US3159736A

US3159736A - Control circuit for electric furnace

Info

Publication number: US3159736A
Application number: US211172A
Authority: US
Inventors: Loule O Wicks
Original assignee: PORTLAND COPPER AND TANK WORKS Inc
Current assignee: PORTLAND COPPER AND TANK WORKS Inc
Priority date: 1962-07-20
Filing date: 1962-07-20
Publication date: 1964-12-01
Anticipated expiration: 1981-12-01

Description

DCC' 1 1964 l.. o. wlcKs 3,159,736

CONTROL CIRCUIT FCR ELECTRIC FCRNACE Filed July 2C, 1962 ATTORNEY 5' United States Patent 3,159,736 CGNTRL CIRCUIT FR ELECTRIC FURNACE Louie 0. Wicks, South Windham, Maine, assigner to Portland Copper d: Tanti Works, Inc., South Portland, Maine, a corporation of Maine Filed July 2o, 1962, Ser. No. 211,172 Claims. (Ci. 219-492) This invent-ion relates to electric furnaces and more particularly to improvements in circuits utilized for controlling the amount of current passing through the heater elements of such furnaces.

In the employment of electric turn-aces in industry an import-ant maintenance problem involves the replacement of heater elements. It has been discovered that the life of these elements is markedly decreased by the application of full line voltages to these elements when they are cold. These relatively high voltages applied to the cold elements cause surges of current through them which are many times greater than the normal operating currents. This is due to the fact that when cold, the heater elements have relatively low resistances compared with their resist-ance Values after they have been heated. The result of these current surges through the elements is that the life of an element is greatly decreased.

This shortcoming is overcome by the improved control circuit of this invention which applies reduced Voltages to the heater elements during warm up and thereafter applies full line voltage to them, thus greatly diminishing the initial drastic current surges which injure the elements.

Another object of the invention is to utilize the heater elements themselves as the current controlling devices with a minimum of external current control devices being required.

A further object of the invention is to provide control circuitry which is compatible with electric furnaces operating on direct current, single phase alternating current or three phase alternating current.

The foregoing objects are accomplished by providing a relay circuit for controlling the opening and closing a plurality of contacts in the heater element circuit to change the electrical relationship of the heater elements with respect to one another.

Other objects and advantages will be apparent from the following description of the invention, pointed out particularly in the appended claims, when the foregoing are taken in connection with the `accompanying drawings of illustrative embodiment of the invention in which:

FIGURE l is a diagram of the preferred form of the relay portion of the contr-ol circuit according to this invention;

FIGURE 2 is a schematic diagram of a three element heater operating on a three phase supply, the elements being interconnected under the control of the circuit of FIGURE 1;

FIGURE 3 is a schematic diagram of a two element heater operating on a single phase alternating current supply or on direct current, the elements being interconnected under the control of the circuit of FIGURE 1; and,

FIGURE 4 is a schematic diagram of a three element heater operating on a three phase supply, the elements being thans-former coupled from a primary circuit which is interconnected under the control of the circuit of FIGURE 1.

Referring to FIGURE 1 the relay portion of the control circuit may ybe described. Across la source of control voltage is connected a series arrangement comprising thermostat contact 12, a normally closed relay contact 14, and the winding ot a relay 16. In parallel with relay 16 is the winding of a timing relay 18, having an adtact 14 and relay I6 .are contact ZIP .and the winding of a relay 22. Contact 2@ operates on energization of timing relay 13. In parallel with contact 2t) is another con-l tact 24, which along with contact ld, is operatively related to relay 22.

In operation thermostat 12 closes when power is to be supplied to the furnace. This completes a circuit through normally closed relay contact l@ and relay 16. The energization of relay 16 occurs simultaneously with that of timing relay I3 in parallel therewith. On completion of a timing cycle commenced by the energization of relay 18, Contact 2l) is closed allowing relay 22 to be energized. This closes contact Z4 and opens Contact I4 thereby cutting ott the voltage supply to Irelays 16 and 1S. The deenergization of relay It opens contact Ztl and current ilows through the circuit comprising contacts l2 and 24 and relay 22. This condition exists until Contact 12 is opened at the end of the heating period.

