Automatic Electric Cooker

Abstract

An automatic electric cooker having a heating unit consisting of a metallic alloy heater having a lower resistance and a metallic alloy heater having a higher resistance which are electrically connected in series, and a positive temperature coefficient of resistance ceramic element made of barium titanate semiconductor material electrically connected in parallel with said metallic alloy heater having a higher resistance. The circuit of said heating unit first cooks the material by heat supplied mainly from said lower resistance alloy heater, and then the resistance of said ceramic element abruptly rises to reduce the current flow so as to keep the cooked material warm by heat supplied mainly from the higher resistance alloy heater.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to an electrical cooker capable of automatically carrying out both of cooking and warming, and more particularly to an electrical cooker characterized by having a heating system which comprises two metallic alloy heaters, one of which has a lower resistance and the other of which has a higher resistance, and a semiconducting ceramic element the electrical resistance of which rises abruptly at a certain temperature.

2. Description of the Prior Art

Conventional automatic cookers keep the temperature of the cooked material at a constant temperature by using a relay such as a bimetallic switch. However, the contact of the relay is frequently apt to break down and fail to operate the automatic cooker.

Recently, U.S. Pat. No. 3,375,774 has disclosed an improved automatic cooker having a heating system which comprises a Ni-Cr alloy heater connected in series with a positive temperature coefficient of resistance ceramic heater made of barium titanate semiconductor material. In operation at room temperature, the resistance of the ceramic heater is lower than that of the alloy heater, and therefore the alloy heater supplies almost all the heat that is used for cooking. During the cooking, the temperature of ceramic heater mounted on the outer bottom of cooking container increases due to self-heating. When the temperature of the ceramic heater reaches a certain value, the resistance of the ceramic heater abruptly rises, thereby reducing the current flow. The resistance of the ceramic heater is then higher than that of the alloy heater. Therefore, the smaller amount of heat which is supplied mainly from the ceramic heater, is used for keeping the already cooked material warm. However, the conventional automatic cooker has a disadvantage in that the heat supplied from the ceramic heater depends on the resistance-temperature characteristic of the positive temperature coefficient of resistance ceramic heater. Usually, the positive temperature coefficient of resistance of semiconducting barium titanate ceramic is very high as shown in FIG. 2. Therefore, an undesirable feature of the conventional automatic cooker is that the amount of heat supplied mainly from the ceramic heater is too small to keep the cooked material warm.

In order to obtain the desired temperature-resistance characteristic for the above-mentioned warming, the semiconducting ceramic element must be produced by atmosphere control firing. The mass production of uniform ceramic elements by atmosphere control firing is not practical. Fluctuation of the resistance-temperature characteristics of the ceramic can not be avoided. A less steep resistance-temperature characteristic results in warming only that portion near the ceramic heater or in overcooking in the portion near the ceramic heater. Another disadvantage of the conventional automatic cooker is the inability to vary the warming temperature.

SUMMARY OF THE INVENTION

An object of the invention is to provide an automatic temperature controlled cooker which has no relay contact.

Another object of the invention is to provide an electric cooker capable of automatic operation for both cooking and warming after cooking, and more particularly capable of keeping the cooked materials warm at a desired temperature which can be varied by adjusting the resistance of the alloy heater of higher resistance.

The objects of the invention are realized by an automatic electric cooker having a heating unit consisting of the semiconducting ceramic element mounted in thermal contact on the outer bottom of a cooking container, and a lower resistance-alloy heater and a higher resistance-alloy heater being electrically connected in series, the higher resistance-alloy heater electrically connected in parallel with the semiconducting ceramic element and both alloy heaters being mounted on the cooking container for supplying heat to the material in the container. The semiconducting ceramic element is a barium titanate ceramic which has an electrical resistance lower than that of the alloy heaters at room temperature and shows a rapid increase in the electrical resistance near the ferroelectric transition temperature of the ceramic element. The voltage-current characteristics of the heating unit allow the current-flow necessary for cooking and then cause an abrupt decrease in the current flowing through the heating system so as to keep the cooked material warm at a given temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in connection with the accompanying drawings in which:

