Patent ID: 12231012

Like reference numbers are used for like elements throughout the description and figures.

DETAILED DESCRIPTION

An embodiment of the invention will now be described with reference to an axial flux motor100. While a motor100is described, it should be appreciated that the invention could equally be implemented in other types of axial flux electrical machines such as generators.

Overview of an Axial Flux Machine

FIG.1AandFIG.1Billustrate the main components of an axial flux motor100. The axial flux motor100includes a stator assembly1, two rotors2a,2bdisposed on opposite sides of the stator assembly1, and a shaft3. The shaft includes a drive end3aand non-drive end3b. The rotors2a,2bare fixedly mounted to the shaft3. In use, the stator1of the axial flux motor100remains stationary and the rotors2a,2band shaft3rotate together relative to the stator1. It should be appreciated that various components typically present in a motor100, such as rotor cover plates and means for connecting the stator to a source of power, have been omitted fromFIGS.1A and1Bfor clarity.

WhileFIGS.1A-1Bshow two rotors2a,2band a single stator1, it will be appreciated that other configurations are possible. For example, one of the rotors2a,2bcould be shared between two axially-aligned stators. That is, there may be two stators and three rotors, with one of the three rotors shared between the two stators.

FIG.2AandFIG.2Billustrate the rotors2a,2band the shaft3of the motor100without the stator assembly1. As is particularly clear fromFIG.2B, each rotor2a,2bincludes a plurality of circumferentially distributed permanent magnets21,22,23,24. The magnets21,22,23,24are, for example, rare-earth magnets such as NdFeB magnets. Circumferentially adjacent magnets, such as permanent magnets21and22have opposite polarity. That is, each north pole23is circumferentially adjacent to two south poles22,24, and each south pole22is circumferentially adjacent to two north poles21,23.

Although it cannot be seen inFIGS.2A and2B, the rotors2a,2bare mounted such that opposing permanent magnets have opposite poles. That is, a north pole on rotor2afaces a south pole on rotor2band vice versa. Consequently, the magnets of the two rotors2a,2bgenerate a magnetic field with axial lines of magnetic flux between the two rotors2a,2b.

As will be understood by those skilled in the art, the stator assemblies1described herein are yokeless but not ironless. A yoke is an additional structural element present in some stators for guiding lines of magnet flux between opposite poles of the rotor magnetic field. That is, the yoke completes the magnetic circuits within the stator. Since the axial flux machines100described herein utilize a pair of opposed rotors2a,2bwhose opposed permanent magnets have opposite polarity, there is no need for a yoke to complete the magnetic circuits because the flux is unidirectional. Having a yokeless stator reduces the overall weight of the axial flux machine, which is greatly beneficial in many practical applications. In addition, it improves efficiency since there are no losses attributed to a varying flux density in a yoke region.

The circumferential (angular) separation α of the centres of two adjacent permanent magnets21,22of the rotor2a,2bdefines the pole pitch of the axial flux motor100. It is noted that the average span of the permanent magnets β may be the same as or less than the pole pitch α of the motor100. InFIGS.2A-2B, adjacent magnets are separated by a non-magnetic spacer and so the average span β of the permanent magnets21-24is less than the pole pitch α of the motor100. In an example, β is approximately ¾ of α. The ratio of β to α can be chosen to reduce the circumferential, spatial harmonic distortion of the permanent magnet flux density in the stator1. As will be appreciated, it is not essential to provide non-magnetic spacers to enable the span β of the permanent magnets21-24to be less than the pole pitch α of the motor100. For example, the permanent magnets21-24can be affixed to the rotor using adhesive, or the like, in their required spaced apart positions.

The rotors2a,2billustrated inFIGS.2A-2Bhave sixteen circumferentially distributed permanent magnets21-24and therefore have sixteen poles. However, this is merely an example and in practice there may be greater or fewer than sixteen poles, partly depending on the intended application. For example, the poles typically exist in pairs (so there is typically an even number of poles) and the number of poles is to some extent limited by the radius of the rotors2a,2b, which will depend on the size of motor suitable for the intended application. The rotor2a,2bcould, for example, have eight or thirty-two poles.

Turning toFIG.3, this shows a cross-sectional view of the axial flux motor100ofFIGS.1-2with additional detail. As the inventions described herein principally concern the conductive components10of the stator assembly1, which will be described in more detail below with reference toFIGS.4-12, only a brief overview of the components ofFIG.3will be provided. Those skilled in the art will be familiar with the components of an axial flux machine such as an axial flux motor100, and will also appreciate that not all of the features shown inFIG.3are essential to an axial flux machine, and that features which are present can be implemented in a variety of different ways.