The application of power to the heater elements of an electric furnace under the control of the sequence just outlined will be described by referring to FIGURE 2. In this embodiment the ends ot each of three heater elements 2.6, ZS `and 3h are connected to a three phase power supply by leads L1, L2 and L3 respectively. The other ends of the heaters are connected through contacts R152 to a common junction point. Between input line L1 and the contact side of element Sil is connected a first contact Rza. Similarly, a second contact Rza is provided between line L2 and the Contact side oi heater 26, and a thi-rd contact Rza is joined between line L3 and the Contact side of element 28. Contacts R261 operate simultaneously with contacts ld and @d shown in FIGURE l.

During the operation of the relay portion of the control circuit as described with reference to FIGURE 1, the energization of relay 16 causes contacts R1a to close providing power to the three heater elements yfrom the three phase supply. With these contacts closed and contacts Rza open, the elements 2o, 28 and 3@ are joined in a star connection. As is well known, such a connection permits only a portion of the line voltage to appear across each heater element. This allows the heaters to gradually warm up without exposure to the severe current surges which would exist if full line voltage were initially applied to the elements.

It will be recalled that the energiaation of relay 22 of FIGURE 1 occurs essentially simultaneously with the deenergization of relay 16, the time of both actions being determined by the timing cycle established by relay 1S. With timing relay It adjusted to complete a timing cycle equal in time with the heater elements reaching a desired condition of gradual heating, contacts Rza are closed on energization of relay 22- while contacts R111 open due to the de-energization of relay 16. The connection between elements Z6, 2.3 and il@ is thereby changed from a star conguration to a delta connection under which full line voltage is applied to the individual elements to effect increased heat generation by the elements.

Referring to FIGURE 3 a two heater element unit is shown for use with the circuit of FIGURE 1 and with a direct current (D.C.) supply or a single phase alternating current (A.C.) source. More specifically, the heater elements are supplied by input lines L11 and L12. Elements 32 and 314 are connected in series with contact R121 across the supply, contact R15 being positioned between one end of element 32 and line L11. Between line L11 and the junction of elements 32 and 34 there is provided a lirst contact Rgb. An additional contact REI; is connected between line L12 and the heater side of Contact R1b. During initial operation of the circuit of FIGURE 1, the energization of relay 16 closes contact R11) connecting elements 32 and 34 in series with the supply 3 causing the voltage to be divided across each element. When relay 16 is dre-energized and relay 22 is simuitaneously energized, contact R opens and contact B2b closes connecting both elements in parallel across the supply. Thus, the heater elements are gradually heated by the initial low voltage condition existing thereacross before being subjected to full line voltage.

The embodiment of FIGURE 4 is very similar in operation to the arrangement shown in FIGURE 2. A three phase power supply is coupled by input lines L21, L22 and L23 and a three phase transformer d2 to

heater elcments

36, 38 and 4d, connected in delta configuration across the secondary windings of the transformer. fn the primary circuit there are provided contacts Rlc and Rgc as shown. During the operation of the relay control circuit of FliGURE l, energization of relay i6 closes contacts Rlc to complete a star connection of the trans former primary windings. Thus, only a portion of full line voltage appears across each primary winding resulting in a coupling of partial vol-tage to the heater elements allowing them to gradually warm up. When relay 16 is de-energized, contacts Rlc open and contacts R close due to the simultaneous energization of relay 22. The transformer primary windings are now connected in a delta configuration whereby they receive full line voltage which is coupled to the heater elements to increase the heat generation thereof. it'can be appreciated that in this embodiment the electrical configuration of the heater elements is varied without a direct mechanical change thereto.

As can be seen from the foregoing, an improved electric furnace heater element control arrangement is provided whereby the heater is initially supplied less than full line voltage to gradually warm up to a condition at which full line voltage may be applied to complete the heating cycle. By such an arrangement, the difficulties caused by initial high current surges in the elements which reduce their life have been appreciably diminished. The improved control system differs from the prior art in utilizing the heater elements themselves with a minimum requirement of external temperature control devices.

The structural arrangements shown are examples of control arrangements in which the inventive features of this disclosure may be utilized, and it will be readily apparent to one skilled in the art that certain modifications may be made to the arrangements within the spirit of `the invention as defined by the appended claims.

