ITS OPERATION Field of the Invention

The present invention relates to electrodynamic machines and, in particular, to a switched reluctance electrodynamic machine of the type disclosed in International Patent Application No. PCT/AU02/00081 (published under No. WO 02/060035) and assigned to the present applicant. As at the priority date of this application, machines as disclosed in the above PCT application have not been sold.

Background Art

In the abovementioned prior art specification, a U-shaped permanent magnet is included in a first magnetic path and is used to supply magnetic flux to a second permeable path about which a coil is wound. Two air gaps separate the permanent magnet from the second permeable path. A third permeable path which is movable relative to the permanent magnet and second permeable path, is introduced into the air gaps in order to shunt the magnetic flux produced by the permanent magnet through the third permeable path. As a consequence, when the third permeable path is introduced into the air gap the magnetic flux linking the coil collapses. However, as the third permeable path is removed from the air gap the magnetic flux linking the coil is re-established.

With the third permeable path located in the two air gaps, the machine is in a low energy state and thus as the third permeable path approaches the air gaps the third permeable path is magnetically attracted into the air gap. Conversely, withdrawing the third permeable path from the air gaps requires a magnetic retarding force to be overcome. It has been found particularly advantageous to permit the coil to conduct current as the third permeable path approaches the air gaps, but to prevent the coil conducting current as the third permeable path is withdrawn from the air gaps.

The abovedescribed arrangement suffers from the problem that the air gaps separating the first and second permeable paths must be relatively large in order to be able to accommodate the third permeable path. In addition, the third permeable path must be relatively narrow in order that the air gaps not be too large. If the air gaps are

too large, this unnecessarily increases the reluctance of the second permeable path and thereby lowers the magnitude of the flux linking the coil when the third permeable path is not located in the air gaps. The magnitude of the magnetic flux in the second permeable path which links the coil, and its rate of change with time, have an influence on the amount of power able to be generated by the coil.

The genesis of the present invention is a desire to seek to ameliorate the abovementioned problem and provide an apparatus and method whereby the rate of change of the magnetic flux in the coil carrying permeable path can be increased.

Summary of the Invention

In accordance with a first aspect of the present invention there is disclosed a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetically permeable path having a first reluctance, a coil wound about a second magnetically permeable path having a second reluctance, said second path being positioned relative to said first path to either receive magnetic flux from said mmf source, or to have said magnetic flux substantially by-pass said second path, and a third magnetically permeable path movable relative to said first and second paths to shunt said magnetic flux through said third path whilst maintaining the cumulative reluctance of the totality of said first and second paths substantially constant to either substantially by-pass said second path, or substantially divert said magnetic flux into said second path respectively, whereby the magnetic flux linking said coil is changed by said relative movement of said third path.

In accordance with a second aspect of the present invention there is disclosed a method of operating a switched reluctance electrodynamic machine, said method comprising the steps of:

(i) providing a first magnetically permeable path including a source of mmf, (ii) positioning a coil wound about a second magnetically permeable path,

(iii) positioning said second path relative to said first path to either receive magnetic flux from said mmf source, or to have said magnetic flux substantially bypass said second path, and

(iv) moving a third magnetically permeable path relative to said first and second paths to shunt said magnetic flux through said third path whilst maintaining the cumulative reluctance of the totality of said first and second paths substantially constant to either substantially by-pass said second path, or substantially divert said magnetic flux into said second path respectively, whereby the magnetic flux linking said coil is changed by said relative movement of said third path.

In accordance with a third aspect of the present invention there is disclosed a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path moveable relative to said first and second paths and arranged to temporarily shunt said second path, wherein said third path has a flux carrying capability at least of similar order of magnitude to that of said second path.

In accordance with a fourth aspect of the present invention there is disclosed a method of operating a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path moveable relative to said first and second paths and arranged to temporarily shunt said second path, said method comprising the step of providing said third path with a flux carrying capability of at least a similar order of magnitude to that of said second path.

In accordance with a fifth aspect of the present invention there is disclosed a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path moveable relative to said first and second paths and arranged to temporarily shunt said second path, wherein a first air gap means separates said first and second magnetic paths and said third magnetic path does not enter into said first air gap means.

In accordance with a sixth aspect of the present invention there is disclosed a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path moveable relative to said first and second paths and arranged to temporarily shunt said second path, wherein a first air gap means separates said first and second magnetic paths and the reluctance of said first air gap means remains substantially constant, notwithstanding the movement of said third magnetic path relative to said first and second paths.

