Rotor assembly

Abstract

A rotor assembly comprises a shaft 105 having a longitudinal axis and comprising a second portion located intermediate first and third portions along the longitudinal axis. A magnet assembly 110 is mo

Description

BACKGROUND The rotor assembly of an electric motor is typically mounted to a static frame by bearings. The rotor assembly may comprise overhangs, in which components of the rotor assembly are mounted to the shaft at positions beyond the span of the bearings. SUMMARY A first aspect of the present disclosure provides a rotor assembly for an electric motor. The rotor assembly comprises a shaft having a longitudinal axis and comprising a first portion, a second portion, and a third portion. The second portion is located intermediate the first portion and the third portion along the longitudinal axis. A magnet assembly is mounted to the first portion, the magnet assembly comprising a permanent magnet. A bearing assembly is mounted to the second portion, the bearing assembly comprising a first bearing and a second bearing spaced along the longitudinal axis. An impeller is mounted to the third portion. The first portion has a first diameter, the second portion has a second diameter, and the second diameter is larger than the first diameter. By locating the bearings between the impeller and the magnet assembly, an axially compact rotor assembly may be achieved. However, this arrangement produces an overhung design in which the impeller and the magnet assembly are located outside the span of the bearings. As a result, the bearings may be subject to increased loading when the rotor assembly is dropped or otherwise subjected to an impact. The magnet assembly may represent a significant proportion of the total mass of the rotor assembly. By reducing the diameter of the first portion of the shaft, the stiffness of the first portion of the shaft is reduced. Consequently, when the rotor assembly is dropped or subjected to an impact, the first portion of the rotor assembly (i.e., that portion comprising the magnet assembly) flexes more and therefore the bearings experience less radial loading. The second diameter may be at least 30% greater than the first diameter. Moreover, the second diameter may be 40-70% greater than the first diameter. The reduction in the diameter of the first portion is therefore significant. As a result, greater flexing of the first portion of the shaft may be achieved in response to a drop or impact, and therefore less radial loading may be imparted to the bearings. Additionally, a lower critical speed for the first portion of the rotor assembly may be achieved. This may be desirable where the rotor assembly operates at supercritical speeds, i.e., when the operating speed range of the rotor assembly is higher than the critical speed of the first portion of the rotor assembly. The first diameter may be no greater than 3mm. As a result, a relatively low bending stiffness may be achieved for the shaft. Additionally or alternatively, the first diameter may be no less than 1.5mm. As a result, a lower bending stiffness may be achieved whilst also ensuring that fatigue may be avoided. The magnet assembly may have a mass Ml, the first portion may have a diameter DI, and the ratio of Ml / Dl may be no less than 1 g / mm. As the ratio of Ml / Dl decreases, the first portion of the rotor assembly flexes less. By ensuring that the ratio is no less than 1 g / mm, excessive loading of the bearings in response to a drop or impact may be avoided. Additionally or alternatively, should the rotor assembly be required to operate at supercritical speeds, having a ratio that is no less than 1 g / mm may result in a lower critical speed that is sufficiently spaced from the operating speed range. The ratio may be no greater than 3 g / mm. As the ratio of Ml / Dl increases, the first portion of the rotor assembly flexes more. By ensuring that the ratio is no greater than 3 g / mm, excessive flexing of the first portion of the rotor assembly, which may lead to premature fatigue, may be avoided. The ratio of Ml / Dl may be between 2.0 g / mm and 2.5 g / mm. This may result in good but not excessive flexing of the first portion of the rotor assembly. The magnet assembly may have a mass no greater than 10 g and / or an outer diameter no greater than 10 mm. As a result, a relatively compact magnet assembly and therefore a relatively compact rotor assembly may be achieved. The magnet assembly may have a mass no greater than 6 g and / or no less than 3 g. A relatively compact arrangement may therefore be achieved without adversely affecting the electromagnetic