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Title:
ELECTRICAL MACHINES HAVING COMPENSATED FIELD REACTION
Document Type and Number:
WIPO Patent Application WO/1997/014209
Kind Code:
A1
Abstract:
An electrical machine such as a generator or motor includes a rotor portion (47, 49), a stator portion (44, 51) and a field circuit (42, 50) for generating a magnetic flux associated with one of the rotor portion and the stator portion. The field circuit is arranged such that field reaction voltage components induced in one part(s) of the field circuit substantially cancel field reaction voltage components induced in another part(s) of the field circuit. Where the field circuit is associated with the stator portion, the rotor portion is adapted to modulate the magnetic flux generated in the stator portion by the field circuit. Advantageously the machine can include an armature circuit associated with the stator portion wherein the field and armature circuits are common. The field circuit can include groups of related subcircuits arranged such that phasors associated with complex field reaction voltages induced in the related subcircuits sum vectorially to zero when the subcircuits are subjected to a modulated magnetic flux. In a preferred form each group may comprise a pair of related subcircuits. Each pair of related subcircuits may be located on the machine such that the field reaction voltage induced in one subcircuit of each related pair is substantially in anti-phase with respect to the field reaction voltage induced in the other subcircuit of the same related pair.

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Inventors:
Celik
Vladimir, Rutkovski
Pavel
Application Number:
PCT/AU1996/000629
Publication Date:
April 17, 1997
Filing Date:
October 09, 1996
Export Citation:
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Assignee:
MNK RESEARCH AND SOFTWARE PTY
LTD
Celik, Vladimir Rutkovski Pavel
International Classes:
H02K7/14; H02K7/18; H02K19/24; H02K19/26; H02K35/06; H02K41/03; H02K55/02; H02K16/00; H02K19/06; H02K19/10; H02K37/02; (IPC1-7): H02K21/00; H02K19/00; H02K41/03; H02K55/02
Foreign References:
US4716329A
CA965827A
US4464617A
US4426592A
GB1379598A
EP0237778A2
EP0134827A1
US4577126A
Other References:
DERWENT ABSTRACT, Accession No. 83-806933/44, Class X11; & SU,A,967 762 (ELEKTROTYAZHMASHWK) 7 January 1983.
DERWENT ABSTRACT, Accession No. 83-745584/34, Class X11; & SU,A,966 820 (GORKI POLY) 17 October 1982.
DERWENT ABSTRACT, Accession No. 92-356328/43, Class X11; & SU,A,1 695 456 (MOSC AVIATION INST) 1 November 1990.
DERWENT ABSTRACT, Accession No. 87-028703/4, Class V06; & SU,A,1 236 589 (KIEV et al.) 07 June 1986.
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Claims:
CLAIMS:
1. An electrical machine including a rotor portion and a stator portion and a field circuit for generating a magnetic flux associated with one of said rotor portion and said stator portion, said field circuit being arranged such that field reaction voltage components induced in one part of said field circuit substantially cancel field reaction voltage components induced in another part of said field circuit.
2. An electrical machine according to claim 1 , wherein said field circuit is associated with said stator portion, and said rotor portion is adapted to modulate the magnetic flux generated in said stator portion by said field circuit.
3. An electrical machine according to claim 2 including an armature circuit associated with said stator portion wherein said field and armature circuits are common.
4. An electrical machine according to claim 1, 2 or 3 wherein said field circuit includes one or more groups of related subcircuits arranged such that phasors associated with complex field reaction voltages induced in the or each group of related subcircuits sum vectorially to zero when said subcircuits are subjected to modulated magnetic flux.
5. An electrical machine according to any one of the preceding claims wherein said field circuit includes pairs of related subcircuits and wherein one subcircuit of each related pair is arranged to substantially cancel field reaction voltage induced in the other subcircuit of the same related pair, whereby a net null field reaction voltage is induced in each pair of related subcircuits when said subcircuits are subjected to modulated magnetic flux.
6. An electrical machine according to claim 5 wherein each subcircuit of a related pair of subcircuits is located on said machine such that field reaction voltages induced in one subcircuit of each related pair is substantially in anti¬ phase with respect to field reaction voltages induced in the other subcircuit of the same related pair.
7. An electrical machine according to claim 5 or 6 wherein the subcircuits of each related pair are located on said stator part immediately adjacent one another.
8. An electrical machine according to claim 5 or 6 wherein the subcircuits of each related pair are located on said stator part other than immediately adjacent one another.
9. An electrical machine according to any one of the preceding claims wherein said rotor part is substantially hollow and includes a plurality of magnetic circuit elements adapted to provide effective modulation of the magnetic flux generated by said field circuit.
10. An electrical machine according to any one of claims 1 to 8 wherein said rotor part is formed from a substantially nonmagnetic material and includes a plurality of magnetic circuit elements adapted to provide effective modulation of the magnetic flux generated by said field circuit.
11. An electrical machine according to claim 10 wherein said nonmagnetic material includes a dielectric.
12. An electrical machine according to claim 10 wherein said nonmagnetic material includes aluminium.
13. An electrical machine according to any one of the preceding claims wherein said rotor part includes laminated magnetic material for reducing losses due to currents in said magnetic material.
14. An electrical machine according to any one of claims 3 to 13 including means for separating currents flowing in said common field and armature circuits.
15. An electrical machine according to claim 14 wherein said separating means includes at least one capacitor.
16. An electrical machine according to any one of claims 4 to 15 wherein each subcircuit includes a winding on the associated part of the stator portion.
17. An electrical machine according to claim 16 wherein each winding and the associated part of said stator portion constitutes a stator cell.
18. An electrical machine according to claim 17 wherein each stator cell is substantially in the shape of a U with the two legs of the U forming respective North and South poles and being disposed towards said rotor portion.
19. An electrical machine according to claim 17 wherein each stator cell is substantially in the shape of a W with the outer legs of the W forming alternating North and South poles and being disposed towards said rotor portion.
20. An electrical machine according to claim 18 or 19 wherein said rotor portion includes magnetic elements adapted to periodically open and close the gaps at the legs of each stator cell to modulate magnetic flux in each cell.
21. An electrical machine according to claim 17 wherein the stator cells are arranged to participate in cancellation of said field reaction voltages in pairs and wherein the angular distance between the two cells of each pair participating in said cancellation of field reaction voltages is such as to provide for substantially opposite phases of alternating components of magnetic flux in the respective cells of each pair of cells.
22. An electrical machine according to any one of claims 17 to 21 wherein said cells are organized in blocks with each block comprising a plurality of said cells having common legs.
23. An electrical machine according to claim 22 wherein said blocks are arranged to participate in cancellation of said field reaction voltages in pairs of related blocks.
24. An electrical machine according to claim 22 wherein the two blocks of each pair of related blocks of cells are arranged to provide for substantially opposite phases of alternating components of magnetic flux in the respective blocks of the same related pair of blocks.
25. An electrical machine according to any one of claims 3 to 24 wherein said common field and armature circuits include a closed ring made of a material which is superconductive at a relatively high temperature.
26. An electrical machine according to claim 25 wherein said ring is placed inside a pair of nested nonmagnetic tubes and the space between one of the pair of tubes and said ring is adapted to be filled with liquid Nitrogen.
27. An electrical machine according to claim 26 wherein the space between the pair of tubes is evacuated to promote thermal insulation.
28. An electrical machine according to claim 25 wherein said common field and armature circuits include plural rings formed as a closed torroidal winding.
29. An electrical machine according to any one of the preceding claims wherein said armature circuit is adapted to support a single phase.
30. An electrical machine according to any one of claims 1 to 28 wherein said armature circuit is adapted to support multiple phases.
31. An electrical machine according to claim 30 wherein said rotor portion includes a plurality of rotors mounted for coaxial rotation.
32. An electrical machine according to any one of claims 1 to 30 wherein said rotor portion is substantially in the form of a conveyor belt.
33. An electrical machine according to any one of claims 1 to 30 wherein said rotor portion includes a plurality of magnetic elements mounted in a roadway.
34. An electrical machine according to any one of claims 1 to 30 wherein said rotor portion is mounted for reciprocating movement relative to said stator portion.
35. An electrical machine according to claim 17 wherein said machine is a generator and said cells are arranged to facilitate shaping of voltage waveform produced by said generator.
36. An electrical machine according to claim 35 wherein voltage components produced by some cells are shifted in phase relative to voltage components produced by other ceils and wherein said voltage components are supeφosed to facilitate the shaping of said voltage waveform.
37. An electrical machine substantially as herein described with reference to any one of Figs. 2 to 19 of the accompanying drawings.
38. An electrical machine according to any one of claims 1 to 34 and 37 wherein said machine is a generator.
39. An electrical machine according to any one of claims 1 to 34 and 37 wherein said machine is a motor.
40. A wind powered generator incorporating an electrical machine according to claim 35, 36 or 38 wherein said rotor portion is substantially annular and said stator portion is located inside said annular rotor portion.
41. A wind powered generator according to claim 40 wherein the axis of rotation of said annular rotor portion is substantially vertical.
42. A wind powered generator according to claim 41 including sail elements attached to said annular rotor portion for catching the wind.
43. A wind powered generator according to claim 42 wherein said sail elements extend substantially parallel to said axis.
Description:
ELECTRICAL MACHINES HAVING COMPENSATED

