GB2491880A

GB2491880A - Non-uniformly spaced claw-pole stator

Info

Publication number: GB2491880A
Application number: GB201110180A
Authority: GB
Inventors: Rajesh Deodhar; Adam Pride
Original assignee: IMRA Europe SAS
Current assignee: IMRA Europe SAS
Priority date: 2011-06-16
Filing date: 2011-06-16
Publication date: 2012-12-19
Anticipated expiration: 2031-06-16
Also published as: GB201110180D0; GB2491880B

Abstract

A claw pole stator comprises plural stator poles that are non-uniformly spaced around the circumference of the stator. The claw stator can comprise adjacent stator portions 2a, 2b each having plural poles 3 defining circumferential spaces between them to receive poles of the adjacent portion, which are offset circumferentially with respect to a centre point of the spaces. The stator portions 2a, 2b can be rotatable relative to each other to adjust the offset to a position where the poles are uniformly spaced around the circumference. An offset angle measured from a centre point of adjacent poles can be between 3 and 12 degrees. Adjacent poles can be separated by different circumferential distances. A claw pole stator can have poles arranged in groups around its circumference, each group comprising two or three poles and the separation of consecutive groups can be larger than the separation of adjacent poles within a group. A toroidal winding (fig 3, 5) can be located in a cavity formed between stator portions 2a, 2b. The claw poles can have a tapered tip. A motor with a rotor can comprise a stack of non-uniformly spaced pole stators, each being offset with respect to an adjacent stator. Such a motor can be three phase and have three stators (fig 8b), each offset by 120 degrees from another. The offset claw pole stator reduces cogging and torque ripple without losing starting torque and be used in a motor, for a power steering mechanism, or in an alternator.

Description

CLAW-POLE STATOR

Field of Invention

The present invention relates to an improved electric motor. In particular the present invention relates to an improved permanent magnet motor with reduced cogging torque and torque ripple.

Background

Electrical motors, such as permanent magnet motors, have been used in an increasing number of applications. Motors of this type in particular advantageously provide high torque outputs whilst having a relatively simple construction and reliable operation.

Although existing motors provide good performance they do not always have a smooth rotational movement. A variety of factors can have a detrimental effect on the operation of a motor. One such factor is the effect of cogging torque.

Cogging torque in motors is caused by variation in magnetic circuit permeance as the rotor passes the non-uniform geometry of the slot openings in the stator. In most applications cogging torque is disadvantageous since it causes torque ripple, noise and vibration.

A number of different methods have previously been used to reduce the effect of cogging.

These include varying the lamination profile in order to reduce permeance variation and altering the magnet.

However the above described techniques for reducing cogging torque also reduce the motor counter-electromotive force and so reduce the resultant running torque.

The present invention overcomes the problems associated with prior art motors and provides a means of reducing cogging torque without reducing the resultant running torque.

Summary of Invention

Aspects of the invention are defmed in the accompanying claims.

According to a first aspect of the invention there is provided a claw pole stator comprising a plurality of stator poles wherein the stator poles are non-uniformly spaced around the circumference of the stator.

Thus, according to the present invention there is provided a stator having a plurality of poles spaced around the inner circumference of the stator and wherein adjacent poles arc separated by unequal spaces i.e. some poles are closer together than other poles. The effect of the non-uniformity is that the magnetic field coupling the stator to the rotor cannot stabilise in the same way it can with a conventional uniformly spaced stator arrangement.

Destabilising the coupling of the stator and rotor in this way surprisingly improves the performance of the motor because of a reduction in cogging torque. The motor is then able to operate is an improved manner. For example, the non-uniformity of stator pole spacing increases average torque as well as reducing torque ripple.

Spacing the claw poles unevenly is counterintuitive because in effect it would be expected to create an un-balanced machine with high torque ripple which is undesirable. However, the inventors have established that the arrangement actually surprisingly enhances performance.