In addition to the stator1, drive-end rotor2a, non-drive-end rotor2band shaft3,FIG.3shows the drive-end and non-drive-end rotor cover plates4a,4bwhich enclose the rotors2a,2band generally seal the motor100to prevent the ingress of outside material. Rotor spacer ring4cspaces apart the rotors2a,2b. O-ring seals8a,8band dynamic seal9further seal the internals of the motor100. Rotation of the rotors2a,2bis assisted by the drive-end and non-drive end bearings6a,6b, which maintain the airgaps5between the permanent magnets of the rotors2a,2band the stator1. An encoder assembly7that includes an encoder mount71, an on-axis position encoder72and an associated encoder sensor magnet73is also shown.

Conductive Coils and Stator

The conductive components10, including the conductive coils12, of a stator assembly1will now be described with reference toFIGS.4-12. It should be appreciated that although specific examples are described, with specific numbers of stator poles11, conductive coils12and current phases, this is not intended to limit the scope of the claims.

Briefly turning toFIGS.12A-12C, there is illustrated a stator assembly1which can be seen to include an annular or ring-shaped stator housing20which houses the conductive components10of the stator1. The core of the stator assembly1, where the axial flux provided by the rotor magnets interacts with the radially flowing current flowing through the conductive components10to generate the torque that causes the rotors2a,2bto rotate, includes radially extending active sections of the conductive components10of the stator and flux guides30in the form of lamination packs. The flux guides30, in the form of lamination packs, which may comprise grain-oriented electrical steel sheets surrounded by electrical insulation, are positioned in spaces between the radially extending active sections of the conductive components10of the core. The flux guides30, in the form of lamination packs, act to channel the magnetic flux produced by the permanent magnets21-24between the current carrying conductors.

Now turning toFIGS.4A-4C, the conductive components10(which from now on will be simply referred to as the “stator10”) are shown without the stator housing20or the flux guides30, in the form of lamination packs. As is best appreciated from the top-down view ofFIG.4C, the stator10has distributed windings and comprises a plurality (in this case sixteen) of circumferentially distributed stator poles11a,11b, . . . ,11p, each of which comprises a plurality of conductive coils12. Each conductive coil12is connected to one phase of a multi-phase power supply via connection means15,16which in this example take the form of busbars. In this specific example, the stator10is configured for use with a three-phase power supply so there are three conductive coils12per pole11a-11pof the stator.

It will be appreciated that with sixteen poles11a-11pand three conductive coils12per pole, the stator10ofFIGS.4A-Chas a total of 48 circumferentially distributed conductive coils12. However, it can be seen from the top-down view ofFIG.4Cthat this stator10actually has 96 radially extending active sections. Further, it can be seen from the side-on view ofFIG.4Bthat there are two axially offset layers of radially extending active sections, giving a total of 192 radially extending active sections. The reasons for this will become apparent from the description ofFIGS.5-9. In summary, each conductive coil12includes one or more conductive elements120, each of which includes a pair of axially offset radially extending active sections. Each conductive coil12of the stator10ofFIGS.4A-4Bincludes two such conductive elements120, and since each conductive element120includes a pair of axially offset radially extending sections, the total of 192 radially extending active sections is accounted for.

The conductive components of stator10may be made of any combination of one or more conductive materials. However, the conductive components10are preferably made from copper.

FIGS.5A-5Dare various views of a single conductive element120. As noted above and as will be explained in more detail below, each conductive coil12is made up of one or more conductive elements120. It will be appreciated that in the case of one conductive element120per conductive coil12, a conductive coil12and a conductive element120are equivalent.FIGS.6A-6Dillustrate a conductive coil12which is made up of two conductive elements120and120′, and will be described below.

Returning toFIGS.5A-5D, as is best appreciated from the top-down views ofFIG.5Ain which the axis of rotation is perpendicular to the plane of the page, a conductive element120includes a pair of circumferentially pitched apart, radially extending active conducting sections121a,121b. These radially extending active sections121a,121bare referred to as “active” sections because, when the conductive coils12are positioned in the stator, they are disposed within the stator core and so interact with the magnetic field provided by the magnets of the rotors2a,2b. It will be appreciated that since the active sections extend in a generally radial direction, which is approximately perpendicular to the axial flux in the core, the flux linkage is at least close to maximized.

The angle γ by which the two active sections121a,121bare pitched apart will be referred to as the coil span. The coil span can be the same as or different (less or more) than the pole pitch α (defined by the angle between the centres of the permanent magnets of the rotor). Preferably the coil span γ is less than the pole pitch α. For example, γ may be approximately ⅚ of α. By making γ less than α, short-chording of the winding can be implemented, which reduces the spatial harmonic content of the winding magnetomotive force (mmf).

Turning toFIGS.5E and5F, these show a sixteen-pole, three-phase stator10′ which is similar to the stator10ofFIGS.4A-4C, but differs in that each coil12of stator10′ has only one conductive element120(one pair of active sections121a,121b). That is, inFIGS.5E and5F, a coil12and a conductive element120are equivalent. Like stator10, conductive coils120a,120b,120cof stator10′ are circumferentially distributed around the stator and circumferentially adjacent coils circumferentially overlap.

