Toroidal electrical machine and an annular winding carrier therefor

A toroidal AC generator includes an outer casing and an annular stator core of ferromagnetic material which is encased in a hollow annular cover of plastics material and which is fixed to the casing between the discs of a twin disc rotor. Each rotor disc carries a circular array of alternately polarised permanent magnets which face the stator core. The hollow annular cover is a two-part moulding and is formed of radially inner and outer ring portions which are joined by circular arrays of radial ribs which extend across the opposed radial faces of the annular core and form open-ended radial passages for the electrical phase windings which are wound around the cover. The ribs project radially beyond the radially outer ring portion and form hooks around which a winding may be retained temporarily during winding.

This invention relates to a toroidal electrical machine including an outer casing, an annular winding carrier which is fixed to the outer casing and a rotor which is rotatable within the outer casing, which is adapted to be coupled with a rotary drive member of a prime mover and which carries a circumferential array of permanent magnets on a radial face which faces a radial face of the annular winding carrier, and to an annular winding carrier for such a toroidal electrical machine.

U.S. Pat. No. 5,334,898 relates to high-density discoidal brushless open frame DC or AC motors and generators in which rare earth permanent magnets are arranged about a disc shaped rotor. A rectangular toroidal stator or annular windings carrier serves as the mounting for numerous flat wound armature coils or phase windings.

JP-A-04364336 discloses an annular stator for a radial flux induction motor wherein a synthetic resin moulded insulator having a profile substantially corresponding to that of the stator core is fitted to the stator core to cover the parts of the core to which a winding is applied.

An object of this invention is to enable the annular winding carrier to be used as a guide for winding phase windings which pass through an air gap formed between the radial faces of the rotor and the annular winding carrier.

According to one aspect of this invention there is provided a toroidal electrical machine including an outer casing, an annular winding carrier which is fixed to the outer casing and which has an annular radial face, a rotor which is rotatable within the outer casing, which is adapted to be coupled with a rotary drive member of a prime mover and which carries a circular array of alternately polarised permanent magnets on a radial face which faces the radial face of the annular winding carrier wherein the annular winding carrier has an annular core of ferromagnetic material which is encased in a hollow annular cover of plastics material, the annular cover being formed of radially inner and outer annular support portions which are joined together across the radial face of the annular core by a circular array of spaced radial ribs which form open ended radial passages, the radially inner and outer annular support portions and the circular array of spaced radial ribs bounding a respective circular array of apertures which are formed in the hollow annular cover over the radial face of the annular core, and electrical phase windings which are wound around the annular core and the annular winding carrier so that they extend radially through the open ended passages around the faces of the inner and outer annular support portions that are remote from the core.

Preferably the annular radial face of the annular winding carrier is one of an axially spaced pair of such annular radial faces, the radial face of the rotor that faces one of the radial faces of the annular winding carrier is one of two such radial faces, the other of the two radial faces of the rotor facing the other of the radial faces of the annular winding carrier so that the annular winding carrier is between the two radial faces of the rotor and the rotor carries two such circular arrays of alternately polarised permanent magnets, each of the circular arrays of alternately polarised magnets being on a respective one of the two radial faces of the rotor, wherein the circular array of spaced ribs which form open ended radial passages and which extend across one of the radial faces of the annular core and join the inner and outer annular support portions together is one of two such circular arrays of spaced ribs, the other circular array of spaced ribs also forming open ended radial passages and extending across the other of the radial faces of the annular core and joining the inner and outer annular support portions together.

According to another aspect of this invention there is provided an annular winding carrier for a toroidal electrical machine, the carrier including a hollow annular cover of plastics material which is to encase an annular core of ferromagnetic material, the annular cover being formed of radially inner and outer annular support portions which are joined together by at least one circular array of spaced radial ribs which extend across a respective opposed radial face of the annular core when that core is encased in the hollow annular cover and which form open ended radial passages for electrical phase windings which are wound around the annular core and the annular winding carrier so that they extend around opposed faces of the inner and outer annular support portions when the annular core is encased in the hollow annular cover and the annular winding carrier is fixed in the toroidal electrical machine, the radially inner and outer annular support portions and the at least one circular array of spaced radial ribs bounding a respective circular array of apertures which are formed in the annular winding carrier.