FIG. 1 is a circuit diagram of the temperature sensitive heating system of the present invention,

FIG. 2 is a graph showing the temperature dependency of the resistance of a semiconducting ceramic element having a positive temperature coefficient of resistance, and which is composed of barium titanate semiconductor material,

FIG. 3 is a graph showing the characteristic current vs, voltage curves of the higher resistance-alloy heater and the ceramic element connected in parallel at several ambient temperatures, and

FIG. 4 is a graph showing various operating points of the heating system at various temperatures.

DESCRIPTION OF PREFERRED EMBODIMENTS

Before proceeding with a detailed description of the construction of the automatic element cooker according to the invention, the novel heating system will be explained with reference to FIGS. 1-4 of the drawings.

Referring to FIG. 1, terminals adapted to be electrically connected to current source are connected to a lower resistance-alloy heater 1, a higher resistance-alloy heater 2, and the ceramic element 3, wherein the lower resistance-alloy heater 1 and the higher resistance-alloy heater 2 are connected in series, and the higher resistance-alloy heater 2 and the ceramic element 3 are connected in parallel. "The warming system" is defined by the circuit system consisting of the higher resistance-alloy heater 2 and the ceramic element 3 connected in parallel. "The heating system" is defined by the circuit system consisting of the lower resistance-alloy heater 1. Said alloy heaters 1 and 2 are made of a conventional alloy resistor such as a Ni-Cr alloy resistor. Said ceramic resistor 3 is made of a so called positive temperature coefficient resistance (hereinafter abbreviated to PTC) barium titanate ceramic body, the resistance of which has temperature dependency as shown in FIG. 2. The electrical resistance of the semiconducting ceramic element is lower than those of both the alloy heaters at room temperature but abruptly increases above a certain temperature to be much higher than those of both the alloy heaters. Such PTC ceramic resistors are disclosed in the prior literature. (e.g., U.S. Pat. Nos. 3,044,968 and 2,981,699)

Referring to FIG. 3, reference characters 4, 5, 6, and 7 designate the characteristic voltage-current curves for said semiconducting ceramic element with respect to various ambient temperature thereof. The ambient temperatures increase in the order of curves, 4, 5, 6 and 7. Reference character 12 designates the voltage-current line for the higher resistance-alloy heater connected in parallel to said ceramic element.

Current flowing through the higher resistance-alloy heater increases linearly with an increase in the applied voltage and is almost constant with respect to the temperature of a higher resistance-alloy heater itself. As a current flowing through the ceramic element increases with an increase of the applied voltage, the temperature of the ceramic element itself rises. When the temperature exceeds a specified temperature which depends upon the composition of the ceramic element, the current flowing through the ceramic element decreases even with increasing applied voltage due to its PTC characteristics. The characteristic voltage-current curves of the ceramic element at the ambient temperatures curve upwards at a low applied voltage as shown in FIG. 3. When the ambient temperature is high a specified temperature is achieved by a low applied voltage. Therefore, the characteristic voltage-current curves of the ceramic element shift down with an increase in ambient temperature as shown in FIG. 3. Reference characters 8, 9, 10 and 11 represent cross points between the curve 12 and the characteristic voltage-current curves 4, 5, 6 and 7. When voltage lower than the voltage at the cross point is applied to the warming system consisting of both of the higher resistance-alloy heater and the ceramic element which are electrically connected in parallel, the voltage-current curves of the warming system are approximately equal to the curves 4, 5, 6 and 7. When the applied voltage is higher than the cross point in FIG. 3, the voltage-current curves of the warming system are changed and are approximately equal to the straight line 12.

