FIELD OF INVENTION

This invention relates to dynamoelectric machines and specifically to a multi-layer electric generator.

BACKGROUND OF INVENTION

The primary object of the invention is to improve the efficiency of electrical energy generation. The most widely used conventional generators consist of a single rotor, containing magnets, which rotates within a hollow cylindrical stator containing coils. The magnetic field lines emitted by the rotor intersect the coils in the stator thereby inducing an EMF in the coils as per the Lorentz Force and Faraday's Law of electromagnetic induction. The formula (1 ) shown below, derived from Lorentz and Faraday's Laws, forms the basis for calculating the output voltage in a conventional generator.

EMF = v. l. B. sin(0) (1 )

Where v is the relative velocity vector between the conductor and the moving magnetic field,

I is the length of the conductor exposed to the magnetic field, B is the magnetic field density vector, and Θ is the angle between v and B From this formula, it can be seen that by increasing v, I, and B, and, by keeping Θ at 90°, serve to maximise induced EMF. Increasing v and / have physical design limitations, thus increasing B and keeping Θ at 90° are better options for raising induced EMF.

One of the methods for increasing B and keeping Θ at 90°, disclosed in Patent CN201207588Y, is to pass a conductor in between two unlike magnetic poles instead of the conventional setup whereby a conductor passes the surface of a single magnet. When two unlike magnetic poles face each other, the magnetic field in between the two magnets straightens as well as increases in density. Thus, the relative movement of a conductor in between the two magnets, as appose to the surface of a single magnet in the conventional case, would cause that conductor to be intersected by a denser magnetic field at an angle close to the optimal 90°. In Patent CN201207588Y a double layer rotor structure with an annular grove separating the inner and outer rotor member is defined. The grove allows for a stator containing coils to be placed in between the magnets contained in the inner and outer rotor members of the double layer rotor structure.

Refer to prior art FIG. 1 for more detail. The straightening of the field causes there to be a constant magnetic field density in the annular grove within the double layer rotor. This constant magnetic field causes an undesirable square wave output voltage to be produced. In order to resolve this, Patent CN201207588Y discloses a magnet shaping method that can be used to vary the magnetic field within the annular grove thereby causing a desirable sinusoidal output voltage to be produced. The magnets are curve shaped with a decreasing radius of curvature towards its ends so that the radial distance between the inner and outer rotor member increases as you traverse the surface of the magnet. Refer to prior art FIGS 2 and 3 which illustrate the inventive step more clearly. Increasing the radial distance between the inner and outer rotor members increases the reluctance of the magnetic field path which decreases the magnetic flux density for the same Magnetomotive force input. Thus, although the disclosed magnet shaping method successfully produces a sine wave output, it is an inefficient use of the available magnetism in the machine. PCT Patent Applications WO 2008/055416 A1 , WO 03/075439 A2 and WO 201 1/021769 A1 disclose multi-layer rotor concepts similar to Patent CN201207588Y however they also disclose a multi-layer stator concept which is used in conjunction with the multi-layer rotor. In order to integrate the multi-layer rotors and stators within a single machine, a "meshing comb-like structure" design is disclosed. The basic design details mounting multiple coaxial cylinders to the normal face of a solid disk at one end. Another set of coaxial cylinders are then mounted on one end to a separate disk. The diameters of the coaxial cylinders on the separate disks are different to ensure that the open ends of the two cylinder-disk assemblies can be interlaced with each other thereby forming the "meshing comb-like structure". Refer to prior art FIGS 4-6 for more detail. One of the cylinder-disk assemblies contains circumferentially displaced permanent magnets and is connected to a rotational mechanical input, thereby forming the multi-layer rotor, while the other cylinder-disk assembly contains circumferentially displaced coils and is held stationary, thereby forming the multi-layer stator. A mechanical flaw in the design, however, is that the coaxial cylinders are only supported at one end which would lead to flexing of the rotational layers at high speeds. Although the design successfully achieves the complex interlaced multi-rotor multi- stator concept it does have limitations in terms of mechanical viability for scaling the design towards larger units.