Preferably each radial rib of the annular winding carrier included in or for the toroidal electrical machine projects radially beyond the radially outer annular support portion, each of the radially projecting rib portions forming a hook around which a winding may be retained temporarily during winding. Conveniently each radially projecting rib portion forms two hooks which are spaced from one another circumferentially. Where the annular winding carrier includes two axially-spaced circular arrays of radial ribs, each of those radial ribs may project radially beyond the radially outer annular support portion and thereby bound an annular path which extends circumferentially around the annular winding carrier.

Conveniently the windings are held within the respective open-ended passages by being embedded within thermoset electrically insulating material.

The radial ribs may be tapered radially inwardly so the open ended radial passages are substantially parallel sided. Conveniently each radial rib projects radially inwards beyond the radially inner ring portion.

Preferably the windings are twisted multi-stranded wire. If the electrical phase windings that were to be wound around the annular winding carrier were to be wound in the form of a multi-stranded cable, the individual conductors or wires of which were arranged so as to run longitudinally in the cable, each conductor would be in a different position in the respective airgap that would be formed between the annular winding carrier and the facing circular array of alternately polarised permanent magnets of the torodial electrical machine in which the annular winding carrier is to be fixed. Some conductors might be in a position in which they are subject to a different level of flux than that to which other conductors are subject. This would result in different currents being induced in different conductors and would lead to electrical imbalance. The twisted nature of the strands of the wire that is wound around the annular winding carrier of a preferred embodiment of the invention, however, results in the position of each strand in the air-gap changing along the section of the multi-stranded wire. Thus, each strand is subject to differing levels of flux along its length. The overall result of this is that, taking an aggregate value over the section of the multi-stranded wire, each strand is subject to a more similar level of flux and therefore has a more similar current induced therein than would otherwise be the case. This reduces electrical imbalance between the strands of the multi-stranded wire.

Conveniently the twisted multi-stranded wire is covered with insulant impregnated sheet wrapping material such as paper whilst it is being wound on the annular winding carrier. The strands of the multi-stranded wire may be impregnated with electrically insulating material. Where the electrically insulating material with which the sheet wrapping material is impregnated and the multi-stranded wire is impregnated is a curable resin, the windings are preferably wound on the annular cover with the resin in its uncured form, the resin being cured and hardened after the windings have been wound onto the annular winding carrier whereby to imbue the windings with strength and to enable the windings to hold the annular stator assembly together.

Preferably the windings are wound on the annular winding carrier in a single layer of juxtaposed turns. The windings may be arranged to provide a plurality of multiphase outputs, for example, a dual three-phase output. The windings of each phase may be wound on the annular winding carrier in groups of juxtaposed turns, each group being spaced circumferentially from the juxtaposed group of that phase by a group of juxtaposed turns of the or each other phase. Conveniently there are the same number of turns in each group of turns of each winding of each phase. The spaced groups of juxtaposed turns of multiple multiphase windings may be arranged so that pairs of leads at the ends of all the windings are adjacent one to another.

In a preferred embodiment of this invention, the hollow annular cover is an interlocked pair of similar annular mouldings of plastics material which are joined together along joint lines which extend circumferentially around the radially inner and outer annular support portions. The circular edge of each of the annular mouldings that forms the joint line that extends circumferentially around the radially outer annular support portion conveniently forms alternate generally rectangular tabs and recesses, each tab being an interference fit in a respective one of the recesses that is formed by the other annular moulding whereby the two annular mouldings are interlocked against relative circumferential movement. The annular core may be an interference fit in each of the parts of the radially outer annular support portion that are formed by the annular mouldings. Conveniently the opposed ends of each of the tabs and of each of the recesses converge radially outwards so that the tabs are urged into the recesses with a wedge action due to the interference fit of the annular core in the outer annular parts of the annular mouldings.

In a preferred embodiment, the multi-stranded wire has twelve strands and there are eight groups of four turns in the windings of each of the six phases of dual three-phase windings.

