WO2002009260A1

WO2002009260A1 - A permanent magnet ac machine

Info

Publication number: WO2002009260A1
Application number: PCT/GB2001/003323
Authority: WO
Inventors: Lawrence Haydock; Neil Brown
Original assignee: Newage International Limited
Priority date: 2000-07-24
Filing date: 2001-07-24
Publication date: 2002-01-31
Also published as: AU2002224585A1

Abstract

A permanent magnet AC generator (10) that includes two disc-shaped rotors (20), each with permanent magnets (22) mounted thereon, and an annular stator core (14) sandwiched between the two rotors (20) and positioned coaxially therewith. A stator winding (16) is wound around the annular stator core (14) such that ends of the stator winding 16 serve as output terminals (18) for the generator (10). In operation, the magnets (22) set up an excitation field such that rotation of the two rotors (20) relative to the stator core (14) and the winding (16) results in an emf being induced in the winding (16). If current is drawn from the winding (16) by an electrical load connected across the terminal (18) thereof, such a current sets up a reaction field around the winding (16) that interacts with the excitation field and tends to alter the magnitude of the induced emf. The generator (10) also includes a control winding (30) wound around the annular stator core (14). Current flow in the winding (30) sets up a control field that tends to oppose the reaction field so as to reduce the effects thereof.

Description

A PERMANENT MAGNET AC MACHINE

The invention relates to electrical machines, such as motors and generators. In

particular, this invention relates to alternating current (AC) generators wherein an excitation field is produced, at least partly, by permanent magnets. Such machines are referred to as "permanent magnet AC machines".

Wu et al in "Design of slotless torus generators with reduced voltage regulation"

[IEE Proc.-Electr. Power Appl., Vol. 142, No. 5, September 1995] disclose a

permanent magnet AC generator that includes a toroidal stator sandwiched

between a pair of coaxial rotor discs. The rotor discs are rotatably mounted on a common axle so as rotate together relative to the stator. Each rotor disc includes a plurality of permanent magnets mounted thereon in a respective circular array. The stator, which is ferrous, has a stator winding wound helically therearound, ends of the winding corresponding to output terminals of the generator. The arrangement is such that the permanent magnets set up an excitation magnetic field that passes through the winding and through the stator. Rotation of the rotor discs and hence the excitation field relative to the stator and the winding causes an electromotive force (emf) to be induced in the winding, such that a current may be drawn therefrom. Wu et al acknowledge the problematic nature of a phenomenon known as

"armature reaction": should a load be connected across the output terminals of

the stator winding so as to draw a current therefrom, the existence of this current in the winding results in a field being set up around that winding. This field is referred to as the "armature reaction field". Its orientation depends on the power factor of the load. For common resistive and inductive loads, or a combination thereof, the effect is to react against and oppose the action of the excitation field so as to effectively reduce the strength thereof. This results in a reduction in the

size of the induced emf, and hence the voltage across the load, with increased load current. Capacitative loads tend to have the opposite effect and result in an

increase in the voltage thereacross when the load current increases. Both situations are undesirable.

Wu et al state that "the use of permanent-magnet excitation precludes excitation control" and go on to propose a solution in which a thin stator is provided such that flux passing therethrough is saturated. Although such an arrangement might address the problems associated with armature reaction, it would tend to result in

increased iron losses in the stator.

It is an object of this invention to address this problem. According to one aspect of this invention there is provided a permanent magnet

AC machine including a first rotating part, a second rotating part and a stator core, the stator core being between the first rotating part and the second rotating

part and including a stator winding therearound, wherein at least one of the first and second rotating parts includes a series of permanent magnets mounted

thereon, the magnets producing an excitation magnetic field whereby rotation of the first and second rotating parts and hence of the excitation magnetic field

induces an emf in the stator winding and allows a current to be drawn from the stator winding by a load, that current setting up a reaction magnetic field that interacts with the excitation magnetic field to affect the magnitude of the induced emf such that variations in the current bring about variations in the induced emf, wherein the machine includes at least one control winding which is adapted such that current flow therein sets up a control magnetic field that interacts with the excitation and the reaction magnetic fields to minimise the variations in the induced emf.

A further disadvantage of known permanent magnet AC generators is that permanent magnets are expensive. This is due to a number of factors. Firstly, the

rare-earth materials used to make such magnets (for example, Neodymium-Iron- Boron) are expensive. Secondly, forming such material into the required shape is not straight-forward. Thirdly, the formed material is brittle and must be handled carefully, particularly once magnetised.

It is a subsidiary object of this invention to address these disadvantages.