As is particularly clear fromFIG.5E, the circumferential overlap of the coils120a,120b,120cdefines circumferential spaces between active sections of the coils. These circumferential spaces, which are elongated in the radial direction, can receive flux guides30. Spaces such as the labelled spaces141a,141b,141cwill be referred to as spaces of the first type. As can be seen, spaces of the first type141a,141b,141care defined between active sections of different coils. For example, space141bis between one of the two active sections of coil120aand one of two active sections of coil120c. However, it is to be appreciated that the two coils that define a particular space of the first type141a,141b,141ccan depend on various factors, including the number of phases per stator pole, the number of poles and the selected coil span γ.

Now returning toFIGS.5A-5D, as can be seen fromFIGS.5B and5D, the two active sections121a,121bare axially offset from each other. This facilitates stacking of the conductive coils12in the circumferential direction, and also facilitates the circumferential stacking of conductive elements120where there are multiple conductive elements120per conductive coil12. As will be discussed in more detail with reference toFIG.14, this allows for more stator poles and more slots per pole per phase, both of which can provide for greater efficiency. Furthermore, the winding may be readily short chorded.

As can be seen in each ofFIGS.5B,5C and5D, each conductive element120is formed from a continuous length of wound conductor. The outermost winding of the length of conductor terminates at a first connection portion128, which will be referred to as the outer tail128. The outer tail128extends substantially parallel to the axial direction. As will be described in more detail below, this facilitates convenient connection of the coils12to the multi-phase power supply. The innermost winding turn portion terminates at a second connection portion129, which will be referred to as the inner tail129.

As can also be seen in each ofFIGS.5B,5C and5D, the length of conductor that forms the conductive element120is wound such that there are a plurality of winding turn portions131a,131bstacked parallel to the axis of rotation of the electrical machine. The resulting cross-section of the conductive element120that is perpendicular to the radial direction of each active section121a,121bis elongate with a major dimension parallel to the axis of rotation. In the example ofFIGS.5A-5D, there are fourteen axially stacked winding turn portions131a,131b, though this is not intended to limit the invention as other numbers are equally possible.

FIGS.5G,5H and5Iillustrate how the conductive element120may be formed by winding a length of conductor. As illustrated inFIG.5G, the conductor is wound around a pair of support elements301,302(which protrude perpendicularly out of the plane of page) in a single plane so as to form a flat, planar winding with a number (in this case fourteen) of turns or layers. That the winding is flat is best appreciated fromFIGS.5H and5I. The innermost winding terminates at the inner tail129and the outermost winding terminates at the outer tail128.

Having formed the flat winding shown inFIGS.5G-5I, the three-dimensional shape of the conductive element120is formed by bending or deforming the flat winding into the shape shown inFIGS.5A-5D. The bending can be performed using a bending tool, as is known in the art. For example, a bending tool with axially offset inner male profile blocks may push against outer female forms to bend the flat winding so that the active sections are axially offset from each other. The outer tail128and inner tail129may be separately bent as desired.

To make the bending process easier, the flat winding may first be imparted with additional strength so that the winding maintains its shape during the bending. In one example, the conductor has a heat- or solvent-activated outer bond layer so that after winding, the turns/layers can be bonded together to maintain the shape.

It should be appreciated, particularly fromFIGS.5G-5I, that the conductive element120can be wound in a variety of different ways, and the particular winding that is illustrated is not intended to limit the invention. Some alternatives include:While the winding inFIG.5Ghas been wound around the support elements301,302in an anti-clockwise sense, the length of conductor could equally be wound in the clockwise sense.While the outermost turn of the winding terminates such that that outer tail128leads into an active section121a,121bof the conductive element120, this need not be the case. The outer turn could terminate at any point of the turn, for example so that the outer tail128leads into a loop section of the turn rather than an active section.While fourteen axially stacked winding turns are illustrated inFIG.5, there could be more than or fewer than fourteen turns.While the winding is one turn/layer thick (seeFIG.5Hin particular), it could be more than one turn/layer thick. In this case, each conductive element120will comprise a plurality of circumferentially stacked winding turn portions. While any number of circumferentially stacked winding turn portions is possible, the number will preferably be less than the number of winding turn portions in the axial direction, such that the cross-section of the conductive element120that is perpendicular to the radial direction of each active section121a,121bstill has a major dimension that is parallel to the axis of rotation. For example, the ratio of the number of axially stacked turns to the number of circumferentially stacked turns may be greater than three, and may preferably be greater than five.