In accordance with a seventh aspect of the present invention there is disclosed a method of operating a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path moveable relative to said first and second paths and arranged to temporarily shunt said second path, said method comprising the steps of providing a first air gap means separating said first and second magnetic paths, and not entering said third magnetic path into said first air gap means.

In accordance with a eighth aspect of the present invention there is disclosed a method of operating a switched reluctance electrodynamic machine comprising a source of mmf located in a first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path moveable relative to said first and second paths and arranged to temporarily shunt said second path, said method comprising the steps of providing a first air gap means separating said first and second magnetic paths, and maintaining the reluctance of said first air gap means substantially constant, notwithstanding the movement of said third magnetic path relative to said first and second paths.

In accordance with a ninth aspect of the present invention there is disclosed a switched reluctance electrodynamic machine comprising a source of mmf located in a

first magnetic path, a coil wound about a second magnetic path positioned relative to said first path to permit magnetic flux from said mmf source to flow through said second path, and a third magnetic path movable relative to said first and second paths and arranged to temporarily shunt said second path, wherein there is no air gap between said first and second paths.

In accordance with a tenth aspect of the present invention there is disclosed a method of operating as a motor any one of the abovementioned switched reluctance electrodynamic machines, said method comprising the steps of: (i) supplying pulses of unidirectional current to said coil,

(ii) the polarity of said pulses and the sense of said coil being arranged to generate a pulsed magnetic flux which increases the magnetic flux in said coil created by said mmf source,

(iii) switching said pulses on as said third magnetic path commences to shunt said second magnetic path, and

(iv) switching said pulses off as said third magnetic path ceases to shunt said second magnetic path

Brief Description of the Drawings Several embodiments of the present invention will now be described with reference to the drawings in which:

Fig. 1 is a schematic magnetic circuit diagram of the prior art arrangement and essentially reproduces in three dimensions Fig. 2 of the abovementioned PCT specification, Fig. 2 is a magnetic circuit diagram of a first embodiment of the present invention having a single coil,

Fig. 3 is a magnetic circuit diagram of a second embodiment of the present invention having a pair of coils,

Fig. 4 is a circuit diagram showing the interconnection of the coils of Fig 3, Fig. 5 is a schematic perspective view of an electrodynamic machine of a third embodiment,

Fig. 6 is a view similar to Fig. 3 but illustrating a machine of a fourth embodiment,

Fig. 7 is a diagram similar to Fig. 2 but of another embodiment,

Fig. 8 illustrates axial movement of the stator to effect control,

Fig. 9 is a magnetic circuit diagram of a still further embodiment of the present invention, Fig. 10 is a schematic perspective view of another embodiment of an electrodynamic machine incorporating the present invention,

Fig. 11 is a schematic perspective view of an electrodynamic machine of yet another embodiment of the present invention,

Fig. 12 is a schematic perspective view of an electrodynamic machine of a still further embodiment,

Fig. 13 is a magnetic circuit diagram similar to Fig. 9 but illustrating an alternative arrangement, and

Fig. 14 is a magnetic circuit diagram similar to Figs. 9 and 13 but of yet another embodiment.

As seen in Fig. 1, in the prior art arrangement a U-shaped permanent magnet 3 constitutes a first magnetic path. The magnet 3 can be U shaped as illustrated and without any pole pieces, or can be rectangular with two L-shaped pole pieces. A C- shaped permeable core 1 constitutes a second magnetic path and a thin slab 2 of permeable material constitutes a third magnetic path. The U-shaped permanent magnet 3 is arranged opposite the core 1 so as to form a pair of air gaps 4A and 4B. A coil 6 is wound around the core 1 and is connected to a diode Dl and resistor R _L which constitutes a resistive load for the machine. The polarity of the diode Dl and the sense of winding of the coil 6 are selected so that the diode Dl permits current to flow in the coil 6 and resistor R _L as the slab 2 approaches the air gaps 4A and 4B as indicated by arrow A, but prevents conduction of current by the coil 6 as the slab 2 leaves the air gaps 4A and 4B as indicated by arrow B.