performance. The outer diameter of the magnet assembly may be no greater than the outer diameter of the bearing assembly. The rotor assembly may then be assembled and balanced as a complete unit. Once assembled and balanced, the rotor assembly can then be inserted into a frame or the like, again as a complete unit. The first portion may have a first length, the second portion may have a second length, and the first length may be greater than the second length. Moreover, the first length may be at least 5% greater than the second length. A comparatively shorter second portion can provide a more axially compact arrangement. A comparatively longer first portion enables a longer, narrower magnet assembly to be used. As a result, a more radially compact arrangement may be achieved. Additionally or alternatively, a longer spacing may be achieved between the centre of mass of the magnet assembly and the bearing assembly. By having a longer spacing, the first portion of the rotor assembly may flex more for a given mass of magnet assembly and / or diameter of shaft. The first length may be no greater than 30 mm. The second portion, being shorter than the first portion, is then less than 30 mm. By employing these lengths for the shaft portions, an axially compact rotor assembly may be achieved. The first bearing may be proximal the magnet assembly and the second bearing may be distal the magnet assembly. The magnet assembly may have a mass Ml, the first portion may have a diameter DI, and the magnet assembly may have a centre of mass spaced longitudinally from a centre of the first bearing by a spacing SI, where Sl=kl*(Dl / Ml) and kl is no less than 3 g and / or is no greater than 10 g. The longitudinal spacing between the centre of mass of the magnet assembly and the bearing assembly also influences flexing of the first portion of the rotor assembly. As the longitudinal spacing decreases (i.e., as kl decreases), flexing decreases. If kl is less than 3 g, the load on the bearing assembly, when dropped or subjected to an impact, may be excessive. Additionally or alternatively, if the rotor assembly is intended to operate at supercritical speeds, then a kl value of less than 3g may result in a critical speed that is too high. In particular, the critical speed may sit within or be close to the operating speed range of the rotor assembly. If, on the other hand, kl is greater than 10 g, flexing of the first portion of the rotor assembly may be excessive and / or the length of the rotor assembly may be excessive. Optionally, kl may be between 4 g and 7 g, and / or SI may be between 9 mm and 15 mm. The third portion may have a third diameter, and the third diameter may be greater than the first diameter. Since the bearing assembly is provided between the first and third portions of the shaft, each of the first and third portions have respective flexible modes. The third diameter being greater than the first diameter can increase the stiffness of the third portion of the shaft and therefore increase the critical speed of the third portion of the rotor assembly. This can help ensure that the critical speed of the third portion is sufficiently distanced from the operational speed range of the rotor assembly. The third diameter may be greater than or equal to the second diameter. As a result, a relatively high stiffness may be achieved for the third portion of the shaft, which in turn further increases the critical speed of the third portion. The third diameter may be greater than the second diameter. The third diameter may be no greater than 20% of the second diameter. As a result, a higher critical speed for the third portion may be achieved without unduly increasing the overall mass of the rotor assembly. The impeller may have a mass M3, the third portion may have a diameter D3, and the ratio of M3 / D3 may be no less than 0.2 g / mm. As the ratio of Ml / Dl decreases, the third portion of the rotor assembly flexes less. By ensuring that the ratio is no less than 0.2 g / mm, a good balance may be struck between the desire for a higher critical speed and the desire for a lighter rotor assembly. Additionally or alternatively, the ratio of M3 / D3 may be no greater than 0.6 g / mm. As the ratio of M3 / D3 increases, the third portion of the rotor assembly flexes more. By ensuring that the ratio is no greater than 0.6 g / mm, a relatively high critical speed may be achieved for the third portion, and therefore the rotor assembly may operate at relatively high operating speeds that are nevertheless sufficiently distanced from the critical speed. The first bearing may be distal and the second bearing