US20060210414A1

US20060210414A1 - Turbomolecular vacuum pump

Info

Publication number: US20060210414A1
Application number: US10/554,804
Authority: US
Inventors: James Haylock; Ulrike Martina; Nigel Schofield
Original assignee: BOC Group Ltd
Current assignee: Edwards Ltd
Priority date: 2003-04-28
Filing date: 2004-04-01
Publication date: 2006-09-21
Also published as: WO2004097223A1; GB0309644D0

Abstract

A turbomolecular pump comprises a pumping mechanism and an electric motor (10) for driving the pumping mechanism. The electric motor comprises a rotor (14), a stator and stator windings (20). The stator comprises a yoke [18(1), 18(2)] and a plurality of teeth (16) projecting from the yoke and carrying the stator windings. The yoke and the teeth are made of a non-sintered magnetic powder material in which the powder particles are bonded by an electrically insulating binder.

Description

The invention relates to turbomolecular vacuum pumps and to high-speed electric motors for driving turbomolecular vacuum pumps.
High-speed motors for turbomolecular vacuum pumps must be capable of operating at very high speeds in difficult operating conditions. Although the motor speed may be as low as 20,000 rpm, typically, the motor speed will be in excess of 50,000 rpm and may be up to 100,000 rpm.
An electric motor driving a turbomolecular pump is mounted within the pump casing in an area which experiences high vacuum. This gives rise to difficulties in cooling the rotor since the sole heat path for conduction of the heat generated when in use, is through the bearings of the shaft carrying the rotor and only a small amount of heat can be dissipated by radiation into the process gas. For this reason, brushless permanent magnet motors are usually used. In order to reduce eddy current losses and smooth the output of such motors, their stators conventionally have six or more teeth with distributed windings. These motors are specifically designed for application in turbomolecular vacuum pumps and their relatively complicated structure makes them expensive to produce, especially as the volumes required for use with turbomolecular pumps are relatively low. In particular, forming the windings in the limited spaces available within conventional stator designs is a difficult and expensive process involving dedicated machinery.
An important consideration in the design of motors for turbomolecular pumps has been the need to minimise the thickness of the rotor magnet so as to maintain a relatively large diameter shaft to provide the stiffness necessary for high-speed rotation.
A further problem with motors for turbomolecular pumps is outgassing from the components of the motor. It is known that gas becomes lodged on or below the surfaces of the pump components and when the surfaces are placed under vacuum, gas evolves from those surfaces. This generation of gas by desorbtion is known as known outgassing. Outgassing is an increasingly important factor when pumps work at higher levels of vacuum and it is desirable to reduce outgassing levels to the extent this may be possible.
It is an object of the invention to provide an electric motor suitable for use in a turbomolecular pump that is more economical to produce than conventional motors and/or provide improved outgassing performance compared with conventional motors and/or at least provide an alternative choice of motor design.
The present invention provides a turbomolecular vacuum pump comprising a pumping mechanism and an electric motor for driving the pumping mechanism, the electric motor comprising a rotor, a stator and stator windings, the stator comprising a yoke and a plurality of teeth projecting from the yoke and carrying the stator windings, the yoke and the teeth being made of a non-sintered magnetic powder material in which the powder particles are bonded by an electrically insulating binder.
The invention also includes an electric motor for a turbomolecular vacuum pump, the motor comprising a stator having a plurality of teeth that have respective radially innermost surfaces that define an inside diameter of the stator, a motor shaft and a rotor comprising a permanent magnet mounted on a portion of the motor shaft, wherein a ratio of an outside diameter of the motor shaft portion and the inside diameter of the stator is less than or substantially equal to 2:3.
The invention also includes an electric motor for a turbomolecular vacuum pump, the motor comprising a stator having a plurality of teeth that each have a curved radially innermost surface and are arranged so as to define a substantially circular through-passage, a motor shaft and a rotor disposed in the through-passage and mounted on the motor shaft, the rotor having an outside diameter and comprising a permanent magnet that defines a through hole by which the rotor is mounted on the motor shaft and wherein the through hole defines an inside diameter of the rotor and a ratio of the inside diameter of the rotor and the outside diameter of the rotor is less than or substantially equal to 2:3.