As will be appreciated from the above, in use, current will flow along the two active sections121a,121bof the conductive element120in opposite directions (that is, inward and outward parallel to the radially extending direction). The reversal of the current direction is provided by outer loop sections122of the winding turn portions131a,131band by inner loop sections125of the winding turn portions131a,131b. Each of the outer loop sections122includes a first portion123and a pair of second portions124a,124b(one for each of the pair of active sections121a,121b) which connect the active sections121a,121bto the first portion123. Similarly, each of the inner loop sections125includes a first portion126and a pair of second portions127a,127b(one for each of the pair of active sections121a,121b) which connect the active sections121a,121bto the first portion126.

As can be seen fromFIGS.5B,5C and5D, the outer first portions123together form an outer part133of the coil element120with a surface that is substantially parallel to the axis of rotation. In the specific example ofFIGS.5A-5D, the outer first portions123are substantially semi-circular and so the outer part133is a substantially flat half-disk133, but other shapes are possible. For example, each of the outer first portions123may have a shape corresponding to three sides of a rectangle, such that they together form an outer part133which has a flat rectangular surface. As another example, the outer part133of the conductive element120formed by the outer first portions123need not be flat or planar: this is illustrated inFIG.5J, which shows a conductive element120″ with an outer part133″ with a curved profile and therefore curved surface.FIG.5Killustrates a plan view of a stator10″ comprising such conductive elements, which can be compared toFIG.4C(though note that stator10″ does not show any connecting means15,16).

The surface133formed by the outer first portions123can be used to facilitate cooling due to its relatively large surface area. Further, since the outer part133of the coil120is substantially parallel to the axis is rotation, a stator housing20may be provided with axially extending apertures25which axially receive the outer part133of the coil element120′,120″ to provide mechanical locking and improved cooling. This will be explained in more detail below.

The inner first portions126together form an inner part136of the coil element120. The inner part136illustrated inFIGS.5B-5Dis substantially the same as the outer part133described above, and like the outer part133described above may be parallel to the axis of rotation and may be of various shapes and profiles. However, the inner part136will generally play less of a role in cooling and stacking of the coils12, and so the inner portions126may be configured so as to reduce the overall quantity of conductor per conductive element120to reduce costs.

With regards to the outer second portions124a,124band the inner second portions127a,127b, while they appear substantially straight inFIGS.5A-5D, they are in fact slightly curved. Specifically, the shape of each of the outer first portions124a,124bis a section of a first involute, and so the first portions124a,124btogether form outer substantially involute parts134a,134bof the coil element120. Similarly, the shape of each of the inner second portions127a,127bis a section of a second involute, and so the first portions127a,127btogether form inner substantially involute parts137a,137bof the coil element120. The significance of the involutes will be described with reference toFIGS.6A-6D.

While it has been described above that the conductive element120is formed by winding a length of conductor, this is not essential. The conductive element120could be manufactured in other ways, including by being formed integrally.

Further, while the illustrated elements120are wound from a length of conductor and comprise a stack of winding turn portions131a,131b, this is preferred but not essential. For example, rather than axially extending stack of winding turn portions131a,131b, each conductive element120could be formed by a single axially extending conductive strip. In some cases a single axially extending conductive strip may be preferable to a plurality of axially stacked winding turn portions131a,131bbut, as will be described below, the use of stacked winding turn portions131a,131badvantageously helps mitigate the skin and proximity effects which can otherwise lead to increased losses.

As noted above, each conductive coil12may include only one conductive element120. However, for reasons which will be explained in more detail below, each conductive element preferably includes two or more circumferentially overlapping conductive elements. An example of a conductive coil that includes two circumferentially overlapping conductive elements120,120′ will now be described with reference toFIGS.6A-6D.

FIG.6Ashows above and below views of a conductive coil12which includes two conductive elements120,120′. The features of each of the two conductive elements120,120′ are the same as those of the single conductive element120described above with reference toFIGS.5A-5D, and so their features will not be described again.

To form the conductive coil12, two identical conductive elements120,120′ are electrically connected together in series at their inner tails129,129′. In the examples illustrated herein, the inner tails129,129′ are connected using a ferrule130. However, there are other ways of connecting the inner tails129,129′, such as brazing or welding. To connect the two elements120,120′, one of the two conductive elements120,120′ is rotated 180° about the axis running vertically in the plane of the page inFIG.6Aso that the outer tails128,128′ of the two conductive elements120,120′ are in opposite directions and the inner tails129,129′ are adjacent and therefore readily connected by a ferrule130. Alternatively, the conductive coil12comprising two conductive elements could be integrally formed as a single piece.

The resulting conductive coil12has two pairs of circumferentially overlapping, pitched apart pairs of active sections121a,121b;121a′,121b′. Notably, the overlap of the two pairs of active sections defines two spaces142a,142b. The first space142ais defined between one (a first) active section121aof a first of the conductive elements120of the coil12and between one (a first) active section121a′ of the second of the conductive elements120′ of the coil12. The second space142bis defined between the other (the second) active section121bof the first conductive element120of the coil12and between the other (the second) active section121b′ of the second conductive element120′ of the coil12. That is, the two spaces142a,142bare circumferential spaces between adjacent active sections121a,121a′;121b,121b′ of two different pairs of active sections121a,121b;121a′,121b′ of the same coil12. Spaces of this type will be referred to as spaces of the second type. Like the spaces of the first type, spaces of the second type142a,142bprovide spaces for flux guides30, such as lamination packs. This makes it easier to construct the stator assembly1, and also increases the number of slots per pole per phase of the stator assembly1, which can increase the motor's efficiency.

