GB2479719A

GB2479719A - Stator teeth for an electrical machine

Info

Publication number: GB2479719A
Application number: GB1006316A
Authority: GB
Inventors: Shinichiro Iwasaki; Rajesh Pranay Deodhar
Original assignee: IMRA Europe SAS
Current assignee: IMRA Europe SAS
Priority date: 2010-04-15
Filing date: 2010-04-15
Publication date: 2011-10-26
Anticipated expiration: 2030-04-15
Also published as: GB201006316D0; GB2479719B

Abstract

A stator 30 for an electrical machine such as a motor comprises a body 31, and a plurality of teeth 32 extending from the body in a plane and separated by slots 33. The teeth have a base adjacent the body, a tip remote from the body and narrower than the base, a height from the base to the tip, and sides which are tapered inwardly from the base over a lower portion which is between one-sixth and one-half the height of the teeth, such as one-third the height of the teeth. The ratio of teeth base width to tip width is in the range 1:1.2 to 1:1.4, preferably 1:1.3 and the ratio of slot base width to teeth base width is in the range 1:2.5 to 1:9, preferably 1:4.3. The area in the plane of the teeth is in the range 2% to 22% greater than the slot area.

Description

TITLE OF THE INVENTION

STATOR FOR AN ELECTRICAL MACHINE

BACKGROUND OF THE INVENTION

The present invention relates to stators for electrical machines, with specific regard to the teeth of stators. The invention may be applied to motors or generators.

Figure 1 shows a simplified schematic cross-sectional view of an electric motor.

The motor 10 comprises a rotor 12 mounted for rotation about a shaft 14. The rotor has a hub portion with a plurality of rotor teeth 16 projecting outwardly from the hub.

Surrounding the rotor 12 is a stator 18, which comprises an annular rim 19 supporting a plurality of stator teeth 20 projecting inwardly towards the rotor teeth 16. The stator teeth are evenly spaced around the rim 19. The spaces between the stator teeth 20 define slots 22. Coils or windings 24 are wrapped around each stator tooth. In this example, the motor 10 is a three-phase motor, so that the coils 24 are arranged in three groups for energisation of the stator teeth by three voltage phases VA, VB and V. In Figure 1, the coil 24 for each stator tooth 20 is shown as a single winding for clarity of illustration. In practice, each coil 24 typically comprises many windings of, for example, copper wire, so that most or all of the space in the slots 22 is filled with windings. In the example of Figure 1, the stator teeth 20 are each straight-edged, with parallel sides.

Stator teeth with parallel sides, such as those in Figure 1, have a substantially uniform cross-section. This provides a uniform magnetic flux density through each tooth so that the whole tooth reaches magnetic saturation at the same time. Once saturated, the flux cannot be further increased. The torque produced by the motor is a function of the total magnetic flux in the energized stator teeth, so it is desirable for the saturation level to be high, so that more flux and more torque are available. For example, wider teeth can carry a higher magnetic flux before reaching saturation, but if the teeth are too wide and too close together, the magnetic flux leakage from the sides of the teeth will begin to limit the torque. More teeth also increases the total amount of flux available, but again, the teeth can become too close together. Close tooth spacing also limits the size of the slots and hence the amount of windings that can be accommodated. Thus, good motor design is often a balance between competing factors.

Among other parameters, the number and shape of the rotor teeth 16 and the stator teeth 20 and the configuration of the coils 24 can be varied to optimise the performance of a motor or other electrical machine for a given application. * -2-

For example, US 4,647,802 describes a variable reluctance motor designed to provide a reduced torque ripple. The stator teeth are shaped to be wider at the base than at the tip, so that the teeth have tapered sides. The taper angle is such that the base is about twice as wide as the tip. The taper extends over most of the height of the tooth, leaving a small portion at the tip which is untapered, having straight parallel sides.

A low number of teeth is used to avoid the widened bases from causing overcrowding.

The tooth shape means that the small tip of the tooth becomes magnetically saturated at a lower flux than the rest of the tooth. When the tip is saturated, the magnetic reluctance in the tooth is about constant at all times as the rotor rotates. This gives a more uniform torque because torque is related to reluctance.

