WO2006012690A1

WO2006012690A1 - Charged stator alternator

Info

Publication number: WO2006012690A1
Application number: PCT/AU2005/001155
Authority: WO
Inventors: Dennis Charles Earnshaw
Original assignee: Precursor Engineering Pty Ltd
Priority date: 2004-08-03
Filing date: 2005-08-03
Publication date: 2006-02-09

Abstract

A charged stator alternator (100) comprising a stator (3), a rotor (2) and switching means (11a, 11b, 18a, 18b), the stator comprising an excitation winding (5) having a first and a second input and comprising a plurality of stator charging coils (5a - 5l), each stator charging coil being associated with at least one pole (4a - 4l) of an even plurality of poles associated with said excitation winding ( 5), the excitation winding (5) being adapted when energized to cause magnetization of said even plurality of poles (4a - 4l), the switching means being adapted to be associated with a DC voltage source to switch the output thereof to the first and a second input of the excitation winding (5), the DC voltage source providing a low voltage output, a high voltage output and an intermediate voltage output having an electrical potential intermediate the electrical potentials of the high voltage output and the low voltage output, wherein in use the intermediate voltage output is continuously connected to the first input of said excitation winding (5) and the second input is switched in a cyclic operation by said switching means between connection with the high voltage output and the low voltage output and wherein the stator (3) further comprises a power output winding comprising a plurality of power output coils (20a - 20l) wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

Description

"Charged Stator Alternator"

Field of the Invention

The present invention relates to rotating electricity generating machines. In particular it discloses a new way of arranging and operating an electrical alternator or generator.

Background Art

The advent of high-current solid-state devices such as diodes and thyristors or SCRs has enabled significant changes from conventional designs in both AC and DC machines, allowing important improvements to be obtained for various applications. In the area of DC motors, such devices, coupled with accurate positional sensing devices have enabled motors to be designed without the use of the commutators, thereby significantly reducing maintenance requirements of such motors. The disclosures of Wilkinson (US 3,025,443) and Fausto Guastadini in US 4,678,974 and WO 86/06564 are examples of only a few of such designs. Nevertheless, such devices have relied on the maintenance of magnetic fields according to conventional models for their design.

The present applicant has previously disclosed in International application PCT/AU2004/001643 published as WO 2005/055400 a switched dc electrical machine which departs from conventional field maintenance resulting in some important improvements over conventional machines. The principles disclosed in WO 2005/055400 are incorporated in the present invention and therefore the disclosure WO 2005/055400 is hereby incorporated by reference.

Nevertheless, the present invention develops further novel and inventive advances applicable to generating machines beyond those disclosed in WO 2005/055400.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia or elsewhere as at the priority date of the application.

Disclosure of the Invention

Accordingly, the invention resides in a charged stator alternator comprising a stator, a rotor and switching means, the stator comprising an excitation winding having a first and a second input and comprising a plurality of stator charging coils, each stator charging coil being associated with at least one pole of an even plurality of poles associated with said excitation winding, the excitation winding being adapted when energized to cause magnetization of said even plurality of poles, the switching means being adapted to be associated with a DC voltage source to switch the output thereof to the first and a second input of the excitation winding, the DC voltage source providing a low voltage output, a high voltage output and an intermediate voltage output having an electrical potential intermediate the electrical potentials of the high voltage output and the low voltage output, wherein in use the intermediate voltage output is continuously connected to the first input of said excitation winding and the second input is switched in a cyclic operation by said switching means between connection with the high voltage output and the low voltage output and wherein the stator further comprises a power output winding comprising a plurality of power output coils wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

According to a preferred feature of the invention, the cycle of the cyclic operation also including segments of time when the second input is disconnected from either of said low voltage or high voltage outputs.

According to a preferred feature of the invention, the excitation winding is configured to energize adjacent poles associated with said excitation winding with opposite magnetic polarity. According to a preferred feature of the invention, the winding of each power output coil is wound in the opposite direction to the winding of the corresponding stator charging coil.

