WO2024184751A1

WO2024184751A1 - A rotor assembly

Info

Publication number: WO2024184751A1
Application number: PCT/IB2024/051951
Authority: WO
Inventors: Marco FORNO; Louis PENROSE
Original assignee: Dyson Technology Limited
Priority date: 2023-03-03
Filing date: 2024-02-29
Publication date: 2024-09-12
Also published as: GB202303141D0; GB2627813A

Abstract

A rotor assembly for a brushless permanent magnet motor. The rotor assembly comprises a rotor body and a first balancing element. The first balancing element has a first density and provides a first balance correction to the rotor assembly. The rotor assembly also includes a second balancing element having a second density, lower than the first density, and providing a second balance correction to the rotor assembly. The first balance correction being greater than the second balance correction.

Description

A ROTOR ASSEMBLY

Field of the Invention

The present invention relates to a rotor assembly for a brushless permanent magnet motor, and to a brushless permanent magnet motor comprising such a rotor assembly.

Background of the Invention

Electric machines include rotor assemblies which rotate about an axis of rotation. Manufacturing tolerances mean that it is very difficult to produce a rotor assembly which is perfectly distributed about the axis of rotation. When rotor assemblies rotate at very high speeds, any imperfections in the weight distribution may result in vibrations. Vibrations can cause noise, energy loss, and cause parts to fail. There is a need to produce rotor assemblies which are better balanced to reduce these issues.

Summary of the Invention

According to a first aspect of the present invention there is provided a rotor assembly for a brushless permanent magnet motor, the rotor assembly comprising: a rotor body; a first balancing element having a first density and providing a first balance correction to the rotor assembly; and a second balancing element having a second density, lower than the first density, and providing a second balance correction to the rotor assembly, the first balance correction being greater than the second balance correction.

A balancing ring can be used to compensate of imbalance in a rotor assembly. Material is removed from the ring at a particular point on the ring’s circumference to provide required degree of compensation. By using a balancing ring that includes two materials of different densities the overall dimensions of the ring can be reduced, and the accuracy of the balance operation improved. Alternatively, two rings can be used, each having a different density. High density materials, such as brass, allow the balancing ring to be smaller, but are difficult to adjust accurately. Low density materials, such as PEI GF30 can provide very accurate compensation, but take up a lot of space. By using both materials in one balance assembly, the high-density material can be used for the majority of the required compensation. The low-density material can be used for the final compensation. This provides the accuracy of the low-density material, with the space savings of the high-density material.

The first and second balancing elements may each have a mass being distributed non-uniformly around the axis of rotation of the rotor assembly to provide the first and second balance correction.

The non-uniform distribution of the masses may be provided by a portion of the first and second balancing elements having been removed.

The first and second balance correction may provide a total balance correction, and the first balance correction may provide at least 80% of the total balance correction.

The first and second balancing elements may be first and second balancing rings, each having have their centre axis aligned with the axis of rotation of the rotor assembly.

The removed portions may be formed at a circumferential edge of each balancing ring, and extend only partially around each balancing ring. The removed portions extend only partially in a direction parallel to the axis of rotation of the rotor assembly. The balancing rings may be cylindrical, and the removed portions may be horizontal cylindrical segments. Alternatively, the removed portions may be axially aligned cylindrical segments or radially aligned cylindrical segments. Such segments and portions are easy to remove with normal tooling.

The first and second balancing elements may be positioned directly adjacent or proximal to each other. This makes it possible to produce the balance assembly as a single element.

The rotor body comprises a shaft and the first and second balancing elements may be positioned on the shaft.

The first balancing element may have a minimum density of 2.5 kg/m³, and the second balancing element may have a maximum density of 2 kg/m³.

The first balancing element may be made of brass, and the second balancing element may be made of PEI GF30.

According to a second aspect of the present invention there is provided a method of balancing a rotor assembly, comprising: providing a rotor assembly, the rotor assembly comprising a rotor body, a first balancing element having a first density, and a second balancing element having a second density, lower than the first density; performing a first balance correction by removing material from the first balancing element; and performing a second balance correction by removing material from the second balancing element.

The method further comprises: determining a degree of balancing correction required by the rotor assembly; determining, based on the degree of balancing correction required, the amount of material to remove from the first and second balancing elements, and the location from which the material should be removed. Each of the first and second balance correction steps may involve a single material removal step.

The balancing elements may be balancing rings having their centre axes aligned with a rotational axis of the rotor assembly.

Each removal step may be performed by cutting the respective balancing elements. Alternatively, the removed portions may be obtained by drilling in a direction parallel or orthogonal to the axis of rotation of the rotor assembly.

