GB2511082A

GB2511082A - Reluctance machines

Info

Publication number: GB2511082A
Application number: GB201303165A
Authority: GB
Inventors: Rajesh Pranay Deodhar
Original assignee: IMRA Europe SAS
Current assignee: IMRA Europe SAS
Priority date: 2013-02-22
Filing date: 2013-02-22
Publication date: 2014-08-27
Anticipated expiration: 2033-02-22
Also published as: GB2511082B; GB201303165D0

Abstract

A reluctance machine comprises a stator with an annular winding 22. Plural stator flux guides 24b are arranged around the annular coil 22 and guide flux generated by current flowing in the winding in a plane perpendicular to the current direction. A rotor has flux guides 26 which, when at least partially aligned with the stator flux guides 24 guide flux between their ends 60b, 62b. The stator and rotor flux guides can be around their respective circumferences. Flux can be guided either in a radial direction in the stator and an axial direction in the rotor, axially in the stator and radially in the rotor (fig 9) or at an angle to the rotation axis (fig 10) with the rotor flux guides lying on a virtual cone. The stator flux guides can be C shaped or T shaped (fig 14) and continuously formed (fig 16). Flux guides can comprise metallic powder, soft magnetic composite or metal laminations. There can be plural sets of independent windings and flux guides that can be circumferentially either aligned or offset and be continuous or have gaps between them. The machine can be a motor or a generator and single or three phase and be part of a pump in whose impellor comprises the rotor flux guides.

Description

RELUCTANCE MACHINES

The present disclosure relates to reluctance machines and in particular, but not exclusively, to reluctance machines such as reluctance motors and reluctance generators, and pumps comprising reluctance motors.

Reluctance motors typically tend to be low-cost and have a high power density. As such, they are becoming more popular in applications where these qualities are desirable.

A typical switched reluctance motor comprises 6 stator poles and 4 rotor poles (a so-called 6/4 motor). The rotor poles typically do not have any windings. The 6 stator poles are typically driven with a unipolar current supplied by a 3-phase inverter, each stator pole typically being driven using a square pulse of time period t/3, where t is the time period for three pulses.

A switched reluctance motor can be thought of as being driven by DC (direct current) pulses.

For switched reluctance motors, control circuitry is usually arranged to generate the unipolar drive current and power the stator poles in an appropriate sequence to cause the rotor to rotate by magnetic reluctance. In this context, "unipolar" is taken to mean flow of current in a coil only in one direction when the motor is in operation.

However, the number of windings used can mean that they can be relatively complex to manufacture, especially to maintain uniformity of windings between each of the windings for each phase. Additionally, typical reluctance motors can suffer from torque ripple at low speed.

Aspects of the invention are defined in the appended claims.

In a first aspect, there is provided a reluctance motor comprising: a stator comprising an annular winding through which an electrical current can pass so as to generate a magnetic field around the annular winding, the stator comprising a plurality of first flux guiding elements each having a respective first end and second end, each first flux guiding element being arranged around a portion of the annular winding so that magnetic field can be guided around the annular winding in a plane substantially perpendicular to a direction of current in the annular winding; and a rotor arranged in use to rotate with respect to the stator about an axis of rotation, the rotor comprising a plurality of second flux guiding elements, the second flux guiding elements being arranged so that, when at least partially aligned with at least one of the first flux guiding elements, the second flux guiding elements can guide magnetic flux between the first end and second end of respective first flux guiding elements.

In a second aspect, there is provided a reluctance motor comprising: a stator having an annular winding and a plurality of first flux guides spaced around the circumference of the annular winding, said first flux guides extending around a portion of the winding and each defining a flux path in a plane perpendicular to the direction of movement of a current along the winding; and a rotor comprising a plurality of second flux guides spaced around the circumference of the rotor, said rotor being arranged in use to rotate with respect to the stator and to bring the second flux guides into proximity with the first flux guides and to complete a flux path around the winding at each first flux guide.

Examples of the present disclosure will now be described by way of example with reference to the accompanying drawings, throughout which like references refer to like elements1 and in which: Figure 1 schematically illustrates a reluctance motor according to examples of the

disclosure;

Figure 2 schematically shows elements of a reluctance motor according to examples of

the disclosure:

Figure 3 schematically illustrates an annular winding together with a first flux guiding element and a second flux guiding element according to examples of the disclosure; Figure 4 schematically illustrates a cross section A-A through the annular winding of

Figure 3 according to examples of the disclosure;

Figure 5 schematically illustrates features of the reluctance motor of Figure 1 according

to examples of the disclosure;

Figure 6 schematically illustrates features of the reluctance motor of Figure 1 according

to examples of the disclosure;

Figure 7 schematically illustrates an exploded view of the reluctance motor of Figure 6

according to examples of the disclosure;

Figures 8a to 8c schematically illustrate power circuits which may be used to drive a reluctance motor according to examples of the disclosure; Figures 9a and 9b schematically illustrate a reluctance motor having an axial flux

topology according to examples of the disclosure;

Figures ba to Thc schematically illustrate a reluctance motor having a conical flux

topology according to examples of the disclosure;

Figure 11 schematically illustrates a reluctance motor with circumferentially offset sets of first flux guiding elements according to examples of the disclosure; Figure 12 schematically illustrates a reluctance motor with circumferentially offset sets of second flux guiding elements according to examples of the disclosure; Figure 13 illustrates a graph of simulated performance of a reluctance motor according

to examples of the disclosure;

Figure 14 schematically illustrates an annular winding together with a first flux guiding element and a second flux guiding element according to examples of the disclosure; Figure 15 schematically illustrates a reluctance motor according to examples of the

disclosure:

Figure 16 schematically illustrates a reluctance motor according to examples of the

disclosure;

Figure 17 schematically shows a rotor of a reluctance motor which can act as an impeller of a pump according to examples of the disclosure; and Figure 18 illustrates a graph of simulated performance of different drive arrangements of

a reluctance motor of examples of the disclosure.

S A reluctance motor according to examples of the disclosure will now be described with reference to Figure 1 Figure 1 schematically illustrates a reluctance motor 10 according to examples of the disclosure. The motor 10 comprises a stator and a rotor 14 arranged in use to rotate with respect to the stator about an axis of rotation (indicated by dashed line 16). In examples, the rotor is concentric with the stator. In the example shown in Figure 1, the reluctance motor 10 is a 3-phase motor although it will be appreciated that any appropriate number of phases could be used for example depending on design requirements.

The stator comprises a first annular winding 18, a second annular winding 20, and a third annular winding 22. Each of the windings 18, 20, and 22 correspond to a phase and are arranged so that current can be driven through the windings independently from each other. For example, an electrical current can be passed through the annular winding 18 so as to generate a magnetic field around the annular winding 18. Similarly, an electrical current can be passed through the annular winding 20 and/or or annular winding 22 so as to generate a magnetic field around the respective annular winding.

The stator comprises a plurality of first flux guiding elements (first flux guides) such as first flux guiding elements 24. The rotor 14 comprises a plurality of second flux guiding elements (second flux guides) such as second flux guiding elements 26. The arrangement of the first flux guiding elements and second flux guiding elements will now be described in more detail with reference to Figures 2 to 5.

Figure 2 schematically shows elements of a reluctance motor 50 according to examples of the disclosure. In the example of Figure 2, the reluctance motor 50 is a single phase motor.

