US2474891A - Argentiferous-copper turbogenerator rotor winding - Google Patents

Description

July 5, 1949. E. A. DAVIS ARGENTIFEROUS-COPPER TURBOGENERATOR ROTOR WINDING Filed April 50, 1947 INVENTOR I [van A. Davis.

wnmzssas; W wM ATTORNEY Patented July 5, 1949 UNITED STAT ES PATENT QFF-I C E ARGENTIFEROUS-COPPER TURBOGEN- ERATOR ROTOR WINDING.

Evan A. Davis; Forest Hills, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, REA, a corporation. of Pennsylvania.

Application April 30, 1947, Serial No. 745,021.

(Cl. 1-'7-l-252) 4 Claims. 1

My invention relates: to a special copper alloy, having minute traces of silver in it, incertain definite proportions, for use as-the" conductor-material for the rotor-windings in turbine generators.

ivlylnvention is an improvement over the invention which is described and claimed in the Rose Patent 2,103,795, granted December 28, 1937, in which a small amount of cold-working was applied to at least the coil-sideswhichlie in the rotor-slots, for the purpose of increasing the life of the rotor-winding from a matter of three to five years, more or less; to a matter of twenty years, more or less.

The difficulty which was being combated' arises from the fact that a turbine generator may be started and stopped a hundred times or a thousand times during its life; Each time the generator is started, the centrifugal force holds the rotor coil-sides in frictionalengagement with the slot-wedges, thus preventing the copper from expanding, and if the copper is soft, the copper is usually stressed beyond its yield-point, at the normal operating-temperatures, thus causing the copper to sufier a permanent set. Each time the generator is stopped, the centrifugal force which has been holdingv the rotor coil-sides-in frictional engagement with the slot-wedges is released while the winding is hot, so that, when the winding cools, it contracts morethanthe iron. or steel of the rotor-core. On account of the. permanent set suffered during. the previous. starts, each stop causes the copper to draw in a. small fractional percentage of its length,,behindits previous condition; and this keeps up until a ground, or shortcircuit is produced. at the. place. where, the endturn portion of, the. winding bends. away from the straight coil-sideportion.v This difficulty has been so acute that it has been seriously proposed, both before and after the Rose invention, particularly in connection with machines not using the Rose invention, to attempt the difficult-task of preheating the turbo-rotor before putting, the machine in operation, after eachperiod of non-operation.

Ithas long been. known. that. copper, in. common with other metals, exhibits a phenomenon known as creep, which is a plastic strain or deformation which increases slowly with time, when the copper is under any stress of either compression or tension, the creep-rate increasing with the temperature ofthe copper. Ithasalso been known, for a number of years, that this creep-phenomenon is present in turbo generator rotors, and that it partially nullifies the benefits, of a small amount of cold-working, depending upon the length of time the machine is operated between its stop}- ping-periods. It has also been known, for anunrber of years, that an extremely small amount: of silver, alloyed with the copper, will materially decrease its creep-rate, although it has not been known how much silver should" be used, as the reduction in creep rate, with the addition oi: silver, is not a continuous process; but there is an optimum amount of silver, which; if exceeded, will result in beginning to increase the creep-rate; instead of furtherreducing it.

Much of the copper in commercial use is derived from the melting of concentrates, and nearly all of the copper thus derived contains silver in vary,- ing amounts. The American Society for Testing Materials has established specifications for copper for electrical purposes, requiring that the metal contain 99.90% copper (plus silver). The aver:- age quality of commercial grades of electrical copper is much better than this. Coppers, containing various small amounts of silver, within the range in which my present invention is concerned. have long been known, and available on the market, and their electrical and mechanical properties have been extensively investigated, except in the matter of the creep-rate, which requires extended tests, extending over long periods of time.

The prior knowledge on the subject may be roughly summarized as follows. In r944, C. M; Laffoon predicted that the use of some unnamed alloy of copper will improve the resistance of copper to creep in the rotors of turbine generators. In the same year, P. H. Brace published some early, unrevised findings of the present applicant, showing that a decreased creep-rate is obtainable by the addition of 0.078% of silver to substantially pure copper, corresponding to 25 avoirdupois ounces of silver per ton. Some years previously, in 1932, H. J. Tapsell and A. E. Johnson indicated that silver-bearing arsenical copper (specifically .072% silver), particularly after 51% cold-work, is more resistant to creep than arsenical copper without silver.