may be proximal the impeller, the impeller may have a mass M3, the third portion may have a diameter D3, and the impeller may have a centre of mass spaced longitudinally from a centre of the second bearing by a spacing S3, where S3 = k3*(D3 / M3) and k3 is no greater than 4 g. The longitudinal spacing between the centre of mass of the impeller and the bearing assembly also influences flexing of the third portion of the rotor assembly. As the longitudinal spacing increases (i.e., as kl increases), flexing increases. By ensuring that k3 is no greater than 4 g, a relatively high critical speed may be achieved for the third portion. Optionally, k3 may be no greater than 2 g and / or S3 may be no greater than 6 mm. The bearing assembly may project into an underside of the impeller. This can reduce the longitudinal spacing between the centre of mass of the impeller and the bearing assembly, which can result in a higher critical speed for the third portion of the rotor assembly. The impeller may have a mass no greater than 2 g and / or an outer diameter no greater than 30 mm. An impeller having such properties can facilitate a higher critical speed for the third portion of the rotor assembly. Optionally, the impeller may have a mass no greater than 1.5 g and / or an outer diameter no greater than 25 mm. The magnet assembly may comprise one or more balance rings. Optionally, the magnet assembly may comprise a first balance ring and a second balance ring mounted on opposite sides of the permanent magnet. The balance ring(s) may serve as a sacrificial mass from which material may be removed in order to balance the rotor assembly. Additionally, the balance ring(s) may serve to improve the securement of the permanent magnet to the shaft. For example, the permanent magnet may be affixed to both the shaft and the balance ring. The bearing assembly may comprise a spring located between the first bearing and the second bearing, and the spring may apply a preload to the first bearing and the second bearing. Preloading the bearings can improve high-speed operation by reducing excessive skidding, improving the running accuracy and / or increasing the stiffness of the bearings. The shaft may be monolithic, i.e., the first portion, the second portion and the third portion may be formed from a single body of material. The first portion of the shaft may have a diameter in the region of 1.5-3 mm and a length in the region of 10-30 mm, and the magnet assembly may have a mass in the region of 3-7 g. This then provides a relatively compact rotor assembly that is capable of operating at relatively high speeds. In particular, the rotor assembly may be used over an operating speed range in the region of 80-250 krpm. A second aspect of the present disclosure provides an electric motor comprising the rotor assembly of the first aspect of the present disclosure. The electric motor may comprise a stator assembly comprising one or more windings which, when energised, generate a magnetic field that interacts with the magnet assembly to rotate the rotor assembly relative to the stator assembly. The electric motor may have a first critical speed at which a first flexible mode of the first portion of the shaft is excited, the electric motor may be configured to operate over an operating speed range having a minimum speed, and the minimum speed may be greater than the first critical speed. Optionally, the minimum speed may be at least 25% greater than the first critical speed. The minimum speed may be greater than 80 krpm. Optionally, the minimum speed may be greater than 100 kprm. The rotor assembly may have a second critical speed at which a first flexible mode of the third portion of the shaft is excited, the electric motor may be configured to operate at an operating speed range having a maximum speed, and the second critical speed may be greater than the maximum speed. Accordingly, the electric motor can be configured to operate in an operating speed range which lies between the first critical speed and the second critical speed. Optionally, the second critical speed is at least 25% greater than the maximum speed The maximum speed may be greater than 200 krpm. Optionally, the maximum speed may be greater than 240 kprm. A third aspect of the present disclosure provides a domestic appliance comprising an electric motor according to the second aspect of the present disclosure. Optionally, the domestic appliance is a vacuum cleaner. Optionally, the domestic appliance is a haircare appliance. The domestic appliance of the third aspect of the present disclosure may be more resilient to damage from mishandling by a user. For example, should the user inadvertently drop the domestic appliance, excessive loading of the bearing assembly may be avoided, thereby improving the lifespan of the domestic appliance. