US1638857A - Electric furnace - Google Patents

Description

' @638,857 Aug.16, 1927 A, D, KEaNE ELECTRIC FUHNACE Filed Nov. 14, 1925 ATTORNEY Patented Aug. 16, 1927.

UNITED frSTATES PATENT OFFICE.

ALVIN I). KEENE, 0F PITTSBURGH, PENNSYLVANIA, ASSIGNLOR TO WESTINGHOU'SE ELECLRC MANUFACTURING' COMPANY A CORPORATION OF PENNSYIQU'ANIL.

ELEGTRC FURNACE. v

Application med November 14, 1925. serieu no. 39,033.

My invention relates to electric furnaces and particularly to control systems Jfor such furnaces. A

An object of my invention is to provide means for controlling the .power input to the resistors ci" an electric 'furnace .whereby all of the resistors are operatively energized from the power supply at ell times during normal operation of the furnace.

Another object of my' invention is to'provide means 'lier controlling the temperature obtainingin an electric furnace by so selectively decreasing the power input into the resistors in 'accordance with the radiation losses ot' the 'furnace as to maintain the furnace at a predetermined temperature when that temperature is obtained.

Another object oli my invention Vis to pro vide means 'lor varying the voltage im-v pressed upon the resistors of an electric furnace in accordance with the radiation losses of said furnace when a predetermined tem` perature obtains, whereby the heat energyv obtained troni the resistors is substantially identical at all voltages. j

ln operating industrial furnaces, and particularly electrically lieatedurnaces, it is desirable to control the power input to them so that the temperature desired will be maintained substantially at teriniiied Value, heat energy generated in the resistors' shall be unil'ornily distributed throughout the furnace and that all of the .resistors shall be operatively energized from tlie power siip ply at all `times. during normal operation in order to obtain substantially identical heat distribution Jtor all amounts el' power supplied.

By my invention l have provided a means for controlling the voltage impressed on the resistor members of furnace distributed within a Vfurnace chandler fin accordance with the degree of lieatdistribution desired. The resistor members `'are initially electrically connected in single `delta and controlled by means wl neby nected with respect lto the tliata large amount oil' lieat energy l, obtained from the resistors during the' heat ing up period olf the furnace in order that predeterniined temperature may be obtained in a sliort time.

a constant preder' It is also desirable that the,

the resistors may be recon-A Wlien the predetermined temperature has been obtained., the controllingynleans are operative to again lchange the connection of the resistors so that the voltage impressed on the various members is decreased, `thereby decreasing the rate et heat energy generated within the furnace 4 so that the thermal inertia oit the resistors may be reduced to 'substantially prevent the temperature resulting therein from overtrayelnig or exceeding the predetermined value.

Fig. l is a diagrammatic View in vertical section et an electric furnace having a plurality of resistor members electrically rconnected'in delta to a source of power supply.

Fig. 2 is a diagrammatic illustration. of

the control system embodying my invention. i, b`1g.3 1s a diagrammatic View ot a double delta."onnection of the resistorswhenthe niaxiinhin 'power is applied, and l Figflis a diagrammatic View of adouble star connection of thercsistors.

*Referring more particularly to Fig. 1, a furnace-structure 8 comprises a plurality of refractory heat insulating walls 9 enclosing a furnace chamber 10. lNithin the turnace chamber 10 are'` disposed a pair of resistor members 11 and 12 evenly distributed along opposite walls. Terminals 13 and 11i of the resistors 11 and 12 are connected by, a conductor 1 5 to supply conductor 17. Termis nals 18 and 19 of' the resistors 11 and 12 are connected by a conductor 21. Supply oonductor 22"'rs connected to a onetliird point 23 ot' the resistor 24 of the resistor 11 .is connected to supply conductor 25. lfVith these connections as illustrated'in Fig. 1, we have a singledelta" arrangement as illustrated in Fig. 2, with Ition with a controlling ineens for changing 'the single delta of Fig. 1 to either a double delta or a double star connection.

In order to' change from the single delta ,connection to the double, delta connection, power supply so may be,

the movable 4member of a thermostat 26 engages aV contact member 27 whereby coil 28 is energized by the `voltage impressed thereM on between conductors 17 and'ilQ. After the coil 28 is energized, contractors 29,31 and 32 are closed as illustrated in Fig. 2," where- 12 and a one-tliird point.

have shown the resistors otV by the midpoints of the sides of the equilateral triangle of the resistor shown in Fig 2, are electrically connected to the opposite corners of the equi-lateral triangle ot resistors. The resultingr double delta connection is shown in Fig.