FIELD REACTION

The present invention relates to electrical machines such as generators and motors including stepper motors.

A serious drawback which arises in the design of electrical machines especially synchronous machines such as generators is the so called field reaction problem. The field reaction problem is essentially a parasitic phenomenon characterized by strong alternating voltages induced in the field circuit or winding of the machine. The induced voltages are due to alternating components of magnetic flux which are present in the rotor.

Depending upon the internal impedance of the field current source the induced voltages may give rise to relatively high current swings in the rotor circuit requiring an elaborate and relatively expensive high impedance current source to supply the excitation current. The induced voltages and currents arising therefrom are undesirable because they interfere with a steady state excitation current which is supplied to the rotor winding and may overload the field current source.

Traditional machine designs address the field reaction problem in a number of ways including use of a solid and conductive rotor material such as iron and/or a relatively large gap between the rotor and stator of the machine. The former promotes a low impedance path which effectively short circuits alternating currents in the rotor material whereas the latter keeps the rotor away from regions of alternating magnetic flux, thereby dramatically reducing flux linkage between the rotor and the stator. Reduced flux values require an increase in the rotational speed to make a machine more efficient. Consequently prior art machines are relatively inefficient in the task of converting mechanical energy to electrical energy (or vice versa) when the revolution rate of the machine is relatively low. With the exception of large scale hydroelectric-generators which operate at approximately 100 rpm, there are no prior art generators, particularly compact generators of which applicant is aware, which are effective at low rotational speeds. Although rotational speed can be reduced by increasing the number of poles, this is made difficult when the gap between the rotor and stator is relatively large because it

extends the area of interaction between the pole of the rotor and stator. This makes prior art generators a poor choice when the source of the mechanical energy is provided at low rates of revolution as in the case of wind or hydro power. Wind power generators typically utilize gears to step up the revolution rate of the generator but this gives rise to transmission losses which may detract significantly from the efficiency of such wind power generators. The prior art machines also are not easily convertible from generator to motor and vice versa.