The stator may be formed in a variety of different ways. Generally the stator comprises a circumferentially extending support ring supporting a plurality of inwardly radially extending stator poles. As discussed above according to the present invention the stator poles are not uniformly spaced around the inner circumference of the stator as is the case with conventional stators.

In order to facilitate manufacture and construction the stator may be formed of two halves.

Thus, the stator may comprise a first stator portion and a second stator portion adjacent to the first adjacent portion. Each stator portion may comprise a plurality of stator poles arranged in opposing directions.

The spacing of adjacent poles of the first stator portion defines spaces extending circumferentially between adjacent poles. These spaces receive the poles of the second half of the stator. In effect the poles interdigitate when the two opposing halves of the stator are brought together.

According to the present invention the poles of the second stator portion are advantageously offset circumferentially with respect to the centre point of a respective space i.e. the second stator half may be rotated about the central axis with respect to the first stator portion such that the poles of the second stator portion are not centrally located within the space. That is they are offset with respect to the centre of the space.

The stator poles of the second stator portion may be positioned at an offset angle measured from the centre point of a circumferential arc between a first adjacent pole and a second adjacent pole of the fir st stator portion. Altering the position of the poles by a predetermined angle (i.e. rotation of one portion with respect to another) provides a convenient way of achieving the benefits of the present invention.

Put another way, the circumferential distance or separation from a first pole to a first adjacent pole in a first circumferential direction is larger than the distance or separation from the first pole to a second adjacent pole in a second opposing circumferential direction.

The offset angle is defined as the angle measured between the centre position of the space between adjacent poles of the first stator portion and the edge of a respective pole of the second stator portion. An offset angle of 0 degrees indicates evenly spaces poles.

It will be recognised that the angular separation of immediately adjacent pole faces of the first and second portion could alternatively be used to define an offset angle.

Positioning the poles immediately adjacent to each other has the effect of creating a single pole. In such a situation the upper limit of angular offset will be dependent on the total number of poles. The inventor has established an optimal angular offset between adjacent poles.

The offset angle may advantageously be between 3 and 12 degrees depending on the particular configuration. Conveniently the offset angle may be approximately nine degrees.

This results in a minimal amount of cogging torque. Advantageously the offset angle may be approximately six degrees. This results in a maximum average torque and a minimum torque ripple.

in order to define a space for the toroidal winding the two stator portions may be adapted to defme a circumferentially extending cavity or recess when the two portions are brought together. Thus, a toroidal winding may be located in the cavity between stator portions.

S

Such an arrangement provides a convenient way of locating the winding in the correct position. Advantageously the circumferentially extending recess may be provided on one of the two portions such that the toroidal winding can be placed in position before the second portion of the stator is brought into contact with the first portion. Thus manufacture can be simplified.

The stator poles may be claw poles having a generally L shaped profile. Additionally the claw poles may comprise a tapered portion at a tip of the claw pole. Thus adjacent poles can be brought into closer proximity. T5

Any number of stator poles may be selected, Advantageously for an automotive water pump or similar such automotive application 10 poles may be provided.

The stator may advantageously be adapted to allow the pole separation to be changed. Thus, the first stator portion may be rotatably moveable between a first position and a second position relative to the second stator portion. The first position may be a position wherein consecutive stator poles are non-uniformly spaced around the circumference of the stator and the second position may be a position wherein the stator poles are uniformly spaced around the circumference of the stator.

Such an arrangement provides the user of a motor with flexibility as to the application for which it can be used. Where smooth operation with low torque ripple is required the user can move the first portion to a first position where consecutive stator poles are non-uniformly spaced around the circumference of the stator and achieve the required benefits. If it is desirable for the motor to be able to hold a particular position without any motor excitation then high cogging torque is an advantage and therefore the first stator portion can be moved to a second position where stator poles are spaced equally around the circumference of the stator.

According to a second aspect of the invention there is provided a stator comprising a plurality of stator poles wherein the stator poles are arranged in groups around the circumference of the stator. Arranging the stator poles in groups can also result in reduced cogging torque and increased average torque.

