Sharp Kabushiki Kaisha Corporation

Hiroshi Minakawa

Masayuki Uno

Hiroshi Kinoshita

Yoshikazu Kitaguchi

Masatugu Fukui

Eiji Fukuda

A microwave oven includes a heating chamber, a turntable provided in the oven, a magnetron, a waveguide for guiding the microwave generated by the magnetron into the heating chamber, an electric field sensor provided at the inner wall face of the waveguide or the heating chamber, a metal reflector provided rotatably inside the waveguide for adjusting the impedance of a load at the heating chamber side by its rotation angle, and a circuit responsive to an output of the electric field sensor for controlling the rotation angle of the metal reflector. The control circuit includes a first program for controlling the rotation angle of the metal reflector according to a first method within a predetermined time period from the start of operation of the microwave oven, and a second program for controlling the rotation angle of the metal reflector according to a second method different from the first method after elapse of the predetermined time period. For example, in the first method, the rotation angle of the metal reflector is sequentially controlled so that the output of the electric field sensor is minimum, and in the second method, the rotation angle of the metal reflector is fixed to an average value of the rotation angle of the metal reflector of the first method. The initial rotation angle of the metal reflector is determined so that impedance matching can be obtained when load of approximately 300-500 cc calculated in water load is placed at the center of the heating chamber.

Microwave oven having a function for matching impedance

A microwave oven with an invertor control power source produces a microwave by the use of the magnetron in relation to the high frequency power generated by the invertor control power source. The heat sensitive ferreit beads provided on the lead wire for transmitting the high frequency power to the magnetron changes the inductance of the wire from the high level to the low level at around a threshold temperature lower than the working equilibrium temperature of the magnetron. The cathode filament current remains within the allowable range, and the moding of the magnetron is prevented. The magnetron can operate in a long life time.

Microwave oven with inverter controlled power source

A hot air exhaustion opening is formed in an oven wall of a microwave oven. A heat-sensitive film is disposed at the hot air exhaustion opening to close the hot air exhaustion opening when the microwave oven is in the normal operation mode. When the oven cavity temperature rises to an abnormal value, the heat-sensitive film is fused to conduct hot air toward the outside of the microwave oven cavity through the hot air exhaustion opening. A heat-sensitive fuse is disposed on the downstream side of the heat-sensitive filter film so that the heat-sensitive fuse receives the hot air exhausted through the hot air exhaustion opening when the heat-sensitive film is fused. The heat-sensitive fuse is associated with a microwave generation control circuit so that the microwave generation is suppressed when the oven cavity temperature rises to the abnormal value.

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Higashiosaka, Japan

Hiroshi Minakawa