Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-11-12
2001-01-23
LaBalle, Clayton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S216006, C310S261100, C310S049540
Reexamination Certificate
active
06177749
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electric motors in general, and specifically to laminated rotors for induction motors.
The use of laminated rotors for induction motors is well known. See, for example, U.S. Pat. No. 5,767,607, the disclosure of which is herein incorporated by reference. Typically, such a laminated rotor is comprised of a shaft with a center axis and a set of axially-mounted laminations constructed of a ferro-magnetic material.
The laminations of these types of rotors are quite versatile in that they may be sized and configured with punch patterns. Prior art laminations have been punched or stamped with circular shaft holes. Prior art laminations have also been punched with polygonal center holes that are then further machined into circular shaft holes.
Advantages of the laminations, however, are somewhat offset by the difficulties encountered in mounting them to a rotor shaft. Typically, the laminations are provided with a circular shaft hole centered in the lamination disks. A stack of laminations is applied to a circular rotor shaft by a cold press process. The force-fit cold press process presents several challenges to assembling the rotors.
First, the interference between the shaft and the shaft hole is typically about 0.0005 inches. It has been found that this is the largest interference, and hence produces the tightest fit, that permits a cold press application to the shaft. A larger interference will prevent the shaft from being pressed into the shaft hole of a stack of laminations. To achieve this precise fit, the inner diameter of the laminations and/or the outer diameter of the shaft require time consuming and relatively expensive finishing. Consistent machining then becomes a premium, as deviations from optimum finishing causes excessive press forces and mispresses.
Second, the tolerance of the shaft hole in the laminations is typically about 0.0007 inches. These close tolerances also present a challenge in press fitting the laminations onto the shaft so that the laminations are “straight” or perpendicular relative to the shaft. This is typically ensured via employment of a straightness plug gage which is 0.0002 inches below the minimum diameter of the shaft hole. If the laminations are not sufficiently straight relative to the shaft, they will not pass the gage. The assemblies that fail to pass the gage are scrapped.
Third and finally, the success of a cold press fit is critically dependent on the relative temperatures of the rotor shaft and the laminations that the shaft is pressed into. Because of the relative coefficients of thermal expansion between the two materials, variations in temperature between the shaft and the rotor may result in excessively tight or excessively loose fits, both of which have undesirable consequences. Whether the fit is too tight or too loose, the integrity of the rotor may be compromised, the performance of the motor may fluctuate, and the probability of failure of the rotor may be increased.
Therefore, the laminations of the prior art are disadvantaged in that they result in considerable scrap in production of the rotors, produce a wide variety of torque retention of the rotor to the shaft, and consequently produce variation in the performance of the rotor.
SUMMARY OF THE INVENTION
Among the several advantages of the present invention may be noted the provision of a laminated rotor where the shaft hole of the laminations may be punched to size without the need for expensive, time consuming finishing; the provision of a laminated rotor where the torque retention of the laminations to the rotor shaft is increased; the provision of a laminated rotor wherein the forces required to press the rotor laminations onto the shaft are reduced; and the provision of a laminated rotor that reduces the scrap generated in manufacturing and assembly of the rotor.
Generally the present invention comprises a laminated rotor in which the ferro-magnetic laminations are punched with a central aperture in the shape of a polygon, as opposed to the circular apertures of the laminations of the prior art. The polygonal aperture has an odd number of straight sides of substantially equal length, each of which extends between vertices formed by the intersecting sides of the polygon. A plurality of laminations are stacked upon one another to form a rotor shell. Each lamination is slightly rotated about the axis of the rotor shaft from the next lamination in the stack.
In the preferred embodiment, the laminations are of at least two different groups stacked along the shaft. Every other lamination of the stack belongs to one of the groups, while the remaining laminations belong to the other group. Thus, the stack of laminations consists of pairs of laminations, one from each of the groups. One of the groups of laminations is rotated relative to the other group so that the central polygonal apertures of the groups of laminations are directly opposed (i.e. rotated 180° from one another). In other words, considering a single pair of adjacent laminations, the two laminations are rotated relative to one another so that the vertices of the polygonal apertures of one of the laminations are positioned in between, and approximately equidistant from, adjacent vertices of the other lamination. Thus, the interior of the rotor shell has a uniform honeycomb-like appearance due to the relative rotations of the laminations.
In alternate embodiments, the laminations are of three or more different groups stacked along the shaft in a sequential arrangement. The laminations of each group are rotated relative to the laminations of the other groups so that the vertices of the polygon shaft holes of each group are positioned in between adjacent vertices of the polygon shaft holes of the other groups.
The stack of laminations is die cast, forming a squirrel cage winding through slots near the outer periphery of the lamination disks. End rings are then placed on the ends of the stack and the rotor shell is completed.
The rotor shaft is then press fit into the rotor shell via a cold press process. Due to the relative material hardness between the laminations and the steel rotor shaft, the laminations yield as the shaft is pressed into the interior of the rotor. The honeycomb-like structure of the interior of the rotor shell provides relatively little resistance to the shaft. As the shaft is inserted, the greatest interference occurs at the smallest radius of the shell interior (i.e., at the midpoints of the sides of the polygonal apertures). At these particular points, the laminate material yields relatively easily as only a single thickness of a laminate opposes the insertion of the shaft. The yielded laminate material fills in the spaces or cells of the honeycomb created by the vertices of the next lamination immediately below. Thus, the force required to insert the shaft is noticeably less. However, this yielding process occurring all around the shaft actually produces a stronger bond than is achieved with the conventional press fit process using laminates with circular apertures as in the prior art. The nominal interference between the shaft and the rotor shell is dramatically increased.
Also because of the yielding of the laminates into the honeycomb structure of the rotor shell, expensive machining of the laminations is avoided. Thus, comparatively speaking, a rotor of the present invention can be manufactured in less time with lesser expense than the rotors of the prior art. Further, because the force necessary to insert the rotor shaft is reduced, the instances of mispresses due to the generation of excessive forces will decline, and the amount of scrap generated during manufacture of the rotor will be accordingly reduced. Finally, the yielding of the laminations reduces the impact of the tolerances in the central apertures of the laminations, and further reduces the impact of relative temperature differences between the rotor shaft and the laminations.
Other objects and features of the invention will be in part apparent and in part pointed out h
Hussey John H.
Meystrik Jeffrey J.
Rose John Scott
White Kent Lee
Emerson Electric Co.
Howell & Haferkamp LC
LaBalle Clayton
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