It is the object of my invention to build turbogenerator rotor-windings with copper containing preferably more silver than 0:078:95, or-25 avoi-rdupois ounces of silver per ton oi copper, but pref erably not more than 0.312%, or I00 avoirdupois ounces of silver per ton, with the partial 0016'.- working substantially like that which. was recommended by Rose for substantially pure copper;

An exemplary turbine generator embodying invention is shown in the accompanying drawing, wherein:

Figure 1 is a longitudinal sectional view of. a-

3 portion of a turbo-generator embodying my invention; and

Fig. 2 is a perspective view of an end of a. rotor member, with certain parts removed, the omitted parts including the retaining ring, the end plate and the blower.

The illustrated turbine generator comprises a stator member H, and a rotor member i2. The stator member comprises a stator-core l3 carrying a three-phase winding I4.

The rotor member l2 comprises a drum-type. slotted rotor-core it carrying a two-pole fieldwinding 20) having straight coil-sides 2| lying in rotor-slots 22, which are closed by slot-wedges 21. The rotor-core is at least four feet long, in machines to which my invention is applicable. The Helm-winding also has end-turns 28, which extend out beyond the rotor-core l6. It is at the bends in the end-turn portions 28, where they join onto the straight coil-side portions 2|, where the diiiiculty has been experienced because of the cumulative contractions of the length of the straight coil-side portions 2|.

Sometimes, as pointed out in the Rose patent, a turbine generator will have four poles, though usually it has two poles; and these generators generally operate at a primary-winding frequency of 50 to 60 cycles.

In accordance with my present invention, the conducting material of the field-windings 2G, or at least the straight coil-sides 2! which lie in the slots 22, are of argentiferous copper having more than 25 and less than 100 avoirdupois ounces of silver per ton. It is desirable to have an amount of silver which is somewhere near to a close approximation of the optimum amount from the standpoint of producing the smallest amount of creep, or the smallest creep-rate. When the copper is subjected to a stress, it begins to creep, at a rate which is somewhat faster at first, but gradually tapers off, until, after a month or two, the creep-rate becomes a constant amount which will be the minimum creep-rate for any particular stress at any particular temperature. It is obviously desirable to utilize an amount of silver which will give a sumciently close approximation to the minimum creep-rate at the temperature and stress conditions which prevail in the turbogenerator rotor.

The minimum creep-rate is obtained, at the operating-conditions in a turbo-generator rotor;

when silver is present in the copper in an amount approximating 5O avoirdupois ounces of silver per ton; or a little more than this figure, possibly something like from 56 to 64 ounces, or say 60 ounces, may be a more accurate optimum figure, although the precise amount of silver is not too critical, because the curve of the minimum creeprate plotted against silver-content is substantially constant at the lowest minimum creep-rate portion of the curve. Silver-amounts approximating 50 avoirdupois ounces, within the limits plus or minus 40%, per ton, are acceptable; or between 30 and 90 avoirdupois ounces of silver per ton; or between 50 and 75 avoirdupois ounces of silver per ton; depending upon the amount of departure from the absolute optimum conditions which is to be tolerated, and also depending upon the operating-temperatures which are going to be maintained.

Before the field-windings 2!! are placed in the slots of the rotor-core, and in fact before the coils of the field-winding are even wound or formed, the copper straps which are used in the ileld-Winding are cold-Worked so as to cause a 6% reduction in the cross-section, or say from 4% to 10% reduction, precisely as described, in the Rose patent, for the substantially pure copper straps, and for the same reason, namely, to increase the yield point to a value higher than the compressional stress to which the winding will be subjected by reason of the thermal expansions in the rotor-core, without, however, increasing the yield point to such a high value as to interfere with the bending and/or straightening of the conductor in the process of forming the coils.

During the use of a turbo-generator having an argentiferous-copper rotor-winding, in accordance with my invention, the rotor-copper and the rotor-iron usually reach a certain temperature which is somewhat above the room-temperature, during each shut-down period when the machine is not in operation. Each time the machine is started up, it is first brought up to speed, which imposes high centrifugal forces on the :roton-windings, pressing the straight coil-sides it hard against the retaining-wedges 21 of the slots, these being sufilcient to develop a considerable frictional force resisting any subsequent expansion of the straight coil-sides, due to heating. After the machine is brought up to speed and synchronized, load is applied, the rotor begins to heat up, and in due course it reaches its steady-state temperature, which is usually of the order of C. for the iron or steel of the rotor-core I6, and C. for the copper of the coil-sides 2|. Since the thermal coefiicient of expansion of the copper is higher than that of the iron, and since the temperature of the copper is also higher, the copper tends to expand, but the friction developed by the centrifugal force is so great that the central portions of the copper, in the middle of the straight coil-side portions, cannot expand.

