Work coil for use in an induction cooking appliance

Abstract

An induction cooking appliance for operation at ultrasonic frequencies and including an improved work coil coupled in a solid state inverter circuit to generate an ultrasonic frequency magnetic field which is coupled to a metallic utensil through a non-electrically conductive cooktop. Coil losses are minimized, and coupling efficiency increased by forming the coil of solid conductor wound in a loose spiral, with the pitch of the spiral being related to the radial thickness of the conductor by a factor falling within a predetermined range. The loosely wound coil is held in a grooved dielectric support which insures proper turn spacing for the life of the appliance and which is immune to high temperatures which may be developed during operation of the coil. Means are provided under the support to minimize stray radiation and limit inductance variations which might otherwise result from nearby metallic members. Means are provided for holding the coil adjacent the underside of the cooktop for optimum coupling and to utilize heat developed in the coil. The loosely wound configuration is advantageously fabricated by methods including interleaving the coil conductor with an insulative strip. Alternatively, an open coil may be formed by winding the conductor on a frusto-conical mandrel whose degree of taper is related to the desired pitch of the coil, and compressing the coil into a planar configuration.