US3540817A - Electric ignition system - Google Patents

Description

Nov. 17, 1970 H. J. TYLER 3,540,817

ELECTRIC IGNITION SYSTEM L Filed Feb. 28, 1969 FUEL l8 BURNER 2 L OOOOOOO SATURABI-E LAW l4 CONTROL 5/ TRANSFORMER ""W e fi n 24 t 227 ELECTROMAGNETIC FUEL VALVE FUEL INPUT INVENTOR HUGH .1. TYLER BY ATTORNEYS United States Patent F 3,540,817 ELECTRIC IGNITION SYSTEM Hugh J. Tyler, Santa Ana, Calif., assignor to Robertshaw Controls Company, Richmond, Va., a corporation of Delaware Filed Feb. 28, 1969, Ser. No. 803,298 Int. Cl. F23n 5/00 11.5. Cl. 431-66 6 Claims ABSTRACT OF THE DISCLOSURE An electric ignition system comprising an electric fuel ignitor coupled in series with the primary winding of a saturable control transformer across an electric power supply. The secondary winding of the transformer is coupled in series with a thermistor type heat sensor, a control switch, and an electromagnetic fuel valve. The control transformer is designed to saturate with only a few volts applied to the primary winding, whereby nearly full line power is applied to the electric ignitor and a constant control voltage is supplied to the transformer secondary circuit. The constant control voltage induced in the secondary winding energizes the fuel valve when the control switch is closed and the heat sensor resistance is low indicating sufficient ignitor temperature.

BACKGROUND OF THE INVENTION The present invention relates to electric ignition systems and, more particularly, to a fail-safe electric ignition system wherein a saturable control transformer is utilized to provide effective fuel flow control.

Fuel burner systems under automatic control require an ignition system which will ignite the fuel when heat is called for and also prevent the flow of fuel to the burner when combustion does not occur. Conventional devices presently in use are controlled by thermoelectric elements or bimetallic devices which are relatively slowacting. This characteristically long reaction time allows raw fuel to escape into the area surrounding the burner where it mixes with air to create a dangerously explosive condition.

To compensate for the above mentioned hazard, fast acting circuits have been developed to control the flow of fuel to the system. While such circuits have reduced the danger of raw fuel leakage, they too have proven to be unsatisfactory under certain conditions of operation. For example, designers have had to resort to complex voltage regulating circuits in order to cope with adverse control circuit conditions created by fluctuating currents in the line used to supply electrical power to the system. In addition, control circuits presently employed in conjunction with electric ignitor elements load down the ignitor which causes increased heat-up time and an increased overall system current drain. Attempted solutions to this problem have rendered the system circuitry even more complex and have limited the development of practical I and economical systems for many important applications.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to construct an electric ignition system having all the advantages of similarly employed prior art systems while improving initial ignitor heat-up time and system control accuracy.

Another object of the invention is to provide an electric ignition system having rapid fuel flow shut-off upon fuel burner flame outage.

This invention has the further object in the provision of an electric ignition system which prevents the flow of fuel to a burner when the ignitor temperature is below fuel igniting temperatures.

An advantage of this invention is the provision of a simple, yet reliable, electric ignition system having improved fail-safe characteristics.

The present invention is summarized in that an electric ignition system for a fuel burner includes an electrical power source coupled to energize an electric ignitor which is located adjacent the burner, and a saturable control transformer. The transformer includes a primary winding which is coupled in series with the ignitor and a secondary winding which provides an induced control voltage, and the transformer is operated in its saturated condition by the power source, whereby the primary winding has negligible impedance and the control voltage is maintained constant. Control means are coupled to the secondary winding of the transformer and located adjacent to the burner for sensing the ignitor temperature and for enabling a flow of fuel to the burner when said ignitor is at fuel ignition temperatures.

These and other objects and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiment of the invention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrates a schematic circuit diagram of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, there is shown an electric ignitor element 10 which is located adjacent to a fuel burner, shown schematically as 12, and is coupled in series with the primary winding of a saturable control transformer 14. The series combination of ignitor 10 and transformer 14 is further coupled to a source of electrical power, represented by power lines 16, through an overload fuse 18 and a main power switch 20. The series circuit thus far described provides energization for the ignitor element and the control circuitry to be described below, and will be referred to herein as the primary circuit of the system. 7

The secondary winding of saturable control transformer 14 provides a control voltage for regulating the flow of fuel to the burner 12 is coupled in series with the energization winding of an electromagnetic fuel valve 22, a negative temperature coetficient thermistor 24, and a. control switch 26. Electromagnetic fuel valve 22, when energized, establishes a flow of fuel from a fuel input source (not shown) through the fuel valve to fuel burner 12 via fuel conduit 28.

Although many varied applications of the circuitry disclosed herein are contemplated, the operation of this invention can best be described by considering a typical installation thereof in a heating system.

When operation of a heating system employing the invention is desired, main power switch 20 will be closed thereby permitting current to flow from power source 16 to the primary circuit, comprising ignitor 10, fuse 18, and the primary Winding of transformer 14. Transformer 14 is a saturable control transformer and is designed to operate in a saturated condition upon the application of only a few volts to the primary winding thereof.

It is important to note at this point that the impedance of the primary winding of saturable control transformers of the type employed by the present invention varies in an inversely proportional relationship with the saturation level thereof. Thus, when such transformers are operated in a fully saturated state, the impedance of the primary winding becomes quite small and consequently reduces the overall current drain of the system as well. as the potential drop across the primary winding.