In one embodiment of this invention, there is provided a permanent magnet AC

machine wherein the first rotating part includes a first series of pole pieces

thereon and the second rotating part includes a second series of pole pieces

thereon, the first series of pole pieces being arranged opposite the second series of pole pieces such that material of the stator is therebetween, the first series of

pole pieces being alternately permanent magnets of one polarity and cores of ferrous material, and the second series of pole pieces being alternately permanent magnets of the opposite polarity and cores of ferrous material, wherein the first series of pole pieces is staggered with respect to the second series of pole pieces such that cores of the first series are substantially opposite magnets of the second series, and wherein the control winding is operable to set up the control field through the moving part, the cores of the first series, the stationary part and the cores of the second series such that the control field enhances or opposes the excitation field in the stator.

Preferably the cores are of a material of high permeability, such as soft iron,

thereby providing paths of low reluctance between the moving part and the stationary part.

Preferably the rotating parts are arranged such that ferrous material thereof

provides a return magnetic path between the second series of pole pieces and the first series of pole pieces through that material, for flux of the control field.

The permanent magnet AC electrical machine may be a rotary or linear motor or

generator. Preferably flux of the excitation field travels between the rotating

parts and the stationary part in a substantially axial direction. The first rotating

part may be a first rotor and the second rotating part may be a second rotor, the two rotors being spaced apart and mounted on a ferrous axle that is arranged for rotation relative to the stator about an axis. The stator may be sandwiched between the first and second rotors. The stator may be substantially annular and surround the axle so as to be coaxial therewith. The stator may include at least one stator winding wound therearound. Preferably the or each winding is located in slots formed in the surface of the stator, thereby minimising the respective air gap that is necessary between the stator and each rotor. This is advantageous in reducing the reluctance of each air gap and encouraging flux to pass thereacross.

Each rotor may be substantially disc-shaped and be coaxially mounted on or affixed to the axle. The first and second series of pole pieces may each be mounted on a face of the

respective rotor that is adjacent the stator. The pole pieces may be mounted on

that face in a circular array of substantially constant angular pitch.

Preferably, the control winding is stationary, thereby avoiding the need for slip- rings, brushes or any such other arrangement for transferring electricity from a stationary to a moving member. The control winding may be supported by the

stator. Preferably, the control winding surrounds the axle of the rotor and is

arranged such that during operation, flux of the control field passes through the

axle, through the first rotor and through the cores thereof, across the respective air gap into the stator, across the other air gap, into the cores of the second rotor and back into the axle.

Additional current compounding may be provided for fast reaction to transient

load fluctuations by providing a separate auxiliary winding in the slots of the stator, the auxiliary winding being excited so as to further reduce the effects of armature reaction.

According to another aspect of this invention there is provided a permanent magnet AC machine having an annular stator core with a stator winding wound therearound, the stator core supported adjacent to a rotor, the rotor having a plurality of excitation magnets mounted thereon and the magnets producing an excitation magnetic field, whereby rotation of the rotor and hence of the excitation magnetic field induces an emf in the stator winding and allows a current to be drawn from the stator winding by a load, the current setting up a

reaction magnetic field which opposes or enhances the excitation magnetic field and consequently tends to reduce or increase the induced emf, wherein the machine includes a control winding wound around the annular stator core such

that conductors of the control winding extend substantially radially, the control winding being adapted such that current flow therein sets up a control field which opposes the reaction magnetic field when the load current increases so as

to prevent a reduction in the induced emf, and/or which enhances the reaction magnetic field when load current reduces so as to prevent an increase in the induced emf .

According to a further aspect of this invention there is provided an AC machine having a stator core with a stator winding wound therearound, the stator core being supported adjacent to a rotor, the rotor having a plurality of pole pieces, the AC machine being operable to set up an excitation magnetic field, whereby rotation of the rotor and hence of the excitation magnetic field induces an emf in

the stator winding and allows a current to be drawn from the stator winding by a load, the current setting up a reaction magnetic field which opposes or enhances the excitation magnetic field and consequently tends to reduce or increase the induced emf, wherein the machine includes at least one control winding wound around a respective one of the pole pieces, the control winding being adapted such that current flow therein sets up a control field which opposes the reaction magnetic field when the load current increases so as to prevent a reduction in

the induced emf, and/or which enhances the reaction magnetic field when load current reduces so as to prevent an increase in the induced emf .

