WO2019224552A1

WO2019224552A1 - Rotor

Info

Publication number: WO2019224552A1
Application number: PCT/GB2019/051433
Authority: WO
Inventors: Michael DEWHIRST; Simon ODLING
Original assignee: Lentus Composites Limited
Priority date: 2018-05-25
Filing date: 2019-05-24
Publication date: 2019-11-28
Also published as: GB201907338D0; GB2575906B; GB2575906A; GB201808657D0

Abstract

A rotor (10) for an electrical machine is described comprising a rotor body (12), a plurality of magnets (24) carried upon the body (12), and a sleeve (26) encircling the magnets (24) and the body (12) and retaining the magnets (24) in position upon the body (12), wherein the body (12) includes a fluid passage (16, 18) through which a fluid can be supplied to expand the sleeve (26) so that the sleeve (26) assumes a pre-stressed condition.

Description

ROTOR

This invention relates to a rotor for an electrical machine such as a motor, and in particular to a rotor suitable for use in a brushless electric motor.

The rotor of a brushless electric motor comprises a body supported by bearings for rotation about a fixed axis of rotation relative to an associated stator. The rotor carries a series of permanent magnets, the magnetic fields of which interact with magnetic fields generated by electrical coils carried by the stator to result in the rotor being driven for rotation.

In a number of applications the rotor is arranged to rotate at very high speeds, and consequently the magnets thereof experience high centripetal forces urging the magnets radially outward relative to the body of the rotor. The manner in which the magnets are mounted to the body of the rotor must be able to withstand such loadings, in use.

As the performance of the motor depends, at least to some extent, upon the spacing of the magnets from the coils of the associated stator, it is desirable to minimise the spacing therebetween. Obviously, reducing the spacing between the magnets and the coils of the stator reduces the space available in which to locate the means by which the magnets are secured in position.

One mounting technique by which the magnets may be secured in position is to use bands or a sleeve encircling the body of the rotor and the magnets, and so retaining the magnets in position. By way of example, the bands or sleeve may be of metallic or composite materials. The bands or sleeve are conveniently pre-stressed to compress the magnets against the body of the rotor, the pre-stressing being such that the centripetal loadings experienced by the bands or sleeve in retaining the magnets in position in normal used do not exceed the pre-stressing of the bands or sleeve, and so that magnets are firmly secured to the rotor against movement relative thereto, in use. Several pre-stressing techniques are known for use in this type of application. By way of example, the bands or sleeve may be expanded before introduction of the body and magnets into the band or sleeve. Subsequent relaxation of the bands or sleeve results in contraction thereof. The dimensions of the bands or sleeve and the body are chosen so that the bands or sleeve remain in a pre-stressed condition, firmly securing the magnets in position. Another known pre-stressing technique involves inserting wedges into the rotor, the interaction between the wedges, when moved relative to one another, expanding parts of the rotor and pre-stressing the bands or sleeve. Yet another technique, where the bands or sleeve are of a wound composite material form, is to form the bands or sleeve using a high tension winding technique so that the bands or sleeve are/is formed in a pre-stressed condition.

These techniques are relatively complex and may require the use of specialist equipment to achieve the pre-stressing. Manufacture has to be accurately controlled in order to ensure that the required tolerances are achieved to result in the formation of a rotor suitable for use in a high performance, high speed electric motor. In particular, high levels of accuracy are required in achieving the desired levels of roundness and fit between the radially outer faces of the magnets and the inner surfaces of the bands or sleeve to ensure that the magnets are correctly positioned and properly supported against movement relative to the body of the rotor.

It is an object of the invention to provide a rotor for an electrical machine such as a motor, and a method of manufacture thereof, in which at least some of the disadvantages associated with at least some known arrangement are overcome or are of reduced impact. In particular, it is an object of the invention to provide a rotor and associated method of manufacture whereby the manufacture of the rotor can be simplified.

According to the present invention there is provided a rotor for an electrical machine comprising a rotor body, a plurality of magnets carried upon the body, and a sleeve encircling the magnets and the body and retaining the magnets in position upon the body, wherein the body includes a fluid passage through which a fluid can be supplied to expand the sleeve so that the sleeve assumes a pre-stressed condition.

