AU2019101680A4 - An electric generator with secondary coils - Google Patents

Abstract

- 14 An electric generator includes a rotor with a magnet arrangement configured to provide a primary magnetic field a stator with armature coils, the rotor being rotatable around the stator such that the primary magnetic field induces a current in the armature coils. The electric generator further includes secondary windings provided around or outside the stator, the secondary windings configured to generate a secondary magnetic field which interacts with, and increases or enhances, the primary magnetic field a switching arrangement configured to switch at least one of an intensity or a polarity of the magnetic field of the secondary windings based on a position of the rotor relative to the secondary windings. 6c 6a 6b \ 481 s

Description

An Electric Generator with Secondary Coils

FIELD OF DISCLOSURE

This disclosure relates to electric machines and more specifically it relates to an electric generator which generates its magnetic field in a rotating part of the generator.

BACKGROUND OF DISCLOSURE

In a conventional electric machine, e.g. an electric generator, a magnetic field is produced by a rotor by supplying field coils wound around the rotor with a DC current. In a particular type of generator, as the rotor spins, the current to the field coils is supplied via a brush and slip ring system. The field coils may be subject to large centrifugal forces due to the rotation of the rotor. In order to secure the field coils, they are often coated in a resin and lodged into coil slots on a rotor body.

A number of turns in the rotor coil, which relates to strength of the magnet and in turn an amount of induced EMF in the stator coils, is limited by the size of the rotor 20 cavity. In summary, using the rotor as the source of magnetic flux in the generator is workable but has practical limitations, namely, the need for a brush and slip ring system, the need to secure the field coils in order to withstand the centrifugal forces, and lastly the physical space within the rotor cavity which in turn limits the magnet size and strength.

The Applicant has noted that there have been efforts to control or influence the field coils by the inclusion of secondary coils.

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US4887020 (Self-compensating brushless alternator) discloses secondary stator coils whose function is to generate power which then gets fed back to the rotor coils. US9912206 (Motor with Damping Means) discloses secondary stator coils whose function is to generate a damping torque on the rotor via its own magnet field produced by the induced currents that flow through it. In both these cases, the secondary coils have currents induced in them to serve a certain function.

US7545056 (Saturation Control of Electric Machine) and US20040239202 (Architecture for electric machine) both describe generators that contain secondary coils in the stator which are supplied with a DC current. The generators described in these patents are both permanent magnet generators. Permanent magnet generators are limited in the way that their output voltage can only be controlled by the speed of the machine. These patents deal with increasing the controllability of the output of permanent magnet generators. This is done by introducing secondary control windings within the stator of the machine. The control windings produce a magnetic field which is magnetically isolated from the magnetic field produced by the permanent magnets in the rotor. The function of the control windings’ magnetic field is to vary the saturation level of the stator material. Varying the saturation level varies the resistance of the magnetic circuit thereby influencing the output voltage induced by the power windings. This method gives a certain degree of control over the output generated by the permanent magnet machine albeit quite inefficient as it requires additional power to saturate a return path thereby decreasing the output power. Generally, the more input power that is put into the machine, the less nett output power it produces.

The Applicant desires an improved electric generator. The Applicant proposes realising this improved electric generator by providing secondary coils configured differently from the prior art disclosures of which the Applicant is aware.

A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known

-32019101680 23 Dec 2019 or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

SUMMARY OF DISCLOSURE

Accordingly, the disclosure provides an electric generator including:

a rotor with a magnet arrangement configured to provide a primary magnetic field; and a stator with armature coils, the rotor being rotatable around the stator such that the primary magnetic field induces a current in the armature coils, wherein the electric generator further includes:

secondary windings provided around or outside the stator, the secondary windings configured to generate a secondary magnetic field which interacts with, and increases or enhances, the primary magnetic field; and a switching arrangement configured to switch at least one of an intensity or a polarity of the magnetic field of the secondary windings based on a position of the rotor relative to the secondary windings.

The magnet arrangement of the rotor may be in the form of an electromagnet with field coils or a permanent magnet

The secondary coils may thus be configured to increase or enhance the current induced in the armature coils from around or outside the stator - in other words, not from within the stator where the rotor may be housed. This may lead to higher current induction (and hence generation) without increasing, or even with reducing, rotor 25 complexity. This reduced rotor complexity may come at the expense of power needed to drive the secondary windings to generate the secondary magnetic field.

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The electric generator may be configured supply a current, e.g., a DC (Direct Current) current, to the secondary coils. The current may be provided or controlled by the switching arrangement.