The invention also includes an electric motor for a turbomolecular pump, the motor comprising a stator, a rotor and a motor shaft, the stator comprising three teeth made of a powdered magnetic material and each carrying an electrical winding, the teeth having concave radially innermost surfaces and being arranged such that the concave surfaces define a substantially circular passage for the rotor, the passage having a diameter, the rotor being mounted directly on the motor shaft and comprising a core made of a polymer bonded magnetic material and defining an axially extending through-passage for the motor shaft, the rotor further comprising a reinforcing sleeve for the core, the rotor being mounted on the motor shaft, which motor shaft has a diameter, and wherein a ratio of the motor shaft diameter to the passage diameter is less than or substantially equal to 2:3.
The invention also includes a turbomolecular vacuum pump comprising a casing having a pump inlet and a pump outlet, a pumping mechanism housed in the casing and connected with the inlet and the outlet and an electric motor housed in the casing and connected with the pumping mechanism, the electric motor comprising a motor shaft having a diameter, a rotor mounted directly on the motor shaft and consisting of an annular core and a reinforcing sleeve for the core, the core being a polymer bonded magnet and the sleeve being made of a material having low electrical conductivity, and a stator comprising three teeth held in a two-piece yoke and each carrying a wound non-distributed electrical winding, the teeth being made from a magnetic powder material and each comprising an arcuate portion having a concave surface and a convex surface opposite the concave surface and a projection projecting radially from the convex surface, the concave surfaces being arranged to define a substantially circular passage having an axis and a diameter and the rotor and motor shaft being coaxially disposed in the passage, wherein a ratio of the motor shaft diameter to the passage diameter is less than or equal to 2:3.
The invention also includes an electric motor for a turbomolecular pump, the motor comprising a stator having a plurality of teeth for carrying respective electrical windings, a motor shaft comprising a hollow member made of a material having low electrical conductivity and a rotor comprising a permanent magnet housed in the hollow member.
The invention also includes an electric motor for a turbomolecular pump, the motor comprising a stator having a plurality of teeth for carrying the motor windings and a tooth holding unit by which the teeth are supported, the teeth and the tooth holding unit being made of a magnetic powder material.
In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the drawings, in which:
FIG. 1 is a schematic representation of a turbomolecular pump;
FIG. 2 is an exploded perspective view of a motor for a turbomolecular pump;
FIG. 3 is a perspective view of the motor in an assembled condition with the motor shaft omitted; and
FIG. 4 is a section on line IV-IV in FIG. 2.
FIG. 1 shows a turbomolecular vacuum pump 2 that comprises a casing 4 having an inlet 5 and an outlet 6. The casing 4 houses a pumping mechanism 8 connected with the inlet 5 and the outlet 6 and an electric motor 10 for driving the pumping mechanism 8. Pumping mechanisms for turbomolecular vacuum pumps are know per se and will therefore not be described in any detail herein. The pumping mechanism could, for example, be a mechanism such as that in the EXT250 made by BOC Edwards of the United Kingdom.
Referring to FIG. 2, the motor 10 comprises a motor shaft 12, a rotor 14, a stator and stator windings 20. The stator comprises three teeth 16 and a two-piece-split yoke 18(1), 18(2).
The teeth 16 are pressed, non-sintered, components made of soft magnetic powder, in which the powder particles are bonded by an electrically insulating binder. An example of a suitable powder material provided with such a binder is Somaloy 500 made by Hoganas of Sweden. It is to be noted that although heat may be applied during the pressing process, it should be insufficient to cause fusion of the particles, which are held together by the binder.
Each tooth 16 comprises an arcuate segment 22 and a stepped projection 24 projecting from the convex side 26 of the arcuate segment. The stepped projections 24 provide a locating point for the windings 20 in the form of a first portion 28 that is shaped such that it can be received in an elongate slot 30 defined by the windings 20. The first portion 28 of the projection 24 is positioned on and extends radially outwardly from the convex side 26 of the arcuate segment 22. A second portion 32 extends radially outwardly from the first portion 28. The second portion 32 is generally circular and smaller in cross-section than the first portion 28.
The two-piece split yoke comprises two annular members 18(1), 18(2). Like the teeth 16, the annular members 18(1), 18(2) are pressed, non-sintered components made of soft magnetic powder, in which the powder particles are bonded by an electrically insulating binder. Each annular member 18(1), 18(2) has three semi-circular cut-outs 34 at its axially inner end. The cut-outs 34 are equi-spaced about the circumference of the annular members 18(1), 18(2) at 120° intervals. The arrangement is such that when the innermost side faces 36 of the annular members are brought into abutting relationship, three circular holes 40 (FIG. 2) are provided for receiving the second portions 32 of the stepped projections 24. It will be appreciated that the second portions 32 and the holes 40 are circular as a matter of convenience and that other shapes could be used if desired.