The present invention proposes a significant departure from prior art designs in the field of such machines. The present invention addresses the field reaction problem by treating the cause of the problem at its source. In a basic form the present invention proposes a field or rotor circuit arranged such that alternating voltages induced in one part or element of the field circuit substantially cancel or compensate alternating voltages induced in another part or element of the field circuit. The manner in which the basic principle of the present invention may be brought into effect in practical machines may vary according to the type of machine to which it is being applied, eg., generator, motor, synchronous, single phase, three phase, etc. Nevertheless, significant benefits may be derived by addressing the field reaction problem according to the present invention, facilitating construction of what may be considered a new class of electrical machines.

For example, electrical machines including synchronous machines may be constructed according to the present invention with extremely small gaps between the rotor and stator, eg. of the order of or less than 0.1 mm for a compact generator such as the prototype described at page 19. In prior art designs this gap was limited by the abovementioned field reaction problem and other technical and design constraints.

Use of a very small gap in an electrical machine may increase significantly the magnetic link between the rotor and stator. This may not only sharply increase the efficiency of the machine but may also make the machine easily and effectively convertible from generator to motor and vice versa. The machine can also be used as a stepper motor by connecting it to a step control unit instead of an ac network. This may expand the background and flexibility for revolutionary

new designs as described below. Smaller gaps may additionally lead to improvements in overall size and weight of the machine.

A strong magnetic link between the rotor and stator may also mean that it is no longer necessary to place the field winding on the rotor. The field winding may instead be placed on the stator alongside the armature winding. In some embodiments the field winding may be eliminated altogether and its function including the function of compensating the field reaction may be carried out by the armature winding. By placing both field and armature windings on the stator, the role of the rotor may change radically. Instead of being a provider of rotating flux in the case of a generator, it may become a mechanically driven modulator of the stator's flux. This also means that the role of the stator changes. Because the stator creates a magnetic field or flux which is stationary (not rotating) only the magnitude and/or direction of the flux created by the stator may vary to generate an ac armature voltage. Placement of the field winding on the stator may give rise to a number of important benefits. Firstly it may eliminate the need to pass a field current into/from rotating parts of the machine. This may avoid the need for brush contacts or complex and expensive brushless systems. Secondly, it may provide a background for employing a light rotor as described below. Thirdly, it may create ac components in both armature and field windings having the same frequency and which are in phase or 180° out of phase. As will be seen later this may enhance and provide a convenient way to compensate or cancel undesirable voltage components in the field/armature winding(s) whilst reinforcing or adding desirable ones. Elimination of the field winding and passing its function to the armature winding may create more space for the armature winding.

Electrical machines including synchronous machines may be constructed according to the present invention without a solid rotor. In some embodiments the rotor may be laminated to dramatically reduce losses in the rotor's bulk material. Alternatively the rotor may be formed from a substantially non-magnetic material such as aluminium or a dielectric material or it may be substantially hollow as described below. A non-magnetic or hollow rotor (hereinafter referred to as a light rotor) may include a plurality of magnetic circuit elements adapted to

provide effective flux modulation in the stator. The magnetic circuit elements may be formed from laminated iron or magnetic oxide composite or other suitable material that substantially prevents electric currents in its volume such as ferromagnetic or ferrimagnetic material. Where the rotor is formed substantially from a magnetic material, it preferably also comprises a material that prevents electric currents in its volume.

To provide a stable generated frequency in the case of a generator, the rotor circuit elements preferably are identical and may be placed uniformly on the outer surface of a light rotor. Because the rotor may not need to carry windings the circuit elements can be made relatively small. This means that a large number of circuit elements may be placed on the rotor. This fact is highly significant because it may allow construction of very slowly rotating ac generators suitable for wind or hydro energy conversion and other important applications. Applicant believes that synchronous speeds well below 100 rpm (eg. 10 rpm or less, especially in large scale generators) may be easily achievable. It may also allow construction of easily convertible machines suitable for use as wheel- motors/generators for use in electric motor vehicles.

In some embodiments simplification of windings of machines according to the present invention may make them highly suitable for use with high temperature superconductor (HTS) technology. A draw-back in the design of HTS generators is that mechanical properties of HTS make it difficult to build a superconductor winding having a complex shape. Using the principles of the present invention windings can be made into an extremely simple ring shape making an HTS generator practical. Absence of brush contacts and simplicity of rotor design makes it possible to insulate major parts of a machine relatively easily against ingress of aggressive or conductive media such as sea water, oil, gas or chemicals etc. making such a machine suitable for use in such media.

According to the present invention a field circuit or winding may be notionally divided into several parts or elements. Each part or element may include a subcircuit of the field circuit. Each respective element or part of a field circuit may be grouped with one or more related elements or parts in the field circuit which is/are capable of substantially cancelling or compensating field

reaction voltage components induced in the group of related elements or parts. The field winding may be considered to be notionally divided into plural groups of elements or parts. Each group of elements or parts may be arranged such that a null net field reaction voltage is induced in the field circuit or optionally in each group of elements or parts when the elements or parts are subjected to alternating magnetic flux. In one form the elements or parts may be connected in series.

Where the elements or parts of the field circuit or winding are grouped in pairs of related elements or parts, they may be positioned or otherwise arranged such that the phase of field reaction voltage induced in one element or part of a related pair is 180° out of phase (hereinafter referred to as antiphase) with respect to voltage induced in the other element or part of the same pair. The respective elements or parts of a related pair may be positioned such that the two elements or parts experience respective flux linkages associated with alternating components which are substantially in antiphase. In this case a series connection of windings of the respective elements or parts may provide that a net null field reaction voltage is induced in the pair of elements or parts. In some embodiments it may be possible to position the elements or parts of a related pair such that both elements or parts experience a flux linkage which is other than substantially in antiphase. However in this case a phase shifting circuit should be used to connect the windings of the respective elements or parts so that a net null field reaction voltage is induced in the pair of elements or parts.