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa perspective view of a rotor assembly. Figure 2 is a cross-sectional schematic view of an electric motor comprising the rotor assembly. Figure 3 is an exploded perspective view of the rotor assembly. Figure 4 is a side view of a shaft of the rotor assembly. Figure 5 is a cross-sectional view of the rotor assembly. Figure 6 is a perspective view of a vacuum cleaner comprising the electric motor. DETAILED DESCRIPTION A rotor assembly 100 according to the present disclosure is illustrated in Figure 1. A motor 10 comprising a frame 50, a stator assembly 40 and the rotor assembly 100 is illustrated schematically in Figure 2. The rotor assembly 100 is mounted to a portion of, and is rotatable relative to, the frame 50. The stator assembly 40 is also attached to the frame 50 but remains in fixed position relative to the frame 50. The rotor assembly 100 comprises a shaft 105, an impeller 120, a bearing assembly 30, and a magnet assembly 35. The bearing assembly 30 comprises a first bearing 52 a second bearing 54, and a spring 56 located between the two bearings, which applies a preload to each of the bearings 52,54. The magnet assembly 35 comprises a permanent magnet 110 located between a first balance ring 112 and a second balance ring 114. Considering the rotor assembly 100 in further detail, the rotor assembly 100 is depicted in exploded view in Figure 3, and the shaft 105 is illustrated in isolation in Figure 4. The shaft 105 comprises, in sequence along a longitudinal axis L of the shaft 105, a first portion 105a, a second portion 105b adjacent the first portion 105a, and a third portion 105c adjacent the second portion 105b. The second portion 105b is intermediate the first portion 105a and the third portion 105c. The first portion 105a comprises a first end 108a of the shaft 105 and the third portion 105c comprises a second, opposite end 108b of the shaft 105. The shaft 105 is monolithic and is formed of stainless steel, such that the first portion 105a, second portion 105b and third portion 105c are integrally formed. The shaft 105 is generally circular in cross-section in a plane perpendicular to the longitudinal axis L. The first portion 105a of the shaft 105 has a first diameter DI of around 2.0 mm. The second portion 105b of the shaft 105 has a second diameter D2 of around 3.0 mm, and so is 50% greater than the first diameter DI. The third portion 105c of the shaft 105 has a third diameter D3 of around 3.3 mm. The third diameter D3 is the largest diameter of the shaft 105. The first diameter DI is the smallest diameter of the shaft. The shaft 105 can be considered to be a stepped cylinder due to the different diameters of the portions 105a-c of the shaft 105. The first portion 105a has a length LI of around 17.3 mm, the length being measured in a direction parallel to the longitudinal axis L. The second portion 105b has a length L2 of around 16.1 mm, and the third portion 105c has a length L3 of around 10.8 mm. The first portion 105a is therefore longer than the second portion 105b, which in turn is longer than the third portion 105c. In this example, the first portion 105a is approximately 7.5% longer than the second 105b portion. The impeller 120 is a centrifugal impeller mounted to the third portion 105c of the shaft 105. In this example, the impeller 120 is a semi-open, mixed flow impeller. The impeller 120 is formed of plastic (e.g., by injection moulding a PEEK material) and is pressed onto the third portion 105c of the shaft 105. The impeller has a mass of around 1.3 g, and has a maximum diameter of around 23.6 mm. The bearing assembly 30 is used to rotatably mount the rotor assembly 100 to the frame 50 of the motor 10. During use, the rotor assembly 100 rotates about the longitudinal axis L. The bearing assembly 30 is mounted to the second portion 105b of the shaft 105. The first bearing 52 is spaced from the second bearing 54 in a direction along the longitudinal axis L, the spacing between the first 52 and second bearing 54 defining a span of the bearing assembly 30. The first bearing 52 is located proximate a first end of the second portion 105b of the shaft 105, and the second bearing 54 is located proximate a second, opposite end of the second portion 105b. The spring 56 is a helical spring arranged between, and in contact with, the first 52 and second bearing 54. The spring 56 applies a preload to the first 52 and