GB971315A - Electric oscillation generating system - Google Patents

Abstract

971,315. Radio-frequency couplings; valve oscillator circuits. COMPAGNIE FRANCAISE THOMSON-HOUSTON. March 28, 1962 [March 28, 1961], No. 11872/62. Headings H3F and H4A. Relates to the circuit arrangements between a self-excited valve or transistor oscillator 16, 10 Fig. 1, and a load impedance 1, 2 having a widely-varying resistive component due to coupling with, for example, a workpiece in eddycurrent or dielectric heating apparatus. According to the invention reactances 6, 7 are of 'opposite sign, the primary circuit incorporating reactances 6, 7 and the secondary circuit incorporating a reactance 5 and impedance 1, 2 are over-coupled, reactance 5 is capacitative when impedance 1, 2 is inductive and vice-versa, and reactance 6 or 7 is adjustable to vary the tuned frequency of the circuit 6, 7 to encompass the tuned frequency of the secondary circuit. A predetermined power output can thereby be maintained; the adjustment of reactance 6 or 7 may be manual, or automatic by known means not described. The circuit operation is described in the Specification with particular reference to the effects of the over-coupling, Fig. 2 (not shown). In one embodiment, Fig. 3, the variable reactance is a ferrite-cored inductor 7a and the ferromagnetic workpiece is in the field of inductor 1a; operation is at audio or radio frequencies and the oscillations are maintained by coupling 10. In operation core 23 is first pushed in to give a low oscillation frequency, Fig. 2 (not shown), before applying anode voltage, the core is then pulled out, after applying anode voltage, until full output power is achieved. As the Curie point of the workpiece is reached the output load impedance falls and the core is pulled out further to maintain maximum output power. Similarly any load impedance changes due to additional charges of material can be compensated. The manual adjustment may be made in response to temperature readings. Graphs illustrating the performance of the embodiment are described, Fig. 4 (not shown); the oscillation frequency is stated to vary over a relatively narrow range. In the dielectric heating embodiment, Fig. 5, the additional secondary circuit reactance is inductor 5b and the variable primary circuit reactance is capacitor 7b, which is initially set to a relatively low capacitance value. To hold the oscillation frequency constant a variable inductor 5b and/or a variable capacitor 28 is added. In the third embodiment two frequencies of operation of eddy-current heating apparatus are provided, Fig. 6 (not shown).