According to a preferred embodiment, each power output coil and its corresponding stator charging coil are wound about a single stator pole.

According to a preferred embodiment, each stator charging coil and its associated power output coil are wound relative to a pole group comprising a group of three adjacent poles comprising a central pole and a pair of side poles.

According to a preferred embodiment, the power output coil associated with a pole group is wound around a central pole of the pole group and the associated stator charging coil is wound about the pair of side poles.

According to a preferred feature of the invention, the voltage differential between the low voltage output and the intermediate voltage output is substantially the same as the voltage differential between the intermediate voltage output and the high voltage output.

According to a preferred feature of the invention, wherein the rotor comprises an even plurality of poles.

According to a preferred feature of the invention, the second input is switched to the high voltage output or the low voltage output when a pole of the rotor is positioned in opposed relationship to a pole of the stator.

According to a preferred feature of the invention, the second input is switched to a disconnected state substantially at a predetermined moment selected to control transient currents.

According to a preferred feature of the invention, the second input is disconnected from the DC voltage source for a substantial proportion of the cyclic period. According to a preferred feature of the invention, the switching of the switching means is synchronised with the rotation of the rotor.

According to a preferred feature of the invention, the switching means comprises sensing means adapted to cause switching of the switching means according to the rotational position of the rotor.

According to a preferred embodiment, the sensing means comprises a photoelectric sensor.

According to a preferred embodiment, a timing wheel is associated with the sensing means to provide a reference for the rotational position of the rotor.

Accordingly to a further aspect, the invention resides in a switched DC rotating electrical machine comprising a stator, a rotor and switching means, the stator being configured with a stator set of poles comprising a plurality of magnetic pole group and the rotor being configured with a rotor set of poles comprising a plurality of magnetic pole groups, the rotor set being configured to provide a magnetic field and the stator set being configured with an excitation coil associated with each pole group of said stator set, said coils being adapted to be excited by a DC voltage source by means of a first input and a second input to thereby induce a magnetic field in association with each pole, said coils being configured to cause said magnetic fields of adjacent poles to be magnetized to opposite polarity, connection to said DC voltage source being controlled by said switching means whereby in use, by the rotation of the rotor with respect to the stator, the magnetic field of the rotor set is adapted to move relative to the pole groups of the stator set, the DC voltage source having a low voltage output, a high voltage output and an intermediate voltage output having an electrical potential intermediate the electrical potentials of the high voltage output and the low voltage output, the intermediate voltage output being adapted in use to be continuously connected to a first input of the said coils and the second input being adapted to be cyclically switched by said switching means between said low voltage output and said high voltage output, and wherein the stator further comprises a power output winding comprising a set of power output coils, wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

Accordingly to a further aspect, the invention resides in a switched DC rotating electrical machine comprising a stator, a rotor and switching means, stator comprising an excitation winding having a first and a second input, the excitation winding having a plurality of stator charging coils being adapted when energized to cause magnetization of a first even plurality of poles associated with said excitation winding and being configured to energize adjacent said associated poles with opposite magnetic polarity, the rotor comprising a second even plurality of poles, the switching means being adapted to be associated with a DC voltage source to switch the output thereof to a first and a second input of the excitation winding in cyclic operation, the switching means being configured to cause switching of the excitation winding to an energized state when a pole of the rotor is positioned in opposed relationship to a pole of the stator and wherein the stator further comprises a power output winding comprising a plurality of power output coils wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

According to a preferred embodiment, the electrical machine is an electric motor. According to a preferred embodiment, the excitation coil is associated with the stator. According to a preferred embodiment, the rotor comprises a permanent magnet.

According to a preferred embodiment, the electrical machine is an electric generator.

