GB2408154A - Stator/rotor arrangement and exciter for an axial flux AC machine - Google Patents

Abstract

An axial flux AC machine has a rotor 10 comprises a shaft 15 which extends through the central aperture of the annular stator 11 and which carries two discs 13 and 14 of ferrous metal; one on either side of the annular stator 11. Each rotor disc 13, 14 carries a circular array of ferrous salient pole pieces 16 which project towards the stator core 11. Field windings 18 and 20 are wound on each salient pole piece 16 and on the annular stator core 11. An exciting arrangement comprises a stationary pole assembly 28,30,31 wound with a static coil 25 which interacts with three phase windings 26 mounted in slits 24 formed in a spiral wound core 23 secured to one rotor disks 14.to energise windings 18. An automatic regulator supplies winding 25.

Description

2408 1 54 AC Machines This invention relates to AC machines more

particularly, although not exclusively, to axial flux AC machines and to an excitation arrangement for such an axial flux AC machine.

The most commonly used AC alternators, motors or generators have a similar topology in that they include a wire-wound rotor which is journalled to rotate coaxially within a generally tubular cylindrical stator which has windings which run axially from end to end of the stator in semi-closed slots which are formed at circumferentially-spaced locations in the inner cylindrical surface of the stator. The bodies of the rotor and the stator are formed of a ferrous material and that material is usually laminated to reduce the influence of any currents. These machines may be provided with brushes for enabling connection to external facilities or they may be brushless. Either way these machines are an assembly of a large number of individual parts or components and they are bulky so that they occupy a large volumetric space when they are installed.

Alternative topologies for AC Machines have been proposed. These include axial flux machines such as are described in a paper by Wu et al entitled "Design of slotless torus generators with reduced voltage regulation" (IKE PROC.-Electr Power Appl., Vol. 142, No. 5, September 1995). This paper discloses a permanent magnet AC generator which includes a toroidal stator sandwiched between a pair of coaxial rotor discs. The rotor discs are ratably mounted on a common axle so to rotate together relative to the stator. Each rotor disc includes a plurality of permanent magnets mounted thereon in a respective circular array. This stator, which is ferrous, has a stator winding wound helically therearound, ends of the winding corresponding to output terminals of the generator. The arrangement is such that the permanent magnets set up an excitation magnetic field which passes through the winding and through the stator. Rotation of the rotor discs and hence the excitation field relative to the stator and winding causes an electromotive force (emf) to be induced in the winding, such that a current may be drawn therefrom.

Permanent magnets are expensive.

An object of this invention is to provide an AC machine topology which will result in an AC machine having an overall volume which is somewhat smaller than that of the most commonly used equivalent AC machines referred to hereinbefore and which is an assembly of a somewhat smaller number of individual parts or components than are the most commonly used AC machines referred to hereinbefore, with a consequent beneficial reduction in cost.

According to one aspect of this invention there is provided an axial flux AC machine including a rotor disc and a stator core, the rotor disc being formed of a ferrous metal and being journalled for rotation about an axis relative to the stator core from which it is spaced in the direction of the axis of rotation, the rotor disc having salient pole pieces which are formed of a ferrous metal and which project towards the stator core so that an air gap is formed between the stator core and the surfaces of the salient pole pieces that are nearest to it, there being field windings wound on the salient pole pieces and on said stator core such that the magnetic field caused by an emf in the field windings on one of said rotor disc and said stator core links with the field windings on the other of said rotor disc and said stator core to induce an emf in the field windings on said other of said rotor disc and said stator core, the arrangement being such that each juxtaposed pair of salient pole pieces together with structure of said rotor disc that extends between the respective salient pole pieces of each pair, with a respective portion of said stator core that is opposite to the salient pole pieces of each juxtaposed pair at any one instant and with the air gaps between the salient pole pieces of each pair and said stator core form a respective magnetic circuit for a magnetic field which is established by or which induces an emf in the respective field windings wound on the salient pole pieces of each pair.

Preferably the rotor disc, including the salient pole pieces, is formed in one piece as a single homogeneous body.