Where the elements or parts of the field circuit or winding are grouped with more than one related element or part, they may be positioned such that the phasors associated with the complex (amplitude plus phase) field reaction voltages sum vectorially to zero. For example, where the elements or parts of the field circuit are in groups of three, the phasors of field reaction voltages induced in the respective elements or parts may be separated by 120°. Where the elements or parts are in groups of six, the phasors may be separated by 60°. However, such groups of six may be equally considered as two groups of three or three pairs. The latter assumes that the magnitudes of the voltages induced in the respective elements or parts are the same.

The elements or parts of a related group of elements or parts may be positioned immediately adjacent to one another or they may be positioned elsewhere on the machine subject to the requirement that the respective elements or parts of each related group produce null net reaction voltages. Each element or part of the field circuit or winding may be associated with a respective element or part of the stator. The element or part of the field circuit or winding and the associated element or part of the stator will hereinafter be referred to as a cell. The cells of a machine may be grouped into blocks of cells.

Each block may be as small as one ceil or as large as the whole stator containing many cells. The blocks may be designed so that compensation takes place between individual cells in each block. However, this approach may require cells and in particular magnetic poles which are not uniformly spaced, leading to less efficient use of stator space when compared to cells or poles which are uniformly spaced. In a block with uniformly spaced poles or cells, all cells may operate with the same phase shift relative to adjacent poles or cells. In one form all cells may operate synchronously and cannot compensate each others reaction. If such synchronous blocks are used, compensation should be applied between respective blocks rather than between cells of one block.

Alternatively, if phase shifts between adjacent cells in a block are such that all cells in the block can be grouped to achieve compensation, compensation may be applied to the cells within the block as well as to the blocks as a whole.

One possibility for grouping cells in the blocks provides for a 180° phase shift between adjacent cells (eg. Refer Fig. 14). In such blocks compensation may be applied to cells that operate in antiphase, or altematively, to the blocks as a whole (refer to Fig. 15).

One possibility for compensation utilizing a block with uniformly spaced poles or cells lies in adopting a different angular period for the stator than for the rotor. For example a three phase machine may be designed by adopting a stator with 6 poles and a rotor with 5 poles (or multiples thereof), to achieve a 60° phase shift between the cells of the stator. The latter may be combined vectorially in any suitable manner to produce null net reaction voltages.

Alternatively the field reaction problem may be addressed by including a low pass ac filter such as an inductor or transformer in the field circuit. The

inductance associated with the ac filter may be sufficiently large to pass the dc excitation current and to substantially attenuate current due to the induced ac field reaction voltage. However, except in the case of high temperature semiconductor (HTS) applications, this approach may not be viable generally because it leads to unacceptable increase in the size, weight and cost of a machine as well as losses associated with practical inductances.

According to the present invention there is provided an electrical machine including a rotor portion and a stator portion and a field circuit for generating a magnetic flux associated with one of said rotor portion and said stator portion, said field circuit being arranged such that field reaction voltage components induced in one part of said field circuit substantially cancel field reaction voltage components induced in another part of said field circuit.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Figs. 1(a) and 1(b) show a classical generator design and associated voltage waveforms;

Figs. 2(a) and 2(b) shows two classical generators with field reaction compensation and associated voltage waveforms;

Figs. 3(a) and 3(b) show two generators with field reaction compensation, field and armature windings on the stator and associated voltage waveforms; Fig. 4 shows the operating principle of a light rotor;

Fig. 5 shows the operating principle of a generator with field reaction compensation and common field and armature windings;

Fig. 6 shows a single phase generator with a light rotor and separated field and armature windings;

Fig. 7 shows a single phase generator with a light rotor and common field and armature windings;

Fig. 8 shows a single phase generator with field reaction compensation applied to two stators and rotors; Fig. 9 shows a basic design of a three phase generator with field reaction compensation and common field and armature windings;

Fig. 10 shows a three phase generator with field reaction compensation applied to blocks of cells;

Figs. 11(a) - (d) show a basic design of a convertible three phase machine which may function as a motor or generator and associated connections and waveforms;

Fig. 12 shows a generator incorporating a conveyor belt topology design; Fig. 13 shows a basic design of a generator employing oscillating magnetic elements;

Figs. 14(a) to (g) show a stator incorporating a modular design of regularly spaced cells in various electrical phase relationships to the rotor;

Fig. 15 shows a basic design of a generator employing blocks of modular cells connected in a U-cell configuration;

Figs. 16(a) to (d) show a basic design of a generator employing blocks of modular cells connected in a W-cell configuration and associated voltage waveforms;

Figs. 17(a) and 17(b) show a design of an HTS generator; Fig. 18(a) shows a schematic representation of the HTS generator of Fig.

17(a) and Fig. 18(b) shows a modification of the HTS generator for wave shaping purposes; and

Figs. 19(a) and 19(b) show a wave shaping technique utilizing superposition of phase shifted components. Fig. 1a shows a classical design of a single phase ac generator or alternator. The alternator includes field circuit 10 having field or rotor winding 11 and excitation source 12. Excitation source 12 is connected to field winding 11 via brush contacts 13, 14. The alternator includes armature circuit 15 having armature or stator windings 16, 17 and load 18. The ac voltage ~U(ab) induced in the armature circuit 15 and the ac voltage ~U(cd) induced in the field circuit 10 are shown in Fig. 1b. The dc voltage U(cd) represents the steady state excitation voltage generated by the excitation source 12. The ac voltage ~U(cd) represents the field reaction voltage which is induced in the field circuit. It may be seen that in the classical design, the frequency of the ac field reaction voltage is double the frequency of the ac voltage in the armature circuit. This occurs because the ac armature voltage is caused by rotation of the rotor's magnetic flux which is synchronized to the rotor's revolution rate, whereas the ac field reaction voltage is caused by flux modulation due to a

non-constant gap reluctance which completes a full cycle each one half-revolution of the rotor.

