Patent Application
20110244178
Various heavy duty high-current industrial equipment dissipate excess energy through resistor grids in the form of large amounts of heat. For example, resistor grids are used for controlling loads in cranes, for load testing of generators, for harmonic filtering in electric substations, for neutral grounding in industrial AC distribution, for dynamic braking on locomotives and so forth.
A resistor grid is a large, usually air or oil cooled grid of metal alloy ribbons or plates, formed as a serpentine structure. The ribbons may have pins at each end for mounting onto an insulation board. The insulation board provides a sturdy frame for the resistor grid and maintains a fixed, safe separation between ribbons, as well as between successive grids when used in a grid stack configuration. The insulation board may be made of a suitable insulating material such as fiber glass, silicon-bonded mica, thermoplastic or thermoset polymers, including silicones and polyesters, all of which may be filled with higher temperature compounds like glass, fiber glass, mica, alumina, silica, and the like. The resistor grid provides little electrical resistance and may carry currents as large as a several hundred or even thousands of amperes. Neighboring ribbons may have a potential difference of a few volts. Such operating parameters may cause arcing between neighboring ribbons or thermal runaway if the ribbons are too close, and especially if they are allowed to touch. Therefore, the structural integrity of the insulation board is critical.
Under normal operating conditions, the resistor grids are typically subject to air temperatures between 200 and 400 degrees centigrade, but may be higher. These high temperatures may cause thermal degradation and/or distortion of the insulation board. If the insulation board distorts or degrades, then pin-out of ribbons may occur. This may further lead to relative motion of the ribbons, electrical arcing, thermal runaway, and subsequent deterioration and ultimate failure of the resistor grid. Furthermore, the failures can produce sparks and molten steel which may be ejected in the air cooling stream. These ejected particulates pose a safety hazard and may cause wayside fires, in the case of locomotive dynamic braking grids.
Insulation boards made of materials that can withstand higher temperatures are expensive.
For these and other reasons, there is a need for the current invention.
An insulation board for a resistor grid and a method of constructing the same are disclosed. The insulation board includes one or more longitudinal voids. Longitudinal structural members are disposed within the longitudinal voids, wherein the cross section of the longitudinal structural members conforms to the profile of the longitudinal voids. The insulation board also includes one or more rows of transverse pin holes for engaging one or more resistive elements of the resistor grid, disposed along the length of the insulation board.
Embodiments of the present invention provide an improved design of an insulation board for resistor grids and methods of manufacturing the insulation board.
The longitudinal voids 202, 204, and 206 may or may not run the entire length of the insulation board. In various embodiments, the longitudinal voids 202, 204, and 206 may be absent at the ends of the insulation board. In other embodiments, the longitudinal voids 202, 204 and 206 may run the entire length of the insulation board.
In the embodiment illustrated in
In the embodiment illustrated in
Structural strength of the insulation board may be improved by disposing one or more longitudinal structural members within the voids 202, 204, and 206. In some embodiments, a longitudinal structural member may be disposed only within the longitudinal voids 204 and 206. The longitudinal void 202 may be left empty. In other embodiments, longitudinal structural members may be disposed within each of the voids 202, 204, and 206. In various embodiments, the longitudinal structural members may be standard tube stock. The gauge and wall thickness of the tube stock may be chosen according to structural strength requirements for the insulation board. In other embodiments, the longitudinal structural members may be standard rod stock. In yet other embodiments, the longitudinal structural members may be beams, angles, or channels. The dimensions of the beams, angles, and channels may be chosen according to the structural strength requirements for the insulation board.
In various embodiments, multiple resistor grids may be placed close to each other to form a stacked resistor grid. In such embodiments, considerations for electrical creepage path between the insulation boards of adjacent resistor grids may prescribe that longitudinal structural members of reduced or different cross section be used. For instance, a C section channel (as shown in
Further, in various embodiments, the longitudinal structural members may not run up to the ends of the insulation board. In one embodiment, the longitudinal voids 202, 204, and 206 may run the entire length of the insulation board, however the longitudinal structural members disposed therein may not run up to the ends of the longitudinal voids 202, 204, and 206. In other embodiments, the longitudinal structural members may run the entire length of the insulation board.