Having now described spaces141a-cof the first type (that is, spaces defined between active sections of different coils) and spaces142a-bof the second type (that is, spaces defined between active sections of the same coil but different pairs), it is noted that when a plurality of coils12which define spaces of the second type are provided in a stator10so as to define spaces of the first type, the spaces of the first and second types may coincide. This can be seen most clearly inFIG.11A, which illustrates a sixteen-pole, three-phase stator in which each coil12comprises two conductive elements120,120′. Only half of the conductive coils12are shown inFIGS.11A-Bso that the spaces can be clearly seen. Whether spaces of the first and second type coincide may depend on a number of factors, including the selected coil span γ, the number of stator poles and the number of phases.

Returning toFIGS.6A-6D, it can also be seen fromFIGS.6A and6Bthat there is a gap143abetween the second portions124a,124a′ of the outer loop sections122,122′ which form one pair of outer involute parts134a,134a′ of the two conductive elements120,120′. Likewise, there is a gap143bbetween the second portions124b,124b′ of the outer loop sections122,122′ which form the other pair of outer involute parts134b,134b′. There is also a gap144abetween the second portions127a,127a′ of the inner loop sections125,125′ which form one pair of inner involute parts137a,137a′. Finally, there is also a gap144bbetween the second portions127b,127b′ of the inner loop sections125,125′ which form the other pair of outer involute parts137b,137b′. Due to the geometric properties of involutes, the width of these gaps143a,143b,144a,144bremains substantially constant along the length of the involute sections of the conductive elements120,120′. This advantageously reduces the resulting diameter of the motor for a given rating and losses in the coils.

While a conductive coil12with two conductive elements120,120′ has been described, it should be appreciated that a conductive coil12could have any integer number of conductive elements120, including more than two. Increasing the number of conductive elements per conductive coil12will increase the number of spaces of the second type defined by the circumferentially adjacent active sections of the conductive elements120, which in turn increases the number of slots per pole per phase in the stator1. This can lead to the generation of a stator magnetic field with a more accurately sinusoidal magnetic flux density, with less significant harmonic distortion. This advantageously reduces the development of eddy currents in the permanent magnets of the rotors2a,2b, which in turn reduces heating losses and therefore provides a higher motor efficiency. However, it will be appreciated that the number of conductive elements120per conductive coil12will generally be limited by size constraints. For example, for a given cross-section of conductor (that is, the cross-section of the wire from which the windings are wound) and a given radius of the stator, the number of conductors which can be circumferentially fit into a single coil span γ is limited.

If a coil12is to have more than two conductive elements, there may be several further considerations. For example:If the coils are to be formed by connecting multiple conductive elements120(by ferrules130, for example), it may be preferable to provide several types of conductive elements to facilitate simpler connection of adjacent conductive elements. For instance, the conductive elements120described above may be used for the two circumferentially outer conductive elements, since their outer tails128will be connected to the power-supply. However, the one or more inner conductive elements that are between the outer conductive elements will be connected to conductive elements at both their inner tails129and outer tails128, so a second type of conductive element with outer tails128adapted in a similar fashion to the inner tails129may be provided for ease of connection. Alternatively, each coil12may be formed as an integral unit, rather than by the connection of three or more separate conductive elements.Integer multiples of two conductive elements120per coil12may be preferable to an odd number of conductive elements120per coil12. If an integer multiple of two elements120are used, the outer tails128of the two circumferentially outermost elements120will be directed in opposite parallel directions, as inFIG.6A-6D. While this is not essential, it provides for a more straightforward connection of the coils12using the connection means which will be described below with reference toFIGS.7-10.

While a stator10with a single axial layer of circumferentially distributed coils12(the single layer having coils12with axially offset active sections) has been described, it will be appreciated that there may be multiple axially-stacked layers of coils per stator. In this case, the spaces of the first type and/or the spaces of the second type of each layer may advantageously substantially circumferentially coincide. This would advantageously allow for the insertion of axially-longer flux guides30which could extend through the axial length of the multiple axially-stacked layers, providing further gains in terms of ease and speed of assembly.

