WO2018172224A1

WO2018172224A1 - A rotor for an electrical machine

Info

Publication number: WO2018172224A1
Application number: PCT/EP2018/056757
Authority: WO
Inventors: José SOLER; Jesus-Angel HUMANES
Original assignee: Safran Electrical & Power
Priority date: 2017-03-20
Filing date: 2018-03-16
Publication date: 2018-09-27
Also published as: GB2562704B; GB2562704A; GB201704395D0

Abstract

There is disclosed a novel arrangement of a rotor for an electrical machine. The rotor comprises one or more slots located between adjacent pairs of magnetic poles and one or more windings located in the one or more slots. A two part retainer assembly is provided in between the poles to retain the windings in place. The retainer comprises a root portion preferably attached to the core using a fir tree root type connection, and a cover portion for attachment to the root portion to retain the windings in place on the core. The cover can preferably be radially advanced toward the root portion to compress windings on the core. A corresponding method of assembly is also provided.

Description

A Rotor for an Electrical Machine

FIELD OF THE INVENTION

The present invention relates to rotors for electrical machines. In particular, the invention relates to an improved mechanical construction of a rotor, for retaining windings on the rotor, and related methods of manufacture of such a rotor.

BACKGROUND TO THE INVENTION

As will be well understood by a person skilled in the design and construction of electrical machines, synchronous electrical machines in particular generally include a stator including a plurality of windings for interacting with electromagnetic fields created on a corresponding rotor. Particularly in high-speed electromagnetic machines, the combination of rotational speeds present, diameter of the windings and weight of the windings can result in extreme centripetal forces acting on the windings of the rotor as they rotate. Generally, the windings are wound around a magnetic core structure and are made of relatively ductile materials such as copper. If not held in place by a mechanical support, the windings would otherwise deform under the action of the centripetal forces generated in high speed rotation and so would fail. Mechanical solutions are therefore required to retain the windings on the magnetic core. Solutions are known in which retainers are placed outside of the windings and held in place by pole tips of the magnetic core, such as that shown in EP 3046229 Al . There are limitations to such designs, since the forces reacted through the tips of the core are large and the required mechanical structure can result in a magnetically sub-optimal structure. The core tips also are subject to high magnetic flux as well, resulting in potentially conflicting magnetic and mechanical design requirements. CN201839179U shows an alternative solution in which anchoring is also provided in between the magnetic pole tips.

There is therefore a need for improved rotors for electrical machines. SUMMARY OF THE INVENTION

There is disclosed a novel arrangement of a rotor for an electrical machine. The rotor comprises one or more slots located between adjacent pairs of magnetic poles and one or more sets of windings located in the one or more slots. A two part retainer assembly is provided in between the poles to retain the windings in place. The retainer comprises a root portion preferably attached to the core using a fir tree root type connection, and a cover portion for attachment to the root portion to retain the windings in place on the core. The cover can preferably be radially advanced toward the root portion to compress windings on the core. A corresponding method of assembly is also provided .

Thus, in addressing the drawbacks of known rotors for electrical machines, there is provided a rotor for an electrical machine the rotor comprising :

a rotational axis; and

a magnetic core, the magnetic core comprising :

a plurality of magnetic poles; and

at least one slot disposed between at least one pair of adjacent poles of the plurality of magnetic poles;

at least one retainer configured to retain windings in the slot of the core;

the retainer comprising a root portion and a cover portion; the root portion and a radially inner part of the slot being configured to mate so as to retain the root portion radially attached to the core;

the cover portion and the root portion being configured to be attached to one another by a radially-engageable connecting interface; and

the cover portion being configured to retain the windings in the slot when the cover portion is connected to the root portion via the connecting interface.

The claimed rotor construction results in more efficient distribution of centripetal forces through the core, allowing the core pole tip in particular to be designed and manufactured with more emphasis on magnetic properties than their mechanical properties, and results in an improved rotor.

The magnetic core and retainer may be configured so that windings are retained in circumferentially offset positions relative to the root portion. The slot or slots may be configured to accommodate first and second sets of windings, the first and second sets of windings being offset in opposing circumferential directions relative to the root portion.

The root portion may be configured to be slideable into position in an elongate opening in the core in a longitudinal direction, preferably substantially parallel to the rotational axis of the rotor, for connection to the cover portion.

At least a part of the root portion may have a longitudinal cross-section which includes a radially projecting stem and a plurality of laterally-projecting lobes configured to engage with corresponding recesses in the magnetic core.

