Electric heating – Metal heating – Of cylinders
Patent
1981-05-18
1984-08-21
Envall, Jr., Roy N.
Electric heating
Metal heating
Of cylinders
219 1049R, 363 21, H05B 608
Patent
active
044671652
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an induction heating apparatus having a transistor as a switching element for generating high frequency energy for heating utensils for cooking foodstuff.
TECHNOLOGICAL BACKGROUND
Over the past, thyristor inverters or transistor inverters have been employed as a high frequency energy source of induction heating apparatus in which a magnetic material is heated by eddy currents generated therein as a result of a high frequency magnetic field produced by a heating coil. Output power control methods which have been proposed are classified broadly into three categories. A first method involves carying the d.c. power voltage of the inverter, a second method involves periodically inhibiting the inverter which is called "duty cycle control", and the third method is to control the oscillating frequency of the inverter (ON-OFF duty control). However, the first method has resulted in a high apparatus cost, the second method has presented a lamp flicker problem and the problem of lengthened warmup periods, and the third method has also presented problems in that it required the use of a heavy duty power switching transistor to bear the burden of transient surge currents and high potentials. In the case of thyristor inverters, in particular, output power is variable as a function of the oscillating high frequency. Theoretically, the power loss of the switching power transistor can be decreased by lowering the oscillating frequency. However, this is achieved only at the expense of a noise generated when the frequency becomes lower than the inaudible limit. Therefore, the inaudible frequency limit sets the lower limit of power control range. Whereas, the allowable values of the surge current and high potential for the thyristor set the upper limit on the variable frequency range and hence the upper limit of the power control range. Thus, a wide range of power control was impossible with the prior art thyristor inverters. On the other hand, the problem associated with transistor inverters is concerned with difficulty in providing a wide range power control without imposing heavy burden on the switching transistor.
DISCLOSURE OF THE INVENTION
The present invention is to eliminate the aforesaid problem by varying the on-off ratio of the switching transistor without imposing heavy burden thereon. In accordance with the invention, the inverter output power is varied as a function of the conduction period of the switching transistor. The apparatus is characterized by the fact that when the power level is decreased the transistor's current and voltage decrease therewith while the oscillating frequency increases.
Another object of the invention is to achieve a quick response power control in a continuously variable range and to set the input current, i.e., the input power to a user's desired setting and the output power is delivered at an 80 per cent of the input power. The apparatus of the invention is capable of providing power control continuously over a range of 50 watts to 1500 watts, for example, without producing lamp flicker. The apparatus can be manufactured in a simplified circuit design which renders it economical.
Another object of the invention is to provide an induction heating apparatus which is stable in operation under conditions of varying loads by detecting the inverter input current and the collector voltage of the switching transistor and controlling the power level in response to the detected operating parameters through a feedback loop.
Additional object of the invention is to provide an induction heating apparatus which employs a pulse width control circuit including a synchronous ramp generator which permits wide range power control.
The invention will now be described with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an embodiment of the induction heating apparatus of the present invention;
FIG. 2 is a detailed circuit diagram of the induction heating apparatus of FIG. 1;
FIG. 3 is a waveform di
REFERENCES:
patent: 3786219 (1974-01-01), Kornrumpf et al.
patent: 3898410 (1975-08-01), Peters, Jr.
patent: 4209683 (1980-06-01), Kiuchi et al.
patent: 4277667 (1981-07-01), Kiuchi
patent: 4356371 (1982-10-01), Kiuchi et al.
Kiuchi Mitsuyuki
Mizukawa Takumi
Envall Jr. Roy N.
Lateef M. M.
Matsushita Electric - Industrial Co., Ltd.
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