The invention will be more full understood in light of the following description of one specific embodiment. Brief Description of the Drawings

The description is made with reference to the accompanying drawings of which:-

Figure 1 is a rear isometric view of a stator, rotor and brush assembly of a charged stator alternator in accordance with first embodiment;

Figure 2 is a front isometric view of a stator, rotor and brush assembly of the charged stator alternator shown in Figure 1 ;

Figure 3 is an isometric view of the rotor of Figure 1 ;

Figure 4 is a front view of the rotor shown in Figure 3;

Figure 5 is a partial front view of the charged stator alternator shown in Figure 1 showing the rotor and timing wheel in a first position (position A);

Figure 6 is a partial front view of the charged stator alternator shown in Figure 1 showing the rotor and timing wheel in a second position (position B);

Figure 7 is a partial front view of the charged stator alternator shown in Figure 1 showing the rotor and timing wheel in a third position (position C);

Figure 8 is a partial front view of the charged stator alternator shown in Figure 1 showing the rotor and timing wheel in a fourth position (position D);

Figure 9 is a perspective view of the stator assembly of the switched DC electric motor shown in Figure 1 ;

Figure 10 is a sectional view of the stator assembly of the switched DC electric motor shown in Figure 1 ;

Figure 11 depicts a diagrammatic arrangement of the circuit of the embodiment; and Figure 12 depicts the timing of connection of the second input of the stator winding of the embodiment to the DC voltage source via the switching means; and

Figure 13 is a diagrammatic representation of the stator of a charged stator alternator according to a second embodiment.

Detailed Description of Preferred Embodiments

The embodiment of the invention comprises a rotating electrical machine in the form of an driven alternator controlled by switching means. The embodiment is described with reference to Figures 1 to 12.

As shown in the drawings, the alternator 100 of the embodiment comprises a stator assembly 3, a brush assembly 8, and a shaft 1 supporting a rotor assembly 2, slip rings 6a and 6b, a timing wheel 9 and switching devices signalled by photo-electric sensors 11a and 11b.

The rotor assembly 2 comprises a first set of "fixed" magnetic poles 16a, 16b, 16c, 16d, 16e, 16f, and second set of "fixed" magnetic poles 17a, 17b, 17c, 17d, 17e, 17f of opposite polarity, energized by a suitable rotor winding 4 to provide a magnetic field.

The brush assembly 8 comprises a pair of brushes (not shown) adapted to bear on the slip rings 6a and 6b and convey energizing current to the rotor coils from a suitable rotor power supply 21. In the embodiment, the rotor power supply 21 provides DC power and it will be appreciated that in operation, the magnetization will be of constant polarity, although it is possible to change the strength according to the strength of the magnetizing current. The brush assembly 8 and photo electric sensors 11a and 11b are supported from the motor housing (not shown) or alternatively from the stator assembly. In an adaptation of the embodiment, the magnetic field of the rotor is provided by one or more permanent magnets. The stator assembly 3 comprises a set of stator poles 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4i, 4j, 4k, 41, stator charging coils 5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h, 51 , 5j, 5k, 5I and power output coils 20a, 20b, 20c, 2Od, 2Oe, 2Of, 2Og, 2Oh, 20i, 2Oj, 20k, 20I. Stator charging coils 5a, 5c, 5e, 5g, 5i, 5k, have their windings configured to provide an excitation current in a clockwise direction, and stator charging coils 5b, 5d, 5f, 5h, 5j, 5I, have their windings configured to provide an excitation current in an anti-clockwise direction so that adjacent poles have opposite magnetic polarity. The stator charging coils may be connected to each other either in "parallel" or in "series" to provide the stator excitation winding 5 so that a two wire input is required to energize all of the stator coils.

