The present disclosure relates to a rotor for an electrical machine. In particular, a rotor for an electrical machine configured to cooperate with a further rotor. The rotor is particularly of benefit to axial flux electrical machines.

Background

Electrical machines, including electric motors and electric generators, are already very widely used. However, concerns over our reliance on and the pollution caused by the fossil fuels that power internal combustion engines is creating political and commercial pressures to extend the use of electrical machines to new applications, and to expand their use in existing ones. Electrical machines are increasingly being used in vehicles, such as electric cars, motorbikes, boats and aircraft. They are also used in energy generation applications, for example generators in wind turbines.

In order to meet the needs of these applications, it will be necessary to design electrical machines that have both suitable performance properties, such as speed and torque, and high efficiency. The efficiency of electrical machines is critically important in almost all applications: it can, for example, both increase an electric vehicle’s range and decrease the required battery capacity. Decreasing the required battery capacity can in turn decrease the weight of the vehicle, which leads to further efficiency gains.

One known type of electrical machine is the axial flux electrical machine. As the name suggests, the direction of the lines of magnetic flux that are cut during the operation of an axial flux machine is parallel to the axis of rotation of the machine. Although less commonly used than radial magnetic flux machines, the use of axial flux machines is expanding. This expansion has resulted in development of varied and improved designs of the components of the electrical machines such as the rotor.

The present inventor has appreciated that it would be desirable to provide a rotor for an electrical machine that enables more efficient manufacture and assembly, while maintaining or improving consistency between electrical machines and thereby enabling potential efficiency and performance improvements of the electrical machine.

Summary of the Disclosure

Examples described herein provide a rotor for an electrical machine. The present disclosure provides a rotor for an electrical machine as defined in the appended independent claim, to which reference should now be made. Preferred or advantageous features of the disclosure are set out in the dependent sub-claims.

Throughout this disclosure, unless otherwise qualified, terms such as “radial”, “axial”, “circumferential” and “angle” are used in the context of a cylindrical polar coordinate system (r, 3, z) in which the direction of the axis of rotation of the electric motor or drive wheel is parallel to the z-axis. That is, “axial” means parallel to the axis of the rotation (that is, along the z-axis), “radial” means any direction perpendicular to the axis of rotation, an “angle” is an angle in the azimuth direction 3, and “circumferential” refers to the azimuth direction around the axis of rotation.

Terms such as “radially extending” and “axially extending” should not be understood to mean that a feature must be exactly radial or exactly parallel to the axial direction. To illustrate, while it is well-known that the Lorentz force experienced by a current carrying conductor in a magnetic field is at a maximum when the direction of the current is exactly perpendicular to the direction of the magnetic flux, a current carrying conductor will still experience a Lorentz for angles less than ninety degrees. Deviations from the parallel and perpendicular directions will therefore not alter the underlying principles of operation.

According to a first aspect of the present disclosure, there is provided a rotor for an electrical machine. The rotor comprises a rotor plate configured to receive a plurality of circumferentially distributed permanent magnets. The rotor further comprises a shaft portion comprising an axial through hole configured to engage with an output shaft of the electrical machine. A distal end of the shaft portion comprises an interlocking portion configured to cooperate with a corresponding interlocking portion of a further rotor of the electrical machine.

It will be appreciated that the present disclosure relates to a rotor for an electrical machine where, in the broadest definition, it is not essential that is comprises a plurality of circumferentially distributed permanent magnets. Therefore, it will be appreciated that the “rotor” in the broadest definition could be referred to as a “back iron”; the skilled person would understand that these terms are used interchangeably.

The rotor provides a rotor for an electrical machine that is capable of being interlocked or coupled with a further rotor of the electrical machine. The rotor advantageously provides an interlocking portion that provides a means for interlocking of the rotor to a further rotor to provide a poka-yoke feature. Poka-yoke is a term that refers to a mechanism that provides a physical indication to an equipment assembler when the equipment has been incorrectly assembled to avoid mistakes and defects. More specifically, the rotor comprises a shaft portion where the distal end of the shaft portion comprises an interlocking portion that provides a means for interlocking the rotor with a further rotor. This means for interlocking the rotor with a further rotor may allow the rotors to be more easily rotationally aligned, and may allow the axial separation of the rotor plates to be better controlled, during the assembly. This is advantageous because securing the rotor and a further rotor together in the required relative orientation and alignment may lead to improved efficiency of the electrical machine. This ensures the required magnetic flux is arranged between the plurality of permanent magnets of the rotor and a further rotor.