The sleeve may be of a metallic material. Alternatively, it may be of a composite material. Where of a wound fibre composite material, the sleeve may be wound in situ upon the body.

In one embodiment, the body may define a thin walled region adjacent the position at which the magnets are located, the application of fluid under pressure to the fluid passage applying an outwardly directed load to the thin walled region, urging the thin walled region and the magnets located adjacent thereto in an outward direction to expand the sleeve. The application of fluid under pressure preferably permanently plastically deforms the thin walled region such that the sleeve is permanently pre-stressed.

The body is preferably provided with a recess in its outer surface in which the magnets are located. Preferably, a filler or adhesive material is located, with the magnets, in the recess. The filler or adhesive material is able to fill any gaps that would otherwise be present between the body, the magnets and the sleeve, ensuring that the magnets are rigidly and securely mounted in position. The need to manufacture the body, the magnets and the sleeve in such a manner as to avoid the formation of such gaps is thus reduced, and so manufacture of the rotor is simplified and the cost of manufacture may be reduced.

For convenience, in this arrangement, the body may be of multi-part form, for example being of two part form. However, this need not always be the case, and one piece arrangements may be possible without departing from the scope of the invention.

In an alternative arrangement, fluid supplied via the flow passage is able to act directly upon the sleeve, applying a load thereto to expand and pre-stress the sleeve. In this arrangement, the fluid conveniently comprises a resin, for example a thermosetting or thermoplastic resin, which not only serves to pre-stress the sleeve but also fills any gaps between the magnets and the sleeve. The resin is preferably maintained under pressure until curing or solidification thereof has taken place with the result that the cured or solidified resin material serves to permanently pre-stress the sleeve.

The invention also relates to a method of manufacture of a rotor for an electrical machine comprising providing a rotor body, a plurality of magnets carried upon the body, and a sleeve encircling the magnets and the body and retaining the magnets in position upon the body, the body including a fluid passage through which a fluid can be supplied to apply a load to the sleeve to cause expansion thereof, and applying fluid under pressure to the said fluid passage to expand the sleeve.

The rotor body, magnets and sleeve are preferably located within a mould prior to the application of the fluid under pressure. The mould may serve as a support, limiting the degree by which the sleeve is expanded during the pre-stressing and aiding in ensuring that the shape and size of the sleeve, once pre-stressed, are as desired.

The rotor may be of any of the forms recited hereinbefore.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic cross-sectional view illustrating a rotor in accordance with an embodiment of the invention;

Figure 2 is a simplified diagrammatic view illustrating a step in the manufacture of the rotor of Figure 1; and

Figure 3 is a view illustrating a rotor in accordance with another embodiment of the invention. Referring firstly to Figure 1, a rotor 10 is illustrated for use in a high rotary speed, high performance electrical motor. Whilst the invention is particularly suitable for use in such applications, the invention is not restricted in this regard and may be employed in other applications. Motors of this type are well known, and the description herein is restricted to the nature and manufacture of the rotor 10.

The rotor 10 comprises a rotor body 12 of, in this case, metallic form. By way of example, it may be of a suitable steel, or lighter weight metals may be used if desired. As shown, it is conveniently of two part form, comprising a first part 12a and a second part 12b, the first and second parts 12a, 12b being welded or otherwise rigidly secured to one another. The first part 12a includes a region 14 of enlarged diameter, the region 14 being of hollow form defining a chamber 16. The wall of the first part 12a of the body 12 defining the chamber 16 is of relatively thin form. By way of example, this thin walled region may have a wall thickness of less than 3mm, preferably less than 2mm, and is conveniently in the region of 0.5-1.5mm or less.

The second part 12b of the rotor 12 takes the form of a plug closing the open end of the chamber 16. A bore 18 extends through the second part 12b, providing a flow path whereby fluid can be supplied to the chamber 16.

The outer surfaces of the first and second parts 12a, 12b are conveniently shaped for cooperation with bearings or the like so that the rotor 10 can be supported for rotation about its axis. The manner in which the rotor 10 is supported is not of relevance to the invention and so will not be described herein in further detail.