The power output coils 20 a - I are magnetically coupled with the stator charging coils by being formed around the stator poles 4a - I. Other than being magnetically coupled, the power output coils 20 a - I are electrically independent and isolated from the stator charging coils 5 a - I. While, there may seem to be no advantage in the orientation of winding of the power output coils 20 a - I relative to the stator charging coils 5 a - I, testing has revealed a performance advantage where the windings of the power output coils 20 a — I are all contra- wound, that is, the winding of a particular power output coil is in the opposite orientation to that of the stator charging coil mounted on the same stator pole. The reason for this advantage is not properly understood at this time.

The timing wheel 9 is provided with a plurality of timing tags 10a, 10b, 10c, 10d, 10e, 10f matching the number of pole pairs of the rotor. The timing wheel is fixed to the shaft to be rotatable with the shaft.

The switching means comprises the photo electric sensors 11a and 11 b mounted to cooperate with the timing wheel 9 and being connectable to electronic switching devices 18a and 18b to trigger switching devices 18a and 18b in a predetermined manner. The photo electric sensors 11a and 11 b are positioned to cooperate with the timing tags of the timing wheel 9. The photoelectric sensors 11a, 11b are energized by a suitable power source. When the timing wheel is rotated, to the position shown in Figure 5, light from the photoelectric sensor 11a is reflected back to 11a from timing wheel tag 10a, closing the internal circuit of photoelectric sensor 11a, sending a signal to an electronic switch set 18a. When the timing wheel is rotated to the position shown in Figure 6, light from the photoelectric sensor 11a is no longer reflected back to 11a from timing wheel tag 10a, so that the internal circuit of photoelectric sensor 11a opens, ending the signal to an electronic switch set 18a. Likewise, when the timing wheel is rotated to the position shown in Figure 7, light from the photoelectric sensor 11b is reflected back to 11 b from timing wheel tag 10a, closing the internal circuit of photoelectric sensor 11b, sending a signal to an electronic switch set 18b. When the timing wheel is rotated to the position shown in Figure 8, light from the photoelectric sensor 11b is no longer reflected back to 11b from timing wheel tag 10a, so that the internal circuit of photoelectric sensor 11 b opens, ending the signal to an electronic switch set 18b.

As shown in Figure 11 , the stator excitation winding 5 of the alternator 100 is adapted to be connected to a DC power source 19 having output switched by high speed electronic switching devices 18a and 18b triggered by the photo electric sensors 11a and 11 b. The embodiment requires a power source having three voltage levels, whereby an intermediate voltage is connected continuously to a first connection of the excitation winding 5. The DC power source 19 provides a 3-wire supply providing voltages of +V, OV and -V respectively where the value of the voltage between the OV and +V outputs is substantially the same as from -V to OV. In use, the OV wire is continuously connected to a first connection of the two-wire input to the stator coils. The second connection of the two-wire input to the stator coils is adapted in use to be switched by switching devices 18a and 18b between the outputs +V, disconnected, -V, disconnected and +V in cyclic manner as described in more detail below. Thus it can be seen that the second input has relatively substantial periods between each of the voltage pulses when it is disconnected from either of the high or low voltages.

Such a voltage source might be provided in a range of ways including a battery of cells, with a takeoff from an intermediate cell providing the intermediate voltage. It will be seen that switch set 18a is wired to deliver dc current from power source 19, through stator coils, 5a - 5I in the forward direction, while switch set 18b, is wired to deliver dc current from power source 19 through stator coils 5a - 51, in the reverse direction.

Direct current from power source 19 is also fed via an appropriate circuit through the photoelectric sensors 11a and 11b, and then (at the correct timing) to the electronic switch sets 18a or 18b, to turn them on and off, powering stator coils 5a - 51.

In the embodiment as described, the stator assembly is provided with twelve stator poles 4a - 4I and the rotor assembly is provided with twelve rotor poles

16a - 16f and 17a - 17f. In operation, six cycles are performed by the stator coils

5a - 5I, each revolution of the shaft 1 and rotor assembly 2. Each cycle then is made up of four parts, they being: -

(A. ) A charge and current from dc power source! 9 is fed through switch set 18a to stator charging coils 5a -5I in the forward direction, magnetizing stators 4a - 4I. (Figure 5) Meanwhile voltage in the power output coils

20a - 20I, begins to rise from zero (0) volts, to that of plus (+) volts.

