US1983242A - Induction furnace - Google Patents

Description

Dec. 4,1934. w 1,983,242

' INDUCTION FURNACE I Filed Aug. 15, 1951 3 SheetS-Shet 1 [nVe ntor flli'arnay- Dec. 4, w ROHN INDUCTION FURNACE Filed Au fls; 1931 s Sheets-Sheet 2 r 0 i n 6 V .m

k 5 C Jim/"nay De. 4, 1934. w. ROHN 1,983,242

' NDUC ION FU NACE Filed Aug. 15, 1931 3 Sheets-Sheet 3 Figia. I L

Inventorlliarncy Patented Dec. 4, 1934 INDUCTION FURNACE Wilhelm Rohn, Hanau-on-the-Main,

Germany Application August 15, 1931, Serial No. 557,234 In Germany August 28, 1930 Claims.

This invention relates to improvements in and relating to induction furnaces.

Since induction furnaces without a core have, during recent years, been introduced into practice 6 to an increasing extent, there have been numerous experiments in order to so construct this furnace system as to enable it to be actuated with the usual frequencies. Furnaces are also known which are operated with 50-cycle single-phase l0 alternating currents. Endeavours have been made to diminish by a specific construction or dimensioning of the parts, the eddy current which occurs in such furnaces.

Operation with a 50-cycle single-phase alterlli nating current does not, however, solve the problem, as it is always necessary to have a separate generator, even though this is of usual construction. Moreover, single-phase generators are less economical than three-phase current generators.

For this reason, coreless induction furnaces have, therefore, been actuated with a three-phase current on the Scott system. In this case, however, it is very difficult to secure continuously'a complete balance of the phases.

Finally, furnaces have been actuated by a three-phase current, in which case, however, it was necessary to use a rotary spark gap for supplying the current to the three sections of the coils, as this arrangement would otherwise produce a short circuiting of the'phases. In these arrangements there is the disadvantage that the magnetic flux must pass for considerable portions of its path through air, so that the power factor is very unsatisfactory. I

86 From a metallurgical point of view, previous arrangements are disadvantageous for many purposes, as the bath surface is small and the depth of the bath large so that the lower part of the lining has to withstand a considerable hydrostatic pressure, the shape of the hearth being comparatively unfavorable and the slag action not sufficiently eflective.

The invention will'now be described with reference to the accompanying drawings, wherein,

smelting chamber of this furnace being cylindrical. Fig. 6 shows a cross-section taken on the line VI-VI of Fig. 5.

Fig. 7 is a fractional perspective view showing a part of an. iron yoke and a hat coil surrounding one of the poles.

Fig. 8 is a vertical cross-sectional view taken on the line VIIIVIII of Fig. '7.

Figure 9 is a, diagram showing the arrangement 5 of the coils and the flow of the metal bath in a furnace-corresponding to Figures 1 and 2.

Figure 10 is a diagram Showing the coils and the flow-of the metal bath in a, furnace provided with six coils all of which are passed by the cur- :0 rent in. the same sense of circulation.

'Figure 11 is a diagram showing the coils and the flow of the metal in a furnace provided with nine coilsall of which are passed by the current in the same sense of circulation.

Figure 12 is a diagrammatical panorama view of the arrangement of coils according to Figure 10, seen from the center of the furnace.

Figure 13 is a diagram showing the coils an the flow of the metal in a furnace provided with 50 six coils which are alternately passed by different phases of the primary current, the direction of the current varying in the coils connected to one phase.

Figure 14 is a diagram showing the arrangeg5 ment of the coils and the flow of the metal in a furnace provided with 12 coils, the coils which are connected to one phase being alternately passed by the current in opposite directions.

Figure 15 is a diagram showing the arrangeo .ment of the coils and the flow of the metal in a furnace in which pairs of adjacent coils are passed bythe same phase of current the direction "of the current being opposite in the two coils of each pair.

Figure 16 shows a diagrammatical panorama of the coils according to Figure 15, seen from the center-of the furnace.

In Figure 1 is shown diagrammatically a furnace with a shallow bath shape suitable even for large charges of from 5 to 10 tons and more.