In one small scale operational prototype machine in accordance with the above described prior art arrangement, the narrowest dimension of each air gap 4A, 4B was 4mm and the corresponding dimension (Z in Fig. 1) of the third permeable path was 3.5mm meaning that the air gap between the permanent magnet 3 and the third permeable path 2 when the third permeable path 2 was inserted in the air gaps 4A, 4B

was approximately 0.25mm. It will be appreciated by those skilled in the electrodynamic arts that if one dimension (ie Z) of the third permeable path is constrained to be, say, 3.5mm then the flux carrying cross sectional area is 3.5mm multiplied by the dimension X in Fig. 1. This cross-sectional area is necessarily small. For example if the cross sectional area of the second permeable path carrying the coil 6 (i.e. X x Y) was approximately 20m x 20m equals 400 square mm, the cross sectional area for the magnetic flux of the third permeable path was only 3.5mm x 20mm equals 70 square mm. As a consequence, the flux carrying capability of the third permeable path is approximately an order of magnitude less than the flux carrying capability of the second permeable path. Furthermore, whilst both the dimensions X and Y in Fig. 1 can be increased (eg by a factor of 10 or more), the dimension Z in Fig. 1 cannot be increased (even by 10%) without seriously reducing the flux through the second permeable path 2 and hence the flux linking the windings of the coil 6. In addition, the flow of magnetic flux through the third permeable path 2 is not directly along the longitudinal axis of the path 2. Thus the path 2 should not be fabricated from normal steel laminations since this will not avoid eddy current losses. Instead, the third permeable path 2 should be sintered or otherwise fabricated from electrically insulated but magnetically permeable particles. Although this avoids eddy current losses, it makes the second permeable path 2 structurally weak. Therefore the entire geometric arrangement becomes to some extent impracticable, as well as being difficult to build on an increased scale.

Turning now to Fig. 2, the magnetic circuit diagram of a first embodiment is illustrated. This takes the form of a short bar magnet 10 of substantially rectangular cross sectional shape which has two pole pieces 11, 12 which together constitute a first magnetic path. The coil 6 is essentially as before being wound on a permeable bar 21, again of rectangular cross sectional configuration. The bar 21 constitutes a second magnetic path. The ends of the bar 21 abut the pole pieces 11, 12 and form a first pair of air gaps 14, 24 or equivalent reluctance increasing joints. Alternative constructions include an air gap centrally located in the bar 21, or magnetically impermeable shims between the bar 21 and pole pieces 11,12 to increase the size of the air gaps 14, 24.

Illustrated in Fig. 2 is a rotatable magnetic shunt 30 of cruciform configuration having four arms 31, 32, 33 and 34. In the shunting position illustrated in Fig. 2 a pair of the arms (31 and 32) shunt the pole pieces 11, 12 thereby creating two air gaps 44 and 54 which constitute a second pair of air gaps.

When the shunt 30 has none of its arms 31-34 extending across the second air gaps 44, 54 (not illustrated), the flux from the magnet 10 passes as indicated by solid arrow A through the pole piece 11, across the air gap 14, through the bar 21, thereby linking the coil 6, across the air gap 24 and back to the magnet 10 through the pole piece 12. The flux path A is illustrated in a solid line in Fig. 2.

However, when the shunt 30 has a pair of its arms 31-34 bridging the pole pieces 11, 12 as illustrated in Fig. 2, a second flux path B (which has a lower reluctance than the first flux path A) is present. In the second flux path B magnetic flux passes from the magnet 10 through the pole piece 12, across the air gap 44, through the shunt 30, across the air gap 54 and into the pole piece 11 to return to the magnet 10. Thus the second flux path B indicated by a dashed line in Fig. 2 does not link the coil 6. The reluctance of path B is made lower than the reluctance of path A by ensuring that the air gaps 14, 24 are larger than the air gaps 44, 54.

It will be seen that in the prior art arrangement of Fig. 1, there is a fundamental magnetic circuit comprising the loop formed by the magnet 3, the core 1 and the two air gaps 4A and 4B. In the position illustrated in Fig. 1 the cumulative reluctance of this fundamental magnetic circuit is the reluctance of the core 1, the reluctance of the two air gaps 4 A and 4B and any reluctance present in the first path 3. However, this reluctance changes when the opposite ends of the slab 2 enters the air gaps 4A and 4B. The presence of the magnetically permeable material in the air gaps 4A and 4B substantially reduces their reluctance. As a consequence, the cumulative reluctance of the totality of this fundamental magnetic circuit is substantially reduced by the (temporary) presence of the slab 2 which (temporarily) shunts the magnet 3.

