Electric heating – Metal heating – Of cylinders
Patent
1984-08-21
1985-12-03
Albritton, C. L.
Electric heating
Metal heating
Of cylinders
219 1055B, H05B 664
Patent
active
045567735
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a monitor circuit for a door safety switch used in high frequency heating appliances such as microwave ovens having an electric wave emission start circuit.
High frequency heating appliances such as microwave ovens generally use a magnetron as a means for generating microwaves and a magnetic leakage type transformer as a means for supplying a high voltage to operate the magnetron. Because of the nature of heating appliances, this transformer must be a high-input power type ranging from 1 KW to 5 KW. When such a large transformer of the magnetic leakage type is used, a very large excitation rush current will flow depending upon the turn-on phase of the power. If .phi..sub.A is the magnetic flux in the core during rated operation, and .phi..sub.MAX is the saturated magnetic flux, and .phi..sub.B is the residual magnetic flux, then a magnetic flux of up to (2 .phi..sub.A +.phi..sub.B -.phi..sub.MAX) will be generated through the air-core effective cross-sectional area of the coil. The amount of rush current corresponding to this magnetic flux will act as a rush current, whose value is very large, culculated from ##EQU1## where .mu..sub.0 is the permeability of air; n is the number of turns of the primary winding; A is the air-core effective cross-sectional area. As an actually measured value, there is a current value of about 150 A recorded during a period ranging from a half-cycle to 2 cycles.
This rush current has various adverse effects on the appliance itself and on the power source connected thereto. For example, it imposes an excessive load on the switch or relay contacts, sometimes actuating the breaker associated with the power source. Therefore, it is necessary to take measures to avoid rush current.
FIG. 1 shows a circuit conventionally used as such a measure. In the figure, power is supplied through a fuse 1, a timer switch 2 is closed, a door is closed, a door switch 3 is opened, a door switch 4 and a door switch 5 are closed, and a start button is pressed to close a start switch 6, whereupon a current flows through the primary coil of a transformer 8 via a resistor 7 for rush current prevention purposes and a voltage is induced between the 8-a terminal and 8-b terminals of the primary coil to produce a current flowing through the coil 9-a of a relay, closing the contact 9-b of the relay to complete the main circuit, with the relay 9 self-holding to supply power to a magnetron 10, thereby generating microwaves.
In this arrangement, since the transformer 8 is excited by a voltage which is lower than the value for normal operation applied thereto, said voltage being supplied for about 20 msec from the time the start switch 6 is closed until the time the relay contact 9-b is closed, the rush current at the time when the relay contact 9-b is closed so as to complete the main circuit is reduced to a low value. Furthermore, an arrangement for ensuring that upon the opening of the door, the electric waves do not leak, i.e., an arrangement for preventing generation of microwaves upon opening of the door is essential, and its reliability must be very high.
Thus, generally, as shown in FIG. 1, the door switch 4 and door switch 5, adapted to be opened and closed when the door is opened and closed, are provided at different positions on the door to monitor the opening and closing of the door. When the door is opened during operation, however, it cannot absolutely be denied that the load which results from cutting off the circuit and which acts on the door switch 4 or door switch 5 upon opening of the door will never cause troubles such as a fusion of the contact. Thus, in order to further increase reliability so as to ensure that the high frequency heaing appliance will operate satisfactorily throughout its life, an arrangement is made for detecting troubles with the switch 4 so as to cut off the power circuit to thereby stop high frequency oscillation. That is, in this arrangement, if the door switch 4 is damaged and shorted, opening the door closes the door
REFERENCES:
patent: 3699300 (1972-10-01), Buerki
patent: 3816688 (1974-06-01), Fritts
patent: 4054769 (1977-10-01), Anderson et al.
patent: 4321447 (1982-03-01), Lamb
patent: 4345135 (1982-08-01), Harmon
Haruhisa Tamura
Yoshiyuki Takada
Albritton C. L.
Lateef M. M.
Matsushita Electric - Industrial Co., Ltd.
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