in any preceding claim in which each infill as a linear coefficient of expansion of about 1O-s per Kelvin.

15. A rotor as claimed in any preceding claim in which the linear coefficient of expansion of each infill is more than 85% and less than 115%

( of the linear coefficient of expansion of the material of each magnet pole.

16. A rotor as claimed in any preceding claim in which each infill is of material having electrical resistivity greater than 10 Ohm metres.

17. A rotor as claimed in any preceding claim in which each infill is of material adapted to maintain substantially constant physical characteristics up to at least about 160 C.

10 18. A rotor as claimed in any preceding claim in which each infill is of material having compressive strength greater than 200 MPa.

19. A rotor as claimed in any preceding claims in which the density of each infill is less than 4 g/cc, preferably being about 2.5 g/cc.

20. A rotor as claimed in any preceding claim in which each infill is of grindable material.

21. A rotor as claimed in any preceding claim in which the central shaft 20 has a middle portion thereof adjacent the magnet poles which has a substantially regular substantially polygonal section and a said magnet pole is provided with a flat surface thereof adjacent a flat surface of said polygonal section. 25 22. A rotor as claimed in claim 21 in which said middle portion of said central shaft has a square or hexagonal section and four or six said magnet poles are provided, one on each of the four or six flat surfaces of the middle

portion, respectively.

23. A rotor as claimed in claim 22 in which each said magnet pole has a loaf shape cross section consisting of a flat base surface located adjacent the 5 central shaft, respective flat side surface is being substantially perpendicular to the base surface, and a part-circularly cylindrical outer surface, the respective part-circularly cylindrical surfaces of said magnet poles lying on a common imaginary cylindrical surface.

10 24. A rotor as claimed in claim 22 or claim 23 in which a said infill is provided between each two adjacent said magnet poles.

25. A rotor as claimed in claim 24 in which each said instill has a generally quadrant shaped or sextant shaped section.

26. A rotor as claimed in claim 25 in which each said infill has a section comprising two mutually perpendicular surfaces, each being located adjacent a said magnet pole.

20 27. A rotor as claimed in any one of claims 22 to 26 in which the rotor has a central axis and each infill has longitudinal edges in the region of the circumference of the rotor and in which an angle formed between two imaginary lines between the axis and each said edge subtends at an angle of equal to or greater than 10 .

28. A rotor as claimed in claim 27 in which the subtended angle is about 20

29. A rotor as claimed in claim 27 in which the subtended angle is about 19 or 28 .

5 30. A rotor as claimed in any one of claims 1 to 20 in which the central shaft has a portion thereof adjacent the magnet poles which has a circular cross section, and in which the magnet poles and infills each have arc-like cross sections.

10 31. A rotor as claimed in any preceding claim in which each said infill has a curved surface, the curved surface having a radius substantially equal to that of a curved surface of a magnet pole adjacent in said infill.

32. A rotor as claimed in any preceding claim in which a plurality of said 15 magnet poles are provided and a plurality of said infills are provided arranged around said central shaft, radially outwardly facing surfaces of the magnet poles and infills forming a substantially cylindrical outer surface.

33. A rotor as claimed in any preceding claim which includes a tubelike 20 retention sleeve located around a plurality of said magnet poles and infills.

34. A rotor as claimed in claim 33 in which said retention sleeve is formed of carbon fibre material.

25 35. A rotor as claimed in any preceding claim in which a said magnet pole comprises a plurality of magnet segments extending in line axially along the rotor.

( 36. An electromagnetic machine including a rotor as claimed in any preceding claim.

5 37. A method of assembling a rotor for an electromagnetic machine comprising providing a rotor body having at least one magnet, and a pre formed retention sleeve, applying a fluid pressure differential to cause a relative expansion of the sleeve compared to the rotor body, and locating the sleeve around the rotor body, wherein the rotor body is a radial field rotor

10 body. 38. A method of assembling a rotor for an electromagnetic machine comprising providing a radial field rotor body having at least one magnet,

and a pre-formed retention sleeve, expanding the sleeve and then inserting 15 the body into the sleeve.

39. A method of assembling a rotor for an electromagnetic machine comprising providing a rotor body, the rotor body having a magnet portion axially defined by the axial extent of one or more magnet segments, and 20 locating a pre-formed retention sleeve around the magnetic rotor body to cover all or substantially all of the magnet segments of the rotor of an electromagnetic machine in an assembled operational state of the machine.

40. A method of assembling a rotor for an electromagnetic machine 25 comprising providing a rotor body having at least one magnet and a pre formed retention sleeve, applying a fluid pressure differential to cause a relative expansion of the sleeve compared to the rotor body, and locating the

sleeve around the rotor body, wherein the expansion of the sleeve is monitored while the sleeve is in an expanded state.

41. A method as claimed in claim 39 or claim 40 which includes 5 providing the rotor body as a radial magnetic field rotor body.

