US3182112A - Current balancing means for multiple electrodes in electrically heated glass meltingunits - Google Patents

Description

May 4, 1965 J. J. TOROK 3,182,112

CURRENT BALANCING MEANS FOR MULTIPLE ELECTRODES IN ELECTRICALLY HEATED GLASS MELTING UNITS Filed July 5, 1962 FIG. I

INVENTOR JULIUS J. TOROK zgzizwwaxw A TTORNE Y3,

United States Patent 3,182,112 CURRENT BALANCING MEANS FOR MULTIPLE ELECTRODES IN ELECTRICALLY HEATED GLASS MELTING UNITS Julius J. Torok, Toledo, Ohio, assignor, by mesne assignments, to Owens-Illinois Glass Company, Toledo, 01110, a corporation of Ohio Filed July 5, 1962, Ser. No. 207,652 13 Claims. (Cl. 13-6) This invention relates to distribution of current among heating electrodes of resistance-heated furnaces and more particularly to the compensation of heating irregularities consequent upon passing a heating current through a material of negative temperature coefficient of resistance, such as in the resistance heating of a glass melting furnace.

Furnaces for melting glass and other materials of similar nature have generally been heated by oil or gas flames passing over the surface of the material, especially when the heat may not be conducted through the walls of the container or furnace. While direct electrical heating and electrical booster heating of a furnace of this type are highly desirable the use of electricity for heating has been limited by one of the characteristics of glass and similar materials which exhibit a negative variation of resistance as the temperature is increased with the result that the conductivity increases with the increasing temperature to produce an unlimited or run-away local heating. This eifect is not observed in metals and most other materials, which generally have positive coefficients of resistance. As metals are heated, they exhibit a rising specific resistivity as the temperature increases. One result is that a number of circuits connected in parallel to a common power source will rise in temperature at a simliar rate for the reason that if any particular one of the parallel circuits conducts an increasing current there results a greater heating and a greater temperature rise, but the resistance in that circuit tends to increase to cause a decrease in current drawn. This provides a self-correcting array of parallel circuits and no control problem is seen. But in Carborundum, glass and certain ceramics a large negative temperature coeflicient of resistance results in an opposite effect. In a case of glass, this temperature effect may cause a reduction of the resistance of 50% or more when the temperature is increased If it is assumed that we have electrodes conducting heating currents through a batch of molten material, it will be evident that the current passing between particular points on opposite electrodes will cause a heating effect in the material between these points. One region or path of the current may take more current than a nearby region and will be heated in proportion to the current passing by that path of the current. It will be evident that the introduction of current in one region not matched by an equal current in other regions will result in unequal heating which further reduces the resistance between those two points on the electrodes causing still greater heating in that locality, usually with the result that heating elsewhere in the tank is decreased. This produces an unstable relationship and results in run-away temperatures previously limited only by the liquid convec tion mixing action in the material. This effect has often resulted in the rapid destruction of the electrodes by overheating and in the consequent introduction of electrode contamination into the batch of material being melted. It is the characteristic of materials having negative temperature coefficients which is directly responsible for the runaway heating in specific localities in the batch which has prevented widespread use of electrical heating and booster heating by electricity for these materials.

It will also be evident that in direct electric heating of such materials by current conducted thereto from electrodes immersed in the material large electrodes may not be employed because of a tendency for the current to concentrate at some point at which the current momentarily becomes higher than average, with the result that this region will ultimately take nearly all of the current and other regions will remain unheated. When a number of smaller electrodes are used to avoid this effect, some means of preventing one pair of electrodes from taking nearly all of the current must be found, which should be automatic to prevent run-away conditions and extreme unevenness of heating.

It is accordingly an object of this invention to provide an automatic control mechanism for the equalization of current between multiple electrodes of a directly heated furnace.

A further object of the invention is to provide a compensating circuit for control of the distribution of heating currents passing through different regions of the furnace melt to prevent unstable resistance heating.

A still further object of the invention is to provide for controlled variations of heating in a current distributing circuit for a plurality of parallel connected units energized from a common source. Other objects and advantages of the invention will be understood by reference to the drawings in which:

FIG. 1 is a diagram of a furnace control circuit according to this invention; and

FIG. 2 shows in a schematic vertical section of a glass furnace employing this invention.

