Abstract:
A combination of a rotor and a stator assembly, the stator assembly having a first part and a second part, the first and second parts being generally concentric, the rotor being rotatably mounted in the stator assembly, the combination further including a plurality of elongate resilient damping elements which extend generally in an axial direction, and being located between and spacing apart the first part and the second part of the stator assembly.

Description:
BACKGROUND TO THE INVENTION 
     The present invention relates to a combination of a rotor and stator assembly, such as an electric motor or generator. More particularly but not exclusively the invention relates to a switched reluctance motor or generator. 
     All types of electric motors produce vibration when operating, and hence noise. In some applications the noise produced may be acceptable but in other applications it may be a significant problem. For example when a switched reluctance motor is used to power an air cycle air conditioning unit for a railway carriage, the noise is typically transmitted through the fixings into the railway carriage producing an unacceptable level of noise for the occupants of the carriage. 
     DESCRIPTION OF THE PRIOR ART 
     Efforts have been made to reduce the transmission of the noise into the carriage by the use of soundproof lagging, but has been found not to be effective on its own, since in some applications there is insufficient space for sufficient lagging to provide effective noise suppression. Also, noise can still travel along air ducting and other pipework. 
     An alternative approach is to try to reduce the vibrations generated by the motor, rather than to prevent the transmission of noise. To do so it is necessary to understand the processes which lead to the vibration. It is believed the following explains how the vibrations arise in a motor. 
     A rotor comprises a plurality of rotor pole portions which project outwardly in a radial direction and extend in an axial direction. The rotor is disposed within a stator assembly which comprises a plurality of stator pole portions which project inwardly in a radial direction and extend axially. During use of the motor the rotor rotates and each of the rotor pole portions moves in and out of alignment with each of the stator pole portions, although a clearance is always maintained between the rotor and stator pole portions. Coils are wound on each of the stator pole portions, with coils on opposing pairs connected in series to each other. Thus, when current is supplied to the coils a magnetic flux is generated between each pair of stator pole portions. This results in a magnetic attractive force between the rotor pole portions and stator pole portions as they approach one another, which can be controlled by switching the supply current in accordance with the rotational orientation of the rotor. 
     In a switched reluctance motor the current supplied to one, or more, pairs of stator pole portions is switched, or pulsed, on and off. The current is generally switched on as a pair of rotor pole portions approaches alignment with a pair of stator pole portions, but is switched off again just before alignment is achieved. Thus the magnetic attractive force is increased as the rotor and stator pole portions approach alignment, but disappears just before alignment is achieved. This sequence produces the motoring torque desired. 
     However, the one, or more, stator pole portions which are switched as described above are, as a result of the magnetic attractive force, attracted to the rotor pole portions producing inward strain within the stator assembly and the housing. When the current is switched off, and the magnetic attractive force disappears, the inward strain on the stator suddenly ceases and the housing moves back outwardly to its original position. As the switching is periodic, the forces acting on the housing are also period and the housing vibrates. 
     One known way of reducing the vibration and hence noise, is to increase the external diameter of the housing or of the stator assembly or both, however as this adds weight and size to the motor it is undesirable in many applications, and may also have cost implications. 
     Another prior art method of addressing this problem in switched reluctance motors is described in UK patent application published under number GB 2 303 745 A. In this case a plurality of stiffening rods pass through portions of the stator and penetrate into end brackets which support the rotor. 
     Another known approach to this problem in switched reluctance motors uses a semi-active vibration reduction system. Multiple pulses are applied to each phase of the motor at a period which produces anti-phase vibrations in the stator, and hence reduces the total vibrations produced. However, this approach is not suitable for motors with a high operating speed, e.g. in excess of 24,000 rpm, as in such cases there is insufficient time available to apply the correcting signals to the stator pole coils. In addition semi-active systems degrade the optimum efficiency of the motor which in some circumstances, is undesirable. 
     It is an object of the present invention to provide an improved electric motor which mitigates the above described problems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention we provide a combination of a rotor and a stator assembly, the stator assembly having a first pail and a second part, the first and second parts being generally concentric, the rotor being rotatably mounted in the stator assembly, the combination further including means located between and spacing apart the first part and the second part of the stator assembly characterised in that the means located between and spacing apart the first part and the second part includes a plurality of elongate resilient damping elements which extend generally in an axial direction. 
     The invention provides the advantage that the transmission of electromagnetic vibrations generated is reduced without degradation of the motor or generator performance. 
     Preferably the first part of the stator assembly is of generally circular internal cross-section, the second part of the stator assembly is of generally circular external cross-section, and the resilient damping elements are spaced apart between the first and second parts to maintain a generally annular space therebetween, and wherein the first and second parts and resilient damping elements extend in an axial direction. 
     Conveniently the first and second parts of the stator assembly provide a plurality of axially extending grooves each of which is adapted to receive one of the plurality of resilient damping elements. For example the first and/or second parts of the stator assembly may have respectively, internal and external surfaces which may comprise a plurality of axially extending grooves each of which is adapted to receive one of the plurality of resilient damping elements. 
     Preferably the resilient damping means is held in compression between the first and second parts of the stator assembly such that it is maintained in position, e.g. in grooves which retain the resilient damping means. 
