Inductor type furnace

Application filed October 16,

My invention relates to combined heating and stirring connections for electric furnaces whereby both the heating and the stirring may be performed to the best advantage.

β One purpose of my invention is to heat and to stir the furnace by electric currents of very considerably different frequency.

A further purpose is to pass higher frequency heating current and lower frequency 10 stirring current through the same furnace inductor coils, preferably at the same time, to perform thereby the preferably coincident double function of heating and stirring to the best advantage.

A further purpose is’to. connect higher and lower frequency currents to the same inductor coils, using frequency traps where needed to prevent the current of either of the frequencies from shunting through the connec20 tion of the other.

A further purpose is to connect higher and lower frequency coils to the same inductor coils, connecting one to flow through the coils in parallel and the other to flow through 25 the coils in series, and using frequency traps to prevent one current from short-circuiting through the connections of the other current.

A further purpose is to supply higher and lower frequency to the same inductor coils SO applying the current of one of the frequencies by connections to the coils in parallel from oiie set of coil terminals and the other through the coils in series and preventing leakage from the series connections through the par35 allel connections.

A further purpose is to apply low or medium frequency stirring to an existing furnace inductor' coil utilizing a higher frequency.

A further purpose is to apply currents of higher and lower frequencies to a furnace inductor for purposes of heating and stirring molten metal in the furnace, and to separate the currents to cause them to pass through 45 their respective paths by appropriate condensers or inductance to block flow of current of one frequency through the connections for application of currant of the other frequency.

A further purpose is to connect higher and 50 lower frequency currents to the turns of two

1,862,215 diagrammatic, and, except for Figure 1 which represents a prior art connection, all show different circuits by which my invention may be carried out.

,. In the drawings similar numerals indicate ^u like parts.

Current of frequency above commercial frequencies is quite desirable for the purpose of inducing heating current in the molten pool , ₀ of a coreless induction furnace because of the higher rate of change of the current induced in the secondary. The frequency is selected usually with a view both to economical generator manufacture and use and economical ,₅ condenser power factor correction.

The heating effect of the current varies· with the frequency and also with the square of the ampere turns. On the other hand the stirring effect of the current varies with the ₂₀ square of the ampere turns and not with the frequency.

At higher frequencies it is possible to obtain the desired heating effect with relatively low current values but with correspondingly 25 lowered stirring effect. It is my purpose to secure highly advantageous stirring also without interfering with the heating effect, nor with the selection of any frequency which may be desired for it wholly independently 30 of the desired stirring. I secure this result by applying two different frequencies which may be applied concurrently or successively. It is much more advantageous to apply the two frequencies together than successively, in 35 order not to lose the time in heating or stirring while current of one frequency is withdrawn in order that the other frequency may be used to stir or heat.

I believe that I am the first to apply current of different frequencies to an inductor furnace for the purpose of heating with a higher frequency and stirring with a lower or commercial frequency and that I am the first also to apply these frequencies coin₄₅ cidently, to apply them to an inductor either wholly or partly surrounding a molten charge, or to apply them either through the same inductor coil turns or through different inductor coil turns. I believe that I am also _&0 the first to apply two different frequencies to the same inductor coils while protecting against the current of one frequency passing through the connections by which the other frequency is applied, the first to use frequency 55 traps for such protection, the first to apply inductance for such a purpose so wound that it does not interfere with passage of high frequency current but greatly reduces or blocks passage of low frequency current, and the BO first to apply condensers to prevention of passage of the lower of two frequencies so used through the connections for applying the other frequency.

In my drawings, I have shown the two C5 frequencies of supply applied in a variety of ways. Where applied concurrently they are shown as passing through the same coils in series for both frequencies, or through the coils in series for one and in parallel for the other, using frequency traps to prevent 70 intermingling of the currents of the two frequencies except as they pass through the inductor coils and many of them balancing the coils as in a Wheatstone bridge. The two frequencies are also shown using independ- 75 ent coils in which they may be applied either concurrently or successively, and as passing through the same coils alternately as distinguished from keeping both heating and stirring currents on concurrently. 80

As it is more convenient to describe the invention beginning with those forms in which the windings and connections are balanced, which may not inaptly be compared with the balanced arms of a Wheatsone 85 bridge I will begin with this form.