Preferably, the shaft portion is of a substantially cylindrical shape. An output shaft of the electrical machine is preferably configured to engage with the axial through hole of the shaft portion and as such the axial through hole will generally conform to the shape and dimensions of the output shaft. The distal end of the shaft portion comprises the interlocking portion. The proximal end of said shaft portion surrounds an axial hole of the rotor plate. The diameter of the axial through hole of the shaft portion may be substantially equal to the diameter of the axial hole of the rotor plate.

Preferably, the interlocking portion is configured to rotationally align the rotor plate with a rotor plate of a further rotor. More specifically, the interlocking portion is configured to rotationally align the plurality of circumferentially distributed permanent magnets of the rotor with a plurality of circumferentially distributed permanent magnets of a further rotor such that the opposing rotors have opposite poles facing one another i.e. the north pole of the rotor faces a south pole of a further rotor and vice versa. This advantageously generates a magnetic field with axial lines of magnetic flux between the rotor and the further rotor. Further, this alignment may improve the efficiency of the electrical machine. Alternatively, the interlocking portion may be configured to rotationally misalign the rotor plate with the rotor plate of a further rotor. As such, the corresponding permanent magnets of the opposing rotors may be circumferentially offset. A circumferential offset between the rotors of an electrical machine may advantageously reduce cogging torque.

Optionally, the interlocking portion comprises at least one slot and at least one projection. The at least one slot and the at least one projection advantageously provide a means for interlocking the rotor to a further rotor. The at least one slot and the at least one projection are at the distal end of the shaft portion. The interlocking portion may comprise a plurality of slots and a plurality of projections circumferentially distributed about the distal end of the shaft portion. However, where a plurality of slots and a plurality of projections are provided, they are arranged such that the rotor and the further rotor can only be coupled in a required relative rotational alignment. The at least one slot and the at least one projection provide a means for ensuring the rotor and the further rotor will not assemble when they are not correctly aligned. Correct alignment is where the slot of the rotor cooperates with a corresponding projection of a further rotor and vice versa. The interlocking portion comprising at least one slot and at least one projection is preferably an integral to the shaft portion. Advantageously, this may reduce the manufacturing time and may reduce the number of parts required.

Optionally, the at least one slot extends over 20 to 60 degrees of the circumference of the interlocking portion. Preferably, the at least one slot extends over 30 to 45 degrees of the circumference of the interlocking portion. Additionally, the at least one projection may extend over 20 to 60 degrees of the circumference of the interlocking portion. Preferably, the at least one projection extends over 30 to 45 degrees of the circumference of the interlocking portion. More preferably, the at least one slot and the at least one projection extend over the circumference of the interlocking portion by the same degree.

Optionally, the at least one slot is of a substantially ‘U’ shape. The slot being of a substantially ‘U’ shape provides curved corners at the points where the corresponding interlocking portion of a further rotor interacts and/or fits. This is advantageous because the curved corners will increase the strength of the part and reduce the stress concentrations at the curved corners compared to the stress concentrations that may arise at a sharp internal corner. Minimising the stress concentrations may contribute to improving the lifetime of the rotor. Further, the at least one slot may comprise rounded edges at the points where the slot intersects with the distal end of the shaft portion. That is, the at least one slot may comprise fillets at said points which will also advantageously minimises stress concentrations. Further, said fillets advantageously allow for easier coupling and/or engagement of the interlocking portion of the rotor with the corresponding interlocking portion of a further rotor as the rotor and the further rotor, to at least some extent, self-align on engagement.

