Motor

The current invention relates to a magnetic pole arrangement comprising a plurality of magnetic pole assembles arranged back-to-back along a longitudinally extending axis of rotation X. Each providing flux to an air gap G. Each magnetic pole assembly comprising one or more magnetic poles pieces and two components of magnetic flux. The first component of magnetic flux provided by a plurality of axially magnetised axially displaced magnets arranged in circumferentially extending arrays. The second component of magnetic flux provided by a plurality of circumferentially magnetised magnets circumferentially spaced around the axis of rotation X.

BACKGROUND OF THE INVENTION

The present invention is related to rotating electrical machines and relates particularly but not exclusively to a radial field electric motor and a rotor arrangement for such a motor which focuses the magnetic flux produced and which is able to allow for the axial stacking of multiple rotor assemblies.

Magnetic flux focusing in rotors of rotating electric machines employing magnets as a source of excitation can be realised using various magnetic circuit arrangements (topologies, designs). The purpose of flux focusing is to achieve high magnetic flux density in the air gap between the stator and rotor, possibly even higher than remanent flux density of magnets. Boulder Wind Power, Inc. (US2016247616 (A1): 2016, Aug. 25) introduced several different magnet and pole piece arrangements, utilizing concepts of flux focusing, which can be incorporated in various electrical machine topologies. Traditionally, flux focusing in radial field electrical machines is realized using one or more magnet arrays embedded in rotor made of laminated electrical steel. Well known practical realisations are spoke or V type rotor topologies where consecutive magnets are magnetised so that they oppose each other. In spoke arrangement, the magnets are magnetised in circumferential direction while in V type rotor, flux leaving the magnets has circumferential and radial component. Additional magnets magnetised in radial direction could be added to both topologies in order to limit flux leakage and further boost the air gap flux density. Examples of radial flux focused rotor are disclosed in: Meidensha Electric Mfg Co. Ltd (JP2016082733 (A): 2016, May 16), Wolfgang Volkrodt (U.S. Pat. No. 4,127,786 (A): 1978, Nov. 28), Samsung Electronics Co. Ltd. (US2014375162 (A1): 2014, Dec. 25) and JTEKT CORP (JP2017055493 (A): 2017, Mar. 16). While spoke type topology is an example of one dimensional flux focusing having magnets providing flux only circumferential direction, V type rotor can be seen as two dimensionally focused since magnets provide flux in circumferential as well as radial direction. Two-dimensional flux focusing can also be realised by combining an array of circumferentially magnetised magnets with and array of magnets magnetised in the direction of rotor's axis of rotation (axially magnetised magnets). This concept is disclosed by K. Atallah and J. Wang (A rotor with axially and circumferentially magnetized permanent magnets, IEEE Transactions on Magnetics, November 2012). Magnetic flux generated by both arrays of magnets is guided towards the electrical machine air gap by magnetic pole pieces. Since the magnetic flux enters the pole piece in circumferential and axial direction and leaves it in radial direction, the flux path is distinctly three dimensional. Similarly, magnetic end plates, providing return path for flux generated by the axially magnetised magnets, guide flux in three-dimensional fashion.

It was shown by K. Atallah and J. Wang (A rotor with axially and circumferentially magnetized permanent magnets, IEEE Transactions on Magnetics, November 2012) that for the two-dimensional flux focusing arrangements, the flux density in the air gap is highest when the axial length of the rotor is short. This is because of the contribution from axially magnetised magnets. For many applications, it is required to have a rotor with small outer diameter and high axial length (for example, if low inertia is required).

The present invention introduces a concept of three-dimensional flux focusing for rotors of electrical machines. Magnetic field in the pole pieces of an electrical machine rotor is excited by magnetic flux sources providing flux in all three directions in such a way that three-dimensional flux focusing is achieved. Detailed description of various three-dimensional flux focused arrangements is disclosed in this document. Due to the three-dimensional flux focusing, air gap flux density can be substantially higher than remanent flux density of magnets employed in the rotor, thus making it possible to use low cost non-rare earth magnets with low remanent flux density.

Furthermore, sources of magnetic flux prevent flux leakage from pole pieces in their respective direction since they actively oppose it. This is of particular benefit in case of flux leaking in radial direction through rotor hub/shaft which would normally need to be made of non-magnetic and potentially expensive material. The three-dimensional flux focusing eliminates this problem by minimizing radial flux leakage. Since the pole pieces and rotor hub can be made of the same magnetic material, they can also be manufactured as a single body, reducing the number of components in the assembly. It is also shown how a single piece magnet can be used to provide flux in all the three directions, with a benefit of significantly reducing the number of components to assemble and constrain.