Connecting the Coils to a Multi-Phase Power Supply

Ways of connecting a plurality of circumferentially distributed conductive coils12to a multi-phase power supply will now be described. It should be appreciated that in practice there are many different ways which this could be accomplished, and many different ways will occur to one skilled in the art. The invention is therefore not limited to any particular connection arrangement. However, the described ways of connecting the conductive coils12, which utilize connection means15,16which are provided axially above/below a plane that is perpendicular to the axis of rotation and axially above/below the conductive coils, provides a particularly neat and well-organized set of connections. Further, the connections are easy to make, which reduces the likelihood of a poor connection, and the stator may be resin impregnated without impregnating the connection means, which allows connections to be checked and fixed even after impregnation of the stator assembly.

First referring toFIG.4B, there is a first connection means15that is provided axially above a plane that is perpendicular to the axis of rotation of the motor100and that is axially above the conductive coils12. There is also a second connection means16that is provided axially below a plane that is perpendicular to the axis of rotation of the motor100and that is axially below the conductive coils12. In the case of the stator10, which is configured for use with a 3-phase power supply, the connection means15and16include provision for each of the 3-phases. However, this could be extended to a multi-phase power supply with any number of phases.

In the particular connection arrangement ofFIGS.4A-4C, which will be referred to as a parallel connection arrangement, each of the connection means15,16includes three phase-connections and one star-connection. That is, the first connection means15includes a first phase connection151for a first phase of the power supply, a second phase connection152for a second phase of the power supply, a third phase connection153for a third phase of the power supply, and a star connection154. Similarly, the second connection means16includes a first phase connection161for the first phase of the power supply, a second phase connection162for the second phase of the power supply, a third phase connection163for the third phase of the power supply, and a star connection164.

In the described examples, the phase connections151-153,161-163and star connections154,164are in the form of annular busbars whose outer circumference (though equally this could be the inner circumference) substantially coincides with the axially extending outer tails128,128′ of the conductive coils. The phase connection busbars151-153,161-163are themselves connected to the power supply via inputs1510-1530,1610-1630.

In the illustrated parallel connection arrangement, each conductive coil12is connected to one phase of the power supply by connecting the coil12to one of the phase connections of one of the connection means15,16(as an example, phase connection151) and to the star connection of the other of the connection means15,16(in the example, star connection164). The connection of one conductive coil12to one phase connection151and one star ring164is illustrated in and will now be described with reference toFIGS.7A-7C.

FIGS.7A-7Cshow one conductive coil12that has two conductive elements120,120′ connected to a first phase connection151from the first connection means15, and to the star connection164from the second connection means16. Since the outer tails128,128′ of the conductive coil12extend axially and in opposite directions, and since the circumference of the busbars151,164coincides with the axial extending outer tails128,128′, the outer tails128,128′ are easily connected to the connections151,164.

In order to make the connection even easier, the annular busbars151,164are provided with circumferentially spaced apart receiving means151a-h,164a-xfor receiving the axially extending outer tails128,128′ of the coils12. In the 3-phase parallel connection arrangement shown, each star connection154,164will be connected to half of all coils12, whereas each phase connection151-153,161-163will only be connected to one in six coils12. Consequently, in this example, the star connection164has three times as many equally spaced receiving means164a-xthan the first phase connection151.

Returning toFIGS.4A-4C, each pole11a-11pof the stator10consists of one conductive coil12for each phase (i.e. three conductive coils12per pole11a-pbecause the stator is configured for use with a 3-phase supply), and circumferentially adjacent conductive coils12are connected to different phases. This is illustrated inFIGS.11A and11Bfor a sixteen pole stator10which is connected to a 3-phase power supply but for which only half of the conductors are shown, and so has only 24 circumferentially distributed conductive coils12can be seen.

In view of this, in the 3-phase parallel connection arrangement illustrated inFIGS.4,7-9and11-12, every sixth conductive coil12will be connected to the connection means15,16in the same way. This is illustrated inFIGS.8A and8B. It can be that there are eight equally spaced conductive coils12a-gconnected to the same phase connection151and the same star ring164. Although not shown inFIGS.8A-8B, it will be appreciated that half way between each of the coils will be another coil12connected to the same phase of the power-supply, but by the complimentary set of bus bars. That is, to the phase connection161and the star connection154.

The conductive coils12corresponding to the other phases of the power-supply will be connected in essentially the same way as described above for one phase. To illustrate this,FIGS.9A-9Cshow how two circumferentially adjacent conductive coils12are connected in the parallel connection arrangement.

FIGS.9A-9Cshow two circumferentially adjacent conductive coils12a,12b. Conductive coil12ais connected in a similar way as conductive coil12inFIGS.7A-7C. That is, coil12ais connected to the second phase connection152and the star connection164. Coil12b, being circumferentially adjacent to coil12a, is connected to a different phase of the power supply and is therefore connected to a different pair of busbars. Specifically, but without loss of generality, circumferentially adjacent coil12bis connected to the third phase connection163of the second connection means16and to the star connection154of the first connection means.

The connections of the conductive coils12have been described above with reference to a parallel connection arrangement. However, other connection arrangements are possible. To illustrate this,FIG.10shows an alternative arrangement, which will be referred to as a series connection arrangement.