When the at least one slot is of a substantially ‘U’ shape, the at least one projection is of a corresponding substantially ‘n’ shape. Similarly to the at least one slot, the substantially ‘n’ shape of the projection provides rounded corners at the points where the corresponding interlocking portion of a further rotor interacts and/or fits. Specifically, the shape of the projection provides a fillet at said points. Said fillets, or rounded corners, advantageously allow for easier coupling and/or engagement of the interlocking portion of the rotor with the corresponding interlocking portion of a further rotor as the rotor and the further rotor, to at least some extent, self-align on engagement. Further, the at least one projection may comprise curved corners at the points where the projection protrudes from the distal end of the shaft portion. That is, the at least one projection may comprise fillets at said points, these fillets may also contribute to reducing the stress concentrations, improve the strength of the part, and provide easier coupling and/or engagement. This is advantageous because the fillets will increase the strength of the part and may reduce stress concentrations compared to those produced at a sharp internal corner. Minimising the stress concentrations may contribute to improving the lifetime of the rotor.

Preferably, the interlocking portion comprises one slot and one projection. More preferably, the one slot and the one projection are substantially diametrically opposed. The one slot and the one projection being substantially diametrically opposed advantageously provides a secure interlocking fit with the corresponding interlocking portion of a further rotor. The slot and the projection being diametrically opposed rather than circumferentially adjacent enables two identical such rotors to engage, thus reducing the number of different parts to manufacture

Alternatively, the interlocking portion may comprise a keyed recess configured to engage with a corresponding keyed projection on the further rotor or the interlocking portion comprises a keyed projection configured to engage with a corresponding keyed recess on the further rotor. As such, it is clear the rotor of the present disclosure may comprise a number of different interlocking portions and a variation of means for interlocking the rotors. The present disclosure is not limited to one specific type of interlocking portion.

Optionally, the inner surface of the axial through hole comprises a plurality of keyways, or splines, configured to engage with corresponding projections, or splines, on said output shaft of an electrical machine. This provides a means fixedly mounting the rotor to the output shaft. This ensures that the rotor and the shaft rotate together relative to the stator. In other words, the rotor and said shaft are mechanically coupled such that relative rotation between the rotor and the shaft is restricted. Further, the plurality of keyways, or splines, may cover only a portion of the axial length of the axial through hole. Alternatively, the plurality of keyways may substantially extend along the entire axial length of the axial through hole.

According to a second aspect of the present disclosure, there is provided a rotor assembly for an electrical machine. The rotor assembly comprises a first rotor according to the first aspect and a second rotor according to the first aspect. The interlocking portion of the first rotor and the interlocking portion of the second rotor are configured to cooperate.

Providing a rotor assembly wherein the first rotor comprises an interlocking portion and the second rotor comprises an interlocking portion that are configured to cooperate provides an effective and simple means for coupling and aligning the two rotors. Providing means for aligning the two rotors and allows the electrical machine to be assembled with a high degree of accuracy which in turn improves the efficiency of the electrical machine. Optionally, the first rotor and the second rotor are opposed such that the shaft portion of the first rotor and the shaft portion of the second rotor are configured to set the axial distance between the permanent magnets of the opposed rotors. The shaft portions of the first rotor and the second rotor provide a means for directly controlling the axial distance between the permanent magnets of the opposed rotors. This is advantageous because it provides a simple means for directly controlling the air gap between each rotor, and therefore the permanent magnets, and the stator. The air gap is a physical gap in the electrical machine that separates each rotor and the stator and it is fundamental to the efficiency and performance of the electrical machine. The respective shaft portions are able to control the size of the air gap by the varying the length of the shaft portions. This provides a simple and effective means for controlling the size of the air gap that does not require extensive manufacturing or time. Further, the shaft portions also ensure there is a uniform air gap. Providing a uniform air gap can reduce the vibration throughout the electrical machine. This is advantageous because excess vibration can reduce the performance of the electrical machine and in turn reduce the life time of the parts of the machine.

Optionally, the interlocking portion of the first rotor comprises at least one slot and the interlocking portion of the second rotor comprises at least one projection. The at least one slot of the first rotor and the at least one projection of the second rotor are configured to cooperate to form interlocking rotors of an electrical machine. The at least one slot of the first rotor and the at least one projection of the second rotor provides a means for engaging the first rotor and the second rotor. The at least one slot and the at least one projection advantageously restrict relative rotational movement between the opposing rotors. Further, the at least slot and the at least one projection provide a means for ensuring the first rotor and the second rotor are configured to cooperate only when the respective rotors are in the required orientation. That is, the at least one slot of the first rotor and the at least one projection of the second motor are distributed on the shaft portion of the respective motor such that the slot and corresponding projection will only engage when the rotors are both in a pre-determined orientation. As discussed, the orientation will be such that the permanent magnets of opposing rotors have opposite poles or alternatively when the permanent magnets of the opposing rotors are circumferentially offset.