42. A method as claimed in claim 37, claim 38 or claim 40 in which the rotor body has a magnet portion axially defined by the axial extent of one or more magnet segments, and the sleeve, once located on the rotor body, 10 covers all or substantially all of the magnet segments of the rotor of an electromagnetic machine in an assembled operational state of the machine.

43. A method as claimed in claim 37 or claim 38 or claim 39 which includes expanding the sleeve compared to the rotor body and inserting the 15 rotor body into the sleeve, and in which the expansion of the sleeve is monitored while the sleeve is in an expanded state.

44. A method as claimed in any preceding claim which includes pre forming the retention sleeve as a carbon fibre sleeve.

45. A method as claimed in claim 40 or claim 43 in which the expansion of the sleeve is monitored with a monitoring device at one or more axial points spaced from each end of the sleeve.

25 46. A method as claimed in claim 45 in which the expansion of the sleeve is monitored at an axial point substantially half way between the ends of the sleeve.

f 47. A method as claimed in any one of claims 37 to 46 in which an inner diameter of the sleeve is less than an axial length of a region of a magnet portion of the rotor body defined by the axial extent of one or more magnet 5 segments. 48. A method as claimed in claim 47 in which an inner diameter of the sleeve is more than twice as small as the axial length of the magnet portion.

10 49. A method as claimed in any of claims 37 to 48 which includes pre fonning the retention sleeve as a circularly cylindrical hollow body.

SO. A method as claimed in any of claims 37 to 49 which includes providing the rotor body with a generally regular polygon-sectioned central 15 shaft. 51. A method as claimed in claim 50 which includes locating at least one magnet segment on a generally flat surface of the central shaft.

20 52. A method as claimed in claim 50 or claim S 1 which includes providing the magnet with a generally flat surface adjacent the central shaft and a radially outwardly convex surface.

53. A method as claimed in claim 52 which includes providing each 25 magnet segment with a generally loaf-shaped cross-section.

54. A method as claimed in claim 53 which includes providing the central

( shaft with a square or hexagonal section and providing four or six magnet poles, each magnet pole being located on a respective side of the central shaft and comprising one or more said magnet segments located extending axially along the rotor body.

55. A method as claimed in any one of claims 37 to 49 which includes assembling a plurality of magnet poles around a central shaft of the rotor body, each magnet pole defining a part-circularly-cylindrical surface, the part-circularly cylindrical surfaces of the magnet poles lying on a common 10 imaginary circularly-cylindrical surface, each magnet pole comprising one or more magnet segments extending axially along the rotor body.

56. A method as claimed in claim 54 or claim 55 which includes providing an infill between each two adjacent magnet poles.

57. A method as claimed in any one of claims 37 to 56 which includes expanding the pre-formed sleeve under the pressure of a pressurised liquid for location thereof on the rotor body.

20 58. A method as claimed in claim 57 which includes maintaining the sleeve in an expanded condition thereof during location of the sleeve on the rotor body.

59. A method as claimed in claim 57 or claim 58 which includes 25 providing the rotor body as a cylindrical body with a first diameter, pre forming the sleeve with an inner diameter which is a second diameter which is smaller than the first diameter, and expanding the sleeve to a diameter

equal to or greater than the first diameter.

60. A method as claimed in any of claims 37 to 59 which includes inserting the rotor body into the sleeve while the sleeve is in an expanded 5 state thereof.

61. A method as claimed in any one of clarions 57 to 60 which includes expanding the sleeve under the pressure of a fluid applied to interior surfaces of the sleeve.

62. A method as claimed in claim 61 which includes removing the pressure of the fluid once the sleeve is located on the rotor body.

63. A method as claimed in claim 61 or claim 62 which includes sealing 15 end regions of the sleeve to interior surfaces of a housing while applying fluid pressure to the sleeve.

64. A method as claimed in claim 63 which includes measuring expansion of the sleeve while the sleeve is under fluid pressure.

65. A method as claimed in any one of claims 37 to 64 which includes leaving material of the sleeve under tension, thereby providing a compression force on the rotor body.

25 66. A method as claimed in any one of claims 37 to 65 which includes forming the sleeve by winding a component thereof on a cylindrical body having a diameter less than an outer diameter of the rotor body.

( 67. A method as claimed in claim 66 in which the component comprises a carbon fibre filament.

5 68. A method as claiTned in claim 66 in which the component comprises a multi-filament carbon fibre ribbon or tow.

69. A method as claimed in any one of claims 66 to 68 including providing a matrix material for filling interstices between the material of the 10 ribbon or tow; and preferably which includes employing pre-impregnated filament material as the component and/or which includes employing a PEEK material as a filler between the filaments.

70. A method as claimed in any of claims 37 to 69 in which the IS electromagnetic machine comprises a generator or motor.

71. A method as claimed in any of claims 37 to 70 which includes working the cylindrical sleeve prior to location of the sleeve on the rotor body. 72. A method as claimed in claim 71 which includes working a cylindrical inner surface of the sleeve before location of the sleeve on the rotor body.

73. A method as claimed in claim 71 or claim 72 which includes 25 machining an end surface of the sleeve to have an internal entrance chamfer before locating the sleeve on the rotor body; and/or providing the rotor body with an external chamfer to assist insertion thereof into the sleeve.