What is claimed is:

1. A circuit for controlling the heating cycle of an electric furnace comprising a plurality of interconnected heater elements, a source of control voltage, a series circuit including a thermostat contact, a normally closed contact and the winding of a first relay connected across said source, an adjustable timing relay winding in parallel with said first relay winding, a series circuit including a first normally open contact controlled by the timing relay and a second relay winding connected in parallel with the series connection of said normally closed contact and said first relay Winding, a second normally open Contact in parallel with said first normally open contact, the energization of said first relay winding by said source resulting in the interconnection of said elements in a first configuration for period of time during the heating cycle determined by said timing relay, and the energization of said second relay controlling said normally closed contact and said second normally open contact resulting in the interconnection of said elements in a second configuration during the remaining portion of said heating cycle to efiect the application of a first voltage value to said heater clement when connected in the first configuration and a second voltage Value of greater magnitude than said first value when said elements are connected in the second configuration.

2. A control circuit for an electric furnace comprising a source of control voltage, a first relay means energized by said source, a first contact means conditioned in response to the energization of said first relay means, a plurality of interconnected heater elements, said heater elements being interconnected in a first configuration on conditioning of said first contact means to apply a first voltage value to said elements; a timing relay energized simultaneously with said first relay means, a second relay means operatively related to said timing relay for connection to said source at a time after the energization of said timing relay dependent on the cycle of the timing relay, means for de-energizing said first relay means on energization of said second relay means; a second contact means conditioned in response to the energization of said second relay means, said heater elements being interconnected in a second configuration on conditioning of said second contact means to apply a second voltage of greater magnitude than said first voltage to each of said heater elements.

3. A control circuit as set forth in claim 2 wherein said heater elements comprise three members, said heater clements being connected in a star configuration when the first contact means are conditioned and in a delta configuration when the second contact means are conditioned.

4. A control circuit as set forth in claim 2 wherein said heater elements comprise two members, said heater eiements being connected in series when the first contact means are conditioned and in parallel when the second contact means are conditioned.

5. A control circuit as set forth in claim 2 further comprising transformer means, said transformer means having primary and secondary winding circuits, said primary winding circuit connected with said first and second contact means and said secondary winding circuit connected to said heater elements, the conditioning of said contact means determining the mechanical configuration of the primary circuit and the electrical configuration of the heater elements.

References Cited by the Examiner UNITED STATES PATENTS 1,094,534 4/14 Conrad 318-226 1,467,745 9/ 23 Zederbohrn 318-226 1,625,380 4/27 Tally a- 13-24 1,638,857 8/27 Keene 13-24 1,798,678 3/31 Keller 13--24 2,568,411 9/51 Reed 219-503 X 2,748,240 5/56 McArthur 2l9-l0.75

RICHARD M. WOOD, Primary Examiner.

Claims

2. A CONTROL CIRCUIT FOR AN ELECTRIC FURNACE COMPRISING A SOURCE OF CONTROL VOLTAGE, A FIRST RELAY MEANS ENERGIZED BY SAID, A FIRST CONTACT MEANS CONDITIONED IN RE SPONSE TO THE ENERGIZATION OF SAID FIRST RELAY MEANS, A PLURALITY OF INTERCONNECTED HEATER ELEMENTS, SAID HEATER ELEMENTS BEING INTERCONNETED IN A FIRST CONFIGURATION ON CONDITIONING OF SAID FIRST CONTACT MEANS TO APPLY A FIRST VOLTAGE VALUE TO SAID ELEMENTS; A TIMING RELAY ENERGIZED SIMULTANEOUSLY WITH SAID FIRST RELAY MEANS, A SECOND RELAY MEANS OPERATIVE RELATED TO SAID TIMING RELAY FOR CON NECTION TO SAID SOURCE AT A TIME AFTER THE ENERGIZATION OF SAID TIMING RELAY DEPENDENT ON THE CYCLE OF THE TIMING RELAY, MEANS FOR DE-ENERGIZING SAID FIRST RELAY MEANS ON ENERGIZATION OF SAID SECOND MEANS; A SECOND CONTACT MEANS CONDITIONED IN RESPONSE TO THE ENERGIZATION OF SAID SECOND RELAY MEANS, SAID HEATER ELEMENTS BEING INTERCON NECTED IN A SECOND CONFIGURATION ON CONDITIONING OF SAID SECOND CONTACT MEANS TO APPLY A SECOND VOLTAGE OF GREATER MAGNITUDE THAN SAID FIRST VOLTAGE TO EACH OF SAID HEATER ELEMENTS.