The longitudinal cross section of at least a part of the root portion may comprise a plurality of opposed pairs of laterally-projecting lobes.

The root portion may comprise a fir tree root connection to the magnetic core.

The root portion may comprise a soft magnetic material such as Steel or Cobalt Iron so as to minimise disturbance of the magnetic flux through the rotor core, in particular when the core is made from a material having same or similar magnetic properties.

The radially-engageable connecting interface may comprise one or more shafts provided on one of the root portion and the cover portion, and one or more openings for receiving the one or more shafts, provided on the other of the root portion and the cover portion. The root portion may comprise one or more shafts and the cover portion may comprise one or more openings for receiving the one or more shafts to retain the cover portion to the root portion. The radially-engageable connecting interface may be configured to allow compression of windings located in the slot upon assembly of the root portion and the cover portion.

The radially-engageable connecting interface may comprise a graduated attachment feature configured to compress the windings located in the slot upon progressive engagement of the graduated attachment feature.

The graduated attachment feature may comprise a thread, a bayonet-type attachment or a graduated slope configured to gradually engage a corresponding projection. The cover portion and/or the root portion may comprise a substantially constant longitudinal cross section.

The invention further provides a method of assembling a rotor for an electrical machine, comprising the steps of:

providing a magnetic core, the magnetic core comprising :

a plurality of magnetic poles; and

winding at least one conductor within the at least one slot to provi electrical windings of the rotor;

providing at least one retainer configured to retain windings in the slot of the core, the retainer comprising a root portion and a cover portion;

placing the root portion in a radially inner part of the slot such that the root portion is radially attached to the core; and

attaching the cover portion to the root portion such that the cover portion retains the windings in the slot.

The method may further comprise the step of compressing the windings in the at least one slot with the cover portion. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which :

Figure 1 shows an illustration of a rotor incorporating a magnetic core assembly of the present invention;

Figure 2 shows a perspective view of an end of a magnetic core assembly according to the invention;

Figure 3 shows an enlarged section of the core assembly of figure 2;

Figure 4 shows an end view of the enlarged section of figure 3; Figure 5 shows an example of a root portion suitable for use in the present invention;

Figure 6 shows an example of a cover portion suitable for use in the present invention.

DETAILED DESCRIPTION OF EMBODIMENT(S) Figure 1 shows an example of a rotor 1 for use in an electrical machine. As the skilled reader will appreciate, electrical machines can generally comprise motors and/or generators. As is well known to the skilled person, rotation of the rotor 1 relative to a stator (not shown) causes interaction of magnetic fields generated by windings in the rotor and/or stator. Where a drive is input to the rotor 1 to drive it in rotation, then electrical currents can be generated to generate an electrical output from corresponding windings on the stator. Conversely, an electrical current can be driven in the stator to induce movement in the rotor 1 to create rotational motion, in a motor.

The following examples relate to electrical machines of the kind having windings on the rotor 1, such that when an electrical current is induced in those windings, a non-permanent magnetic field is generated around the rotor. Asynchronous machines, or induction machines, are examples of such machines, as will be evident to the skilled reader. However other types of machine, such as brushed machines can also benefit from the features described herein, although the main benefits are realised in high speed machines having rotor windings. In particular, the following examples relate to a novel method of retaining electrical windings on the rotor 1. This can facilitate an improved method of assembly of the rotor, to retain the windings in place on the rotor.

The rotor 1 comprises a first end 10, a second end 12 and a rotational axis X about which the rotor rotates, in use, within a stator. The rotor also comprises a magnetic core assembly 100, which includes a magnetic core carrying one or more windings for generating a moving magnetic field relative to the stator (not shown).

Figure 2 shows a view of the magnetic core assembly 100 comprising a magnetic core 101. The core 101 is constructed from a magnetically conductive material. Such magnetic cores are commonly constructed from laminated plate sections, each having a cross section as shown at the first end 102 of the core assembly 100, but other constructions, such as being constructed from a uniform, homogeneous piece of solid magnetically conductive material, are possible. The core 101 is constructed to provide one or more magnetic poles 111, 112, 113 and 110. Between each magnetic pole there may be provided a slot, comprising one or more channels for receiving windings of the core. At its circumferential extremity or extremities, each pole may comprise one or more pole tips 110a, 110b, 111a, 111b, 112a, 112b, 113a, 113b.