second bearing 54. The impeller 120 comprises a recess 122 in an underside of the impeller 120, dimensioned to accommodate a portion of the second bearing 54 such that the impeller 120 is rotatable about the longitudinal axis with the second bearing 54 positioned in the recess 122. The bearing assembly 30 thereby projects into the underside of the impeller 120 which allows a centre of mass of the impeller 120 to be closer to the bearing assembly 30, illustrated also in Figure 5. The magnet assembly 35 is mounted to the first portion 105a of the shaft 105. As noted, the magnet assembly 35 comprises a permanent magnet 110 and two balance rings 112, 114. The first balance ring 112 is located towards the first end 108a of the shaft 105. The second balance ring 112 is located towards the first bearing 52. The permanent magnet 110 is located between the first and second balance rings 52, 54. The permanent magnet 100 has a mass of around 4.0 g. The first balance ring 112 has a mass of around 0.21 g, and the second balance ring 114 has a mass of around 0.28 g. In having a bearing assembly 30 that is located between the impeller 120 and the magnet assembly 35, the rotor assembly 100 has a double-overhung design. More particularly, the rotor assembly 100 comprises a first portion (comprising the first portion 105a of the shaft 105 and the magnet assembly 35) which overhangs the bearing assembly 30 at one end, and a third portion (comprising the third portion 105c of the shaft 105 and the impeller 120) that overhangs the bearing assembly 30 at the opposite end. The magnet assembly 35 has a centre of mass which is located a distance SI of around 12.1 mm from the first bearing 52, as measured along the longitudinal axis L and illustrated in Figure 5. The impeller 120 has a centre of mass which is located at a distance S3 of around 4.1 mm from the second bearing 54, as measured along the longitudinal axis L and illustrated in Figure 5. The stator assembly 40 comprises windings (not shown), which are energised to generate a magnetic field that interacts with the permanent magnet 110 of the magnet assembly 35 to cause rotation of the rotor assembly 100. The bearing assembly 30 is mounted to the frame 50 to secure the rotor assembly 100 in position. In use, the rotor assembly 100 rotates about the longitudinal axis L and the impeller 120 generates a flow of air, or more generally a fluid flow. In use, the electric motor 10 is configured to operate at an operating speed or over an operating speed range. In this example, the electric motor 10 is configured to operate over an operating speed range of between 110 krpm and 250 krpm. The rotor assembly 100 has flexible modes of vibration which are excited when the rotor assembly 100 rotates at certain critical speeds. In particular, the first portion of the rotor assembly 100 which overhangs the first bearing 52 has a first critical speed, and the third portion of the rotor assembly 100 which overhangs the second bearing 54 has a second critical speed. When a flexible mode of the rotor assembly 100 is excited for a prolonged period, the resulting displacement or flexing places increased load on the bearing assembly 30. Additionally, the rotor assembly 100 may contact or collide with surrounding components of the electric motor 10, such as the frame 50 or stator assembly 40. It is therefore desirable that the operating speed range does not encompass the first critical speed or the second critical speed. A rotor assembly comprising a double overhang is typically designed such that the first critical speed and the second critical speed are greater than the operating speed range. This is typically be achieved by ensuring that the shaft is sufficiently stiff (e.g., the diameter is sufficiently large) such that the flexible modes of the first portion and the third portion of the rotor assembly 100 occur at speeds greater than the operating speed range. However, by employing a shaft that is relatively stiff, radial displacement of the rotor assembly 100 relative to the frame 50 can place excessive loading on the bearings 52, 54. For example, if the electric motor 10 is dropped or otherwise subjected to a sudden impact, the force transferred to the bearings 52, 54 by the rotor assembly 100 can be significant and may damage or otherwise reduce the operational lifetime of the bearings 52, 54. The mass of the magnet assembly 35, and in particular the permanent magnet 100, is relatively heavy (e.g., compared to that of the impeller 120). Accordingly, when the electric motor 10 is dropped or subjected to an impact, the first portion of the rotor assembly 100 can