As shown in FIG. 4, when the motor 10 is assembled, the second, free end, portions 32 of the stepped projections 24 are held in the respective holes 40 defined by the opposed cutouts 34. The teeth 16 are held such that the concave radially innermost surfaces 42 of the arcuate segments form a substantially continuous circular passage extending axially through the stator. The passage defined by the concave surfaces 42 has a diameter corresponding to the radial distance R of the surfaces from the longitudinal axis of the stator and this diameter can be considered the inside diameter of the stator.
The electrical windings 20 are formed by entirely separate coils of wound wire that may be wound onto bobbins made of plastic to provide electrical insulation. The windings each have tails (not shown) by which they are connected to an a.c. electrical supply.
When the motor is assembled, the stator parts (the annular members 18(1), 18(2) and teeth 16) are preferably bonded as a unit using a suitable adhesive. This sub-assembly is then assembled into the pump and preferably then set in resin. However, if desired, other assembly techniques can be used.
The rotor 14 comprises an annular core 44 made of a magnetic material having low electrical conductivity. The core 44 may be made of ferrite, but in a presently preferred embodiment is made of a polymer bonded magnetic material. Polymer bonded magnetic materials are composites of non-conductive polymer and embedded magnetic particles. Such materials will be well known to those skilled in the art and will not be described detail herein. An example of a suitable material is Vacobond (Trade Name) made by Vacuumschmelze of Hanau, Germany. The core 44 has an axially extending through-hole 46 for receiving the motor shaft 12. The rotor 14 additionally comprises a sleeve 48 for the core 44. The sleeve serves as a containing, or reinforcing, member for the rotor and may be made of any suitable material. The sleeve is made of a material having low electrical conductivity. Preferably, in practical terms, the material is electrically non-conductive. In a preferred embodiment, the sleeve 48 is thin and made of a carbon-fibre reinforced plastic (CFRP). The sleeve need only be sufficiently thick as to provide the desired reinforcing and it is preferable that it is kept as thin as is practicable.
The diameter of the through-hole 46 essentially corresponds to the outside diameter of the motor shaft 12 so that the core 44 is a close sliding fit on the motor shaft and can be securely fixed thereon. The motor shaft diameter is selected such that the ratio of the inside diameter of the stator and the outside diameter of the motor shaft, or at least the portion of the shaft on which the rotor 14 is mounted, is not greater than 3:2. In an embodiment of the motor 10, the shaft diameter is 14 mm and the outside diameter of the rotor 14 is 21 mm. Since there is just a very small running clearance between the surfaces 42 of the teeth 16 and the outer surface of the sleeve 48 (exaggerated in FIG. 3), the inside diameter of the stator corresponds substantially to the outside diameter of the rotor. Accordingly, when the outside diameter of the rotor is 21 mm and the shaft diameter is 14 mm, this essentially corresponds to a 2:3 ratio of shaft to stator inside diameter. In another embodiment the motor shaft diameter is 13.7 mm, while in yet another embodiment, the motor shaft diameter is 10 mm, with the stator inside diameter being 21 mm in each case. In the examples given above, the sleeve thickness will be approximately 1 mm and is preferably as thin as is possible within the constraints of providing the reinforcing desired.
By reducing the diameter of the motor shaft as much as possible within the allowable limits of required stiffness, it is possible to provide a deeper magnetic core 46 (by deeper it is meant that the thickness of the core in the radial direction is increased). Increasing the distance between the stator and motor shaft in this way serves to minimise the rotor losses arising from high-speed operation of the motor. If such losses are not minimised, significant heating of the shaft can occur. In the vacuum environment found within a turbomolecular pump, it is very difficult to cool the motor shaft. A small amount of heat can be dissipated via the process gases passing through the pump, but the only other heat path is through the bearings. The consequent temperature difference imposed on the bearings is detrimental to them and results in a reduction of their useful life. By minimising the losses and therefore, the heating of the shaft, bearing life can be extended thereby potentially reducing the servicing requirement for the pump.
The use of a deeper polymer bonded magnet together with the configuration and relative positioning of the other components in the described embodiment provides the advantage of minimising eddy current losses found in the motor shaft arising from the space harmonics associated with the three tooth configuration of the motor. By using a three tooth configuration, instead of six or more teeth, as is conventional in high speed motors for turbomolecular vacuum pumps, the motor design is greatly simplified making it easier and more economic to produce.