The rotor assembly 100 is also provided with one or more sets of windings 120a, 120b, 121a, 121b, 122a, 122b, 123a, 123b. As a skilled person will appreciate, the windings 121a to 123b can be wound around the respective pools 111, 112, 113 and 110 of the magnetic core 101 during manufacture of the magnetic core assembly 100. Then details of how the windings are formed and connected to one another are not provided here, since this will be carried out in a standard manner already familiar to the skilled reader. As has been discussed above, in the absence of any physical retainer, the windings 120 to 123 may be displaced or dislodged, or in the extreme may disintegrate, due to the very high centripetal forces present at high rotational speeds of the rotor rotating about the axis of rotation X.

Therefore, one or more retainers 131, 132, 133, 134 may be provided and may preferably be physically connected to the magnetic core 101 in order to retain the windings 120a to 123b in place. The one or more retainers 131 to 134 can act as retainers for retaining the windings in place on the core 101. The retainers described herein are sometimes termed "wedges" due to their wedgelike form when viewed from the first end 102 of the core assembly 100.

As has been discussed, prior art methods of retaining these wedges in place can either result in complex assembly methods, and/or may transfer the centripetal loads, tending to bias the windings radially away from the centre of the core 101, to a less substantial part of the magnetic core 101, such as the pole tips 110a to 113b listed above. The pole tips 110a to 113b already carry a significant amount of the centripetal force of the windings in any case. This is because they are located radially outward of at least a portion of the windings 120a to 123b. Radial forces on at least a part of the windings are therefore directly transferred to the pole tips from the windings. These pole tips also experience some of the highest magnetic fields in the rotor and so in prior designs, designing them with sufficient mechanical and magnetic properties to carry such forces could be challenging. Materials such as cobalt-iron alloys may be employed in the rotor core and are advantageous for such uses due to their ability to take high magnetic fields without saturating . In order to provide sufficient strength to the core, particularly in the pole tips when they carry loads associated with the windings, the materials of the core may be heat- treated to give the material greater mechanical strength. However, this heat treatment can be detrimental to the magnetic properties of the alloy. The mechanical strength which can be provided in the pole tips is therefore limited and the strength of the system of wedges or retainers in a rotor has in the past been one of the main limiting factors in how high a rotational speed the electrical machine can safely operate. Removing mechanical loads from the pole tips is therefore advantageous. A further problem which has been identified with prior art methods of retaining the windings on the rotor is that it can be difficult to provide any significant degree of pre-compression to the windings without significant weight penalties or additional complexity and expense. However, without pre-compression, on the first spinning up of the rotor, the windings may tend to settle and redistribute themselves within the respective slots and this can upset the balance of the rotor, causing excessive vibrations. Even a very small amount of movement can result in severe vibrations at high speed and so sufficient compression to eliminate all movement of the windings is preferred. As will be described in more detail in relation to later figures, the new form of retainer provided by the present invention overcomes a number of these drawbacks.

Figure 3 shows an enlarged view of the an exemplary retainer 133 in place on the rotor 101 to illustrate how it acts to retain the windings 121b and 122a in place in the slot between poles 111 and 112. As will be appreciated, such an arrangement can be reproduced one or more times around the periphery of the core assembly 100. The following description therefore relates to one pair of windings in a particular slot, but the arrangement can be repeated for one or more, or each adjacent pair, of poles 111 to 113 of the core 101. As illustrated in Figure 3 with respect to windings 121b and 122a, any or all of the windings may be electrically insulated from the core by an insulator 151 or 152, respectively. Further, any or all of the windings may be electrically insulated from the retainer by an insulator 161 or 162, respectively. The insulators 151 and 161, or 152 and 162, may be integrally formed for each winding or, as illustrated, they may be formed as separate items assembled around each winding. It will also be appreciated in light of the present disclosure that more than the four pairs of poles illustrated can be employed, as necessary or desirable in view of magnetic, electrical or mechanical design requirements.