Referring now to the drawings more in detail there is shown in FIGS. 1 and 2 a tank 16 containing material which it is desired to heat by means of an electric current passing therethrough in a number of paths to cause a distributed heating effect throughout the mass of material in the tank. While other means for heating the tank are not shown it is to be understood that the tank may be initially heated by gas or oil fires particularly when the tank is of large size. Because of the negative temperature coeflicient which is exhibited by some material contained in tank It it becomes necessary to provide an overall control for the large currents conveyed to the liquid from a source 11. For this purpose it is preferable to supply current through a transformer 12 having a second ary winding 13 connected by a pair of buses 14 and 15 to an overall current regulating device, indicated at 16, for the control of the overall heating effect.

Sets of current conducting terminals are immersed in the material to be resistively heated and the heating is effected within the material itself. Several sets of terminals or electrodes are preferred. Electrodes 17 are illustrated as disposed along one side of the tank 10 with corresponding electrodes 18 at the opposite side of the tank, such that the electrodes 17 and 18 form pairs of electrodes to provide separate current paths through the liquid therebetween. The electrodes may be of molybdenum, carbon or platinum for use in a glass melting unit. In order to provide uniform distribution or regulation of current between the several pairs of electrodes, similar transformer primaries are connected from bus 14 to each of the electrodes 17 and from bus 15 to each of the electrodes 18. As illustrated in FIG. 1 a number of pairs of electrodes may thus be supplied by means of two transformers for each pair of electrodes. Transformers thus having primary windings 19 in series with electrodes 17 are illustrated at 21, 22, 23, 24 and 25.

Electrodes 17 are preferably distributed along one side of the tank 10 at approximately equal intervals, or are spaced according to the heating required. Corresponding electrodes 13 are preferably distributed along the opposite side of the tank. While electrodes 18 might, for example, be directly connected to a current controlling device 16 by way of bus 15, it is preferably that each of electrodes 18 be connected by way of a primary winding 2d of a set of transformers 26, 27, 23, 29 and 39, like the corresponding transformers for electrodes 1'7.

Each of the transformers 2140 has a secondary winding, being generally alike for all transformers. Transformers 21-25 have secondary windings 31 connected together in series and in like polarity, such that an increased current in the windings l9 induces an increased electromotive force in the windings 31, which are connected in series aiding. When so connected the secondaries 31 form a complete closed loop, such that a common current will be induced therein whenever a current flows through the paralleled primaries l9 and through the electrodes 17. Similarly, transformers 263tl have primary windings 20 connected in parallel to the bus 15 and secondaries 32 connected in series aiding, as in the case of the secondaries 31, to form a closed loop.

Because of space requirements for a large tank it is preferable to form a separate circuit for the secondaries 3i and 32. It will be realized that the secondaries 31 and 32 might be connected together in a single loop with proper attention to the polarity if all electrode currents were to be equalized or compensated. However, as illustrated in FIG. 1, a shorting bar 34 connects opposite ends of the series connected transformer secondaries 31, and shorting bar 35 connects opposite ends of the secondaries 32, while inner connections may provide turns variation.

in FIG. 2 there is illustrated a tank 10 of the type which would be employed for the melting of glass and controlling the temperature prior to pouring or drawing glass therefrom. A pair of power leads 3d and 37 extend from a source of alternating current to the electrode transformers and to a suitable return path from terminals at the opposite side of the tank 10. The tank itself is prefera'oly formed-of a substantial thickness of insulating material as at '38 which provides suitable thermal insula-" tion and has resistance to decomposition. The tank is preferably equipped with a spout 39 for the drawing of portions of the liquid glass after is has been heated. A cover extends over almost the entire surface of the tank except for a supply port 43 into which the material to be melted is introduced by means of a spout 44 and a conveyor 45. The material brought into the spout 44 would normally be composed of the ingredients in weighed quantities to comprise the several batches of material introduced by Way of the port 43 for melting together to form the glass product of the furnace. The molten glass shown at 46 might, for example, be of several feet depth and of as much as several thousand square feet area.

The spout 39 normally has a closure member 42 which I serves to cut off the flow of glass from the tank.

Transformers 21, 22, 23, 24 and 25 are shown connected to terminals 17, which may be identical for equal heating effect but are not necessarily alike, since different amounts of heating may be desired.

