Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
2002-12-16
2004-08-24
Paschall, Mark (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S486000, C219S483000, C219S508000, C219S494000, C219S414000
Reexamination Certificate
active
06781097
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to controllers for ovens and, more particularly, to methods and systems for controlling the power radiated by heater elements in a single oven.
BACKGROUND OF THE INVENTION
Temperature controls in ovens are well known. Typically, two rotary switches are provided for oven control. One switch allows a user to select a operational mode for an oven, such as bake, broil, or time bake; and the other switch regulates the temperature of the oven. Detents are typically provided in cams or disks that are mounted to the shaft of a rotary switch for function selection. These detents provide tactile and auditory feedback that confirms the user has selected a particular function by aligning an index marker on the switch with a mode identifier located proximate the switch or vice versa. The mode identifier is typically indicia such as words that are printed along the circumference of a rotary mode selection switch. Alternatively, a pushbutton or membrane switch may be provided for each operational mode on the control panel of a cooking appliance. Depressing a switch activates the mode of operation identified by the indicia located proximate the switch. The temperature control is typically a rotary switch that may include a potentiometer that is operated by rotating the shaft to provide a continuously variable control signal that corresponds to a temperature range.
In most previously known ovens, two heater elements are disposed in the oven cavity. One heater element, sometimes called a baking element, is typically located proximate the floor of the oven cavity and a second heater element, sometimes called a broiler element, is typically located proximate the ceiling of the oven cavity. When the function selector is set to a baking mode, typically both heater elements are activated to quickly heat the air in the oven cavity to a temperature set on the temperature switch and then electrical power to the broiler element is removed. The operation of the heater elements is typically regulated by a thermostatic switch that has its regulation temperature set by the potentiometer of the temperature control. Electrical power may be selectively applied to the heater elements when the thermostatic switch is closed and the power source is decoupled from the heater elements when the switch opens at the regulation temperature. In response to the function selector being set to “Broil,” the temperature selector is typically disabled and the upper heater element alone is operated at full power.
The dissipation of heat from the two heater elements has different effects upon food. The upper heater element has more of a direct effect upon the surface of a dish because most foods placed in an oven cavity are not covered. Thus, the pan bottom and sides insulate the food from the bottom heater element to some degree but the upper heater element may brown the top of an uncovered food more quickly. For this reason, some oven controls have been developed that alter the application of electrical power to the heating elements or that alter the electrical load that the heating elements present to the power source.
U.S. Pat. No. 3,393,295 to Jepson et al. discloses a cooking device having a proportional heater element control. The proportional control is provided by a circuit that couples one heating element of the device to the electrical power source during a portion of a time interval and then couples the other heating element to the power source for the remainder of the time interval. This alternating coupling of electrical power to the two heating elements occurs for each time interval during the entire time that the cooking device is operated for cooking food that is placed within the heating cavity. For example, if the time interval is set to one minute and the cooking device is operated for thirty minutes to cook some food item, the alternating coupling of the heater elements to the electrical source occurs during each of the thirty one minute intervals. The device of this patent does not have a mode switch as the position of the proportional control switch determines whether the device is operating in the frying mode (bottom element alone is continuously coupled to power), baking mode (both elements coupling to power in an alternating manner), or broiling mode (upper element alone is continuously coupled to power).
U.S. Pat. No. 4,538,049 to Ryckman, Jr. discloses a toaster oven that has two upper heating elements coupled together in series and two lower heating elements coupled together in series. The upper heating elements have the same resistance value and this value is less than the resistance value of the lower heating elements. The toaster oven also includes a mode selector switch that may be used to select a toast, baking, or broiling mode as well as a special bake mode switch. The special bake mode switch couples one of the upper heating elements to one of the lower heating elements in series while the other upper heating element is coupled to the other lower heating element in series. Thus, both series combinations equally divide the current and the increase in the resistance of the upper element leg reduces the power dissipated by the upper heating elements. Consequently, the power ratio between the upper and lower heating elements is reduced and the food may be cooked with less intense heat on the upper surface of the item.
The temperature control switch of U.S. Pat. No. 2,832,878 to Baird incorporates a variable resistor with the temperature control switch so the resistance in one leg of a Wheatstone bridge corresponds to a temperature setting on a dial mounted in association with the temperature control switch. This variable resistance alters the point of balance of the Wheatstone bridge to vary the operation of a relay that applies electrical power to the heating elements of an oven. However, the signal generated by the potentiometer does not change the proportion of time that an upper element is operated with respect to a lower heating element nor does it alter the level of power applied to either heating element.
However, the oven controls disclosed by these patents suffer from one or more of the following limitations. For one, the device of the '295 Patent does not provide the simultaneous operation of both heating elements. The repetitive coupling and decoupling of the heating elements during each occurrence of a timing interval of a baking period may cause arcing at the contacts and degrade the operational life of the coupling switch. Also, the repetitive application and removal of electrical power causes variations in the current required by the elements and may increase power consumption of the device. The proportional control circuit of the '295 Patent requires a complicated cam follower arrangement that is driven by a timing mechanism. Such electromechanical circuits are relatively expensive to manufacture. The provision of a special baking mode switch, such as the one disclosed in the '049 Patent, requires additional cost for the special mode switch and its incorporation in the electrical circuit of the cooking device. The special mode switch also fails to provide continuously variable variation in the contribution of the upper and lower heating elements to the heat generated for the oven cavity. Finally, the temperature control of the '878 Patent provides a continuously variable signal over a temperature range but it does not alter the contribution of heat in the oven cavity from the lower and upper heating elements.
Another limitation of previously known oven controls is the use of a special circuit that is coupled to only one input line and a neutral line to reduce the input voltage for the heater element from the line-to-line voltage. This mechanism not only requires an additional circuit but it also only provides one alternative voltage setting for operation of the upper heater element.
What is needed is a system and method for operating two heater elements in an oven at different rates or power levels within
Emerson Electric Co.
Maginot Moore & Beck
Paschall Mark
LandOfFree
System and method for proportional control of oven heating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method for proportional control of oven heating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method for proportional control of oven heating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3288390