The cutting steps each remove a portion of the balancing rings that at the circumferential edge of the balancing rings. The cutting step is performed by cutting into an axial end of each balancing ring. The cuts are either straight or circumferential cuts which extend only partially through the axial length of each respective balancing ring.

In the case of drilling, the drilling process removes a portion of the balancing rings from any one of the balancing ring faces. The drilling direction may be orthogonal to the plane of the balancing ring faces, or in radial direction.

The balancing rings may be cylindrical, and the removed portions are either horizontal cylindrical/tubular segments or axial/radial cylinders. The first and second balance corrections provide a total balance correction, and the first balance correction provides at least 80% of the total balance correction.

According to a third aspect of the present invention there is provided rotor assembly balanced in the above method.

According to a fourth aspect of the present invention there is provided a brushless permanent magnet motor comprising a rotor assembly described above. According to a fifth aspect of the present invention there is provided a haircare appliance comprising the brushless permanent magnet motor described above.

According to a sixth aspect of the present invention there is provided vacuum cleaner comprising a brushless permanent magnet motor described above.

Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate.

Brief Description of the Drawings

Figure 1 is a perspective view of a rotor assembly;

Figures 2A is a perspective view of a first balance assembly;

Figure 2B is a side view of the first balance assembly;

Figures 3A is a perspective view of a second balance assembly;

Figure 3B is a side view of the second balance assembly;

Figure 4 is a flow chart showing a first method of balancing the rotor assembly;

Figure 5 is a flow chart showing a second method of balancing the rotor assembly;

Figure 6A is a cross-section of the first balance assembly after a balancing operation;

Figure 6B is an axial end view of the first balance assembly after a balancing operation; Figures 7A shows a first balancing ring having material removed by a cutting operation;

Figures 7B shows a second balancing ring having material removed by an alternative cutting operation;

Figures 8A shows a third balancing ring having material removed by a drilling operation;

Figures 8B shows a fourth balancing ring having material removed by an alternative drilling operation;

Figure 9 is a schematic illustration of a brushless permanent magnet motor comprising the rotor assembly of Figure 1 ;

Figure 10 is a schematic illustration of a vacuum cleaner comprising the brushless permanent magnet motor of Figure 9; and

Figure 11 is a schematic illustration of a haircare appliance comprising the brushless permanent magnet motor of Figure 9.

Detailed Description of the Invention

A rotor assembly 10 is illustrated schematically in Figure 1. The rotor assembly 10 includes a rotor body 12 which is mounted on a shaft 14. The rotor body 12 and shaft 14 may be separate components which are joined together during a manufacturing process. Alternatively, the rotor body 12 and the shaft may be formed as one integral component. The rotor body 12 includes a plurality of magnets 16 provided in fixed locations around an axis of rotation 18 of the rotor assembly 10. The rotor assembly includes two bearings 20A, 20B which can be placed anywhere on the rotor. The bearings are for mounting the rotor within a motor assembly. The rotor assembly 10 also includes a balance assembly 22. The balance assembly 22 is also located on the shaft 14. The balance assembly 22 is a separate component which is joined to the shaft 14 during the manufacturing process. The rotor assembly also includes an impeller 23, located at the opposite end of the shaft 14 to the balance assembly 22. In this example, the rotor is for use in a vacuum cleaner or a hair care appliance. Further details of the balance assembly 10 and the manufacturing process will be provided below.

In one example of a brushless permanent magnet motor for medium size vacuum cleaner, the shaft is long 70mm, the impeller 23 and magnets 16 weights are 3g and 7g respectively. The contribution to the rotor unbalance from these components may be, in one example, 150mg.mm (milligrams x millimetres) from the impeller and 200mg.mm from the magnets.

In one example of a brushless permanent magnet motor for small size vacuum cleaner or for hair care appliance, the shaft is long 50mm, the impeller 23 and magnets 16 weights are 1 g and 5g respectively. The contribution to the rotor unbalance from these components may be, in one example, 60mg.mm from the impeller and 150mg.mm from the magnets.

