US3759654A

US3759654A - Thermal operator for ignition proving system

Info

Publication number: US3759654A
Application number: US00198142A
Authority: US
Inventors: J Clifford
Original assignee: Robertshaw Controls Co
Current assignee: Robertshaw Controls Co
Priority date: 1971-11-12
Filing date: 1971-11-12
Publication date: 1973-09-18
Anticipated expiration: 1990-09-18

Abstract

An improved operator is disclosed which greatly amplifies the relatively small expansion and contraction movements of a heat expansible wire connected in series with an electric igniter having a positive coefficient of resistance. As the igniter changes temperature its electrical resistance also changes causing a difference in current flow through the wire and accordingly an expansion or contraction thereof. Lever means attached to the wire amplify this movement and utilize it to selectively actuate switches controlling energization of both the electric igniter and an electrically actuated fuel valve providing fuel to a burner.

Description

United States Patent 1 Clifford [451 Sept. 18,1973

THERMAL OPERATOR FOR IGNITION PROVING SYSTEM [75] Inventor: Joseph Clifford, Corona, Calif.

[73] Assignee: Robertshaw Controls Company,

Richmond, Va.

[22] Filed: Nov. 12, 1971 [21] Appl. No.: 198,142

[52] 0.8. Cl. 431/66, 431/73 [51] Int. Cl. F23n 5/00 [58] Field of Search 431/66, 67, 70, 73

[56'] I References Cited UNITED STATES PATENTS 3,619,096 11/1977] Krueger et al. 431/66 3,660,005

5/1972 "Wilson 431/66 Primary Examiner-Carroll B. Dority/Jr. Attorney-Anthony A. OBrien [57] ABSTRACT An improved operator is disclosed which greatly amplifies the relatively small expansion and contraction movements of a heat expansible wire connected in series with an electric igniter having a positive coefficient of resistance. As the igniter changes temperature its electrical resistance also changes causing a difference in current flow through the wire and accordingly an expansion or contraction thereof. Lever means attached to, the wire amplify this movement and utilize it to selectively actuate switches controlling energization of both'the electric igniter and an electrically actuated fuel valve providing fuel to a burner.

5 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains to an electric ignition mechanism and more particularly to a thermally responsive actuator capable of sensing minor movements of a hot wire and amplify these movements sufficiently to provide a useful function.

2. Description of the Prior Art A recent trend in the field of igniting devices for fuel burners is to utilize electric igniters in place of the wellknown standing pilot flames. Bumer'operation, it has been learned, can be monitored by this type of igniter if the igniter element is constructed of a material having a positive coefficient of resistance, namely a resistance which varies with temperature. The resistance of such an igniter varies as a function of the temperature and by sensing the changing resistance, the operation of the igniter and burner may be controlled.

lnherently dangerous ,conditionscan arise in fuel burners resulting from improper operation of ignition systems. It is necessary that the systems be capable of being shut down and fuel flow stopped upon the failure of any system component or interruption of electric power to the system. More specifically, it is important that the valve supplying fuel to the burner be open onlywhen igniting temperatures exist to prevent a build-up or raw fuel. It is also desirable to have the system recycle when igniting temperatures are not timely provided, once electricity is restored after an interruption or when the valve is closed due to other failures in the system in order to have another attempt made to initiate ignition when the system is in proper operation.

The resistance of electric igniters has been used in previously known electric ignition systems, but such known systems have lacked sophistication and have not provided the required comprehensive failsafe or recycling characteristics. Furthermore, the known ignition systems have not taken full advantage of the temperature variable resistance characteristics'ofthe electric igniter element.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to construct a novel electric ignition mechanism utilizing the electrical characteristics of an electric igniter which vary with temperature to provide'a controlling function.

Another object of the present invention is to utilize an electric igniter having a positivetemperature coefficient of resistance as a sensor and control element as well as for ignition purposes.

A further object of the present invention is to provide an electric ignition mechanism having electrical sensing means connected to an igniter to sense changing electrical characteristics and control operation of a fuel valve in response to these changes. A

Yet another object of the present invention is to provide a thermally expansive wire connected in series with an igniter, to be heated by the passage of current therethrough, and a lever attached to the wire in a manner which will greatly amplify the expansion and contraction movements of the wire to perform useful work in a control function.