(B.) The charge and current from dc power source 19, and fed through switch set 18a to stator charging coils 5a - 5I is turned off, demagnetizing stators 4a - 4I. (Figure 6) Voltage on the power output coils 20a - 20I, begins to fall from plus (+) volts, back to zero (0) volts. (C.) A charge and current from dc power source 19 is fed through switch set 18b to stator charging coils 5a -5I in the reverse direction, magnetizing stators 4a - 4I in the opposite order of polarity. (Figure 7) Meanwhile voltage in the power output coils 20a - 201, begin to fall from zero (0) volts, to that of minus (-) volts. (D.) The charge and current from dc power source 19 and fed through switch set 18b to stator charging coils 5a - 5I is turned off, demagnetizing stators 4a - 4I. (Figure 8) Voltage in the power output coils 20a - 20I, begin to rise from minus (-) volts, to that of zero (0) volts.

This cycle of the switching signal sent by the photo-electric sensors 11a and 11b to the solid state switches 18a and 18b to switch the second input to the DC voltage source 19 is represented by the graph shown in Figure 12.

During one complete cycle, the excitation winding 5 is first excited by the voltage applied by the first switch set 18a, the voltage across the stator excitation winding 5 rises from 0 volts to +V volts, almost instantaneously, is held at +V volts for a predetermined period at which time the switching means disconnects the second input. The voltage is thereafter maintained by back emf substantially at the level +V while the rotor poles approach the stator poles. At a point when the rotor poles and the stator poles are aligned, second switch set 18b is closed. The second input is then switched almost instantaneously to -V volts, is held at -V volts for a second predetermined period, being substantially of the same duration as said first predetermined period. After the second predetermined period, the second switch set 18b is opened, thereby disconnecting the second input. The voltage across the second excitation winding 5 is maintained substantially at -V by back emf.

It is to be appreciated that the voltage across the excitation winding 5 may be switched almost instantaneously from the +V level to the -V level or vice versa when the rotor poles are aligned with the stator poles without causing a large transient current. This is a result of the delay between to the opening of the respective switch set and the alignment of the rotor poles with the stator poles.

The switching to a high or low voltage should occur as closely as possible to the moment when poles of the rotor are in directly opposed relationship to corresponding poles of the stator. This is necessary so that the electron current flow in the excitation winding 5 immediately after switching is minimized. Testing has shown that the efficiency of the device is affected by the precision and speed with which the switching can be made to occur.

The disconnection of the second connection from the high or low voltage should also occur as closely as possible to a precise moment during the cyclic period. This moment appears to be a characteristic of the configuration of the machine and the parameters which determine it and the reasons for it are not fully understood at this time. However, deviations of switching from this optimal moment will cause significant current transients which can be sufficient to destroy the switching devices in some configurations. In tests, the stator excitation winding 5 according to the embodiment required to be disconnected approximately 30% of the cyclic period between pulses of connection to either of the high or low voltage outputs.

The operation of the charged stator alternator of the embodiment may be better understood by further reference to Figures 5 to 8. In operation, the rotor coils are energized so that rotor poles 16a - 16f are energized north, while rotor poles 17a - 17f are energized south (Figure 4). This polarity is not reversed during operation.