According to a still further aspect of this invention there is provided a method for regulating output voltage of a permanent magnet AC machine comprising a

stator with a stator winding supported adjacent to a rotor, wherein the stator winding comprises a series of output connections at different points along its length, the method comprising the steps of: a) connecting an electrical load to a first length of the stator winding so as to draw a current from that section of the stator winding and maintaining that

connection for as long as the load current remains substantially the same; b) in the event that the current drawn by the load increases, connecting the load to a greater length of the stator winding to offset a reduction in output voltage; and c) in the event that the current drawn by the load decreases, connecting the load to a lesser length of the stator winding to offset an increase in output voltage.

According to another still further aspect of this invention there is provided a

method for varying the power output of a permanent magnet AC motor

comprising a stator with a stator winding supported adjacent to a rotor, wherein the stator winding comprises a series of input connections at different points along its length, the method comprising the steps of:

a) connecting an electrical supply across a first length of the stator

winding so as to supply a current to that section of the stator winding, thereby i operating the motor;

b) in the event that the power output of the motor is to be increased, connecting the electrical supply to a greater length of the stator winding; and c) in the event that the power output of the motor is to be reduced, connecting the electrical supply to a lesser length of the stator winding.

The control winding may be controlled by current compounding wherein the

output current of the stator winding is fed through a transformer primary and the

secondary of the transformer supplies current to the control winding; or alternatively the control winding may be fed from an automatic voltage regulator (AVR) which uses power electronics to sense the output current of the stator winding and supplies a corresponding AC voltage to the control winding. Where the control winding is provided on one or more of the excitation magnets, it may be fed with a DC current via slip rings fitted to the rotor; or

alternatively the auxiliary magnet windings may be fed by inducing an alternating emf on the rotor from a stationary magnetic field, which can then be

fed into a rectifier which in turn supplies a DC current to the or each control winding.

According to still yet another aspect of this invention there is provided a

permanent magnet AC motor arranged so as to be substantially similar to one or

more of the permanent magnet AC machines of the previously ^'described aspects

of this invention, wherein an AC electrical supply is connected across the or each stator winding thereby setting up a rotating magnetic field around the stator that interacts with the excitation magnetic field set up by permanent magnets to cause rotation of the rotating parts, or rotors, relative to the stationary parts, or stators, and wherein the control winding is adapted such that current flow therein serves to enhance or oppose one or both of the rotating magnetic field and the excitation magnetic field, thereby increasing or reducing the power output of the motor and controlling the power factor of the motor.

Specific embodiments of this invention are now described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is an exploded diagrammatic perspective view of one form an

AC generator in which an auxiliary stator winding is used;

Figure 2 is an exploded diagrammatic perspective view of another form

of an AC generator in which auxiliary rotor windings are used; Figure 3 is an enlarged fragmentary view of detail A of Figure 2;

Figure 4 is an axial view of a stator and stator winding of a further AC generator;

Figure 5 is an exploded perspective view of a further AC generator;

Figure 6 is a sectional view of the further generator; and

Figure 7 is a schematic plan view of the further generator.

Figure 1 shows a three-phase permanent magnet axial flux generator 10. The generator 10 includes a stator 12 and two rotors 20. The stator 12 includes a ferrous core 14 and stator windings 16. The stator winding for each phase is a length of copper wire wound around the ferrous core 14. The ends of the copper wires are the output terminals 18 of the generator.

The rotors 20 are ferrous discs that are rotatably mounted and coaxial with the stator 12. The rotors 20 and the stator 12 are shown spaced apart axially in Figure 1 for the sake of clarity, but in practice, the stator 12 would be closely sandwiched by the rotors 20 with an airgap between the stator 12 and each rotor 20.

Six permanent magnets 22 are mounted on the inside face of each rotor 20. The

magnets 22 are arranged so as to be facing the stator windings 16 and are circumferentially distributed with a pitch of 60 degrees. Furthermore, the magnets are arranged in an alternating N-S configuration so that flux travels

from the north-seeking faces of the magnets on the rotor through the adjacent

airgap and into the stator 12. Flux then travels circumferentially through the ferrous core 14 and returns across the respective airgap to the south-seeking faces of the adjacent magnets, closing the flux path through the respective rotor

20. This flux path constitutes the excitation field of the generator 10.

The effect of the armature reaction is reduced in this first example by the inclusion of a control winding 30 on the stator 12, as shown in Figure 1. The control winding is wound around the ferrous core 14 of the stator 12 in a similar way to the stator winding 16. Although shown thicker than the stator winding 16 for clarity, it is preferred that the control winding 30 be of the same form as the stator winding 16 and be three-phase. When a current exists in this control winding 30, a corresponding control field is set up. The direction of the current in the control winding 30 can be chosen such that the corresponding field

opposes or enhances the secondary field that exists due to the emfs that are induced in the stator windings 16. The net result is a reduction jn the effect of

the armature reaction and a consequential near-constant output voltage, irrespective of the load current.

