Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-03-28
2004-06-15
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S091000
Reexamination Certificate
active
06750587
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to an alternator assembly and more particularly to an alternator rotor assembly with improved RPM and output capabilities.
BACKGROUND OF THE INVENTION
Alternators are utilized in a wide variety of applications within a wide variety of fields. The automotive industry makes frequent usage of these systems within their vehicles. A popular style of alternator within the automotive field utilizes a Lundell style alternator rotor to provide the magnetic field. The use of this traditional style alternator rotor, however, has come into conflict with industry trends calling for increased electrical power, improved power density, and increased speed capability from their alternators.
The apparent conflict arises from well known methods of increasing electrical power output from a Lundell alternator design. One method commonly involves spinning the rotor faster within the alternator since the power output of the alternator increases with rotor speed. The increase in rotor speed is commonly accomplished through a reduction in the size of the alternator pulley on creating a high pulley ration between the engine's crank pulley and the alternator pulley. Another well-known approach to increasing electrical power output is to reduce the air-gap between the outside diameter of the rotor and the inside diameter of the stator. However, this air-gap can serve to limit the speed capability of the rotor.
Centrifugal forces act on the rotor and deflect the fingers of the pole toward the stator, effectively closing the air-gap between the two. The pole fingers deflect outwardly due to the centrifugal forces until they contact the stator core inside diameter causing an immediate alternator failure.
FIG. 2
illustrates the movement of the fingers of one pole from low to high RPMs. The pole fingers deflect outwardly due to the cantilever nature of the design under the influence of both centrifugal force and magnetic attraction between the rotor outside and the stator inside. Therefore, when the air-gap is reduced, the RPM capability must be reduced to insure the fingers do not deflect into the stator. Thus, the rotor RPM and the air-gap can work against each other in a traditional Lundell style design.
There are several known methodologies for attempting to increase RPM while minimizing air-gap. One such approach has been to increase the strength of the fingers. The fingers can be strengthened by increasing their cross-sectional thickness. This method, however, can result in less room for the field coil windings yielding less field strength. The field strength capability of the rotor is proportional to the amount of room dedicated for the field wire winds. Therefore, increasing the thickness of the fingers results in decreased field strength and lower electrical output for the alternator. Additionally, the thicker fingers decrease the air-gap between features on opposing magnetic poles resulting in additional leakage flux, lowering the electrical output ability. Thus the approach of increasing finger cross-section alone is often not a satisfactory approach to increasing alternator output.
An alternate known approach attempts to restrain the fingers from flexing outward during high RPM operation. This approach utilizes bands around the outside diameter of the rotor to restrain such deflection. The bands, however, are generally positioned in the air-gap between the rotor and the stator and thereby requiring an increase in the air-gap resulting in a diminished output of the alternator. This is often true whether the bands are simply wrapped around the outside diameter or if they are placed in cut grooves. The band material must be non-magnetic (since they touch fingers of both pole pieces having opposite magnetic polarities during operation) and therefore any portion of the air-gap taken up by the bands acts effectively as additional air-gap. If the bands were to be magnetic they would provide a direct leakage path for flux to “short circuit” between the two poles.
Materials known to be utilized for such bands include stainless steel, fiber reinforced wound composites, and aluminum. Stainless steel and fiber reinforced wound compounds are both expensive and can be difficult to process. Aluminum bands need to be large to sustain the mechanical stress from the fingers. The increase dimensions of the aluminum bands further increases the amount of the air-gap taken up my non-magnetic bands and can result in inferior performance. Additionally the use of any bands wrapped around the outside diameter of the rotors can add a substantial amount of rotating inertia to the rotor assembly. Rotating inertia can limit the pulley ratio between the alternator and the engine due to the potential for belt slip. In addition, the fingers and bands often still deflect under high RPM and therefore require additional air-gap spacing.
Finally, an additional undesirable aspect of traditional Lundell style alternator designs arises from the vibration of the fingers during operation. Finger vibration amplitude is excited due to magnetic forces acting on the finger face and the ends of the stator teeth. These attraction forces causes the rotor to excite when a natural frequency is reached and can cause undesirable audible noise.
It would, therefore, be highly desirable to have an alternator rotor assembly with improved capacity for increased RPM usage, a capacity for a decreased air-gap, and that provided a reduction in audible noise caused by finger vibration.
SUMMARY OF THE INVENTION
In accordance with the present invention, an alternator rotor assembly is provided. The alternator rotor assembly includes a rear pole piece having a rear pole base portion and a plurality of rear pole finger portions each having a rear pole fingertip. The plurality of rear pole finger portions are positioned to form a plurality of rear pole finger gaps. The alternator rotor assembly further includes a front pole piece having a front pole base portion and a plurality of front pole finger portions each having a front pole fingertip. The plurality of front pole finger portions are positioned to form a plurality of front pole finger gaps. The rear pole piece and the front pole piece are assembled such that the rear pole finger portions are positioned within the front pole finger gaps and the front pole finger portions are positioned within the rear pole finger gaps.
The alternator rotor assembly further includes a plurality of link elements. The link elements are positioned within the finger gaps and connect the front pole fingertips to the rear pole base portion and the rear pole fingertips to the front pole base portion. In this fashion the deflection of the fingertips due to centrifugal forces is reduced.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and the appended claims.
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Militello Anthony
York Michael Timothy
Kajander, Esq. John E.
Nguyen Tran
Visteon Global Technologies Inc.
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