Microwave oven

Abstract: A microwave oven comprises a heating chamber (1) in which to put food to be heated, a high frequency heating circuit for supply­ ing high frequency energy to the heating chamber, two thermal heaters (2, 3) each provided in the upper and lower areas of the heating chamber, a switching circuit (12) for selecting the high frequency heating circuit or the thermal heaters, a first switching circuit (18) for permitting intermittent power supply to the high frequency heating circuit when the high frequency heating circuit is selected by the swithcing circuit, a second switching circuit (22) connected in parallel with the first switching circuit so as to provide a bypass of the first switching circuit when closed, a first contact mounted in the first switching circuit to permit intermittent power supply to one of the two thermal heaters in the heating chamber, and a second contact mounted in the second switching circuit to provide a bypass of the first contact.

Description: Background of the Invention The present invention relates to a microwave oven with toasting and baking functions having two upper and lower heaters in the heating chamber, or particularly to a microwave oven having a circuit for periodically energizing and de-energizing one of the two heaters. Typically, a microwave oven with toasting and baking functions comprises the circuitry shown in Fig. 4, and controls, according to the selected cooking mode, power supply to upper and lower heating elements (50) and (51) in the heating chamber (not shown) and to high frequency heating means (56) comprising a magnetron (52), a high voltage transformer (53), a high voltage capacitor (54) and a high voltage diode (55). The cooking mode is selected from among toasting, baking, defrosting and high-frequency heating (hereinafter referred to as microwave heating) modes. The operation of the electrical elements in the baking mode is described with reference to Fig. 4. When baking mode is selected, the N.O contact (57a) of the selector switch (57) is closed. The toasting switch (58) and the defrost-cancelling switch (59) are then closed interlocking with the N.O contact (57a). To control the temperature in the heating chamber (not shown), the oil thermostat (60) provided in the heating chamber is adjusted to set the heating temperature. When heating time is set by the timer (61), the timer contact (61a) closes so that power is supplied to the upper and lower heating elements (50) and (51) arranged in the heating chamber. The oil thermostat (60) is turned OFF when the temperature in the heating chamber rises above the setting, and turned ON when the heating temperature drops to the setting, thus maintaining the heating temperature at the preset value. When the preset heating time has elapsed, the timer contact (61a) opens, terminating the cooking operation. However, in the microwave oven with the above circuitry in which power supply to the two heaters is turned ON and OFF around the preset heating temperature, the radiant heat is too large for baking, say, cake, often causing cracks in the cake surface. The possible measure against this is to lower the heating temperature or to keep the heaters away from the food. These measures have the following problems, however. Cooking operation takes longer time as the heating temperature is lowered. The remoter the heaters are from the food, the longer it takes in toasting food, sacrificing the food taste. Patent Application US-A-3 569 656 describes a combined thermal and microwave cooker having a thaw mode wherein both a thermal heater and a microwave heater provide a pulsed output, this being stated to reduce power consumption. The possibility of thawing using pulsed thermal heating alone is also described. In this case two thermal heaters may be alternately and simultaneously connected to the power source. As described, the thermal heater and microwave heater are pulsed at different duty cycles, which requires two separate switching devices. This document additionally describes heating modes wherein both thermal and microwave heating takes place, the thermal heating being continuous and the microwave heater being energised for a relatively short period within the heating cycle. Patent Application DE-A1-2,911,929 also describes a combined thermal and microwave cooker wherein thermal heating elements are provided in the top and bottom of the oven. Cooking occurs in three phases in each of which a different combination of thermal elements and microwave elements are energised. Patent Application FR-A-2,210,072 describes a microwave oven having a defrosting mode in which the microwave generator is intermittently energised by means of a cyclically operated switch. During a continuous mode of operation this switch is shunted by a by-pass circuit path. German Patent DE-C1-2,857,367 describes a combined thermal and microwave oven wherein the oven is thermally heated at the start of operation. When the temperature in the heating chamber reaches a predetermined level, thermal heating is stopped and high frequency heating commences. Thermal heating is resumed once the temperature within the heating chamber falls to less than a predetermined level. Summary of the Invention In view of the above, it would be desirable: (a) to provide a microwave oven capable of changing the energization period of the heaters according to the cooking mode while keeping the oven temperature constant at a preset value; and (b) to provide a mircowave oven capable of baking food such as cake optimally without increasing the cooking time. Other features of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. According to the present invention there is provided a microwave oven having a heating chamber for accommodating food to be cooked, high frequency heating means for supplying high frequency energy to the chamber, first and second thermal heaters for heating the food, and a switch means for controlling the operation of said high frequency heating means and said first and second thermal heaters, characterised in that said switch means comprises a first switch and is selectively operable to provide first, second, third and fourth selectable heating modes, said first heating mode employing only said high frequency