Another consistent observation between PCT Patent Applications WO 2008/055416 A1 , WO 03/075439 A2 and WO 201 1/021769 A1 is that they describe hollow cylindrical cavities in the centre of their designs. This is illustrated in FIG. 6. This hollow cylindrical cavity can be seen as an unused volume of space that, with the correct flux channelling techniques, could be used to generate electricity. The hollow cylindrical cavity has a high magnetic reluctance and therefore prevents flux linkage between diametrically opposite magnets within the design. SUMMARY OF INVENTION

According to one aspect of the invention, there is provided an electronic control system for an electrically excited electric generator comprising a rotor with electromagnets and a stator with windings configured to produce an electrical output, the electronic control system comprising: a controller having at least one control input and a control output, the control input being configured to receive a position signal indicative of a rotary position of the rotor, the control output being connected or connectable to the electromagnets of the rotor, wherein the controller is configured to provide the control output based, at least partially, on the rotary position of the rotor and a difference between a desired output voltage and an actual output voltage for the rotary position of the rotor, the control output being configured to control a strength of the electromagnets which, in turn, controls the actual output voltage.

The control input may be configured to receive a feedback signal indicative of the actual output voltage.

The electronic control system may include at least one sensor to provide the control input. The electronic control system may include a position sensor configured to measure the rotary position of the rotor. The electronic control system may include a voltage sensor configured to measure the actual output voltage.

The controller may include a memory module having stored thereon waveform data which may include a correlation between two or more of (a) the rotary position or the position signal, (b) the control output and (c) the desired output voltage.

The control output may have a varying duty cycle. The control output may be configured to vary a duty cycle of the electromagnets. The control output may be a PWM (Pulse Width Modulated) signal. The duty cycle of the output signal may be configured by the controller to compensate for the difference between the desired output voltage and the actual output voltage.

The invention extends to a generator comprising the electronic control system as defined above.

The generator may have a plural stator and plural rotor configuration. The generator may have a dual-rotor, tri-stator configuration. An inner stator of the generator may comprise a solid stator core to enhance flux linkage between diametrically opposite magnetic poles. In the generator, magnetically resistive slits may be provided in at least one of the stators to guide magnetic flux, thereby reducing tangential leakage flux.

The electronic control system may be connected to plural generators having respective electrical outputs, thereby to control the respective actual voltage outputs. Controlling the respective actual outputs may comprise matching the actual output outputs to one another.

The invention extends to a power generation facility comprising plural generators which are controlled by the electronic control system as defined above.

The invention extends to a method of controlling an electrically excited electric generator comprising a rotor with electromagnets and a stator with windings configured to produce an electrical output, the method comprising: receiving, by a controller, a position signal indicative of a rotary position of the rotor; providing, by the controller, a control output to the electromagnets of the stator, the control output being based, at least partially, on the rotary position of the rotor and a difference between a desired output voltage and an actual output voltage for the rotary position of the rotor, the control output being configured to control a strength of the electromagnets which, in turn, controls the actual output voltage of the generator.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings:

FIG. 1 shows a PRIOR ART double layer rotor structure with an annular grove adapted from CN 201207588;

FIG. 2 shows a PRIOR ART constant magnetic field density between two coaxial magnets with constant radial air gap adapted from CN 201207588;

FIG. 3 shows a PRIOR ART distributed magnetic field due to varying radial airgap adapted from CN 201207588;

FIG. 4 shows a PRIOR ART multi-layer stator adapted from WO 03/075439;

FIG. 5 shows a PRIOR ART multi-layer rotor adapted from WO 03/075439;

FIG. 6 shows a PRIOR ART assembled multi-stator multi-rotor machine adapted from WO 201 1 1/021769;

FIG. 7 shows a PRIOR ART multi-rotor generator with constant perpendicular magnetic field lines that intersect the stator;

FIG. 8 shows a PRIOR ART square wave output voltage induced in multi-rotor generators;

FIG. 9 shows a sinusoidally varying electromagnet input voltage as a function of rotor position resulting in a sinusoidal induced stator output voltage, in accordance with the invention; FIG. 10 shows a schematic view of a generator assembly including an electronic control system in accordance with the invention;

FIG. 11 shows a varying duty cycle as a function of rotor position resulting in an apparent sinusoidally varying electromagnet input voltage, in accordance with the invention;

FIG. 12 shows a compensated electromagnetic input waveform for induced output voltage amplitude and shape correction, in accordance with the invention;

FIG. 13 shows pseudo code flow chart for the electromagnet input voltage adjustment technique employed by a microcontroller, in accordance with the invention;

FIG. 14 shows an embedded system block diagram, in accordance with the invention;

FIG. 15 shows a schematic representation of magnetic field in the multi-layer machine, in accordance with the invention;

FIG. 16 shows redirection of flux due to flux channelling slits, in accordance with the invention;

FIG. 17 shows a multi-layer machine mechanical structure, in accordance with the invention; and

