Electrical motors are widely used for many different applications and are commonly used in domestic appliances. For example, in a vacuum cleaner a motor is used to drive a fan that causes dirty air to be sucked through a dirty air inlet. The dirty air passes through some form of separation device such as a cyclonic or bag separator that separates dirt and dust from the airflow, and finally the air is exhausted from an air outlet.

Switched reluctance machines have become increasingly popular in recent years. In a switched reluctance motor, a stator has sets of poles that are sequentially energised to rotate a rotor into line with the energised pair of poles, under the influence of the magnetic fields associated with each set of poles. By rapidly switching between different pairs of poles, it is possible to cause the rotor to rotate at a very high speed.

Switched reluctance machines have an advantage in that they do not use carbon brushes, which need to be replaced periodically and which emit particles of carbon into the atmosphere as they wear down. Furthermore, the motor has a relatively long life and its speed is not limited by the need to maintain a reasonable brush life.

A problem which may be encountered with conventional switched reluctance machines, is that, owing to the very high speeds of rotor rotation achievable, the bearings that support the rotor in the stator are prone to wear. This may have a detrimental effect on the reliability, or even the lifetime, of the machine.

The invention provides a rotor assembly comprising a rotor on a shaft having a bearing on each end, the bearings being arranged to rotatably support the rotor and shaft, the assembly further comprising resilient means associated with the bearings.

The provision of resilient means associated with the bearings permits the rotor to rotate about its own centre of mass, particularly above the resonant speed of the rotor assembly. Thus, the rotor assembly may be arranged to rotate at speeds above the resonant speed of the rotor assembly with reduced wear on the bearings.

Furthermore, the positioning of the bearings at the extreme ends of the rotor shaft enables dynamic balancing of the complete rotor assembly in a plurality of planes.

This feature provides the benefit of smooth, quiet running and increased bearing life.

Preferably, each bearing is located in a housing made of a thermally conductive material. In conventional bearing assemblies, the bearings can get hot in use and, at very high rotational speeds, may even overheat. This has traditionally limited the rotational speed at which such bearings can be run. The provision of a thermally conductive housing for the bearing permits heat generated by the bearing to be dissipated. Thus, the bearing can be run at speeds above the resonant speed of the rotor assembly.

Preferably, the rotor assembly also comprises an impeller fixedly mounted on the shaft, to enable the rotor assembly to be employed as fluid-impelling apparatus in, for example, a vacuum cleaner. The impeller is located between the bearing housings, preferably adjacent one of the housings. In use, fluid pumped by the impeller is drawn over at least one of the housings before being drawn onto the impeller. This has a cooling effect on the housing and further helps to dissipate heat.

Advantageously, each housing which contains means for supplying lubrication to the bearings, to ensure smooth running of the rotor assembly throughout the lifetime of the bearings.

The resilient means may take the form of at least one resilient mount for, for example at least one o-ring attached to each housing. Preferably, a pair of o-rings is provided on each housing, to enable equal load distribution, with one ring attached to each end of each housing.

In use in an electrical machine, the rotor assembly is located in a stator assembly, the resilient means being located so as to provide a soft mounting of the rotor assembly in the stator assembly.

The invention is applicable to switched reluctance machines, and is particularly useful in such machines that operate at high speeds of, say, 100,000 revolutions per minute.

While the following embodiments describe the invention as applied to motors which are used to drive a fan in a vacuum cleaner, it will be appreciated that the invention can be applied to both motors and generators, for any type of application, and is not limited to vacuum cleaners or the field of domestic appliances.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 shows a rotor assembly constructed in accordance with the invention; Figure 2 is an exploded view of the rotor assembly of Figure 1;

Figure 3 is a sectional view of the rotor assembly of Figures 1 and 2; Figure 4 is a sectional view of a motor incorporating the rotor assembly of Figures 1 to 3; Figure 5 is a side view of a vacuum cleaner incorporating the motor of Figure 4; and Figures 6a and 6b schematically illustrate rotation of the rotor assembly of the present invention, both below and above critical speed.

Like reference numerals refer to like parts throughout the specification.

Figures 1 to 3 show a rotor assembly constructed according to the invention and indicated generally by the reference numeral 1. The rotor assembly 1 comprises a rotor shaft 2 having a rotor member 3. The rotor member 3 comprises an axially laminated stack of steel plates, arranged to form a pair of poles 3a, 3b. The shaft 2 also carries a coaxial impeller 4 having a plurality of blades 5 arranged to direct fluid flow from the shaft 2 to the periphery of the impeller in tangential directions. The shaft 2 also carries a position indicator in the form of an optical encoder disc 6, to enable the rotational position of the rotor member 3 to be determined in use.