At position A (Figure 5) stator poles 4a - 4I, having their stator charging coils energized, begin to oppose the rotor poles 16a - 16f and 17a - 17f, and induces rotor 2 to move in a clockwise direction. When rotor 2 reaches position B (Figure 6) current from photoelectric sensor 11 a is turned off, circuits to electronic switch set 18a, and to stator coils 5a - 5I are opened and current flow from dc power source 19 through them ceases. Back emf then continues to energize stator charging coils 5a - 5I, to substantially maintain the potential across the excitation winding 5. Rotor poles 17a - 17f, 16a - 16f, meanwhile are attracted to the stator poles 4a - 4I inducing the rotor 2 to continue in it's clockwise direction between position B (Figure 6) and position C (Figure 7). Upon the rotor 2 and timing wheel tab 10a reaching position C, (Figure 7) the timing photoelectric sensor 11b, is turned on. Current from dc power source 19 flows to electronic switch set 18b closing it's internal circuit to enable current from the dc power source 19 to flow through the stator coils 5a - 5I in the reverse direction charging them to -V volts and inducing a reverse order of polarity within stator poles 4a - 4I. Rotor poles 17a - 17f, 16a - 16f, now being opposed by stators 4a - 4I, continue to move in a clockwise direction away from the opposing stator poles 4a - 4I.

When rotor 2 reaches position D (Figure 8) current from photoelectric sensor Ma is turned off, circuits to electronic switch set 18b, and to stator coils 5a - 5I are opened and current flow from dc power source 19 through them ceases. Back emf then continues to energize stator coils 5a - 5I, to substantially maintain the potential across the excitation winding 5. Rotor poles 16a - 16f, 17a - 17f, meanwhile are attracted to the stator poles 4a - 4I inducing the rotor 2 to continue in it's clockwise direction between position D (Figure 8) and position E, where the cycle repeats itself.

It should be noted that the rotor power supply mentioned above may be independent from the DC power source 19 used to provide electric power to the stator winding, as shown in Figure 11 or alternatively power may be taken from the DC power source 19 to excite the rotor.

It should also be noted that the embodiment comprises an equal number of poles on the rotor as is present on the stator. The actual number on each may differ from the number used in the embodiment described, but must be even to provide an equal number of north and south magnetised poles. It is understood that the number of poles selected will be one of the factors contributing to the performance characteristics of a particular design. It is believed that it would be possible for the rotor of the embodiment to be configured with a number of poles which is different to the number on the stator, although it is expected that some complications may result. It will be appreciated that the manner of energizing of the stator excitation winding 5 in the present embodiment is substantially the same as that used in the motor described in the applicant's previous application published as WO 2005/055400 as described above, and that as a result the rotor is caused to be driven in the same manner. The present invention adds to that disclosure the power output coils 20 a - I. It will be appreciated that the flux in each stator pole varies during the cycle of operation and that as a result, an alternating voltage is induced in each power output coil. Because this output is electrically independent of the input, it may be used as an independent power source. In particular, it may be fed back to the power supply to the stator charging windings and/or the rotor windings by appropriate circuitry. Alternatively, or in addition, it may be utilized independently to power other circuits.

While the arrangement shown in Figure 9 and 10 discloses one means of magnetically coupling the power output coils 20 a - I to the corresponding stator charging coils 5 a - I, it will be appreciated that other configurations are possible. Figure 13 shows a second embodiment of a stator according to the invention wherein the power output coils are magnetically coupled with the stator charging coils in a different configuration to that of the first embodiment. As shown in Figure 13, the stator 103 comprises a set of stator poles 104, in number being a multiple by 3 of the number of pole groups on the stator, that is, each pole group is associated with a set of three poles, a central pole 121 and pair of side poles 122 disposed parallel to either side of the central pole 121. A power output coil 120 is wound around each central pole 121 while a corresponding stator charging coil 105 is wound around each pair of side poles 122. The power output coil 120 is wound to be substantially co-planar with that of the stator charging coil 105, the normal central planar axis of each coil of the corresponding pair of coils thereby being concurrent. The direction of winding of the power output coils 120 is opposite to that of the corresponding stator charging coil 105, as discussed in relation to the first embodiment. The direction of winding of the adjacent stator charging coils is the opposite of each other. In other respects, the alternator of the second embodiment is identical to that of the first embodiment and is not described herein any further.