The stator comprises an annular winding 52 through which an electrical current can pass so as to generate a magnetic field around the annular winding 52. For example, referring to Figure 3, a current passing through the annular winding 52 (annular coil) causes a magnetic field (e.g. indicated by dashed lines 54) to be formed around the annular winding with magnetic flux lines lying in a plane perpendicular to the direction of current. In examples, the annular winding comprises a plurality of turns of electrically conductive wire wound so as to lie in a plane substantially perpendicular to the axis of rotation of the rotor. However, it will be appreciated that the annular winding 52 could be a single winding (e.g. formed from a solid conductor), formed from a plurality of different windings wired together for example in series, or other appropriate configurations.

In examples, the annular winding 52 is in electrical communication with suitable drive and control circuitry for controlling the current passing through the annular winding so as to drive the motor.

The stator comprises a plurality of first flux guiding elements (such as first flux guiding elements 56), and the rotor comprises a plurality of second flux guiding elements (such as second flux guiding elements 58). The first flux guiding elements can be thought of as first flux guides, and the second flux guiding elements can be thought of a second flux guides. These will be described in more detail with reference to Figures 3 and 4.

Figure 3 schematically illustrates an annular winding together with a first flux guiding element and a second flux guiding element according to examples of the disclosure. Figure 4 schematically illustrates a cross section A-A through the annular winding of Figure 3.

In particular, Figure 3 shows the annular winding 52, a first flux guiding element 56 and a second flux guiding element 58. Referring to Figure 4, the first flux guiding element 56 has a first end 60 and a second end 62. The first flux guiding element 56 is arranged around a portion of the annular winding 52 so that magnetic flux of the magnetic field can be guided around the annular winding in a plane substantially perpendicular to a direction of current in the annular winding 52. In examples, the annular winding 52 has a rectangular cross section and the first flux guiding element 56 has a c-shaped cross section. However, it will be appreciated that other cross sections could be used.

In examples, the first flux guiding element 56 is arranged around the annular winding 52 on three sides. However, it will be appreciated that the annular winding 52 could have any appropriate cross section (such as circular, ellipsoid, square, hexagonal and the like) and a surface of the first flux guiding element which faces the annular winding having a similar or the same profile as the surface of the annular winding. In other words, in examples, the inner profile of the first flux guiding elements corresponds with the surface of the annular winding. In examples, the first flux guiding element 56 is in physical contact with the annular winding 52.

However, it will be appreciated that they need not be in physical contact and the inner profile of the first flux guiding element could be different from the cross sectional profile of the annular winding 52. In examples, the annular winding comprises copper, although it will be appreciated that any other appropriate electrically conductive material could be used.

For example, referring to Figure 4, the current is indicated as into the page with the magnetic flux (indicated by dashed line 64) being guided around the annular winding 52 in the clock-wise direction. In other words, for example, each of the first flux guides (first flux guiding elements) extends around a portion of the winding and each defines a flux path in a plane perpendicular to the direction of movement of a current along the winding.

Referring to Figure 4, the second flux guiding element 58 can guide magnetic flux between the first end 60 and the second end 62 of the first flux guiding element 56 when at least partially aligned with the first flux guiding element 56. In other words, for example, when the rotor rotates so as to bring the second flux guiding element 58 into proximity with the first flux guiding element a flux path around the annular winding 52 is completed at the first flux guiding element 56.

Referring back to Figure 2, in examples, the first flux guiding elements 56 are spaced around the circumference of the annular winding 52. By appropriate switching of current through the annular winding 52 the rotor can be caused to rotate with respect to the stator and the second flux guiding elements brought into proximity with the first flux guiding elements so as to complete a flux path around the winding at each first flux guiding element. In other words, for example, the second flux guiding elements guide flux between the first end and second end of respective first flux guiding elements so that variation of the current in the winding causes the rotor to rotate with respect to the stator. The reluctance motor of the examples of Figures 2 to 4 tends to help reduce part count and manufacturing complexity.

A 3-phase reluctance motor according to examples of the disclosure will now be described in more detail with reference to Figures 5 to 7.

Figure 5 schematically illustrates the reluctance motor 10. As mentioned above, the reluctance motor 10 comprises a plurality of annular windings arranged so that electrical current can pass through each of the windings independently from each other. In examples, each of the windings 18, 20 and 22, the first flux guiding elements 24 and the second flux guiding elements 26 have the same configuration as those described above with respect to Figures 2 to 4. In examples, the first flux guiding elements are arranged in a plurality of first guiding element sets of first flux guiding elements, with the first flux guiding elements of each first guiding element set each being located circumferentially around a respective annular winding.

For example, referring to Figure 5, first flux guiding elements 24a, first flux guiding elements 24b and first flux guiding elements 24c are arranged as respective different first guiding element sets. In the example shown in Figure 5, second flux guiding elements 26a, second flux guiding elements 26b, and second flux guiding elements 26c are arranged as respective different second guiding elements sets. In other words, the second flux guiding elements can be thought of as being arranged in sets of second flux guiding elements.

In examples, each second guiding element set is associated with a respective first flux guiding element set so that the second flux guiding elements of each second guiding element set can guide magnetic flux between the first end and second end of the first flux guiding elements in respective first guiding element sets. For example, referring to Figure 5, the first flux guiding elements 24a are associated with the second flux guiding elements 26a, the first flux guiding elements 24b are associated with the second flux guiding elements 26b and the first flux guiding elements 24c are associated with the second flux guiding elements 26c. For example, when a position of a second flux guiding element corresponds with that of an associated first flux guiding element (e.g. when brought into alignment by rotation of the rotor) the second flux guiding element can guide flux between the first and second end of the first flux guiding element.

In examples each of the first and second flux guiding element sets which are associated with each other can be thought of being the same as the first and second flux guiding elements described above with respect to Figures 2 to 4 and as corresponding to a phase for driving the reluctance motor. In other words, in examples, each of the annular windings can be driven independently from each other so as to carry out control of the motor and cause the rotor to rotate with respect to the stator.

Referring to Figure 5, in examples, the first flux guiding elements for each of the first guiding elements sets are offset from each other in a circumferential direction with respect to the axis of rotation. In examples, the second flux guiding elements for each of the second guiding elements sets are arranged in the circumferential direction so as to substantially correspond with each other. For example, the first flux guiding elements 24a are offset circumferentially from the first flux guiding elements 24b and the first flux guiding elements 24b are circumferentially offset from the first flux guiding elements 24c.

In examples, the first flux guiding elements of each of the first guiding element sets are separated from the first flux guiding elements of another guiding element set by a gap. In other words, for example, the first flux guiding elements of each first guiding element set are spaced apart from each other in the axial direction with respect to the axis of rotation. This may help improve magnetic, electrical, and physical isolation of the first flux guiding elements from each other and so help improve performance of the reluctance motor for example due to reduced flux leakage between the annular windings of different phases. In examples, the gap is an air gap, although it will be appreciated that the first flux guiding elements of each set could be separated from each other by other suitable materials, such as non-magnetic and/or non-conducting materials (e.g. plastics materials, ceramic materials and the like). In examples, the gap width in the axial direction is of the order of a few millimetres (e.g. 0.5mm, 1.0mm, 1.5mm, 2mm, 2.5mm, 3mm, although it will be appreciated that any other suitable gap could be used). Additionally, it will be appreciated that the gap between sets could be different from each other or could be the same.

In the example shown in Figure 5, the second flux guiding elements of each of the second guiding elements sets are separated from the second guiding elements of another second guiding element set by a gap. For example the second flux guiding elements 26a are spaced apart from the second flux guiding elements 26b in the axial direction (for example by a gap 28).

Figure 6 schematically illustrates the reluctance motor 10 and Figure 7 schematically illustrates an exploded view of the reluctance motor 10. In these examples, the reluctance motor is the same as that described with reference to Figure 5 but with the second flux guiding elements of different second guiding elements sets being continuously formed with each other.

In other words, for example, the second flux guiding elements of the different second guiding element sets can be integrally formed so as to extend continuously in the axial direction.