According to another aspect of this invention there is provided an AC machine including two parts, one of the two parts being movable laterally relative to the other, a first of the two parts having salient pole pieces which project towards the second of the two parts so that an air gap is formed between said second part and the surfaces of the salient pole pieces that are nearest to it, there being field windings wound on the salient pole pieces and on said second part such that a magnetic field caused by an emf in the field windings on one of said first and second parts links with the field windings on the other of said first and second parts to induce an emf in the field windings on said other of said first and second parts, the arrangement being such that each juxtaposed pair of the salient pole pieces together with structure of said first part that extends between the respective salient pole pieces of each pair, with a respective portion of said second part that is opposite to the salient pole pieces of each juxtaposed pair at any one instant and with the air gaps between the salient pole pieces of each pair and said second part form a respective magnetic circuit for a magnetic field which is established by or which induces an emf in the respective field windings wound on the salient pole pieces of each pair, wherein said first part, including the salient pole pieces, is formed in one piece as a single homogeneous body.

Preferably the rotor disc of an axial flux AC machine which embodies this invention is one of two similar rotor discs which are mounted on a common shaft for conjoint rotation about said axis and the stator core is annular with a central aperture and is positioned between the two similar rotor discs which are both orientated similarly so that their salient pole pieces project towards the annular stator core, said common shaft extending through said central aperture. Such use of two similar rotor discs, one on either side of the stator core, enhances the power factor of the resultant axial flux AC machine.

In one form of axial flux AC machine which embodies this invention, each salient pole piece has a respective coil wound on it, the coils on the salient pole pieces being connected together to form the respective field windings. Conveniently each coil wound on a salient pole piece is wound onto a bobbin which is fitted onto the respective salient pole piece. This simplifies the assembly process.

In another form of axial flux machine which embodies this invention, the field windings wound onto the salient pole pieces of the or each rotor disc are flywound around two or more of the respective salient pole pieces.

The field windings on the annular stator core may be toroidally wound around that core.

Wu et al acknowledged the problematic nature of a phenomenon known as "armature reaction" whereby should a load be connected across the output terminals of the stator winding so as to draw a current therefrom, the existence of this current in the winding results in a field being set up around that winding. This field is referred to as the "armature reaction field". Its orientation depends on the power factor of the load. For common resistive and inductive loads, or a combination thereof, the effect is to react against and oppose the action of the excitation field so as to effectively reduce the strength thereof. This results in a reduction in the size of the induced emf, and hence the voltage across the load, with increased load current. Capacitive loads tend to have the opposite effect and result in an increase in the voltage thereacross when the load current increases. Both situations are undesirable.

A subsidiary object of this invention is to provide a fixed speed axial flux alternator. Hence a preferred embodiment of this invention is an axial flux alternator which is provided with automatic voltage regulation means lo which are responsive to load variations at the output of the alternator and which are operable to energise excitor windings and thereby to vary current flow in the field windings wound on the salient pole pieces whereby to counter the phenomenon that is know as "armature reaction" and to maintain the magnetic field that is established between the rotor disc or discs and the stator core at a constant level. Where such a control is provided, it is desirable for the field windings on the stator core to be wound into slots formed in the stator core. The field windings on the stator core may be flywound into the slots that are formed in the stator core.

In an embodiment, exciter windings are provided on the side of said rotor disc, or on the side of one of the two similar rotor discs which is remote from said stator core, said exciter windings being connected to the field windings on the respective rotor disc through rectifier means, there being a static winding which is located relative to said exciter windings such that a magnetic field established by an emf in the static winding links with said exciter windings and thereby induces an emf in the exciter windings, the static winding being adapted to be connected to automatic voltage regulation means which are responsive to any variation in load applied to the output of the AC machine and which are operable to supply a unidirectional control current to the static winding whereby to maintain flux in the air gaps of the AC machine substantially constant. The exciter windings may be multi-phase windings, conveniently three phase windings. The exciter windings may be flywound into slots which are formed in a laminated ferous metal component which is mounted on the respective side of the respective rotor disc. Conveniently the static windings are wound on claw-shaped magnetic poles which are mounted on a back iron support.

According to another aspect of this invention there is provided an excitation arrangement for an axial flux AC machine, the excitation arrangement including exciter windings for connection to field windings of the AC machine through rectifier means' a static winding located relative to said excitor windings such that a magnetic field established by an emf in the static winding induces an emf in the exciter windings, the static winding being adapted to be connected to automatic voltage regulation means which are responsive to any variation in load applied to the output of the AC machine and which are operable to supply a unidirectional control current to the static winding whereby to maintain flux in the air gaps of the AC machine substantially constant, wherein the exciter windings are mounted on a rotary element of the axial flux AC machine, the static winding is substantially coaxial with the axis of rotation of the rotary element and is wound around the structure of ferromagnetic material which projects from a backplate of ferromagnetic material and which forms one circular array of pole pieces, there being another circular array of other pole pieces of ferromagnetic material which are mounted on the backplate, wherein the pole pieces of said one circular array are oppositely polarised as compared to the polarization of the pole pieces of said other circular array by flow of current through the static winding, the flux path between the two circular arrays of pole pieces being through the rotary element and the return flux path being through the backplate on which the sets of pole pieces are mounted.

