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  Turbomolecular vacuum pumpUSPTO Application #: 20060210414Title: Turbomolecular vacuum pump 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. (end of abstract)
Agent: The Boc Group, Inc. - Murray Hill, NJ, US Inventors: James Alexander Haylock, Ulrike Martina, Nigel Paul Schofield USPTO Applicaton #: 20060210414 - Class: 417423400 (USPTO) Related Patent Categories: Pumps, Motor Driven, Electric Or Magnetic Motor, Rotary Motor And Rotary Nonexpansible Chamber Pump, Turbomolecular Pump The Patent Description & Claims data below is from USPTO Patent Application 20060210414. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The invention relates to turbomolecular vacuum pumps and to high-speed electric motors for driving turbomolecular vacuum pumps. [0002] 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. [0003] 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. [0004] 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. [0005] 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. [0006] 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. [0007] 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. [0008] 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. [0009] 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. [0010] 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. [0011] 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. [0012] 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. [0013] 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. [0014] 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: [0015] FIG. 1 is a schematic representation of a turbomolecular pump; [0016] FIG. 2 is an exploded perspective view of a motor for a turbomolecular pump; [0017] FIG. 3 is a perspective view of the motor in an assembled condition with the motor shaft omitted; and [0018] FIG. 4 is a section on line IV-IV in FIG. 2. [0019] 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. [0020] 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). [0021] 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. [0022] 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. [0023] 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.degree. 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. [0024] 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. Continue reading... Full patent description for Turbomolecular vacuum pump Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Turbomolecular vacuum pump patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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