Figures 2A and 2B show the balance assembly 22 in further detail. Here, the balance assembly 22 is shown on its own, without the rotor assembly 10 or shaft 14. However, for reference, the axis of rotation 18 is shown. The balance assembly 22 includes a first balancing element 24A and second balancing element 24B. In this example, the balancing elements are balancing rings. More accurately, the balancing rings are cylinders which are axially aligned and either bonded together or simply stacked. The balancing rings 24A, 24B may be attached to the shaft using an interference fit. Each balancing element 24A, 24B has a cylindrical hole 26A, 26B formed along the axis of rotation, with the hole 26A being aligned with the hole 26B and having the same radius. To fix the balancing elements to the shaft 14 using an interference fit, the diameter of the holes 26A, 26B may be slightly less than the diameter of the shaft 14. The cylindrical hole 26A, 26B is for mounting the balance assembly on the shaft 14. In one example, the balance assembly has an outer diameter of 8mm and an inner diameter of 3mm. However, it will be appreciated that the exact size of the balance assembly will depend on the size of the rotor assembly, amongst other considerations.

The first balancing element 24A is made from a first material having a first density. The second balancing element 24B is made from a second material having a second density. The first density is higher than the second density. In one example, the first balancing element 24A is made from brass, with a typical density of 8.47g/cm³. The second balancing element 24B may be made from PEI GF30 (30% glass fiber reinforced polyetherimide), with a typical density of 1.51 g/cm³. The balancing elements may be made from different materials with appropriate densities. For example, the first balancing element 24A may have a minimum density of 2.5 kg/m³, and the second balancing element 24B has a maximum density of 2 kg/m³. Materials other than brass and PEI GF30 may be used, depending on the application.

In an alternative embodiment, the first and second balancing elements 24A, 24B can be integrally formed as a single element. In this case, single element would have two halves, each having a different density to the other.

Figures 3A and 3B show an alternative embodiment of the present invention. A balance assembly 32 is shown in further detail. The balance assembly 32 is the same as the balance assembly 22 in most respects. For example, the balance assembly 32 includes a first balancing element 34A and second balancing element 34B. The second balancing element 34B is a balancing ring, as before. However, the first balancing element 34A includes a ring portion 38 and a flange portion 40. The flange portion is itself ring-shaped and extends away from the ring portion 38 in the direction of the axis of rotation. The ring portion 38 and the flange portion 40 share the same inner diameter and are formed integrally as one component. The outer diameter of the flange portion 40 is substantially the same as the inner diameter of the second balancing element 34B. The flange portion 40 sits within the second balancing element 34B. The first and second balancing elements 34A and 34B are bonded together such that their axial faces abut each other.

Each balancing element 34A, 34B has a cylindrical hole 36A, 36B formed along the axis of rotation, with the hole 36A being aligned with the hole 36B. As the flange portion 40 first within the cylindrical hole 36B, the cylindrical hole 36A is for mounting the balance assembly on the shaft 14.

The process of balancing the rotor assembly 10 using the balance assembly 22, 32 will now be described.

Figure 4 show the steps taken during the balancing process. The rotor assembly 10, including the first and second balancing elements 24A, 24B, is provided (step 400). A first mass is then removed from the first balancing element 24A (step 402). This may be regarded as a coarse or gross correction. The remaining mass is then removed from the second balancing element 24B (step 404). This may be regarded as a fine or refinement correction. Following these steps, the rotor assembly 10 will be completely or substantially balanced. The degree of correction should be sufficient to reduce the amount of vibration or noise to acceptable levels. For example, it is desirable to reduce the amount of unbalance to 10/20mg.mm or less.

Figure 5 shows the steps taken during the balancing process in an alternative embodiment. The rotor assembly 10, including the first and second balancing elements 24A, 24B, is provided (step 500). The steps of producing the rotor assembly 10 and mounting the balance assembly 22 on the shaft 14 are a separate process and will not be described here in detail. At this stage of production, the balance assembly 22 is located on the shaft 14, but the balance assembly has not been modified to provide any balance correction to the rotor assembly 22.

The degree of unbalance in the rotor assembly 10 is determined (step 502). This process is well known to the person skilled in the art, and uses balancing machines that are known to the person skilled in the art. The balancing machine (not shown) determines the amount of mass to remove from the balancing assembly, and where that mass should be removed (step 504). For example, the balancing machine might determine that a correction of 230 mg. mm is required on a first side of the balancing assembly. The first side may be defined as an angular distance in relation to a reference point on the rotor assembly.

The majority of the mass that is required to be removed is removed from the denser first balancing element 24A. Because the first balancing element is denser, most of the correction can be performed by removing less material than would be required with the less dense second balancing element 24B. This part of the process may be regarded as a coarse or gross correction. Because the first balancing element 22A is relatively dense, fine correction of the imbalance is difficult. The correction error is too high, and it is difficult to remove the exact amount of mass required. The remaining mass is therefore removed from the less dense second balancing element 24B. This may be referred to as a fine or refinement correction. Being less dense, the second balancing element has a lower correction error, and the precise amount of mass can more easily be removed. Conversely, the second balancing element 24B would not be very useful for the first mass removal, as the amount of material, dimensionally speaking, that would need to be removed would be excessive.