FIGS. 1to4show an AC generator which has a rotor10which is rotatable in a casing11relative to a stator assembly12which is mounted in the casing11. The rotor10is adapted to be mounted on the drive shaft of a diesel engine in place of the usual flywheel. The rotor10is fixed to the drive shaft in the same way as the flywheel would be. The casing11is provided with fixing blots13by which it is fixed to the engine block of the diesel engine.

The rotor10comprises a co-axial pair of rotor discs14and15. Each rotor disc14,15has a hub portion16,17and an annular disc portion18,19which projects radially outwardly from the respective hub portion16,17. The hub portion16of the rotor disc14is the part of the rotor10that is adapted to be coupled with the diesel engine drive shaft. The hub portion17of the other rotor disc15is annular and is spiggotted into a correspondingly shaped recess21which is formed in the hub portion16on the side of the rotor disc14opposite to the side thereof that is to be mated with the diesel engine drive shaft. The two rotor discs14and15are fastened together by setscrews22which extend through the annular hub portion17of the rotor disc15and which are screwed into the hub portion16of the rotor disc14.

Each rotor disc14,15has a circular array of alternately polarised permanent magnets23,24fixed to and positioned on the radial face25,26of the respective annular disc portion18,19that faces the other annular disc portion18,19. Each rotor disc14,15with the respective magnets23,24mounted thereon which conveniently is as described and illustrated in our co-pending International patent application No. PCT/GB02/00092 filed 10 Jan. 2002 (International Publication WO 02/056443) and which designates the priority of British patent application No. 0100635.2 filed 10 Jan. 2001.

The casing11comprises a shallow cup-shaped annular component27. The fixing bolts13are mounted in the base of the annular component27at circumferentially spaced locations therein. The stator assembly12is annular.FIG. 3shows that a pair of retainer rings28and29embrace the radially outer peripheral portion of the stator assembly12and are fixed to the brim of the cup-shaped component27by a circular array of setscrews31which pass through the retainer rings28and29so that the stator assembly12is supported by the cup-shaped component27between the opposed circular arrays of permanent magnets23and24that are carried by the rotor10and from which it is spaced. The remainder of the casing11is a dished cover32which is fitted over the retainer ring29that is spaced from the brim of the annular component27so as to enclose the rotor10and the stator assembly12. Ventilation louvres are formed in the cover32and ventilation holes are formed in the side wall of the annular component27.

The stator assembly12comprises an annular core33of ferromagnetic material, a two-part moulding34of an electrically insulating plastics material which encases the annular core33, and stator windings (not shown) which are wound around and carried by the two part moulding34. It follows that the two part moulding is an annular winding carrier. The two annular parts35of the two part moulding34are substantially similar.

The annular core33is formed by rolling a continuous strip of ferromagnetic material lined with a thin sheet of a loaded epoxy resin electrically insulating material such as Nomex, on itself into a tightly wound flat spiral laminated structure. The laminations extend parallel to the axis of the annular core33. Each side of the annular core33is lined with a sheet of loaded epoxy resin electrically insulating material such as Nomex which is bonded to the core33.

The stator windings are dual three phase windings arranged so that rotation of the rotor19relative to the stator assembly12generates two three phase AC outputs. The AC outputs are fed to a control system which conveniently is as described with reference toFIG. 7of the drawings of our International application GB01/00169 filed 16 Jan. 2001 (International Publication WO 01/56133). The control system is operable to control the speed of the engine on which the rotor10is mounted when in use such that the speed at which the AC generator is driven is varied automatically so as to vary the AC outputs to match variations in an external load.

