Electric motor

Details Electric motor Electric motor

1999-03-30

2001-11-20

Waks, Joseph (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S254100, C310S091000

Reexamination Certificate

active

06320287

ABSTRACT:

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.


REFERENCES:
patent: 1668889 (1928-10-01), Spreen
patent: 3465182 (1969-09-01), Church et al.
patent: 4082974 (1978-04-01), Yamamoto et al.
patent: 4173724 (1979-11-01), Mujsil
patent: 4990809 (1991-02-01), Artus et al.
patent: 5142179 (1992-08-01), Nakamura et al.
patent: 5521447 (1996-05-01), Bertolini et al.

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