In one embodiment said other circular array of other pole pieces is a circular array of axial projections from the backplate. In another embodiment said other circular array of other pole pieces is mounted on other structure of ferromagnetic material which projects from the backplate on a side of the static winding opposite the structure which forms said one circular array of pole pieces, the pole pieces extending radially from the structure on which they are mounted between the static winding and said rotary element and being arranged so that the pole pieces of said one circular array physically alternate with pole pieces of said other circular array.

One form of axial flux alternator and three forms of excitation arrangement for use with such an axial flux alternator which is to be operated at fixed speed will be described now by way of example with reference to accompanying drawings, of which: Figure 1 is a perspective view of a rotor and a stator core of the axial flux alternator; Figure 2 is a perspective view of the rotor shown in Figure 1; Figure 3 is a perspective view of the alternator including the rotor and stator core shown in Figure 1 housed in a casing; Figure 4 is circuit diagram of an excitation arrangement to which the field windings on the rotor discs of the alternator shown in Figures 1 to 3 are connected; Figure 5 is a perspective view of an excitation arrangement mounted on and between an endplate of the casing of the alternator shown in Figure 3 and the nearer rotor disc of the rotor of that alternator.

Figure 6 is a diagrammatic representation in linear form of part of the excitation arrangement shown in Figure 5; Figure 7 is a view in elevation of the components of the excitation arrangement shown in Figure 5 as seen looking along the axis of rotation from the left in Figure 5 with the rotor shaft and rotor disc that carries the exciter windings removed; Figure 8 is a section on the line VIII - VIII in Figure 7 with an alternative arrangement for mounting three phase windings of the excitation arrangement on the rotor disc; and Figure 9 is a view similar to Figure 5 illustrating an alternative arrangement of exciter pole pieces mounted on the endplate of the alternator outer casing of the alternator shown in Figures 1 to 3.

Figure 1 shows the alternator has a rotor (to) and an annular stator core (11) which has a central aperture. Figure 2 shows the rotor (10) is formed of two similar rotor discs (13) and (14) which are mounted coaxially, one adjacent either end of a rotor shaft (15). The rotor shaft (15) extends through the central aperture of the annular stator core 11 so that the annular stator core (11) is between the two rotor discs (13) and (14) as is shown in Figure 1.

Each rotor disc (13, 14) is formed in one piece of a ferrous metal, say by casting, with a circular array of salient pole pieces (16) which project towards the annular stator core (11) from the radial face of the rotor disc (13,14) that faces the annular stator core (11). Hence each rotor disc (13,14) and its circular array of salient pole pieces (16) is a single homogenous body of ferrous metal.

Each salient pole piece (16) has a bobbin fitted onto it, that bobbin carrying a coil (18) which has been wound around it. The coils (18) are interconnected in series to serve as rotor windings. Alternatively the rotor windings may be flywound around the salient pole pieces (16) of each rotor disc (13,14), each turn of the rotor windings on each rotor disc (13,14) being flywound around a selected group, say two or three of the salient poles (16) on the respective rotor disc (13,14).

The stator core (11) is formed by rolling up a strip of ferrous metal.

The strip has notches punched in its edges at appropriately indexed intervals before it is rolled up. The intervals are selected so that the notches align with rows of other such notches when the strip is rolled up so that the resultant annular body has radially extending slots (19) formed by the rows of aligned notches in each of its radially extending side faces. The slots (19) receive stator windings (20) which are torridally wound around the annular stator core (11). Each turn of the stator windings (20) is led along a slot (19) on one side of the stator core (11), across the outer periphery of the annular stator core (11), along a respective slot (19) on the other side of the stator core (11) and back through the central aperture of the annular stator core (11) . In an alternative arrangement, the stator windings are flywound on each side face of the annular stator core (11), each turn of the stator windings being flywound around a selected group of the segmented portions of the annular stator core (11) that are bounded by adjacent slots (19) in the respective side face. The stator windings (20) are connected to the output terminals of the alternator whereby an emf induced in the stator windings is supplied to an external load as is usual.