Fig. 2(a) shows two classical ac generators 20, 21 mounted on the same drive shaft. The rotors of the generators are mounted with a 90 degree relative phase shift relative to the respective stators. It may be seen in Fig. 2(b) that U(fe), the ac field reaction voltage induced in the field circuit of generator 21 is 180° out of phase with U(ge), the ac field reaction voltage induced in the field circuit of generator 20. By connecting the field circuits of generators 20, 21 in series as shown in Fig. 2(a) the ac field reaction voltages are compensated by cancelling each other. No net ac field reaction voltage is present at the ends of the compensated field or rotor circuits of generators 20, 21. Although the simple scheme shown in Fig. 2 illustrates the principle for compensating the field reaction voltage according to the present invention, it does not provide a practical implementation thereof because a 90° phase shift occurs between the voltages generated by the armature windings of generators 20, 21. This phase shift complicates utilization of generated ac power because the two armature windings cannot be simply connected to one load. However, a multiphase (multiple of four) generator could be designed utilizing a similar arrangement.

Fig. 3(a) shows one embodiment of a generator design without a rotor winding. In this embodiment a pair of ac generators 22,23 are mounted on the same drive shaft. The rotors of the respective generators are mounted with a 90° relative phase as in Fig. 2. However, the field winding of each generator is fixed on the stator alongside the armature winding. The rotor modulates the magnetic flux generated by the fixed field winding thereby generating an ac voltage in the armature winding.

As may be seen in Fig. 3(b), the ac voltages [U(ab), U(cd)] in the armature windings and the ac voltages [U(fe), U(ge)] in the field windings of this embodiment have the same frequency because both sets of voltages are generated by modulation of magnetic flux each one half revolution of the rotor. Moreover the armature and field voltages are exactly in antiphase (or in phase depending on how the ends of the windings are connected), facilitating compensation or cancellation of ac reaction voltages induced in the field windings

and reinforcement or addition of ac voltages induced in the armature windings by appropriately connecting the respective field and armature windings.

Although Fig. 3(a) shows a two pole machine, the same principles may be readily extended to a multipole machine. However, an even number of poles should be used to ensure that a field reaction voltage induced in a winding or cell associated with any given pole may be cancelled by a field reaction voltage induced in a winding or cell associated with another pole.

The operating principle of a light rotor 24 is shown in Fig. 4. A source 25 of field current gives rise to a circulating current in field circuit 26 including winding 27 associated with inverted U shaped cell 28 which forms part of the stator to create a magnetic flux in U shaped cell 28. As magnetic shorts or strips 29 move past cell 28 they periodically open and close the gap at the open end of U shaped cell 28. This causes modulation or variation in time of the magnetic flux in cell 28. Because armature circuit 29 including winding 30 intercepts the time varying flux in cell 28 it causes an ac current to be induced in armature circuit 29 including winding 30 and load 31.

A light rotor 24 as shown in Fig. 4 effectively confines magnetic interaction between the stator and rotor to the surface of the latter where the magnetic shorts 29 are located. One consequence of this is that the inner body of the rotor 24 may be dispensed with if required. In some embodiments the body of the rotor may be substantially hollow or may contain elements for driving the rotor (in the case of a generator) or for delivering torque (in the case of a motor). For example in the case of a wind generator, a hollow rotor may contain the vanes or blades for catching the wind directly. Applicant envisages an embodiment in which the rotor may be annular and of the order of 100 meters or more in diameter. In a preferred embodiment the stator may be located inside the annular opening of the rotor. The rotor may include a plurality of wind sails mounted around its perimeter for catching the wind, in one form the plane of the rotor may be horizontal and the sails may project vertically therefrom. The sails may be steered by computer for optimum wind engagement.

Moreover, each magnetic short 29 is itself separate from other shorts and because the rotor/stator gap is small, effectively interacts only with those cells of the stator which are immediately adjacent to it. This means that the various cells

of the stator operate substantially independently to a high degree and may be considered to form separate generating cells. This consideration suggests that a modular design for the stator cells is feasible. It is to be appreciated that rotor 24 may adopt any convenient topology such as circular or linear or may be in the shape of a conveyor belt or the like. Hence drawings herein which show a linear topology may be readily interchanged with a circular topology and vice versa.

Fig. 5 shows a generator utilizing a common field and armature circuit 32. The generator includes a rotor 33 and a pair of identical U-shaped magnetic cells 34, 35. A dc voltage source 36 provides dc excitation current in series windings 37, 38 associated with magnetic cells 34, 35 respectively. Capacitors C1 , C2 are adapted to separate out the ac current which is generated in the dc circuit. Capacitors C1 , C2 also provide a parallel connection of windings 37, 38 to the load 39.

Because magnetic cells 34, 35 and shorts 40, 41 and 42 are disposed such that magnetic cell 34 is fully open (ie. flux linkage is at a minimum) when adjacent magnetic cell 35 is fully closed (flux linkage is at a maximum), the ac current generated in winding 37 will be in antiphase to the ac current generated in winding 38. Opposite phases of ac currents generated in windings 37, 38 causes cancellation of ac current in the field voltage source 36 which is connected in series with windings 37, 38. However, the ac currents in the load 39 which is connected in parallel with windings 37, 38 are reinforced. It is to be emphasized that the currents in windings 37, 38 will only be in antiphase if the relative position between the shorts 40-42 and cells 34, 35 is correctly chosen as shown in Fig. 5, ie. at the instant cell 35 operates at maximum flux, cell 34 should operate at minimum flux and vice-versa.