For example, referring to Figure 7, a second flux guiding element 26 comprises a first flux guiding portion 26a, a second flux guiding portion 26b and a third flux guiding portion 26c.

In examples, the flux guiding portions 26a, 26b and 26c can be thought of as being the same as the second flux guiding elements of each second guiding element set as described with reference to Figure 5 but without any gap between the second flux guiding elements in the axial direction. In examples, the second flux guiding elements are continuously formed with each other (e.g. for each phase), this arrangement may help reduce manufacturing costs.

An arrangement of the first flux guiding elements with respect to the second flux guiding elements is schematically illustrated in Figure 7. In this example, the first flux guiding element 24a comprises a first end 60a and second end 62a, the first flux guiding element 24b comprises a first end 60b and second end 62b, and the first flux guiding element 24c comprises a first end 60c and a second end 62c.

In operation, a current is passed through the first annular winding 18 so that the first flux guiding elements 24a guide magnetic field around the annular winding. In examples, when a current is passed through one of the windings, there is zero applied current in the other windings. This may occur for example where a square wave drive sequence is employed.

However, in other examples, for example using a sine wave drive current for each phase (or other non-square wave alternating current source), some current may be present in all of the annular windings simultaneously but with the maxima of the current in each winding being offset from each other with respect to time.

As a current is applied to the annular winding 18, the rotor 14 is caused to rotate with respect to the stator to reduce magnetic reluctance between the first flux guiding elements and the second flux guiding elements (e.g. between the first flux guiding element 24a and the first flux guiding portion 26a of the second flux guiding element 26). For example, the second flux guiding elements are caused to be brought into proximity with the first flux guiding elements 24a. In other words, the rotor rotates so that the second flux guiding elements 26 (such as first flux guiding portion 26a) can guide magnetic flux between the first end (for example first end 60a) and the second end (for example second end 62a) of respective first flux guiding elements 24a. In examples, the term "being brought into proximity" can be thought of as minimising the distance between the first flux guiding elements and the second flux guiding elements in the radial direction, for example reducing the distance to less than a threshold distance. The first flux guiding elements of a first guide element set can therefore be thought of as aligning with the respective second flux guiding elements of a second guide element set in the circumferential direction when a current is applied to the respective annular winding (for example when the current in that annular winding is a maximum for the current being passed through that winding).

A current is then passed through the annular winding 20 so that the rotor 14 is caused to rotate to reduce magnetic reluctance between the first flux guiding elements and the second S flux guiding element (e.g. between the first flux guiding element 24b and the second flux guiding portion 2Gb of the second flux guiding element 26). In other words, the rotor rotates so that the second flux guiding elements 26 (such as the second flux guiding portion 26b) can guide magnetic flux between the first end (for example first end 60b) and the second end (for example second end 62b) of respective first flux guiding elements 24b.

A current is then passed through the annular winding 22 so that the rotor 14 is caused to rotate to reduce magnetic reluctance between the first flux guiding elements and the second flux guiding element (e.g. between the first flux guiding element 24c and the third flux guiding portion 26c of the second flux guiding portion 26). In other words, the rotor rotates so that the second flux guiding elements 26 (such as the third flux guiding portion 26c) can guide magnetic flux between the first end (for example first end GOc) and the second end (for example second end 62c) of respective first flux guiding elements 24c.

Accordingly, by suitable control of electrical current in the annular windings 18, 20 and 22, the rotor can be caused to rotate with respect to the stator by magnetic reluctance.

Figures 8a to Sc schematically illustrate power circuits which may be used to drive the examples of the reluctance motor of the disclosure.

Figure Ba schematically illustrates a power circuit for supplying current to the reluctance motor 10 in a three phase switched reluctance configuration with a conduction angle of 120 degrees between each phase. The power circuitry is illustrated on the left hand side of Figure Ba-Sc and a schematic waveform for one of the phases is illustrated on the right hand side of Figures 8a-8c. In Figures Ba to Sc "T" indicates the time period of one drive period of the rotor with respect to the stator (e.g. one rotation of the rotor with respect to the stator in the case of synchronous drive).

Referring to Figure Ba, the power circuit comprises a power source 70, such as a battery or rectified AC source, although it will be appreciated that any other appropriate power source could be used. In examples, the power circuit comprises a capacitor 72 for example for smoothing any voltage or current spikes caused by switching of the windings 18, 20, or 22. The power circuit comprises switching elements SI and S2 for controlling switching of current through the first winding 15, switching elements S3 and S4 for controlling switching of current through the second winding 20, and switching elements S5 and 86 for controlling switching of current through the third winding 22. In examples, the switching elements 81-56 are MOSFETs (metal oxide silicon field effect transistors) although it will be appreciated that any other type of switching element could be used. In examples, the switching elements are controlled by a suitable microprocessor to control operation of the motor 10.

The power circuit of Figure Sb is the same as that of Figure Ba. However, in the example of Figure 8b, the switching elements are arranged to be controlled in a three phase switched reluctance configuration with a conduction angle of 180 degrees between each phase.

The power circuit of Figure 8c is similar to that of Figures 8a and Sb. However, in the example of Figure 8c, the first winding 18, second winding 20 and third winding 22 are electrically connected together at points A, B, and C (in a so-called 3-phase star configuration).

In the example of Figure 8c, the switching elements are bipolar junction transistors. The power circuit of the example of Figure Bc is arranged as a standard 3-phase AC inverter for supplying a sinusoidal bipolar current to each of the windings with a 120 degree phase shift between each phase.

Although Figures Ba to Sc illustrate examples of power circuits for supplying current to the winding to control operation of the motor, it will be appreciated that other types of power circuit and control configuration could be used as appropriate, for example depending on the number of phases. It will be appreciated that any number of phases could be used as appropriate. For example, a one phase reluctance motor such as that described with respect to Figure 2 or a three phase reluctance motor such as that described with respect to Figures 5 could be used.

In examples of a reluctance motor described with respect to Figure 1 to 7, the first flux guiding elements are positioned so as to guide magnetic flux through the first ends and second ends in a radial direction with respect to the axis of rotation. In these examples, the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements is guided through the second flux guiding elements in an axial direction with respect to the axis of rotation. In other words, for example, magnetic flux of the stator can* be thought of as being guided in a radial direction. The examples of Figures 1 to 7 can thus be thought of as having a radial flux topology.

Some other examples of reluctance motors according to examples of the disclosure will now be described with reference to Figures 9 to 17.

Figures 9a and 9b schematically illustrate a reluctance motor having an axial flux topology. In particular, Figure 9a schematically illustrates a reluctance motor 90 having an axial flux topology and Figure 9b schematically illustrates an exploded view of the reluctance motor 90.

The reluctance motor 90 comprises a rotor 92 and a stator. In use, the rotor 92 is arranged to rotate with respect to the stator about an axis of rotation (indicated by dashed line 91). The stator comprises a first annular winding 94, a second annular winding 96 and a third annular winding 98. In examples, the annular winding 94, 96, and 98 are arranged concentrically with each other and lie in substantially the same plane. In other words, for example, the annular windings are arranged to form a disc. The stator comprises a plurality of first flux guiding elements (such as first flux guiding elements 100). Each first flux guiding element has a first end and a second end, in a similar manner to the first flux guiding elements described above with respect to Figure 1 to 7. Each annular winding corresponds to a phase, in a similar manner to that described above with respect to Figures ito 7.

In the example of the reluctance motor having an axial flux topology, the first flux guiding elements are positioned so as to guide magnetic flux through the first ends and second ends in an axial direction with respect to the axis of rotation. In other words, for example, the first flux guiding elements are arranged to guide magnetic flux through the first and second ends substantially parallel to the axis of rotation of the rotor.

