1. Field of the Invention
The present invention relates generally to techniques for forming laminations for magnetic cores such as the magnetic cores (rotors and stators) typically found in dynamoelectric machines. More particularly, the field of the invention is that of centrally interlocked lamination stacks used to manufacture magnetic cores and methods and machines (progressive dies) for processing the material (coil stock) which forms the laminations for such magnetic cores.
2. Description of the Related Art
Typically, the laminas are blanked from continuous strip stock and then stacked and bound together to form the rotor or stator. Progressive die assemblies for producing stator or rotor laminations are well known, wherein a strip of lamination material is fed through a sequence of punching steps to progressively form the laminas to the desired end configuration. It is also well known to form arcuately spaced interlock tabs in the laminas which extend below the lamina lower surface and engage a slot formed in the next lower lamina. In this manner, a plurality of laminas may be formed from a single strip stock and interconnected by means of the interlock tabs.
A lamina stack may be held together by an interlocking structure. Each lamina, excepting the bottom lamina of the stack, may have a plurality of arcuately spaced interlock tabs (typically 4 or 6 circumferentially disposed) depressed from the lamina lower surface adjacent to apertures formed in the lamina below. Each interlock tab engages a corresponding aperture in the next lower lamina of the stack, preferably by the entire thickness of the tab and the aperture in the adjacent lamina. The bottom lamina of the stack may have the interlock tabs blanked and removed to avoid interlocking of the lowermost lamina with an adjacent lamina which forms the top lamina of the prior stack.
Stator stacks include openings around the inner periphery of the stack which are shaped to receive the stator windings, with the openings extending longitudinally straight down the bound stator stack. The laminas of the rotor, however, include a plurality of skewed conductor slots which are formed around the periphery of the rotor stack in arcuately spaced relation to one another, by rotationally indexing the laminas with respect to the rotor stack. Indexing involves rotating the rotor stack with respect to the last produced lamina by a predetermined rotational increment so that, when the laminas are combined in a stack, the space defined by adjacent conductor slots are skewed or slanted relative to the stack axis. Skew inaccuracies and/or excessive adjustment time results from many prior art systems.
One system of forming a stack involves loosely stacking the laminas in a barrel as they are formed and blanked from the stock material in a progressive die assembly. After all the laminas are put into the barrel, the barrel is shuttled to a pressing station and the laminas are pressed together to engage the interlock tabs and thereby form the lamina stack. However, this system does not consistently lock adjacent laminas together because of the loose stacking, the shuttling mechanism is slow and thereby slows production times, and the system lacks a mechanism for creating a desired skew angle for the conductor slots.
Also, variations in the thickness of the stock material of the laminas may cause unbalanced stacks to be formed. In order to compensate for these problems, a system for compensating for the nonuniform stock thickness was developed which rotates the stacked laminas to compensate for variations in thickness while still properly skewing the conductor slots, as described in U.S. Pat. Nos. 4,619,028; 4,738,020; 5,087,849 and 5,123,155, all assigned to the assignee of the present invention and which disclosures are incorporated herein by reference. In the system disclosed in the aforementioned patents, the barrel holding the lamination stack is automatically rotated before each lamina is blanked from the strip stock and interlocks its circumferentially disposed tabs with the lamination stack.
In the apparatus and method disclosed in the aforementioned patents, the individual laminas are rotated by an angle of 180°. Although the laminas may be rotated by other angles, the angle must be at least 360°/(number of interlock tabs) so that the interlocking tabs and slots are properly aligned. The time needed to perform the rotation of the laminas may increase total manufacturing time, and thus increase the cost.
Certain particular structures require great care and expense to manufacture. For example, rotors having conductor slots with a herringbone skew cannot easily be manufactured using conventional interlock techniques. The herringbone skew requires that the first half of the laminas be rotated in one radial direction and that the last half of the laminas be rotated in the other radial direction. The interlocking tabs for each half of the laminas would require an opposite orientation, and would require an additional processing station in a progressive die assembly. The additional processing station entails further expense and complication. Also, rotors having straight (zero skew) conductor slots are difficult to form with interlocking tabs because the interlocking tabs inherently skew the laminas because of the shape of the tab and the aperture in the adjacent lamina which receives it.
Another type of rotor or stator structure which requires great care and expense to manufacture involves applications requiring cores with smaller diameters. For example, a rotor lamina about an inch in diameter includes very little surface area in which to form interlocking tabs.
What is needed is an improved method and apparatus for manufacturing and interlocking laminations.
Also needed is a method and apparatus for manufacturing and interlocking laminations more quickly than known in the prior art.
A further need exists for a method and apparatus for manufacturing laminations that provides an interlock for relatively small lamina stacks.