US1625380A - Electric heating device - Google Patents

Description

i 192 Apr 1 7 R. E. TALLEY ELECTRIC HEATING DEVICE Filed Feb. 24, 1926 2 Sheets-Shet 1 I INVENTOR fianc/a/ E TaHey A TORNEY B NY/LQM 192 Apnl 7 R. E. TALLEY ELECTRIC HEATING DEVICE Filed Feb 24, 1926 2 Sheets-Sheet 2 INVENTOR Rando/ f, Ta/Agy A TO NEY SWVWRM zilT/NES Patentiid A3. 19, 1921.

UNITED STATES 1,625,380 PIA-TENT OFFICE.

RANDAL a. TALLEY, or NOIBTH mwm; ranusxnvami. assmuon-ro GEORGE J. moan couranx, or rrr'rsnunen, rnnusynvama, A cbnrom'nox or rm- SYLVANIA.

ELECTRIC 'KEATING DEVICE.

Application filed February 2-1, 1928. Serial No. 90,898.

My invention relates to electric heating devices and it has particularrelation to arrangements and means for controlling the connections of the elements used in electric furnaces, and the like.

In certain operations of electric heating furnaces, two conditions have to be distinguished. First, the placing of afresh, relatively cold, charge in the furnace which requires a relatively large energy input into the furnace in order to promptly bring the charge up to the desired temperature. Second, after the furnace with the charge has reached the normal temperature, at which the furnace is desired to be maintained, the energy input need only be suflicient to cover the heat losses due to radiation, convection, etc. In most cases the energy input may be red'ucedto one-third of the initial value or less. p

In commercialfurnaces for industrial purposes, utilizing metallic ribbon heating elements, the energy input of the furnace is varied by suit-ably controlling the connections of the heating element to the source of electrical energy from which they are supplied. However, the proportioning of the heating elements which constitutes the mostvital element of the furnaces is a very delicate matter, it being on one hand required to have the same of sufficient mechanical strength and of such shape asflto be readily supported in the interior of the furnace and secure efficient heating action and, on the other hand, to have the cross section small enouglrto give the resistance necessary topermit the connection of the unit'directly across commercial voltages such as 110 or 220 volts and still have the current sufficiently low in value to prevent overheating of the unit.

In'addition to the foregoing, apparently contradictory, requirements with respect to the strength and shape of the heating elements and the resistance of the same, it is, of course, important that the control arrangements for varying the input into the furnace shall be as simple as possible.

According to my invention, the variations of the input into the furnace having a plurality of heating elements is obtained by at times connecting the several elements in star across a. source of polyphase currents and at other times connecting the several elements in series across one phase of the supply source.

A more particular object of my invention is to provide a furnace arranged to be supplied from a three phase line, the furnace having three heating elements and means for at times connecting said elements in star to the source of three-phase currents and at other times connecting the three elements in series across one phase of the line, thus varying the input into the furnace from full load to one-third full load.

The foregoing and other objects of my invention will be best understood by reference to the accompanying drawings wherein,

Figure 1 illustrates diagrammatically the connections of a furnace embodying my invention.

Fig. 2 is a diagram illustrating a modification of my invention in which the in ut into the furnace may be varied to oneird or one-ninth full load.

Fig. 3 is a. diagram of a furnace control system embodying my invention and provided with automatic means for controlling the furnace temperature.

Fig. 4 is a view similar to Fig. 3 illustrating a further modification of my invention.

Referring toFig. 1, a furnace of the box type comprises three heating elements 1, 2 and 3, of approximately equal resistance, two of the heatlng elements 1 and 3 being supported on the side walls of the furnace, while the third heating element 2 is mounted on the bottom wall of the furnace. The furnace is supplied with electric current from a balanced three-phase line 4 by means ofswitches 7, 8 and 9 two of the heating elements 1 and 2 being directly connected to the switches 8 and 9, the connection to the hird element 3 leading through a double-poledouble-throw switch 10. The line switches 7, 8 and 9 are shown 0 erated by a regulator 5 which is controlle by a pyrometer 6 in the interior of the furnace to connect the furnace to the line or cut it off therefrom so as to maintain the furnace chamber at a redetermined temperature. When the dou lethrow switch 10 is in the right-hand position, the bottom heating element 2 is connected to the neutral point between the serially connected wall elements 1 and 3, the other terminal of the wall element 3 bein connected to the switch 9 of the supply line. be three heating elements are thus connected in star across the three phase line and the energy input into the furnace is equal to g) where E is 1 are connected in series across one phase of the line between the switches 8 and 9, the

2 input into the furnace now being that is, one-third 'of the former input.