Two ways in which the current in the control winding 30 and hence the strength of the control field that opposes the secondary field can be controlled are current compounding and automatic voltage regulation (AVR) control.

In current compounding, the output current of the stator windings 16 is fed

through a transformer primary coil. The secondary coil of this transformer is used to supply the current to the control winding 30. The latter current is

therefore determined by the former current so that when the current that is drawn from the stator windings 16 changes, the current that is fed to the control winding 30 will change accordingly.

In AVR control, the generator is provided with power electronics operable to sense the output voltage of the stator windings 16. The power electronics have an output that is connected to the control winding 30 where an AC current is applied to the control winding 30, that AC current being a signal indicative of the output voltage of the stator winding. The output current is such that it causes the field set up around the control winding 30 to most closely cancel out, or to maintain at a substantially constant level, the secondary field that exists due to the current drawn from the stator windings 16. The net result is a reduction in

the effect of the armature reaction and a consequential near-constant output voltage, irrespective of the load current.

Figures 2 and 3 show another example of voltage regulation in a permanent magnet AC generator 10. The main components are the same as in figure 1, except for the auxiliary stator winding which is absent. Instead, an auxiliary excitation winding 40 is wound around each permanent magnet 22. Current is

fed to the auxiliary excitation windings in one of two ways.

The first way is to include two slip rings on the outside face of one of the rotors 20 to which each excitation winding 40 is connected in series. Series connection

is preferred for two reasons: in order to reduce the total number of connections; and to ensure uniformity of current through each excitation winding and thereby uniformity of the corresponding fields that are set up. DC current is supplied from a stationary source to the auxiliary excitation coils via stationary brushes that are in contact with the slip rings.

Alternative slip ring arrangements are possible, including slip rings on a shaft on i which the rotors 20 are mounted. The second way is to induce an AC signal on one or both of the rotors. This

signal is then rectified to produce a DC current suitable for the auxiliary

excitation windings 40.

When a current exists in the auxiliary excitation windings 40, fields are set up that either boost or oppose the excitation field of the permanent magnets 22. The excitation field is boosted in this way to reduce the effect of the armature reaction at high current for a resistive or inductive load, and is opposed in this way to reduce the effect of the armature reaction at low or no load current,

thereby improving the voltage regulation of the generator 10.

Each magnet 22 of the permanent magnet AC generator 10 may protrude only

10mm or so from the surface of the rotor 20 to which it is attached. As a result, the positioning of each control winding 40 around a respective magnet 22 may prove difficult, the available space being inadequate to accommodate the control winding 40. An alternative arrangement of the other example of voltage regulation of a permanent magnet AC generator described with reference to Figures 2 and 3 is therefore envisaged. In this alternative, one of the rotors 20 is as described previously with reference to Figure 1 and does not include a control winding 40 on any of the magnets 22 mounted thereon as described with reference to Figures 2 and 3. The other rotor 20, rather than including magnets 22, substitutes a core (not shown) for each magnet. It is envisaged that each core

is situated on the other rotor 20 opposite a respective magnet 22 on the one rotor

20. the two rotors 20 being joined so as to rotate together. Each core is of a

magnetisable material and may be integral with the other rotor plate 20, which is

ferrous. Each core includes a respective control winding 40 therearound as described previously with reference to Figures 2 and 3, charge being caused to flow in each control winding 40 either by connecting a potential difference

across slip rings (not shown) which feed each control winding 40 or by inducing a current on the other rotor plate 20 from a stationary winding (not shown)and

also as described previously.

Although each core may be shaped and arranged as the respective magnet that it

replaces, it is envisaged that each core may be shaped and arranged differently

and may not be integral with the other rotor plate 20, but may instead be attached thereto. It will be understood that the arrangement of each core is dictated primarily by function, that function being to concentrate a flux path axially through that core, circumferentially through the other rotor 20, axially through one or both neighbouring cores, axially across the adjacent airgap, circumferentially through the stator, and axially back across the airgap and into

that core. The provision of cores that differ in shape to that of the magnets may be necessary in order to effectively guide and maximise flux in, the required direction. In such circumstances the provision of a dedicated other rotor equipped with the control windings and cores, the dimensions of which are dictated solely by function, may be advantageous.