Alternatively, the interlocking portion of the first rotor comprises at least one projection and the interlocking portion of the second rotor comprises at least one slot.

Further alternatively, the interlocking portion of the first rotor and the interlocking portion of the second rotor each comprise at least one slot and at least one projection, wherein the at least one slot of the first rotor is configured to corporate with the at least one projection of the second rotor and wherein the at least one projection of the first rotor is configured to cooperate with the at least one slot of the second rotor to form interlocking rotors of an electrical machine. The interlocking portion of the first rotor and interlocking portion of the second rotor each comprising at least one slot and at least one projection may advantageously provide a more secure fit of the respective rotors. This in turn may provide an improved means for restricting the relative rotational movement between the first rotor and the second rotor.

Preferably, the depth of the slot and the height of the projection are substantially equal. As such, the slot and the projection are configured to cooperate such that they form a close fit.

Preferably, the first rotor and the second rotor are substantially identical. Advantageously, providing a rotor assembly comprising two identical rotors may reduce the manufacturing time, reduce the number of parts required and in turn minimise the cost of the overall assembly. Further, providing a first rotor and a second rotor which are substantially identical ensures the respective rotors cooperate to provide a secure fit when the rotors are in the required relative orientation. More preferably, the first rotor and the second rotor each comprise one slot and one projection, where the one slot and the one projection are diametrically opposed.

Alternatively, the interlocking portion of the first rotor comprises a keyed recess and the interlocking portion of the second rotor comprises a keyed projection, wherein the keyed recess of the first rotor and the keyed projection of the second rotor are configured to engage to form interlocking rotors of an electrical machine.

Optionally, the interlocking portions of the first rotor and the second rotor are configured to engage when the first rotor and the second rotor are in a single relative rotational orientation. The interlocking portions of the respective rotors ensure the first rotor and the second rotor only fit together properly in a single orientation and as a result ensures the required alignment between the permanent magnets of the first rotor and the permanent magnets of the second rotor.

Further, the interlocking portions provide a simple means for detecting undesired misalignment between the first rotor and the second rotor.

Optionally, the interlocking portions of the first rotor and the second rotor are configured to engage such that the plurality of circumferentially distributed permanent magnets of the first rotor and the opposing plurality of circumferentially distributed permanent magnets of the second rotor align. The term align refers to the relative rotational alignment between the rotor plate of the first rotor and the rotor plate of the second rotor. Preferably, the opposing rotors align such that the plurality of circumferentially distributed permanent magnets of the first rotor and the plurality of circumferentially distributed permanent magnets of the second rotor have opposite poles. This provides a magnetic field with axial lines of magnetic flux between the first rotor and the second rotor. Further, by providing opposing rotors whose opposed permanent magnets have opposite polarity, there is no need for the electrical machine to comprise a stator with a yoke. This is because the flux is unidirectional and as such there is no need for a yoke to complete the magnetic circuit. Having an electrical machine comprising a yokeless stator reduces the overall weight of the electrical machine, which is greatly beneficial in many practical applications. In addition, it improves efficiency since there are no losses attributed to a varying flux density in a yoke region.

Alternatively, the opposing rotors align such that the plurality of circumferentially distributed permanent magnets of the first rotor and the plurality of circumferentially distributed permanent magnets of the second rotor are circumferentially offset. The plurality of circumferentially distributed permanent magnets of the first rotor and the plurality of circumferentially distributed permanent magnets of the second rotor may circumferentially offset by a pitch angle of about 1 .875 degrees to 3.75 degrees. The skilled person will appreciate that this angle may vary depending on the size and number of permanent magnets. A circumferential offset between the rotors can advantageously reduce cogging torque.

According to a third aspect of the present disclosure, there is provided an electrical machine comprising: a stator comprising a plurality of conductive coils; a rotor assembly according to the second aspect of the present disclosure; and an output shaft coupled to said rotor assembly through said respective axial through holes of the first rotor and the second rotor. The electrical machine is preferably an axial flux electrical machine.