The three phases of a three-phase current are supplied to flat wound coils m, an, a: shaped in the form of bowls, which in view of the favourable shape of the hearth b can be placed very close '10 to the metal bath, so that eddy currents become fairly low. If, as shown in Figures 3 and 4, a starshaped iron yoke c or a triangular one (I or a yoke of any other suitable shape is arranged outside the coils, the part of the way which the magnetic lines'have to go through air is reduced to a mini-'- mum. The entire magnetic flux passes practically through the iron of the yoke or the smelting bath, so that a satisfactory power factor is obtained. In order to protect the surfaces of the yokes from the transmission of heat from the melting bath, the coils may be wound in the usual manner from copper pipes through which water ismade to flow or from fiat copper strips cooled by a device in contact with the coil. In this case the inner part of the coil can cover the front surface of the poles, whilst the supply of current need only be connected beyond the front face of the poles. Separate flat coils of thin copper tubing may also be placed in front of the front faces of the yokes, these coils not being passed by current, butbeing solely for the purpose of shielding ofi the heat.

In Figs. 5 and 6 B is a cylindrical smelting chamber. A1 A3, A3 are the coils wound round the poles C1, C2, C3. In Fig. 5 only one of these coils and poles is shown.

Figs. 7 and 8 show, in pe spective and vertical cross-sectional view respectively, a fraction of the iron yoke e and one of the poles f projecting towards thesmelting vessel b. The poles are surrounded by fiat copper coils 9 containing a watercooled path h.

' Water-cooled coils i not traversed by the electric current are to protect the outer surfaces of the poles f against the heat radiating from the smeltingvessel.

The operation of such furnaces may either be efiected by separate three-phase current generators of the usual construction of such a size that they are capable of taking up the necessary wattless current, or the current may be supplied by means of transformers, with which there is arranged in parallel a synchronous machine running as a phase regulator. The latter arrangementcan'be built somewhat more cheaply. In a number of cases the supply, by reason of the favourable power factor of this arrangement, may also be efiected directly from the mains, with or without connecting condenser banks in parallel.

advantage of the construction described herein consists in the fact that all sizes of furnaces having a capacity of from about 1 to tons can be controlled with a frequency range of from about. 250 to 20, which are frequencies for which the usual construction of three-phase current generators can be used without substantial modiflcations. Even furnaces having a capacity as small as 1' ton can be operated successfully with 50 cycles.

A further development of my invention consistsin shaping suitable coils also for furnaces requiring a small surface and a great depth of the bath. Such furnaces are especially used in connection with the duplex process in which slag reactions are carried out in a first operation and. the smelt is refined or settled in a second separate operation. 7

According to the present invention furnaces having a cylindrical melting chamber are heated by means of three-phase alternating current in such a manner that the primary current flows through three fiat coils which are symmetrically distributed around the melting vessel to the surface of which they are conformed. These coils may either surround, wholly or partly, only the side wall of the cylinder or they may extend wholly or partly underneath the bottom of the melting vessel which bottom may be convex if desired. v

While the invention hitherto has been described merely in view of furnaces in which each phase of the three-phase current is only connected to one single coil, now an arrangement shall be described in which a greater number of coils divisible by three are used. For better understanding the above described arrangement is once more illustrated in Fig; 9 of the drawings by way of a diagram. The circle A is to show in whole bath around a vertical axis takes place.

Over this movement is superposed another move-.-

ment whichtakes place in front of the single coils and consists in a rotation around an axis. which is perpendicular to the plane of each coil in its center. When the melting hearth is hemispheroidal, these axes of the rotation in front of the single coils are slightly inclined upwards when regarded from the outside to the inside. With a 50-cycle alternating current the two rotary movements and the resulting total movement are relatively vehement so that such a furnace is especially adaptedfor reactions such as, for instance, puddlingiron, converting raw iron to steel, and slag-reactions (removing sulfur and phosphorus from metals). In furnaces which are especially determined for mere melting work this movement is too violent, but it may be moderated in the following manner.

Around the melting chamber several groups of three coils are provided. Fig. 10, for instance, shows an arrangement of twice three=six coils, Fig. 11 an arrangement of three times three-=9 coils. According to the desired strength of the resulting movement of the bath and to the size of the furnace a greater number of groups of three coils may be provided.

The numbers 1, 2, and 3 correspond to the phasesof the three-phase current. The several groups of coils are distinguished by the addition of indexes. Now supposed that with the arrangement shown in Fig. 9 the rotation in the horizontal plane takes place at the rate of p .turns per minute, the rate is 2 respectively with arrangements according to Figures 10 and 11. Thus, by increasing the number of groups of coils the number of the revolutions of thebath in the horizontal plane may be reduced to any desired extent.