The Applicant believes that a technical advantage of the present disclosure over the prior art disclosures mentioned in the background is that, in the prior art disclosures, there is no magnetic linkage between the rotor’s magnetic field and the magnetic field produced elsewhere, e.g. in the secondary stator coils. The present disclosure intends to improve on this by ensuring that the secondary magnetic field (or magnetic flux) produced outside the rotor, e.g., by the secondary coils consistently and constructively adds to the magnetic flux produced by the rotor in order to increase the output of the generator.

Stated differently, the present disclosure proposes the idea of shifting some (or even most) of the magnetic flux production away from the rotor and instead producing magnetic flux outside the rotor, e.g., in or by the stator, in the return path and even outside the machine. The fields (the primary field and the secondary field) generated from two different sources (the field coils and the secondary coils) may work constructively.

There may be many advantages to producing flux outside the rotor, e.g. within the stator and outside the stator instead of the in the rotor, namely, the stator is stationary, therefore, there may be no need for the lossy brush and slip ring system and there are no centrifugal forces to counter. The stator is on the outside of the rotor, 25 thus there may be fewer space or size restrictions for the secondary stator coils, unlike the space limitation placed on the rotor coils. The other advantages are that there may be less complicated requirement for cooling, easy of repair and maintenance compared to rotor as the source of magnetic field. By increasing the magnetic field generated outside the rotor, this may reduce the volume and space needed inside the 30 machine in the rotor.

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The switching arrangement may be configured to provide a switching algorithm as a function of the rotor position.

The secondary coils may be provided at one or more of the following places:

An inner section of the stator;

An outer section of the stator; and/or

Outside the stator entirely.

When the magnetic field is generated outside the stator, a high magnetic permeable material may be employed to link and guide the secondary magnetic field to the stator. The high magnetic permeable material may comprise mu-metal, purified iron, or supermalloy.

BRIEF DESCRIPTION OF DRAWINGS

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

In the drawings:

FIG. 1 shows a schematic cross-sectional view of a simplified conventional two 20 pole generator;

FIG. 2 shows a schematic cross-sectional view of an electric generator in accordance with an example embodiment of the disclosure;

FIG. 3

FIG. 4 shows a schematic view of a secondary coil switching as a function of rotor position in the electric generator of FIG. 2;

shows a graphical view of a function graph of secondary coil current vs rotor position in the electric generator of FIG. 2;

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FIG. 5 shows a graphical view of a smoothed step function graph of secondary coil current versus rotor position in the electric generator of FIG. 2;

FIG. 6 shows a schematic cross-sectional view of an alternate embodiment of an electric generator in accordance with the disclosure;

FIG. 7 shows a diagrammatic representation of a secondary coil switching as a function of rotor position in the electric generator of FIG. 2;

FIG. 8 shows a schematic view of another embodiment of an electric generator in accordance with the disclosure, having two outside electromagnets for a four-pole generator;

FIG. 9 shows a schematic view of another embodiment of an electric generator in accordance with the disclosure, with two north poles at both connecting to a stator;

FIG. 10 shows a schematic view of another embodiment of an electric generator in accordance with the disclosure, with two south poles and a middle north pole connecting to a stator;

FIG. 11 shows a schematic view of another embodiment of an electric generator in accordance with the disclosure, with secondary coils comprising two north poles and a two south poles;

FIG. 12 shows a schematic view of another embodiment of an electric generator in accordance with the disclosure, with two north poles and no south poles linking to a stator; and

FIG. 13 shows a schematic view of another embodiment of an electric generator in accordance with the disclosure, with two north poles and two south poles with a magnetic barrier for a larger outer magnet surface area coverage 25 linking to a stator.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

The following description of the disclosure is provided as an enabling teaching of the disclosure. Those skilled in the relevant art will recognise that many changes can

-72019101680 23 Dec 2019 be made to the embodiment described, while still attaining the beneficial results ofthe present disclosure. It will also be apparent that some of the desired benefits of the present disclosure can be attained by selecting some of the features of the present disclosure without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present disclosure are possible and can even be desirable in certain circumstances, and are a part of the present disclosure. Thus, the following description is provided as illustrative ofthe principles of the present disclosure and not a limitation thereof.

Referring to FIG. 1, a conventional two pole electric machine can be seen. The conventional machine consists of a stator 1 containing armature coils 2 and a rotor 3 containing field coils 4. A DC current is supplied to the rotor field coils 4. The direction of the current through the rotor coils 4 is depicted by a cross and a dot index with the cross representing current into the page and the dot representing current out of the page. The illustrated current direction in the rotor field coils 4 results in the observed magnetic field 5 in the machine.