It should be appreciated that particular combinations of the various features described and defined in any aspects of the disclosure can be implemented and/or supplied and/or used independently.

Description of Specific Embodiments of the Disclosure

The disclosure will be further described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a rotor according to the present disclosure;

Figure 2 shows a further perspective view of the rotor;

Figure 3 shows a side view of the rotor;

Figure 4a shows an exploded view of the rotor assembly;

Figure 4b shows a further exploded view of the rotor assembly; Figure 5a shows a side view of the rotor assembly; and

Figure 5b shows a top view of the rotor assembly.

Specific Description

Figure 1 illustrates the main components of a rotor 100 for an electrical machine. The rotor 100 comprises a rotor plate 102 configured to receive a plurality of circumferentially distributed permanent magnets. The rotor 100 further comprises a shaft portion 106 comprising an axial through hole configured to engage with an output shaft of the electrical machine. The distal end of the shaft portion 106 comprises an interlocking portion 108. The interlocking portion 108 extends in a generally axial direction from shaft portion 106. The interlocking portion 108 is configured to cooperate with a corresponding interlocking portion 108 of a further rotor.

It will be appreciated that the present disclosure relates to a rotor for an electrical machine where, in the broadest definition, it is not essential that is comprises a plurality of circumferentially distributed permanent magnets. Therefore, it will be appreciated that the “rotor” in the broadest definition could be referred to as a “back iron”; the skilled person would understand that these terms are used interchangeably.

The interlocking portion 108 is configured to rotationally align the rotor plate 102 with a rotor plate of the further rotor. The interlocking portion 108 of the example embodiment comprises one slot 110 and one projection 112. The slot 110 and the projection 112 are diametrically opposed. The slot 110 extends towards the planar surface of the rotor plate 102. As shown clearly, the slot 110 is of a substantially U-shape. The slot 110 comprises rounded corners where the slot 110 meets the distal end of the shaft portion 106. The rounded corners form a fillet. The slot 110 extends over generally 40 degrees of the circumference of the interlocking portion 108. The projection 112 protrudes axially from the shaft portion 106. The projection 112 extends over generally 40 degrees of the circumference of the interlocking portion 108. The projection 112 is of a substantially ‘n’ shape. The points at which the projection 112 protrudes from the second longitudinal end of the shaft portion 106 are curved to form a fillet. Figure 1 further illustrates that the edges of the second longitudinal end of the shaft portion 106 are rounded and form a continuous fillet around the outer and inner circumferential edge of the shaft portion 106. The depth of the slot 110 and the height projection 112 are substantially equal.

The shaft portion 106 of the rotor 100 is of a substantially cylindrical shape. As shown in Figure 2, the proximal end of the shaft portion 106 surrounds an axial hole 200 of the rotor plate 102. The cylindrical shaft portion 106 extends in an axial direction from the axial hole 104 of the rotor plate 102. The inner diameter of the shaft portion 106 is substantially equal to the diameter of the axial hole 200 of the rotor plate 102. In this example, the inner surface of the axial hole 200 and a section of the shaft portion 106 comprise a plurality of splines 202. The plurality of splines 202 are configured to engage with corresponding splines on an output shaft of the electrical machine. Axial flux electrical machines can comprise a single shaft mechanically coupled to each rotor 100 of each axial flux electrical machine. The plurality of splines 202 on inner surface of the axial hole 200 and the shaft portion 106 assist in fixedly mounting the rotor 100 to said output shaft. As a result, the rotor 100 and the shaft will rotate together.

As is particularly clear from Figure 3, the slot 110 and the projection 112 are diametrically opposed. This enables the rotors to be substantially identical.

Figure 4a and 4b illustrate a rotor assembly 400. The rotor assembly 400 comprises a first rotor 402 and a second rotor 404. The first rotor 402 and the second rotor 404 are identical. In this example the first rotor and the second rotor are identical to the rotor of Figures 1 to 3. As seen clearly in Figure 4b, the first rotor 402 comprises one slot 406 and one projection 408 and the second rotor 404 comprises a corresponding one slot 412 and one projection 410. As such, the rotor assembly 400 comprises a pair of identical opposed rotors 402 and 404. Providing identical rotors 402 and 404, in particular the identical interlocking portions 414 and 416 improves the efficiency and ease of manufacture as there is no requirement to make a number of different variations of rotors to form the rotor assembly.