As is well known, the Wheatstone bridge symbolized in Figure 1 as having arms a, δ, c and d may have the source of current 15 in the middle connecting with terminals 00 16 and 17 and the current indicator 18 in the place where the applied current is generally located, connecting with terminals 19 and 20. This is a mere rearrangement of the diagram. 05

If the source of current can thus be placed in either position without disturbing the indicator, two sources of current can be used in the two positions 15 and 18 without the current from either source passing through 100 the other.

In Figure 2 the bridge arrangement has been applied to a furnace construction, and sources of current 15' and 18' of markedly different frequency have been located at the 105 two positions. The arms b and d are here two oppositely wound inductors 21 and 22 making up together a total furnace winding adapted to surround the melt. They are separately compensated for power factor by no bridged condenser capacity 23 and 24. The arms a and c may contain either capacity, inductance or resistance, of which I have illustrated capacity at 25 and 26. The use of resistance is wasteful of energy but is in- 115 eluded in order that the discussion may be complete.

From Figure 1 it will be evident that if the arms be properly balanced current from the higher frequency source 18' will not be 120 transmitted to or through the lower frequency source 15', and likewise current from the lower frequency source 15' will not be transmitted to nor effect the higher frequency source 18'. The lower frequency may 125 advantageously be a commercial frequency such as 60 cycles.

The higher frequency current from source 18' passes through the arms a and c and through the inductors 21 and 22 in parallel. i2

1,862,215 shown, in that they are oppositely wound upon the same magnetic core 30 (Figure 12) to neutralize each other with passage of high frequency current through both of the branches a and c in parallel, but to add their inductances to prevent short-circuiting leakage of lower frequency current through them which should pass through the furnace inductors. ...

The tap connections provided in Figure 3, by which larger or lesser parts of the inductors may be included in the parallel connection, provide variation of the voltage upon the condensers 23', 24' with consequent variation in the current passing through the mductors. It is quite desirable to tune, or approximately tune, the inductor circuits with the result that the higher frequency, current passing through the tuned circuit containing induction and condenser power 35 factor correction, will have many times the value of the current passing through the higher frequency generator.

It is evident that any of the other higher frequency connections shown may be made 90 to the inductors through taps corresponding generally with those in Figure 3 where it is desirable to vary the number of inductor turns bridged by the connection.

Adjustability of the condenser power factor 05 correction is shown for the purpose, chiefly, of indicating that the power factor correction can be accommodated to any conditions of the melt or types of melt,, or to any adjustments of the number of inductor turns 10bridged by the higher frequency supply.

Both of these forms of Figures 2 and 3 as well as forms appearing in Figures 5, 6, 7 and 8 have connections of what I have referred to as the bridge form, and use two icj inductors which may be wound in the same direction or in opposite directions as seen. All but the form of Figure 7. are particularly suited to concurrent application of the higher and lower frequencies of current 11c supply. As the two circuits operate independently, in the sense that neither one disturbs the operation of the other, either supply can be cut off at will when the application of the other one alone is desired, and the form 115 of Figure 7 is capable of concurrent, application of the two frequencies when desired.

It is necessary of course, to limit the sum of the two currents passed through any inductor to that current which the inductor will carry.

I have referred to the lower frequency and the higher frequency respectively to point the fact that what is called lower frequency need not necessarily be commercial fre- 1-5 quency, and what is called higher frequency may not come within the measure of what by some would be called high frequency. There must, of course, be a decided difference.in the frequencies or there is no advantage in apSince the inductors are oppositely wound the heating currents induced in the melt by the current in the two inductors will be in the same direction.