The balance correction is made by performing a cutting or drilling operation in the first and second balancing elements 24A, 24B. The cutting or drilling operation is a 2-stage operation, in that one cut or drilling operation is made in the first balancing element 24A, and one cut or drilling operation is made in the second balancing element 24B. The first stage is performed in the first balancing element 24A (step 506). As noted above, this may be regarded as a coarse or gross cut. In this example, 200 mg. mm of material are removed from the first balancing element 22A. This represents removal of 86.96% of the total required mass. In an alternative embodiment, the first cut may involve removal of at least 80% of the required mass.

The second stage is performed in the second balancing element 24B (step 510). As noted above, this may be regarded as a fine or refinement cut. Optionally, after the first stage, the residual unbalance is measured. The amount of material that needs removing from the second stage can then be confirmed or adjusted accordingly. In this example, 30 mg. mm of material are removed from the second balancing element 24B. This represents removal of 13.04% of the total mass. In an alternative embodiment, the second cut may involve removal of 20% or less of the required mass.

Figure 6A shows a cross-section through the balance assembly 10 after the balancing process is complete. Figure 6B shows an axial end view of the same balance assembly looking from the left of Figure 6A. These figures are representative of the sizes and dimensions of the balance assembly 10 and the cuts made therein and are not intend to accurately reflect these measurements.

The first balancing element 24A includes a first cut 40A. The second balancing element 24B includes a second cut 40B. As can be seen in Figure 6B, the cuts are straight cuts, formed along a chord of the circular axial end-face of each balancing element. The removed portions are horizontal cylindrical segments. These cuts may be formed from the axial end of each balancing element, or from the circumferential edge, depending on the cutting tool used. It will be appreciated that different material removal techniques may be used depending on the material of the balance assembly and the tools used. For example, as noted above, drilling may be used. Furthermore, the shape of the cut and the shape of the removed material need not be as described. Other shapes may be used, so long as the correct mass is removed at the correct location. For example, the removed portions may be axially aligned cylindrical segments or radially aligned cylindrical segments.

In this example, to remove 200 mg. mm of material from the first balancing element 24A, which is made of brass, a cut having an axial length of 0.85mm is required. The cut depth is (i.e. in the radial direction) is 2mm. To remove 30 mg. mm of material from the second balancing element 24B, which is made of PEI GF 30, a cut having an axial length of 0.72mm is required. The total axial cut length is therefore 1.57mm. This is the minimum axial length of the balance assembly 20. In practice, as the actual cut length is not known until the balancing operation, the balance assembly would need to be longer (in the axial direction) than these measurements.

These numbers can be compared with those for a balancing ring made from a single material. For example, a brass balancing ring would require a cut of 0.98mm in the axial direction. The correction error on such a cut, assuming a cut tolerance of 0.1 mm, would be 34.6 mg. mm, which is very high. A PEI GF 30 balancing ring would require a cut of 5.5mm, which is significantly longer that that achieved in the present invention.

Furthermore, providing a coarse and fine cut with a single material presents issues when that material is brass. If the first cut was intended to remove 200 mg. mm, and the second cut was intended to remove 30 mg. mm, the cut lengths (axially speaking) would be 0.85mm and 0.13mm. Given a cut tolerance of 0.1 mm, it would be very difficult to achieve the desired accuracy with brass alone. Figures 7A, 7B, 8A and 8B show various examples of cuts and holes formed in a balancing ring in order to balance the rotor assembly 10. Figure 7A shows a perspective view of the arrangement shown in Figures 6A and 6B. Figure 7B shows a balancing ring 24C, having a cut 40C formed there. Cut 40C is not a horizontal cylindrical segment. The cut does not extend along an entire chord of the circular end-face of the balancing ring. Instead, the cut 40C is a circumferential notch. The circumferential extent of the notch may vary depending on the degree of balance required.

Figure 8A shows a balancing ring 24D having drill-holes 40D formed in an axial end-face. In this example, there are three holes. The diameter, location, depth and number of holes may vary depending on the amount of balancing required. Figure 8B shows another example of a balancing ring 24E having drill-holes 40E formed in the circumferential-face of the cylinder of the balancing ring. In this example, there are three holes. Again, the diameter, location, depth and number of holes may vary depending on the amount of balancing required.

Table 1 shows the mathematical explanation of why a bi-material balance assembly provides a greater correction capability for the same total cut width than a single-material ring.