It is envisaged that a triangular plate 49 (FIG. 3) may be fitted to an end of the two-piece yoke 18(1), 18(2) by means of screws or the like. The triangular plate would be provided with a through-hole for the motor shaft 12 and carry a sensor arranged to detect the rotational position of the shaft 12. The sensor would provide signals for use in controlling the switching of the electrical windings 20. The sensor may, for example, be a Hall sensor ring. It will, however, be understood that other sensors could be used and it is not essential that the sensor is carried by the two-piece split yoke 18(1), 18(2). One alternative that could be used if the electrical windings 20 are wound onto bobbins made of plastic, would be to provide one or more of the bobbins with a projection by means of which as sensor, such as a Hall sensor ring, can be supported.
It has been found that a stator consisting of parts made of a magnetic powder material in which the powder particles are bonded by an electrically insulating binder, is capable of providing the necessary electrical properties, while at the same time providing improvements in outgassing performance. Tests have been carried out in which the outgassing performance of an embodiment provided with such a stator was compared with that of a pump having a stator comprising stacked, or laminated steel plates. It was found that the powdered metal stator provided a five-fold improvement in outgassing performance. That is, the amount of gas evolving from the stator made of magnetic powder material was approximately one-fifth of that evolving from the equivalent stator constructed as a lamination of steel plates.
It will be appreciated that the modular construction of the stator makes the motor 10 more economic to produce than conventional motors, particularly if production levels are low, and provides considerable design flexibility. For example, forming the teeth 16 as separate components from magnetic powder material allows automated production using proven powder technology and the tailoring of the tooth shape to provide optimum electromagnetic properties. The modular construction also allows the electrical windings to be produced as individual coils 20 and then when complete, simply positioned on the teeth 16 prior to assembly of the stator. Once the coils are seated on the respective teeth, the stator can be readily assembled by clamping the required number of teeth between the annular is members of the two-piece yoke 18(1), 18(2). Although it is preferred that a three tooth configuration is used, it will be recognised that the modular construction makes it straight forward to produce motors having a different number of teeth so that again the freedom to design a motor having particular performance characteristics is enhanced. Thus, when compared with conventional motors used in turbomolecular vacuum pumps, the motor 10 has the twin advantage that it can be produced economically in small volumes and the design details can be easily modified to tailor the performance characteristics to the particular requirements of a given application.
It will be understood that while the yoke of the embodiment is a two-piece split unit as shown, other configurations are possible. For example, the yoke could be made up of a plurality of arcuate segments that combine to form an annular part. In this case, instead of providing the cutouts in the axially facing side faces of the segments, as in the embodiment, the recesses for the teeth would be provided between the abutting end surfaces of the segments.
While it is preferred that the stator simply consists of a yoke and teeth made of a non-sintered magnetic powder material in which the powder particles are bonded by an electrically insulating binder, it is possible that the parts defining the yoke and teeth may be held in other parts not made of such a material. For example, in the embodiment, the members defining the yoke might be clamped in a sleeve made of any other suitable material. However, this is not preferred since it may reduce the improvement in outgassing performance demonstrated when the stator consists of components made of the powder material.
In the illustrated embodiment, the sleeve 48 is described as being as thin as practicable within the constraints of providing the reinforcement for the core 44 made of polymer bonded magnetic material. In an alternative embodiment, the sleeve could be extended to serve as the motor shaft while still housing the magnetic core 44. In this case, the sleeve thickness would be increased to provide the necessary shaft stiffness for high speed motor operation.
In another alternative arrangement instead of having an electrically low conductive magnet housed in an electrically low conductive sleeve, the rotor magnet may be housed in a sleeve made of an electrically highly conductive sleeve made of a material such as aluminium. In this case, it would not be essential that the magnet was made of an electrically low conductive material. With a sleeve made of an electrically highly conductive material such as aluminium, the stator flux is transmitted into the sleeve such that the eddy current losses found in the motor shaft are minimised. Since the resistivity of such a highly conductive sleeve will be low, heating of the sleeve component will be low.