Alternate coils (18) of the series connected rotor windings are wound in the opposite sense one to another so that the magnetic fields established by unidirectional current flow through the rotor windings results in the salient pole pieces (16) on which the coils (18) are wound being alternatively north and south poles. Further, each north pole on the rotor disc (13) is axially aligned with a north pole on the rotor disc (14) and the south poles are similarly axially aligned.

Figure 3 shows the alternator has an outer casing (21) which has an opposed pair of circular end plates at either end of a tubular body. A central annular portion of the tubular body is formed with a circular array of apertures (22). The stator core (l l) is mounted substantially coaxially within the tubular body of the casing (21) so that the apertures (22) surround it for cooling purposes. The rotor (to) is journalled within the casing 21 for rotation relative to the stator core (11).

If the alternator that has been described above with reference to Figures l to 3 is to be operated as a fixed speed alternator, the series connected coils (18) of the rotor windings are connected into an exciter circuit as is illustrated in Figure 4. Also, a separate annular disc 23 is secured to a radially extending surface of one, 14, of the rotor discs 13 and 14 that is opposite to the face of that rotor disc 14 from which the integral salient pole pieces 16 project axially.

The annular rotor disc 23 has a laminated construction and, conveniently, may be formed by rolling up a strip of ferrous metal such as an electrical steel. The strip would have notches punched in one of its edges at appropriately indexed intervals before it was rolled up. The intervals would have been selected so that the notches aligned with rows of other such notches when the strip was rolled up so that the resultant annular body had radially extending slots 24 formed by the rows of aligned notches in one of its radially extending sides. The annular disc 23 would be secured to the rotor disc 14 by any conveninent means such as by bonding or by bolting or by being fitted with an interference fit into a cylindrical recess which had been formed in the face of the rotor disc 14 to which the annular disc 23 is secured. There would be an annular clearance between the radially inner periphery of the annular disc 23 and an end portion of the rotor shaft. The slots 24 would be formed in the radially extending surface of the annular disc 23 that is remote from the rotor disc 14.

The exciter circuit includes a static winding (25) which is connected across the output terminals of an automatic voltage regulator. The static winding (25) is positioned adjacent three phase windings (26) so that current that is caused to flow in the static winding (25) induces current flow in the three phase windings (26). The output of the three phase windings (26) is rectified in a rectifier (27) and the resultant light unidirectional current is fed to the series connected rotor winding coils (18). The automatic voltage regulator is a conventional device which need not be described in detail except to say that it is responsive to the load on the output terminals of the alternator and that the current it supplies to the static winding (25) varies with variations in that load so that the light unidirectional load that is fed to the rotor winding coils (18) varies likewise.

Figure 6 shows that the three phase windings (26) of the exciter circuit are located in the slots (24) that are formed in the radially-extending surface (27) of the annular disc (23) that is opposite to the face of the rotor disc (14) from which the integral salient pole pieces (16) project axially. The rotor disc (14), the salient pole pieces (16) and the three phase windings (26) are not shown in Figure 5. The three phase windings (26) are flywound into the slots (24). The surface (27) of the annular disc (23) faces a static backplate (28) of ferrous metal on which the static windings (25) are mounted. The static backplate (28) is an endplate of the outer casing (21) of the alternator shown in Figure 3.

Figures 5, 7 and 8 show that the static winding (25) is a compressed winding which is wound around a cylindrical body (29) which projects from the backplate (28) towards the rotor disc (14). A number of radial arms (30) project radially outwardly from the cylindrical body (29) at angularly spaced intervals. The cylindrical body (29) and the radial arms (30) are formed in one piece from ferrous metal and the radial arms (30) serve as pole pieces.

The backplate (28) carries a circular array of axially projecting pole pieces (31) of ferrous metal which each project between a juxtaposed pair of the radial arms (30). Current flow in the static winding (25) results in the pole pieces (31) being polarised with a magnetic polarity which is opposite to that of the radial arms (30), say north for the pole pieces (31) and south for the radial arms (30).

In an alternative arrangement, as shown in Figure 8, the three phase windings (26) may be wound onto ferrous cores (26A) which project axially from the surface (27) of the rotor disc (14) towards the backplate (28).