I am aware that it has been proposed to vary the input of a three-phase furnace from full load to one-third load by connecting the furnace elements in delta and in star, respectively, across the three phase supply line. However, in such cases, the heating elements must be so designed as to have the requisite resistance corresponding to the input into the furnace with the delta' connection.

As pointed out above, the requirements with respect to the mechanical strength and shape and mounting of the metallic ribbon resistance elements make it sometimes very difficult, or even impossible, to obtain heating elements having the resistance required for the delta connection or the delta-star connection. Accordingly, the designers of electric furnaces felt a distinctive need for some other way of securing a variation of the furnace input while using the starconnection for full-load operation.

My invention meets this difficulty and enables the operation of the furnace with the star connection at full load and the reduction of the furnace input to one-third by simply connecting the three heating elements in series across one phase of the supply line.

When using m improved connections, balanced load con itions are obtained when full load is taken by the furnace, the three elements being symmetrically connected to the three lines of the supply source. On the other hand, with the three elements connected in series across one phase, the load does not exceed the full-load value across that line, the input into the furnace being then not greater than the load carried by the respective phase under full load conditions. My improved control system thus secures favorable load distribution under both operating conditions.

In the furnace illustrated in Fig. 1, the heating element in the bottom of the furnace constitutes asubstantially closed loop having two rows of convolutions disposed on both sides on the center line of the bottom wall. With both connections of the heating elements, each of the three elements carries-an equal amount of energy, thus securing uniform heating.

Under certain conditions, it may be desirable to operate the furnace with the bottom element deenergized and only the two wall elements connected to one phase of the line by means of switches 7 and 9,the double throw switch being then in. the righthand position and the line switch 8 being open. WVith this connection, the energy input into the furnace is used with advantage after a charge in the furnace has been brought up to temperature and it is desired to hold the same through a suitable soaking period. Since the furnace chamber is uniformly heated on both side walls, there will be no undue temperature distortion in the furnace and the energy will distribute equally to both sides of the charge.

It is, of course, understood that my invention is not restricted to the particular arrangement of the heating elements on the side walls and bottom wall of the furnace and the schematic diagram of Fig. 1 may be regarded as an illustrationof any type of heating device using three heating elements, the heating elements being distributed in any suitable manner. Thus, in many box type furnaces, all of the heating elements are mounted upon the walls, the two side walls carrying each a full phase element and one-half .of the third phase element.

\Vhile, as stated before, the three heating elements 1 to 3 are ordinarily made of approximately equal resistance, my improved control system may with very good results be applied to \furnaces wherein the three heating elements are not of the same resistance. Thus, it is in many cases desirable to make the resistance of the bottom element larger or smaller than that of the side elements.

'Fig. 1 of the drawing. The three heating elements 1, 2 and 3 are supplied from a polyphase line 4 by means of three switches 7, 8 and 9. The various connections of the heating elements are effected by a six-pole double-throw switch 11 and a three-pole Hm I being connected between switches 7 and 8,

element 2 between switches 8 and 9, and element 3 between switches 9 and 7.

To reduce the input into the furnace toone-third of that correspondingto the delta then connected directly in star across the connection, the three-pole switch 12 is thrown to the left while leaving the SIX- pole switch 11 in the right-hand position. The three heating elements 1, 2 and 3 are terminals of the three phase line.

To further reduce the input into the fur nace to one-third of that with the star connection, or one-ninth of that with the delta ,connection, the six-pole switch 11 is thrown to the left, leaving the three-pole switch 12 tothe left, as before. The heating element 3 is then connected in series between the ele ments 1 and 2 across one phase of the suppl line between the line switches 7 and 8. in controlling the temperature of furnaces, it is desirable to automatically regulate. the connections of the heating elements so as to maintain a predetermined temperature at which the furnace is to operate. A system embodying my invention in connection with an automatically controlled furnace is shown in Fig. 3, wherein the furnace 20 is provided with three heating elements 21, 22 and 23, arranged to be energized either in star or in series by means of a magnetically operated main three-pole switch 24 and a magnetically operated star-series twopole switch 25. \Vhen both switches 24 and 25 are energized, the three heating elements 1, 2 and 3 are connected in star across the three phase supply line 4. \Vhen the main three-pole switch 24 is energized, but the star-series two-pole switch 25 is deenergized the middle heating element 3 is connected in series with the heating elements 1 and 2 across one pha:e of the supply line.

The operation of the two' switches 24 and 25 is controlled by means of suitable regulating devices in response to the variations in the temperature of the furnace chamber. To this end, the switches 24 and 25 are provided with operating coils 26 and 27, respectively, the two coils being energized or deenergized by means of relays 31 and 32 in accordance with the operation of pyrometers 33 and 34, respectively.