A further permanent magnet AC generator is described with reference to Figure 4. The components of the further permanent magnet AC generator are substantially the same as those of the AC generator described above with

reference to Figure 1, but without the auxiliary stator winding 30. Furthermore, and primarily for clarity of description, a single-phase stator winding 17 is substituted for the three-phase winding 16. Figure 4 shows the single-phase

stator winding 17 wound around the stator core 14. The single-phase winding 17 includes a number of output terminal connections (or "taps")19,21,21a,21b,21c connected at different points along its length. Two of the connections 19, 21 are end connections, each being at a respective end of the single-phase winding 17, such that the whole length of the single-phase winding 17 extends therebetween.

The remainder of the connections 21a,21b,21c are intermediate connections and are positioned on the single-phase winding 17 between the two end connections 19,21.

In operation, an electrical load (not shown) that is to be powered by the further generator is connected across a first pair of the connections, for example across a first one 19 of the end connections and across one 21b of the intermediate connections. Should the voltage across the load drop, for example due to increased current drawn by a resistive or inductive load, the load is connected

across a second pair of the connections, the second pair including the first one

of the end connections 19, but another one 21a of the intermediate connections.

The other one 21a of the intermediate connections is chosen such that the length

of the single-phase winding 17 between the second pair of connections 19,21a is

greater than that between the first pair of connections 19,21b. This tends to compensate for the drop in voltage across the load due to the higher current drawn thereby and results in a more constant voltage across the load. Similarly, an increase in the voltage across the resistive or inductive load due to a fall in the current drawn thereby can be compensated for by connecting the load across

a further pair 19,21c of the connections. The further pair 19,21c would be

chosen such that the length of the single-phase winding therebetween is less than that between the first pair 19,21b.

Although not illustrated, it is envisaged that other windings, similar to the single-phase winding 17, may be wound around the stator 14 to give a polyphase output. Each other winding would include end connections and intermediate connections thereon, similar to those of the single-phase winding 17, so as to provide voltage regulation for the outputs of those other windings.

A still further example of voltage regulation of a permanent magnet AC generator is to artificially load the generator during no-load and low-load conditions. This arrangement is not illustrated. The artificial load can be reduced as the real load increases; or the artificial load can be increased as the real load

reduces, thereby regulating the output voltage. In order to reduce the I R losses

due to the load, it is advantageous to use an inductor. The inductor is connected

across the output terminals of the stator windings and a switch is connected in parallel to the inductor so that it may be used to vary the current through the inductor. Alternatively, a variable inductor may be used.

Figure 5 shows yet another permanent magnet AC generator 110 of the axial

flux type. The generator 110 includes a stator 120, a first rotor 130 and a second rotor 140. The stator 120 is an annulus formed of ferrous material, such as

electrical steel. End faces of the stator 120 include many radial slots 121 formed therein and distributed with constant angular pitch. The slots 121 are for receiving a three phase distributed stator winding, but this is not shown in Figure 5.

The first rotor 130 and the second rotor 140 are both disc-shaped plates of ferrous material. Each of the rotors 130,140 is positioned adjacent a respective one of the end faces of the annular stator 120. This has the effect of sandwiching the stator 120 between the two rotors 130,140. It should be noted that the rotors 130,140 are shown spaced apart in Figure 5 for clarity. The two rotors 130,140 are connected to each other by a ferrous axle 150 that extends between centres of the rotors 130,140. Although not shown, it is envisaged that the axle 150 and the two rotors 130, 140 would be mounted so as to be coaxial

with, and rotatable relative to, the stator 120.

Each rotor 130,140 includes permanent magnets 132,142 respectively mounted on its innermost face, that is to say, the face of that rotor 130,140 that is adjacent the stator 120. The magnets 132,142 are fabricated from a rare earth material such as neodymium-iron-boron. The magnets 132,142 are segment- shaped and are positioned on the respective rotor 130,140 adjacent the periphery thereof and so as to be coaxial therewith. In this embodiment, two magnets

132,142 are provided on each rotor 130,140, however it should be appreciated that any number of magnets is envisaged.

On the first rotor 130, the two magnets 132 are positioned diametrically opposite

one another with north-seeking faces adjacent the innermost face of the first rotor 130. Also provided on the innermost face of the first rotor 130, are two ferrous cores 134. The cores 134 are of a material of high permeability, such as soft iron. The cores 134 are preferably of the same size and shape as the magnets 132 and are positioned diametrically opposite one another and so as to be adjacent the periphery of the rotor 130 and coaxial therewith. Each core 134 is positioned circumferentially between the two magnets 140 so that, together, the two magnets 132 and the two cores 134 are arranged in a circular array with a constant angular pitch of 90 degrees. In this embodiment, it is envisaged that

the cores 134 are integrally formed with the rotor 130, although the cores 134 may not be integral with the rotor 130 and may be attached thereto. Any number

of cores 134 may be provided, but it is preferred that there be an equal number

of cores 134 and magnets 132 and that these are positioned alternately around the circumference of the first rotor 130.