FIG. 2 shows an embodiment ofthe present disclosure whereby the conventional design has been modified to include secondary coils 6 in the stator. Note that the outer part of the stator 7, which acts as the return path for the machine, has been extruded radially outwards compared to the conventional design. This increases the size ofthe stator slot 8, allowing space for and accessibility to the secondary coils 6. Extruding the return path outwards increases the accessibility of the stator armature coils 9 as well, allowing for ease of cooling ofthe coils.

The secondary coils 6 are excited by an external DC current with the direction of this current depicted by the cross and dot key as well, with the dot representing current direction out of the page and the cross representing into the page. Note that the secondary coils 6a on the left-hand side of the machine have a different current 30 direction to the coils on the right-hand side of the machine 6b. This is done to ensure that the magnetic field produced by the secondary coils 6 compliments the magnetic field produced by the rotor field coils 10.

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The secondary coils along the centre line of the machine 6c are not supplied with any current as this would destructively interfere with the magnetic field within the machine, hence no cross or dot index. In order for the secondary stator coils 6 to constructively add to the magnetic field produced by the rotor field coils 10, the current direction through them needs to be switched as a function of the rotor’s 11 position.

FIG. 3 illustrates how the current through the secondary stator coils 6 is switched as a function of rotor position. Referring to the label A on FIG. 3, which highlights a specific secondary stator coil, it can be seen that the current direction through that coil changes from out of the page to zero to into the page as the rotor moves from position 1 to position 2 to position 3. This ensures that the magnetic field produced by the secondary stator coils 6 changes orientation in order to constantly align itself with the rotating magnetic field produced by the rotor field coils 9 as the rotor 11 rotates.

FIG. 4 is an example graph of the current applied to the secondary coil marked by label A versus the rotor position in degrees in FIG. 3. The purpose of FIG. 4 is to show how the sign of the current, which indicates direction, changes as function of rotor position. Note that this embodiment is not limited to the step function waveform shape shown in FIG. 4.

In practice, a number of waveform shapes could be applied to the secondary coils so long as their sign changes as a function of rotor position to ensure that their magnetic field constructively adds to the rotor’s magnetic field. An example of an 25 alternate waveform shape is shown in FIG. 5. Here, the switching function has been rounded in order smooth the switching transitions. If the rate of change of current is high through a solenoid, it causes a large induced magnetic field resulting in a spike in the induced output voltage waveform. Smoothing the switching transition helps to mitigate these spikes in output voltage.

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FIG. 6 shows another embodiment of the present disclosure. In this embodiment, instead of the secondary stator coils sharing the same slot as the stator armature coils as in the previously described embodiment, the secondary stator coil 12 is extruded away from the generator body 13. The magnetic flux is transferred to the generator body via a flux bridge 14. In order to ensure that the flux produced by the secondary coil 12 actually crosses the air gap 15 and links with the rotor’s magnetic flux 16, an airgap 17 within the stator return path had to be introduced.

This airgap 17 may be a magnetic flow resistance means material which comprises air or non-magnetic material. This airgap 17 ensures that the flux produced by the secondary coil 12 does not by-pass the rotor and in turn forces the flux to intersect the armature windings 18 resulting in a greater induced output voltage.

The advantage of this embodiment is that the secondary stator coil 12 has its own magnetic support structure 19 that is physically removed from that of the generator’s 13. This allows for ease of cooling for the secondary coil 12 as well as ease of maintenance as the generator does not have to be shut down while the secondary coil is being maintained. In order for the secondary coil to constructively add to the magnetic flux produced by the rotor 16, a similar switching algorithm to the previous embodiment has to be used.

FIG. 7 shows how the current direction, described using the dot and cross index, in the secondary stator coil 12 remains the same from positions 1 to 3 but as the rotor 16 goes through the 90° position, the current direction changes, as can be seen by 25 comparing positions 3 and 4. The change in current direction within the secondary coil 12 is necessary in order to ensure that the flux produced by the secondary coil aligns with that produced by the rotor 16. As the orientation of the rotor 16 changes, so does its magnetic flux, hence the required change in orientation of the secondary coil’s 12 magnetic flux, to ensure that they comply with each other.

