CN1853331A - A rotor assembly - Google Patents

Abstract

A rotor assembly (1) for an electric machine such as an electric motor, comprising a rotor (3) on a shaft (2) having bearings (7, 8) on each end portion, said Bearings are arranged to rotatably support the rotor and shaft. The assembly further comprises resilient means in the form of O-rings (15a, 15b, 16a, 16b) on the bearings, the resilient means being arranged so that in use the rotor rotates about its own center of mass. This reduces the wear on the bearings caused by the rotation of the rotor about its geometric center. The rotor assembly may be used in switched reluctance machines, such as electric motors or generators. An air propelling mechanism (4) may be provided on said shaft and arranged to, in use, draw air onto at least one bearing in order to cool the bearing.

Description

rotor assembly

technical field

The present invention relates to rotor assemblies for electric machines such as electric motors or generators.

Background technique

Electric motors are widely used in many different applications and are commonly used in domestic appliances. For example, in a vacuum cleaner, an electric motor is used to drive a fan that draws dirty air through the dirty air inlet. Dirty air flows through some form of separation equipment, such as a cyclone or bag separator, which separates dirt and dust from the airflow, and finally exhausts the air through an air outlet.

Switched reluctance machines (switched reluctance machines) have become more and more popular in recent years. In a switched reluctance motor, the stator has sets of poles that are sequentially energized to rotate the rotor under the influence of the magnetic field associated with each set of poles so that the rotor conforms to the pair of poles that are energized. By quickly switching between different pairs of poles, it is possible to spin the rotor at very high speeds.

The advantage of switched reluctance machines is that they do not use carbon brushes, which need to be replaced periodically and release carbon particles into the atmosphere when they wear out. Additionally, this motor has a reasonably long life and its speed is not limited by the need to maintain reasonable brush life.

A problem that can be encountered with conventional switched reluctance machines is that the bearings in the stator supporting the rotor are prone to wear due to the very high rotor rotational speeds that can be achieved. This can adversely affect the reliability and even lifespan of the machine.

Contents of the invention

The present invention provides a rotor assembly comprising a rotor on a shaft having bearings at each end arranged to rotatably support said rotor and shaft, said assembly further comprising said Elastic mechanism associated with bearings.

The arrangement of the elastic means associated with the bearings allows the rotor to rotate about its own center of mass, especially above the resonant speed of the rotor assembly. Thus, the rotor assembly may be arranged to rotate at a speed above the resonant speed of the rotor assembly while reducing wear on the bearings.

Additionally, bearings are positioned at the extreme ends of the rotor shaft, allowing the entire rotor assembly to be dynamically balanced in multiple planes. This feature provides the benefits of smooth, quiet operation and increased bearing life.

Preferably, each bearing is located in a bearing housing made of thermally conductive material. In conventional bearing assemblies, the bearings heat up during use and can even overheat at very high rotational speeds. This has traditionally limited the rotational speeds at which such bearings can operate. The configuration of the thermally conductive housing allows the heat generated by the bearing to be dissipated. Therefore, the bearings can operate at speeds above the resonant speed of the rotor assembly.

Preferably, the rotor assembly may also include an impeller fixedly mounted on said shaft to allow the rotor assembly to be used as a fluid propulsion device, eg in a vacuum cleaner. The impeller is located between bearing seats, preferably close to one of the bearing seats. In use, fluid drawn by the impeller is drawn onto at least one of said bearing seats before being drawn onto the impeller. This has a cooling effect on the bearing housing and further helps to dissipate heat.

Advantageously, each bearing housing contains means for supplying said bearing with lubricating fluid to ensure smooth operation of the rotor assembly throughout the life of the bearing.

The resilient means may take the form of at least one resilient fixture, such as at least one O-ring mounted to each bearing seat. Preferably, a pair of O-rings are provided on each bearing housing to create equal load distribution, and each O-ring is attached to each end of each bearing housing.

When in use in an electric machine, the rotor assembly is located in the stator assembly, and the elastic mechanism is positioned such that the rotor assembly is soft mounted in the stator assembly.

The invention is applicable to switched reluctance machines, and is particularly suitable for operating machines at high speeds, up to say 100,000 revolutions per minute.