In the illustrated example, the retainer 133 has a root portion 331 and a cover portion 332. As can be seen, the core assembly is arranged such that the root portion 331 can be located between the first and second windings 121b and 122a, in the slot between poles 111 and 112. The root portion 331 is located in a substantially radially inner portion of the slot in the core 101 and is configured to mate with the core at a location in the slot between the adjacent poles 111 and 112. This configuration enables the root portion to be retained in the core such that it is radially attached to the core. This means that while the root portion can during assembly slide longitudinally relative to or parallel to the rotational axis X of the core 101, the construction of the root portion 331 and the channel in which it is received prevents the root portion 331 from being moved away from the axis of the core in a radial direction . Forms which enable such a radial attachment will be discussed in relation to later figures. The cover portion 332 is configured to be attachable to, and preferably detachable from, the root portion 331. In particular, the root portion 331 and the cover portion 332 are configured so that the cover portion can be introduced into engagement with, and connected to, the root portion 331 in a radial direction relative to the core 101. Therefore, during assembly, the root portion 331 can be slid longitudinally or axially, i.e. substantially parallel to rotational axis X of the core 101, into engagement with the core 101 such that it is radially attached to the core and cannot be detached from the core in a radial direction. Then, by introduction of the cover portion 332 in a substantially radial direction, which may be substantially perpendicular to the direction of introduction of the root portion 331 into the core 101, the windings 121b and 122a can be retained in place in the slot, slots or channels provided between poles 111 and 112. The desired level of pre-compression in the windings can then be obtained by increasing the tension in the fastenings used to connect the cover portion 332 to the root portion 331. As can be seen in the figures, any of the windings, such as 121b and 122a, are preferably retained relative to the core in circumferentially offset positions relative to the root portion 331. Otherwise stated, they are located to opposite sides of the root portion 331 when viewed from an axial direction of the rotor assembly 100 as shown in figure 4, for example. The arrangement of the root portion 331 between the windings enables the attachment and the resulting load path, passing from the root portion 331 to the cover portion 332 to pass between the windings to deliver loads directly to a radially inner portion of the core 101. This helps to avoid transferring excessive centripetal forces generated by the mass of the windings 121b and 122a when the rotor is in rotation to the tips 112a and 111b of the poles 111 and 112.

As can be appreciated from figures 3 and 4, either or both of the root portion 331, and preferably the cover portion 332, may have a substantially constant longitudinal cross-section, i.e. the cross-section when viewed in the axial direction of the rotor, i.e. when viewed along the axis of rotation X shown in figure 1. In relation to the root portion, this can help to allow the root portion 331 to be translated into and out of a receiving channel 143 provided in the core 101. The channel 143 is therefore preferably elongate in an axial direction of the rotor and preferably extends through the core in that direction. As can be seen in figure 4, the root portion 331 has a longitudinal cross-section which includes a substantially central and radially-projecting stem, which projects radially towards the axis of rotation X and away from the cover portion 133. At least one laterally projecting lobe 431 may be provided on stem 430, and may be configured to engage with a corresponding lateral recess 441 in the channel 143. One or more further lobes 432, 433, 434, may be provided for engagement with one or more corresponding laterally projecting recesses in the channel 143. Such an arrangement of plural pairs of opposing lobes as illustrated in figure 4 can be sometimes termed a "fir tree root". In preferred forms, one or more lobes 431, 432 at a distal end of the stem 430 have a lesser degree of lateral projection than one or more lobes 433, 434 located nearer the cover portion 432. This form of "tapering" of the lobes, i.e. a narrowing toward a distal end of the root portion, can give the cross-section of the root portion the appearance of a "fir tree", hence the commonly used name of a fir tree root to refer to this form. In preferred forms, the longitudinal cross-section of the root portion may therefore comprise a plurality of pairs of opposed laterally projecting lobes 431, 432, 433, 434. One or more pairs of lobes distal from the cover portion may project less in a lateral direction than one or more pairs of lobes disposed adjacent the cover portion. In this manner, the root portion can comprise a fir tree root connection to the magnetic core 101. All of the above features of the laterally projecting recesses and corresponding lobes can also help to distribute forces over a larger contact area, so reducing stress concentrations in both the root portion 331 and the corresponding parts of the core.

As has been described above, this attachment of the root portion 331 can be combined with a radially-engageable connecting interface for connecting the cover portion 332 to the root portion 331. A radially engagable interface allows the cover portion 332 to be introduced into the slot between poles 111 or 112, and into engagement with the root portion 331 of the retainer in a substantially radial direction relative to the core 101, as illustrated by arrow 40. This combined arrangement allows a number of advantages. As can be seen in Figure 4, the walls of the slot directly engaging the windings are, in the arrangement of figure 4, able to be arranged more closely to one another than the maximum width of the lobes 431 to 434. This allows a secure connection between the root portion 331 and the core 101. This also avoids unnecessarily restricting the proximity of the windings 122a and 121b to one another in that region. This enables an efficient overall packaging of windings on the core assembly 100. Further, the ability to introduce the cover portion 332 in a radial direction of arrow 40, allows compression of the windings 122a and 121b in a radial direction. As described above, this pre-compression allows a reduction in the redistribution of the windings on first spinning up of the rotor to operational speed, and so can improve the overall balancing of the rotor.