Figures 5a and 5b illustrate the rotor assembly 400 with the interlocking portion 414 of the first rotor 402 cooperating with the interlocking portion 416 of the second rotor 404. The slot 406 of the first rotor 402 is configured to cooperate or interlock or couple with the projection 410 of the second rotor 404. The projection 408 of the first rotor 402 is configured to cooperate or interlock or couple with the slot 412 of the second rotor 404. As each projection 408 and 410 and each slot 406 and 412 have the same dimensions, when the first rotor 402 and the second rotor 404 interlock there is a close fit between the respective interlocking portions 414 and 416.

Consequently, relative rotation between the first rotor and the second rotor is restricted.

The first rotor 402 and the second rotor 404 are opposed and when the interlocking portion 414 of the first rotor 402 cooperate with the interlocking portion 416 of the second rotor 404 the first rotor plate 418 and the second rotor plate 420 are a pre-determined axial distance apart. As such, the shaft portion 422 of the first rotor 402 and the shaft portion 424 of the second rotor 404 set the axial distance between the rotor plates 418 and 420 of the opposed rotors 402 and 404. In particular, the respective shaft portions 422 and 424 set the axial distance between the permanent magnets of the opposed rotors 402 and 404. This provides a means for directly controlling the air gap between the stator and each rotor 402 and 404. In particular, controlling the size of the air gap but also ensuring the air gap remains uniform. The dimensions of the shaft portions 422 and 424 can be chosen to achieve the desired air gap and as such ensure an efficient electrical machine is provided with the desired performance.

As shown clearly, the first rotor 402 and the second rotor 404 each have one slot 408 and 412 and one projection 408 and 410. Consequently, the interlocking portion 414 of the first rotor 402 and the interlocking portion 416 second rotor 404 will only interlock or engage when the first rotor 402 and the second rotor 404 are in a relative rotational orientation such that the one slot 406 of the first rotor 402 cooperates with the one projection 410 of the second rotor 404 and the one projection 408 of the first rotor 402 cooperates with the one slot 412 of the second rotor 404. In this manner, the interlocking portions 414 and 416 of the first rotor 402 and the second rotor 404 are configured to engage when the first rotor 402 and the second rotor 404 are in a single relative rotational orientation.

Although not shown in the Figures, the rotor plate 418 of the first rotor 402 and the rotor plate 420 of the second rotor 404 are configured to receive a plurality of circumferentially distributed permanent magnets. The interlocking portions 414 and 416 of the first rotor 402 and the second rotor 404 are configured to engage as shown in Figures 5a and 5b. In particular, the interlocking portions 414 and 416 are configured to engage such that the plurality of circumferentially distributed permanent magnets of the first rotor 402 and the opposing plurality of circumferentially distributed permanent magnets of the second rotor 404 align. In this example embodiment, the interlocking portions 414 and 416 engage such that the opposing permanent magnets have opposite poles. That is, a north pole of the first rotor 402 faces a south pole on the second rotor 404 and vice versa. Consequently, the magnets of the first rotor 402 and the second rotor 404 generate a magnetic field with axial lines of magnetic flux between the opposing rotors 402 and 404.

In practice, a shaft will be present along the axis of rotation and would pass through the axial hole of the first rotor 402 and the axial hole of the second rotor 404. The shaft would typically include a drive end and a non-drive end. The rotors 402 and 404 are typically fixedly mounted to the shaft. In use, the stator of the axial flux motor remains stationary and the rotors 402 and 404 and shaft rotate together relative to the stator. The interlocking portions 414 and 416 and the splines 202 ensure the first rotor 402, the second rotor 404 and the shaft rotate together relative to the stator. The interlocking portions 414 and 416 and the splines 202 ensure synchronous rotation between the rotors 402 and 404 and as a result increase the efficiency of the electrical motor. As clearly shown, due to the configuration of the interlocking portions 414 and 416 there is no need for any further coupling means such as pins to achieve the rotor assembly 400.

It will be understood that the disclosure has been described above purely by way of example, and modifications of detail can be made within the scope of the disclosure. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.