The reduction of torque ripple is also addressed in JP 10 164807 and JP 2002- 078299, both of which make use of stator teeth with straight parallel sides and no pole shoe.

JP 2003-250237 describes a different use of taper in relation to stators. In a stator unit comprising stator teeth projecting from a hub, the base of each slot is formed as a very shallow "V"-shape, tapering upwards to join the sides of each tooth near the base of the tooth. The sides of the tooth are straight and parallel along the height of the tooth. The windings are arranged around the parallel sides of the teeth. The shaping is intended to strengthen the stator so that forces produced when the stator is fitted into an annular yoke can be better withstood.

WO 2007/1 00255 and JP 2008-1 60939 both describe stators in which the teeth are shaped to better receive pre-assernbled coils so that the coils fill the slots between the teeth. Combinations of parallel-sided and tapered-sided teeth are used, in which the tapered teeth have sides which are tapered inwardly towards the tip over the full height of the teeth.

For some applications, it has been found that a desired motor performance, in terms of torque and power, cannot be achieved using a conventional stator tooth design in a desired size of motor. For example, a traction motor for use in the wheel of an electric car has restrictions on size, so that it can fit within the wheel, but is required to give high torque and power performance which are apparently incompatible with the size restriction.

Thus, there is a demand for an improved motor design.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the present invention is directed to a stator comprising a body, and a plurality of teeth extending from the body in a plane and separated by slots, in which the teeth have a base adjacent the body, a tip remote from the body and narrower than the base, a height from the base to the tip, and sides which are tapered inwardly from the base over a lower portion being substantially between one-sixth and one-half the height of the teeth.

A stator having teeth configured in this manner produces greater torque and power when utilised in a motor or similar electrical machine as compared to a stator having the same arrangement of teeth and slots but the teeth having straight sides. The taper increases the size of the teeth, allowing higher magnetic flux before saturation is reached, which in turn gives more torque. Tapers shorter than about one-sixth of the tooth height produce a negligible torque increase, whereas tapers longer than about one-half the tooth height do not produce significantly more torque than a half-height taper.

In some embodiments, the lower portion having the taper is substantially one-third the height of the teeth. This size of taper produces a sizeable improvement in torque over straight-sided teeth, and is not detrimentally less good than a taper extending to half the height of the tooth.

The invention is applicable to any configuration of stator, but in some embodiments, the stator is dimensioned such that if the sides of the teeth were not tapered, in the plane of the teeth the area of each tooth would be substantially equal to the area of each slot.

Other dimensions relevant to the taper can be defined so as to enhance motor performance. For example, in the plane of the teeth, the ratio of the width of the tip of the teeth to the width of the base of the teeth may be in the range 1:1.2 and 1:1.4. In particular, the ratio of the width of the base of the teeth to the width of the tip of the teeth may be substantially 1:1.3.

In some embodiments, the slots may have a substantially flat base between the tapered sides of the adjacent teeth. Thus, the tapers do not extend across the base of the slots. In such embodiments, in the plane of the teeth, the ratio of the width of the base of the slots to the width of the base of the teeth may be in the range 1:2.5 to 1:9.

For example, the ratio of the width of the base of the slots to the base of the teeth may be substantially 1:4.3. *

One may also consider the area of the teeth and the slots. It is desirable for the tapered portions of the teeth not to encroach too much into the slots, as this can reduce the volume of windings that can be accommodated around the teeth. Therefore, according to some embodiments, in the plane of the teeth, the ratio of the area of the slots to the area of the teeth is in the range 1:1.02 to 1:1.22. Areas in this range provide a good balance between increased tooth size, and hence increased torque, and decreased windings. Usefully, the ratio of the area of the slots to the area of the teeth may be substantially 1:1.1. Expressed anotherway, in some embodiments, in the plane of the teeth the area of the teeth is between 2% and 22% greater than the area of the slots, such as, for example, 10% greater.

For use in an electrical machine, the stator may be provided with windings, such that the stator further comprises windings arranged around the teeth, the windings occupying the slots. In some embodiments, the windings occupy substantially all of each slot.