The pyrometers 33 and 34 are shown in the form of thermocouples which control the operation of galvanometers 35 and 36, re-

spectively, which in turn energize the relays 31 and 32 in accordance with the furnace temperature. The galvanometers 35 and 36 are provided with pointers '37 and 38 ar ranged to short circuit pairs of contacts 41, 42, 43, 44, when engaging the same, respectively. When the pointer 37 short circuits the contact pairs 41 or 42, either one or other of operating coils 51 and 52 of the relay v31 is energized, controlling, in turn, the energization'of the operating coil 27 of the twopole switch'25 of the furnace. pointer 38 short circuits the contact pairs 43 or 44, either one of operating coils 53 and 54 of the'relay 32 are energized, controlling,

in turn, the operation of the operating coil 26 ofthe three-pole switch 24 of the furnace.

The relays 31 and 32 are so constructed that, upon energizing their left-hand operating coils 51 and 53, relay- contact arms 61 and 62 are thrown to the right, opening their When the own circuits and bridging pairs of contacts 63, 64, thus energizing the operating coils 26 and 27 of the furnace control switches 24' and 25, and also establishing operating circuits for the right-hand operating coils 52 and 54 of the two relays. \Vhen the right-hand operating coils 52 and 54 of the two relays 31 and 32 are energized, the relay- contact arms 61 and 62 are thrown to the left, deenergizing their own circuits, bridging contact pairs '65, 66 and deenergizing the operating coils of furnace switches 24 and 25.

The automatic control of the furnace switches 25 and 24 is so arranged that, in

the first instance, the double-pole switch 25,

changing the connection of the heating elements from star to series, is energized or deenergized in response to the variations of the temperature and, in the second instance, with the two-pole switch 25.de'energized and the heating elements connected in series, the three-pole switch 24 is operated to maintain the furnace at the desired temperature by cutting in or cutting out the energy supply to the three serially-connectedheating eletemperature of the furnace has reached the maximum temperature of 610 F., while the switch 25 will be raised if the furnace temperature is less than 606 F., and lowered it the temperature is above 608 F.

' I may also provide a suitable interlocking circuit to prevent the opening of the main three-pole switch 24 as long as the starseries switch 25 is in raised position and,

conversely, to prevent the closing of the star- 'series switch as long as the main three- Assuming that the furnace is operating at a temperature of 607 F., then the several parts comprising the controlling system have the positions shown in the drawing.

The pointer 38 of the galvanometer 36 is in a position intermediate the contact pairs 43 and 44 and the relay that is controlled thereby is to the left, deenergizing the operating coil 27 of the star-series switch 25, thus conill necting the three heating elements in series across one phase of the supply source. Should the furnace temperature drop, say, to 606, the pointer 38 will short circuit the contact pair 43, energizing the left-hand operating coil 53 of relay 32, throwing the contact arm 62 of the relay to the right and thereby energizing the operating coil 27 of the star-series switch 25 and connecting by the heating elements of the furnace in star across the source of supply for full-load operation. I

If the furnace temperature now rises above 608 F., the pointer 68 of the galvanometer 36 moves to the right, short-circuiting the contact pair 44 and energizing therethrough the right-hand operating coil 54 of the relay 32, thereby throwing the contact arm 62 to the left and deenergizing the operating coil 27 of the star-series switch 25, the furnace now again being connected for onethird full load input, with the three heating elements in series across one phase of the source of supply.

Throughout all the time, the galvanometer 36 was exercising its controlling action within the temperature range between 606 and 608, the pointer 37 of the galvanometer 36. which controls the operation of the threepole main furnace switch 24 was outside the range embraced by the two contact pairs 41 and 42, corresponding to the temperature range of 608 and 610. As the temperature has now gone up to 608, the po nter moves into its regulating range and if, with the reduced input into the furnace, the temperature reaches 610, the pointer 37 short circuits the contact pair 42, thereby energizing the right hand operating coil 52 of the relay 31, throwing the contact arin 61 to the left and thereby deenergizing the operating coil 56 of the main furnace switch 24. thus entirely cutting off the energy supply. If, after some time, the temperature of thefurnace chamber again drops down. the pointer 37 moves to the left, short-circuiting the contact pair 41; the left hand operating coil 51 of the relay 31 then becomes energized, throwing the contact arm 61 to the right, thereby again energizing the operat- In the foregoing manner, the temperature of the furnace is regulated within very close limits by cutting in and out the. energy supply to the furnace in relatively small steps without undue disturbance of the electrical conditions in the supply line and without undue stresses or disturbances in the furnace itself.

