US3393039A - Burner control system - Google Patents

Description

July 16, 1968 Rfw. ELDRIDGE, JR.. ET AL 3,393,039

' BURNER CONTROL SYSTEM Filed May 1l. 1966 United States Patent O 3,393,039 BURNER CONTROL SYSTEM Raymond W. Eldridge, Jr., and Chung L. Feng, St. Louis County, James A. Wright, Webster Groves, and John A. Ancona, Atl'ton, Mo., assignors to Emerson Electric Company, St. Louis, Mo., a corporation of Missouri Filed May 11, 1966, Ser. No. 549,313 8 Claims. (Cl. 431-70) ABSTRACT;` OF THE DISCLOSURE A burner control system operating on an A.C. power source, in which an SCR is rendered conductive in the absence of burner flame to effect through relay closure the opening of a fuel valve, to heat a bimetal safety switch, and to operate a spark igniter by effecting the discharge every half cycle of an R-C circuit which includes the primary winding of an ignition transformer, in which the SCR is rendered non-conductive upon the appearance of burner flame, and in which a current limited holding circuit holds open the fuel valve when the SCR is rendered non-conductive.

This invention relates to automatic control systems for fluid fuel burners which include ignition means for igniting the fuel and safety means operative to supply fuel to the burner for a predetermined short trial period during which the igniter is operated and thereafter provided combustion of the fuel occurs within the trial period, and to cut off the supply of fuel if combustion fails to occur within the trial period or having occurred subsequently fails.

An object of the invention is to provide a particularly simple, economical, and reliable electrical control system for the operation of gas burners.

A further object is'to provide an electrical control system for gas burners having a spark igniter with a high voltage transformer and a bimetalsafety time switch with an electrical resistance heater controlling operation of electrically operated fuel supply means, in which a single solid state switching device under control of combustion llame responsive means is rendered conductive in the absence of burner flame to effect energization of the time switch resistance heater and to supply intermittent energy pulses to the primary winding of the ignition transformer to effect operation of thel spark igniter and is rendered non-conductive when -bu'ruer flame appears, whereby the safety switch heater and igniter are deenergized. l

A further object is to provide a Iburner control system, as in the foregoing paragraph, in which conduction through the solid state switching device effects the opening of an electromagnetic fuel control valve as well as the energization of thesafety switch heater and the operation of the igniter.

A further object is to provide in a burner control system for vgas burners adapted to operate on an A.C. power supply a spark igniter having a high voltage transformer and means including acapacitor and a parallel connected solid state switching device responsive to burner 3,393,039 Patented July 16, 1968 flame to effect the periodic discharge of the capacitor through the primary winding of the igniter transformer.

Other objects and advantages will appear from the following description when read in connection with the accompanying drawing.

In the drawing:

FIG. l is a diagrammatic illustration of a preferred form of the present invention; and

FIG, 2 illustrates a modification of the means shunting the current limiting resistor.

Referring to FIG. l, a gas burner is indicated at 10 having a gas supply line 12 leading thereto in which is interposed a normally closed, electromagnetically opened fuel valve 14 having a solenoid winding 16 which when sufficiently energized opens valve 14. The solenoid winding 16 is connected across terminals 18 and 20 of an alternating current power source through a circuit which may be traced as follows: from terminal 18 through lead 21, a system switch 22, lead 24, lead 26, resistor 28, lead 29, a normally closed safety switch 30, lead 32, a thermostat 34 responsive to the temperature of a space or body being heated by burnerl 10, and lead 36 to terminal 20.

The value of resistor 28 is such that the level of energization of solenoid winding 16 effected through the justdescribed circuit will hold valve 14 in an open position once it is moved to an open position, but will not move the valve open from a closed position. A sensitive relay, preferably a reed-type relay, having a winding 38 and normally open contacts 40 is operative when energized to complete the shuntng of resistor 28 and thereby permit sufficient energization of solenoid winding 26 to enable it to open valve 14 from a closed position.

A circuit connecting relay winding 38 across power source terminals 18 and 20 extends from junction 42 through a lead 44, a diode 46, leads 48, 50, 52, and 54, a safety switch resistance heater 56, lead 58, a siliconcontrolled rectier (SCR) 60, and lead 62 to junction 64. When the SCR 60 is conducting, suiiicient half wave energization of relay winding 38 is effected to close relay contacts 40 to complete a shunt including leads 33 and 35 around resistor 28, and suiiicient energization of series connected resistance heater 56 is effected to cause bimetal safety switch 30 to open in a predetermined short period of time. A capacitor 66 and the primary winding 68 of an ignition transformer 70 are connected in series arrangement across the SCR 60 by leads 63, 65, and y67. The SCR '60 has a gate lead 61 which is connected to the anode side of the SCR through series-arranged resistor 72 and neon bulb 74 by leads 76, 78, and 80, and the gate lead 61 is connected to the cathode side through parallel arranged lresistor 82 and flame responsive switch 84 by leads 86, 88, 90, and 92, respectively.