A second aspect of the present invention is directed to an electrical machine comprising a stator according to the first aspect. The electrical machine may be a motor, although the invention may also be applied to the stators of electrical generators.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: Figure 1 shows a schematic cross-sectional view of an electrical motor of conventional design, comprising a stator having teeth with parallel sides; Figures 2A and 2B show respectively a plan view and a perspective view of a stator for an electrical machine to which the present invention may be applied; Figure 3 shows a schematic diagram of stator teeth having a tapered lower portion in accordance with embodiments of the present invention; Figure 4 is a graph showing an improvement in motor torque and power produced by tapering the teeth of the motor's stator in accordance with embodiments of the invention; Figure 5 is a graph of magnetic flux density distribution inside stator teeth having either straight sides or sides tapered in accordance with embodiments of the invention; Figure 6 is a graph of torque in a motor having stator teeth with either straight sides or sides tapered in accordance with embodiments of the invention; Figure 7 is a graph of torque in a motor having stator teeth with either straight sides or sides tapered in accordance with embodiments of the invention, and different tooth base widths; and Figure 8 is a schematic diagram of stator teeth illustrating the concepts of tooth area and slot area used in defining embodiments of the present invention. S -6-

DETAILED DESCRIPTION

Figure 2A shows a simple outline plan view drawing of an example conventional stator 30 for use in an electrical machine such as a motor or a generator. The stator 30 comprises a central annular hub or body 31, and a plurality of teeth 32 extending radially outwardly from the body 31. The teeth 32 are evenly positioned around the circumference of the body 31, and spaced apart from each other to define spaces or slots 33 between the teeth 32. The slots 33 provide space for the stator 30 to accommodate windings of copper wire around each tooth 32, in the conventional manner. The windings are not shown in Figure 2A. Each tooth 32 has straight parallel sides. The stator 30 is an internal stator, configured with its teeth radiating outwardly to fit inside an annular rotor (not shown) having teeth extending inwardly. By contrast, the motor 10 shown in Figure 1 has an external stator 18, having teeth extending inwardly from an annular rim, which fits around a central rotor with teeth radiating outwardly.

Figure 2B shows a perspective view of the stator of Figure 2A.

The present invention proposes to modify the conventional straight-sided tooth shape of stators such as that shown in Figures 2A and 2B.

Figure 3 shows a schematic plan view of a portion of a stator according to an embodiment of the present invention. Teeth 32 are separated by slots 33, and extend from a body 31. For simplicity, the teeth are shown extending in parallel from a straight body 31, but in reality the body would be curved, either convexly for an internal stator having teeth extending radially outwards, or concavely for an external stator having teeth extending radially inwards.

Each tooth 32 has a base portion adjacent to and connected with the body 31, the base having a width b. The tooth 32 extends from the base over a height h to terminate at a tip portion having a width t. The two sides of the tooth are tapered over a lower portion of its height, the taper comprising straight edges sloping inwardly from the base for a height T. Beyond the taper, the sides of the tooth extend parallel and straight to the tip. According to this configuration, the base of the tooth is wider than its tip, so that b>t, and the tapered portion extends over less than the height of the tooth, so that T<h. The taper is sized so that the base of each tooth 32 is spaced apart from its neighbours, giving a flat portion of width s at the base of each slot 33.

Tapering the teeth in this manner improves the performance of a motor incorporating the stator. The taper increases the size of each tooth, making the magnetic flux saturation threshold higher. Thus, flux saturation is relieved, since more flux can be S -7-achieved before saturation. Torque is proportional to flux, so increased flux gives increased torque, increased power and hence improved motor performance.

Figure 4 is a graph illustrating the improvements that can be obtained using the invention. A stator having the configuration of that in Figure 2A was modified so that each tooth had a taper extending over the lower third of its height. The width of the taper was chosen so that the ratio of the width of the tooth base to the width of the tooth tip (b:t) was 9:7.

The graph includes specified requirements for torque (line Al) and power (line Bi) for operation of a motor over an operating range of 0 to 1600 rpm. Using a stator with straight-sided teeth gives the torque and power performance shown by the lines A2 and B2. For both torque and power, the performance is below specification at all operating speeds. Modifying the teeth by tapering them as described above improves both the torque (line A3) and power (line B3) compared to the straight teeth. At lower operating speeds, the torque and power are closer to the specification. At speeds above about 1100 rpm, the specification is exceeded for both torque and power The improved performance arises from an increase in magnetic flux, made available by the increase in tooth size given by the taper. A larger taper increases the available flux, because the tooth size is increased.