Bearing assemblies 7,8 are provided on the shaft 2. Each bearing assembly 7,8 comprises a bearing 9,10 supported on the shaft 2 by a housing 11,12. The bearings 9,10 are arranged to press-fit onto the shaft and into their respective housings 11,12. Each bearing 9,10 comprises an inner race 9a, 9b, an outer race 10, lOb and a plurality of ball bearings (not shown) held between the races. The bearings 9,10

permit the rotor 3 to be rotatably supported in a stator 13, such as is shown in Figure 4.

The stator 13 comprises a stack of steel laminations arranged to have four inwardly projecting salient poles. Two of the poles 13a, 13b, diametrically opposite each other, are shown in Figure 4. Each pole supports a winding 14a, 14b which together form a first phase. The other diametrically opposite poles (not shown) similarly accommodate respective windings, which represent a second phase. Each winding 14 comprises a large number of turns (e. g. 50+ turns) of an insulated electrical conductor around the respective stator pole.

In accordance with the invention, the bearing assemblies 7,8 are supported by resilient means 15, 16. In this embodiment, the resilient means is provided in the form of o-rings 15a, 15b, 16a, 16b carried by the housings 11,12. Each of the housings 11,12 carries a pair of o-rings 15a, 15b and 16a, 16b. The o-rings of each pair are located at positions corresponding approximately to the end portions of the bearing within the respective housing. This soft mounting of the rotor assembly 1 against the stator assembly permits the rotor assembly 1 to find its own centre of rotation in use. Thus, the rotor assembly 1 rotates about its own centre of mass, with little excursion.

Figures 6a and 6b illustrate the general principles behind the invention. The unbroken thick line of Figure 6a represents the rotor assembly 1, having bearing assemblies 7,8 at its ends. The rotor assembly 1, including the bearings, is arranged to rotate inside a stator assembly. Figure 6a shows the condition of the rotor assembly when the rotor is spinning below critical speed. As the centre of mass of the rotor assembly is located off-centre, the rotor shaft tends to flex slightly as the rotor assembly rotates. Of course, the degree of flexing of the shaft has been exaggerated here for clarity.

With reference to Figure 6b, when the rotor assembly of the invention exceeds a predetermined speed of resonance, the resilient means on the bearings come into play. The resilient means permits the ends of the shaft to describe an orbital motion, as shown by the arrows. Thus, the rotor assembly rotates about its own centre of mass whilst retaining a straight shaft, with relatively little excursion.

In conventional rotor assemblies having hard mounted bearing assemblies, the rotor rotates about its geometric centre above critical speed. As the bearings in conventional rotor assemblies are rigidly mounted and unable to orbit, the shaft then tends to flex. Large out-of-balance forces are exerted on the rotor assembly, which, in turn, cause radial stress on the bearings, thereby reducing their lifetime.

This arrangement of resilient means associated with the bearing assemblies also provides a reduction in vibration transmitted between rotating and static components and, hence, also a reduction in noise generated by the machine in use. Furthermore, the rotor assembly passes through the resonance condition much more smoothly than was achievable hitherto.

The bearing assemblies 7,8 are located at the extreme end portions of the rotor shaft 2. This feature aids the balancing of the shaft 2, particularly at the high speeds experienced by the rotor assembly 1. In conventional arrangements, in which the shaft is unsupported at its ends, the ends of the shaft tend to be pushed outwardly when the rotor operates at high speed. This effect is particularly pronounced when the rotor is driven at speeds beyond the resonant speed of the rotor assembly. This also causes the central portion of the shaft to flex. Thus, in order to prevent the rotor from contacting or even grinding against the stator, conventional machines have a relatively large clearance between the rotor and stator. This, in turn, has a detrimental effect on the efficiency of the machine.

In the rotor assembly of the present invention, the excursion of the rotor is smaller than was achievable hitherto, and so the clearance between the rotor poles and the stator can be made smaller than in conventional electric machines. The smaller the gap, the smaller the magnetic reluctance between the stator and the rotor and hence the more power that can be generated by the motor with a given electrical input.

Thus, the efficiency of the machine is improved.

A problem that has previously been encountered with causing a rotor assembly to rotate above critical speed is that the bearings get very hot. Therefore, the housings 7,8, for the bearings 9,10 are thermally conductive. Heat generated by the bearings 9,10 is dissipated by the housings 7,8. Thus, the rotor assembly can be rotated at very high speeds for prolonged periods without the bearings overheating.