It is to be appreciated that this requirement can be met even if magnetic cells 34, 35 are not physically adjacent to each other in the stator as shown in Fig. 5. Hence cells 34, 35 may be spaced some distance apart and can even belong to different blocks of cells. This may provide for a degree of flexibility in the design of the stator. Importantly the angular distance between the two cells participating in compensation should be chosen to provide for substantially opposite phases of the alternating component of magnetic flux in the two respective cells.

Because there exist many relative locations which satisfy the above conditions, there is not just one exact location for the cells in the stator. This may provide additional flexibility for designing the machine. It may also allow for convenient grouping of cells into blocks. Compensation may be applied to pair of cells individually or to respective blocks having several cells.

Fig. 6 shows one embodiment of a single phase generator 40 according to the present invention utilizing a light rotor 41 and having a field winding 42 and separate armature winding 43. Generator 40 includes a stator 44 with respective blocks of U shaped cells 45, 46 for generating north(N) and south(S) poles around the periphery of the light rotor 41. Compensation is applied between blocks 45, 46 as a whole rather than between individual cells in the blocks.

Light rotor 41 includes a non-magnetic cylindrical body 47 and a plurality of longitudinal (shown in cross-section in Fig. 6) laminated iron shorts or strips 48 fitted in corresponding grooves formed in non-magnetic body 47. The cross section of strips 48 may be tapered and/or otherwise contoured to obtain a close- to-sinusoidal shape of generated voltage. The latter assumes that rotor 41 is rotated at a constant angular speed. Alternatively or additionally strips 47 may at least partly follow the shape of a helix for the purpose of shaping the waveform of the generated voltage. It is to be noted that strips 48 are spaced around the periphery of stator 41 such that when one pair of North/South poles in stator blocks 45, 46 is fully closed by a strip 48 an adjacent pair of South/North poles are fully open. The latter is to ensure that the adjacent North/South poles experience flux linkages which are substantially in antiphase.

Fig. 7 shows another embodiment of a single phase generator utilizing a light rotor 49 and including a common field and armature winding 50. The stator 51 includes respective blocks of cells 52, 53 with compensation applied between the blocks 52, 53 as a whole rather than between individual cells in the blocks. Capacitors C1 , C2 separate the armature current flowing in the common field and armature winding 50 to the load, which should be connected between the points marked a, b.

Fig. 8 shows a single phase generator employing two stators and rotors. Each stator 74, 75 includes respective blocks of cells 76, 77 each of which may occupy a whole stator. Compensation in this case can be applied to blocks 76,

77 notwithstanding that they are associated with different rotors. The load in figure 8 should be connected between the points marked a, b. It may be noted that the placement of magnetic shorts on rotor 74 relative to the cells of block 76 in Fig. 8 is similar to the placement of magnetic shorts on rotor 73 relative to the cells of block 71 in Fig. 7. Also, the placement of magnetic shorts on rotor 75 relative to the cells of block 77 in Fig. 8 is similar to the placement of magnetic shorts on rotor 73 relative to the cells of block 72 in Fig. 7. These placements provide for anti-phase of the alternating components of flux.

The principles of the present invention may be readily applied to multiphase machines. Fig. 9 shows the basic design of three phase generator incorporating field reaction compensation according to the present invention. Desired angular relationships between the three phases may be achieved by selecting appropriate angular distances between cells or block of cells belonging to the three phases ψ 1 ( ψ 2 , ψ 3 . Field reaction compensation may be applied to respective individual pair of cells or to block of several cells in the manner shown in Fig. 7. Field reaction compensation preferably is organized for each phase separately. In the embodiment shown in Fig. 9 cells having common field/armature windings are used. Operation of the cells in the first phase ψ., is similar to the embodiment described with reference to Fig. 6. In Fig. 9 cells 78, 79 associated with the first phase ψ 1 t are positioned relative to magnetic shorts 84 such that they operate at minimum and maximum flux respectively. Cells 80, 81 associated with the second phase ψ 2 are positioned relative to magnetic shorts 84 such that they operate 120° out of phase relative to cells 78, 79 respectively. Cells 82, 83 associated with the third phase ψ 3 are positioned relative to the magnetic shorts 84 such that they operate 120° out of phase relative to cells 80, 81 respectively.

Fig. 10 shows one embodiment of a three-phase generator in which the stator is made up of six blocks 90-95 of magnetic cells. Compensation for the first phase ψ, is arranged between blocks 90 and 93. Compensation for the second phase ψ 2 is arranged between blocks 91 and 94 and compensation for the third phase ψ 3 is arranged between blocks 92 and 95. The loads in this case should be connected between the points marked ψ-,, ψ 2 , ψ 3 respectively and ground.