The concept of three-dimensional flux focusing and axial stacking presented in this document makes it possible to achieve high air gap flux densities in electrical machines equipped with this rotor despite using weak sources of magnetic flux. Consequently, low cost permanent magnets such as ferrites can be employed instead of high-performance rare earth based permanent magnets which are currently prevalent but suffer from supply chain issues and are substantially more expensive. Electrical machines using this concept show potential to achieve similar efficiency and power density to rare earth based electrical machines, and improved performance over current state-of-the-art (SoA) non-rare earth technologies. Because of this, the presented rotor technology is particularly suitable for applications where high performance, low cost and robustness is required. Additionally, stable supply of ferrite magnets enables relatively low risk high volume production. Due to all these benefits, the invention has the potential to accelerate wide spread adoption of environmentally friendly technologies. Among applications which are most likely to benefit from the present invention are reduced or zero emission automotive traction and renewable power generation.

The invention discloses the concept of flux focusing in all the three dimensions around the magnetic poles of the rotor in order to improve the density of flux through the poles and hence the performance of the rotor. This is achieved using novel magnet topology or arrays of magnets such that when put together, provide flux in the circumferential, axial as well as radial directions of the rotor and focuses the flux through the magnetic poles of the rotor.

The invention also discloses a method of reducing the radial dimensions of the rotor without compromising the performance, and potentially improve the generated torque. This method involves stacking of the flux-focusing units in the rotor's axial direction.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the current invention a magnetic pole arrangement having a longitudinally extending axis of rotation X and comprising a plurality of magnetic pole assemblies is arranged back-to-back along said longitudinally extending axis of rotation X. Each magnetic pole assembly providing flux to an air gap G. Each magnetic pole assembly may comprise one or more magnetic pole pieces and two components of magnetic flux. Each magnetic pole piece may include a first axial face, a second axial face a first circumferential face, a second circumferential face, a radially inner surface and a radially outer surface. Said first component of magnetic flux may comprise a plurality of first and second axially displaced axially magnetised magnets. Said axially magnetised magnets having a north side N and a south side S may be arranged in respective circumferentially extending arrays adjacent respective first and second axial faces of said one or more magnetic pole pieces. Said second component of magnetic flux may comprise a plurality of circumferentially magnetised magnets. The circumferentially magnetised magnets each having a north side N and a south side S may be circumferentially spaced around said axis X relative to each other. The circumferentially magnetised magnets may lie adjacent respective first circumferential faces and respective second circumferential faces of each magnetic pole piece.

According to a further aspect of the current invention each magnetic pole assembly may include first and a second circumferentially and radially extending magnetic plates. Said first plate may contact and extend between the respective first circumferentially extending axially magnetised magnets. Said second plate may contact and extend between respective second circumferentially extending axially magnetised magnets.

Said first plate of a first pole assembly may also form the second plate of a next adjacent pole assembly.

The magnetic pole arrangement may further include a plurality of third components of magnetic flux. Each third components of magnetic flux may comprise a radially magnetised magnet having a north side N and a south side S, and each may be provided adjacent said respective radially inner surfaces or said respective radially outer surfaces of the magnetic pole pieces.

The magnetic pole arrangement may further include a ferromagnetic tube attached to said radially magnetised magnets.

Each of said first component of magnetic flux may comprise a plurality of axially magnetised circumferentially spaced central magnets and a plurality of circumferentially magnetised circumferentially spaced side magnets. Wherein said central magnets and respective side magnets may be arranged in a Halbach array.

According to a further aspect of the present invention the magnetic pole arrangement may comprise circumferentially adjacent pole pieces in which each magnetic pole assembly is arranged in alternating North and South magnetic polarity.

Axially adjacent pole pieces in each magnetic pole assembly may be arranged in alternating North and South magnetic polarity.

Axially adjacent pole pieces in each magnetic pole assembly may be circumferentially skewed or offset relative to each other.

The axially and radially extending circumferentially magnetised second component of magnetic flux may extend axially past the inner faces of said plurality of first and second axially displaced axially magnetised magnets.

The arrangement may include a combined source of said first and second components of magnetic flux. Said combined source of first and second components of magnetic flux may comprise a triangular cross-sectioned structure in which said triangular cross-sectioned structure may have an axially, radially and circumferentially extending first surface confronting an adjacent pole piece and the pole piece may have a circumferential width W which varies along axial direction A.

An alternative arrangement may include a combined source of said first and second components of magnetic flux which may comprise a quadrilateral cross-sectioned magnet having first and second oppositely facing axial surfaces where the surfaces confront adjacent pole pieces and the combined source of said first and second components of magnetic flux may have circumferential width W which varies along axial direction A. The plurality of pole pieces may each having a hexagonal cross-sectioned structure having oppositely facing axially, radially and circumferentially extending end surfaces each confronting a respective face of an adjacent component of magnetic flux.