Each group may include two stator poles. Arranging the stator poles in groups of two allows for ease of construction of the stator and allows any amount of offset to be achieved.

Alternatively each group may include three stator poles.

The circumferential distance between consecutive groups of stator poles may be larger than the circumferential distance between adjacent stator poles within each group of stator poles.

Such an arrangement ensures that certain poles are offset from a central position between two adjacent poles and thus ensures the advantageous effects of the invention are realised.

According to a third aspect of the invention there is provided an electrical motor comprising a stator stack and a rotor rotatable around an axis with respect to the stator. The stator stack comprises a toroidal winding, two opposing stator portions and a plurality of stator poles non-uniformly spaced around the inner circumference of the stator stack.

According to another aspect there is provided an electrical motor comprising a rotor rotatable about an axis of the motor and a plurality of stators dcfming a stator stack. Each of the stator stacks comprises a plurality of non-uniformly spaced stator poles.

Thus, an electric motor (or indeed a generator) can be provided with reduced cogging torque as described above. The stator stack may advantageously correspond to the stator arrangement described for example with reference to the first aspect of the invention.

The stacks may advantageously be offset with respect to one another by a predetermined angle to optimise performance. For example the motor may comprise three stators wherein each stator in the stack is offset with respect to an adjacent stator by i 20 degrees. The angles may for example be 0 degrees, 120 degrees and 240 degrees for use with a three phase electrical machine.

It will be recognised that references to offset angles between three stator stacks of 0, 120 and 240 are electrical degrees as opposed to mechanical degrees. Depending on the number of claw-poles the mechanical angle will be differenl, but the electrical angle will remain the same.

Description of the Figures

The invention will now be described by way of example only and with reference to the following drawings, in which: Fig. 1A shows a single stack assembly corresponding to one phase of a multi-phase machine; Figure lB shows two stator portions being brought together and the associated separation; Fig. 2 shows an exploded view of the stator according to an embodiment of the present invention; Fig. 3 shows an enlarged view of a section of the stator according to an embodiment of the present invention; Fig. 4 is a graph showing cogging torque against offset angle in accordance with embodiments of the invention; Fig. 5 is a graph of torque ripple and average torque against offset angle in accordance with embodiments of the invention; Fig. 6 shows an end view of a motor incorporating a stator according to an embodiment of the present invention where the stator poles are offset; Figure 7 shows an end view of a motor incorporating a stator where the stator poles are not offset and stator poles are equally spaced; and Figs. SA, 8B and 8C show a plurality of stators according to the present invention in a stack arrangement.

While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail.

It should be understood however that drawings and detailed description attached hereto are not intended to limit the invention to the particular form disclosed but rather the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention

Detailed Description

The construction and operation of a conventional motor comprising a stator and rotor is known to the person skilled in the art and will therefore not be described in detail. It will be known for example that a motor comprises a rotor and circumferentially located stator. The rotor supports a series of permanent magnets and the stator one or more electrical windings and associated poles.

The present invention is concerned with a particular motor; a claw pole motor.

Turning to the figures, Figure 1A shows a stator 1 according to an embodiment of the present invention. The stator I is constructed from two stator portions 2a and 2b which, as shown, are brought together to form the stator.

Both stator portions 2a, 2b arc of the same size and diameter and each carry a plurality of stator poles 3.

Each stator portion includes a plurality of stator poles 3 located around the inner circumference of the respective stator portion 2a, 2b. In the example shown in Figure 1A each stator portion has 5 stator poles, thus the assembled stator is a 10 pole! stator.

The stator portions may be selected from any suitable material. Such materials include, but arc not restricted to, steel or soft magnetic composites (SMC).

Each stator portion 2a, 2b comprises a circumferentially extending ring or disc from which the statQr poles extend. As shown the stator poles extend in a generally radially inward direction towards the rotor (not shown). The rotor would rotate about axis x passing through the centre of the stator assembly as shown in figure 1A.