exert a significant force on the bearing assembly 30. This force can by reduced by reducing the mass of the magnet assembly 35 and in particular the comparatively heavy permanent magnet 110. However, reducing the mass of the permanent magnet 110 is likely to impact the performance of the electric motor 10. Rather than reducing the mass of the permanent magnet 110, the mass of the shaft 105 of the rotor assembly 100 is instead reduced. More particularly, the mass of the first portion 105a of the shaft 105 is reduced by reducing the diameter of the shaft 105 over the first portion 105a. Reducing the mass and the diameter of the first portion 105a of the shaft 105 has two benefits. First, the total mass of the first portion of the rotor assembly 100 is reduced. Second, the stiffness of the first portion 105a of the shaft 105 is reduced, i.e., the first portion 105a of the shaft 105 is more flexible. Consequently, in response to a drop or sudden impact, the first portion 105a flexes more and therefore less radial force is transferred to the bearing assembly 30. However, reducing the diameter of the first portion 105a of the shaft 105 is not without its challenges. By reducing the stiffness of the first portion 105a of the shaft 105, the first critical speed of the first portion of the rotor assembly 100 is reduced. The rotor assembly 100 is therefore designed such that the first critical speed is less than the operating speed range, whilst the second critical speed is greater than the operating speed. The electric motor 10 therefore operates in a supercritical regime, in which the operating speed range is greater than the first critical speed of the rotor assembly 100, but lower than the second critical speed. This is achieved through a number of design features of the rotor assembly 100, which will now be described. As noted, the first portion 105a of the shaft 105 has a smaller diameter than that of the second portion 105b. This in turn reduces the first critical speed. It is not only desirable that the first critical speed is lower than the operating speed range, but that there is a sufficient separation or spacing between the two. The diameter of the first portion 105a of the shaft 105 may be further reduced in order to further reduce the first critical speed. However, as the diameter of the shaft 105 is reduced, fatigue of the shaft 105 can be increasingly likely. Accordingly, in order to further reduce the first critical speed, the magnet assembly 35 is designed such that the centre of mass of the magnet assembly 35 is spaced further the bearing assembly 30. This is achieved by having a relatively long and narrow magnet assembly 35. In this particular example, the outer diameter of the magnet assembly 35 is no greater than the outer diameter of the bearing assembly 30. This then has the benefit that the rotor assembly 100 can be fully assembled and balanced and then inserted into the frame 50 of the electric motor 10. It will be appreciated from the discussion above that both the mass of the magnet assembly 35 and the diameter of the first portion 105a of the shaft 105 influence the degree of flexing of the first portion of the rotor assembly 100 and therefore the first critical speed. A heavier magnet assembly 35 will increase flexing and decrease the first critical speed. Conversely, a larger diameter shaft will decrease flexing and increase the first critical speed. The first critical speed is therefore defined in part by the ratio of these two factors, namely the mass Ml of the magnet pack 35 and the diameter DI of the first portion 105a of the shaft 105. As the ratio of Ml / Dl decreases, flexing of the first portion of the rotor assembly decreases. Consequently, the load on the bearing assembly 30 (when the electric motor 10 is dropped or subjected to an impact) increases, and the first critical speed increases. If the ratio is too low (e.g., less than 1 g / mm), the load on the bearing assembly 30 may be excessive and / or the first critical speed may be too high and may sit within or be close to the operating speed range. If, on the other hand, the ratio is too high (e.g., greater than 3 g / mm), flexing of the first portion of the rotor assembly 100 may be excessive, and could lead to premature fatigue of the shaft 105. Flexing and the first critical speed are also defined in part by the distance or spacing SI between the centre of mass of the magnet assembly 35 and the first bearing 52. As the spacing increases (i.e., as the centre of mass of the magnet assembly 35 moves further from the first bearing 52), flexing increases and the first critical speed decreases. There