The function of ame responsive switch 84, which is to shunt resistor 82, may be accomplished by any suitable temperature responsive or light responsive switching means or temperature responsive or light responsive variable resistance means. In the present form, the normally open switch contacts are closed as the result of eXpansion of an expansible chamber 96 which is connected by a capillary tube 98 to a bulb 100 containing a thermally expansible or vaporizing liquid. The -bulb 100 is positioned so as to be impinged by burner flame. In the ab- The neon bulb 74 is arranged to fire and become con-` ductive when the voltage applied thereacross reaches a predetermined value during the conductive half cycle of diode 48 and SCR 60, and the relative values of resistors 72 and 82 are such that a firing signal is applied to the SCR gate when the neon bulb fires and the switch 94 .is open. When llame appears at the burner 10, switch 94 closes in response thereto and resistor 82 is shunted. Under these conditions the voltage applied to the SCR gate falls considerably below the firing signal and conduction through the SCR is cut off upon reversal of current in the succeeding non-conductive half cycle.

During the initial portion of the conducting half cycle, prior to the firing of the neon bulb 74, a charge is built up on capacitor 66, and under conditions in which switch 94 is open, the SCR 60 fires and effects the discharge of the capacitor through the transformer primary winding 68. This energy pulse induces a high voltage across the transformer secondary winding 69 and an arc occurs across igniter electrodes 71. A diode 102 is connected across the capacitor 66 by leads 103 and 104, in opposite polarity to diode 46 and SCR 60', for the purpose of diverting capacitor reactance which would otherwise result in a reverse current swing that would cut off the SCR.

In the absence of diode 102, t-he reactance of capacitor 66 would effect the cutoff of the SCR with the first reverse current swing and would, at the higher power supply voltages, effect its re-firing upon the following forward swing so that the SCR would fire some number of times each conductive half cycle, depending upon the power supply voltage. Inasmuch as commercial A.C. power supply voltage may vary from 90 to 130 volts in a nominal 115 volt supply, the total time per half cycle during which the SCR is conducting may at 130 volts be many times that which it would be conducting at 90 volts. That is to say that any circuit time constant which would result in the capacitor 66 charging fast enough at 130 volts to result in the SCR firing six or eight times per half cycle would in all probability result in only one firing of the SCR per half cycle at 90 volts.

It will be appreciated that such variation in total current passing the SCR will considerably affect the heat output of safety s-witch heater 56 and, consequently, the time required to effect opening of the safety switch 30. The provision of diode 102 considerably reduces this variation by insuring that the SCR 60, when once fired, will continue conducting regardless of line voltage variation until cut off by reversal of supply current.

The .purpose of diode 46 is to block passage of alternating current through capacitor 66 under conditions when SCR 60 is not conducting. The passage of current through capacitor 66 would heat safety switch resistance heater 56 when there is fiame at the burner and when SCR 60 is not conducting. A relatively small capacitor 106 is connected in parallel with relay winding 38 by leads 108 and 110 and discharges during the non-conductive half cycle to prevent chattering of relay contacts 40.

The transformer primary winding 68 has relatively low impedance and the SCR 60, primary winding 68, capacitor 66, and connecting leads 62, 32, 67, 65, 63 and 58 form a low impedance circuit through which capacitor 66 discharges. Inasmuch as the capacitor discharge current and supply current flow in the same direction through lead 58, SCR 60, and lead 62, the primary winding 68 may be positioned at any point within the described low impedance capacitor discharge circuit without appreciably affecting the performance of the igniter or the system as a whole. While it is true that the positioning of the primary winding in series with SCR 60, with respect to the power source, would reduce the current flow through rel-ay winding 38 and safety switch heater 56 slightly, it is to be understood that the impedance of primary winding 68 may be made quite'small, and the exclusive use of an arrangement in which primary winding 68 is positioned in series with the SCR with respect to the power supply, but within the low impedance capacitor discharge circuit, is contemplated.