Although the following examples describe the invention as being applied to an electric motor for driving a fan in a vacuum cleaner, it should be understood that the invention can be applied to both an electric motor and a generator, for any type of application, and is not limited to a vacuum cleaner or The field of household appliances.

Description of drawings

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 present invention;

Figure 2 is an exploded view of the rotor assembly of Figure 1;

Figure 3 is a cross-sectional view of the rotor assembly of Figures 1 and 2;

Figure 4 is a cross-sectional view of an electric motor incorporating the rotor assembly of Figures 1-3;

Figure 5 is a side view of a vacuum cleaner incorporating the motor of Figure 4; and

Figures 6a and 6b schematically illustrate the rotation of the rotor assembly of the present invention below and above the critical speed.

Detailed ways

Like reference numerals refer to like parts throughout the specification.

1-3 show a rotor assembly constructed in accordance with the present invention, generally designated by the reference numeral 1 . The rotor assembly 1 comprises a rotor shaft 2 with a rotor element 3 . The rotor element 3 comprises axially stacked steel plates arranged to form a pair of magnetic 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 tangentially around the impeller. 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 element 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 bearing housing 11 , 12 . The bearings 9 , 10 are arranged to be press-fit on the shaft and into their respective bearing housings 11 , 12 . Each bearing 9, 10 comprises an inner race 9a, 9b, an outer race 10, 10b and a plurality of ball barings (not shown) held between said races. The bearings 9 , 10 allow the rotor 3 to be rotatably mounted in the stator 13 , as shown for example in FIG. 4 .

The stator 13 comprises a set of stacked steel sheets arranged to have four inwardly projecting salient poles. Two of the poles 13a, 13b are diametrically opposed to each other as shown in FIG. 4 . Each pole supports a winding 14a, 14b which together form a first phase. Another diametrically opposite pole (not shown) likewise houses a respective winding representing the second phase. Each winding 14 includes a large number of turns (eg, more than 50 turns) of insulated electrical wire around a respective stator pole.

According to the invention, the bearing assemblies 7 , 8 are supported by elastic means 15 , 16 . In the embodiment described, the elastic means are configured in the form of O-rings 15 a , 15 b , 16 a , 16 b carried by the bearing housings 11 , 12 . Each bearing housing 11, 12 supports a pair of O-rings 15a, 15b and 16a, 16b. The location of each pair of O-rings generally corresponds to the end of the bearing within the respective housing. This flexible mounting of the rotor assembly 1 relative to the stator assembly allows the rotor assembly 1 to find its own center of rotation in use. Thus, the rotor assembly 1 rotates about its own center of mass with little offset.

Figures 6a and 6b illustrate the general principle behind which the invention is based. The continuous thick line of Figure 6a represents the rotor assembly 1 having bearing assemblies 7, 8 at its ends. The rotor assembly 1 including bearings is arranged to rotate inside the stator assembly. Figure 6a shows the state of the rotor assembly when the rotor rotates below the critical speed. Due to the off-center center of mass of the rotor assembly, the rotor shaft tends to bend slightly as the rotor assembly rotates. Of course, the extent to which the shaft is bent is exaggerated here for clarity.

Referring to Fig. 6b, when the rotor assembly of the present invention exceeds a predetermined resonance speed, the elastic mechanism on the bearing starts to work. The resilient mechanism causes the end of the shaft to orbit as indicated by the arrow. In this way, the rotor assembly rotates about its center of mass while maintaining a straight axis with less offset.

In conventional rotor assemblies with hard-mounted bearing assemblies, the rotor rotates about its geometric center above a critical speed. Since the bearings in conventional rotor assemblies are rigidly mounted and cannot orbit, the shaft tends to bend. Large unbalanced forces are exerted on the rotor assembly, which in turn cause radial stress on the bearings, thereby reducing their life.

This arrangement of the elastic means associated with the bearing assembly also results in a reduction of vibrations transmitted between the rotating and stationary parts, thus also reducing the noise generated in use of the machine. Furthermore, the passage of the rotor assembly through the resonance state is smoother than hitherto achievable.