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The electric generators of FIGS 2-3 have the secondary windings arranged circumferentially around the stator. The stator may function as a back-iron to provide a return path for the magnetic field/flux. However, in FIGS 6-13, the secondary windings are provided outside and spaced from the stator. Arms or cores of magnetically permeable material are then included to provide a return path from the secondary windings to the stator so that the secondary magnetic field may interact with the rotor.

Some embodiments will have bigger magnetic field strength generated outside the rotor when there are two North poles feeding magnetic field into the return path, and others will have bigger magnetic field generated in the rotor to encourage the flow of the magnetic flux to the rotor. Where there are two north poles feeding magnetic field into the return path, the electromagnet will have two coils with two different current directions to create the two north poles at the two ends with a south pole in the middle whose field inside the magnet will flow/orientate towards the north poles in each direction, the opposite of above where there are two south poles on both ends connecting to north pole in the return path and one north pole in the middle connecting to the south pole in the return path where both the south pole field will flow to.

Some embodiments are electric generator with more poles e.g. an 8-pole electric generator with four north poles of outside electromagnets connecting to the four south poles of the electric generator. The magnetic field generated outside the machine must be axial and along the permeable material into the return path. The path of the magnetic field when generated from outside the machine must be as short as possible 25 to allow for the optimum operation of the electric machine.

To make the magnetic field path of travel shorter the electromagnet coil must optimally end very close to the return path so that the magnetic field does not have a long path of travel. To guide the magnetic flux into the rotor and avoid short circuit of 30 the Magnetic flux, the return path will have multiple air-gaps as illustrated by FIG. 7 which has two air-gaps in this case. For a four-pole machine there will be four airgaps. The number of poles will match the number of air-gaps points.

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Where any or all of the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components.

CLAUSES

1. An electric generator having some of the magnetic fields generated outside the rotor with an electronic control that changes the direction of the current with respect to rotor position, into secondary coils and electromagnets to change their polarity to constructively and consistently add magnetic fields into magnetic circuit through the return path.

2. The electric generator according to clause 1 where the secondary coils to generate secondary magnetic field are situated in the outer region of the stator, where the stator is the outer member around the rotor.

3. The electric generator according to clause 1 where the secondary coils to 20 generate secondary magnetic field are situated in the inner region of the stator, where the stator is the inner member around the rotor.

4. The electric generator according to clause 1 where the (electromagnets) secondary coils to generate secondary magnetic field are situated outside the electric machine.

5. The electric Generator according to clause 4 where when the machine has X number of poles the return path will have X number of magnetic resistance means

- 122019101680 23 Dec 2019 comprising air-gaps that is the return path for a two-pole machine has two air-gaps and for a four-pole machine has four air-gaps.

6. The four-pole electric Generator according to clause 5 where there are two outside electromagnets with two North pole ends connecting to the two south poles of the electric machine.

7. The four-pole electric Generator according to clause 5 where there are two electromagnets with four north pole ends connecting to the two south poles of the electric machine.

8. The multi-pole generator with the north poles of the outside electromagnets feeding magnetic field into the generator south poles.

9. The electric generator according to clause 4 where the magnetic field generated outside the rotor is bigger than the magnetic field generated in the rotor electromagnets to enhance magnetic field flow into windings.

10. The electric generator according to clause 4 where the outside electromagnets with magnetic barrier between the electromagnets where the surface contact area with return path is covering a bigger surface area on the return path.

11. The electric generator according to clause 4 where the magnetic fields is guided by high magnetic permeable material comprising mu metal, supermalloy with bigger surface area.

12. The electric generator according to clause 1, where the electromagnets poles end very close to the return path and have high magnetic permeable material comprising mu metal.

Claims (3)

1. What is claimed is:

   1. An electric generator including:

   a rotor with a magnet arrangement configured to provide a primary magnetic field; and a stator with armature coils, the rotor being rotatable around the stator such that the primary magnetic field induces a current in the armature coils, wherein the electric generator further includes:

   secondary windings provided around or outside the stator, the secondary windings configured to generate a secondary magnetic field which interacts with, and increases or enhances, the primary magnetic field; and a switching arrangement configured to switch at least one of an intensity or a polarity of the magnetic field of the secondary windings based on a position of the rotor relative to the secondary windings.
2. 2. The electric generator as claimed in claim 1, wherein the secondary windings are configured to increase or enhance the current induced in the armature coils from around or outside the stator which operatively leads to higher current induction (and hence power generation) without increasing rotor complexity.
3. 3. The electric generator as claimed in claim 1, which is configured to supply a current to the secondary windings and wherein the current is provided or controlled by the switching arrangement.