In examples, the first flux guiding elements are grouped in first guiding element sets, each associated with a respective annular winding, in a similar manner to that described above with respect to Figure 5.

For example, referring to Figure 9b, first flux guiding elements bOa of a first guiding element set are associated with the annular winding 94 so as to guide flux through the first and second ends in the axial direction. First flux guiding elements lOOb are associated with the annular winding 96 so as to guide flux through the first and second ends of the first flux guiding elements lOOb in the axial direction. First flux guiding elements lOOc are associated with the annular winding 98 so as to guide flux through the first and second ends of the first flux guiding elements iOOc in the axial direction. Although in the example of Figure 9b only some of the first flux guiding elements are indicated with lOOa, lOOb or lOOc, it will be appreciated that all of the first flux guiding elements that are each arranged around a portion of an annular winding could be considered to be associated with that annular winding and form a first guiding element set.

In examples, the rotor 92 comprises a plurality of second flux guiding elements (such as second flux guiding elements 102). In the example illustrated in Figures 9a and 9b, the second flux guiding elements are integrafly formed so that they are continuous in the radial direction across all of the first flux guiding elements of the different annular windings (e.g. annular windings 94, 96 and 98). However, it will be appreciated that the second flux guiding elements could be spaced apart from each other in the radial direction so that each second flux guiding elements is associated with a respective first flux guiding element in a similar manner to that described above with reference to Figure 5 for example.

The second flux guiding elements of the example of Figures 9a and 9b are positioned so that magnetic flux through the first and second ends of the first guiding elements can be guided through the second flux guiding elements in a radial direction with respect to the axis of rotation.

Figures IDa to bc schematically illustrate a reluctance motor with a topology referred to as a conical flux topology in accordance with examples of the disclosure. In general, in examples of the conical flux topology, positions of the first ends of the first flux guiding elements are offset in the radial direction from positions of the respective second ends of the first flux guiding elements. In these examples the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements is guided through the second flux guiding elements in a direction at a non-zero angle to the axial direction with respect to the axis of rotation.

In particular, Figure ba schematically illustrates a reluctance motor 110, Figure lOb schematically illustrates an exploded view of the reluctance motor 110, and Figure bc schematically illustrates a partial cross section through the reluctance motor 110.

Referring to Figures ba and lob, the reluctance motor 110 comprises a rotor 112 and a stator. In use, the rotor 112 is arranged to rotate with respect to the stator about an axis of rotation (indicated by dashed line 111). The stator comprises a first annular winding 114, a second annular winding 116 and a third annular winding 118. In examples, the annular winding 114, 116, and 118 are arranged concentrically with each other and lie in different planes from each other, with each of the planes being normal to the axis of rotation. In other words, for example, the annular windings can be thought of as being arranged to lie on the surface of a virtual cone. In these examples, the annular windings have different diameters from each other.

Each annular winding 114, 116, 118 corresponds to a phase, in a similar manner to that described above with respect to Figures 1 to 7.

In examples, the stator comprises a plurality of first flux guiding elements (such as first H flux guiding elements 120). Each first flux guiding element has a first end and a second end, in a similar manner to the first flux guiding elements described above with respect to Figure 1 to 7. H However, in the examples of Figures IDa-iDe, the first and second ends are offset from each other in the radial direction. This will be described in more detail later below.

For example, referring to Figure lOb, first flux guiding elements (such as first flux guiding element 120a) of a first guiding element set are associated with the annular winding 114 so as to guide flux through the first and second ends of the first flux guiding elements in the radial direction. First flux guiding elements (such as first flux guiding element 120b) of a first guiding element set are associated with the annular winding 116 so as to guide flux through the first and second ends of the first flux guiding elements in the radial direction. First flux guiding elements (such as first flux guiding element 120c) of a first guiding element set are associated with the annular winding 118 so as to guide flux through the first and second ends of the first flux guiding elements in the radial direction. Although in the example of Figure lOb only some of the first flux guiding elements are indicated with 120a, 120b or 120c, it will be appreciated that all of the first flux guiding elements that are each arranged around a portion of an annular winding could be considered to be associated with that annular winding and form a first guiding element set.

In examples, the rotor 110 comprises a plurality of second flux guiding elements (such as second flux guiding elements 122) and a support member 119. The support member 119 is arranged to restrict motion of the second flux guiding elements with respect to each other so that torque provide by the second flux guiding elements can be transferred to an appropriate output member (such as a drive shaft).

In the example illustrated in Figures ba and lob, the second flux guiding elements are integrally formed so that they are continuous in the radial direction across all of the first flux guiding elements of the different annular windings (e.g. annular windings 114, 116 and 118).

However, it wilt be appreciated that the second flux guiding elements could be spaced apart from each other in the radial direction so that each second flux guiding elements is associated with a respective first flux guiding element in a similar manner to that described above with reference to Figure 5 for example.

The second flux guiding elements of the example of Figures IDa and lOb are positioned so that magnetic flux through the first and second ends of the first guiding elements can be guided through the second flux guiding elements in a direction at a non-zero angle to the axial direction with respect to the axis of rotation.

This will now be described in more detail with reference to Figure bc.

Figure bc is a schematic illustration of a partial cross section of the reluctance motor in a plane passing through the axis of rotation and coplanar with the axis of rotation. Figure lOc schematically illustrates the first annular winding 114, the second annular winding 116, and the third annular winding 118 together with the first flux guiding element 1 20a, first flux guiding element 12Db, first flux guiding element 12Cc and second flux guiding element 122.

For example, the first flux guiding element 120b comprises a first end 124a and a second end 124b. The first flux guiding element 120b is arranged around a portion of the annular winding 116 so as to guide magnetic field around the annular winding 116 in a plane substantially perpendicular to a direction of current in the annular winding 116. As illustrated in Figure bc, the first end 124a is offset in the radial direction from the second end 124b. In other words, a distance between the first end 124a and the axis of rotation 111 is different from a distance between the second end 124b and the axis of rotation 111. When at least partially rotationally aligned with the first flux guiding element 12Db, the second flux guiding element 122 is arranged to guide magnetic flux (indicated by dashed line 126) between the first end 124a and the second end 124b in a direction at a non-zero angle to the axial direction with respect to the axis of rotation 111.

In some examples, a portion of each of the second flux guiding elements is arranged parallel to the axis of rotation. This may help reduce the overall diameter of the motor 110. For example, referring to Figures 1Db and 1 0, a portion 128 of the second flux guiding elements is arranged parallel to the axis of rotation. In examples, the portion 128 is positioned axially so as to correspond with at least a part of the second flux guiding element 120c. However, it will be appreciated that the portion 128 could be positioned to correspond with at least a part of more than one second flux guiding portion 128 of different annular windings. For example, the portion 128 could be arranged to extend axially over the second flux guiding element 12Db and the second flux guiding element 1 20c.

In other examples, the second flux guiding elements for each phase are separate from each other and spaced apart from each other in the axial direction. Referring to Figure bc, in examples, the second flux guiding elements are separated by an air gap positioned between the second flux guiding elements (for example located at dashed lines 129 in Figure bc).

In the examples described above, the first flux guiding elements of each first guiding element set are offset from each other in the circumferential direction. Additionally, in examples, the second flux guiding elements of each second guiding element set (for example if integrally formed) are aligned with each other in the radial direction so that the second flux guiding elements in each second guiding element set are not offset in the circumferential direction from second guiding elements in another second flux guiding element set.