FIG. 18 shows a cross-section about line A-A in FIG. 17.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiment described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

An example embodiment describes a dual-rotor, tri-stator, five-layer generator. In order to achieve a sinusoidal output voltage, the electromagnet strength within the rotors is varied as a function of rotor position via a position sensor and a microcontroller. The microcontroller compares the actual output voltage to the ideal output voltage for that specific rotor position and adjusts the electromagnet strength accordingly so that the output waveform shape, phase and level can be controlled. The example embodiment also discloses the use of a solid stator core to maximise flux linkage throughout the machine and slits within the stator's laminations to guide the field towards the coil slots. To ensure that the example embodiment can be scaled towards commercial sized units, careful attention has been made to ensure that the individual layers within the machine are all supported at both ends to minimize flexing at high rotational speeds.

As stated in the background of the invention, multi-rotor generators with unlike pole faces radially aligned, shown in FIG. 7, induce square wave output voltages, shown in FIG. 8, due to the constant perpendicular magnetic field lines between the inner and outer magnetic members that intersect the stator coils.

The example embodiment of the present invention discloses a method for achieving a varying magnetic flux density so that a sinusoidal output voltage waveform can be achieved without the draw-backs of the method described in CN201207588. Instead of keeping the magnet strength constant and varying the radial airgap, as done in CN201207588, the present invention varies the magnet strength while keeping the radial airgap constant. The present invention varies the supplied voltage to the electromagnets sinusoidally with respect to the rotors actual position. This creates a sinusoidally varying magnetic flux density as a function of the rotor position which in turn induces a sinusoidal stator output voltage, as shown in FIG. 9. FIG. 10 illustrates a generator assembly 100 which comprises a generator 101 and an electronic control system 102 in accordance with the invention. The generator 101 is an electrically excited electric generator and comprises a rotor with electromagnets and a stator with windings configured to produce an electrical output (see FIGS. 15, 17, 18). The generator 101 may itself be conventional or may be modified for use with the control system 102.

The control system 102 comprises a controller 103 having at least one control input 104 and a control output 106. The control input 104 can receive a position signal indicative of a rotary position of the rotor. The control output 106 is connected or connectable to the electromagnets of the rotor. The controller 103 is configured to provide the control output 106 based, at least partially, on the rotary position of the rotor and a difference between a desired output voltage and an actual output voltage for the rotary position of the rotor. The control output 106 is configured to control a strength of the electromagnets which, in turn, controls the actual output voltage.

The control system 101 includes sensors (only one sensor 108 is illustrated) to measure characteristics 1 10 of the generator 101 and to provide the control input 104. The sensor 108 includes a position sensor configured to measure the rotary position of the rotor. The sensor 108 also includes a voltage sensor configured to measure the actual output voltage of the generator 101 .

Less electrical input power for the electromagnets is required when their voltage is varied compared to when it is held constant at the maximum voltage. Thus, the present invention is advantageous over prior multi-layer generators as less input power is required for the same output power. The present invention efficiently uses the perpendicular magnetic field uniquely obtained in multi-rotor generators, unlike Patent CN201207588 which purposefully degrades the perpendicular magnetic field thereby losing the advantage that multi-rotor technology has over conventional single-rotor technology in the first place. Cogging is directly proportional to the applied magnetic field strength of the electromagnets. Thus, varying the electromagnet strength as a function of rotor position causes the net cogging in a single rotation to decrease compared to when the electromagnetic strength is kept constant. This results in lower mechanical input power requirements for the present invention compared to prior art.

In order to create a sinusoidally varying electromagnet input voltage as a function of rotor position, digital control is required via a micro-controller. A position sensor relays the current rotor position to the micro-controller, which in turn outputs a control signal to a bank of MOSFET's which then drive the electromagnets accordingly. Varying duty cycle Pulse Width Modulation (PWM) is a known and efficient method used for emulating an analogue sinusoidal output voltage from a high frequency digital output. The micro-controller will employ this technique to control its output signal level, with the duty cycle of the output pulse stream being determined by the rotor's position. Thus, the duty cycle of the high frequency pulse stream, and hence the apparent electromagnet input voltage, will become a function of the rotor's position, as shown in FIG. 1 1 .