In the arrangements described hereinbefore, I have shown the use of the star-series connections in connection with three ele ments only. By using these connections in combinations of 9 heating elements as shown in Fig. 4, it is possible to so arrange the control of the connections of the elements that'all the phases of the supply line will be uniformly loaded, both under full-load and fractional-load conditions. To this end, the heating'elements are arranged in three groups 71, 72, 73 of three elements each,

each group being provided with switches 7 5,

76, by means of which the elements of each group may be connected elther in star, for

full-load operation, or with all three elements in series across one phase, for fractional-load operation. The sequence of the connections of the individual groups to the phases of the supply line 1 are, however,

-cyclically different so that when the three groups of elements are connected in series across a phase, each of the groups 71, 72 and 7 3 is connected across a differentphase of the supply line.

The operation of the switches 7 5 to 76 is controlled by regulators 81, 82 which may be arranged similar to the regulating system shown in Fig. 3, or in any other suitable manner.

I claim as my invention:

1. In a heating device the combination with a plurality of heating elements, of means for connecting the heating elements in star when using a three phase current or in series when only one phase of the current is to be used.

2. In a heating device the combination with a plurality of heating elements, of means for connecting the heating elements in star when using a three phase current or in series when only one phase of the current is to be used, and means responsive to the temperature of said heating device for selectively controlling the operation of said connecting means.

3. In a heating device, the combination with a plurality of heating elements, two of which are permanently connected in series. of means for connecting a third element, either to the neutral point between the serially connected elements for polyphase operation, or in series with said two serially can nected elements for single-phase operation. 4. In a heating device, the combination with a pair of serially connected heating elements symmetrically disposed in said heating device to secure uniform heating, of a third heating element symmetrically disposed with respect to the same, and means for, at times, connecting said third element to the neutral point between said pair of elements to operate said heating device with polyphase currents from a polyphase source and, at other times, connecting said third element in series with said pair of elements to operate said heating device with singlephase currents at reduced input.

5. The combination with a three-phase source of alternating current, of a heating device comprising a pair of serially connected heating elements disposed in said heating device to secure uniform heating, a third heating element, symmetrically disposed with respect. to the same, means for connecting an end terminal of said pair of elements and of said third element to two terminals of said source, respectively, and means for either connecting the free second end-terminal of said pair of elements to the third terminal of said source and thefree' second end terminal of said third element to the neutral interconnection between said pair of elements, for three-phase operation of said heating device, or connecting the free end terminal of said pair of elements to the free end-terminal of said third element for operating all three elements in series across one phase of said source.

6. The combination with a source of polyphase currents, of heating means comprising a plurality of groups of heating elements corresponding in number to the number of phasesof'said source, each group comprismg a plurality of heating-elements of approximately the same resistance, the number of heating elements in each group being equal to the number of groups, high-input switching means for, at times, establishing balanced polyphase connections between the elements of each group and the phases of said source and, at other times, reconnecting the elements of each group to draw unbalanced currents from said source, the sequence of the connections of the several groups to said source being such that the total currents drawn from the source-are balanced under all conditions of operation.

7. The combination with a source. of three-phase currents, of heatin means comprising three groups of heating elements, each group comprising three heating elements of approximately the same resistance, Q

high-input switching means for, at times, connecting the elements in star to said source, and, at other times, connecting the elements of each group in' series across one phase of said source, the serially connected elements of the several groups being connected to different phases of said source, respectively. y 8. The combination with a source of threephase currents, of heating means comprising three groups of heating elements, each group comprising three heating elements of approximately the same resistance, highinput switching means for, at times, connecting the elements of each group in star to said source, and, at other times, connecting the elements of each group in series across one phase of said'source, the sequence of the connections of the corresponding elements of the several groups to the phases of said source being cyclically difi'erent.

9. The combination with a three-phase source of alternating current, of a heating device comprising three heating elements of approximately equal capacity, low-input switching means for connecting said three elements in series across one phase of said source, medium-input switching means for connecting said three elements in star to the three phases of said source, and big -input switching means for reconnecting sai three elements in delta to the three phases of said source.

10. The combination with a three-phase source of alternating current, of a heating device comprising a pair of serially connected heating elements symmetrically disposed in said heating device to secure uniform heating. a third heating element symmetrically disposed with respect to said pair of elements, means for connecting an end terminal of said pair of elements and of said third element to two terminals of said source, respectively, and means for either connecting the free second end-terminal of said pair of elements to the third terminal of said source and the free second end terminal of said third element -to the neutral interconnection between said pair of elements, for three-phase operation of said heating device, or connecting the free end terminal of said pair of elements to the free end-terminal of said third element for operating all three elements in series across one phase of said source.

In testimony whereof, I have hereunto subscribed my name this 26th day of January, 1926.

I RANDAL E. TALLEY.

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