The housings 7,8 also contain respective reservoirs 17,18 of fluid, such as grease, which are arranged to provide lubrication to the bearings 9,10 in use. Typically, the ball bearings are coated with grease that, over time, gets pushed out of the races. The reservoirs 17,18 of grease supply the ball bearings with lubrication throughout their lifetime.

The o-rings 15a, 15b, 16a, 16b have a limited resilience to ensure that the rotor rotates within the aperture 19 provided for it and does not contact the stator 13, which would cause damage to the rotor member 3, the stator, or both. The clearance between the rotor member 3 and stator 13 is necessarily small to ensure torque is imparted to the rotor member efficiently.

The o-rings are manufactured from a synthetic rubber material, such as Ethylene Propylene Diene Monomer (EPDM) or silicone rubber. Other suitable materials will be apparent to the skilled person.

The stator 13 and windings 14 are encapsulated by plastics material 20 by means of an injection-moulding process, by which plastic granules are melted, then injected into a mould cavity under pressure to create the required shape. During this process, the aperture 19 for the rotor assembly 1 and an end cap 21 for receiving one of the bearing housings 11 are formed simultaneously.

An optical encoder disc 6, or chopper, is disposed on the rotor shaft 2. The disc is associated with an optical sensor arranged to detect the rotation position of the disc and, hence the rotor member 3. Signals from the optical sensor are transmitted to a controller (not shown). The encoder disc 6 has a diameter smaller than that of the rotor member 3, which facilitates manufacture of the rotor assembly. During manufacture, the components of the rotor assembly are assembled on the shaft, and the entire rotor assembly is simply slotted into the aperture 19 provided for the rotor member 3, with the housing 11 abutting the end cap 21. Previously, the individual components of the rotor assembly were balanced separately before being incorporated into the motor or generator, produced a less than ideal balance condition of the completed rotor assembly. However, the rotor assembly of the present invention may be completed before final assembly of the motor, so that the complete rotor assembly may be balanced in one operation.

The controller is electrically connected to the drive circuit, to which the windings on each of the stator pole portions are connected. Torque is produced by switching current on in each phase winding in a sequence, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position.

The impeller 4 rotates with the rotor shaft 2 and thus draws air into the motor. The bearing assembly 8 forms a nose cone located at the end of the shaft 2, upstream of

the impeller 4. Hence, air being drawn in by the impeller 4 will firstly flow over the bearing assembly 8. Heat generated by the bearing 10 is dissipated by the thermally conductive bearing housing 12. The airflow over the bearing assembly 8 serves to cool the bearing housing 12.

There is also provided an inlet 22 for a second airflow for the bearing assembly 7 at the other end of the shaft. Heat generated by the bearing 9 is dissipated by the thermally conductive housing 11, which is cooled by the flow of air from the inlet 22.

Figure 5 shows one example of a vacuum cleaner 30 in which the motor may be used. The motor-driven impeller 4 draws dirty air into the cleaner 30 via a nozzle 31 and a hose and wand assembly 32. The dirty air enters a separator 33, which serves to separate dirt and dust from the dirty air. The separator 33 can be a cyclonic separator, as shown here, or some other separator, such as a dust bag. Cleaned air leaves the separator 33 before entering the motor housing located within the main body 34 of the cleaner. A pre-motor filter is typically placed in the airflow path before the impeller to filter any fine dust particles that were not separated by separator 33.

In use, the motor rotates the impeller 4 at a very high speed (of around 100,000rpm).

The pumping action of the impeller 4 draws air through the cleaner. The air then flows over the bearing housings and is redirected by the impeller blades 5 through diffusion outlets 23 into the scroll 24.

A post-motor filter may be placed in the airflow path after the scroll 24. However, the provision of a brushless motor reduces the requirement for such a filter. The cleaned air is then exhausted from the cleaner to the atmosphere via a suitable outlet.

Variations to the described embodiments will be apparent to a skilled person and are intended to fall within the scope of the invention. For example, while a four-pole

stator, two-pole rotor machine has been described, the invention can be equally applied to machines having other numbers of poles on its stator and rotor and with motors having other dimensions.

Alternative resilient means may be provided in the form of, for example, resilient sleeves for the housing, compression springs or dampers. The resilient means may be integral with the bearing housing or may be provided between the bearing and the respective housing.

The rotor assembly of the invention is equally applicable to motors and generators, not necessarily of the switched reluctance type, and may be employed in appliances other than domestic vacuum cleaners, such as lawn mowers, air conditioners, hand dryers and water pumps.