Numerous different types of radially-engageable connecting interfaces can be envisaged. A particular arrangement and some alternatives will now be described in relation to figures 5 and 6. Figure 5 shows an example of a root portion 500 which can be employed in examples of the arrangements described in figures 1 to 4. The root portion 500 can preferably comprise a fir-tree-root- type stem and lobe arrangement as described in relation to figure 4. The root portion 500 preferably extends in a longitudinal direction, and the fir tree root type connection is preferably constant along its length as shown in the figure. At intervals along the length of the root portion, one or more shafts 510, 511, 512, 513, 514, 515 may be provided.

Figure 6 illustrates an example of a cover portion 600 which may be used in conjunction with the root portion 500 of figure 5. The cover portion 600 may comprise a plurality of openings 610, 611, 612, 613, 614, 615, configured to correspond with the shafts 510 to 515. As the skilled reader will appreciate, placing the cover portion 600 onto the root portion 500 can be carried out in a radial direction of the core 101 such that the shafts 510 to 515 are located in openings 610 to 615. The one or more shafts 510 to 515 may be threaded and then a nut arrangement may be threaded onto one or more of the shafts. The nut arrangement may be received in the opening 610 to 615 of the cover portion 600. As will be appreciated, openings of a smaller diameter provided in a radially inner region 620 of the cover portion 600 can enable the shafts 510 to 515 to pass through those radially inner openings and engage the nut arrangements in the openings 610 to 615 and the nut arrangements can be tightened to retain the cover portions 600 to the root portion 500. The nut arrangements can therefore be accommodated within the cover portion 600. As will be appreciated, it would be possible to provide a series of holes in the root portion 500 in place of the shafts 510 to 515, and those holes could be threaded, such that bolts may be located through the openings 610 to 615 to bolt the cover portion 600 to the root portion 500. Such threaded arrangements are one manner of providing a graduated attachment feature, where the threads allow progressive engagement of the graduated attachment feature to progressively compress the windings rotated in the slot between the poles of the rotor. Other arrangements can be envisaged which allow such compression and attachment to take place. For example, a bayonet-type attachment could be envisaged, in which in a first step, the cover portion 600 is advanced towards the root portion 500 to compress the windings, and then upon rotation of a bayonet fixing device, the cover portion may be locked in place to hold the windings in compression. Other forms of connection including graduated slopes for rotational engagement with corresponding features can be envisaged to apply this graduated compression of the windings.

As can be seen in figures 5 and 6, at least some elements of the root portion 500 and the cover portion 600 may have a substantially constant longitudinal cross-section in an axial direction of the rotational axis X of the rotor. This can enable the components to be extruded. The shafts and holes 510 to 515 and 610 to 615 may be provided in secondary manufacturing operations, such as by drilling the holes and/or by providing the shafts 510 to 515 by welding or otherwise fixing them onto an extruded root portion 500, for example. Another possible operation for connecting the cover portion 600 to the root portion 500 is a riveting-type operation, where the cover portion 600 may be held in place by tolling to provide compression of the windings. A deformation operation can then be performed on the tips of the shaft 510 to 515 to retain the cover portion 600 in place.

Exemplary materials which may be used for the retainer include, for example steel for the root portion. The mechanical properties of the root portion may preferably be greater in terms of strength and stiffness, than the cover portion 600. This is because the relatively small dimensions of the lobes on the root portion 500 can result in relatively high stress concentrations in these areas, in view of the forces being concentrated in this relatively small cross-section. This small area is smaller than the large cross-sectional area of the cover portion 600, which reacts forces generated by the centripetal acceleration of the windings. The cover portion may be manufactured from other materials, such as titanium or aluminium, or alloys thereof, for example.

Alternative arrangements of the cover portion 600 can be envisaged, in which it can be configured to extend over and outside at least a portion of the pole tips 111b and 112a. In these examples, the retainer arrangement can in fact provide mechanical support to the pole tips, allowing those pole tips to be designed for manufacture for magnetic efficiency, with the structural burden placed on the retainer arrangement. A further advantage of the examples described, is that there is not necessarily any structural requirement for a rotor sleeve to be provided outside of the core assembly 100. This has been a solution in some prior art examples, where a sleeve is provided over the whole rotor to retain the "wedges" in place. However, this prior art method has a packaging disadvantage, since the thickness of the sleeve must be accommodated in the overall diameter of the rotor. Further advantages of the arrangements described herein are that very tight tolerances or interference fits are not required. The components can be slid and located in place, and then tightened into place by, for example, the screwing, bolting or riveting operation described. This, combined with the static compression of the windings, can result in a fast and efficient manufacturing operation, which allows the windings and retainer to be quickly and efficiently assembled to the required tolerances by mere tightening of the threaded engagements or riveted operations.