FIGS. 6 and 7respectively show opposite sides of one of the annular parts35of the two-part moulding34or annular winding carrier. The moulded annular component which comprises an annular part35comprises co-axial inner and outer ring portions36and37joined together by a circular array of spaced radial ribs38and39. The ring portions36and37project in the same direction from the array of radial ribs38and39, that direction being substantially parallel to the axis of the annular part35. The circular edge of the radially inner ring portion36that is remote from the radial ribs38and39is flat, lying in a notional plane which is normal to the axis of the annular part35. The corresponding circular edge of the radially outer ring portion37is stepped at circumferentially spaced locations around it to form alternate generally rectangular tabs41and generally rectangular recesses42, there being eight tabs41and eight recesses42and each tab41being spaced from the juxtaposed recesses42. The circumferentially spaced ends of each tab41and of each recess42slope at a small angle to a notional line which is parallel to the axis of the annular part35, the angle of the slope at either end of each tab41preferably being slightly less than the angle of the slope at each end of each recess42. Further the opposed ends of each tab41and of each recess42converge radially outwardly, the angle included between the opposed ends of each tab41being substantially the same as the angle included between the opposed ends of each recess42.

The radially inner surface of the radially outer ring portion37of each annular part35is stepped, that portion of the inner surface that is nearer to the circular array of radial ribs38and39having a smaller diameter than does the remainder.

The two annular parts35are placed one on either side of the annular core33with the ring portions36and37of each annular part35extending towards those of the other annular part35and with their edges in contact so that the two part moulding34is a hollow annular body. The annular core33is an interference fit in the smaller diameter portion of the radially inner surface of the radially outer ring portion37of each of the annular components35. This and the trapezoidal cross-section of the tabs41and recesses42causes each tab41to be urged radially outwardly with a wedge action into the respective radially aligned recess42formed by the other annular part35. Each tab41preferably is a press fit in the corresponding one of the recesses42. Hence the two annular parts35are interlocked against circumferential movement one relative to the other. The loaded epoxy resin electrically insulating material that lines and is bonded to each side face of the annular core33is also bonded to the inner surfaces of the ribs38and39whereby the annular core33is fixed to the annular winding carrier that is the two part moulding34.

A circumferential array of elongated apertures43is formed in the two part moulding34on either side of the stator core33between the radially inner and outer ring portions36and37and adjacent ones of the radially extending ribs38,39.

Each rib38,39tapers radially inwardly and projects radially beyond each of the inner and outer rings36,37. The inner end of each rib38,39is thin and generally rectangular, extending to the circular edge of the radially inner ring portion36. The radially outer projecting portion44,45of each rib38,39is formed as a hook by having a chamfer46, formed on either side. Furthermore each radially outer projecting portion44,45is thicker than the portion of the respective rib38,39that extends between the inner and outer portions36and37in a direction parallel to the axis of rotation of the rotor10so that a step50is formed in the radial surface that faces the adjacent circular array of magnets23,24, that step50being located substantially at the root of each radially outer projecting portion44,45.

Each chamfer46extends radially from the radially outer face of the radially outer ring portion37to a shoulder47which is formed at a short distance from the radially outer end of the respective rib38,39. Due to the chamfer46the radially outer projecting portion44,45of each rib38,39tapers in an axial direction towards the other annular part35on the other side of the annular stator core33.

Every third rib38differs from the pair of ribs39therebetween. Each rib38has a recess48formed in it between its two chamfered sides. Hence the radially outer end of each rib38is U shaped, the sides of the U being generally perpendicular to the base of the U so that the edges formed by the sides of the U are spaced apart to a greater extent than are the adjacent chamfered surfaces46on each side of the respective radially outer projecting portion44. The recess48communicates with an opening49which is formed in the radially outer ring portion37, that opening49extending into the radially outer projecting portion44at right angles to the part of the opening49that is formed in the radially outer ring portion37. The opening49exposes the underlying portion of the annular stator core33. In contrast, the radially outer projecting portion45of each rib39is solid having a rectangular end portion51which is wider than the adjacent chamfered portions46between it and the radially outer ring portion37. The chamfered surfaces46and the wider end portion51of the radially outer projecting portions45form the hooks as do the corresponding parts of the radially outer projecting portion44of each recessed rib38.

Each recessed rib38is adapted to receive a respective one of sixteen equally spaced radially inwardly projecting tongues52which are formed on the respective one of the retainer rings28,29. Each tongue52abuts the exposed portion of the annular core33. In that way, the retainer rings28and29that are fastened to the rim of the cup-shaped annular component27react directly against the annular core33. Being so located with respect to both the annular core33and the cup-shaped component27, the retainer rings28and29together with the annular winding carrier34thereby provide protection for the windings that are carried by the annular winding carrier34.