The outer surface of the first part 12a is formed with a pair of outwardly protruding flanges 20a, 20b located adjacent opposite ends of the region 14, and hence at opposite ends of the thin walled region. The flanges 20a, 20b and region 14 therefore together define a recess 22 within which magnets 24 are located. A sleeve 26 closes the recess 22 and so secures the magnets 24 in position. Whilst the sleeve 26 could be of a range of materials, for example it could be of metallic form, in this embodiment it is of a wound fibre composite material, the fibres of the composite material being wound in situ upon the rest of the rotor 10, and cured in situ to form the sleeve 26. The winding process used is such that the sleeve is not, in this condition of the rotor 10, significantly pre-stressed. By winding the sleeve 26 in situ over the rotor body 12 and the magnets 24, it will be appreciated that the sleeve 26 conforms closely to the outer surface shape of the rotor body 12 and the magnets 24.

Conveniently, the sleeve 26 is of a carbon fibre composite material. However, it will be appreciated that other composite materials may be used, depending upon the application in which the rotor or motor are to be used. Where used in high temperature application, it may be preferred for the fibre to be of a metallic form.

Prior to winding of the fibres forming the sleeve 26, the recess 22 containing the magnets 24 is preferably filled with a filler or adhesive material. Consequently, in the finished rotor 10, any gaps that would otherwise be present between the rotor body 12 and the magnets 24 or between the magnets 24 and the sleeve 26, or between adjacent ones of the magnets 24, are filled with the filler or adhesive material, ensuring that the magnets 24 are firmly and rigidly supported relative to the rotor body 12, relative movement therebetween being resisted.

After winding of the fibres of the composite material, the composite material is cured. Any excess resin from the composite material may also aid in filling the aforementioned gaps. If desired, the outer surface of the sleeve 26 may be machined to a desired shape and size, for example to ensure that it is of good roundness.

It will be appreciated that, in this condition, the sleeve 26 is not pre-stressed. In order to pre stress the sleeve 26, fluid under pressure is applied to the fluid passage defined by the chamber 16 and the bore 18, the fluid under pressure applying a load to the thin walled region of the rotor 12 urging that part of the rotor 12 to expand radially outwardly. The expansion of the thin walled region pushes the magnets 24 outward which, in turn, forces the sleeve 26 to expand, thereby pre-stressing the sleeve 26. The degree by which the thin walled region expands during this pre-stressing operation is such that plastic deformation thereof occurs. Consequently, upon subsequent release of the fluid pressure applied to the fluid passage, the thin-walled region will remain in its expanded condition, and so the sleeve 26 will be maintained in its expanded, pre stressed condition.

As illustrated in Figure 2, prior to the application of the fluid under pressure to the rotor 10 to pre-stress the sleeve, the rotor assembly is placed within a mould 30 having a cavity shaped to cooperate with the exposed parts of the rotor body 12, but spaced slightly from the outer surface of the sleeve 26. During the pre-stressing operation, the sleeve 26 expands radially, but expansion thereof is limited or restricted by engagement between the sleeve 26 and the wall of the mould 30 defining the cavity. Consequently, the level of pre-stressing can be controlled, and the size and shape of the pre-stressed sleeve 26 can be accurately controlled. The elastic strain on the sleeve 26 is thus controlled or restricted, preventing rupture of the sleeve 26 during this operation despite the application of a very high pressure to the fluid passage, sufficient to cause plastic deformation of the thin walled region. After pre-stressing has be completed, the rotor 10 can be removed from the mould and assembled into the remainder of the motor with which it is to be used.

It will be appreciated that the rotor 10 is advantageous in that manufacture, including pre stressing, thereof can be achieved relatively simply without requiring the use of complex, specialist equipment. The magnets 24 of the rotor 10 are well supported, this being achieved without the need to manufacture the rotor body 12 or sleeve 26 to a high degree of accuracy. One particular advantage is that there is no need to achieve a high degree of roundness and an accurate fit at the point of engagement between the magnets and the sleeve through the use of a composite material sleeve wound upon the rotor, and through the use of a filler or adhesive material which can fill any gaps between the magnets and the sleeve. In the arrangement described hereinbefore, the high pressure fluid used to pre-stress the sleeve 26 does not itself contact the sleeve 26, but rather is isolated therefrom, being contained within the rotor body 12, and it removed from the rotor 10 after completion of the pre-stressing operation. This need not always be the case, and other arrangements are possible. By way of example, Figure 3 illustrates an arrangement that, in many ways, is similar to that of Figure 1 but in which the fluid used to pre-stress the sleeve 26 acts directly upon the sleeve 26 and remains in situ in the finished rotor 10. Only the significant differences between the embodiments are described herein.