In the absence of a frequency trap or other means to prevent it the lower frequency current would divide through the inductors, in series and through the arms a and c in series. It is prevented from doing so in the illustration by the condensers. 25 and 26 which have been designed for high frequency and which act as frequency traps for the lower frequency. The lower frequency, therefore, is held to its path through the inductors 21 ₁₅ and 22 in series.

For the stirring purpose, the fact that the current passes through the turns of the two reversely wound inductors in opposite directions, so as to induce current in the melt 2Q also in opposite directions, makes little difference. Motor effect circulation, is not dependent upon the absolute direction of flow of current but depends upon the fact, true in regard tc both of the inductors 21 and 25 22,.that the (secondary) current induced in the bath by each inductor will be opposite in direction to that of the primary inducing the current.

However, if it be desired to avoid a slight :jo overlapping of the fluxes of the low frequency field of the two inductors in order to get a maximum stirring effect from them, with a minimum of intermingling of the fields due to the inductors, the coils may be >>- spaced a short distance longitudinally of the furnace. The spacing may desirably approximate twice the distance from the molten metal to the coil. ...

In the form of Figure 2 the capacities m _tl) arms a and c could be replaced by inductances as shown in other figures. Resistances could be used here and would serve to cut down the flow of the current of lower frequency in arms a and c and hence to increase the flow 45 of this current through the inductors but are very much less desirable than capacities and inductances because of the PR losses of the currents of both frequencies which pass through the resistances. They are included 50 to complete the discussion.

The same arrangement of parts is shown in Figure 3 as in Figure 2, with three exceptions. Inductances 27 and 28 in branches a and c afford the· frequency traps, as distin55 guished from the traps shown by condensers 25 and 26 in Figure 2; the current supply of higher frequency as shown is adapted for connection to the inductors through taps 29, 29', so as to vary the number of turns of the 60 inductors included in the parallel connection; and the condensers 23', 24' are also shown as adjustable.

The use of inductances instead of capacities is of further advantage in this form because ββ of the relation between the inductances

1,852,215 plying both, since the second frequency would not improve the heating or stirring, as the case may be, sufficiently otherwise to justify its use. My invention will of course find its greatest utility in places where the lower frequency current supplied is taken from commercial alternating current feed lines, and in which the heating current will be of a frequency very much higher than this.

The advantage of applying the heating current and stirring current concurrently lies in the saving of time which would be required to apply them successively, and avoidance of any question of undue stirring or heating 15 while, the heating or stirring is effected. At the same time, my construction makes it possible to operate from one source of supply alone, permitting application of heating current alone or concurrent supply of heating 20 and stirring currents up to the point where a stirring operation is required, with subsequent stirring alone, or heating alone, as the case may be, during the treating.

In Figure 4 a form is shown which is im25 portant because it is a good form with a view to construction and is also well adapted for use in conjunction with a high frequency melting furnace to which it is desirable to apply ^an intermediate or low frequency stir30 ring current.

In this Figure 4 a single inductor 21² only is shown, adapted to be supplied with taps 29, 29', if desired, and to which inductor the higher frequency is connected through con35 densers 25 and 26. Power factor correction is provided by condenser capacity at 23². Lower frequency current is provided through any suitable source 31_r preferably of commercial frequency—say, 60 cycles—connected 40 through transformer 32, so' as to apply the current to the terminals 33 and 34 of the inductor.