In a still further alternative arrangement, said magnetic pole pieces may comprise a hexagonal cross-sectioned structure having first and second oppositely facing axially displaced end surfaces and first and second oppositely facing circumferentially displaced side surfaces and said combined source of said first and second components of magnetic flux may comprise a hexagonal cross-sectioned structure having oppositely facing axially, radially and circumferentially extending end surfaces and oppositely facing radially and circumferentially extending blank ends and further include a plurality of radially and axially extending circumferentially spaced supplemental magnets each having first and second oppositely facing axial ends and oppositely facing side surfaces.

Said blank ends of the source of magnetic flux may confront respective first and second axial ends of said supplemental magnets and said side surfaces of said pole pieces may confront respective oppositely facing side surfaces of said supplemental magnets.

Each pole piece may include a pole shoe extending away from a radial surface. The pole shoe may not have a radial source of magnetic flux adjacent thereto. The said pole shoe may include a pole face which is both radially and circumferentially offset relative to the pole piece.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is related to three-dimensional flux focused poles able to create flux density in excess of 1 Tesla in the air gap of electrical machines.

FIG. 1aillustrates the concept of three-dimensional flux focusing as a method of providing magnetic flux in three different directions to a magnetic guiding element called pole piece30, which combines the three-directional flux into a single stream and guides the flux towards the air gap G of an electrical machine. The magnetic pole piece in the preferred embodiment is formed of a ferromagnetic material. In another embodiment the pole piece can be formed of any soft magnetic material. Flux is provided in one or more of x and −x, y, and z and −z directions. Opposing axial faces32of the pole piece30(z axis) are subject to flux in opposing directions. Opposing circumferential faces34of the pole piece30(x axis) are subject to flux in opposing directions. Of the radial faces36(y axis) a first face may be subject to flux and a second face provides output flux to the air gap G. At the faces flux is provided, the direction relative to the face at which it is provided will be the same. The sources of flux40,50,60in all the directions are in parallel from an equivalent magnetic circuit point of view such that the total flux is the sum of fluxes provided by each source. It is therefore possible to utilize several relatively weak sources of flux and still achieve high air gap flux density. Each source of magnetic flux40,50,60for the pole piece30must be oriented so that it contributes to the total output flux20for the pole piece30. Each source of magnetic flux also actively supresses leakage of flux in its respective direction. For instance, a source of magnetic flux providing flux in axial direction A acts against leakage of flux in axial direction A.

The three-dimensional flux shown inFIG. 1ais provided in direction of x, y and z axis with reference to a Cartesian coordinate system. If referring to a cylindrical or polar coordinate system the flux is provided in circumferential C, radial R, and axial A directions equivalent to x, y and z respectively, as shown inFIG. 1b.

FIG. 1bshows a plurality of magnetic pole pieces30arranged in a circular pattern or array. The embodiment shown comprises 8 magnetic pole pieces, but it will be appreciated that one or more pole pieces are possible. Magnetic pole pieces30can have radially and circumferentially extending axial faces32comprising a first axial face32A and a second axial face32B. Magnetic pole pieces30can also have axially and radially extending circumferential faces34comprising a first circumferential face34A and a second circumferential face34B. Magnetic pole pieces30can also have axially and circumferentially extending radial surfaces36comprising a radially inner surface36A and a radially outer surface36B.

FIG. 2ashows components of a simple three dimensionally flux focused pole. It comprises a guiding element (magnetic pole piece30) and 5 block shaped magnets40,50,60attached to 5 sides of the magnetic pole piece30. There are two magnets providing flux in axial and circumferential directions (axially magnetised magnets40and circumferentially magnetised magnets50) and one providing flux in radial direction (radially magnetised magnet60). The sixth side of the pole piece30with no magnet attached is the side adjacent to the air gap. In an electrical machine, such configuration would represent a single pole and would be combined with other similarly looking elements into a circular array providing number of magnetic poles, as depicted inFIG. 2b. As shown inFIG. 2b, the polarity (N or S) of the magnetic pole depends on the direction of magnetisation of the magnets40,50,60. For the purpose of clarity in this document, we term this circular array as ‘rotor unit’120. In some embodiments shown later, several of these ‘rotor units’120are stacked in the axial direction. This stacked assembly is termed as ‘rotor’ in this document.

FIG. 2aandFIG. 2dshow possible embodiments of sources of magnetic flux40,50,60arranged around a magnetic pole piece30to form a magnetic pole assembly110for providing focused flux to an airgap G. The first source of magnetic flux40comprises a first radially and circumferentially extending segment40A and a second radially and circumferentially extending segment40B which is axially displaced along axis X from the first40A. Respective sources of magnetic flux40A,40B lie adjacent respective axial faces32A,32B of a pole piece30. The second source of magnetic flux50comprises a first and second axially and radially extending circumferential segment50A,50B which lie adjacent respective circumferential faces34A,34B of the pole piece30. The third source of magnetic flux60comprises circumferentially and axially extending first and second radial segments60A,60B which are radially displaced relative to each other and lie adjacent respective first and second radial faces36A,36B of the pole piece30. The arrangement described above allows flux to flow from the one or more sources of magnetic flux40,50,60into the one or more magnetic pole pieces30in a manner that concentrates the magnetic flux in the manner illustrated in multiple figures by way of outlined arrows. It will be understood that said arrows represent a flow of flux within a magnet from a south pole to a north pole with the arrow head representing the north direction. InFIG. 2athe radial segment60A is provided on a radially inner surface of the pole piece30whilst inFIG. 2dthe radial segment is provided on the radially outer surface of the pole piece30. It will be appreciated that one or other of these two arrangements may be used depending on the layout of the assembly but both cannot be used together as this would not allow for flux focussing.