Operation When the temperature of the space or body being heated by burner 10 is sufficiently low to cause thermostat 34 to close and it is desired to operate burner 10, the system switch 22 is closed. Closure of thermostat 34 and switch 22 completes the described holding circuit for solenoid winding 14 through resistor 28 and the parallel circuit -between junctions 42 and 64 which includes relay winding 38, safety switch resistance heater 56, and SCR 60. There being no fiame at burner 10, switch 94 is open and therefore as the A.C. supply voltage increases during the first conductive half cycle to a predetermined value neon bulb 74 fires and effects the firing of SCR 60'. During that portion of the conductive half cycle prior to the firing of neon `bulb 74, the capacitor 66 is charged, and upon firing of the SCR 60, the capacitor discharges through the series connected transformer primary winding 68, thereby initiating operation of the spark igniter 70. The SCR 60 continues to conduct during the remainder of the conductive half cycle due to the shunting diode 102. The SCR now continues to fire Iat a predetermined point in each conducting half cycle, determined by the firing of neon bulb 74, and the igniter continues to spark once each cycle.

Conduction through SCR 60 energizes sensitive relay coil 38 and closes contacts 40 and energizes safety switch heater 56. The closing of contacts 40` shunts resistor 28 and permits full energization of solenoid winding 16 and the opening of normally closed valve 14 so that fuel now fiows to burner 10. Under normal conditions of operation, burner 10 will ignite in a very short period of time. When burner flame appears bulb 100 is rapidly heated, and as a result, switch 84 is closed. Closure of switch 84 shorts out resistor 82, and as a result, SCR 60` is rendered non-conductive.

When as a result of occurrence of burner flame conduction through SCR 60 is cut off, relay winding 38, safety switch heater S6, and igniter are de-energized. De-energization of relay winding 38 causes contacts 40 to open so that now solenoid winding 16 is energized at a lower level throu-gh resistor 28, that is, at a level of energization which will hold valve 14 open, but will not open it. The burner 10 will now continue to burn under normal operating conditions until thermostat 34 opens or system switch 22 is opened.

If under abnormal conditions the burner 10 upon starting fails to ignite in a predetermined short period of time the resistance heater 56 will cause bimetal safety switch 30 to open, thereby completely de-energizing solenoid winding 14 and effecting closure of valve 14. The resistance heater 56 and igniter 70 will, however, continue to be energized to hold open safety switch 30 and to provide ignition under these conditions until the systern is opened. When the control system is used in connection with gas-fired domestic clothes dryers, a timer operated system is usually employed and opens to deenergize the entire system at the termination of a timed drying period.

If burner fiame is extinguished during normal operation after switch 94 has opened, the switch 94 will again close, and resistance heater 56 will again be energized and igniter 70 again placed in operation. If the electrical power source fails momentarily causing the fuel to be cut off, the valve 14 will not re-open upon restoration of the power supply until ybulb has cooled and switch 94 has closed. Under these conditions, the burner is restarted after closure of switch 94 in the normal manner of starting.

If the SCR becomes defective at any timeso as to remain conductive at all times, the safety switch heater ,y 56, being in series therewith, willeffect opening of the safety switch 30 to cut off the flow of fuel, or if the SCR 60 becomes faulty and fails to conduct, the fuel valve 14 will not be opened.

FIG. 2 shows a second form of means for shunting the limiting resistor 28 under certain conditions and comprises a vnormally open switch 57 including a bimetal strip, which when heated by the safety switch heater 56 closes to complete with leads 33 and 35 a shunt which renders limiting resistor 28 ineffective and permits sufficient energization of electromagnetic coil winding 16 to open valve 14. The bimetal blade of switch 57 is so constructed and positioned in heat exchange relationship with resistance heater 56 that it responds to close its contacts in a considerably shorter period of time than that required to heat safety switch 30 sufficiently to effect its opening. When the SCR 60 is rendered non-conductive upon the appearance of burner flame, the bimetal blade of switch 57 cools rapidly and the switch re-opens.

The foregoing description is intended to .be illustrative and not limiting, the scope of the invention being set forth in the appended claims.

We claim:

1. In a burner control system including a burner, a thermostat, a normally closed electromagnetic fuel valve including a winding, spa-rk ignition means including a voltage step-up transformer having a primary Winding, a normally closed heat-opened safety switch and a resistance heater for heating the safety switch; in which the thermostat, the electromagnetic valve winding, the safety switch, and a current limiting resistor are series connected across an A.C. power source to effect the hold-open but less than pull-open energization of said electromagnetic valve winding when Said thermostat is closed; in which the winding of a normally open relay having contacts which when closed effect the shunting of s-aid current limiting resistor, said resistance heater, and an SCR having a flame responsive gate control network are series connected across said power source in parallel with said electromagnetic valve winding, said safety switch, and said current limiting resistor, but in series with said thermostat; and in which upon closure of said thermostat said SCR is rendered conductive in the absence of burner flame to energize said relay winding, thereby shunting said limiting resistor to permit suicient energization of said electromagnetic valve winding to open said fuel valve and the energization of said resistance heater to heat said safety switch, and is rendered nonconductive when flame appears, thereby rendering said shunt and safety switch heater inoperative; the improvement which consists in providing a capacitor and circuit connections connecting said capacitor across said SCR to form a low impedance discharge circuit for said capacitor, and in connecting said ignition transformer primary winding in series with said capacitor and said SCR in said low impedance capacitor discharge circuit whereby said capacitor discharges through said primary winding, and in providing voltage responsive switching means in said ame responsive gate control network responsive to a predetermined voltage to become conductive, thereby to delay the application of a tiring signal to said SCR du-ring its conductive half cycle until said capacitor is sufficiently charged to effect upon its discharge the operation of said ignition means.