Many other designs of a three phase generator utilizing the principles of the present invention are possible. For example, six identical rotors and stators may be used in tandem with the cells of each stator placed equidistantly around its respective rotor. The number of cells in each stator may correspond or be equal to the number of shorts in the associated rotor. The stators may be positioned angularly relative to one another to provide field reaction compensation between pairs of stators. The stators may also be positioned to provide phase angles of 120° between voltages generated by each of the three pairs of stators. Although the embodiments described with reference to Figs. 1-10 refer to generators it is to be appreciated that the same machines may function as motors essentially without modification. That is, they are convertible from generator to motor and vice versa. In some embodiments standard start up arrangements may be required. Fig. 11(a) shows a basic design of a convertible three phase machine which may function as a synchronous motor or generator or stepper motor. With the connections shown in Fig. 11(b) the machine can be used as a synchronous generator or motor. With the connections shown in Fig. 11(c) the machine can be used as a stepper motor. The pulse sequences necessary to produce stepped movement of the rotor are shown in Fig. 11(d). Relative independence in the operation of respective cells of the machines described herein provides an opportunity to design machines which do not rely on rotational symmetry. Hence machines having any desired topology may be readily constructed eg. linear, curved including combinations thereof. As an example of what is possible, Fig. 12 shows a machine incoφorating a conveyor- belt topology design. For convenience the magnetic shorts 96 are shown on the linear part only of the conveyor belt rotor, although it is to be appreciated that they extend right around the conveyor belt. A linear motor/generator may be constructed by transposing one of the blocks 97, 98 of magnetic cells shown in Fig. 12 so that it lies alongside the other of blocks 97, 98 such that blocks 97, 98 become colinear. This may form the basis of a linear drive system such as a transport system. For example, magnetic blocks 97, 98 may be carried by a vehicle and shorts 96 may be part of a rail or road way supporting the vehicle or vice versa.

It is to be appreciated that flux modulation in the magnetic cells can be provided by movement other than relative rotational or linear movement between the magnetic shorts and cells. For example Fig. 13 show an oscillating generator in which oscillating or vibrating movements of magnetic elements 99, 100 are used to obtain flux modulation is respective magnetic cells 101 , 102. In this design a crankshaft 103 is used to convert rotary movement of a prime mover (not shown) into reciprocating movement of magnetic elements 99, 100.

Whilst the embodiments described herein refer to a stator having U-shaped magnetic cells or blocks of such U-shaped cells, it is to be appreciated that the present invention is not limited to cells having such configuration. Rather the magnetic cells of the stator may take any configuration which exhibits an incomplete or open magnetic circuit and which can be selectively closed by circuit elements such as magnetic shorting elements or shorts on a rotor as described herein. For example, the or each cell may be designed as a W-shaped magnetic circuit having a central leg or rod and a pair of adjacent side legs or rods. The armature winding may be placed on the central leg of the W-shaped cell. The field windings may be placed on the side legs of the W-shaped cell. It will be appreciated that the side legs will have opposite directions of magnetization. W-ceils also will require separate field windings whereas U-cells may be designed either with separate field windings or with common field/armature windings if desired.

Figs. 14(a) to (g) show a stator 104 incorporating a modular design of regularly spaced cells which may be connected either in a U-type or a W-type cell configuration. Figs. 14(a) to 14(g) show positions of rotor 105 relative to stator 104 at electrical phases 0°, 90°, 180°, 270°, 360°, 450° and 540° respectively in the case when the cells are connected in a U-type cell configuration. The arrow indicates the direction of motion of rotor 105 relative to stator 104. A significant advantage of U type cells is that all generated voltages are in phase (0 or 180 degrees) and have the same frequency. In the case of W-type cells the generated voltage will be half the frequency of the field reaction voltage giving rise to the utilization problems discussed with reference to Fig. 2.

Windings can be placed virtually anywhere on a cell's or block's magnetic circuit(s) subject to the need to provide sequentially alternating North and South

poles. Placement of the windings should also be such that voltages of equal magnitudes are generated in cells which participate in mutual compensation of field reaction. Subject to the need to connect some cells which participate in mutual compensation in series, the direction or sense of windings and/or order of connection of the windings may not be important.

Fig. 15 shows a basic design of a generator employing the modular design of regularly spaced cells shown in Fig. 14. The cells in Fig. 15 are connected in a configuration exhibiting U-type cells.

Fig. 16(a) shows a further design of a generator employing the modular design of cells shown in Fig. 14 in which the cells are connected in a configuration exhibiting W-type cells. Figs. 16(b) to (d) show waveforms associated with the generator of Fig. 16(a) for a W-type cell configuration.

It is to be appreciated that reference to U-shaped or W-shaped magnetic cells does not limit the configuration to essentially planar designs as shown in the drawings. Both U-shaped and W-shaped cells can adopt relatively complex 3-D designs. For example whilst the free ends of the pole creating legs in a W-cell may be disposed towards the rotor, the legs and common bases of the cells need not lie in the same plane as the free ends but may instead lie in a different plane or planes (see Fig. 17). The pole faces associated with the free ends of the pole creating legs may be substantially planar. The pole faces may exhibit a slight curvature in a plane of rotation of the machine consistent with the angle subtended by the pole faces at the axis of rotation of the machine. In some embodiments the pole faces may be other than substantially planar. This may be desirable for reasons of compactness or uniqueness of the design adopted. For example, the pole faces may be curved (concave or convex) or exhibit otherwise complex shapes providing that the shorts associated with the rotor adopt a complementary shape which closely matches or follows the shape of the pole faces. In one embodiment interaction between the stator and rotor may be arranged as between coplanar discs. One disc surface may carry the pole faces and the other disc surface may carry the magnetic shorts. A capacity to adopt a complex 3-D shape for the magnetic cells may increase compactness or packing density of the stator. It may also provide additional flexibility for designing the associated windings. Packing density may also be enhanced by grouping the

magnetic cells in blocks such that the side legs or rods of adjacent cells are merged into common side legs or rods.