The movable member of the thermostat 26 is preferably of bi-metal and is adapted to engage either the contact'member 27 or the member 33. When the movable member of the thermostat 26 is in engagement with the contact member 33 as illustrated in Fig. 2, the power input to the resistor furnace as shown in Fig. 2 is four times the input for the' single delta connection illustrated in Fig. 1 This connection will be useful in bringing the furnace up to ten'iperature in a comparatively short period ot time depending upon the valueof the resistance in the resistors and the voltage impressed thereon.

The thermostat 26 is preterably adjusted to operate at a temperature slightly lower than the maximum predetermined temperature to be obtained. lfVhen the lower temperature has been obtained, the movable member of the thermostat 26 discngages the 'Contact member and the coil 2b is deenergized, thereby opening conl'actors 2l, Il and 32. The thermostat 26 engages contactmember 27 and coil 33 is energized by the voltage between the conductors 22 and 25 to close contactors 36 and 37.

When the contactors 36 and 37 are closed, the midpoints 38, 35) and 41 ol the resistors, as illustrated in Fig. 2, are electrically connected to form a double star connection, as shown in Fig. 4. The power input to the furnace by reason of the double star connection is reduced as compared with the double delta connection but is 33% more than l'or the single delta connection.

When the maximum temperature has been .reached the thermostat 42, which is secured to the support 43, operates to disengage the contact member 44 whereby the coil 33 is deenergized to open the contactors 36 and 37 thereby leavin the resistors of the furnace chamber 10 o? Fig. l connected in single delta.

The power input for the single delta is taken as unity and is therefore the connection having the lowest power consumption.

In 1"ig. 2,.the resistor member between the power supply lines 17 and 22 is divided into sections 45 and 46 at the midpoint Likewise the twoI opposite resistor members are divided into lequal segments 47 and 48 having a midpoint 41 and two members 35 and 36 having a midpoint 39.

When the double delta connection is made by the closing of the contactors 29, 31 and 32 by reason of the energization of the coil 28, which is energized by the voltage be# tween .lines 17 and 22, each half of the re* sisters illustrated by the equidateral triangle in Fig. 2, iselectrically connected in parallel with one half of another resistor member as illustrated in Fig' 3. The members 35 and 47 are connected in parallel across the power supply lines 17 and The members 46 and 48` are connected in parallel across the supply lines 22 and 25 and the resistor members 45 and 36 are electrically connected in parallel between .the power supply lines 17 and 25,. Obviously for this connection, the voltage impressed across each of the halt members is double the-value ot the voltage impressed across the entire members of the single delta connection as shown in Fig. 1. l

Likewise in Fig. 4, where a double star connection is shown, the .midpoint of the sides ot the equi-lateral triangle illustrated in Fig. 2, which are also the miifllpoints ol the resistor members are electrically connected so to form the double star illustrated in Fig. 4. In this double star connection, the half resistor members 36 and 48, 45 and 47, 35 and 46 are connected in parallel. The voltage impressed across the parallel resistor members illustrated in li'ig. 4 is 161 higher 'than the voltage impressed across the members 'when in the single delta connection, therefore, the power input to the double star connection will. be 33% higher than the single delta. rlhis connection is very useful when the }.ncdeter1nineil operating temperature has been obtained because the value otl the power input may be apportioned according' to the .resistor design so that substantially enough power will be plied to compensate .tor the radiation lo.V s thereby rlleeting a. constant temperature within the furnace 'chamber as' illustratfl in lilig'. The mii'lpoint 38 illustrated in Fig'. electrically connected to thepoint 24 of the delta connection illustrated in Fig. 2, by reason ol. the conductor 62 and the contacter The mid-p int 35) is connected to the point 16 as illustrated in Fig. 2, by reason 51., the contacter 2f) and the el: the conductor conductor 6l. The midpoint 4l, is connect ed to the point 23 of the delta d a'grnm shown in Fig. contacter 31.

The midpoints 3S,

2, by the conductor 52 and the- 39 and 4 1 are joined electrically to form the double star connection as shown in Fig. 4, by the reason of the conductors a3, 54, andy the. contactors 36 and 37.

The coil is member of the thermostat engages the contact member 33 by reason of the connection across the power supply lilies 17 and s 22 `through theconductors 50, 60 and 6l. The coil 33 is energized by the voltage dif. ference between the lines 22 and 25 when the movable member of th thermostat 26'engages the eontaetmem r 27 whereby an .encrgized'when the movable 'a plurality of thern ially actuated members lor "aryingthe voltage impressed 'on said Iresistor members in accordance with the predetermined Steps in temperature rise above 5 a desired temperature so as to maintain all "of said resistor mem -theipower supply at bers in connection with all times and to effect

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