Figure 11 schematically illustrates the reluctance motor 10. The reluctance motor 10 of the example of Figure ills substantially the same as that described above for Figure 5. In the example of Figure 11, the first flux guiding elements 24 of each first guiding element set are offset from each other in the circumferential direction. For example, the first flux guiding elements 24a are circumferentially offset from the first flux guiding elements 24b and the first flux guiding elements 24b are circumferentially offset from the first flux guiding elements 24c. In this example, the second flux guiding elements of each second guiding element set are aligned with each other (i.e. not circumferentially offset). For example, each of the second flux guiding elements 26a is aligned with a respective second flux guiding element 26b so that they are aligned in the axial direction.

However, in other examples, the first flux guiding elements of each first guiding element set are not offset from those in another first guiding element set (e.g. they are aligned in the axial. direction). In these examples, the second flux guiding elements in a second guiding element set are offset in the circumferential direction from those in another second guiding element set.

Figure 12 schematically illustrates another example of the reluctance motor 10. The reluctance motor of Figure 12 is substantially the same as the reluctance motor described above with respect to Figure 5. However, in this example, the first flux guiding elements of each first guiding element set are aligned with each other in the axial direction and the second flux guiding elements of a second guiding element set are offset in the circumferential direction with respect to second flux guiding elements of another second guiding element set. For example, the first flux guiding elements 24a are aligned with the first flux guiding elements 24b and first flux guiding elements 24c. The second flux guiding elements 26a are circurnferentially offset with respect to the second flux guiding elements 26b (and also the second flux guiding elements S 26c which are not shown in Figure 12).

In other words, in some examples, the first flux guiding elements of the stator are offset in the circumferential direction between first guiding element sets with the second flux guiding elements of the rotor being aligned between second guiding element sets. In other examples, the first flux guiding elements of the stator are aligned in the circumferential direction between first guiding element sets, and the second flux guiding elements of the rotor are offset in the circumferential direction between second guiding element sets.

As mentioned above, it is thought that an axial air gap between the first flux guiding elements and/or the second flux guiding elements may improve the torque performance of the reluctance motor. For example, Figure 13 illustrates a graph of simulated performance of the reluctance motor 10 described above with respect to Figure 5. Inter-stack axial air gap distance in millimetres (mm) (that is the gap between the first and/or second flux guiding elements of each guide element set associated with each annular winding) is plotted on the x-axis, and simulated torque in newton-metres (Nim) is plotted on the y-axis. As can be seen from Figure 13, the predicted torque of the motor is thought to increase as the distance of the air gap increases. It is also thought that the axial air gap can help improve physical, electrical and magnetic isolation between the phases, which may help improve the fault tolerance of the reluctance motor.

Referring back to the example of Figure 11, the first flux guiding elements (e.g. first flux guiding elements 24a, 24b, and 24c) do not have an axial airgap between them but the second flux guiding elements (e.g. second flux guiding elements 26a, 26b, and 26c) do have an axial airgap between the second flux guiding elements of each phase (e.g. between second flux guiding elements 26a and 26b). As another example, Figure 12 schematically illustrates an example of the reluctance motor 10 in which there is an axial air gap between first flux guiding elements for each of the different phases (e.g. between first flux guiding elements 24a and 24b and between first flux guiding elements 24b and 24c), and between the second flux guiding elements for each different phase (e.g. between second flux guiding elements 26a and 26b). In other words, they may be an axial air gap between the flux guiding elements of each phase on the rotor, on the stator, or both.

For example, by having an axial air gap between the flux guiding elements of different phases only on the rotor but not the stator, it is thought that the performance benefit of the having the air gap can be maintained, whilst also maintaining the overall size (or at least not increasing it significantly). In examples, this may be selected based on the aspect ratio of the motor (for example, a "drum type" machine or a "pancake" type machine). For example, a "drum type" machine is a reluctance machine in which the axial length of the machine is longer than the diameter of the stator, and a "pancake type" machine is one in which the axial length of the machine is shorter than the diameter of the stator. It is thought that having an axial air gap S between the rotor flux guiding elements (e.g. second flux guiding elements) of the different phases but not between the stator flux guiding elements (e.g. first flux guiding elements) for a "pancake type" machine may be especially important in helping minimise the size of the machine.

In examples, such as those described above, the first flux guiding elements have a substantially c-shaped" cross section. In the examples of reluctance motors having a "radial flux topology" (such as those of Figures 1-7, 11 and 12), the first flux guiding elements have a c-shaped cross section in a plane parallel to the axis of rotation and coplanar with the axis of rotation. Similarly, in the examples of reluctance motors having an "axial flux topology" (such as Figures 9a and 9b) the first flux guiding elements have a c-shaped cross section in a plane parallel to the axis of rotation and coplanar with the axis of rotation. However, in the radial flux examples, the open part of the "c' shape is directed towards the axis of rotation whereas in the axial flux examples, the open part of the c" shape is directed along a line parallel to the axis of rotation. In this context a "c-shape" can be thought of as a loop in which a break is formed, with the break corresponding to the open part loop. In examples (such as those described with respect to Figures 1 to 7) the first and second ends are located at ends of linear elongate portions joined by a linear bridging portion which is arranged at right angles with the linear elongate portions. However, it will be appreciated that a "c-shape" could be a combination of linear and/or arcuate portions.

In some examples, a portion of a first flux guiding element located further from the axis of rotation than the first and second ends in a plane perpendicular to the axis of rotation is larger than a portion of the first flux guiding element at the first and/or second end. In other words, in examples, a cross section through the first flux guiding elements is substantially "1-shaped".

This will be described in more detail with reference to Figures 14 to 16.

Figure 14 schematically illustrates an annular winding together with a first flux guiding element and a second flux guiding element according to examples of the disclosure. The example of Figure 14 is similar to that of Figure 3. For example, the cross section A-A is the same as that described above with respect to Figure 4. In particular, Figure 14 schematically illustrates the annular winding 52, a first flux guiding element 110 and the second flux guiding element 58. The first flux guiding element comprises a proximal portion 112 and a distal portion 114. In examples, the proximal portion is closer to the axis of rotation than the distal portion. In examples, a dimension of the distal portion in the circumferential direction is greater than a dimension of the proximal portion in the circumferential direction. In the example of Figure 14, the first flux guiding element 110 has a substantially T-shaped cross section (e.g. a dimension of the distal portion in the circumferential direction is greater than a dimension of the proximal portion in the circumferential direction) in a plane perpendicular to the axis of rotation, although it will be appreciated that other configurations where the distal portion is wider than the proximal S portion in the circumferential direction could be used. In examples, one or more of the "c-shaped" first flux guiding elements has a substantially rectangular cross section in a plane perpendicular to the axis of rotation. However, it will be appreciated that other cross sections could also be used.

The use of "T-shaped" first flux guiding elements may help improve performance (such as torque performance) by coupling in more magnetic flux of the annular winding than for a c-shaped" flux guiding element with a substantially rectangular cross section in a plane perpendicular to the axis of rotation. However, the use of "T-shaped" first flux guiding elements may increase weight over that of the "c-shaped" first flux guiding elements.

Figure 15 schematically illustrates a reluctance motor according to examples of the disclosure, In particular, Figure 15 illustrates a reluctance motor 120. The reluctance motor 120 is substantially the same as that described above with respect to Figure 11. However, in the example of Figure 15, the reluctance motor 120 comprises a plurality of first flux guiding elements with a "T-shaped" cross section in a plane perpendicular to the axis of rotation (such as flux guiding elements 11 Oa, 11 Ob and 1 bc). In the example shown in Figure 15, the first flux guiding elements of each first guiding element set and the second flux guiding elements of each second guiding element set are separated from each other by an axial gap (e.g. air gap 122, 124, and 126), in a similar manner to that described above with respect to Figure 11. However, it will be appreciated that the first and/or second flux guiding elements of each phase (first and/or second guiding elements sets) could abut each other in the axial direction. Additionally, it will be appreciated that any suitable material could be used in the gap.