This is to be contracted with the arrangement of Fig 2. in which the reluctance of the first and second paths (11, 14, 21, 24 and 12) does not change. So the

cumulative reluctance of the totality of the first and second paths in Fig. 2 remains substantially constant.

It will be appreciated that the shunt 30 of Fig. 2 constitutes a third magnetic path which is moveable relative to the first magnetic path (11, 12) and the second magnetic path (21). With the above in mind, it is convenient hereafter to refer to the magnet 10, first magnetic path (11, 12) and coil 6 and second magnetic path (21) collectively as constituting a stator component 60 whereas the shunt 30 constitutes a rotor 40. As the rotor 40 shunts the stator 60, the flux in the magnetic path 21 is transferred through the coil conductors to the shunt 30 and this rapid change thereby generates a current in the coil 6 if its diode (not illustrated in Fig. 2) is connected with the correct polarity to permit conduction.

A second embodiment of an electrodynamic machine in accordance with the present invention is illustrated in Fig. 3. Here there are two stators 60 each essentially as in Fig. 2. However, a rotor 41 has three individual magnetically permeable shunt arms 35, 36 and 37 set in a matrix 39 which is not magnetically permeable. As with the arrangement of Fig. 2, as the rotor 41 of Fig. 3 rotates, so the flux path B is alternatively created and destroyed. This moves the magnetic flux alternately between the second and third magnetic paths thereby repeatedly changing the flux through the coils 6.

In the circuit diagram of Fig. 4 it will be seen that each coil 6 of Fig. 3 is connected in series with a corresponding diode Dl or D2 respectively. The sense of the coils 6, the polarity of the magnets 10, and the direction of rotation of the rotor 41 are all selected so that, as before, the coils 6 only conduct current as one of the arms 35, 36 or 37 is being drawn into the space between the air gaps 44, 54. As a result, the coils 6 each produce unidirectional pulses of current, thereby causing a unidirectional flow of current through the load impedance ZL of Fig. 4.

Turning now to Fig 5, a still further embodiment is illustrated with a coil 6, in a stator 62 and a generally crescent-shaped shunt 38 formed from magnetically permeable material, forming the third magnetic path as before, and being carried by a

rotor 42 (illustrated in phantom in Fig. 5). The stator 62 of Fig. 5 is formed by two magnets 110, 210 and the first magnetic path is formed by pole pieces 111,112 and a yoke 113. The second magnetic path is formed by a C-shaped bar 121. As before, there is a first pair of air gaps 114, 124 formed between the pole pieces 111 and 112 and bar 121 respectively. Similarly, there is a second pair of air gaps 144 and 154 formed between the pole piece 111 and shunt 38, and between the pole piece 112 and shunt 38 respectively. As before the reluctance of the magnetic path through the coil 6 is greater than the reluctance of the shunting magnetic path through the rotor 42.

A fundamental distinction between the prior art of Fig. 1 and the above described embodiments of Figs. 2-5, is that the reluctance of the (first) air gaps between the magnetic path of the source of magnetomotive force (mmf) and the second magnetic path carrying the coil, is unchanged irrespective of the relative position of the shunting third magnetic path. This is because the shunt 30, 41, 42, instead of entering the (first) air gaps 14, 24 or 114, 124 in the same manner as the slab 2 enters the air gaps 4A, 4B, instead shunts the flux in a different manner by switching the magnitude of the reluctance in the two flux paths. Thus the relative size of the (first) air gaps 14, 24, or 114, 124 can be selected at will relative to the size of the (second) air gaps 44, 54 or 144, 154. This distinction can be expressed in another way by observing that in the present arrangements the moving magnetically permeable members do not move through the first air gaps (14, 24 or 114, 124) but instead create shunting air gaps (44, 54 or 144, 154) to utilize the necessary flux diversion. As explained above, the cumulative reluctance of the total of the first and second magnetic paths is not changed by the movement of the third magnetic path.