While it will be understood that a number of different furnace arrangements may benefit from the employment of the current compensating arrangement of this invention, the very high negative temperature coefficient of re sistance of molten glass makes mandatory the. controlled current distribution between the electrodes of the heating system when adirect resistive electrical heating of glass is employed. In the apparatus illustrated it will be appreciated that the process may be continuous such that the molten material 46 is never allowed tocool or solidify, and the 'ingredientsmay be deposited in the tank in a solid form, usually powdered, and as deposited would be a very poor conductor of electricity. It is thereforecommon practice to employ initial gas heating in which a flame is directed over the surface of the material. tinuous process the gas flame may be employedor may be omitted for the reason that the large mass of glass td once melted will receive batches of materials from the conveyor 45 which will mix with the much larger mass of glassalready contained in the tank and will be brought In a conthat current.

quickly to a molten condition and to a temperature at which satisfactory resistance-temperature conditions permit electrical heating. Thus, this apparatus may be employed either as purely an electrical heating system or as an electrical booster to increase the melting capacity of a furnace. Overall control of heat input is accomplished by means of a current controlling device as at 16 and it is feasible to exercise a close control of the temperature of the melt as it leaves the furnace by the spout 39.

The principles of the operation of the invention will be seen by considering first, that even though the overall current to a furnace is controlled, it is not feasible to employ a pair of large electrodes, one on either side of the tank, when the material is of large negative temperature coeflicient, since the current between the electrodes along any particular path causes a local heating and the decreased resistance causes an increased flow of current in the path already carrying too large a current. Other portions of the tank will receive a reduced heating because of limited current capacity and increased relative resistance. A further effect of this local heating is that a uniform temperature cannot be established throughout the tank. Excessive current passing between two points of the pair of electrodes causes destruction of electrodes, or so shortens their life as to make this method of heating impracticable.

To correct these difiiculties a large number of electrodes are separately supplied from a power source arranged in pairs to have currents therebetween isolated from currents between other electrodes. It is preferable that cross-currents between electrodes of differing pairs be largely eliminated. For this reason it is desirable to separately control the currents in each of the electrodes on one side of the tank and also to separately control the currents in the electrodes at the other side of the tank. This arrangement is illustrated in FIG. 1 by the employment of duplicate transformers for the primaries of each pair of electrodes.

Buses 1 and 15 are supplied from a transformer 12 with an AC. voltage building up in like polarity on each of the terminals 17 and in the opposite polarity on terminals l8 twice for each cycle of the alternating current supply. As current builds up between each pair of electrodes there is induced in transformer secondary windings 3i and 32 like electromotive forces. For example, transformer 22 has an EMF. built up in its secondary 31 and a similar in secondary 32 of the transformer 27, the transformers 22 and 27 forming the supply for one pair of electrodes 17 and 18. In each of the other pairs ofelectrodes the same circumstance obtains so long as .the circuits are alike and the temperature is similar throughout the tank. A current will thus flow through both of the secondary circuits to generate a counter electromotive force in a direction to oppose the passage of current in the primary windings, which is of like effect in all parallel circuits. If one portion of the'molten material 33 is at higher temperature than other portions a larger current would tend to flow between the associated electrodes for the reason that the resistance of the material 33 is there decreased. However, such an increase in current in one pair of transformers such as 22 and 27 results in an increased current in all series connected secondaries 31 and 32 of which a larger than average contribution is now made by transformers 22 and 27. By well known theory a counter electromotive force is always generated in an induction coil in a direction tending to oppose the current which produced it and in amount proportional to are no longer contributing equally to thesecondary currents and the counter electrornotivef forces therein are reduced and a greater voltage passed to the electrodes fed 7 a If the secondary coil currents should now i therethrough. Thus, this circuit serves both to decrease the current in paths of lowered resistance and to increase current in paths of above average resistance.

The larger the discrepancy of current the greater is the compensating effect both in the electrode carrying the larger current and in those other electrodes having less than average current. An opposite result is obtained if the temperature is decreased at one of the pairs of electrodes, as for example, when a new batch of material is loaded into the furnace at port 43. The effect described is analogous to a negative feedback in an amplifier or in a servo system, and the negative feedback tends to eliminate the discrepancy in current between the electnodes of each group whose secondary transformer windings are connected together in a series closed loop.