In the middle of each coil-side portion 2|, the copper thus compresses in an amount equal to the difference in thermal expansion between copper and steel, developing a compressive stress S represented by the equation,

where l4.5 10 =the modulus of elasticity of copper in pounds per square-inch,

l7 l0 the thermal coefiicient of expansion of copper per C.,

11 10 =the thermal coefficient of expansion of steel,

Tc=the final temperature of the copper,

Ti=the final temperature of the iron, and

T0=the initial or cold temperature.

When the starting temperature To=50, the copper-temperature Tc=l25, and the iron-temperatures Ti=85,

(2) S=12,900 pounds per square-dnch, which is a fair estimate of operating-conditions at present practices in regard to temperatures.

At points away from the middle of the rotorcore, that is, near the ends of the straight coilside portions 2|, the frictional forces resulting from the centrifugal force pressing the coil-sides out against the retaining-wedges diminish, reaching zero at the extreme ends of the coils. At any distance at from each end of a coil-side, or from each end of the wedge, the frictionally imposed stress, which is operative on the copper by reason of the internal portion of the copper trying to expand thermally, will be represented by the equation where p=centrifugal forc of a unit length of coil-side, f=friction coefficient, and w=distance from end of coil.

This formula for the frictionally developed compressive force S holds true for a distance a: in from each end of the coil, until S becomes equal to S, after which, the compressive stress on the copper remains constant at S for all larger values of :13, that is, for the internal portions of the coilside, as previously explained.

The effect of the addition of an optimum amount of silver to substantially pure copper, for the rotor-conductors of my invention, is to materially reduce the creep-rate of the copper, that is, the very small fraction of the total length of the coil-side by which it creeps, or releases, or plastically or permanently compresses during each hour when it is being subjected to a compressive stress at its operative temperature. As the copper creeps, it relaxes the thermal compressive stresses to which it is subjected, so that, in a few months, say in a years continuous operation of the turbo-generator without a shutdown, the relaxation of the compressive stress S operating on the central portions of each coilside of my argentiferous-copper rotor-winding will reduce the stress to something like or /2 of the original value S which is shown in Equations 1 or 2. With the pure-copper windings of the Rose patent, the efiect of relaxation due to creepage reduced the compressive stresses much more.

It should be noted that the rate of relaxation is the most rapid at first, and this rate slows down in the later stages. Thus, in a machine for which there are many startings and stoppings, the accumulated relaxation is greater than in a machine which runs continuously. The permanent shorting of a coil, upon unloading, that is, during each stopping of the machine, is proportional to the amount by which the stress relaxed while under compression.

The addition of 50 avoirdupois ounces of silver per ton, to pure copper or electrolytic copper, makes the creep-rate 5 to times slower, at present operating temperatures, thus making a very material increase in the number of years during which the turbine generator can be kept operative without encountering a failure due to shortening of the rotor coil-sides.

I claim as my invention:

1. In a turbo-generator having a stator member and a drum-type, slotted, iron rotor member having slots of at least 4 feet in length, the combination, with said rotor member, of a winding having coil-sides lying in the rotor-slots, at least substantially all of those portions of the winding which lie in said slots being composed of argentiferous copper which has been coldworked so as to reduce its cross-section between 4 and 10 per cent, said argentiferous copper having more than 25 and less than 100 avoirdupois ounces of silver per ton.

2. In a turbo-generator having a stator member and a drum-type, slotted, iron rotor member having slots of at least 4 feet in length, the combination, with said rotor member, of a winding having coil-sides lying in the rotor-slots, at least substantially all of those portions of the winding which lie in said slots being composed of argentiferous copper which has been coldworked so as to reduce its cross-section between 4 and 10 per cent, said argentiferous copper having approximately 50 avoirdupois ounces of silver, within the limits plus or minus 40 per cent, per ton.

3. In a turbo-generator having a stator member and a drum-type, slotted, iron rotor member having slots of at least 4 feet in length, the combination, with said rotor member, of a winding having coil-sides lying in the rotor-slots, at least substantially all of those portions of the winding which lie in said slots being composed of argentiferous copper having betwen 30 and avoirdupois ounces of silver per ton.

4. In a turbo-generator having a stator member and a drum-type, slotted, iron rotor member having slots of at least 4 feet in length, the combination, with said rotor member, of a winding having coil-sides lying in the rotor-slots, at least substantially all of those portions of the winding which lie in said slots being composed of argentiferous copper having between 50 and '70 avoirdupois ounces of silver per ton.

EVAN A. DAVIS.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,028,985 Behrend June 11, 1902 2,103,795 Rose Dec. 28, 1937 OTHER REFERENCES AIEE Transactions, 1944, page 1353. Journal IEE, April 1946, page 209. Journal IEE, April 1946, pages 210-211. Journal IEE, April 1946, page 212.

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