heating means, said second heating mode employing only said thermal heaters and including a period of continuous operation of both said first and second thermal heaters, said third heating mode employing only said first and second thermal heaters and including a period of continuous operation of the first thermal heater and intermittent operation of the second thermal heater, and said fourth heating mode involving intermittent operation of the high frequency heating means, and wherein the intermittent operation of the second thermal heater in said third heating mode and the intermittent operation of the high frequency heating means in the fourth heating mode are each effected using said first switch, said switch means further comprising a by-pass switch which can by-pass the first switch to provide said first heating mode in which, despite the intermittent switching of the first switch, the high frequency heating means is energized continuously, said switch means further comprising a selector switch, said by-pass switch having contacts which, upon closure when heating by the first and second thermal heaters has been selected by said selector switch, causes the continuous operation of the second thermal heater in said second heating mode. Since the second heater is energized intermittently because of the function of the switch means, in the preferred embodiment it generates smaller radiant heat than it does when energized continuously, which prevents food from being heated excessively. Brief Description of the Drawings The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and wherein: Fig. 1 is a front view of a microwave oven as an embodiment of the present invention; Fig. 2 is an electrical circuit diagram of the microwave oven of Fig. 1; Fig. 3 is a graph showing temperature variation in the heating chamber in the baking mode; and Fig. 4 is an electrical circuit diagram of the conventional microwave oven, for comparison with the circuit diagram of the microwave oven of the present invention. Detailed Description of the Invention An embodiment of the present invention is described in detail below with reference to the accompanying drawings. Referring to Fig. 1, food to be heated is placed in a heating chamber (1). Respective heating elements such as sheathed heaters (hereinafter called heaters) (2) and (3) are provided in the upper and lower parts of the heating chamber (1). A turntable (4) on which to put food is positioned over the lower heater (3). The turntable (4) has holes through which the radiant heat of the lower heater (3) reaches the food. An outer casing (6) had an operation panel (5) with various switches (not shown) and indicators (not shown) provided thereon. Fig. 2 shows the electrical circuit of the embodiment. Referring to Fig. 2, high frequency heating means such as a microwave heating means (7) comprises a high voltage transformer (8), a high voltage capacitor (9) connected to the secondary side of the high voltage transformer (8), a high voltage diode (10) and a magnetron (11). High frequency energy such as microwaves is supplied through a wave guide (not shown) from the high frequency heating means (7) to the heating chamber (1). Selector switch means (12) (hereinafter referred to as a selector switch) has a common contact (12a) connected to a first power line (13), an N.C contact (12b) connected to the primary side of the high voltage transformer (8) of the high frequency heating means (7), and an N.O contact (12c) connected through an oil thermostat (14) to the heaters (2) and (3) and to a cement resistor (15). The oil thermostat (14) is mounted in the heating chamber (1) to control the temperature of the heating chamber (1) in the baking mode. It can be set from the exterior. The cement resistor (15) is mounted adjacent to a thermal reed switch (17) connected to a timer motor (16b) of a timer (16) with which heating time is set. The cement resistor (15) serves to control the heating period in the toasting mode, preventing the food from being toasted irregularly due to the temperature variation in the heating chamber (1). First switch means (herein after referred to as a defrost switch) (18) comprises a motor (19) and a contact system (20) made of a micro switch, for example, which opens or closes as a cam mounted on the motor shaft rotates. The contact system (20) comprises an N.O contact (20a) connected to the primary side of the high voltage transformer (8), and N.C contact (20b) connected to the heater (2) and a common contact (20c) connected to a second power line (21). A second switch means (hereinafter called a defrost-cancelling switch) (22) is connected in parallel with the contact system (20). The defrost-cancelling switch (22) comprises an N.O contact (22a) connected to the primary side of the high voltage transformer (8), an N.C contact (22b) connected to the heater (2) and a common contact (22c) connected to the second power line (21). A toast switch (23) is connected in parallel with the thermal reed switch (17) and opens in the toasting mode. (24) is a fan motor for cooling the entire microwave oven (25). (26) is an oven lamp for illuminating the heating chamber (1). (27), (28) and (29) are safety switches operating when the oven door (30) is opened. A timer contact (16a) is connected to the first power line (13). The selector switch (12), the defrost-cancelling switch (22) and the toast switch (23) operate as shown in Table 1 below, interlocking mechanically with one another. Operation of the present invention is described now for each cooking mode. In the baking mode, when the heating time is set, the N.O contact (12c) of the selector switch (12) is closed. Then the timer contact (16a) is closed so that power is supplied to the lower heater (3). The motor (19) of the defrost switch (18) is also energized, so that the N.C contact (20b) of the contact system (20) of the defrost switch (18) is closed, permitting power to be supplied for a specified period to the heater (2). Then, the N.C contact (20b) is opened and the N.O contact (20a) is closed so that the power supply to the heater (2) is cut off. The period of an energization and de-energization cycle of the heater (2) is determined by the configuration of the cam mounted on the motor (19). The above energization and de-energization cycle is repeated periodically until the preset heating time elapses, permitting intermittent power supply to the heater (2). Consequently, the temperature in the heating chamber (1) is maintained around the value set for the oil thermostat (14) in such a manner that the temperature rises with the N.C contact (20b) ON, and drops with the N.C contact (20b) OFF while the oil thermostat (14) is turned ON, and drops when the oil thermostat (14) is turned OFF, as shown in Fig. 3, whereby the food is baked desirably. Operation of the present invention for other cooking modes is as follows. When the defrosting mode is selected, the N.C contact (12b) of the selector switch (12) is closed. The toast switch (23) and the N.C contact (22b) of the defrost-cancelling switch (22) are closed, interlocking with the selector switch (12). When heating time is set by the timer (16), the timer contact (16a) is closed so that the voltage is applied through the selector (12) to the high voltage transformer (8), oscillating the magnetron (11). Since the N.O contact (22a) opens and closes periodically as the motor (19) of the defrost switch (18) rotates, oscillation of the magnetron (11) is intermittent. When the preset heating time elapses, the timer contact (16a) opens, terminating the defrosting operation. When the microwave heating mode is selected, the N.C contact (12b) of the selector switch (12) and the toast swatch (23) are closed as in the defrosting mode. The N.O contact (22a) of the defrost-cancelling switch (22) is closed, providing a bypass of the contact system (20). Accordingly, regardless of the position of the defrost switch (18), the magnetron (11) is oscillated continuously until the timer contact (16a) opens. In the toasting mode, the N.O contact (12c) of the selector switch (12) and the N.C contact (22b) of the defrost-cancelling switch (22) are closed while the toast switch (23) is opened. With this state, if the timer contact (16a) is closed by setting the timer (16), the timer rotor (16b) will not be operated although the heaters (2) and (3) are energized. This makes it possible to control the toasting time so as to toast food uniformly for the initial and the subsequent toasting operation. Specifically, in the initial toasting operation, the thermal reed switch (17) adjacent to the heating time-collecting cement resistor (15) is open in the early stage because the temperature is below the operating temperature. When the temperature of the cement resistor (15) rises to the operating temperature of the thermal reed switch (17) , the thermal reed switch (17) is closed, thus actuating the timer motor (16b). When the preset heating time has elapsed, the timer contact (16a) is opened, terminating the toasting operation. In the second toasting operation, the temperature of the cement resistor (16) should already be sufficiently high enough to close the thermal reed switch (17). The timer motor (16b) is therefore actuated from the beginning, supplying power to the heaters (2) and (3) until the preset heating time elapses. Therefore, when the user wants to cook several slices (loaves) of bread in continuous toasting operations, he (or she) needs to set a heating time only in the beginning. With the same heating time setting, the heating time is controlled automatically for each toasting operation so that all the slices (loaves) of bread are toasted uniformly. In the toasting mode, intermittent energization of the heater (2) and (3) would result in longer heating time, causing bread to be dried to the interior and thus impairing the taste. To avoid this, the oil thermostat (14) of the present invention is designed to be set to the maximum value when the toasting mode is selected, so that the oil thermostat (14) is not actuated in the toasting mode. The upper heater is intermittently energized at a specified frequency in the baking mode, preventing the temperature of the heating chamber from rising excessively. Therefore, the microwave oven of the present invention is capable of baking food such as cake optimally without causing surface cracks.

Claims: A microwave oven having a heating chamber (1) for accommodating food to be cooked, high frequency heating means (7) for supplying high frequency energy to the chamber, first and second thermal heaters (3,2) for heating the food, and a switch means (12,20,22) for controlling the operation of said high frequency heating means (7) and said first and second thermal heaters (3,2), characterised in that said switch means (12,20,22) comprises a first switch (20) and is selectively operable to provide first, second, third and fourth selectable heating modes, said first heating mode employing only said high frequency heating means (7), said second heating mode employing only said thermal heaters (3,2) and including a period of continuous operation of both said first and second thermal heaters (3,2), said third heating mode employing only said first and second thermal heaters (3,2) and including a period of continuous operation of the first thermal heater (3) and intermittent operation of the second thermal heater (2), and said fourth heating mode involving intermittent operation of the high frequency heating means (7), and wherein the intermittent operation of the second thermal heater (2) in said third heating mode and the intermittent operation of the high frequency heating means (7) in the fourth heating mode are each effected using said first switch (20), said switch means (12,20,22) further comprising a by-pass switch (22) which can by-pass the first switch (20) to provide said first heating mode in which, despite the intermittent switching of the first switch (20), the high frequency heating means (7) is energized continuously, said switch means (12,20,22) further comprising a selector switch (12), said by-pass switch (22) having contacts (N.C., 22b) which, upon closure when heating by the first and second thermal heaters (3,2) has been selected by said selector switch (12), causes the continuous operation of the second thermal heater (2) in said second heating mode.

Inventors: IKUYASU KAMINAKA, IKUYASU, KAMINAKA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: H05B6
IPC codes: H05B6, H05B11, F24C7