are therefore at least three factors that influencing flexing and the first critical speed: the spacing SI, the mass Ml of the magnet pack, and the diameter DI of the first portion 105a of the shaft. The spacing SI may be defined as Sl=kl*(Dl / Ml). As kl decreases, flexing of the first portion of the rotor assembly 100 decreases. Consequently, for the same reasons as noted in the previous paragraph, if kl is too low (e.g., less than 3 g), the load on the bearing assembly 30 (when dropped or subjected to an impact) may be excessive and / or the first critical speed maybe too high. If, on the other hand, kl is too high (e.g., greater than 10 g), flexing of the first portion of the rotor assembly 100 may be excessive and / or the length of the rotor assembly 100 may be excessive. For similar reasons to that identified above, the mass M3 of the impeller 120, the diameter D3 of the third portion 105c of the shaft, and the spacing S3 of the centre of mass of the impeller from the bearing assembly 30 all influence the second critical speed. However, rather than seeking a second critical speed that is lower than the operating speed range, the rotor assembly 100 is designed such that the second critical speed is higher than the operating speed range. This is achieved by having a comparatively low ratio of M3 / D3 and by having a relatively short spacing S3. If the ratio M3 / D3 is too high (e.g., greater than 0.6 g / mm), the second critical speed may be too low and may sit within or be close to the operating speed range. If, on the other hand, M3 / D3 is too low (e.g., less than 0.2 g / mm), this suggests that the third portion 105c of the shaft 105 is perhaps thicker (and therefore more massive) than that required for the given mass of impeller 120 and desired critical speed. The spacing S3 may be defined as S3 = k3*(D3 / M3). If k3 is too high (e.g., greater than 4 g), the second critical speed may be too low. In the present example, a relatively short spacing S3 is achieved by projecting the bearing assembly into the underside of the impeller 120. In this particular example, the electric motor 10 is configured to operate at an operating speed range of between 110 krpm and 250 krpm. The rotor assembly 100 has a first critical speed of 72 krpm and a second critical speed of 350 krpm. The first and second critical speeds are therefore spaced sufficiently far from the operating speed range. In particular, the minimum operating speed (i.e., 110 krpm) is around 53% greater than the first critical speed, and the second critical speed is around 40% greater than the maximum operating speed (i.e., 250 krpm). Although alternative operating speed ranges and critical speeds are possible, it is may be desirable to design the rotor assembly 100 such that the difference between each of the critical speeds and the operating speed range is at least 25%. It will be appreciated that a specific example of rotor assembly 100 has been described above, and that the design of the rotor assembly (e.g., the diameters of the shaft portions 105a-c, the masses of the impeller 120 and magnet assembly 35, and the spacings from the centre of masses to the bearing assembly 30) will depend on the operating speed range of the electric motor 10. In the example described above, the diameter of the third portion 105c of the shaft 105 is larger than the second portion 105b of the shaft 105. In other examples, the diameter of the third portion 105c may be the same as that of the second portion 105b. Moreover, depending on the operating speed range, the mass M3 of the impeller 120 and the spacing S3 from the bearing assembly 30, the diameter of the third portion 105c could conceivably be smaller than that of the second portion 105b. The electric motor 10 may be employed in a domestic appliance. By way of example, Figure 6 illustrates a vacuum cleaner 200 having a casing 202 that houses the electric motor 10. It will be appreciated that the electric motor 10 may be employed in other types of domestic appliance, where the electric motor is required to move a fluid, such as in a haircare appliance or a fan appliance. The above examples are to be understood as illustrative examples of the present disclosure, and further examples are envisaged. By way of example, the magnet assembly 35 may comprise just the permanent magnet or it may comprise a single balance ring rather than a pair of balance rings. In other examples, rather than a mixed-flow, semi-open centrifugal impeller, the rotor assembly may comprise a different type of impeller, such as a radial impeller or axial impeller, which may be open, semi-open, or closed.