The present invention is intended for use with an ignition proving system for a fuel burner comprising a burner, an electric igniter means with a positive temperature coefficient of resistance in igniting proximity to the burner, and an electrically actuated valve connecting the burner to a source of fuel. The invention is characterized by a current sensing thermally expansive BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a schematic diagram of an electric ignitor mechanism embodying the present invention and showing the mechanism in a ready condition;

FIG. 2 is a schematic diagram similar to FIG. 1 showing the mechanism after successful ignition; and

FIGS is a schematic diagram of the mechanism shown in FIG. l but with the mechanism shown in a fail-safe state. i I

I DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of the present thermal operator for an ignition proving system is illustrated in FIGS. 1 through 3 for use with a fuel burner 10. An electrically operated fuel valve 12 connects burner 10 with source of fuel, not shown, to control the flow of fuel to burner 10. The valve 12 may be controlled by any suitable electric operator, such as a solenoid. An electric ignitor 14 is disposed in igniting proximity to the ports of burner 10. The ignitor14 is constructed of material havinga positive temperature coefficient of resistance, such as molybdenum disilicide. A housing 16, shown in broken lines, has a pair of

input terminals

18 and 20 connected with secondary winding 22 of a transformer 24 having a primary winding 26 connected to a suitable source of electric power, not shown, through a thermostatic switch 28. Terminal 18 is connected to one end of a stationary contact 30 which is secured to housing 16 and carries a fixed contact 32 on the other end. A movable contact 34 cooperates with fixed contact 32 to form a switch generally indicated at 36.

Movable contact 34 is carried by a movable contact arm 38 which has one end pivotably mounted on a pin' 54. Depending leg 56 of T-shaped lever 52 engages a mid point of an electrically conductive hot wire 58 mounted between

insulated portions

60 and 62 of housing 16 with one end of wire 58 electrically connected with terminal 20. Hot wire 58 has a thermal expansion characteristic such that it expands as it heats and contracts as it cools. At normal room temperature the hotwire 58 has a relatively contracted state, as shown in FIG. 1.

A helical bias spring 64 is mounted in compression between housing 16 and the free end of lever 52. An upstanding finger 66 is secured to the upper side of lever 52 and has a rounded free end 68 adapted to abut the underside of movable contact arm 38. A disc 70 is fixed adjacent the free end of finger 66 and is adapted to abut the underside of a second movable contact arm 72 which is pivotably mounted in housing 16 on a pin 74. A bias spring 76 is mounted in compression between the underside of first movable contact arm 38 and the upper side of second movable contact arm 72. The second movable contact arm 72 carries on its underside a movable contact 78 which cooperates with a fixed contact 80 to form a second switch generally indicated at 82. Fixed contact 80 is carried on one end of second stationary contact 84 which is fixed in housing 16.

Housing 16 has three

output terminals

86,88 and 90. Terminal 86 is electrically connected with

movable contact arms

38 and 72 within the housing 16 and is connected through a lead 92 to terminal 94 of ignitor 14. The other terminal 96 of ignitor 14 is connected to a terminal 88 through lead 98, and terminal 88 is connected with hot wire 58 within the housing 16 through a lead 100. Terminal 90 is connected to second stationary contact 84 within housing 16 and is connected through a lead 102 to a terminal 104 of fuel valve 12. The other terminal 106 of fuel valve 12 is connected through lead 108 to input terminal 20.

Holding coil 1 has two legs which are connected in parallel with ignitor 14 through

leads

112 and 114. The center tap 116 for coil 110 is coupled to lead 112 through an input terminal 118 of housing 16. Lead 114 is connected through an input terminal 120 of housing 16 and oppositely poled full-wave rectifying

diodes

122 and 124 to the two legs of coil 110. An armature 126 is controlled by coil 110 and is secured to the upper side of movable contact arm 38.

In operation, when thermostatic switch 28 is open, the ignition proving system will be in a ready state, as illustrated in FIG. 1, with no power supplied to the system, fuel valve 12 closed, and ignitor 14 deenergized. I-Iot wire 58 will be cold since no current will be flowing therethrough and will therefore be contracted to hold lever 52 up against the force of helical bias spring 64 and finger 66 will hold contact arm 38 up against the force of helical bias spring 42 to close

contacts

32 and 34 of switch 36.

Contacts

78 and 80 of switch 82 will be open at this time due to contact arm 72 engaging with the disc 70 on finger 66.

Once the thermostatic switch 28 is closed, signalling a call for burner operation, current will flow with terminal 18 through closed switch 36, lead 92, ignitor 14, leads 98 and 100, and hot wire 58 to terminal to energize ignitor l4 and begin heating hot wire 58. Since ignitor 14 has a positive temperature coefficient of resistance, the initial voltage drop across it is small and not sufficient to pull in coil 110 so that most of the initial voltage drop will be across hot wire 58. Coil 110 must be very sensitive to voltage across ignitor 14 and therefore is arranged so that it does not have to pull in armature 126 to close switch 36 but instead merely holds armature 126, since switch 36 is closed at the start of operation.