The second rotary movement around axes which are perpendicular to. the planes of the coils in their centers will be the more violent, the greater the product of the number of the windings of the single coil and the intensity of the current flowing through the coil. 4 total amount of electric energy as to a furnace according to Fig. 9 is supplied to a furnace sc- 2 and 3 When the same cording to Fig. 10, the energy of phase 1, which in the case of Fig. 9 is supplied merely to coil 1, is divided among coils 1 and 1. Consequently in a furnace according to Fig. 10 the vehemence of rotation around axes perpendicular to the planes of the coils in their centers will be essentially reduced as compared with the vehemence of rotation in front of coil 1 in Fig. 9. Thus, with the increasing numbers of groups each of which comprises three coils the vehemence of the stirring of the bath is. also continuously decreased while the furnace absorbs the same total energy.

Thereby one is able to control the vehemence of movement of the bath to any desired extent.

Fig. 12 shows diagrammatically a panorama view of the coils of a furnace according to Fig. 10 as obtained by looking from the center of the furnace in the direction to its wall. The axes which are perpendicular to the planes of the coils in their centers appear as the centers of the circles B1, B2, B3, B1, B2, B3. The rotation of the bath around these axes takes place in the direction of the arrows indicated within the circles 1, 2, 3, 1', 2', and 3'. The surface of the bath is marked in this figure by a dotted line C--D. It appears that, for instance, in the interval be tween the coils 2 and 3 the liquid metal has a tendency of moving from top to bottom under the action of the coil 2, and a tendency of moving from bottom to top under the action of the coil 3. It is obvious that both these movements must compensate one another to a certain extent. These partial movements of the bath do not completely counterbalance one another, as in addition still anoiher movement in a horizontal plane exists and the different movements together always unite to form a resultant. is likewise obvious for the above mentioned reasons that the vehemence of the stirring action, regarded as a whole as well as from point to point, mustbecome the weaker the more the whole primary windings of the furnace are subdivided into groups of three coils per group.

The subdivision of the whole furnace-winding into more than one group of three coils per group results in a series of further possibilities for influencing the movement of the bath. This is shown by way of example in Fig. 13. The coils 1, 2, 3 are so connected to the three phases of the three-phase current, that they are passed, for instance, by the current in the clockwise or positive sense of circulation. On the contrary, the coils of the second groups 1, 2', 3' are so connected with the three phases of the current, that they are transversed by the current in the negative sense of circulation. The result of this arrangement is that in the right half of the bath the rotation in the horizontal plane tends to take place in clock-wise direction, and in the left half in the opposite direction. Both the movements partially counterbalance each other and produce a further reduction of the movement ofthe bath.

The same method of connection is diagrammatically illustrated in .Fig. 14 for a furnacewinding of four groups of three coils each, the movements of the bath in the horizontal plane under the action of the rotary field being represented by two-pointed arrows, whereas the movements of the bath which take place around approximately horizontal axes (i. e. axes passing through the coil centers perpendicular to the However, it

A further possibility of connection is illustrated in Fig. 15 in which three pairs of neighboring coils are connected to the three phases respectively, the circulation of the current in the single coils of each pair, taking place in opposite directions. Obviously, as seen from the panorama view of this connection in 16, in this case the movements of the bath around axes perpendicular to the planes of coils 1 and 1' strengthen each other between both coils 1, l &c., it being possible by determining the direction of the currentin the single coils in one or the other sense, to produce a movement of the bath between the two coils 1 and 1' either from bottom to top or from top to bottom. However, the impulses of rotation in the horizontal plane counteract one another as the rotating field produced by the coils l, 2, 3 has just the opposite direction of rotation to that of the rotating field produced by the coils 1,'2', 8'.