     Alternatively, or in addition, the combination may include retaining means to retain the resilient damping means in position relative to the first and second parts of the stator assembly. For examples, the retaining means may comprise end plates with recesses to receive the ends of the resilient damping means thus to support the resilient damping means, at their ends. 
     Preferably the resilient damping elements comprise elongate elements. 
     Preferably the elongate elements are substantially circular in cross section. Alternatively the elongate elements may be substantially elliptical in cross section, or polygonal in cross section. 
     Preferably the elongate elements comprise tubes made from metal or fibre reinforced plastics material. The tubes may be filled with a resilient material. 
     Preferably the combination includes three resilient damping elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An example of an electric motor according to the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a cross sectional view of an electric motor according to the present invention, from which the rotor has been omitted for the sake of clarity; 
     FIG. 2 is a side sectional view of the motor of FIG. 1 with a rotor 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings there is shown an electric motor  10 , of the switched reluctance kind. The motor  10  has a stator assembly  11  having a first part or housing  12 , and a second part or stator  16 . The housing  12  has an internal surface  13  which defines a generally cylindrical chamber with a first radial dimension, and the chamber extends in an axial direction A. The stator  16 , has an external surface  17  and is of generally circular cross section, with a second radial dimension smaller than the first radial dimension of the chamber. The external surface  17  of the stator  16  has formed therein three axially extending grooves  18  spaced apart around the circumference, and corresponding grooves  14  are provided in the internal surface  13  of the housing  12 . Each pair of grooves  18 ,  14  provides a channel in which is located an elongate resilient damping element  20 . The grooves  18 ,  14  and the elongate resilient damping elements  20  are dimensioned so as to space apart the housing  12  and stator  16  and maintain a generally annular space  22  therebetween. 
     The elongate resilient damping elements  20  take the form of, in this embodiment metallic tubes of generally circular cross-section and of substantially the same length as the stator  16 . Contact between the tubes  20  and the internal and external surfaces  13 ,  17  of the housing  12  and stator  16  is tangential point contact, along the lengths of the tubes  20 . 
     The tubes  20  have a spring rate which is determined by their external diameter, their wall thickness and the material from which they are made. The form of the tubes  20  used in any particular motor  10  is selected to provide the appropriate spring rating required to produce the damping effect necessary for that motor. For example, having consideration to the axial lengths of the tubes  20  and the number of tubes  20  used. In a three tube arrangement, in which each tube  20  has an outside diameter of 10 mm and an internal diameter of 9.1 mm, and a length of 100 mm, each tube may have a spring rate of about 10 kN mm −1 . Each tube  20  may be made of commercially available high tensile steel. 
     In the examples, the tubes  20  are in compression between the housing  12  and the stator  16  of the stator assembly  11 , and are retained in position as a result. The grooves  17 , and  13  of the external and internal surface  18 ,  14  are dimensioned to ensure this. The cross-sections of the tubes  20  may become distorted slightly once the motor  10  is assembled. 
     In order to assemble the motor  10  in order to achieve compression of the tubes  20 , the housing  12  is heated, the stator  16  is cooled, and then the stator  16  is placed within the housing  12  and the tubes  20  inserted between the two, before the whole is allowed to reach ambient temperature. The holding of the tubes  20  in compression results in an inward force on the stator assembly  16  which positions the stator  16  concentrically within the housing  12 . This in itself is advantageous as it is known that truly concentric stator/rotor assemblies produce less vibration. 
     This method of mounting reduces the transmission in use, of electromagnetically induced vibration, i.e. noise, from the stator  16  to the housing  12 , and hence of noise out of the motor  10 . It also provides the advantage in this embodiment, that the annular space  22  can be used for the passage of cooling fluid, either gas or liquid as appropriate, to provide motor cooling as required. 
     The motor  10  further comprises end plates  30 ,  31  which provide bearings  32  for the mounting of a rotor  34  as is well known in the art. 
     Although in the embodiment described above three resilient damping elements  20  are provided, embodiments according to the invention may comprise differing numbers in excess of three. 
     Furthermore, whilst in the embodiment described the internal surface  13  of the housing  12  has grooves  14 , the internal surface  13  may be smooth. Alternatively both the internal surface  13  of the housing  12  and the external surface  17  of the stator  16  may be smooth. In this case the contact of the tubes  20  on both surfaces  13 ,  17  will be solely tangential point contact, along the lengths of the tubes  20 . 
     The tubes  20  described above are of generally circular cross-section, but tubes of elliptical, or polygonal (e.g. triangular or rectangular) cross-section may used in their place. The material from which the tubes are made may be metal, or for some applications fibre reinforced plastics material, the fibre reinforcement typically being glass or carbon fibre. The tubes  20  may be filled with a resilient material to tailor the characteristic of the damping achieved, to a particular use. The tubes  20  may comprise an assembly of an outer tube e.g. of metal, and an inner generally concentric tube e.g. of metal, with a tube of e.g. resilient material between the inner and outer tubes. 
     As described, the tubes  20  in the motor  10  are retained in position simply by being held in compression between the housing  12  and stator  16  between the end plates  30 ,  31 . However, if desired retaining means such as brackets, pins, screws or the like may be used or the tubes may simply be supported at their ends, e.g. in cut-outs  33  provided in the motor end plates  30 ,  31 . 
     Although the invention has been described with reference to a switched reluctance motor, the invention may be applied to a switched reluctance generator, or to any other kind of motor or generator which has a rotor and a stator assembly.