The second rotor 140 also includes two magnets 142 and two cores 144 on the innermost surface thereof and may be considered a mirror image of the first rotor 130. However, the arrangement of magnets 142 and cores 144 on the second rotor 140 is rotationally displaced from that of the first rotor by the amount of the pitch angle, ie by 90 degrees. This has the effect of positioning the magnets 132 of the first rotor 130 opposite the cores 144 of the second rotor

140. Furthermore, the magnets 142 on the second rotor 140 are positioned with

their north-seeking faces furthermost from the second rotor 140.

Figure 6 shows the generator 110 in cross-section, the section being taken through a radial plane. Also shown is the stator winding 122 on the stator 120. In addition to the stator winding 122, the stator 120 carries a control winding 124. The control winding 124 is shown schematically as a single conductor in order to aid clarity. The control winding 124 is annular and is positioned radially inside the stator 120, around the periphery of the hole therethrough so as to encircle the axle 150 that connects the two rotors 130,140. The control winding

is fixedly located relative to the stator 120.

Figure 7 is a schematic representation of the generator 1 10, converted into linear form for clarity. The stator winding 122 of the stator 120 may be seen

positioned within the slots 121 of the stator 120. This enables a first air gap 160

between the first rotor 130 and ferrous material of the stator 120, and a second air gap 170 between the second rotor 140 and ferrous material of the stator 120. to be minimised.

In operation, the permanent magnets 132,142 set up an excitation magnetic field

135. As stated above, the north-seeking faces of the magnets 142 of the second rotor 140 are positioned adjacent the stator 120. Flux of the excitation magnetic field 135 passes from each magnet 142 of the second rotor 140, across the second airgap 170 and into the stator 120, circumferentially through the stator

120 in both directions, out of the stator 120 and across the first airgap. into the respective south seeking face of each of the respective two adjacent magnets 132 on the first rotor 130. The flux path continues through the first rotor 130, through the ferrous axle 150, through the second rotor 140 and back into the magnets 142 mounted thereon, thereby forming a closed loop.

Rotation of the first and second rotors 130,140 causes the excitation field 135 to rotate, subjecting the stator winding 122 to a changing magnetic field. This

results in an emf being induced in the winding 122, such that a current may be drawn therefrom by a load (not shown) connected thereto.

If a current is drawn from the winding 122, a reaction field (not shown) is set

up around that winding 122. The direction of the reaction field may be such that

it tends to oppose the excitation field 135 in the stator 120 and therefore tends to reduce the size of the emf induced in the winding 122. Flux of the reaction field, in following a path of least reluctance, passes from the cores 144 of the

second rotor plate 140, across the second airgap 170 and into the stator 120, circumferentially through the stator in both directions so as to oppose the

excitation magnetic field 135, out of the stator 120 and across the first airgap 160, into the cores 134 of the first rotor 130. The flux path of the reaction field

is completed through the first rotor 130, the axle 150 and the second rotor 140 so as to form a closed loop. Thus, the reaction field opposes the excitation field 135 in the stator core, but enhances the excitation field in the first 130 and second 140 rotors and in the axle 150. Both of these characteristics are undesirable: the former results in variation in the output voltage of the generator 110 with variations in the current drawn therefrom; the latter requires the axle

150 to be of large cross-sectional area.

In such circumstances, it is envisaged that the control winding 124 would have a potential difference connected thereacross so as to cause a current to flow therein. This would result in a control field being set up around the control

winding 124. Figure 6 shows the direction of the control field 125 in a radial

plane. Flux of this field 125 passes through the axle 150, into the first rotor 130,

across the first air gap 160, into and through the stator 120, across the second air

gap 170, into the second rotor 140 and back into the axle 150. The direction of the control field 125 in the first rotor 130, the second rotor 140 and the axle 150 is such that it serves to oppose both the excitation field 135 and the reaction field. As a result, the net flux in the axle 150 may be negligible. This is advantageous in removing the requirement for an axle 150 of large cross- sectional area.