With an increasing number of groups of three coils per group an increasing number of connections results, a certain number of which influence the movement of the bath in the desired sense. The upper limit of the number of groups is merely given by the increasing difficulty of practical performance, and especially by the fact that the distance between the melting bath and the coils cannot be reduced indefinitely, because the refractory lining must have a certain minimum thickness. In the same proportion as the average winding radius of the separate coils approximates the thickness of the lining, the coupling coefiicient between'the coils and the bath becomes worse and thereby the power factor of the furnace is impaired. On the other hand. it is not desirable to reduce too much the stirring of the bath, as otherwise the constituents of the alloy to be produced. or the refining additions, would not be thoroughly mixed. Furthermore one is able by suitably varying the connections of the coils to work with strong stirring during certain periods of operation and with weak stirring during other periods; For many metallurgical operations, for instance, the following method is advisable: As long as the charge is'solid and it is only desired to supply per unity of time as much energy as possible, a connection may be chosen by which a molten bath would be stirred too vehemently. This connection then gives the optimum power factor. As soon as the bath begins to melt, a connection is made which results in a reduced stirring, and this connection is kept until the entire bath is liquefied and is heated to the desired temperature above its melting point. From this time the supply of energy to the furnace may be reducednas henceforth only the loss of heat of the bath is to be compensated, and no further increase of temperature is to be effected. The reduction of the energy results from a reduction of the current intensity in the coils, and because of the reduction of the current only a moderate whirling is effected, even with a connection which would produce an undesirably strong stirring with As mentioned above the power factor depends upon the ratio of the winding radius of the sepa-' rate coils to the thickness of the lining. On the other hand, the loss of heat increases with decreasing thickness of the lining. Therefore one is able to operate the same furnace with as high a power factor as possible and with somewhat increased loss of heat, or with a loss of heat as low as possible and with a somewhat reduced power factor. It must be taken into consideration that an increased loss of heat requires an increased supply of energy, i. e. an increased current intensity which results in an intensified whirling.

According to my invention the desired stirring may be obtained by starting with a primary connection producing a slight whirling of the melting bath, and thereafter disconnecting several coils and supplying still the same total energy to the remaining coils to produce a more violent stirring. When, for instance, the furnace is operated with 12 coils subdivided into 4 groups, each containing 3 coils, each of which is connected to one phase, the direction of the current varying from group to group, as indicated in Fig. 14 only a slight movement of the bath is effected provided that all coils are connected with the source of energy. Now when-disconnecting one of the sub-groups consisting of three coils each, the symmetry of the magnetic fields is disturbed and an essentially stronger stirring of the bath is produced. According to the desired strength of the stirring and the desired powerinput, also two or three groups may be disconnected. A corresponding procedure is to be adopted when the whole of the coils is subdivided into another number of sub-groups.

1. An induction furnace the charge of which is to be heated by means of a three phase alternating current, in which each of the three phases of the primary current flows through a different group of "vaulted pan-cake coils shaped according to the form of the bath and comprising one coil at least, the coils of each of the three groups of coils being arranged symmetrically to the coils of the other two groups.

2. Induction furnace in accordance with claim 1, in which each individual coil traversed by a certain phase of the current is arranged between two coils through which the two other phases of the current are .fiowing.

3. Induction furnace in accordance with claim 1, in which each individual group of coils traversed by a certain phase of the current is arranged between two groups of coils through which the two other phases of the current are flowing.

4. An induction furnace in accordance with claim 1, in which a star-shaped iron yoke is provided outside the coils, the poles of the yoke being situated near the centers of the coils.

5. An induction furnace in accordance with claim 1, in which a triangular yoke is provided outside the coils, the poles of the yoke being situated near the centers of the coils.

6. An induction furnace in accordance with claim 1, in which an iron. yoke is provided outside the coils, the poles of the yoke being situated near the centers of the coils and the front faces of the poles being covered bythe coils.

7. An induction furnace in accordance with claim 1, in which an iron yoke is provided outside the coils, the poles of the yoke being situated near the centers of the coils, and cooled flat coils to which no current is supplied being located in front of the front faces of the poles.

8. An induction furnace in accordance with claim 1, in which the coils connected'with one phase of the primary current are passed by the current alternately in opposite directions.

9. An induction furnace in accordance with claim 1, in which the primary current flows in the same direction through the coils connected to one phase and the first, fourth, seventh etc. coil are connected to the first phase, the second, fifth, eighth etc. coil to the second phase, the third, sixth, ninth etc. coil to the third phase of the current.

10. An induction furnace in accordance with claim 1, in which the current flows through the 00115 connected to one phase of the primary cur-.

rent alternately in different directions, each one coil connected to a determined phase being arranged between two coils through which the two other phases of the current flow in the same direction.

WILHELM ROHN.

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