The claw poles of the stator portions comprise a generally L shaped profile, that is a portion extending generally radially inwards and a distal portion (with respect to the stator ring) extending parallel with the rotor axis x.

In the example shown in Figure IA the tip of the claw which extends in the axial direction is tapered.

Adjacent claws on the first portion 2a are separated by circumferentially extending spaces.

On constructing the stator 1 these spaces receive corresponding claw poles of the opposing stator portion 2b. In effect the poles of each portion 2a, 2b interdigitate.

Thus when the two stator portions 2a, 2b are assembled together a plurality of claws are arranged around the inner circumference of the stator 1 with alternate claws belonging to either the first or second stator portion 2a, 2b. This is illustrated in Figure 1A.

Figure lB shows the positioning of a stator pole on a second stator portion 2b between two stator poles of the first stator portion 2a. Also illustrated is the spacing of the poles where the following may be true: d1 > or d1cd2. but

d1!= d2 When the stator portions 2a, 2b are assembled together to form the stator a circumferentially extending space, cavity or recess 4 is formed on the inner circumference of the stator portion.

This is shown in Figure 2. In the example in figure 2 the space is defmed between the stator poles 3 and the outer ring of the stator portion 2a. A corresponding space is defined on the opposing stator portion.

A toroidal winding 5 is located in this space 4 as shown in Figure 2. Thus, when the two opposing stator portions are brought together the toroidal winding is located within the stator I between an outer ring of the stator and the stator poles 3.

The portion of the claw poles in the axial direction (i.e. extending parallel to the axis x of the rotor) overlaps the cavity 4 between the stator portions 2a, 2b and thus overlaps the toroidal winding 5 located therein.

As can be seen in Figure lA and lB the claw poles 3 are unequally spaced around the inner circumference of the stator 1. That is, the spaces between adjacent poles 3 are not equal around the stator 1. The poles can be considered to be arranged in pairs or groups of two where the separation between pairs of poles is greater than the separation between the poles making the pair.

Figure 3 shows an end view along the axis x in Figure 1A.

Specifically, as can be clearly seen in Figure 3, the stator poles 3 are arranged such that the distance between a particular stator pole X on one stator portion and its first adjacent pole Y on the other stator portion is smaller than the distance between the particular stator pole X and its second adjacent pole Z on the other stator portion. As shown in Figure 3 according to this configuration, the circumferential distances between stator poles 3 are not equal around the circumference of the stator, so that b>a.

This positioning of the poles is achieved by offsetting alternate poles 3 by a specified angle measured from a central line or axis extending from the centre of axis x of the rotor through the centre of the space between adjacent poles on the first stator portion 2a.

In a position where the spaces between poles 3 around the inner circumference of the stator are equal, the poles 3 of the second stator portion are received at the centre of the circumferential space between first and second adjacent poles 3 of the first stator portion. In such a position the angular offset is zero degrees.

However, as discussed above the inventor has established a surprising performance advantage of offsetting the stator poles from this equal spacing position (zero offset) by a particular and predetermined angle.

if the angle of offset is too great the poles are in effect merged together and therefore the advantageous result of reduced cogging is not achieved. There is therefore an optimal separation which results in minimal cogging whilst still retaining distinct separate poles. This is shown in the graphs of Figures 4 and 5.

Figure 4 shows a graph illustrating the reduction in cogging torque with the amount of offset of the stator poles 3. As can be clearly seen from the graph where the reduction in peak to peak cogging decreases with the amount of offset of the stator poles 3. The value continues to decrease up to an offset of approximately 9 degrees.

Figure 5 shows a graph illustrating the effect of offset angle on average torque and torque ripple. It can be clearly seen that torque ripple decreases with offset up to an offset of 6 degrees. After this point any further increase in offset angle results in the torque ripple starting to rise again.