In a still further embodiment illustrated in Fig. 6, eight stators 260 are provided each of which has a pair of magnets 310, 410 and two pole pieces 211 and 212. A yoke 213 completes the first magnetic path. The second magnetic path is formed by a cross member 221 which carries a coil 206. A rotor 240 has six individual shunts 235 set in a matrix 239 which is not magnetically permeable. As the rotor 240 rotates, each pair of pole pieces 211, 212 is shunted by one of the shunts 235 in succession. In one embodiment there is no air gap between the first magnetic path (211, 212, 213) and the second magnetic path 221 in which case the reluctance of the

magnetic path through the coil 206 is adjusted to be larger than that through the rotor 240 by adjusting the cross-section of the cross-member 221. However, an air gap 214 can be provided in the second magnetic path and preferably interior of the coil 206. This air gap 214 is preferably larger than the air gaps between the rotor 240 and the pole pieces 211, 212.

The above described arrangements of Figs. 2-6 also ensure that the transverse cross sectional area of the flux carrying shunt (30, 31-34, 35-37, 38 or 235) of the third magnetic path can be made any dimension as desired and preferably substantially equal to the equivalent transverse cross sectional area of the first magnetic path (11, 12 or 111, 112, 113 or 211, 212, 213) and the second magnetic path (21, 121 or 221). As a consequence, the reluctance which determines the size of the magnetic flux A (Fig. 2) which links the coil 6, 206 is determined by the size of the first air gaps (14, 24 or 114, 124) which can be closely controlled (or eliminated in the case of Fig. 6). Similarly, the ability of the shunt 30, 38, 41, 235 to shunt the magnetic flux created by the magnet(s) 10, 110, 210, 310, 410 so as to follow the flux path B (Fig. 2) is determined by the reluctance introduced by the second air gaps (44, 54 or 144, 154). With the above-described geometries, these last mentioned air gaps can be made exceedingly small and thus the efficiency of the shunting procedure is substantially increased, thereby giving rise to a very high efficiency.

A further very substantial advantage is that each of the magnetic circuits of Figs. 2, 3, 5, and 6 is able to be fabricated from conventional insulated (eg. varnished) steel laminations as illustrated in Fig. 5. As a consequence, eddy current losses are very much diminished and fabrication is simplified.

The electrodynamic machine of the above-described embodiments can be operated as a generator by rotating the rotor and allowing the diode(s) Dl, D2 etc (or equivalent mechanical or electronic switches) to steer or direct the current generated in the coil(s) 6, 206. The electrodynamic machine is also able to operate as a motor by providing a pulse of unidirectional current to the (or each) coil 6, 206 which is of polarity to maximise the magnetic flux which flows through the rotor (i.e. the third magnetic path). Furthermore, these current pulses are timed so that the extra flux is

generated as the shunt (30, 35, 38, or 235) is approaching the stator. The current to the coil is then switched off as the shunt is moving away from the stator.

Turning now to Fig. 7, a still further embodiment of the present invention is illustrated which is similar to Fig. 2, save that the position of the coil is moved. In the arrangement of Fig. 7 the stator 760 includes a magnet 710 and two pole pieces 711 and 712 which form a C-shaped first permeable path including an air gap 714. The pole pieces 711 and 712 are provided with respective legs 721 and 722 which together constitute a second permeable path. The leg 722 carries a coil 706. A rotor 740 includes a number of third magnetically permeable paths in the form of shunts 730 only one of which is illustrated in Fig. 7.

In the embodiment of Fig. 7 a fourth magnetically permeable path 799 is provided for the purposes of control. The path 799 is formed by two stationary legs 797 and 798, a stationary bar 796 and a movable bar 795. An air gap 794 is formed between the opposed ends of the bars 795,796. A magnetic flux Φ3 flows through the fourth permeable path 799 which shunts the magnet 710.

In understanding the operation of the arrangement of Fig. 7, the fourth permeable path 799 can initially be ignored on the basis that the air gap 794 is large and thus the reluctance of the fourth path 799 is so large that the flux Φ3 is negligible. In this case substantially all the flux created by the magnet 710 flows through the pole pieces 711 and 712.

If the shunt 730 is not across the legs 721 and 722 substantially all the flux of the magnet 710 crosses the air gap 714 and constitutes flux Φl as shown in Fig. 7. Substantially no flux flows through the coil 706.

However, when the rotor 740 has moved so that the legs 721 and 722 are bridged by the shunt 730, the flux from the magnet 710 passes through the coil 706 as flux Φ2 and substantially no flux crosses the air gap 714 (since its reluctance is much larger than the reluctance of the air gaps formed between the legs 721, 722 and the

shunt 730). That is, in this embodiment it is the diverted or shunted flux which links the coil, not the undiverted (or unshunted) flux as in the previous embodiments,

Further, the movable bar 795 is able to be moved between two extreme positions as illustrated in Fig. 7. In one extreme as illustrated by dot-dash lines in Fig. 7, the movable bar 795 is moved to the left as seen in Fig. 7 so as to increase the size of the air gap 794. Beyond a certain size, the increased reluctance of air gap 794 is so large that the flux Φ3 is negligible and the fourth permeable path can be ignored.