Figure 7 shows how the control field 125 may interact with the excitation field

135. Flux tends to follow a path of least reluctance. The flux of the control field 125 therefore passes from the first rotor 130 into the ferrous cores 134. rather than into the permanent magnets 132, the cores 134 being of greater permeability. The flux of the control field 125 passes through the cores 134 and then crosses the first air gap 160 into the stator 120. It passes circumferentially through the stator 120 in the same directions as does the flux of the excitation field 135, thereby enhancing the excitation field 135. The flux of the control field 125 then passes out of the stator 120 and across the second air gap 170, into the cores 144 of the second rotor 140. Again, the flux of the control field tends to pass into the cores 144, rather than the permanent magnets 142 of the second rotor 140 in accordance with its tendency to follow the path of least

reluctance. The flux path of the control field 125 is then closed via the second rotor 140 and the axle 150 as described above with reference to Figure 6. The

path of the control field 125 is therefore the same as the path of the reaction

field, but the respective directions are such that the control field 125 serves to oppose reaction field.

It should be noted that the minimisation of the first and second air gaps 160,170 is advantageous in reducing the reluctance thereof and encouraging the flux of the control field 125 (as well as that of the excitation field 135) to pass thereacross. Furthermore, the ferrous axle 150 that connects each of the ferrous rotors 130,140, and the ferrous rotors 130,140 themselves, provide a path of low

reluctance for flux of the control field 125 to follow.

It will be appreciated that north-seeking pole pieces of the first rotor 130 are opposite north-seeking pole pieces of the second rotor 140; and that south- seeking pole pieces of the first rotor 130 are opposite south-seeking pole pieces of the second rotor 140. This is a preferred arrangement as it results in flux 135 of the excitation field cutting the stator 120 and the stator winding 122 in alternate circumferential directions at different angular positions. During operation, rotation of this excitation field 135 results in a rapidly changing flux in the stator winding 122 and in the induction of an emf therein.

Having settled on the arrangement described in the foregoing paragraph, it is

further advantageous to arrange each core 134 of the first rotor 130 opposite a respective one of the magnets 142 of the second rotor 140, rather than opposite a respective one of the cores 144 thereof. This offsetting of the respective cores

134,144 of each rotor 130,140 results in the cores 134 of the first rotor 130

being of a different polarity to the cores 144 of.the second rotor 140 (it already having been stated that opposite poles of a like polarity is desirous). This provides paths of low reluctance along which flux of the control field 125 may pass. These low-reluctance paths are from the cores 134 of the first rotor 130 to cores 144 of the second rotor 140 circumferentially through the stator and in a

direction such that flux of the control field 125 that passes therealong enhances the excitation field 135.

From the forgoing description of the flux path of the control field 125, it is evident that this field tends to interact with the excitation field 135 to enhance the latter in the stator 120. This results in a reduction in the effect of the reaction field, which in turn results in a prevention of the reduction in the level of the emf induced in the stator winding 122 that would otherwise occur.

However, it is also envisaged that the current in the control winding 124 may be reduced or have its direction reversed so that the control field 125 set up thereby acts to oppose the excitation field 135 and prevent the size of the induced emf from increasing should the load current be reduced, or a capacitative load be

connected across the stator winding 122.

Although not shown, it is preferred that an automatic voltage regulator (AVR)

be employed to sense the size of the output voltage across the stator winding

122 and to vary the current in the control winding 134 accordingly, thereby maintaining the emf induced in the stator winding 122, and hence the voltage across the outputs thereof, at a near-constant level, irrespective of load current.

Claims

1. A permanent magnet AC machine including a first rotating part, a second rotating part and a stator core, the stator core being between the first rotating part and the second rotating part and including a stator winding therearound, wherein at least one of the first and second rotating parts includes a series of

permanent magnets mounted thereon, the magnets producing an excitation magnetic field whereby rotation of the first and second rotating parts and hence of the excitation magnetic field induces an emf in the stator winding and

allows a current to be drawn from the stator winding by a load that current setting up a reaction magnetic field that interacts with the excitation magnetic

field to affect the magnitude of the induced emf such that variations in the current bring about variations in the induced emf, characterised in that the machine includes at least one control winding which is adapted such that current flow therein sets up a control magnetic field that interacts with the excitation and the reaction magnetic fields to minimise the variations in the induced emf.

2. A permanent magnet AC machine according to claim 1 wherein the control winding is wound around the stator core.

3. A permanent magnet AC machine according to claim 2 wherein the control

winding is provided in slots formed in the stator core.