In the series connection arrangement ofFIG.10, the first connection means15′ which is above the conductive coils12differs from the connection means15ofFIGS.4,7-9and11-12in that it does not include a star connection: it only includes a first phase connection151′, a second phase connection152′ and a third phase connection153′. However, the second connection means16′ is the same as the second connection means16ofFIGS.4,7-9and11-12in that it has three phase connections161′,162′,163′ and a star connection164′. To compensate for the lack of star connection in the first connection means15′, the conductive coils12are connected in a different way. The phase connections151′-153′ of the first connection means15′ also serve twice as many conductive coils12, and therefore have additional receiving means compared to the receiving means of the second connection means16′ and the first and second connection means15,16of the parallel connection arrangement.

FIG.10illustrates the series connection arrangement for two circumferentially adjacent stator poles11and11′. Like the parallel connection arrangement, each pole11,11′ includes one conductive coil per pole, giving three coils per pole: pole11consists of conductive coils12a,12band12c, and pole11′ consists of conductive coils12a′,12b′ and12c′. Also like with the parallel connection arrangement, circumferentially adjacent coils are connected to different phases. However, while the coils of the same phase but adjacent poles (12aand12a′, for example) in the parallel connection arrangement are essentially independently connected and form separate current paths, in the series connection arrangement their connections are related and they are part of the same current path.

Considering only coils12a,12a′ which are connected to the same phase, the coil12aof the first pole11is connected by its outer tails to the phase connection153′ of the first connection means and to the phase connection163′ of the second connection means. The coil12a′ of the second, adjacent pole11′ is connected to the phase connection153′ of the first connection means15′ and to the star connection164′ of the second connection means. The current path can therefore be considered to run from the phase connection163′ through the coil12a, then along phase connection153′ and then through coil12a′ to the star connection164′.

Different connection arrangements may be used for different practical applications. For example, the series connection arrangement described above theoretically provides a machine Torque Constant (measured in Nm/A) that is twice as high as than that provided by the parallel connection arrangement described above. This will be better for some, though certainly not all, practical applications.

While the connection means15,15′ have been described as being above the coils12and the connection means16,16′ have been described as being below the coils, it should be appreciated that both pairs15,16;15′,16′ may be above the coils or both pairs15,16;15′,16′ may be below the coils. In this case, it may be preferable to produce coils12whose outer tails128,128′ extend in the same axial direction rather than opposite axial directions.

Further, while the connection means15,16,15′ and16′ have been described as continuous, annular busbars, this is merely one way of implementing the connection means. For example, the connection means may not be continuous or annular, and may instead take the form of a series of two or more circumferentially distributed busbar sections. Many other kinds of connection means will occur to those skilled in the art.

Stator Manufacture

The features and construction of the conductive coils12described above provide for particularly efficient and effective manufacture of a stator that includes a plurality of circumferentially distributed coils12. Of particular significance is the fact that the coils12themselves provide a structure into which flux guides30, for example in the form of lamination packs, can be provided. This makes placing of the flux guides30in the stator assembly1a comparatively straightforward and precise exercise, especially compared to many known manufacturing techniques which may involve winding coils around bobbin-like structures which house lamination packs, and then separately securing (using glue, for example) the wound bobbin-like structures into a stator housing. Various other advantages will be described.

FIG.13is a flow-chart illustrating a method500for manufacturing a stator.

The method500includes providing510a plurality of conductive coils, such as the conductive coils12described above. Preferably the conductive coils12have a plurality of circumferentially overlapping pairs of circumferentially pitched apart radially extending active sections (as in the coil12ofFIGS.6A-6D) such that each coil12provides spaces of the second type. However, the coils12may only have one pitched apart pair of active sections (as in the coil ofFIGS.5A-5D). The conductive coils12may have been formed as a single integral piece, by connecting multiple conductive elements120in series, or in any other way.

At520, the method500includes positioning a plurality of the conductive coils12in a stator housing so that the plurality of coils are circumferentially distributed around the stator housing. Preferably the conductive coils are positioned so that circumferentially adjacent conductive coils circumferentially overlap and thereby define spaces of the first type for receiving flux guides. The circumferential overlap of circumferentially adjacent coils12can be ensured by providing an appropriate number of coils12of an appropriate coil span γ within the housing. As noted above, where the coils12have multiple pairs of active sections such that the coils each define spaces of the second type, the spaces of the first and second types may coincide with each other.

The stator housing20may be provided with a plurality of circumferentially spaced apart axially extending apertures25for receiving the coils12. This makes the positioning of the coils12in the stator housing easier and more precise. Advantageously, if the coils12are formed so as to have an axially extending outer part133, the axially extending outer part133can be received within the axially extending apertures25. Since the axially extending outer part133have a large surface area, they provide good mechanically locking of the coils12in the stator housing for assembly without the need for glue (for example) and also provide a source of cooling of the stator. Circumferentially distributed apertures25for receiving the coils12can most clearly be seen inFIGS.12A-12C.