However, if the movable bar 795 is moved to the right as seen in Fig. 7 into the position illustrated by dashed lines in Fig. 7, then the reluctance of the air gap 794 is reduced and Φ3 becomes appreciable relative to either of either flux Φl or flux Φ2, both of which are correspondingly reduced. In this way movement of the movable leg 795 can be used to control or adjust the magnitude of the flux linking the coil 706 and thus the overall performance of the machine.

An additional, or alternative, method of control is illustrated in Fig. 8. Here the rotor (42, 240, 740) is mounted on a splined shaft 770 and thus is reciprocable in an axial direction as indicated by double headed arrow R. This movement can be brought about by a yoke 771 in the manner used to slide gears along a shaft in an automotive gearbox, by moving the rotor (42, 240, 740) axially relative to the stator (62, 260, 760). Movement of the rotor in this way reduces the magnitude of the magnetic flux flowing in the rotor and thus also controls the overall performance of the machine.

Turning now to Fig. 9, the magnetic circuit diagram of another embodiment is illustrated. This takes the form of a bar magnet 813 of substantially rectangular cross sectional shape which has two pole pieces 811,821 which together constitute a first magnetic path. The coil 86 is essentially as before being wound on a permeable bar 812 again of rectangular cross sectional configuration. The ends of the bar 812 abut two further bars 822, 832 so that the bars 812, 822, 832 constitute a second permeable path. Between the pole piece 811 and bar 822 is an air gap 814 and between the pole

piece 821 and bar 832 is another air gap 824. The air gaps 814 and 824 constitute a first pair of air gaps.

Illustrated in phantom in Fig. 9 is a magnetic shunt 820 also of rectangular configuration which when in the shunting position illustrated in Fig. 9 lies alongside the bar magnet 813 and the pole pieces 811, 821 thereby creating two air gaps 841, 842 which constitute a second pair of air gaps.

When the shunt 820 is absent, the flux from the magnet 813 passes as indicated by solid arrow 819 through the pole piece 811, across the air gap 814, through the bar 822, through the bar 812 thereby linking the coil 86, through the bar 832, across the air gap 824 and back to the magnet 813 through the pole piece 821. The flux path 819 is illustrated by a solid line in Fig. 9.

However, when the shunt 820 is present, a second flux path 829 (which is preferred over the first flux path 819) is present. In the second flux path 829 magnetic flux passes from the magnet 813 through the pole piece 811, across the air gap 841, through the shunt 820, across the air gap 842 and into the pole piece 821 to return to the magnet 813. Thus the second flux path 829 indicated by a dashed arrow in Fig. 9 does not link the coil 86.

It will be appreciated that the shunt 820 constitutes a third magnetic path which is moveable relative to the first magnetic path (811, 821) and the second magnetic path (812, 822, 832). Although it is preferable to have the first and second magnetic paths stationary and move the third magnetic path in the form of shunt 820, it is of course possible to have the shunt 820 stationary and move the first and second magnetic paths, or to move both the shunt 820 and the first and second magnetic paths. With the above in mind, it is convenient hereafter to refer to the magnet 813, first magnetic path (811, 821) and coil 86 and second magnetic path (812, 822, 832) collectively as constituting a stator component 840.

Turning now to Fig. 10, an electrodynamic machine of yet another embodiment is illustrated in schematic form which takes the form of three stator

components 840 arranged in a generally Y-shaped configuration inside a hollow cylindrical rotor 830 which is fabricated from non-magnetically permeable material such as fibreglass reinforced resin. Embedded in the rotor 830 are a multiplicity of shunts 820 each of which is aligned with the longitudinal axis of the rotor 830. As the stator components 840 are similarly aligned, it will be apparent to those skilled in the art that rotation of the rotor 830 relative to the stator components 840 brings the shunts 820 alongside each of the magnets 813 in succession, and then removes them from that position.