In examples, the first flux guiding elements of each first guiding element set are integrally formed with each other. In other words, the distal portion of each first flux guiding element in each set is continuous with that of a neighbouring first flux guiding element in that set. In the example shown in Figure 15, the first flux guiding elements in a first guiding element set are offset circumferentially from those in another first guiding element set, and the second flux guiding elements of neighbouring second guiding element sets are axially aligned with each other.

However, in other examples, the first flux guiding elements (e.g. first flux guiding elements 110) may be separate from each other (for example where indicated by the dashed lines in Figure 15). In examples, the first flux guiding elements of each first guiding element set abut each other circumferentially, although it will be appreciated that they could be separated from each other by a gap (such as an air gap).

Figure 16 schematically illustrates a reluctance motor according to examples of the disclosure. In particular, Figure 16 illustrates a reluctance motor 130. The reluctance motor 130 is substantially the same as that described above with respect to Figure 12. However, in the example of Figure 16, the reluctance motor 130 comprises a plurality of first flux guiding elements with a "T-shaped" cross section in a plane perpendicular to the axis of rotation (such as flux guiding elements llOa, hUb and hOc). In the example shown in Figure 16, the first flux guiding elements of each first guiding element set abut each other axially. In the example of Figure 16, the second flux guiding elements of each second guiding element set are separated from each other by an axial gap (e.g. air gap 126), in a similar manner to that described above with respect to Figure 11. However, it will be appreciated that any suitable material could be used in the gap.

However, in other examples, the first flux guiding elements (e.g. first flux guiding elements 110) may be separate from each other (for example where indicated by the dashed lines in Figure 15). In examples, the first flux guiding elements of each first guiding element set abut each other circumferentially, although it will be appreciated that they could be separated from each other by a gap (such as an air gap). Additionally, it will be appreciated that some first and/or second flux guiding elements of respective phases may abut each other (e.g. circumferentially and/or axially) and others may be positioned so as to be separated by a gap.

In examples, the stator comprises a carrier (not shown) on which the annular winding(s) and/or the first flux guiding elements are mounted. The carrier helps maintain the relative positions of the annular windings and/or the first flux guiding elements, for example by resisting mechanical stress in the windings due to the magnetic field in the windings. However, it will be appreciated that any other appropriate arrangement could be used. For example, the annular windings and first flux guiding elements could be potted in a suitable epoxy. Additionally, it will be appreciated that a similar carrier arrangement to that of the stator could be used for the rotor, although they could have different carrier arrangements.

Although the examples of the stator and rotor have been described as comprising first and second flux guiding elements respectively which may be separated by a gap between those of different phases, it will be appreciated that a gap may be used between some of those flux guiding elements of phases and not others. Additionally, it will be appreciated that aspects of different topologies (e.g. radial flux topology, axial flux topology and conical flux topology) as well as different configurations of first and/or second flux guiding elements may be combined as appropriate.

In examples, the number of first flux guiding elements is the same as the number of second flux guiding elements. However, it will be appreciated that the number of first flux guiding elements could be different from the number of second flux guiding elements.

Although in the examples described above one of the first flux guiding elements of each phase and the second flux guiding elements of each phase are circumferentially offset from those of another phase, it will be appreciated that both the first flux guiding elements of each phase and the second flux guiding elements of each phase may be offset circumferentially with respect to those of another phase. By suitable positioning and selection of the amount of offset it is thought that torque ripple may be reduced.

Examples of the disclosure may help simplify reluctance motor structure. The examples of the disclosure may also help reduce part count in manufacturing a reluctance motor for example by using metallic powder or soft magnetic composite for some of the parts. In examples, the first flux guiding elements and/or the second flux guiding elements comprise one or more of: metallic powder; soft magnetic composite; and metallic laminations, although it will be appreciated that other suitable materials could be used. For example, in the examples in which "c-shaped" first flux guiding elements are used, metallic laminations can be used so as to help improve performance, although it will be appreciated that a combination of any of metallic powder (e.g. iron powder), soft magnetic composite and metallic laminations or other material could be used.

Additionally for example, where the examples of the reluctance motor include "T-shaped" first flux guiding elements, metallic powder (e.g. iron powder) or soft metallic composite could be used so as to help reduce manufacturing steps. However, it will be appreciated that a combination of any of metallic powder (e.g. iron powder), soft magnetic composite and metallic laminations or other material could be used.

In examples, the reluctance motor is coupled to a cooling system for cooling the annular windings. This can help improve efficiency of operation of the motor. For example, in the examples where "c-shaped" first flux guiding elements are used, cooling may be achieved by directly cooling the annular windings between the first flux guiding elements. For example, gas convection cooling (such as air cooling) could be used in which gas flows between circumferential gaps between the first flux guiding elements so that the gas can directly cool the annular winding(s). In this context, "directly" is for example taken as meaning as "being in contact with". As another example, the cooling system comprises a cooling liquid jacket (e.g. a water jacket, although it will be appreciated that any suitable cooling liquid could be used) in which the cooling liquid jacket is in direct contact with the annular winding(s) between the first flux guiding elements. In other examples, the cooling system comprises one or more cooling elements (such as peltier cooling elements, although it will be appreciated that any suitable type of cooling element could be used) which are arranged to be in direct contact with the annular winding(s) between the first flux guiding elements in the circumferential direction. These examples can help improve thermal coupling between the annular windings and the cooling elements and so help improve cooling efficiency of the cooling system. Similar arrangements may be applied where "1-shaped" first flux guiding elements are used which are separated from each other by gap.

Although some of the examples described above illustrate the rotor being inside the stator, it will be appreciated that the rotor could be outside the stator with appropriate modifications to the orientation of different elements such as the first and second flux guiding elements. For example, where the rotor is insider the stator, the rotor can be considered to be closer to the axis of rotation than the stator. However, in other examples where the rotor is outside the stator, the rotor can be considered to be further away from the axis of rotation than the stator It will be appreciated that where a gap (such as an air gap) between flux guiding elements is referred to, the gap could be an air gap, although it will be appreciated that the flux guiding elements could be separated from each other by other suitable materials, such as non-magnetic and/or non-conducting materials (e.g. plastics materials, ceramic materials and the like).

In examples, the reluctance motor of the examples described herein is included in a pump. In particular, in examples, the pump comprises the reluctance motor, arid an impeller, which is integrally formed with the rotor. For example, referring to Figures lOa and lOb, the second flux guiding elements 122 could be shaped so as to act as an impeller of the pump (e.g. a centrifugal pump). In other words, for example, the impeller and the rotor may be directly integrated together. As another example, referring to Figures 9a and 9b, the irpeller could comprise the second flux guiding elements 102 so that the motor can act as a pump (such as a centrifugal pump). More generally, in examples, the impeller comprises the second flux guiding elements.

In some examples, the second flux guiding elements are integrally formed so that the portions of the second flux guiding elements which align with each of the first flux guiding elements of different phases are offset circumferentially from each other. These may help shaping of the impeller to achieve desired fluid flow.

Figure 17 schematically shows a rotor of a reluctance motor which acts as an impeller of

a pump according to examples of the disclosure.

In particular, Figure 17 schematically shows a rotor 150 comprising a plurality of substantially arcuate second flux guiding elements (such as second flux guiding elements 152, 158). In the example of Figure 17, the rotor comprises 8 second flux guiding elements, although it will be appreciated that any appropriate number Gould be used. In the example of Figure 17, the rotor cooperates with a stator having an axial flux topology such as that described above with respect to Figures 9a and 9b. In examples, the second flux guiding elements are shaped so that the motor can act as a pump. Although the second flux guiding elements of Figure 17 are substantially arcuate, it will be appreciated that other shapes and configurations are possible.