The average torque increases with offset angle up to an offset of 6 degrees. Again any further increase in offset angle results in a slight reduction in average torque. It is therefore clear that in terms of torque ripple and average torque the largest improvements in motor performance are seen at an offset angle of 6 degrees.

It is therefore possible to optimise the performance of a motor either in terms of cogging torque or in terms of torque ripple and average torque. In order to achieve a minimal amount of cogging torque an offset angle of 9 degrees is used. In order to achieve a maximum average torque an offset angle of 6 degrees is used.

Offsetting the stator poles 3 in this manner thus improves the performance of an electrical machine incorporating a stator according to the present invention. Such a machine may for

example be a motor.

An example of such a motor 6 is shown in Figure 6 which illustrates a view along the axis of the rotor 7. The motor 6 comprises a rotating rotor 7 which itself comprises a plurality of permanent magnets 8. In the view of figure 6 the toroidal winding 5 can be seen within the stator 1.

The non-uniform spaces between adjacent stator poles disrupts the equilibrium resulting in a reduced cogging torque and therefore provides improved motor performance.

In use the electric motor is activated and the toroidal winding energised. The resultant magnetic field causes the rotor to rotate in conventional manner.

Cogging torque in motors is caused by variation in magnetic circuit permeance as the rotor passes the non-uniform geometry of the slot openings in the stator. Since the number of claw-poles and magnet poles is equal in each stack, when the claw-poles are uniformly spaced, all magnet poles align with all claw-poles at a particular rotor position, resulting in a strong detent force at that position and high cogging torque. On the other hand, when the claw-poles are non-uniformly spaced, all magnet poles never align with all claw-poles resulting in no strong detent force at any particular rotor position and low cogging torque. I0

In a second embodiment of the invention it is possible to change the offset angle depending on the application of the motor.

For example in some applications the motor may have a mode of operation where cogging torque is desirable and a second mode where cogging torque is not desirable. It may therefore be advantageous to provide a motor with a stator assembly capable of stator movement during the operation of the motor. In such an arrangement optimal performance may be achieved for different modes of operation as discussed above.

Such relative movement could be achieved in a range of ways. For example a simple rack and pinion arrangement could be provided with a rack coupled to the outer periphery of one half of the stator and a motor and pinion coupled to the other. Operation of the motor would then cause the half of the stator coupled to the rack to move relative to the first part. It will be recognised that other mechanisms could include a suitably coupled linear actuator and lever

for example.

Figure 7 shows a stator 20 in a second position where the stator poles 23 have no offset angle and are positioned in the centre of each space between adjacent poles 23 thus resulting in equal spacing around the inner circumference of the stator 20.

Alternatively if a motor with low torque ripple, noise and vibration is required then minimum cogging torque is advantageous and therefore the stator poles should be arranged at an optimal offset angle as discussed above and as shown in Figure 1A.

According to such an embodiment of the invention a stator is provided in which it is possible to rotate one of the stator portions with respect to the other stator portion in order to change the offset angle. In one example a motor could be used to move the stator portion. In another example a lever could be used.

In the example embodiments discussed each stator portion includes half the total number of stator poles of the assembled stator. However it is possible to achieve the benefits of the invention by having different numbers of poles on each stator. For example all of the stator poles being on one stator portion with none on the other stator portion. In such an arrangement the second portion of the stator would in effect be a plate or disc portion arranged to cover the toroidal winding once in-situ.

The benefits of a motor with reduced cogging torque are particularly advantageous in power steering where they result in a smooth, quiet movement with decreased vibration and noise.

The advantages of a motor with reduced cogging torque are not only limited to the use of such a motor in power steering but can also provide beneficial effects in car alternators.

The invention may also bc applied to a multi-phase electrical machine. In such an arrangement the motor may comprise a plurality of stators as illustrated in figures 1 to 6. The stators may be arranged immediately adjacent to one another in a stack arrangement. This is illustrated in Figures 8A and SB.

Figure 8A shows a plurality of stators in an exploded view. Three stators Sa, Sb and Sc are shown.