It will be understood that the same casting can be used for both the rotor plate (13) and the rotor plate (14), the latter being modified subsequently to secure the annular disc (23) or by adding ferrous cores (26A), for the three phase windings (26).

Figure 6 shows in dotted lines, the magnetic circuit for the magnetic field produced by current flow in the static winding (25) and which links with the three phase windings (26) as well as for the magnetic fields produced by current flow in the rotor winding coils (18) which link with the stator windings (20) on the annular stator core (11). The path of magnetic flux induced by current flow in the static winding (25) passes from the axially- extending pole pieces (31) across the air gaps between them and the adjacent annular disc (23) into that annular disc (23). That flux path extends circumferentially around the annular disc (23) in either direction to a location opposite the adjacent radial arm (30) and then extends from that location to the opposite radial arm (30) through the intervening air gap. The flux path in the radial arms (30) extends radially inwardly to the cylindrical body (29) from where it passes axially to the backplate (28) within which it extends circumferentially in either direction back to the adjacent axially extending pole pieces (31).

It will be understood that the control provided by the automatic voltage regulator in combination with the static windings (25) and the three phase windings (26) of the exciter circuit counters armature reaction and keeps the magnetic flux in the air gaps that are formed between the salient pole pieces (16) and the annular stator core (11) at a substantially constant level.

Figure 9 shows a more compact alternative to the circular array of axially projecting pole pieces (31) of the excitation arrangement described above with reference to Figure 5. Parts shown in Figure 9 which are similar to corresponding parts shown in Figure 5 are identified by the same reference numerals and are not described again in detail.

The compressed winding (25) is surrounded by another cylindrical body (32) which projects towards the rotor disc (14) from the back plate (28) on S which it is mounted. Another circular array of radial arms (33) projects radially inwardly from the other cylindrical body (32), each arm (33) projecting into the gap, between a respective juxtaposed pair of the radially outwardly projecting radial arms (30). The other cylindrical body (32) and the other circular array of radial arms (33) are formed in one piece from ferrous metal and the radial arms (33) serve as pole pieces in place of the axially projecting pole pieces (31) of the excitation arrangement described above with reference to Figure 5.

Claims (20)

1. An axial flux AC machine including a rotor disc and a stator core, the rotor disc being formed of a ferrous metal and being journalled for rotation about an axis relative to the stator core from which it is spaced in the direction of the axis of rotation, the rotor disc having salient pole pieces which are formed of a ferrous metal and which project towards the stator core so that an air gap is formed between the stator core and the surfaces of the salient pole pieces that are nearest to it, there being field windings wound on the salient pole pieces and on said stator core such that a magnetic field caused by an emf in the field windings on one of said rotor discs and said stator core links with the field windings on the other of said rotor disc and said stator core to induce an emf in the field windings on said other of said rotor disc and said stator core, the arrangement being such that each juxtaposed pair of the salient pole pieces together with structure of said rotor disc that extends between the respective salient pole pieces of each pair, with a respective portion of said stator core that is opposite to the salient pole pieces of each juxtaposed pair at any one instant and with the air gaps between the salient pole pieces of each pair and said stator core form a respective magnetic circuit for a magnetic field which is established by or which induces an emf in the respective field windings wound on the salient pole pieces of each pair.

2. An axial flux AC machine according to claim 1, wherein the rotor disc, including the salient pole pieces, is formed in one piece as a single homogenous body.

3. An AC machine including two parts, one of the two parts being movable laterally relative to the other, a first of the two parts having salient poles pieces which project towards the second of the two parts so that an air gap is formed between said second part and the surfaces of the salient pole pieces that are nearest to it, there being field windings wound on the salient pole pieces and on said second part such that a magnetic field caused by an emf in the field windings on one of said first and second parts links with the field windings on the other of said first and second parts to induce an emf in the field windings on said other of said first and second parts, the arrangement being such that each juxtaposed pair of the salient pole pieces together with structure of said first part that extends between the respective salient pole pieces of each pair, with a respective portion of said second part that is opposite to the salient pole pieces of each juxtaposed pair at any one instant and with the air gaps between the salient pole pieces of each pair and said second part form a respective magnetic circuit for a magnetic field which is established by or which induces an emf in the respective field windings wound on the salient pole pieces of each pair, wherein said first part, including the salient pole pieces, is formed in one piece as a single homogenous body.