It will be recognized that it would be possible to wind the stator charging coils 105 about the central pole 121 and the power output coils 120 about the pairs of side poles 122 but it is not believed that this would result in improved performance and indeed it is believed that reduced performance is likely to result.

Those skilled in the art will recognize that the design of the embodiment may be adapted in many ways while still incorporating the essential features of the invention. For instance, the number of pole pairs in the stator and rotor may be changed from the embodiment. Also a rotor having an excitation coil may be replaced with a permanent magnet. In that event, the need for slip rings to the rotor will be avoided. Many alternative switching means are possible instead of the photo-electric sensors and timing wheel arrangement described above. For example, magnetic or Hall-effect sensors may replace the photo-electric sensors. It is to be appreciated that all such adaptations as well as others apparent to the skilled addressee are to be considered within the scope of the invention.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

The Claims Defining the Invention are as Follows:

1. A charged stator alternator comprising a stator, a rotor and switching means, the stator comprising an excitation winding having a first and a second input and comprising a plurality of stator charging coils, each stator charging coil being associated with at least one pole of an even plurality of poles associated with said excitation winding, the excitation winding being adapted when energized to cause magnetization of said even plurality of poles, the switching means being adapted to be associated with a DC voltage source to switch the output thereof to the first and a second input of the excitation winding, the DC voltage source providing a low voltage output, a high voltage output and an intermediate voltage output having an electrical potential intermediate the electrical potentials of the high voltage output and the low voltage output, wherein in use the intermediate voltage output is continuously connected to the first input of said excitation winding and the second input is switched in a cyclic operation by said switching means between connection with the high voltage output and the low voltage output and . wherein the stator further comprises a power output winding comprising a plurality of power output coils wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

2. A charged stator alternator as claimed at claim 1 wherein the cycle of the cyclic operation includes segments of time when the second input is disconnected from either of said low voltage or high voltage outputs.

3. A charged stator alternator as claimed at claim 1 or claim 2 wherein the excitation winding is configured to energize adjacent poles associated with said excitation winding with opposite magnetic polarity.

4. A charged stator alternator as claimed at any one of the previous claims wherein the winding of each power output coil is wound in the opposite direction to the winding of the corresponding stator charging coil.

5. A charged stator alternator as claimed at any one of the previous claims wherein each power output coil and its corresponding stator charging coil are wound about a single stator pole.

6. A charged stator alternator as claimed at any one of claims 1 to 4 wherein each stator charging coil and its associated power output coil are wound relative to a pole group comprising a group of three adjacent poles comprising a central pole and a pair of side poles.

7. A charged stator alternator as claimed at claim 6 wherein the power output coil associated with a pole group is wound around a central pole of the pole group and the associated stator charging coil is wound about the pair of side poles.

8. An charged stator alternator as claimed at any one of the previous claims wherein the voltage differential between the low voltage output and the intermediate voltage output is substantially the same as the voltage differential between the intermediate voltage output and the high voltage output.

9. An charged stator alternator as claimed at any one of the previous claims wherein the rotor comprises an even plurality of poles.

10. A charged stator alternator as claimed at any one of the previous claims wherein the switching of the switching means is synchronised with the rotation of the rotor.

11. A charged stator alternator as claimed at claim 10 wherein switching means comprises sensing means adapted to cause switching of the switching means according to the rotational position of the rotor.

12. A charged stator alternator as claimed at claim 11 wherein the sensing means comprises a photoelectric sensor.

13. A charged stator alternator as claimed at claim 11 or claim 12 wherein a timing wheel is associated with the sensing means to provide a reference for the rotational position of the rotor.

14. A charged stator alternator as claimed at any one of claims 10 to 13 wherein the second input is switched to the high voltage output or to the low voltage output when a pole of the rotor is positioned in opposed relationship to a pole of the stator.