Each magnet 813 is preferably formed from rare earth magnetic material such as Neodymium Iron Boron. In order to reduce eddy current losses, the pole pieces 811, 821, bars 812, 822, 832 and shunt 820 are all preferably manufactured from sintered soft magnetic material such as SOMALLOY 550 manufactured by HOGANAS AB of Sweden.

Another embodiment of an electrodynamic machine in accordance with the present invention is illustrated in Fig. 11. Here the rotor 830 is essentially as in Fig. 10, however, it is illustrated provided with a circular gear 835 with which a pinion gear 836 meshes, the pinion gear 36 being driven by an electric motor 837 or equivalent prime mover.

Arranged around the rotor 830 are three stator components 840 as before, and each of which is aligned with the longitudinal axis of the rotor 830. As schematically indicated by broken lines in Fig. 11, it is also possible to include stator components 840 inside the rotor 830 in the same manner as illustrated in Fig. 10.

A still further embodiment is illustrated in Fig. 12. In this embodiment based upon a rotating disc geometry, a disc rotor 850 in which the shunts 820 are radially embedded is provided. The stator components 840 are radially aligned and positioned to either side of the disc rotor 850 and preferably circumferentially staggered so that each shunt 820 does not simultaneously carry flux from two magnets 813 (if it did the shunt transverse cross-sectional area would need to be doubled).

Turning now to Fig. 13, an alternative magnet circuit arrangement is illustrated in which equivalent components are designated by a number decreased by 400 relative to the equivalent designations in Figs. 9-12. The coil 86, magnet 813 and shunt 820 are unchanged. However, the stator component 440 has a first magnetic path formed by bars 401, 411, 421 and 431. The second magnetic path is formed from a single bar 412 which defines a first pair of relatively large air gaps 414 and 424 across which the first flux path 419 (indicated by a solid line in Fig. 13) crosses to link the coil 86.

The second flux path is only formed when the shunt 820 is present thereby defining a second pair of relatively small air gaps 441 and 442 across which the second flux path 429 (indicated by a broken line in Fig. 13) passes. By making the first air gaps 414, 424 larger than the second air gaps 441, 442 the shunt 820 can be made to satisfactorily shunt the flux previously passing through the coil 86.

The stator 440 can be substituted for the stators 840 in the arrangements of Figs. 10-12. The stators 840 and 440 can also be intermingled in the arrangements of Figs. 10-12.

In Fig. 14 yet another magnet circuit arrangement is illustrated in which equivalent components are designated by a number decreased by 200 relative to the equivalent designations in Figs. 10-12. The coil 86 and magnet 813 are unchanged. However, the stator component 640 has a first magnetic path formed by L-shaped pole pieces 601 and 621. The second magnetic path is formed from a C-shaped core 612 which carries the coil 86 and which defines a first pair of relatively large air gaps 614 and 624 across which the first flux path 619 crosses to link the coil 86.

The shunt 620 is rotatably mounted on a central pivot and is rotatable in the direction of arrow F. In the position indicated by solid lines, the shunt 620 does not play any significant role. However, in the position indicated in phantom in Fig. 14, the shunt 620 shunts the flux created by magnet 813.

That is, the second flux path 629 is only formed when the shunt 620 is in the position illustrated in phantom thereby defining a second pair of relatively small air gaps 641 and 642 across which the second flux path 629 passes. By making the first air gaps 614, 624 larger than the second air gaps 641, 642 the shunt 620 can be made to satisfactorily shunt the flux through the coil 86.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the electrodynamic arts can be made thereto without departing from the scope of the present invention. For example, the permanent magnet(s) 10, 110, 210 can be replaced by an electromagnet. It is possible to turn such an electromagnet off when, or just before, the shunt 37 moves away from the air gaps 44, 54 in the arrangement of Fig. 3, for example.

Similarly, the entire machine can be located within an evacuated enclosure so that the machine operates in a vacuum. This not only reduces windage losses but is thought to improve the magnetic flux flow.

In addition, the rotor will come to rest at its lowest energy point. The minimum energy state is provided if a shunt (eg 37) is immediately opposite each of the corresponding pole pieces (11,12). In order to ensure that the rotor is not hard to start, it is desirable that the number and distribution of the shunts does not exactly equal the number and distribution of the stator pole pieces.

The term "comprising" (and its grammatical variations) as used herein is used in the inclusive sense of "including" or "having" and not in the exclusive sense of

"consisting only of. Similarly, the term "magnetic path" is intended to imply a path which is permeable to magnetic flux rather than a path which is magnetised.