The two annular arrays of radially outer projecting portions44and45of the two annular parts35are spaced from one another axially so that they bound a circular path through which wires may be lead.

The dual 3-phase stator windings are arranged in a single layer of turns of a multi-stranded wire. The multi-stranded wire comprises twelve strands of thin copper wire twisted together like a rope, each strand having been pre-coated with a layer of an electrically insulating polymeric material. Also the multi-stranded wire is infiltrated with an electrically insulating resin, such as an epoxy resin, in powder form. The particles of resin, which are disposed on the outer surface of each of the strands of copper wire, are heatable so as to melt and subsequently to cure and harden. The resultant bundle of twelve wires is held together by a tape of resin impregnated paper which has been spirally wound around the bundle along its length. The helically wound tape serves to preserve the twisted arrangement of the copper wires and provides further electrical insulation, the resin impregnated paper being an electrical insulator The resultant paper covered stranded wire is stiff.

The stiff paper covered multi-stranded wire is wound by a robot machine onto the annular winding carrier34. The robot machine includes a C-shaped cassette in which the stiff wire is stored. The cassette is caused by operation of the robot machine to travel along an orbital path and is operable to dispense the wire as it travels. The stator assembly12is held stationary at a winding station in the robot machine whilst the multi-stranded wire is wound around it. One turn of the orbital path along which the cassette is caused to travel extends axially across the radially outer surface of the outer ring portion37between two juxtaposed opposed pairs of the radially-outer projecting portions44,45, then radially inwards through the radially aligned open-ended passage that extends between the respective juxtaposed pair of the ribs38,39of the respective one of the annular moulding parts35, then axially through the central aperture of the stator assembly12between a juxtaposed pair of the radially inwardly projecting portions of the ribs38,39and then radially outwards through the respective one of the open-ended radial passages of the other annular moulding part35to complete the turn around the annular winding carrier34.

The orbital travel of the C-shaped cassette is continued until four turns of the stranded wire have been wound side-by-side around the annular winding carrier34between the same two opposed pairs of the radially outer projecting portions44,45between the same juxtaposed pair of radially-inwardly projecting projections and through the two open-ended passages that extend therebetween on either side of the annular winding carrier34. The cassette precesses in a circumferential direction with respect to the annular winding carrier34as the four windings are wound.

Once the fourth turn has been wound, wire is hooked round the hook formed by the chamfered portion of the adjacent one of the radially outer projecting portions44,45and the shoulder47at the radially outer end of that projecting portion44,45so that the wire is held under tension whilst the stator assembly12is indexed in one angular sense to move six juxtaposed opposed pairs of ribs38,39passed the cassette and to bring the pair of open-ended passages that are circumferentially spaced from the four turns just wound by five other such pairs of open-ended passages to the winding station. Once that sixth pair of open-ended passages has been indexed to the winding station, the cassette is caused to travel again to wind a further four turns around the stator assembly12, those turns passing through that sixth opposed pair of open-ended passages.

The procedure by which four turns of the multi-stranded wire are wound side-by-side around the annular winding carrier34is repeated eight times. The portion of the wire extending from the last turn of the eighth group of turns is cut off to form one of the two leads of the respective phase of the stator winding, the other lead of that phase being the input end of the first turn of the first group of four turns.

The stator assembly12is then indexed in the same angular sense to bring the seventh pair of opposed open-ended passages to the winding station so that the cassette can be operated to wind four turns of the stiff wire around the annular stator assembly12side-by-side within those opposed pair of open-ended passages. It will be understood that the group of four turns so wound will be juxtaposed to the first group of four turns of the previously wound phase of stator windings.

The procedure is continued until six phases of groups of four turns of such stiff wire with leads at either end have been formed around the stator assembly12, the leads at either end of the wire of each phase being juxtaposed with leads at either end of a juxtaposed phase so that all the leads are adjacent.

The wire is led through the circular path defined by the two annular arrays of radially outer projecting portions44and45as the stator assembly12is indexed to bring different opposed pairs of open-ended passages to the winding station. When all the windings have been wound on the stator, the leads at either end of each of the six phases of winding can be led along the circular path to the terminals to which they are to be connected.