The longitudinal structural members may have substantially equal stiffness. Structural members having substantially equal stiffness may help in distributing the load evenly across the insulation board, and reduce or prevent the warping or buckling of the insulation board due to mechanical load and heat. The longitudinal structural members may have substantially higher stiffness than the electrical insulation material used in the insulation board, to maintain the required structural integrity of the insulation board, specially at elevated temperatures, where the electrical insulation material is prone to degradation and distortion.
The longitudinal structural members may be made of an inexpensive material, such as metals including, without limitation, iron and steel. Alternatively, the longitudinal structural members may be made of non-metallic materials such as, but not limited to, fiber glass, weave board, carbon fiber and so forth.
In step 1002 a profiled block is provided. The profiled block is made of a high temperature electrical insulator such as, but not limited to, electrical grade silicone resin. The profiled block may be made by molding the high temperature electrical insulator using molding techniques such as, but not limited to, injection molding, compression molding, and so forth. In some embodiments, the profiled block may be formed using fiber glass or weave board, and over molded with electrical grade silicon resin. In various embodiments, the profiled block may further have one or more longitudinal voids. The longitudinal voids may or may not run the entire length of the profiled block. Further, the longitudinal voids may be placed entirely within the profiled block, or may be placed on the outer longitudinal sides of the profiled block.
In step 1004 of one or more longitudinal structural members are inserted in at least one of the voids of the profiled block. In various embodiments, the cross section of the longitudinal structural members may conform to the profile of the voids in which the longitudinal structural members are inserted. The longitudinal structural members may simply be inserted into the voids. Alternatively, the longitudinal structural members may be cooled down first such that the longitudinal structural members contract, thus facilitating easy insertion into the voids.
The longitudinal structural members may be any one of, but not limited to, a beam, a channel, an angle, a tube or a rod. The longitudinal structural members are inserted for providing additional mechanical strength to the profiled block. The longitudinal structural members may have substantially equal stiffness and mechanical strength. In an embodiment of the present invention, the longitudinal structural members may be made of metal. In an alternate embodiment of the present invention, the longitudinal structural members may be made of glass fiber.
In step 1006, one or more rows of transverse pin holes are provided on the profiled block. The pin holes engage the resistive elements of the resistor grid. The number of rows of pins holes on the profiled block may vary depending on the number of fastening pins disposed on the said resistive elements. In one embodiment, the pin holes are machined into the profiled block. In other embodiments, the provision for pin holes is made in the mold used for providing the profiled block in step 1002.
In step 1102, one or more longitudinal structural members are provided. The longitudinal structural members may be, without limitation, beams, channels, angles, tubes, or rods. In some embodiments, the longitudinal structural members may have a complex section. The longitudinal structural members may have substantially equal stiffness. In an embodiment, the longitudinal structural members may be made of a metal such as, but not limited to, iron and steel. In another embodiment, the longitudinal structural members may be made of non-metallic materials such as, but not limited to, fiber glass, weave board, carbon fiber and so forth.
In step 1104 a block of high temperature electrical insulator is molded over the longitudinal structural members. The high temperature electrical insulator may be, without limitation, an electrical grade silicone resin. The block may be made molding using techniques such as, but not limited to, injection molding, compression molding, and so forth. The longitudinal structural members may be positioned within the mold prior to molding.
In step 1106, one or more rows of transverse pin holes are provided on the molded block. The pin holes engage the resistive elements of the resistor grid. The number of rows of pins holes on the molded block may vary depending on the number of fastening pins disposed on the said resistive elements. In one embodiment, the pin holes are machined into the molded block. In other embodiments, the provision for pin holes is made in the mold used for molding the block of electrical grade insulator in step 1104.
The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.