FIG. 2aandFIG. 2bshow the magnetic flux provided by the pole pieces30and the sources of magnetic flux40,50,60in an assembled state. The pole pieces30each have a north side N and a south side S radially displaced from one another. The first one or more axial sources of magnetic flux40comprise axially magnetised magnets having a north side N and a south side S displaced axially along Axis X from each other. The second one or more circumferential sources of magnetic flux50comprise circumferentially magnetised magnets having a north side N and a south side S displaced circumferentially around axis X relative to each other. The third one or more circumferential sources of magnetic flux60comprise radially magnetised magnets having a north side N and a south side S displaced radially relative to each other. Said sources of magnetic flux40,50,60lying with same side adjacent to the pole piece30as the pole piece presents to the air gap i.e. if the pole piece presents its north side N to the air gap the sources of magnetic flux will lie with their north sides adjacent to the pole piece.

The one or more sources of magnetic flux40,50,60together with the pole piece30create a single magnetic pole assembly110. A plurality of the magnetic pole assemblies110arranged in a circular pattern or array around an axis of rotation X creates a radial field rotor unit120. In the preferred embodiment the plurality of magnetic pole assemblies110comprising a plurality of pole pieces30, the consecutive pole pieces having alternate magnetic polarity as displayed inFIG. 2b. In an alternative arrangement the plurality of magnetic pole assemblies110comprising a plurality of pole pieces30, the magnetic polarity of the pole pieces alternating every other pole piece. Pole pieces30are typically employed in electrical machines in this fashion to provide electromagnetic excitation

FIG. 2ashows a magnetic pole assembly110suitable for use in the radial field internal rotor unit shown inFIG. 2b. In this embodiment one or more sources of magnetic flux lie adjacent to the radially inner surface36A but not the radially outer surface36B. The radially outer surface36B being adjacent to the air gap G when used in an electric motor as shown inFIGS. 9aand9b.

FIG. 2dshows a magnetic pole assembly110suitable for use in the radial field external rotor unit shown inFIG. 2e. In this embodiment one or more sources of magnetic flux lie adjacent to the radially outer surface36B but not the radially inner surface36A. The radially inner surface36A being adjacent to the air gap G when used in an electric motor as shown inFIGS. 10aand10b.

The sources of magnetic flux are arranged to focus the magnetic flux of the pole piece30towards and out of the radial surface36not having a source of magnetic flux adjacent there to into the air gap G.

The one or more sources of magnetic flux40,50,60utilized in the 3D flux focused magnetic pole assemblies of the preferred embodiment are permanent magnets as shown inFIG. 2a-e. However, in an alternative arrangement, as shown inFIG. 2f-h, the one or more sources of magnetic flux40,50,60are electro-magnets comprising coils400,500,600carrying electric current. In another embodiment the source of magnetic flux40,50,60is a combination of coils400,500,600carrying electric current and permanent magnets or any other source of magnetic flux may be used. The coils400,500,600may be wrapped around a core of magnetic material410,510,610. The coils400,500,600may be supplied by electric current from a source of electric current700. Said electrical current passing from the stationary source of electric current700to the coils400,500,600on the rotating magnetic pole assembly110by way of a carbon brush810and slip ring820assembly800.

Although, a radial field internal rotor unit120is shown inFIG. 2bandFIG. 2c, the same concept of three-dimensional flux focusing can be used to construct a radial field external rotor unit120as shown inFIG. 2e. InFIG. 2d, one pole for the radial field external rotor unit is depicted where the face at the inner radius of the pole piece30is adjacent to the air gap and the radially magnetised magnet60is placed on the face located at outer radius of the pole piece30. A circular array of such poles with alternating magnetic polarity creates the rotor unit120shown inFIG. 2e.