Figure 5 is a graph illustrating this point. The flux density inside three different teeth is shown, plotted as the variation in flux density along the radial centre line of the tooth, moving from the base b to the tip t of the tooth. The line A represents a straight- sided tooth with no taper, the line B is a tooth with the same tip width as the straight-sided tooth and having a taper over its lower half, and the line C is a tooth having the same base width as the half-tapered tooth, but having a taper extending over its full height, from base to tip. The straight-sided tooth shows the least variation in flux density over the tooth height, and has the highest flux density for the lower parts of the teeth, near their bases. As expected from their changing width, the tapered teeth show more variation in flux density with tooth height. In the upper half of the tooth, the tapered teeth have a higher flux density than the straight-sided tooth, with the half-tapered tooth having the highest density in the middle portion of the tooth.

Figure 6 is a graph showing the increase in torque provided by tapering the stator teeth. Data for three tooth shapes are shown: a taper over the lower one-sixth of the tooth, a taper over the lower half of the tooth, and a taper over the full height of the tooth.

The tooth base width b and tip width t is the same in each case. From Figure 6, it is clear that a longer taper gives more torque. The full height taper gives about 13% more torque than the one-sixth taper. Note, however, that the relationship is not directly proportional. S -8-

Increasing the taper height from one-sixth to one-half increases the torque much more than increasing the taper height from one-half to full height. Taper at lower parts of the tooth offers a greater benefit than taper higher up.

From this, one can conclude that confining the taper to a lower portion of the tooth is an advantageous arrangement, since the additional benefit in torque obtained by extending the taper over the full tooth height is small. Hence, the present invention proposes that stator teeth be provided with a taper over their lower portion, extending from the base to a height of between approximately one-sixth and one-half of the full height of the tooth.

One may consider the effect of the width of the taper as well as the height of the taper. The width of the taper can be defined by the width of the tapered tooth at its base.

A wider taper of the same height gives an overall larger tooth, thereby increasing the potentially available flux and hence the available torque. However, the width cannot be increased indefinitely, since the taper will at some point abut the taper of the adjacent teeth. A larger slot width gives more scope for increasing the taper width.

Figure 7 shows the results in graphical form of an investigation by modelling into the effect of increasing the taper width, by increasing the width of the tooth base. The graph plots the torque obtained for different heights and widths of taper, for teeth having a tip width of 7 mm. The diamond data point shows the torque for an untapered tooth (having a constant width of 7 mm) for the purpose of comparison. The square data points are for teeth with a one-sixth taper height, the cross data points are for teeth with a one-third taper height, the triangle data points are for teeth with a one-half taper height and the circle data points are for teeth with a full taper height. In each case, the tooth base width is increased over the range of 7.5 mm to 10 mm. This corresponds to a increase over the untapered width of between 7% and 43%.

For the one-sixth height tapered teeth, there is no increase in torque over the 8 mm to 10 mm base width range. Thus taper width has little effect for this short taper height. This suggests that taper heights below one-sixth are of little interest for increasing the torque and power available from a motor. The lower limit of one-sixth in the useful taper height range of one-sixth to one-half mentioned above is thus reinforced, For the larger taper heights, Figure 7 shows little increase in torque for increasing the tooth base width from 7.5 mm to 8 mm, a much larger increase in torque for increasing the tooth base width from 8 mm to 9 mm, and a smaller increase for increasing the tooth base width from 9 mm to 10 mm. This suggests that any further increase in tooth base width is unlikely to provide much further benefit. Thus, the limit on width necessarily imposed by the presence of adjacent teeth is not of undue concern; a significant improvement in performance can be obtained for smaller amounts of width increase. There does not appear to be any particular advantage in increasing tooth base width beyond 50% of the untapered width.