Table 2 shows the mathematical explanation of why a bi-material balance assembly provides a greater correction resolution than a greater density singlematerial ring.

The rotor assembly 10 may be paired with a stator assembly 42 to form a brushless permanent magnet motor 44, as illustrated schematically in Figure 9. The stator assembly 42 includes coils (not shown) which when driven with an appropriate voltage (here of up to around 400V), the stator assembly 42 generates a magnetic field that interacts with the magnets 16 to rotate the rotor assembly 10.

A vacuum cleaner 600 comprising the brushless permanent magnet motor 44 is illustrated schematically in Figure 10.

A haircare appliance 700 comprising the brushless permanent magnet motor 44 is illustrated schematically in Figure 11.

Claims

1. A rotor assembly for a brushless permanent magnet motor, the rotor assembly comprising: a rotor body; a first balancing element having a first density and providing a first balance correction to the rotor assembly; and a second balancing element having a second density, lower than the first density, and providing a second balance correction to the rotor assembly, the first balance correction being greater than the second balance correction.

2. A rotor assembly according to claim 1 , wherein the first and second balancing elements each having a mass being distributed non-uniformly around the axis of rotation of the rotor assembly to provide the first and second balance correction.

3. A rotor according to claim 3, wherein the non-uniform distribution of the masses is provided by a portion of the first and second balancing elements having been removed.

4. A rotor according to any preceding claim, wherein the first and second balance correction provide a total balance correction, and the first balance correction provides at least 80% of the total balance correction.

5. A rotor assembly according to claim 3, wherein the first and second balancing elements are first and second balancing rings, each having have their centre axis aligned with the axis of rotation of the rotor assembly.

6. A rotor assembly according to claim 5, wherein the removed portions are formed at a circumferential edge of each balancing ring, and extend only partially around each balancing ring.

7. A rotor assembly according to claim 6, wherein the removed portions extend only partially in a direction parallel to the axis of rotation of the rotor assembly.

8. A rotor assembly according to claims 6 or 7, wherein the balancing rings are cylindrical, and the removed portions are horizontal cylindrical segments, axially aligned cylindrical segments or radially aligned cylindrical segments.

9. A rotor assembly according to any preceding claim, wherein the first and second balancing elements are positioned directly adjacent or proximal to each other.

10. A rotor assembly according to any preceding claim, wherein the rotor body comprises a shaft and the first and second balancing elements are positioned on the shaft.

11. A rotor assembly according to any preceding claim, wherein the first balancing element has a minimum density of at least 2.5 kg/m³, and the second balancing element has a maximum density of 2 kg/m³.

12. A rotor assembly according to any preceding claim, wherein the first balancing element is made of brass, and the second balancing element is made of PEI GF30.

13. A method of balancing a rotor assembly, comprising: providing a rotor assembly, the rotor assembly comprising a rotor body, a first balancing element having a first density, and a second balancing element having a second density, lower than the first density; performing a first balance correction by removing material from the first balancing element; and performing a second balance correction by removing material from the second balancing element.

14. A method according to claim 13, further comprising: determining a degree of balancing correction required by the rotor assembly; determining, based on the degree of balancing correction required, the amount of material to remove from the first and second balancing elements, and the location from which the material should be removed.

15. A method according to claims 13 or 14, wherein each of the first and second balance correction steps involve a single material removal step.

16. A method according to any of claims 13 to 15, wherein the balancing elements are balancing rings having their centre axes aligned with a rotational axis of the rotor assembly.

17. A method according to claim 16, wherein each removal step is performed by cutting or drilling the respective balancing elements.

18. A method according to claim 17, wherein the cutting steps each remove a portion of the balancing rings that are at the circumferential edge of the balancing rings.

19. A method according to claim 18, wherein the cutting step is performed by cutting into an axial end of each balancing ring.

20. A method as claimed in claim 19, wherein the cuts are straight cuts which extend only partially through the axial length of each respective balancing ring.

21. A method according to claims 19 or 20, wherein the balancing rings are cylindrical, and the removed portions are horizontal cylindrical segments.

22. A method according to any of claims 13 to 21 , wherein the first and second balance corrections provide a total balance correction, and the first balance correction provides at least 80% of the total balance correction.

23. A rotor assembly balanced in accordance with the method of any of claims 13 to 22.

24. A brushless permanent magnet motor comprising a rotor assembly according to any of claims 1 to 12.

25. A haircare appliance comprising a brushless permanent magnet motor according to Claim 24.

26. A vacuum cleaner comprising a brushless permanent magnet motor according to Claim 24.