In addition to the sources of magnetic flux40,50,60and the flux guiding element or magnetic pole piece30, further magnetic components can be provided for the purpose of further improving effectiveness of three-dimensional flux focusing. For instance, ferromagnetic plates100(shown inFIGS. 2cand 9a) can provide return path for flux generated by the axial sources of magnetic flux or magnetised magnets40and effectively reduce reluctance of flux path for any of the flux sources. In one embodiment said ferromagnetic plates100extend radially and circumferentially. The ferromagnetic plates100can comprise a first ferromagnetic plate100A and a second ferromagnetic plate100B. Said first and second ferromagnetic plates100A,100B can each have a first face100A1,100B1. The first face100A1,100B1of the respective first and second plates100A,100B adjacent respective first and second segments40A,40B. Further additional components such as a ferromagnetic axially extending tubular structure101may be included to provided return path for flux and to reduce reluctance of the flux path. For instance, in the embodiment shown inFIG. 2c,FIG. 10aandFIG. 10b, ferromagnetic axially extending tubular structure101lies adjacent to radial sources of flux60. The plates100and the tube101must be dimensioned so that the flux passing through them will not cause excessive saturation.

In addition, a single component or structure can provide multiple sources of magnetic flux40,50,60in one or more directions.

The further embodiment shown inFIG. 3apresents alternative arrangement to that with 5 sources of magnetic flux. A single piece, unitary structure71, provides a second circumferential source of flux50and a third radial source of flux60and replaces three out of 5 individual sources of magnetic flux, thus reducing the number of components providing a source of magnetic flux to 3 in order to create a magnetic pole. Here, the single piece, unitary structure71provides flux in two of the three directions. Separate axial sources of magnetic flux40provide flux in axial direction for each pole, thus realizing the three-dimensional flux focusing for the pole piece30. This arrangement can be put together in a circular array to construct a radial field internal rotor unit120as depicted inFIG. 3b.

Similarly,FIG. 4apresents another further embodiment of a magnetic pole assembly110when a single piece, unitary structure72is used to provide a first axial source of flux40and a third radial source of flux60. The source of flux in the circumferential direction is provided by separate sources of circumferential magnetic flux50. A circular array of this arrangement is shown inFIG. 4b.

Another alternative embodiment of a magnetic pole assembly110is shown inFIG. 5awhere a single piece, unitary structure73provides a first axial source of magnetic flux40and a second circumferential source of magnetic flux. A separate radial source of magnetic flux60provides flux in radial direction for each pole. A circular array of this arrangement is shown inFIG. 5b.

The further embodiment shown inFIG. 7presents an alternative arrangement where the topology and direction of magnetisation of magnets in the magnetic pole assembly110is such that a single unitary structure74provides a source of magnetic flux40,50,60in all the three directions: axial, radial and circumferential. The shown topology encloses the magnetic pole pieces30across the faces at the inner radius and rotor unit's110circumferential and axial directions.

The advantage of the concepts with the embodiments shown inFIG. 3,FIG. 4FIG. 5andFIG. 7is that it tends to reduce loss of useful magnetic flux due to field fringing near the magnet edges. This phenomenon is shown inFIG. 6. Single piece magnet providing flux in two directions has only two edges where field fringing occurs (FIG. 6a), unlike similar arrangement with 3 separate magnet blocks which has 6 edges where field fringing occurs (FIG. 6b). Field fringing results in increased reluctance and a loss of flux density and a reduction in the flux focusing effect.

Although, a radial field internal rotor unit120is shown in these embodiments (FIG. 3b,FIG. 4bFIG. 5bandFIG. 7), the same concept of three-dimensional flux focusing can be used to construct a radial field external rotor unit120similar to that illustrated inFIG. 2eby changing the shape or placement of magnets such that the focused radial flux is provided on a face at external radius of the pole piece30.

Although, a radial field internal rotor unit120is shown in this embodiments ofFIG. 2,FIG. 3,FIG. 4FIG. 5andFIG. 7, the same concept of three-dimensional flux focusing can be used to construct a radial field external rotor unit120similar to that illustrated inFIG. 2eby changing the shape or placement of magnets such that the radial flux is provided on a face at external radius of the pole piece30.

FIG. 11ashows a simple example of stacking of the individual rotor units120shown inFIG. 2bwhen stacked in the axial direction X and comprising a plurality of magnetic pole assemblies110arranged back-to-back along said longitudinally extending axis of rotation X.

The embodiment of the magnetic pole arrangement300includes radially and circumferentially extending plates100A,100B made of magnetic or ferromagnetic material. said first plate100A contacting and extending between respective first circumferentially extending axially magnetised magnets40A and said second plate100B contacting and extending between respective second circumferentially extending axially magnetised magnets40B. The active length of the rotor is defined as the portion of the rotor through which magnetic flux enters the air gap. This active length in (illustrated inFIG. 11b) is provided by the axial dimension of the magnetic pole piece30. The axial length containing plates100and axially magnetised magnets40does not provide radial flux in the air gap. To increase the active length of a rotor130relative to its overall length it is possible for the first plate100A to also form the second plate100B of a next adjacent magnetic pole assembly110as inFIG. 12a.

In any embodiment it is beneficial for a magnetic pole arrangement300if a magnetic return path is provided for the third plurality of radial components of magnetic flux60generated by radially magnetised magnets60. This return path can be provided by axially extending tubular structure101, hub or shaft made of magnetic or ferromagnetic material. Said axially extending tubular structure101can be included in an internal rotor embodimentFIG. 2cor an external rotor embodimentFIGS. 9aand9b.