Optionally, at530, the method500includes positioning flux guides30, such as lamination packs, in the spaces (of the first and/or second type) defined by the coils12. As explained above, the overlap of adjacent coils creates spaces of the first type141a,141b,141cbetween active sections of different coils. If the coils12each comprise more than one pair of radially extending active sections (as inFIGS.6A-6D), pairs of spaces142a,142a′ of the second type will also be defined within each conductive coil12. In either case, flux guides can also be positioned within the spaces. Since the coils12themselves provide a structure with defined spaces, positioning the lamination packs into the structure is straightforward, fast and precise. In combination with the provision of apertures25in the stator housing20for receiving the coils12, this means that both the components of the stator core (the active sections of the coils12and the flux guides30) can be quickly and very accurately positioned compared to many known techniques. It will be appreciated that accurately positioned core components reduces losses and therefore improves machine efficiency.

Optionally, at540, the method500includes connecting the plurality of coils12to connecting means15,16so that the coils can be connected to the multi-phase power supply. This may be done in any desired way, for example as described above using busbars in the parallel or series connection arrangements.

Optionally, at550, the method500includes impregnating at least part of the stator assembly1in a bonding compound such as a resin. This strengthens the stator structure and therefore protects the stator assembly1against the electromagnet and mechanical forces it experiences in use. Furthermore, it can improve the conduction of heat between the stator constituents if the bonding compound has a heat transfer coefficient significantly higher than air.

If the connecting means15,16are provided axially above and/or below the coils12as described above, the impregnation of the stator can take place before or after the coils are connected to the connecting means. Further, and advantageously, if the connecting means15,16themselves are not impregnated, the connections can be tested, altered, and if necessary replaced after impregnation. This is highly desirable because a faulty connection in a resin-impregnated stator may otherwise render the entire stator unusable and unfixable.

Machine Efficiency

Axial flux machines100comprising the stator assembly1described herein have been found to provide not only a high peak efficiency, but a high efficiency over a broad range of operating parameters. While high peak efficiencies are often quoted, they are in practice rarely achieved, especially in applications where the machine is required to perform over a range of operating parameters. Efficiency over a broad range of parameters is therefore a more practically meaningful measure for many applications.

To illustrate this,FIG.14is an efficiency map showing the measured efficiency of an axial flux machine comprising the stator assembly ofFIGS.12A-12Cfor a range of torque and speed values that are commonly-used in many applications. Contours of constant efficiency are included on the efficiency map. As can be seen, as well as a high peak efficiency (93%), the efficiency remains very high for almost all of the area of the efficiency map and high (over 80%) even at a relatively low speed of 500 rpm up to a torque of 30 Nm.

There may be a number of different reasons for the high efficiencies which the stator assembly1is able to achieve. Some of these will now be described.

First, as explained above, the almost self-forming structure of the conductive components of the stator10that is provided by the geometry of the coils12allows for the very accurate placement of components of the stator core. The accurate placement of the components of the core means that there is better coupling of the stator and rotor fields, and a high degree of symmetry around the circumference of the stator which improves the generation or torque.

Another significant advantage is the generation of a stator field with a more accurately sinusoidal magnetic flux density. As will be understood by those skilled in the art, the higher the number of slots per pole per phase in the stator, the more sinusoidal the magnetic flux density can be. The coils12and stator10described above can provide an increased number of slots per pole per phase by increasing the number of conductive elements120per conductive coil12, and this number can easily be scaled up (if, for example, the radius of the stator can be increased for a particular application). An advantage of a highly sinusoidal magnetic flux density is that the flux density has a relatively low harmonic content. With a low harmonic content, more of the coupling the rotor and stator fields involves the fundamental components of the flux density, and less involves the interaction with the harmonic components. This reduces the generation of eddy currents in the rotor magnets, which in turn reduced losses due to heating. In contrast, many known axial flux motors utilize a concentrated winding arrangement which only provides for a limited number (e.g. fractional) slot per pole per phase, which generates a much more trapezoidal flux density with more significant harmonic components.

While the coils12can be implemented using axially extending strips, they are preferably implemented using axially stacked winding arrangement illustrated inFIGS.5A-5D and6A-6D. While many motor manufacturers may consider this a disadvantage because it may be considered to reduce the fill factor in the stator core, the inventors have found this disadvantage is compensated for by the reduction in the skin and proximity effects which causes currents to flow around the outside of the conductor cross-section and predominantly the axially-outer portions of the active sections. The number of windings in the axial direction may be selected to balance these two considerations.

Described above are a number of embodiments with various optional features. It should be appreciated that, with the exception of any mutually exclusive features, any combination of one or more of the optional features are possible.