Furthermore, the second flux guiding elements need not be coplanar with each other. For example, they could be arranged as a cone (for example in a similar manner to that described above with respect to Figures IDa and lob).

For example, fluid can enter the motor (pump) through a fluid port located within a first threshold distance of an axis of rotation 156 of the rotor 150. On rotation of the rotor 150 with respect to the stator about an axis of rotation 154, the second flux guiding elements cause fluid to be moved away from the axis of rotation (for example as indicated by arrow 156) towards a fluid outlet port located within a second threshold distance of the outer circumference of the rotor 150. Accordingly rotation of the rotor with respect to the stator about the axis of rotation can cause fluid to be pumped through the motor. In other words, the reluctance motor can act as a centrifugal pump for example. In examples, the second flux guiding elements can therefore act as fluid guides.

In examples, each second flux guiding element 152, 158 is continuously formed and comprises a plurality of flux guiding portions. In examples, each flux guiding portion corresponds to a phase and is arranged to guide flux from corresponding first flux guiding elements. For example, the second flux guiding element 158 comprises a first flux guiding portion 160a, a second flux guiding portion 160b, and a third flux guiding portion 160c.

Referring to Figures 9a and 9b, in examples, the rotor 150 replaces the rotor 92. In these examples, the first flux guiding portion 160a is arranged so that flux can be guided between respective first ends and second ends of the first flux guiding elements boa of the annular winding 94, the second flux guiding portion 160b is arranged so as that flux can be guided between respective first ends and second ends of the first flux guiding elements 10Db of the annular winding 96, and the third flux guiding portion 160c is arranged so that flux can be guided between respective first ends and second ends of the first flux guiding elements 1 OOc of the annular winding 98. The other second flux guiding elements of the rotor 150 are arranged in the same manner.

Accordingly, in examples, elements of the reluctance rotor structure can be combined with that of a pump so that the dimensions of the pump may be reduced, and construction, design and manufacture simplified.

Figure 18 illustrates a graph of simulated performance of different drive arrangements of a reluctance motor of examples of the disclosure. In particular, Figure 18 illustrates a simulation of performance of a reluctance motor such as that described above with reference to Figure 5. Phase current (in root mean square amps (ARMS)) is plotted on the x-axis, and torque in newton-metres (Nm) is plotted on the y-axis. In this example "Ct-core SR' refers to a configuration of a switched reluctance motor such as that of Figure 5. The simulated performance for three different drive configurations is shown. Points indicated with a cross (X) H S indicate simulated performance with a square wave unipolar drive with a conduction angle of degrees such as that described above with reference to Figure 8a. Points indicated with a triangle (A) indicate simulated performance with a square wave unipolar drive with a conduction angle of 180 degrees such as that described above with reference to Figure 8b. Points indicated with a square (0) indicate simulated performance with a sine wave drive such as that described above with reference to Figure Bc. As can be seen from Figure 18, a square wave unipolar drive with a conduction angle of 120 degrees is thought to offer better torque performance than square wave unipolar drive with a conduction angle of 180 degrees, or sine wave drive.

Although the reluctance motors of the examples of Figures 1 and 5 to 18 have been described as three phase reluctance motors, it will be appreciated that they could be operated as single phase or two phase motors with appropriate control circuitry, for example by driving two or more of the annular windings at the same time so as to simulate operation of a one phase or two phase motor. However, it will be appreciated that whether this is possible may depend on the relative offset of the flux guiding elements of the different annular windings.

It will be appreciated that although the examples relate to reluctance motors, these can be thought of as examples of an electrical machine. Additionally, although the examples described herein relate to reluctance motors, it will be appreciated that they could also be operated as reluctance generators with appropriate known control circuitry such as that described in "Switched Reluctance Motors and Their Control" T.J.E. Miller, Magna Physics Publishing (Mentor, Ohio, USA) and Oxford University Press (Oxford, UK), 1993, Magna Physics ISBN 1-881855-02-3 I OUP ISBN 0-19-859387-2, (see for example pages 92 to 97), the entire contents of which is hereby incorporated by reference. In other words, in examples, the reluctance machine is a reluctance motor. In other examples, the reluctance machine is a reluctance generator. More generally, examples of the disclosure relate to reluctance machines such as reluctance motors and reluctance generators.

In conclusion, although a variety of examples have been described herein, these are provided by way of example only, and many variations and modifications on such examples will be apparent to the skilled person and fall within the spirit and scope of the present invention, which is defined by the appended claims and their equivalents.