The helical bias spring '64 urges lever 52 to rotate in clockwise direction about pivot 54 thereby holding the depending leg 56 firmly against the hot wire 58. When current is passed through terminal 88, lead and hot wire 58 to terminal 20, the hot wire 58 expands and allows lever 52 to rotate under the influence of spring 64 in a clockwise direction about pivot 54.

The rotational movement of lever 52 moves finger 66 downward allowing contact arm 72 to be pivoted clockwise about pivot 74 by spring 76 and contact arm 38 is free to pivot clockwise about pivot 40, as shown in FIG. 2 which illustrates the state of the ignition proving system after successful ignition. If ignitor 14 has reached ignition temperature by this time, end 68 of finger 66 moves away from movable contact arm 38, the resistance of ignitor 14 will be high due to the ignitors positive temperature coefficient of resistance and, accordingly, the voltage drop thereacross will be sufficient to energize holding coil and maintain switch 36 closed. Since coil 110 is connected in parallel with ignitor 14, it experiences the same voltage drop and thus coil 110 will be sufficiently energized to hold armature 126 only when ignitor 14 is at ignition temperature.

Diodes

122 and 124 provide full wave rectified current to holding coil 110 to eliminate noise and chatter when the coil is near its dropout point.

Switch 82 is closed by the clockwise movement of contact arm 72, and valve 12 is energized by current flowing from terminal 18 through closed

switches

36 and 82 and leads 102 and 108 to terminal 20. Thus, when ignitor 14 reaches ignition temperature within the predetermined time between closure of thermostatic switch 28 and expansion of hot wire 58, valve 12 will be open to supply fuel to burner 12.

Iffor any reason ignitor 14 fails to reach ignition temperature by the time the current through hot wire 58 heats the wire sufficiently for its expansion to permit spring 64 to move lever 52 clockwise, switch 36 will be opened since contact arm 38 will follow finger 66 due to the force from bias spring 42, see FIG. 3. Once switch 36 is opened, all electric power from second winding 22 is interrupted and, accordingly, ignitor 14 is de-energized and valve 12 is closed. Thus, it can be seen that valve 12 depends on the operation of both

switches

36 and 82, which are in series, and that valve 12 will not open unless ignitor 14 is at ignition temperature.

Springs

42 and 76 are selected so that

contacts

32 and 34 of switch 36 will break prior to the making of

contacts

78 and 80 of switch 82, thereby assuring that upon failure of ignitor 14 to reach ignition temperature within the predetermined time, valve 12 will not be opened.

Opening of switch 36 also prevents current from flowing through hot wire 58 thereby permitting hot wire 58 to cool and contract moving lever 52 counterclockwise about pivot 54 and finger 66 to close

contacts

32 and 34 of switch 36. Thus, the system is returned to the ready state shown in FIG. 1 and will recycle until ignitor l4 properly reaches ignition temperature within the predetermined time, assuming thermostatic switch 28 remains closed.

If during normal operation of the ignition proving system any type of electrical interruption occurs, holding coil 110 will be de-energized to drop out armature 126 and permit the force from bias spring 42 to open switch 36 thereby deenergizing ignitor 14, closing valve 12 and stopping the flow of current through hot wire 58. The system is thus placed in fail-safe state as shown in FIG. 3.

Since no current passes through hot wire 58 in this I state, the wire will cool and contract causing lever 52 to move counterclockwise to place the system in the ready state shown in FIG. 1 and thus permit recycling, again assuming thermostatic switch 28 remains closed when power is restored. The burner will be reignited in the same manner previously described.

If ignitor I4 is ruptured once the system is in the operating state shown in FIG. 2, the voltage across holding coil 1 will increase substantially tohold armature 126 and prevent switch 36 from opening. However, since hot wire 58 is in series with ignitor lid, the current flowing through hot wire 5% will be interrupted to cool the wire. As previously mentioned, hot wire 58 will contract as it cools to move lever 52 counterclockwise and cause finger 66 to lift contact arm 72 and open switch 82 thereby closing valve 12 and stop flow of fuel to the burner. The system will remain in this condition until ignitor M is replaced.

If ignitor 14 should become short-circuited, the voltage there across will decrease to zero'and coil 110 will allow armature 126 to drop out toopen switch 36 and close valve 12. If hot wire 58 breaks, the current path through ignitor 14 will be opened to allow holding coil 110 to dropout armature 126 and permit contact arm 38 to follow lever 52 under the force of spring 42 to open switch 36. If a short-circuit occurs across hot wire 58, the wire will cool and contract thereby maintaining switch 82 open due to the lifting of contact arm 72 by disc 70. i