74. A rotor assembly for an electromagnetic machine comprising a rotor body, the rotor body having a magnet portion axially defined by the axial extent of one or more magnet segments, and a retention sleeve located 5 around the rotor body, wherein the sleeve covers all or substantially all of the magnet segments of the rotor of an electromagnetic machine in an assembled operational state of the machine.

75. A rotor assembly for an electromagnetic machine comprising a rotor 10 body having a magnet portion axially deemed by the axial extent of one or more magnet segments, and a retention sleeve located around the magnet portion, wherein an imer diameter of the retention sleeve is less than the axial length of the magnet portion.

15 76. A rotor assembly for an electromagnetic machine comprising a rotor body, the rotor body having a magnet portion axially defined by the axial extent of one or more magnet segments, and a pre-formed unitary retention sleeve located around the rotor body, wherein the rotor body is a radial field

magnetic rotor body.

77. A rotor assembly as claimed in claim 75 or in claim 76 in which the sleeve covers all or substantially all of the magnet segments of the rotor of an electromagnetic machine in an assembled operational state of the machine.

25 78. A rotor assembly as claimed in claim 74 or claim 76 in which an inner diameter of the sleeve is less than the axial length of the magnet portion.

79. A rotor assembly as claimed in claim 78 in which the inner diameter of the sleeve is less than half the axial length of the magnet portion.

80. A rotor assembly as claimed in claim 74 or claim 75 in which the 5 rotor body is a radial field magnetic rotor body.

81. An assembly as claimed in any of claims 74 to 80 in which the material of the sleeve is under tension, thereby providing a compressive force on the rotor body.

82. An assembly as claimed in any one of claims 74 to 81 in which the retention sleeve has a worked inner surface.

83. An assembly as claimed in any one of claims 74 to 82 in which the 15 retention sleeve comprises a carbon fibre sleeve.

84. An assembly as claimed in any one of claims 74 to 83 in which the retention sleeve comprises a circularly cylindrical hollow pre-form.

20 85. An assembly as claimed in any one of claims 74 to 84 in which the rotor body has a generally regular polygon-sectioned central shaft.

86. An assembly as claimed in claim 85 in which at least one magnet segment is located on a generally flat surface of the central shaft.

87. An assembly as claimed in claim 86 in which each said magnet segment has a generally flat surface adjacent the central shaft and a radially

outwardly convex surface.

88. An assembly as claimed in claim 87 in which each said magnet has a generally loaf-shaped cross section.

s 89. An assembly as claimed in any one of claims 85 to 88 in which each said magnet segment defines a part circularly cylindrical surface, the circularly cylindrical surfaces of a plurality of said magnet segments lying on a common imaginary circularly cylindrical surface.

90. An assembly as claimed in any one of claims 85 to 89 in which the central shaft has a square or hexagonal section and four or six magnet poles are provided, respectively, each magnet pole being located on a respective flat surface of the central shaft and comprising one or more magnet segments 15 extending axially along the rotor body.

91. An assembly as claimed in any one of claims 74 to 90 in which an inner surface of the sleeve engages each said magnet segment.

20 92. An assembly as claimed in any one of claims 74 to 91 in which the electromagnetic machine comprises a generator or motor.

93. A generator or motor including a rotor assembly as claimed in any one of claims 74 to 92.

94. Apparatus for use in assembling a rotor for an electromagnetic i machine comprising: a pressure housing, the housing having interior seals

for sealing on end portions of a rotor body retention sleeve, an inlet for application of fluid pressure into the housing, and a positioning element for pushing a rotor body axially into a rotor body retention sleeve held in the housing, and at least one measurement port located in the housing.

95. An apparatus as claimed in claim 94 in which at least one port being substantially half way between two sealing regions of the apparatus for sealingly containing a retention sleeve.

10 96. An apparatus as claimed in claim 95 in which the sealing regions are adapted to sealingly engage a radially outer surface of a retention sleeve in the housing.

97. An apparatus as claimed in claim 94 or claim 95 or claim 96 in which 15 the housing includes a removable lid, the lid being removable for insertion and removal of rotor sleeves and bodies to and from the apparatus.

98. An apparatus as claimed in claim 97 in which the lid has an annular internal sealing ledge having the same diameter as a spaced sealing portion 20 of an internal bore of the housing.

99. A method of assembling a rotor for a electromagnetic machine substantially as described herein with reference to the accompanying drawings. 100. A rotor assembly substantially as described herein with reference to the accompanying drawings.

101. A generator or motor substantially as described herein with reference to the accompanying drawings.

5 102. Apparatus for use in assembling a rotor for an electromagnetic machine, the apparatus being substantially as described herein with reference to the accompanying drawings.

103. A generator or motor including a rotor as claimed in any of claims 1 10 to 35.

104. An electromagnetic machine substantially described herein with reference to the accompanying drawings.

15 105. A micro-turbine substantially as described herein with reference to the accompanying drawings.