S The numerical values used in the modelling that correspond to the most significant increase in torque shown in Figure 7 are the 8 mm to 10 mm tooth base width range, which for a tooth of untapered width 7 mm corresponds to an increase in tooth base width of between approximately 14% and 43%. Since the tooth tip width is 7 mm for all teeth in this example, this range is equivalent to having tapered teeth in which the ratio of tip width to base width is in the range 1:1.14 to 1:1.43. A useful range for configuring tapered stator teeth to improve motor performance is therefore to have a tip width to base width ratio substantially between 1:1.2 and 1:1.4.

The 9 mm base width teeth are of particular interest, since the improvement in torque is substantially greater for a 9 mm base compared to a 8 mm base than for a 10 mm base compared to a 9 mm base. The 9 mm base is an increase in tooth base width over the 7 mm untapered tooth of 28.6%. Thus, within the above range of useful tip width to base width ratios, a tapered tooth having a ratio of approximately 1:1.3 is advantageous.

Note also that in Figure 7, the teeth having a one-third taper height perform almost as well as the teeth having a one-half taper height. This indicates that for the taper height range of one-sixth to one-half already determined, the value of one-third is advantageous, since taper height increase beyond this value gives only a small improvement in performance.

The graphs and data discussed so far have indicated that while increasing stator tooth size using a taper improves torque output, there is little additional benefit produced by increasing the taper height and taper width beyond certain limits. A taper height of over one-half the tooth height provides little extra torque, and a tooth base width more than about 1.4 times greater than the tip width is similarly of little additional value.

A further important factor to consider when designing the teeth of a stator is the slots between the teeth. The stator teeth receive the windings (coils) used to energise the motor by voltage application. In the present invention, it is intended that the windirigs are passed around both the tapered and untapered parts of the teeth. The windings, usually of copper wire, are densely wound to provide a large coil volume, and occupy the space between the teeth defined by the slots. For a given number of teeth and slots, increasing the tooth size by adding a taper necessarily reduces the slot size by a corresponding amount. This reduces the space available for the windings, giving a S -10-smaller coil volume. A reduction in coil volume will reduce the motor performance.

Hence, optimising the motor performance requires a balance to be found between the increased tooth size and the corresponding reduction in slot size.

These parameters can be considered in terms of the area of the teeth and the area of the slots in the plane occupied by the teeth, which is the plane of the circumference of the stator, orthogonal to the central axis of the stator. This is because the stator has a constant thickness (as is apparent from Figure 2A), so the volumes of the teeth and the slots are proportional to these areas.

Figure 8 illustrates the areas in question, for teeth 32 with area AT and slots 33 with area As. The dotted lines indicate the boundaries of the areas. For simplicity, as in Figure 3 the teeth are shown extending from a straight body 31, whereas in reality the body will be curved and the teeth will not be parallel to each other.

The relationship between torque increase from increased tooth side and the corresponding reduction in slot size is investigated using the examples of tapered teeth discussed above, with a 7 mm tip width, tooth base widths of 8, 9 and 10 mm and taper lengths of one-sixth, one-third and one-half of the tooth height, and a full tooth-height taper. Table 1 below shows the amount of torque determined from computer-modelling of a motor having a stator with these different tooth shapes. The percentage value given with each calculated torque value is the increase over the amount of torque achieved with an untapered tooth shape of the same tip width (7 mm), shown in Table I as being 376 Nm.

________ Torque [Nm] ________ _____ ______ Taper length _______ Tooth base No 1/6 1/3 1/2 1/1 wcdth taper [mml ____ ____ _____ _____ _____ 7 376 ______ ______ ______ ______ 8 405 381 379 _______ ____ (+8%) ______ (+1%) (+1%) 405 441 444 458 _______ ____ (+8%) (+17%) (+18%) (+22%) 1 405 442 446 471 0 _____ (÷8%) (+18%) (+19%) (25%)

Table I

Table 2 below shows, for the same selection of tooth shapes, the corresponding slot area (where the area is in the plane of the teeth, as illustrated in Figure 8). In the model used, the stator is configured such that for the untapered tooth shape, the tooth area and the slot area are substantially the same. As shown in Table 2, for this situation the slot area is 234 mm2. The percentage values in Table 2 are the decrease in slot area caused by tapering the teeth, as compared to the 234 mm2 untapered tooth value.