Claims

1. A rotor assembly comprising:a shaft having a longitudinal axis and comprising a first portion, a second portion, and a third portion, the second portion located intermediate the first portion and the third portion along the longitudinal axis;a magnet assembly mounted to the first portion, the magnet assembly comprising a permanent magnet;a bearing assembly mounted to the second portion, the bearing assembly comprising a first bearing and a second bearing spaced along the longitudinal axis; andan impeller mounted to the third portion;wherein the first portion has a first diameter, the second portion has a second diameter, and the second diameter is larger than the first diameter.

2. The rotor assembly of claim 1, wherein the second diameter is at least 30% greater than the first diameter, and wherein optionally the second diameter is 40-70% greater than the first diameter.

3. The rotor assembly of claim 1 or 2, wherein the first diameter is no greater than 3 mm, and wherein optionally the first diameter is no less than 1.5 mm.

4. The rotor assembly of any previous claim, wherein the magnet assembly has a mass Ml, the first portion has a diameter DI, and the ratio of Ml / Dl is no less than 1 g / mm and / or is no greater than 3 g / mm, and wherein optionally the ratio of M1:D1 is between 2.0 g / mm and 2.5 g / mm.

5. The rotor assembly of any previous claim, wherein the magnet assembly has a mass no greater than 10 g and / or an outer diameter no greater than 10 mm, and wherein optionally the magnet assembly has a mass no greater than 6 g and / or no less than 3 g.

6. The rotor assembly of any previous claim, wherein the outer diameter of the magnet assembly is no greater than the outer diameter of the bearing assembly.

7. The rotor assembly of any previous claim, wherein the first portion has a first length, the second portion has a second length, and the first length is greater than the second length.

8. The rotor assembly of claim 7, wherein the first length is at least 5% greater than the second length.

9. The rotor assembly of claim 7 or 8, wherein the first length is no greater than 30 mm.

10. The rotor assembly of any previous claim, wherein the first bearing is proximal andthe second bearing is distal the magnet assembly, the magnet assembly has a mass Ml, the first portion has a diameter DI, the magnet assembly has a centre of mass spaced longitudinally from a centre of the first bearing by a spacing SI, SI = kl*(Dl / Ml), and kl is no less than 3 g and / or is no greater than 10 g, and wherein optionally kl is between 4 g and 7 g.

11. The rotor assembly of any previous claim, wherein the third portion has a third diameter, and the third diameter is greater than the first diameter.

12. The rotor assembly of claim 11, wherein the third diameter is greater than or equal to the second diameter, wherein optionally the third diameter is greater than the second diameter, and wherein optionally the third diameter is no greater than 20% of the second diameter.

13. The rotor assembly of any previous claim, wherein the impeller has a mass M3, the third portion has a diameter D3, and the ratio of M3 / D3 is no less than 0.2 g / mm and / or is no greater than 0.6 g / mm, and wherein optionally the ratio of M3 / D3 is between 0.3 and 0.5.

14. The rotor assembly of any previous claim, wherein the first bearing is proximal and the second bearing is distal the magnet assembly, the impeller has a mass M3, the third portion has a diameter D3, the impeller has a centre of mass spaced longitudinally from a centre of the second bearing by a spacing S3, S3 = k3*(D3 / M3), and k3 is no greater than 4 g, and wherein optionally k3 is no greater than 2 g.

15. The rotor assembly of any previous claim, wherein the bearing assembly projects into an underside of the impeller.

16. The rotor assembly of any previous claim, wherein the impeller has a mass no greaterthan 2 g and / or an outer diameter no greater than 30 mm, and wherein optionally the impeller has a mass no greater than 1.5 g and / or an outer diameter no greater than 25 mm.

17. The rotor assembly of any previous claim, wherein the magnet assembly comprises one or more balance rings, and wherein optionally the magnet assembly comprises a first balance ring and a second balance ring mounted to the first portion on opposite sides of the permanent magnet.

18. The rotor assembly of any previous claim, wherein the bearing assembly comprises a spring located between the first bearing and the second bearing, and the spring applies a preload to the first bearing and the second bearing.

19. An electric motor comprising the rotor assembly of any previous claim.

20. The electric motor of claim 19, wherein the rotor assembly has a first critical speed at which a first flexible mode of the first portion of the shaft is excited, the electric motor is configured to operate over an operating speed range having a minimum speed, and the minimum speed is greater than the first critical speed, and wherein optionally the minimum speed is at least 25% greater than the first critical speed.

21. The electric motor of claim 20, wherein the minimum speed is greater than 80 krpm, and wherein optionally the minimum speed is greater than 100 krpm.

22. The electric motor of claim 20 or 21, wherein the rotor assembly has a second critical speed at which a first flexible mode of the third portion of the shaft is excited, the electric motor is configured to operate over an operating speed range having a maximum speed, and the second critical speed is greater than the maximum speed, and wherein optionally the second critical speed is at least 25% greater than the maximum speed.

23. The electric motor of claim 22, wherein the maximum speed is greater than 200 krpm, and wherein optionally the maximum speed is greater than 240 krpm.5 24. A domestic appliance comprising an electric motor as claimed in any one of claims19 to 23, wherein optionally the domestic appliance is a vacuum cleaner or a haircare appliance.

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