FIG. 4 shows the operating points of the present novel heating system comprising said ceramic element, said higher resistance-alloy heater and said lower resistance-alloy heater when a voltage 19 is applied to the heating system. The voltage-current curves of the ceramic element and high resistance-alloy heater are designated by the same reference numbers as in FIG. 3. The load line of said lower resistance-alloy heater is represented by the reference character 13. As a practical matter, the load line varies slightly with the ambient temperature. However, the variation is negligible compared with that of the ceramic element. An operating point is defined as the intersection of the load line of said lower resistance-alloy heater with a voltage-current curve of said warming system consisting of said ceramic element and said higher resistance-alloy heater. The operating point can be determined by current flowing in the heating system and the voltage divided between the lower resistance-alloy heater and the higher resistance-alloy heater. The voltage-current curves of said warming system vary in the order 4, 5, 6 and 7 with increasing ambient temperature under an applied voltage lower than that of the cross points 8, 9, 10 and 11, but the voltage-current line 12 of said warming system is almost constant regardless of ambient temperature under a voltage higher than that of the cross points. Therefore, the operating point varies successively in order from 14, 15, 16 and 17 while there is an accompanying decrease of the flowing current as shown in FIG. 4 until the point 17. Since the characteristic curve 7 is tangent to the load line at the point 17, the operating point immediately moves from the point 17 to the point 18 as soon as the ambient temperature exceeds the temperature for the characteristic curve 7. Consequently, the flowing current decreases abruptly. The abrupt decrease in the current results in a decrease in the amount of heat radiated from the heating system. Before the abrupt decrease, the lower resistance-alloy heater supplied the heat necessary for cooking, and after the abrupt decrease, the high resistance-alloy heater supplied the heat necessary for warming. The point 18 can be determined by the intersection of the curve 12 and the curve 13 regardless of the characteristic curve of the ceramic element. Therefore, if the higher resistance-alloy heater is a variable resistance, the desired warming temperature can be varied.

An electric cooker having this novel heating system can automatically control the temperature of a given amount of the materials to be cooked in such a way that the material is heated up to the cooking temperature, kept at the cooking temperature for a time interval sufficient for cooking, and then kept warm at a desired temperature without overcooking.

The novel heating system according to the invention comprises a ceramic element mounted on the outer bottom of a cooking container and two Ni-Cr alloy heaters. The electrical resistance of the ceramic element is much lower than that of the alloy heaters at room temperatures of 10.degree. to 30.degree. C. and becomes much higher than that of the alloy heaters above the temperature at which the material is kept warm after cooking. A desirable ratio of the resistance of the lower resistance alloy heater to the resistance of the higher resistance-alloy heater is in the range from one-half to one-tenth. For example, the resistance value of lower resistance heater and the higher resistance heater may be 20.OMEGA. and 120.OMEGA. respectively, and the ceramic element may have the resistance-temperature characteristic as shown in FIG. 2, wherein its resistances at room temperature and at 200.degree. C. are 0.8.OMEGA. and 8,000.OMEGA., respectively. In this heating system, the alloy heater having the resistance of 20.OMEGA. is the primary heat source until cooking is finished, whereas the alloy heater having the resistance of 120.OMEGA. is the primary source of heat for warming after completing the cooking. When a voltage of 100 v. is first applied to this heating system, an electric power of about 500 w. is consumed by the 20.OMEGA. alloy heater and is used for cooking. When the resistance of the ceramic element increases, thereby reducing the current, 10 w. of power are consumed by the 20.OMEGA. alloy heater and of 60 w. of power are consumed by the 120.OMEGA. alloy heater, both being used for warming. The electric power consumed by the semiconducting ceramic element is negligible, since very little current flows through the ceramic element due to its resistance of 800.OMEGA.. Substitution of a variable resistance of 40 to 200.OMEGA. for the fixed resistance of 120.OMEGA. makes it possible to vary the electrical power for warming from 170 w. to 45 w.