The three phases forming the stator stack are shown in an exploded view in figure 8A and brought together in figure SB. Figure SC also shows the angles of the three phases in the form of an end view of the stator stack. In this example the three phases are equally spaced at 0, 120 degrees and 240 degrees.

Although the invention has been described in reference to the use of the improved stator in a motor the improved stator provides the same advantages when used in other types of electrical machine for example generators.

Claims

Claims 1. A claw pole stator comprising a plurality of stator poles wherein the stator poles are non-uniformly spaced around the circumference of the stator.
2. A claw pole stator according to claim 1, further comprising a first stator portion and a second stator portion adjacent to the first adjacent portion; wherein each stator portion comprises a plurality of stator poles; adjacent poles of the first stator portion define circumferential spaces therebetween arranged to receive the stator poles of the second stator portion; and wherein the poles of said second stator portion are offset circumferentially with respect to a centre point of a respective space.
3. A claw pole stator according to claim 2, wherein each stator portion comprises half of the total number of stator poles of the stator.
4. A claw pole stator according to any of claims I to 3, wherein the stator poles of the second stator portion are positioned at an offset angle measured from a centre point of a circumferential arc extending between a first adjacent pole and a second adjacent pole of the first stator portion.
5. A claw pole stator according to claim 4, wherein the offset angle is between three and twelve degrees.
6. A claw pole stator according to claim 4, wherein the offset angle is approximately nine degrees.
7. A claw pole stator according to claim 4, wherein the offset angle is approximately six degrees.
8. A claw pole stator according to any of claims 2 to 7, wherein the two stator portions form a cavity therebetween.
9. A claw pole stator according to claim 8, wherein a toroidal winding is located in the cavity between stator portions.
10. A claw pole stator according to claim 1, wherein adjacent poles are separated by different circumferential distances.
11. A claw pole stator according to claim 10, wherein the circumferential distance from a first pole to a first adjacent pole in a first circumferential direction is larger than the distance from the fir st pole to a second adjacent pole in a second opposing circumferential direction.
12. A claw pole stator according to any preceding claim, wherein the stator poles are claw poles.
13. A claw pole stator according to claim 12, wherein the claw poles comprise a tapered portion at a tip of the claw pole.
14. A claw pole stator according to any preceding claim, wherein the total number of stator poles is ten.
15. A claw pole stator according to any preceding claim, wherein the fir st stator portion is rotatably moveable relative to the second stator portion between a first position and a second position.
16. A claw pole stator according to claim 15, wherein the first position is a position wherein consecutive stator poles are non-uniformly spaced around the circumference of the stator and the second position is a position wherein the stator poles are uniformly spaced around the circumference of the stator.
17. A claw pole stator comprising a plurality of stator poles wherein the stator poles are arranged in groups around the circumference of the stator.
18. A claw pole stator according to claim 17, wherein each group comprises two stator poles.
19. A claw pole stator according to claim 17, wherein each group comprises three stator poles.
20. A claw pole stator according to any of claims 17-19, wherein the circumferential separation of consecutive groups of stator poles is larger than the circumferential separation of adjacent sUitor poles within each group of stator poles.S
21. A claw pole motor comprising a stator according to any of claims 1-20.
22. A method of making a claw pole machine comprising the steps of: A) providing a first claw pole stator having a plurality of equally spaced poles; B) providing a second opposing claw pole stator having a plurality of equally spaced poles; C) locating a toroidal winding between said claw poles; and D) locating the first and second claw poles adjacent to each other and rotating one relative to the other such that the stator poles are non-uniformly spaced around the circumference of the stator.
23. A electrical motor comprising: a rotor rotatable about an axis of the motor; and a plurality of claw pole stators defining a stator stack; wherein each stator comprises a plurality of non-uniformly spaced stator poles.