In the arrangement shown in Figure 3, no thin walled region is provided and, instead, the fluid passage opens into the recess 22. Seals 32 are provided between the sleeve 26 and the rotor body 12 such that fluid delivered to the recess 22 via the fluid passage is retained within the recess 22 and does not escape therefrom between the rotor body 12 and the sleeve 26. After positioning of the magnets 24 and formation or location of the sleeve 26 to close the recess 22, a filler or adhesive in the form of a thermosetting or thermoplastic resin is supplied under pressure to the fluid passage. The fluid supplied under pressure acts upon the sleeve 26 causing radial expansion thereof, and so induces the required level of pre-stressing therein. With the fluid pressure maintained, curing or setting of the resin is undertaken such that, once cured or set, the pre-stressing of the sleeve 26 is locked in.

Preferably, the resin used is one that sets or cures without the application of heat. By way of example, the resin may be injected into the rotor with the rotor at high temperature, and subsequent cooling of the rotor results in curing or setting of the resin.

As with the arrangement described with reference to Figures 1 and 2, a mould may be used during the pre-stressing operation to limit expansion of the sleeve 26.

It will be appreciated that, as with the embodiment shown in Figures 1 and 2, the rotor 10 of Figure 3 is relatively simple to manufacture. It will be appreciated that whilst the description hereinbefore is of two specific example embodiments of the invention, a wide range of modifications and alterations may be made thereto without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. A rotor for an electrical machine comprising a rotor body, a plurality of magnets carried upon the body, and a sleeve encircling the magnets and the body and retaining the magnets in position upon the body, wherein the body includes a fluid passage through which a fluid can be supplied to expand the sleeve so that the sleeve assumes a pre-stressed condition.

2. A rotor according to Claim 1, wherein the sleeve is of a metallic material.

3. A rotor according to Claim 1, wherein the sleeve is of a composite material.

4. A rotor according to Claim 3, wherein the composite material is a wound fibre composite material, wound in situ upon the body.

5. A rotor according to any of the preceding claims, wherein the body defines a thin walled region adjacent the position at which the magnets are located, the application of fluid under pressure to the fluid passage applying an outwardly directed load to the thin walled region, urging the thin walled region and the magnets located adjacent thereto in an outward direction to expand the sleeve.

6. A rotor according to Claim 5, wherein the application of fluid under pressure permanently plastically deforms the thin walled region such that the sleeve is permanently pre stressed.

7. A rotor according to any of the preceding claims, wherein the body is provided with a recess in its outer surface in which the magnets are located.

8. A rotor according to Claim 7, wherein a filler or adhesive material is located, with the magnets, in the recess.

9. A rotor according to any of Claims 1 to 4, wherein fluid supplied via the flow passage is able to act directly upon the sleeve, applying a load thereto to expand and pre-stress the sleeve.

10. A rotor according to Claim 9, wherein the resin is maintained under pressure until curing or solidification thereof has taken place with the result that the cured or solidified resin material serves to permanently pre-stress the sleeve.

11. A method of manufacture of a rotor for an electrical machine comprising providing a rotor body, a plurality of magnets carried upon the body, and a sleeve encircling the magnets and the body and retaining the magnets in position upon the body, the body including a fluid passage through which a fluid can be supplied to apply a load to the sleeve to cause expansion thereof, the method including a step of applying fluid under pressure to the said fluid passage to expand the sleeve.

12. A method according to Claim 11, wherein the rotor body, magnets and sleeve are located within a mould prior to the application of the fluid under pressure.

13. A method according to Claim 12, wherein the mould serves as a support, limiting the degree by which the sleeve is expanded during the pre-stressing and aiding in ensuring that the shape and size of the sleeve, once pre-stressed, are as desired.

14. A method according to any of Claims 11 to 14, wherein the rotor is as defined by any of

Claims 1 to 10.