To see the operation of the invention from a voltage transfer viewpoint one may consider the case of three transformers feeding three electrode pairs and drawing a total current of 100 amperes from a 300 v. supply. If one electrode path is of ohms resistance, a second of 9 ohms and a third a little over 8 ohms we find the voltages necessary to equalize the currents to be 333 v., 300 v. and 267 v., thus providing an average of 300 volts. If the secondaries are in closed series loop and alike, there is seen the requirement that the ampere turns be alike. The turns are made alike and the currents must then be alike in the primaries since the flux in the cores is proportional to ampere times turns in each case, and since the flux is assumed to be fully coupled to both primary and secondary windings in each case. This provides the conclusion that the secondary for the l0wer-than-average resistance between electrodes must absorb 33 v. (300 v.267 v.) which is applied at the 10 ohm path transformer to supply the needed 33 v. excess above the 300 v. supply.

'This is the general effect of the closed loop secondary connection, modified somewhat by the fact that current must flow in the secondary windings in order to transfer the voltage, and resulting in an accompanying average voltage drop across the primary windings by virtue of the counter electromotive forces as previously noted.

It will be noted that each of the transformers 21-30 has been assumed to be like each other transformer of the group in order to supply equal heating through the various pairs of electrodes. In an actual construction it may be found that more heating is required at certain locations, such as at the input end of the tank, or less heating at other locations. The control circuit of this invention provides a means of distributing current between pairs of electrodes as may be desired. When each of the transformers has like windings like voltages are transferred to the corresponding electrodes of the group. If the secondaries are made alike for each transformer of a group the ampere turns will be alike since they are in series and must conduct the same current. When the ampere turns are alike in the secondaries of the group the ampere turns will be alike in the corresponding primaries, assuming the transformers to be close-coupled. If it is desired that more current be passed through the electrode supplied from the transformer 21 this may be accomplished by providing that a larger current shall pass through the winding 19 of the transformer 21 than in the primaries of the other transformers of the group to produce like ampere turns in conformity to the like ampere turns of the secondary windings of the group. This may be accomplished according to well known transformer theory, by reducing the number of turns in the primary 19 of the transformer 21 with respect to the secondary windings of that transformer. A reduction of the turns ratio to one-half would thus require twice the current in primary 19 of transformer 21 over that required for the other transformers. It might also be desirable to reduce the current through one pair of electrodes, for example, near the output end of the furnace. This may be done by increasing the turns ratio of the transformer 25, such that a smaller current is required in its transformer primary to produce the ampere turns of its secondary winding which will be common to all of the transformers of the group. Because the secondary windings are all in series within one group the reflected impedance in the primary winding of each of the transformers will be in proportion to the turns ratios of the several transformers. Instead of varying the primary windings 19 the secondary windings 31 of the group may be varied in order to provide a primary current through each transformer which is inversely proportional to the turns ratios of the transformers of the group.

It will ordinarily be desirable to eifect similar changes in the turns ratios for the transformers in series with electrodes 17 and 18 of each pair. Thus, if transformer 21 has a changed turns ratio to effect a greater current in electrode 17 a similar change in turns ratio of transformer 26 will ordinarily be made in order that the current shall pass exclusively between the electrodes of the pair. Likewise, when a reduction is desired in the current at the electrode associated with transformer 25 a similar reduction in current will be appropriate for the electrode asso ciated with transformer 30. Each winding may have taps to vary the turns ratio.

As illustrated in FIG. 1 transformers are employed in pairs of which the primaries are both in series with the electrodes carrying heating current through the material 33. It might be satisfactory for some installations to employ a single set of transformers as in FIG. 2 connected to a power supply line 37 of which the return path is through a common central electrode or some other arrangement, and returned to the power supply by way of the power connection 36, here shown diagrammatically. It will also be understood that the electrodes 17 may be disposed vertically to pass through the bottom of the tank, or may be arranged around the sides thereof, so long as the current from the various electrodes 17 to the return path 4-7 is caused to take paths separated by sufficient distance to efiect even distribution of current.