By sampling the stator's output voltage and the rotors current position simultaneously, wave form shape correction and voltage regulation can be achieved via the micro-controller. An example of such a correction is shown in FIG. 12. The first plot in FIG. 12 shows how the actual induced voltage's amplitude and shape have changed due to loading of the machine. In order to correct the output waveform, the input voltage into the electromagnets has to compensate for the changes due to loading. The peak of the waveform in the actual output voltage occurs after where it should occur, thus the electromagnet input voltage must peak earlier to pull the induced output peak towards where it should be. The amplitude of the actual output voltage is lower than what it is required to be, thus the overall electromagnetic input voltage has to increase as well. An example of the compensated input voltage is shown in the second plot of FIG. 12. This compensated input waveform is calculated by the microcontroller as it knows what the current output voltage is, what the current output voltage should be, what the current input voltage is as well as the current rotor position. With this information, the microcontroller timeously updates the duty cycle of the PWM stream used to control the electromagnet input voltage.

An example of the Pseudo code that could be used to achieve this is shown in FIG. 13. The microcontroller can also use the described input voltage compensation technique to synchronise other generators within a power-station to all produce exactly the same output voltage waveforms. The embedded system block diagram used to achieve this control is shown in FIG.14. This control system can be scaled to control a n-layer design and synchronise n generators within a power station.

FIG. 15 illustrates a schematic cross of the dual-rotor, tri-stator, five-layer example embodiment. Building on prior art, the present invention has a solid stator core with slots to hold coils. This stator's purpose is twofold. Firstly, it contains coils, thus acting as another output for the machine. Secondly, it is made from a magnetically conductive material so that flux linkage can occur between diametrically opposite magnets, as illustrated in FIG. 15. Flux linkage is a phenomenon whereby the magnetic flux from two magnetic sources interacts constructively with each other resulting in a net magnetic flux increase in the magnetic circuit. As a result of the solid stator core inducing flux linkage between diametrically opposite magnets, the net magnetic field density throughout the machine increases, thereby maximising the induced EMF as per the electromagnetic induction formula (1 ). Not only does the outer most stator in the example embodiment, layer 5, act as another generating output, it also has a positive effect on the magnetic field within the machine. Layer 5 acts as low reluctance return path for the magnetic flux within the machine. This results in flux linkage between the diametrically opposite outwards facing magnetic surfaces of layer 4 thereby further increasing the induced EMF in the machine.

In order to direct the flux towards the back of the slots in Layer 5, thereby causing the entirety of the coil in layer 5 to be intersected by magnetic flux, slits are cut into the laminations of the present invention. These slits are filled with air which has a large magnetic reluctance. FIG. 16 illustrates how the slits direct the unused tangential flux towards the coil slot. Referring to FIG. 17, a simplified schematic representation of the complex mechanical structure needed to achieve the multi-layered design is illustrated. It should be noted that air gaps have been exaggerated for ease of differentiation between the multiple layers. A drive shaft 1 which can be connected to any source of rotational mechanical power is connected to a double layered rotor 2. This rotor 2 consists of an inner cylindrical member 3 and an outer cylindrical member 4 (respectively, Layer 2 and Layer 4 of the multi-layered generator). This configuration allows for two jointly connected, concentrically rotating layers, namely the inner rotor member 3 and the outer rotor member 4, respectively known as Layer 2 and Layer 4 in the multi-layer machine, which function as inner and outer magnetising layers.

Referring to FIGS 17-18, the inner and outer rotor members 3, 4 contain electromagnets which supply the excitation field to the multi-layered generator. The electromagnets comprise: an electromagnet core 5 made from a highly magnetically permeable metal; dispersion layers 6 also made from a magnetically permeable metal; and coils of wire 7 which supply the excitation current to the electromagnets.

At the centre of the machine is a solid stator core 8 which is connected to the supporting frame 9. The stator core 8 supports laminates 10 that contain coil slots 1 1 . The combination of these parts forms the inner most generating layer within the machine, respectively known as Layer 1 of the multi-layer machine.

The middle stator, referred to as layer 3 of the multi-layer machine, consists of a support member 12 and laminates 13 with coil slots 14. Layer 3's support member 12 is connected to the central support member 8 via endplates 15. In order to take advantage of the outer magnetic surface of Layer 4, another stator was added to the machine. The outer most stator, referred to as Layer 5 in the multilayer machine, consists of a support member 16 and laminates 17 that contain coil slots 18. The support member 16 is connected directly to the supporting frame 9.

The rotating and stationary parts of the generator are interfaced via a multitude of bearings 19 throughout the design. From FIGS 17-18 it can be seen that the disclosed structure for the multi-layer machine is mechanically viable for scaling towards large commercial sized generators due to the fact that, unlike prior art, the individual layers within the machine are supported at both ends.