It is also possible to implement skewing of the rotor units120such that the poles in the neighbouring rotor units are not axially aligned. This is can be implemented by rotation of the whole rotor unit120about the axis of the rotor. This skew angle can be adjusted depending on the desired performance of the rotor.

Each rotor unit120in this embodiment has an array of radially magnetised magnets60placed at the inner radius (below the magnetic pole pieces30shown inFIG. 11b) of the magnetic pole pieces30as shown inFIG. 11a. This would further improve the flux density in the air gap between rotor and stator.

Although, a radial field internal rotor is shown in this embodiment (FIG. 11aandFIG. 11b), same concept of three-dimensional flux focusing can be used to construct a radial field external rotor using rotor unit120similar to that illustrated inFIG. 2eby changing the placement of radially magnetised magnets60such that the radial component of flux60is provided on a face at external radius of the pole piece30.

The concept of Halbach array can be used to reduce axial length of the rotor consisting of two or more rotor units120and to improve ratio of magnetically active to passive length of the rotor. Halbach array is an array of magnets arranged in such manner that magnetic field on one side (strong side) is strengthened and on the opposite side (weak side) weakened to near zero flux density. This is achieved by having a spatially rotating pattern of magnetisation. A Halbach array comprises a rotating pattern of permanent magnets that can be continued indefinitely and have the same effect. The sequence rotates the field of the magnet 90 degrees for each permanent magnet. The effect is illustrated by the lines96inFIG. 12awhere lines94show the flow of flux through a Halbach Array. Each of said first components of magnetic flux40A,40B may comprise a plurality of axially magnetised circumferentially spaced central magnets97F or97S and a plurality of circumferentially magnetised circumferentially spaced side magnets98F,98S arranged in a Halbach array such that circumferentially adjacent pole pieces30in each magnetic pole assembly110are arranged in alternating North and South magnetic polarity. As shown, said side magnets98F,98S are interposed between respective ones of said central magnets97F or97S and are each magnetised in opposite circumferential directions E1, E2whilst central magnets97F,97S are alternately magnetised in different axial directions D1, D2. The combined magnetic effect of said central magnets (97F,97S) and said side magnets (98F,98S) is applied to respective adjacent pole pieces30in the direction of arrows96ofFIG. 12aor12b. The plurality of central magnets97F or97S and a plurality of side magnets98F,98S may be arranged in a repeating circumferential sequence. The sequence may comprise alternating centre magnets97F,97S and side magnets98F,98S. The centre magnets97F,97S may be arranged with the north side adjacent a magnetic pole piece30presenting a north side to the air gap G. The centre magnets97F,97S may be arranged with the south side adjacent a magnetic pole piece30presenting a south side to the air gap G. The side magnets98F,98S in said sequence may be orientated to guide flux from centre magnets97F,97S with a south side adjacent a magnetic pole piece30to centre magnets97F,97S with a south side adjacent a magnetic pole piece30. In said sequence the magnetic field of each sequential magnet rotates 90 degrees to the last and 180 degrees to the magnet before. Since flux density on the weak side of the Halbach array is nearly zero, there is no need for a plate100made of magnetic material to provide return path for the flux generated by the magnets91in the array. Because of this, the plates100can be eliminated thus reducing axial length of the rotor. While plates100can be eliminated, they still may be included to provide mechanical support for the rotor. However, the plate's100thickness is not dictated by the need to guide magnetic flux. Additionally, the thickness (axial dimension) of Halbach array magnets91can be less than the thickness of axially magnetised magnets40shown in the previous embodiment (FIG. 11aandFIG. 11b) because the field on the strong side of Halbach array91A is boosted compared to field produced by only axially magnetised magnets40.

A magnetic pole arrangement300can also implement Halbach array concept using an axially longer circumferentially magnetised magnets50as shown inFIG. 12b. in this arrangement said axially and radially extending circumferentially magnetised second component of magnetic flux50may extend axially past inner faces40iof said plurality of first40A and second40B axially displaced axially magnetised magnets40. Although not shown in these embodiments, radially magnetized magnets can also be replaced by radial Halbach array having the strong side adjacent to the pole piece.

Each rotor unit120in this embodiment can have an array of radially magnetised magnets60(as shown inFIG. 11a) placed at the inner radius (below the magnetic pole pieces30shown inFIG. 12aandFIG. 12b) of the magnetic pole pieces30. This would further improve the flux density in the air gap between rotor and stator.

Although, a radial field internal rotor is shown in these embodiments (FIG. 12aandFIG. 12b), same concept of three-dimensional flux focusing can be used to construct a radial field external rotor using rotor unit120similar to that illustrated inFIG. 2eby changing the placement of radially magnetised magnets such that the radial flux is provided on a face at external radius of the pole piece30.