Since transformer 14 is always operated in a fully saturated condition, the impedance of the primary winding thereof will be small thereby permitting the application of substantially full line voltage to the electric ignitor. Since the ignitor element of the present invention is therefore operated at full voltage, it will reach fuel ignition temperatures more rapidly than heretofore accomplished. This feature of the invention assures initial ignition, provides rapid reignition after a momentary electrical failure in the primary circuit, and thus increases the inherent fail-safe characteristics of the overall system.

Continuing with the description of the operation of the invention, after the system has been turned on by main power switch 20 and energization of the primary circuit has been initiated, a control voltage will be induced in the secondary winding of transformer 14 to energize the electromagnetic fuel valve 22 and establish the flow of fuel to the system burner. Before the fuel valve can be actuated, however, two conditions must be satisfied. The first is that control switch 26 be closed as to complete the circuit connection between the fuel valve and the secondary winding of the transformer. Switch 16 may be any of several devices depending upon individual installation requirements and is shown as a single-pole singlethrow switch solely for the sake of clarity. In a conventional heating system, for example, switch 16 would be responsive to a thermostat (not shown) which monitors the temperature of the area to be heated and automatically actuates the switch when a demand for heat exists.

The second condition is that the resistance of thermistor 24 be sufliciently low to allow enough current to flow through the energization winding of the electromagnetic fuel valve for actuation thereof. Thermistor 24 is located adjacent to the ignitor element so that its temperature closely follows the ignitor temperature, and is designed to allow sufficient current to flow to the fuel valve only after the ignitor has reached fuel ignition temperatures.

Therefore, the burner will receive fuel for combustion only when (a) a demand for heat exists and (b) the ignitor element is at fuel ignition temperatures. In addition, since the transformer is maintained in a saturated condition at all times during operation, the control voltage pro: vided by the secondary winding of saturable transformer 14 remains constant and is independent of any fluctuations in line voltage from power source 16. Since the volt-. age across fuel valve 22 determines whether it will "be open or closed, it is extremely important to prevent line fluctuations from influencing the control circuit voltage. Thus, by utilizing a saturable control transformer as shown, the effect of power line voltage changes is minimized so that the only variations in the control circuit voltage of the present invention will be those produced by the thermistor 24 in response to the temperature of ignitor 10, as desired. This feature allows more accurate design of the control circuit and increases the safety of the system. In describing the fail-safe characteristics of the invention, it can be seen from the figure that a failure in any component causing an open circuit will immediately remove actuating potential from the fuel valve to rapidly stop the flow of fuel to the burner. In addition, a short circuit produced by a failure in the fuel valve 22, switch 26 or transformer 14 will also remove the actuating potential from the valve. Finally, a short circuit in the ignitor 10 will cause an overload condition in the primary circuit, due to the low impedence of the primary winding of transformer 14 when saturated, which will cause fuse 18 to rapidly break the energizing current path to the ignition system from power source 16.

Thus, there is provided an electric ignition system which is simple, and yet includes additional fail-safe features not found in present systems. The invention therefore makes possible the design of many useful appliances requiring only minimal maintenance and at the same time providing increased consumer safety.

Inasmuch as the present invention is subject to many variations, modifications and changes in detail, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: 1. An electric ignition system for a fuel burner, comprising:

an electrical power source; an electric ignitor coupled to said power source for energization thereby and located adjacent said burner; I

a saturable control transformer coupled to said power source and including a primary winding coupled in series with said electric ignitor, and a secondary winding for providing an induced control voltage, said transformer being operated in its saturated condition by said power source whereby said primary winding has negligible impedance and said control voltage is maintained constant; and

control means coupled to the secondary winding of said transformer and located adjacent said burner for sensing the temperature of said electric ignitor and for enabling a flow of fuel to said burner when said ignitor is at fuel ignition temperatures.

2. The invention as recited in claim 1, wherein said control means includes a thermistor having a negative temperature coefficient of resistance.

3. The invention as recited in claim 2, whereinsaid control means further includes an electromagnetic fuel valve electrically coupled in series with said thermistor to form a series network, said series network being coupled in parallel with the secondary winding of said transformer.

4. The invention as recited in claim 3, wherein said control means further includes a thermostatically actuated electrical switch coupled in series with said fuel valve and said thermistor.

5. A rapid ignite fail-safe electric ignition system, comprising:

a fail-safe saturable control transformer having a saturationlevel dependent upon applied voltage, including a primary Winding having an impedance which is inversely proportional to the saturation level of the transformer, and a secondary winding producing an induced control voltage which is constant when the transformer is fully saturated;

an electric ignitor coupled in series with the primary winding of said transformer;

an electromagnetic fuel valve;

heat sensing means coupled to said fuel valve and the secondary winding of said saturable transformer to form a closed-loop series circuit, for sensing the temperature of said ignitor and for enabling said fuel valve when said ignitor is at fuel igniting temperatures; and 1 electrical power supply means coupled to said ignitor and said transformer for energizing said system and for maintaining said transformer in its fully saturated state.

6. The invention as recited in claim 5, wherein said heat sensing means comprises a thermistor having a nega tive temperature coefiicient of resistance.

References Cited UNITED STATES PATENTS 3,454,345 7/1969 Dyre 43l-66 3,480,374 11/1969 Lamb et al. 43166 EDWARD G. FAVORS, Primary Examiner US. Cl. X.R. 31798; 431-254

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