An electric cooker having a novel heating system according to the invention has the advantages that the current flowing through the heating system can be automatically controlled without the use of a relay contact, that any desired warming temperature can be easily selected by using a variable resistor as the high resistance-alloy heater, and also that a ceramic element having a wide tolerance of resistance-temperature characteristics can be used since the heat necessary for warming is determined by the resistance of the high resistance-alloy heater independent of the resistance-temperature characteristics of said ceramic element.

Claims

What is claimed is:

1. An automatic heating control system comprising low resistance resistor means for primarily performing a heating function, high resistance resistor means permanently connected in series with said low resistance resistor means for primarily performing a warming function, and ceramic semiconducting resistor means having a relatively steep positive temperature coefficient of resistance permanently connected in parallel with said high resistance resistor means for automatically changing from said heating function to said warming function as a result of a change in resistance of said ceramic semiconducting resistor means.

2. An automatic heating control system as claimed in claim 1, wherein the ratio of the resistance of said low resistance resistor means to the resistance of said high resistance resistor means is in the range from one-half to one-tenth.

3. In an electric cooker, an automatic heating control system comprising low resistance resistor means for primarily performing a heating function, high resistance resistor means permanently connected in series with said low resistance resistor means for primarily performing a warming function, and ceramic semiconducting resistor means having a relatively steep positive temperature coefficient of resistance permanently connected in parallel with said high resistance resistor means for automatically changing from said heating function to said warming function as a result of a change in resistance of said ceramic semiconducting resistor means, said resistor means having a positive temperature coefficient of resistance being thermally coupled with said cooker.

4. In an automatic cooker as claimed in claim 3, wherein the ratio of the resistance of said low resistance resistor means to the resistance of said high resistance resistor means is in the range from one-half to one-tenth.

5. In an electric cooker, an automatic heating control system comprising a low resistance resistor, a high resistance resistor permanently connected in series with said low resistance resistor, and a resistor having a positive temperature coefficient of resistance permanently connected in parallel with said high resistance resistor, said resistor having a positive temperature coefficient of resistance being thermally coupled with said cooker, said high resistance resistor comprising a variable resistor for keeping the cooked material warm at a desired temperature, the resistance of said variable resistor being higher than the resistance of said low resistance resistor or the resistance of said resistor having a positive temperature coefficient of resistance at room temperature, but being lower than the resistance of said resistor having a positive temperature coefficient of resistance after cooking.

6. In an electric cooker, an automatic heating control system comprising a low resistance resistor, a high resistance resistor permanently connected in series with said low resistance resistor, and a resistor having a positive temperature coefficient of resistance permanently connected in parallel with said high resistance resistor, said resistor having a positive temperature coefficient of resistance being thermally coupled with said cooker, said resistor having a positive temperature coefficient of resistance comprising a semiconducting barium titanate ceramic element which has a resistance lower than that of said low resistance resistor at room temperature and which has a resistance higher than that of said high resistance resistor at the ferroelectric transition temperature of said resistor having a positive temperature coefficient of resistance.

7. An automatic heating control system comprising a low resistance resistor, a high resistance resistor permanently connected in series with said low resistance resistor, and a resistor having a positive temperature coefficient of resistance permanently connected in parallel with said high resistance resistor, said high resistance resistor being a variable resistor and the resistance of said resistor having a positive temperature coefficient of resistance being lower than the resistance of said low resistance resistor at a first ambient temperature and higher than the resistance of said high resistance resistor at a second ambient temperature.

8. An automatic heating control system comprising a low resistance resistor, a high resistance resistor permanently connected in series with said low resistance resistor, and a resistor having a positive temperature coefficient of resistance permanently connected in parallel with said high resistance resistor, said resistor having a positive temperature coefficient of resistance comprising a semiconducting barium titanate ceramic element which has a resistance lower than that of said low resistance resistor at room temperature and which has a resistance higher than that of said high resistance resistor at the ferroelectric transition temperature of said resistor having a positive temperature coefficient of resistance.