Bearing assemblies 7 , 8 are located at extreme parts of the rotor shaft 2 . This feature helps to balance the shaft 2, especially when the rotor assembly 1 is at high speed. In conventional arrangements, where the shaft is unsupported at its ends, the ends of the shaft tend to be pushed outward when the rotor operates at high speeds. This effect is especially pronounced when the rotor is driven at speeds beyond the resonance of the rotor assembly. This also causes the central portion of the shaft to bend. Therefore, in order to prevent the rotor from contacting or even rubbing against the stator, conventional machines have a considerable gap between the rotor and the stator. As a result, this has an adverse effect on the efficiency of the machine.

In the rotor assembly of the present invention, the deflection of the rotor is less than hitherto achievable, thus enabling smaller gaps between the rotor poles and the stator than in conventional electric machines. The smaller the gap, the lower the reluctance between the stator and rotor, and thus the more power the motor can produce with a given electrical input. Thus, the efficiency of the machine is increased.

A problem previously encountered with spinning rotor assemblies above a critical speed was that the bearings became very hot. Therefore, the bearing housings 7, 8 for the bearings 9, 10 are thermally conductive. The heat generated by the bearings 9,10 is dissipated by the bearing housings 7,8. As a result, the rotor assembly can spin at very high speeds for longer periods of time without overheating the bearings.

The bearing housings 7, 8 also contain respective reservoirs 17, 18 of fluid, eg grease, arranged to provide lubrication to the bearings 9, 10 in use. Typically, the balls are coated with grease that is forced out of the races over time. Reservoirs 17, 18 of grease provide lubrication to the balls throughout their life.

The O-rings 15a, 15b, 16a, 16b have limited elasticity to ensure that the rotor rotates within the opening 19 provided therefor and does not come into contact with the stator 13 which could lead to damage to the rotor element 3, the stator or both. The gap between the rotor element 3 and the stator 13 must be small to ensure efficient torque application to the rotor element.

O-rings are manufactured from synthetic rubber materials such as ethylene propylene diene monomer (EPDM) or silicone rubber. Other suitable materials will be apparent to the skilled artisan.

The stator 13 and windings 14 are encapsulated by plastic material 20 by an injection molding process in which plastic particles are melted and then injected under pressure into the cavity to create the desired shape. During this process, the bore 19 for the rotor assembly 1 and the end cap 21 for receiving one of the bearing seats 11 are formed simultaneously.

An optical encoder disk 6 or shutter is placed on the rotor shaft 2 . The disc is associated with an optical sensor for detecting the rotational position of the disc and thus the position of the rotor element 3 . Signals from the optical sensors are transmitted to a controller (not shown). The encoder disc 6 has a smaller diameter than the rotor element 3, which facilitates the manufacture of the rotor assembly. During manufacture the elements of the rotor assembly are fitted on the shaft and the whole rotor assembly is simply inserted into the opening 19 provided for the rotor element 3 with the bearing housing 11 adjoining the end cover 21 . Previously, the individual components of the rotor assembly were individually balanced before being installed into a motor or generator, resulting in a less than ideal balance of the finished rotor assembly. However, the rotor assembly of the present invention can be completed prior to final assembly of the electric motor so that the entire rotor assembly can be balanced in one operation.

The controller is electrically connected to a drive circuit to which the windings on each stator pole section are connected. Torque is produced by sequentially passing current in each phase winding, thereby creating an attractive magnetic force between rotor and stator poles approaching each other. In each phase, current is cut off before the rotor pole closest to the stator pole for that phase rotates past the aligned position.

The impeller 4 rotates with the rotor shaft 2, thereby drawing air into the motor. The bearing assembly 8 forms a nose cone which is located at the end of the shaft 2, upstream of the impeller 4. Therefore, the air sucked by the impeller 4 will first flow through the bearing assembly 8 . The heat generated by the bearing 10 is dissipated by the thermally conductive bearing housing 12 . The air flow over the bearing assembly 8 acts to cool the bearing housing 12 .

An inlet 22 for the second air flow of the bearing assembly 7 is also provided at the other end of the shaft. The heat generated by the bearing 9 is dissipated by the heat transfer bearing housing 11 which is cooled by the air flow from the inlet 22 .