While one of the objects of the invention is to provide a means for heating a glass melting furnace the invention here disclosed has other applications and may be employed whenever it is desirable to resistively heat a mass of material having a negative temperature coefficient of resistance. Various ceramic materials and other semiconductor materials exhibit this negative temperature coefficient to a degree which renders the resistive heating thereof diflicult or uncertain in the absence of a current compensating device as here disclosed. In overcoming this effect the action is wholly automatic, requires no moving parts, no maintenance, no make-and-break contacts, and is eco nomical of capital cost.

Another application of this invention concerns the em ployment of carborundum rods which serve either as electrodes in .a heating unit or themselves serve as the resistive elements of a heater. When a number of carborundum rods are to be employed in the same unit and it is desired to maintain the heating effect from each at an approximate average value the carborundum rods may be connected to the power supply by way of transformer primaries as illustrated in the drawings for the present example. In this case the rods themselves have a negative temperature coefiicient of resistance and would tend to heat unequally over the surface thereof unless the rods were kept small. Furthermore, when several pairs of such rods are employed it may be found that one pair of rods, or a single rod, might become heated more than others and thus have a lowered resistance to the passage of current, and would therefore continuously heat to a hotter and hotter temperature until the material surrounding it became contaminated with decomposition products, or the rod destroyed by extraordinary local heating. Similarly, carbon arcs exhibit a degree of negative temperature coefiicient of resistance and may be connected in a similar manner to equalize the current between several such carbon arc circuits.

7 a Illustrative of one embodiment of the invention a glass furnace was heated by a number of pairs of electrodes -lar transformer winding primaries were then connected in each circuit to a pair of electrodes and the similar secondary windings thereof were connected in series as illustrated. In this installation the transformer cores were of suilicient flux capacity to permit a voltage differential of approximately 12 volts between the primary terminals in each transformer. Under this circumstance it might be said that the transformers thus had a voltage rating of approximately 4% of the employed voltage. The result, however, was a reduction in the current disparity between the electrode pairs to well below 10% of the current flow. When the coils were not used, the disparity was as high as a ratio of 5 to 1 from one electrode to another. It will, of course, be understood that a complete balance may be obtained by increasing the voltage H rating capacity of the transformer.

In general, a voltage transfer is usually adequate for complete control. The transformers are accordingly employed in very much smaller size than might be expected and produce much larger regulation effects than would be expected from the voltage ratios which could be calculated.

In accordance with transformer theory and practice, each of the transformers of a group may be of step-down type or of step-up type, in which the relative ratios between the transformers of the group determines the relative distribution of the current between the electrodes of the group. In the case of a glass melting furnace the currents drawn are frequently in the hundreds of amperes per electrode. In such service a smaller current may be employed in the secondary loop if the step-up type transformer is employed, requiring much lighter wiring and all the cost and attendant handling advantages.

These controls or balancing effects are based upon Well known transformer operating principles in which the basic relationship is ampere turns and volts per turn. if different voltages and currents are to be assigned to individual electrodes of the multiple series, then the individual transformer primary and secondary turns can be computed on these principles.

While the invention has been described in connection with this single application thereof, it will be appreciated by those skilled in the art that it may be otherwise employed and it is intended that various equivalents and modifications be included within the scope of the appended claims.

What is claimed is:

l. A device for equalizing the resistive heating in a molten mass of negative temperature coefiicient material, comprising a source of electrical current,

a plurality of electrodes immersed in said mass being energized in parallel from saidsource, a plurality of primary transformer windings each in series with one said electrode and each energized in like phase from said source, 7

a plurality of secondary transformer windings individually associated with said primary windings, respectively, and i means connecting said secondary windings in a closed series circuit for like phase response to currents in said primary windings, whereby common secondary current flows in all said secondarywindings.

2. A current compensating arrangement for an electric furnace directly heated by current supplied through multiple electrodes immersed in a body of material of negativetemperature coefficient, comprising a source of heating current, a multiplicity of electrodes connected in at least three parallel circuits through said material to said source,

Cit

Q 1;) I transformer means including a primary winding seriesconnected in each said electrode circuit, transformer means including a secondary winding indi vidual to each said primary winding and closely coupled thereto, and

means including said secondary winding for coupling current in each said electrode circuit to each other said electrode circuit in a sense to oppose deviation of current in any said circuit from a preferred relation to the average current in said circuits.

3. A current compensating arrangement according to claim 2, in which said coupling means comprises means connecting said secondary windings in a series-connected closed loop.

