Analysis and design of a ferroresonant regulator for microwave cooking appliances

Abstract

Ferroresonant leakage transformer is used for microwave cooking appliances in order to regulate the power fluctuation in the magnetron. This becomes feasible by using the saturation characteristics of the ferromagnetic core and providing a leakage flux path between the primary and the secondary windings. To supply a pulsed direct current to the magnetron, a diode is used in the secondary circuit of the transformer. A capacitor connected in series with the secondary winding of the transformer makes ferro-resonance with the secondary ferromagnetic core. The flux due to the secondary capacitive current saturates the secondary core and returns through the leakage path. In order to understand the mechanism of the ferroresonant regulator and provide its simple design procedures, nonlinear characteristics of the transformer and the load (the magnetron and the diode) are idealized to be piecewise linear. For this approximation, the transformer is represented by an equivalent pi-circuit which is more convenient in analyzing the nonlinearity than a well-known equivalent T-circuit. Dipped peak shape of the magnetron current is explained qualitatively by considering the fundamental and third harmonic components in the nonlinear equivalent circuit. Design equations providing the values of the leakage inductance, turn ratio of the transformer, and the capacitance are derived analytically by considering the fundamental frequency component only. For a required power regulation and input power factor, these three parameters are determined from the derived design equations and compared with the computer simulation and experiment. For the computer simulation, more accurate magnetization curve is used for the ferromagnetic core. Design examples are presented.