_ A circuit 35 is .shown, comprising a coil 36 high in inductance and very low in resistance, · 15 and a capacity 37. The circuit is tuned to the higher frequency and constitutes a fre- i quency trap to prevent leakage of higher fre- i quency current through it though the lower i frequency passes through the inductance coil i 50 36. The condensers 25 and 26 form frequency 1 traps for the lower frequency current, 1 Neither current can thus short-circuit i through the connections for the other. i I have inserted Figure 5 for the purpose ^e - of showing that even where, it is desired to 1 utilize the general connections, of Figures 2 s and 3, for example, in which the two indue- < tors are oppositely wound, it is not necessary f that these inductors be in fact wound ορρό- i sitelv. The connections of Figure 5 are ef- i fectivelv those of Figure 4, except as to the i connection by which the high frequency is a run through the inductors 21³ and 22³ in par- c allel, so as to produce heating flux in the same v ⁶⁵ direction within the furnace pool, and the q stirring current is passed through the two e inductors in series with the effect of producer ing flux in the pool in opposite directions, s The connections for the higher frequency r supply are made at 16 and 17 at one end and ⁷⁰i 20, 20 (the “intermediate” connection com!, mon to both inductors) at the other, and the f lower frequency supply is connected across terminals 16, 17, so that the parallel flow of

- h’gher frequency (heating) current passes ⁷⁵i through the inductors from the top downl wardly in the same direction, giving uniform : direction of flux throughout the furnace. On > the other hand, the lower frequency stirring connections are made with the upper end of ⁸⁰ the upper inductor, through the length of this inductor and from it to the lower end of the lower inductor, upwardly through this : coil and retnrnmg to the generator, giving ; reverse direction of magnetic flux through ⁸⁵the two inductors.

The form shown in Figure 6 corresponds generally with the forms shown in Figure 5, except that the connections of the higher and lower frequency current supplies to the in- ⁹⁰ductors are relatively reversed so that the higher frequency current passes through both inductors in series, from terminal 16 to term nal 17. and the lower frequency supply is connected to pass through the two inductors ⁰⁵in parallel by connecting one lead from the lower frequency supply to the terminal 20 between the inductors and the other lead from the lower frequency supply through counterbalancing induction coils 27' and 28' ¹⁰⁰in parallel to the inductors 21*, 22⁴ and through them in parallel.

Here the winding of the coils 27', 28' should be in opposite directions as before, in order that they may be non-inductive to ¹⁰⁵passage of current of lower frequency through them in parallel and highly inductive for passage of current of higher frequency through them in series.

Where for any reason it is desirable to ¹¹⁰apply currents of the different frequencies to the furnace one at a time rather than concurrently, a construction can be used corresponding generally with any of the other figures, but with means for interrupting or checlrng ¹¹⁵the flow of current of one of the frequencies while current of the other frequency is being applied, and vice versa.

. In Figure 7 for completeness of illustration I have shown a form in which high re- ¹²⁰sistances 38 and 39 are used to prevent shortcircuiting of a low frequency current supply applied to the terminals 16 and 17 of the reversely wound inductors 21 and 22. The current is supplied through a transformer 32 from any suitable lower frequency source 31 and short-circuiting of the lower frequency current through the branches a and c is prevented by the resistances when the lower frequency is applied by closing switch 40. ^{; 3} s

1,852,215 but complementary main purposes of heating and stirring and that in the absence of some special reason why these should be successive there is advantage in making them concurrent where the time element is important or where the stirring effect of the heating current used, or the heating effect of the stirring current used is insufficient to. accomplish the purposes required at the time.

My broad invention is effective for heating _7δand stirring to the best advantage and with the greatest economy whether the connections’ be bridge connections or apply both currents at the same, or approximately the same inductor terminals, or use wholly dif- ₈o ferent inductors. It will be evident moreover that I can accommodate any desired conditions of heating and stirring, using, a particularly high frequency if it be desired to heat to a maximum while keeping the metal ₈₅as quiet as possible and obtaining any proportion of the heating to the stirring by selection of the frequency or by applying the currents concurrently and reducing the strength of one of the currents. This makes go possible a very nice adjustment to the needs of treating operations covering the entire range from mere holding nearly' free from circulation while maintaining or even varying the temperature to treating operations ₀₅requiring high stirring effect while holding or while varying the temperature. .

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless _]nobecome evident to others skilled in the. art, to obtain all or part of the benefits of my invention without copying the structure shown, and I, therefore, claim all such in. so far as they fall within the reasonable spirit 105 and scope of my invention.