4; A current compensating arrangement according to claim 2, said transformer means associated with each said circuit being of like turns ratio to equalize the current in said circuits.

5. A current compensating arrangement according to claim 2, said transformer means associated with each said circuit having a turns ratio between each associated primary and secondary variable between said circuits to provide selectably different degrees of heating from the electrode currents in said parallel circuits. i

6. A current compensating arrangement according to claim 2 wherein said primary windings have equal turns and said secondary windings each have active turns in series in proportion to the desired currents in the associated electrodes, respectively.

7. A current compensating arrangement according to claim 2, wherein said secondary windings have equal turns and said primary windings have actively energized turns in inverse proportion to the desired electrode current therethrough, respectively.

8. An electrically controlled melting furnace for elec trically resistive material of negative temperature coefficient of resistance, comprising a melting chamber containing said material,

at least three electrode pairs disposed at intervals within said material,

means parallel-connecting each said pair of electrodes to a current source,

a transformer primary winding in series with each said electrode pair, a transformer secondary winding coupled to each said primary winding, and a means interconnecting said secondary windings to cause increases of current in one said electrode pair from a mean current'to induce greater electromotive forces in said primary winding thereof proportioned to said increases and lesser electrornotive forces in said primary windings of each other said pair, whereby current is decreased in said one pair and increased in each other pair. a

9. A furnace according to claim 8 wherein said means for interconnecting said secondary windings. includes a series connection therebetween to form a closedloop of secondary windings responsive in like polarityto like changes of current in said primary windings.

10.'A furnace according to claim 8 wherein the ratio of primary and associated secondary winding turns is adjusted in inverse proportion to the desired heating currents supplied to the electrode pair in series with said primary windings, respectively. 7 a

'11. A method of controlling the current in a plurality of paths through a material 'resistively heated thereby comprising, i

' supplying heating current at separate terminals to said paths in said material,

inductively coupling current variations in each said path to a common circuit in like phase to induce therein variations ofcurrent proportional to the sum of the current variations of all said paths, and V inductively coupling variations of. current in said common circuit to each of said paths to reduce current flow in a said path of greater than average current and to increase current flow in a said path of less than average current.

12. The method of controlling the current in a plurality of paths through a material resistively heated thereby, comprising supplying heating current for said material in a plurality of parallel-connected paths, inductively transforming current in said paths to produce voltages proportional thereto, respectively,

applying said voltages in series to produce a combined secondary current varying with the sum of said voltages,

inductively coupling back said combined secondary current into said paths individually to induce therein counter-electromotive forces proportional in each instance to the portion of said combined current resulting from said voltage induced by current variations of the individual path, whereby larger than average currents are reduced and smaller than aver- References Cited by the Examiner UNITED STATES PATENTS 1,696,177 12/28 Evans 323-85 X 2,100,313 11/37 Fitzgerald 323-85 X 2,659,764 11/53 Konig 13-6 2,830,254 4/58 Davis 323-50 X RICHARD M. WOOD, Primary Examiner. JOSEPH V. TRUHE, Examiner.

Claims (1)

1. A DEVICE FOR EQUALIZING THE RESISTIVE HEATING IN A MOLTEN MASS OF NEGATIVE TEMPERATURE COEFFICIENT MATERIAL, COMPRISING A SOURCE OF ELECTRICAL CURRENT, A PLURALITY OF ELECTRODES IMMERSED IN SAID MASS BEING ENERGIZED IN PARALLEL FROM SAID SOURCE, A PLURALITY OF PRIMARY TRANSFORMER WINDINGS EACH IN SERIES WITH ONE SAID ELECTRODE AND EACH ENERGIZED IN LIKE PHASE FROM SAID SOURCE, A PLURALITY OF SECONDARY TRANSFORMER WINDINGS INDIVIDUALLY ASSOCIATED WITH SAID PRIMARY WINDINGS, RESPECTIVELY, AND MEANS CONNECTING SAID SECONDARY WINDINGS IN A CLOSED SERIES CIRCUIT FOR LIKE PHASE RESPONSE TO CURRENTS IN SAID PRIMARY WINDINGS, WHEREBY COMMON SECONDARY CURRENT FLOWS IN ALL SAID SECONDARY WINDINGS.

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