4. An axial flux AC machine according to claim 1, claim 2 or claim 3, wherein the rotor disc is one of two similar rotor discs which are mounted on a common shaft for conjoint rotation about said axis and the stator core is annular with a central aperture and is positioned between the two similar rotor discs which are both oriented similarly so that their salient pole pieces project towards the annular stator core, said common shaft extending through said central aperture.

5. An axial flux AC machine according to any one of claims 1 to 4, wherein each salient pole piece has a respective coil wound on it, the coils on the salient pole pieces being connected together to form the respective field windings.

6. An axial flux AC machine according to claim 5, wherein each coil wound on a salient pole piece is wound onto a bobbin which is fitted onto the respective salient pole piece.

7. An axial flux AC machine according to any one of claims 1 to 4, wherein the field windings wound onto the salient pole pieces of the or each rotor disc are flywound around two or more of the respective salient pole pieces

8. An axial flux AC machine according to claim 4 or any one of claims S to 7 when appended to claim 4, wherein the field windings on the annular stator core are toroidally wound around that core.

9. An axial flux AC machine according to any one of claims 1 to 8, wherein exciter windings are provided on the side of said rotor disc, or on the side of one of the two similar rotor discs which is remote from said stator core, said exciter windings being connected to the field windings on the respective rotor disc through rectifier means, there being a static winding which is located relative to said exciter windings such that a magnetic field established by an emf in the static winding links with said exciter windings and thereby induces an emf in the exciter windings, the static winding being adapted to be connected to automatic voltage regulation means which are responsive to any variation in load applied to the output of the AC machine and which are operable to supply a unidirectional control current to the static winding whereby to maintain flux in the air gaps of the AC machine substantially constant.

10. An axial flux AC machine according to claim 9, wherein the exciter windings are multi-phase windings.

11. An axial flux AC machine according to claim 10, wherein the exciter windings are three phase windings.

12. An axial flux AC machine according to anyone of claims 9 to 11, wherein the exciter windings are fly-wound into slots which are formed in a laminated ferrous metal component which mounted on the respective side of the respective rotor disc.

13. An axial flux AC machine according to any one of claims 9 to 12, wherein said static windings are wound on claw-shaped magnetic poles which are mounted on a back-iron support.

14. An axial flux AC machine according to any one of claims 9 to 13, wherein the field windings on the annular stator core are wound into slots formed in the annular stator core.

15. An axial flux AC machine according to claim 14 when appended to any one of claims 1 to 6, wherein the field windings on the annular stator core are flywound into the slots that are formed in the annular stator core.

16. An excitation arrangement for an axial flux AC machine, the excitation arrangement including exciter windings for connection to field windings of the AC machine through rectifier means, a static winding located relative to said exciter windings such that a magnetic field established by an emf in the static winding induces an emf in the exciter windings, the static winding being adapted to be connected to automatic voltage regulation means which are responsive to any variation in load applied to the output of the AC machine and which are operable to supply a unidirectional control current to the static to winding whereby to maintain flux in the air gaps of the AC machine substantially constant, wherein the exciter windings are mounted on a rotary element of the axial flux AC machine, the static winding is substantially coaxial with the axis of rotation of the rotary element and is wound around the structure of ferromagnetic material which projects from a backplate of ferromagnetic material and which forms one circular array of pole pieces, there being another circular array of other pole pieces of ferromagnetic material which are mounted on the backplate, wherein the pole pieces of said one circular array are oppositely polarised as compared to the polarisation of the pole pieces of said other circular array by flow of current through the static winding, the flux path between the two circular arrays of pole pieces being through the rotary element and the return flux path being through the backplate on which the sets of pole pieces are mounted.

17. An excitation arrangement according to claim 16, wherein said other circular array of other pole pieces is a circular array of axial projections from the backplate.

18. An excitation arrangement according to claim 16, wherein said other circular array of other pole pieces is mounted on other structure of ferromagnetic material which projects from the backplate on a side of the static winding opposite the structure which forms said one circular array of pole pieces, the pole pieces extending radially from the structure on which they are mounted between the static winding and said rotary element and being arranged so that the pole pieces of said one circular array physically alternate with pole pieces of said other circular array.

19. An axial flux AC machine substantially as described hereinbefore with reference to and as shown in the accompanying drawings.

20. An excitation arrangement for an axial flux AC machine substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figures 4 to 8 or in Figures 4 and 6 to 9 of those drawings.