Figure 5 shows an example of a vacuum cleaner 30 in which an electric motor may be used. The motor drives the impeller 4 to draw dirty air into the cleaner 30 through the nozzle 31 and the hose and wand assembly 32 . Dirty air enters a separator 33 which is used to separate dirt and dust from the dirty air. Separator 33 may be a cyclone separator, as shown here, or some other separator, such as a dust bag. The clean air leaves the separator 33 before entering the motor housing located within the main body 34 of the cleaner. In the air flow path, the motor pre-filter is usually placed before the impeller to filter any fine dust particles not separated by the separator 33 .

In use, the electric motor rotates the impeller 4 at a very high speed (approximately 100,000 rpm). The suction action of the impeller 4 draws air through the cleaner. The air then flows through the bearing housing and is redirected by the impeller blades 5 through the diffuser outlet 23 and into a scroll 24 .

In the air flow path of the motor, the after-motor filter can be placed after the scroll tube 24. However, the configuration of the brushless motor reduces the requirement for such filters. The cleaned air is then exhausted from the vacuum cleaner to atmosphere via a suitable outlet.

Variations of the described embodiments will be apparent to the skilled person and are intended to fall within the scope of the invention. For example, although a four pole stator, two pole rotor machine has been described, the invention is equally applicable to machines having other numbers of poles on their stators and rotors, and machines having other sizes of motors.

Alternative resilient mechanisms may be configured in the form of resilient sleeves, compression springs or shock absorbers such as for the housing. The elastic mechanism may be integrated with the bearing housing, or may be provided between the bearing and the respective bearing housing.

The rotor assembly of the present invention applies equally to electric motors and generators, not necessarily the switched reluctance type, and can be used in appliances other than household vacuum cleaners, such as lawnmowers, air conditioners, hand dryers and water pumps.

Claims (19)

1. A rotor assembly comprising a rotor on a shaft having bearings on each end arranged to rotatably support the rotor and shaft, the assembly further comprising a Associated elastic body. 2. The rotor assembly of claim 1, wherein said bearings are located at each extreme end of said shaft. 3. A rotor assembly as claimed in claim 1 or 2, wherein the bearings are located in respective bearing housings and the resilient means is attached to each bearing housing. 4. The rotor assembly of claim 3, wherein at least one of said bearing seats is thermally conductive. 5. A rotor assembly as claimed in claim 3 or 4, wherein at least one of said bearing housings further contains a reservoir of lubricating fluid for the respective bearing. 6. A rotor assembly as claimed in claim 3, 4 or 5, further comprising an impeller mounted in fixed relation on said shaft between said bearing housings such that in use pumped by said impeller Suction fluid is attracted to at least one of said bearing seats. 7. A rotor assembly as claimed in any one of claims 3-6, wherein the resilient mechanism comprises an O-ring on each bearing seat, the O-ring being integrally concentric with the shaft. 8. A rotor assembly as claimed in any one of claims 3-6, wherein said resilient mechanism comprises a pair of O-rings on each bearing housing, said O-rings being integrally concentric with said shaft, And each pair of rings is arranged at a position substantially corresponding to a respective end of the bearing within the housing. 9. A rotor assembly as claimed in any preceding claim, wherein the resilient means is of rubber material. 10. A rotor assembly substantially as hereinbefore described with reference to or as shown in the accompanying drawings. 11. An electric machine comprising a rotor assembly as claimed in any preceding claim. 12. An electric machine as claimed in claim 11, further comprising a stator in which said rotor assembly is located, said resilient mechanism interposed between said bearing and said stator. 13. An electric machine as claimed in claim 11 or 12, further comprising means for detecting the rotational position of the rotor relative to the stator. 14. An electric machine as claimed in claim 13, further comprising electrical windings on the stator and a control mechanism arranged to energize the windings in dependence on a signal from the position detection mechanism. 15. An electric machine as claimed in any one of claims 11-14 in the form of a high speed electric motor. 16. An electric machine substantially as hereinbefore described and as hereinbefore described with reference to or as shown in the accompanying drawings. 17. A cleaning appliance comprising a rotor assembly as claimed in any one of claims 1-10. 18. A cleaning implement comprising an electric machine as claimed in any one of claims 11-16. 19. A cleaning implement substantially as hereinbefore described with reference to or as shown in the accompanying drawings.

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