In the event of a failure due to ignitor 14 being cooled, such as by the fuel stream, the voltage across the ignitor l4 and holding coil 110 will be reduced due to the drop in resistanceof ignitor 14 as it is cooled. It has been found that the ignitor cooling can occur to theextent that ignition is prevented while the decreased voltage drop across ignitor I4 is insufficient to dropout holding coil 110. In order to make the system of the present invention fail safe for such ignitor cooling, spring 48 is mounted in tension between lever 52 and movable contact arm 38. The decrease in the resistance of ignitor14 upon cooling causes the current flowing through hot wire 58 to increase and this increase causes further heating and expansion of the hot wire thus permitting lever 52 to increase its movement in the clockwise direction. This increased movement' of lever 52 stretches spring 48 to increase the clockwise force on movable contact arm 38 until armature 126 is released by holding coil 110, even though the voltage across holding coil 110 is at a higher than normal value. Thus, it can be seen that the system of the present invention is extremely sensitive to external conditions that might cool ignitor M since armature l26responds to the voltage across ignitor M and the current through ignitor 14. The system of the present invention can thus assume a fail safe state in response to internal or external thereacross to provide fail-safe operation. It may also be seen that the present invention provides a high mechanical advantagev lever system which will amplify small hot wire movements that are usually difficult to detect and amplify. A relatively low force, large movement is taken as a mechanical output from the side of ahot wire so that the ignitior proving system can be operated through a low mechanical advantage lever system. Thus, the inventive construction provides an essentially stiff, friction-free mechanical switching arrangement which solves the problem of mechanical motion loss due to deflection, hysteresis and mechanical wear.

' The present invention may be embodied in other specific forms with many variations, modifications and changes in detail. It is therefore intended that the foregoing description be interpreted as illustrative and not in a limiting sense.

What is claimed is:

i. in an ignition proving system for a fuel burner comprising input means connected to a source of electricity;

, electric igniter means positioned in igniting proximity to the burner, said electric igniter means having a positive temperature coefficient of resistance such 7 that current flow therethrough is dependent on the temperature on the electric igniter;

electrically actuated valve means adapted to control flow of fuel to the burner; and

switch means connected between said input means and both said electric igniter means and said valve means; I

an improved thermal actuator means comprising current sensing means including a heat expansible wire connected in series with said electric igniter to be heated by current passing therethrough; said wire being attached at each end to a stationary support; and

lever means pivotally connected to a support at one end, said lever means including an integral leg portion depending from a point intermediate the ends of said lever means and engaging said wire, and means for selectively actuating said switch means in response to expansion and contractionof said wire.

2. The invention as claimed in claim 1 wherein said wire has a cool contracted state defining a first position for said lever means and a hot expanded state defining a second position for said lever means,

said switch means including first and second switches, said first switch connected to energize said igniter means and said first and second switches connected in series to energize said valve means, said first switch being normally closed and said second switch being normally opened when said lever means is in said first positionwhereby said valve means will not be energized to open until after said igniter means reaches ignition temperature.

3. The invention as claimed in claim 1 wherein said electric ingiter means has a high resistance at ignition temperatures, further comprising voltage responsive means connected in parallel with said igniter means and operatively connected to hold said switch means in a predetermined condition.

4. The invention as claimed in claim 3 wherein said switch means comprises first and second switches, said voltage responsive means includes a holding coil and an armature controlled by said coil and connected to hold said first switch in closed condition.

i t i l i

Claims

1. In an ignition proving system for a fuel burner comprising input means connected to a source of electricity; electric igniter means positioned in igniting proximity to the burner, said electric igniter means having a positive temperature coefficient of resistance such that current flow therethrough is dependent on the temperature on the electric igniter; electrically actuated valve means adapted to control flow of fuel to the burner; and switch means connected between said input means and both said electric igniter means and said valve means; an improved thermal actuator means comprising current sensing means including a heat expansible wire connected in series with said electric igniter to be heated by current passing therethrough; said wire being attached at each end to a stationary support; and lever means pivotally connected to a support at one end, said lever means including an integral leg portion depending from a point intermediate the ends of said lever means and engaging said wire, and means for selectively actuating said switch means in response to expansion and contraction of said wire.

2. The invention as claimed in claim 1 wherein said wire has a cool contracted state defining a first position for said lever means and a hot expanded state defining a second position for said lever means, said switch means including first and second switches, said first switch connected to energize said igniter means and said first and second switches connected in series to energize said valve means, said first switch being normally closed and said second switch being normally opened when said lever means is in said first position whereby said valve means will not be energized to open until after said igniter means reaches ignition temperature.

4. The invention as claimed in claim 3 wherein said switch means comprises first and second switches, said first switch being operatively connected with said lever means and said voltage responsive means, said second switch being connected to be actuated by said lever means only.

5. The invention as claimed in claim 3 wherein said voltage responsive means includes a holding coil and an armature controlled by said coil and connected to hold said first switch in closed condition.