Inductor devices – With temperature modifier
Reexamination Certificate
2000-11-16
2003-05-13
Enad, Elvin (Department: 2832)
Inductor devices
With temperature modifier
C336S057000, C336S058000, C336S059000, C336S060000, C336S061000
Reexamination Certificate
active
06563410
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
BACKGROUND FIELD OF INVENTION
This invention relates generally to small footprint transformers equipped with heat dissipators and, more particularly, to improved transformer constructions adapted to the more efficient cooling arrangements for dissipating heat generated in the winding structure of power transformers.
BACKGROUND DISCUSSION OF PRIOR ART
Transformers, as most electric apparatus and equipment, do not have specific rating: their load carrying capacity is limited only by their temperature. In transformer windings, due to their resistance, losses are generated proportionally to the square of the load currents and eddy currents, warming up the windings. Their temperature, however, depends on the efficiency of the cooling arrangement used for removing the generated losses.
In the present practice, natural convection plays the largest role in cooling via the surface of the winding. Tubular windings are in use almost exclusively. If the outside surface of the winding does not provide sufficient heat transfer, the present practice is to create cooling ducts between winding layers by separating the layers with spacers. These ducts are not very efficient, because the cooling medium moves slowly in narrow spaces, and warms up considerably before finally exits at the top of the duct. Consequently, the temperature at the top portion of the winding is much higher than at the bottom portion.
Including wider ducts increases the mean turn length of the winding. Thus the weight of the winding also increases, and the losses. Using longer core legs and longer windings to increase the cooling surface, but the losses further increase. Deviating the configuration more from the optimum format, the toroid—which has the minimum material content, but inferior cooling surfaces for natural convection creates this increase. Generally, a large part of the gain expected from enlarging the cooling surfaces of the winding is canceled by increased weight and losses.
Several attempts are documented in the prior art to improve the cooling process by including highly heat conductive metal sheets into windings. None of the prior art uses a dissipator displaying features of the present invention and achieves significant improvement except one: U.S. Pat. No. 3,659,239 to Marton, Apr. 25, 1972. This patent, however, limits the use of heat dissipators to tubular layer-wound winding structures mounted on vertical core legs. The layers of the windings are interleaved with contiguous portions of dissipators wound into the windings alternating with the winding layers. A louver-like structure is prefabricated on an extended portion of the dissipator sheets, and arranged outside the winding. The louver-like structures are cut into segments containing a group of fins. The segments are bent into horizontal position disposed in planes at both ends of each layer. The segments build up several levels of fins. The major surfaces of the fins are oriented close to vertical. With this orientation the channels are wide, and the resistance to the flow of the cooling medium is small.
With the heat dissipators in this configuration, substantial improvement can be achieved: Keeping the costs and materials the same, the winding losses and temperature rise can be reduced. These values are less than half of the conventional values. Keeping the same losses, 30% winding material, and 12% core steel can be saved with 15% less temperature rise.
Between 1968 and 1976, four small companies in a row manufactured about 3000 units with tubular heat dissipators according to this patent. These units are still in flawless operation. This small scale production has been discontinued only because of lack of interest in energy saving, lack of honest cooperation between partners, unfair competition, and lack of adequate working capital.
During the elapsed 32 years, this technology has been offered five times to every U.S. transformer manufacturer. All of them rejected it. In 1978, it was submitted to the invention evaluation program sponsored by the U.S. Department of Energy. Two independent engineering companies evaluated it with positive recommendations. In 1980, the Department of Energy still refused to offer meaningful support. Thus, in the past twenty-five years, the substantial improvements introduced by this technology remain unused.
All present transformer production uses the conventional 100-year-old technology.
This presently unused technology of U.S. Pat. No. 3,659,239 uses layer-wound tubular windings with wound-in heat dissipators. It has several drawbacks. Some of the drawbacks emerge in the production. In this process the dissipators are incorporated into the winding structure at the winding operation. First, the dissipator sheet is bent to follow the curvature of the designated winding layer, wrapped in the proper insulating sheet and placed over the layer. After securing the heat dissipator in its correct position, the next layer is wound over it. Special attention is required to wind very tightly to eliminate any gaps between the layers and the dissipators to keep the internal temperature gradient low. Winding tightly is a slow process.
Tubular winding structures generate leakage flux inside windings; this flux is oriented parallel to the axis of core legs. This flux orientation makes heat dissipator application very difficult when the winding is built up from discs. In flat contiguous dissipators, heavy eddy-currents would develop. To prevent this problem by splitting up the inserted portion of the dissipator into narrow sections, the tooling becomes prohibitively expensive, and the assembly gets complicated. Furthermore, the method described in the prior art cannot be used with dissipators having longer fins. The contour of the windings has a large variety of curvatures, and a separate tool would be required for every different curvature. Thus, the application of heat dissipators in tubular windings built up from discs is limited to short fins, usable only in liquid cooling. Considering the expensive tooling costs and the additional labor costs this version requires, dissipator cooling for discs in tubular winding systems is not economical.
Further drawbacks in layer wound windings become apparent after removing the completed winding from the winding machine. The several levels of louver-like structures on the curved extensions are hand-cut into uneven smaller segments. This type of subdivision is necessary to allow the 90 degree outward bending of the cut-up irregular fin groups. The cut up segments are bent into their final horizontal radial position. Several levels are built up on both ends of the vertical tubular winding.
The combined work of tight winding, dissipator implantation, and the subsequent cutting and bending operations of the dissipators require additional skilled labor time and extra care. Due to the uneven hand-cutting of the bent louver-like structure, the finished transformers don't have a smooth professional appearance. This aspect tends to diminish the acceptability of the product for some customers.
Another shortcoming emerged in the practice. When during assembly or cleaning, the fin segments have been bent up and down three times, they have the tendency to break off. This failure can be remedied only by replacing the winding. After impregnation, there is no remedy possible.
When building transformers with higher kVA rating, the efficiency of the dissipator arrangement diminishes. This occurs due to larger internal temperature gradients developing along the longer layers. There is difficulty of accommodating more levels of louver-like structures crowding at both ends of the windings. This difficulty can be alleviated by assigning extra space along the leg for the louver-like structures. This can be done by interrupting the winding, subdividing it into sections. This solution leads to longer legs, thus heavier units andincreased losses. If the interruption is applied only to the upper layers, some of the louver-like structures have to be cut i
Enad Elvin
Poker Jennifer A.
LandOfFree
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