FIG. 13shows an embodiment aiming to reduce axial length of rotor consisting of more than one rotor units120. In this case, axially magnetised magnets and circumferentially magnetised magnets are replaced by a combined source92of said first and second magnetic flux40,50. This combined source92may have a triangular cross-sectioned structure radially. Said combined source92may be magnetised to provide both first axial40and second circumferential50components of magnetic flux as depicted by arrows inFIG. 13. The combined source92may have an axially, radially and circumferentially extending first surface92A confronting an adjacent pole piece31. Wherein the pole piece31may have a circumferential width W which varies along the axial direction A and, in this embodiment, may be diamond shape. The triangular cross-section magnets92may have magnetisation direction perpendicular to the pole face to which they provide flux. This means that the vector of flux density entering the magnetic pole pieces31has circumferential and axial components, thus providing flux in two directions. This arrangement eliminates the need for plates100(as shown inFIG. 11) since magnetic flux is guided from one pole to another through triangular cross-section magnets92and therefore doesn't need additional return path. While plates100are not necessary from magnetic point of view, they may be included in an alternative embodiment to improve mechanical robustness of the rotor. However, their thickness is not dictated by the need to guide magnetic flux. Each rotor unit120in this embodiment can have an array of radially magnetised magnets60(as shown inFIG. 11a) placed at the inner radius (below the magnetic pole pieces31shown inFIG. 13) of the magnetic pole pieces31. This would further improve the flux density in the air gap between rotor and stator.

Although, a radial field internal rotor is shown in this embodiment (FIG. 13), the same concept of three-dimensional flux focusing can be used to construct a radial field external rotor using rotor unit120similar to that illustrated inFIG. 2eby changing the placement of radially magnetised magnets such that the radial flux is provided on a face at external radius of the pole piece31.

Similar to the previous embodiment of magnetic pole arrangement300shown inFIG. 13, further alternative embodiments depicted inFIG. 14aandFIG. 14bshow a magnetic pole arrangement300having more than one rotor units120comprising a plurality of magnetic pole pieces33each having a hexagonal cross-section structure (circumferential width varying with axial length). The magnetic pole piece33may also having oppositely facing axially, radially and circumferentially extending end surfaces33A,33B and first and second oppositely facing circumferentially displaced side surfaces (33C,33D). In these embodiments the first and second components of magnetic flux40,50are provided by a combined source.

An alternative arrangement of a magnetic pole arrangement300is shown inFIG. 14aand in which said combined source93comprises a quadrilateral cross-sectioned magnet93having circumferential width W which varies along axial direction X. Said magnet93may have first and second oppositely facing axial surfaces93A,93B confronting adjacent pole pieces33. The magnetic pole piece33end surfaces33A,33B each confronting a respective face93A or93B of an adjacent component of magnetic flux93.

In a still further alternative arrangement of a magnetic pole arrangement300is shown inFIG. 14band in which said combined source94comprises a hexagonal cross-sectioned structure magnet94. Said hexagonal cross-sectioned structure94may have oppositely facing axially, radially and circumferentially extending end surfaces94A,94B and oppositely facing radially and circumferentially extending blank ends94C,94D. The combined source of magnetic flux may further include a plurality of radially and axially extending circumferentially spaced supplemental magnets51. Said magnets51each having first and second oppositely facing axial ends51A,51B and oppositely facing side surfaces51C,51D. The blank ends94C,94D of the source of magnetic flux94may confront respective first and second axial ends51A,51B of said supplemental magnets51whilst said side surfaces33C,33D of said pole pieces33may confront respective oppositely facing side surfaces51C,51D of said supplemental magnets51. The magnets93,94are only magnetised in circumferential direction but due to their orientation and the way they interface with magnetic pole pieces33, vector of flux density entering magnetic pole pieces33has axial and circumferential component.

A circumferential gap between neighbouring magnetic pole pieces33is necessary to limit pole to pole leakage of magnetic flux.FIG. 14ashows this circumferential gap. It is possible to place circumferentially magnetised magnets50in the gap between neighbouring magnetic pole pieces33, as shown inFIG. 14b. The additional circumferentially magnetised magnets50will limit pole to pole flux leakage and also provide additional flux, boosting magnetic performance of the rotor.

While plates100(as shown inFIG. 11) are not necessary from magnetic point of view, they can be included to improve mechanical robustness of the rotor. However, their thickness is not dictated by the need to guide magnetic flux.

The rotor unit120in any embodiment can have an array of radially magnetised magnets60(as shown inFIGS. 5 & 6) placed at the inner radius of the magnetic pole pieces30. This would further improve the flux density in the air gap between rotor and stator.

Although, a radial field internal rotor is shown in some embodiments, the same concept of three-dimensional flux focusing can be used to construct a radial field external rotor using rotor unit120similar to that illustrated inFIG. 2eby changing the placement of radially magnetised magnets such that the radial flux is provided on a face at external radius of the pole piece30.

