Inductor devices – Core joint structure
Reexamination Certificate
1999-02-17
2001-04-17
Mai, Anh (Department: 2832)
Inductor devices
Core joint structure
C336S217000, C336S212000, C336S234000
Reexamination Certificate
active
06218927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to magnetic core structures and, more specifically to laminated magnetic transformer core structures having center leg curvilinear S-joints.
2. Description of the Prior Art
Stacked magnetic cores for large electrical power transformers of the core-form type conventionally use the butt-lap type of joint disclosed in U.S. Pat. No. 2,300,964. In the butt-lap joint the ends of the leg and the yoke laminations are mitred and butted together to form diagonal joints between the laminations, in each layer of laminations. In principal, the joints and alternate layers are aligned and offset from aligned joints and the intervening layers. In practice to reduce handling, the joints in three adjacent layers of laminations are usually aligned, and the joints in the next three adjacent layers are aligned, but offset from the joints of the adjacent group of three laminations. While the butt-lap construction can form a good magnetic surface, it has disadvantages. One is the great care with which laminations must be stacked in order to optimize magnetic performance. Another disadvantage is the amount of power loss at the joints which increases the excitation current required and increases the sound level.
Laminated magnetic step-lap transformer cores are also well known in the art. Step-lap joints and transformers reduce core losses, reduce the excitation current requirements, and reduce the sound level, compared with a similarly rated transformer constructed with a butt-lap joint. In a step lap joint, the joints created by the abutting laminations of each layer are successively offset in succeeding layers in the same direction to create at least three “steps” and preferably at least six or seven, before the step pattern is repeated. An example of transformer cores with step-lap joints is disclosed in U.S. Pat. No. 4,200,854. While such prior art cores with step-lap joints improve the magnetic properties of the core, they have left something to be desired in the way of minimizing scrap saving in their construction.
Accordingly, it would be desirable and it is an object of this invention, to provide a laminated magnetic step-lap transformer core with center leg curvilinear S-joints which may be fabricated from less material than present state of the art cores. It is a further object of this invention to improve the no-load performance of a transformer, to lower the cost and to increase the productivity of core manufacturing.
SUMMARY OF THE INVENTION
This invention discloses a new and improved arrangement for constructing a stacked magnetic core for a transformer. A novel arrangement is provided for the middle leg lamination geometry to provide a curvilinear S-joint between the middle leg and the yoke laminations. The present invention also provides cutting sequences of laminations to ensure most efficient steel usage.
In accordance with one aspect of the invention there is provided in a stacked magnetic core for a transformer, the core including a plurality of layers of magnetic laminations, each lamination including an upper yoke element, a lower yoke element, a central leg element interconnecting the upper and lower yoke elements and a pair of outer leg elements interconnecting the opposite ends of the yoke elements, the improvement in the core lamination geometry to improve the conditions for magnetic flux transfer across the joint areas wherein the ends of the center leg element have a curvilinear S-shape configuration with interfits with a complementary curvilinear S-shape configuration intermediate the ends of the yoke elements to form curvilinear S-joints therebetween, said ends of said center leg element being of reverse configuration, and the ends of the yoke elements and the ends of the pair of outer leg elements are cut at an angle so as to form step-lap mitre joints in the corners of the core when assembled with each other. Further in accordance with the invention the core is stacked with no more than three laminations per layer and the orientation of the curvilinear S-joints is altered in the even and odd layers of the core.
In accordance with a further aspect of the invention, the core includes at least one group of the layers of magnetic laminations, each group includes a plurality of the layers of magnetic laminations, the upper yoke elements of each layer of laminations in each group being shifted longitudinally in one direction so that the step lap mitre ends overlap a predetermined distance, and the lower yoke elements of each layer of laminations in each group being shifted longitudinally in the opposite direction so that the step lap mitre ends overlap a distance corresponding to the overlap distance of the layers of upper yoke elements. In a preferred form of the invention, the step lap mitre ends overlap a distance of about 2 to 20 mm.
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ABB Power T&D Company Inc.
Mai Anh
Woodcock Washburn Kurtz Mackiewicz & Norris
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