The chamfers46on the radially outer projecting portions44,45allow the wires to be hooked around those projecting portions44,45and thereby kept under tension so that the wire stays positioned in the open-ended radial passages while the stator assembly12is turned to bring another pair of open-ended passages to the winding station. The arrangement of the chamfers46and the adjacent shoulders47provides the necessary radius for hooking the wire around.

The stator assembly12with the six phases of multi-stranded wire thereon is clamped in a press which presses the windings through the apertures43formed on either side of the stator assembly12between juxtaposed pairs of the ribs38,39and onto the layer of electrically insulating material that is bonded to both the respective annular part35of the annular winding carrier34and the annular core33. Direct current is passed through the windings whilst a light pressure is applied to the stator assembly12by the press. Preferably the direct current is “pulsed” through the windings. The heat generated by passing direct current through the windings melts and cures the epoxy resin with which the multi-stranded wire and its paper wrapping is impregnated, that lies between adjacent turns of the flat spiral core assembly33, that lines the opposed faces of the annular core33and which is bonded to the inner surfaces of the annular winding carrier34so that, when the resin melts, it seeps in-between and around the wires, the light pressure being maintained by the press until the resin has been cured. As a result, the windings are incorporated in a rigid structure imbued with both mechanical strength and electrical stress withstand strength and are constrained into the open-ended radial passages formed between the juxtaposed pairs of ribs38,39where they are held by the cured resin. Hence, since each of the windings is a single layer and the windings are physically constrained in the radial passages of the annular winding carrier34of the stator assembly12, the axial extent by which the windings project towards the magnets23and24is minimised so that the desired dimensions of the air gap are maintained.

The thickness of the plastics material from which the two annular moulding parts35that are assembled together to form the annular winding carrier34and the material from which those annular moulding parts35are formed are chosen so that the annular winding carrier34does not bow or otherwise distort when subjected to the light pressure by the press.

The windings that are wound on the annular winding carrier34hold the stator assembly12together.

FIG. 17shows an AC generator which has a stator assembly12which is similar to that of the AC generator shown inFIGS. 1to15. Diametrically opposed pairs of the tongues52that project radially inwards from the retainer rings28and29are shown engaged with opposed faces of the annular stator core33. The stator windings53are shown in FIG.17.

The principal differences between the AC generator shown in FIG.17and that described above with reference to and shown inFIGS. 1to15relate to the form of the rotor discs which are modified to enhance airflow around them for cooling. An aperture54is formed in the hub portion17A of the non-drive end rotor disc15A. Axially extending apertures (not shown) are formed in the annular disc portion18A of the drive end rotor disc14A to provide a path through that rotor disc14A for air that emerges from the radially outer side of the aperture54and that circulates around the radially inner portions of the stator windings53.

A further modification which is contemplated is to make the outside diameter of the annular disc portions18and19less than the pitch circle diameter of the steps50of the radially outer projecting portions44which in turn is less than the pitch circle diameter of the steps50of the outer radially outwardly projecting portions45. That modification would allow the air gaps between the permanent magnets23and24and the stator assembly12to be reduced which in turn would allow the thickness of those magnets23and24to be reduced without reducing the magnetic flux linkage with the stator windings53and with consequent savings in cost and weight which follow from the reduction in the amount of magnetic material used. There would also be a reduction in response time. Furthermore there would be a reduction in the amount of leakage flux that flows through the tongues52.

The stator winding arrangement described above in which there are four turns of the twisted multi-stranded wire between each juxtaposed pair of radially-outer projecting portions44,45is designed for higher power applications. Simple single conductor wire could be used instead of the twisted multi-stranded wire for lower power applications in which case there may be a greater number of turns of the wire (say 6 turns) between each juxtaposed pair of radially-outer projecting portions44,45. In addition, there may be an auxiliary winding which is provided for a certain purpose. Such an auxiliary winding may be wound over or under the single layer of turns of the stator winding arrangement or may be interleaved with the turns of the stator winding arrangement.