In the further alternative embodiment shown inFIG. 15acomprising magnetic pole pieces30, axially magnetised magnets40and circumferentially magnetised magnets50the axially adjacent magnetic pole pieces30in each magnetic pole assembly110belonging to two different rotor units120may be connected by only one axially magnetized magnet40. Therefore, axially adjacent pole pieces30may be arranged in alternating south and north polarities. This creates a rotor130with stepped skew. Such step skewing of rotors130is a well-known method of reducing torque ripple in electrical machines.

The embodiment inFIG. 15bshows that parallelogram cross-section magnets95can be used to connect rotor units120, providing axially adjacent pole pieces30in each magnetic pole assembly110that are circumferentially skewed or offset relative to each other. Thus, allowing for shift of rotor units120in circumferential direction with respect to each other, thus reducing skew angle. In extreme case, the shift angle can be such that north and south poles in all rotor units120are perfectly aligned with skew angle ˜0. Depending on desired performance, this skew angle can be varied.

The concept of skewing is further extended in the embodiment shown inFIG. 15cwith three rotor units120. Between the rotor units120, the magnets95have a parallelogram cross-section in the radial view. The array of parallelogram cross-section magnets95between first and second rotor units120aand120bhas different orientation and magnetisation direction compared to the array of parallelogram cross-section magnets95between second and third rotor units120band120c. Such arrangement again changes the skew angle between the rotor units. Having only one array of magnets95between rotor units120reduces axial length of the rotor130and improves active to passive axial length ratio.

Plates100made of magnetic material are only needed to provide return path for flux generated by axially magnetised magnets40at axial ends of the rotor130. Plates can also be included between the rotor units120to improve mechanical robustness of the rotor.

Embodiment depicted inFIG. 8acomprises a first axial source of magnetic flux40, a second source of circumferential magnetic flux50and each magnetic pole piece30includes a pole shoe38. Since magnetic pole pieces30belonging to two neighbouring rotor units120aand120bare connected by only one array of axially magnetized magnets40, alternate south and north poles are formed in axial direction. The pole shoes38inFIG. 8are used to offset pole face38F with respect to the magnetic pole piece30in both circumferential and radial directions so that north and south poles created by the rotor units120aand120bare aligned. Axial alignment of poles can be either perfect or can have skewed. The partial axial cross-sectional views are shown inFIG. 8bandFIG. 8cfor rotor units120aand120b, respectively. In the axial direction, alternate poles are created. Since the pole shoes are offset in the circumferential direction, the poles of neighbouring rotor units are aligned at the outer radius of the rotor, as depicted inFIG. 11b.

It will be appreciated that this embodiment allows the advantages of reduced axial length of the rotor130and improved active to passive axial length ratio given by the rotor arrangement ofFIG. 15to be maximised, with the benefit of independently being able to vary the skew of the flux presented to air gap G by sequential rotor units120.

Each rotor unit120in this embodiment can have an array of radially magnetised magnets60(as shown inFIG. 2b) placed at the inner radius (below the magnetic pole piece30shown inFIG. 8) of the magnetic pole piece30. This would further improve the flux density in the air gap between rotor and stator.

FIG. 9ashows the radial field internal rotor unit120shown inFIG. 2cas part of an electrical machine. In addition to the radial field internal rotor unit120previously described the machine comprises a stator200. Said stator200comprises a plurality of electro magnets220. The plurality of electro magnets include a plurality of fingers224of the core210around which a plurality of coils222which can be supplied with electric current. The electromagnets220are disposed in a circumferential direction C around the axis of rotation X.FIG. 9bshows an axial section of the electric machine ofFIG. 9a. The electric machine ofFIG. 9aandFIG. 9bshows four electromagnets comprising four sets of coils but it will be appreciated that not all the electromagnets220of the particular embodiment of the electrical machine are shown in said figures. In further embodiments it is possible for the stator200to comprise any plurality of electromagnets220.

FIG. 10ashows the radial field internal rotor unit120shown inFIG. 2ewith the addition of a cut away view of the axially extending tubular structure101, as part of an electrical machine. In addition to the radial field external rotor unit120previously described the machine comprises a stator200. Said stator200comprises a plurality of electro magnets220. The plurality of electro magnets comprise a plurality of fingers224of the core210around which a plurality of coils222which can be supplied with electric current sit. The electromagnets220are disposed in a circumferential direction C around the axis of rotation X.FIG. 10bshows an axial section of the electric machine ofFIG. 10a. The electric machine ofFIG. 10aandFIG. 10bshows four electromagnets comprising four sets of coils, it will be appreciated that not all the electromagnets220of the particular embodiment of the electrical machine are shown in said figures. In further embodiments it is possible for the stator200to comprise any plurality of electromagnets220.