4. A permanent magnet AC machine according to claim 1 wherein the first rotating part includes a first series of pole pieces thereon and the second rotating part includes a second series of pole pieces thereon, the first series of pole pieces being arranged opposite the second series of pole pieces such that material of the stator is therebetween, the first series of pole pieces being

alternately permanent magnets of one polarity and cores of ferrous material,

and the second series of pole pieces being alternately permanent magnets of the opposite polarity and cores of ferrous material, wherein the first series of pole

pieces is staggered with respect to the second series of pole pieces such that cores of the first series are substantially opposite magnets of the second series, and wherein the control winding is operable to set up the control field through the moving part, the cores of the first series, the stationary part and the cores of the second series such that the control field enhances or opposes the excitation field.

5. A permanent magnet AC machine according to claim 1 wherein a respective

control winding is provided around one or more of the permanent magnets.

6. A permanent magnet AC machine according to claim 1 wherein one rotating part includes a plurality of permanent magnets mounted thereon and the other

rotating part includes a plurality of control windings mounted thereon, each

control winding of the other rotating part being opposite a respective one of the magnets of the one rotating part.

7. A permanent magnet AC machine according to claim 6 wherein each control winding of the other rotating part is wound around a respective core of

magnetisable material of high permeability.

8. A permanent magnet AC machine having an annular stator core with a stator winding wound therearound, the stator core supported adjacent to a rotor, the rotor having a plurality of excitation magnets mounted thereon and the magnets producing an excitation magnetic field, whereby rotation of the rotor and hence of the excitation magnetic field induces an emf in the stator winding and allows a current to be drawn from the stator winding by a load, the current setting up a reaction magnetic field which opposes or enhances the excitation magnetic field and consequently tends to reduce or increase the induced emf,

characterised in that the machine includes a control winding wound around the annular stator core such that conductors of the control winding .extend substantially radially, the control winding being adapted such that current flow therein sets up a control field which opposes the reaction magnetic field when

the load current increases so as to prevent a reduction in the induced emf,

and/or which enhances the reaction magnetic field when load current reduces so as to prevent an increase in the induced emf .

9. An AC machine having a stator core with a stator winding wound therearound, the stator core being supported adjacent to a rotor, the rotor

having a plurality of pole pieces, the AC machine being operable to set up an excitation magnetic field, whereby rotation of the rotor and hence of the

excitation magnetic field induces an emf in the stator winding and allows a current to be drawn from the stator winding by a load, the current setting up a reaction magnetic field which opposes or enhances the excitation magnetic field and consequently tends to reduce or increase the induced emf, characterised in that the machine includes at least one control winding wound around a respective one of the pole pieces, the control winding being adapted such that current flow therein sets up a control field which opposes the reaction magnetic field when the load current increases so as to prevent a reduction in the induced emf, and/or which enhances the reaction magnetic field when load current reduces so as to prevent an increase in the induced emf .

10. An AC machine according to claim 9 wherein at least some of the pole pieces are permanent magnets and some or all of those permanent magnets

have at least one respective control winding wound therearound.

11. An AC machine according to claim 9 or claim 10 wherein at least some of the pole pieces are cores of magnetisable material of high permeability and some of all of those cores have at least one respective control winding wound therearound.

12. A permanent magnet AC machine or an AC machine according to any one

of the preceding claims, wherein at least one control winding is controlled by current compounding such that the output current of the stator winding is fed through a transformer primary and the secondary of the transformer supplies

current to the control winding;

13. A permanent magnet AC machine or an AC machine according to any one of the preceding claims, wherein at least one control winding is fed from an automatic voltage regulator (AVR) which uses power electronics to sense the output current of the stator winding and supplies a corresponding AC voltage to the control winding.

14. A method for regulating output voltage of a permanent magnet AC machine comprising a stator with a stator winding supported adjacent to a rotor,

characterised in that the stator winding comprises a series of output connections

at different points along its length, the method comprising the steps of: a) connecting an electrical load to a first length of the stator winding so as to draw a current from that section of the stator winding and maintaining

that connection for as long as the load current remains substantially the same;

b) in the event that the current drawn by the load increases, connecting the load to a greater length of the stator winding to offset a reduction in output voltage; and

c) in the event that the current drawn by the load decreases, connecting the load to a lesser length of the stator winding to offset an increase in output voltage.

15. A method for varying the power output of a permanent magnet AC motor comprising a stator with a stator winding supported adjacent to a rotor, characterised in that the stator winding comprises a series of input connections at different points along its length, the method comprising the steps of: a) connecting an electrical supply across a first length of the stator winding so as to supply a current to that section of the stator winding, thereby operating the motor; b) in the event that the power output of the motor is to be increased, connecting the electrical supply to a greater length of the stator winding; and c) in the event that the power output of the motor is to be reduced,

connecting the electrical supply to a lesser length of the stator winding.