_________ Slot_area_[mm2] _________ _____ ______ Taper length ________ Tooth base No 1/6 1/3 1/2 1/1 width taper [mm] ___ ____ ____ _____ _____ 7 234 _____ ______ _______ _______ 231 228 226 8 ____ (-1 %L (-3%) (3%) ______ 228 222 217 201 ________ _____ (-3%) (-5%) (-7%) (-14%) 209 184 _____ ______ (-11%L (-21%)

Table 2

As already discussed, the tooth shapes of interest are in the range of one-sixth to one-half taper height and 1:1.2 and 1:1.4 tooth tip width to base width ratio (corresponding roughly to the 8 mm to 10 mm base range in the modelled examples). Within these ranges, the largest tooth size is given by a one-half taper height and a 1:1.4 width ratio, and the smallest is given by a one-sixth taper height and a 1:1.2 width ratio.

Considering these ranges for the above tooth dimensions and slot area of 234 mm2, where the tooth area for an untapered tooth is substantially the same as the slot area, one can derive a useful range for the ratio of slot size to tapered tooth size of substantially 1:1.02 to 1:1.22. In other words, the tooth area can be in the range of 2% to 22% larger than the slot area.

Returning to Tables 1 and 2, the figures therein can be used to identify an optimum design from the ranges derived so far. Recall that there is a balance to be made between increasing the torque (by increasing the tooth area) and decreasing the coil volume (by decreasing the slot area as the tooth area is increased). The tooth configuration of one-third taper height and a 9 mm base width (tip to base width ratio of about 1:1.3) gives the largest increase in torque (+17%) for the lowest loss of slot area (- 5%), considered in terms of the difference between the values (22%). This height and width specification corresponds with the taper height and tip to base width ratio previously identified as advantageous from the graphical data. Hence, taking the winding volume into account when optimising the stator design does not result in design parameters that might be considered sub-optimal from other analyses. It is therefore possible to achieve a good balance between the conflicting interests of the tooth volume and the slot volume, leading to an improved motor performance.

For this optimum design choice (one-third taper height and 9 mm base width), the ratio of the slot area to the tooth area is substantially equal to 1:1.1, so that the tooth area is approximately 10% larger than the slot area.

The balance of the design features requires that the slot size not be reduced too much, so as to preserve the largest possible amount of coil volume. The feature which reduces the slot size is the increased width of the teeth given by the taper. Hence it is useful to consider the relationship of the slot width to the tooth width. For this purpose, consider the tooth base width and the slot base width (respectively b and s in Figure 3), where the slot base is the flat portion at the base of each stot bounded by the tapers of the adjacent slots. For the examples under discussion (teeth having a base width in the range of 6 mm to 10 mm), the ratio of the slot base width to the tooth base width can be shown to be substantially in the range 1:2.5 to 1:9. For the preferred tooth base width of 9mm, the ratio is substantially 1:4.3.

The data presented above relate to an example stator having a design based on that shown in Figures 2A and 2B. The stator has 36 teeth, and hence also 36 slots, the tooth area and the slot area are substantially equal in the absence of any taper being applied to the teeth, the hub or body from which the teeth radiate has a diameter of 127.4 mm, and the teeth have a height of 33.3 mm. However, the advantages of the invention are applicable to stators of other configurations. The stator may have any number of teeth, and be of any diameter. Any stator having straight teeth may be improved by tapering the lower portion of the teeth, in particular over a height of between one-sixth and one-half. The scope for widening the teeth at the base will be constrained by chosen slot width, but as has been shown, a large amount of widening is not significantly better than a smaller amount (<50% increase in width compared to no taper). Hence, most stators should be able to benefit from a taper in accordance with the width range described herein, where the tip to base width ratio is between 1:1.2 and 1:1.4.

The various embodiments of the invention may be applied to internal or external stators. After selection of the tooth shape and dimen&ons in accordance with the invention, the stator can be provided with windings in the usual manner, and utilised in an electrical machine, such as a motor or a generator.

While the invention is susceptible to various modifications and alternative forms, as will be apparent to the person skilled in the art, specific embodiments are shown by way of example in the drawings and herein described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular forms disclosed. On the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.