24. A motor according to claim 23, wherein the stack comprises three stators; and wherein each stator in said stack is offset with respect to an adjacent sUitor by 120 degrees.
25. A motor as claimed in claim 24, wherein the three stators are offset by 0 degrees, 120 degrees and 240 degrees.
26. A motor according to claim 24 or 25, wherein the motor is a three phase motor.
27. A power steering mechanism comprising a motor according to 23.
28. An alternator comprising a stator according to any of claims 1 to 20.
29. A claw pole stator as substantially hereinbeforc described with reference to Figures 1 to 6 and BA to 8C.Amendment to the claims have been filed as follows Claims 1. A claw pole stator comprising a plurality of suitor poles wherein the stator poles are selectively uniformly or non-uniformly spaced around the circumference of the stator.said suitor further comprising a first stator portion and a second suitor portion adjacent to the first adjacent portion; wherein each stator portion comprises a plurality of stator poles: adjacent poles of the first stator portion define circumferential spaces therebetween arranged to receive the suitor poles of the second suitor portion: wherein the poles of said second suitor portion are selectively offset circumferentially with respect to a centre point of a respective space, and wherein the first stator portion is rotatably moveable relative to the second suitor portion bthveen a first position and a second position.CJ 2. A claw pole stator according to claim 1, wherein each suitor portion comprises half of the total number of stator poles of the stator 2O 0 3. A claw pole suitor according to any of claims I or 2, wherein the stator poles of the CC) second suitor portion are selectively positioned at an offset angle measured from a centre point of a circumferential arc extending between a first a4jacent pole and a second adjacent pole of the first stator portion.4. A claw pole stator according to claim 3, wherein the offset angle is bthveen three and twelve degrees.S. A claw pole stator according to claim 3, wherein the offset angle is approximately nine degrees.6. A claw pole stator according to claim 3, wherein the offset angle is approximately six degrees.7. A claw pole suitor according to any of claims I to 6 wherein the two suitor portions form a cavity therebetween.8 A claw pole statot accordina to claim 7, wherein a (ovoidal winding is located in the ca' ity between stator portions.9.. A claw pole stator according to claim 1. wherein adjacent Poles are sel)arated by difièrent circumferential distances.0. A cla\v pole stator according to claim 9, wherein the circumferentiaL distance from a first pole to a first adjacent pole in a first circumfCrential direction is larger than the distance 0 from the first pole to a second adjacent pole in a second opposing circumferential direction.II A claw pole statoi according to any preceding claim, wherein the stator poles are claw poles.12 A claw pole stator according to claim 11, wherein the claw poles comprise a tapered 01 tion at a tip of the claw pole 1 3. A claw pole stator according to any preceding claim, wherein the total number of r stator poles is ten.0 14. A claw pole stator accord ag to claim I. wherein the first position is a position cQ wherein consecutive stator poles are non-uniformly spaced around the circumference of the r stator arid the second position is a position wherein the stator poles are uniformly spaced around the circumference of the stator 15. A claw pole motor comprising a stator according to any ofclainis 1-14.1 6. A method of making a claw pole machine compiising the steps of A) providing a first claw pole stator having a plurality of equally spaced poles; B) providing a second opposing claw pole stator having a plurality of equally spaced poles: C) locating a toroidal winding between said claw poles; arid D) locating the first and second claw poles adjacent to each other and selectively rotating one relative to the other such that the stator poles arc non-uniformly spaced around the circumference of the stator 17. A electrical motor comprising: a rotor rotatable about an axis of the moton a plurality of claw pole stators defining a stator stack: wherein each suitor comprises a plurality of selectively uniform and non-uniformly spaced suitor poles 18. A motor according to claim 17. wherein the stack comprises three stators: and wheicin each suitor in said stack is offset with respect to an adjacent suitor by an electrical angle ol 120 degrees.19 A motor according to claim 18, wherein the motor is a three phase motor.20. A power steering mechanism comprising a motor according to 17.21. An alternator comprising a stator according to any of claims I to 14.C'J 22. A claw pole stator as substantially hereinbefore described with reference to Figures 1 to6and8Ato8C. -2o Co r