Figs. 17(a) and (b) show a generator embodying the principles of the present invention utilizing high temperature superconductor (HTS) technology. As noted earlier, the demands of HTS technology make it difficult to produce superconductor windings having a complex shape. Using the principles of the present invention an HTS generator may adopt a relatively simple ring shaped winding 106 as shown in Figs. 17(a) and (b). Two blocks of U-type cells 107, 108 each as shown in Fig. 17(b) embrace the ring winding 106 such that legs of cells 107, 108 which terminate in south poles pass over one side of ring winding 106 and alternate legs which terminate in north poles pass under the other side of ring winding 106. This is one example of a complex 3-D shape for the magnetic cells discussed above in which the legs of the cells lie in different planes to that of the poles. Ring winding 106 is shown placed inside a pair of nested non-magnetic torroidal (copper) tubes 109, 110. The space between the inner tube 110 and ring winding 106 is filled with liquid Nitrogen (temperature 77°K). The space between outer tube 109 and inner tube 110 is preferably evacuated to promote good thermal insulation. Ring winding 106 is used as a common field and armature winding with generated ac voltage being taken up from opposite sides of the ring 106 between the terminals marked ~U and ground. The dc field excitation current may be created in ring winding 106 by means of induction or any other suitable means. Field reaction compensation is provided between the blocks of U type cells 107, 108 in the manner described herein. It should be appreciated that as a result of compensation of the field reaction, use of torroidal copper tubes containing liquid nitrogen becomes feasible, otherwise very strong circular currents would be generated in those tubes. The necessary phase relationships between the blocks of cells 107, 108 is provided by appropriately locating blocks 107, 108 with respect to rotor 111 and associated magnetic shorts 112.

In some embodiments a plurality of ring windings similar to ring winding 106 may be used. In one form the rings may be formed as a closed torroidal winding. It may be appreciated that torroidal or ring shaped windings are not

necessarily confined to HTS technology. Such designs may also be used in conjunction with more conventional windings eg. copper.

Fig. 18(a) shows a schematic representation of the ring-shaped winding 106 and blocks of cells 107, 108 in Fig. 17(a). Referring to Fig. 18(a), it will be appreciated that an ac voltage (U) is induced in winding 106 only at locations which are under the influence of the blocks of cells 107, 108. No ac voltage is induced (U = 0) in locations which are not under the influence of the blocks of cells. This feature of the HTS generator facilitates placement of more than 2 blocks of cells around winding 106. For example, 6 blocks of cells may be placed around winding 106 to generate 3 phase current from a single unit.

A further important use of multiple blocks of cells may be to improve the shape or waveform of the ac voltage produced. The idea is illustrated by reference to Fig. 18(b) which shows the design of a single phase HTS generator having 8 blocks of cells ie. 4 pairs of blocks 113, 114; 115,116; 117, 118; and 119, 120 generating 4 distinct phases Ψ 1 ? Ψ 2 . ψ 3 and ψ of voltage.

The phases of voltage produced by any one pair of blocks may be shifted slightly relative to other pairs (typically by approximately 10 to 30°, although theoretically by as much as 90°). The respective phase shifts may be optimized by means of a computer to cancell or compensate unwanted harmonic components. The respective phases may be superposed by being applied to a common load, to cancell or compensate high harmonic components of induced ac currents, and to reinforce the main harmonic component. Fig. 19(b) shows how addition of several voltages (3 in Fig. 19(a)) leads to better shape of the resulting voltage. Figs. 19(a) and 19(b) show voltages produced by separate pairs of cells as square waves, which is a worst case scenario. In reality, the shapes of individual voltage waveforms are not that square, and by using just several slightly phase shifted pairs of blocks in one phase produces a well shaped sinusoidal waveform.

The concept of using multiple pairs of blocks whose generated voltages are shifted in phase and then superposed to shape the waveform of the generated voltages may be applied to all of the designs disclosed in the present application.

It is possible to achieve a similar result by introducing a predetermined misalignment of the cells within each block of cells to create the desired phase shifts. Thus, cells within the same block need not be placed exactly uniformly or in such positions as would produce the same electrical phase. On the contrary a predetermined misalignment from an exact in phase position may lead to positive results as described above.

It is important to stress that although a predetermined misalignment of cells within the blocks, or between pairs of blocks is permitted for wave shaping purposes, the requirement for strict compensation of field reaction (meaning substantially opposite phases) still applies to the cells or blocks of cells forming the pairs. That is, it is only after complete compensation is provided within pairs, that such compensated pairs can be manipulated to achieve additional advantages.

To test the principles of the present invention, a prototype generator was constructed. The prototype generator includes a light rotor 260 mm in diameter and carrying 30 magnetic shorts. The prototype includes a laminated stator made of steel E310 (0.5mm thickness) and 50 mm in width. The stator was assembled from two similar blocks of generating U-cells, each having 28 poles, which is two poles less than the number of shorts on the stator. This was done to provide gaps between the blocks of U cells for adjusting relative angular positions of the blocks within the stator to obtain necessary phase relations between the generated voltages for compensation purposes. The gaps between stator's poles and rotor's shorts were 0.1 mm, an incredible figure for conventional generators. The parameters for the prototype generator are as follows: nominal ac frequency - 50 Hz rotor's rotation speed - 50 rev per min weight of active part of generator

(blocks, windings, rotor's outer part) - 9 kg weight of the body etc. - 17 kg total weight - 26 kg generated power in synchronous mode - 230 W

The purpose of the prototype was to check basic design concepts, accordingly the design parameters used were not optimized for maximum

performance. Nevertheless prototype testing has established that the design concepts disclosed herein are valid. Specifically, it has established that (i) field reaction compensation is achievable, (ii) the gap between the rotor and the stator can be made very small, (iii) it is possible to use the same winding for both dc field and ac armature currents, (iv) a very slowly rotating small size synchronous ac generator is achievable, and (v) the generator is easily convertible into a motor with a very high torque. Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.