REFERENCES

[1] US 4,647,802 [2] JP10164807 [31 JP 2002-078299 [4] JP 2003-250237 [5] WO 2007/1 00255 [6] JP200B-160939

Claims

S -14-CLAIMS1. A stator comprising a body, and a plurality of teeth extending from the body in a plane and separated by slots, in which the teeth have a base adjacent the body, a tip remote from the body and narrower than the base, a height from the base to the tip, and sides which are tapered inwardly from the base over a lower portion being substantially between one-sixth and one-half the height of the teeth.
2. A stator according to claim 1, in which the lower portion is substantially one-third the height of the teeth.
3. A stator according to claim I or claim 2, wherein the stator is dimensioned such that if the sides of the teeth were not tapered, in the plane of the teeth the area of each tooth would be substantially equal to the area of each slot.
4. A stator according to any one of claims 1, 2 or 3, in which, in the plane of the teeth, the ratio of the width of the tip of the teeth to the width of the base of the teeth is in the range 1:1.2 and 1:1.4.
5. A stator according to claim 4, in which the ratio of the width of the base of the teeth to the width of the tip of the teeth is substantially 1:1.3.
6. Astator according to any preceding claim, in which the slots have a substantially flat base between the tapered sides of the adjacent teeth.
7. A stator according to claim 6, in which, in the plane of the teeth, the ratio of the width of the base of the slots to the width of the base of the teeth is in the range 1:2.5 to 1:9.
8. A stator according to claim 7, in which the ratio of the width of the base of the slots to the base of the teeth is substantially 1:4.3.
9. A stator according to any preceding claim, in which, in the plane of the teeth, the area of the teeth is in the range of 2% to 22% greater than the area of the slots.
10. A stator according to claim 9, in which the area of the teeth is substantially 10% greater than the area of the slots.
11. A stator according to any preceding claim, further comprising windings arranged around the teeth, the windings occupying the slots.
12. A stator according to claim 11, in which the windings occupy substantiatly all of each slot.
13. An electrical machine comprising a stator according to any one of claims ito 12.
14. An electrical machine according to claim 13, in which the machine is a motor.
15. A stator substantially as described herein with reference to Figures 3-8 of the accompanying drawings.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWS1 A stator comprising a body, and a pluraRty of teeth extending from the body in a plane and separated by slots, in which the teeth have a base adjacent the body, a tip s remote from the body and narrower than the base, a height from the base to the tip, and sides which are tapered inwardly from the base over a lower portion being substantially between one-sixth and one-half the height of the teeth and which, beyond the taper, extend parallel and straight to the tip Ic 2,, A stator according to claim 1, in which the lower portion is substantially one-third the height of the teeth 3. A stator according to claim 1 or claim 2, wherein the stator is dimensioned such that if the sides of the teeth were not tapered, in the plane of the teeth the area of each }5 tooth would be substantially equal to the area of each slot.4. A stator according to any one of claims 1, 2 or 3, in which, in the plane of the teeth, the ratio of the width of the tip of the teeth to the width of the base of the teeth is in the range 1:1.2 and 1:14.A stator according to claim 4, in which the ratio of the width of the base of the teeth to the width of the tip of the teeth is substantially 1: 1.3.6 A stator according to any preceding claim, in which the slots have a substantially flat base between the tapered sides of the adjacent teeth, 7, A stator according to claim 6, in which, in the plane of the teeth, the ratio of the width of the base of the slots to the width of the base of the teeth is in the range 1:2,5 to 1:9, B, A stator according to claim 7, in which the ratio of the width of the base of the slots to the base of the teeth is substantially 1:4.3 9. A stator according to any preceding claim, in which, in the plane of the teeth, the area of the teeth is in the range of 2% to 22% greater than the area of the slots.10. A stator according to claim 9, in which the area of the teeth is substantially 10% greater than the area of the slots.11. A stator according to any preceding claim, further comprising windings arranged around the teeth, the windings occupying the slots.12. A stator according to claim 11, in which the windings occupy substantiatly all of each slot.13. An electrical machine comprising a stator according to any one of claims ito 12.14. An electrical machine according to claim 13, in which the machine is a motor.15. A stator substantially as described herein with reference to Figures 3-8 of the accompanying drawings.