2. A burner control system as set forth in claim 1 in which said normally non-conductive voltage responsive switching means in said flame responsive gate control circuit is a neon bulb connected between the SCR gate electrode and the anode side of the SCR.

3. A burner control system as set forth in claim 1 in which said ame responsive gate control network includes a first resistor connected between the SCR gate electrode and the anode side of the SCR and a second resistor connected between the SCR gate electrode and the cathode side of the. SCR, the relative values of said resistors being such as to render the SCR conductive, and circuit connections including anormally open ame `responsive switch ,paralleling said second resistor, which switch closes in response `to yburner flame to effect the shunting of said second resistor, thereby to prevent the application of a firing signal to said SCR gate.

4.A burner control system as set forth yin claim which further includes a diode connected in series relationship with said capacitor, thereby to block the passage of A.C. current through said safety switch heater.

5. A burner control system as set forth in claim 1 which further includes a diode connected across said capacitor and arranged in opposite polarity to said SCR to preclude a reverse current swing through said low impedance capacitor discharge circuit.

6. In a burner control system, an A.C. power source, a burner, a normally closed electromagnetic fuel valve including a winding, a first circuit connecting said winding across said A.C. power source including in series arrangement a space thermostat, said winding, a current limiting resistor operative to limit the energization of said winding to that which will hold said fuel valve open but will not open it, and a normally closed heat-opened bimetal safety switch, a normally open bimetal shunting switch connected across said limiting resistor and operative to shunt said limiting resistor when closed, thereby to permit suicient energiz-ation of said electromagnetic winding to open said valve, a resistance heater operative when energized to heat said normally closed bimetal safety switch and said normally open bimetal shunting switch to elfect the opening of the former and the closing of the latter, and said bimetal switches being so constructed and having such relative heat exchange relationships with said resistance heater that said normally open shunting switch is caused to close before said safety switch is caused to open, and a second circuit connecting Said resistance heater across said power source in parallel with said electromagnetic winding, said limiting resistor, said normally open bimetal shunting switch, and said normally closed safety switch, solid state switching means controlling conduction through said second circuit and iiame responsive means controlling said solid state switching means and operative to render said solid state switching means conductive in the absence of burner arne and non-conductive in the presence of flame.

7. A burner control system as set forth in claim 6 which further includes a spark ignition device having a voltage step-up transformer and a primary winding therefor, in which said solid state switching device is an SCR, in which a capacitor is connected across said SCR to form a low impedance capacitor discharge circuit, in which said ignition transformer secondary winding is connected with said SCR and said capacitor in said low impedance circuit, and in which said flame responsive means comprises a flame responsive SCR gate control network including ame responsive switching means operative to effect conduction of said SCR in the absence of burner ame and to cut off conduction therethrough in the presence of burner ame.

8. In a burner control system, an A.C. power source, a normally closed electromagnetic fuel valve including a winding, a iirst circuit connecting said valve winding across said A.C. power source and including in series arrangement a space thermostat, said winding, and a normally closed, heat opened,` bimetal safety switch, a resistance heater operative when energized to heat said normally closed safety switch and effect its opening, a second circuit connecting said resistance heater, an SCR, and a first diode of the same polarity across said power source in parallel with said valve winding, spark ignition means including a voltage step-up transformer having a primary winding, and a capacitor, circuit connections connecting saidcapacitor and said primary winding across said SCR and in series relationship with said resistance heater and said first diode, a second diode, circuit connections connecting said second diode across said SCR in opposite polarity thereto and in series relationship with said resistance heater and said rst diode, and an SCR gate control network including burner ame responsive means operative to effect the conduction of said SCR in the absence of ame and to cut off conduction thereof in the presence of llame.

CII

YReferences Cited UNITED STATES PATENTS McNulty et al. 315-183 Patrick 158-125 Forbes 158-28 Eldridge et al. 158-128 Potts 158-125 X Walker 158-128 X FREDERICK KETTERER, Primary Examiner.

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