Claims

CLAIMS1. A reluctance machine comprising: a stator comprising an annular winding through which an electrical current can pass so as to generate a magnetic field around the annular winding, the stator comprising a plurality of first flux guiding elements each having a respective first end and second end, each first flux guiding element being arranged around a portion of the annular winding so that magnetic field can be guided around the annular winding in a plane substantially perpendicular to a direction of current in the annular winding; and a rotor arranged in use to rotate with respect to the stator about an axis of rotation, the rotor comprising a plurality of second flux guiding elements, the second flux guiding elements being arranged so that, when at least partially aligned with at least one of the first flux guiding elements, the second flux guiding elements can guide magnetic flux between the first end and second end of respective first flux guiding elements.
2. A reluctance machine according to claim 1, in which the annular winding lies in a plane substantially perpendicular to the axis of rotation of the rotor.
3. A reluctance machine according to claim 1 or claim 2, in which the number of first flux guiding elements is the same as the number of second flux guiding elements.
4. A reluctance machine according to claim 1 or claim 2, in which the number of first flux guiding elements is different from the number of second flux guiding elements.
5. A reluctance machine according to any preceding claim, in which: the first flux guiding elements are positioned so as to guide magnetic flux through the first ends and second ends in a radial direction with respect to the axis of rotation; and the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements is guided through the second flux guiding elements in an axial direction with respect to the axis of rotation.
6. A reluctance machine according to any of claims ito 4, in which; the first flux guiding elements are positioned so as to guide magnetic flux through the first ends and second ends in an axial direction with respect to the axis of rotation; and the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements is guided through the second flux guiding elements in a radial direction with respect to the axis of rotation.
7. A reluctance machine according to any of claims 1 to 4, in which: positions of the first ends of the first flux guiding elements are offset in the radial direction from positions of the respective second ends of the first flux guiding elements; and the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements can be guided through the second flux guiding elements in a direction at a non-zero angle to the axial direction with respect to the axis of rotation.
8. A reluctance machine according to claim 7, in which a portion of each of the second flux guiding elements is arranged parallel to the axis of rotation.
9. A reluctance machine according to any preceding claim, in which a cross section through the first flux guiding elements is substantially c-shaped.
10. A reluctance machine according to any of claims 1 to 8, in which a cross section through the first flux guiding elements is substantially T-shaped.
11. A reluctance machine according to any preceding claim, comprising: a plurality of annular windings arranged so that electrical current can pass through each of the windings independently from each other, in which: the first flux guiding elements are arranged in a plurality of first guiding element sets of first flux guiding elements, the first flux guiding elements of each first guiding element set each being located circumferentially around a respective annular winding.
12. A reluctance machine according to claim 11, in which the second flux guiding elements are arranged in a plurality of second guiding element sets of second flux guiding elements, each second guiding element set being associated with a respective first flux guiding element set so that the second flux guiding elements of each second guiding element set can guide magnetic flux between the first end and second end of the first flux guiding elements in respective first guiding element sets.
13. A reluctance machine according to claim 11 or claim 12, in which the first flux guiding elements for each of the first guiding elements sets are offset from each other in a circumferential direction with respect to the axis of rotation.
14. A reluctance machine according to claim 12 or 13, in which the second flux guiding elements for each of the second guiding elements sets are offset from each other in a circumferential direction with respect to the axis of rotation.
15 A reluctance machine according to claim 12 or 13, in which the second flux guiding elements for each of the second guiding elements sets are arranged in the circumferential direction so as to substantially correspond with each other.
16. A reluctance machine according to any of claims 11 to 14, in which the first flux guiding elements of each of the first guiding element sets are separated from the first flux guiding elements of another first guiding element set by a gap.
17. A reluctance machine according to any of claims 12 to 16, in which the second flux guiding elements of each of the second guiding element sets are separated from the second flux guiding elements of another second guiding element set by a gap.
18. A reluctance machine according to any of claims 12 to 16, in which the second flux guiding elements of different second guiding elements sets are continuously formed with each other.
19. A reluctance machine according to any preceding claim in which the first flux guiding elements and/or the second flux guiding elements comprise one or more of: metallic powder; soft magnetic composite; and metallic laminations.
20. A reluctance machine according to any preceding claim, in which the rotor is closer to the axis of rotation than the stator.
21. A reluctance machine according to any of claims I to 19, in which the rotor is further away from the axis of rotation than the stator.
22. A reluctance machine comprising: a stator having an annular winding and a plurality of first flux guides spaced around the circumference of the annular winding, said first flux guides extending around a portion of the winding and each defining a flux path in a plane perpendicular to the direction of movement of a current along the winding; and a rotor comprising a plurality of second flux guides spaced around the circumference of the rotor, said rotor being arranged in use to rotate with respect to the stator and to bring the second flux guides into proximity with the first flux guides and to complete a flux path around the winding at each first flux guide.
23. A reluctance machine according to any preceding claim, in which the reluctance machine is a reluctance motor.
24. A reluctance machine according to any of claims 1 to 22, in which the reluctance machine is a reluctance generator.
25. A reluctance machine according to any preceding claim, in which the reluctance machine is a three-phase reluctance machine.
26. A reluctance machine according to any preceding claim, in which the reluctance machine is a single phase reluctance machine.
27. A pump comprising: a reluctance machine according to any preceding claim; and an impeller, in which the impeller is integrally formed with the rotor.
28. A pump according to claim 27, in which the impeller comprises the second flux guiding elements.
29. A reluctance machine substantially as hereinbefore described.
30. A pump substantially as hereinbefore described.Amendments to the claims have been filed as followsCLAIMS1. A reluctance machine comprising: a stator comprising an annular winding through which an electrical current can pass so as to generate a magnetic field around the annular winding, the stator comprising a plurality of first flux guiding elements each having a respective first end and second end, each first flux guiding element being arranged around a portion of the annular winding so that magnetic field can be guided around the annular winding in a plane substantially perpendicular to a direction of current in the annular winding; and a rotor arranged in use to rotate with respect to the stator about an axis of rotation, the rotor comprising a plurality of second flux guiding elements, the second flux guiding elements being arranged so that, when at least partially aligned with at least one of the first flux guiding elements, the second flux guiding elements can guide magnetic flux between the first end and second end of respective first flux guiding elements, in which: positions of the first ends of the first flux guiding elements are offset in the radial direction from positions of the respective second ends of the first flux guiding elements; and the second flux guiding elements are positioned so that magnetic flux through the first and o second ends of the first guiding elements can be guided through the second flux guiding elements in a direction at a non-zero angle to the axial direction with respect to the axis of rotation.2. A reluctance machine according to claim 1, in which the annular winding lies in a plane substantially perpendicular to the axis of rotation of the rotor.3. A reluctance machine according to claim 1 or claim 2, in which the number of first flux guiding elements is the same as the number of second flux guiding elements.4. A reluctance machine according to claim 1 or claim 2, in which the number of first flux guiding elements is different from the number of second flux guiding elements.5. A reluctance machine according to any preceding claim, in which: the first flux guiding elements are positioned so as to guide magnetic flux through the first ends and second ends in a radial direction with respect to the axis of rotation; and the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements is guided through the second flux guiding elements in an axial direction with respect to the axis of rotation.6. A reluctance machine according to any of claims 1 to 4, in which: the first flux guiding elements are positioned so as to guide magnetic flux through the first ends and second ends in an axial direction with respect to the axis of rotation; and the second flux guiding elements are positioned so that magnetic flux through the first and second ends of the first guiding elements is guided through the second flux guiding elements in a radial direction with respect to the axis of rotation.7. A reluctance machine according to claim 1, in which a portion of each of the second flux guiding elements is arranged parallel to the axis of rotation.8. A reluctance machine according to any preceding claim, in which a cross section through the first flux guiding elements is substantially c-shaped.9. A reluctance machine according to any of claims 1 to 7, in which a cross section through the first flux guiding elements is substantially T-shaped.o 10. A reluctance machine according to any preceding claim, comprising: a plurality of annular windings arranged so that electrical current can pass through each of the windings independently from each other, in which: the first flux guiding elements are arranged in a plurality of first guiding element sets of first flux guiding elements, the first flux guiding elements of each first guiding element set each being located circumferentially around a respective annular winding.11. A reluctance machine according to claim 10, in which the second flux guiding elements are arranged in a plurality of second guiding element sets of second flux guiding elements, each second guiding element set being associated with a respective first flux guiding element set so that the second flux guiding elements of each second guiding element set can guide magnetic flux between the first end and second end of the first flux guiding elements in respective first guiding element sets.12. A reluctance machine according to claim 10 or claim 11, in which the first flux guiding elements for each of the first guiding elements sets are offset from each other in a circumferential direction with respect to the axis of rotation.13. A reluctance machine according to claim 11 or 12, in which the second flux guiding elements for each of the second guiding elements sets are offset from each other in a circumferential direction with respect to the axis of rotation.14. A reluctance machine according to claim 11 or 12, in which the second flux guiding elements for each of the second guiding elements sets are arranged in the circumferential direction so as to substantially correspond with each other.15. A reluctance machine according to any of claims 10 to 13, in which the first flux guiding elements of each of the first guiding element sets are separated from the first flux guiding elements of another first guiding element set by a gap.16. A reluctance machine according to any of claims 11 to 15, in which the second flux guiding elements of each of the second guiding element sets are separated from the second flux guiding elements of another second guiding element set by a gap.17. A reluctance machine according to any of claims 11 to 15, in which the second flux o guiding elements of different second guiding elements sets are continuously formed with each other.18. A reluctance machine according to any preceding claim in which the first flux guiding elements and/or the second flux guiding elements comprise one or more of: metallic powder; soft magnetic composite; and metallic laminations.19. A reluctance machine according to any preceding claim, in which the rotor is closer to the axis of rotation than the stator.20. A reluctance machine according to any of claims 1 to 18, in which the rotor is further away from the axis of rotation than the stator.21. A reluctance machine according to any preceding claim, in which the reluctance machine is a reluctance motor.22. A reluctance machine according to any of claims 1 to 20, in which the reluctance machine is a reluctance generator.23. A reluctance machine according to any preceding claim, in which the reluctance machine is a three-phase reluctance machine.24. A reluctance machine according to any preceding claim, in which the reluctance machine is a single phase reluctance machine.25. A pump comprising: a reluctance machine according to any preceding claim; and an impeller, in which the impeller is integrally formed with the rotor.26. A pump according to claim 25, in which the impeller comprises the second flux guiding elements.27. A reluctance machine substantially as hereinbefore described.28. A pump substantially as hereinbefore described. (4