Efficiencies in terms of maintenance can also be achieved, since, particularly in the case of threaded engagements between the cover portions 600 and the root portion 500, the threaded connection of the bolts or nuts can be easily undone for repair or replacement of the windings, root portion or cover portion.

As will be appreciated from the above description, the invention therefore provides an improved structure for a rotor for an electrical machine, and, as has been described above, an improved method of assembling such a rotor is also provided .

In such an improved method, as will be appreciated from the above description of the components and their means for assembly, a magnetic core can be provided having one or more poles. One or more electrical windings can be provided around at least one of the poles of the core. A root portion of the retainer can be provided in a radially inner a part of a slot between poles of the core, such that the root portion is radially attached to the core and prevented from radial detachment from the core. Then, the cover portion can be attached to the root portion, so that circumferentially projecting portions of the cover portion act to retain the windings within the slot of core. As has been described, it is preferable that the assembly method includes compressing the windings in the at least one slot of the core with the cover portion. This can preferably be implemented by attachment means, such as nut and/or bolt arrangements for attaching the cover portion 600 to the root portion 500. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

A rotor for an electrical machine the rotor comprising :

a rotational axis; and

a magnetic core, the magnetic core comprising :

a plurality of magnetic poles; and

at least one retainer configured to retain windings in the slot of the core;

the retainer comprising a root portion and a cover portion;

the root portion and a radially inner part of the slot being configured to mate so as to retain the root portion radially attached to the core;

the cover portion being configured to retain the windings in the slot when the cover portion is connected to the root portion via the connecting interface;

wherein at least a part of the root portion has a longitudinal cross-section which includes a radially projecting stem and a plurality of laterally-projecting lobes configured to engage with corresponding recesses in the magnetic core.

A rotor according to claim 1, wherein the magnetic core and retainer are configured so that windings are retained in circumferentially offset positions relative to the root portion.

A rotor according to claim 1 or claim 2, wherein the slot is configured to accommodate first and second sets of windings, the first and second sets of windings being offset in opposing circumferential directions relative to the root portion.

4. A rotor according to any of the preceding claims, wherein the root portion is configured to be slideable into position in an elongate opening in the core in a longitudinal direction, preferably substantially parallel to the rotational axis of the rotor, for connection to the cover portion.

5. A rotor according to any of the preceding claims, wherein the longitudinal cross section of at least a part of the root portion comprises a plurality of opposed pairs of laterally-projecting lobes.

6. A rotor according to any of the preceding claims, wherein the root portion comprises a fir tree root connection to the magnetic core.

7. A rotor according to any of the preceding claims, wherein the root portion comprises a soft magnetic material such as Steel or Cobalt Iron so as to minimise disturbance of the magnetic flux through the rotor core.

8. A rotor according to any of the preceding claims, wherein the radially-engageable connecting interface comprises one or more shafts provided on one of the root portion and the cover portion, and one or more openings for receiving the one or more shafts, provided on the other of the root portion and the cover portion.

9. A rotor according to claim 8, wherein the root portion comprises one or more shafts and the cover portion comprises one or more openings for receiving the one or more shafts to retain the cover portion to the root portion.

10. A rotor according to any of the preceding claims, wherein the radially-engageable connecting interface is configured to allow compression of windings located in the slot upon assembly of the root portion and the cover portion.

11. A rotor according to claim 10, wherein the radially-engageable connecting interface comprises a graduated attachment feature configured to compress the windings located in the slot upon progressive engagement of the graduated attachment feature.

A rotor according to claim 11, wherein the graduated attachment feature comprises a thread, a bayonet-type attachment or a graduated slope configured to gradually engage a corresponding projection.

A rotor according to any preceding claim, wherein the cover portion and/or the root portion comprise a substantially constant longitudinal cross section.

A method of assembling a rotor for an electrical machine, comprising the steps of:

providing a magnetic core, the magnetic core comprising :

a plurality of magnetic poles; and

winding at least one conductor within the at least one slot to provide electrical windings of the rotor;

The method of claim 14, further comprising the step of compressing the windings in the at least one slot with the cover portion.