Claims

1. A turbomolecular vacuum pump comprising a pumping mechanism and an electric motor for driving the pumping mechanism, the electric motor comprising a rotor, a stator and stator windings, the stator comprising a yoke and a plurality of teeth projecting from the yoke and carrying the stator windings, the yoke and the teeth being made of a non-sintered magnetic powder material in which particles in the powder material are bonded by an electrically insulating binder.

2. The pump according to claim 1 wherein the stator comprises three of the teeth.

3. The pump according to claim 1, wherein the stator windings are non-distributed windings.

4. The pump according to claim 1 wherein the rotor comprises a permanent magnet.

5. The pump according to claim 4 wherein the permanent magnet comprises a material having low electrical conductivity.

6. The pump according to claim 5 wherein the permanent magnet is a polymer bonded magnet.

7. The pump according to claim 6 wherein the rotor comprises a reinforcing sleeve for the polymer bonded magnet.

8. The pump according to claim 7 wherein the sleeve comprises a material having low electrical conductivity.

9. The pump according to claim 8 wherein the sleeve comprises a carbon reinforced plastics material.

10. The pump according to claim 4 wherein the rotor further comprises a sleeve comprising an electrically highly conductive material.

11. (canceled)

12. The pump according to claim 1 wherein at least one of the plurality of teeth has a concave radially innermost surface and a projection projecting radially outwardly with respect to a corresponding one of the radially innermost surfaces.

13. The pump according to claim 12; wherein plurality of yoke members have side surfaces disposed in abutting relationship so that a plurality of recesses are formed between the side surfaces.

14. The pump according to claim 13 wherein each of the plurality of yoke members comprises a first annular part having a side surface and a second annular part having a side surface wherein the recesses for the projections Beware defined by at least one of the side surfaces of the first annular part and the second annular part.

15. The pump according to claim 12 further comprising a motor shaft; and wherein the substantially circular passage defines an inside diameter of the stator, the rotor has an outside diameter and defines a through hole by which the rotor is mounted on the motor shaft, the through hole defines an inside diameter of the rotor and wherein the ratio of the outside diameter of a portion of the motor shaft on which the rotor is mounted to the inside diameter of the stator is less than or substantially equal to 2:3.

16. The pump according to claim 15 wherein the rotor outside diameter is less than the stator inside diameter and wherein the inside diameter of the rotor is substantially equal to the outside diameter of the portion of the motor shaft on which the rotor is mounted.

17. The pump according to claim 12 wherein the substantially circular passage defines an inside diameter of the stator, the rotor has an outside diameter and defines a through hole by which the rotor is mounted on a motor shaft and the through hole defines an inside diameter of the rotor, wherein the ratio of the inside diameter of the rotor and to the outside diameter of the rotor is less than or equal to 2:3.

18. The pump according to claim 1; wherein the electric motor has speed from 20,000 to 100,000 rpm.

19. The pump according to claim 1 wherein at least one of the plurality of teeth has a concave radially innermost surface and a projection projecting radially outwardly with respect to a corresponding one of radially innermost surfaces, and wherein the radially innermost surfaces are arranged to form a substantially circular passage and wherein the yoke is adapted to have recesses for receiving the projections.