15. A charged stator alternator as claimed at any one of claims 10 to 14 wherein the second input is switched to a disconnected state substantially at a predetermined moment selected to control transient currents.

16. A charged stator alternator as claimed at claim 15 wherein the second input is disconnected from the DC voltage source for a substantial proportion of the cyclic period.

17. A charged stator alternator comprising a stator, a rotor and switching means, the stator being configured with a stator set of poles comprising a plurality of magnetic pole group and the rotor being configured with a rotor set of poles comprising a plurality of magnetic pole groups, the rotor set being configured to provide a magnetic field and the stator set being configured with an excitation coil associated with each pole group of said stator set, said coils being adapted to be excited by a DC voltage source by means of a first input and a second input to thereby induce a magnetic field in association with each pole, said coils being configured to cause said magnetic fields of adjacent poles to be magnetized to opposite polarity, connection to said DC voltage source being controlled by said switching means whereby in use, by the rotation of the rotor with respect to the stator, the magnetic field of the rotor set is adapted to move relative to the pole groups of the stator set, the DC voltage source having a low voltage output, a high voltage output and an intermediate voltage output having an electrical potential intermediate the electrical potentials of the high voltage output and the low voltage output, the intermediate voltage output being adapted in use to be continuously connected to a first input of the said coils and the second input being adapted to be cyclically switched by said switching means between said low voltage output and said high voltage output, and wherein the stator further comprises a power output winding comprising a set of power output coils, wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

18. A charged stator alternator comprising a stator, a rotor and switching means, stator comprising an excitation winding having a first and a second input, the excitation winding having a plurality of stator charging coils being adapted when energized to cause magnetization of a first even plurality of poles associated with said excitation winding and being configured to energize adjacent said associated poles with opposite magnetic polarity, the rotor comprising a second even plurality of poles, the switching means being adapted to be associated with a DC voltage source to switch the output thereof to a first and a second input of the excitation winding in cyclic operation, the switching means being configured to cause switching of the excitation winding to an energized state when a pole of the rotor is positioned in opposed relationship to a pole of the stator and wherein the stator further comprises a power output winding comprising a plurality of power output coils wherein each power output coil is magnetically coupled with a corresponding stator charging coil.

19. A charged stator alternator as claimed at claim 18 wherein the cycle of the cyclic operation also includes segments of time when the second input is switched to a state disconnected from the DC voltage source.

20. A charged stator alternator as claimed at claim 19 wherein the second input is switched to said disconnected state substantially at a predetermined moment selected to minimize transient currents.

21. A charged stator alternator as claimed at any one of claims 18 to 20 wherein the switching of the switching means is synchronised with the rotation of the rotor.

22. A charged stator alternator as claimed at claim 18 wherein switching means comprises sensing means adapted to cause switching of the switching means according to the rotational position of the rotor.

23. A charged stator alternator as claimed at claim 18 wherein the sensing means comprises a photoelectric sensor.

24. A charged stator alternator as claimed at claim 18 or claim 19 wherein a timing wheel is associated with the sensing means to provide a reference for the rotational position of the rotor.

25. A charged stator alternator as claimed at any one of claims 18 to 24 wherein the DC voltage source provides a low voltage output, a high voltage output and an intermediate voltage output having an electrical potential intermediate the electrical potentials of the high voltage output and the low voltage output, wherein in use the intermediate voltage output is continuously connected to the first input of said excitation winding and the second input is switched in said cyclic operation by said switching means between connection with the high voltage output and the low voltage output

26. A charged stator alternator as claimed at claim 24 wherein the rotor comprises a winding energized from a DC power supply via slip rings.

27. A charged stator alternator as claimed at claim 24 wherein the rotor comprises a permanent magnet.

28. A charged stator alternator as claimed at any one of claims 17 to 27 wherein the winding of each power output coil is wound in the opposite direction to the winding of the corresponding stator charging coil.